Decoding emf. Basics of electrocardiography Vector emf of the heart its construction clinical significance

Details

Electrical and mechanical processes occur in the heart.
Electrical processes: automaticity, excitement, conduction. Studied using ECG.
Mechanical processes: contraction, relaxation. They are studied using numerous methods for measuring the pressure and volume of blood in the cavities of the heart.

ELECTROCARDIOGRAPHY.

ECG is a recording of biopotentials (which arise in the heart during the spread of excitation) using electrodes located on the surface of the body. An ECG helps determine the location of the impulse (pacemaker) and the nature of the spread of excitation throughout the myocardium of the atria and ventricles.

GENESIS OF TEETH:(See ECG diagram):

• the P wave reflects the process of atrial depolarization;
• the PQ segment (isoelectric line) reflects the conduction time through the AV node (atrioventricular delay);
• the QRS wave complex reflects the process of ventricular depolarization;
• ST segment (isoelectric line) – complete excitation of all ventricular cardiomyocytes (coincides with the “plateau” phase of the action potential);
• the T wave reflects the process of ventricular repolarization.

DIPOLE CONCEPT.

The surface of the excited area of ​​the myocardium is negatively charged, the surface of the non-excited area of ​​the myocardium is positively charged. At the interface between excited and non-excited areas of the myocardium, many dipoles are formed.

Dipole is a collection of two point electric charges (equal in magnitude and opposite in sign), located at a vanishingly small distance from each other. The dipole vector has a direction from (-) to (+).

The dipole vectors can be summed up:

(1) if the vectors are directed in the same direction, the second is added to the first vector;

(2) if the vectors are directed in opposite directions, the smaller one is subtracted from the larger vector;

(3) if the vectors are directed at an angle to each other, they are added according to the “parallelogram” rule.

As a result of adding the vectors of all dipoles, the total torque vector (vector of the cardiac emf) is obtained. The projection of the total moment vector onto the lead axis corresponds to a specific wave on the ECG curve.

An ECG lead is the placement of two electrodes on the surface of the body(at certain points). The line connecting two electrodes is called abduction axis. The lead axis has a certain polarity: one of the electrodes is “negative” (-), i.e. the signal from it is fed to the negative “input” of the electrocardiograph, the other electrode is “positive” (+), i.e. the signal from it is fed to the positive “input” of the electrocardiograph.

When examining patients, at least 12 leads: 3 standard limb leads (I, II and III); 3 enhanced limb leads (AVR, AVL, AVF) and 6 chest leads (V1 – V6).

Standard limb leads.

Bipolar (two-pole) – both electrodes are active. The axes of these leads represent the sides of Einthoven's triangle:
1 standard answer: right hand (-) and left hand (+)
II standard answer: right hand (-) and left leg (+)
III standard answer: left arm (-) and left leg (+)

Reinforced limb leads.

Unipolar (single-pole)– one electrode is active and the other is passive (indifferent, reference electrode, zero).

AVR: active electrode on the right hand (+); the electrodes of the other two limbs are connected and through additional resistance they supply a signal (the potential is close to zero) to the negative “input” of the electrocardiograph.

AVL: active electrode on the left hand (+); the electrodes of the other two limbs are connected and through additional resistance they supply a signal (the potential is close to zero) to the negative “input” of the electrocardiograph.

AVF: active electrode on the left leg (+); the electrodes of the other two limbs are connected and through additional resistance they supply a signal (the potential is close to zero) to the negative “input” of the electrocardiograph.

The axes of all limb leads are located in the frontal plane. For ECG analysis, they can be combined into a common six-axis coordinate system.

Chest leads: unipolar (unipolar) - one active electrode, located at a certain point on the surface of the chest (+); the other is a reference electrode (zero) obtained by connecting all three electrodes of the limbs. The signal from it is fed through an additional resistance to the negative “input” of the electrocardiograph.
The axes of the chest leads are located in the horizontal plane.

Vectors cardiac emf.

• Vector P – atrial vector – is directed from top to bottom, from right to left. Vector Q - the 1st vector of ventricular depolarization - is directed from bottom to top, from left to right (0.02 seconds from the beginning of ventricular depolarization; excitation of the lower part of the interventricular septum).
• Vector R - the 2nd vector of ventricular depolarization - is directed from top to bottom, from right to left (0.04 seconds from the beginning of ventricular depolarization; excitation spreads from the apex of the heart to the base of the ventricles, and from the endocardium to the epicardium).
• Vector S - the 3rd vector of ventricular depolarization - is directed from bottom to top, from left to right, (0.06 seconds from the beginning of ventricular depolarization; excitation of the base of the left ventricle).

Vector T is directed from top to bottom, from right to left (repolarization occurs in all parts of the ventricles, from the epicardium to the endocardium).

Projection of the total moment vector(P,Q,R,S,T) on the lead axis corresponds to a specific wave on the ECG curve. If the projection of the vector is directed to the (+) pole of the lead axis, the ECG wave is directed upward from the isoelectric line (positive wave). If the projection of the vector is directed to the (-) pole of the lead axis, the ECG wave is directed downward from the isoelectric line (negative wave). The amplitude of the wave is proportional to the length of the vector projection on the lead axis. If the vector runs parallel to the lead axis, its projection onto the axis of a given lead (and therefore the amplitude of the wave in a given lead) is maximum. If the vector passes perpendicular to the axis of the lead, its projection onto the axis of this lead is zero (which means there is no tooth in this lead).

Electrical axis of the heart.

This is the projection of the average resulting vector of ventricular depolarization onto the frontal plane. The average resulting vector of ventricular depolarization is obtained by summing three moment vectors – Q, R and S. The directions of the electrical and anatomical axes of the heart in an adult healthy person coincide. For asthenics this direction is more vertical (pravogramma), for hypersthenics it is more horizontal (levogramma).

Among the numerous instrumental methods for studying a cardiac patient, the leading place belongs to electrocardiography (ECG). This method is indispensable in everyday clinical practice, helping the doctor to timely diagnose cardiac rhythm and conduction disturbances, myocardial infarction and unstable angina, episodes of silent myocardial ischemia, hypertrophy or overload of the ventricles of the heart and atria, cardiomyopathies and myocarditis, etc.

Methods for recording a 12-lead electrocardiogram and the basic principles of analyzing a traditional ECG have not changed much recently and are fully applicable to the assessment of many modern methods for studying the electrical activity of the heart - long-term Holter ECG monitoring, results of functional stress tests, automated systems for recording and analyzing electrocardiograms and other methods.

Keywords: electrocardiography, rhythm and conduction disorders, ventricular and atrial myocardial hypertrophy, coronary heart disease, myocardial infarction, electrolyte disorders.

METHOD FOR REGISTRATION OF ELECTROCARDIOGRAM

Electrocardiographic leads. An electrocardiogram is a recording of oscillations in the potential difference that occur on the surface of excitable tissue or the conductive medium surrounding the heart as an excitation wave propagates through the heart. An ECG is recorded using electrocardiographs - devices that record changes in the potential difference between two points in the electrical field of the heart (for example, on the surface of the body) during its excitation. Modern electrocardiographs are distinguished by high technical perfection and allow both single-channel and multi-channel ECG recording.

Changes in the potential difference on the body surface that occur during heart activity are recorded using various ECG lead systems. Each lead records the potential difference that exists between two specific points in the electrical field of the heart at which the electrodes are installed. The latter are connected to the galvanometer of the electrocardiograph: one of the electrodes is connected to the positive pole of the galvanometer (this positive, or active, lead electrode), the second electrode - to its negative pole (negative, or indifferent, lead electrode).

Currently, 12 ECG leads are most widely used in clinical practice, the recording of which is mandatory for each electrocardiographic examination of a patient: 3 standard leads, 3 enhanced unipolar limb leads and 6 chest leads.

Standard leads

Standard bipolar leads, proposed in 1913 by Einthoven, record the potential difference between two points of the electric field, remote from the heart and located in the frontal plane - on the limbs. To record these leads, electrodes are placed on the right arm (red marking), left arm (yellow marking) and left leg (green marking) (Fig. 3.1). These electrodes are connected in pairs to the electrocardiograph to record each of the three standard leads. The fourth electrode is installed on the right leg to connect

ground wire (black marking). Standard limb leads are recorded with the following pairwise connection of electrodes:

Lead I - left hand (+) and right hand (-);

Lead II - left leg (+) and right arm (-);

III lead - left leg (+) and left arm (-).

The signs (+) and (-) here indicate the corresponding connection of the electrodes to the positive or negative pole of the galvanometer, i.e. The positive and negative poles of each lead are indicated.

Rice. 3.1. Scheme of formation of three standard electrocardiographic leads from the limbs.

Below is Einthoven’s triangle, each side of which is the axis of one or another standard lead

As can be seen in Fig. 3.1, three standard leads form an equilateral triangle (Einthoven triangle), the vertices of which are the right arm, left arm and left leg with electrodes installed there. In the center of Einthoven's equilateral triangle is the electrical center of the heart, or a single point cardiac dipole, equally distant from all three standard leads. The hypothetical line connecting the two electrodes involved in the formation of the electrocardiographic lead is called the lead axis. The axes of standard leads are the sides of Einthoven's triangle. Perpendiculars drawn from the center of the heart, i.e. from the location of a single

cardiac dipole, to the axis of each standard lead, each axis is divided into two equal parts: positive, facing the positive (active) electrode (+) lead, and negative, facing the negative electrode (-).

Reinforced limb leads

Enhanced limb leads were proposed by Goldberger in 1942. They record the potential difference between one of the limbs on which the active positive electrode of this lead is installed (right arm, left arm or left leg), and the average potential of the other two limbs (Fig. 3.2) . Thus, the so-called combined Goldberger electrode, which is formed when two limbs connect through additional resistance. The three enhanced unipolar limb leads are designated as follows:

AVR - enhanced abduction from the right hand;

AVL - enhanced abduction from the left arm;

AVF - increased abduction from the left leg.

The designation of enhanced limb leads comes from the first letters of English words: “a” - augemented (reinforced); “V” - voltage (potential); “R” - right (right); “L” - left (left); “F” - foot (leg).

Rice. 3.2. Scheme of the formation of three reinforced unipolar leads from the limbs.

Below - Einthoven's triangle and the location of the axes of three reinforced unipolar limb leads

As can be seen in Fig. 3.2, the axes of reinforced unipolar leads from the limbs are obtained by connecting the electrical center of the heart with the location of the active electrode of this lead, i.e. in fact, from one of the vertices of the Einthoven triangle. The electrical center of the heart, as it were, divides the axes of these leads into two equal parts: positive, facing the active electrode, and negative, facing the combined Goldberger electrode.

Six-axis coordinate system

Standard and enhanced unipolar limb leads make it possible to record changes in cardiac EMF in the frontal plane, i.e. in the plane in which the Einthoven triangle is located. For a more accurate and visual determination of various deviations of the EMF of the heart in this frontal plane, the so-called six-axis coordinate system. It is obtained by combining the axes of three standard and three reinforced leads from the limbs, drawn through the electrical center of the heart. The latter divides the axis of each lead into positive and negative parts, facing respectively the active (positive) or negative electrode (Fig. 3.3).

Rice. 3.3. Six-axis coordinate system according to Bayley. Explanation in the text

Electrocardiographic deviations in different limb leads can be considered as different projections of the same cardiac EMF on the axis of these leads. Therefore, by comparing the amplitude and polarity of electrocardiographic complexes in various leads that are part of the six-axis coordinate system, it is possible to quite accurately determine the magnitude and direction of the EMF vector of the heart in the frontal plane.

The direction of the lead axes is usually determined in degrees. The reference point (0) is conventionally taken to be a radius drawn strictly horizontally from the electrical center of the heart to the left towards the positive pole of standard lead I. The positive pole of standard lead II is located at an angle of +60°, lead aVF is at an angle of +90°, standard lead III is at an angle of +120°, aVL is at an angle of -30°, and aVR is at an angle of -150° to the horizontal. The axis of lead aVL is perpendicular to axis II of the standard lead, axis I of the standard lead is perpendicular to the axis aVF, and the axis aVR is perpendicular to axis III of the standard lead.

Chest leads

Single-pole chest leads, proposed by Wilson in 1934, record the potential difference between an active positive electrode installed at certain points on the surface of the chest (Fig. 3.4) and Wilson's negative combined electrode. The latter is formed by connecting three limbs (right arm, left arm and left leg) through additional resistances, the combined potential of which is close to zero (about 0.2 mV).

Typically, 6 positions of active electrodes on the chest are used to record an ECG:

Lead V1 - in the IV intercostal space along the right edge of the sternum;

Lead V2 - in the IV intercostal space along the left edge of the sternum;

Lead V3 - between the second and fourth positions (see below), approximately at the level of the V rib along the left parasternal line;

Lead V4 - in the V intercostal space along the left midclavicular line;

Lead V5 - at the same horizontal level as V4, along the left anterior axillary line;

Lead V6 - along the left midaxillary line at the same horizontal level as the electrodes of leads V4 and V5.

Rice. 3.4. Places of application of 6 chest electrodes

Unlike standard and enhanced limb leads, chest leads record changes in cardiac EMF predominantly in the horizontal plane. As shown in Fig. 3.5, the axis of each chest lead is formed by a line connecting the electrical center of the heart with the location of the active electrode on the chest. The figure shows that the axes of leads V1 and V5, as well as V 2 and V 6 are approximately perpendicular

each other.

Additional leads

The diagnostic capabilities of electrocardiographic examination can be expanded with the use of some additional leads. Their use is especially advisable in cases where the usual program for recording 12 generally accepted ECG leads does not allow one to reliably diagnose a particular electrocardiographic pathology or requires clarification of some quantitative parameters of the identified changes.

Rice. 3.5. Location of the axes of 6 chest electrocardiographic leads in the horizontal plane

The method of recording additional chest leads differs from the method of recording 6 conventional chest leads only in the localization of the active electrode on the surface of the chest. A combined Wilson electrode is used as an electrode connected to the negative pole of the cardiograph.

Unipolar leads V7-V9 is used for more accurate diagnosis of focal myocardial changes in the posterobasal regions of the LV. Active electrodes are installed along the posterior axillary (V7), scapular (V 8) and paravertebral (V9) lines at the horizontal level, on which electrodes V4-V6 are located (Fig. 3.6).

Rice. 3.6. Location of electrodes of additional chest leads V7-V9 (a) and the axes of these leads in the horizontal plane (b)

Bipolar leads according to Neb. To record these leads, electrodes are used to record three standard limb leads. Electrode usually placed on the right hand (red wire marking), placed in the second intercostal space along the right edge of the sternum; left leg electrode (green marking) moved to the position of chest lead V 4 (at the apex of the heart), and the electrode located on the left arm (yellow marking), placed at the same horizontal level as the green electrode, but along the posterior axillary line (Fig. 3.7). If the electrocardiograph lead switch is in position I of the standard lead, the “Dorsalis” lead (D) is recorded. By moving the switch to standard leads II and III, the “Inferior” (I) and “Anterior” (A) leads are recorded, respectively. Neb leads are used to diagnose focal changes in the myocardium of the posterior wall (lead D), anterolateral wall (lead A) and upper parts of the anterior wall (lead I).

Leads V3R-V6R, the active electrodes of which are placed on the right half of the chest (Fig. 3.8), are used to diagnose hypertrophy of the right heart and focal changes in the RV.

Rice. 3.7. Location of electrodes and axes of additional chest leads according to Neb

Rice. 3.8. Location of electrodes of additional chest leads

Electrocardiogram recording technique

To obtain a high-quality ECG recording, you must strictly adhere to some general rules for its registration.

Conditions for conducting the study. The ECG is recorded in a special room, remote from possible sources of electrical interference: physiotherapy and X-ray rooms, electric motors, electrical distribution panels, etc. The couch must be located at a distance of at least 1.5-2 m from the electrical wires. It is advisable to shield the couch by placing a blanket under the patient with a sewn-in metal mesh, which must be grounded.

The study is carried out after 10-15 minutes of rest and no earlier than 2 hours after eating. An ECG is usually recorded with the patient lying on his back, which allows for maximum muscle relaxation. The patient's last name, first name and patronymic, his age, date and time of the study, medical history number and diagnosis are preliminarily recorded.

Application of electrodes. 4 plate electrodes are applied to the inner surface of the shins and forearms in the lower third using rubber bands or special plastic clamps, and one or more are installed on the chest (if

multi-channel recording) chest electrodes using a rubber suction bulb or adhesive disposable chest electrodes. To improve the contact of the electrodes with the skin and reduce interference and inducted currents in the areas where the electrodes are applied, it is necessary to first degrease the skin with alcohol and cover the electrodes with a layer of special conductive paste, which allows you to minimize the interelectrode resistance.

When applying electrodes, you should not use gauze pads between the electrode and the skin, moistened with a solution of 5-10% sodium chloride solution, which usually dry out quickly during the study, which sharply increases the electrical resistance of the skin and the possibility of interference during ECG recording.

Connecting wires to electrodes. Each electrode, installed on the limbs or on the surface of the chest, is connected to a wire coming from the electrocardiograph and marked with a certain color. The following marking of input wires is generally accepted: right hand - red; left hand - yellow; left leg - green; right leg (patient grounding) - black; chest electrode - white.

If you have a 6-channel electrocardiograph that allows you to simultaneously record an ECG in 6 chest leads, a wire with a red tip marking is connected to electrode V1; to electrode V2 - yellow, V3 - green, V4 - brown, V5 - black and V6 - blue or purple. The markings of the remaining wires are the same as in single-channel electrocardiographs.

Selecting the electrocardiograph gain. Before you start recording an ECG, you must set the same amplification of the electrical signal on all channels of the electrocardiograph. To do this, each electrocardiograph has the ability to supply a standard calibration voltage of 1 mV to the galvanometer (Fig. 3.9).

Typically, the gain of each channel is selected so that a voltage of 1 mV causes a deflection of the galvanometer and recording system of 10 mm. To do this, in the lead switch position “0”, the gain of the electrocardiograph is adjusted and the calibration millivolt is recorded. If necessary, you can change the gain: reduce it if the amplitude of the ECG waves is too large (1 mV = 5 mm) or increase it if their amplitude is small (1 mV equals 15 or 20 mm).

Rice. 3.9. ECG recorded at 50 mm? with -1 (a) and 25 mm? s -1 (b).

A reference millivolt is shown at the beginning of each ECG recording.

Modern electrocardiographs provide automatic gain calibration.

Recording an electrocardiogram. ECG recording is carried out during quiet breathing. First, the ECG is recorded in standard leads (I, II, III), then in enhanced limb leads (aVR, aVL and aVF) and chest leads (V1-V6). At least 4 cardiac cycles are recorded in each lead. ECG is usually recorded at a paper speed of 50 mm? s -1 . A lower speed (25 mm? s -1) is used when longer ECG recordings are necessary, for example, to diagnose rhythm disturbances.

ELECTROCARDIOGRAM ANALYSIS

To avoid errors in the interpretation of electrocardiographic changes, when analyzing any ECG, it is necessary to strictly adhere to a certain decoding scheme, which is given below.

General scheme (plan) of ECG decoding

I. Heart rate and conduction analysis:

assessment of heart rate regularity;

Counting the number of heartbeats;

Determining the source of excitation;

Conduction function assessment.

II. Determination of heart rotations around the anteroposterior, longitudinal and transverse axes:

determination of the position of the electrical axis of the heart in the frontal plane;

Determination of heart rotation around the longitudinal axis;

Determination of heart rotation around the transverse axis.

III. Atrial P wave analysis.

IV. Analysis of the ventricular QRS-T complex:

QRS complex analysis;

RS-T segment analysis;

T wave analysis;

QT interval analysis.

V. Electrocardiographic report.

Heart rate and conduction analysis

The regularity of heartbeats is assessed by comparing the duration of R-R intervals between successively recorded cardiac cycles. Regular, or correct, the heart rhythm is diagnosed if the duration of the measured R-R intervals is the same and the spread of the obtained values ​​does not exceed ± 10% of the average duration of the R-R intervals (Fig. 3.10 a). In other cases, an incorrect (irregular) heart rhythm is diagnosed (Fig. 3.10 b, c).

At wrong rhythm count the number of QRS complexes recorded over a certain period of time (for example, 3 s). Multiplying this result in this case by 20 (60 s: 3 s = 20), the heart rate is calculated. If the rhythm is incorrect, you can also limit yourself to determining the minimum and maximum heart rate. The minimum heart rate is determined by the duration of the longest R-R interval, and the maximum by the shortest R-R interval.

For determining the source of excitation, or the so-called pacemaker, it is necessary to evaluate the course of excitation in the atria and establish the ratio of the R waves to the ventricular QRS complexes (Fig. 3.11). In this case, you should focus on the following signs:

1. Sinus rhythm(Fig. 3.11 a):

a) PII waves are positive and precede each ventricular QRS complex;

b) the shape of all P waves in the same lead is the same.

2. Atrial rhythms(from the lower sections) (Fig. 3.11 b):

a) waves PII and PIII are negative;

b) each P wave is followed by unchanged QRS complexes.

3. Rhythms from the AV connection(Fig. 3.11 c, d):

Rice. 3.11. ECG for sinus and non-sinus rhythms:

a - sinus rhythm; b - lower atrial rhythm; c, d - rhythms from the AV connection; d - ventricular (idioventricular) rhythm

a) if the ectopic impulse simultaneously reaches the atria and ventricles, there are no P waves on the ECG, which merge with the usual unchanged QRS complexes;

b) if the ectopic impulse first reaches the ventricles and only then the atria, negative RP and RS are recorded on the ECG, which are located after the usual unchanged QRS complexes.

4. Ventricular (idioventricular) rhythm(Fig. 3.11 d):

a) all QRS complexes are widened and deformed;

b) there is no regular connection between QRS complexes and P waves;

c) the number of heart contractions does not exceed 40-60 beats. per minute). Conduction function assessment. For preliminary assessment

conductivity function (Fig. 3.12) it is necessary to measure the duration:

1) the P wave, which characterizes the speed of electrical impulse transmission through the atria (normally no more than 0.1 s);

2) P-Q(R) intervals in standard lead II, reflecting the overall conduction velocity in the atria, AV junction and His system (normally from 0.12 to 0.2 s);

3) ventricular QRS complexes (conduction of excitation through the ventricles), which normally ranges from 0.08 to 0.09 s.

An increase in the duration of these waves and intervals indicates a slowdown in conduction in the corresponding part of the conduction system of the heart.

Rice. 3.12. Assessment of conduction function using ECG. Explanation in the text

After this they measure internal deviation interval in the chest leads V1 and V6, indirectly characterizing the speed of propagation of the excitation wave from the endocardium to the epicardium of the right and left ventricles, respectively. The internal deviation interval is measured from the beginning of the QRS complex in a given lead to the apex of the R wave.

DETERMINATION OF HEART ROTATIONS AROUND ANTEROPOSTERIOR, LONGITUDINAL AND TRANSVERSE AXES

Determination of the position of the electrical axis of the heart

Rotations of the heart around the anteroposterior axis are accompanied by a deviation of the electrical axis of the heart (the average resulting vector A QRS) in the frontal plane and a significant change in the configuration of the QRS complex in standard and enhanced unipolar limb leads.

There are the following options for the position of the electrical axis of the heart (Fig. 3.13):

Rice. 3.13. Various options for the position of the electrical axis of the heart

1) normal position, when the angle α is from +30° to +69°;

2) vertical position - angle α from +70° to +90°;

3) horizontal - angle α from 0° to +29°;

4) axis deviation to the right - angle α from +91° to ±180°;

5) axis deviation to the left - angle α from 0° to -90°.

To accurately determine the position of the electrical axis of the heart graphical method it is enough to calculate the algebraic sum of the amplitudes of the QRS complex waves in any two leads from the limbs, the axes of which are located in the frontal plane. Typically, standard leads I and III are used for this purpose. The positive or negative value of the algebraic sum of the QRS complex waves on an arbitrarily selected scale is plotted on the positive or negative part of the axis of the corresponding lead in the Bayley six-axis coordinate system. Typically, charts and tables given in special manuals on electrocardiography are used for this purpose.

A simpler, although less accurate way to assess the position of the electrical axis of the heart is visual angle determinationα. The method is based on two principles:

1. Maximum positive (or negative) the value of the algebraic sum of the teeth of the QRS complex is recorded in the electrocardiographic lead, the axis of which approximately coincides with the location of the electrical axis of the heart and the average resulting QRS vector is deposited on the positive (or, accordingly, negative) part of the axis of this lead.

2. Complex type RS, where the algebraic sum of the teeth is equal to zero (R = S or R = Q + S), is written in the lead whose axis is perpendicular to the electrical axis of the heart.

Table 3.2 shows the leads in which, depending on the position of the electrical axis of the heart, there is a maximum positive, maximum negative algebraic sum of the teeth of the QRS complex and an algebraic sum of the teeth equal to zero.

Table 3.2

Configuration of the QRS complex depending on the position of the electrical axis of the heart

As an example, Figures 3.14-3.21 show ECGs at different positions of the electrical axis of the heart. From the table and figures it is clear that when:

1) normal position of the electrical axis of the heart (angle α from +30° to +69°), amplitude Rh > Ri > Rm, and in leads III and/or aVL the R and S teeth are approximately equal to each other;

2) horizontal position of the electrical axis of the heart (angle α from 0° to +29°), amplitude Ri > Rh > Riii, and in leads aVF and/or III an RS type complex is recorded;

3) vertical position of the electrical axis of the heart (angle α from +70° to +90°), amplitude Rn > Rm > Ri, and in leads I and/or aVL an RS type complex is recorded;

4) deviation of the electrical axis of the heart to the left(angle α from 0° to -90°) the maximum positive sum of waves is recorded in leads I and/or aVL (or aVL and aVR), in leads aVR, aVF and/or II or I an RS type complex is recorded and there is a deep S wave in leads III and/or aVF;

5) when deviation of the electrical axis of the heart to the right(angle α from 91° to ±180°) the maximum R wave is fixed in leads aVF and/or III (or aVR), the RS type complex is in leads I and/or II (or aVR), and the deep S wave is in leads aVL and/or I.

Rice. 3.14. Normal position of the electrical axis of the heart. Angle α +60°

Rice. 3.15. Normal position of the electrical axis of the heart. Angle α +30°

Rice. 3.16. Vertical position of the electrical axis of the heart. Angle α +90°

Rice. 3.17. Horizontal position of the electrical axis of the heart. Angle α 0°

Rice. 3.18. Horizontal position of the electrical axis of the heart. Angle α +15°

Rice. 3.19. Deviation of the electrical axis of the heart to the left. Angle α -30°

Rice. 3.20. A sharp deviation of the electrical axis of the heart to the left. Angle α -60°

Rice. 3.21. Deviation of the electrical axis of the heart to the right. Angle α +120°

Rice. 3.22. The shape of the ventricular QRS complex in the chest leads when the heart rotates around the longitudinal axis (modification of diagram A.3. Chernov and M.I. Kechker, 1979)

Determination of heart rotation around the longitudinal axis

The rotations of the heart around the longitudinal axis, conventionally drawn through the apex and base of the heart, are determined by the configuration of the QRS complex in the chest leads, the axes of which are located in the horizontal plane. To do this, it is usually necessary to establish the localization of the transition zone, as well as evaluate the shape of the QRS complex in the lead

At normal heart position in the horizontal plane (Fig. 3.22a), the transition zone is most often located in lead V3. In this lead, R and S waves of equal amplitude are recorded. In lead V 6, the ventricular complex usually has the shape of qR or qRs.

When the heart rotates around its longitudinal axis clockwise(if you monitor the rotation of the heart from below from the apex), the transition zone shifts slightly to the left, to the region of lead V4, and in lead V 6 the complex takes the form RS (Fig. 3.22b). When the heart rotates around its longitudinal axis counterclockwise, the transition zone may shift to the right to lead V2. In leads V6, V5, a deepened (but not pathological) Q wave is recorded, and the QRS complex takes the form qR (Fig. 3.22c).

Rice. 3.23. Combination of rotation of the heart around the longitudinal axis clockwise with rotation of the electrical axis of the heart to the right (angle α +120°)

Rice. 3.24. Combination of rotation of the heart around the longitudinal axis counterclockwise with a horizontal position of the electrical axis of the heart (angle α +15°)

It should be remembered that rotations of the heart around the longitudinal axis clockwise often combined with a vertical position of the electrical axis of the heart or deviation of the heart axis to the right (Fig. 3.23), and counterclockwise turns with a horizontal position or deviation of the electrical axis to the left (Fig. 3.24).

Determination of heart rotation around the transverse axis

Rotations of the heart around the transverse axis are usually associated with deviation of the apex of the heart forward or backward relative to its normal position. When the heart rotates around the transverse axis with the apex forward (Fig. 3.25 b), the ventricular QRS complex in standard leads takes on the form qRi, qRn, qRm. On the contrary, when the heart rotates around the transverse axis with the apex backward, the ventricular complex in standard leads has the shape RS I, RSn, RSiii (Fig. 3.25 c).

Rice. 3.25. The ECG shape in three standard leads is normal (a) and when the heart rotates around the transverse axis with the apex forward (b) and the apex backward (c)

Atrial P wave analysis

P wave analysis includes:

Measuring the amplitude of the P wave (normally no more than 2.5 mm);

Measuring the duration of the P wave (normally no more than 0.1 s);

Determination of the polarity of the P wave in leads I, II, III;

Determination of the shape of the P wave.

1. When normal in the direction of movement of the excitation wave along the atria (from top to bottom and slightly to the left), the P waves in leads I, II and III are positive.

2. When the movement of the excitation wave is directed along the atria down up(if the pacemaker is located in the lower parts of the atria or in the upper part of the AB node) the P waves in these leads are negative.

3. Split with two peaks, the P wave in leads I, aVL, V5, V6 is characteristic of severe hypertrophy of the left atrium, for example, in patients with mitral heart defects (P-mitrale). Pointed high amplitude P waves in leads II, III, aVF (P-ri1topa1e) appear with hypertrophy of the right atrium, for example in patients with cor pulmonale (see below).

Analysis of the ventricular QRST complex The analysis of the QRS complex includes.

1. Assessment of the ratio of the Q, R, S waves in 12 leads, which allows you to determine the rotation of the heart around three axes.

2. Measuring the amplitude and duration of the Q wave. The so-called pathological Q wave is characterized by an increase in its duration of more than 0.03 s and an increase in the amplitude of more than Y4 of the amplitude of the R wave in the same lead.

3. Assessment of R waves with measurement of their amplitude, duration of the interval of internal deviation (in leads V 1 and V 6) and determination of possible splitting of the R wave or the appearance of a second additional R wave (γ) in the same lead.

4. Assessment of S waves with measurement of their amplitude, as well as determination of possible broadening, jaggedness or splitting of the S wave.

Analysis of the RS-T segment. Analyzing the state of the RS-T segment, you must:

Measure positive (+) or negative (-) deviation connection points(j) from the isoelectric line;

Measure the magnitude of what is possible RS-T segment offsets at a distance of 0.08 s to the right of connection point j;

Define form possible displacement of the RS-T segment: horizontal, oblique-downward or oblique-ascending displacement.

At T wave analysis should:

Determine the polarity of the T wave;

Assess the shape of the T wave;

Measure the amplitude of the T wave.

Normally, in most leads, except V1, V2 and aVR, the T wave is positive, asymmetrical (has a flat ascending bend and a slightly steeper descending bend). In lead aVR, the T wave is always negative; in leads V1-V2, III and aVF it can be positive, biphasic or weakly negative.

QT interval analysis includes its measurement from the beginning of the QRS complex (Q or R wave) to the end of the T wave and comparison with the proper value of this indicator, calculated using the Bazett formula:

where K is a coefficient equal to 0.37 for men and 0.40 for women; R-R - duration of one cardiac cycle.

Electrocardiographic report

The electrocardiographic report indicates:

1) main pacemaker: sinus or non-sinus (which one) rhythm;

2) regularity of heart rhythm: correct or incorrect rhythm;

3) number of heartbeats (HR);

4) position of the electrical axis of the heart;

5) the presence of four electrocardiographic syndromes: a) heart rhythm disturbances;

6) conduction disorders;

c) hypertrophy of the myocardium of the ventricles and/or atria, as well as their acute overload;

d) myocardial damage (ischemia, dystrophy, necrosis, scars, etc.).

LONG-TERM ECG MONITORING BY HOLTER

In recent years, long-term Holter ECG monitoring has become widespread in clinical practice. The method is used mainly for diagnostics transient heart rhythm disturbances, identifying ischemic ECG changes in patients with coronary artery disease, as well as for assessing heart rate variability. A significant advantage of the method is the possibility of long-term (within 1-2 days) ECG recording under conditions familiar to the patient.

A device for long-term Holter ECG monitoring consists of a lead system, a special device that records the ECG on magnetic tape, and a stationary electrocardiac analyzer. A miniature recording device and electrodes are attached to the patient's body. Typically, two to four precordial bipolar leads are used, corresponding, for example, to standard chest electrode positions V1 and V5. ECG recording is carried out on magnetic tape at a very low speed (25-100 mm? min -1). During the study, the patient keeps a diary in which data is entered on the nature of the load performed by the patient and on the patient’s subjective unpleasant sensations (pain in the heart, shortness of breath, interruptions, palpitations, etc.) indicating the exact time of their occurrence.

After the study is completed, a cassette with a magnetic recording of the ECG is placed in an electrocardiac analyzer, which automatically analyzes the heart rhythm and changes in the final part of the ventricular complex, in particular the RS-T segment. At the same time, an automatic printout of 24-hour ECG episodes identified by the device as rhythm disturbances or changes in the process of ventricular repolarization is performed.

Modern systems for long-term Holter ECG monitoring provide data presentation on a special paper tape in a compressed, compact form, which allows you to get a visual representation of the most significant episodes of heart rhythm disturbances and RS-T segment displacements. Information can also be presented in digital form and in the form of histograms reflecting the distribution of various heart rates, QT interval durations and/or arrhythmia episodes during the day.

Detection of arrhythmias

The use of long-term Holter ECG monitoring is part of a mandatory program for examining patients with cardiac arrhythmias or suspected of having such disorders. This method is most important in patients with paroxysmal arrhythmias. The method allows:

1) establish the fact of the occurrence of paroxysmal cardiac arrhythmias and determine their nature and duration, since many patients retain relatively short episodes of paroxysmal arrhythmias, which cannot be recorded for a long time using a classic ECG study.

2) study the correlation between paroxysms of rhythm disturbances and subjective and objective clinical manifestations of the disease (interruptions in the heart, palpitations, episodes of loss of consciousness, unmotivated weakness, dizziness, etc.).

3) to form an approximate idea of ​​the basic electrophysiological mechanisms of paroxysmal cardiac arrhythmias, since it is always possible to register the beginning and end of an attack of arrhythmias.

4) objectively evaluate the effectiveness of antiarrhythmic therapy.

Diagnosis of coronary heart disease

Long-term Holter ECG monitoring in patients with coronary artery disease is used to record transient changes in ventricular repolarization and cardiac arrhythmias. In most patients with coronary artery disease, the Holter ECG monitoring method allows obtaining additional objective confirmation temporary transient myocardial ischemia in the form of depression and/or elevation of the RS-T segment, often accompanied by changes in heart rate and blood pressure. It is important that continuous ECG recording is carried out under normal activity conditions for the patient. In most cases, this makes it possible to study the relationship between episodes of ischemic ECG changes and various clinical manifestations of the disease, including atypical ones.

The sensitivity and specificity of diagnosing ischemic heart disease using the 24-hour Holter ECG monitoring method depends primarily on

in total from the selected criteria for ischemic changes in the final part of the ventricular complex. Usually the same ones are used objective criteria for transient myocardial ischemia, as during stress tests, namely: displacement of the RS-T segment below or above the isoelectric line by 1.0 mm or more, provided that this displacement is maintained for 80 ms from the connection point (j). The duration of diagnostically significant ischemic displacement of the RS-T segment should exceed 1 minute.

An even more reliable and highly specific sign of myocardial ischemia is horizontal or oblique depression of the RS-T segment by 2 mm or more, detected within 80 ms from the beginning of the segment. In these cases, the diagnosis of IHD is practically beyond doubt, even in the absence of an angina attack at that moment.

Long-term Holter ECG monitoring is an indispensable research method for identifying episodes of the so-called asymptomatic myocardial ischemia, which are found in the majority of patients with coronary artery disease and are not accompanied by attacks of angina. In addition, it should be remembered that in some patients with verified coronary artery disease, displacement of the RS-T segment during daily life activities always occurs asymptomatically. According to the results of some studies, the predominance of asymptomatic episodes of myocardial ischemia in patients with documented coronary artery disease is a very unfavorable prognostic sign, indicating a high risk of acute repeated disturbances of coronary blood flow (unstable angina, acute myocardial infarction, sudden death).

The Holter ECG monitoring method is especially important in the diagnosis of the so-called variant Prinzmetal angina(vasospastic angina), which is based on spasm and short-term increase in the tone of the coronary artery. The cessation or sharp decrease in coronary blood flow usually leads to deep, often transmural, myocardial ischemia, a decrease in cardiac muscle contractility, contraction asynergy and significant electrical instability of the myocardium, manifested by rhythm and conduction disturbances. On the ECG, during attacks of variant Prinzmetal angina, a sudden rise in the RS-T segment above the isoline (transmural ischemia) is most often observed, although in some cases it may

depression (subendocardial ischemia) can also occur. It is important that these changes in the RS-T segment, as well as attacks of angina, develop at rest, more often at night, and are not accompanied (at least at the beginning of the attack) by an increase in heart rate by more than 5 beats per minute. This fundamentally distinguishes vasospastic angina from exertional angina attacks caused by increased myocardial oxygen demand. Moreover, an attack of vasospastic angina and ECG signs of myocardial ischemia may disappear, despite an increase in heart rate caused by a reflex response to pain, awakening and/or taking nitroglycerin (the phenomenon of “going through pain”).

Continuous ECG recording allows us to identify another important distinguishing feature of Prinzmetal's angina: the displacement of the RS-T segment at the beginning of the attack occurs very quickly, spasmodically, and also quickly disappears after the end of the spastic reaction. Angina pectoris, on the contrary, is characterized by a smooth gradual shift of the RS-T segment with an increase in myocardial oxygen demand (increase in heart rate) and an equally slow return to its original level after the attack has stopped.

One more area of ​​application of Holter ECG monitoring should be mentioned, the results of which can be used to assess effectiveness of antianginal therapy in patients with ischemic heart disease. This takes into account the number and total duration of recorded episodes of myocardial ischemia, the ratio of the number of painful and non-painful episodes of ischemia, the number of rhythm and conduction disturbances that occur during the day, as well as daily fluctuations in heart rate and other signs. Particular attention should be paid to the presence of paroxysms of asymptomatic myocardial ischemia, since it is known that in some patients who have undergone treatment, there is a decrease or even disappearance of angina attacks, but signs of silent ischemia of the heart muscle remain. Repeated studies using Holter ECG monitoring are especially advisable when prescribing and selecting the dose of β-adrenergic receptor blockers, which are known to affect heart rate and conductivity, since the individual response to these drugs is difficult to predict and is not always easy to identify using traditional clinical and electrocardiographic research methods .

Electrical phenomena in the heart muscle

There is no potential difference on the surface of a muscle fiber that is at rest (the resting current can only be recorded using an intracellular electrode). When connected to opposite ends of the galvanometer cage, the needle will not deviate; a straight line will be written - an isoelectric line. When excited or depolarized, the excited areas become electronegative, while the unexcited areas retain a positive charge. If the trim electrode is facing the positive charge of the dipole, then the upward deviation of the curve from the isoline is recorded. If the trim electrode is facing a negative charge, the deflection is downward. The amplitude of the wave increases as the excitation spreads through the cell. When the entire cell has become excited, its entire outer surface has acquired a negative charge, the potential difference has disappeared, and the isoelectric line begins to be recorded again. When excitation is released, repolarization occurs, a potential difference again arises between the already released and positively charged areas and the still excited, negatively charged areas. This is accompanied by the appearance of the next tooth. The direction of recording of this wave depends on which areas are adjacent to the electrode: still excited - a negative wave, those already out of excitation - positive. Complete exit from the state of excitation leads to polarization of the cell, the entire outer surface of its membrane is positively charged, there is no potential difference, and the isoelectric line is again recorded.

So, during the period of propagation of excitation, the myocardial cell has two oppositely charged poles and is, as it were, a small generator of electric current.

The surface of the ventricles of the heart can be considered as an extensive polarized membrane covering a single huge cell. The magnitude and direction of the electrical potentials of the heart, which naturally change during the excitation of the heart, are accompanied by changes in potentials on the surface of the human body. The orientation of electrical charges in the tissues of the body obeys general laws in accordance with the cardiac total dipole.

In the main process of excitation, the electrical axis of the heart is directed downward to the left - from the negative pole to the positive. Therefore, from the surface of the body it is always possible to register the potential difference from different points of the electric field of the heart.

Formation of ECG elements

The ECG records the total potential difference from all myocardial cells, or, as it is called, the electromotive force of the heart (cardiac emf). The electrocardiograph records the voltage (electrical potential difference) between 2 points, that is, in some lead. In other words, the ECG device records on paper (screen) the magnitude of the projection of the cardiac EMF onto any lead.

A standard ECG is recorded in 12 leads:

3 standard (I, II, III);

3 reinforced from limbs (aVR, aVL, aVF);

6 chest (V1, V2, V3, V4, V5, V6).

1) Standard leads (proposed by Einthoven in 1913). I - between the left hand and the right hand, II - between the left foot and the right hand, III - between the left foot and the left hand.

2) Reinforced limb leads (proposed by Goldberger in 1942).

The same electrodes are used as for recording standard leads, but each of the electrodes in turn connects 2 limbs at once, and a combined Goldberger electrode is obtained. In practice, recording of these leads is done by simply switching the handle on a single-channel cardiograph (i.e., there is no need to rearrange the electrodes).

aVR - enhanced abduction from the right hand (short for augmented voltage right - enhanced potential on the right). aVL - enhanced abduction from the left arm (left - left) aVF - enhanced abduction from the left leg (foot - leg)

3) Chest leads (proposed by Wilson in 1934) are recorded between the chest electrode and the combined electrode from all 3 limbs. The chest electrode locations are located sequentially along the anterolateral surface of the chest from the midline of the body to the left arm.

V1 - in the IV intercostal space along the right edge of the sternum. V2 V3 V4 - at the level of the apex of the heart. V5 V6 - along the left mid-axillary line at the level of the apex of the heart.

Rice. 1

The 12 leads indicated are standard. If necessary, additional leads can be recorded. It is no coincidence that there is such a large number of leads. The EMF of the heart is the vector of the EMF of the heart in the three-dimensional world (length, width, height) taking into account time. On a flat ECG film we can see only 2-dimensional values, so the cardiograph records the projection of the EMF of the heart on one of the planes in time.

Rice. 2

Each lead records its own projection of the cardiac EMF. The first 6 leads (3 standard and 3 reinforced from the limbs) reflect the EMF of the heart in the so-called frontal plane (see figure) and allow you to calculate the electrical axis of the heart with an accuracy of 30° (180° / 6 leads = 30°). The missing 6 leads to form a circle (360°) are obtained by continuing the existing lead axes through the center to the second half of the circle.

6 chest leads reflect the EMF of the heart in the horizontal (transverse) plane. This makes it possible to clarify the localization of the pathological focus (for example, myocardial infarction): interventricular septum, apex of the heart, lateral parts of the left ventricle, etc.

When analyzing an ECG, projections of the EMF vector of the heart are used, therefore this ECG analysis is called vector.

During the electrical activity of the heart, numerous and multidirectional forces arise and interact in a certain order, reflecting the many emerging dipoles. If we record this process under the condition that the electrodes are directly approaching the surface of the heart, then the formation of an ECG will depend on how the resulting vector of all simultaneous forces is oriented in relation to the trim electrode. Let's imagine that the differential electrode is located at the bottom left of the mass of the excited myocardium, and the indifferent electrode is located at the top right (this principle of electrode placement is the most common in electrocardiography).

The sinus node has the highest automaticity, so normally it is the pacemaker of the heart. However, due to the too small magnitude of the resulting potential difference, the electrical activity of the sinus node is not recorded on the ECG. Excitation of the atrial myocardium begins in the region of the sinus node and spreads along the surface of the myocardium in all directions. Multidirectional depolarization vectors, interacting with each other, are partially neutralized. Since the sinus node is located in the upper part of the right atrium, most vectors are oriented downward and to the left. The resulting vector of atrial excitation is directed, due to this, down and to the left. This direction of the depolarization wave is also facilitated by the accelerated conduction of the impulse down and to the left along the internodal and interatrial specialized tracts. The trim electrode located at the bottom left faces the positive charge of the dipole during atrial depolarization, therefore a positive deviation is recorded - the P wave, the duration of which normally reaches 0.1 s. During the first 0.02 - 0.03 s of its formation, the P wave reflects the excitation of only the right atrium, after that - the total activity of both atria, and the last 0.02 - 0.03 s of the P wave are associated with depolarization of only the left atrium, i.e. To. the right atrium is already fully excited by this time.

After the end of atrial depolarization, their repolarization begins, which occurs in the same sequence as the excitation occurred. The positive resting potential is restored first in the region of the sinus node, so the resulting vector of atrial repolarization is directed upward to the right, away from the trim electrode. This causes the formation of a negative Ta wave, reflecting the final phase of atrial repolarization. It is very small in amplitude and coincides in time with the ventricular ECG complex, so under normal conditions it cannot be isolated and analyzed.

Rice. 3

After 0.02 - 0.04 s from the beginning of atrial depolarization, the excitation wave already reaches the area of ​​the atrioventricular node. Here, the speed of propagation of excitation sharply decreases, after which the impulse quickly spreads along the His bundle and intraventricular pathways, reaching the ventricular myocardium. The ECG identifies the P segment - Q(R) - a segment of the recording line from the end of the P wave to the beginning of the ventricular QRS complex. The P - Q (R) interval reflects the time of atrioventricular impulse conduction and is normally 0.12 - 0.19 s. Normal fluctuations in the duration of P - Q(R) depend on changes in the duration of the atrioventricular delay.

Rice. 4

Excitation of the ventricles, in contrast to excitation of the atria, spreads not from one center, but from many foci located mainly in the subendocardial layers of the myocardium. The sources of depolarization are Purkinje fibers - the terminal branches of the intraventricular pathways. the spread of excitation of the ventricular wall is directed from multiple foci in the subendocardial sections to the subepicardial sections, i.e. perpendicular to the outer surface of the heart. For a detailed analysis of the electrical forces reflecting ventricular depolarization, it is convenient to divide this continuous process into three stages.

The first - initial - is associated with the appearance of foci of depolarization in the left part of the interventricular septum, where the wave of excitation arrives first along the branches of the left bundle branch. The depolarization vector is directed from the left to the right surface of the interventricular septum. When the active electrode is located on the left, the initial stage of ventricular depolarization is reflected by a small negative deflection (Q wave), the duration of which is 0.02 s. Following the depolarization of the left surface of the interventricular septum, depolarization of its right sections begins, where excitation comes along the right bundle branch. The direction of the vector of this depolarization from right to left neutralizes the initially generated electric field, and therefore the initial stage of ventricular excitation is reflected by a small and short-lived wave.

The next - main - stage reflects the spread of excitation through the myocardium of the free walls of the ventricle. The total vector of depolarization of the left ventricle is oriented to the left. The equidirectionality of these vectors leads to partial neutralization of electrical forces. The large muscle mass of the left ventricle determines its electric field above the electric field of the right ventricle, so the resulting vector of ventricular depolarization is oriented to the left. When the active electrode is located on the left, this main stage of ventricular depolarization, corresponding to 0.03 - 0.05 s, is recorded as a positive deflection (R wave).

The final stage of ventricular depolarization reflects the excitation of the posterobasal interventricular septum and ventricles. The depolarization vector is oriented upward and often to the right; the direction of terminal depolarization varies significantly. When the trim electrode is located to the left of the heart, the terminal stage of depolarization is more often reflected by a small negative wave (S).

Thus, successive changes in the magnitude and direction of the resulting electric field vector during excitation of the ventricles lead to the fact that this single process is reflected by the QRS complex, consisting of teeth of different sizes and different polarities. Depending on the position of the electrodes, the teeth, reflecting the initial, main and terminal stages of depolarization, can have different directions (and, as a result, different letter designations). The Q wave indicates the first deviation of the ventricular complex if it is directed downward from the isoline. The deviation of a recording upward from the isoline, regardless of when it is recorded (i.e. whether it is the first or subsequent) is called an R wave. A negative deviation following a positive one is designated as an S wave. Thus, there can only be one Q wave per ventricular complex, and in cases where the complex begins with a positive deviation, the Q wave is absent. If there are several positive waves, then they are called R waves, but each subsequent one is designated as R?, R? ?etc. There can also be several S waves, and then they are designated as S?, S? ?etc. the total duration of the QRS complex, reflecting the intraventricular conduction time, is 0.06 - 0.10 s.

Unlike the atria, the ventricular myocardium of different layers and sections has different durations of electrical processes. The action potential of the subepicardial layers has a shorter duration than the action potential of the subendocardial layers; The action potential of myocardial fibers at the apex of the heart is shorter than at the base of the heart. This leads to the fact that in the ventricular wall, repolarization processes begin earlier in the subepicardial layers and in the apex region, while the subendocardial layers and the base of the ventricles retain negative charges longer. During repolarization, the resulting vector is therefore directed to the left, i.e., in the same direction as the main depolarization vector. The greatest electromotive force occurs in the final repolarization phase, this process is reflected by the appearance of the T wave. When the trim electrode is located on the left, the vector of ventricular repolarization is directed towards this electrode and the T wave is registered as positive. Between the end of the QRS complex and the beginning of the wave, the S-T segment is located: it corresponds to the second phase of repolarization of the ventricular myocardium, during which the potential almost does not change its value. There is almost no potential difference, so the S - T segment is located on the isoline. The different duration of the action potential in different parts of the ventricular myocardium leads to a slight asynchronism of the repolarization phases and the appearance of a small potential difference, which gives the S-T segment some curvature with a smooth transition into the T wave. The time interval from the beginning of the QRS complex to the beginning of the T wave reflects the entire period of the electrical ventricular activity (electrical systole). Normally, Q - T is 0.36 - 0.44 s and depends on gender, age and rhythm frequency. Following the T wave, another positive deviation of small amplitude is usually recorded - the U wave. The mechanisms of its appearance have not been precisely established and, apparently, are not always unambiguous.

Rice. 5

In the process of studying all waves, segments and intervals recorded by the electrocardiogram, an electrocardiographic conclusion is drawn, which should include:

1. Source of rhythm (sinus or not).

2. Regularity of rhythm (correct or not). Usually sinus rhythm is normal, although respiratory arrhythmia is possible.

4. Position of the electrical axis of the heart.

5. Presence of 4 syndromes:

rhythm disturbance

conduction disturbance

hypertrophy and/or overload of the ventricles and atria

myocardial damage (ischemia, dystrophy, necrosis, scars)

Body as a volumetric conductor of electrical phenomena

The tissues and organs surrounding the heart play the role of conductors that transmit electrical charges to the surface of the body. The magnitude of the potentials decreases with distance from the heart. In a homogeneous conducting medium, the potential value of any point is inversely proportional to the distance from it to the source of the potential difference. Body tissues have different electrical conductivity, which introduces significant distortions into the distribution and magnitude of potentials on the surface of the body. The ECG may change under the influence of conditions such as obesity, cachexia, body edema, fluid accumulation in the pleura and pericardium, emphysema and pulmonary consolidation, etc.

Electrocardiography is a method of graphically recording the potential difference in the electrical field of the heart that arises during its activity. Registration is carried out using a device - an electrocardiograph. It consists of an amplifier that allows it to capture currents of very low voltage; a galvanometer that measures voltage; power systems; recording device; electrodes and wires connecting the patient to the device. The waveform that is recorded is called an electrocardiogram (ECG). Registration of the potential difference in the electric field of the heart from two points on the surface of the body is called lead. As a rule, the ECG is recorded in twelve leads: three bipolar (three standard leads) and nine unipolar (three unipolar enhanced limb leads and 6 unipolar chest leads). With bipolar leads, two electrodes are connected to the electrocardiograph; with unipolar leads, one electrode (indifferent) is combined, and the second (different, active) is placed at a selected point on the body. If the active electrode is placed on a limb, the lead is called unipolar, limb-amplified; if this electrode is placed on the chest - with a unipolar chest lead.

To record an ECG in standard leads (I, II and III), cloth napkins moistened with saline are placed on the limbs, on which metal electrode plates are placed. One electrode with a red wire and one raised ring is placed on the right, the second - with a yellow wire and two raised rings - on the left forearm, and the third - with a green wire and three raised rings - on the left shin. To record leads, two electrodes are connected to the electrocardiograph in turn. To record lead I, the electrodes of the right and left hands are connected, lead II - electrodes of the right hand and left leg, lead III - electrodes of the left hand and left leg. Switching leads is done by turning the knob. In addition to the standard ones, unipolar reinforced leads are removed from the limbs. If the active electrode is located on the right arm, the lead is designated as aVR or UP, if on the left arm - aVL or UL, and if on the left leg - aVF or UL.

Rice. 1. The location of the electrodes when registering the anterior chest leads (indicated in numbers corresponding to their serial numbers). The vertical stripes crossing the numbers correspond to the anatomical lines: 1 - right sternal; 2 - left sternal; 3 - left parasternal; 4-left midclavicular; 5-left anterior axillary; 6 - left middle axillary.

When recording unipolar chest leads, the active electrode is placed on the chest. The ECG is recorded in the following six electrode positions: 1) at the right edge of the sternum in the IV intercostal space; 2) at the left edge of the sternum in the IV intercostal space; 3) along the left parasternal line between the IV and V intercostal spaces; 4) along the midclavicular line in the 5th intercostal space; 5) along the anterior axillary line in the 5th intercostal space and 6) along the middle axillary line in the 5th intercostal space (Fig. 1). Unipolar chest leads are designated by the Latin letter V or in Russian - GO. Less commonly recorded are bipolar chest leads, in which one electrode is located on the chest and the other on the right arm or left leg. If the second electrode was located on the right arm, the chest leads were designated by the Latin letters CR or Russian - GP; when the second electrode was located on the left leg, the chest leads were designated by the Latin letters CF or Russian - GN.

The ECG of healthy people is variable. It depends on age, physique, etc. However, normally it is always possible to distinguish certain teeth and intervals on it, reflecting the sequence of excitation of the heart muscle (Fig. 2). According to the available time stamp (on photographic paper the distance between two vertical stripes is 0.05 sec., on graph paper at a broaching speed of 50 mm/sec 1 mm is 0.02 sec., at a speed of 25 mm/sec - 0.04 sec. ) you can calculate the duration of ECG waves and intervals (segments). The height of the teeth is compared with a standard mark (when a 1 mV pulse is applied to the device, the recorded line should deviate from the original position by 1 cm). Excitation of the myocardium begins from the atria, and the atrial P wave appears on the ECG. Normally, it is small: 1-2 mm high and lasting 0.08-0.1 seconds. The distance from the beginning of the P wave to the Q wave (P-Q interval) corresponds to the time of propagation of excitation from the atria to the ventricles and is equal to 0.12-0.2 seconds. During excitation of the ventricles, the QRS complex is recorded, and the size of its waves in different leads is expressed differently: the duration of the QRS complex is 0.06-0.1 seconds. The distance from the S wave to the beginning of the T wave - the S-T segment, is normally located at the same level with the P-Q interval and its displacement should not exceed 1 mm. When excitation in the ventricles fades, a T wave is recorded. The interval from the beginning of the Q wave to the end of the T wave reflects the process of excitation of the ventricles (electrical systole). Its duration depends on the heart rate: when the rhythm increases, it shortens, when it slows down, it lengthens (on average it is 0.24-0.55 seconds). The heart rate can be easily calculated from an ECG, knowing how long one cardiac cycle lasts (the distance between two R waves) and how many such cycles are contained in a minute. The T-P interval corresponds to the diastole of the heart; at this time the device records a straight (so-called isoelectric) line. Sometimes after the T wave a U wave is recorded, the origin of which is not entirely clear.

Rice. 2. Electrocardiogram of a healthy person.

In pathology, the size of the waves, their duration and direction, as well as the duration and location of ECG intervals (segments), can vary significantly, which gives rise to the use of electrocardiography in the diagnosis of many heart diseases. Using electrocardiography, various heart rhythm disturbances are diagnosed (see), inflammatory and dystrophic lesions of the myocardium are reflected on the ECG. Electrocardiography plays a particularly important role in the diagnosis of coronary insufficiency and myocardial infarction.

Using an ECG, you can determine not only the presence of a heart attack, but also find out which wall of the heart is affected. In recent years, to study the potential difference in the electric field of the heart, the method of teleelectrocardiography (radioelectrocardiography), based on the principle of wireless transmission of the electric field of the heart using a radio transmitter, has been used. This method allows you to register an ECG during physical activity, in motion (for athletes, pilots, astronauts).

Electrocardiography (Greek kardia - heart, grapho - writing, recording) is a method of recording electrical phenomena occurring in the heart during its contraction.

The history of electrophysiology, and therefore electrocardiography, begins with the experiment of Galvani (L. Galvani), who discovered electrical phenomena in the muscles of animals in 1791. Matteucci (S. Matteucci, 1843) established the presence of electrical phenomena in an excised heart. Dubois-Reymond (E. Dubois-Reymond, 1848) proved that in both nerves and muscles the excited part is electronegative relative to the resting part. Kolliker and Muller (A. Kolliker, N. Muller, 1855), applying a frog neuromuscular preparation consisting of a sciatic nerve connected to the gastrocnemius muscle to the contracting heart, obtained a double contraction during heart contraction: one at the beginning of systole and the other (non-constant ) at the beginning of diastole. Thus, the electromotive force (EMF) of the naked heart was recorded for the first time. Waller (A. D. Waller, 1887) was the first to record the EMF of the heart from the surface of the human body using a capillary electrometer. Waller believed that the human body is a conductor surrounding the source of EMF - the heart; different points of the human body have potentials of different magnitudes (Fig. 1). However, the recording of cardiac EMF obtained by a capillary electrometer did not accurately reproduce its fluctuations.

Rice. 1. Scheme of the distribution of isopotential lines on the surface of the human body, caused by the electromotive force of the heart. The numbers indicate the potential values.

An accurate recording of the EMF of the heart from the surface of the human body - an electrocardiogram (ECG) - was made by Einthoven (W. Einthoven, 1903) using a string galvanometer, built on the principle of devices for receiving transatlantic telegrams.

According to modern concepts, cells of excitable tissues, in particular myocardial cells, are covered with a semi-permeable membrane (membrane), permeable to potassium ions and impermeable to anions. Positively charged potassium ions, which are in excess in cells compared to their surrounding environment, are retained on the outer surface of the membrane by negatively charged anions located on its inner surface, impenetrable to them.

Thus, a double electrical layer appears on the shell of a living cell - the shell is polarized, and its outer surface is charged positively in relation to the internal contents, which are negatively charged.

This transverse potential difference is the resting potential. If microelectrodes are applied to the outer and inner sides of the polarized membrane, a current arises in the outer circuit. Recording the resulting potential difference gives a monophasic curve. When excitation occurs, the membrane of the excited area loses its semi-permeability, depolarizes, and its surface becomes electronegative. Registration of the potentials of the outer and inner shell of the depolarized membrane with two microelectrodes also gives a monophasic curve.

Due to the potential difference between the surface of the excited depolarized area and the surface of the polarized one, which is at rest, an action current arises - an action potential. When excitation covers the entire muscle fiber, its surface becomes electronegative. The cessation of excitation causes a wave of repolarization, and the resting potential of the muscle fiber is restored (Fig. 2).

Rice. 2. Schematic representation of polarization, depolarization and repolarization of a cell.

If the cell is at rest (1), then on both sides of the cell membrane there is an electrostatic equilibrium, consisting in the fact that the surface of the cell is electropositive (+) in relation to its inner side (-).

The excitation wave (2) instantly disrupts this balance, and the surface of the cell becomes electronegative with respect to its interior; This phenomenon is called depolarization or, more correctly, inversion polarization. After the excitation has passed through the entire muscle fiber, it becomes completely depolarized (3); its entire surface has the same negative potential. This new equilibrium does not last long, since the excitation wave is followed by a repolarization wave (4), which restores the polarization of the resting state (5).

The process of excitation in a normal human heart - depolarization - proceeds as follows. Arising in the sinus node, located in the right atrium, the excitation wave propagates at a speed of 800-1000 mm per 1 second. radially along the muscle bundles of first the right and then the left atrium. The duration of excitation coverage of both atria is 0.08-0.11 seconds.

The first 0.02 - 0.03 sec. Only the right atrium is excited, then 0.04 - 0.06 seconds - both atria and the last 0.02 - 0.03 seconds - only the left atrium.

Upon reaching the atrioventricular node, the spread of excitation slows down. Then, at a high and gradually increasing speed (from 1400 to 4000 mm per 1 second), it is directed along the bundle of His, its legs, their branches and branches and reaches the final ends of the conduction system. Having reached the contractile myocardium, excitation spreads through both ventricles at a significantly reduced speed (300-400 mm per 1 second). Since the peripheral branches of the conduction system are scattered mainly under the endocardium, the inner surface of the heart muscle is the first to become excited. The further course of excitation of the ventricles is not related to the anatomical location of the muscle fibers, but is directed from the inner surface of the heart to the outer. The time of excitation in the muscle bundles located on the surface of the heart (subepicardial) is determined by two factors: the time of excitation of the branches of the conduction system closest to these bundles and the thickness of the muscle layer separating the subepicardial muscle bundles from the peripheral branches of the conduction system.

The interventricular septum and the right papillary muscle are the first to be excited. In the right ventricle, excitation first covers the surface of its central part, since the muscle wall in this place is thin and its muscle layers are in close contact with the peripheral branches of the right leg of the conduction system. In the left ventricle, the apex is the first to become excited, since the wall separating it from the peripheral branches of the left leg is thin. For various points on the surface of the right and left ventricles of a normal heart, the period of excitation begins at a strictly defined time, and most of the fibers on the surface of the thin-walled right ventricle and only a small number of fibers on the surface of the left ventricle become excited first due to their proximity to the peripheral branches of the conduction system (Fig. .3).

Rice. 3. Schematic representation of normal excitation of the interventricular septum and outer walls of the ventricles (according to Sodi-Pallares et al.). Excitation of the ventricles begins on the left side of the septum in its middle part (0.00-0.01 sec.) and then can reach the base of the right papillary muscle (0.02 sec.). After this, the subendocardial muscle layers of the outer wall of the left (0.03 sec.) and right (0.04 sec.) ventricles are excited. The last to be excited are the basal parts of the outer walls of the ventricles (0.05-0.09 sec.).

The process of cessation of excitation of the muscle fibers of the heart - repolarization - cannot be considered fully studied. The process of atrial repolarization coincides for the most part with the process of depolarization of the ventricles and partly with the process of their repolarization.

The process of ventricular repolarization is much slower and in a slightly different sequence than the process of depolarization. This is explained by the fact that the duration of excitation of the muscle bundles of the superficial layers of the myocardium is less than the duration of excitation of the subendocardial fibers and papillary muscles. Recording the process of depolarization and repolarization of the atria and ventricles from the surface of the human body gives a characteristic curve - an ECG, reflecting the electrical systole of the heart.

The EMF of the heart is currently recorded using slightly different methods than those recorded by Einthoven. Einthoven recorded the current generated by connecting two points on the surface of the human body. Modern devices - electrocardiographs - directly record the voltage caused by the electromotive force of the heart.

The voltage caused by the heart, equal to 1-2 mV, is amplified by radio tubes, semiconductors or a cathode ray tube to 3-6 V, depending on the amplifier and recording apparatus.

The sensitivity of the measuring system is set so that a potential difference of 1 mV gives a deviation of 1 cm. Recording is done on photographic paper or film or directly on paper (ink, thermal recording, inkjet recording). The most accurate results are obtained by recording on photographic paper or film and inkjet recording.

To explain the peculiar shape of the ECG, various theories of its genesis have been proposed.

A.F. Samoilov considered the ECG as the result of the interaction of two monophasic curves.

Considering that when two microelectrodes record the outer and inner surfaces of the membrane in states of rest, excitation and damage, a monophasic curve is obtained, M. T. Udelnov believes that the monophasic curve reflects the main form of bioelectrical activity of the myocardium. The algebraic sum of two monophasic curves gives the ECG.

Pathological ECG changes are caused by shifts in monophasic curves. This theory of the genesis of the ECG is called differential.

The outer surface of the cell membrane during the period of excitation can be represented schematically as consisting of two poles: negative and positive.

Immediately before the excitation wave at any point in its propagation, the cell surface is electropositive (resting state of polarization), and immediately after the excitation wave, the cell surface is electronegative (depolarization state; Fig. 4). These electric charges of opposite signs, grouped in pairs on one side and the other of each place covered by the excitation wave, form electric dipoles (a). Repolarization also creates an innumerable number of dipoles, but unlike the above dipoles, the negative pole is in front and the positive pole is behind in relation to the direction of wave propagation (b). If depolarization or repolarization is complete, the surface of all cells has the same potential (negative or positive); dipoles are completely absent (see Fig. 2, 3 and 5).

Rice. 4. Schematic representation of electric dipoles during depolarization (a) and repolarization (b), arising on both sides of the excitation wave and the repolarization wave as a result of changes in the electrical potential on the surface of the myocardial fibers.

Rice. 5. Diagram of an equilateral triangle according to Einthoven, Faro and Warth.

The muscle fiber is a small bipolar generator that produces a small (elementary) EMF - an elementary dipole.

At each moment of cardiac systole, depolarization and repolarization occurs of a huge number of myocardial fibers located in different parts of the heart. The sum of the resulting elementary dipoles creates the corresponding value of the EMF of the heart at each moment of systole. Thus, the heart represents, as it were, one total dipole, changing its magnitude and direction during the cardiac cycle, but not changing the location of its center. The potential at different points on the surface of the human body has different values ​​depending on the location of the total dipole. The sign of the potential depends on which side of the line perpendicular to the dipole axis and drawn through its center the given point is located: on the side of the positive pole the potential has a + sign, and on the opposite side it has a - sign.

Most of the time the heart is excited, the surface of the right half of the torso, right arm, head and neck has a negative potential, and the surface of the left half of the torso, both legs and left arm has a positive potential (Fig. 1). This is a schematic explanation of the genesis of the ECG according to the dipole theory.

The EMF of the heart during electrical systole changes not only its magnitude, but also its direction; therefore, it is a vector quantity. A vector is depicted as a straight line segment of a certain length, the size of which, given certain data from the recording apparatus, indicates the absolute value of the vector.

The arrow at the end of the vector indicates the direction of the cardiac EMF.

The EMF vectors of individual heart fibers that arise simultaneously are summed up according to the vector addition rule.

The total (integral) vector of two vectors located parallel and directed in one direction is equal in absolute value to the sum of its component vectors and is directed in the same direction.

The total vector of two vectors of the same magnitude, located parallel and directed in opposite directions, is equal to 0. The total vector of two vectors directed to each other at an angle is equal to the diagonal of a parallelogram constructed from its constituent vectors. If both vectors form an acute angle, then their total vector is directed towards its constituent vectors and is greater than any of them. If both vectors form an obtuse angle and, therefore, are directed in opposite directions, then their total vector is directed towards the largest vector and is shorter than it. Vector analysis of an ECG consists of determining from the ECG waves the spatial direction and magnitude of the total EMF of the heart at any moment of its excitation.

Electrocardiography I Electrocardiography

Electrocardiography is a method of electrophysiological study of heart activity in normal and pathological conditions, based on recording and analysis of the electrical activity of the myocardium spreading throughout the heart during the cardiac cycle. Registration is carried out using special devices - electrocardiographs. The recorded curve - () - reflects the dynamics during the cardiac cycle of the potential difference at two points of the electric field of the heart, corresponding to the places on the body of the subject of two electrodes, one of which is the positive pole, the other is negative (connected respectively to the + and - poles of the electrocardiograph). A certain relative position of these electrodes is called an electrocardiographic lead, and a conditional straight line between them is called the axis of this lead. On a normal basis, the magnitude of the electromotive force (EMF) of the heart and its direction, changing during the cardiac cycle, are reflected in the form of the dynamics of the projection of the EMF vector onto the lead axis, i.e. on a line, and not on a plane, as happens when recording a vectorcardiogram (see Vectorcardiography), reflecting the spatial dynamics of the direction of the EMF of the heart in projection onto the plane. Therefore, an ECG, as opposed to a vectorcardiogram, is sometimes called a scalar one. In order to use it to obtain spatial information about changes in electrical processes in, it is necessary to take an ECG at different positions of the electrodes, i.e. in different leads whose axes are not parallel.

Theoretical foundations of electrocardiography are based on the laws of electrodynamics applicable to electrical processes occurring in connection with the rhythmic generation of an electrical impulse by the heart pacemaker and the spread of electrical excitation through the conduction system of the heart (Heart) and myocardium. After generating an impulse in the sinus node, it first spreads to the right, and after 0.02 With and to the left atrium, then after a short delay in the atrioventricular node it passes to the septum and synchronously covers the right and left ventricles of the heart, causing them. Each excited one becomes an elementary dipole (two-pole generator): the sum of the elementary dipoles at a given moment of excitation constitutes the so-called equivalent dipole. The spread of excitation throughout the heart is accompanied by the appearance of an electric field in the volumetric conductor (body) surrounding it. The change in the potential difference at 2 points of this field is perceived by the electrocardiograph electrodes and recorded in the form of ECG waves directed upward (positive) or downward (negative) along the isoelectric line, depending on the direction of the EMF between the poles of the electrodes. In this case, the amplitude of the teeth, measured in millivolts or millimeters (usually recording is done in a mode where the standard calibration potential lmv deflects the recorder pen by 10 mm), reflects the magnitude of the potential difference along the axis of the ECG lead.

The founder of E., the Dutch physiologist W. Einthoven, proposed recording the potential difference in the frontal plane of the body in three standard leads - as if from the vertices of an equilateral triangle, for which he took the right hand, left hand and pubic (in practical E. as the third vertices the left one is used). The lines between these vertices, i.e. The sides of the triangle are the axes of standard leads.

Normal electrocardiogram reflects the process of excitation spreading through the conduction system of the heart ( rice. 3 ) and contractile myocardium after generating an impulse in the sinoatrial node, which is normally the pacemaker of the heart. On the ECG ( rice. 4, 5 ) during diastole (between the T and P waves), a straight horizontal line, called isoelectric (isoline), is recorded. The impulse in the sinoatrial node spreads through the atrial myocardium, which forms the atrial P wave on the ECG, and at the same time along the internodal fast conduction pathways to the atrioventricular node. Thanks to this, it enters the atrioventricular chamber even before the end of atrial excitation. It travels slowly through the atrioventricular node, so after the P wave before the start of the waves reflecting the excitation of the ventricles, isoelectric is recorded on the ECG; During this time, mechanical atrial function is completed. Then the impulse is quickly conducted along the atrioventricular bundle (bundle of His), its trunk and legs (branches), the branches of which through the Purkinje fibers transmit excitation directly to the fibers of the contractile myocardium of the ventricles. () of the ventricular myocardium is reflected on the ECG by the appearance of Q, R, S waves (QRS complex), and in the early phase - the RST segment (more precisely, the ST or RT segment, if the S wave is absent), almost coinciding with the isoline, and in the main (fast ) phase - the T wave. Often the T wave is followed by a small U wave, the origin of which is associated with repolarization in the His-Purkinje system. First 0.01-0.03 With The QRS complex is due to excitation of the interventricular septum, which is reflected by the Q wave in the standard and left chest leads, and by the beginning of the R wave in the right chest leads. The normal duration of the Q wave is no more than 0.03 With. In the next 0.015-0.07 With the apices of the right and left ventricles are excited from the subendocardial to subepicardial layers, their anterior, posterior and lateral walls, lastly (0.06-0.09 With) excitation spreads to the bases of the right and left ventricles. Integral vector of the heart between 0.04 and 0.07 With complex is oriented to the left - to the positive pole of leads II and V 4, V 5, and in the period 0.08-0.09 With- up and slightly to the right. Therefore, in these leads, the QRS complex is represented by a high R wave with shallow Q and S waves, and in the right chest leads a deep S wave is formed. The ratio of the values ​​of the R and S waves in each of the standard and unipolar leads is determined by the spatial position of the integral vector of the heart of the electrical axis of the heart) , which normally depend on the location of the heart in the chest.

Thus, the ECG normally reveals an atrial P wave and QRST, consisting of negative Q, S waves, a positive R wave, as well as a T wave, positive in all leads except VR, in which it is negative, and V 1 -V 2 , where the T wave can be either positive or negative or slightly pronounced. The atrial P wave in lead aVR is also normally always negative, and in lead V 1 it is usually represented by two phases: positive - larger (excitation predominantly of the right atrium), then negative - smaller (excitation of the left atrium). The QRS complex may lack Q and/or S waves (forms RS, QR, R), and also have two R or S waves, with the second wave designated R 1 (forms RSR 1 and RR 1) or S 1 .

The time intervals between the same teeth of neighboring cycles are called inter-cycle intervals (for example, P-P, R-R intervals), and between different teeth of the same cycle - intra-cycle intervals (for example, P-Q, O-T intervals). ECG segments between waves are designated as segments if their duration is not described, but in relation to an isoline or configuration (for example, ST, or RT, a segment extending from the end of the QRS complex to the end of the T wave). Under pathological conditions, they can shift upward (elevation) or downward () relative to the isoline (for example, the ST segment upward in myocardial infarction, pericarditis).

Sinus rhythm is determined by the presence in leads I, II, aVF, V 6 of a positive P wave, which normally always precedes the QRS complex and is spaced from it (P-Q interval or P-R interval if there is no Q wave) by at least 0 ,12 With. With pathological localization of the atrial pacemaker close to the atrioventricular junction or in it itself, the P wave in these leads is negative, approaches the QRS complex, can coincide with it in time and even be detected after it.

The regularity of the rhythm is determined by the equality of intercycle intervals (P-P or R-R). With sinus arrhythmia, the P-P (R-R) intervals differ by 0.10 With and more. The normal duration of atrial excitation, measured by the width of the P wave, is 0.08-0.10 With. The normal P-Q interval is 0.12-0.20 With. The time of propagation of excitation through the ventricles, determined by the width of the QRS complex, is 0.06-0.10 With. Duration of electrical systole of the ventricles, i.e. The Q-T interval, measured from the beginning of the QRS complex to the end of the T wave, normally has the proper value depending on the heart rate (proper Q-T duration), i.e. on the duration of the cardiac cycle (C), corresponding to the R-R interval. According to Bazett's formula, the proper Q-T duration is equal to k, where k is a coefficient of 0.37 for men and 0.39 for women and children. An increase or decrease in the Q-T interval by more than 10% compared to the expected value is a sign of pathology.

The amplitude (voltage) of normal ECG waves in different leads depends on the physique of the subject, the severity of the subcutaneous tissue, and the position of the heart in the chest. In adults, the normal P wave is usually the highest (up to 2-2.5 mm) in lead II; it has a semi-oval shape. PIII and PaVL - positive low (rarely shallow negative). with a normal location of the electrical axis of the heart, it is presented in leads I, II, III, aVL, aVF, V 4 -V 6 shallow (less than 3 mm) the initial Q wave, the high R wave and the small final S wave. The highest R wave is in leads II, V 4, V 5, and in lead V 4 the amplitude of the R wave is usually greater than in lead V 6, but does not exceed 25 mm (2,5 mV). In lead aVR, the main wave of the QRS complex (S wave) and the T wave are negative. In lead V, the rS complex is recorded (lowercase letters indicate waves of relatively small amplitude, when it is necessary to specifically emphasize the amplitude ratio), in leads V 2 and V 3 - the RS or rS complex. The R wave in the chest leads increases from right to left (from V to V 4 -V 5) and then decreases slightly towards V 6. The S wave decreases from right to left (from V 2 to V 6). The equality of the R and S waves in one lead determines the transition zone - a lead in a plane perpendicular to the spatial vector of the QRS complex. Normally, the transition zone of the complex is located between leads V 2 and V 4. The direction of the T wave usually coincides with the direction of the largest wave in the QRS complex. It is positive, as a rule, in leads I, II, Ill, aVL, aVF, V 2 -V 6 and has a greater amplitude in those leads where the R wave is higher; and the T wave is 2-4 times smaller (with the exception of leads V 2 -V 3, where the T wave can be equal to or higher than R).

The ST segment (RT) in all limb leads and in the left chest leads is recorded at the level of the isoelectric line. Small horizontal displacements (down to 0.5 mm or up to 1 mm) ST segment are possible in healthy people, especially against the background of tachycardia or bradycardia, but in all such cases it is necessary to exclude the nature of such displacements by dynamic observation, functional tests or comparison with clinical data. In leads V 1, V 2, V 3, the RST segment is located on the isoelectric line or is shifted upward by 1-2 mm.

Variants of a normal ECG, depending on the location of the heart in the chest, are determined by the ratio of the R and S waves or the shape of the QRS complex in different leads; in the same way, pathological deviations of the electrical axis of the heart are identified with hypertrophy of the ventricles of the heart, blockade of the branches of the His bundle, etc. These options are considered conventionally as rotations of the heart around three axes: anteroposterior (the position of the electrical axis of the heart is defined as normal, horizontal, vertical, or as its deviation to the left, right), longitudinal (rotation clockwise and counterclockwise) and transverse (rotation of the heart by the apex forward or backward).

The position of the electrical axis is determined by the value of the angle α, constructed in the coordinate system and axes of abduction from the limbs (see. rice. 1, a and b ) and calculated from the algebraic sum of the amplitudes of the QRS complex teeth in each of any two limb leads (usually in I and III): normal position - α from + 30 to 60°: horizontal - α from 0 to +29°; vertical α from +70 to +90°. deviation to the left - α from -1 to -90°; to the right - α from +91 to ±80°. When the electrical axis of the heart is horizontal, the integral vector is parallel to the T axis of the lead; the R I wave is high (higher than the R II wave); R III SVF. When the electrical axis deviates to the left, R I > R II > R aVF

When the heart rotates around the longitudinal axis clockwise, the ECG shows an RS shape in leads I, V 5.6 and a qR shape in lead III. When rotated counterclockwise, the ventricular complex has a qR shape in leads I, V 5.6 and an RS shape in lead III and a moderately increased R in leads V 1 -V 2 without displacement of the transition zone (in lead V 2 R

In children, a normal ECG has a number of features, the main of which are: deviation of the electrical axis of the heart to the right (α is +90 - +180° in newborns, +40° - +100° in children aged 2-7 years); the presence in leads II, Ill, aVF of a deep Q wave, the amplitude of which decreases with age and becomes close to that in adults by 10-12 years; low voltage of the T wave in all leads and the presence of a negative T wave in leads III, V 1 -V 2 (sometimes V 3, V 4), shorter duration of the P waves and the QRS complex - on average 0.05 each With in newborns and 0.07 With in children from 2 to 7 years old; shorter P-Q interval (average 0.11 With in newborns and 0.13 With in children from 2 to 7 years old). By the age of 15, the listed ECG features are largely lost, the duration of the P wave and the QRS complex averages 0.08 each With, P-Q interval - 11.14 With.

Electrocardiographic changes in the state and activity of the heart is based on an analysis of the size, shape, direction in different leads and repeatability in each cycle of all ECG waves, measurement data of the duration of the P, Q waves, the QRS complex and the P-Q (P-R), Q-T intervals, R-R, as well as deviations from the isoline of the RST segment with subsequent interpretation of the identified features as pathological or as a variant of the norm. The protocol part of the ECG report must characterize the heart rhythm (sinus, ectopic, etc.) and the position of the electrical axis of the heart. The conclusion contains characteristics of a specific pathological ECG syndrome. In a number of forms of heart pathology, the totality of ECG changes has a certain specificity, and therefore E. is one of the leading diagnostic methods in cardiology.

Dextrocardia due to a mirror change in the topography of the heart relative to the sagittal plane and its displacement to the right, it determines the orientation of the main vectors of excitation of the atria and ventricles of the heart to the right, i.e. to the negative pole of lead I and to the positive pole of lead III. Therefore, on the ECG in lead I, a deep S wave and negative P and T waves are recorded; the R III wave is high, the P III and T III waves are positive; in the chest leads, the QRS voltage is reduced in the left positions with an increase in the depth of the S wave towards leads V 5 -V 6 . If you swap the electrodes of the right and left hands, then the ECG shows waves of the usual shape and direction in leads I and III. Such replacement of electrodes and registration of additional chest leads V 3R, V 4R, V 5R, V 6R make it possible to confirm the conclusion and identify or exclude other myocardial pathology in dextrocardia.

With dextroversion, unlike dextrocardia, the P wave in leads I, II, V 6 is positive. the initial part of the ventricular complex has a qRS shape in leads I and V 6 and an RS shape in lead V 3R.

Hypertrophy of the atria and ventricles of the heart is accompanied by an increase in the EMF of the hypertrophied section and a deviation in its direction of the vector of the total EMF of the heart. On the ECG, this is reflected in certain leads by an increase and (or) change in the shape of the P waves with atrial hypertrophy and the R and S waves with ventricular hypertrophy. There may be a slight widening of the corresponding tooth and an increase in the so-called internal deviation, i.e. time from the beginning of the P wave or ventricular complex to the moment corresponding to the maximum of their positive deflection (the top of the P or R wave). With ventricular hypertrophy, the final part of the ventricular complex may change: the RST moves downward and becomes lower or the T wave in leads with high R is inverted (becomes negative), which is designated as (multidirectional) ST segment and T wave in relation to the R wave. A segment is also observed RST and T wave relative to the S wave in leads with a deep S wave.

With left atrial hypertrophy ( rice. 7 ) the P wave expands to 0.11-0.14 With, becomes double-humped (P mitrale) in leads I, II, aVL and left thoracic, often with an increase in the amplitude of the second apex (in some cases the P wave is flattened). Time of internal deviation of the P wave in leads I, II, V 6 more than 0.06 With. The most common and reliable sign of left atrial hypertrophy is an increase in the negative phase of the P wave in lead V1, which becomes larger in amplitude than the positive phase.

Right atrial hypertrophy ( rice. 8 ) is characterized by an increase in the amplitude of the P wave (more than 1.8-2.5 mm) in leads II, Ill, aVF, its pointed form (P pulmonale). The electrical axis of the P wave acquires a vertical position, less often deviated to the right. A significant increase in the amplitude of the P wave in leads V 1 -V 3 is observed with congenital heart defects (P congenitale).