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Electrocardiography (ECG)

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

To register an ECG in standard leads (I, II, and III), cloth napkins moistened with saline solution are placed on the limbs, and metal plates of electrodes are placed on them. One electrode with a red wire and one relief ring is placed on the right forearm , the second with a yellow wire and two relief rings on the left forearm and a third with a green wire and three relief rings on the left lower leg. To register leads to the electrocardiograph, two electrodes are connected in turn. For recording I leads, electrodes of the right and left hands are connected, leads II - electrodes of the right hand and left leg, leads III - electrodes of the left hand and left leg. Switching leads by turning the knob. In addition to the standard, unipolar reinforced leads are removed from the extremities. 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 YL, and if on the left leg - aVF or yN.

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Fig. 1. The location of the electrodes when registering the front chest leads (indicated by the numbers corresponding to their ordinal 1 numbers). The vertical stripes crossing the figures correspond to the anatomical lines: 1 - the right sternum; 2 - left sternum; 3 - left okologrudinnoy; 4 — left midclavicular; 5 — left anterior axillary; 6 - the left middle axillary.

When registering single pole chest leads, the active electrode is placed on the chest. ECG is recorded in the following six positions of the electrode: 1) at the right edge of the sternum in the fourth intercostal space; 2) at the left edge of the sternum in the fourth intercostal space; 3) on the left okolovrudnoy line between the IV and V intercostal spaces; 4) on the midclavicular line in the V intercostal space; 5) along the anterior axillary line in the V intercostal space; and 6) along the mid-axillary line in the V intercostal space (Fig. 1). Monopolar chest leads represent the Latin letter V or Russian - GO. Less commonly, bipolar chest leads are recorded, in which one electrode was located on the chest and the other on the right arm or left leg. If the second electrode was located on the right hand, chest leads were designated by Latin letters CR or by Russians - GP; at the location of the second electrode on the left leg, the chest leads were designated by Latin letters CF or by Russians - GN.

The ECG of healthy people is different variability. It depends on age, physique, etc. However, normally, it is always possible to distinguish between certain teeth and intervals, reflecting the sequence of excitation of the heart muscle (Fig. 2). According to the current time mark (on photo paper, the distance between two vertical stripes is 0.05 sec., On graph paper, at a drawing speed of 50 mm / s, 1 mm is 0.02 s., And at a speed of 25 mm / s - 0.04 s. ) you can calculate the duration of the teeth and the intervals (segments) of the ECG. The height of the teeth is compared with the standard mark (when a pulse of 1 mV is applied to the device, the recorded line should deviate from the initial position by 1 cm). The excitation of the myocardium begins with the atria, and the atrial tooth R appears on the ECG. Normally, it is small: 1–2 mm in height and 0.08–0.1 sec. In duration. The distance from the beginning of the P wave to the Q wave (P-Q interval) corresponds to the time of excitation propagation from the atria to the ventricles and is 0.12—0.2 sec. During excitation of the ventricles, the QRS complex is recorded, and the magnitude of its teeth in different leads is expressed differently: the duration of the QRS complex is 0.06– 0.1 sec. The distance from the S wave to the beginning of the T wave is the S — T segment, normally located on the same level with the interval P – Q and its displacement should not exceed 1 mm. With the fading of excitation in the ventricles, the 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 frequency of the heart rhythm: when the rhythm increases, it shortens, and when it slows down, it becomes longer (on average, it is 0.24-0.55 sec.). The heart rate is easy to calculate by ECG, knowing how long one heart cycle lasts (the distance between two R waves) and how many such cycles are contained in a minute. The interval T — P corresponds to the diastole of the heart, the apparatus at this time records a straight (so-called isoelectric) line. Sometimes after the T wave, the U wave is recorded, the origin of which is not quite clear.

ECG
Fig. 2. Electrocardiogram of a healthy person.

In pathology, the size of the teeth, their duration and direction, as well as the duration and location of the intervals (segments) of an ECG, can vary considerably, which gives grounds to use electrocardiography in the diagnosis of many heart diseases. Using electrocardiography, various cardiac rhythm disorders are diagnosed (see Cardiac arrhythmias ), inflammatory and dystrophic myocardial lesions are reflected on the ECG. A particularly important role is played by electrocardiography in the diagnosis of coronary insufficiency and myocardial infarction.

The ECG 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 of the electric field of the heart, the teleelectrocardiography (radioelectrocardiography) method is used, based on the principle of wireless transmission of the heart’s electric field using a radio transmitter. This method allows you to register an ECG during exercise, on the move (in athletes, pilots, astronauts).

Electrocardiography (Greek kardia - the heart, grapho - I write, I write down) is a method of recording electrical phenomena that occur in the heart during its contraction.

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The history of electrophysiology, and hence electrocardiography, begins with the experience 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 the excised heart. Dubois-Reymond (E. Dubois-Reymond, 1848) proved that both the nerves and muscles are the excited part of the electronegative with respect to those in rest. Kelliker and Muller (A. Kolliker, N. Muller, 1855), imposing on the contracting heart the neuromuscular preparation of the frog, consisting of the sciatic nerve connected to the gastrocnemius muscle, was obtained with the contraction of the heart a double contraction: one at the beginning of systole and the other (non-permanent ) at the beginning of diastole. Thus, the electromotive force (EMF) of a naked heart was first recorded. For the first time, Waller (AD Waller, 1887) was able to register the EMF of the heart from the surface of the human body with 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 various sizes (Fig. 1). However, the recording of the EMF of the heart obtained by a capillary electrometer did not accurately reproduce its oscillations.


Fig. 1. Diagram of the distribution of isopotential lines on the surface of the human body, due to the electromotive force of the heart. Numbers denote potential values.

An accurate recording of the EMF of the heart from the surface of the human body - the electrocardiogram (ECG) - was made by Einthoven (W. Einthoven, 1903) by means of a string galvanometer, built on the principle of apparatus 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 the cells compared to their environment, are retained on the outer surface of the membrane by negatively charged anions located on its inner surface, impermeable to them.

Thus, a double electric layer appears on the shell of a living cell - the shell is polarized, and its outer surface is positively charged with respect to the inner contents, which are negatively charged.

This lateral potential difference is a resting potential. If microelectrodes are applied to the outer and inner sides of the polarized membrane, a current arises in the outer circuit. Writing the resulting potential difference gives a monophasic curve. When excitation occurs, the membrane of the excited area loses its semi-impenetrability, is depolarized and its surface becomes electronegative. Registering 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 at rest, there is a current of action - action potential. When excitement covers the entire muscle fiber, its surface becomes electronegative. Termination of excitation causes a wave of repolarization, and the resting potential of the muscle fiber is restored (Fig. 2).


Fig. 2. Schematic representation of polarization, depolarization and repolarization of the cell.

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

The excitation wave (2) instantly breaks this equilibrium, and the surface of the cell becomes electronegative with respect to its inner side; Such a 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 after the excitation wave there follows a repolarization wave (4), which restores the polarization of the rest state (5).

The process of excitation in a normal human heart - depolarization - goes as follows. Arising in the sinus node, located in the right atrium, the excitation wave propagates at a speed of 800-1000 mm in 1 sec. ray-shaped along the muscle bundles of the first right and then the left atrium. Duration of excitation coverage of both atria is 0.08–0.11 sec.

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

Upon reaching the atrio-ventricular node, the spread of excitation slows down. Then, with a large and gradually increasing speed (from 1400 to 4000 mm in 1 sec.), It is directed along the bundle of His, his legs, their branches and forks, and reaches the final terminations of the conductor system. Having reached the contractile myocardium, the excitation with a significantly reduced rate (300–400 mm in 1 sec.) Spreads through both ventricles. Since the peripheral branches of the wiring system are scattered mainly under the endocardium, the inner surface of the cardiac muscle is the first to be excited. The further course of the excitation of the ventricles is not associated with the anatomical location of the muscle fibers, but is directed from the inner surface of the heart to the outer. The time of onset 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 branch system’s conductor system closest to these bundles and the thickness of the muscle layer separating the subepicardial muscle bundles from the peripheral branches of the conductor system.

First of all, the interventricular septum and right papillary muscle are excited. In the right ventricle, the excitation first covers the surface of its central part, since the muscular wall in this place is thin and its muscular layers closely adjoin the peripheral branches of the right leg of the conduction system. In the left ventricle, the apex comes first of all, as the wall separating it from the peripheral branches of the left leg is thin. For different points of the surface of the right and left ventricles of the normal heart, the period of excitement begins at a strictly defined time, and most of the fibers on the surface of the thin-walled right ventricle and only a small amount of fibers on the surface of the left ventricle due to their proximity to the peripheral ramifications of the wiring system (the fig. 3).


Fig. 3. Schematic representation of the normal excitation of the interventricular septum and the outer walls of the ventricles (according to Sodi-Palareres et al.). The 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 that, the subendocardial muscular layers of the outer wall of the left (0.03 sec.) And right (0.04 sec.) Ventricles are excited. The basal parts of the external walls of the ventricles are excited last (0.05–0.09 sec.).

The process of stopping the excitation of the muscle fibers of the heart - repolarization - cannot be considered fully studied. The process of atrial repolarization coincides mostly 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 depolarization process. This is explained by the fact that the duration of excitation of muscle bundles of the surface layers of the myocardium is less than the duration of excitation of subendocardial fibers and papillary muscles. The recording of 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.

Recording of the EMF of the heart is currently being done by several different methods than that recorded by Einthoven. Einthoven recorded the current resulting from connecting two points on the surface of the human body. Modern devices - electrocardiographs - register directly the voltage caused by the electromotive force of the heart.

The voltage due to the heart, equal to 1–2 mV, is amplified by radio tubes, semiconductors, or a cathode ray tube up 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. The recording is made on photo paper or photographic film or directly on paper (ink-writing, thermal recording, inkjet recording). The most accurate results are recorded on photo paper or film and inkjet recording.

To explain the peculiar form of ECG, various theories of its genesis were proposed.

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

Considering that when two microelectrodes register the outer and inner surface of the membrane in states of rest, excitement and damage, a monophasic curve is obtained, M. T. Udelnov believes that the monophasic curve reflects the basic form of the bioelectric activity of the myocardium. The algebraic sum of two monophasic curves gives an ECG.

Pathological changes in the ECG 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 in the period of excitation can be represented schematically as consisting of two poles: negative and positive.

Immediately before the excitation wave in any place of its propagation, the cell surface is electropositive (state of polarization at rest), and directly behind the excitation wave, the cell surface is electronegative (depolarization state; Fig. 4). These electric charges of opposite signs, grouped in pairs on one and the other side of each place covered by the excitation wave, form electric dipoles (a). Repolarization also creates an incalculable number of dipoles, but, unlike the above dipoles, the negative pole is in front, and the positive pole is in the back relative to the direction of wave propagation (b). If the 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).


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


Fig. 5. The scheme of an equilateral triangle according to Einthoven, Faro and Wart.

Muscle fiber is a small bipolar generator producing a small (elementary) EMF - an elementary dipole.

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

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

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

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

The EMF vectors that appeared simultaneously at the individual fibers of the heart are summarized according to the rule of vector addition.

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

The total vector of two vectors of the same size, arranged in 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 the 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 larger 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 shorter than it. Vector ECG analysis is to determine the spatial direction and magnitude of the total EMF of the heart at any time of its excitation by the teeth of an ECG.