Analysis and decoding of ECG
The ECG displays the processes of the onset of excitation and its conduct. The teeth are recorded when there is a potential difference between the parts of the excitable system, i.e. One part of the system is engulfed by excitation, and the other is not. The isopotential line occurs when there is no potential difference, i.e. When the entire system is not excited or, on the contrary, is enveloped in excitement. From the standpoint of electrocardiology, the heart consists of two excitable systems: the atria and the ventricles. Transmission of excitation between them is carried out by the conduction system of the heart. Due to the fact that the mass of the conducting system is small, the potentials arising in it under ordinary amplifications are not captured by a standard electrocardiograph, so the ECG reflects successive coverage by excitation of the contractile myocardium of the atria and ventricles.
In atria, excitation is spread from the sinoatrial node to the atrioventricular node. Normally, the velocity of propagation of excitation along the conductive atrial beams is approximately equal to the propagation velocity of the contractile myocardium of the atria, so the coverage by its excitation is displayed monophasically with the tooth P. The spread of excitation through the ventricular myocardium occurs by transferring excitation from the elements of the conducting system to the contractile myocardium, which causes the complex nature of the QRS complex . In this case, the Q wave corresponds to the excitation of the apex of the heart, the right papillary muscle and the inner surface of the ventricles, and the R tooth to the excitation of the base of the heart and the external surface of the ventricles. The process of propagation of excitation in the basal part of the interventricular septum, the right and left ventricles forms a tooth S on the ECG. The ST segment reflects the state of complete excitation of both ventricles, normally located on the isopotential line, since there is no potential difference in the excitable ventricular system. Tine T reflects the process of repolarization, i.e. Restoration of the membrane resting potential of myocardial cells. This process in different cells occurs asynchronously, so there is a potential difference between still depolarized portions of the myocardium that have a negative charge, and parts of the myocardium that have restored their positive charge. The indicated potential difference is recorded as a T wave . This tooth is the most variable part of the ECG. Between the tooth T and the subsequent tooth P , an isopotential line is recorded, since at this time there is no potential difference in the myocardium of the ventricles and atria.
The total duration of the electric systole of the ventricles (QRST) almost coincides with the duration of the mechanical systole (the mechanical systole begins somewhat later than the electric systole).
ECG makes it possible to evaluate the nature of disturbances in the excitation in the heart
Thus, in terms of the magnitude of the PQ interval (from the origin of the P wave and to the onset of the Q wave), it can be judged that excitation is performed from the myocardium to the myocardium of the ventricles. Normally this time is 0.12-0.2 s. The total duration of the QRS complex reflects the rate of coverage by excitation of the contractile ventricle myocardium and is 0.06-0.1 s.
Depolarization and repolarization processes occur in different parts of the myocardium non-simultaneously, so the potential difference between the different parts of the cardiac muscle changes throughout the cardiac cycle. The conditional line connecting at each moment two points with the greatest potential difference is usually called the electric axis of the heart. At each time, the electrical axis of the heart is characterized by its length and direction, i.e. Is a vector quantity. Changing the direction of the electrical axis of the heart can be important for diagnosis.
The ECG makes it possible to analyze in detail the changes in the heart rate. Normally, the heart rate is 60-80 per minute, with a more rare rhythm - bradycardia - 40-50, and with more frequent - tachycardia - exceeds 90-100 and reaches 150 per minute or more.
In some pathological conditions of the heart, the correct rhythm is occasionally or regularly disturbed by an extraordinary contraction - the extrasystole. If an extraordinary excitation occurs in the sinoatrial node at the moment when the refractory period is over, but another automatic impulse has not yet appeared, there is an early contraction of the heart - a sinus extrasystole. The pause following such extrasystole lasts the same time as usual.
Extraordinary excitation, which has arisen in the myocardium of the ventricles, does not affect the automatics of the atrioventricular node. This node timely sends another impulse reaching the ventricles at a time when they are in a refractory state after extrasystoles and therefore do not respond to another impulse. At the end of the refractory period, the ventricles can again respond to irritation, but it takes some time until the next impulse comes from the sinoatrial node. Thus, the extrasystole caused by the impulse that appeared in one of the ventricles (ventricular extrasystole) leads to a long-term so-called compensatory pause of the ventricles with an unchanged rhythm of atrial work.
Extrasystoles may appear in the presence of foci of irritation in the myocardium, in the region of atrial or ventricular pacemakers. Extrasystolia can also cause impulses coming into the heart from the central nervous system.
The ECG reflects changes in the magnitude and direction of action potentials, but does not allow one to evaluate the features of the cardiac delivery function. The potentials of the action of the membrane of myocardial cells are only a trigger mechanism of myocardial contraction, which includes a certain sequence of intracellular processes terminating with shortening of myofibrils. These successive processes are called conjugation of excitation and contraction.
Defeat of the myocardium to some extent can be observed with any generalized infection and affect the severity of the course and the outcome of the disease. At the same time, it is assumed that persistent infectious agents, and especially viruses, can lead to the development of chronic cardiac damage. The most clinically important causes of myocardial damage are enteroviruses, Epstein-Barr virus (EBV), cytomegalovirus (CMV), HIV, meningococcal, b-hemolytic group A streptococcus, Yersinia, botulinum toxin Corynebacterium diphtheriae (diphtheria), Borrelia burgdorferi (Lyme borreliosis ), Toxoplasma gondii (toxoplasmosis), etc.
Despite the fact that each infectious disease has its own etiology, pathogenesis and clinical manifestations, there are general patterns of myocardial damage and the corresponding ECG changes in acute and distant periods.
The most frequent infectious diseases on the ECG are changes in the final part of the ventricular complex in the form of depression or elevation of the ST segment and a decrease in the amplitude of the T wave . The severity of myocardial damage may indicate conduction abnormalities in the form of various atrioventricular blockades (AV blockade), left bundle branch blockade and disturbances of excitability in the form of ventricular tachycardia or ventricular extrasystole of high gradations.
ECG signs of blockade of the right bundle of the bundle, His polytropic atrial extrasystole, ST segment elevation usually accompany pericardial damage and / or increased pressure in the small circulation.
The conduction system of the heart in infectious diseases is less often affected than the contractile myocardium, which is manifested on the ECG by a more rare finding of ECG signs of conduction disturbance compared with the change in the ST segment . In the case of infectious pathology, the sensitivity of the ECG is higher than that of the clinical examination method.
ECG makes it possible to evaluate the nature of disturbances in the excitation in the heart
Thus, in terms of the magnitude of the PQ interval (from the origin of the P wave and to the onset of the Q wave), it can be judged that excitation is performed from the myocardium to the myocardium of the ventricles. Normally this time is 0.12-0.2 s. The total duration of the QRS complex reflects the rate of coverage by excitation of the contractile ventricle myocardium and is 0.06-0.1 s.
Depolarization and repolarization processes occur in different parts of the myocardium non-simultaneously, so the potential difference between the different parts of the cardiac muscle changes throughout the cardiac cycle. The conditional line connecting at each moment two points with the greatest potential difference is usually called the electric axis of the heart. At each time, the electrical axis of the heart is characterized by its length and direction, i.e. Is a vector quantity. Changing the direction of the electrical axis of the heart can be important for diagnosis.
The ECG makes it possible to analyze in detail the changes in the heart rate. Normally, the heart rate is 60-80 per minute, with a more rare rhythm - bradycardia - 40-50, and with more frequent - tachycardia - exceeds 90-100 and reaches 150 per minute or more.
Read also: ECG in pathology
In some pathological conditions of the heart, the correct rhythm is occasionally or regularly disturbed by an extraordinary contraction - the extrasystole. If an extraordinary excitation occurs in the sinoatrial node at the moment when the refractory period is over, but another automatic impulse has not yet appeared, there is an early contraction of the heart - a sinus extrasystole. The pause following such extrasystole lasts the same time as usual.
Extraordinary excitation, which has arisen in the myocardium of the ventricles, does not affect the automaticity of the atrioventricular node. This node timely sends another impulse reaching the ventricles at a time when they are in a refractory state after extrasystoles and therefore do not respond to another impulse. At the end of the refractory period, the ventricles can again respond to irritation, but it takes some time until the next impulse comes from the sinoatrial node. Thus, the extrasystole caused by a pulse that appeared in one of the ventricles ( ventricular extrasystole ) leads to a long-term so-called compensatory pause of the ventricles with an unchanged rhythm of atrial work.
Extrasystoles may appear in the presence of foci of irritation in the myocardium, in the region of atrial or ventricular pacemakers. Extrasystolia can also cause impulses coming into the heart from the central nervous system.
The ECG reflects changes in the magnitude and direction of action potentials, but does not allow one to evaluate the features of the cardiac delivery function. The potentials of the action of the membrane of myocardial cells are only a trigger mechanism of myocardial contraction, which includes a certain sequence of intracellular processes terminating with shortening of myofibrils. These successive processes are called conjugation of excitation and contraction.
Defeat of the myocardium to some extent can be observed with any generalized infection and affect the severity of the course and the outcome of the disease. At the same time, it is assumed that persistent infectious agents, and especially viruses, can lead to the development of chronic cardiac damage. The most clinically important causes of myocardial damage are enteroviruses, Epstein-Barr virus (EBV), cytomegalovirus (CMV), HIV, meningococcal, b-hemolytic group A streptococcus, Yersinia, botulinum toxin Corynebacterium diphtheriae (diphtheria), Borrelia burgdorferi (Lyme borreliosis ), Toxoplasma gondii (toxoplasmosis), etc.
Despite the fact that each infectious disease has its own etiology, pathogenesis and clinical manifestations, there are general patterns of myocardial damage and the corresponding ECG changes in acute and distant periods.
The most frequent infectious diseases on the ECG are changes in the final part of the ventricular complex in the form of depression or elevation of the ST segment and a decrease in the amplitude of the T wave . The severity of myocardial damage may indicate conduction abnormalities in the form of various atrioventricular blockades (AV blockade), left bundle branch blockade and disturbances of excitability in the form of ventricular tachycardia or ventricular extrasystole of high gradations.
ECG signs of blockade of the right bundle of the bundle, His polytropic atrial extrasystole, ST segment elevation usually accompany pericardial damage and / or increased pressure in the small circulation.
The conduction system of the heart in infectious diseases is less often affected than the contractile myocardium, which is manifested on the ECG by a more rare finding of ECG signs of conduction disturbance compared with the change in the ST segment . In the case of infectious pathology, the sensitivity of the ECG is higher than that of the clinical examination method.
ECG criteria for clinically significant myocardial damage
- ST segment depression is more than 2 mm from the isoline in three leads or more;
- any disturbance of conductivity, detected for the first time;
- ventricular extrasystole of high gradation.
ECG criteria for severe myocardial damage
- conduction disturbances in the form of AV dissociation with idioventricular rhythm, AV blockade of II degree of Mobits II type, first detected;
- ventricular tachycardia.
Medical expert editor
Portnov Alexey Alexandrovich
Education: Kiev National Medical University. A.A. Bogomolets, Specialty - "General Medicine"