Atrioventricular block - causes, symptoms, diagnosis and treatment

Atrioventricular (atrioventricular) blockade (AV-blockade) is a violation of the conduction function, expressed in slowing down or stopping the passage of an electrical pulse between the atria and ventricles and leading to a disorder of the heart rhythm and hemodynamics. AV blockade may be asymptomatic or accompanied by bradycardia, weakness, dizziness, angina attacks and loss of consciousness. Atrioventricular block is confirmed by electrocardiography, Holter ECG monitoring, EFI. Treatment of atrioventricular blockade can be medical or cardiac surgery (implantation of an electrocardiostimulator).

General information

Atrioventricular blockade is based on the slowing down or complete cessation of the passage of the pulse from the atria to the ventricles due to damage to the AV node itself, the Gis bundle or the legs of the Gis bundle. At the same time, the lower the level of damage, the more severe the manifestations of the blockade and the more unsatisfactory the prognosis. The prevalence of atrioventricular blockade is higher among patients suffering from concomitant cardiopathology. Among people with heart diseases, grade I AV block occurs in 5% of cases, grade II - in 2% of cases, grade III AV block usually develops in patients older than 70 years. Sudden cardiac death, according to statistics, occurs in 17% of patients with complete AV block.

The atrioventricular node (AV node) is part of the conduction system of the heart, which provides a consistent contraction of the atria and ventricles. The movement of electrical impulses coming from the sinus node slows down in the AV node, providing the possibility of atrial contraction and pumping blood into the ventricles. After a short delay, the impulses propagate along the Gis bundle and its legs to the right and left ventricles, contributing to their excitation and contraction. This mechanism provides alternate contraction of the myocardium of the atria and ventricles and maintains stable hemodynamics.

Classification of AV blockades

Depending on the level at which the violation of the electrical impulse develops, proximal, distal and combined atrioventricular blockades are distinguished. With proximal AV blockades, pulse conduction may be disrupted at the level of the atria, AV node, trunk of the Gis bundle; with distal ones - at the level of branches of the Gis bundle; with combined ones, multi–level conduction disturbances are observed.

Taking into account the duration of development of atrioventricular blockade, its acute (with myocardial infarction, drug overdose, etc.), intermittent (intermittent – with coronary artery disease, accompanied by transient coronary insufficiency) and chronic forms are distinguished. According to electrocardiographic criteria (deceleration, periodicity or complete absence of an impulse to the ventricles), there are three degrees of atrioventricular blockade:

• Grade I - atrioventricular conduction through the AV node is slowed down, but all impulses from the atria reach the ventricles. It is not clinically recognized; on the ECG, the P-Q interval is extended > 0.20 seconds.
• Grade II - incomplete atrioventricular block; not all atrial impulses reach the ventricles. On the ECG - periodic loss of ventricular complexes. There are three types of AV blockade of the II degree according to Mobitz:

1 Type I Mobitz - the delay of each subsequent pulse in the AV node leads to a complete delay of one of them and the loss of the ventricular complex (Samoilov - Wenkebach period).

2 Mobitz Type II - critical pulse delay develops suddenly, without prior prolongation of the delay period. At the same time, the absence of every second (2:1) or third (3:1) pulse is noted.

• Grade III - (complete atrioventricular blockade) - complete cessation of the passage of impulses from the atria to the ventricles. The atria contract under the influence of the sinus node, the ventricles - in their own rhythm, less than 40 times a minute, which is not enough to ensure adequate blood circulation.

Atrioventricular blockades of the I and II degrees are partial (incomplete), the blockade of the III degree is complete.

Reasons for the development of AV blockades

Functional and organic atrioventricular blockades differ in etiology. Functional AV blockades are caused by an increase in the tone of the parasympathetic nervous system. Atrioventricular blockade of the I and II degrees in isolated cases is observed in young physically healthy individuals, trained athletes, pilots. It usually develops during sleep and disappears during physical activity, which is explained by the increased activity of the vagus nerve and is considered as a variant of the norm.

AV blockades of organic (cardiac) genesis develop as a result of idiopathic fibrosis and sclerosis of the conduction system of the heart in its various diseases. The causes of cardiac AV blockades can be rheumatic processes in the myocardium, cardiosclerosis, syphilitic heart disease, ventricular septal infarction, heart defects, cardiomyopathy, myxedema, diffuse connective tissue diseases, myocarditis of various genesis (autoimmune, diphtheria, thyrotoxic), amyloidosis, sarcoidosis, hemochromatosis, heart tumors, etc. With cardiac AV blockades, partial blockade may be observed at first, however, as cardiopathology progresses, grade III blockade develops.

Various surgical procedures can lead to the development of atrioventricular blockages: prosthetics of the aortic valve, plastic surgery of congenital heart defects, atrioventricular RF of the heart, catheterization of the right parts of the heart, etc.

Quite rarely in cardiology there is a congenital form of atrioventricular blockade (1:20,000 newborns). In the case of congenital AV blockages, there is a lack of sections of the conducting system (between the atria and the AV node, between the AV node and the ventricles or both legs of the Gis bundle) with the development of an appropriate level of blockade. In a quarter of newborns, atrioventricular block is combined with other congenital cardiac abnormalities.

Among the causes of atrioventricular blockades, intoxication with medications is often found: cardiac glycosides (digitalis), beta-blockers, calcium channel blockers (verapamil, diltiazem, less often - corinthar), antiarrhythmics (quinidine), lithium salts, some other drugs and their combinations.

Symptoms of AV blockades

The nature of clinical manifestations of atrioventricular blockages depends on the level of conduction impairment, the degree of blockade, etiology and severity of concomitant heart disease. Blockades that have developed at the level of the atrioventricular node and do not cause bradycardia do not manifest themselves clinically. The clinic of AV blockade with this topography of violations develops in cases of pronounced bradycardia. Due to a low heart rate and a drop in the minute release of blood by the heart under physical exertion, such patients experience weakness, shortness of breath, and sometimes angina attacks. Due to a decrease in cerebral blood flow, dizziness, transient feelings of confusion and fainting may occur.

With atrioventricular blockade of the II degree, patients feel the loss of the pulse wave as interruptions in the heart area. With type III AV block, seizures of Morgan-Adams-Stokes occur: a decrease in the pulse to 40 or less beats per minute, dizziness, weakness, darkening of the eyes, short-term loss of consciousness, pain in the heart, cyanosis of the face, possibly convulsions. Congenital AV blockades in children and adolescents may be asymptomatic.

Complications of AV blockades

Complications in atrioventricular blockades are mainly due to a pronounced slowing of the rhythm, developing against the background of organic heart damage. Most often, the course of AV blockades is accompanied by the appearance or aggravation of chronic heart failure and the development of ectopic arrhythmias, including ventricular tachycardia.

The course of complete atrioventricular blockade may be complicated by the development of Morgagni-Adams-Stokes seizures associated with brain hypoxia as a result of bradycardia. The onset of the attack may be preceded by a feeling of heat in the head, attacks of weakness and dizziness; during the attack, the patient turns pale, then cyanosis and loss of consciousness develop. At this point, the patient may need to perform an indirect heart massage and ventilator, since prolonged asystole or the addition of ventricular arrhythmias increases the likelihood of sudden cardiac death.

Repeated episodes of loss of consciousness in elderly patients can lead to the development or aggravation of intellectual and mnestic disorders. Less often, with AV blockades, arrhythmogenic cardiogenic shock may develop, more often in patients with myocardial infarction.

In conditions of insufficient blood supply during AV blockades, the phenomena of cardiovascular insufficiency (collapse, fainting), exacerbation of coronary heart disease, kidney diseases are sometimes observed.

Diagnosis of AV blockades

When assessing the patient's anamnesis in case of suspicion of atrioventricular blockade, the fact of past myocardial infarction, myocarditis, other cardiopathologies, taking medications that violate atrioventricular conduction (digitalis, beta-blockers, calcium channel blockers, etc.) is clarified.

During auscultation of the heart rhythm, the correct rhythm is listened to, interrupted by long pauses indicating the loss of ventricular contractions, bradycardia, the appearance of a cannon I tone of Strazhesco. An increase in the pulsation of the cervical veins is determined in comparison with the carotid and radial arteries.

On an ECG, AV block of the I degree is manifested by an elongation of the interval P-Q > 0.20 sec.; II degree - by a sinus rhythm with pauses, as a result of loss of ventricular complexes after the P wave, the appearance of Samoilov-Wenkebach complexes; III degree - by a decrease in the number of ventricular complexes 2-3 times compared to atrial (from 20 to 50 per minute).

Daily monitoring of the Holter ECG during AV blockades allows you to compare the patient's subjective sensations with electrocardiographic changes (for example, fainting with sudden bradycardia), assess the degree of bradycardia and blockade, the relationship with the patient's activity, medication intake, determine whether there are indications for implantation of a pacemaker, etc.

With the help of electrophysiological examination of the heart (EFI), the topography of the AV block is clarified and indications for its surgical correction are determined. In the presence of concomitant cardiopathology and for its detection during AV block, echocardiography, MSCT or MRI of the heart are performed.

Additional laboratory tests with AV block are indicated in the presence of concomitant conditions and diseases (determination of the blood level of electrolytes in hyperkalemia, the content of antiarrhythmics in their overdose, enzyme activity in myocardial infarction).

Treatment of AV blockades

With atrioventricular blockade of the I degree, proceeding without clinical manifestations, only dynamic observation is possible. If the AV block is caused by taking medications (cardiac glycosides, antiarrhythmic drugs, beta-blockers), it is necessary to adjust the dose or completely cancel them.

With AV-blockades of cardiac genesis (with myocardial infarction, myocarditis, cardiosclerosis, etc.), a course of treatment with beta-adrenostimulators (isoprenaline, orciprenaline) is carried out, further implantation of a pacemaker is indicated.

First aid drugs for the relief of Morgagni-Adams-Stokes seizures are isoprenaline (sublingually), atropine (intravenously or subcutaneously). In cases of congestive heart failure, diuretics, cardiac glycosides (with caution), vasodilators are prescribed. As symptomatic therapy in the chronic form of AV blockades, theophylline, belladonna extract, and nifedipine are treated.

A radical method of treating AV blockades is the installation of an electrocardiostimulator (EX), which restores the normal rhythm and heart rate. Indications for implantation of endocardial EX are the presence in the anamnesis of attacks of Morgagni-Adams-Stokes (even once); the frequency of ventricular rhythm less than 40 per minute and periods of asystole of 3 or more seconds; AV block of II degree (II type according to Mobitz) or III degree; complete AV block accompanied by angina pectoris, congestive heart failure, high arterial hypertension, etc. To resolve the issue of surgery, a consultation with a cardiac surgeon is necessary.

Prognosis and prevention of AV blockades

The effect of the developed atrioventricular blockade on the patient's future life and ability to work is determined by a number of factors and, above all, the level and degree of blockade, the underlying disease. The most serious prognosis is for the III degree of AV blockade: patients are incapacitated, the development of heart failure is noted.

The prognosis is complicated by the development of distal AV blockades due to the threat of complete blockade and a rare ventricular rhythm, as well as their occurrence against the background of acute myocardial infarction. Early implantation of an electrocardiostimulator can increase the life expectancy of patients with AV blockades and improve their quality of life. Complete congenital atrioventricular blockages are prognostically more favorable than acquired ones.

As a rule, atrioventricular blockade is caused by the underlying disease or pathological condition, therefore, its prevention is the elimination of etiological factors (treatment of cardiac pathology, exclusion of uncontrolled intake of drugs that affect the conduction of impulses, etc.). To prevent the aggravation of the degree of AV blockade, implantation of an electrocardiostimulator is indicated.