atrioventricular block



Overview.

Definition

Atrioventricular (AV) block is an abnormally delayed transmission of an impulse from the atria to the ventricles, resulting in a partial or complete failure to transmit the impulse.

The site of the block can be the AV node, the Hitchcock’s bundle, or the right and left bundle branches.

The impulse to beat originates in the sinoatrial node, and the conduction pathway is “sinoatrial node → inter-nodal bundle → atrioventricular node → Hippocampal bundle → right and left bundle branches → Purkinje fibers network”, resulting in a regular heart beat. If there is an abnormality in the conduction system, conduction block occurs.

Classification

Classification according to the degree of block

Atrioventricular block: Incomplete atrioventricular block is characterized by delayed conduction of impulses, but all atrial impulses are transmitted down to the ventricles. It can be seen in healthy people, but is more common in people with heart disease.

Second-degree AV block

Incomplete atrioventricular block occurs when part of the atrial impulse does not travel down to the ventricles.

It can be further divided into second-degree type I and second-degree type II AV block.

Third-degree atrioventricular block: all atrial impulses cannot be transmitted down to the ventricle, and the atria and ventricles move independently of each other, which is complete atrioventricular block.

Classification according to the cause

Congenital atrioventricular (AV) block: a congenital disease that causes conduction block.

Primary atrioventricular block: chronic atrioventricular block of unknown origin.

Secondary AV block: AV block caused by damage to the conduction system due to some heart diseases (e.g. coronary atherosclerotic heart disease, various types of cardiomyopathy, myocarditis, etc.).

Epidemiology

Epidemiologic information on AV block is not clear.

One study showed that the incidence of one degree of AV block in a young, asymptomatic population without organic heart disease was 0.5%.

In patients with acute myocardial infarction, varying degrees of atrioventricular block are seen in 11.6% to 17.7%.

In patients with rheumatic heart disease, about 26% have transient first or second degree type I AV block.

Causes

Causes

Intracardiac lesions

Ischemic heart disease: coronary heart disease, acute myocardial infarction (the most common cause of acute atrioventricular block, about 14% of the block is caused by infarction of the lower wall of the myocardium, and about 2% is caused by infarction of the anterior wall of the myocardium), and coronary artery spasm.

Myocarditis: acute and chronic infectious (viral, bacterial, helminthic, etc.) and non-infectious (rheumatic, due to physical and chemical factors, drug-related) myocarditis, etc., are the main causes of atrioventricular block in children and adolescents.

Degenerative changes in the conduction system or myocardium

Fibrosis of the conduction system of unknown origin; myocardial fibrous degeneration caused by coronary artery disease, cardiomyopathy (dilated cardiomyopathy is the most common), inflammatory or injurious lesions.

Calcification of the aortic valve and myocardial degeneration due to tumor compression (especially pericardial mesothelioma). Degenerative changes of the conduction system of unknown origin are more common in the elderly. If the pathologic changes are reversible, the block often recovers in the short term, otherwise it is persistent.

Congenital heart disease: may be isolated congenital AV block or combined with other cardiac malformations (e.g., large vessel malposition, septal or endocardial cushion defects, etc.).

Other factors

Cardiac surgical injury: mitral or aortic valve replacement, radical surgery for tetralogy of Fallot, repair of large ventricular septal defects, etc.

Vagal hyperfunction, electrolyte disorders (e.g. hyperkalemia), hyperthyroidism, hypoxia or drug effects (e.g. digitalis). Among these, some healthy individuals or athletes can develop first or second degree type I AV block, mostly associated with elevated vagal tone, usually at night.

Risk factors

Old age, male gender, history of myocardial infarction, and history of heart failure are associated with AV block.

Systolic blood pressure and blood glucose are independently associated with AV block. For every 10 mmHg increase in systolic blood pressure, the risk of atrioventricular block increases by 22%; for every 1.1 mol/L (20 mg/dl) increase in fasting blood glucose, the risk increases by 19%.

Mechanism of occurrence

AV block may be associated with pathologic alterations of the atrioventricular (AV) conduction system due period, such as significant prolongation of the absolute due period (cardiomyocytes do not generate an action potential no matter how strong a stimulus is given) or prolongation, shortening, or loss of the relative due period (cardiomyocytes are given a threshold stimulus that causes them to generate an action potential).

In the case of prolongation of the relative period of inactivity (seen in second-degree type I AV block), the speed of conduction depends on the moment at which the excitation arrives at the site of the block. If successive excitations fall earlier and earlier, one by one, within the relative phase of atrioventricular node, until an excitation falls within the absolute phase of atrioventricular node area and cannot be transmitted downward.

With a shortened or absent relative phase (as seen in second-degree type II AV block), the excitation can be transmitted essentially one at a time at the same rate, and thus the PR interval is the same. Whether the excitation falls in early or late diastole, the PR intervals for downbeat conduction are equal as long as it is outside the absolute period of inappropriateness, whereas when the excitation falls within the absolute period of inappropriateness, it cannot be downbeat.

Symptoms

First degree AV block

Usually asymptomatic.

Second degree AV block

May be asymptomatic or have symptoms such as palpitations and chest tightness.

Third degree AV block

Symptoms depend on the speed of the ventricular rate and concomitant diseases.

A low ventricular rate can cause a decrease in blood flow to the heart, brain, and kidneys, resulting in dizziness, fatigue, dyspnea, blackouts (darkness and blurred vision), or even A.S. Syndrome (i.e., an abnormally slowed or dramatically increased heart rate causing a sudden decrease in blood flow to the heart, leading to insufficient blood supply to the brain, acute ischemic attacks, and fainting and convulsions as clinical syndromes) and sudden cardiac death.

Consultation

Department of Medicine

Cardiovascular Medicine

Physical examination reveals atrioventricular block or symptoms such as palpitations, chest discomfort, dizziness, and fatigue, etc. It is recommended to consult a doctor promptly.

Emergency Department

Loss of consciousness, convulsions, respiratory arrest, etc., it is recommended to go to the Emergency Department or call 120 emergency immediately.

Preparation for medical treatment

Preparing for medical treatment: registration, preparation of documents, common problems

Tips for seeking medical treatment

Some patients with atrioventricular block may have no obvious symptoms, and should seek prompt medical attention for a clear diagnosis after physical examination.

Preparation List

Symptom list

Pay special attention to the time of onset of symptoms, special manifestations, etc.

How do you feel at present? Are there palpitations, dizziness, blackouts, shortness of breath, etc.?

Are there any triggers for the attacks?

What is the frequency, duration and mode of relief of the attacks?

List of medical history

Is there any organic heart disease? Such as coronary heart disease, myocarditis, cardiomyopathy, congenital heart disease, etc.?

Has any recent heart-related surgery been performed?

What tests have been performed? What are the test results?

Has it ever been treated? How was it treated? What is the effect of the treatment?

Checklist

Results of tests in the last 6 months, which can be brought to the doctor’s office

Esophageal electrocardiogram

Intracardiac electrophysiologic examination

Echocardiogram

Routine electrocardiogram

Diagnosis

Diagnostic Basis

Medical history

There may be a history of coronary artery disease, myocarditis, or cardiomyopathy.

Clinical manifestations

Symptoms

In mild cases, there may be no symptoms, or only mild palpitations, chest tightness and other manifestations.

In severe cases, there may be cardiac and cerebral ischemia manifestations, such as dizziness, fatigue, dyspnea, chest pain, blackout, or even fainting.

Signs and symptoms

First-degree atrioventricular block: the intensity of the first heart sound is weakened during auscultation.

Second degree atrioventricular block

Second-degree type I: the intensity of the first heart sound gradually decreases, and there is a leakage of heart beat.

Second-degree type II: constant intensity of the first heart sound with intermittent beat leakage.

Third degree atrioventricular block: due to atrioventricular separation, the intensity of the first heart sound changes frequently, the second heart sound may be normal or abnormally split, and a loud and hyperactive first heart sound (cannon sound) is occasionally heard.

Routine electrocardiogram

A routine electrocardiogram (ECG) is a non-invasive test that can usually diagnose and deduce the location of the block.

First degree AV block

Each P wave is followed by a QRS wave.

Prolonged PR interval

PR interval >0.20 seconds in adults and >0.18 seconds in children.

May originate from the AV node and/or Hitchcock’s bundle-Purkinje fibers.

The QRS wave trains have a normal morphology and time frame.

Second-degree AV block

Second-degree type I: Also known as ventricular block, this is the most common type of second-degree AV block and rarely progresses to third-degree AV block.

P waves appear regularly.

The PR interval is progressively prolonged until the downward transmission of the P wave is blocked and 1 QRS wave cluster is missed to form a cycle. The most common atrioventricular conduction ratios are 3:2 (i.e., 2 QRS waves following every 3 P waves) and 5:4 (i.e., 4 QRS waves following every 5 P waves).

Site of block: In most cases, the block is located in the AV node (about 80%) with a normal QRS wave group.

Second degree type II

Constant PR interval, some P waves followed by no QRS wave clusters.

The long RR interval (with QRS wave miss) is equal to twice the short RR interval.

Site of block: If the QRS wave cluster is normal, the block may be located in the AV node; if the QRS wave cluster is widened, the block is mostly located in the Hirschsprung’s bundle-Purkinje system.

High atrioventricular block: two or more consecutive P waves cannot be transmitted down to the ventricle. The ratio of atrioventricular conduction is mostly (3~8):1, and it often evolves into third-degree atrioventricular block in response to severe lesions of the conduction system.

2:1 atrioventricular block

Every 2 P waves are followed by 1 QRS wave.

Site of block: either in the AV node, Hitchcock’s bundle or bilateral bundle branches.

Third degree AV block

No fixed relationship between P waves and QRS wave clusters: all P waves do not travel down to the ventricles, the atria and ventricles are controlled by separate pacing points, and the atria are separated from the ventricles.

The atrial rate is faster than the ventricular rate, and the atrial impulse comes from the sinus node or ectopic atrial rhythms (atrial tachycardia, flutter, or fibrillation).

The ventricular pacing point is usually slightly below the block site.

If located in the Hitchcock bundle and its immediate neighborhood, the ventricular rate is 40 to 60 beats per minute, the QRS wave trains are normal, and the rhythm is stable.

If it is located distal to the intraventricular conduction system, the ventricular rate may be as low as 40 beats/minute or less, the QRS waveforms are widened, and the ventricular rhythm is often unstable.

Suspension of spontaneous rhythm at the ventricular pacing point results in ventricular arrest, which is characterized by a series of P waves on the electrocardiogram.

Esophageal electrocardiogram

The posterior wall of the left atrium is adjacent to the esophagus. By utilizing the anatomical location of the heart and the esophagus, the electrodes are fed into the esophagus through the nasal cavity and placed at the level of the atria, and a clear picture of the atrial potentials can be recorded.

It is a noninvasive cardiac electrophysiologic examination method.

The esophageal ECG clearly identifies atrial and ventricular electrical activity, facilitating the determination of atrioventricular separation, and also allows for rapid atrial pacing or programmed electrical stimulation.

Intracardiac electrophysiology

Atraumatic intracardiac electrophysiology is necessary when the site of block cannot be determined by routine ECG analysis and the site of block is critical to patient management.

Indications: Symptomatic, suspected Hippocampal system block, but the diagnosis cannot be confirmed; second- or third-degree atrioventricular block still symptomatic after pacemaker treatment, and suspected other arrhythmias causing symptoms.

Recording or stimulating intracardiac electrical activity by implanting a multielectrode catheter in a vein or in the atrial system.

Echocardiography

To examine the heart for structural lesions.

Differential Diagnosis

Sinus bradycardia

Similarities

Both may be asymptomatic or present only with palpitations and chest tightness.

In second degree type II AV block with 2:1 conduction, the P wave that is not transmitted is easily misdiagnosed as sinus bradycardia if the P wave is heavy in the T wave.

Difference: In case of sinus bradycardia with long RR interval, T wave should be observed to see whether there is a trace of T wave, and if necessary, let the patient to do up and down activities in bed, such as sinus heart rate is still 1:1 atrial response after the increase in heart rate, can be excluded from the second degree of type II atrioventricular block.

Treatment

First-degree and second-degree type I atrioventricular block: asymptomatic patients do not need special treatment.

Second-degree type II and third-degree atrioventricular block: if the ventricular rate is significantly slow, accompanied by obvious symptoms or hemodynamic changes, pacemaker treatment should be given.

Treatment of etiology

According to the different causes of the disease, treatments such as improving myocardial blood supply, increasing myocardial nutrition and correcting electrolyte balance should be adopted.

Discontinue the drugs that cause atrioventricular block, such as β-blockers, calcium channel blockers, digoxin, etc.

Actively treat metabolic diseases, such as hypothyroidism.

Symptomatic treatment

Drug therapy

If the block occurs in the AV node, drugs such as isoproterenol can be applied adjunctively, only in emergency situations without cardiac pacing conditions.

Isoprenaline increases AV node conduction velocity and inhibits subnodal conduction.

The use of isoprenaline in acute myocardial infarction may result in severe ventricular arrhythmias and should be used with caution.

Pacemaker Implantation

In symptomatic patients with a slow ventricular rate, temporary or permanent cardiac pacing should be given early.

Implantation of a pacemaker, which is an instrument placed in the heart to control the heart rate, temporarily or permanently raises the heart rate and reduces the risk of serious conditions.

Most patients with AV block require a dual-chamber pacemaker to maintain atrial synchronization, prevent pacemaker syndrome, and reduce the risk of atrial fibrillation.

Precautions for Pacemaker Use

Try to stay away from electronic devices such as cell phones.

Stay away from magnetic cards, magnets, induction cookers, and other items with magnetic properties.

Avoid magnetic resonance imaging.

Seek medical attention if there is any abnormality in function.

Prognosis

Cure

The prognosis depends on the degree and location of AV block and the combination of underlying diseases.

Asymptomatic patients generally have a good prognosis.

In symptomatic patients, atrioventricular block can be basically corrected after pacemaker implantation, but the prognosis is poor in patients with severe underlying cardiac diseases.

Hazards

Symptoms, if not treated in time, may lead to syncope, etc., affecting the patient’s life safety.

In severe cases, A. S. syndrome may occur and even lead to death.

After the installation of pacemakers, there are restrictions on the places of activities, daily necessities and so on.

Daily

Daily Management

Dietary management

Ensure adequate intake of protein, unsaturated fat, vitamins, minerals, dietary fiber and water. You may choose soy products, dairy, lean meat, fish (may be 1~2 times per week), fresh fruits, fresh vegetables and nuts.

Control sugar intake appropriately, and choose cereals and potatoes instead of refined rice and noodles.

Avoid diets high in salt and fat. Control salt intake to less than 6 grams per day, and avoid deep-fried, pickled and barbecued foods.

Avoid raw, cold, hard or too hot food, chew slowly and avoid overeating.

Eat less irritating foods such as chili peppers, coffee, strong tea and mustard.

Life management

Quit smoking and stay away from “second-hand smoke”.

Quit drinking.

Ensure adequate sleep and avoid staying up late.

Exercise in moderation and avoid over-exercise; if you feel unwell during exercise, stop immediately and take a rest.

Pay attention to safety

If dizziness occurs, sit down or lie down immediately to avoid falling.

Prevention

Active treatment of pre-existing diseases

Actively treat heart diseases such as coronary heart disease, myocarditis, cardiomyopathy, and extracardiac diseases such as hyperkalemia.

Actively treat high blood pressure and maintain normal blood sugar.

Reasonable use of medication

Use the medication according to the doctor’s requirements. If there is any discomfort during the use of medication, consult a doctor promptly.

Reasonable diet to prevent heart disease

Improve dietary structure to ensure adequate nutrition; avoid overeating and maintain appropriate body weight.

Avoid diets high in salt, sugar and fat, and try to avoid pickled, smoked, barbecued and fried foods.

Eat less stimulating foods, such as chili peppers, coffee, strong tea and mustard.

Eat more foods rich in dietary fiber, such as whole grains, fresh vegetables and fruits, etc. Food types should be diversified and alternated.