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Atrioventricular Block

(AV Block)

By

L. Brent Mitchell

, MD, Libin Cardiovascular Institute of Alberta, University of Calgary

Last full review/revision Jan 2021| Content last modified Jan 2021
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Atrioventricular (AV) block is partial or complete interruption of impulse transmission from the atria to the ventricles. The most common cause is idiopathic fibrosis and sclerosis of the conduction system. Diagnosis is by electrocardiography; symptoms and treatment depend on degree of block, but treatment, when necessary, usually involves pacing.

The most common causes of AV block are

  • Idiopathic fibrosis and sclerosis of the conduction system (about 50% of patients)

  • Ischemic heart disease (40%)

The remaining cases of AV block are caused by

  • Drugs (eg, beta-blockers, calcium channel blockers, digoxin, amiodarone)

  • Increased vagal tone

  • Valvulopathy

  • Congenital heart, genetic, or other disorders

Atrioventricular block may be partial or complete. First-degree and second-degree blocks are partial. Third degree blocks are complete.

First-degree AV block

First-degree atrioventricular block

For 1st-degree block, conduction is slowed without skipped beats. All normal P waves are followed by QRS complexes, but the PR interval is longer than normal (> 0.2 sec). For 3rd-degree block, there is no relationship between P waves and QRS complexes, and the P wave rate is greater than the QRS rate.

First-degree atrioventricular block

First-degree AV block may be physiologic in younger patients with high vagal tone and in well-trained athletes. First-degree AV block is rarely symptomatic, and no treatment is required. Further investigation may be indicated when 1st degree AV block accompanies another heart disorder or appears to be caused by drugs.

Second-degree AV block

Some normal P waves are followed by QRS complexes, but some are not. Two types exist:

  • Mobitz type I

  • Mobitz type II

In Mobitz type I 2nd-degree AV block, the PR interval progressively lengthens with each beat until the atrial impulse is not conducted and the QRS complex is dropped (Wenckebach phenomenon); AV nodal conduction resumes with the next beat, and the sequence is repeated (see figure Mobitz type I 2nd-degree AV block Mobitz type I 2nd-degree atrioventricular block Atrioventricular (AV) block is partial or complete interruption of impulse transmission from the atria to the ventricles. The most common cause is idiopathic fibrosis and sclerosis of the conduction... read more Mobitz type I 2nd-degree atrioventricular block ).

Mobitz type I 2nd-degree atrioventricular block

The PR interval progressively lengthens with each beat until the atrial impulse is not conducted and the QRS complex is dropped (Wenckebach phenomenon); AV nodal conduction resumes with the next beat, and the sequence is repeated.

Mobitz type I 2nd-degree atrioventricular block

Mobitz type I 2nd-degree AV block may be physiologic in younger and more athletic patients. The block occurs at the AV node in about 75% of patients with a narrow QRS complex and at infranodal sites (His bundle, bundle branches, or fascicles) in the rest. If the block becomes complete, a reliable junctional escape rhythm typically develops. Treatment is therefore unnecessary unless the block causes symptomatic bradycardia Bradyarrhythmias The normal heart beats in a regular, coordinated way because electrical impulses generated and spread by myocytes with unique electrical properties trigger a sequence of organized myocardial... read more Bradyarrhythmias and transient or reversible causes have been excluded. Treatment is pacemaker insertion Cardiac Pacemakers The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic... read more Cardiac Pacemakers , which may also benefit asymptomatic patients with Mobitz type I 2nd-degree AV block at infranodal sites detected by electrophysiologic studies done for other reasons.

In Mobitz type II 2nd-degree AV block, the PR interval remains constant. Beats are intermittently nonconducted and QRS complexes dropped, usually in a repeating cycle of every 3rd (3:1 block) or 4th (4:1 block) P wave (see figure Mobitz type II 2nd-degree AV block Mobitz type II 2nd-degree atrioventricular block Atrioventricular (AV) block is partial or complete interruption of impulse transmission from the atria to the ventricles. The most common cause is idiopathic fibrosis and sclerosis of the conduction... read more Mobitz type II 2nd-degree atrioventricular block ).

Mobitz type II 2nd-degree atrioventricular block

The PR interval remains constant. Beats are intermittently nonconducted, and QRS complexes dropped, usually in a repeating cycle of every 3rd (3:1 block) or 4th (4:1 block) P wave.

Mobitz type II 2nd-degree atrioventricular block

Mobitz type II 2nd-degree AV block is always pathologic; the block occurs at the His bundle in 20% of patients and in the bundle branches in the rest. Patients may be asymptomatic or experience light-headedness, presyncope, and syncope, depending on the ratio of conducted to blocked beats. Patients are at risk of developing symptomatic high-grade or complete AV block, in which the escape rhythm is likely to be ventricular and thus too slow and unreliable to maintain systemic perfusion; therefore, a pacemaker Cardiac Pacemakers The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic... read more Cardiac Pacemakers is indicated.

Second-degree atrioventricular block (high grade)

Every second P wave is blocked in this example of high-grade 2nd-degree AV block.

Second-degree atrioventricular block (high grade)

The distinction between Mobitz type I and Mobitz type II block is difficult to make because 2 P waves are never conducted in a row. Risk of complete AV block is difficult to predict, and a pacemaker is usually indicated.

Third-degree AV block

Third-degree atrioventricular block

There is no relationship between P waves and QRS complexes (AV dissociation) in 3rd-degree AV block. The P wave rate is greater than the QRS rate. The underlying rhythm here is a junctional escape rhythm with narrow QRS complexes and a rate of approximately 65 beats/minute.

Third-degree atrioventricular block

There is no electrical communication between the atria and ventricles and no relationship between P waves and QRS complexes (AV dissociation). Cardiac function is maintained by an escape junctional or ventricular pacemaker. Escape rhythms originating above the bifurcation of the His bundle produce narrow QRS complexes, relatively rapid (> 40 beats/minute) and reliable heart rates, and mild symptoms (eg, fatigue, postural light-headedness, effort intolerance). Escape rhythms originating below the bifurcation produce wider QRS complexes, slower and unreliable heart rates, and more severe symptoms (eg, presyncope, syncope, heart failure). Signs include those of AV dissociation, such as cannon a waves, blood pressure fluctuations, and changes in loudness of the 1st heart sound (S1). Risk of asystole-related syncope and sudden death is greater if low escape rhythms are present.

Most patients require a pacemaker Cardiac Pacemakers The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic... read more Cardiac Pacemakers . If the block is caused by antiarrhythmic drugs Drugs for Arrhythmias The need for treatment of arrhythmias depends on the symptoms and the seriousness of the arrhythmia. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic... read more , stopping the drug may be effective, although temporary pacing may be needed. A block caused by acute inferior myocardial infarction usually reflects atrioventricular nodal dysfunction and may respond to atropine or resolve spontaneously over several days. A block caused by anterior myocardial infarction usually reflects extensive myocardial necrosis involving the His-Purkinje system and requires immediate transvenous pacemaker insertion with interim external pacing as necessary. Spontaneous resolution may occur but warrants evaluation of AV nodal and infranodal conduction (eg, electrophysiologic study, exercise testing, 24-hour ECG).

Most patients with congenital 3rd-degree AV block have a junctional escape rhythm that maintains a reasonable rate, but they require a permanent pacemaker before they reach middle age. Less commonly, patients with congenital AV block have a slow escape rhythm and require a permanent pacemaker at a young age, perhaps even during infancy.

Drugs Mentioned In This Article

Drug Name Select Trade
CORDARONE
ATROPEN
LANOXIN
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