(See also Overview of Arrhythmias.)
The most common causes of AV block are
The remaining cases of AV block are caused by
Atrioventricular block may be partial or complete. First-degree and second-degree blocks are partial. Third degree blocks are complete.
All normal P waves are followed by QRS complexes, but the PR interval is longer than normal (> 0.20 second—see figure 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, but further investigation may be indicated when it accompanies another heart disorder or appears to be caused by drugs.
Some normal P waves are followed by QRS complexes, but some are not. Two types exist:
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 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 and transient or reversible causes have been excluded. Treatment is pacemaker insertion, 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 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 is indicated.
In high-grade 2nd-degree AV block, every 2nd (or more) P wave is blocked (see figure Second-degree AV 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.
Patients with any form of 2nd-degree AV block and a structural heart disorder should be considered candidates for permanent pacing unless there is a transient or reversible cause.
Heart block is complete in 3rd-degree AV block (see figure Third-degree AV block).
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. If the block is caused by antiarrhythmic drugs, 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.
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