Atrioventricular (AV) block can cause intermittent failure of atrial impulses to reach the ventricles or complete independence of atrial and ventricular activity.
Second-degree AV block patterns
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In Mobitz type I, also called the Wenckebach phenomenon, the PR interval progressively increases until a P wave is not conducted; the cycle then repeats.
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The illustrated Mobitz type I recording has 4:3 conduction, producing groups of three ventricular complexes.
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In the illustrated Mobitz type II recording, conducted beats have a normal, constant PR interval, while some P waves are not conducted. The constant PR interval distinguishes this pattern from Wenckebach.
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In fixed 2:1 AV block, alternate P waves are not conducted. This can result from either Mobitz type I or type II, so the two usually cannot be distinguished from this pattern alone.
Complete AV block and escape rhythms
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In third-degree AV block, conduction fails completely and the atria and ventricles beat independently, producing AV dissociation.
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An escape rhythm arising in the AV node or bundle of His maintains ventricular activity with narrow QRS complexes.
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An escape rhythm arising in distal Purkinje tissues or myocardium produces broad QRS complexes. Distal escape rhythms tend to be slower and less reliable.
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The illustrated complete AV block recording has an atrial rate of 80/min and a ventricular rate of 38/min.
Clinical findings in complete AV block
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The pulse is slow, at 25–50/min, and regular. It does not vary with exercise, except in congenital complete AV block.
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A compensatory increase in stroke volume usually produces a large-volume pulse.
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Cannon waves may be visible in the neck, and first heart sound intensity varies because AV synchrony is lost.