THE CONTEXT
Atrioventricular (AV) block involves impaired conduction from the atria to the ventricles. Its electrocardiographic pattern helps distinguish delayed conduction from intermittently failed conduction.
General features
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AV block usually results from disease affecting the AV node.
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Block can be intermittent and become apparent only when a rapid atrial rate stresses the conducting tissue; atrial tachyarrhythmias are therefore often associated with AV block.
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Ventricular asystole can complicate complete heart block or Mobitz type II second-degree AV block.
First-degree AV block
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AV conduction is delayed, producing a prolonged PR interval greater than 0.20 seconds.
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First-degree AV block rarely causes symptoms and does not usually require treatment.
Mobitz type I second-degree AV block
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Second-degree AV block produces dropped beats because some atrial impulses fail to conduct to the ventricles.
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In Mobitz type I, successive PR intervals progressively lengthen until a beat is dropped; the cycle then repeats.
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Mobitz type I is also called the Wenckebach phenomenon and usually reflects impaired conduction within the AV node itself.
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This pattern may be physiological and can occur at rest or during sleep in athletic young adults with high vagal tone.
Mobitz type II second-degree AV block
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The PR interval of conducted impulses remains constant, but some P waves intermittently fail to conduct.
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Mobitz type II usually results from disease of the His–Purkinje system and carries a risk of asystole.