Type I (Wenckebach or Mobitz I) second-degree atrioventricular block occurs when there is intermittent failure of the atrial beats to reach the ventricle. It is characterized by progressive lengthening of the PR interval with subsequent shortening of the RR interval until a P wave is not conducted. The PR interval is not constant, and the longest interval (between the non-conducted and conducted beats) is less than twice the normal sinus interval. The periodic pauses create the impression of group beating (Fig, 3). Physical examination reveals regular A waves with progressive softening of the first heart sound. Wenckebach periodicity is usually asymptomatic. It is exacerbated by drugs (digoxin, b-blockers, calcium channel blockers) and heightened vagal tone, but in the setting of acute myocardial infarction, particularly inferior infarction, it often responds to atropine.
Fig. 3 Type I (Wenckebach) second-degree atrioventricular block. The rhythm strip shows progressive lengthening of the PR interval, with progressive shortening of the RR interval until a dropped beat occurs. The longest interval is less than twice the sinus interval and group beating, characteristic of the Wenckebach pattern, can be seen.
Type II (Mobitz II) second-degree atrioventricular block occurs when there is intermittent failure of the atrial beats to reach the ventricle and there is a sudden non-conducted P wave unassociated with any change in PR or RR intervals. Most often a sign of block in the bundle of His or lower in the conduction system, it is more likely than type I to progress to complete heart block and can be caused by ischemia. When it occurs with 2:1 periodicity, surface electrocardiography cannot distinguish it from type I and so invasive testing may be necessary. However, if the rate is fast and well tolerated, carotid sinus massage may slow it and lead to 1:1 conduction. Similarly, atropine or atrial pacing can change the atrial rate or the degree of atrioventricular node refractoriness and alter the periodicity in type I atrioventricular block. Although patients with Mobitz II block may be asymptomatic, it is a warning of potentially severe conduction system disease. In acute myocardial infarction a pacemaker is indicated for type II block because of the likelihood of progression to complete heart block.
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