Second degree atrioventricular block - Mobitz type I (Wenckebach phenomenon)

Second degree atrioventricular block – Mobitz type I (Wenckebach phenomenon)

Definition

Second degree atrioventricular block is intermittent failure of AV transmission. In Mobitz type I (Wenckebach phenomenon) the missed beat is preceded by a progressive lengthening of the PR interval.

Aetiology

Occurs most commonly in association with underlying acute coronary pathology such as post-MI. Wenckebach phenomenon may result from digoxin, verapamil or Î²-blocker overdose and a benign form exists in the young and athletes due to high vagal tone.

Pathophysiology

The site of pathology of Mobitz type I is the AV node itself (in contrast to Mobitz II where infra nodal pathology is thought to be the cause). Complete AV block may develop, when a ventricular escape rhythm must occur for cardiac output to be maintained.

Clinical features

Patients are usually asymptomatic; however, an irregular pulse is detected on examination.

Investigations

The ECG reveals the progressive lengthening of the PR interval until a beat is missed after which the PR interval returns to normal and the cycle recurs (see Fig. 2.12).

Complications

With the exception of cases occurring in the young and athletes, Wenckebach carries similar risks to Mobitz II with increased risk of ventricular bradycardia and sudden death. The greatest risk is at time of progression from second to third degree block as the ventricular escape rhythm is most unreliable at this time.

Management

Post-MI atropine may be used intravenously to reduce AV block and increase ventricular rate. Cardiac pacing may be required either as a temporary measure or permanently in persistent cases.

Other symptomatic patients should be paced permanently.