Vagally Mediated AV Block

Case

A 20-year-old woman, without any comorbidities, presented to the out-patient department with complaints of presyncope. Her clinical examination was unremarkable, and baseline ECG showed no abnormalities (PR = 160 ms, QRS = 90 ms, no bundle branch block). What is the interpretation of the Holter recording (Figure 1), and what are the clinical implications?

Figure 1.

12 Lead Holter Monitoring (Recording at 3:59 am)

Discussion

The Holter recording (Figure 1) revealed a Mobitz type 1 AV block (prolongation of PR interval) associated with simultaneous slowing of the sinus rate (prolongation of PP interval). There is a single nonconducted P wave (black arrow) associated with a shortening of the subsequent PR interval, suggestive of a typical Wenckebach block (Figure 2). This phenomenon occurred more often during the night than during the day. These findings were suggestive of a vagally mediated AV block. She was advised lifestyle modification and reassurance regarding the benign nature of the condition.

Figure 2.

Simultaneous Prolongation of PR and PP Intervals

Vagally mediated AV block is defined as a paroxysmal first-, second- or third-degree AV block associated with slowing of the sinus rate. A simultaneous depression of the sinus node function and AV nodal conduction suggests that the mechanism of block does not lie within, but above, the two nodes, and the parasympathetic system is probably involved. ¹ In support of this, few authors have confirmed normal findings of AV conduction (AH and HV intervals) during electrophysiological studies in these patients. ² A common differential diagnosis for this condition is a bradycardia-dependent (intrinsic) AV block. The following features are more in favor of a bradycardia dependent AV block : presence of a bundle branch or bifascicular block, triggered by a premature supraventricular or ventricular beat, absence of sinus slowing. ³ Making a diagnosis of vagally mediated and AV block has therapeutic implications, as these patients do not require pacing therapy and can be managed conservatively with lifestyle modifications.