Idiopathic paroxysmal atrioventricular block – yet another cause of syncope?

Idiopathic paroxysmal atrioventricular block – yet another cause of syncope?

Idiopathic paroxysmal atrioventricular block without any preceding PP interval or PR interval prolongation and mostly a negative electrophysiological study is being proposed as a new entity which can cause syncope by Brignole M et al [J Am Coll Cardiol, 2011; 58:167-173]. They could identify eighteen such patients from four centres spread across three countries. They all had a normal baseline ECG, no structural heart disease and documentation of abrupt onset of paroxysmal AV block at the onset of syncope by prolonged ECG monitoring – an implantable loop recorder (ILR), Holter monitoring or in hospital telemetry. They also had a delayed emergence of an adequate escape rhythm and absence of other rhythm disturbances before or during the AV block. Seventeen of them accepted permanent dual chamber pacing and had no recurrence of syncope. An interesting observation was a lower plasma level of adenosine in these patients. Fifteen of them also developed AV block following an intravenous bolus of adenosine triphospate (ATP). The authors propose that low plasma levels of adenosine may upregulate A1 receptors causing a higher response of the AV node to adenosine. A transient release of endogenous adenosine may be the cause of the paroxysmal AV block, though the reason for such release if actually present, is unknown. They recommend further studies for confirming these observations.