"Paradoxical" prolongation of sinus nodal recovery time after atropine in the sick sinus syndrome.

A case of symptomatic sick sinus syndrome is presented with confirmation of sinus nodal dysfunction established by functional testing. The validity of such provocative testing and the criteria for abnormality are discussed. A newly recognized, seemingly "paradoxical" and potentially detrimental effect of atropine noted in this patient is examined. Despite an increase in sinus rate and an improvement in sinoatrial conduction time after administration of atropine, a markedly prolonged sinus recovery time after rapid atrial pacing occurred, and atrial quiescence for more than 10 seconds was seen. Possible electrophysiologic mechanisms for this phenomenon, such as decreased atriosinus entrance block, concealed sinoatrial reentry or enhanced intranodal depolarization, are discussed and potential clinic correlates are made.