Transient high degree AV block as a cause of Stokes-Adams syndrome--clinical observation and experimental study.

A case of Stokes-Adams syndrome, caused by a high degree AV block due to repetitive concealed conduction in the AV node, was presented. Experimental study in dogs with impaired AV conduction by verapamil showed that the favourable conditions for appearance of "repetitive" concealed conduction were as follows: 1) prolonged effective refractory period (ERP) of the AV node, 2) atrial stimulations applied at or just inside of ERP of the AV node successively (deeper penetration of concealed conduction), 3) a prolonged preceding PQ interval (slower speed of concealed conduction) and 4) overdrive suppression of subsidiary pacemaker(s). In clinical cases with apparently normal AV conduction but with prolonged ERP of the AV node, atrial excitations with suitable timing may cause repetitive concealed conduction, resulting in a high degree AV block and Stokes-Adams syndrome.