The anatomic electrophysiologic basis of AV node re-entry tachycardia is dual AV node conduction pathways. AV node re-entry tachycardia is most likely to occur with which of the following electrophysiologic aberrations?

C. Proximal antegrade block in the fast conduction pathway.

DISCUSSION: A retrograde conduction block in either the slow or fast pathway would be likely to prevent a re-entrant circuit from developing. A proximal antegrade block in the slow conduction pathway is extremely unusual because of the short refractory period of the slow conduction pathway. The most common conduction block that occurs in patients with dual AV node physiology is a proximal antegrade conduction block in the fast pathway because of its longer refractory period. This antegrade block in the fast conduction pathway allows AV conduction to occur via the slow pathway and to return in retrograde fashion up the fast pathway to establish the re-entrant circuit responsible for AV node re-entry tachycardia.