Anterograde block in accessory pathways with retrograde conduction in reciprocating tachycardia

3 patients with reciprocating tachycardia are described. In all 3, tachycardia was due to a reentrant mechanism. Anterograde conduction was via the A V node while conduction was over an accessory (Kent) pathway. In none of the 3 patients did this pathway sustain anterograde conduction despite pacing the atrium close to its location. With the advent of surgical modes of therapy, it is important to recognize such 'silent' bypasses. The observations which allow this diagnosis of unidirectional anterograde block in an accessory pathway and the exclusion of AV nodal reentry, the main differential diagnostic possibility, are discussed.