Accessory pathway with two conduction times.

A 48-year old man was referred for frequent paroxysmal narrow QRS tachycardias. Either a rapid orthodromic tachycardia (220 b/min) using a fast-conducting left lateral concealed atrioventricular (AV) accessory pathway (AP) for its retrograde conduction or a slow orthodromic tachycardia (125 b/min) using the same concealed AP which was slow-conducting, were induced. One application of radiofrequency energy at the earliest site of retrograde conduction suppressed both forms of tachycardias. Reciprocating tachycardias presenting with different retrograde conduction times were related to a single reentrant circuit.