[His bundle block 2:1 during paroxysmal supraventricular tachycardia involving an occult accessory atrio-His pathway].

A high degree AV block during paroxysmal supraventricular tachycardia is an uncommon and short-lasting finding in patients with intranodal re-entry circuits. The AV block is the result of temporary inadequacy of the infranodal conduction system--which is not part of the circuit--to cope with the sudden increase in heart rate in terms of refractory periods and conduction velocity. In a patient with paroxysmal reciprocating tachycardia at a very high rate (230-250 bpm), 2:1 intra-hisian AV block was constantly observed after arrhythmia initiation by atrial extrastimuli. This persisted for 1-4 minutes and then progressively subsided passing through a period of 3:2 intra- infra-hisian AV block until it reached 1:1 conduction with transient left bundle branch block. The latter occasionally disappeared as result of retrograde activation of the area of functional anterograde block and of subsequent noncompensatory pause, following a premature ventricular depolarization. Tachycardia-dependent AV block was abolished by verapamil and flecainide, because of lengthening of the tachycardia cycle length. Functional and electropharmacological features of retrograde conduction were consistent with an extranodal concealed atrio-hisian accessory pathway acting as the retrograde limb of the re-entry circuit. Moreover, His bundle electrogram was prolonged and polyphasic even in sinus rhythm. Therefore, tachycardia-dependent advanced AV block can occur not only in truly intranodal re-entry but also in very fast atrio-hisian re-entry tachycardias with evidence of transient impairment of intra-hisian conduction, distal to the insertion of the accessory pathway. Localized morphofunctional pathological changes in the His bundle are probably involved in the mechanism of this uncommon pattern.