Intermittent AV conduction disturbances in patients with AV nodal bypass tracts. Possible mechanisms of unusual variant of tachycardia-bradycardia syndrome.

Abstract

His bundle recordings were performed in 2 patients in whom AV nodal bypass tracts coexisted with intermittent A V conduction disturbances occurring below the sitefrom which the His bundle deflection was recorded. Case 1 had: (a) tachycardia dependent right bundle-branch block, (b) persistent HV prolongation, and (c) bradycardia dependent A V block. Case 2 showed: (a) intra-atrial conduction delay, (b) tachycardia dependent left bundle-branch block with HV prolongation, (c) bradycardia dependent HV conduction disturbance, (d) tachycardia-bradycardia syndrome of an unusual type; the latter presumably resulted, during atrial.flutter, from the alternation of rapid AH conduction through the bypass tract with intermittent (complete) distal His bundle block or bilateral bundle-branch block.In a previous communication in this journal , we discussed the occurrence of bundle-branch block and normal PR intervals in patients with AV nodal bypass tracts. It appeared justified, in view of the very few existing publications (Krishnaswani and Geraci, 1974;Brechenmacher, 1975), to report the association of AV conduction disturbances with AV nodal bypass tracts (in absence of coexisting Mahaim tracts). In addition the possible mechanisms of an unusual type of tachycardia-bradycardia syndrome are discussed. Subjects and methodsHis bundle recordings were obtained in 2 patients referred to the Cardiovascular Laboratory for evaluation of recurrent supraventricular tachycardias. The procedure was explained and informed consent obtained. The normal values in our department for the various conduction intervals are as follows (Castellanos et al., 1975;Befeler et al., 1976): PA=20-45 ms; AH=55-120 ms; HV= 35-55 ms.