[Nodoventricular Mahaim fibres. 2 cases confirmed by electrophysiological study]

Two adolescents with no obvious cardiac were investigated for paroxysmal tachycardia at a rate of about 200/mn with wide QRS complexes showing left bundle branch block, reduced by vagal manoeuvres. Atrial pacing showed shortening of the HV interval associated with widening of the QRS complex to appearances identical to those of the tachycardia, suggesting right ventricular preexcitation masked in sinus rhythm. The progressive prolongation of the atrio-ventricular conduction time with increasing prematurity of the extrastimulus and atrioventricular block after intravenous injection of ATP were more in favour of right nodo-ventricular Mahaim fibres (NV) than a Kent bundle. These properties of the NV pathway and the recording of a "gap" phenomenon during the measurement of the refractory period of the accessory pathway are explained by the proximal part of the AV node not being short-circuited. Paroxysmal tachycardia was easily induced by atrial and ventricular stimulation; the tachycardia QRS complexes always showed appearances of left bundle branch block with a His potential at the onset of the ventricular complex. The NV pathway therefore always depolarised in the anterograde direction in tachycardia; the retrograde pathway is discussed: His, latent Kent bundle or purely intranodal reentry.