[Electrophysiological study of 2 cases of ventricular pre-excitation compatible with the presence of Mahaim's fibers].

Report of two cases of ventricular pre-excitation in which the presence of Mahaim's fibres was suggested by the electro-physiological studies. In both cases, the short-circuit of the His bundle was responsible for the abnormally short HV interval. In a patient, atrial acceleration induced an increase of rh at first, related to an A-V delay without HV variation. For a cycle of 360 ms, HV lengthened suddenly from 15 to 40 ms while QRS changed configuration, which is in favour of a functional exclusion of an accessory pathway. Combination with the pre-excitation of a left bundle-branch block complicated, in this case, the problems of identification. The other case responded also to cardiac pacing by an increase of the A-V delay due mainly to intra-nodal conduction slowing. In the same time HV increases from 20 to 30 ms (for a cycle change from 480 to 315 ms) and the delta wave decreased progressively. For a higher rate there was a 2:1 A-V block proximal to H. The extrastimulus method produced initially the same results. On the contrary, when the coupling-interval decreased to 290 ms, HV increased suddenly from 25 to 45 ms, with disappearance of the pre-excitation pattern. At a still shorter interval, HV shortened again (25 ms) in parallel with a marked lengthening of the A-V nodal conduction times, thus revealing a "gap" phenomenon of the accessory pathway.