His bundle electrocardiography in the Wolff-Parkinson-syndrome. Evidence for combination of James and Mahaim conduction.

His bundle electrographic studies with atrial pacing in 2 of 8 patients with the Wolff-Parkinson-White (WPW) syndrome demonstrated evidence for the combination of a James tract and Mahaim fibers. During normal sinus rhythm these 2 patients had P-H intervals of 65 and 75 msec and H-Q intervals of 15 and 35 msec, respectively. On atrial pacing, the P-H intervals showed a small increase with the H-Q intervals remaining constant. The QRS duration, delta-wave duration and P-J intervals remained relatively constant. These observations strikingly contrasted with the findings typical of Kenty-type conduction in the remaining 6 patients in whom during sinus rhythm, the His depolarization occurred simultaneously with the onset of the delta-wave on the surface ECG. During atrial pacing (ranges 85-174/min), in these 6 patients, there was a progressive increase in the duration of the delta-wave, gradual increase in the P-H and Q-H intervals, and an increase in the QRS duration. 5 of these 6 patients showed a progressive increase in the P-J intervals. These observations suggest a practival method for determining in the individual patient whether anomalous conduction is occurring via a Kent-type pathway or a combination of a James tract and Mahaim fibers arranged in series.