Unusual mechanism in the initiation of the paroxysmal supraventricular tachycardia in a patient with WPW syndrome.

A heretofore unreported unusual mechanism in the initiation of the paroxysmal supraventricular tachycardia (PSVT) in a patient with WPW (Wolff-Parkinson-White) syndrome was observed using His bundle recordings. The patient initially had some degree of AV conduction disturbance at the level of the AV node. A premature atrial impulse initially activated the ventricle exclusively through the accessory pathway and the same impulse re-excited the ventricle via the AV nodal-His axis after finishing the pure pre-excitation with marked prolongation of the AH and the HV intervals. After finishing this double ventricular response it traversed to the atrium to produce the PSVT.