Abstract
In a patient with idiopathic hypertrophic subaortic stenosis, propagated responses followed testing stimuli (St2) delivered within the wide QRS complexes elicited by driving stimuli (St1). This phenomenon was not seen at all St1-St2 which produced the minimal V1-V2 interval. It appeared at St1-St2 intervals which resulted in the largest difference between St1-St2 intervals, probably because of the marked asymmetry of propagation occurring very early in the cycle. More studies are required to determine whether this was due to a shift of the vulnerable period toward early systole (suigenesis to idiopathic hypertrophic subaortic stenosis) or to more complex types of intraventricular reentry. In other clinical conditions a similar mechanism could explain the multiple responses produced by pacemaker stimuli falling at the end of ventricular depolarization and also by "properly synchronized' low intensity transthoracic electrical discharges.
