Origin of initial escape beat during graded vagal stimulation

Graded vagal stimulation was applied to the distal vago-sympathetic trunk in anesthetized dogs. The effects of vagal stimulation were expressed as cardiac delay time, which is specifically measured as the R-R interval of the asystolic period or bradycardial beat, minus the control R-R interval. Graded vagal effects were best obtained by stimulation with varying pulse duration at fixed voltage and frequency. As pulse duration was increased, progressive bradycardia and brief cardiac arrest occurred. With further increase in pulse duration, cardiac delay time increased markedly and remained essentially unchanged over a wide range of still greater pulse durations. ECG's recorded stimultaneously showed initial sinus bradycardia, atrial pacemakers and A-V junctional beats during brief cardiac delays. The initial escape beat following prolonged cardiac delay was always A-V junctional or ventricular ectopic in origin. It is concluded that the initial escape beat following prolonged cardiac delay arises from latent ventricular pacemakers not subject to vagal influence.