Ventricular ‘Echoes’

Abstract

In dogs anesthetized with pentobarbital, with the heart denervated; the adrenals ligated, and the S.A. node crushed, stimuli were applied to the ventricle. Whenever the V-A delay lengthens sufficiently there is an additional ventricular impulse that returns from the auricle; these impulses are designated as ‘echoes.’ If the stimulated ventricular impulse is blocked before reaching the auricle no echo ensues. The V-A lengthenings were obtained by stimulation at appropriate rates, by the application of pairs of stimuli with short delays, and by vagal stimulation. Adrenaline may cancel echoes by shortening the V-A interval. Ventricular or auricular stimulation may give rise to automatic discharges of the A.V. node. Echoes are not automatic discharges. They are similar to the ‘re-entrant’ or ‘reciprocating’ discharges. The hypothesis usually accepted for the mechanism of these discharges is shown to fail to account for the time relations between the ventricular and auricular impulses in the responses followed by echoes. An alternative explanation is suggested, based on the hypothesis that A-V and V-A propagation follow different paths.