THE LOCUS AND THE NATURE OF THE A-V PAUSE IN THE SPREAD OF CARDIAC ACTIVATION

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Abstract
The exposed heart of the turtle, P. elegans, was used in an exptl. study involving electrical recording of impulses propagated across the atrio-ventricular junction and visual observations made with the aid of the dissecting microscope, in response to electrical stimuli applied above., below, or at the junction. The locus of the normal A-V delay is the anatomical A-V junction. By use of slowly rising electrical currents it was possible to elicit responses of the ventricle after excitation times of 20 sees, or more. Consideration of the electrical current lines set up on the ventricular side of the junction leads to the conclusion that ventricular activation normally occurs not during or because of the arrival or development of the atrial potential at the junction but during the period of electrical recovery of the atrium. Preps, made to show very long A-V times produce serial responses of several re- sponding increments, with intervening delays of about normal duration. The Wolff-Parkinson-White syndrome with short P-R interval is cited as a possible situation in which there may be A-V conduction without the normal delay.