Evidence Against the “Latency Theory” of Partial A-V Block.

Abstract

There are two fundamentally opposing explanations of the characteristic groupings of beats separated by a pause when occasionally the ventricle fails to follow the auricle in low grade heart block (Wenckebach's periods). The “latency theory”, Mobitz, 1 holds that the velocity of conduction through the muscular elements is invariable with constant auricular rate, and that the gradual prolongation of A-V interval following the pause in partial block is due to the arrival of the sinus impulse at the A-V junction at progressively earlier times during the recovery in the excitability on the ventricular side of the junction. Thus the impulse is regarded as arriving at the junction, pausing a moment, and then going forward. The duration of the pause is supposed to depend upon the excitability of the tissue immediately beyond the boundary plane.

That such a conception is inadequate is regarded as certain by some, and yet it is difficult to secure direct evidence against it.

The first evidence herein outlined is obtained from excised turtle hearts, rendered quiescent by removal of the sinus and driven at varying rates by break induction shocks applied to an auricle. In one experiment, after a 10 second rest, the auricle was twice stimulated, the interval between auricular responses measuring 3.026 seconds. The first A-V conduction time was 0.526 second; the second, 0.538. A 3 second rest was, therefore, almost sufficient for complete recovery of conductivity. In a second test after a 10 second rest the interval between the 2 transmitted auricular impulses was again practically 3 seconds, hut an auricular response which was not transmitted to the ventricle was interpolated between the two, 1.156 second before the following transmitted auricular response. Although the ventricle did not respond to the interpolated impulse, the conduction time for the next impulse increased from the expected 0.540 second to 0.652 second.