Abstract
One of the most characteristic features of low grade partial block, as seen clinically, is the gradual prolongation of the A-V interval up to the time of the dropping out of a single ventricular response. This prolongation may be regarded as a fatigue effect. Furthermore, cases of partial heart block have been reported in which the recovery of conductivity passes through a supernormal phase, 1 , 2 and the course of the supernormal recovery has been traced in compressed heart muscle of the turtle by one of us. 3 Therefore, since both supernormal recovery and fatigue are known to occur, separately at least, in clinical heart block cases, it appeared desirable to study the two phenomena as they occur simultaneously in the compressed turtle muscle. The results seem to us to have justified the effort, since certain relationships between the two have been discovered which could scarcely have been predicted from a study of the two phenomena separately.
In some experiments we used the atropinized, excised, bloodless hearts of Chrysemys elegans and C. troosti rendered quiescent by removal of the sinus. In other cases, the unatropinized ventricle alone was employed, the muscle being split from the base to near the apex. A heart clamp was placed in the A-V groove, or at the apex when the ventricle was used, and the desired degree of compression applied. The muscle was then caused to respond to induction shocks at any desired frequency. Simultaneous electrograms and myograms of the responses were recorded. Since the weather was warm, marked fatigue effects were invariably obtained. The supernormal phase was not so constant in its appearance, although shown by both types of preparation.
In the figure is shown a recovery curve of conductivity obtained when the clamp was in the A-V groove.
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