“The effects of extra stimuli upon the heart in the several stages of block,together with a theory of heart-block”

This research was undertaken with the object primarily of testing the statement made by Hering that the absence of a compensatory pause following an extra stimulation of the ventricles of the warm-blooded heart suffices to prove that the ventricles are beating independently of the auricles.

The author's experiments on the dog's heart have shown that in partial, as well as in complete heart-block, extra systoles of the ventricles are not followed by compensatory pauses. This results from the tendency for the same number of auricular beats to elapse between the extra contraction and the next following natural contraction as intervene between two natural ventricular beats in any stage of partial block. The following may be taken as an average example: If the auriculoventricular rhythm is 3:1, a ventricular extra cycle will last through any part of such auricular cycle as may have been unfinished at the moment of stimulation, plus two more auricular cycles if more than one half of the first auricular cycle was unfinished, or plus three or more auricular cycles if less than half of the first auricular cycle was unfinished.

In partial heart-block extra systoles of the auricules do not cause contractions of the ventricles excepting, occasionally, when such extra systoles fall close to the end of a ventricular cycle; and extra contractions of the ventricles never cause contractions of the auricles.

The irritability of the ventricles in partial and complete heart-block is not reduced but rather it is increased over that which obtains in the normal heart. Furthermore, in each ventricular cycle of partial and complete heart-block the irritability of the ventricles probably increases until they pass into the refractory state which develops with their contraction.

In order to determine the significance of these results, a strip of terrapin ventricle was arranged so that rhythmic stimuli as well as extra stimuli could be thrown into either end as desired.