A SURVEY OF SOME ELEMENTS OF CARDIAC EXCITABILITY

¹ The Physiological Laboratory, Oxford

1. Reversible changes of cardiac excitability are of two modes.

(a) Changes which come and go concomitantly with the substance producing them.

(b) Changes showing a variable rate of development, always hysteresis of subsidence and sometimes overswing. The two changes are quantitatively as well as qualitatively distinct.

2. The second mode of change has been suggested as causally connected with changes of aggregation in the colloids concerned in excitation Evidence is given of interchangeability in the external mode of expression of this change. A given amount of this change can appear as augmentation of contraction or as quickening or tonus according to circumstances.

3. The same substance can produce both modes of change and to opposite ends. When such is the case the manifest action of the substance exhibits a variety of phases. One extreme of such double action is mimicry of inhibition and the inhibitory rebound.

4. The inhibitory rebound is shown to be an excitable state of the heart of the second mode of change.

5. It is shown that a cardiac state favouring hyperactivity can be developed and exist in the heart without any external manifestation of its presence, provided there is a concomitant interference with the functions of calcium.

6. The evidence is given that there is an interference with the functions of calcium during inhibition and that this interference belongs to the first mode of change.