THE EFFECTS OF ANOXEMIC ANOXIA ON EXCITABILITY, CONDUCTION AND REFRACTORINESS OF MAMMALIAN CARDIAC MUSCLE

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Abstract
Methods for the production of a slowly developed and accurately controlled anoxia and for testing the excitability, conduction changes, and the duration of re-fractoriness are described. During moderate anoxia, the excitability of the dogs'' ventricles to brief shocks delivered at a constant moment in diastole was found to be increased (threshold lowered). In some expts., the maximal change in threshold was near 50%. In severe anoxia, after the blood pressure curve entered its steep decline, the threshold rose rapidly reaching levels 2 or 3 times the control within a brief period. Changes in conduction rate were indicated by changes in the duration of various intervals which include conduction time. Stimulus-response intervals (from local leads), P-R intervals and the QRS of the electrocardiogram all were shortened somewhat during moderate anoxia. All were lengthened rapidly and markedly when the "crisis" was approached in severe anoxia. The duration of refractoriness was shortened in severe anoxia by about 30 msec. or 15%. In some expts., a smaller degree of shortening was recorded at moderately anoxic levels. No ventricular ectopic systoles were observed or recorded in this series of expts. This together with the smallness of the reduction in the refractory period account for the absence of ventricular fibrillations in anoxemic anoxia. Because of the suddenness of the collapse in severe anoxia in many expts. and the lack of specific signs warning of imminent cardiac failure, it is regarded as dangerous.

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