The effects of severe hypoxia on the canine electrocardiogram with particular reference to the QRS complex

Abstract

Summary The effects of progressive hypoxia upon the canine electrocardiogram, with particular reference to the QRS complex, have been investigated. The two mechanisms previously held responsible for all or most of the decrease in QRS amplitude seen during hypoxia are (1) increase in chest girth and (2) change in heart size. Data are presented herein showing that during hypoxia the R wave regularly decreases by a minimum of 50 per cent, which far exceeds that produced by even the greatest changes in chest girth. Furthermore, the R wave diminishes steadily and progressively with the known early decrease in heart size and continues to diminish as the heart becomes larger with further hypoxia. The possible contributory factor of altered skin resistance due to systemic hypoxemia has been eliminated by implanting electrodes subcutaneously. By recording drops in voltage of similar magnitude in epicardial leads, any effect upon QRS caused by changes in the volume conductor has been made less likely. Thus, with the control of factors known to influence the amplitude of QRS during severe hypoxia, present evidence points primarily to an absolute decrease in the electromotive force generated by the heart.