STUDIES ON TURTLE HEARTS—THE END OF SYSTOLE, THE DURATION OF THE REFRACTORY PERIOD, THE LATENT PERIOD OF EXTRASYSTOLES AND THE INFLUENCE OF HEART RATE ON AORTIC BLOOD PRESSURE
Exptl. procedures disturb the relationship between the end of mechanical systole and the T wave of the electrocardiogram. The form of the T wave is changed, it starts earlier and it is prolonged. The duration of systole differs to a greater extent in various portions of the ventricle. Ventricular tissue is absolutely refractory to stimuli until the end of electrical systole in some portion of the ventricle. As the excitation current was applied earlier in diastole, the duration of the extrasystole decreased, the QRS and T waves were prolonged, the myograms and the pressure pulses showed an increase in the latent period, and contrary to the accepted understanding the electrocardiogram showed that the latent period of ventricular tissue did not increase. In the intact turtle, increasing the heart rate from 2-70 beats per min. markedly increased the systolic and diastolic blood pressure and the minute cardiac output. Important contributing factors are: increased venous return, heart rate and speed of the cardiac contraction.