STUDIES ON THE OVERDISTENDED HEART

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Abstract
I. The effects of venesection on the non-distended and the previously acutely overdistended heart were compared in an unstated number of anesthetized dogs. In the non-distended heart venesection caused a reduction in the stroke volume of the ventricles (especially left), a decrease in the maximum pressure and pressure excursion of the ventricles during systole and a fall in the pressure level in the aorta and pulmonary artery. During the next 20 min. the results varied; pressure and stroke volume in some cases increasing, in others decreasing and in others becoming stabilized. In the acutely overdistended heart, the results (apart from a constant decrease in diastolic volume and in initial pressure, generally greater in the left ventricle) were variable: in some cases continued decline in ventricular stroke volume, maximum pressure and pressure excursion during systole, and in pressure level and pulse pressure in aorta and pulmonary artery; in other cases, temporary or lasting improvement, evidenced by an augmentation of the stroke volume, by an elevation of the maximum pressure, and pressure excursion of the ventricles during systole, or both, and by a rise in the systolic and pulse pressures in the aorta and pulmonary artery, occasionally accompanied by a rise in the diastolic pressure level and by a fall in the pressures in the systemic and pulmonary veins. The changes in heart rate following venesection were variable both in the non-distended and overdistended heart. An asynchrony in the pressure rise of the curves obtained from the 2 ventricles was observed, as well as in the onset and end of the ejection period.[long dash]II. In these experiments a diastolic dip was seen, resembling the one described in the isolated turtle heart, when the heart was distended and overdistended. It was always observed in the right ventricular pressure curve and occasionally in the left. Its size in the right ventricular curve was a direct function of the diastolic size of the heart. The diastolic dip is not an instrumental artefact but expresses the pressure changes in the ventricle at this time. The dip is an exaggeration of the normal drop in pressure present in the mammalian heart in the rapid filling phase and expresses an augmentation in the disparity between the rate of ventricular expansion and rate of filling. An intraventricular pressure below atmospheric was never found. The diastolic dip indicates that ventricular relaxation plays a role in filling the mammalian heart by exerting an aspirating action during the rapid inflow phase.

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