Abstract
The relations of the left ventricular volumes of the intact canine hearts, such as stroke volume (SV), end-diastolic volume (EDV) and end-systolic volume (ESV), to the heart rates were studied. After the atrio-ventricular node was blocked by injecting a small amount of formalin, ventricular pacing was induced. Varying the pacing rates, the left ventricular volumes were measured by means of thermal dilution using a thermistor with a very short time constant. The results showed that under left atrial pressure clamp, increases in pacing rates ranging from 50 to 160 beats/min caused decreases in EDV to considerable extents but no consistent and large changes in ESV. Measurements of the left ventricular end-diastolic pressure (LVEDP) under left atrial pressure clamp demonstrated that reductions in EDV at higher pacing rates were caused by lowering of LVEDP. From these results, some mathematical expressions on the control of SV are presented.
