Effects of changes in the aortic input impedance on systolic pressure-ejected volume relationships in the isolated supported canine left ventricle.

We examined the effects of aortic input impedance alteration on left ventricular pressure, aortic flow and ejected volume (integral value of aortic flow), in an isolated blood perfused ejecting canine heart, with special reference to end-systolic values. A hydraulic model which stimulates an aortic input impedance was attached to the aortic root of an excised heart. Left ventricular end-diastolic pressure was kept constant by electrical pacing. Three coronary arteries were perfused with arterial blood from support dogs. When the peripheral resistance in the hydraulic model was changed, there were inverse linear relationships between stroke volume and mean left ventricular systolic pressure and between ejected volume and pressure at end-systole. Time interval from the onset of contraction to end-systole did not change. Thus the relation between stroke volume and mean left ventricular pressure obtained by changes in peripheral resistance is governed by a source resistance, which can be considered as the contractile state of the ventricle. When the capacitance (arterial compliance) was changed, there was no inverse linear relation between stroke volume and mean systolic pressure. In many cases, there was an inverse linear relationship between ejected volume and pressure at end-systole. However, an increase in capacitance prolonged the time interval from the onset of contraction to end-systole. We conclude that the end-systolic pressure-ejected volume relationship in the ejecting heart is governed not only by contractility but also by arterial capacitance.