Pressure and Flow Generated by the Left Ventricle against Different Impedances

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Abstract
To study selective changes in capacitive and resistive load under constant left atrial filling pressure, isolated cat hearts were loaded with a hydraulic model simulating the input impedance of the systemic arteries. The model was constructed so that resistive (peripheral resistance) and capacitive (total arterial compliance) characteristics could be changed independently. Aortic and left ventricular pressure and aortic flow, as generated by the left ventricle against the different impedances, were measured. An increase in resistance resulted in an increase in systolic, diastolic, and mean aortic pressure. A decrease in capacitance caused a small increase in systolic pressure and a decrease in diastolic and mean aortic pressure. Mean left ventricular pressure increased when resistance increased or capacitance decreased. Both peak flow and mean flow decreased when resistance increased or capacitance decreased. We attempted to explain these observations by the concept of source impedance. This concept had been, until now, incorrectly approached, because the nonlinearity arising from the aortic valves had been neglected. The correct computations met with difficulties, but it was shown that the isolated heart, with constant atrial filling pressure, behaved as neither a flow source nor a pressure source.