THE EFFECTS OF HALOTHANE ON THE INTERACTIONS BETWEEN MYOCARDIAL CONTRACTILITY, AORTIC IMPEDANCE AND LEFT VENTRICULAR PERFORMANCE;II: AORTIC INPUT IMPEDANCE, AND THE DISTRIBUTION OF ENERGY DURING VENTRICULAR EJECTION.

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Abstract
SUMMARYAortic input impedance, representing the effective resistance to oscillatory flow offered by the systemic vascular bed, has been measured in dogs before and during the administration of halothane, and during the infusion of phenylephrine and trimetaphan. Simultaneously, estimates of pulsatile and steady energy losses were obtained. Halothane administration had no consistent effects on aortic input impedance, but significantly reduced peak aortic power and left ventricular external (systolic) stroke work. Phenylephrine markedly increased the mean term of aortic input impedance, but had little effect on the impedance to oscillatory flow. Trimetaphan markedly decreased the mean term of aortic input impedance, but caused a moderate increase in the impedance to oscillatory flow associated with a consistent increase in the ratio of pulsatile to total work. The low values of pulsatile work by comparison with the steady and total work of the left ventricle, together with the flat pattern of aortic input impedance are in accordance with the concept that the left ventricle is effectively “decoupled” from its load resistance imposed by the systemic vascular bed, thus minimizing the work done in maintaining pulsatile flow. Within the limits of the methods used to evaluate aortic input impedance and pulsatile energy losses in the aorta, we conclude that neither the inductive nor the capacitative characteristics of the aorta and peripheral vascular bed could play a significant role in the haemodynamic responses to halothane anaesthesia.

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