Assessment of aortic pressure-volume relationships with an impedance catheter

Volume measurements obtained with an impedance catheter (impedance volume measurements) have been used previously in determining pressure-volume relationships in the right and left ventricles and in the right atrium. The purpose of the present study was to determine the utility of impedance volume techniques in the assessment of aortic pressure-volume relationships in animals and humans. Experiments to develop this methodology were first performed in ten anesthetized dogs, with simultaneous measurements of aortic pressure (micromanometer), diameter (ultrasonic crystals), and impedance volume. Transient (20 sec) inferior vena cava (IVC) occlusion and nitroprusside infusion resulted in a reproducible series of aortic pressure-volume and pressure-diameter points. In each case changes in impedance volume closely paralleled changes in diameter. From the wide range of pressure-volume points, it was possible to construct pressure-volume plots for a single cardiac cycle, as well as longer loops over a 20-40 mm Hg pressure range. End-systolic and end-diastolic pressure-volume points could subsequently be identified, which outlined a more precise pressure-volume relationship that was linear in six cases and curvilinear in four cases, depending on the level of aortic pressure. We extended these techniques to human subjects undergoing diagnostic cardiac catheterization. Studies using the impedance catheter in the aorta were performed in 11 patients. Aortic pressure-volume curves were elicited during Valsalva maneuver to obtain data over a range wider than that of a single cardiac cycle. Measurements of relative volume were calibrated with simultaneous M-mode echocardiography in three patients. We conclude that pressure-volume relationships in the aorta can be assessed with impedance volume techniques, making possible in vivo measurements of arterial compliance. Potential nonlinearities in aortic P-V relationships require that the assessment of aortic compliance encompass as much as possible of the physiologic range of pressure and volume.