Aortic input impedance in normal man: relationship to pressure wave forms.

The relationship between the shape of the ascending aortic pressure wave form and aortic input impedance was studied in 18 patients who underwent elective cardiac catheterization but in whom no heart disease was found. Ascending aortic flow velocity and pressure were simultaneously recorded from a multisensor catheter with an electromagnetic velocity probe and a pressure sensor mounted at the same location. Another pressure sensor at the catheter tip provided left ventricular pressure or a second aortic pressure to determine pulse-wave velocity. Fick cardiac outputs were used to scale the velocity signal to instantaneous volumetric flow. Input impedance was calculated from 10 harmonics of aortic pressure and flow. For each patient, impedance moduli and phases from a minimum of 15 beats during a steady state were averaged. Peripheral resistance was 1137 ± 39 dyn-sec-cm' (4 SEM) and characteristic impedance was 47 4 4 dyn-sec-cm; pulsewave velocity was 6.68 ± 0.32 m-sec-'. In all patients, a well-defined systolic inflection point divided the aortic pressure wave form into an early and late systolic phase. The patients were classified into three groups: group A (n = 7) patients whose late systolic pressure exceeded early systolic pressure; group B (n = 7) patients whose early and late systolic pressures were nearly equal; group C (n = 4) patients whose early systolic pressure exceeded late systolic pressure. Group A and B patients all demonstrated oscillations of the impedance moduli about the characteristic impedance. Group C patients demonstrated flatter impedance spectra. Thus, a larger secondary rise in pressure was associated with a more oscillatory impedance spectrum. These results suggest that the differences in pressure wave forms are due to differences in reflections in the arterial tree and not secondary to differences in cardiac function. Using pulse-wave velocity, the "effective" reflection site distance was determined from both pressure (48 cm) and impedance (44 cm) data, implying that the region of the terminal abdominal aorta acts as the major reflection site in normal adult man.