High-fidelity pressure measurements in man have revealed major differences in the configuration of central pressures, even in absence of cardiovascular disease. To discriminate between cardiac and arterial factors that could underlie these differences, evaluation of the ascending aortic input impedance was undertaken at our institution since impedance selectively describes the arterial system. Ascending aortic pressure and flow velocity were simultaneously recorded via a Millar multisensor catheter, with an electromagnetic velocimeter and a micromanometer mounted at the same location. Another micromanometer at the catheter tip provided 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. The impedance spectrum was derived for 10 harmonics. For each subject, moduli and phases from a minimum of 15 beats during a steady state were averaged, to suppress the effects of noise. Pressure and velocity signals were low-pass filtered with corner frequencies at 100 Hz. Digital processing was obtained using a 13-bit A-to-D converter at 200 Hz coupled to a minicomputer.