ABSTRACTTotal left ventricular external power and aortic input impedance spectra were calculated from recordings of pulsatile pressure and flow in the ascending aorta of 22 human subjects undergoing cardiac catheterization. Eleven subjects had increased aortic pressure (systolic 153 ± 3.8[SEM] mm Hg, p < .001; diastolic 91 ± 2.4 mm Hg, p < .03; mean 118 ± 2.4 mm Hg, p < .001) and constituted the group with mild hypertension (average age 50 ± 1.9 years). The other 11 (age-matched) subjects had normal arterial pressures and constituted the control group. Cardiac output in the hypertensive group was abnormally high (6.9 ± 0.3 liters/min, p < .04) compared with that in control subjects (6.1 + 0.2 liters/min), so that peripheral resistance was similar. Characteristic aortic impedance (index of aortic elastance) was increased in the hypertensive group (142 + 19 vs 72 ± 4.5 dyne-sec-cm-5, p < .002), as was the fluctuation of impedance moduli and phase. These elevated pulsatile components of arterial load were associated with a significant (p < .002) increase in pulsatile left ventricular external power (89%), and the increased cardiac output was associated with a significant (p < .001) increase in steady flow power (31%). The ratio of pulsatile to total power was also increased (38%) in the hypertensive group (p < .001). Increased characteristic aortic impedance in the hypertensive group suggests that the human aorta is stiffer, and fluctuations in the impedance spectra suggest increased or less dispersed wave reflections. These alterations in the systemic arterial tree suggest that factors other than blood pressure and peripheral resistance impose an additional, functionally important hydraulic load on the left ventricle of subjects with mild or borderline hypertension.