Attenuated aortic–cerebral pulsatile transmission with posture-related increases in stroke volume in healthy young adults

Excessive pulsatile hemodynamics increased under higher stroke volume (SV) impose mechanical stress on the cerebrovasculature, requiring attenuation before reaching the brain. SV is greater in the supine than upright, yet the characteristics of aortic-to-cerebral pulsatile transmission under such conditions are unclear. To address this, we compared the transfer function gain from aortic pressure (AoP) to middle cerebral artery blood velocity (MCAv, measured by transcranial Doppler) across postures that altered SV. Fifteen young adults underwent four postures: semi-sitting at 60° and 30° backrest, supine, and supine with leg elevation. AoP was estimated from the radial arterial pressure through the general transfer function. AoP and MCAv spectra were analyzed by heart rate harmonic analysis to exclude the effect of postural difference in heart rate. SV (via the Modelflow method) was significantly greater in the supine than in the semi-recumbent positions (60° vs. supine, P < 0.05 for both). There was a significant difference in AoP spectra among postures (main effect of condition, P < 0.01). However, transfer function gain was significantly attenuated in the supine positions (60° vs. supine positions P < 0.05 for both), resulting in no significant increase in MCAv spectra across postures (main effect of condition P > 0.05). Transfer function gain decreased with decreasing systemic vascular resistance (r(rm) = 0.67, P < 0.01). Therefore, cerebral pulsatility may not increase despite greater SV, owing to an attenuation in aortic–cerebral pulsatile transmission associated with changes in systemic vascular resistance in healthy young adults.