Impact of Mild Orthostatic Stress on Aortic‐Cerebral Pulsatile Flow Transmission: Insight from the Frequency Domain

Abstract

Normally, pulsatile flow fluctuation generated from the left ventricle is damped effectively along central elastic arteries. However, the impact of mild orthostatic stress on hemodynamic transmission from the aorta to the brain is unknown. To clarify this question, a stepwise lower body negative pressure (LBNP: -10 mmHg, -20 mmHg, -30 mmHg) was applied on 9 young healthy men. Frequency-domain relation between aortic blood pressure and cerebral hemodynamics (via transcranial Doppler ultrasonography) was assessed by spectral and transfer function analysis. Aortic blood pressure was estimated from radial arterial pressure waveforms by a validated software (SphygmoCor). Mean arterial pressure (MAP) and CBFV were not changed but aortic pulse pressure (PP) and pulsatile CBFV (peak velocity – end-diastolic velocity) gradually and significantly deceased throughout the LBNP protocol. Interestingly, steady-state component of transfer function gain (between MAP and mean CBFV) in the low frequency range (0.07-0.20Hz) was maintained throughout the LBNP protocol, but pulsatile component of transfer function gain (between aortic PP and pulsatile CBFV) was significantly greater during -20 mmHg and -30 mmHg LBNP (1.74±0.35 and 1.80±0.26 cm/s/mmHg) compared with the baseline (1.10±0.19 cm/s/mmHg). Mean values of coherence function were greater than 0.5 during baseline and LBNP protocol in either steady-state or pulsatile component. These results suggest that central arterial damping function on pulsatile flow fluctuation is partly diminished during mild orthostatic stress.