Characterization of cerebral macro- and microvascular hemodynamics during transient hypotension

Abstract

The aim of the current study was to establish the interplay between blood flow patterns within a large cerebral artery and a downstream microvascular segment under conditions of transiently reduced mean arterial pressure (MAP). We report data from nine young, healthy participants (5 women; 26 ± 4 years) acquired during a 15 s bout of sudden-onset lower body negative pressure (LBNP; -80 mmHg). Simultaneous changes in microvascular cerebral blood flow (CBF) and middle cerebral artery blood velocity (MCAvmean) were captured using diffuse correlation spectroscopy and transcranial Doppler ultrasound (TCD), respectively. Brachial blood pressure (finger photoplethysmography) and TCD waveforms were extracted at baseline and during the nadir BP response to LBNP and analyzed using a modified Windkessel model to calculate indices of cerebrovascular resistance (Ri) and compliance (Ci). Compared to baseline, rapid-onset LBNP decreased MAP by 22 ± 16% and Ri by 14 ± 10% (both P ≤ 0.03). Ci increased (322 ± 298%; P <0.01) but MCAvmean (-8 ± 16%; P = 0.09) and CBF (-2 ± 3%; P = 0.29) were preserved. The results provide evidence that changes in both vascular resistance and compliance preserve CBF, as indexed by no significant changes in MCAvmean or DCS microvascular flow, during transient hypotension.

A Quantitative Assessment of Cerebral Hemodynamic Perturbations Associated with Long R-R Intervals in Atrial Fibrillation: A Pilot-Case-Based Experience

“…This partial "deflation", presumably concerning the large cerebral blood arteries (which are more exposed to the blood pressure drop) in particular, depends on the compliance of the arterial network and is likely to account for the Windkessel effect, which may attenuate the ensuing changes in cerebral blood flow. In fact, it was recently proposed that this effect may almost completely compensate for the transient drops in ABP as produced by lower body negative pressure [19], thigh-cuff deflation, and the sitting-to-standing transition [20]. However, this hypothesis is refuted by the present data, in which a drop in tissue oxygenation is consistently observed.…”