Cerebrovascular Compliance is Affected by Posture

Abstract

The mechanical properties of the arterial vascular wall influence the oscillatory component of flow through a vascular bed. Windkessel modeling has often been used to non‐invasively estimate these mechanical properties. While the systemic and coronary circulation have received considerable attention, the cerebral circulation represents a unique challenge because the oscillatory component of cerebral blood flow occurs within a pressurized and rigid skull. The present investigation evaluated the compliance of the cerebral vascular bed in comparison to that of the forearm vascular bed, a bed without significant extravascular pressure. Six young and healthy adults (24 ± 3 years, 5 females) were instrumented with a 3‐lead electrocardiogram, a finger photoplethysmograph (Finapres Medical Systems), and transcranial Doppler ultrasound (Multigon Industries) for continuous measures of heart rate, arterial blood pressure from the brachial artery, and cerebral blood flow velocity from the middle cerebral artery, respectively. Brachial artery blood flow velocity was also measured (duplex Doppler ultrasound; GE Healthcare). Following stabilization of hemodynamic variables, one to two minutes of data were collected in the supine posture. Measures were repeated after participants transitioned to a sitting posture, following re‐stabilization of variables. A modified Windkessel model was used to calculate the values of vascular compliance, as well as viscoelasticity and inertance effects, that are required to match the measured flow waveform to that predicted from the measured pressure waveform. Estimates of compliance were calculated for the forearm and cerebral vascular beds. In the supine posture, compliance of the cerebral vascular bed was lower than that of the forearm vascular bed by more than one order of magnitude (0.0006 ± 0.0005 vs. 0.002 ± 0.0007 mL/mmHg; P≤0.001; d=2.6). However, when seated, cerebrovascular compliance and forearm vascular compliance were not significantly different (0.002 ± 0.001 vs. 0.003 ± 0.002 mL/mmHg; P=0.10; d=1.0). The compliance in the brain increased considerably from the supine to sitting posture (P=0.03; d=0.91) but no significant difference was observed in the forearm (P=0.13; d=−0.65). The results suggest that intracranial pressure may prevent the cerebral vessels from exercising their compliance in supine. Further, as cerebrovascular compliance is increased with sitting, we speculate that posture‐induced reductions in cerebral venous volume affect cerebrovascular mechanics.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.