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The aim of this study was to determine the effect of posture changes on vascular compliance in intracranial (brain) versus extracranial vascular beds (forearm). Eighteen young adults (nine females) performed a supine-to-seated-to-standing protocol involving 5 min of rest in each position. Continuous blood pressure, middle cerebral artery (MCA) blood velocity, and brachial artery blood velocity were recorded at each posture. Three to five consecutive steady-state cardiac cycles at each posture were analyzed by a four-element lumped parameter modified Windkessel model to calculate vascular compliance. Mean arterial pressure (MAP) increased from supine to seated (76(9) vs. 81(12) mmHg; P = 0.006) and from supine to standing (76(9) vs. 82(13) mmHg; P = 0.034). Mean blood flow was greater in the MCA relative to the forearm (forearm: 40(5) mL·min −1 , MCA: 224(17) mL·min −1 ; main effect P < 0.001). Conversely, vascular resistance (forearm: 3.25(0.50) mmHg −1 ·mL·min −1 , brain: 0.36(0.04) mmHg −1 ·mL·min −1 ; main effect P < 0.001) and compliance (forearm: 0.010(0.001) mL·min −1 ·mmHg −1 , brain: 0.005(0.001) mL·min −1 ·mmHg −1 ; main effect P = 0.001) were greater in the forearm compared to the brain. Significant main effects of posture were observed with decreasing values in upright positions for mean blood flow ( P = 0.001) in both vascular beds, but not for resistance ( P = 0.163) or compliance ( P = 0.385). There were no significant interaction effects between vascular bed and posture for mean flow ( P = 0.057), resistance ( P = 0.258), or compliance ( P = 0.329). This study provides evidence that under steady-state conditions, posture does not affect cerebrovascular compliance.
Kells et al. (2024) studied this question.