No influence of steady-state postural changes on cerebrovascular compliance in humans.

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;  = 0.006) and from supine to standing (76(9) vs. 82(13) mmHg;  = 0.034). Mean blood flow was greater in the MCA relative to the forearm (forearm: 40(5) mL·min, MCA: 224(17) mL·min; main effect  < 0.001). Conversely, vascular resistance (forearm: 3.25(0.50) mmHg·mL·min, brain: 0.36(0.04) mmHg·mL·min; main effect  < 0.001) and compliance (forearm: 0.010(0.001) mL·min·mmHg, brain: 0.005(0.001) mL·min·mmHg; main effect  = 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 ( = 0.001) in both vascular beds, but not for resistance ( = 0.163) or compliance ( = 0.385). There were no significant interaction effects between vascular bed and posture for mean flow ( = 0.057), resistance ( = 0.258), or compliance ( = 0.329). This study provides evidence that under steady-state conditions, posture does not affect cerebrovascular compliance.