Rapid changes in vascular compliance contribute to cerebrovascular adjustments during transient reductions in blood pressure in young, healthy adults.

Characterization of dynamic cerebral autoregulation has focused primarily on adjustments in cerebrovascular resistance in response to blood pressure (BP) alterations. However, the role of vascular compliance in dynamic autoregulatory processes remains elusive. The present study examined changes in cerebrovascular compliance and resistance during standing-induced transient BP reductions in nine young, healthy adults (3 women). Brachial artery BP (Finometer) and middle cerebral artery blood velocity (BV; Multigon) waveforms were collected. Beginning 20 beats before standing and continuing 40 beats after standing, individual BP and BV waveforms of every second heartbeat were extracted and input into a four-element modified Windkessel model to calculate indexes of cerebrovascular resistance (Ri) and compliance (Ci). Standing elicited a transient reduction in mean BP of 20 ± 9 mmHg. In all participants, a large increase in Ci (165 ± 84%; P < 0.001 vs. seated baseline) occurred 2 ± 2 beats following standing. Reductions in Ri occurred 11 ± 3 beats after standing (Ci vs. Ri delay: P < 0.001). The increase in Ci contributed to maintained systolic BV before the decrease in Ri. The present results demonstrate rapid, large but transient increases in Ci that precede reductions in Ri, in response to standing-induced reductions in BP. Therefore, Ci represents a discreet component of cerebrovascular responses during acute decreases in BP and, consequently, dynamic autoregulation.NEW & NOTEWORTHY Historically, dynamic cerebral autoregulation has been characterized by adjustments in cerebrovascular resistance following systematic changes in blood pressure. However, with the use of Windkessel modeling approaches, this study revealed rapid and large increases in cerebrovascular compliance that preceded reductions in cerebrovascular resistance following standing-induced blood pressure reductions. Importantly, the rapid cerebrovascular compliance response contributed to preservation of systolic blood velocity during the transient hypotensive phase. These results broaden our understanding of dynamic cerebral autoregulation.