Age‐related impairment in dynamic cerebrovascular compliance responses to standing despite preserved cerebrovascular resistance responses

Abstract

During transient reductions in blood pressure induced by standing upright, a rapid increase in cerebrovascular compliance represents a fundamental response in addition to reductions in cerebrovascular resistance. Age‐related reductions in central arterial compliance are known but how ageing affects the cerebrovascular compliance response to standing remains unknown. Therefore, the present study tested the hypothesis that older adults would demonstrate attenuation of cerebrovascular compliance during standing relative to young adults. Ten young healthy adults (24 ± 3 years, 5 females) and 10 older healthy adults (57 ± 11 years, 4 females) completed a sit‐to‐stand protocol involving at least three minutes of sitting and at least two minutes of standing while brachial artery blood pressure (BP; Finapres) and middle cerebral artery blood velocity (BV; Multigon) were collected. Individual BP waveforms were time aligned to corresponding BV waveforms and were input into a four‐element lumped parameter Windkessel model to calculate an index of cerebrovascular compliance (Ci). Cerebrovascular resistance (Ri) was calculated as the quotient of BP and BV. Beginning 20 beats prior to standing and continuing 40 beats following the stand, every other heartbeat was extracted so that a total of 30 beats were analyzed (i.e., 10 beats during sitting, 20 beats during standing). Seated baseline Ci was calculated as the average of the 10 beats of sitting. Peak Ci (maximum value) minus seated baseline Ci determined the change in (∆) Ci. In the sitting posture, older adults demonstrated similar baseline Ci compared with young adults (7.2e‐4 ± 2.2e‐4 vs. 7.3e‐4 ± 1.5e‐4 cm∙s‐1·mmHg‐1, P= 0.97). Upon standing, Ci increased in all participants, but peak Ci was attenuated in older adults relative to young adults (1.4e‐3 ± 3.2e‐4 vs. 2.2e‐3 ± 4.8e‐4 cm∙s‐1·mmHg‐1, P< 0.001, d= 1.96). Therefore, the change in Ci from sitting to standing was less in older adults compared with young adults (7.1e‐4 ± 2.9e‐4 vs. 1.5e‐3 ± 4.3e‐4 cm∙s‐1·mmHg‐1, P< 0.001, d= 2.15; 109 ± 56 vs. 205 ± 71 %, P= 0.004, d= 1.50) despite both groups showing no difference in the drop in BP (older: ‐21 ± 7 mmHg, young: ‐22 ± 8 mmHg; P= 0.76). Further, no differences were observed between groups in the change in Ri between sitting and standing (older: ‐0.4 ± 03 mmHg∙cm‐1∙s‐1, young: ‐0.3 ± 0.2 mmHg∙cm‐1∙s‐1; P= 0.67). Therefore, older adults demonstrate similar alterations to cerebrovascular resistance upon standing but attenuated cerebrovascular compliance responses. As such, age‐related changes to the cerebral vasculature may be masked by evaluating cerebrovascular resistance alone.