Sex Differences in Baseline and Reactive Cerebrovascular Compliance

Abstract

Adjustments in cerebrovascular resistance represent the primary characterization of cerebral autoregulation to date. However, recent studies have demonstrated that cerebrovascular compliance contributes to the relationship between BP and cerebral blood velocity (BV) during dynamic changes in BP. Therefore, the present study tested the hypotheses that i) hypotension‐induced autoregulatory processes involve increases in cerebrovascular compliance and ii) sex differences exist in cerebrovascular compliance. Twenty young healthy adults (24 ± 3 years, 10 females) completed a sit‐to‐stand protocol involving three minutes of sitting and three minutes of standing while brachial artery BP (Finapres Finometer) and middle cerebral artery BV (Multigon Neurovision) were collected. Individual BP (with temporal shift) and corresponding BV waveforms were input into a four‐element lumped parameter Windkessel model to calculate indices of cerebrovascular compliance (Ci) and resistance (Ri). 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 minus seated baseline Ci determined the change in (Δ) Ci. Females demonstrated higher Ci than males during seated baseline (1.1e−3 ± 4.4e−4 vs. 7.1e−4 ± 2.1e−4 cm·s−1·mmHg−1; P = 0.04, d = 1.3). Upon standing, large and rapid increases in Ci were observed in both groups but females exhibited a lesser increase when compared with males (114 ± 48 vs. 202 ± 50 %; P < 0.01, d = 1.8). No differences were observed in peak Ci with standing (females: 2.1e−3 ± 6.7e−4, males: 2.1e−3 ± 5.5e−4 cm·s−1·mmHg−1; P = 0.89). While the drop in mean BP with standing was attenuated in females compared with males (−14 ± 5 vs. −25 ± 9 mmHg; P < 0.01, d = 1.5), no relationship existed between Δ Ci and Δ mean BP (b = −7.0e−7 cm·s−1·mmHg−2; r2 = 3.0e−4; b P = 0.94). Importantly, the increase in Ci preceded reductions in Ri (2 ± 2 vs. 11 ± 4 beats following standing; P < 0.01, d = 2.8). Therefore, large and rapid increases in Ci occur with standing‐induced hypotension that contribute to dynamic cerebral autoregulation. Further, females demonstrate attenuated Ci responses to standing despite exhibiting higher seated baseline Ci compared with males.Support or Funding InformationSupported by the Natural Sciences and Engineering Research Council of Canada.