Baseline Hypocapnia Blunts Steady-state But Not Transient Hypercapnia-induced Shear-mediated Dilation Of The Internal Carotid Artery

Abstract

PURPOSE: Shear-mediated dilation of the internal carotid artery (ICA) induced by transient or steady-state hypercapnia (TR and SS tests, respectively) potentially reflects cerebrovascular endothelial function. During the baseline phase prior to elevating PETCO2, increased ventilation and/or hypocapnia associated with hyperventilation may attenuate the shear rate (SR), resulting in suppression of ICA dilation. This study aimed to elucidate whether hyperventilation with or without hypocapnia blunts shear-mediated dilation of the ICA and whether this effect differs between the TR and SS tests. METHODS: Shear-mediated dilation was measured using the TR and SS tests in 12 healthy men (25 ± 3 years, mean ± SD) on separate days. In each test, subjects breathed under three conditions: spontaneous breathing (SB), hypocapnic hyperventilation (HV), and isocapnic hyperventilation (IHV) with a tidal volume of 1.5 times SB. Shear-mediated dilation was induced by an increase in PETCO2 (10 mmHg above the baseline value) for 30 sec and 3 min in the TR and SS tests, respectively, and was calculated as percent rise in peak diameter relative to the baseline diameter. SR area under the curve (SRAUC) was calculated as SR from PETCO2 elevation to the peak dilation. RESULTS: Baseline hypocapnia attenuated baseline SR, and SRAUC was lower during HV than during SB in both tests (both P < 0.05). Shear-mediated dilation corrected by baseline diameter and SRAUC was not altered by an increase in ventilatory drive in the TR test (SB: 3.3% ± 2.1%, HV: 4.5% ± 2.3%, and IHV: 4.0% ± 1.9%; P > 0.05 for all). In contrast, corrected shear-mediated dilation in the SS test was lower during HV (3.6% ± 1.8%) than during SB (5.3% ± 1.7%, P < 0.05), but not during IHV (4.7% ± 1.7%, P > 0.05). Shear-mediated dilation was positively correlated with SRAUC during SB in both TR and SS tests (r = 0.80, P < 0.01 and r = 0.68, P = 0.02, respectively), but not during IHV (r = 0.36, P = 0.28 and r = −0.20, P = 0.55, respectively). During HV, there was a positive relationship between shear-mediated dilation and SRAUC in the TR test (r = 0.69, P = 0.02), but not in the SS test (r = −0.41, P = 0.21). CONCLUSIONS: Baseline hypocapnia, but not increased ventilation, blunts shear-mediated dilation of the ICA in the SS test, but not in the TR test. Supported by JSPS KAKENHI Grants (21J22042 and 20 K11186).