Cyclic intermittent hypoxia (IH) increases cerebral blood velocity. This enhanced velocity augments the commensurate shear stimulus and may subsequently increase cerebrovascular endothelial function. This study aimed to examine the effects of cyclic IH on hypercapnia-induced shear-mediated dilation of the internal carotid artery (ICA), a potential index of cerebrovascular endothelial function. Shear-mediated dilation was measured in nine adults (22 ± 4 yr) before as well as after 50 min of cyclic IH [5 cycles, 4 min of normoxia, followed by 6 min of hypoxia (target 80% [Formula: see text]) per cycle] and control normoxia (sham, 50 min of continuous normoxia) on separate days (≥72 h apart). ICA diameter and velocity were measured using Doppler ultrasound during cyclic IH and hypercapnia. Shear-mediated dilation was induced by 3 min of hypercapnia (Δ[Formula: see text]; IH: pre 10.1 ± 1.0 mmHg, post 10.8 ± 1.3 mmHg; sham: pre 10.5 ± 1.5 mmHg, post 10.8 ± 1.5 mmHg) and was calculated as the percent rise in peak relative to baseline diameter. Hypoxia increased ICA blood flow and shear rate (SR) during each cycle [blood flow: 322 ± 90 to 406 ± 74 mL/min, P < 0.01; SR: 179 ± 42 to 207 ± 55/s, P = 0.06, baseline to hypoxia (average of last minute of each cycle)], which was normalized during the succeeding normoxic period (blood flow: 322 ± 90 to 329 ± 68 mL/min, P = 0.54, SR: 179 ± 42 to 176 ± 32/s, P = 0.56). As such, shear-mediated dilation increased following cyclic IH (4.6 ± 1.3% to 6.2 ± 2.2%, P < 0.01), but not control normoxia (4.9 ± 1.4% to 4.9 ± 1.4%, P = 0.92). Our data indicate that increased blood flow and SR during cyclic IH enhance shear-mediated dilation of the ICA in young adults. These results suggest that cyclic IH could be used to optimize cerebral vascular health.NEW & NOTEWORTHY We explored the effects of cyclic intermittent hypoxia (IH) on shear-mediated dilation of the internal carotid artery (ICA), a potential index of cerebral endothelial function, in young adults. Cyclic IH increased blood flow and shear rate in the ICA and, as a result, increased shear-mediated dilation of the ICA. These data suggest that cyclic IH could potentially be applied as a nonpharmacological therapy to optimize cerebral vascular health.
Keywords: cyclic intermittent hypoxia; hypercapnia; internal carotid artery; shear-mediated dilation.