Intermittent hypoxia enhances shear-mediated dilation of the internal carotid artery in young adults.

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±4yrs) before as well as after 50min of cyclic IH [5 cycles, 4min normoxia followed by 6min hypoxia (target 80% SpO2) per cycle] and control normoxia (sham, 50min continuous normoxia) on separate days (≥72hrs apart). ICA diameter and velocity were measured using Doppler ultrasound during cyclic IH and hypercapnia. Shear-mediated dilation was induced by 3min of hypercapnia (ΔPETCO2, IH; pre 10.1±1.0mmHg, post 10.8±1.3mmHg, sham; pre 10.5±1.5mmHg, post 10.8±1.5mmHg) 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±74ml/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 normalized during the succeeding normoxic period (blood flow; 322±90 to 329±68ml/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 enhances shear-mediated dilation of the ICA in young adults. These results suggest that cyclic IH could be used to optimize cerebral vascular health.