Shear-mediated dilation of the internal carotid artery occurs independent of hypercapnia

Evidence for shear stress as a regulator of carotid artery dilation in response to increased arterial CO₂ was recently demonstrated in humans during sustained elevations in CO₂(hypercapnia); however, the relative contributions of CO₂and shear stress to this response remains unclear. We examined the hypothesis that, after a 30-s transient increase in arterial CO₂tension and consequent increase in internal carotid artery shear stress, internal carotid artery diameter would increase, indicating shear-mediated dilation, in the absence of concurrent hypercapnia. In 27 healthy participants, partial pressures of end-tidal O₂ and CO₂, ventilation (pneumotachography), blood pressure (finger photoplethysmography), heart rate (electrocardiogram), internal carotid artery flow, diameter, and shear stress (high-resolution duplex ultrasound), and middle cerebral artery blood velocity (transcranial Doppler) were measured during 4-min steady-state and transient 30-s hypercapnic tests (both +9 mmHg CO₂). Internal carotid artery dilation was lower in the transient compared with steady-state hypercapnia (3.3 ± 1.9 vs. 5.3 ± 2.9%, respectively, P < 0.03). Increases in internal carotid artery shear stress preceded increases in diameter in both transient (time: 16.8 ± 13.2 vs. 59.4 ± 60.3 s, P < 0.01) and steady-state (time: 18.2 ±14.2 vs. 110.3 ± 79.6 s, P < 0.01) tests. Internal carotid artery dilation was positively correlated with shear rate area under the curve in the transient (r² ± 0.44, P < 0.01) but not steady-state (r² ± 0.02, P < 0.53) trial. Collectively, these results suggest that hypercapnia induces shear-mediated dilation of the internal carotid artery in humans. This study further promotes the application and development of hypercapnia as a clinical strategy for the assessment of cerebrovascular vasodilatory function and health in humans. NEW & NOTEWORTHY Shear stress dilates the internal carotid artery in humans. This vasodilatory response occurs independent of other physiological factors, as demonstrated by our transient CO₂ test, and is strongly correlated to shear area under the curve. Assessing carotid shear-mediated dilation may provide a future avenue for assessing cerebrovascular health and the risk of cerebrovascular events.