Dilatory responsiveness of the internal carotid artery to shear stimulus is constant under different levels of transient hypercapnia

Peripheral endothelial function, which accounts for the variability in shear stimulus, can be assessed using shear-mediated dilation normalized to the increased shear stimulus. Similarly, shear-mediated dilation of the internal carotid artery (ICA), an index of cerebrovascular endothelial function, should be normalized to increased shear stimulus. However, this approach has not yet been validated. Thus, the shear-mediated dilation of the ICA was assessed in 14 young adults during three levels of transient hypercapnia, induced by elevating the partial pressure of end-tidal carbon dioxide for 30 s by 6, 9, and 12 mmHg. The ICA shear rate (SR) was calculated using the ICA diameter and velocity, both measured by Doppler ultrasound. The total vasodilator stimulus was quantified as the SR area under the curve from the onset of hypercapnia to peak dilation, including and excluding baseline values [(SR_AUC) and delta SR_AUC (_DSR_AUC), respectively]. Shear-mediated dilation was calculated as the percent increase in diameter from baselines. ICA dilation was positively associated with _DSR_AUC [r_(rm) = 0.47, <i>P</i> < 0.01] but not with SR_AUC [r_(rm) = 0.32, <i>P</i> = 0.09]. Consequently, ICA dilation normalized to _DSR_AUC did not differ among trials (main effect of rial, <i>P</i> = 0.77). Contrarily, the difference in ICA dilation among trials remained significant when normalized to SR_AUC (main effect of trial, <i>P</i> = 0.02). Therefore, normalized shear-mediated dilation using _DSR_AUC can reduce variability associated with increased shear stimulus during ICA dilation assessment, thereby enhancing the validity of evaluating cerebrovascular endothelial function.<b>NEW & NOTEWORTHY</b> This study demonstrated that shear-mediated dilation of the internal carotid artery (ICA), an index of cerebrovascular endothelial function, increased with the increase of shear stimulus induced by different degrees of transient hypercapnia. However, when ICA dilation was normalized to the total increased shear stimulus above baseline, the vasodilation became comparable across different hypercapnia levels. Thus, normalizing ICA dilation to the total shear stimulus increased from baseline may enhance the validity of assessing cerebrovascular endothelial function.