Using a 30-second Transient Hypercapnic Douglas Bag Technique to Assess the Impact of Sex on Internal Carotid Artery Shear-mediated Vasodilation

Hypercapnia has been demonstrated to cause vasodilation in the extracranial internal carotid artery (ICA). Studies using transient end-tidal forcing have demonstrated that cerebral hypercapnic vasodilation is mediated by endothelial shear stress. Previous techniques have used a Douglas bag to provoke a prolonged steady-state hypercapnic vasodilation have not demonstrated an endothelial-mediated technique. Objective: This study aimed to integrate these methodologies, creating an affordable and safe transient Douglas bag hypercapnic endothelial function test. We hypothesized that a transient hypercapnic Douglas bag approach would elicit shear-mediated vasodilation. Results: Sixteen young healthy adults (female = 7) completed a 30-second transient hypercapnic (9% Fi CO 2 ) paced-breathing (7.5 breaths per minute) protocol which using a 100 L Douglas bag. ICA blood flow was measured using a vascular duplex ultrasound. Peak ICA diameter (0.52±0.05 cm) was observed 75.22±12.33 seconds after the onset of CO 2 (p <0.0001). ICA velocity, flow, and shear rate increased throughout the transient CO 2 test (51.55 ±11.70 cm*sec-1; 547.67±126.61 mL*min-1; 443.65±138.25 s-1) and at the time of peak dilation (47.54±11.91 cm*sec-1; 588.41±122.58 mL*min-1; 412.90±132.27 s-1) compared to during rest and paced room-air breathing (p<0.005; p<0.001; p<0.0001). Pooled analysis (males and females) demonstrated a strong positive (r = 0.60) correlation between peak diameter and shear-rate area under the curve (SR AUC). After stratifying by sex, it was shown that there is a greater positive correlation (p <0.05) between SR AUC and percent dilation in females (r = 0.73) compared to males (r = 0.42). When accounting for the effect of baseline ICA diameter on the amount of shear-mediated dilation occurring from transient hypercapnia, only the males were found to have a significant, strong negative (r=-0.79, p=0.01) correlation between baseline ICA diameter and shear. The results demonstrate that the transient hypercapnic Douglas bag technique provides an economically friendly option for assessing shear-mediated vasodilation. Dr. Kurt Smith is funded through a National Sciences and Engineering Research Council (NSERC) of Canada RGPIN-2020-06269 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.