Using a transient hypercapnic cerebrovascular douglas bag protocol to examine shear-rate and vasodilation in the internal carotid artery

Aims Transient hypercapnia has been used to examine shear-mediated vasodilation in the internal carotid artery (ICA) in humans. Previously, a 30-second transient hypercapnic protocol using an end-tidal forcing machine has been used. We aimed to investigate if we could simplify this technique by using a Douglas bag instead of an end-tidal forcing machine, thus creating the transient hypercapnic Douglas bag (THC DB ) protocol. We hypothesized that by increasing the hypercapnic intensity when using a Douglas bag, a 30-second transient test would elicit shear mediated vasodilation in the internal carotid artery. Methods 16 participants (7 female) completed the THC DB protocol, breathing 9% CO 2 . Internal carotid artery diameter (ICAd), velocity, flow, and shear rate were recorded using vascular Duplex ultrasound, with the change in shear rate area under the curve ( D SR AUC ) between baseline and the time of peak dilation being calculated from these measures; while mean arterial pressure (MAP) was recorded using finger photoplethysmography before, during, and after the THC DB protocol. Results The THC DB protocol increased shear rate in the ICA (p < 0.0001) and led to significant peak dilation of the ICA (Baseline: 4.9 mm; Peak: 5.2 mm; p < 0.0001). Peak dilation of the ICA occurred 75.9± 13.2 seconds following the onset of CO 2 inhalation. The peak %ΔICAd was positively correlated with the change in D SR AUC (R 2 = 0.36; p = 0.01). Discussion The THC DB protocol resulted in shear-mediated vasodilation in the ICA akin to that seen in other transient CO 2 tests and therefore represents a novel technique to assess shear-mediated vasodilation of the ICA. While THC DB protocol is reasonably effective at investigating the effects of transient hypercapnia on cerebral hemodynamics, there are several methodological factors that require further experimentation to refine the technique.