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

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.

16 participants (7 female) completed the THC_DB protocol, breathing 9% CO₂. 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 (_DSR_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.

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 s following the onset of CO₂ inhalation. The peak %ΔICAd was positively correlated with the change in _DSR_AUC (R² = 0.36; p = 0.01).

The THC_DB protocol resulted in shear-mediated vasodilation in the ICA akin to that seen in other transient CO₂ 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.