Background: Although the brachial artery is appreciated as a representative of the arterial tree for endothelial function tests, it would be favourable to estimate flow mediated dilation (FMD) at clinically relevant locations e.g. the common carotid artery (CCA). Hypercapnia strongly stimulates cerebral blood volume flow, with steady state achieved after 2 mins, inducing FMD in the CCA.

Objective: To develop a complete stimulus-response profile (baseline, peak, steady state) of flow velocity, diameter and heart rate during and after hypercapnia, also to evaluate their interrelationships.

Methods: The CCA was visualised with an ultrasound beam at 60° in Doppler-mode with a P350 ultrasound system. Hypercapnia was induced with a 6.8% CO2 inhalation for a period of 2 mins. Processing of received signals from baseline, during hypercapnia and return to baseline (total 10 mins) clearly revealed in beat-to-beat changes in systolic and end diastolic diameters, blood flow velocity (BFV), blood flow rates, and carotid peripheral resistance.

Results: Continuous long-term measurements indicate a peak at 3 mins, with a steady decay over the remaining follow up time. The BFV (+25%), carotid diameter (+12%), distension and heart rate increase significantly during and post hypercapnia. No significant changes are seen in blood pressure.

Conclusions: Inhalation of CO2 for 2mins increases blood flow and induces carotid artery dilatation. CCA FMD exposes the dynamic interrelationship between blood flow and CCA diameter and is a good tool to test directly endothelial function in atherosclerotic prone arteries.