Background: An increase in blood velocity-associated shear stress results in release of endothelial factors, causing endothelium-dependent flow mediated dilatation (FMD). Hypercapnia strongly stimulates cerebral blood flow velocity in the common carotid artery (CCA).

Objective: Test the reliability of hypercapnia induced FMD of the CCA and evaluate the stimulus response relationship (changes in blood velocity and diameter) for different age populations.

Methods: Hypercapnia was induced with inhalation of a gas mixture of 6.8% CO2, 74.5%N2, and 18.6% O2 for a period of 2 minutes in 19 healthy young (24±6y) and 11 old (60±12y) subjects. Continuous beat-to-beat changes in CCA diameter were used to determine FMD magnitude in percentage change in end diastolic diameter (ΔD/D) and mean centre stream blood velocity (ΔV/V). Endothelial function (EF) is estimated by the relative response of ΔD/D to ΔV/V.

Results: Heart rate increases significantly during and post hypercapnia. No significant changes are seen in peripheral blood pressure. Hypercapnia stimulus induces significant increases in flow velocity and diameter in both populations, reaching a steady state after 3 minutes. EF=0.6±0.03 (young), and EF=0.4±0.08 (old).

Conclusions: CO2 stimulated FMD response at the CCA exposes the dynamic interrelationship between blood velocity and diameter. It provides a direct and well-tolerated tool to quantify endothelial function in atherosclerotic prone arteries.