Novel methodology to comprehensively assess retinal arteriolar vascular reactivity to hypercapnia

(1) Describe a new methodology that permits the comprehensive assessment of retinal arteriolar vascular reactivity in response to a sustained and stable hypercapnic stimulus. (2) Determine the magnitude of the vascular reactivity response of the retinal arterioles to hypercapnic provocation in healthy, young subjects.

Eleven healthy subjects of mean age 27 years (SD 3.43) participated in the study and one eye was randomly selected. A mask attached to a sequential rebreathing circuit, and connected to a gas delivery system, was fitted to the face. To establish baseline values, subjects breathed bottled air for 15 min and at least 6 blood flow measurements of the supero-temporal arteriole were acquired using the Canon Laser Blood Flowmeter (CLBF). Air flow was then decreased until a stable increase in fractional end-tidal CO(2) concentration (F(ET)CO(2)) of 10-15% was achieved. CLBF measurements were acquired every minute (minimum of 6 measurements) during the 20-minute period of elevated F(ET)CO(2). F(et)CO(2) was then reduced to baseline levels, and 6 further CLBF measurements were acquired. Respiratory rate, blood pressure, pulse rate and oxygen saturation were monitored continuously.

Retinal arteriolar diameter, blood velocity and blood flow increased during hypercapnia relative to baseline (p=0.0045, p<0.0001 and p<0.0001, respectively). Group mean F(ET)CO(2) showed an increase of 12.0% (SD 3.6) relative to baseline (p<0.0001).

This study describes a new methodology that permits the comprehensive assessment of retinal arteriolar vascular reactivity in response to a sustained and stable hypercapnic stimulus. Retinal arteriolar diameter, blood velocity and blood flow increased significantly in response to a hypercapnic provocation in young, healthy subjects.