Response of retinal blood flow to CO₂-breathing in humans

PURPOSE. To investigate the effect of systemic hypercapnia on retinal hemodynamics in humans. METHODS. We studied the effect of breathing a mixture of normal air with 5% CO₂ for 13 minutes in ten healthy young male volunteers, using the Zeiss retinal vessel analyzer for continuous measurement of retinal vessel diameter and the blue-field entoptic technique to quantify retinal white blood cell flux. In eight other subjects the effect of hypercapnia was measured with the Zeiss retinal vessel analyzer and by laser Doppler velocimetry to establish retinal blood flow velocity. RESULTS. Retinal arterial and venous vessel diameters increased by a maximum of 4.2% and 3.2%, respectively. Peak effect was observed after 3 minutes of breathing the mixture of normal air with 5% CO₂. During hypercapnia red blood cell velocity increased 11.7% and, accordingly, retinal blood flow increased 19.1%. White blood cell density and velocity rose significantly during hypercapnia, resulting in an increase in white blood cell flux (19.2%). CONCLUSIONS. Our data indicate that CO₂ induces vasodilation in retinal arteries and retinal veins. Retinal blood flow and perimacular white blood cell flux increased to the same extent in subjects breathing a mixture of normal air with 5% CO₂.