EFFECTS OF OXYGEN AND CARBON-DIOXIDE ON HUMAN RETINAL CIRCULATION

Purpose. Carbogen, a gas mixture of 95% O2 and 5% CO2, is given to patients with retinal artery obstruction in an attempt to improve retinal oxygenation. The purpose of this study was to compare the effects of carbogen and 100% O2 breathing on retinal blood flow. Methods. On two separate occasions, 12 normal, healthy volunteers breathed air and then either 100% O2 or carbogen while laser Doppler velocimetry measurements and monochromatic fundus photographs were taken. Retinal vessel diameter, maximum velocity of red blood cells, and volumetric blood flow rate were determined in a main temporal vein. Results. Both 100% O2 and carbogen caused significant average reductions in vessel diameter (14.1% and 10.6%, respectively), maximum red blood cell velocity (42.1 % and 27.3%, respectively), and blood flow (56.4% and 42.2%, respectively). The average vasoconstriction of the large retinal veins caused by carbogen was not significantly smaller than that caused by 100% O2. The average reductions in maximum red blood cell velocity and blood flow caused by carbogen were significantly smaller than those caused by 100% O2 (P < .001 and P < .01, respectively). Conclusions. In normal subjects, inhalation of carbogen leads to less reduction in blood flow than inhalation of 100% O2, presumably by reducing the vasoconstriction of small arterioles induced by elevated oxygen levels.