Effect of inhalation of different mixtures of O2 and CO2 on retinal blood flow

Aim: To determine the effects of various mixtures of O-2 and CO2 on retinal blood flow in healthy subjects. Methods: A randomised, double masked, four way crossover trial was carried out in 12 healthy male non-smoking subjects. Gas mixtures (100% O-2, 97.5% O-2 + 2.5% CO2, 95% O-2 + 5% CO2, and 92% O-2 + 8% CO2) were administered for 10 minutes each. Two non-invasive methods were used: laser Doppler velocimetry (LDV) for measurement of retinal blood velocity and fundus imaging with the Zeiss retinal vessel analyser (RVA) for the assessment of retinal vessel diameters. Arterial pH, pCO(2), and pO(2) were determined with an automatic blood gas analysis system. Retinal blood flow through a major temporal vein was calculated. Results: Retinal blood velocity, retinal vessel diameter, and retinal blood flow decreased during all breathing periods (p < 0.001 each). Administration of 92% O-2 + 8% CO2 significantly increased SBP, MAP, and PR (p < 0.001 each, versus baseline), whereas the other gas mixtures had little effect on systemic haemodynamics. Addition of 2.5%, 5%, and 8% CO2 to oxygen caused a marked decrease in pH and an increase in pCO(2) (p < 0.001 versus pure oxygen). Conclusions: Breathing of pure oxygen and oxygen in combination with carbon dioxide significantly decreases retinal blood flow. Based on these data the authors speculate that hyperoxia induced vasoconstriction is not due to changes in intravascular pH and cannot be counteracted by an intravascular increase in pCO(2).