Retinal blood velocities during carbogen breathing using scanning laser ophthalmoscopy

Hyperoxic-hypercapnic gas mixture ('carbogen': 6% CO2-94% O2) is widely used clinically. Its influence on retinal perfusion remains unclear, since past work suggests that high O2 may reduce and high CO2 may increase retinal blood flow. To examine the changes in retinal blood velocity during 'hyperoxic-hypercapnic' gax mixture breathing, we quantified retinal blood velocity indices. Twenty-eight healthy subjects were studied using scanning laser video fluorescein angiograms at baseline and after gas inhalation. Retinal arteriovenous passage time and mean arterial dye velocity were measured by means of a digital image processing system. Arterial diameter measurements showed no change (4%) during carbogen breathing. Increased arterial Pco2 and Po2 resulted in small but statistically significant increases in systolic and diastolic blood pressure and heart rate (each c. 8%, p < 0.05), and a dramatic increase in mean dye velocity (27%, p < 0.01), with parallel reduction in arteriovenous passage time (40%, p < 0.01). The substantial acceleration of retinal dye velocity and transit under combined hyperoxia and hypercapnia strongly suggests that this clinically standard gas mixture may indeed improve oxygenation without reducing retinal perfusion.