The effect of caffeine on the human macular circulation.

The acute effect of caffeine on the retinal circulation was studied in 14 healthy volunteers using the blue field simulation technique, which provides measurements of the velocity of leukocytes flowing within the macular capillaries. Subjects adjusted the mean velocity (Vm) of computer-simulated leukocytes moving on a cathode ray tube screen to match that of their own entoptically perceived leukocytes before and 1 hr after a double-masked, randomized administration of 200 mg caffeine or placebo. Caffeine produced an average 13% +/- 5% (SEM) decrease in Vm (P less than 0.05) and a 9% +/- 3% increase in diastolic blood pressure (P less than 0.05). The decrease in Vm and, presumably, blood flow occurring despite the increased diastolic blood pressure probably is attributable to retinal vasoconstriction. This effect may result from caffeine's known inhibitory effect on adenosine, a potent vasodilator of the retinal vasculature.