Ocular oxygenation: the effect of phenylephrine on anterior chamber oxygen tension

Phenylephrine preparations are widely used in medicine for their vasoconstrictive and mydriatic effects. The effect of phenylephrine, a powerful alpha-receptor stimulant, on anterior chamber oxygen tension (Po2) was studied using a polarographic electrode inserted into the mid anterior chamber of cats. A commercial 10% solution was applied topically. A decrease in anterior chamber Po2 was usually observed in 8-24 minutes and declined steadily thereafter. Prior to treatment, the mean anterior chamber Po2 was 26 +/- 3 torr; 1/2 hour after treatment it decreased to 20 +/- 7 torr. After 1 hour it dropped to 13 +/- 8 torr and was further reduced to 11 +/- 6 torr by 2 hours. By 90 minutes, the drug had caused a 58% reduction in anterior chamber oxygen tension. This drop is similar to that reported for epinephrine. It is suggested that the mechanism for most of this decrease is reduced blood flow, mediated by the direct vasoconstrictive effect of the drug in addition to compression of the iris vasculature induced by dilation. The resulting change (reduction) in caliber of the iris arteries produces a parallel change (reduction) in blood volume and flow resulting in a diminished Po2. The clinical implications of this conclusion in disorders where hypoxia is felt to play a causal role such as neovascular glaucoma, diabetes, pregnancy and hyphema in sickle cell disease are explored.