Pneumatic measurement of intraocular pressure: interp´retaion of hypobaric and hypoxic effects

Abstract

Using a non-contact tonometer, we previously demonstrated that intraocular pressure (IOP) was reduced after exposure to 446 mm Hg for two hours, as well as during 15 days of residence at Pikes Peak (4300 m — 446 mm Hg). Although a definite acclimatization effect was show during the latter exposure, the nature of the aprubt 20% reduction in IOP (15.8 ± 0.6 vs. 12.9 ± 0.9 mm Hg, promedio ± SE, n = 11) during the 2- h altitude simulation was of concern. We hypothezized that the reduction could be due either to the reduction in barometric pressure (P_B) and/or to the physiological effects of hypoxia. In two, 2-h series of experiments separated by one week, seven subjects were exposed to either normobaric hypoxia (P_B = 760 mm Hg; 12.8% O₂) (NH) or hypobaric normoxia (P_B = 446 mm Hg; 37.3% O₂) (NH).  During NH, IOP gradually decreased, reaching statistical significance after two hours (P < 0.009). During HN, IOP_s were reduced at 0.5 hours (P < 0.0001) and remained decreased for the entire two hours. Within two minutes of return to normobaria after HN, IOP returned to 17.14 ± 1.0 mm Hg, demostrating an inmediate pressure effect on the measurement of IOP. These results indicate that the reduction in IOP observed during altitude exposure is the result of the combination of hypobaria and a physiological hypoxic effect manifests itself after a minimun of two hours of exposure. The previously-observed IOP changes with exposure remain valid.