Vascular response of retinal arteries and veins to acute hypoxia of 8,000, 10,000, 12,500, and 15,000 feet of simulated altitude.

Computerized scanning microdensitometry of fundus photographs was used to study the effect of acute hypoxia on vessel diameters of 238 retinal arteries and 227 veins in 40 men aged 18-23 years. Fundus photography was performed at sea level and at simulated altitudes in a low-pressure chamber, randomly selecting 10 men to each of four altitudes--8,000, 10,000, 12,500, and 15,000 ft (2,400, 3,000, 3,800, and 4,500 m). After 15 min of hypoxic exposure, the caliber of small arteries (less than 75 microns) increased 4% (p less than 0.01) and that of large arteries (greater than or equal to 75 microns) 7% (p less than 0.05), already at 8,000 ft (2,400 m). The caliber of large veins did not change until an increase of 6% (p less than 0.01) was observed at 15,000 ft (4,500 m). Although large arteries increased 7% (p less than 0.05) and 8% (p less than 0.01) in diameter at 10,000 ft (3,000 m) and 15,000 ft and small veins 9% (p less than 0.05) and 12% (p less than 0.01) at the same altitudes, respectively, no change was observed at 12,500 ft (3,800 m). Thus, the dilating effect of hypobaric hypoxia was nonlinear from sea level to 15,000 ft of simulated altitude. As an overall response to the four simulated altitudes the vasodilation was negatively correlated to the vessel diameter at sea level (r = -0.20) in both arteries and veins (p less than 0.01). The variability of hypoxic vascular response was as great within different parts of individual retinas as between different retinas.