VISUAL THRESHOLDS AS AN INDEX OF THE MODIFICATION OF THE EFFECTS OF ANOXIA BY GLUCOSE

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Abstract
The measurement of differential intensity thresholds for vision at low brightness levels provides a sensitive and objective index of the impairment caused by anoxia. This test was applied to the study of the modification of this impairment by glucose. Three fasting subjects were used at simulated altitudes of 12,700-17,200 ft. The ingestion of 50 gs. of glucose during exposure to low O2 tensions (simulated high altitude) resulted in *a considerable decrease in the im-pairment due to anoxia. At a simulated altitude of 13,800 ft. the adm. of glucose diminished the visual impairment of one subject to an amt. corresponding to an altitude of only 8,000 ft. The "physiologic altitude" was thus 42% lower than the actual altitude. In the other 2 subjects, the lowering of physiologic altitude was 25% and 48%, respectively. The ingestion of glucose1/2 hr. before exposure to low O2 tensions likewise prevented a large part of the im-pairment expected from anoxia. Control expts., in which a saccharin soln. was given at simulated high altitudes instead of glucose, showed no effects on the impairment caused by anoxia. The adm. of glucose to fasting subjects at a normal atmospheric O2 tension resulted in no improvement of visual sensitivity. An improvement occurred only if visual sensitivity had first been impaired by anoxia. The amt. of improvement of visual sensitivity during anoxia after glucose adm., in relation to time, was approx. parallel to the blood sugar curve during its rise and subsequent fall. The increase in CO2 production after glucose ingestion lags behind the rise in blood sugar and declines considerably later than the fall in blood sugar. The "anti-anoxic" effect of glucose therefore seems to depend directly on the blood sugar level rather than on the secondary increase in CO2 production.