Acute mountain sickness at 4500 m is not altered by repeated eight-hour exposures to 3200-3550 m normobaric hypoxic equivalent.

A lightweight device, designed to supply inspired air at 12.8% O2 concentration (PO2 equivalent to 3960 m altitude) by recirculating a portion of each expired breath after CO2 removal, was tested at sea-level for its ability to induce altitude acclimation. Twelve young men (experimental group) breathed from the device for 7.5-8 h each day for 10 successive days. On the morning of day 1, inspired O2 concentrations averaged 12.8%, as intended, but increased by noontime and remained elevated thereafter. This raised the average hypoxic stimulus to 13.8 +/- 0.9% (PO2 equivalent to 3370 +/- 517 m altitude) for the entire 10-d period. Ten other young men (control group) breathed normoxic air from a placebo device of identical appearance on the same schedule. On day 10, both groups were exposed for the next 2 d to 4500 m altitude in a hypobaric chamber to assess the effect of the treatment on acute mountain sickness (AMS). After the sea level treatment, the experimental group showed no significant differences from control in resting ventilatory rate, respiratory frequency or end tidal PO2, but end-tidal PCO2 was lower; there was no indication of hemoconcentration. At altitude, both groups showed the expected decreases in end-tidal PO2 and PCO2, and increases in hemoglobin concentration and hematocrit indicative of hemoconcentration, with no differences between them. Neither incidence nor severity of AMS differed significantly between groups, but the experimental group had a lower incidence rate than historical controls.(ABSTRACT TRUNCATED AT 250 WORDS)