ACUTE VENTILATORY RESPONSE TO SIMULATED ALTITUDE, NORMOBARIC HYPOXIA,AND HYPOBARIA

Background: Some reports claim that ventilation (VE) is greater in hum an subjects in normobaric hypoxia than at altitude following an equiva lent drop in inspired PO2 (PIO2) It has been suggested that reduced ba rometric pressure (PB) may decrease chemoreceptor sensitivity and acco unt for these results. In this pilot study we tested the hypothesis th at VE and hypoxic chemoresponsiveness would not be different after 30 min of normobaric hypoxia and altitude. Methods: We exposed three male and three female subjects to four conditions in an environmental cham ber, varying the order. The four conditions were: air (PB = 640, FIO2 = 0.204), hypobaria (434, 0.298), hypoxia (640, 0.14l)and altitude (43 4, 0.203). We measured VE, end-tidal O-2 and CO2 and arterial O-2 satu ration (SpO(2)) after 30 min in each environment, and while breathing 100% O-2 for 1 min immediately thereafter. Results: The mean increase in VE relative to air was 14%, 20% and 26% for hypobaria, hypoxia and altitude, respectively, with corresponding reductions in PETCO(2) in t he three conditions. The reduction in VE with 100% O-2 was inversely p roportional to the rise in SpO(2) in all cases, indicating that chemor esponsiveness was unchanged by PB. When hypobaria preceded altitude, t he VE at altitude increased less, relative to air, than when altitude was given first (not significant). Conclusions:The VE and chemosensiti vity are about the same after 30 min of altitude and equivalent hypoxi a. However, when the drop in PIO2 is not synchronous with the drop in PB, like at altitude, the VE values may be altered. Air density, hypox ic pulmonary vasoconstriction and circulating microbubbles may interac t to account for the observed findings.