Human Responses to a Simulated 35,000-Foot Instantaneous Decompression and the Subsequent Descent Profile Required by FAA Policy

Sudden decompression of an airliner passenger cabin due to structural failure or damage is unlikely, but it poses a potentially life-threatening event for occupants. The authors investigated a worst-case scenario, where the passenger fails to receive supplemental oxygen during a rapid decompression (RD), and the subsequent emergency descent to 25,000 ft required by Federal Aviation Administration (FAA) policy. The research question was whether an individual’s oxygen stores will be depleted prior to the aircraft descending to an altitude that will permit inward fluxes of oxygen that exceed the resting oxygen consumption requirement. The authors exposed 24 subjects to normobaric instantaneous decompressions to a simulated altitude of 35,000 ft. The peak altitude was maintained for 10 s and then followed by a 5000 ft/min descent to 25,000 ft. Resting oxygen consumption was measured prior to the hypoxia exposure. During each trial, tidal volume, respiratory rate, breath-by breath inhalation, and end-tidal O₂, CO₂, and N₂ tensions were measured and net directional oxygen flux computed. All subjects had an initial reversal of the direction of oxygen flux following the RD that persisted until after the descent commenced with outward flux predominating at higher altitudes of the profile. Return to net inward flux almost always occurred near 29,000 ft, the altitude at which the mixed venous and alveolar PO₂ gradient approximates nil. The inward flux of oxygen approached but never surpassed each subject’s resting oxygen consumption as the altitude approached the 25,000 ft endpoint. Based on the data, the authors used computational methods to predict the O₂ fluxes that would have occurred during normobaric exposures to 40,000 and 45,000 ft, along with Boyle’s law effects expected during an actual rapid decompression. These data are unique, as they are the first to result from actual human exposure to the descent profile required by FAA policy. This research serves to quantitatively define this risk associated with a high altitude decompression, and may be useful in future policy decisions.