DETECTION OF ENDOGENOUS GAS PHASE FORMATION IN HUMANS AT ALTITUDE

Abstract

Air travel routinely exposes passengers to ambient pressure transients. Work in decompression during undersea operations has established that these pressure changes may produce small, mostly inert gas bubbles which become widespread throughout tissues and blood. It was previously thought that bubbles would not form unless a specific supersaturation ratio of tissue pressure to ambient pressure was exceeded. However, detectable gas phases have been identified in humans after surprisingly small decompressions. The purpose of the study was to identify the magnitude and duration of gas phase production at altitude to establish a dose response relationship between bubble formation and hypobaric exposures using Doppler ultrasound. Data from preliminary work for this study suggested the decompression required to produce bubbles in 50% of male subjects was from air saturation at only 10 fswg. Extrapolation to altitude suggested that pressure reductions commonly attained in commercial and military aircraft might be sufficient to form a gas phase in humans. We examined the magnitude and duration of gas phase production in male and female human volunteers at altitudes to 12,000 feet in an altitude chamber to establish a dose response relationship between bubble formation and hypobaric exposures using Doppler ultrasound. Subjects were exposed to hypobaric conditions up to 10,000 ft at 1000 ft increments. At each altitude Doppler ultrasound was used to detect mobile gas phase at several venous sites. We found bubbling in 75% of male subjects at 10,000 feet which is sometimes reached as an equivalent cabin pressure in commercial air flights, and is routinely achieved in unpressurized passenger flights and some military aircraft. The rate of ascent was found to relate to diving decompression stops. The data may be applicable to flying after diving schedules, and may contribute to understanding of a bubble component to altitude sickness.