Rapid Decompression to 45,000 ft:Results of Human Trials and Implications for Future Cockpit Oxygen Systems

Introduction: The success story of civil air transportation is closely related to the provision of a pressurised environment during flight; typical flight altitudes of 30,000+ feet for efficient air traffic are only possible with a reliable protection of passengers and crew against a hostile and life-threatening outside. In case of failure of this protecting cocoon, oxygen masks are provided to support flight deck crew during immediate emergency descent. For decompression in less than 5s at maximum flight altitude prebreathing of oxygen enriched gas is necessary to prevent transient unconsciousness of pilots (45,000 ft: 52% oxygen). However, in large transportation aircraft a typical depressurisation time of 10 - 20 s minimum can be expected, allowing to top up alveolar oxygen by several breaths if pure oxygen is provided from the beginning. Material und Methods: Tests were performed on 10 subjects aged 29-71 yrs making use of the barochamber complex of the DLR-Institute of Aerospace Medicine by pressure equilibration from 8,000 to 45,000 ft. cruising altitude within 20 s via large tubing with computer controlled valves between different chambers. Subjects initially breathed air via a modified cockpit mask, after start of depressurisation the mask was switched to 100% oxygen. Subject oxygenation status was monitored via pulse oxymetry and oxygen partial pressure in the mask. Results: Minimum values during decompression and stay at maximum altitude for 10 s were an end tidal oxygen partial pressure of 43 hPa (41-62) and an arterial saturation of 77% (62-92). Middle ear pressure equilibration was performed without problems, none of the subjects showed any incapacitation during runs. Conclusion: By immediate switch (t = 2 s) of the test mask to pure oxygen in the beginning of a 20-s depressurisation prebreathing of an oxygen enriched gas is not necessary as several breaths with pure oxygen are possible, leading to sufficient alveolar oxygen enrichment; this scenario is not possible with current equipment. The data can be used for model validation in this highly dynamic process, as proposed in the SAE A-10 Aircraft Oxygen Equipment Committee, opening the way to estimate crew and passenger hypoxia protection with different types of equipment. The results can also be seen as a stimulus for new hardware development making use of an early switch from air to oxygen when a rapid decompression is detected, instead of altitude dependent air dilution. With this approach, pre-wearing a mask at very high flight altitude without oxygen prebreathing, a considerable gain in safety may be achieved, eliminating the threat of unconsciousness in the cockpit