Variations on Ernsting's Post-Decompression Hypoxia Prevention Model

INTRODUCTION: In the event of decompression using an isobaric differential cockpit pressurization system, oxygen concentration breathed pre-decompression must be greater than required for the given cockpit altitude in order to prevent hypoxia. The model for determining oxygen concentration requirements advanced by Dr. John Ernsting, when graphed against cockpit altitude, creates a hypoxia safety "notch" which has become a standard requirement for aircraft oxygen systems. Although variables in the Ernsting notch model are not fixed, they are often presented as such.METHODS: Model equations are presented to evaluate the effects of different cockpit pressurization, oxygen regulator PBA schedules, and changes to the physiological state of the aircrew.RESULTS: Increased cockpit differential pressure, regulator breathing pressure, and aircrew respiratory quotient decreased pre-decompression oxygen concentration requirements by up to 6%, eliminating the hypoxia safety "notch." Although effects were small, reducing alveolar carbon dioxide pressure decreased oxygen concentration requirements while reducing respiratory quotient increased oxygen concentration requirements. A 10-mmHg increase in the minimal oxygen hypoxia threshold increased the pre-decompression oxygen concentration requirement 8 to 12% depending on cockpit altitude.CONCLUSION: Variation in cockpit and regulator pressure schedules which stray outside the parameters used by Ernsting need to be independently calculated. During flight, an individual's physiological "notch" will be dynamic, wavering in response to changes in metabolic load, respiratory dynamics, and environmental conditions. Consideration of aircrew activity should be factored in when considering minimal oxygen concentration for pre-decompression hypoxia protection in the design of aircrew life support systems.Dart TS, Morse BG.