Prevention of Hypoxia by a Continuous Flow of Product Gas
John Ernsting & Claire Peckover
Aircraft oxygen systems employing a continuous flow of oxygen with or without a reservoir have been employed widely in aviation for over a hundred years. With the widespread adoption of demand regulators in military aircraft the continuous flow technique was employed for the delivery of oxygen from the emergency/bail out supply. Recently, however, the provision of a continuous flow of the breathing gas from a molecular sieve oxygen concentrator has been used as a method of continuing to provide breathing gas in the event of a failure of the main demand regulator to pass gas in a high performance, low cabin differential pressure aircraft. In this system the continuous flow of product gas is delivered to the cavity of the pressure demand mask through the normal inlet hose whilst, when required, air is drawn into the mask cavity through an anti-suffocation valve mounted in the wall of the mask. The excess flow of product gas over the inspiratory demand passes to the environment through a compensated dump valve at the outlet of the regulator.
A simple model of this delivery system was developed to predict the concentration of oxygen in the inspired gas provided at specified altitudes with various breathing patterns by a given flow of product gas containing a given concentration of oxygen. The model predicted that the breathing patterns produced by speech or by performing the anti G straining manoeuvre would cause a large decrease in the inspired oxygen concentration from that provided during normal breathing.
The performance of the system described above was investigated using human subjects in a hypobaric chamber at pressure altitudes up to 25,000 feet. The subject exercised on a cycle ergometer and spoke whilst at rest and during exercise. Records of the partial pressures of oxygen and carbon dioxide in the subject's mask were obtained using a respiratory mass spectrometer. The "effective" inspired oxygen concentration was calculated using the alveolar gas equation. Speech produced a rapid and profound reduction of the alveolar Po2 especially at the higher levels of pulmonary ventilation. There was a very satisfactory correlation between the concentration of oxygen in the inspired gas predicted by the model of the system and the "effective" inspired oxygen concentration obtained in the studies using human subjects.
Both the model and the studies using human subjects demonstrate that speech and presumably the performance of the anti G straining manoeuvre can rapidly produce significant hypoxia in a system employing a continuous flow of product gas.
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