Combined altitude depleted oxygen vis-à-vis hypobaric hypoxia: Efficacy in hypoxia indoctrination

Abstract

Introduction: The Combined Altitude and Depleted Oxygen (CADO), as an alternate method for hypoxia indoctrination, has the dual advantages of exposing to an altitude less than the threshold for decompression sickness, a known risk in training using Hypobaric Hypoxia (HH) while accounting for the pressure changes due to altitude, a limitation of using normobaric hypoxia. Objectives: This study aimed to evaluate the efficacy of CADO in hypoxia indoctrination by comparing it with the time-tested gold standard method of HH. Material and Methods: Forty subjects were exposed to both CADO and HH, simulating 25,000 ft for a maximum period of 5 min. CADO was achieved by combining exposure to an altitude of 10,000 ft in the hypobaric chamber and breathing a hypoxic gas mixture of 10.3% oxygen and 89.7% nitrogen. Physiological parameters (oxygen saturation, heart rate, and respiratory rate) and psychomotor performance (dual task test component of pSuMEDhA) were compared between the two exposures. The incidence and severity of subjective symptoms were also compared at the end of exposures. Results: No significant difference was observed in the physiological parameters and psychomotor performance during the two exposures. Out of the 24 common symptoms of hypoxia assessed, there was a higher incidence of 20 symptoms in subjects exposed to HH compared to CADO. The severity of 15 symptoms was also found to be significantly greater (P < 0.05) in subjects exposed to HH. Conclusion: The similarity of physiological changes in CADO and HH shows the potential application of CADO as a tool for hypoxia demonstration. However, in view of decreased incidence and severity of subjective symptoms in CADO compared to HH, CADO cannot be considered equivalent to the gold standard (HH) for hypoxia indoctrination for high-risk individuals, namely, aircrew and combat free fall personnel. CADO as a modality can be used as a tool for hypoxia demonstration for persons not involved in flying duties and for high-altitude research.