Acute hypoxia in a simulated high altitude airdrop scenario due to oxygen system failure

High Altitude High Opening (HAHO) is a military operational procedure in which parachute jumps are performed at high altitude requiring supplemental oxygen, putting personnel at risk of acute hypoxia in the event of oxygen equipment failure. This study was initiated by the Norwegian Army in order to evaluate potential outcomes during failure of oxygen supply, and to explore physiology during acute severe hypobaric hypoxia. A simulated HAHO without supplemental oxygen was carried out in a hypobaric chamber with decompression to 30 000 ft (9144 m) and then recompression to ground level with a descent rate of 1000 ft/min (305 m/min). Nine subjects were studied. Repeated arterial blood gas samples were drawn throughout the entire hypoxic exposure. Additionally, pulse oximetry, cerebral oximetry and hemodynamic variables were monitored. Desaturation evolved rapidly and the arterial oxygen tensions are amongst the lowest ever reported in volunteers during acute hypoxia. PaO₂ decreased from baseline 18.4 (17.3 - 19.1) kPa, 138.0 (133.5 - 143.3) mmHg, to a minimum value of 3.3 (2.9 - 3.7) kPa, 24.8 (21.6 - 27.8) mmHg, after 180 (60 - 210) sec, [median (range)], N=9. Hyperventilation with ensuing hypocapnia was associated with both increased arterial oxygen saturation and cerebral oximetry values, and potentially improved tolerance to severe hypoxia. One subject had a sharp drop in heart rate and cardiac index and lost consciousness 4 min into the hypoxic exposure. A simulated high altitude airdrop scenario without supplemental oxygen results in extreme hypoxemia and may result in loss of consciousness in some individuals.

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