Decompression Sickness Risk at 6553 m Breathing Two Gas Mixtures

Introduction: The risk of severe decompression sickness (DCS) increases rapidly above 6248 m (20,500 ft) and is greater when breathing higher proportions of inert gas. Contemporary aircrew may be exposed to higher cabin altitudes while breathing molecular sieve oxygen concentrator (MSOC) product gas containing variable concentrations of oxygen, nitrogen, and argon. This study assessed the risk of DCS at 6553 m (21,500 ft) breathing two simulated MSOC product gas mixtures. Methods: In a hypobaric chamber, 10 subjects each undertook 2 4-h exposures at 6553 m breathing either 75% O 2 :21% N 2 :4% Ar or 56% O 2 :42% N 2 :2% Ar. Subjects undertook regular activities simulating inflight movements of fast jet aircrew. Venous gas emboli (VGE) "bubble" load was graded every 15 min using 2D and Doppler echocardiography by experienced operators blinded to breathing gas composition. Results: DCS occurred in five exposures (25%), the earliest after less than 90 min at altitude. All were minor, single-site, uncomplicated limb bends that resolved with recompression. VGE occurred in 85% of exposures with some early-onset, heavy loads. Survival (Probit) analysis indicated that breathing 56% oxygen significantly decreased VGE latency relative to breathing 75% oxygen (relative potency 3.05). Conclusions: From 20 experimental exposures, the risk of DCS at 6553 m is estimated at 5% by 90 min and 20% at 3 h. Exploiting the negative predictive value of VGE latency as a surrogate measure of protection from DCS, at high cabin altitudes better MSOC performance (higher product gas oxygen concentrations) will protect more aircrew for longer.