Thermal regulation of emergency oxygen supplies in commercial space vehicles

INTRODUCTION: While NASA requires that commercial spaceflight vehicles provide onboard emergency oxygen supplies for crew, there are currently no requirements in place regarding thermal constraints of delivered gas. The question has been raised whether or not onboard emergency oxygen supplies must be warmed prior to administration to the crew, as inclusion of warming capabilities will increase the complexity and mass of life support systems in the vehicle. We sought to identify the risk of various inhaled oxygen temperatures and resultant pulmonary inflammatory response in potentially injured crewmembers. METHODS: A systematic review of published literature was conducted concerning thermal regulation of inhaled gases, reactive airway response, and inflammatory reactions. In particular, we sought literature that correlated inhaled gas temperature to airway response to identify a temperature threshold that would avoid deleterious sequelae. RESULTS: Cold air inhalation can induce acute bronchoconstriction, increased respiratory rate, and associated dyspnea and hypoxia. Physiological response to cold air varies between healthy lungs and injured tissues, and increased inflammation is associated with increasing airway reactivity. Most studies suggest that inhaled gas temperatures below 10°C may induce deleterious physiological sequelae. DISCUSSION: Best practices would include maintenance of inhaled gas temperatures to > 10°C to avoid poor physiological response, preferably as close to physiological norms as possible. Given that inhaled gas temperature may be altered by transit through an oxygen delivery system, measurement of actual delivered gas temperature should occur at the point of crewmember inhalation.