Recovery from acute hypoxia: A systematic review of cognitive and physiological responses during the 'hypoxia hangover'

Recovery of cognitive and physiological responses following a hypoxic exposure may not be considered in various operational and research settings. Understanding recovery profiles and influential factors can guide post-hypoxia restrictions to reduce the risk of further cognitive and physiological deterioration, and the potential for incidents and accidents. We systematically evaluated the available evidence on recovery of cognitive and basic physiological responses following an acute hypoxic exposure to improve understanding of the performance and safety implications, and to inform post-hypoxia restrictions. This systematic review summarises 30 studies that document the recovery of either a cognitive or physiological index from an acute hypoxic exposure. Titles and abstracts from PubMed (MEDLINE) and Scopus were searched from inception to July 2022, of which 22 full text articles were considered eligible. An additional 8 articles from other sources were identified and also considered eligible. The overall quality of evidence was moderate (average Rosendal score, 58%) and there was a large range of hypoxic exposures. Heart rate, peripheral blood haemoglobin-oxygen saturation and heart rate variability typically normalised within seconds-to-minutes following return to normoxia or hyperoxia. Whereas, cognitive performance, blood pressure, cerebral tissue oxygenation, ventilation and electroencephalogram indices could persist for minutes-to-hours following a hypoxic exposure, and one study suggested regional cerebral tissue oxygenation requires up to 24 hours to recover. Full recovery of most cognitive and physiological indices, however, appear much sooner and typically within similar to 2-4 hours. Based on these findings, there is evidence to support a 'hypoxia hangover' and a need to implement restrictions following acute hypoxic exposures. The severity and duration of these restrictions is unclear but should consider the population, subsequent requirement for safety-critical tasks and hypoxic exposure.