Objective: Identify the protocols used in normobaric hypoxia and the main symptoms in that context. Theoretical Framework: Hypoxia, particularly hypoxic hypoxia, is a critical challenge in aviation, caused by reduced atmospheric pressure at high altitudes. Controlled hypoxia training allows aviators to recognize symptoms, assess the time of useful consciousness, and practice emergency responses. This study synthesizes key parameters in normobaric hypoxia training, addressing gaps in oxygen concentrations, exposure times, and symptom monitoring. Method: Systematic electronic searches were conducted in MEDLINE and Embase databases. Inclusion criteria required original studies involving aircraft crews, descriptions of gas mixtures used for altitude simulation, and details of simulated altitudes during training. Results and Discussion: From 295 identified articles, eight met the inclusion criteria. Sample sizes ranged from 8 to 89 participants, predominantly male. Protocols varied widely, with O₂ concentrations of 6%, 7%, and 8% frequently used to simulate altitudes of 25,919 feet (7,900 m) to 20,341 feet (6,200 m). Considerable heterogeneity was found in protocols for the same simulated altitude, likely due to differences in equipment calibration. Continuous exposure periods of three to five minutes were common, and diverse hypoxia symptoms were reported, emphasizing the importance of symptom recognition for operational safety. Research Implications: This study offers practical insights for standardizing hypoxia training protocols, enhancing safety procedures, and developing guidelines for normobaric environments. Originality/Value: By synthesizing hypoxia training parameters, this research highlights the need for protocol standardization to ensure aviator safety and operational efficiency.

Hypobaric hypoxia is caused by insufficient oxygen in the crew due to low atmospheric pressure. It causes hypoxemia and the symptoms of hypobaric hypoxia progressively develop, making the crew unable to perform executive functions. Aircraft accidents due to the development of hypobaric hypoxia in the crew have caused hundreds of casualties in the modern era of aviation, involving different aircraft and in other countries. To prevent such accidents, the aircrews perform simulated hypobaric chamber exercises periodically, in this way, it is possible to introduce the symptoms of hypobaric hypoxia to them in a safe and controlled atmosphere. Consequently, this allows them to remember hypoxia symptoms during an eventual aviation incident. The interval between hypobaric chamber exercises varies between countries. Although they are not mandatory, it is suggested that an interval of between three and six years between hypoxia exercises is appropriate to maintain flight safety concerning hypoxia symptoms. However, it is important to be aware that there is intra- and inter-personal variability in hypobaric hypoxia symptoms.

The comprehension of the symptoms caused by hypobaric hypoxia in air travelers allows aviation safety actions to be efficiently carried out. In this context, it is also important to clarify the behavior of these symptoms between exposure sessions. This registered systematic review (PROSPERO: CRD42021283758) aims to identify possible changes in the symptoms of hypobaric hypoxia reported by air travelers submitted to repeated exposure in a hypobaric chamber throughout their careers. The following inclusion criteria were adopted: re-exposure to hypobaric hypoxia; and the following exclusion criteria: mountain sickness, decompression sickness, experimental animal models, and medical or dental outcomes. The search was conducted in Scopus, BVS/VHL, Cochrane, Medline, Web of Science, and Science Direct and Google Scholar databases. The selected studies were analyzed for risk of bias using the Newcastle-Ottawa Scale. Of the 430 titles found, five articles met the inclusion and exclusion criteria and were selected for the systematic review, totaling a sample of 1,569 participants. It was found that the participants' hypoxia symptoms tend to repeat themselves in the same way in the other sessions of exposure to hypobaric hypoxia. However, it was identified that such symptoms are influenced by personal factors that can lead to variations in symptoms for the same individual between sessions of exposure to hypobaric hypoxia. Therefore, with aviation safety as a priority, it is prudent to interpret that the symptoms of hypobaric hypoxia are malleable and that they may vary upon re-exposure of the subject to it.