Predicting hypoxia in cystic fibrosis patients during exposure to high altitudes

BackgroundFor patients with cystic fibrosis (CF)-related partial respiratory insufficiency and reduced arterial oxygen tension at ground level, the mild hypobaric environment on commercial jet aircraft poses the risk of severe hypoxemia. Thus, physicians should be able to estimate the extent of in-flight hypoxia.ObjectivesTo derive tools for estimating the expected drop in arterial oxygen partial pressure (paO2) and oxygen saturation (saO2) in young adult CF patients with mild to moderate airway obstruction during exposure to the hypobaric conditions aboard commercial aircraft and to test the predictive power of a hypobaric chamber simulation.MethodsBlood gases of 12 CF patients were measured at ground level, at two altitudes in a hypobaric chamber (2000 and 3000 m) and during two 3.5-h flights at cabin altitudes of 1855 m and 1700 m. The altitude dependence of paO2 and saO2 in the chamber and during the flights was calculated and results were used to derive estimation equations for in-flight values.ResultsIn the chamber, saO2 decreased by 0.33% per 100 m vertical ascent, and this rate increased significantly at altitudes > 2000 m. Predicted saO2 differed from in-flight value by < 5%, and agreement between in-flight saO2 decrease rate and chamber data was good. paO2 decreased at a rate of 0.99 mm Hg/100 m in the chamber and by 1.33 mm Hg/100 m during flights. None of the subjects showed any clinical symptoms during the flights and the chamber simulation.ConclusionDuring our worst-case scenario, i.e. the hypobaric chamber simulation at 3000 m, 90% of patients tolerated paO2 values below the commonly recommended threshold of 50 mm Hg, probably due to adaptation to chronic hypoxemia and lung function impairment. We propose the following equations for an estimation of the expected extent of in-flight hypoxemia in CF patients with mild to moderate airway obstruction and a flight duration of up to 3.5 h:–paO2 [Alt] = paO2 [ground] − 1.33 × Alt [mm Hg], and–saO2 [Alt] = saO2 [ground] − 0.33 × Alt [%], with Alt = altitude in 100 m.In addition to the overall clinical situation of a patient, these equations will serve as a practical supportive tool for the assessment of the fitness to fly in the primary care setting.