Predicting hypoxia in cystic fibrosis patients during exposure to high altitudes☆

Abstract 

Background

For 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.

Objectives

To 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.

Methods

Blood 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.

Results

In 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.

Conclusion

During 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:

–paO_{2 [Alt]}=paO_{2 [ground]}−1.33×Alt [mm Hg], and

–saO_{2 [Alt]}=saO_{2 [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.

Abbreviations: a.s.l., above sea-level, FEV1, forced expiratory volume in the first second [l], FVC, forced vital capacity [l], HAST, hypoxia-altitude simulations tests, paO₂, arterial oxygen partial pressure [mm Hg], p_b, barometric pressure [mm Hg], pCO₂, arterial carbon dioxide partial pressure [mm Hg], pI_O2, inspired oxygen pressure [mm Hg], saO₂, oxygen saturation [%], S.E.M., standard error of the mean

Keywords: Hypobaric hypoxemia, Oxygen saturation, Oxygen partial pressure, Commercial flights, Cystic fibrosis (CF)