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

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 (paO(2)) and oxygen saturation (saO(2)) 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 paO(2) and saO(2) 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, saO(2) decreased by 0.33% per 100 m vertical ascent, and this rate increased significantly at altitudes >2000 m. Predicted saO(2) differed from in-flight value by <5%, and agreement between in-flight saO(2) decrease rate and chamber data was good. paO(2) 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 paO(2) 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 x Alt[mm Hg], and -saO2[Alt]=saO2[ground] -0.33 x 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.

Respiratory function and blood gas variables in cystic fibrosis patients during reduced environmental pressure.

Patients with cystic fibrosis (CF) suffer from hypoxaemia even under normobaric conditions and the reduction of inspiratory PO2 (O2 partial pressure) during air travel corresponding to an altitude of 1,800-2,450 m might be a problem for these patients. Ten CF patients and 27 healthy control subjects were investigated in a chamber where the ambient pressure was reduced to that found at 2,000 and 3,000 m. The respiratory function was reduced in the CF patients with a vital capacity of 3.1 (0.3) l [vs 4.9 (0.2) l in controls; mean (SEM)] and a forced expiratory 1-s volume of 2.1 (0.3) l [vs 4.3 (0.20 l in controls], unrelated to the reduction in ambient pressure. Mean arterial PO2 decreased from 75 (4) mmHg [85 (1) mmHg in controls, P<0.01] at sea level to 58 (3) mmHg at 580 mmHg and to 46 (1) mmHg [58 (1) mmHg and 49 (2) mmHg in controls, n.s.] at 513 mmHg ambient pressure. These results indicate that during air travel with a cabin pressure that corresponds to an altitude of 2,500 m, the arterial PO2 of CF patients is likely to remain above the accepted critical value of 50 mmHg. However, a further reduction of the pressure to that found at 3,000 m altitude may lead to severe hypoxia in patients with moderate airway obstruction.