Abstract
Introduction
The predictors of in-flight hypoxia in interstitial lung disease remain unknown. The hypoxic challenge test (HCT) is widely used to evaluate fitness to fly. We assessed the determinants of a positive HCT in patients with ILD.
Methods
183 patients underwent HCT in 2005-2011 (74 males; mean age 57.6; 73 ex-smokers) of which 126 had idiopathic interstitial pneumonia, 26 hypersensitivity pneumonitis, 23 sarcoid and 8 other ILDs. 164 had lung function tests on the same day, 124 had an echocardiogram within 1 year. The HCT was performed using an inhaled gas mixture containing 15% oxygen. From BTS guidelines, the HCT was positive (in-flight oxygen required) if PaO2 < 6.6 kPa on 15% oxygen.
Results
Median PaO2 on air was 10.3 kPa (range 8.2-14.3), with median percent predicted FVC 68.1% and DLCO 38.3%. On univariate logistic regression, variables associated with a positive HCT were age (p=0.04), PaO2 on air (p<0.0001), FVC%, DLCO% and composite physiologic index (CPI) (p for all<0.00001). Pulmonary hypertension (PH) on echocardiogram was also predictive, although less strongly (p=0.015). On multivariate analysis, variables remaining significantly associated with a positive HCT were PaO2 on air (p<0.0001), lung function markers including CPI (p<0.0001), or in separate models DLCO (p<0.0001), and FVC (p=0.006), while PH on echo was no longer predictive (p=0.13). On ROC analysis, area under the curve was 0.80 for PaO2 on air, 0.75 for DLCO%, 0.72 for CPI, 0.83 for combined CPI and PaO2, and 0.85 for combined DLCO and PaO2.
Conclusion
Our findings highlight the potential of PaO2 on air, DLCO and CPI levels as non-invasive predictors of fitness to fly. Identifying the best combination requires further prospective evaluation.