Abstract
BACKGROUND: Fibrotic interstitial lung disease (ILD) is frequently associated with abnormal oxygenation; however, little is known about the accuracy of oxygen saturation by pulse oximetry (SpO(2)) compared with arterial blood gas (ABG) saturation (SaO(2)), the factors that influence the partial pressure of carbon dioxide (PaCO(2)) and the impact of PaCO(2) on outcomes in patients with fibrotic ILD. STUDY DESIGN AND METHODS: Patients with fibrotic ILD enrolled in a large prospective registry with a room air ABG were included. Prespecified analyses included testing the correlation between SaO(2) and SpO(2), the difference between SaO(2) and SpO(2), the association of baseline characteristics with both the difference between SaO(2) and SpO(2) and the PaCO(2), the association of baseline characteristics with acid-base category, and the association of PaCO(2) and acid-base category with time to death or transplant. RESULTS: A total of 532 patients with fibrotic ILD were included. Mean resting SaO(2) was 92±4% and SpO(2) was 95±3%. Mean PaCO(2) was 38±6 mmHg, with 135 patients having PaCO(2) <35 mmHg and 62 having PaCO(2) >45 mmHg. Correlation between SaO(2) and SpO(2) was mild to moderate (r=0.39), with SpO(2) on average 3.0% higher than SaO(2). No baseline characteristics were associated with the difference in SaO(2) and SpO(2). Variables associated with either elevated or abnormal (elevated or low) PaCO(2) included higher smoking pack-years and lower baseline forced vital capacity (FVC). Lower baseline lung function was associated with an increased risk of chronic respiratory acidosis. PaCO(2) and acid-base status were not associated with time to death or transplant. INTERPRETATION: SaO(2) and SpO(2) are weakly-to-moderately correlated in fibrotic ILD, with limited ability to accurately predict this difference. Abnormal PaCO(2) was associated with baseline FVC but was not associated with outcomes.