Abstract
BackgroundFibrotic interstitial lung disease (ILD) is frequently associated with abnormal oxygenation; however, little is known about the accuracy of oxygen saturation by pulse oximetry (SpO2) compared with arterial blood gas (ABG) saturation (SaO2), the factors that influence the partial pressure of carbon dioxide (PaCO2) and the impact of PaCO2on outcomes in patients with fibrotic ILD.Study design and methodsPatients with fibrotic ILD enrolled in a large prospective registry with a room air ABG were included. Prespecified analyses included testing the correlation between SaO2and SpO2, the difference between SaO2and SpO2, the association of baseline characteristics with both the difference between SaO2and SpO2and the PaCO2, the association of baseline characteristics with acid-base category, and the association of PaCO2and acid-base category with time to death or transplant.ResultsA total of 532 patients with fibrotic ILD were included. Mean resting SaO2was 92±4% and SpO2was 95±3%. Mean PaCO2was 38±6 mmHg, with 135 patients having PaCO2<35 mmHg and 62 having PaCO2>45 mmHg. Correlation between SaO2and SpO2was mild to moderate (r=0.39), with SpO2on average 3.0% higher than SaO2. No baseline characteristics were associated with the difference in SaO2and SpO2. Variables associated with either elevated or abnormal (elevated or low) PaCO2included 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. PaCO2and acid-base status were not associated with time to death or transplant.InterpretationSaO2and SpO2are weakly-to-moderately correlated in fibrotic ILD, with limited ability to accurately predict this difference. Abnormal PaCO2was associated with baseline FVC but was not associated with outcomes.