Characteristics of pulse oximetry and arterial blood gas in patients with fibrotic interstitial lung disease

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 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.