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 (PaCO2) and the impact of PaCO2 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 PaCO2, the association of baseline characteristics with acid-base category, and the association of PaCO2 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 PaCO2 was 38 +/- 6 mmHg, with 135 patients having PaCO2 <35 mmHg and 62 having PaCO2 >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) PaCO2 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. PaCO2 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 PaCO2 was associated with baseline FVC but was not associated with outcomes.