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 (SpO₂) compared with arterial blood gas (ABG) saturation (SaO₂), 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 SaO₂ and SpO₂, the difference between SaO₂ and SpO₂, the association of baseline characteristics with both the difference between SaO₂ and SpO₂ 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 SaO₂ was 92±4% and SpO₂ was 95±3%. Mean PaCO₂ was 38±6 mmHg, with 135 patients having PaCO₂ <35 mmHg and 62 having PaCO₂ >45 mmHg. Correlation between SaO₂ and SpO₂ was mild to moderate (r=0.39), with SpO₂ on average 3.0% higher than SaO₂. No baseline characteristics were associated with the difference in SaO₂ and SpO₂. 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: SaO₂ and SpO₂ 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.