Resting PaO2 and 6MWT as diagnostic index for nocturnal oxygen desaturation in diffuse parenchymal lung diseases

Introduction: Despite of normal daytime oxygen saturation many patients of diffuse parenchymal lung disease (DPLD) desaturate during night time. Such patients may develop signs of pulmonary arterial hypertension (PAH) which increases the mortality and morbidity in DPLD. Our aim was to document the magnitude of nocturnal oxygen desaturation and find its predictors in DPLD. Methods: It was a cross sectional and observational study. 48 consecutive patients of DPLD were recruited. Idiopathic pulmonary fibrosis (IPF) was diagnosed by ATS/ERS guidelines 2002 and other DPLD by their clinical and radiological presentation. Arterial blood gas analysis, six minute walk test (6MWT) and overnight pulse oximetry were done and variables such as time spent with nocturnal oxygen saturation (SpO2) below 90%, mean SpO2, worst SpO2 and apnea - hypopnea index were recorded. Results: Amongst 48 subjects, 35 (72.9%) patients had IPF, 9 (18.8%) patients had hypersensitivity pneumonitis and 4 (8.3%) patients had stage 4 sarcoidosis. Nocturnal desaturation (>10% of sleep time with SpO2<90%) was observed in 20 (41.7%) subjects. The desaturators were found to have resting daytime PaO2 of less than 61mm of Hg (p=0.0015), end SpO2 after 6MWT ≤83.5% (p=0.0077) and distance walked during 6MWT ≤380.4m (p=0.0051). Based on cut off values of these variables an index called desaturation index was evolved which had 95% sensitivity. Conclusion: Significant nocturnal oxygen desaturation occurs in many patients of DPLD despite of normal resting SpO2. A desaturation index based on PaO2, end SpO2 after 6MWT and distance walked during 6MWT was found to be 95% sensitive in diagnosing nocturnal desaturation.