Spirometry in the supine position improves the detection of diaphragmatic weakness in patients with amyotrophic lateral sclerosis

Study objectives: To determine which respiratory function tests best predicted diaphragmatic strength in patients with amyotrophic lateral sclerosis. Patients and methods: Patients referred for pulmonary evaluation were included (n = 25) if they underwent measurement of transdiaphragmatic pressure (Pdi) and one or more of the following on the same day: upright FVC, supine FVC, upright FEV₁, supine FEV₁, maximal inspiratory pressure (MIP), maximal expiratory pressure (MEP), and PaCO₂. Abdominal paradox and use of accessory muscles were also assessed. Bivariate analyses were performed using simple linear regression. Sensitivity and specificity of the potential predictors to detect an abnormal Pdi (< 70 cm H₂O) were calculated. Setting: Pulmonary function laboratory of an academic medical center. Results: Upright FVC, FEV₁, and MEP were all significantly correlated with Pdi, while MIP and PaCO₂ were not. Supine FVC was the most highly correlated predictor of Pdi (R² = 0.76). A cutoff of supine FVC that was < 75% predicted was 100% sensitive and specific for predicting an abnormally low Pdi. Accessory muscle use and abdominal paradox were both significantly associated with Pdi, and the presence of accessory muscle use had a sensitivity of 84% and a specificity of 100% for detecting a low Pdi. Conclusions: Our findings suggest that supine FVC is an excellent and simple test of diaphragmatic weakness.