Spirometry in the Supine Position Improves the Detection of Diaphragmatic Weakness in Patients With Amyotrophic Lateral Sclerosis

To determine which respiratory function tests best predicted diaphragmatic strength in patients with amyotrophic lateral sclerosis. 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. Pulmonary function laboratory of an academic medical center. 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. Our findings suggest that supine FVC is an excellent and simple test of diaphragmatic weakness.