Spirometry in the Supine Position Improves the Detection of Diaphragmatic Weakness in Patients With Amyotrophic Lateral Sclerosis* Noah Lechtzin, MD, MHS; Charles M. Wiener, MD, FCCP; David M. Shade, BA, JD; Lora Clawson, MSN, CRNP; and Gregory B. Diette, MD, MHS, FCCP 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 FEV 1 , supine FEV 1 , maximal inspiratory pressure (MIP), maximal expiratory pressure (MEP), and PaCO 2 . 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 H 2 O) were calculated. Setting: Pulmonary function laboratory of an academic medical center. Results: Upright FVC, FEV 1 , and MEP were all significantly correlated with Pdi, while MIP and PaCO 2 were not. Supine FVC was the most highly correlated predictor of Pdi (R 2  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 diaphrag- matic weakness. (CHEST 2002; 121:436 – 442) Key words: amyotrophic lateral sclerosis; diaphragm strength; pulmonary function tests; respiratory muscles; spirometry; transdiaphragmatic pressure Abbreviations: ALS = amyotrophic lateral sclerosis; BMI = body mass index; FVC = change in FVC from upright to supine; MEP = maximal expiratory pressure; MIP = maximal inspiratory pressure; Pdi = transdiaphragmatic pressure; Pdi-sniff = transdiaphragmatic pressure during maximal sniffing; Pes = esophageal pressure A myotrophic lateral sclerosis (ALS) is a fatal neu- rodegenerative disease. Respiratory muscles at- rophy over the course of the disease, leading to respiratory failure. 1–3 Though ALS is uniformly fatal, the rate of progression varies widely between pa- tients. 1 One clinically relevant prognostic factor in patients with ALS is the timing of onset of respira- tory muscle weakness. Patients with respiratory mus- cle weakness as the initial manifestation of ALS have very poor prognoses, with median survival from diagnosis of only 2 months. 1 It is crucial, therefore, to accurately detect respiratory muscle involvement in order to estimate prognosis, provide patient coun- seling, and make treatment decisions. The diaphragm is the primary inspiratory muscle, and assessment of transdiaphragmatic pressure (Pdi) is the “gold standard” measure of diaphragmatic strength. 4 Esophageal pressure (Pes) is a measure of global inspiratory strength that closely parallels Pdi. 5 Pes has been shown to closely correlate with survival in patients with ALS. 6 Unfortunately, measurement *From the Department of Medicine (Drs. Lechtzin, Wiener, and Shade), Division of Pulmonary and Critical Care Medicine, and Department of Neurology (Ms. Clawson), Johns Hopkins Uni- versity School of Medicine, and the Department of Epidemiology (Dr. Diette), Johns Hopkins School of Hygiene and Public Health, Baltimore, MD. This research was supported by National Heart, Lung, and Blood Institute grant 2T32HL07534, and an Amyotrophic Lateral Scle- rosis Association Clinical Management Research Grant. Manuscript received February 8, 2001; revision accepted July 25, 2001. Correspondence to: Noah Lechtzin, MD, MHS, Division of Pulmonary and Critical Care Medicine, Johns Hopkins Hospital, Blalock 910, 600 N Wolfe St, Baltimore, MD 21287; e-mail: nlechtz@welch.jhu.edu 436 Clinical Investigations Downloaded From: http://publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/21973/ on 06/17/2017