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Background: There is no gold standard for the differential diagnosis of acute dyspnea despite the usefulness of N-terminal pro-B-type natriuretic peptide (NT-proBNP) and lung ultrasound. No study has evaluated the contribution of bioelectrical impedance vector analysis (BIVA) in discriminating between cardiac and noncardiac dyspnea. We sought to determine whether a relationship exists between ultrasound detection of lung congestion, NT-proBNP, and BIVA in patients with acute dyspnea., Methods and Results: Eligible patients were between 50 and 95 years, with an estimated glomerular filtration rate of ≥30 mL min(-1) 1.73 m(-2), who presented to an emergency department with dyspnea. Dyspnea was classified by reviewers blinded to BIVA as cardiac or noncardiac based on physical examination, electrocardiogram, chest X-ray, NT-proBNP, and B-lines of lung congestion on ultrasound. Overall, 315 patients were enrolled (median age 77 years, 48% male). An adjudicated diagnosis of cardiac dyspnea was established in 169 (54%). Using BIVA, vector positions below -1 SD of the Z-score of reactance were associated with peripheral congestion (χ(2) = 115; P < .001). BIVA measures were reasonably accurate in discriminating cardiac and noncardiac dyspnea (69% sensitivity, 79% specificity, 80% area under the receiver operating characteristic curve)., Conclusions: In patients presenting with acute dyspnea, the combination of BIVA and lung ultrasound may provide a rapid noninvasive method to determine the cause of dyspnea., (Copyright © 2012 Elsevier Inc. All rights reserved.)