BACKGROUND: This study investigated the respiratory function and mechanics of patients with orthopnea caused by acute left ventricular failure (ALVF).METHODS: The study comprised 40 patients with ALVF and 15 control subjects. All patients underwent lung function tests and impulse oscillometry in both sitting and supine positions. In a subgroup of 22 patients, isosorbide dinitrate was administered and impulse oscillometry was performed 15 minutes later in the supine position.RESULTS: No patient reported dyspnea while seated, and the orthopnea score was 2.9 ± 1.4. Left ventricular ejection fraction was 43% ± 10%. Patients demonstrated restrictive spirometric pattern in the sitting position, whereas functional residual capacity was comparable to that of the control group. In the supine position, all pulmonary volumes decreased, except inspiratory capacity which increased. Respiratory reactance (Xrs5) was higher in patients in both sitting (421.8 ± 630.6%pred vs 147.2 ± 72.8%pred, P = .01) and supine (699.8 ± 699.9%pred vs 251.2 ± 151.6%pred, P ≤ .001) positions. Respiratory resistance (Rrs5) (10.6% ± 17.8% mean decrease) and Xrs5 (17.2% ± 39.4% mean decrease) improved after nitrates administration. Orthopnea was better correlated with Xrs5%pred in the supine position (r = .42, P = .007). Ejection fraction was positively correlated with inspiratory capacity %pred (r = .42, P = .007) in the sitting position.CONCLUSION: Patients with ALVF demonstrated increased respiratory reactance that correlated with orthopnea severity and improved after nitrates administration.