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To investigate the relationship between alterations in lung mechanics and acute pulmonary vascular congestion, repeated measurements of the respiratory system impedance (Zrs) were performed in 11 patients with and in seven without acute left ventricular failure. Indexes of Zrs were obtained by calculating the average and slope of the resistance and reactance in low (10 to 20 Hz) and high (20 to 50 Hz) frequency intervals. Zrs indexes in patients with ventricular failure differ significantly from those in patients without failure. Pulmonary vascular congestion is regularly associated with an abnormal frequency dependence of resistance at low frequencies and with an increased resonant frequency. Discriminant analysis of Zrs indexes allows 92% correct classification of pulmonary capillary wedge pressures lower than and those equal to or higher than 18 mm Hg. Zrs differences between patients with and without left ventricular failure are consistent with the presence of a small airways obstruction even in patients with mild left ventricular failure. Furthermore, use of Zrs indexes permits moderate and severe pulmonary vascular congestion to be distinguished from one another and this is probably due to a significant narrowing of the large airways during severe left ventricular failure.