In the acute respiratory distress syndrome (ARDS), lower lobes appear essentially non-aerated in contrast to upper lobes whose aeration can be preserved in some patients. The aim of this study was to assess the mechanical compression exerted by the heart on lower lobes in patients with ARDS. Fourteen healthy volunteers and 38 patients with ARDS free of left ventricular failure were studied. Cardiorespiratory parameters were recorded and the cardiac dimensions, the pressure exerted by the heart on subjacent lower lobes, and the gas tissue ratio of lower lobes in the supine position were measured using computer tomography and Lungview, a specifically designed software. In patients with ARDS, the heart was larger and heavier than in healthy volunteers. The enlargement of the heart was mainly related to a left cardiac protrusion and the pressure exerted by the left heart on the lower lobes was higher in patients with ARDS than in healthy volunteers (8 +/- 3 g. cm(-)(2) versus 6 +/- 1 g. cm(-)(2), p < 0.01). As a consequence, the faction of gas represented 62% of the left lower lobes in healthy volunteers and 12% only in patients with ARDS. The present study demonstrates that apart from the already known anteroposterior and cephalocaudal gradients of pressure depending on the lung weight and abdominal pressure, the heart plays an important role in the dramatic loss of aeration characterizing lower lobes of patients with ARDS lying in the supine position.