The ventilatory and hemodynamic effects of continuous positive airway pressure (CPAP) delivered via a face mask (at 0, 5, and 10 cm H2O, and after a return to 0 cm H2O) were studied in nine patients with acute left heart failure (pulmonary artery occlusion pressure [PAOP] > or = 18 mm Hg, and cardiac index [CI] < or = 2.8 L/min/m2). CPAP at 10 cm H2O induced an improvement in lung compliance (60 +/- 10 ml/cm H2O to 87 +/- 20 ml/cm H2O, p < 0.05) and in lung and airway resistance (5.7 +/- 1.0 cm H2O/L/s to 3.4 +/- 1.0 cm H2O/L/s, p < 0.05), a reduction in work of breathing (18 +/- 3 J/min to 12 +/- 2 J/min, p < 0.05), and in the pressure-time index of the respiratory muscles (279 +/- 22 cm H2O/s/min to 174 +/- 25 cm H2O/s/min, p < 0.05), without significant changes in breathing pattern. Despite a significant reduction in the negative swings in intrathoracic pressure (15.2 +/- 1.9 cm H2O to 10.8 +/- 1.8 cm H2O, p < 0.001), no significant change was observed in CI or stroke volume during CPAP. However, mean transmural filling pressures decreased significantly with CPAP, suggesting a better cardiac performance. Neither the level of stroke volume nor of PAOP, was predictive of changes in CI or in stroke volume. In patients with respiratory insufficiency caused by congestive heart failure (CHF), CPAP reduces respiratory muscle effort without altering cardiac output. The slight decrease in mean transmural left and right atrial pressures suggests an improvement in cardiac performance.