The present study was designed to investigate hemodynamics and gas exchange during weaning from mechanical ventilation (assist/control mode) to spontaneous breathing with continuous high flow (chf)-CPAP, and to study the predictive value of these parameters in respect of longterm success or failure of weaning. Hemodynamic and gas exchange parameters were obtained in 10 patients without severe pulmonary and cerebral dysfunction at -240, -60, -30, and -15 min before, and at +15, +30, +45, +60, +120, +180, +240 min after chf-CPAP. During chf-CPAP significant increases in heart rate/min (92 +/- 17 to 103 +/- 20), cardiac index (3.9 +/- 0.7 to 4.4 +/- 1.0 1/min.m2), respiratory rate/min (15 +/- 1 to 28 +/- 7), PaCO2 (36.7 +/- 3.0 to 41.2 +/- 5.9 torr) and oxygen delivery (12.2 +/- 2.7 to 13.9 +/- 2.3 ml/min.kg) were found. Arterial and pulmonary artery pressures rose only briefly within the first hour. All other parameters changed non-significantly. In the 4 patients who required mechanical ventilation 12 to 34 hours after the end of the study we found a significantly more pronounced increase in heart rate than in those who where weaned successfully (114 +/- 19 vs 89 +/- 9). Increases in heart rate, respiratory rate, cardiac index, PaCO2 and oxygen delivery can therefore be expected during weaning from mechanical ventilation to spontaneous breathing with CPAP. A pronounced increase in heart rate may suggest a weaning failure.