The aim of the present study was to investigate the effects of CPAP (continuous positive airway pressure) support on myocardial energetics in patients with CHF (congestive heart failure). CPAP has been shown to decrease left ventricular afterload and to produce favourable short- and long-term haemodynamic and neurohormonal benefits in CHF patients. The mechanisms responsible for these actions are not completely understood. We measured the haemodynamic and myocardial metabolic response to the acute (10 min) application of CPAP in CHF patients. Myocardial VO(2) (O(2) consumption) and VCO(2) (CO(2) production) were measured by simultaneous arterial and coronary sinus blood sampling. The application of CPAP resulted in a significant decrease in left ventricular stroke work (97+/-12 to 83+/-9 g.m; P <0.05) and myocardial VO(2) (0.32+/-0.03 to 0.25+/-0.01 ml of O(2)/beat; P <0.05). Myocardial mechanical efficiency, however, was unchanged. CPAP application decreases myocardial work and VO(2). This effect on myocardial energetics could account for some of the favourable effects of CPAP in CHF patients.