Nasal continuous positive airway pressure (NCPAP) during sleep may be a useful adjunct to medical therapy in patients with stable severe congestive heart failure (CHF), particularly when there is a coexisting respiratory sleep disorder. However, the direct haemodynamic effects of NCPAP in patients with severe stable CHF have not yet been adequately assessed. Right heart catheter studies were performed in seven awake males (aged 51-75 yrs) with stable CHF, before, during and after the application of 5 cmH2O NCPAP over 3 h. All patients had left ventricular ejection fractions < or = 30% and baseline pulmonary capillary wedge pressures > 12 mmHg, and six patients were in atrial fibrillation. Cardiac index fell from baseline in all patients whilst on NCPAP, with the greatest fall at 2 h (from 3.3 +/- 0.3 (mean +/- SEM) at baseline to 2.8 +/- 0.2 l.min-1.m-2) and rose back to baseline after NCPAP withdrawal. Systemic vascular resistance (SVR) increased during NCPAP application (1,268 +/- 108 to 1,560 +/- 82 dyn.s-1.cm5), with baseline SVR showing a significant negative correlation vs percentage fall in cardiac index (CI) at 2 h on multiple linear regression analysis (r2 = 0.8). These data indicate that domiciliary nocturnal NCPAP should not be prescribed as part of the therapy in severe CHF without first determining the individual patient's cardiac response to such therapy.