Mixed venous oxygen saturation (SVO2) is an haemodynamic parameter of cardiovascular function. The object of this study was to measure SVO2 during percutaneous transluminal coronary angioplasty (PTCA) with prolonged balloon inflation in order to evaluate haemodynamic tolerance more precisely in two groups of patients. Twenty-six patients undergoing PTCA were divided into two groups: Group I, N = 15, with single vessel disease and good left ventricular function (EF: 61.63 +/- 10.1%); Group II, N = 11, with triple vessel disease and poor left ventricular function (EF: 48.05 +/- 14%; p < 0.05). Continuous monitoring of SVO2 with an oximetrix fiber optic catheter was performed in all patients during PTCA. Irrespective of the duration of balloon inflation, a significant correlation was observed between the changes in cardiac output and VSO2. Given that arterial oxygen pressure, global oxygen consumption and haemoglobin concentrations remained constant during balloon inflation, SVO2 was an indicator of cardiac output.