Mixed venous oxygen saturation (SvO2) is a physiologic parameter reflecting cardiac output (CO) and tissue oxygen utilization. An implantable oxygen sensor incorporated in a right ventricular pacing lead has been developed to assess the feasibility of ambulatory monitoring of SvO2 to predict cardiorespiratory parameters. Eight patients with a mean age of 62 +/- 2 years and sinoatrial disease received an SvO2-driven dual-chamber rate-adaptive pacemaker capable of continuous SvO2 measurement. During graded maximal exercise with measurement of oxygen consumption (Vo2), arterial oxygen saturation (SaO2), and telemetered derived SvO2 data, CO was assessed using the Fick principle. The accuracy of the derived CO was compared with CO measured directly by continuous-wave Doppler assessment of the ascending aortic flow. The maximum changes in SvO2 and SaO2 during exercise were 25 +/- 5 and 3 +/- 1%, respectively. SvO2 was significantly correlated with Vo2 (r = 0.9 +/- 0.1, p < 0.001), work done (r = 0.8 +/- 0.1, p < 0.05), and minute ventilation (r = 0.9 +/- 0.1, p < 0.05). Doppler-derived CO was significantly correlated with CO estimated from SvO2 measured (r = 0.8 +/- 0.1, p < 0.05) and is expressed as 46x derived CO +300. Although continuous SvO2 sensing was originally developed to increase pacing rate during exercise, its use can be extended for monitoring cardiopulmonary performance on an ambulatory basis. This may be useful as a direct assessment of cardiopulmonary status in diseased states and also as an objective means to evaluate cardiac response to medical therapy in patients with heart failure.