Introduction. Mixed venous oxygen saturation (SvO2) is a standard invasive measure used in the management of congestive heart failure patients. The reliability of a long-term SvO2 sensor remains unproven. Methods. Nine patients (NYHA Class I/II, n=2/7) were implanted with a dual chamber pacemaker modified to utilize a right ventricular SvO2 lead (Medtronic Inc., Models 8007/4327A IPG/Lead). Invasive studies compared sensor SvO2 to reference (Optical Swan–Ganz catheter) at 0, 3 and 9 months. Symptom limited tests (Bikemax) with metabolic assessment and arterial oxygen saturation measurements performed 1–7 days, 3.5 and 9.5 months post-implant allowed for cardiac output calculations. Long-term sensor performance was confirmed by submaximal tests, Bikesubm in years 1–3, and Walkin-place every 6 months for the duration of follow-up. Results. Sensor SvO2 readings were stable over time when compared to the Swan–Ganz Catheter. Non-invasive CO measured during Bikemax was in normal ranges for this patient population, 3.7±0.9 l/min at rest and 8.4±2.2 l/min at peak-exercise. Resting SvO2 values from Bikesubm and Walkin-place did not change significantly over time (P>0.1 vs. 1 year) and neither did the change from rest to peak exercise during Bikesubm (P>0.05 vs. 1 year) or Walkin-place (P>0.05 vs. 4 year). Conclusion. While limited in size, this small pilot study suggests that long-term monitoring of SvO2 by implanted devices may be feasible. The clinical value remains to be proven in future studies. [Copyright &y& Elsevier]