Introduction: Mixed venous oxygen saturation (SvO 2 ) is a standard invasive measure used in the management of congestive heart failure patients. The reliability of a long-term SvO 2 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 SvO 2 lead (Medtronic Inc., Models 8007/4327A IPG/Lead). Invasive studies compared sensor SvO 2 to reference (Optical Swan -Ganz catheter) at 0, 3 and 9 months. Symptom limited tests (Bike max ) 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, Bike subm in years 1 -3, and Walk in-place every 6 months for the duration of follow-up. Results: Sensor SvO 2 readings were stable over time when compared to the Swan -Ganz Catheter. Non-invasive CO measured during Bike max was in normal ranges for this patient population, 3.7F0.9 l/min at rest and 8.4F2.2 l/min at peak-exercise. Resting SvO 2 values from Bike subm and Walk in-place did not change significantly over time ( P > 0.1 vs. 1 year) and neither did the change from rest to peak exercise during Bike subm ( P>0.05 vs. 1 year) or Walk in-place ( P>0.05 vs. 4 year). Conclusion: While limited in size, this small pilot study suggests that long-term monitoring of SvO 2 by implanted devices may be feasible. The clinical value remains to be proven in future studies.