In some embodiments, an implantable medical device (IMD) system may include one or more of the following elements: (a) an oxygen sensor for measuring oxygen extraction from blood flowing through a coronary sinus of a patient's heart, (b) an oxygen signal generated by the oxygen sensor, (c) an IMD coupled to the oxygen sensor, wherein the IMD is configured to output pacing pulses as a function of the oxygen signal, and (d) an atrial and a ventricular pacing lead coupled to the IMD to deliver the pacing pulses to the patient's heart, wherein the IMD generates the pacing pulses as a function of the oxygen signal, wherein the pacing pulses are adjusted by the IMD as a function of the oxygen signal, wherein the IMD is configured to adjust the pacing pulses to increase oxygen in the blood flow through the coronary sinus.