A lead system is provided including a coronary sinus lead extending from a proximal end to a distal end, where the lead includes at least one thermal sensor, or optionally, a thermistor. The thermal sensor is positionable within a coronary sinus of a heart when the coronary sinus lead is implanted in the heart to measure the temperature of a myocardium. The method includes coupling at least one thermal sensor with the implantable lead, including coupling a first thermal sensor and a second thermal sensor with the lead, placing the implantable lead and at least one thermal sensor within a coronary sinus of a heart and positioning the first thermal sensor within the coronary sinus, and positioning the second thermal sensor within a right atrium of a heart. The method further includes coupling the implantable lead with the implantable electrical stimulation source, and measuring a myocardium temperature.