An improved system for measuring changes in blood flow, particularly in the context of an automatic arrhythmia treatment device. The system includes a heater, exposable to a bloodstream, a driver providing power to the heater, a temperature sensor, isolated from the blood stream and thermally coupled to the temperature of the heater and a measurement circuit coupled to the temperature sensor, providing signals indicative of blood flow. The system may include an intravascular lead having a lead body wherein the heater is mounted to the lead body so as to dissipate its generated heat into the blood. The heater and temperature sensor may be thermally insulated from the lead body and the heater and the temperature sensor may be on substantially a single isotherm during operation.
