An implantable medical device comprises a connector connectable to an implantable oxygen sensor configured to generate a sensor signal representative of oxygen concentration in coronary sinus blood in a subject's heart. An ischemia detector is connected to the connector and configured to detect an ischemic event in the heart if the sensor signal indicates a temporary decrease in oxygen concentration in the coronary sinus blood below a normal level followed by a temporary increase in oxygen concentration in the coronary sinus blood above the normal level.
