An implantable medical device (100) comprises a connector (110) connectable to an implantable oxygen sensor (40) configured to generate a sensor signal representative of oxygen concentration in coronary sinus blood in a subject's heart (15). An ischemia detector (132) is connected to the connector (110) and configured to detect an ischemic event in the heart (15) 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.