In general, the invention is directed to techniques for using an external defibrillator to detect a presence of an implantable medical device (IMD) implanted within a patient, and providing therapy to the patient through communication between the external defibrillator and the IMD. An external defibrillator provides prompts to a user of the external defibrillator to determine the presence of an IMD implanted within the patient. For example, the external defibrillator may prompt the user to visually inspect the patient's chest for signs that an IMD was implanted, such as a scar or raised portion of skin near the patient's clavicles. As another example, the external defibrillator may prompt the user to place a detection device on the patient's chest. The detection device may be coupled to the external defibrillator, and may employ a magnet to initiate telemetry by the IMD to detect the presence of the IMD.