Systems, devices, and techniques that enable medical devices to integrate and interoperate with one another are provided. In some examples, a wearable cardiac defibrillator (WCD) advantageously interoperates with an implanted pacemaker to provide a variety of benefits. For instance, in some examples, the WCD oversees execution of an antitachycardia (ATP) protocol by the implanted pacemaker and intervenes as needed. In other examples, the WCD drives an ATP protocol in which internal pacing pulses are provided by the implanted pacemaker under the control of the WCD. In other examples, the WCD monitors the activity of the implanted pacemaker to identify potential maintenance issues affecting the implanted pacemaker. The WCD and the implanted pacemaker may also interoperate to classify and act upon particular arrhythmia conditions.