A subcutaneous implantable cardioverter defibrillator (SICD) for delivering a defibrillation shock to a heart of a patient, the SICD comprising: two or more electrodes; a charge storage device for storing a charge that can be delivered to shock the heart via the two or more electrodes; a controller operatively coupled to two or more of the electrodes and to the charge storage device, the controller configured to: monitor cardiac activity of the heart of the patient via cardiac signals received via two or more of the electrodes; detect an occurrence of a cardiac arrhythmia based on the cardiac activity; in response to detecting an occurrence of a cardiac arrhythmia: send an instruction via two or more of the electrodes for reception by a Leadless Cardiac Pacemaker to initiate application of ATP therapy; after sending the instruction, determine a type of the detected cardiac arrhythmia from two or more types of cardiac arrhythmias; if the determined type of cardiac arrhythmia is of a first set of one or more types of cardiac arrhythmias, wait to initiate charging of the charge storage device to see if application of ATP therapy terminates the detected cardiac arrhythmia; and if the determined type of cardiac arrhythmia is not of the first set of one or more types of cardiac arrhythmias, initiate charging of the charge storage device without waiting to see if application of ATP therapy terminates the detected cardiac arrhythmia. two or more electrodes; a charge storage device for storing a charge that can be delivered to shock the heart via the two or more electrodes; a controller operatively coupled to two or more of the electrodes and to the charge storage device, the controller configured to: monitor cardiac activity of the heart of the patient via cardiac signals received via two or more of the electrodes; detect an occurrence of a cardiac arrhythmia based on the cardiac activity; in response to detecting an occurrence of a cardiac arrhythmia: send an instruction via two or more of the electrodes for reception by a Leadless Cardiac Pacemaker to initiate application of ATP therapy; after sending the instruction, determine a type of the detected cardiac arrhythmia from two or more types of cardiac arrhythmias; if the determined type of cardiac arrhythmia is of a first set of one or more types of cardiac arrhythmias, wait to initiate charging of the charge storage device to see if application of ATP therapy terminates the detected cardiac arrhythmia; and if the determined type of cardiac arrhythmia is not of the first set of one or more types of cardiac arrhythmias, initiate charging of the charge storage device without waiting to see if application of ATP therapy terminates the detected cardiac arrhythmia. monitor cardiac activity of the heart of the patient via cardiac signals received via two or more of the electrodes; detect an occurrence of a cardiac arrhythmia based on the cardiac activity; in response to detecting an occurrence of a cardiac arrhythmia: send an instruction via two or more of the electrodes for reception by a Leadless Cardiac Pacemaker to initiate application of ATP therapy; after sending the instruction, determine a type of the detected cardiac arrhythmia from two or more types of cardiac arrhythmias; if the determined type of cardiac arrhythmia is of a first set of one or more types of cardiac arrhythmias, wait to initiate charging of the charge storage device to see if application of ATP therapy terminates the detected cardiac arrhythmia; and if the determined type of cardiac arrhythmia is not of the first set of one or more types of cardiac arrhythmias, initiate charging of the charge storage device without waiting to see if application of ATP therapy terminates the detected cardiac arrhythmia. send an instruction via two or more of the electrodes for reception by a Leadless Cardiac Pacemaker to initiate application of ATP therapy; after sending the instruction, determine a type of the detected cardiac arrhythmia from two or more types of cardiac arrhythmias; if the determined type of cardiac arrhythmia is of a first set of one or more types of cardiac arrhythmias, wait to initiate charging of the charge storage device to see if application of ATP therapy terminates the detected cardiac arrhythmia; and if the determined type of cardiac arrhythmia is not of the first set of one or more types of cardiac arrhythmias, initiate charging of the charge storage device without waiting to see if application of ATP therapy terminates the detected cardiac arrhythmia.