Implantable medical devices (IMDs), including implantable pacemakers and implantable cardioverter-defibrillators (ICDs), record cardiac electrogram (EGM) signals for sensing cardiac events, e.g., P-waves and R-waves. IMDs detect episodes of bradycardia, tachycardia and/or fibrillation from the sensed cardiac events, and respond to the episodes as needed with pacing therapy or high-voltage anti-tachyarrhythmia shocks, e.g., cardioversion or defibrillation shocks. Types of pacing therapy include bradycardia pacing, cardiac resynchronization therapy (CRT), anti-tachycardia pacing (ATP), which may attempted prior to resorting to an anti-tachyarrhythmia shock to terminate a tachyarrhythmia, and post-shock pacing, which may be delivered to help the heart recover from successful termination of a tachyarrhythmia by an anti-tachyarrhythmia shock.
Some IMDs include a can or housing that is implanted subcutaneously or submuscularly, and coupled to one or more intracardiac leads. Such IMDs may be capable of providing cardiac pacing and/or anti-tachyarrhythmia shock therapies via the one or more leads. However, other IMD configurations that avoid the use of intracardiac leads have been proposed. For example, an intracardiac pacing device (IPD) may be fully implantable within the heart, i.e., may include a housing and electrodes configured to be implanted in the heart. The IPD may be configured to deliver one or more types of pacing.
As another example, an extracardiovascular ICD system may include a can or housing that is implanted at a subcutaneous or submuscular location, and at least one lead implanted extracardiovascularly, e.g., subcutaneously or substernally. Extracardiovascular ICD systems deliver anti-tachyarrhythmia shocks, and may also be configured to deliver pacing pulses, using one or more electrodes on the lead and/or housing. However, the greater distance between the electrodes and the cardiac tissue, e.g., relative to the electrodes of an intracardiac lead or IPD, may necessitate greater pacing pulse magnitudes to capture the heart. Consequently, co-implantation of an extracardiovascular ICD system and IPD to provide shock and pacing therapies, respectively, has been proposed.