Your doctor will monitor the battery as part of your regular device checkup. Most device batteries last three to six years, although it depends on the device and how often it paces your heart or has to correct rapid heart rhythms with rapid pacing or shocks.
When the battery starts to run low, your doctor will arrange for a replacement. Implantable devices are sealed shut, so the batteries are not replaceable. Instead, your doctor will implant a new device. Typically, this procedure does not last as long as your original device implant because the doctor will simply plug the new ICD or pacemaker into your already existing heart leads.