Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. The most common condition for which pacemakers have been used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate and/or artificially restoring AV conduction. Pacing therapy may also be used in treatment of cardiac conduction disorders in order to improve the coordination of cardiac contractions, termed cardiac resynchronization therapy. Other cardiac rhythm management devices are designed to detect atrial and/or ventricular tachyarrhythmias and deliver electrical stimulation in order to terminate the tachyarrhythmia in the form of a cardioversion/defibrillation shock or anti-tachycardia pacing. Certain combination devices may incorporate all of the above functionalities. Any device with a pacing functionality will be referred to herein simply as a pacemaker regardless of other functions it may be capable of performing.
Cardiac rhythm management devices such as described above monitor the electrical activity of heart via one or more sensing channels so that pacing pulses or defibrillation shocks can be delivered appropriately. Such sensing channels include implanted leads which have electrodes disposed internally near the heart, which leads may also be used for delivering pacing pulses or defibrillation shocks. The signals generated from the sensing channels are intra-cardiac electrograms and reflect the time course of depolarization and repolarization as the heart beats, similar to a surface electrocardiogram (ECG). A device may also employ other types of sensing modalities such as an accelerometer and/or a minute ventilation sensor for measuring the patient's exertion level. In rate-adaptive pacing modes, the pacing rate is adjusted in accordance with a measured exertion level.
Various means are commonly used to monitor the condition of the sensing and pacing systems of an implantable device after implantation. Clinicians traditionally utilize the surface ECG in a follow-up setting to verify the functionality of an implantable cardiac device. The surface ECG offers a far-field view of cardiac electrical activity, producing larger and higher fidelity signals than available from most device electrograms, which aids in verifying capture of the heart by pacing pulses. Additionally, the surface ECG provides the only independent view of the implanted device behavior. This can be critical when reconciling confusing device behavior or diagnosing a malfunctioning device or broken lead. Because of this, many clinicians insist on a surface ECG to confirm device operation.