When Implantable Cardiac Defibrillators (ICDs) Malfunction

Internal defibrillators are commonly confused with pacemakers. However, the two medical devices are placed for different purposes. Pacemakers literally pace the heart at every beat, keeping it in a healthy rhythm. Internal defibrillators, in contrast, only send electrical shocks when a specific arrhythmia is detected. Internal defibrillators are also called implantable cardioverter defibrillators, implantable cardiac defibrillators or ICDs.

ICDs sense the rate and rhythm of the heart. ICDs can detect a particular arrhythmia called ventricular fibrillation or V-fib. Ventricular fibrillation is a heart rhythm that cannot sustain life. Each chamber of the heart normally contracts in a coordinated rhythm to pump blood through the body. In V-fib, the heart is a quivering mass unable to efficiently pump any blood.

When an internal cardiac defibrillator detects a V-fib rhythm, it delivers an electrical stimulus that depolarizes the entire myocardium (heart muscle). This essentially resets the heart in the hopes that the myocardium will return to normal coordinated contractions.

Normal cardiac rhythm is usually restored if the sinoatrial node of the heart is the first area to reach threshold after myocardial repolarization. In this manner, ICDs are effective in preventing ventricular fibrillation, as well as sudden cardiac death, in individuals with heart failure.

However, implantable defibrillators are not fail-proof. ICDs are associated with complications. The most common complication is inappropriate detection of a rhythm and subsequent delivery of an unneeded shock. This ICD malfunction usually resolves with device reprogramming or adjustment of anti-arrhythmic drugs.

The most common cause for inappropriate multiple shocks from an ICD is atrial fibrillation with rapid ventricular rates. The heart rhythm, known as A-fib with RVR, can be interpreted by the ICD as ventricular fibrillation. Atrial fibrillation can often go unnoticed, particularly in older individuals with sedentary lives. However, atrial fibrillation greatly increases the risk of clot formation and stroke such that A-fib patients require anticoagulant therapy with warfarin or aspirin.

An individual who experiences multiple shocks from his or her ICD should go to the hospital immediately. While multiple shocks may be the result of A-fib with RVR, it is also possible that the ICD is accurately detecting V-fib episodes. Multiple appropriate ICD shocks are delivered with an electrical arrhythmic storm. An electrical storm of the heart is defined as repeated episodes of ventricular tachycardia or V-fib occurring within a 24 hour period.

Cardiac electrical storm is a rare occurrence most commonly associated with acute myocardial infarction (MI). However, an arrhythmic storm can also be seen in people with dilated cardiomyopathy. In people with heart failure, electrical storm can be an indicator of progressive myocardial decompensation, suggesting that these patients need to be followed closely.