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Implantable cardioverter-defibrillator

An implantable cardioverter-defibrillator (ICD) is a small battery-powered electrical impulse generator which is implanted in patients who are at risk of sudden cardiac death due to ventricular fibrillation. The device is programmed to detect cardiac arrhythmia and correct it by delivering a jolt of electricity. In current variants, the ability to revert ventricular fibrillation has been extended to include both atrial and ventricular arrhythmias as well as the ability to perform biventricular pacing in patients with congestive heart failure or bradycardia.

The process of implantation of an ICD is similar to implantation of a pacemaker. Similar to pacemakers, these devices typically include electrode wire/s which pass through a vein to the right chambers of the heart, usually being lodged in the apex of the right ventricle. The difference is that pacemakers are more often temporary and generally designed to consistently correct bradycardia, while AICDs are often permanent safeguards against sudden abnormalities.

ICDs constantly monitor the rate and rhythm of the heart and can deliver therapies, by way of an electrical shock, when the electrical manifestations of the heart activity exceeds the preset number. More modern devices can distinguish between ventricular fibrillation and ventricular tachycardia (VT), and may try to pace the heart faster than its intrinsic rate in the case of VT, to try to break the tachycardia before it progresses to ventricular fibrillation. This is known as fast-pacing, overdrive pacing, or anti-tachycardia pacing (ATP). ATP is only effective if the underlying rhythm is ventricular tachycardia, and is never effective if the rhythm is ventricular fibrillation.

Many modern ICDs use a combination of various methods to determine if a fast rhythm is normal, ventricular tachycardia, or ventricular fibrillation.

Rate discrimination evaluates the rate of the lower chambers of the heart (the ventricles) and compares it to the rate in the upper chambers of the heart (the atria). If the rate in the atria is faster than or equal to the rate in the ventricles, then the rhythm is most likely not ventricular in origin, and is usually more benign. If this is the case, the ICD does not provide any therapy.

Rhythm discrimination will see how regular a ventricular tachycardia is. Generally, ventricular tachycardia is regular. If the rhythm is irregular, it is usually due to conduction of an irregular rhythm that originates in the atria, such as atrial fibrillation.

Morphology discrimination checks the morphology of every ventricular beat and compares it to what the ICD believes is a normally conducted ventricular impulse for the patient. This normal ventricular impulse is often an average of a multiple of beats of the patient taken in the recent past.

History

The development of the ICD was pioneered at Sinai Hospital in Baltimore by a team including Stephen Heilman, Alois Langer, Morton Mower, Michel Mirowski, and Mir Imran, with the help of industrial collaborator Intec Systems of Pittsburgh. Mirowski teamed up with Mower and Staewen, and together they commenced their research in 1969 but it was 11 years before they treated their first patient. Imran was the young engineer who was able to turn the benchtop idea into a working device. Using analogue tape recordings of patients heartbeats, Imran developed algorithms and hardware that are the foundation of today's ICD's and Cardiac Rhythm Management industry. Similar developmental work was carried out by Schuder and colleagues at the University of Missouri.

More than a decade of research went into the development of an implantable defibrillator that would automatically sense the onset of ventricular fibrillation and deliver an electric countershock within 15-20 seconds, converting the rhythm to sinus rhythm. Improved versions were programmed to be able to detect ventricular tachycardia, often a forerunner of ventricular fibrillation. These were then called implantable cardioverters.

The work was commenced against much skepticism even by leading experts in the field of arrhythmias and sudden death. There was doubt that their ideas would ever become a clinical reality. In 1972 Bernard Lown, the inventor of the external defibrillator, stated in the journal Circulation - "The very rare patient who has frequent bouts of ventricular fibrillation is best treated in a coronary care unit and is better served by an effective antiarrhythmic program or surgical correction of inadequate coronary blood flow or ventricular malfunction. In fact, the implanted defibrillator system represents an imperfect solution in search of a plausible and practical application".

The problems to be overcome were the design of a system which would allow detection of ventricular fibrillation or ventricular tachycardia. Despite the lack of financial backing and grants, they persisted and the first device was implanted in February 1980 at Johns Hopkins Hospital by Dr. Levi Watkins, Jr. Modern ICDs do not require a thoracotomy and possess pacing, cardioversion, and defibrillation capabilities.

Internal cardioverter defibrillators have also been used twice in dogs to prevent sudden death from arrhythmia. The first defibrillator was implanted at Washington State University by a team of cardiologists led by Dr Lynne Johnson in 2003. The patient was a Boxer dog with life threatening arrhythmias from arrhythmogenic right ventricular cardiomyopathy, an inherited disease. On July 21rst 2008, a second ICD was implanted in a 6-month-old German Shepherd dog with inherited ventricular arrhythmias. The 5-hour long surgery took place at Louisiana State University and was led by Dr Romain Pariaut. So far, these pets are the only two client-owned dogs that have received such a high-tech treatment.

Clinical trials

A number of clinical trials have demonstrated the superiority of the ICD over AAD (antiarrhythmic drugs) in the prevention of death from malignant arrhythmias. The SCD-HeFT trial (published in 2005) showed a significant all-cause mortality benefit for patients with ICD. Congestive heart failure patients that were implanted with an ICD had an all-cause death risk 23% lower than placebo and an absolute decrease in mortality of 7.2 percentage points after five years in the overall population.¹ Reporting in 1999, the Antiarrhythmics Versus Implantable Defibrillators (AVID) trial consisted of 1,016 patients, and deaths in those treated with AAD were more frequent (n=122) compared with deaths in the ICD groups (n=80, p < 0.001)

Initially ICDs were implanted via thoracotomy with defibrillator patches applied to the epicardium or pericardium. The device was attached via subcutaneous and transvenous leads to the device contained in a subcutaneous abdominal wall pocket. The device itself acts as an electrode. Most ICDs nowadays are implanted transvenously with the devices placed in the left pectoral region similar to pacemakers. Intravascular spring or coil electrodes are used to defibrillate. The devices have become smaller and less invasive as the technology advances. Current ICDs weigh only 70 grams and are about 12.9 mm thick.

A recent study by Birnie et al at the University of Ottawa Heart Institute has demonstrated that ICDs are underused in both the United States and Canada. An accompanying editorial by Dr. Chris Simpson of Queen's University explores some of the economic, geographic, social and political reasons for this.

References

Bardy GH, Lee KL, Mark DB, et al for the Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT) Investigators. Amiodarone or an implantable cardioverter-defibrillator for congestive heart failure. N Engl J Med 2005; 352:225-237
Stevenson W, Chaitman B, Ellenbogen K, Epstein A, Gross W, Hayes D, Strickberger S, Sweeney M (2004). "Clinical assessment and management of patients with implanted cardioverter-defibrillators presenting to nonelectrophysiologists". Circulation 110 (25): 3866–9. Full text