Implantable Defibrillator

Implantable Defibrillator: Definition

An implantable defibrillator or implantable cardioverter defibrillator (ICD) is a device that is implanted under the skin of patients that are at risk ofsudden cardiac death due to ventricular fibrillation. The rudiments of cardiac arrhythmia detection and treatment are incorporated into the implantable device.

The device was designed primarily to deal with ventricular fibrillation. Its current use has however extended to include atrial and ventricular arrhythmias as well as the ability to perform biventricular pacing in patients with congestive heart failure and to pace should there be any marked bradycardia.

Implantable Defibrillator: The Operation

The process of implantation of an implantable defibrillator or ICD is similar to implantation of a pacemaker. Similar to pacemakers, these devices typically include a wire that runs through the right chambers of the heart, usually ending in the apex of the right ventricle.

Implantable Defibrillator: How They Work

Implantable defibrillators or ICDs constantly monitor the rate and rhythm of the heart and can deliver therapies when the heart rate goes over a set number. All implantable defibrillators or ICDs are programmed to deliver an electrical shock when the ventricles of the heart go faster than the set rate. 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 implantable defibrillators or 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.

Implantable Defibrillator: History

The development of the implantable defribrillator or ICD was pioneered by Michel Mirowski’s team working at Sinai Hospital in Baltimore with the help of industrial collaborator Intec Systems of Pittsburgh. This team comprised Stephen Heilman, Alois Langer, Morton Mower and Michael Mirowski, all of whom are now in the National Inventors Hall of Fame. Mirowski teamed up with Mower and Staewen. Together they commenced their research in 1969 but it was 11 years before they treated their first patient. 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 scepticism 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 implantable 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. Current devices now do not require a thoracotomy, and are multiprogrammable devices with pacing, cardioversion, and defibrillation capabilities.

Implantable Defibrillator: Clinical trials

A number of clinical trials have demonstrated the superiority of the implantable defribrillator 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 an implantable defibrillator or ICD. Congestive heart failure patients that were implanted with an implantable defibrillator or 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. Reporting in 1999, the Antiarrhythmics Versus Implantable Defibrillators (AVID) trial consisted of 1,016 patients, and deaths in those treated with AAD where more frequent (n=122) compared with deaths in the ICD groups (n=80, p<0.001)

Initially implantable defibrillators or 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 pectoral region similar to pacemakers. Intravascular spring or coil electrodes are used to defibrillate. The devices have gotten smaller and less invasive as the technology advances. Current ICDs weigh only 70 grams and are about 12.9 mm thick.