Implantable cardioverter defibrillator

Overview
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 and ventricular tachycardia. 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.

Implantation
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 ICDs are often permanent safeguards against sudden abnormalities.

The most recent development is the subcutaneous ICD (S-ICD). Current state-of-the-art electronics and batteries have enabled an implantable device to deliver enough energy to defibrillate the heart without the need for a lead in or on the heart. This prevents lead-related problems and the risk of dangerous infections in or near the heart. This ICD is positioned just under the skin and outside the ribcage. It can be placed during a minor procedure under conscious sedation. A study of 300 patients is in progress for US approvals.

Working mechanism
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 Michel Mirowski, Morton Mower, and William Staewen. 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. Similar developmental work was carried out almost coincidentally 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 21, 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.

Living with an ICD
People that have an implanted cardioverter-defibrillator can live full and happy lives. Usually the ICD improves the living conditions of a patient significantly. As with a pacemaker, living with an ICD does impose some restrictions on the person's lifestyle.

Quality of Life
Implantable cardioverter defibrillators have demonstrated clear life-saving benefits but concerns about patient acceptance and psychological adjustment to the ICD have been the focus of much research. Researchers including those from the field of cardiac psychology have concluded that the QoL of ICD patients is at least equal to, or better than those taking anti-arrhythmic medications. The largest study of examined 2,521 patients with stable heart failure in the SCD-HeFT trial. Results indicated that there were no differences between ICD treated and medication-treated groups at 30 months in patient reported QoL. Psychological adjustment following ICD implantation has also been well-studied. Anxiety is a common psychological side effect with approximately 13-38% of ICD patients reporting clinically significant anxiety. The primary etiological factors contributing to anxiety in ICD patients has not been determined, however. Depressive symptoms are also common but incidence of these problems have shown to be similar to those observed in other cardiac patient groups with approximately 24-41% of patients with ICDs experiencing depressive symptoms.

Problems in psychosocial adjustment to ICDs, including the experience of anxiety, among spouses or other romantic partners are also prevalent. This phenomenon may be related, at least in part, to shared shock anxiety and avoidance of physical and sexual contact.

Physical activities
Almost all forms of physical activities can be performed by patients with an ICD. All forms of sports that do not pose a risk of damaging the ICD can be enjoyed by the patient. Special care should be placed not to put excessive strain on the shoulder, arm and torso area where the ICD is implanted. Doing so may damage the ICD or the leads going from the unit to the patient's heart.

Electronic equipment
As a general rule, all electronic equipment is safe to use for patients with an ICD if kept at relative small distance from the unit. Most electronic equipment such as cellphones, Devices that generate radio waves or radio interference should be kept at least 15 cm from the ICD.

Electronic equipment used in a professional environment or equipment using large magnets or generating magnetic fields must be avoided by patients with an ICD. Both the magnetic fields and the EMI (Electromagnetic Interference) in an MRI scanner can interfere with the correct working of the ICD. As with other metallic objects, an ICD is a contraindication to the use of magnetic resonance imaging. Experiments are on the way for solving this kind of problem. For example Medtronic showed interesting results with a pacemaker.

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). In 2002 the MADITII trial showed benefit of ICD treatment in patients after myocardial infarction with reduced left ventricular function (EF<30).

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.

==ACC / AHA Guidelines- Recommendations for Implantable Cardioverter Defibrillators (DO NOT EDIT) == {{cquote|

Class I
1. ICD therapy is indicated in patients who are survivors of cardiac arrest due to VF or hemodynamically unstable sustained VT after evaluation to define the cause of the event and to exclude any completely reversible causes. (Level of Evidence: A)

2. ICD therapy is indicated in patients with structural heart disease and spontaneous sustained VT, whether hemodynamically stable or unstable. (Level of Evidence: B)

3. ICD therapy is indicated in patients with syncope of undetermined origin with clinically relevant, hemodynamically significant sustained VT or VF induced at electrophysiological study. (Level of Evidence: B)

4. ICD therapy is indicated in patients with LVEF less than 35% due to prior MI who are at least 40 days post-MI and are in NYHA functional Class II or III. (Level of Evidence: A)

5. ICD therapy is indicated in patients with nonischemic DCM who have an LVEF less than or equal to 35% and who are in NYHA functional Class II or III. (Level of Evidence: B)

6. ICD therapy is indicated in patients with LV dysfunction due to prior MI who are at least 40 days post-MI, have an LVEF less than 30%, and are in NYHA functional Class I. (Level of Evidence: A)

7. ICD therapy is indicated in patients with nonsustained VT due to prior MI, LVEF less than 40%, and inducible VF or sustained VT at electrophysiological study. (Level of Evidence: B)

Class IIa
1. ICD implantation is reasonable for patients with unexplained syncope, significant LV dysfunction, and nonischemic DCM. (Level of Evidence: C)

2. ICD implantation is reasonable for patients with sustained VT and normal or near-normal ventricular function. (Level of Evidence: C)

3. ICD implantation is reasonable for patients with HCM who have 1 or more major{dagger} risk factors for SCD. (Level of Evidence: C)

4. ICD implantation is reasonable for the prevention of SCD in patients with ARVD/C who have 1 or more risk factors for SCD. (Level of Evidence: C)

5. ICD implantation is reasonable to reduce SCD in patients with long-QT syndrome who are experiencing syncope and/or VT while receiving beta blockers. (Level of Evidence: B)

6. ICD implantation is reasonable for non hospitalized patients awaiting transplantation. (Level of Evidence: C)

7. ICD implantation is reasonable for patients with Brugada syndrome who have had syncope. (Level of Evidence: C)

8. ICD implantation is reasonable for patients with Brugada syndrome who have documented VT that has not resulted in cardiac arrest. (Level of Evidence: C)

9. ICD implantation is reasonable for patients with catecholaminergic polymorphic VT who have syncope and/or documented sustained VT while receiving beta blockers. (Level of Evidence: C)

10. ICD implantation is reasonable for patients with cardiac sarcoidosis, giant cell myocarditis, or Chagas disease. (Level of Evidence: C)

Class IIb
1. ICD therapy may be considered in patients with nonischemic heart disease who have an LVEF of less than or equal to 35% and who are in NYHA functional Class I. (Level of Evidence: C)

2. ICD therapy may be considered for patients with long-QT syndrome and risk factors for SCD. (Level of Evidence: B)

3. ICD therapy may be considered in patients with syncope and advanced structural heart disease in whom thorough invasive and noninvasive investigations have failed to define a cause. (Level of Evidence: C)

4. ICD therapy may be considered in patients with a familial cardiomyopathy associated with sudden death. (Level of Evidence: C)

5. ICD therapy may be considered in patients with LV noncompaction. (Level of Evidence: C)

Class III
1. ICD therapy is not indicated for patients who do not have a reasonable expectation of survival with an acceptable functional status for at least 1 year, even if they meet ICD implantation criteria specified in the Class I, IIa, and IIb recommendations above. (Level of Evidence: C)

2. ICD therapy is not indicated for patients with incessant VT or VF. (Level of Evidence: C)

3. ICD therapy is not indicated in patients with significant psychiatric illnesses that may be aggravated by device implantation or that may preclude systematic follow-up. (Level of Evidence: C)

4. ICD therapy is not indicated for NYHA Class IV patients with drug-refractory congestive heart failure who are not candidates for cardiac transplantation or CRT-D. (Level of Evidence: C)

5. ICD therapy is not indicated for syncope of undetermined cause in a patient without inducible ventricular tachyarrhythmias and without structural heart disease. (Level of Evidence: C)

6. ICD therapy is not indicated when VF or VT is amenable to surgical or catheter ablation (e.g., atrial arrhythmias associated with the Wolff-Parkinson-White syndrome, RV or LV outflow tract VT, idiopathic VT, or fascicular VT in the absence of structural heart disease). (Level of Evidence: C)

7. ICD therapy is not indicated for patients with ventricular tachyarrhythmias due to a completely reversible disorder in the absence of structural heart disease (e.g., electrolyte imbalance, drugs, or trauma). (Level of Evidence: B)}}

==ACC / AHA Guidelines- Recommendations for Implantable Cardioverter-Defibrillators in Pediatric Patients and Patients With Congenital Heart Disease (DO NOT EDIT) == {{cquote|

Class I
1. ICD implantation is indicated in the survivor of cardiac arrest after evaluation to define the cause of the event and to exclude any reversible causes. (Level of Evidence: B)

2. ICD implantation is indicated for patients with symptomatic sustained VT in association with congenital heart disease who have undergone hemodynamic and electrophysiological evaluation. Catheter ablation or surgical repair may offer possible alternatives in carefully selected patients. (Level of Evidence: C)

Class IIa
1. ICD implantation is reasonable for patients with congenital heart disease with recurrent syncope of undetermined origin in the presence of either ventricular dysfunction or inducible ventricular arrhythmias at electrophysiological study. (Level of Evidence: B)

Class IIb
1. ICD implantation may be considered for patients with recurrent syncope associated with complex congenital heart disease and advanced systemic ventricular dysfunction when thorough invasive and noninvasive investigations have failed to define a cause. (Level of Evidence: C)

Class III
1. All Class III recommendations found in Section 3, "Indications for Implantable Cardioverter-Defibrillator Therapy," apply to pediatric patients and patients with congenital heart disease, and ICD implantation is not indicated in these patient populations. (Level of Evidence: C) }}