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Automatic Internal Cardiac Defibrillator

Editor: Syed F. Jamal Updated: 5/22/2023 9:46:36 PM


The automated internal cardiac defibrillator or shock box is the common name given to the Implantable Cardioverter Defibrillator (ICD).

ICD is a state-of-the-art device that treats arrhythmias specifically those of ventricular origin like ventricular tachycardia and fibrillation. It has become the first line of defense in patients who are at high risk for sudden cardiac death (SCD) and has shown consistent survival benefit in cardiac arrest survivors (SCA), in patients with Heart failure and severe systolic dysfunction (left ventricular ejection fraction-LVEF less than or equal to 35%) as well as in patients with hypertrophic cardiomyopathy (HCM).[1][2]

ICD is essentially a pacemaker with the ability to recognize abnormally fast cardiac rhythm and provide an immediate treatment which can be in the form of overdrive pacing called anti-tachycardia pacing (ATP) or shock therapy which could be synchronized or asynchronized, depending on the recognized rhythm and the pre-programmed rhythm detection algorithm. It comes in three systems, a single lead or single chamber device, a dual lead or a dual-chamber device, and one that is coupled with cardiac resynchronization therapy (CRT-D) that is essentially a bi-ventricular device with leads in the right atrium (RA), Right ventricle (RV) and coronary sinus (CS) lead. Due to its proximity, CS lead is also called the LV lead.

In 2012, the US Food and Drug Administration (FDA) approved the sub-cutaneous device (S-ICD) which used sub-cutaneous leads rather than intra-cardiac leads. Recently, wearable defibrillators (WCD) have also been introduced for short-term usage.


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Indications are usually secondary where the patient has already suffered and survived cardiac arrest due to ventricular fibrillation/ventricular tachycardia, or primary when the patient is at high risk of sudden cardiac death due to VF/ VT but has never had any such event.

Secondary prophylaxis usually involves the event of cardiac arrest due to ventricular fibrillation (VF) or hemodynamically unstable, also known as pulseless, ventricular tachycardia (VT). Adequate workup and exclusion of reversible causes should be done first before deciding to put the device in, as is endorsed by the guidelines laid down by Heart Rhythm Society (HRS) and American College of Cardiology (ACC).[3]

Primary prophylaxis, as described above, involves implanting the device in patients who have not had sudden cardiac death due to VF or pulseless VT but are at high risk of having such an event. These have become the most common reasons for device implantation in recent times.

Class (I) recommendations for ICD implantation include[3]:

  • LV dysfunction with ejection fraction less than or equal to 35% and NYHA II/III symptoms
  • LV dysfunction with ejection fraction less than or equal to 35% due to previous Myocardial Infarction (MI) or at least 40 days post-MI and NYHA II/III symptoms
  • LV dysfunction with ejection fraction less than or equal to 30% due to previous Myocardial Infarction (MI) or at least 40 days post-MI
  • LV dysfunction with ejection fraction less than or equal to 40% due to previous Myocardial Infarction (MI) or at least 40 days post-MI and inducible VT/VF on EP study (EPS)
  • Syncope of unknown etiology and inducible VT/ VF on EPS
  • Sustained VT in the presence of structural heart disease


ICD is contraindicated in situations where there are reversible causes (like myocardial ischemia, sepsis, hypoxia, electrolyte imbalance, electrocution, etc.) leading to VT/VF.

Atrial arrhythmias with no concomitant VT/VF and incessant VT/VF are also contra-indications for ICD.


ICD system consists of three components;

  1. The pulse generator or generator is the ovoid/circular pod that houses the sophisticated machinery which reads, recognizes, and stores cardiac rhythm tracings as well as decides for the need and delivery of appropriate therapy (ATP or defibrillation) according to a pre-programmed algorithm. It is implanted in the subcutaneous pocket usually in the pectoral region. It also contains the battery and defibrillator which have finite charge and capacity to defibrillate and pace the heart, as the need be. This part also includes the manufacturer code which is readable on the magnified plain radiogram. The device programmer magnet is placed over Pulse-generator for purposes of magnet inhibition, interrogation, and external programming or therapy. Moreover, these generators also have wireless communication ability with compatible programmers, within a limited radius.
  2. Leads are the cables or silicone coated wires that connect the generator to the myocardium and house the electrodes which facilitate conduction between the device and myocardium. They remain inside the heart. They have screws at one end hooked to the device, and the cardiac end has either screws or tines by which the lead is secured in place inside the respective cardiac chamber.
  3. Shocking coil. Inside the ICD, the lead has a shocking coil that delivers the necessary charge for defibrillation/electrical cardioversion. ICD leads can be identified on plain radiograms by the presence of shocking coils which appear as localized and well-demarcated spiral thickenings on the lead usually in the region of SVC on atrial lead and near the ventricular end of RV lead.


The decision for ICD implantation rests with the patient's primary physician and usually requires referral to a cardiac electrophysiologist and rhythm device specialist for appropriate screening and procedural planning.


Patients are required to undergo a whole-body wash usually 24 hours before and should be kept NPO for at least 4 hours prior to the procedure.

Blood workup consisting of platelet count and checking the INR  is run and addressed appropriately. HbA1c is also checked to ensure adequate glycemic control as this is essential for diabetic individuals to minimize the risk of infection.

In some cases, the implanting physician might also advise discontinuing anticoagulants (warfarin, NOACs, DTIs, etc.) for a short period.

Technique or Treatment

The patient is usually draped and prepped in a sterile manner similar to that for pacemakers with the pectoral region exposed, and the procedure is generally performed under local anesthesia along with conscious sedation.

The device is implanted in a subcutaneous pocket usually made in the pectoral region where the device generator is placed, the leads are then hooked to the generator at one end, and the other end of the lead is then advanced, via the subclavian vein, and positioned in the respective cardiac chamber. While there is no consensus regarding defibrillation threshold testing (DFT), some operators perform DFT after putting in the device to check for defibrillation thresholds and device optimization and then close the pocket after DFT.[4][5]

The procedure typically takes up to 90 minutes.

Although it is a daycare procedure, patients are usually retained over the night and are discharged the next day after device interrogation and obtaining a plain chest radiogram to check for lead placement.


Complications divide into short-term and long-term.

Short term complications (2 to 3%) are usually immediate and relate to the procedure; they are[6][7]:

  1. Access complications like bleeding, thrombosis of subclavian/ axillary vein, and inadvertent puncture of lung tissue causing pneumothorax or hemothorax
  2. Pocket complications like pain, pocket hematoma, and even Twiddler syndrome which is the twisting and misplacement of the device, usually due to pressure exerted by expanding hematoma, that causes device malfunction usually by disrupting the lead and generator connections
  3. Device-related infection (1 to 2%), including endocarditis, have also been reported
  4. Serious complications (less than 1%) like pulseless electrical activity (PEA) and death can also occur during DFT

Long term complications (up to 4%) include[7][8]:

  1. Device-related pain
  2. Anxiety
  3. Lead fracture
  4. Inappropriate shock delivery
  5. Phantom shock (which means that the patient perceives as receiving shock therapy, but device interrogation does not reveal any such shock or event)
  6. Device erosion through the skin
  7. Device Infection - more so in cases of replacement or generator change, than new device implantation[8]
  8. Immunologic rejection - rare

Clinical Significance

ICDs have proven survival benefits. Since their inception in the late 1980s, they continue to improve in functionality and design and have provided a lifeline for high-risk patients who, can seek help in case of cardiac arrest, otherwise would not have made it to the hospital.

Patients with structural heart disease as well as cardiomyopathies, both ischemic and non-ischemic, fare equally from the device especially when it is coupled with resynchronization therapy that provides not only mortality benefit but improves the quality of life (QOL) and has also shown potential to aid in the recovery of LVEF.

Now, there is an ever-growing list of patients who can live up to their date of transplant with ICDs/CRT-Ds watching over them while they wait for a suitable donor.

Enhancing Healthcare Team Outcomes

With advances in cardiovascular healthcare and new strategies and therapies in the management of coronary heart disease and heart failure, the overall life expectancy of patients has increased and has led to an ever-growing population of patients with LV systolic dysfunction and an increased risk of sudden cardiac death. This has opened up avenues for device therapy not only to save but also to improve life.

The National Cardiovascular Data Registry (NCDR) also has setup device registry which helps physicians and hospitals to pool their data, share their experiences and help each other out which, not only, is translating into better outcomes for these patients but also reassuring patients that there is hope, and there are health professionals for them, working for them.

There remains a taboo, a psychological component with these devices, the fear of not returning to their previous lives, and the fear of getting shocked during treatment. However, this could be readily overcome by adequate support and proper patient counseling by the physicians, electrophysiologists, cardiac rehab staff, and the heart failure staff that are involved in the management of such patients.

Effective communication and timely referral are essential amongst physicians, cardiologists, electrophysiologists as well as rehab and cardiac staff and patient caretakers as these patients require a lot of medical and psychosocial support to return to and maintain an adequate quality of life.

All in all, these devices have become a valuable tool for the heart failure team.



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