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Implantable Defibrillator

Editor: Kunal Mahajan Updated: 10/3/2022 8:43:28 PM

Practitioners use the terms sudden cardiac arrest (SCA) and sudden cardiac death interchangeably; however, their definitions are distinct. Sudden cardiac arrest is the “sudden cessation of cardiac activity so that the victim becomes unresponsive, with no normal breathing and no signs of circulation.” If the condition is not addressed immediately, sudden cardiac arrest progresses to sudden cardiac death. Sudden cardiac death is defined “as a natural death due to cardiac causes, heralded by abrupt loss of consciousness.” Out-of-hospital cardiac arrest occurs outside of the hospital, and emergency medical services personnel most commonly attend to patients these situations. There are approximately 356,500 out-of-hospital cardiac arrests per year in the United States.[1][2][3]

Anatomy and Physiology

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Confirmed by series of trials (Antiarrhythmics versus Implantable Defibrillators (AVID), Cardiac Arrest Study Hamburg (CASH), and the Canadian Implantable Defibrillator Study (CIDS)), implantable cardioverter-defibrillators or ICDs have been proven significantly to reduce the risk of sudden cardiac death attributable to:

  • In patients with New York Heart Association (NYHA) class I, II, or III Ventricular tachyarrhythmias caused by a prior myocardial infarction
  • In patients with New York Heart Association class II or III ventricular tachyarrhythmias caused by non-ischemic cardiomyopathy despite optimal medical therapy, and in whom survival with good functional capacity is anticipated to extend beyond 1 year.

The ICD improved survival, even despite beta-blockade, surgical revascularization, or presenting arrhythmia (VT or VF). There was no difference in benefit gained from ICD implantation between those patients with coronary disease and those with non-ischemic cardiomyopathies. Primary prevention trials also played an important role in proving the unique benefit of ICD implantation even before the incidence of lethal arrhythmia, patients with left-ventricle (LV) dysfunction and post-myocardial infarction and a history of non-sustained VT underwent electrophysiology (EP) testing to identify higher risk patients with inducible, non-suppressible, ventricular tachyarrhythmias. This population was the target for the primary prevention trials, including:

  • The famous Multicenter Automatic Defibrillator Trial (MADIT) which compared ICDs to conventional therapy (mainly amiodarone) (ejection fraction (EF) =35%)
  • Multicenter Unsustained Tachycardia Trial (MUSTT), which compared EP-guided therapy (ICDs or drug therapy) vs. no EP-guided therapy (EF =40%).

The Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT) confirmed the benefit of ICDs in ischemic patients. This validated findings in MADIT II, as well as the findings of a previous smaller study of non-ischemic cardiomyopathy patients, the Prophylactic Defibrillator Implantation in Patients with Non-ischemic Dilated Cardiomyopathy (DEFINITE) trial.

Doctors use ICDs to prevent sudden cardiac death in other high-risk patient subsets, such as patients with ion-channel abnormalities such as Brugada syndrome, long QT syndrome (LQTS) or other structural heart diseases such as RV dysplasia and hypertrophic cardiomyopathy. Although prospective randomized trials in these rare conditions are not likely to be pursued, case series showed the efficacy of the ICD in patients with LQTS, Brugada syndrome, hypertrophic cardiomyopathy, and arrhythmogenic RV dysplasia.[4][5][6]

Indications of ICD according to American College of Cardiology/American Heart Rhythm Society (ACC/AHRS) 2008:[7][8][9][10]

Class I in patients having any of the following:

  1. Survivors of cardiac arrest due to hemodynamically unstable sustained ventricular tachycardia (VT) or FF after evaluation and exclusion of any reversible cause
  2. Structural heart disease, with spontaneous sustained VT whether hemodynamically stable or not
  3. Unexplained syncope, with hemodynamically significant sustained VT or ventricular fibrillation (VF), induced at electrophysiology study (EPS)
  4. Ischemic cardiomyopathy patients with left-ventricular ejection fraction (LVEF) less than 35% due to prior MI who a minimum of 40 days post-MI and are in NYHA functional class II or III.
  5. Non-ischemic dilated cardiomyopathy (DCM) with LVEF 35% or less and in NYHA functional class II or III.
  6. Patients with LV dysfunction due to prior MI who are a minimum of 40 days post-MI, and LVEF less than 30%, with New York Heart Association functional class I
  7. Non-sustained VT due to prior MI, LVEF less than 40%, and sustained VT or inducible VF at EPS.

Class IIa

  1. Unexplained syncope, non-ischemic DCM, and significant LV dysfunction
  2. Patients with sustained VT and normal or near-normal ventricular function
  3. Hypertrophic cardiomyopathy who have one or more major risk factors for sudden cardiac death
  4. Prevention of sudden cardiac death in patients with arrhythmogenic right ventricular dysplasia (ARVD/C) who have one or more risk factors for sudden cardiac death
  5. Patients with Long QT syndrome, to reduce sudden cardiac death, who are suffering syncope or Ventricular tachycardia
  6. Nonhospitalized patients are awaiting transplantation
  7. Brugada syndrome patients who have had syncope or have documented VT that has not resulted in cardiac arrest
  8. Catecholaminergic polymorphic VT who have syncope and/or documented sustained VT while receiving beta blockers
  9. Patients with cardiac sarcoidosis, Chagas disease, and giant cell myocarditis.

Class IIb

  1. Non-ischemic heart disease with LVEF 35% and who are in NYHA functional class I.
  2. Long-QT syndrome patients and risk factors for sudden cardiac death
  3. ICD therapy may be considered in patients with sudden cardiac death and advanced structural heart disease in whom without a defining cause by invasive or noninvasive methods
  4. Patients with a familial cardiomyopathy associated with sudden death
  5. Patients with LV noncompaction.

Contraindications include the following:

  1. ICD therapy is not indicated in those who do not have a reasonable expectation of survival with an acceptable functional status for at least 1 year, even if they meet implantation criteria for ICD specified in class I, IIa, and IIb recommendations above. (Level of Evidence: C)
  2. ICD therapy is not recommended for patients with incessant VT or VF. (Level of Evidence: C)
  3. ICD therapy is not recommended in 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 recommended in NYHA Class IV 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 recommended for syncope of undetermined cause in a patient without inducible ventricular tachyarrhythmias and structural heart disease. (Level of Evidence: C)
  6. ICD therapy is not recommended when VF or VT is amenable to surgical or catheter ablation (e.g., atrial arrhythmias associated with the Wolff-Parkinson-White syndrome, LV, or RV RV outflow tract VT, idiopathic VT, or fascicular VT in the absence of structural heart disease). (Level of Evidence: C)
  7. ICD therapy is not recommended for patients with ventricular tachyarrhythmias as a result of an entirely reversible disorder in the absence of structural heart disease (e.g., drugs, electrolyte imbalance, or trauma). (Level of Evidence: B)

ICD Components

The ICD Generator

The ICD generator has the necessary elements to coordinate all pacing, sensing, and defibrillation functions. These elements include the battery and electronic circuitry (brains and capacitor).

  • The Battery: Each ICD has a battery whose lifetime is dependent on how much pacing and defibrillating the device is called upon to perform. Typical longevities fall between about 4 to 7 years.
  • The Circuitry: The ICD circuitry determines how and when both bradycardia pacing and anti-tachycardia therapies are delivered. This facilitates a wide range of programming options so that each device can be tailored to function in a manner most appropriate for each individual. The ICD capacitor(s) is (are) a critical electrical element allowing for defibrillation. Capacitors perform this task by first storing energy (charging) that can then be released in a path across the heart (defibrillation/ cardioversion).

The ICD Header and Can

The ICD generator communicates with the heart through a ventricular defibrillator ± atrial and LV electrode or lead. The leads are connected to the ICD via a header. The header provides holes for insertion of all aspects of the ventricular defibrillator lead and if also needed, an atrial and LV lead in specific ICD models. As in the pacemaker, set screws in the header may be tightened to fix the leads in place or loosened to allow their removal. The metal casing of the ICD generator is called the “can.” The can is similar in composition to that of the pacemaker and will protect the ICDs electrical elements from fluids and many external electrical sources.

Ventricular ICD Leads

A ventricular defibrillator lead consists of multiple internally separated metal wires that are externally encased in silicone rubber or polyurethane insulation. This allows the lead to function similarly to a standard pacemaker lead, in other words, transmission of electrical pacing/sensing signals between the heart and generator, but also structurally provides a separate pathway that participates in the delivery of shocks. This pathway includes what are commonly referred to as the lead “coil(s).” Shocks are delivered across the heart between the coil(s) and potentially the ICD can. Each element of the lead has its own pin that may connect to the ICD header. Ventricular ICD leads are commonly described by having the presence of one (single) or two (dual) coils. Both types have a coil distally, but a dual coil lead also has a proximal one along the length of the lead that preferably winds up in the superior vena cava (SVC) when this type of lead is desired. Thus, each ventricular defibrillator lead has a total of either two or three pins that connect to the ICD, one for pacing and sensing and one or two additional for defibrillating, depending on whether it is a single or dual coil lead. The ventricular ICD lead is essentially a bipolar lead; it is either an integrated bipolar configuration utilizes the lead tip and the distal coil for pacing and sensing purposes or a true bipolar configuration that utilizes the tip and a ring electrode.


Complications are mainly related to ICD implantation and are uncommon but may include:

  • Infection at the implant site that may extend from a mild reaction and seroma up to suppuration and necessitates lead extraction
  • Allergic reaction to medications used during the procedure
  • Bleeding or bruising where your ICD was implanted
  • Damage to the vein where your ICD leads are placed
  • In rare cases, upper limb deep vein thrombosis (DVT)
  • Bleeding around the heart, which can be life-threatening. May include pericardial effusion and may lead to tapping and surgical intervention
  • Pneumothorax during the procedure of implantation.

Clinical Significance

ICD functions


Sensing in implantable devices refers to the ability to pick up intrinsic signals from the heart and interpret them properly, in such a way that allows the device to respond appropriately. The ICD system needs to be sensitive enough to detect the VF wavelength that occurs in a tenth of millivolts, and also not to over-sense the T wave, far fields, or myopotential. This is usually achieved through a digitally modified signal algorithm. The peak signal amplitude of the sensed ventricular event during refractory period is stored to determine the threshold start, and when the sensed refractory period expires, the threshold start (which is a percentage of the peak sensed ventricular event, usually 50%) determines the sensitivity setting of the next cycle. Three maximum sensitivity settings are used in ICD, two are used for the pacemaker function of the ICD, and the third is the threshold start for the defibrillation function.

Arrhythmia Detection

Proper ICD therapy is a matter of life. Most of the ICDs have three broad detection categories: normal sinus rhythm, ventricular tachycardia, and ventricular fibrillation. These rhythms are defined by rate; the device looks at the corresponding cycle lengths or millisecond (ms) intervals to make its determinations.

Most ICDs now offer three-tiered therapy to terminate the tachycardia:

  • Anti-tachycardia pacing (ATP)
  • Cardioversion or low-energy shocks
  • Defibrillation or high-energy shocks.

Detection is usually defined according to the ability of the ICD to detect the rate of the arrhythmia.

  1. Defibrillation only configuration detects only the sinus rhythm as a rate less than 200 bpm and VF with a rate more than 200 bpm. It offers the only defibrillation to rates above 200 bpm.
  2. Defibrillation and single tachycardia zone: In this kind of configuration, the ICD is able to identify three detection rates, sinus rhythm with a rate less than 120 bpm, VT as a rate between 120bpm to 200 bpm, and VF at a rate of more than 200 bpm. It offers ATP for VT detection zone and defibrillation for VF detection zone.
  3. Defibrillation with two tachycardia zones:
  • The ICD has the sinus rate of less than 120 bpm, slow VT at tachycardia zone A with a rate between 120bpm to 160 bpm, fast VT 160 bpm to 200 bpm, and the VF zone at a rate of more than 200 bpm.The ICD usually uses the average interval together with the current interval. If the current interval meets the average rate, then the interval is binned; if the current rate does not meet the average interval, then the rate is binned to the faster rate. This helps in overcoming the occasional single-beat rhythm variation from resulting in therapy delivery. 
  • There are two methods for counting the sensed events and to bin them to the accurate rate zone. The first method is the consecutive interval counting, determined by a programmable number of events needed to satisfy detection, within the tachycardia zone, each interspersed non-tachycardia beat causes counting to be reset to zero. The other method is X of Y counting method, the continuous rolling window of detection that is updated with each new ventricular heartbeat. Detection is initially satisfied when X number out of the total number of beats in the window Y are classified to a tachycardia zone. X of Y scheme is the counting method of choice for VF zone detection. Also, VT can have some variation in its cycle length. An X of Y counting method may compensate for potential transient drops below the rate cutoff and prevent delayed detection and delivery of therapy.

SVT/VT Discrimination

In addition to rate detection, rhythm detection is also crucial in therapy-delivering. Heart rate is not the only factor that detection in VT zones may be reliant upon. SVT discriminators are algorithms available in Tach A and Tach B zones. (SVT discriminators cannot be used in the fib zone).

  • Rate branch: Rate Branch is one of the discrimination methods, by simultaneous counting of both atrial and ventricular events, in case of supraventricular events the atrial events are more than ventricular events, then the device diagnoses this as atrial flutter or AF. This is an SVT, and the ICD would automatically inhibit therapy. On the other hand, if V is greater than A, that is, the ventricular rate was more rapid than the atrial rate, the ICD would categorize this as VT and advance to deliver therapy.
  • Interval stability: Interval stability is another method to overcome rapid ventricular rate associated with the chronic Afib patients, tachycardia originating in the ventricles usually has a more regular, albeit rapid rhythm. Thus, interval stability is a special SVT discriminator which assesses the stability of the ventricular rate. Interval stability in the form of regular R–R intervals is diagnosed as a VT (therapy is delivered), while instability in the R–R intervals is diagnosed as AF with rapid ventricular response, inhibiting therapy. The programmable setting for the interval stability of the R-R interval may be a delta or an interval of 80 msec, if R-R interval changes by  more than 80 msec the diagnosis of Afib is set up, and thus, therapy is inhibited, while if R-R variability is less than the programmable delta, then the diagnosis of VT is established, and therapy is delivered.
  • Sudden onset: This algorithm is used in case of confusing VT and sinus tachycardia, sinus tachycardia is characterized by gradual onset due to the physiological demand, on the other hand, VT starts abruptly. Sudden onset algorithm looks at the change (delta) in interval measurement from the non-tachycardia to the binned tachycardia event. VT is diagnosed when the change is more than the programmed value.
  • Morphology discrimination: This algorithm needs a highly expert clinician who uses the own patient's rhythm as a template during programming session to discriminate between supraventricular ORS morphology that uses the His-Purkinje conduction system to match it to the arrhythmia interval detected to make a matching score of the arrhythmic ORS complex.


Ventricular tachyarrhythmia therapy in an ICD may be programmed for each zone of detection. For VF zone, therapy occurs in the form of defibrillation. Shocks to terminate tachycardias in this zone may occur regardless of the timing in the cardiac cycle, and up to a maximum of 25 J to 42 J stored energy in contemporary devices. Some ICD models may deliver up to eight shocks per arrhythmia episode.

ATP may end arrhythmia in VT zone; the ventricular pacing is delivered in a burst at a cycle length shorter than that of the tachycardia (typically 10% to 20% shorter). The tachycardia may be terminated by rendering its circuit refractory at a critical point. ATP burst options include pacing at a fixed cycle length, pacing at a progressively shorter cycle length (“ramp”) with each impulse, and pacing with a combination of both. The number of pacing impulses within each burst and the number of burst attempts are also programmable. Cardioversion is the next level for VT termination; each shock is synchronized to occur on R wave in the cardiac cycle, the initial shock energy is usually low and is increased in subsequent shocks.

Bradycardia Pacing

ICD systems have similar bradycardia pacing capabilities as a conventional pacemaker. Whether or not backup pacing function is required for the defibrillator patient may frequently depend on the reason the ICD was implanted, and the future needs for medication that may precipitate significant Bradyarrhythmias.

Enhancing Healthcare Team Outcomes

ICD implantation is often performed by the cardiologist or cardiac surgeon. However, the care of these patients is often done by the primary care provide or nurse practitioner. These healthcare professionals should closely monitor the patients for shocks and arrhythmias. Any adverse event should be referred promptly to a cardiologist. In addition, any patient who is a survivors of cardiac arrest, has structural heart disease, Unexplained syncope, Ischemic cardiomyopathy, Non-ischemic dilated cardiomyopathy, LV dysfunction due to prior MI or has Non-sustained VT should be referred to a cardiologist to determine if they are candidates for an ICD.



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