Hospital Medicine

Defibrillators

I. Prevention of Sudden Cardiac Death with Implantable Cardioverter-Defibrillator

Sudden cardiac death (SCD), defined as death from cardiac causes occurring within one hour of an acute change in cardiovascular status, is a significant public health problem, with the majority of these cases resulting from a structural cardiac abnormality, such as coronary artery disease. These abnormalities can lead to the development of a potentially fatal arrhythmia, typically ventricular tachycardia (VT) that deteriorates into ventricular fibrillation (VF), resulting in inadequate tissue perfusion to maintain life.

Implantable devices, capable of detecting and reverting potentially fatal ventricular arrhythmias, have demonstrated survival benefit compared to optimal medical therapy in patients at high risk for SCD. Based on this evidence, implantable cardioverter-defibrillators (ICD) have become the preferred treatment option for secondary prevention in survivors of cardiac arrest at risk for recurrence and for primary prevention in populations at high risk of SCD due to VT and VF.

What is an implantable cardioverter defibrillator (ICD)?

An ICD looks very similar to a pacemaker, except that it is slightly larger. It has a generator, one or more leads, and an electrode for each lead. These components work very much like a pacemaker. However, the ICD is designed to deliver two levels of electrical energy: a low energy shock that can convert a beating heart that is in an abnormal rhythm back to a normal heartbeat, and a high energy shock that is delivered only if the arrhythmia is so severe that the heart is only quivering instead of beating.

An ICD senses when the heart is beating too fast and delivers an electrical shock to convert the fast rhythm to a normal rhythm. Many devices combine a pacemaker and ICD in one unit for people who need both functions. After the shock is delivered, a "back-up" pacing mode is available if needed for a short while.

The ICD has another type of treatment for certain fast rhythms called anti-tachycardia pacing, a fast-pacing impulse sent to correct the rhythm.

II. Identifying evidence based practices for implantable cardioverter-defibrillator use

It is critical to incorporate evidence-based care into clinical practice so that patients with heart disease who are at a high risk of SCD - and thus, would benefit from ICD placement - can receive appropriate treatment. The general practitioner is commonly involved in the care of patients with coronary artery disease, prior myocardial infarction, and heart failure and should be able to recognize the major risk factors for sudden death, understand the importance of quantifying the left ventricle ejection fraction (LVEF) in risk stratification, and be familiar with the current guidelines for ICD therapy so that these patients can be referred to cardiology.

It is important to understand the relevant elements of the history in the evaluation of patients with an ICD, as well as the common issues encountered by these patients while hospitalized.

III. Describe a Step-by-Step approach on the important issues involving implantable cardioverter-defibrillator use

Approach to risk stratification for sudden cardiac death

The ability to identify patients at high risk for SCD due to ventricular tachyarrhythmia using either clinical risk profiling or diagnostic testing is imperfect based on our current understanding. The major risk factors for SCD include coronary artery disease (which accounts for up 80% of cases of SCD), reduced ejection fraction due to either ischemic or non-ischemic cardiomyopathy, and prior ventricular tachyarrhythmias. The best predictor of SCD and most widely used criteria for risk stratification for ICD placement is left ventricular ejection fraction (LVEF).

A reduced LVEF is the most powerful independent predictor of sudden death in patients with ischemic and non-ischemic cardiomyopathy although the specificity and sensitivity are low. Multiple randomized controlled trials have shown that ICD therapy in patients with a reduced ejection fraction (EF) (often combined with another criteria such as prior myocardial infarction, non-sustained ventricular tachycardia, and inducible VT on electrophysiological testing, or symptomatic heart failure) result in a mortality benefit.

In patients with an increased risk of SCD - such as those with a history of coronary artery disease, prior myocardial infarction, or heart failure - the recommended initial test for risk stratification is measurement of the ejection fraction. Guidelines note that there is no gold standard for determination of LVEF so clinicians should use whatever method they believe to be the most accurate and appropriate at their institution (e.g., trans-thoracic echocardiogram or ventriculography).

In patients with an EF of less than 35%, referral should be made to a cardiologist or electrophysiologist for further risk stratification. Similarly, patients with a history of ventricular arrhythmia, syncope of undetermined origin, inherited structural heart disease (e.g., hypertrophic cardiomyopathy), or arrhythmia syndromes (e.g., congenital long QT syndrome or Brugada syndrome) should also be referred for further evaluation.

Risk stratification for sudden cardiac death

  1. Identify patients with a history of coronary artery disease, prior myocardial infarction, or heart failure

  2. Evaluate the left ventricular ejection fraction (e.g., using trans-thoracic echocardiogram)

  3. Patients with an ejection fraction of 35% or less OR history of ventricular arrhythmia, syncope of undetermined origin, inherited structural heart disease, or arrhythmia syndrome should be referred to cardiology for further risk stratification and consideration of ICD therapy

Indications for implantable cardioverter-defibrillator therapy

The American College of Cardiology, American Heart Association, and Heart Rhythm Society (ACC/AHA/HRS) has developed joint guidelines regarding the evidence-based use of ICD therapy (summarized in Table I). Recommendations are considered secondary prevention in those who have survived a prior cardiac arrest or demonstrated spontaneous sustained ventricular tachycardia and primary prevention for those who have not yet experienced a clinically significant ventricular tachycardia but are considered to be at increased risk.

Table 1.

ACC/AHA/HRS guidelines for implantable cardioverter-defibrillator therapy
Primary prevention - ICD therapy 1. Ejection fraction less than or equal to 35% due to prior myocardial infarction (MI) who are >40 days post-MI and in NYHA functional class II or III (class I) 2. Ejection fraction less than or equal to 30% due to prior myocardial infarction who are > 40 days post-MI and NYHA functional class I (class I) 3. Ejection fraction less than or equal to 35% in patients with nonischemic dilated cardiomyopathy and are NYHA functional class II or III (class I) 4. Ejection fraction less than or equal to 35% in patients with nonischemic dilated cardiomyopathy and are NYHA functional class I (class IIB) 5. Ejection fraction less than 40% with non-sustained VT due to prior myocardial infarction and inducible VT or VF at electrophysiology study (class I) 6. Patients with underlying disorders at high risk for VT/VF (such as hypertrophic cardiomyopathy and 1 risk factor for SCD) (class II)
Secondary prevention - ICD therapy 1. Survivors of cardiac arrest due to VF or hemodynamically unstable VT after excluding reversible causes (class I) 2. Patients with structural heart disease and spontaneous sustained VT (class I) 3. Sustained VT with normal or near normal ventricular function (class IIA) 4. Syncope of undetermined origin with sustained VT or VF induced at electrophysiology study (class I) 5. Syncope of undetermined origin with significant LV dysfunction and non-ischemic dilated cardiomyopathy (class IIA)
Situations where ICD therapy is not recommended 1. Patients who do not have a reasonable expectation of survival with an acceptable functional status for at least 1 year 2. Incessant VT or VF 3. Patients with significant psychiatry illnesses that may be aggravated by device implantation or that may preclude follow-up 4. Syncope of undetermined cause in a patient without inducible VT and without structural heart disease 5. VT or VF that is amenable to catheter ablation in the absence of structural heart disease 6. VT or VF due to a completely reversible disorder in the absence of structural heart disease (such as electrolyte abnormality or drugs)

The majority of ICDs today are placed for primary prevention. Guidelines apply to patients who are receiving optimal medical therapy for at least three months (e.g., beta blocker and ACE inhibitor therapy) and have a reasonable expectation of survival with a good functional status for more than 1 year.

Knowing about ICDs versus Cardiovascular Resynchronization Therapy with Defibrillator (CRT-D):

CRT-D is also implantable defibrillator. An ICD can have one or two wires, called leads, where one lead goes in the right ventricle, and if a second lead is needed, it will be placed in the right atrium. A CRT-D system adds a third lead, attaching a lead to the left ventricle so it can help both sides beat in synchrony, and thus pump more efficiently. CRT-D has been shown to be more helpful in heart failure patients.

The evaluation of a patient with an implantable cardioverter-defibrillator

There are several important questions to ask patients who have an ICD to ensure appropriate care:

  • Type of device (ICD or cardiac resynchronization therapy with defibrillator)

  • Manufacturer of device

  • Indication for ICD placement (primary or secondary prevention)

  • Underlying cardiac disease (coronary artery disease, congenital heart disease, congenital arrhythmia)

  • Left ventricular ejection fraction and when this was last assessed

  • Underlying rhythm (is the patient pacemaker dependent)

  • Optimal medical therapy for heart failure

  • Symptoms such as palpitations, syncope, or heart failure symptoms

  • History of ICD shocks

    • Patients receiving ICD shocks are a population with an increased risk of heart failure hospitalizations and mortality and require more vigilant surveillance and optimization of heart failure management.

    • Appropriate electrophysiology evaluation is necessary after an ICD shock to identify the cause and reduce the incidence of future shocks. Patients experiencing multiple shocks or shocks associated with a change in clinical status (such as chest pain or decompensated heart failure symptoms) require urgent (same day) electrophysiology evaluation while patients experiencing a single shock without a change in clinical status should be evaluated within one week.

    • Shocks may be appropriate or inappropriate. Patients with an appropriate discharge in response to VT or VF should be assessed for reversible causes, electrolyte abnormalities, medications reviewed, potentially have their ICD programming optimized, and consideration made of anti-arrhythmic therapy or VT ablation if appropriate. An inappropriate discharge most often is due to a supraventricular tachyarrhythmia (90%) with atrial fibrillation being the most common although lead fracture and dislodgement should also be considered.

  • Timing of last device interrogation

    • Patients with ICDs must be followed on a regular basis, typically at three to six month intervals. The typical lifespan of an ICD generator is from five to nine years depending on the degree of pacemaker dependency and frequency of shocks delivered.

Approach to the hospitalized patient with an implantable cardioverter-defibrillator

  • Magnetic resonance imaging (MRI) compatibility

    • A common question is whether it is possible to perform a MRI. MRI is a source of electromagnetic interference (EMI), which is a signal that is within the frequency spectrum detectable by the sensing circuitry of the ICD. The risk of exposure to EMI includes inappropriate delivery of therapy, resetting of programmed parameters, failure to provide pacing, and a theoretical concern for heating the conductor coil resulting in direct injury to the heart. Due to these concerns the American Heart Association considers MRI use to be a relative contraindication in the general population. While there are published studies where MRI has been safely performed in centers with expertise in MR imaging and electrophysiology, cardiology consultation is certainly recommended if MRI is being considered in situations where the benefits are thought to outweigh the risk. At the time of this writing, there are no MRI-conditional ICD devices that have been approved for use by the US Food and Drug Administration.

  • Peri-operative management

    • Another common question is how to manage an ICD during an operative procedure. The concern here is also due to electromagnetic interference, caused by electrocautery. Prior to the procedure, the device should be interrogated and deactivated. During the time when the defibrillator therapy is not available, defibrillation paddles should be placed in the anteroposterior position at least four inches from the generator. Following the procedure, the device should be re-interrogated and re-activated.

IV. Common Pitfalls in implantable cardioverter-defibrillator use

One of the main pitfalls in the prevention of SCD with ICD therapy is the failure to recognize patients at risk for SCD, resulting in under-utilization of ICD placement among eligible patients. Studies have reported less than 50% of patients eligible for ICD placement undergo implantation even after accounting for patient preferences and co-morbid conditions. Women and African Americans have been found to have a disproportionately lower rate of ICD placement compared to their male and white counterparts (respectively).

Although many physicians may be aware that ACC/AHA/HRS guidelines on device placement exist, a study has shown that physician knowledge may not be sufficient enough to refer all individuals who would benefit from ICD therapy. For example, in one scenario a patient with a history of myocardial infarction one year ago and ejection fraction of 30% but no symptoms of heart failure is admitted to the hospital with a urinary tract infection. The patient is high risk for SCD and should be referred to cardiology for further discussion of ICD placement based on guidelines. However, too often this does not happen due to either lack of knowledge of the indication or possibly the perception of the general provider regarding his/her role in the referral process in a patient hospitalized for an unrelated diagnosis. Thus it is important to improve physician awareness of the current guidelines so that better and more equally distributed care can be provided to patients at risk for SCD.

V. National Standards, Core Indicators and Quality Measures.

The Joint Commission indicates that heart failure patients require that left ventricular systolic function be evaluated before arrival, during hospitalization, or is planned for after discharge. National guidelines advocate for the evaluation of LVEF as the most important diagnostic test in the management of all patients with heart failure.

What's the evidence?

Epstein, AE, DiMarco, JP, Ellenbogen, KA. "ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines". Circulation. vol. 117. 2008. pp. 350-4081.

(The published guidelines providing evidence-based recommendations for the appropriate use of ICDs.)

Moss, AJ, Hall, WJ, Cannom, DS. "Improved survival with an implanted defibrillator in patients with coronary disease at high risk for ventricular arrhythmia. Multicenter Automatic Defibrillator Implantation Trial Investigators". NEJM. vol. 335. 1996. pp. 1933-194.

(Landmark randomized trial demonstrating that ICD therapy can improve survival in patients with previous myocardial infarction, LVEF of 35% or less, nonsustained VT, and nonsuppressible VT on electrophysiology study compared to conventional medical therapy alone.)

Moss, AJ, Zareba, W, Hall, WJ. "Multicenter Automatic Debrillator Implantation Trial II Investigators. Prophylactic implantation of a defibrillator in patients with myocardial infarction and reduced ejection fraction". NEJM. vol. 346. 2002. pp. 877-83.

(Randomized trial demonstrating that ICD therapy can improve mortality in patients with previous myocardial infarction and LVEF of 30% or less compared to conventional medical therapy.)

Brady, GH, Lee, KL, Mark, DB. "Sudden Cardiac Death in Heart Failure (SCD-HeFT) Investigators. Amiodarone or an implantable cardioverter-defibrillator for congestive heart failure". NEJM. vol. 352. 2005. pp. 225-37.

(Randomized trial demonstrating that ICD therapy can improve mortality in patients with either ischemic or non-ischemic cardiomyopathy with an LVEF of 35% or less and NYHA class II or III symptoms compared to conventional medical therapy.)

Hohnloser, SH, Kuck, KH, Dorian, P. "DINAMIT Investigators: Prophylactic use of an implantable cardioverter-defibrillator after acute myocardial infarction". N Engl J Med. vol. 251. 2004. pp. 2481.

(Randomized trial demonstrating that ICD therapy in patients who were between 6 and 40 days out from a myocardial infarction, with an LVEF of 35% or less, and impaired cardiac autonomic function [depressed heart rate variability or increased mean 24 hour heart rate] did not improve overall mortality. While ICD therapy was associated with a reduction in the death rate due to arrhythmia, there was an offset increase in the death rate due to non-arrhythmia causes such as heart failure.)

Levine, GN, Gomes, AS, Arai, AE. "AHA Scientific statement. Safety of Magnetic Resonance Imaging in patients with cardiovascular devices". Circulation. vol. 116. 2007. pp. 2878-2891.

(Scientific statement summarizing the safety issues of MRI in patients with cardiac devices with a general recommendation against the routine use of MRI in patients with an ICD while recognizing that there may be individual cases where the diagnostic benefit outweighs the risk and what can be done to minimize risk in this situation.)

Gehi, AK, Mehta, D, Gomes, JA. "Evaluation and management of patients after implantable cardioverter-defibrillator shock". JAMA. vol. 296. 2006. pp. 2839-47.

(Review article aimed at the non-electrophysiologist to provide a basic understanding of the evaluation for the patient reporting an ICD shock.)

http://www.hopkinsmedicine.org/healthlibrary/conditions/cardiovascular_diseases/overview_of_pacemakers_and_implantable_cardioverter_defibrillators_icds_85,P00234/.

Alan, Kadish, Mandeep, Mehra. "Heart Failure Devices: Implantable Cardioverter-Defibrillators and Biventricular Pacing Therapy". Circulation. vol. 111. 2005. pp. 3327-3335.

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