Implantable Cardiac Defibrillator

CHAPTER 39

IMPLANTABLE CARDIAC DEFIBRILLATOR Paul Schurmann, Moises Rodriguez-Manero

1. What are the components of an implantable cardioverter defibrillator? An implantable cardioverter defibrillator (ICD) is composed of a pulse generator (typically implanted in the left upper chest) and one or more leads. The generator uses lithium-vanadium batteries, which are reliable energy sources with predictable discharge curves. An ICD is able to deliver a charge larger than its battery voltage because of a system of internal capacitors. Since many patients in whom an ICD is indicated are also candidates for cardiac resynchronization therapy, many current ICDs also have biventricular pacing capabilities (Fig. 39.1).  2. What are the types of implantable cardioverter defibrillators? There are two types of ICDs: • Transvenous ICDs (having one or multiple leads, with a transvenous lead attached directly to the heart) • Subcutaneous ICDs (S-ICDs) (with only one subcutaneous lead) Transvenous ICDs are commonly used and can work as pacemakers. The transvenous lead contains two distal electrodes (tip and ring) that sense local electrical signals or are paced. The right ventricular lead is essential for defibrillators, but additional leads may be used for atrial and biventricular pacing.

SVC shock coil

Atrial pacing electrodes

CS (LV) pacing electrode

RV shock coil and pacing “ring” electrode RV pacing electrode Fig. 39.1.  A radiograph of a patient with an implantable cardioverter defibrillator (ICD). With this ICD, shock coils are present in both the right ventricle (RV) and the superior vena cava (SVC). There is also a coronary sinus (CS) lead present for biventricular pacing. LV, left ventricle. (From Miller, R., Eriksson, L., Fleisher, L., Wiener-Kronish, J. P., Cohen, N. H., & Young, W. L. [2009]. Miller’s anesthesia [7th ed.]. Philadelphia, PA: Churchill Livingstone.)

352

Chapter 39  Implantable Cardiac Defibrillator  353 The right ventricular lead differs from a pacemaker owing to its ability to deliver high-energy shocks. ICDs may contain one or two coils (located in the right ventricle and the superior vena cava) specifically designed to create a circuit to capture most of the myocardium with a high-energy shock.  3. How does the implantable cardioverter defibrillator work? The ICD is always sensing the electrical signals of the heart and processes them to classify the rhythm. The arrhythmia criteria are met by counting intervals between successive electrograms. If the detected arrhythmia meets the criteria for ventricular tachycardia (VT), the ICD often first tries to restore a normal rhythm by pacing the ventricle faster than the intrinsic rhythm, which is known as antitachycardia pacing (ATP). If the VT is not terminated with ATP, the ICD sends a high-energy shock to the heart to restore a normal rhythm. The device then goes back to its watchful mode. The criteria to detect arrhythmia and therapies can be reprogrammed based on the patient’s clinical needs. The criteria for arrhythmia are based on beat-to-beat intervals and usually consist of three zones: a monitor zone, a VT zone, and a ventricular fibrillation (VF) zone. The monitor zone usually captures the slowest arrhythmia of all three zones. Tachycardia is only recorded and no therapies are delivered. Shorter RR intervals in the arrhythmia would fall in the VT zone and even shorter RR intervals (corresponding to faster heart rates) in the VF zone. Once the arrhythmia is classified into a zone, algorithms can often distinguish whether the rhythm is more likely to be VT, atrial fibrillation with rapid ventricular response, or another supraventricular rhythm (or sinus tachycardia). Criteria used by the algorithms to distinguish VT from supraventricular tachycardia (SVT, or sinus tachycardia) include abrupt versus gradual tachycardia onset, regularity or irregularity of the rhythm, QRS morphology, and QRS vector (Fig. 39.2). If the ICD algorithms determine that the rhythm is ventricular in origin, the programmed therapy for the selected zone will be delivered. Once therapy is delivered, the ICD monitors the following intervals to redetect sinus rate (which means the therapy was successful) or redetect the arrhythmia (which results in additional therapy).  4. How does implantable cardioverter defibrillator therapy improve survival? ICD therapy, compared with conventional or traditional antiarrhythmic drug therapy, has been associated with reductions in mortality of 23% to 55%, with the improvement in survival due almost exclusively to a reduction in sudden cardiac death (SCD). ICD defibrillation can terminate VT, torsades de pointes, and VF (Figs. 39.3 and 39.4). Trials of ICDs are categorized into two types: •  Primary prevention trials, in which the subjects have not experienced life-threatening sustained VT, VF, or resuscitated cardiac arrest but are at risk Table 39.1 •  Secondary prevention trials, involving subjects who have had an abortive cardiac arrest, a lifethreatening VT, or VT as the cause of syncope 

NSR vector

SVT vector

VT vector

Fig. 39.2.  Demonstration of an ICD rhythm-discrimination algorithm based on the vector of the QRS complex. Black arrow are the QRS vector of Normal sinus rhythm and SVT. Blue arrow shows the difference vector from VT. NSR, normal sinus rhythm; SVT, supraventricular; VT, ventricular. (Image from Seifert M. Tachycardia discrimination algorithms in ICDs. In D. Erkapic & T. Bauernfeind [Eds.], Cardiac defibrillation. Heart. 2007 Nov; 93(11): 1478–1483. doi: 10.1136/ hrt.2006.095190intechweb.org.)

354  PART V  ARRHYTHMIAS 5. What are the current class I indications for implantable cardioverter defibrillator implantation for primary prevention of sudden cardiac death? Assuming that patients are receiving guideline-directed medical therapy and have a reasonable expectation of survival with good functional status for more than 1 year, class I indications are as follows: • Patients with left ventricular (LV) ejection fraction (LVEF) less than 35% as a result of prior myocardial infarction (MI) who are at least 40 days post MI and are in New York Heart Association (NYHA) functional class II or III Shock

Fig. 39.3.  Example of successful ICD shock in a patient with torsades de pointes. (Image from Rosenheck, S. Defibrillation shock amplitude, location and timing. In J. J. Harris [Ed.], Cardiac defibrillation—prediction, prevention and management of cardiovascular arrhythmic events. intechweb.org.)

I II III aVR aVL

V1 V3 V4 V5

Fig. 39.4.  An example of ICD termination of ventricular fibrillation with an appropriately delivered shock. (Image from Rosenheck, S. Defibrillation shock amplitude, location and timing. In J. J. Harris [Ed.], Cardiac defibrillation—prediction, prevention and management of cardiovascular arrhythmic events. intechweb.org.)

Chapter 39  Implantable Cardiac Defibrillator  355

• Patients with LV dysfunction as a result of prior MI who are at least 40 days post MI, have an LVEF less than 30%, and are in NYHA functional class I • Patients with nonischemic dilated cardiomyopathy (DCM) who have an LVEF of 35% or less and are in NYHA functional class II or III • Patients with nonsustained VT as a result of prior MI, LVEF less than 40%, and inducible VF or sustained VT at electrophysiologic study 

6. What are the current class I indications for implantable cardioverter defibrillator implantation for secondary prevention of sudden cardiac death? ICDs are indicated for secondary prevention of SCD in patients who • Survive sudden cardiac arrest without completely reversible causes • Have structural heart disease with spontaneous sustained VT • Have syncope of unexplained etiology with clinically relevant inducible sustained VT or VF on electrophysiologic study  7. What other special populations may benefit from implantable cardioverter defibrillator therapy? ICD implantation is performed in other patient populations and those at high risk for SCD including patients with • Hypertrophic cardiomyopathy (HCM) with one or more risk factors for SCD (VF, VT, family history of SCD, unexplained syncope, LV thickness 30 mm or more, abnormal blood pressure response to exercise) • Brugada syndrome with a history of previous cardiac arrest, documented sustained VT, or unexplained syncope • Nonhospitalized patients awaiting transplantation • Noncompaction of the left ventricle • Long and short QT syndromes with previous cardiac arrest or unexplained syncope • Catecholaminergic polymorphic VT who have syncope and/or VT while receiving beta-blockers • Arrhythmogenic right ventricular dysplasia/cardiomyopathy who have one or more risk factors for SCD • Infiltrative cardiomyopathies such as sarcoidosis, amyloidosis, Fabry disease, hemochromatosis, giant cell myocarditis, or Chagas disease • Certain muscular dystrophies  8. What is defibrillator threshold testing? The defibrillation threshold (DFT) is defined as the amount of energy required to reliably defibrillate the heart during an arrhythmia. During implant, the electrophysiologist may elect to perform DFT testing, in which VF is induced, detected, and then terminated by the device with a high-energy shock to find the reliable level of energy needed to achieve defibrillation. Defibrillation efficacy has traditionally been initiated with successive DFT attempts using approximately 20 J in a device that delivered 25 to 30 J. A safety margin of 10 J was required for the implantation of the earliest defibrillators. Because DFT testing involves inducing potentially fatal VF several times, it requires deep conscious sedation. In addition, lack of evidence for routine use has led some electrophysiologists to stop performing DFT testing at implantation and instead to program the device to the highest output settings.  9. What is antitachycardia pacing? Antitachycardia pacing (ATP) has long been recognized as a way to pace-terminate certain types of arrhythmias, particularly slow monomorphic VT involving a reentry circuit. The idea is to deliver a few seconds of pacing stimuli to the heart at a rate faster than the tachycardia. The basic principle is that in most reentrant circuits there is an excitable gap—that is, a time between successive activations when the myocardium is available to respond to excitations. Pacing in a reentrant circuit during the excitable gap introduces new activation wavefronts that collide with one of the preexisting tachycardias and can terminate it. Advantages offered by ATP include the following: • ATP is not painful (sometimes barely noticed by some patients). • ATP may reduce battery drainage by terminating the arrhythmia without the need for shocks. • ATP is an easily programmable feature on most ICDs. On the other hand, ATP has potential disadvantages. If ineffective, it can delay defibrillation therapies and prolong the time during which the patient is in tachycardia, which may lead to syncope. ATP can also accelerate VT into faster rhythms and even VF.

356  PART V  ARRHYTHMIAS

Fig. 39.5.  Schematic example of how antitachycardia pacing (overdrive pacing) disrupts the reentrant circuit in the ventricle, terminating the arrhythmia.

VT

Burst pacing

Fig. 39.6.  Intracardiac tracing showing burst pacing (overdrive pacing or antitachycardia pacing) terminating an episode of ventricular tachycardia. (Image from Raatikainen, M. J. P., & Koivisto, U. M. Remote monitoring of implantable cardioverterdefibrillator therapy. In N. Trayanova [Ed.], Cardiac defibrillation—mechanisms, challenges and implications. intechweb .org.)

For this reason ATP is mostly used in patients who remain stable and asymptomatic during episodes of slow VT, generally below 200 beats per minute. However, more aggressive ATP programming has been shown to decrease ICD shocks, and newer ICDs incorporate ATP while charging for a shock. A schematic example of ATP is shown in Fig. 39.5. An actual intracardiac tracing showing burst pacing terminating VT is shown in Fig. 39.6.  10. How common are inappropriate shocks in patients with implantable cardioverter defibrillators? Inappropriate shocks occur when a device delivers therapy for a fast supraventricular rhythm or for abnormal sensing in the ventricle. They occur in up to 11% of patients with ICDs and can constitute up to a third of all shock episodes a patient experiences. The most common rhythm triggering an inappropriate shock is atrial fibrillation with rapid ventricular response. Other inappropriate shocks can result from other SVTs, including sinus tachycardia. Smoking, atrial fibrillation, diastolic hypertension, young age, nonischemic cardiomyopathy, and prior appropriate shocks increase the chance of receiving an inappropriate shock. Inappropriate shock can also result from what is called ventricular oversensing by the ICD. One example of this is when the ICD mistakes the T waves for QRS complexes, so that the ICD mistakenly calculates the ventricular rate at twice the actual rate. T-wave oversensing remains the most frequent cause of ventricular oversensing and has been reported in up to 14% of patients with ICDs. Ventricular oversensing is associated with increased mortality in patients with ICDs and has important psychological effects in this population. ICD programming options can be used to reduce oversensing.  11. What is a subcutaneous defibrillator? A totally subcutaneous defibrillator (S-ICD) is a recently developed device that is capable of providing the same proven defibrillation protection as a transvenous ICD but without the complications associated with the presence of transvenous endocardial leads. The S-ICD electrode is implanted in the parasternal area in the subcutaneous space and then connected to an active high voltage can (Fig. 39.7). Defibrillation is delivered between the coil on the electrode and the active can. Sensing is accomplished using both or either of the proximal and distal ring electrodes and the electrically conductive pulse generator enclosure. An S-ICD eliminates problems such as failure to achieve vascular access, intravascular injury, or intracardiac injury and makes it possible to avoid radiation

Chapter 39  Implantable Cardiac Defibrillator  357

Fig. 39.7.  A subcutaneous ICD.

Fig. 39.8.  A wearable ICD.

exposure. The disadvantage of the S-ICD is the limited pacing capability up to 30 seconds after a shock at a rate of 50 beats per minute and the lack of ATP.  12. What is a wearable implantable cardioverter defibrillator? A wearable ICD system (also referred to as a LifeVest) is a vest-like system worn by the patient that can deliver defibrillation (Fig. 39.8). The wearable ICD has three defibrillation and four electrocardiographic (ECG) sensing electrodes fitted within a garment worn by the patient. The defibrillation electrodes are self-gelling, and the ECG electrodes are nonadhesive dry tantalum oxide capacitive electrodes. The defibrillator unit is carried on a waist belt. Two ECG channels can be monitored with the two pairs of ECG electrodes from front-to-back and right-to-left lead sets. Microampere alternating current is used to check electrode contacts, as in conventional monitoring systems.

358  PART V  ARRHYTHMIAS Table 39.1.  Primary prevention for sudden cardiac death with implantable cardiac defibrillator TRIAL

Mortality (%) # OF AGE LVEF FOLLOW-UP CONTROL PTS (YEARS) (%) (MONTHS) THERAPY CONTROL ICD P

MUSTT 704 (Multicentre unstable tachycardia trial)

67 + 12

MADIT 196 63 + 9 (Multicentre automatic defibrillator implantation Trial) DEFINITE 458 58 (Defibrillators in nonischemic cardiomyopathy treatment evaluation) ACD-HeFT 2521 60.1 (Sudden cardiac death in heart failure)

30

39

No EP-guided therapy

48

24

.06

26

27

Conventional

38.6

15.7 .009

21

29.0 + 14.4

Optimal medical therapy

14.1

7.9 .08

25

45.5

Optimal medical therapy

36.1

28.9 .007

EP, Electrophysiologic; ICD, implantable cardioverter defibrillator; LVEF, left ventricular ejection fraction.

Once the programmed detection parameters are fulfilled, a sequence of alarms is initiated, starting with vibration in the belt electronics, followed by low- and high-volume two-tone alarms, and finishing with a voice warning to bystanders that a shock may be delivered. The patient can press the response buttons within 20 seconds to withhold the capacitor discharge in cases where he or she is not symptomatic. If the response button is not pressed, an impedance-adapted biphasic truncated exponential shock is delivered. The time elapsing between onset of the tachycardia and shock delivery is 45 to 55 seconds. The LifeVest is able to deliver up to five successive shocks in case the arrhythmia continues after the first shock. At present no study has convincingly demonstrated that utilization of a wearable ICD reduces mortality. Nevertheless there are certain situations in which some practitioners consider the use of prophylaxis with this device. An American Heart Association (AHA) scientific advisory concluded that the use of a wearable ICD is reasonable when there is a clear indication for an implanted permanent device but a transient contraindication to implantation and that use of these devices might be reasonable in patients at risk for SCD but in whom ventricular function may improve after medical therapy or revascularization. Bibliography and Suggested Readings Bardy, G. H., Lee, K. L., Mark, D. B., Poole, J. E., Packer, D. L., Boineau, R., et al. (2005). Amiodarone or an implantable cardioverter-defibrillator for congestive heart failure. New England Journal of Medicine, 352, 225–237. Bardy, G. H., Smith, W. M., Hood, M. A., Crozier, I. G., Melton, I. C., Jordaens, L., et al. (2010). An entirely subcutaneous implantable cardioverter-defibrillator. New England Journal of Medicine, 363, 36–44.

Chapter 39  Implantable Cardiac Defibrillator  359 Daubert, J. P., Zareba, W., Cannom, D. S., McNitt, S., Rosero, S. Z., Wang, P., et al. (2008). Inappropriate implantable cardioverter-defibrillators shocks in the MADIT II study: frequency, mechanisms, predictors, and survival impact. Journal of the American College of Cardiology, 51, 1357–1365. DiMarco, J. P. (2003). Implantable cardioverter-defibrillators. New England Journal of Medicine, 349, 1836–1847. Epstein, A. E., DiMarco, J. P., Ellenbogen, K. A., Mark Estes, N. A., Freedman, R. A., Gettes, L. S., et al. (2008). ACC/AHA/HRS 2008 guidelines for device-based therapy of cardiac rhythm abnormalities: executive summary: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Revise the ACC/AHA/NASPE 2002 Guideline Update for Implantation of Cardiac Pacemakers and Antiarrhythmia Devices). Journal of the American College of Cardiology, 51, 2085–2105. Hohnloser, S. H., Kuck, K. H., Dorian, P., Roberts, R. S., Hampton, J. R., Hatala, R., et al. (2004). Prophylactic use of an implantable cardioverter-defibrillator after acute myocardial infarction. New England Journal of Medicine, 351, 2481–2488. Kadish, A., Dyer, A., Daubert, J. P., Quigg, R., Estes, N. A., Anderson, K. P., et al. (2004). Prophylactic defibrillator implantation in patients with nonischemic dilated cardiomyopathy. New England Journal of Medicine, 350, 2151–2158. Moss, A. J., Zareba, W., Hall, W. J., Klein, H., Wilber, D. J., Cannom, D. S., et al. (2002). Prophylactic implantation of a defibrillator in patients with myocardial infarction and reduced ejection fraction. New England Journal of Medicine, 346, 877–883. Piccini, J. P., Sr., Allen, L. A., Kudenchuk, P. J., Page, R. L., Patel, M. R., & Turakhia, M. P. (2016). Wearable cardioverter-­ defibrillator therapy for the prevention of sudden cardiac death: a science advisory from the American Heart Association. Circulation, 133, 1715–1727. http://dx.doi.org/10.1161/CIR.0000000000000394.