Jose L. Baez-Escudero, MD and Miguel Valderrábano, MD

1. What are the components of an implantable cardioverter-defibrillator (ICD) system?

    ICDs are composed of a pulse generator (typically implanted in the left pectoral region) and one or more intracardiac leads. Typically the right ventricular (RV) lead contains two distal electrodes (tip and ring) that are used to sense local electrical ventricular signals and to pace if necessary. This lead also contains one or two defibrillation coils (one sits in the RV and the other in the superior vena cava) specifically designed to deliver high voltages. Additional leads may include an atrial lead (for pacing, sensing, and arrhythmia discrimination) or a left ventricular (LV) lead for cardiac resynchronization (Fig. 38-1). All transvenous defibrillators are also capable of pacemaker functions.

2. How does an ICD deliver shocking energy?

    ICDs use lithium-vanadium batteries that 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. These hold a charge and then release it all at once when it is needed. Shocking energy travels around a circular circuit, usually formed between the defibrillator lead coils and the device titanium-housed can.

3. How does the ICD detect and define a rhythm?

    The ICD collects intracardiac electrical signals (a process called sensing) and then processes them with dedicated software to classify the rhythm (a process called detection). The ICD identifies an arrhythmia by counting intervals between successive electrograms sensed by the intracardiac leads. In the ventricular lead, beat-to-beat intervals are counted and assigned into a category, usually called a zone. For example, normal sinus rhythm is expected to be slower than approximately 160 beats/min, which corresponds to beat-to-beat intervals longer than 375 ms. Shorter intervals would fall in the ventricular tachycardia (VT) zone and even shorter ones in the ventricular fibrillation (VF) zone. The ICD diagnoses an abnormally fast rhythm when a sufficient number of consecutive beat-to-beat intervals fall into either the VT or VF zone. This rate-based detection scheme can be adjusted to meet the individual patient’s needs by programming for different categories and therapies (e.g., antitachycardia pacing as appropriate therapy for slow VT). Also, detection algorithms can take into account onset of tachycardia (abrupt in VT, versus gradual in sinus tachycardia), stability (regular in VT, versus irregular in atrial fibrillation), the relationship with the atrial activation timings, and even the morphology of the ventricular signal, to maximize accuracy of detection and minimize inappropriate therapies. After 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 ICD therapy improve survival?

    ICD therapy, compared with conventional or traditional antiarrhythmic drug therapy, has been associated with mortality reductions from 23% to 55%, depending on the risk group participating in each trial, with the improvement in survival due almost exclusively to a reduction in sudden cardiac death (SCD). The trials are subcategorized into two types: primary prevention (prophylactic) trials, in which the subjects have not experienced life-threatening sustained VT, VF, or resuscitated cardiac arrest but are at risk; and secondary prevention trials, involving subjects who have had an abortive cardiac arrest, a life-threatening VT, or ventricular tachyarrhythmia as the cause of syncope.

5. What are the key clinical trials that evaluated the benefit of ICD for primary prevention?

    Clinical trials have evaluated the risks and benefits of ICDs in prevention of sudden death and have improved survival in multiple patient populations, including those with prior myocardial infarction (MI) and heart failure caused by either coronary artery disease or nonischemic dilated cardiomyopathy (DCM). Table 38-1 summarizes the important trials that evaluated the mortality benefit of ICDs for primary prevention.

6. What are the current class I indications for ICD implantation for primary prevention of SCD?

    Assuming patients are receiving chronic optimal medical therapy and have a reasonable expectation of survival with good functional status for more than 1 year, the following are the current indications for ICD therapy:

7. What are the current class I indications for ICD implantation for secondary prevention of SCD?

    Secondary prevention refers to prevention of SCD in those patients who have survived a prior sudden cardiac arrest or sustained VT. Evidence from multiple randomized controlled trials supports the use of ICDs for secondary prevention of SCD regardless of the type of underlying structural heart disease. In patients resuscitated from cardiac arrest, the ICD is associated with clinically and statistically significant reductions in SCD total mortality compared with antiarrhythmic drug therapy in prospective randomized controlled trials. Hence, ICDs are indicated for secondary prevention of SCD in patients who survived VF or hemodynamically unstable VT or experienced VT with syncope and have an LVEF 40% or less, after evaluation to define the cause of the event and to exclude any completely reversible causes.

8. What other special populations may also benefit from ICD therapy?

    Certain patients with less common forms of cardiomyopathy, as well as patients with inherited conditions who share genetically determined susceptibility to VT and SCD, also benefit from ICD therapy (especially when a history of arrhythmia or syncope is present, or for primary prevention in patients with a very strong family history of early mortality). These patients meet class II indications for ICD implantation that are outlined in the current guidelines in detail, including the following:

9. 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 in a step-down sequence to find the minimal reliable level of energy needed to achieve defibrillation. Defibrillation efficacy traditionally has been performed with successive attempts to determine the DFT using approximately 20 J in a device that could deliver 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 and requires deep conscious sedation, some electrophysiologists forego DFT testing at implant and program the device to the highest output settings. Additionally, an ICD can be potentially tested without inducing VF, using upper limit of vulnerability (ULV) testing.

10. 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 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 wave fronts that collide with the one of the preexisting tachycardia and can terminate it. Advantages offered by ATP include the following:

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.

For this reason, it is mostly used in patients that remain stable and asymptomatic during episodes of slow VT, generally below 200 beats/min. However, more aggressive ATP programming has been shown to decrease ICD shocks, and newer ICDs incorporate ATP while charging for a shock.

11. How common are inappropriate shocks in patients with ICDs?

    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, followed by other supraventricular tachycardias (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. They are associated with increased mortality in patients with ICDs and have important psychologic effects in this population. ICD programming options currently available should be used to reduce the risk of inappropriate shocks.

12. What is the totally subcutaneous defibrillator?

    The totally subcutaneous ICD (S-ICD) is a newly developed device that is capable of providing the same proven defibrillation protection as conventional ICDs, but without the serious complications associated with the presence of a transvenous endocardial defibrillator lead. The S-ICD electrode is implanted in the parasternal area in the subcutaneous space and then connected to an active high-voltage can. Defibrillation is delivered between the coil on the electrode and the active can. Sensing is accomplished using both or any one of the proximal and distal ring electrodes and the electrically conductive pulse generator enclosure. This device may potentially reduce or eliminate problems such as failure to achieve vascular access, intravascular injury, and lead failure requiring difficult procedures for extraction and replacement. Additional potential benefits of such a device include the preservation of venous access for other uses and the avoidance of radiation exposure during fluoroscopy, which is required for transvenous ICD implantation. These benefits would be especially important for young patients with no pacing indications, in whom leads may fail during the decades that therapy is needed.

13. What are ICD lead failures?

    Although conceptually simple, ICD leads are complicated devices with manufacturer-specific designs that vary from model to model. These design differences may include variations in insulation, cable/conductor, length, diameter, and fixation mechanism. Over time, ICD leads can fail due to several mechanisms, most of them related to lead design and time after implant. The clinical presentation of ICD lead failure varies. A lead failure is rare and most of the time can be diagnosed through routine monitoring of the ICD. A high impedance is a frequent presentation consistent with a lead fracture. However, lead failures can result in significant clinical events, including failure to pace and/or defibrillate, inappropriate shocks, and even death. Most lead failures are managed with lead extraction and replacement.

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