Implantable cardioverter defibrillators
An implantable cardioverter defibrillator (ICD) is a device that is placed subcutaneously or submuscularly, with leads that are positioned within the heart (or, more recently, subcutaneously). In contrast to a pacemaker that primarily delivers pacing stimuli to treat bradyarrhythmias, the primary purpose of an ICD is to prevent tachyarrhythmic death due to ventricular tachycardia (VT) or ventricular fibrillation (VF). This is done by continuously monitoring the heart rhythm and delivering antitachycardia pacing (ATP) stimuli or shocks to terminate VT with ATP or VT/VF with shocks. Current ICDs incorporate fully functional pacing support, thereby also treating bradyarrhythmias, including asystolic pauses that can result from shock delivery. Because of the need for capacitors that store large amounts of electrical energy necessary to defibrillate, an ICD is larger than a pacemaker.
The implant procedure is similar to that for a pacemaker, except for the size of the device and the need for special leads that incorporate defibrillation coils along the body of the leads. The typical device has one to three transvenous leads placed, including a pace/sense defibrillation lead in the right ventricle (RV). Dual-chamber ICDs also have a pacing/sensing lead in the right atrium. Devices that incorporate cardiac resynchronization therapy (CRT) have a lead placed in a ventricular branch of the coronary sinus to stimulate the left ventricle (LV) or a lead placed on the LV epicardial surface.
The device senses the ventricular rhythm through the right ventricular lead and then can rapidly pace or deliver a shock to defibrillate or cardiovert. The shock configuration includes the defibrillation coils on the transvenous lead(s) and often the ICD pulse generator (“can”) itself. Fortunately, modern implantable defibrillators use a biphasic shock, and this is highly effective to stop VT and VF, but for patients who have higher energy requirements for termination of their ventricular arrhythmias or who have older systems with epicardially placed defibrillation patches, additional shocking electrode hardware may include subcutaneous coils or patches and separate coils in the superior vena cava/brachiocephalic vein or azygos vein. The atrial lead (or a sensing electrode in the right atrium) is used to detect atrial electrical activity and pace the atria if necessary. It can help detect and discriminate atrial from ventricular tachyarrhythmias (via automatic detection algorithms or via manual analysis of stored data), thus helping prevent “inappropriate shocks” (i.e., shocks not given to stop otherwise sustained ventricular tachyarrhythmias). The LV lead is used for purposes of cardiac resynchronization to improve ventricular function and symptoms in patients with heart failure who have electrical dyssynchrony due to conduction system abnormalities.
The ICD is multiprogrammable. A series of tachycardia “zones” can be programmed to detect VF and VTs of varying rates. Most devices can be programmed from one to three zones. A slower zone can be programmed “on” for purposes of arrhythmia detection only, but generally zones are programmed to determine the type of therapy that will be delivered. The fastest zone is considered a VF zone. Rate alone is considered the main criterion for arrhythmia “diagnosis,” but a certain number of beats or arrhythmia duration need to be satisfied before therapy is actually delivered.
The way the ICD works is as follows: After tachyarrhythmia detection criteria are satisfied, the capacitors are charged, and the device delivers a shock ( Figs. 8.1 and 8.2 ). Most modern devices take a second look (reevaluation of the heart rhythm) after charging up for a shock (“noncommitted”) to avoid delivering a shock if the arrhythmia is not sustained. The shock energy is programmable and in some instances can exceed 40 J. The shock waveform is biphasic; biphasic shocks need less energy to terminate ventricular arrhythmias than monophasic shocks and are thus more effective. Most modern devices can deliver four to six consecutive shocks as needed before being required to see a normal rhythm and reattempting to shock. ICDs also have the capability to pace for slow and for fast rhythms.
The subcutaneous ICD (SICD) is not a leadless ICD but has leads placed subcutaneously with the ICD. It is larger than the transvenous ICD and cannot detect atrial activity. Nevertheless, it is highly effective with specific algorithms to discriminate atrial from ventricular arrhythmias. It cannot pace the heart for long periods of time with present technology.
ICDs can be programmed in a manner similar to pacemaker programming to deliver pacing stimuli for bradyarrhythmias, but they can also overdrive pace (ATP) in attempts to terminate VT. Some devices are capable of attempting to pace terminate VT while charging to deliver a defibrillating shock if necessary. Some ICDs can also deliver ATP to treat atrial arrhythmias. The time required to detect a tachycardia and the characteristics of the atrial and ventricular relationships are programmable for each tachycardia zone to help discriminate SVT from VT.
ICDs are not perfect in discriminating SVT from VT simply by rate criteria or in determining whether an arrhythmia is nonsustained. As such, some shocks are “inappropriate”—that is, given for non-life-threatening SVTs, for sinus tachycardia faster than the programmed rate criterion, for self-terminating VTs, or for electrical noise (e.g., for noise from a fractured lead; Figs. 8.3 , 8.4 , and 8.5 ). Inappropriate shocks may occur in up to one-third of patients receiving ICDs. To minimize the occurrence of inappropriate shocks, various algorithms have been used to discriminate atrial fibrillation (AF), atrial flutter (AFL), sinus tachycardia, or other SVT from VT. These discriminating algorithms commonly use specific rhythm characteristics, such as atrial and ventricular relationships, irregularity of the rhythm, suddenness of onset, or electrogram morphology. The atrial and ventricular relationships may help discriminate 1:1 A-V relationships (as may be seen in SVT) from V-A dissociation (as may be seen in VT). Irregularity of the intervals between beats is used to distinguish AF from the more regular ventricular tachyarrhythmia. Sudden-onset criteria are used to discriminate gradual-onset sinus tachycardia from a sudden-onset VT characterized by an abrupt increase or jump in rate. Some ICDs have special template-matching algorithms to assess the ventricular electrogram morphology and help discriminate atrial from ventricular rhythms.
Although programming of discrimination algorithms is intended to improve the specificity of shocks, inherent to this strategy is a potential trade-off in sensitivity for true life-threatening ventricular arrhythmias. As the goal of ICD therapy is to treat all life-threatening arrhythmias, and as the consequences of missing out on treating one of these true arrhythmias could be sudden death, the VF rate zone may be programmed without many SVT discriminators, which are reserved for slower VT detection zones. Another strategy is to defer programming of discriminators unless there is a clinical occurrence or history of supraventricular arrhythmias causing inappropriate shocks, or rapid rates that could overlap with a VT zone. Other strategies may also use longer detection intervals. Because devices are multiprogrammable, each device can be tailored to a specific patient’s needs and clinical arrhythmias.
Other functions included in ICDs are automatic capacitor reforms and advanced recordings and diagnostics that generally include the capability of transtelephonic monitoring to send information to clinicians caring for the patient. The average battery life of an ICD is 5 to 10 years. The devices can now automatically maintain the capacitors regularly and can monitor lead functional parameters (e.g., impedance) and intracardiac electrogram characteristics (e.g., ventricular and atrial electrogram size). Modern ICDs can allow this information to be downloaded via a programmer or remotely via an external device that communicates with the ICD and transmits information to a central station. Remote monitoring is highly valuable, as it can provide the clinician with continuous information, should there be failure of a device or a component, such as a lead. Furthermore, it can provide information about the frequency of ventricular pacing and sensing.
Device interrogation can be performed with a wand placed over the device or remotely transtelephonically to determine if a tachyarrhythmia occurred, what type of therapy was delivered, and whether the therapy was successful in terminating the arrhythmia. In addition, interrogation can provide the frequency and duration (“burden”) of atrial arrhythmias, such as AF. It can also provide the heart rate and rate variation over time. Device interrogation is useful to determine the need for pacing and the frequency of pacing at different programmed rates. The relationship between pacing timing is adjustable. These adjustments include lower (base) rate; rate responsiveness; upper tracking rate at which the ventricles are paced in 1:1 relationship to atrial activity; an upper sensor-based rate, if necessary; atrial and ventricular timing relationships; and left and right ventricular pace timing relationships in biventricular devices. Most devices also contain the ability to turn off atrial tracking should an SVT such as AF or AFL occur within a certain rate zone. Magnet application will inactivate tachycardia detection for most ICDs but will have no effect on the antibradycardia function of the pacemaker. ICDs have specific programming characteristics depending on the type, manufacturer, and model, and knowledge of the peculiarities of each device is mandatory to ensure proper understanding of device function and programming.
Implantable Cardioverter Defibrillator Indications
Indications for ICD implantation can be categorized into indications for primary versus secondary prevention of sudden cardiac death (SCD). Current guideline recommendations for ICD indications are summarized in Table 8.1 and Charts 8.1 to 8.5 . An algorithm for primary prevention of SCD is shown in Algorithm 8.1 and for secondary prevention of SCD in Algorithm 8.2 . There are many patients who were not well represented in randomized clinical trials and who are thus not covered by guideline recommendations. These include patients within 40 days of myocardial infarction ( Algorithm 8.3 ), 90 days of coronary revascularization ( Algorithm 8.4 ), and 3 to 9 months of newly diagnosed cardiomyopathy ( Algorithm 8.5 ).
