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.
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:
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 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.
Indications of ICD according to American College of Cardiology/American Heart Rhythm Society (ACC/AHRS) 2008:
Class I in patients having any of the following:
Contraindications include the following:
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 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:
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.
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:
Detection is usually defined according to the ability of the ICD to detect the rate of the arrhythmia.
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).
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.
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.