Implantable Cardioverter-Defibrillators

Chapter 38


Implantable Cardioverter-Defibrillators





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:


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Jun 5, 2016 | Posted by in CARDIOLOGY | Comments Off on Implantable Cardioverter-Defibrillators

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