Implantable cardioverter-defibrillators

Published on 07/02/2015 by admin

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Implantable cardioverter-defibrillators

Efrain Israel Cubillo, IV, MD

Overview

Approximately 300,000 Americans die each year from sudden cardiac arrest, many of whom were taking antiarrhythmic drugs, but drugs alone were insufficient to prevent ventricular tachycardia and fibrillation. The implantable cardioverter-defibrillator (ICD) has revolutionized the treatment of patients at risk for experiencing sudden cardiac death due to these ventricular tachyarrhythmias. The superiority of the ICD device over antiarrhythmic therapy has been confirmed in several randomized trials. Expanding clinical indications for the implantation of these devices arose with the publication of the MADIT (Multi-center Autonomic Defibrillator Implantation Trial), the results of which have been validated by the MUSTT (Multicenter UnSustained Tachycardia Trial). Both studies demonstrated a survival benefit of ICDs over antiarrhythmic medication and placebo in patients with nonsustained ventricular tachycardia. The number of ICD implants continues to increase, with the United States leading the world in both total number and rate per population (434 new implants per 1 million people). In 2009 alone, based on industry statistics, 133,262 ICDs were implanted in the United States. ICD technology has progressed exponentially since its introduction by Michel Mirowski and colleagues in the early 1980s. Early devices were true “shock boxes,” capable of detecting a tachycardia and delivering a shock without the ability to pace.

The ICD system

The ICD system comprises a microprocessor/pulse generator, a battery, and a conducting lead system. The lead system is required for sensing, pacing, and the delivery of therapy. Earlier systems required that the pulse generators be placed abdominally because of their large size. Defibrillation was delivered via two epicardial patches positioned anteriorly and posteriorly. Occasionally, a transvenous spring electrode in the superior vena cava was utilized with an epicardial patch. Sensing was achieved through separate epicardial screw-in electrodes. Initial lead placement required a sternotomy, lateral thoracotomy, or a subxiphoid incision, making early implants quite cumbersome. ICD implantation has evolved quite rapidly due to advancements in lead technology, generator technology, and the development of biphasic defibrillation electric impulses, which lowered the energy requirements necessary for successful defibrillation. The creation of a bipolar lead combining pacing and sensing capabilities with a high-voltage electrode coil allowed for nonthoracotomy system implants, which reduced surgical morbidity and mortality rates. The leads were positioned transvenously via the subclavian vein and fixed to the inside of the right ventricle. However, the leads still had to be tunneled subcutaneously to the abdomen, as the generators remained fairly large. In current practice, generators are fairly small—the smallest commercially available devices today are approximately 7 cm × 5 cm × 1 cm and weigh well under 100 g—allowing for subcutaneous pectoral implantation and simplification of the implantation process. The ICD generator houses the batteries, high-voltage capacitors, and microprocessors necessary to process sensed intrinsic cardiac electrical activity. In essence, the generator is a minicomputer within a hermetically sealed titanium box, which is capable of storing an electric charge that can be delivered, “shocking” the atria and ventricles back to a sinus rhythm. Typically, ICDs deliver no more than 6 shocks per event, although some can deliver as many as 18. Within an event, each successive therapy must be at equal or greater energy than the previous attempt. Once a shock is delivered, no further antitachycardia pacing can take place.

Typical ICDs contain lithium silver vanadium oxide cells that store between 2 and 7 volts. The high voltages necessary for defibrillation are generated with the aid of high-voltage capacitors that are able to generate 700 to 800 volts of defibrillation energy in under 20 sec.

Current devices allow extensive programmability for tiered antitachycardia pacing, tiered high-voltage therapies, bradycardia pacing, supraventricular tachycardia discrimination algorithms, and detailed diagnostics of tachycardic and bradycardic episodes. They also allow physicians to conduct completely noninvasive programmed stimulation. The most recent iterations provide dedicated dual-chamber and antitachycardia pacing as well as options for atrial defibrillation. Diagnostic functions, including stored electrocardiograms, allow for verification of shock appropriateness. Device battery longevity has also increased; early devices lasted 2 years or less, whereas current devices are expected to last 6 years or longer.

ICD placement

Transvenous placement is performed by cardiologists to place ICDs, usually in the left or right infraclavicular area, with the leads tunneled transvenously while the patient receives intravenous sedation using monitored anesthesia care. A deeper level of anesthesia, which most would consider general anesthesia, is provided to the patient for the discomfort that occurs when the unit is tested (i.e., discharged) and an electric shock is delivered to the patient. Defibrillation can lead to prolonged periods of asystole that can result in significant myocardial and cerebral ischemia. Enough time should be allowed between tests to ensure reperfusion and restoration of hemodynamic stability. The anesthesia provider must monitor the duration and frequency of testing and ischemic periods. Vasoactive drugs are often used to stabilize these patients during and immediately after the testing period. Minimum monitoring includes standard American Society of Anesthesiologists monitors and continuous arterial pressure measurement, usually through an arterial cannula placed by the cardiologist.

Function of pacemakers with an ICD

Single-chamber and dual-chamber pacemakers can function in the presence of an ICD as long as the pacing electrodes are bipolar. An ICD with a built-in capability for pacing will begin pacing when the RR interval is greater than previously set limits. Beginning about 1993, most ICDs incorporated backup VVI pacing to protect the patient from the common occurrence of postshock bradycardia. In July 1997, the U.S. Food and Drug Administration approved devices with sophisticated dual-chamber pacing modes and rate-responsive behavior for patients with ICDs who needed permanent pacing (about 20% of patients with ICDs). If a patient with an ICD requires temporary pacing, bipolar leads, the lowest possible amplitude for capture, and the slowest rate associated with adequate hemodynamic status should be used. It is possible for a pacing spike followed by a QRS complex to be interpreted as ventricular tachycardia, causing discharge of the unit. Inactivation of the ICD may be required during temporary pacing if interference occurs. Cardioverter units for atrial fibrillation are presently under investigation. The perioperative management of these devices is unknown, but they will most likely behave similar to ICDs.

Indications for cardiodefibrillator implantation

The American College of Cardiology (ACC) and American Heart Association (AHA), in collaboration with the American Association for Thoracic Surgery and the Society of Thoracic Surgeons, have developed an extensive set of guidelines for cardiodefibrillator implantation. These guidelines represent a consensus statement that is largely evidence based and that summarizes the available clinical evidence as of the time of its initial publication in May 2008 and further revision in October 2012. The latest update emphasizes the role that left bundle branch block with a QRS complex of 150 ms or greater has in sudden death and now considers this to be an indication for implantation of a cardiodefibrillator if the patient’s status is New York Heart Association classification II or higher.

Electromagnetic interference and ICDs

The ability of ICDs to function is dependent on their ability to sense intrinsic cardiac electrical activity. Hermetic shielding, filtering, interference rejection circuits, and bipolar sensing have safeguarded ICDs (and pacemakers) against the effects of common electromagnetic sources. However, exposure to electromagnetic interference (EMI) may still result in oversensing, asynchronous pacing, ventricular inhibition, and spurious ICD discharges. EMI may also lead to loss of output, increased pacing thresholds, and decreased R-wave amplitude. Common sources of EMI include cellular phones, electronic article surveillance (antitheft) devices, and metal detectors. Occupational sources of EMI include high-voltage power lines, electrical transformers, and arc welding. Interference of concern to anesthesia providers can occur during procedures, such as magnetic resonance imaging, or from electrocautery, spinal cord stimulators, transcutaneous electrical nerve stimulator units, radiofrequency catheter ablation, therapeutic diathermy, and lithotripsy.

Inappropriate ICD shocks

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