Paul L. Hess, MD, and Sana M. Al-Khatib, MD, MHS
A 61-year-old man with ischemic cardiomyopathy, a left ventricular ejection fraction of 25%, and an implantable cardioverter defibrillator (ICD) presented for an ICD shock that occurred at home without any warning. The device fired 2 weeks prior to the current presentation without associated symptoms. After a second shock in the emergency department that was clearly delivered for ventricular tachycardia (VT) (Figure 65-1), a bolus of amiodarone was administered followed by a continuous infusion. On arrival to the floor, the patient initially denied symptoms but was concerned about whether he would receive another shock. Device interrogation confirmed two appropriate shocks. A cardiac catheterization demonstrated two patent bare metal stents in the left anterior descending artery and diffuse narrowing in all arteries largely unchanged from a cardiac catheterization done 3 years ago. Baseline thyroid and liver function profiles were obtained, and a 6-g load of amiodarone was completed. Defibrillation threshold testing was performed, and a >10-J safety margin was documented. Upon discharge, oral amiodarone was prescribed, and close follow-up was arranged with his cardiologist and electrophysiologist. He was instructed to call the clinic if he received a single shock and to proceed directly to the emergency department in the event of a second shock or if he had significant symptoms with one shock. During follow-up, thyroid and liver function profiles have remained stable, and no further ICD shocks have been delivered.
FIGURE 65-1 ICD shock delivery. After the ICD detected an arrhythmia with a higher number of ventricular (arrows) than atrial (*) depolarizations, it appropriately delivered a 35-J defibrillation.
• Claiming the lives of more than 400 000 patients in the United States annually, sudden cardiac death is a significant public health hazard.1 The ICD improves the overall survival of many patients with left ventricular systolic function2,3 by terminating malignant arrhythmias. The survival advantage observed in clinical trials translates to a benefit in real-world populations.4
• Broadening indications and technology dissemination have led to a rise in the number of patients with ICDs, and in turn, therapies delivered by devices. The incidence of shocks varies according to patient characteristics, including the original indication for ICD placement, as well as concomitant medical therapies and device programming. In the modern therapeutic era, approximately 10% to 20% of patients experience a shock within 1 year, and 40% to 50% experience a shock within 5 years after ICD placement.5 Two in every three of these are appropriate.5
• The incidence of recurrent shocks is less understood.
ETIOLOGY AND PATHOPHYSIOLOGY
• Shocks delivered for VT or ventricular fibrillation are termed “appropriate,” while those delivered for any other reason such as supraventricular arrhythmias, noise from electromagnetic interference, or a fractured lead are called “inappropriate.”
• Appropriate shocks can occur in the setting of acute ischemia or myocardial scarring. Abrupt ischemia can trigger a number of ventricular arrhythmias, including monomorphic VT (Figure 65-2), polymorphic VT, or ventricular fibrillation. Chronic scarring and temporal dispersion of the surrounding myocardium can lead to VT whose mechanism is typically reentry.
FIGURE 65-2 Monomorphic VT. Electrocardiographic features suggestive of ventricular tachycardia include atrioventricular dissociation, precordial QRS concordance, fusion beats, and capture beats. In this example, fusion beats (arrows) are readily observed in lead II.
• Appropriate shock-inducing arrhythmias are also observed in the setting of decompensated heart failure, electrolyte abnormalities, drug exposures, and genetic disorders.
• Inappropriate shocks are most commonly caused by atrial fibrillation, supraventricular tachycardia, or sinus tachycardia (Figure 65-3A), and abnormal sensing,6 including T-wave oversensing (Figure 65-3B), double counting of the QRS complex, oversensing of diaphragmatic or skeletal myopotentials, electromagnetic interference, or a fractured lead. ICD programming is a key determinant of therapy delivery. Patients’ heart rates must exceed a detection threshold chosen by the treating physician. The duration and number of intervals at heart rates above the threshold also play a role. Programming optimization is imperfect, however, and inappropriate shocks can occur despite it. Mechanical complications related to the ICD and leads per se may also lead to shocks. Problems relating to implantation include a loose set screw, periclavicular lead placement causing a fracture, or lead dislodgment. Manufacturing defects can cause lead fractures or insulation breaks, as has been observed with the Sprint Fidelis and Riata leads respectively. Twiddler’s syndrome often occurs in the setting of psychiatric comorbidity and can cause lead dislodgement or failure.
FIGURE 65-3A Inappropriate ICD shock delivered for sinus tachycardia. Atrial depolarizations (*) preceding ventricular depolarizations (arrows) with a 1:1 ratio, QRS morphology identical to that in normal sinus rhythm, and persistence of this rhythm after a shock indicate this tachyarrhythmia was sinus tachycardia. The patient was exercising heavily at the time of the shock.
FIGURE 65-3B Inappropriate ICD shock delivered for T-wave over-sensing. Markers at the foot of the strip indicate that both QRS complexes and T waves (arrows) were sensed; this double counting led to shock delivery.
• When a shock is reported, an assessment is indicated. When this is performed depends on the number of shocks and the associated symptoms. If the patient does not need immediate medical attention, transtelephonic transmission is useful. Patients experiencing ≥2 shocks or significant symptoms should be assessed promptly. A careful history and physical examination is the cornerstone of any medical evaluation. A history consistent with anginal discomfort may prompt a work-up for ischemia. A history of vomiting or diarrhea may suggest an electrolyte abnormality. Review of prescribed medications may identify a culprit. Device interrogation is critical to differentiate appropriate from inappropriate therapies. VT may be differentiated from a supraventricular cause by comparing the atrial rate with the ventricular rate (see Figure 65-1). If only ventricular markers are available for analysis (such as the case in a single chamber device), a change in R-wave morphology from baseline may provide a clue.
• Noise on the electrogram may indicate electromagnetic interference, a loose set screw, or a lead fracture (Figure 65-4). Software upgrades are available for various lead models that allow early detection and management of lead fractures.
FIGURE 65-4 Electrogram noise. Electrogram noise (arrows) can be caused by electromagnetic interference, a loose set screw, or a lead fracture. The patient in this example had a ventricular lead fracture.
• Potentially offending medications should be stopped. Conversely, evidence-based medications should be initiated or up-titrated barring contraindications, as they reduce mortality and heart failure hospitalizations in patients with left ventricular systolic function. β-Blockers, angiotensin converting enzyme inhibitors, and mineralocorticoid receptor antagonists also favorably impact the risk of sudden death.7-9
• In addition to optimizing heart failure therapies, consideration can be given to initiating antiarrhythmic medications. Antiarrhythmic options are dictated by the underlying cause, the two most common of which are atrial fibrillation and VT. Atrial fibrillation can be treated with amiodarone, sotalol (if LVEF >20%), or dofetilide.10 VT can be treated with sotalol (if LVEF >20%) or amiodarone.
• In a multicenter trial of 302 ICD recipients, 160 to 320 mg of sotalol daily resulted in a 44% relative risk reduction in the composite endpoint of death or ICD shock.11
• In a different trial of 146 patients with inducible sustained VT or ventricular fibrillation, sotalol reduced the incidence of sustained ventricular tachyarrhythmias compared with no antiarrhythmic drug treatment.12
• The isolated enantiomer d-sotalol is associated with worse outcomes among patients with severe systolic dysfunction or advanced heart failure and is thus contraindicated in these instances.13,14 However, the racemic mixture of sotalol (d,l-sotalol) is used in current clinical practice.
• Another clinical trial randomized 412 patients to amiodarone and β-blocker, sotalol alone, or β-blocker alone. Sotalol alone was superior to β-blockade in reducing the number of ICD shocks; amiodarone superimposed on background β-blockade was better still.15
• Amiodarone has been shown to reduce arrhythmic death but not all-cause mortality in several trials.16-18
• If either sotalol or amiodarone prove ineffective, mexiletine is a viable alternative. However, a trend toward increased mortality was observed in one trial.19
• Mounting evidence suggests off-label use of dofetilide20 and ranolazine21 can be considered. The latter is often reserved for patients with ischemic heart disease. The Ranolazine in ICD trial was designed to assess whether ranolazine decreases the likelihood of a composite consisting of VT or ventricular fibrillation requiring antitachycardia pacing, ICD shocks, or death, and is ongoing. Further research on ranolazine and dofetilide is needed.
• In the event of serial shocks, amiodarone in its intravenous form is first-line treatment.
• When using antiarrhythmic drugs, the side effect profiles and potential toxicities must be weighed against the potential therapeutic benefit. Amiodarone toxicities can impact a number of organs, including the eye, thyroid gland, lungs, liver, and skin. Regular monitoring is required, but there are no rigorous data on the best means or optimal frequency. Mexiletine can adversely affect the gastrointestinal and central nervous systems in a dose-dependent fashion. Dofetilide and ranolazine are metabolized by the cytochrome P450 3A enzyme and thus interact with ketoconazole, diltiazem, verapamil, macrolide antibiotics, and grapefruit juice. Sotalol, amiodarone, dofetilide, and ranolazine can prolong the QT interval and potentially cause torsades de pointes. As a general rule, all antiarrhythmics can be viewed as proarrhythmic as well and must be used judiciously. In patients with renal dysfunction, particularly the elderly, sotalol, dofetilide, and ranolazine must be used with caution.
• ICD-antiarrhythmic drug interactions must also be taken into account. Most antiarrhythmic medications have a negative chronotropic effect. Prescribing physicians should therefore optimize bradycardic pacing mode parameters when such medications are started. DDD mode is generally preferred to maintain AV synchrony and minimize the likelihood of pacemaker syndrome. In addition to negative chronotropy, many antiarrhythmic drugs can also affect defibrillation thresholds. Whereas sotalol and dofetilide can decrease defibrillation thresholds, lidocaine and amiodarone increase it. Ranolazine, mexiletine, and propafenone do not appear to significantly affect it. Patients with borderline safety margins may benefit from defibrillation threshold testing after initiating drugs that increase defibrillation thresholds. The same drugs should prompt testing of pacing thresholds and sensing, though this space is less well-characterized.
• Finally, ICD programming should be optimized to prevent further shocks. Increasing the treatment threshold to ≥200 beats per minute reduces shocks and mortality in a primary prevention population.22 This may also hold true among patients who have received an ICD shock. Increasing the number of intervals to detect VT in primary and secondary prevention ICD recipients also reduces the incidence of shocks but not mortality.23 Further, the sensitivity of device leads to myocardial depolarization can be adjusted. If done properly, abnormal sensing can be minimized. Finally, detection algorithms differentiating lethal from nonlethal arrhythmias by taking into account the rapidity of rhythm onset, rhythm stability, and electrogram morphology (Figure 65-5) can also be employed.
FIGURE 65-5 Electrogram morphology detection. ICDs can be configured to detect the degree of mismatch between the observed QRS complex during an arrhythmia and the morphology of the stored QRS template. The device administers a shock when the difference between the two exceeds a programmed value.
LONG-TERM COMPLICATIONS AND FOLLOW-UP
• Appropriate shock recipients are at a higher risk of heart failure and death compared with those who do not experience a shock24; the latter risk doubles with one or more recurrent defibrillations.25Inappropriate shock recipients are also at greater risk of death.25 Whether appropriate or inappropriate, defibrillations are associated with a reduced quality of life.26
• Accordingly, shock recipients should be closely monitored for imminent heart failure exacerbations. During clinical encounters, close attention should be paid to patients’ volume status and evidence-based medication regimen. Further, patients’ psychosocial distress should be carefully managed. This may take the form of education, close follow-up, and concrete plans for management of a future shock. A full complement of health care providers, including a primary care physician, a heart failure specialist, an electrophysiologist, and a mental health specialist, may jointly aid in these undertakings.
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