Patients presenting with implantable cardioverter–defibrillator (ICD) shocks are increasingly common, primarily due to the expansion of ICD-placement indications. The discomfort and pain associated with ICD shocks often result in immediate medical attention. This occurs in emergency rooms and as acute visits in doctor’s offices. With the advances in remote monitoring, these alerts can also occur as emails, faxes, and telephone calls. More importantly, patients who receive shocks from their ICD are at increased risk for death than those who receive no shock irrespective of whether the shock is appropriate or not. This measurable increase in mortality warrants further evaluation of patients who receive shocks. Patients with ICDs are expected to receive shocks. The risk for receiving an appropriate shock in the first one to three years is 20–35% in patients with left ventricular ejection fractions ≤35% who have had no evidence of sustained ventricular arrhythmias. Forty-five percent of patients with an ICD who have survived a cardiac arrest or have documented sustained ventricular arrhythmias will receive an appropriate shock within the first year after implantation. Inappropriate shocks account for about one-third of all shocks. The overall annual inappropriate shock risk for a patient with an ICD is estimated at 7.5%.1 Irrespective of the cause of the ICD shocks, patients also have increased morbidity, with anxiety, depression, and even post-traumatic stress response from multiple shocks. These consequences can lead to further functional decline in the patient and a decreased quality of life.2
Management of frequent shocks starts with determination of the cause of the shock. Is the shock from an artifact or an arrhythmia? Most frequently, the cause of an ICD shock is appropriate and secondary to ventricular arrhythmias. Despite the potentially life-saving shock, sustained ventricular arrhythmias are associated with increased mortality and morbidity. In the Multicenter Automatic Defibrillator Implantation Trial II (MADIT II) study group, patients had a three-fold increase in heart failure hospitalization and death after an appropriate shock.3 In the Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT), the risk for death was increased five-fold after an appropriate shock from the ICD.1 Antitachycardic pacing therapies have been shown in multiple studies to reduce shock number.4 This reduction in shock number by using antitachycardic pacing has recently been shown to correlate with reduced mortality in a retrospective analysis of >2,000 patients.5
Once a patient receives an appropriate shock from the ICD, there should be a full evaluation of the patient’s condition and medical regimen. Changes in the medical regimen that can optimize the reduction of future sustained arrhythmias should be considered. In most cases, patients have ICD for their heart failure condition. Re-evaluation of the patient’s underlying condition, including evaluation for ischemia, should also be considered, as ischemia can certainly lower the arrhythmia threshold. Optimizing the heart failure medical regimen, such as maximizing the dose of beta-blocker, angiotensin-converting enzyme (ACE) inhibitors, and statins, has been shown to reduce sustained arrhythmias.6,7 Once the heart failure regimen has been maximized, the addition of an antiarrhythmic should be considered. Both amiodarone and sotalol have been shown to reduce shock burden and suppress sustained ventricular tachycardia.8,9
Radiofrequency ablation (RFA) should also be considered in reducing shock burden. In the Ventricular Tachycardia Ablation in Addition to Implantable Defibrillators in Coronary Heart Disease (VTACH) trial, RFA was shown to prolong time to recurrence of ventricular tachycardia (VT) in patients with stable VT prior to ICD implantation from 5.9 to 18.6 months and reduce ICD shocks by 43% compared with the control group.10 RFA has been shown to be effective both in patients with recurrent ventricular arrhythmias and as a prophylactic reduction of ICD shocks after a single episode of sustained ventricular arrhythmia or syncope in patients receiving an ICD.11,12
Patients who receive multiple shocks for sustained ventricular arrhythmias (more than three in a 24-hour period) are at the highest risk for future morbidity and mortality. This is also a growing population, with an incidence as high as 10–40% in the secondary prevention of sudden cardiac death population. These patients should be urgently evaluated (frequently emergently) for aggressive antiarrhythmic management. This treatment includes use of antiarrhythmic medications and search for reversible causes of the recurrent arrhythmia. Treatment can also involve supportive care to minimize shock number. Acute management of electrical storm can include sedation with respiratory support both for acute treatment of anxiety and to decrease the dramatic sympathetic nervous system surge. Intra-aortic balloon pumps and other forms of circulatory support can also help manage sustained ventricular arrhythmias, at least in the short term. Evaluation for ischemia should be completed. If nothing reversible is found, moving aggressively to medical management with antiarrhythmics and invasive management with RFA should be considered. RFA in this patient population has been shown to be quite effective, with a short-term success rate of 72% and long-term success in approximately two-thirds of patients in a recent prospective single-center study.13
Most often, the cause of the shock is a sustained arrhythmia. However, if this arrhythmia is not from ventricular tachycardia or ventricular fibrillation, it is considered inappropriate because supraventricular tachycardias are not usually considered a life-threatening arrhythmia. In the case of supraventricular tachycardia causing ICD shock, atrial fibrillation is the most common cause of shock. This arrhythmia is not just a nuisance to the patient by causing the ICD to fire, but is also a marker of worsening mortality and morbidity. Patients with known heart failure have been shown to have further decline when there is also atrial fibrillation. When atrial fibrillation presents in patients resulting in ICD shocks and evaluation, this opportunity for medical optimization should be considered. Atrial fibrillation is a marker of worse prognosis, and perhaps intervening at the time of discovery can help alter this natural history. In addition, episodes of atrial fibrillation have been shown to be associated with the onset of ventricular arrhythmias. In a review of the Inhibition of Unnecessary Right Ventricular (RV) Pacing with Arterioventricular (AV) Search Hysteresis in ICDs (INTRINSIC RV) database, a history of atrial fibrillation and new-onset atrial fibrillation was associated with increased mortality and heart failure hospitalization. A history of atrial fibrillation has been shown to be an independent predictor of appropriate therapy from an ICD.14 Patients with a history of atrial fibrillation with a QRS duration >130ms are five times more likely to receive an appropriate shock from an ICD and those with QTc >440ms are up to 12 times as likely to receive a shock.15 Furthermore, patients with persistent atrial fibrillation are also more likely to receive appropriate shocks.16 This proarrhythmic effect likely contributes to the worse mortality and morbidity in this patient population.
Other supraventricular tachycardias that can cause inappropriate shocks include sinus tachycardia, atrial tachycardia, atrial flutter, and even re-entrant supraventricular tachycardia (SVT). Management of these patients often includes reprogramming of ICD to minimize shocks from SVTs. Such changes in reprogramming can include adding additional zones of therapy with SVT discriminators, increasing the detection time, or increasing the heart rate to receive therapy. In the Primary Prevention Parameters Evaluation (PREPARE) study, simply extending the detection criteria avoided potential inappropriate therapy by about 50% (3.6 versus 7.5%).17 In addition to ICD reprogramming, changes in medications also help to minimize this form of inappropriate shock, including increasing doses of AV nodal blocking agents or adding antiarrhythmic medication. In some cases, catheter-directed RF ablation can be curative in eliminating these arrhythmias and decreasing the chance of this type of inappropriate shock.
Management of frequent shocks starts with determination of the cause of the shock: is the shock from artifact or arrhythmia? Sensed artifacts can occur from both non-physiologic and physiologic mechanisms. Examples of non-physiologic mechanisms include damage somewhere in the ICD system or interference from an external electromagnetic source or from leads placed too closely together. The most common cause of ICD-system failure is from ICD-lead dysfunction, as ICD leads can fracture. Fractures frequently present as inappropriate shocks due to noise. Other sources of noise due to failure of the ICD system include problems with the lead/ICD interface with a loose set screw or from damage to the header portion of the ICD. Usually, a loose set screw will present with noise shortly after implantation, but may present years after implantation. Noise from a faulty ICD system is often reproducible with maneuvers in the office setting. Frequently asking the patient to extend the arm across the chest and extending the hand on the back can reproduce noise. Other maneuvers include placing pressure on the device while monitoring for transient noise. When noise is reproducible in the office setting, it is almost certainly explained by ICD-system malfunction. The solution is to revise the system. This may involve lead or generator replacement.
Other causes of an inappropriate shock can include oversensing of signals within the heart or external to the heart or body. Cardiac sources of oversensing most commonly include T-wave oversensing. Other sources of noise can include electromagnetic interference or even myopotentials from chest-wall or diaphragm muscles. When the shock is from non-physiologic external noise, the solution is simply to remove the source of the noise. For instance, use of transcutaneous electrical nerve stimulation units creates electromagnetic noise that can be interpreted as sustained ventricular tachycardia. Use of these units should be avoided in patients with ICDs. In the case of T-wave oversensing or myopotentials, reprogramming the lead sensitivity can often remedy this cause of inappropriate shocks. Less common is the need for lead or ICD-system revision to minimize T-wave oversensing while maintaining adequate sensing of sustained ventricular arrhythmias.
In summary, management of ICD shocks will become a growing part of any cardiology practice. Evaluation does not stop after determination of whether the shock is appropriate. These shocks are frequently a marker of increasing mortality. Less often, shocks are due to device system failure, which may warrant revision. More often, the shocks are due to arrhythmias that require intervention to reduce future shocks. This management includes reprogramming of the ICD, optimization of heart failure regimen, searching for reversible causes, and direct treatment of the arrhythmia. These interventions are intended to reduce shock number, improve quality of life, and hopefully minimize the increase in mortality.