With the widespread use of implantable cardioverter defibrillators (ICDs), the natural history of patients with advanced heart disease has changed, and physicians are now frequently called upon to manage recurrent ventricular arrhythmias. As many as 50–70% of patients who have an ICD for secondary prevention of sudden cardiac death (SCD) receive appropriate therapy within 2 years of implant, and 10–20% will experience very frequent episodes of ventricular tachyarrhythmias, a situation which has been known as ‘electrical storm’ or ventricular tachycardia storm [1–7]. The incidence of electrical storm is lower when ICDs are placed for primary prevention in the setting of ischemic cardiomyopathy, about 4% over an average of 20.6 months . Rates of appropriate therapy are comparable among patients with nonischemic cardiomyopathy. In a study including dilated cardiomyopathy patients with ICD for secondary prophylaxis, 30 of 106 patients (28.3%) experienced 52 electrical storm events during a mean follow-up of 33 ± 23 months .
Recurrent ventricular arrhythmias and their consequent therapy have been associated with substantial impairment of quality of life and with adverse prognosis [9,10▪,11,12,13▪,14]. A particularly high mortality risk has been associated with electrical storm. The definition of what constitutes electrical storm and its therapies is the subject of this review.
THE DEFINITION OF ELECTRICAL STORM
There has been no widely accepted formal definition of electrical storm. Before the ICD era, the term ‘electrical storm’ referred to the occurrence of two or more ventricular tachycardia/ventricular fibrillation episodes within 24 h . The most commonly accepted definition at present is ‘three or more separate arrhythmia episodes leading to ICD therapies [antitachycardia pacing (ATP) or shock] occurring over a single 24 h time period’ [3,6,7,16▪]. However, there have been a variety of definitions of electrical storm. An alternative definition is ‘at least two episodes of ventricular tachyarrhythmia requiring ICD shock within 24 h’ [17,18]. Other definitions, however, have included ‘two or more hemodynamically relevant ventricular tachycardia in 24 h’, ‘ventricular tachycardia recurring immediately after termination’ ; ‘three or more separate episodes of ventricular tachycardia/ventricular fibrillation within a 24-h period, each separated by more than 5 min’ ; ‘at least twenty ventricular tachycardias in 24 h or at least four ventricular tachycardias in 1 h’ ; ‘two or more shocks for one ventricular tachycardia episode’ ; or even ‘greater than two ventricular tachycardia episodes separated by less than 1 h of sinus rhythm’ . The term ‘ventricular tachycardia Cluster’ has also been used as equivalent to electrical storm . It was defined as at least three different sustained episodes of ventricular tachycardia or ventricular fibrillation terminated by ICD intervention during 24 h, and repetitive ineffective shocks were excluded. ‘Incessant ventricular tachycardia’ [18,20] has been used to indicate ventricular tachycardia which recurs immediately (after ≥1 sinus cycle and within 5 min) after a technically successful therapy.
Thus far, the diagnosis of electrical storm has been defined empirically. The current most commonly accepted electrical storm definition does not include situations which may have similar clinical significance, but slightly different presentations. For example, ventricular tachycardia which sustains more than 30 s, but does not lead to ICD therapy, either because it is under-sensed or because it falls below the lowest programmed therapy zone, would be excluded from this definition. Similarly, one could argue that the clinical significance of two episodes of ventricular tachycardia per day for four consecutive days should warrant inclusion within the definition. Some definitions require at least 5 min of separation between the episodes of ventricular tachycardia , and yet the clinical severity and prognosis of incessant ventricular tachycardia may match or exceed that of recurrent ventricular tachycardia that does fall within the definition. No study to date has examined the threshold burden of ventricular arrhythmias or associated ICD therapies which begin to confer an adverse outcome. Although at present it remains reasonable to use the current commonly accepted definition of ‘three episodes of ventricular tachycardia within 24 h’ [16▪], we would argue that, in most cases, this definition should not depend upon whether ICD or other therapy was applied; ideally an outcome-derived definition would be more clinically relevant.
The definition of the end of an electrical storm event has similarly been open to question. Greene et al.  have defined the termination of electrical storm as a period ‘without ventricular tachycardia recurrence in 2 weeks’. Although intuitive, the rationale for this remains empiric.
THE CLINICAL IMPLICATION OF ELECTRICAL STORM
Although some studies have failed to identify a clearly increased mortality risk with electrical storm , the preponderance of data suggests electrical storm is an adverse prognostic factor [1,2,21,22]. Cohort studies have consistently found that electrical storm is associated with higher mortality in both secondary and primary prophylaxis patients. Electrical storm is also associated with an increased rate of hospitalization and can have an extremely negative impact on patients’ quality of life.
In the Antiarrhythmic Versus Implantable Defibrillators (AVID) trial for secondary prevention, 34 of 90 (38%) electrical storm patients died during follow-up compared to 15% of those without electrical storm. Electrical storm was a significant independent risk factor for subsequent death, independent of ejection fraction and other prognostic variables (relative risk 2.4, P = 0.003), but ventricular tachycardia/ventricular fibrillation unrelated to electrical storm were not. The risk of death was greatest within the first 3 months after electrical storm (relative risk 5.4) and diminished beyond this time . Gatzoulis et al.  studied 32 electrical storm patients who had an ICD for secondary prophylaxis. Electrical storm was defined as the occurrence of three or more episodes of ventricular tachyarrhythmias terminated by either the device or an external shock in a period less than 24 h. Seventeen of 32 patients (53%) died during 3 years of follow-up, compared with 19 of the 137 (14%, P < 0.001) ICD patients without electrical storm, suggesting that electrical storm was a strong independent predictor of poor outcome in ICD patients.
Among patients who have received an ICD for primary prophylaxis, electrical storm has also been associated with higher mortality. In a Multicenter Automatic Defibrillator Implantation Trial II (MADIT-II) substudy , patients who experienced electrical storm had a significantly higher risk of death. The hazard ratio for death in the first 3 months after the storm event was 17.8 in comparison with those with no ventricular tachycardia/ventricular fibrillation. After the first 3 months, the hazard ratio decreased to 3.5. Even in patients with isolated ventricular tachycardia/ventricular fibrillation episodes, there was still an increased risk of death (hazard ratio 2.5) when compared with patients without ventricular tachycardia/ventricular fibrillation episodes .
The mortality rate has also been observed to be increased after electrical storm in patients with nonischemic cardiomyopathy. One study showed the mortality and transplantation rate (54%) were significantly higher in patients with a history of electrical storm compared with those without electrical storm event in 3 years of follow-up . Among a mixed group of patients with either ischemic or nonischemic cardiomyopathy, and heart failure and a prophylactic ICD , those who received shocks for any arrhythmia had a substantially higher risk of death. An appropriate shock was associated with a risk of death that was increased by a factor of 3. Even an inappropriate shock was associated with a trend toward an increased risk (hazard ratio 1.57; P = 0.06) [10▪].
Electrical storm also increases the rate of hospitalization and significantly affects the patients’ quality of life. In a SHock Inhibition Evaluation with azimiLiDe (SHIELD) trial subanalysis , electrical storm led to a 3.1-fold increase in arrhythmia-related hospitalization (P < 0.0001) compared with patients with isolated ventricular tachycardia/ventricular fibrillation. It has been well known that ICD therapies, especially repeated frequent shocks, may have significant psychological effects on both patients and their families, recently reviewed in detail [13▪]. Data from the AVID trial suggested that sporadic shocks and adverse symptoms were both associated with reduced physical and mental well being . Electrical storm can undermine the perception of security provided by the ICD.
Although electrical storm is associated with higher mortality, higher hospitalization and also poor quality of life, it is still not clear whether electrical storm contributes to higher mortality directly or is an epiphenomenon of advanced heart disease or systemic illness . Although no consistent triggers for electrical storm have been identified, clinical observations have suggested that patients with severely compromised left ventricular function, chronic renal failure, ischemia, infection, hypokalemia or hyperkalemia, and older age might have the greatest probability of experiencing electrical storm [4,6,7,18,22,23]. One study has observed that the combination of left ventricular ejection fraction less than 25% and QRS duration greater than 120 ms was a powerful predictor of the occurrence of electrical storm . Furthermore, although the majority of deaths happen in the first 3 months after electrical storm, most of them are non-SCD, which again raises the question of whether electrical storm is just a marker of progression of underlying disease.
On the other hand, electrical storm might directly contribute to increased mortality. In spite of detailed investigation, a triggering mechanism is only identified in 10–25% of patients, while the majority of patients presented with electrical storm without any other perceptible change in baseline cardiovascular health [3,4,6,7,22]. It has been suggested that frequent ventricular tachycardias and, especially, incessant ventricular tachycardias may cause congestive heart failure . It has also been shown that repeated ventricular tachycardia/ventricular fibrillation episodes and ICD shocks may cause myocardial injury with increased troponin and induce intracellular calcium overload, which leads to postshock reinitiation of ventricular fibrillation [27–29]. Multiple shocks delivered for treatment-refractory ventricular tachycardia/ventricular fibrillation might contribute to the progression of heart failure and cardiac injury, which in turn increases the mortality [30–32].
Although the weight of the evidence supports electrical storm as an independent risk factor of mortality, the risk attributable to monomorphic ventricular tachycardia without immediate hemodynamic decompensation, especially when successfully treated with ATP, is less well characterized. Understanding whether some ventricular tachycardia events do not confer higher risk, and whether there is a threshold of arrhythmia or therapy frequency which causes adverse outcome, is an important area of research. Although most clinicians feel (appropriately) compelled to intervene when patients experience ventricular tachycardia, the evidence behind this practice remains weak. Answering these questions would provide important guidance to clinicians: Should we intervene early or only with recurrences? Should we use antiarrhythmic drugs or catheter ablation? Should we intervene at all?
MANAGEMENT OF ELECTRICAL STORM
Electrical storm is a clinical emergency. Despite the lack of identifiable triggers for electrical storm in the majority of patients, triggering causes or exacerbating factors should be sought, including a systematic search for ischemia, decompensation of heart failure, changes to medications, bradycardia-induced tachyarrhythmias, or other systemic illness. It has been estimated that 10–25% of patients may have reversible factors triggering the electrical storm episode . If an apparently responsible trigger can be found, it should generally be treated aggressively. If shocks are frequently recurrent, sedation may help prevent psychological distress . The psychological effects of shocks should be considered both early and subsequent to electrical storm, and psychological intervention should be considered when appropriate [13▪].
Antiarrhythmic drugs may stabilize ventricular rhythm in many electrical storm patients. Therapy may be escalated depending upon the severity of illness.
Electrical storm, particularly if recurrent, elevates sympathetic tone, which may provoke further recurrent ventricular arrhythmias. β-Blockade has a fundamental role in the management of electrical storm, especially with the use of β-blockers which antagonize both β1 and β2 receptors; it has been shown to increase the fibrillation threshold and decrease the incidence of sudden death . Within the MADIT-II study , 691 patients with ischemic cardiomyopathy who received maximal doses of β-blockers (metoprolol, atenolol, or carvedilol) had a 52% relative risk reduction for recurrent ventricular tachycardia/ventricular fibrillation requiring ICD therapy compared with those who did not take β-blockers. Even in electrical storm patients already on oral β-blocker therapy, adding β-blockers intravenously may help further to suppress the electrical storm episode .
Amiodarone has been widely used for the treatment of electrical storm. The Optimal Pharmacological Therapy in Cardiovertor Defibrillator Patients (OPTIC) study compared β-blocker, sotalol, and β-blocker plus amiodarone in the prevention of ICD shocks . A total of 412 patients with ICDs and recent ventricular arrhythmias were followed up for 1 year: frequent shocks (>10 per year) occurred in 10 patients (7.4%) randomized to β-blocker compared with two patients (1.4%) randomized to amiodarone plus β-blocker and three patients (2.3%) randomized to sotalol. Patients treated with sotalol or amiodarone plus β-blocker had a 56% reduction in risk compared with β-blocker alone. Like β-blockers, intravenous amiodarone may be an effective drug even in patients already receiving chronic oral amiodarone .
In a placebo-controlled randomized trial reported by Pacifico et al. , sotalol has been shown to significantly decrease the recurrences of ventricular tachycardia/ventricular fibrillation, all-cause ICD shocks, and all-cause death. In the OPTIC study, sotalol compared with a regular β-blocker only tended to reduce ICD shocks, but this effect did not reach statistical significance . Two other small studies did not observe a significant difference in either mortality or recurrent ventricular tachycardia/ventricular fibrillation requiring ICD therapies attributable to sotalol, and sotalol was not superior to metoprolol [38,39].
In the SHIELD study, the 75 and 125 mg dose of azimilide significantly reduced the recurrence of shocks plus symptomatic arrhythmias treated by ATP. It also reduced emergency department visits and hospitalizations in patients with ICDs . However, in a prospective study , of the 148 patients who experienced at least one episode of electrical storm, azimilide did not significantly reduce the number of patients with electrical storm. Of note, Torsade de pointes (TdP) associated with azimilide treatment was documented in five patients (1.2%).
Dofetilide selectively blocks the rapid component of the delayed rectifier potassium current and is principally used for the treatment of atrial fibrillation . It was shown to be effective in increasing the median time to first all-cause ICD shocks in a study by O’Toole et al. , but was associated with a high incidence of TdP. One small study supported efficacy and safety of dofetilide in the treatment of frequent ventricular tachycardia/ventricular fibrillation after amiodarone intolerance or failure .
Combinations of antiarrhythmic drugs
There has been no randomized study comparing treating ventricular tachycardia with combinations of antiarrhythmic drugs; however, some cases and clinical experience have been reported. Monotherapy with class I drugs for the prevention of ventricular arrhythmia does not appear to be well tolerated . But, it has been used with the combination of class III drugs . Theoretically, the combination of a class I drug (with the exception of a class Ic) and dofetilide might have comparable effects to amiodarone plus β-blocker therapy. Some studies  have suggested a beneficial effect from the combination of mexiletine and amiodarone and procainamide or quinidine with D,L,-sotalol. No randomized trial has compared these drug regimes, however. If mexiletine, procainamide, or quinidine are combined with amiodarone, their doses should be reduced in order to avoid side-effects.
In summary, the decision to prescribe an antiarrhythmic drug to an electrical storm patient should be individualized, taking into account not only the efficacy but also the increased risks of drug-related proarrhythmia and side-effects. Although antiarrhythmic drugs reduce the number of ICD shocks, they are associated with a relatively high incidence of side-effects. In the OPTIC study, sotalol and amiodarone were discontinued during the 1-year follow-up because of side-effects in 18.2 and 23.5% of patients, respectively, while, in a substudy of the Canadian Implantable Defibrillator Study (CIDS) trial, amiodarone was associated with side-effects in 82% of patients during 5.6 years of follow-up , limiting the long-term utility of these medications in a significant number of ICD patients. This, combined with the limited efficacy of antiarrhythmic drugs, has prompted the need for the development of nonpharmacologic treatment strategies.
The vast majority of arrhythmic episodes during electrical storm consist of monomorphic ventricular tachycardia (86–97%) [1,3] consistent with scar-mediated reentry as the main mechanism of electrical storm, which may be a suitable target for catheter ablation. With increasing experience and the rapid advancement of mapping and ablation technologies, catheter ablation of ventricular tachycardia can be performed safely, with a relatively low complication rate [48,49].
Thus far, two randomized trials have compared ICD implant and early prophylactic ablation after ICD implantation for secondary prevention in patients with a history of myocardial infarction (MI). Both showed catheter ablation significantly decreased the ICD therapies. The Substrate Mapping and Ablation in Sinus Rhythm to Halt Ventricular Tachycardia Trial  enrolled 128 patients with ventricular tachycardia who were not treated with antiarrhythmic drugs. Prophylactic substrate-based catheter ablation reduced ICD shocks from 31 to 9% over a mean follow-up of 22.5 ± 5 months (P = 0.003), and reduced ventricular tachycardia from 33 to 12% (P = 0.007). The Ventricular Tachycardia Ablation in Coronary Heart Disease trial [51▪] enrolled 110 patients with prior MI and hemodynamically stable ventricular tachycardia, and randomized them to either catheter ablation or no additional treatment. Within the trial, 35% of patients were treated with amiodarone at baseline, and 25–30% were treated with amiodarone at 1 year. After ablation, the number of appropriate ICD therapy events per patient and per year was significantly lower than in the control group (median 0.2 versus 3.0; P = 0.013).
Catheter ablation has been used to treat electrical storm. Recent reports on ablation for electrical storm have shown not only a reduction in recurrent electrical storm, but also a survival benefit in patients who underwent successful ablation. Sra et al.  first reported 19 electrical storm patients who underwent catheter ablation. The procedure success rate was 79%. There were two (11%) electrical storm recurrences, but no deaths over a 26-week follow-up. A prospective study [53▪] enrolled 95 drug refractory electrical storm patients who all had frequent ICD shocks. After one to three procedures, 85 patients (89%) did not have any inducible clinical ventricular tachycardia(s) by programmed electrical stimulation. Electrical storm was acutely suppressed in all patients with a minimum period of 7 days. At a median follow-up of 22 months, 87 patients (92%) were free of electrical storm and 63 patients (66%) were free of ventricular tachycardia recurrence. Recently, Deneke et al. [54▪] reported 32 electrical storm patients undergoing catheter ablation, of whom 27 underwent ablation within 24 h after admission and five underwent acute ablation within 8 h. The acute success rate was 94%, and electrical storm recurrence or death was observed in 6 and 9%, respectively, during 15 months of follow-up. Two further cohort studies of catheter ablation for electrical storm also showed high acute success rate and significant benefit of electrical storm ablation on mortality [55▪,56].
Overall, based on the limited data available reported by expert centers, electrical storm ablation has an acceptably high success rate (79–94%), low complication rate (5–13%), and low electrical storm recurrence rate (8–12%). Ventricular tachycardia recurrence rate was significantly reduced only in those for whom all (clinical and nonclinical) ventricular tachycardias were acutely rendered noninducible. The incidence of death is significantly higher in patients with electrical storm recurrences (50 versus 8%).
Electrical storm is an emergent life-threatening situation. Despite the lack of high-quality evidence supporting the benefit of intervention, virtually all clinicians would advocate urgent therapy. If pharmacological management fails and a catheter ablation facility with adequate expertise is available, the patient should be rapidly referred. The relative merits of early ablative therapy in comparison to early pharmacologic therapy are unknown. One study [57▪] compared the outcomes of catheter ablation between patients who were referred for ablation early and those who were only referred after drug failure. Late referrals (62 patients) were defined as those with two or more episodes of ventricular tachycardia, separated by at least 1 month. All others (36 patients) were considered early referrals. Fifty-eight percent of 98 patients were in ventricular tachycardia storm. Overall, acute procedural success was achieved in 89%. The early referral group had a significantly superior 1-year ventricular tachycardia free rate, supporting earlier referral for catheter ablation following ICD therapy, although, in this retrospective trial, confounding effects could not be ruled out. Nonetheless, catheter ablation has the potential to improve patient mortality as well as quality of life. Early intervention is also supported by other reports, which note a high mortality rate while awaiting catheter ablation for electrical storm [54▪,55▪]. It appears that early intervention with ablation may be followed by a lower recurrence rate. Patients referred late for ablation might already have received a higher dose of amiodarone, which may suppress ventricular tachycardia during an ablation procedure, in turn leading to less extensive ablation, more residual substrate, and greater risk of recurrence [57▪].
There are still unanswered questions about catheter ablation for electrical storm. So far, most series reported in the literature have included patients with ischemic heart disease. It is not clear whether the outcome would be similar for nonischemic heart disease patients. There has been no randomized controlled trial to date which clarifies the relative merits of catheter ablation in comparison to the pharmacologic management of electrical storm. Likewise, we do not know the optimal timing of catheter ablation or whether ablation has a long-term mortality benefit. The ongoing Ventricular Tachycardia Ablation vs. Enhanced Drug Therapy in Structural Heart Disease (VANISH) study (NCT00905853) will compare the outcomes of aggressive antiarrhythmic drug therapy versus catheter ablation for patients with ischemic cardiomyopathy and ventricular tachycardia.
Surgical management of electrical storm
There are limited data about the surgical management of electrical storm. The antiarrhythmic effects of thoracic epidural anesthesia (TEA) have been demonstrated in animal studies [58,59], and the potential protective effects of left cardiac sympathetic denervation (LCSD) have been demonstrated in patients with long-QT syndrome who continue to experience syncope or cardiac arrest despite blockade and in small numbers of patients with catecholaminergic polymorphic ventricular tachycardia [60–62].
Bourke et al.  have reported 14 patients who had frequent ventricular tachycardia episodes, despite maximal medical therapy and catheter ablation procedures. Twelve patients were in ventricular tachycardia storm; eight patients had prior catheter ablation. Both TEA (nine patients) and LCSD (eight patients) were associated with a subsequent decrease in arrhythmia burden. When LCSD is ineffective in suppressing ventricular tachycardia, adjunctive right cardiac sympathetic denervation may be an option. Ajijola et al. [64▪] reported the result of bilateral cardiac sympathetic denervation. All six patients had ventricular tachycardia/ventricular fibrillation refractory to antiarrhythmic drugs and three had prior catheter ablation. After the surgery, complete response was observed in four patients (decreased to no shocks or episodes in three patients and decreased by >50% in one patient), partial response was seen in one patient, and no response in one patient. At this time, using surgery for the treatment of electrical storm should be reserved to those refractory to drugs and catheter ablation where there is appropriate surgical expertise.
The current definition of electrical storm is empiric, but has been associated with adverse effects on mortality and quality of life; an ideal definition of electrical storm would be derived directly from the outcomes. Although a clear causative role for electrical storm in accelerating mortality is yet unproven, this is biologically plausible, and standard care at present is to intervene aggressively when electrical storm occurs. Future research should clarify the optimal relative roles and timing of both antiarrhythmic drug therapy and catheter ablation.
Conflicts of interest
J.L.S.: Research funding (Biosense Webster, St Jude Medical), Consultant (Biosense Webster).
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
Additional references related to this topic can also be found in the Current World Literature section in this issue (p. 83).
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