Currently RFCA is indicated if pharmacologic therapies

Currently, RFCA is indicated if pharmacologic therapies are unsuccessful [41]. Myocardial substrates for reentrant VT and/or the origin of focal VT are the ablation target for monomorphic VT storms. Although ablating multiple and unstable VTs is challenging, modern computerized mapping and catheter navigation technologies enable them to be treated with a substantial success rate [42]. When the hemodynamics during the VT are sufficiently stable for VT mapping, an ablation target can be localized using the conventional criteria of activation and entrainment mapping [43]. It is important to be aware that some types of monomorphic VT involve the bundle branch (bundle branch reentry) or Purkinje fibers (similar to fascicular VT) in the VT circuit, as the ablation target is very different from that of myocardial VT (i.e., the bundle branch or Purkinje potentials) [32,33]. When the targeted VTs are unmappable owing to hemodynamic instability, difficulty with induction, or unstable morphology in response to attempted entrainment pacing, substrate mapping during sinus rhythm is useful with the assistance of an electroanatomic mapping system [44]. Substrate modification using several RFCA strategies can render the unmappable VTs non-inducible and suppress monomorphic VT storms [45]. If patients with an ICD receive frequent shocks for monomorphic VT, the ICD treatment protocol should be reviewed to determine if device reprogramming is desirable. Programming an ICD to deliver antitachycardia pacing for a fast VT can reduce the need for shocks. In the Pain-Free Rx II trial [46], antitachycardia pacing effectively treated fast VTs (188–250bpm). The Primary Prevention Parameters Evaluation (PREPARE) investigators [47] evaluated the effect of extending the VT duration needed to trigger ICD shocks to prevent repeated shocks for non-sustained VT. The PREPARE study patients were less likely to receive a shock in the first year compared with the control patients (9% vs. 17%), without any increase in arrhythmic syncope. The Multicenter Automatic Defibrillator Implantation Trial-Reduce Inappropriate Therapy (MADIT-RIT) trial [48] showed that programming ICD therapies for VT to ≥200bpm for ≥2.5s or with a prolonged delay in therapy (60s for VT between 170 and 199bpm and 12s for VT between 200 and 249bpm) was associated with a Malonic acid in all-cause mortality.
Polymorphic VT/VF storms in structurally normal hearts Polymorphic VT/VF storms are relatively rare in structurally normal hearts but occur in patients with electrical disorders of primary (genetic) or secondary causes or an unknown etiology, termed idiopathic VF (Table 1). In this category, differential diagnosis is of particular importance because the treatment strategies differ greatly among the causes of polymorphic VT/VF storms. The baseline ECG characteristics have a high diagnostic yield for the primary causes, while gathering the patient׳s information (medical history, drugs, etc.) and identifying extra-cardiac pathologies are the keys to a correct diagnosis of secondary causes. If the QT interval is markedly prolonged, the polymorphic VT is most likely Torsades de points (TdP, a specific form of polymorphic VT in which the QRS complexes seem to twist around the isoelectric line), which is attributable to congenital or acquired LQTS. Congenital LQTS is an ion channel disorder with at least 15 different related genes. Mutations in the KCNQ1 (LQT1), KCNH2 (LQT2), and SCN5A (LQT3) genes represent the most common causes of LQTS and account for an estimated 60–75% of the genotype-positive LQTS cases [49]. On the other hand, acquired LQTS is caused by QT-prolonging drugs (e.g., antiarrhythmic drugs, several antibiotics such as macrolides and fluoroquinolones, antipsychotics, etc.) and an electrolyte imbalance (hypokalemia, hypocalcemia, or hypomagnesemia) with or without genetic susceptibility [50]. In LQTS, triggered activity due to early afterdepolarization (EAD) leads to TdP. Recent studies showed that not only reactivation of the L-type Ca2+ current (ICa,L) but also spontaneous SR Ca2+ release also play a role in the genesis of phase 2 EADs [51]. Furthermore, heterogeneous prolongation of the action potential duration (APD) sets the stage for reentry and phase 3 EADs caused by electrotonic reexcitation at the sites with a steep repolarization gradient [51]. The initial treatment for polymorphic VT storms in LQTS is the discontinuation of possible QT-prolonging medications and rapid correction of an electrolyte imbalance, if present. Beta-blockers are the first-line therapy for congenital LQTS (particularly in LQT1 and LQT2), which is associated with defective ion channels needed to adequately adapt to beta-adrenergic stimulation [49]. Left cardiac sympathetic denervation may be useful for LQTS patients who cannot tolerate or are resistant to beta-blocker therapy [52]. In contrast to congenital LQTS, beta-blockers may worsen the situation in acquired LQTS because bradycardia, which is promoted by beta-blockers, further prolongs the QT interval and heterogeneous repolarization [51]. Thus, temporary pacing is the treatment of choice for patients with bradycardia-dependent polymorphic VT in LQTS. Isoproterenol can be used in the interim while awaiting pacing treatment, but it might aggravate the polymorphic VT if it does not increase the heart rate enough to shorten the QT interval, since phase 2 EADs are enhanced by the isoproterenol-induced increase in Ca2+ influx. Intravenous verapamil blocks ICa,L and effectively suppresses polymorphic VT refractory to beta-blockers in congenital LQTS [53]. Magnesium modulates Ca2+ ions and has been called “nature׳s physiologic Ca2+ blocker.” Intravenous magnesium sulfate can facilitate termination of polymorphic VT associated with LQTS [54]. Magnesium sulfate has few hemodynamic effects and seems safer for hemodynamically unstable patients. If the LQTS genotype is known to be LQT3, which is attributed to a gain-of-function mutation in the SCN5A gene, drugs with late Na+ current blocking effects such as mexiletine, ranolazine, and propranolol are useful [49].