The other uncommon types of idiopathic VT

The other uncommon types of idiopathic VT such as non-reentrant focal Purkinje VT, papillary muscle VT, and mitral/tricuspid annular VT may also present as incessant or repetitive forms of monomorphic VT. Importantly, an incessant nature of the VTs can secondarily decrease the global (not segmental) left ventricular (LV) function, despite the normal heart structure [16,17,26]. The prognosis of such patients is excellent if the VT is controlled by RFCA or medical therapy with complete recovery of the LV function.
Monomorphic VT storms in structural heart disease Monomorphic VT storms associated with structural heart disease are the most common type of electrical storm. Reentry is responsible for most monomorphic VTs. Conduction and repolarization abnormalities lie within heterogeneous areas of scarred myocardium caused by fibrosis and collagen deposition of various etiologies (Table 1). Bundles of surviving myofibrils around the border of the scar provide an anatomic substrate for conduction pathways, allowing electrically stable reentry. Focal mechanisms usually play an important role in initiating reentry, but may also contribute to maintenance of the VT, especially in patients with non-ischemic cardiomyopathy [27]. An experimental study has shown that beta-adrenergic stimulation due to sympathetic activation, which generally follows hemodyamically-unstable VT, contributes greatly to focal ventricular ectopy and VT recurrence through the DAD mechanism [28]. Beta-adrenergic stimulation enhances intracellular Ca2+ overload secondary to rapid activation during VT, and Ca2+/calmodulin-dependent protein kinase II activation may promote Ca2+ handling abnormality and DADs [29]. The genesis of a DAD requires high membrane responsiveness to changes in intracellular Ca2+ (i.e., high intracellular Ca2+-membrane voltage coupling gain) [28], and it is known that Ca2+-voltage coupling gain increases in heart failure via downregulation of the inward rectifier K+ current and upregulation of the Na+–Ca2+ exchange current [30]. Purkinje buy Aminoallyl-dCTP - Cy3 display a greater propensity to develop a Ca2+ handling abnormality than ventricular myocytes [31], and DADs seem to arise predominantly from Purkinje fibers [28]. Furthermore, the Purkinje fiber network itself can serve as a substrate for the VT reentrant circuit by a similar mechanism to idiopathic fascicular VT in patients with prior myocardial infarction [32] and non-ischemic cardiomyopathy [33], leading to a monomorphic VT storm. Intravenous administration of class I antiarrhythmic drugs can be used for the treatment of monomorphic VT storms. Procainamide is reported to be superior to lidocaine for termination of monomorphic VT [34]. However, electrical storms tend to occur in patients with structural heart disease and severely reduced LV function, where class I antiarrhythmic drugs can be harmful. In such cases, class III antiarrhythmic drugs, such as intravenous amiodarone [5] and nifekalant [35], are useful unless the patient has a prolonged QT interval. In light of the role of DADs in triggering monomorphic VT storm, sympathetic blockade is a key treatment to control the VT storm. Beta-blockers that suppress DADs by alleviating intracellular Ca2+ overload rank foremost for this purpose. It should be noted that not all beta-blockers offer the same level of antiarrhythmic effects [36], which might depend on beta-1 selectivity, a lipophilic nature, or other pleiotropic effects. Recent experimental studies have shown that carvedilol inhibits DADs by a direct action on ryanodine receptor type 2 (RyR2) independent of its beta-blocking effect [37]. Future clinical studies are needed to determine which beta-blocker has the most favorable effects. Sedation with short-acting anesthetics such as propofol, benzodiazepines, and some general anesthesia agents is also helpful in suppressing VT storms [38], since physical and emotional stresses in association with electrical storms contribute to the enhancement of sympathetic activity. Sedation may also help prevent any post-treatment psychological distress [39]. Hence, all patients who have electrical storms should be sedated appropriately. If a pharmacologic sympathetic blockade is not sufficient to control the VT storm, neural modulation with a left or bilateral stellate ganglion blockade [8] or thoracic epidural anesthesia [40] may be a potential treatment of choice for VT storms. Owing to limited data, neural modulation should be reserved for patients who are resistant to drugs and RFCA.