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  • SQTS is relatively rare but should be

    2019-05-07

    SQTS is relatively rare, but should be considered if the QT interval is markedly abbreviated and the ST segment is nearly absent with peaked and symmetrical T waves. Thus far, mutations in 5 different genes have been found, but genetic testing remains investigational because these genes account for only a small proportion of patients with SQTS. Heterogeneous abbreviation of the adrenergic receptors in SQTS increases transmural dispersion of repolarization and promotes the reentry responsible for polymorphic VT [55]. Class IA and III antiarrhythmic drugs, including quinidine, disopyramide, nifekalant, and amiodarone, are effective in prolonging the QT interval [55,56]. Bun et al. [13] reported that isoproterenol rapidly suppressed polymorphic VT/VF storms in a patient with SQTS. Isoproterenol reduced the transmural voltage gradients, likely because of an increased heart rate (blunting the transient outward current [Ito] current) and increase in ICa,L. Brugada syndrome is characterized by a distinct ECG morphology, the adrenergic receptors absence of structural heart disease, and a high risk of polymorphic VT/VF and sudden cardiac death. To date, 12 mutations in different genes encoding the proteins of Na+, Ca2+, and K+ channels have been found [57]. Brugada syndrome can present as an electrical storm [58]. Hypokalemia, high vagal tone, bradycardia, and fever are predisposing factors for an electrical storm. The Brugada-pattern ECG is divided into 3 types. The type 1 ECG has prominent coved ST segment elevation with a J-point amplitude of ≥2mm, followed by a negative T wave in ≥1 lead among the right precordial leads (V1 and V2) in the second, third, or fourth intercostal space [59]. A type 1 ECG is a specific finding for the diagnosis of Brugada syndrome. A transmural voltage gradient is physiologically present, which is attributable to an Ito-mediated spike-and-dome morphology or notch in the ventricular epicardium, but not the endocardium. A net outward shift in the membrane currents during phases 1 and 2 of the right ventricular epicardial action potential results in depression or loss of the action potential dome and ST-segment elevation, leading to enhancement of the transmural voltage gradient, phase 2 reentry, and polymorphic VT/VF [57]. Isoproterenol has been proven to normalize ST segment elevation and suppress electrical storms in Brugada syndrome, likely because isoproterenol increases the ICa,L and aids recovery of the action potential dome [11]. Class I antiarrhythmic agents are Na+ channel blockers that shift the net currents during the early phase of the action potential outwardly and aggravate the Brugada-pattern ECG changes, therefore, most are contraindicated because they heighten the risk of VT/VF. However, quinidine prevents polymorphic VT/VF by blocking the Ito. Other drugs that can be useful in Brugada syndrome include denopamine and cilostazol (increasing ICa,L), as well as bepridil (blocking Ito) [11]. Epicardial right ventricular outflow tract RFCA of a fractionated electrogram may be an option for severely affected patients with polymorphic VT/VF storms [57]. If the 12-lead ECG shows a J-point elevation of ≥1mm, QRS slurring or notching in ≥2 contiguous inferior and/or lateral leads in a patent with polymorphic VT/VF storm, early repolarization syndrome is diagnosed [59]. Underlying genetic abnormalities in early repolarization syndrome have not yet been demonstrated, except for a reported case with an ATP-sensitive K+ channel (IKATP) gene mutation [60]. Early repolarization syndrome and Brugada syndrome are considered to share common pathophysiologic mechanisms despite their differing responses to Na+ channel blockade [61]. As in Brugada syndrome, isoproterenol acutely suppresses polymorphic VT/VF storms, and quinidine chronically prevents electrical storms in early repolarization syndrome [12]. One matter of concern is that the early repolarization ECG pattern may be merely an innocent bystander, as the incidence of the early repolarization ECG pattern is high in the general population (5%) [62]. Therefore, if beta-blockers are used in storm patients with early repolarization of unknown significance, it is prudent to use an ultra-short-acting beta-blocker such as esmolol and landiolol [7], in case of subsequent isoproterenol infusion.