The type of ventricular arrhythmia monomorphic VT or polymor
The type of ventricular arrhythmia (monomorphic VT or polymorphic VT/VF) responsible for the electrical storm provides an important diagnostic clue to its pathophysiology (Fig. 1). In this review, we address the diagnosis and specific treatments according to the type of ventricular arrhythmia and the presence or absence of structural fiin disease.
Monomorphic VT storms in structurally normal hearts The majority of patients with monomorphic VT storm have structural heart disease, but monomorphic VT storms can occur in structurally normal hearts on rare occasions [16,17] (Fig. 2). Monomorphic VT occurring in structurally normal hearts is referred to as idiopathic VT. The characteristics of idiopathic VT depend on the origin of the VT. VT arising from the outflow tract is the most common form of idiopathic VT (OT-VT), which is characterized by VT with left bundle branch block (LBBB) and inferior axis QRS morphology. The typical mechanism of OT-VT involves triggered activity due to cyclic adenosine monophosphate (AMP)-mediated delayed afterdepolarizations (DADs) . Beta-adrenergic stimulation increases the intracellular cyclic AMP and intracellular Ca2+ levels, resulting in spontaneous Ca2+ release from the sarcoplasmic reticulum (SR), DADs, and triggered activity. Therefore, OT-VT can be suppressed typically by beta-blockers that lower the stimulated levels of cyclic AMP (and thus decrease intracellular Ca2+) or non-dihydropyridine Ca2+ channel blockers (verapamil or diltiazem) that directly reduce intracellular Ca2+ by inhibiting the inward L-type Ca2+ current. As a second-line therapy, class IC or III antiarrhythmic agents are also effective for suppressing OT-VT . Radiofrequency catheter ablation (RFCA) is a safe and reliable technique for the treatment of OT-VT, unless it has an epicardial origin that may require less safe approaches through the coronary venous system, a subxyphoid approach for RFCA, or surgical ablation (an epicardial origin is suggested by delayed initial precordial QRS activation as quantified by a maximum deflection index ≥0.55 , Q wave amplitude in aVL to aVR >1.4, or S wave amplitude in V1 ≥1.2mV ). If available, RFCA can be the first choice of treatment for OT-VT not only when OT-VT is refractory to medical therapy, because RFCA can be a curative treatment for OT-VT. Activation mapping is useful when seeking the VT origin during an electrical storm. The earliest activation at successful ablation sites typically precedes the surface QRS onset by 20–40ms, with unipolar electrograms exhibiting a QS pattern with a rapid initial downstroke . Fascicular VT, also known as idiopathic left VT, is the second leading cause of idiopathic VT. The mechanism of fascicular VT is supposed to be macro-reentry involving the Purkinje fiber network, which connects to the left fascicle [23,24]. Fascicular VT is sub-classified based on the ECG morphology (a right bundle branch block [RBBB] pattern and superior or inferior QRS axis) and corresponding fascicle coupled to the reentrant circuit: left posterior fascicular VT, left anterior fascicular VT, and left upper septal VT . Left posterior fascicle VT is the most common manifestation. Fascicular VT has a characteristic ECG with a relatively narrow QRS width semiconservative replication reflects the rapid spread of activation using the specialized conduction system. The prognosis of fascicular VT is usually excellent, but a fascicular VT case with tachycardia-induced cardiomyopathy leading to a life-threatening electrical storm has been described . The distinctive feature of fascicular VT is its sensitivity to verapamil. The efficacy of class I antiarrhythmic drugs or beta-blockers is variable (can be harmful ), and intravenous verapamil is the first-line therapy for sustained fascicular VT. As with OT-VT, RFCA is highly effective as a long-term cure. Early activation of the His bundle potential usually preceding the local ventricular septal electrogram confirms the close coupling of the VT circuit with the fascicle. The earliest pre-systolic fascicular potential with or without a late diastolic Purkinje potential is an optimal ablation target for fascicular VT .