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While a left anterior fascicular VT could be a possible diagnosis given the RBBB/left posterior fascicular block (LPFB) pattern, the ECG in Figure Figure1B 1B demonstrates that the A:V ratio is greater than 1, therefore excluding VT. A broad differential includes supraventricular tachycardia (SVT) with aberrancy versus ventricular tachycardia (VT). The ECG in Figure Figure1A 1A shows a regular, wide complex tachycardia (WCT) at a rate of 150 bpm with right bundle branch block (RBBB) morphology (QRS duration of 140 ms) and borderline right axis deviation and as well as a 1:1 atrial (A) to ventricular (V) ratio. What is the differential diagnosis of the ECG shown in Figure Figure1A? 1A? How does the ECG in Figure Figure1B 1B aid in determining the diagnosis? Administration of 6mg, followed shortly by 12mg, of adenosine had no effect, however, an ECG obtained 10 min after intravenous (IV) metoprolol is shown in Figure Figure1B. Bedside echocardiogram demonstrated an ejection fraction of 45% with anterior wall hypokinesis. The patient's BP was stable at 110/70 mm Hg and laboratory results were unremarkable including serum potassium (K) and magnesium (Mg) levels of 4.2 mmol/L and 2.2 mmol/L, respectively. The presenting ECG is shown in Figure Figure1A. American College of Cardiology 6: 14.5.1-7.A 65‐year‐old man with a history of coronary artery disease (CAD) status post percutaneous coronary intervention (PCI) to the left anterior descending artery (LAD) and atrial fibrillation (AF) status post recent (<3 months) pulmonary vein isolation (PVI) presented to the emergency department with symptoms of palpitations for 1 day after admittedly forgetting to take his medications. Adult Clinicall Cardiology Self-Assessment Program (ACCSAP). Variegated left ventricular electrical activation in response to a novel quadripolar electrode: Visualization by non-invasive electrocardiographic imaging. Mechanisms underlying atrioventricular nodal conduction and the reentrant circuit of atrioventricular nodal reentrant tachycardia using optical mapping. Mechanisms underlying the reentrant circuit of atrioventricular nodal reentrant tachycardia in isolated canine atrioventricular nodal preparation using optical mapping. Wu J, Wu J, Olgin J, Miller JM, Zipes DP. Cardiac Electrophysiology: From Cell to Bedside. Autonomic modulation of cardiac arrhythmias. Philadelphia, Pa: Saunders Elsevier 2008. Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine. Libby P, Bonow RO, Mann DL, Zipes DP, eds. Relationship of electro-mechanical remodeling to survival rates after cardiac resynchronization therapy. Kiani J, Agarwal SK, Kamireddy S, Adelstein E, Saba S. Philadelphia, Pa: Lippincott Williams & Wilkins 2005.