Wide QRS Complex Tachycardia Change to Narrow QRS Complex Tachycardia After β-Blocker Administration

A patient in their 80s with a history of paroxysmal atrial fibrillation presented to the emergency department with palpitation, dizziness, nausea, and vomiting. The patient was treated with propafenone 3 × 150 mg/d and discontinued metoprolol use a week ago due to low blood pressure. The patient remained mentally clear. Physical examination revealed a blood pressure of 82/60 mm Hg and heart rate of 160 beats/min. Serum potassium level was 5.2 mmol/L. A 12-lead electrocardiogram (ECG) was obtained (Figure, A).

Questions: What is the ECG diagnosis, and how should this patient’s treatment be managed?

The ECG revealed a regular monomorphic wide QRS complex tachycardia (162 beats/min) with a right bundle-branch block (RBBB) pattern and a reversed (<1.0) R/S ratio in lead V₆. The possible differential diagnoses include (1) ventricular tachycardia (VT) from the mid to apical left ventricle or idiopathic fascicular VT, (2) supraventricular tachycardia (SVT)/atrial flutter (AFL) with aberration, (3) SVT with preexcitation, and (4) SVT/AFL with QRS widening due to electrolyte disturbance or antiarrhythmic drugs (AADs). Several features supported the diagnosis of VT: in lead II, the R wave peak time (80 milliseconds) was greater than 50 milliseconds (specificity, 99%; sensitivity, 93%).¹ In lead aVR, there was an initial dominant R wave (accuracy, 98.6%).² In lead V₆, the R/S ratio was less than 1.0 (specificity, 78.3%; sensitivity, 82.1%).³ However, several features argue against this: in leads V₅ and V₆, the R to S interval (80 milliseconds) was not more than 100 milliseconds.⁴ Atrioventricular (AV) dissociation was not found.