Narrow QRS complex beat or tachycardia is not diagnostic of supraventricular origin. In particular, narrow QRS complexes can arise from the ventricle in close proximity to the His-Purkinje system that activates right and left ventricular muscle in a balanced manner, or owing to ventricular ectopy occurring ipsilateral to bundle branch block and simultaneous with intrinsic conduction.
Karturi SP, Gudmundsson H, Akhtar M, Jahangir A, Choudhuri I. Spectrum of Cardiac Manifestations From Aconitine Poisoning. HeartRhythm Case Reports. 2016; 2(5):415-420.