Is Coumel's Law Always True?

We report a 27-year-old man presenting with paroxysmal palpitations due to alternating wide and narrow QRS tachycardias that transitioned without termination. Based on Coumel's law, an accessory pathway-mediated mechanism was initially suspected for both tachycardias. However, the wide QRS tachycardia was ultimately confirmed to be ventricular tachycardia. Although Coumel's law may aid in the diagnosis of tachycardias with alternating QRS widths, potential confounding factors must be carefully considered to prevent misdiagnosis.