Larissa Araújo de Lucena, Flávia da Costa Fernandes Mesquita, Micheli Rigon Madeira, Ferdinand Gilbert Saraiva da Silva Maia
BACKGROUND: Propafenone, a class Ic antiarrhythmic with a narrow therapeutic window, may cause conduction disturbances. CASE SUMMARY: A 68-year-old man on propafenone presented with dyspnea and dizziness. Electrocardiogram showed QRS widening (160 milliseconds). Angiography excluded occlusion but showed transient ventricular dysfunction. Sodium bicarbonate normalized QRS complex and reversed dysfunction. CONCLUSIONS: New wide-QRS rhythm in patients on class Ic agents suggests sodium channel blocker toxicity. QRS narrowing after bicarbonate is diagnostically important. Early treatment ensures reversibility and prevents arrhythmias. TAKE-HOME MESSAGES: Toxicity in new wide-QRS rhythm on class Ic drugs should be assumed. QRS narrowing after bicarbonate is key. Early treatment prevents fatal complications.