A 45-year-old female with paroxysmal atrial fibrillation is administered propafenone to maintain sinus rhythm. Two weeks later while hiking she has a cardiac arrest and dies. Which of the following would have most likely been present on ECG just prior to death?
A) interval prolongation
(B) PR interval prolongation
(C) QRS interval prolongation
(D) Supraventricular tachycardia
(E) Torsades de Pointes
Answer C (correct answer): Propafenone is a class Ic antiarrhythmic that blocks cardiac sodium channels. This drug exhibits “use dependence” meaning binding to sodium channels increases at fast heart rates as explained in the video. For this reason, propafenone toxicity is most likely to occur during exercise such as hiking.
Through blockade of sodium channels, propafenone may prolong the QRS interval. If QRS interval prolongation is excessive, ventricular tachycardia may occur through a mechanism called ventricular re-entry. Patients on propafenone are usually referred for a cardiac stress test shortly after starting the drug to screen for exercise-induced QRS prolongation. If this occurs, the drug is discontinued or the dose changed.
Answer A (incorrect answer): QT prolongation occurs with class III antiarrhythmic drugs such as amiodarone, dofetilide, and sotalol. It may also be seen in class Ia drugs like quinidine and procainamide.
Answer B (incorrect answer): PR interval prolongation is seen with AV nodal blocking drugs such as beta blockers, calcium channel blockers, and digoxin.
Answer D (incorrect answer): Supraventricular tachycardia does not occur due to propafenone. This type of arrhythmia may develop in patients with AV nodal reentrant tachycardia (AVNRT) and other disorders.
Answer E (incorrect answer): Torsades de Pointes is a form of ventricular tachycardia that occurs in the setting of excessive QT prolongation. This can be seen class III antiarrhythmic drugs such as amiodarone, dofetilide, and sotalol.