Final answer:
Class 1C antiarrhythmic agents like flecainide and propafenone block the fast sodium channels responsible for ventricular depolarization (phase 0), resulting in prolongation of the QRS duration and QT interval.
Step-by-step explanation:
Class 1C antiarrhythmic agents like flecainide and propafenone block the fast sodium channels responsible for ventricular depolarization (phase 0) in the heart. These medications can have an effect on the QRS duration and QT interval.
When the fast sodium channels are blocked, it slows down the ventricular depolarization process. This can result in prolongation of the QRS duration on the ECG. Additionally, blocking the fast sodium channels can also prolong the QT interval, which represents the time it takes for the ventricles to repolarize.