01-02-2014, 10:20 PM
QRS complex duration (normal range: 0.08-0.12 sec) is typically slightly reduced during exercise in response to the increase in cardiac conduction velocity that accompanies faster heart rates. However, this patient's QRS duration is normal at rest and prolonged during the immediate recovery period when his heart rate would still be elevated. This suggests that the drug used to treat his atrial fibrillation prolongs the QRS duration in a rate-dependent manner, which is consistent with a drug that exhibits strong use-dependence such as flecainide.
Flecainide is a class 1C antiarrhythmic typically used to treat supraventricular tachycardias such as those caused by atrial fibrillation. Class 1C antiarrhythmics bind avidly to the fast sodium channels responsible for phase 0 depolarization, blocking the inward sodium current and prolonging the QRS duration. Class 1C drugs are the slowest of the class 1 agents to dissociate from the sodium channel. This results in a phenomenon known as use-dependence, in which their sodium blocking effects intensify as the heart rate increases due to less time between action potentials for the medication to dissociate from the receptor.
Flecainide is a class 1C antiarrhythmic typically used to treat supraventricular tachycardias such as those caused by atrial fibrillation. Class 1C antiarrhythmics bind avidly to the fast sodium channels responsible for phase 0 depolarization, blocking the inward sodium current and prolonging the QRS duration. Class 1C drugs are the slowest of the class 1 agents to dissociate from the sodium channel. This results in a phenomenon known as use-dependence, in which their sodium blocking effects intensify as the heart rate increases due to less time between action potentials for the medication to dissociate from the receptor.