The antiarrhythmic drug which decreases both rate of depolarization (phase 0) as well as rate of repolarization of myocardial fibres is
**Core Concept**
The question is testing the effects of antiarrhythmic drugs on the cardiac action potential, specifically the phases of depolarization and repolarization. The cardiac action potential is a complex process involving the movement of ions across the cell membrane, which can be altered by various drugs to treat arrhythmias.
**Why the Correct Answer is Right**
The correct answer is a Class Ia antiarrhythmic drug, which works by blocking sodium channels and thereby decreasing the rate of depolarization (phase 0) of myocardial fibers. Additionally, Class Ia drugs also block potassium channels, which slows down the rate of repolarization (phase 3) of myocardial fibers. This dual effect on both depolarization and repolarization makes the correct answer effective in treating certain types of arrhythmias.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Class Ib antiarrhythmic drugs primarily block sodium channels, but they do not significantly affect potassium channels, and therefore, do not slow down repolarization.
**Option B:** This option is incorrect because Class II antiarrhythmic drugs, such as beta-blockers, primarily slow the heart rate by blocking beta-adrenergic receptors, but they do not directly affect the rate of depolarization or repolarization of myocardial fibers.
**Option C:** This option is incorrect because Class III antiarrhythmic drugs primarily block potassium channels, which slows down repolarization, but they do not significantly affect the rate of depolarization.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that Class Ia antiarrhythmic drugs can be proarrhythmic in certain situations, such as in patients with a history of myocardial infarction or those with pre-existing conduction system abnormalities.
**Correct Answer:** C. Quinidine