Quinidine's a pharmaceutical agent that acts as a class I antiarrhythmic agent (Ia) in the heart. It's a stereoisomer of quinine, originally derived from the bark of the cinchona tree. Mechanism Like all other class I antiarrhythmic agents, quinidine primarily works by blocking the fast inward sodium current (INa). Quinidine's effect on INa's known as a use dependent block. This means that at higher heart rates, the block increases, while at lower heart rates the block decreases. The effect of… (More on Quinidine)