Quinidine is a pharmaceutical agent that acts as a class I antiarrhythmic agent in the heart. It is a stereoisomer of quinine, originally derived from the bark of the cinchona tree.Like all other class I antiarrhythmic agents, quinidine primarily works by blocking the fast inward sodium current (I_Na). Quinidine's effect on I_Na is 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 blocking the fast inward sodium current causes the phase 0 depolarization of the cardiac action potential to decrease (decreased V_max).Quinidine also blocks the slowly inactivating tetrodotoxin-sensitive Na current, the slow inward calcium current (I_Ca), the rapid (I_Kr) and slow (I_Ks) components of the delayed potassium rectifier current, the inward potassium rectifier current (I_KI), the ATP-sensitive potassium channel (I_KATP) and I_to.The effect of quinidine on the ion channels is to prolong the cardiac action potential, thereby prolonging the QT interval on the surface EKG.The half life of oral quinidine is 6 to 8 hours, and it is eliminated by the cytochrome P450 system in the liver. About 20 percent is excreted unchanged via the kidneys.Qunidine-induced thrombocytopenia (low platelet count) is mediated by the immune system, and may lead to thrombocytic purpura.