What are Class I antiarrhythmic drugs?

• A. Sodium Channel Blockers
• B. Potassium Channel Blockers
• C. Calcium Channel Blockers
• D. Beta Blockers

Answer: (A)

Class I antiarrhythmic drugs are sodium channel blockers. They bind to fast sodium channels in cardiac cells and reduce the inward Na current during the rapid upstroke of the action potential (phase 0). This slows the rate of depolarization and conduction velocity through atrial and ventricular tissue and the Purkinje system, which helps interrupt and prevent tachyarrhythmias. Their effect can depend on heart rate (use-dependent block), and they’re further categorized into subtypes that differ in how much they lengthen or shorten the action potential, but they all share the common feature of blocking fast Na channels. Examples include drugs like quinidine and procainamide (intermediate effects), lidocaine (more effect on inactivated channels), and flecainide or propafenone (strong Na block with marked conduction slowing). Potassium channel blockers, calcium channel blockers, and beta blockers act through different mechanisms and are not Class I.