The document discusses anti-arrhythmic drugs and focuses on quinidine, outlining its historical use, indications for treating atrial flutter and fibrillation and preventing recurrent ventricular arrhythmias, mechanism of action in blocking sodium channels and prolonging the action potential, and common side effects like diarrhea, nausea, dizziness and vision problems. Quinidine is contraindicated in patients with certain conduction defects.