The present description relates to methods of administering effective amounts of an anti-arrhythmic agent, e.g., dantrolene, azumolene or a pharmaceutically acceptable salt thereof, for the acute treatment of cardiac arrhythmias, e.g., atrial fibrillation, premature ventricular contraction, ventricular tachycardia or ventricular fibrillation, and prevention of subsequent cardiac arrhythmias, wherein the methods effectuate a reduction in morbidity and mortality.