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Symptomatic sustained cardiac arrhythmias are frequently observed in in the coronary care unit and often lead to hemodynamic compromise, especially in the presence of multisystem disease. The predominant arrhythmias noted in intensive care units are tachyarrhythmias, particularly atrial fibrillation and flutter, and ventricular tachycardia. Bradycardias, arguably less lifethreatening than tachyarrhythmias, can arise from sinus node dysfunction or atrioventricular conduction block; transient vagally-mediated bradycardias are frequently encountered as well. Prompt diagnosis of the patient with tachycardia is critical as treatment depends on the accurate diagnosis of tachycardia mechanism. The electrocardiogram remains the most important diagnostic tool for the evaluation of both wide and narrow complex tachycardia. The electrocardiographic diagnosis of wide complex tachycardia is based on evaluation of atrioventricular relationship and QRS morphology while the diagnosis of narrow complex tachycardia is based on the location and morphology of P waves. It is important for critical care specialists to understand the principles of cardiac arrhythmia diagnosis and remain current with the recent advances in the pharmacologic and non-pharmacologic management of patients with arrhythmias.