Electrocardiographic monitoring for dysrhythmias is a major responsibility of critical care nurses, and patients with wide QRS complex tachycardias present a challenge. Criteria for differentiating the impulse origin as ventricular or supraventricular are well documented: QRS duration, QRS morphology, QRS axis, and presence of atrioventricular dissociation. However, definitive diagnosis can only be made by invasive electrophysiologic studies. Knowledge of treatment protocols is important to prevent hemodynamic deterioration. Procainamide is effective for treating ventricular and supraventricular dysrhythmias and is the drug of choice unless certainty of impulse origin exists.