Cyclopropane and halothane anesthetics increase cardiac autonomic irritability and therefore seem to sensitize the myocardium to the action of intravenously administered epinephrine or levarterenol. Hence, the use of Norepinephrine during cyclopropane and halothane anesthesia is generally considered contraindicated because of the risk of producing ventricular tachycardia or fibrillation. The same type of cardiac arrhythmias may result from the use of Norepinephrine in patients with profound hypoxia or hypercarbia. Norepinephrine should be used with extreme caution in patients receiving monoamine oxidase (MAO) inhibitors or anti-depressants of the triptyline or imipramine types, because severe and prolonged hypertension may result.