Higher initial and maintenance doses of phenylephrine are required in patients with persistent or untreated severe hypotension or shock. Hypotension produced by powerful peripheral adrenergic blocking agents, chlorpromazine, or pheochromocytomectomy may also require more intensive therapy. The rate of flow is then adjusted until the desired blood pressure level is obtained. Hypertension should be avoided. The blood pressure should be checked frequently.
Arrhythmias are rare.
Routine parenteral use of phenylephrine has been recommended by many investigators for the prophylaxis and treatment of hypotension during spinal anesthesia. It is best administered s.c. or i.m. 3 or 4 minutes before injection of the spinal anesthetic. For hypotensive emergencies during spinal anesthesia, phenylephrine may be injected i.v. beginning with a dose of 0.2 mg. Any subsequent dose should not exceed the previous dose by more than 0.1 to 0.2 mg and should not be more than 0.5 mg.