DOSAGE AND ADMINISTRATION
The addition of phenytoin solution to intravenous infusion is not recommended due to lack of solubility and resultant precipitation.
Not to exceed 50 mg per minute, intravenously in adults, and not exceeding 1 mg/kg/min to 3 mg/kg/min in neonates. There is a relatively small margin between full therapeutic effect and minimally toxic doses of this drug.
The solution is suitable for use as long as it remains free of haziness and precipitate. Upon refrigeration or freezing, a precipitate might form; this will dissolve again after the solution is allowed to stand at room temperature. The product is still suitable for use. Only a clear solution should be used. A faint yellow coloration may develop; however, this has no effect on the potency of the solution.
In the treatment of status epilepticus, the intravenous route is preferred because of the delay in absorption of phenytoin when administered intramuscularly.
Serum concentrations should be monitored and care should be taken when switching a patient from the sodium salt to the free acid form.
Phenytoin sodium injection is formulated with the sodium salt of phenytoin. Because there is approximately an 8% increase in drug content with the free acid form over that of the sodium salt, dosage adjustments and serum level monitoring may be necessary when switching from a product formulated with the free acid to a product formulated with the sodium salt and vice versa.
Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration, whenever solution and container permit.
In adults, a loading dose of 10 mg/kg to 15 mg/kg should be administered slowly intravenously, at a rate not exceeding 50 mg per minute (this will require approximately 20 minutes in a 70 kg patient). The loading dose should be followed by maintenance doses of 100 mg orally or intravenously every 6 to 8 hours.
Recent work in neonates and children has shown that absorption of phenytoin is unreliable after oral administration, but a loading dose of 15 mg/kg to 20 mg/kg of phenytoin intravenously will usually produce plasma concentrations of phenytoin within the generally accepted therapeutic range (10 mcg/mL to 20 mcg/mL). The drug should be injected slowly intravenously at a rate not exceeding 1 mg/kg/min to 3 mg/kg/min.
Parenteral phenytoin should be injected slowly and directly into a large vein through a large-gauge needle or intravenous catheter. Each injection of intravenous phenytoin should be followed by an injection of sterile saline through the same needle or catheter to avoid local venous irritation due to alkalinity of the solution. Continuous infusion should be avoided; the addition of parenteral phenytoin to intravenous infusion fluids is not recommended because of the likelihood of precipitation.
Continuous monitoring of the electrocardiogram and blood pressure is essential. The patient should be observed for signs of respiratory depression. Determination of phenytoin plasma levels is advised when using phenytoin in the management of status epilepticus and in the subsequent establishment of maintenance dosage.
Other measures, including concomitant administration of an intravenous benzodiazepine such as diazepam, or an intravenous short-acting barbiturate, will usually be necessary for rapid control of seizures because of the required slow rate of administration of phenytoin.
If administration of parenteral phenytoin does not terminate seizures, the use of other anticonvulsants, intravenous barbiturates, general anesthesia, and other appropriate measures should be considered.
Intramuscular administration should not be used in the treatment of status epilepticus because the attainment of peak plasma levels may require up to 24 hours.
Prophylactic dosage—100 mg to 200 mg (2 mL to 4 mL) intramuscularly at approximately 4-hour intervals during surgery and continued during the post-operative period.
When intramuscular administration is required for a patient previously stabilized orally, compensating dosage adjustments are necessary to maintain therapeutic plasma levels. An intramuscular dose 50% greater than the oral dose is necessary to maintain these levels. When returned to oral administration, the dose should be reduced by 50% of the original oral dose for one week to prevent excessive plasma levels due to sustained release from intramuscular tissue sites.
If the patient requires more than a week of IM phenytoin, alternative routes should be explored, such as gastric intubation. For time periods less than one week, the patient shifted back from IM administration should receive one half the original oral dose for the same period of time the patient received IM phenytoin. Monitoring plasma levels would help prevent a fall into the subtherapeutic range. Serum blood level determinations are especially helpful when possible drug interactions are suspected.