What is the intravenous (IV) phenytoin dose for seizures in a 68-kilogram male with a subtherapeutic phenytoin level?

From the Guidelines

For a 68 kg male with a subtherapeutic phenytoin level of 5.7 micrograms/mL experiencing seizures, I recommend administering a loading dose of IV phenytoin 1,224 mg (18 mg/kg) as per the guidelines outlined in 1 and 1. This dose is calculated based on the patient's weight and the need to rapidly achieve a therapeutic phenytoin level of 10-20 micrograms/mL. The infusion should be given at a maximum rate of 50 mg/minute, meaning the infusion would take approximately 24 minutes to complete. Some key points to consider when administering IV phenytoin include:

• Using a large vein and normal saline as the diluent since phenytoin precipitates in dextrose solutions
• Monitoring the patient closely during administration for hypotension, cardiac arrhythmias, and infusion site reactions
• Checking the phenytoin level in 2 hours after administering the loading dose, then initiating maintenance dosing of 4-6 mg/kg/day (approximately 272-408 mg daily, divided into 2-3 doses) The loading dose calculation is based on the principle that each 1 mg/kg of phenytoin raises the serum level by approximately 1-1.5 micrograms/mL, and this patient needs to increase his level by at least 4.3 micrograms/mL to reach the therapeutic range, as suggested by 1. It's also important to note that other options like valproate or fosphenytoin may be considered as alternatives to phenytoin, as mentioned in 1, but the recommended dose of IV phenytoin is the most appropriate choice in this scenario.

From the FDA Drug Label

The loading dose should be followed by maintenance doses of oral or intravenous phenytoin every 6 to 8 hours. A loading dose of 10 to 15 mg/kg should be administered slowly The rate of intravenous administration should not exceed 50 mg per minute in adults and 1 to 3 mg/kg/min (or 50 mg per minute, whichever is slower) in pediatric patients. When treatment with oral phenytoin is not possible, IV phenytoin can be substituted for oral phenytoin at the same total daily dose.

To determine the IV phenytoin dose for a 68 kg male, we need to calculate the loading dose.

• The recommended loading dose is 10 to 15 mg/kg.
• For a 68 kg male, the loading dose would be 680 mg to 1020 mg.
• Since the patient's current phenytoin level is 5.7 micrograms/mL, which is below the therapeutic range, a loading dose may be necessary.
• However, the exact dose cannot be determined without more information about the patient's current medication regimen and the desired serum concentration.
• The rate of administration should not exceed 1 to 3 mg/kg/min or 50 mg per minute, whichever is slower.
• For a 68 kg male, the maximum rate of administration would be 68 mg/min to 204 mg/min or 50 mg per minute, whichever is slower.
• Therefore, the maximum rate of administration would be 50 mg per minute. 2

From the Research

Phenytoin Dosing for Seizures

To determine the appropriate IV phenytoin dose for a 68 kg male with a phenytoin level of 5.7 micrograms/mL, we can refer to the study by 3.

• The study suggests an initial loading dose of 15 mg phenytoin acid/kg body weight, given intravenously over 4 hours.
• For a 68 kg male, the loading dose would be 15 mg/kg x 68 kg = 1020 mg.
• The study also provides maintenance doses, which are 175 mg for patients weighing ≤ 70 kg and 202 mg for patients weighing > 70 kg, given intravenously every 12 hours.
• However, the maintenance dose may need to be individualized based on serum trough levels, as demonstrated in the study by 3.

Considerations for Phenytoin Use

When using phenytoin, it's essential to consider the potential risks and side effects, as highlighted in the studies by 4, 5, and 6.

• Phenytoin can cause local cutaneous reactions, severe hypotension, cardiac arrhythmias, and increased risk of severe allergic reactions.
• The studies suggest that levetiracetam may be a safer and more effective alternative to phenytoin for the treatment of status epilepticus and acute repetitive seizures.
• However, the decision to use phenytoin or an alternative medication should be based on individual patient needs and medical history, as well as the specific clinical scenario.

Monitoring and Dose Adjustment

Regular monitoring of phenytoin levels and adjustment of the dose as needed is crucial to ensure therapeutic levels and minimize side effects, as demonstrated in the study by 3.

• The study used a Bayesian forecasting computer program to predict maintenance doses based on non-steady-state serum levels.
• The results showed that the prediction error was 3.95 mg/L, and 70% of the levels were within the 68% confidence interval.
• This approach can help individualize phenytoin dosing and improve patient outcomes.