A 49-year-old, 75kg male with a history of epilepsy is brought to the emergency department in status epilepticus. The team decides to initiate intravenous antiepileptic therapy. The pharmacist is consulted to verify the appropriate dosing and administration.

At a dose of 20mg/kg, which of the following regimens of IV of Dilantin is appropriate?

A. 1500mg in 10mL of NS IV push over 5 minutes

B. 1500mg in 300mL of NS infused over 30 minutes

C. 1500mg in 150mL of NS infused over 30 minutes

D. 1500mg in 300mL of D5W infused over 15 minutes

E. 1500mg in 60mL of D5W infused over 10 minutes

Answer with rationale:

Phenytoin is an antiepileptic drug that works by blocking inactive voltage-gated sodium channels, which stabilizes neuronal membranes and reduces repetitive firing. It can be used in the management of status epilepticus, typically after benzodiazepines, to help prevent seizure recurrence. The pharmacist plays an important role in appropriate administration, as there are important concentration and infusion rate limits for phenytoin. It should be infused at a maximum rate of 50 mg/min in adults, because the IV formulation contains propylene glycol, which can cause hypotension, bradycardia, and other arrhythmias. The maximum concentration for IV phenytoin is 5mg/mL in solution and can only be diluted in normal saline, because it is a very basic drug that is a vesicant and has a high risk for extravasation. This can cause “purple glove syndrome,” where pain, swelling, and potential tissue necrosis can occur, and the extremities in the infusion arm become a purple color. A central line is preferred to avoid some of these effects.

Fosphenytoin is also an option and generally preferred, always dosed in phenytoin equivalents (mg PE) to prevent confusion. It is not formulated with propylene glycol, so it poses much less risk of hypotension and is not a vesicant. It can also be diluted in either normal saline or D5W. These properties allow it to have a max infusion rate of 150mg PE/min and a max concentration of 25mg PE/mL. 

20mg/kg x 75kg = 1500mg of phenytoin

1500mg x 1mL/5mg = 300mL of NS minimum

1500mg x 1min/50mg= 30 mins fastest

Answer A is incorrect: Phenytoin should never be administered faster than 50mg/min to avoid hypotension and arrhythmia risk. In addition, all doses including maintenance are generally avoided as IV push due to adverse effects. 

Answer B is correct: This is the max infusion rate and concentration recommended for a 75kg patient at 20mg/kg.

Answer C is incorrect:  The infusion rate is accurate, however it is too concentrated (10mg/mL).

Answer D is incorrect:  The concentration is accurate, however it would be too fast of an infusion rate, and phenytoin cannot be diluted in D5W.

Answer E is incorrect:  These were calculated based on the maximums for Fosphenytoin (25mgPE/mL and 150mgPE/min) so would be incorrect for phenytoin.

Brand/generics Covered:

Phenytoin (Dilantin), Fosphenytoin (Cerebyx)

Naplex Content Domains Covered

1.C.2,3,5: Calculations of quantities of drugs to be administered, rates of administration, and drug concentrations

3.D.2: Plan development regarding safety