Fosphenytoin loading in status epilepticus: phenytoin equivalents and preparation
A 3-year-old boy weighing 14 kg is being treated in the emergency department for convulsive status epilepticus. The handwritten order reads: 'Fosphenytoin 280 mg IV once now, infuse at 3 mg PE/kg/min.' Stock on the unit is fosphenytoin sodium injection, 10 mL vials labelled 75 mg/mL (50 mg PE/mL). Normal saline and 5% dextrose in water are available as diluents. The nurse has already drawn the dose into a syringe and asks you to check it before it is given, saying the child has been seizing for 22 minutes and has received two doses of lorazepam. The infusion pump is free. Reference information available to you states a maximum infusion concentration of 25 mg PE/mL and a maximum infusion rate of 150 mg PE/min.
1. Is this order unambiguous and safe as written? Show your reasoning and state exactly what you would do before the dose is given.
Read the worked answer
The order is ambiguous in a way that could produce a one-third underdose.
Fosphenytoin is prescribed in phenytoin sodium equivalents (PE), not in milligrams of the fosphenytoin salt. The salt-to-PE ratio is 1.5:1 (75 mg fosphenytoin = 50 mg PE). Therefore 'fosphenytoin 280 mg' has two readings:
- 280 mg PE = the intended 20 mg PE/kg × 14 kg load.
- 280 mg of fosphenytoin salt = 280 × 2/3 = 186.7 mg PE = only 13.3 mg PE/kg, about one-third below the intended loading dose.
Because a subtherapeutic load in ongoing status epilepticus risks a third seizure and further neuronal injury, this must be resolved, not guessed. But resolution must not delay treatment: the child has already had two benzodiazepine doses and a 20-minute seizure burden.
Action: phone the prescriber immediately, state the two interpretations and the resultant mg/kg, and ask for the order rewritten explicitly in mg PE. Carry out the rest of the check (dose, concentration, rate, line compatibility) in parallel so the dose is ready to hang the moment clarification is received. If the prescriber cannot be reached and the order is clearly intended as 20 mg PE/kg (the standard loading dose is stated in the chart), document the clarification attempt, the rationale and the prescriber's verbal confirmation or the standard reference used. Record the final dose in mg PE as well as mg.
2. Calculate the volume to withdraw, how to dilute it, the infusion rate and duration, and the number of vials required.
Read the worked answer
Dose: 20 mg PE/kg × 14 kg = 280 mg PE.
Volume from the vial: 280 mg PE ÷ 50 mg PE/mL = 5.6 mL of the undiluted 75 mg/mL (50 mg PE/mL) product.
Dilution: the final concentration must not exceed 25 mg PE/mL. 280 mg PE ÷ 25 mg PE/mL = 11.2 mL final volume. So withdraw 5.6 mL of drug and add 5.6 mL of normal saline or 5% dextrose in water to reach 11.2 mL at 25 mg PE/mL.
Vials: one 10 mL vial contains 750 mg fosphenytoin = 500 mg PE. A single vial supplies the dose with 220 mg PE spare; a 10 mL vial is the correct size for this prescription, and the remainder must be discarded (no preservative, single use).
Rate: ordered rate 3 mg PE/kg/min × 14 kg = 42 mg PE/min. This is well below the 150 mg PE/min maximum. Duration = 280 mg PE ÷ 42 mg PE/min = 6.7 minutes (round to ~7 minutes minimum).
Practical points: use the infusion pump as offered; a 6.7-minute infusion is short enough that a free-flow error is plausible if run by gravity. Confirm the line is dedicated or flushed - fosphenytoin is incompatible with several drugs and can precipitate in the line. Label the syringe in mg PE, not mg.
3. Fifteen minutes after the infusion the child's blood pressure falls to 78/40 mmHg and he becomes bradycardic. What is your assessment and what should have been in place before the dose?
Read the worked answer
Assessment: rate-related hypotension and bradycardia are recognized adverse effects of fosphenytoin infusion, and they are more likely with rapid administration and in a small, possibly volume-depleted child who has received repeated benzodiazepines. The rate ordered (42 mg PE/min) is within the labelled maximum but is still a fast push; in practice a loading dose is often given over 10–15 minutes, or as a slower infusion with ECG monitoring, to reduce hemodynamic effects.
Immediate management: stop or slow the infusion, reassess airway, breathing, circulation and rhythm, give a fluid bolus per the unit's protocol, and escalate to the resuscitation team. Notify the prescriber. Most such episodes resolve with slowing or stopping the infusion; a fluid bolus is often enough.
What should have been in place beforehand:
- Continuous cardiac monitoring and blood pressure monitoring before, during and after the infusion - standard for any parenteral loading of a hydantoin.
- A controlled infusion device with the rate and volume limits programmed.
- A patent, large-bore IV line with the ability to give fluid rapidly, and a dedicated lumen.
- Clear labelling of the syringe in mg PE and a verbal read-back between pharmacist, nurse and prescriber, because the mg-versus-mg-PE confusion is the dominant system error in this scenario.
- Knowledge that extravasation of fosphenytoin can cause 'purple glove' syndrome; the site should be inspected during infusion.