IV Phenytoin Loading in Status Epilepticus
A 72-year-old man, weight 72 kg, is admitted in status epilepticus. The neurology team orders a phenytoin loading dose of 18 mg/kg IV. The pharmacy stocks phenytoin sodium 250 mg/5 mL ampoules, 0.9% sodium chloride 500 mL bags, and 0.22-micron in-line filters. The patient already has a peripheral IV line running dextrose 5% in water. His blood pressure is 150/90 mmHg, pulse 118/min, and the ECG shows sinus tachycardia. He has no history of heart block, and renal and hepatic function are normal. The nurse asks the pharmacist to prepare the loading dose for infusion over the shortest safe period. The ward has no fosphenytoin. A technician has withdrawn 26 mL of phenytoin concentrate from the ampoules but has not selected a diluent. The patient has no other IV access.
1. Calculate the loading dose, the volume of phenytoin concentrate, and the maximum infusion rate. How would you prepare the final infusion?
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Dose: 18 mg/kg × 72 kg = 1,296 mg, rounded to 1,300 mg. Concentrate: phenytoin sodium 250 mg/5 mL = 50 mg/mL, so 1,300 ÷ 50 = 26 mL, equivalent to 5.2 ampoules. Practically, withdraw 26 mL from six 5 mL ampoules and discard the excess; never pool leftover drug. Rate: the usual maximum adult IV phenytoin rate is 50 mg/min, so 1,300 ÷ 50 = 26 minutes minimum. Because the patient has no cardiac conduction disease, 50 mg/min is acceptable; use 25 mg/min if hypotension or arrhythmia develops. Dilution: dilute in 0.9% sodium chloride to a maximum concentration of 5 mg/mL; 1,300 mg in 500 mL gives 2.6 mg/mL. At 50 mg/min the pump rate is 19.2 mL/min (1,152 mL/hour), delivered over 26 minutes, with a 0.22-micron in-line filter. Confirm concentration and rate independently before starting.
2. The existing IV line contains dextrose 5% in water. Explain the compatibility problem and what you will do.
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Phenytoin sodium injection is alkaline and dextrose 5% is acidic; mixing causes phenytoin to precipitate, and the precipitate can occlude the line or embolize. Stop the dextrose infusion, flush the catheter with 0.9% sodium chloride, and administer phenytoin through a separate line or after adequate saline flushing. Do not add phenytoin to the dextrose bag or co-infuse. Inspect the tubing and filter during infusion; if precipitate appears, stop and restart with new tubing. Use a large peripheral vein and monitor for extravasation because phenytoin can cause severe tissue injury, including purple glove syndrome.
3. What monitoring is required during and immediately after the infusion?
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Monitor ECG continuously and blood pressure every 5 minutes during the 26-minute infusion; stop or slow for hypotension, bradycardia, heart block, or widening QRS. Watch for nystagmus, ataxia, sedation, confusion, and respiratory depression. Inspect the IV site for pain, swelling, or discoloration. After infusion, continue vital signs and neurologic checks; check a total phenytoin level according to protocol, interpreting the timing of the sample, and monitor for delayed toxicity because phenytoin has nonlinear kinetics. Document the exact dose, concentration, rate, start and end times, and site. Have resuscitation equipment available.