Fosphenytoin Loading Dose Calculation
For a 60 kg patient requiring 20 mg/kg phenytoin loading, administer 1200 mg PE (phenytoin equivalents) of fosphenytoin intravenously at a maximum rate of 150 mg PE/min.
Dose Calculation
- The loading dose is 20 mg PE/kg × 60 kg = 1200 mg PE of fosphenytoin 1, 2
- Fosphenytoin dosing must always be expressed in phenytoin equivalents (PE) to prevent 10-fold dosing errors 1, 2
- The FDA-approved loading dose range for status epilepticus is 15-20 mg PE/kg, and your calculation of 20 mg/kg falls at the upper end of this therapeutic range 2
Administration Parameters
- Infuse at a maximum rate of 150 mg PE/min in adults 1, 2
- At this rate, the 1200 mg PE dose will take approximately 8 minutes to infuse 2
- Dilute only in normal saline to a final concentration ≥5 mg PE/mL; never mix with dextrose-containing solutions as this causes precipitation 1, 3
Critical Safety Monitoring During Infusion
Continuous cardiac and blood pressure monitoring is mandatory throughout the infusion: 1, 3
- Monitor ECG continuously for bradycardia, arrhythmias, and heart block 4, 1
- Monitor blood pressure closely for hypotension 4, 1
- Reduce infusion rate immediately if heart rate decreases by 10 beats/min 4, 1
- Never exceed the maximum infusion rate of 150 mg PE/min, as rapid administration causes hypotension, bradyarrhythmias, and cardiac arrest 4, 1
Advantages of Fosphenytoin Over Phenytoin
- Fosphenytoin has significantly fewer adverse cardiovascular events compared to phenytoin due to its faster allowable infusion rate (150 mg PE/min vs 50 mg/min for phenytoin) 5, 1
- Fosphenytoin has fewer local tissue reactions and extravasation injuries compared to phenytoin 1, 3
- The American College of Emergency Physicians notes that head-to-head analysis shows fewer adverse events with IV fosphenytoin compared to IV phenytoin loading 5
Expected Outcomes
- Therapeutic serum phenytoin levels (10-20 mcg/mL) are typically achieved within minutes after completion of the infusion 3
- Recent evidence shows that weight-based loading doses of 19.1 mg/kg achieve therapeutic levels in 91.4% of patients, and higher weight-based dosing is a significant predictor of achieving goal levels 6
Common Pitfalls to Avoid
- Always use actual body weight, not estimated weight - estimation errors can result in doses ranging from 78% to 138% of the intended dose, with 20% of patients achieving supra-therapeutic levels when weights are estimated 7
- Do not administer maintenance doses until after checking the first phenytoin level 6
- Watch for transient paresthesias and pruritus with rapid infusion, which are generally mild but may require slowing the infusion rate 1