Recommended Loading Dose of Phenytoin for Seizure Management
The recommended loading dose of phenytoin for seizure management is 18-20 mg/kg intravenously, administered at a maximum rate of 50 mg/minute in adults and 1-3 mg/kg/minute (or 50 mg/minute, whichever is slower) in pediatric patients. 1, 2, 3
Administration Guidelines
Intravenous Loading Dose
- Adults with status epilepticus: 18-20 mg/kg IV 1, 2
- Adults with non-emergent loading: 10-15 mg/kg IV 3
- Children with status epilepticus: 20 mg/kg IV (maximum initial dose: 1000 mg) 1
- Neonates with status epilepticus: 10 mg/kg IV 1
Administration Rate
- Adults: Maximum 50 mg/minute 3
- Pediatric patients: 1-3 mg/kg/minute or 50 mg/minute, whichever is slower 3
- Elderly (>50 years) or patients with cardiovascular disease: Reduced rate of 25 mg/minute to minimize cardiovascular side effects 4
Solution Preparation
- Phenytoin must be diluted in normal saline (NOT dextrose solutions) 1, 3
- Final concentration should be no less than 5 mg/mL 3
- Use an in-line filter (0.22 to 0.55 microns) for infusion administration 3
Monitoring During Administration
Vital Parameters
- Continuous cardiac monitoring is essential 3
- Monitor blood pressure every 15 minutes during infusion 5
- Monitor respiratory function throughout administration 3
- If heart rate decreases by 10 beats per minute, reduce infusion rate 1
Therapeutic Levels
- Target therapeutic range: 10-20 mcg/mL total phenytoin concentration (1-2 mcg/mL unbound) 3
- Therapeutic levels are typically achieved within 5-30 minutes of completing the infusion 6
- Most patients maintain therapeutic levels for 12-24 hours after loading 5
Potential Adverse Effects
Cardiovascular
- Hypotension and bradycardia, especially with rapid infusion 1, 4
- Higher risk in elderly patients and those with cardiovascular disease 4
Local Reactions
- Burning or irritation at IV site (can be mitigated by reducing infusion rate) 5
- Risk of tissue necrosis with extravasation due to alkalinity of solution 3
Neurological
- Nystagmus, ataxia, and drowsiness may occur with high serum levels 2
- Risk of transient supratherapeutic levels immediately after infusion 7
Important Considerations
Alternative to Phenytoin
- If available, fosphenytoin is preferred due to lower risk of adverse cardiac effects and faster administration rate (150 PE/min) 1, 2
- Fosphenytoin can cause transient hypotension but is generally better tolerated 7
Pitfalls to Avoid
- Never administer phenytoin in dextrose-containing solutions (causes precipitation) 3
- Avoid intramuscular administration due to erratic absorption, pain, and tissue necrosis 3
- Be cautious about overshoot of phenytoin levels immediately after infusion 7
- Follow each injection with sterile saline flush through the same catheter to avoid local venous irritation 3
Special Populations
- For patients with renal or hepatic disease, or hypoalbuminemia, monitoring of unbound phenytoin concentrations is more relevant 3
- Consider lower infusion rates in elderly patients and those with cardiovascular disease 4
The evidence strongly supports that a loading dose of 18-20 mg/kg achieves therapeutic levels rapidly and effectively in most patients, with an acceptable safety profile when administered at appropriate rates and with proper monitoring 1, 6, 5.