Phenytoin (Concepts) Flashcards
Uses of Phenytoin
Generalized tonic-clonic seizures
Complex partial seizures
Prevention following neurosurgery
Phenytoin MOA
Increases efflux or decreases influx of Na.
S of Phenytoin acid
1.0
S of Phenytoin sodium
0.92
Oral Phenytoin
Dilantin Kapseals = Phenytoin Sodium capsules
Generic Phenytoin sodium capsules as well
Dilantin-125 suspension
- per 5mL
Dilantin Infatabs = Phenytoin acid chewable tablets
Can you substitute rapid and sustained-release phenytoin products?
No.
Phenytoin administration considerations
Be consistent in terms of with or without meals.
Pause continuous nasogastric feeding for 1-2 hours before and after dose
Brand-Generic considerations
Don’t switch if no reason.
Contact prescriber before making change.
Maintain generic supplier.
Careful consideration with:
- elderly
- patients requiring levels of 20mg/L or more.
Phenytoin IV products
Dilantin IV
Fosphenytoin (Cerebyx)
Dilantin IV characteristics
- Phenytoin sodium!
- S = 0.92, even though IV
- 30 min infusion
- Monitoring required.
- ONLY NS; 100-150mL
- Infuse within 4 hours of mixing.
Fosphenytoin characteristics
Prodrug: converted to phenytoin by plasma esterases.
- More water soluble.
- Dose expressed in Phenytoin SODIUM equivalents (PE)
- Dilute to [1.5-25]
- 30 min infusion
- Requires monitoring
Which IV phenytoin product is preferred in Peds?
Fosphenytoin
Which IV phenytoin can be given IM?
Fosphenytoin
Which IV phenytoin can be put in NS or D5W?
Fosphenytoin
Which IV phenytoin needs an in-line filter?
Phenytoin sodium (Dilantin IV)
Which IV phenytoin contains propylene glycol?
Phenytoin sodium (Dilantin IV)
Weight based dosing of Fosphenytoin:
15-20mg PE/kg in adults
10-20mg PE/kg in peds
IV phenytoin max rate
50mg/min
IV fosphenytoin max rate
150mgPE/min
IV phenytoin : IV fosphenytoin conversion
1mg phenytoin sodium
1.5mg fosphenytoin
Adverse Events seen with Parenteral Products DURING INFUSION
Can be severe:
- Hypotension
- Bradycardia
- Dysrhythmias
- Lethargy
Adverse Events seen with Parenteral Products depending on SERUM LEVELS
15mg/mL
- drowsy/fatigue
20mg/mL
- nystagmus
30mg/mL
- ataxia, incoordination
40mg/mL
- mental status changes (even coma)
Over 50-60
- drug-induced seizures
Adverse Effects seen with Parenteral Products after LONG TERM use
Hypertrichosis*
Gingival hyperplasia*
Altered facial features Carbohydrate intolerance/hyperglycemia Folate deficiency VitD deficiency SLE (drug-induced lupus)
Purple Glove Syndrome
Seen with multiple large doses of IV phenytoin:
DC it.
Monitoring during IV infusion
Cardiac monitor/telemetry
Someone actually there
Therapeutic Ranges
Total
- Children/Adults: 10-20
- Neonates: 8-15
Free
- 1-2.5 (more important)
3 Reasons to draw free levels
Concern subtherapeutic
Concern toxicity
Renal-failure
Causes of hypoalbuminemia that alter phenytoin plasma protein binding?
Liver disease: dec. synthesis
Nephrotic syndrom: urine-losses
Burns: hypermetabolic
Trauma: hypermetabolic
Malnourishment
Eldery
Causes of displacement from plasma proteins due to ENDOGENOUS compounds
Hyperbilirubinemia
- Jaundice/liver disease
Toxin accumulation
- Renal dysfunction
Causes of displacement from plasma proteins due to EXOGENOUS compunds
Warfarin
Valproic acid
Aspirin > 2g….
NSAIDs
Phenytoin: low E or high E?
Low E.
Css,total with increased Fup
Decreased
Css,free with increased Fup
Decreased
Michaellis-Menten PK
Used b/c enzyme system responsible for extraction/metabolism is saturable.
