Phenytoin

Phenytoin

MOA: Phenytoin sodium is a hydantoin derivative anticonvulsant. It alters sodium flow in the motor cortex reducing neuronal excitability and the conduction of electrical activity.
1. The onset of action after IV administration is 30-60minutes and the effect persists up to 24 hours. Phenytoin is 90% and free phenytoin levels may be altered in those with hypoalbuminaemia, ureamia, after acute trauma and in pregnancy. protein bound
2. AVE:hypotension, cardiac arrhythmia if too fast. Common signs of toxicity=nystagmus, ataxia, slurred speech, decreased coordination, lethargy, mental confusion. Headaches, nausea, vomiting, constipation., Skin rash, haemopoetic complications( thrombocytopenia) Local iiritation,thrombophlebitis, inflammation,necrosis, purple glove syndrome,Dyskineasias =dystonia, tremor, asterixis.
3. Phenytoin levels may be potentiated or decreased by by carbamazepine, phenobarbitone, sodium valproate,diazepam. Acute ETOH intake.
  1. Phenytoin levels can be potentiated by diazepam, methylphenidate, omeprazole, erythromycin, antifunga agents,warfarin, isoniazid, phenothiazines, ranitidine , azoles.
    1. Phenytoin levels may be decreased by calcium, rifampicin, theophylline,methotrexate.
4. Interactions: Increases the effects of warfarin, TCAs,tramadol, MAOIs.
Narrow therapeutic index!!!!!
Indications: Tx status epilepticus: tonis clonic psychomotor seizures. Prevention of seizures during or following neurosurgery.
1. Status Epilepticus dosage: Elderly >65years, low albumin: loading dose=15mg/kg max rate 50mg/min maintenance 100mg Q6-8Hrly according to response and phenytoin levels. Other adults= 20mg/kg max rate 50mg/min, followed by maintenance PO OD.
  1. Phenytoin comes in different formulations- Capules (phenytoin sodium 30mg and 100mg), tablet(infatabs) (phenytoin 50mg), suspension (phenytoin 30mg in 5ml), injection (phenytoin sodium 50mg/ml)
    1. Converting oral to IV dose: 1:1 conversion -capsules: IV,,,0.92:1 infatabs/suspension:IV
      1. Usual daily dose range= 200-500mg
    2. Intermittent IV infusion:
      1. Must be completed within 1 hour when diluted using a 0.22micron inline filter
    3. IV phenytoin is alkaline and irritant. Use large vein ad large bore needle, follow with a 20ml 0.9% NaCl flush, Give slwoly max 50mg/min rapid administration can cause bradycardia, ypotension, asystole,arrhythmias.
      1. Dilute with 0.9% NaCl only
      2. Do not mix with other meds even at luer site.

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