Dose:  10 to 20 mg/kg/dose IV as loading dose   
             2.5 mg/kg/dose IV as maintenance dose 
             See comments for oral dosing
             Use fosphenytoin if using a peripheral IV.

Frequency:  Maintenance dose Q 12 hours

Comments: Phenytoin is an anticonvulsant used to treat seizures refractory to phenobarbital alone.  Phenytoin is also useful in treating ectopy due to digoxin toxicity, especially with AV block since phenytoin rarely depresses AV conduction.  Phenytoin appears to interfere with calcium regulated processes at the cell membrane level. Only free drug is physiologically active. In newborns serum levels tend to be higher resulting in  half-lives ranging from 15 to 105 hours compared to 2 to 7 hours in infants and 18 to 30 hours in adults.   Intravenous loading with 20 mg/kg is usually required to obtain serum levels > 10 mcg/ml (therapeutic level 10 to 20 mcg/ml). The newborn should be carefully monitored during IV infusions because of the possibility of inducing cardiac arrhythmias or hypotension. Infusion should not exceed 1 to 3 mg/kg/min in neonates.   Concurrent use of either diazepam or phenobarbital has resulted in either increased or decreased serum levels of phenytoin. A new formulation of parenteral phenytoin, fosphenytoin, is soluble in water.  It should be used when given through a peripheral IV or IM.   For further information, see comments on phenytoin.

Preparation: 
IV:Phenytoin (Dilantin) is available in a 2 ml vial with a concentration of 50 mg/ml.   If dilution is necessary, add 9 ml of normal saline to 1 ml of the 50 mg/ml concentration to make a 5 mg/ml solution of phenytoin.  Use immediately and discard.   Do not use if precipitates are present in the solution. Observe infusion for the development of precipitates. Do not refrigerate. Do not infuse at greater than 3 mg/kg/min. If the medication is given in a peripheral IV, use fosphenytoin.


PO: Phenytoin (Dilantin) is available as a 125 mg/5 ml suspension. Anecdotal information suggested that the drug was difficult to maintain in suspension.  Sartar (Neurology 1989; 39:207-209) reported that after resuspension (vigorous shaking) before the first dose and minimal agitation prior to each dose was sufficient to maintain the drug in suspension.                           
Phenytoin is also available in a 50 mg tablet (Dilantin Infatab).  The tablet is triangular and scored.  Cut the tablet into fractions to approximate the dose.  Dissolve the tablet in milk and give prior to feeding.

Compatibility: Not compatible with TPN and lipids, D5W, D5 with sodium chloride, LR, or most other drugs.  It is compatible esmolol, famotidine, fluconazole, and sodium bicarbonate. It is equivocally compatible with normal saline (final concentration > 1 mg/ml).  Use of an in-line filter has been recommended to protect against the possibility of microcrystal formation.