Description and indication for use
Phenytoin is an anticonvulsant drug. Its primary site of action appears to be
motor cortex, where the spreading of seizure activity is inhibited. The precise mechanism of anticonvulsant
activity has not been determined. It has
a rapid onset of action and is highly protein bound.
It is used as an anticonvulsant in the
treatment of neonatal seizures unresponsive to Phenobarbitone.
Dose
IV:
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Loading dose:
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15 mg/kg to 20
mg/kg
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Maintenance dose:
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VLBW
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2 mg/kg/dose 12 hourly
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Term
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4 to 5
mg/kg/dose 12 hourly
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Week 2 and over
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4 to 5
mg/kg/dose 6 hourly
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Maintenance dose
to start 12 hours after loading dose.
Maintenance dose can be given orally.
Reconstitution/Dilution
Ampoule = 100 mg/2 mL (50 mg/mL).
IV:
Administer as 5 mg/mL or more dilute to
produce a 5 mg/mL solution.
Withdraw 1 mL of 50 mg/mL solution (50 mg)
and add to 9 mL of sodium chloride 0.9% in a 10 mL syringe = 50 mg/10 mL = 5
mg/mL.
If dose is greater than 50 mg, use double
quantities.
Withdraw required dose (exact dose).
Once prepared, use within 1 hour.
Route and method of administration
IM:
Administration
is not recommended due to pain and may cause tissue necrosis. Absorption is erratic and delayed.
IV:
Give
slowly, no greater than 1 mg/kg/minute. Flush with sodium chloride 0.9% before
administration.
Loading dose:
Should be given over approximately 1 hour
via syringe pump with minimum volume line.
Draw up 3 mLs of Sodium Chloride 0.9% in a
10 mL syringe to use as a flush.
Following administration of exact dose of
Phenytoin infuse 2 mLs of Sodium Chloride 0.9% via the syringe pump at same
rate as the Phenytoin (set volume limit at 2 mL).
Following completion of the infusion and
flush, disconnect and discard syringe and line used for the infusion.
Maintenance doses:
Given over at least 15 minutes via syringe
pump.
Side effects
Rapid IV infusion may cause hypotension,
arrythmias, bradycardia, cardiovascular collapse and/or respiratory distress.
Vomiting, gastric irritation.
Thrombocytopenia, leukopenia,
granulocytosis.
Macrocytosis and megaloblastic anaemia
which respond to folic acid therapy.
Toxicity will cause cardiovascular collapse
and/or CNS depression.
Tissue necrosis and inflammation at injection
site from extravasation.
Skin rash drug
should be discontinued.
Hypoinsulinaemia, hyperglycaemia,
glycosuria.
Special precautions
CAUTION in patients with hyperbilirubinaemia
CAUTION in patients with renal or hepatic impairment - bilirubin will
displace Phenytoin from protein -binding sites, resulting in increased free
drug
Dose may need to be increased after the
third week of life
Contraindicatons
Heartblock.
Blood dyscrasias.
Hypoglycaemic seizures.
Compatible Solutions
Use only Sodium Chloride 0.9% to dilute the
50 mg/mL solution.
Incompatibilities
Glucose 5% and 10%.
TPN and Lipid.
Administer alone or contact Pharmacy if
further information is required.
Drug interactions
Do not mix with other drugs.
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Phenytoin
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Should not be administered during a Dopamine
infusion as the combination may result in profound hypotension, bradycardia
and possibly cardiac arrest. If used
together, monitor blood pressure and use extreme CAUTION.
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Folic Acid, Pyridoxine, Rifampicin and Chloral
Hydrate
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May all decrease serum Phenytoin levels therefore
possible loss of effectiveness.
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Phenobarbitone
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Levels may be increased when given concurrently
with Phenytoin. Monitor serum levels of both drugs.
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Chloramphenicol
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Levels can be increased by Phenytoin, as well as
a possible increase in Phenytoin levels.
Monitor levels of both drugs.
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Theophylline
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Levels are reduced by Phenytoin with possible
loss of effectiveness of both drugs.
Monitor levels of both drugs.
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Pancuronium
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May have a shorter duration of action.
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Frusemide
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May have reduced diuretic effect.
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Corticosteroid
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Metabolism is enhanced by Phenytoin, reducing
effectiveness.
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Digoxin
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Serum levels may be decreased. Monitor serum levels of Digoxin.
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Paracetamol
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Do not give high doses if giving Phenytoin.
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Amiodarone
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May increase serum levels of Phenytoin, with
Phenytoin possibly reducing the effectiveness of Amiodarone.
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Nursing responsibilities
Monitor infant with cardio-respiratory
monitor (cardiac rate and rhythmn).
Observe for arrythmias during
administration.
Record and report effect of drug on seizure
activity.
Observe infant for signs of toxicity and Side
effects.
Monitor infant's blood pressure.
Observe IV site for phlebitis or tissue
inflammation.
If extravasation occurs check with medical
staff regarding the possible use of Hyaluronidase around the periphery of the
affected area.
Ensure that phenytoin serum levels are monitored.
Sample should be taken immediately before
the next dose.
Therapeutic range: 40 – 80 micromol/L.
Phenytoin is
highly unstable in any IV solution.
Avoid using in central lines because of the risk of precipitation.