Anticonvulsant levels and reloading

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Background

  • IV load can be performed with phenobarbital, phenytoin, valproate, levetiracetam
  • Oral loading can be spread over day or more to avoid GI upset

Dose

Dose (mg) = ideal body weight (kg) X Vd X [desired level - current level (mcg/mL)]

Volume of Distribution

Agent
Volume of Distribution
Phenytoin (dilantin) 0.8
Carbamazepine (tegretol) 0.8
Phenobarbital 0.6
Valproate (depakote) 0.2

Desired Level

  • Pt's optimal level = desired level
    • If optimal level unknown target upper end of therapeutic range:
      • Phenytoin = 20 mcg/mL
      • Valproate = 100 mcg/mL
      • Phenobarbital = 40 mcg/mL

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Phenytoin

Zero Level of Initial Load

  • IV: 18mg/kg at max rate of 25mg/min
    • PO: 18mg/kg as single dose or divided into 3 doses q2hr
  • Level above 0 (to avoid over-dosing)

Subtherapeautic level >0

Concentration adjusted = (concentration desired - concentration measured) x (0.7 x adjusted body weight)

Sheiner-Tozer Equation

  • may be needed to correct level in patients with low albumin or abnormal creatinine:
  • Corrected level = measured level/[(adjustment x albumin) + 0.1]
  • adjustment is 0.2 unless patient has Creatinine clearance <20 then it is 0.1


Fosphenytoin

  • IV: 18mg PE/kg at 100-150mg/min

See Also

Source

DONALDSON 3/08 (from emedicine; Treiman 1997; Bourgeois, 1996)