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

Loading

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 (0.8 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

Source

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

Authors: