Anticonvulsant levels and reloading
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
- If optimal level unknown target upper end of therapeutic range:
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