Ketamine Infusion Dosing: Use Ideal Body Weight
For ketamine infusions, dose based on ideal body weight (IBW), not total body weight, particularly in obese patients. This approach reduces opioid requirements without compromising efficacy while avoiding potential overdosing complications.
Dosing Strategy
The evidence strongly supports IBW-based dosing for continuous ketamine infusions:
- Standard ketamine infusion dosing: 0.5 mg/kg bolus followed by 1-2 μg/kg/min infusion 1
- Calculate using IBW formula: Height (cm) - 100 for men; Height (cm) - 105 for women 2
A recent study directly comparing dosing strategies in critically ill obese patients (BMI >30 kg/m²) demonstrated that IBW dosing significantly reduced cumulative opioid requirements (median 485.8 mg vs 914.1 mg morphine equivalents, P=0.03) without requiring increased doses of other sedatives or as-needed medications 3. This confirms that IBW dosing provides adequate analgesia while avoiding the relative overdose risk associated with total body weight dosing.
Rationale for IBW Dosing
The 2015 AAGBI/SOBA guidelines on perioperative management of obese patients explicitly state that "for most anaesthetic agents, dosing to total body weight is rarely appropriate and increases the risk of relative overdose" 4. While ketamine is lipophilic with increased volume of distribution in obesity, the clinical evidence demonstrates that IBW dosing achieves therapeutic goals without excessive drug exposure.
For pediatric patients, ketamine dosing follows actual body weight with standard doses of 0.5 mg/kg bolus followed by 0.1-0.2 mg/kg/h infusion (maximum 0.4 mg/kg/h) 5.
Important Caveats
- Dose to effect: Like most anesthetic agents, ketamine should ultimately be titrated to clinical response (pain relief, sedation level) rather than rigidly adhering to weight-based calculations 4
- Maximum infusion rate: Consider not exceeding 120 mg/h regardless of calculated dose, similar to lidocaine infusion guidelines 2
- Monitor for adverse effects: Psychotropic effects (hallucinations, nightmares) may be dose-dependent, supporting lower IBW-based dosing 6, 1
- Lean body weight alternative: In obese patients, lean body weight (LBW) calculations may also be appropriate, though IBW is simpler and well-validated 4
The evidence does not support using actual body weight, body surface area (Mosteller formula), or other normalization methods for ketamine infusions 7.