Pick the dosing weight per drug: lipophilic loading doses and some antimicrobials by actual weight, most maintenance infusions and neuromuscular blockers by ideal or adjusted weight, and titrate sedatives to effect. Use therapeutic drug monitoring where available.
At the bedside
Timed steps- 1
Measure
WeighRecord actual (total) body weight and height; estimate in bed with weighing scales or bed scales rather than guess.
- 2
Weight scalars
CalculateIBW (Devine): men 50 + 0.91 × (height cm − 152.4); women 45.5 + 0.91 × (height cm − 152.4). Adjusted BW = IBW + 0.4 × (TBW − IBW).
- 3
Titrate
SedationPropofol, fentanyl and midazolam accumulate in adipose tissue with prolonged infusion; start on IBW/adjusted weight and titrate to RASS.
- 4
Load generously
AntimicrobialsHydrophilic drugs (beta-lactams, aminoglycosides) need full or higher loading doses; vancomycin load on actual weight (cap ~3 g) then TDM-guided; aminoglycosides on adjusted weight.
- 5
Intubation
NMBSuxamethonium 1-1.5 mg/kg actual weight; rocuronium 1.2 mg/kg on ideal (or lean) weight.
- 6
Monitor
AnticoagulationUFH by actual weight with local cap and aPTT/anti-Xa; LMWH prophylaxis dose often increased when BMI ≥ 40 (local protocol, anti-Xa if available).
Loading dose in obesity
Vancomycin
- Dose
- 20-35 mg/kg actual body weight, max ~3 g
- Worked example
- 130 kg man: 25 mg/kg = 3250 mg → cap 3 g, infused over ≥ 3 h; then 15-20 mg/kg/dose guided by levels.
Which weight? (common ICU drugs)
| Drug | Loading | Maintenance |
|---|---|---|
| Propofol | Lean/ideal weight (induction) | Adjusted weight, titrate to RASS |
| Fentanyl / midazolam | Ideal weight | Ideal/adjusted weight, titrate |
| Dexmedetomidine | Not usually loaded | Actual or adjusted weight, titrate |
| Suxamethonium | Actual weight | – |
| Rocuronium / atracurium | Ideal weight | Ideal weight |
| Vancomycin | Actual weight (cap ~3 g) | TDM-guided (AUC/trough) |
| Aminoglycosides | Adjusted weight | Levels |
| Unfractionated heparin | Actual weight (capped) | aPTT/anti-Xa |
Conventions differ between sources; follow local protocol and monitor effect or levels.
Calculators for this topic
Predicted (ideal) body weight
ARDSNet predicted body weight from height and sex. Use for tidal volume, not for most drug doses.
Adjusted body weight
IBW + 0.4 × (actual − IBW). Used for some drug doses (e.g. aminoglycosides) and nutrition in obesity.
Body mass index
Weight ÷ height². Asian cut-offs differ (overweight ≥ 23, obese ≥ 25 in Indian guidance).
References
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Choosing a dosing weight
Step 1 of ~3
Decision: Loading dose of hydrophilic antimicrobial or suxamethonium?
Based on Erstad BL, Barletta JF. Educational use; follow local protocol.
Pathophysiology
- Obesity increases volume of distribution for lipophilic drugs and raises lean mass, cardiac output and often renal clearance early; fat-laden liver may alter metabolism.
Evidence
- Erstad & Barletta 2020: review of sedative and analgesic dosing; little high-quality data, favours titration and lean/ideal scalars.
- Pai 2000: origin and limitations of IBW equations.
🇮🇳 India notes
- Asian body composition: higher fat percentage at lower BMI; WHO Asian cut-offs (overweight ≥ 23, obese ≥ 25 kg/m²) are used in India.
References
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Educational reference for trained clinicians. Check doses against your hospital protocol and the patient in front of you.