Predicted Body Weight (Ventilation)

Lung size correlates with height, not weight. An obese patient at 175 cm has essentially the same lung volume (functional residual capacity) as a lean patient at 175 cm. If tidal volume is set on actual body weight in an obese patient, the lungs receive a much larger volume per breath than they can safely accommodate, causing ventilator-induced lung injury (VILI) through overdistension of already-compromised alveoli.

The ARDSNet (ARMA trial, 2000) protocol set tidal volume at 6 mL/kg PBW, with a target plateau pressure ≤30 cmH2O. If plateau pressure exceeds 30 cmH2O, the tidal volume is reduced in 1 mL/kg steps to a minimum of 4 mL/kg PBW. This strategy reduced 28-day mortality from 39.8% (12 mL/kg traditional ventilation) to 31.0% — one of the most significant outcomes improvements in the history of critical care.

PBW and IBW are often used interchangeably in clinical practice and reference the same Devine formula. Strictly, the term 'ideal body weight' has fallen out of favour as it implies a value judgement. 'Predicted body weight' or 'lean body weight' better captures the physiological rationale — this is the body weight the person would have if they had a normal BMI for their height.

The Devine formula was developed from data on adults and performs less reliably at very short statures. For patients below 152 cm, the formula can give negative values below 152 cm by more than a few centimetres, which is clinically nonsensical. In very short patients, use clinical judgement, consult paediatric dosing references if applicable, or use alternative formulas such as the Miller formula, which handles extremes of height better.

For obese critically ill patients, caloric goals in enteral or parenteral nutrition should be based on PBW (or adjusted body weight) rather than actual weight, to avoid overfeeding. Overfeeding in critically ill patients causes hyperglycaemia, hyperlipidaemia, excess CO2 production (worsening respiratory failure), and immune suppression. ESPEN guidelines recommend 25–30 kcal/kg/day using an appropriate body weight estimate.

Drugs that distribute primarily in lean (non-fat) tissue are dosed on PBW: aminoglycosides (gentamicin, tobramycin), vancomycin (loading dose, though maintenance is based on AUC monitoring), digoxin, neuromuscular blocking agents (rocuronium, vecuronium), and many chemotherapy agents. Drugs that distribute significantly into adipose tissue (e.g., benzodiazepines, lipophilic drugs) may require actual or adjusted body weight.

Adjusted body weight accounts for partial drug or nutrient distribution into adipose tissue in obese patients: AdjBW = PBW + correction factor × (actual weight − PBW). The correction factor is drug-specific: 0.4 for aminoglycosides and many drugs (40% of excess adipose tissue participates in distribution). AdjBW is used when neither PBW (too low) nor actual weight (too high) is appropriate — most commonly for drug dosing in obesity.

The original Devine formula was derived from primarily Caucasian populations. Some evidence suggests that individuals of Asian descent may have lower lean body mass for the same height, and alternative formulas or adjustments are sometimes used. In pharmacokinetic studies, Asian populations have been observed to reach therapeutic drug concentrations at lower doses per kg PBW for some agents. Clinical pharmacist consultation is recommended for complex cases.