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เรากำลังจัดทำคู่มือการศึกษาที่ครอบคลุมสำหรับ Predicted Body Weight (Ventilation) กลับมาเร็วๆ นี้เพื่อดูคำอธิบายทีละขั้นตอน สูตร ตัวอย่างจริง และเคล็ดลับจากผู้เชี่ยวชาญ
Predicted Body Weight (PBW), also known as Ideal Body Weight (IBW), is a calculated weight that represents the expected weight of a person of a given height and sex, regardless of their actual weight. It is critically important in clinical medicine — particularly in mechanical ventilation and drug dosing — because many physiological parameters (lung size, tidal volume, renal tubular function) correlate with height rather than actual body weight. The Devine formula, published in 1974, is the most widely used PBW formula in clinical practice: for males, PBW (kg) = 50 + 2.3 × (height in inches − 60); for females, PBW (kg) = 45.5 + 2.3 × (height in inches − 60). An equivalent centimetre-based formula uses 0.91 instead of 2.3 per inch: males = 50 + 0.91 × (height in cm − 152.4); females = 45.5 + 0.91 × (height in cm − 152.4). PBW is most critically applied in mechanical ventilation: lung-protective ventilation protocols for ARDS (ARDSNet) mandate a tidal volume of 6 mL/kg PBW (with acceptable range 4–8 mL/kg PBW) because the lungs of obese patients are not larger than those of normal-weight individuals at the same height. Using actual body weight in obese patients would deliver excessively large tidal volumes, causing ventilator-induced lung injury (VILI) through volutrauma and barotrauma. PBW is also used for medication dosing for drugs distributed in lean body mass (aminoglycosides, digoxin, vancomycin loading doses), and for nutritional calculations where overfeeding of obese critically ill patients is a recognised risk.
PBW Male (kg) = 50 + 2.3 × (Height(in) − 60); PBW Female (kg) = 45.5 + 2.3 × (Height(in) − 60); Or: Male = 50 + 0.91 × (Height(cm) − 152.4); Female = 45.5 + 0.91 × (Height(cm) − 152.4)
- 1Measure patient height accurately (in cm or convert to inches: cm ÷ 2.54).
- 2Identify patient sex (male or female — the formula differs by 4.5 kg at the intercept).
- 3Apply the Devine formula: Male PBW = 50 + 2.3 × (height_inches − 60); Female PBW = 45.5 + 2.3 × (height_inches − 60).
- 4For heights below 152 cm (60 inches), the formula gives values below 50/45.5 kg; use clinical judgement for very short patients.
- 5Use PBW to calculate mechanical ventilation tidal volume: TV = 6 mL/kg PBW (range 4–8 mL/kg for lung-protective ventilation).
- 6Set initial ventilator tidal volume at 6 mL/kg PBW; check plateau pressure (target <30 cmH2O) and adjust as needed.
- 7For obese patients (actual weight much higher than PBW), always use PBW for ventilator settings and lean-body-mass drug dosing.
Using actual weight would deliver 70% more tidal volume — a major cause of VILI
An obese patient has the same lung size as a non-obese person of the same height. Setting tidal volume on PBW prevents volutrauma.
If actual weight used: TV would be 510 mL — inappropriately high
A female patient at 163 cm has a PBW of approximately 55 kg regardless of her actual weight of 85 kg.
For obese patients, aminoglycoside dosing uses 'adjusted body weight' = PBW + 40% of excess weight
Aminoglycosides distribute partially into adipose tissue; adjusted body weight accounts for this partial distribution into fat.
Target plateau pressure <30 cmH2O; minimum TV 4 mL/kg PBW per ARDSNet protocol
Even at 6 mL/kg PBW, some patients with stiff lungs (low compliance) will have high plateau pressures. Reduce tidal volume further to protect lungs.
Mechanical ventilation: calculating tidal volume (6 mL/kg PBW) for lung-protective ARDS ventilation per ARDSNet protocol., where accurate predicted body weight analysis through the Predicted Body Weight supports evidence-based decision-making and quantitative rigor in professional workflows
Aminoglycoside dosing: gentamicin, tobramycin, amikacin — using PBW or adjusted body weight in obese patients., where accurate predicted body weight analysis through the Predicted Body Weight supports evidence-based decision-making and quantitative rigor in professional workflows
Vancomycin loading dose calculation in morbidly obese patients., where accurate predicted body weight analysis through the Predicted Body Weight supports evidence-based decision-making and quantitative rigor in professional workflows across diverse organizational contexts and analytical requirements
Caloric goal setting for enteral and parenteral nutrition in critically ill obese patients to avoid overfeeding., where accurate predicted body weight analysis through the Predicted Body Weight supports evidence-based decision-making and quantitative rigor in professional workflows
Pharmacist-guided drug dosing calculations for chemotherapy, neuromuscular blocking agents, and other body-size-dependent medications.
Morbid Obesity and PBW
In morbidly obese patients (BMI >40), the difference between PBW and actual weight may exceed 50–100 kg. Setting tidal volume on actual weight in these patients could double or triple the safe tidal volume. PBW is non-negotiable for ventilator tidal volume calculation in obese patients. However, PEEP selection in obese ARDS patients often needs to be higher than standard protocols to counteract the increased chest wall weight of abdominal adipose tissue.
Pregnancy and PBW
Pregnant patients gain significant weight during pregnancy (10–15 kg on average), of which a variable amount is lean mass (uterus, fetus, placenta, amniotic fluid) versus fat. For drug dosing in pregnancy, specific pharmacokinetic guidance for each drug is essential. For ventilation of pregnant patients in ARDS, PBW based on pre-pregnancy height is still the appropriate tidal volume reference.
Paediatric PBW
Paediatric PBW is calculated differently from adult formulas. Common paediatric approximations: (Age × 2) + 8 for children 1–12 years (weight in kg). Alternatively, the 50th percentile weight from standardised growth charts for the child's age and sex is used as the reference for drug dosing and ventilation. Paediatric ARDS ventilation also targets 6 mL/kg ideal weight.
Underweight Patients
When actual body weight is less than PBW (underweight or cachectic patients), use actual body weight rather than PBW for ventilation and drug dosing — PBW would overestimate the appropriate dose or tidal volume. Clinical pharmacist review is essential for drug dosing in severely cachectic patients, particularly for chemotherapy, where underdosing may compromise efficacy.
| Height (cm) | PBW Male (kg) | PBW Female (kg) | Tidal Volume 6 mL/kg — Male | Tidal Volume 6 mL/kg — Female |
|---|---|---|---|---|
| 155 | 52.3 | 47.8 | 314 mL | 287 mL |
| 160 | 56.9 | 52.4 | 341 mL | 314 mL |
| 165 | 61.4 | 56.9 | 368 mL | 342 mL |
| 170 | 65.9 | 61.4 | 396 mL | 369 mL |
| 175 | 70.5 | 66.0 | 423 mL | 396 mL |
| 180 | 75.0 | 70.5 | 450 mL | 423 mL |
| 185 | 79.5 | 75.1 | 477 mL | 451 mL |
Why not use actual body weight for tidal volume setting?
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.
What is the ARDSNet protocol for lung-protective ventilation?
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.
Is PBW the same as IBW?
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.
What if the patient is shorter than 152 cm (60 inches)?
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.
How is PBW used in nutritional support?
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.
Which drugs are dosed on PBW?
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.
What is adjusted body weight (AdjBW) and when is it used?
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.
Does the formula change for different ethnic groups?
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.
เคล็ดลับโปร
Memorise the quick reference: for a 170 cm male, PBW ≈ 66 kg; for a 165 cm female, PBW ≈ 57 kg. Use these as anchor points for quick bedside calculations during emergencies. Always double-check ventilator tidal volume settings against PBW in any mechanically ventilated patient — this takes 30 seconds and prevents a potentially fatal error.
คุณรู้ไหม?
The Devine formula was originally published in 1974 by Dr Benjamin J. Devine in a paper titled 'Gentamicin therapy' — not as a general weight formula, but as a practical tool for dosing this nephrotoxic antibiotic appropriately in patients of different sizes. The fact that this empirically derived antibiotic-dosing formula became the global standard for mechanical ventilation tidal volume calculation decades later was entirely unintended by its author.
เอกสารอ้างอิง
- ›ARDSNet — Ventilation with Lower Tidal Volumes (NEJM 2000)
- ›Devine BJ — Gentamicin Therapy (Drug Intelligence Clinical Pharmacy 1974)
- ›Pai MP & Paloucek FP — Ideal Body Weight Equations (Ann Pharmacother 2000)
- ›Fan E et al — Mechanical Ventilation in ARDS — Best Practices (JAMA 2017)
- ›ESPEN Guideline — Clinical Nutrition in ICU (Clin Nutr 2019)