Anesthesia Practice Guidelines

Introduction to Body Weight Calculations

• The American Society of Anesthesiologists recommends using ideal body weight (IBW) for calculating medication doses rather than total body weight, particularly for most anesthetic agents, as dosing to total body weight is rarely appropriate and increases the risk of relative overdose 1
• The formula for calculating IBW is: for males, IBW (kg) = height (cm) - 100, and for females, IBW (kg) = height (cm) - 105, with alternative formulas including IBW (kg) = height (cm) × 0.01 for males and IBW (kg) = height (cm) × 0.0105 for females 1
• Adjusted Body Weight (ABW) can be calculated using the formula: ABW (kg) = IBW (kg) + 0.4 × (TBW - IBW), where the adjustment factor accounts for 40% of excess weight contributing to metabolic and pharmacokinetic considerations 1

Medication Dosing

• For IV lidocaine, the recommended dose calculation is based on IBW, with a maximum of 120 mg/h, and the general principle is that for most anesthetic agents, lean or adjusted body weight should be used rather than total body weight 1, 2
• Hydrophilic drugs should use IBW or LBW, lipophilic drugs should consider adjusted body weight, and anesthetic agents should use LBW or ABW rather than TBW, as recommended by the American Society of Anesthesiologists 1

Special Considerations

• Rapid redistribution of induction agents into larger fat mass means obese patients wake up more quickly after a single bolus dose, and using total body weight for dosing can lead to significant hypotension and increased risk of awareness 1
• Lean Body Weight (LBW) rarely exceeds 100 kg in men and 70 kg in women, regardless of total body weight, and can be calculated using formulas:
• The fifth National Audit (NAP5) found a disproportionate number of obese patients experienced accidental awareness under anesthesia, highlighting the importance of accurate dosing and consideration of body weight in anesthesia practice 1
• Adjusted body weight is essential for accurate medication dosing and nutritional assessments, as it reflects the metabolic needs and dietary requirements of patients with varying body compositions, and should be used for patients with significant weight variations, including obese and underweight patients 1, 3
• For obese patients, use adjusted body weight when actual body weight exceeds 115% of standard/ideal body weight, and for underweight patients, use adjusted body weight when actual body weight is less than 95% of standard/ideal body weight, considering additional nutrients for repletion 3
• For dialysis patients, adjusted body weight should be calculated using postdialysis weight for hemodialysis patients, and weight after drainage of dialysate for peritoneal dialysis patients, considering fluid status and edema 3
• Age and sex significantly affect treatment considerations, with different TBW calculation formulas and dose adjustments required, as noted by the National Kidney Foundation and the American Geriatrics Society 4, 1

Pediatric and Special Populations

• For pediatric patients, the Gehan and George equation for BSA is recommended: BSA (m²) = 0.0235 × (height [cm])^0.42246 × (weight [kg])^0.51456, and sex-specific nomograms should be used for TBW estimation, as recommended by the American Academy of Pediatrics 5
• Bioimpedance spectroscopy (BIS) measurements may be more accurate than the Watson formula in dialysis patients, particularly in those with higher waist circumference, although this is not directly cited, the concept is supported by 4

Calculation Formulas