What is ideal body weight in anesthesia?

Ideal Body Weight in Anesthesia

In anesthesia practice, ideal body weight (IBW) should be used 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

Definition and Calculation of Ideal Body Weight

Ideal body weight represents what a patient should weigh with a normal ratio of lean to fat mass. It varies with age and is typically calculated using simple formulas based on height and sex:

• For males: IBW (kg) = height (cm) × 0.01 2
• For females: IBW (kg) = height (cm) × 0.0105 2

Alternatively, the formula can be expressed as:

• For males: IBW (kg) = height (cm) - 100 1
• For females: IBW (kg) = height (cm) - 105 1

Key Weight Measurements in Anesthesia

Four important weight measurements are used in anesthesia practice:

1. Total Body Weight (TBW): The actual weight of the patient
2. Ideal Body Weight (IBW): Weight with normal lean-to-fat ratio
3. Lean Body Weight (LBW): Weight excluding fat, calculated using Janmahasatian formulas:
   • Males: LBW (kg) = (6680 + 216 × BMI) ÷ 9270 × TBW
   • Females: LBW (kg) = (8780 + 244 × BMI) ÷ 9270 × TBW
4. Adjusted Body Weight (ABW): Accounts for increased lean body mass in obesity
   • ABW (kg) = IBW (kg) + 0.4 × (TBW - IBW) 1

Clinical Applications in Anesthesia

Medication Dosing

• Induction agents: LBW is the most appropriate scalar for propofol dosing in obese patients 3
• IV lidocaine: IBW should be used for dose calculation, with maximum 120 mg/h 1
• General principle: For most anesthetic agents, lean or adjusted body weight should be used rather than total body weight 1

Special Considerations for Obese Patients

• Rapid redistribution of induction agents into larger fat mass means obese patients wake up more quickly after a single bolus dose 1
• Using total body weight for dosing can lead to significant hypotension 1
• LBW rarely exceeds 100 kg in men and 70 kg in women, regardless of total body weight 1

Risk of Accidental Awareness

• The fifth National Audit (NAP5) found a disproportionate number of obese patients experienced accidental awareness under anesthesia 1
• Small doses of induction agents based on lean or adjusted body weight not quickly followed by maintenance anesthesia may increase risk of awareness 1

Practical Guidelines for Specific Medications

• Hydrophilic drugs: Use IBW or LBW for dosing 2
• Lipophilic drugs: Consider ABW for dosing 2
• Propofol:
  • For induction in morbidly obese patients, dosing based on LBW produces similar results to normal-weight patients dosed on TBW 3
  • Appropriate dosage range is 2.31-3.57 mg/kg when calculated based on LBW for morbidly obese patients 4

Common Pitfalls to Avoid

• Using total body weight for all medication calculations can lead to overdosing in obese patients 2
• Failing to quickly follow induction with maintenance anesthesia in obese patients increases risk of awareness 1
• Not accounting for drug-specific changes in volume of distribution in obese patients 1
• Using IBW alone for underweight patients may not provide adequate dosing 2

By using the appropriate weight-based scalar for anesthetic medications, clinicians can optimize drug delivery, minimize adverse effects, and improve patient outcomes during anesthesia.