Epidural Anesthesia Dosing in Obese Patients

Weight-Based Dosing Strategy

The Association of Anaesthetists of Great Britain and Ireland recommends against using total body weight for most anesthetic agents in obese patients, as this increases the risk of relative overdose, and instead suggests using lean body weight or adjusted body weight for calculating initial anesthetic drug doses, with a strength of evidence supporting the use of these weight-based calculations to minimize complications 1, 2, 3, 4
Lean body weight or adjusted body weight should be used as the scalar for calculating initial anesthetic drug doses to avoid relative overdose and associated complications, including hypotension, with a high level of evidence supporting this recommendation 1, 2, 3

Lean Body Weight (LBW) can be calculated using the Janmahasatian formula, with a moderate level of evidence supporting its use: Men: LBW (kg) = [9270 × TBW (kg)] / [6680 + (216 × BMI)], Women: LBW (kg) = [9270 × TBW (kg)] / [8780 + (244 × BMI)], and it is important to note that LBW rarely exceeds 100 kg in men and 70 kg in women regardless of total body weight, with a low risk of error 1, 5
Adjusted Body Weight (ABW) accounts for increased lean body mass in obesity, with a moderate level of evidence supporting its use: ABW (kg) = IBW (kg) + 0.4 × [TBW (kg) - IBW (kg)], and is a useful alternative to LBW for dose calculations 1, 2

Dosing to total body weight in obese patients is rarely appropriate because fat tissue has relatively low blood flow, limiting drug distribution, with a high level of evidence supporting this statement, and volume of distribution changes are drug-specific and unpredictable, making it difficult to determine the optimal dose 3, 4
Using total body weight significantly increases the risk of hypotension, with a high level of evidence supporting this statement, and most anesthetic agents should be titrated to clinical effect rather than fixed weight-based calculations, with a moderate level of evidence supporting this recommendation 1, 3, 4, 5

The American Society of Anesthesiologists recommends calculating the patient's lean body weight or adjusted body weight using the formulas above, with a high level of evidence supporting this recommendation, and using this calculated weight (not total body weight) for initial dose calculations to minimize the risk of complications 1, 2, 3
For obese patients, the American College of Obstetricians and Gynecologists suggests considering using ideal body weight specifically for bupivacaine epidural dosing, with a moderate level of evidence supporting this recommendation, and titrating subsequent doses to clinical effect (sensory level achieved, pain relief) to ensure optimal pain management 6, 7