Morbid obesity can be defined by a BMI > 40, or BMI > 35 with comorbid diseases such as DM and hypertension. Obese patients have increased total body weight, with increases in both fat body weight as well as lean body weight. However, the ratio of lean body weight to total body weight decreases with increase in total body weight. Physiologically, obese patients have an increase in cardiac output, total body volume, GFR, and other regional alterations in blood flow that alter pharmacokinetics and dynamics.
Lean body weight is calculated based on an individual’s height and weight, whereas ideal body weight is based only on height. Medication dosing based on total body weight runs the risk of overdose, whereas dosing based on lean body weight can run the risk of underdosing. Highly lipophilic and polar drugs in particular need adjustment due to changes in Vd.
In general, lean body weight is the most appropriate dose for most anesthetic drugs, with the exception of NDMB, where ideal body weight may be more appropriate. Succinylcholine should be dosed to total body weight.
In very general terms, the following table should serve as a guideline:
Base on Total Body Weidht (TBW) Succinylcholine Base on Lean Body Weight (LBW)
Base on Ideal Body Weight (IBW)
The induction of anesthesia with propofol should be based on LBW; however, the maintenance of anesthesia for a TIVA should be based on TBW.
J Ingrande, H J M Lemmens Dose adjustment of anaesthetics in the morbidly obese. Br J Anaesth: 2010, 105 Suppl 1;i16-23