Influence of Obesity on Anesthetic Drugs.

Author: Cardiac Anesthesiologist

Anesthesia providers should be familiar with the pharmacologic alterations of anesthetics in obese individuals. In general, manufacturer dosing recommendations are scaled to kilograms of actual total body weight (TBW).

However, anesthesia providers rarely use mg/kg dosing in obese patients for fear of administering an excessive dose (e.g., a 136-kg patient does not require twice as much drug as a patient of the same height who weighs 68 kg).

Accordingly researchers have developed several weight scalars in an attempt to avoid excessive dosing or underdosing in this patient population. Some of these scalars include lean body mass (LBM), ideal body weight (IBW), and fat-free mass (FFM).

    Formulas used to estimate these weight scalars:

Samples of the resultant scaled weight for a lean individual and an obese individual:

In general, the aim of weight scalars is to match dosing regimens for obese patients with what is required for normal-size patients. These scaled weights are usually smaller than TBW in obese patients and thus help prevent excessive drug administration. Scaled weights have been used in place of TBW for both bolus (mg/kg) and infusion (mg/kg/hr) dosing and also for target-controlled infusions (TCIs).