Author: Cardiac Anesthesiologist
Generally, in obese patients, the blood distributes more to nonadipose than to adipose tissues, resulting in higher plasma drug concentrations in obese patients with mg/kg dosing than in normal patients with less adipose mass.
Furthermore, propofol clearance increases because of the increased liver volume and liver blood flow associated with obesity (and increased cardiac output).
Changes to volumes of distribution likely influence concentration peaks with bolus dosing, whereas changes in clearance likely influence concentrations during and following infusions.Various weight scalars in propofol bolus and continuous infusion dosing have been studied.
First, use formulas used to estimate weight scalars (click to review)
Of the many available dosing scalars, authors recommend LBM (lean body mass) 24 for bolus dosing (i.e., during induction) and TBW (total body weight in kg) or corrected body weight (CBW) for infusions.
For continuous infusions, other weight scalars are likely to result in inadequate dosing (most worrisome for LBM). One concern with using TBW to dose continuous infusions (i.e., μg/kg/min) is drug accumulation.
Prior investigations, however, do not support this assumption. Servin and colleagues performed pharmacokinetic analyses of propofol administration to normal and obese patients using TBW and CBW. The CBW was defined as the IBW + 0.4 × (TBW − IBW). They found similar concentrations at eye opening in both groups and absence of propofol accumulation in obese patients. However, some reports suggest that dosing infusions according to CBW may underdose morbidly obese patients.