On 2014 Sep 29, John Friesen commented:

This well designed study demonstrates two important things about induction doses of propofol in morbidly obese patients. First, they are best normalized to the lean body weight. Second, the dose required is affected by the rate at which it is administered.

The authors recommend using the lean body weight to estimate the dose for morbidly obese patients. However, any clinically useful weight scalar must be equal to the total body weight for patients of normal weight. Because the lean body weight is always less than the total body weight even for non-obese patients, it cannot be used directly to calculate doses drug doses: it will result in underestimation, and must be scaled upwards(1).

For example: a woman weighing 105 kilograms with a height of 170 centimeters and a BMI of 36.3 presents for bariatric surgery. Her induction dose of propofol is based on her calculated lean body weight5 of 55.2 kilograms. She successfully loses weight, and she returns for another operation having lost 40 kilograms. She is no longer obese, and her induction dose is based on her total body weight of 65 kilograms. Assuming that she is given 2.0 mg/kg of propofol each time, her calculated dose is 110 mg when she weighs 105 kilograms, and 130 mg (20 mg more) when she weighs only 65 kilograms. This is not correct.

The lean-scaled weight is a weight scalar that is proportional to the lean body weight for all obese and non-obese patients(2). It is calculated by multiplying the lean body weight function(3) by a normalization factor such that it is equal to the total body weight at a BMI of 22. This factor is 1.2332 for men and 1.5262 for women: for the 105 kilogram woman in the example, the lean-scaled weight is 84.2 kilograms.

It is interesting to ask what results might have been obtained if this study had included a morbidly obese group administered propofol at a rate proportional to the lean-scaled weight. Combining the two morbidly obese groups the mean total body weight is 131.1 kg and the lean body weight is 67.05 kg. For a BMI of 46.55, the mean lean-scaled weight (given the proportion of males to females) calculates to be 91.09 kg for this imagined group. Using these values to interpolate between the results reported for the infusion rates used in the study, the dose estimated to a linear approximation is about 2.27 mg/kg of lean-scaled weight.

The lean-scaled weight is equal to the total body weight for non-obese patients, and therefore this result can be compared directly to the 2.57 mg/kg reported for the control group. 2.27 mg/kg is close to this value, and well within the reported error. The results of this study are consistent with using the lean-scaled weight when estimating induction doses of propofol, for both obese and non-obese patients.

References

1 Bouillon T, Shafer SL. Does size matter? Anesthesiology. 1998 Sep;89(3):557-60.

2 Friesen JH. Lean-scaled weight: a proposed weight scalar to calculate drug doses for obese patients. Can J Anesth. 2013 Feb;60(2):214-5.

3 Janmahasatian S, Duffull SB, Ash S, Ward LC, Byrne NM, Green B. Quantification of lean bodyweight. Clin Pharmacokinet. 2005;44(10):1051-65.

On 2014 Sep 29, John Friesen commented:

This well designed study demonstrates two important things about induction doses of propofol in morbidly obese patients. First, they are best normalized to the lean body weight. Second, the dose required is affected by the rate at which it is administered.

The authors recommend using the lean body weight to estimate the dose for morbidly obese patients. However, any clinically useful weight scalar must be equal to the total body weight for patients of normal weight. Because the lean body weight is always less than the total body weight even for non-obese patients, it cannot be used directly to calculate doses drug doses: it will result in underestimation, and must be scaled upwards(1).

For example: a woman weighing 105 kilograms with a height of 170 centimeters and a BMI of 36.3 presents for bariatric surgery. Her induction dose of propofol is based on her calculated lean body weight5 of 55.2 kilograms. She successfully loses weight, and she returns for another operation having lost 40 kilograms. She is no longer obese, and her induction dose is based on her total body weight of 65 kilograms. Assuming that she is given 2.0 mg/kg of propofol each time, her calculated dose is 110 mg when she weighs 105 kilograms, and 130 mg (20 mg more) when she weighs only 65 kilograms. This is not correct.

The lean-scaled weight is a weight scalar that is proportional to the lean body weight for all obese and non-obese patients(2). It is calculated by multiplying the lean body weight function(3) by a normalization factor such that it is equal to the total body weight at a BMI of 22. This factor is 1.2332 for men and 1.5262 for women: for the 105 kilogram woman in the example, the lean-scaled weight is 84.2 kilograms.

It is interesting to ask what results might have been obtained if this study had included a morbidly obese group administered propofol at a rate proportional to the lean-scaled weight. Combining the two morbidly obese groups the mean total body weight is 131.1 kg and the lean body weight is 67.05 kg. For a BMI of 46.55, the mean lean-scaled weight (given the proportion of males to females) calculates to be 91.09 kg for this imagined group. Using these values to interpolate between the results reported for the infusion rates used in the study, the dose estimated to a linear approximation is about 2.27 mg/kg of lean-scaled weight.

The lean-scaled weight is equal to the total body weight for non-obese patients, and therefore this result can be compared directly to the 2.57 mg/kg reported for the control group. 2.27 mg/kg is close to this value, and well within the reported error. The results of this study are consistent with using the lean-scaled weight when estimating induction doses of propofol, for both obese and non-obese patients.

References

1 Bouillon T, Shafer SL. Does size matter? Anesthesiology. 1998 Sep;89(3):557-60.

2 Friesen JH. Lean-scaled weight: a proposed weight scalar to calculate drug doses for obese patients. Can J Anesth. 2013 Feb;60(2):214-5.

3 Janmahasatian S, Duffull SB, Ash S, Ward LC, Byrne NM, Green B. Quantification of lean bodyweight. Clin Pharmacokinet. 2005;44(10):1051-65.