TOTAL BODY WEIGHT (TBW) , LEAN BODY WEIGHT (LBW), IDEAL BODY WEIGHT (IBW) & ADJUSTED BODY WEIGHT (ABW) ; THEIR IMPLICATIONS IN ANESTHESIA AND CRITICAL CARE

🏀 Drug administration in obese patients is difficult because recommended doses are based on pharmacokinetic data obtained from individuals with normal weights

🏀 With increasing obesity, fat mass accounts for an increasing amount of TBW, and the LBW/TBW ratio decreases

🏀 TBW is defined as the actual weight 

🏀 IBW is what the patient should weigh with a normal ratio of lean to fat mass 

🏀 IBW can be estimated from the formula: IBW (kg) = Height(cm) − x ( where x = 100 for adult males and 105 for adult females).

🏀 LBM is the patient's weight , excluding fat

🏀Male LBM = 1.1(weight)-128(weight/height)^2 (Weight in Kg and Height in cm)

🏀Female LBM = 1.07 (weight) -148 (weight/height)^2

🏀Regardless of total body weight, lean body weight rarely exceeds 100 kg in men and 70 kg in women 

🏀 Below IBW, TBW and LBM are similar. 

🏀 Adjusted body weight (ABW) Takes into account the fact that obese individuals have increased lean body mass and an increased volume of distribution for drugs.

🏀 It is calculated by adding 40% of the excess weight to the IBW :

ABW (kg) = IBW (kg) + 0.4 (TBW (kg)   IBW (kg)

🏀 Drugs with weak or moderate lipophilicity can be dosed on the basis of IBW or more accurately on LBM. These values are not same in obese; because 20–40% of an obese patient's increase in TBW can be attributed to an increase in LBM. Adding 20% to the 'estimated IBW based dose' of hydrophilic medication is sufficient to include the extra lean mass. Non-depolarizing neuromuscular blocking drugs can be dosed in this manner. 

🏀 In morbidly obese patients, the induction dose of propofol can be calculated on IBW. 

🏀 In case of midazolam, prolonged sedation can occur from the larger initial dose needed to achieve adequate serum concentrations. 

🏀 Remifentanil dosing regimens should be based on IBW or LBM and not on TBW. 

🏀 When using succinylcholine in obese adults or adolescents, dosage should be calculated on TBW

🏀 The antagonism time of neostigmine has been shown to be independent of TBW and BMI. Therefore, TBW can be used to calculate the dose.

−128(weight/height)2

$$\begin{array}{rcl}& & \mathrm{M}\mathrm{a}\mathrm{l}\mathrm{e}\mathrm{L}\mathrm{B}\mathrm{M}1.1\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\\ & & 128{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}^2\end{array}$$

Female LBM = 1.07(weight)

$$\mathrm{I}\mathrm{B}\mathrm{W}\mathrm{k}\mathrm{g}\mathrm{h}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{c}\mathrm{m}x$$

IBW (kg) = height (cm) − xm.ceaccp.oxfordjournals.org/content/4/5/152.full

Ref:Association of Anaesthetists of Great Britain and Ireland. Peri-operative management of the obese surgical patient 2015. Anaesthesia 2015, 70, pages 859–876.

http://bja.oxfordjournals.org/content/105/suppl_1/i16.full