Question 20 from the first paper of 2009 asks the candidates to define ideal body weight, as a segue into a discussion of pharmacokinetic changes in morbid obesity. Apart from the ideal body weight, other measures of body composition play a role in pharmacokinetics. These include the total body weight, the body surface area index, the lean body weight, and the predicted normal weight. Drug dosing in the instructions of the manufacturer may favour one measure over the others, and specific pharmacokinetic properties of various drugs may influence which weight measure you choose to calculate their weight-adjusted dose.
Information regarding this specific topic is quite hard to find. The de Baerdmaeker article from 2004 is probably the most illuminating in terms of discussing ideal body weight and lean body weight. Ingrande and Lemmens (2010) discuss the other measures in brief. These articles were used a sources for the summary offered below.
Gold standard measure of body composition
- Unfortunately, the gold standard is the cadaveric measurement.
- You puree the entire organism and separate the components.
- This is inconvenient in the critical care environment.
Total body weight and pharmacokinetics
- Dosing recommendations are generally based on TBW
- This is fine for normal people, in whom total body weight is similar to ideal body weight.
- With increasing obesity, fat mass accounts for an increasing amount of total body weight. However, the majority of the cardiac output is still directed to the lean tissue groups.
- Therefore, dosing a drug to total body weight in a morbidly obese individual may result in overdose (imagine, 7mg/kg of gentamicin in a 250kg person would produce an ear-ringing 1750mg dose).
Ideal body weight and pharmacokinetics
Body surface area and pharmacokinetics
Lean body weight and pharmacokinetics
- In people of normal weight, TWB = IBW = LBW and no equations are required.
- Most drug dose instructions refer to TBW.
- In morbidly obese people,
- TBW overestimates the dose.
- IBW underestimates the dose.
- LBW is probably the best estimate of dose.
- In the case of most nonlipophilic drugs, IBW is sufficient because Vd does not change.
- In the case of strongly hydrophilic drugs, instead of calculating LBW, 20% can be added to the IBW to account for the increase in lean body tissue content.
- In the case of strongly lipophilic drugs, and in the case of many anaesthetic agents, LBW is the ideal metric.