13.2: Drug Dosing: Obese Patients

In the United States, obesity is a prominent concern. It is linked to heightened mortality rates due to increased occurrences of conditions such as hypertension, atherosclerosis, coronary artery disease, and diabetes compared to nonobese individuals. A patient is classified as obese if their actual body weight surpasses the ideal or desirable body weight by 20%, based on Metropolitan Life Insurance Company data. Ideal body weights consider average weights and heights for males and females relative to age, excluding athletes with greater muscle mass. Obesity is often defined by the body mass index (BMI), which correlates strongly with total body fat and serves as a surrogate measure for total body fat.

Notably, adipose tissue possesses a smaller proportion of water relative to muscle tissue, thereby impacting the apparent volume of drug distribution in obese patients. The apparent volume of distribution of certain drugs, such as antipyrine, exhibits a notable contrast between obese and ideal-body-weight patients, further reinforcing the influence of obesity on pharmacokinetics.

In the context of pharmacokinetic parameters, physiologic alterations in obese patients may affect biotransformation and renal excretion, consequently necessitating a nuanced approach to drug dosing strategies. For instance, polar drugs like gentamicin, which are distributed into extracellular fluids, require dosing based on ideal body weight to avoid overdosing. Lean body weight (LBW) can be estimated using empirical equations for dosage adjustments, particularly in renally impaired patients.

Moreover, the distributional changes in pharmacokinetics due to the higher proportion of body fat in obese patients, as well as the effects on specific drugs, must be carefully considered. Additionally, alterations in total body water per kilogram body weight and the influence of body fat on drug partitioning between lipid and aqueous environments further emphasize the complexity of drug dosing in the context of obesity.

A person exceeding the ideal body weight, IBW, by 20% is considered obese. It can be calculated using the following equations.

Typically, obesity is defined by the body mass index, which correlates strongly with total body fat.

Obesity can lead to hypertension, atherosclerosis, and coronary artery disease, all of which increase mortality rates.

Obese patients with excess body fat will have a smaller proportion of total body water, influencing the apparent volume of drug distribution.

Dosing in obese patients depends on the type of drug being administered.

For instance, polar drugs, such as gentamicin, distribute more into lean tissue than into adipose tissue. These drugs should be dosed based on IBW to avoid overdosing.

In contrast, non-polar drugs such as diazepam are distributed more in adipose tissues than lean tissues, necessitating larger doses based on total body weight.

Some amphiphilic drugs like lidocaine, which distribute equally in lean and adipose tissues, are dosed based on the patient’s total body weight.