![]() Further, IBW is not the best scaler for all drugs. However, there are five published methods available for calculation and poor understanding exists of when and how to calculate IBW among clinicians. Total body weight is a poor size scaler and ideal body weight (IBW), which has a non-linear relationship to clearance (i.e., rate of clearance increase slows as size increases), is currently the only alternative body weight scaler to total body weight (TBW) mentioned in the British National Formulary for Children. The rate of clearance increase slows as size increases consequently dose, when expressed as per kilogram of total body weight, is invariably excessive. Most body weight scalers demonstrate a nonlinear relationship between clearance and size, a relationship that is evident in both obese and lean individuals. Consequently, recommendations for any size scaler are tempered by expert opinion that presumes dose in the obese child will be determined by better pharmacokinetic understanding. There is often confusion as to which metric is best suited for an individual child and that metric may change between phases of anesthesia (e.g., lean body mass for propofol induction dose and total body weight for maintenance dose rate). A smorgasbord of body weight scalers (e.g., total body weight, body surface area, ideal body weight, lean body mass, adjusted body weight, body mass index, fat-free mass, allometry) have been used to determine dose in the obese individual. Although it is recognized that fat mass may influence pharmacokinetic parameters such as volume of distribution (V) or clearance (CL), that the effect of fat mass is drug-specific, that weight-based dosing is a contributor to dose inaccuracies and that obesity influences disease processes, there are few practical dose recommendations for obese children. ![]() Total body weight dosing in obese children contributes to dose errors because the contribution from the fat mass portion of the body composition is not acknowledged. The use of target-controlled infusion pumps, assuming practitioners have a sound understanding of the PKPD within programs, provide the best available guide to intravenous dose in obese children. These models, along with covariates (age, weight, body composition), can be incorporated into programmable target-controlled infusion pumps. Dose is best determined using pharmacokinetic–pharmacodynamic (PKPD) models that account for these varied factors. Obesity is also associated with other morbidity that may also influence pharmacokinetics. Dosing is further complicated by the need for multicompartment models to describe intravenous drug pharmacokinetics and the concentration effect relationship, both beneficial and adverse, is often poorly understood. Normal fat mass, used in conjunction with allometry, may prove a useful size metric but computation by clinicians for the individual child is not facile. Fat-free mass, lean body mass and ideal body mass are not drug specific and fail to recognize the variable impact of fat mass contributing to body composition in children, both lean and obese. Fat mass also has an indirect influence on clearance through both metabolic and renal function that is independent of its effects due to increased body mass. Dosing schedules recognize the curvilinear relationship, described using allometric theory, between clearance and size. Clearance is the key parameter used to calculate infusion rates or maintenance dosing at steady state. Size metrics alternative to total body mass (e.g., fat-free and normal fat mass, ideal body weight and lean body weight) have been proposed to scale pharmacokinetic parameters (clearance, volume of distribution) for size. ![]() Fat mass influences the volume of distribution and the use of total body weight fails to recognize the impact of fat mass on pharmacokinetics in children. Total body weight comprises both fat and fat-free mass. ![]() That dose recognizes the linear relationship between volume of distribution and total body weight. ![]() The intravenous induction or loading dose in children is commonly prescribed per kilogram. ![]()
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