Adverse physiologic changes in obese patients (Table 20–1) present multiple challenges in the operating room, including intravenous line placement, airway management, positioning, surgical exposure, and blood glucose control.10 Perhaps most important is determining the appropriate dose of anesthetic, as it will impact airway management and extubation and pain management. Although manufacturer's dosing recommendations are weight-normalized (mg/kg), obese patients are often excluded from clinical trials during drug development.9 Anesthesiologists rarely use total body weight (TBW) when programming an infusion pump or administering an induction dose for fear of an overdose; they often use something less than TBW to get “close enough” to the desired effect while avoiding significant adverse effects.11,12
Adverse effects of obesity by organ system.
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Table 20–1 Adverse effects of obesity by organ system.
|Organ System ||Adverse Effects |
|Cardiovascular || |
Increased vascular volume and cardiac output1,2 and 3
Decreased myocardial compliance3
Hypertension and left ventricular hypertrophy1
|Respiratory || |
Decreased lung compliance and functional residual activity4
Restrictive lung disease5
Rapid oxygen desaturation following apnea4
Increased risk of sleep apnea6
High airway pressures required to achieve adequate ventilation4
|Airway ||Increased likelihood of difficult ventilation and tracheal intubation6 |
|Hepatic || |
Minimal effect on drug metabolism.
Drug clearance: variable cytochrome P450 enzyme activity with altered drug binding to α1 glycoprotein but not albumin7,8
|Renal ||Glomerular filtration and creatinine clearance may exhibit no change, an increase, or decrease8,9 |
A popular method of administering intravenous anesthetics is as a continuous infusion. To improve the accuracy of infusion delivery, computerized infusion pumps that use pharmacokinetic models have been developed to quickly achieve and maintain desired target effect-site concentrations. This delivery technique, known as target-controlled infusions (TCI), has become popular worldwide. Unfortunately, neither have models used to drive TCI been validated in obese patients nor has agreement regarding the correct weight to input been identified. Most models were developed with normal-sized patients or volunteers and extrapolated for use in larger patients. The main limitation is that the composition of normal size and obese patients is not the same; hence, kinetic model predictions are likely to be less accurate in the obese.
To address these issues and others, clinical pharmacologists have put forth various “scaled weights” for dosing anesthetics and recommended modified or improved kinetic models to drive TCI pumps for selected anesthetics (ie, propofol and remifentanil). For most anesthetics, however, dosing recommendations are extrapolated and not based on studies in obese patients, making them difficult to reliably use.
DOSING “WEIGHTS” FOR OBESE PATIENTS
A simple approach when formulating a dose in obese patients is to use TBW. This approach assumes that the volume of distribution and clearance are the same in all patients, lean or ...