In managing medication doses for obese patients, the process is complicated by several physiological alterations, including diminished total body water, augmented total body fat and lean mass, changes in protein binding due to hypoalbuminemia, expanded blood volume, increased cardiac output (CO), and the drug's affinity for fat (lipophilicity). Consequently, obese individuals may require higher doses of certain induction agents, as their blood volume, muscle mass, and CO tend to rise in proportion to their level of obesity. Given the strong lipophilic nature of many anesthetic drugs, which leads them to distribute extensively into adipose tissue, these medications can exhibit a substantially extended elimination half-life in obese patients.
When determining dosages:
- Using Ideal Body Weight (IBW) or Lean Body Weight (LBW) can result in underdosing, potentially yielding subtherapeutic effects.
- Basing doses on Total Body Weight (TBW) increases the risk of overdosing due to the enhanced volume of distribution in fat tissue.
Lipophilic Drugs
Lipophilic (fat-loving) drugs have a high affinity for fat tissues, leading to rapid distribution into the central nervous system (CNS) and other fatty areas, which contributes to their swift onset of action. These drugs typically have a large volume of distribution and, when combined with a higher cardiac output (CO), experience quicker redistribution throughout the body. This results in lower plasma concentrations, necessitating either a consistent dosing rate to achieve steady-state plasma concentrations or, in the case of a single bolus dose, potentially faster recovery times due to rapid awakening.
The increased redistribution into fat tissues can extend the drug's elimination half-life, prolonging its effect even if the drug's clearance rate remains unchanged or is increased. Despite this, the majority of CO is directed towards vessel-rich or lean tissue groups, making the use of total body weight (TBW) for dosing potentially hazardous in morbidly obese (MO) individuals due to the risk of overdosing.
Hydrophilic Drugs
Hydrophilic (water-loving) drugs, on the other hand, are primarily distributed to lean tissues, such as the muscle compartment. Therefore, the loading dose for these drugs should ideally be calculated based on ideal body weight (IBW). The distribution volume of hydrophilic, water-soluble agents is less influenced by obesity compared to lipophilic drugs, making them less prone to the dosing complications seen with fat-soluble medications in obese patients.
In summary, IDEAL body weight (IBW) or LEAN body weight (LBW) is the most appropriate dose for MOST dosing of intravenous anesthetic drugs. We’ll cover the exceptions below:
Exceptions to IBW and LBW Dosing
Propofol Maintenance Infusions - TBW correlates with increased volume of distribution and clearance during prolonged infusions.
Midazolam Bolus - Midazolam is highly lipophilic; duration and intensity of the bolus is dependent on distribution - and there's a high volume of distribution in patients with obesity.
Succinylcholine Bolus - Higher body weight is linked to greater pseudocholinesterase activity, requiring dosing based on TBW to achieve optimal conditions for intubation.
Sugammadex Bolus - Dosing based on TBW helps avoid insufficient medication, particularly crucial for obese patients whose pulmonary mechanics increase their vulnerability to the harmful consequences of lingering neuromuscular blockade.
Neostigmine Bolus - For all non-depolarizing muscle relaxants, including cisatracurium and others, reversal is effective up to a MAX dose of 5 mg; achieving a train-of-four (TOF) ratio greater than 0.9 occurs more quickly with higher doses.
Cefazolin Bolus - For adults, the standard dose is 2g, with many institutions suggesting a 3g dose for patients over 120 kg. However, it remains unclear whether a higher dose improves outcomes, as the evidence is inconclusive.
Vancomycin - Loading doses typically range from 15-20 mg/kg of TBW, though in obese patients, the loading dose may be less, often under 10 mg/kg of TBW. Subsequent dosing should be guided by drug-level monitoring. The dosing relies significantly on renal function.
Piperacillin-Tazobactam - Non-weight based. Renal function, (eg. estimated glomerular filtration rate), is the primary factor in determining dosage.
| Drug | Dosing basis | Reason |
|---|---|---|
| Most IV anesthetic drugs | Ideal or lean body weight | Total body weight risks overdose from the expanded volume of distribution in fat |
| Propofol maintenance infusion | Total body weight | Correlates with the increased volume of distribution and clearance seen during prolonged infusions |
| Midazolam bolus | Exception to ideal and lean body weight | Highly lipophilic; bolus duration and intensity depend on distribution, and volume of distribution is large in obesity |
| Succinylcholine bolus | Total body weight | Higher body weight is linked to greater pseudocholinesterase activity |
| Sugammadex bolus | Total body weight | Avoids underdosing; obese pulmonary mechanics make residual neuromuscular blockade especially harmful |
| Neostigmine bolus | Maximum dose 5 mg | Reversal of non-depolarizing relaxants is effective up to this ceiling; higher doses reach a train-of-four ratio above 0.9 sooner |
| Cefazolin bolus | 2 g standard; many institutions use 3 g above 120 kg | Evidence that the higher dose improves outcomes is inconclusive |
| Vancomycin | Loading 15–20 mg/kg total body weight, often under 10 mg/kg in obese patients | Subsequent dosing guided by drug levels; relies heavily on renal function |
| Piperacillin-tazobactam | Not weight based | Renal function, such as estimated glomerular filtration rate, is the primary factor |
