All of the following (with the exception of the Ideal Body Weight Calculator and Formula) is from Resource 1, Manual of Emergency Airway Management 4th edition, 2012, RM Walls and MF Murphy. Chapter 39: The Morbidly Obese Patient, RD Zane and AL Erickson, pp 412-417.
Drug dosing for rapid sequence intubation (RSI) in the obese patient is complicated. Some drugs are dosed based on Ideal Body Weight (IBW). Others are based on Lean Body Weight (LBW), and still others are based on Total Body Weight (TBW).
The reason for the different dosing weights is because the different drugs have different volumes of distribution and/or different end organ clearance rates and/or different amounts of protein binding.
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Ideal body weight is calculated based on the patient’s height (if greater than or equal to five feet) from MDCalc Ideal Body Weight Calculator (1)
The Dr. Devine formula:
- Ideal Body Weight (men) = 50kg + 2.3kg * ( Height(in) – 60 )
- Ideal Body Weight (women) = 45.5kg + 2.3kg *( Height(in) – 60 )
Note: This formula is only an approximation, and is generally only applicable for people 60 inches (5 foot) tall or greater.
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The total body weight is the measured weight or the patient’s reported weight.
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The lean body weight is (the difference between the total body weight minus the ideal body weight) times 0.3—that quantity added to the ideal body weight.
In other words, the lean body weight is ideal body weight plus 30% of the difference between the total body weight minus the ideal body weight. Not all of the difference between total body weight and ideal body weight is fat. About 30% is additional muscle.
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The reason for the different dosing weights is because the different drugs have different volumes of distribution and/or different end organ clearance rates and/or different amounts of protein binding.
There is little data on the effect of obesity on RSI drug dosing and so what follows is based on the recommendations of the authors. (2)
The following drugs are dosed based on ideal body weight (IBW) for RSI:
Propofol Midazolam Vecuronium Rocuronium Remifentanil Lidocaine
The following drugs are based on lean body weight (LBW) for RSI:
Thiopental Fentanyl Etomidate
The following drugs are based on total body weight (TBW) for RSI:
Succinylcholine Atracurium
The authors do not discuss the whether ketamine is to be dosed based on IBW, LBW, or TBW.
Resources
(1) Manual of Emergency Airway Management 4th edition, 2012, RM Walls and MF Murphy. Chapter 39: The Morbidly Obese Patient, RD Zane and AL Erickson, pp 412-417.
(2) Dr. Devine’s formula for ideal body weight is explicityly defined on https://www.mdcalc.com/ideal-body-weight/.
