Ideal Body Weight - Master Anesthesia

Ideal body weight (IBW) is the weight associated with the lowest mortality. This is based on height and gender. There are a couple different equations out there to find IBW. The one most often used to calculate adult IBW is Broca’s index:

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IBW (kg) = height(cm) – x

x represents 100 if your male, and 105 if your female. So a patient that was 165 cm would have an ideal body weight of 65 kg (male) and 60 kg (female)

If your dealing with pounds and inches, another way of doing it is for the first 60 inches of height, give 110lbs for males and 100lbs for females. Then for every inch over 60, add an additional 5lbs.

So, if your patient is 65 inches and male, the ideal body weight would be 135lbs (110 + 5*5). If your patient were the same height, but female, the ideal body weight would be 125lbs (100 + 5*5). When converted to kg, this is 61 kg and 57 kg for males and females respectively. You can see that this is pretty close to the results of the other equation! We prefer the highlighted calculation, both because it is easier to calculate, and it’s the one most referenced in our sources.

Obese patients typically have 20-30% more lean body mass. So the IBW for an obese patient would look like this:
Obese IBW = IBW x 1.3

Pediatric patients have their own equation for calculating IBW

Pediatric IBW less than 8 years old:

IBW = 2 x Age(years) + 9

Pediatric IBW greater than 8 years old:

IBW = 3 x Age(years)

IBW is useful in many ways. Most notably, it is used for the calculation of drug doses. An easy way to remember which ones are dosed on IBW:

Water-soluble drugs use ideal body weight. Lipid Soluble drugs use actual body weight.

Most drugs in anesthesia are lipid soluble. These are the common water soluble drugs:
Midazolam
Ketamine
Nondepolarizing Neuromuscular Blockers
Remifentanil

Exceptions:
Opioids and local anesthetics should be based on IBW weight in the obese.

Calculations using ideal body weight:
LMA sizing (when using the manufacturer’s recommended weight guidelines)

Respiratory estimations
-Tidal volume (6-8 ml/kg)

-Anatomic dead space (2 ml/kg)

-FRC (35 ml/kg)

-Vital Capacity (70 ml/kg)

References
Butterworth. Morgan & Mikhail’s Clinical Anesthesiology. 2013.
Chu & Fuller. Manual of clinical anesthesiology. 2012.
Kim. Randomized Comparison of Actual and Ideal Body Weight for Size Selection of the Laryngeal Mask Airway Classic in Overweight Patients. 2015. web link
Nagelhout. Nurse anesthesia. 5th edition. 2014.

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