A simple equation for ideal body weight : European Journal of Anaesthesiology | EJA

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A simple equation for ideal body weight

Oh, Chahyun; Chung, Woosuk; Hong, Boohwi

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European Journal of Anaesthesiology 38(6):p 671-673, June 2021. | DOI: 10.1097/EJA.0000000000001488
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In clinical settings, patient weight is often used to determine the proper dose of medications, to adjust tidal volume during mechanical ventilation and to guide fluid management. However, use of unadjusted weight in these patients can lead to medication overdose1 or lung injury. Fortunately, such complications can be avoided by basing determinations on ideal body weight (IBW).1,2

Devine's formula, used in a landmark study [Acute Respiratory Distress Network (ARDSnet)],3 is often considered the standard formula for calculating IBW.4 Unfortunately, this formula is rather complex and difficult to memorise or calculate. Although simpler formulae have been developed, most remain too difficult to calculate in one's head in emergency situations or have limited consistency with the standard formula5,6 (Figs. 1 and 2). We, therefore, attempted to develop an extremely simple formula that can be applied in clinical practice without causing significant errors (IBW = height − intercept), by finding the optimal intercept, which minimises the deviation from Devine's formula. According to Devine's formula,

Fig. 1:
Modified Bland--Altman plots of ideal body weight predicted by Devine's and Broca's formulae in males and females
Fig. 2:
Modified Bland--Altman plots of ideal body weight predicted by Devine's and BMI-based formula in males and females

IBW = 50 (for males) or 45.5 (for females) + 0.91 × [(height (cm) − 152.4] = 0.91 × height − [88.7 (for males) or 93.2 (for females)]

When considering that the typical range of height is between 140 and 200 cm, the calculated values for the intercept are between 101.3 and 106.7 for males and 105.8 and 111.2 for females. Initially, the mean values [104 for males, 108 (integer for 108.5) for females] were used as the optimal intercept for our simplified formula. We next applied a computer-generated (R software version 4.0.3, R Project for Statistical Computing, Vienna, Austria) set of normally distributed data (n = 1000 for males and females, respectively) from a recent cohort for height in order to adjust the intercept,7 thus keeping the mean difference between the estimated IBWs calculated by Devine's formula and our new formula close to zero. The final version of our newly developed simplified formula is:

IBW (kg) = height (cm) − 105 (for males) or 108 (for females).

The 95% limits of agreement between Devine's formula and our new formula were −1.1 and 1.6 kg in males and −1.2 and 1.2 kg in females (Fig. 3). As the newly derived formula was a simplification of Devine's original formula, it could not overcome the limitations of its origin, such as underestimation of ideal weight in shorter persons and overestimation in taller people.8 However, because of its extreme simplicity and low error rate, our formula can be of use in routine clinical practice, not only for primary calculation of IBW but also as a secondary method to cross-check results obtained from complex formulae or electronic devices.

Fig. 3:
Modified Bland--Altman plots of ideal body weight predicted by Devine's and our new formula in males and females

Acknowledgements relating to this article

Assistance with the letter: none.

Financial support and sponsorship: none.

Conflicts of interest: none.


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