Introduction

In morbidly obese populations the use of total Total Body Weight (TBW), is rarely the correct scalar when calculating doses of drugs. Most drugs are correctly dosed based upon some variation of Lean Body Mass (LBM) in this demographic, but there is a concern that the use of IBW is underestimating drug dosage in obese patients and the implications this may hold e.g. increasing the risk of awareness.

LBM can be calculated using the accepted Janmahasation formula[1]. The Devine formula is a popular equation used for to estimating Ideal Body Weight, using a height measurement only. The Robinson modification of the Devine formula is more robust as it is based on empirical population data rather than estimates of IBW [2].
We have used the above formulae for LBM and IBW to ascertain whether a significant difference exists in a population of males and female patients who underwent primary gastric bypass surgery and its implications on drug dosing. 

Methods
An anaesthetic airway database of bariatric surgical patients operated on at our institution for the past ten years was analysed, using the Janmahasatian and Robinson formulae to calculate LBM and IBW respectively. We have then used GraphPad Prism to create a series of graphics to help visualize the difference in calculated IBWs versus LBMs for this population.

Results

Over the period 2006-17, more than 3700 patients underwent bariatric surgery under our care. Females represented 78% of the group, the median age of both males and females was 44 (IQR 37-51, range 17-74) and the median BMI of the cohort was 49. We found a significant difference between calculated IBWs versus LBMs in this group. The mean difference was 1.833 kg in females and 13.71 kg in males, with the maximum weight difference between scalars being 20.50kg and 37.3kg in females and males respectively. Plotting the weight differences calculated from LBM-IBW against the TBW of our population, demonstrated that at extreme ends of TBW, the difference in calculated LBM and IBW can be significant.

Conclusions

The implications of using IBW as a dosing scalar are obvious – significant under-dosing of drugs in the larger patients. This becomes an issue in all males above 120kg, and in all females above around 150kg. With a rise in obesity in the general population, we all need to be familiar in calculating LBM to prevent adverse outcomes from under dosing of anaesthetic drugs.

10 / Ideal body weight versus lean body mass in bariatric surgical patients. Far from ideal?_x000D_