To the Editor:—
Lundstrøm et al. 1demonstrated that high body mass index (BMI), defined as weight per height squared (kg/m2), is a weak but significant predictor of difficult tracheal intubation. Exact determination of body composition to define the quantity and distribution of muscle and fat requires complex measurements unavailable in the perioperative setting. Teleologically, one hopes that an easily derived parameter like BMI can quantify obesity, which one intuitively expects to correlate with difficult tracheal intubation. Although the World Health Organization has adopted BMI to quantify obesity, BMI remains a misunderstood empiric 19th-century observation that is an illogical parameter for this task.2,3
BMI compares weight (and approximate volume) to surface area, which correlates to useful physical characteristics such as joint loading (force/area) or heat retention (mass of metabolically active tissue per surface area available for heat loss). It may not be apparent that, for objects of identical shape and density, BMI is directly and exactly proportional to height. Using the definition above, one can easily calculate that a 1-cm cube of water has a BMI of 10, a 2-cm cube of water has a BMI of 20, a 1-m cube of water has a BMI of 1,000, and so forth. However, differences in height may overwhelm differences in thickness: Short overweight patients may have a lower BMI than tall thin patients. The ponderal index (PI = kg/m3) is a statistic proportional to the cube of the height instead of the square of the height. Because volume and mass are cubic functions of the linear dimension, the PI depends on shape but is insensitive to height: The PI of a sphere of water is always 523.6, the PI of a cube of water is always 1,000, and normal human PI is within a narrow range of 10 to 14.3The PI is more commonly used in pediatrics when height changes rapidly; during adolescence, the PI may decrease as children become taller and proportionally thinner, even though the BMI may paradoxically increase.3The human body shape index (HBSI) seeks to determine the optimal exponent, which appears to lie between 2.7 and 2.9 over a wide range of heights and ages (HBSI = kg/m2.8).3
For these reasons, Lundstrøm et al. may find better correlations between either the PI or human body shape index and difficult tracheal intubation than between BMI and difficult tracheal intubation. The authors’ data are a valuable resource, and we would like to encourage them to determine if a more significant relationship can be obtained using the PI or the human body shape index.
*Medical College of Wisconsin, Milwaukee, Wisconsin. firstname.lastname@example.org