Estimation of diaphragm depth based on waist circumference in the Thai population

There is a potential risk of causing a pneumothorax during needle electromyography of the diaphragm. The diaphragm depth can be estimated from body mass index (BMI), but this may not be practical in bedbound patients. The objectives of this study were to formulate correlation equations to estimate diaphragm depth based on waist circumference (WC) and determine their validity.

Personal data, weight, height, and WC were collected from healthy volunteers, and the diaphragm depth from the skin and diaphragm thickness of the participants were measured by ultrasonography. These data were used to formulate the equations for estimation of diaphragm depth based on WC, and the equations were validated by comparing the estimated diaphragm depths between BMI-based equations and WC-based equations.

A total of 100 volunteers (48 men and 52 women) were recruited. The median age of the participants was 27 y (interquartile range, 26). The mean WC was 83.3 cm (SD, 7.8 cm). The diaphragm depth ranged between 1.0 and 2.7 cm. Equations for estimation of the diaphragm depth were created for each hemidiaphragm using data from 80 participants: left hemidiaphragm depth = (0.03 × WC) - 0.89 (r²  = 0.56); right hemidiaphragm depth = (0.03 × WC) - 0.94 (r²  = 0.55) with 10%-20% errors in 20 tested participants. The estimated diaphragm depths calculated from BMI and WC were comparable.

The WC-based equation can be used in addition to palpation techniques for guiding needle EMG insertion when ultrasound guidance is not available.