The Response of Dorsal Lung Compliance to Prone Positioning Differs by Body Mass Index in ARDS

Excess adipose tissue compresses dorsal lung regions that PEEP may not fully reverse in the supine position. Prone positioning, in combination with PEEP, can unload the lung, but whether regional responses differ by obesity is unknown.

We measured regional lung compliance using electrical impedance tomography in 14 mechanically ventilated subjects with COVID-19 ARDS (6 with BMI ≥35 kg/m² and 8 with BMI <30 kg/m²). Measurements were taken during decremental PEEP titration in supine and prone positions, following recruitment maneuvers. Regional compliance was derived from pixel-level impedance change within each region of interest, divided by driving pressure and predicted body weight. Bayesian multilevel gamma regression models with patient-level random effects estimated the 3-way interaction between position, BMI group, and lung region on regional compliance, adjusted for age.

The overall cohort had a median P_aO2/F_IO2 of 108 mm Hg and median respiratory system compliance of 29 mL/cm H₂O in the supine position. At the PEEP that produced the highest total respiratory system compliance in each position, the increase in dorsal regional compliance with prone positioning was greater in subjects with BMI ≥35 kg/m² compared with subjects with BMI <30 kg/m² (difference-in-differences: 0.16 mL/cm H₂O/kg, 95% credible interval: 0.01 to 0.37; posterior probability: 98.0%). Regional compliance in ventral and central lung regions did not credibly differ between BMI groups. The direction and magnitude of the change in dorsal lung compliance between BMI groups were consistent across PEEP levels.

Prone positioning was associated with a greater increase in dorsal lung compliance in subjects with BMI ≥35 kg/m² compared with subjects with BMI <30 kg/m². Our findings suggest that the response of regional compliance to prone positioning in ARDS differs by body mass index, identifying obesity as a population in whom positional unloading may be more effective.