Mechanical ventilation and selecting optimal positive end-expiratory pressure (PEEP) in patients across a wide range of body mass indexes (BMI) is challenging. Adjusting PEEP to BMI using the equation BMI divided by 3 ('BMI/3') or setting a PEEP of 10 cmH ₂ O in obesity has been proposed; our objective is to describe the difference between 'BMI/3' and PEEP 10 cmH ₂ O as compared to optimal PEEP by esophageal manometry.

Esophageal manometry was used in patients undergoing laparoscopic abdominal surgery to estimate pleural pressure, transpulmonary pressure, and optimal physiological PEEP across a range of BMIs. Methods of estimating optimal PEEP in patients with normal and elevated BMI, namely 'BMI/3' and PEEP of 10 cmH ₂ O, were compared to estimates of optimal physiological PEEP as measured by an end-expiratory esophageal pressure-based transpulmonary pressure of 0 cmH ₂ O.

A total of 109 patients were included for analysis. Thirty-seven percent had ‘BMI/3’ estimated PEEP values within ±2 cmH ₂ O of optimal physiological PEEP measured by esophageal pressure-based transpulmonary pressure. A set PEEP of 10 cmH ₂ O correctly estimated optimal physiologic PEEP (±2 cmH ₂ O) in only 27% of patients. The mean optimal physiologic PEEP measured by esophageal pressure-based transpulmonary pressure is closely approximated by the mean estimated PEEP derived by 'BMI/3'. However, the ranges of individualized optimal physiologic PEEP are wider than PEEP estimated by 'BMI/3' across BMI categories.

BMI/3' estimated the mean optimal PEEP as measured by esophageal pressure-based transpulmonary pressure and may serve as a starting point for PEEP in patients with increased BMI. However, this purely anthropometric method fails to capture the individual variability of the chest wall and pleural pressure and most often results in inadequate or excessive PEEP as compared to optimal PEEP based on esophageal manometry.