Using body mass index (BMI) to guide positive end-expiratory pressure (PEEP)

What is the optimal amount of PEEP to use on patients? Everyone benefits from a minimum amount of PEEP while undergoing mechanical ventilation, but should PEEP be increased with laparoscopic insufflation, Trendelenburg positioning, or overweight and obesity? It stands to reason that these factors require higher PEEP due to increased compressive forces on the lungs. However, determining the optimal PEEP can be difficult without instruments, such as esophageal manometry, that are not widely available.

A group of researchers set out to determine if a simple PEEP estimating strategy using BMI could give anesthesiologists a more practical means of setting PEEP. They conducted an RCT comparing PEEP of 5 cm cm H₂O (PEEP-5 group) to a PEEP set equal to BMI divided by 3 (PEEP-BMI/3 group) with 30 participants in each arm. Participants were undergoing elective, non-cardiac, non-thoracic surgery under general anesthesia with endotracheal intubation. Laparoscopic surgery and any surgery requiring more than 10° of Trendelenburg or reverse Trendelenburg were excluded. Pregnant patients and those with BMI > 60 kg/m² were also excluded. Ultrasound was used pre- and post-operatively to determine lung aeration and airway pressures were measured.

The PEEP-BMI/3 group had lower driving pressures (PEEP-BMI/3 group median 7.9 cm H₂O compared to PEEP-5 group median 8.9 cm H₂O, p = 0.027) meeting the study’s primary endpoint. The intervention group also had higher compliance (PEEP-BMI/3 group mean 0.95 mL/cm H₂O/kg of predicted body weight compared to PEEP-5 group mean 0.83 mL/cm H₂O/kg, p = 0.02), improved aeration (p ≤ 0.01), and decreased atelectasis (no new atelectasis in PEEP-BMI/3 group compared to 7 new cases in PEEP-5 group, p = 0.011). The median PEEP in the PEEP-BMI/3 group was 9 cm H₂O with a range of 7-13 cm H₂O, which represents a fairly modest increase.

These results may seem promising as a way of estimating PEEP, but the limitations should give us pause. Despite being a positive study, the authors note that the small difference in driving pressures may not be clinically impactful. Furthermore, the mean (SD) BMI across all participants was 27.8 kg/m² (4.8). This represents a population fairly close to normal weight individuals. Whether this BMI PEEP strategy would provide optimal PEEP, or perhaps too much PEEP, in individuals with class 2 and 3 obesity is not clear. Two groups I am particularly interested in, those undergoing laparoscopic surgery and those in steep Trendelenburg positioning, were both excluded. Taken together, these limitations significantly reduce the ability to generalize these results across patient populations.

In a reply to this article, Kozanhan and colleagues pointed out the limitations of using BMI to assess body habits. Body mass index does not distinguish between lean mass and adiposity. Nor does it separate patients who have central versus peripheral obesity—the former likely having a more negative effect on respiratory mechanics. They hypothesized that using waist circumference or waist-to-hip ratio may be more appropriate than using BMI.

While an accurate and accessible way to determine PEEP for all populations remains to be developed, what this paper emphasizes to me is the importance of using higher PEEP values for those with overweight and obesity. Hopefully, in the years ahead, we will have a simple and accurate way to determine optimal PEEP across patients, surgeries, and patient positions.