Adjustment of positive end-expiratory pressure based on body mass index during general anaesthesia: a randomised controlled trial

The Takeaway

In intubated adults under general anesthesia, setting PEEP levels to BMI/3 is a practical initial step towards optimizing individual PEEP settings.

Study Design

• Randomised controlled, patient-blinded, single center superiority trial
• Arms:
  • Standardized PEEP (PEEP = 5 cmH₂O; group PEEP-5)
  • PEEP set according to BMI (PEEP = BMI/3 cmH₂O; group PEEP-BMI/3).
• Volume Control mode with tidal volumes of 7 ml/kg ideal body weight, 10s recruitment maneuver performed after setting PEEP
• Primary endpoint: average driving pressure
• 60 patients
• Inclusion Criteria: Adults under general anesthesia requiring tracheal intubation, duration of anesthesia > 2 h, Non-cardiothoracic, non-neurological procedures performed by the general, vascular, orthopaedic, gynaecological, urological and plastic surgery
• Exclusion Criteria: laparoscopic or thoracic surgery; surgery involving cardiopulmonary bypass; patients in positions exceeding 10° Trendelenburg/reverse-Trendelenburg position; patients who were pregnant; BMI > 60 kg.m^-2; those with acute cardiac decompensation and severe pulmonary conditions

Abstract

Introduction: Lung-protective ventilation is essential for preventing postoperative pulmonary complications. While maintaining a low driving pressure and optimising PEEP is of importance, the ideal strategy remains contentious. This study evaluated whether adjusting PEEP based on BMI, compared with standard PEEP, could reduce driving pressure and peri-operative loss of lung aeration.

Methods: We conducted a randomised controlled, patient-blinded, single-centre superiority trial with two parallel groups. Adult patients undergoing surgery with general anaesthesia who required tracheal intubation were assigned randomly to either standardised PEEP (PEEP = 5 cmH₂O; group PEEP-5) or PEEP set according to BMI (PEEP = BMI/3 cmH₂O; group PEEP-BMI/3). Patients' lungs were ventilated using a volume-controlled mode with tidal volumes of 7 ml.kg^-1 predicted body weight. Lung aeration scores were assessed using ultrasound pre- and postoperatively.

Results: Sixty patients were enrolled and allocated randomly. Adjustment of PEEP according to BMI/3 was associated with a significantly lower driving pressure, with a median (IQR [range]) of 8.9 (7.1-10.4 [5.2-14.9]) cmH₂O in group PEEP-5 and 7.9 (7.2-8.5 [5.9-14.1]) cmH₂O in group PEEP-BMI/3 (p = 0.027) and higher mean (SD) respiratory system compliance (group PEEP-5, 0.83 (0.20) ml cmH₂O^-1 kg^-1 predicted body weight vs. group PEEP-BMI/3, 0.95 (0.17) ml cmH₂O^-1 kg^-1 predicted body weight; p = 0.020). Lung ultrasound revealed a reduced postoperative loss of lung aeration in patients allocated to the BMI/3 group. Patients allocated to the BMI-adjusted group required less supplemental oxygen, had less newly developed atelectasis and had higher oxygen saturations upon arrival in the post-anaesthesia care unit.

Discussion: In patients without major pulmonary disease who were undergoing non-cardiothoracic surgeries with tracheal intubation, adjusting PEEP based on a calculation of BMI/3 improved lung mechanics and reduced postoperative loss of lung aeration. This approach provides a straightforward and pragmatic method for individualising PEEP in patients undergoing general anaesthesia.

Physiology Refresh

Driving Pressure, defined as the difference between plateau pressure and PEEP (ΔP = Plateau Pressure – PEEP), represents the pressure required to deliver a tidal volume to the ventilated, open parts of the lung. In simple terms, it reflects lung compliance. A higher driving pressure means the lungs are stiffer or less compliant, which means greater pressure is needed to move air into the alveoli. This increases the risk of ventilator-induced lung injury, even if the tidal volume or plateau pressure appears within normal limits.

Several forms of lung injury can result from elevated driving pressure. Volutrauma occurs when alveoli are overdistended. Barotrauma results from excessive airway pressure, potentially leading to complications such as pneumothorax. Atelectrauma is caused by the repeated opening and closing of alveoli, which creates shear stress. Finally, biotrauma arises from the inflammatory response triggered by mechanical injury, which can contribute to systemic inflammation and even multi-organ dysfunction.

Research has confirmed the significance of driving pressure. A study published in The New England Journal of Medicine by Amato et al. in 2015 demonstrated that driving pressure was a stronger predictor of mortality in ARDS patients than tidal volume or plateau pressure alone. More recent studies have extended this concept to the intraoperative setting, showing that even patients without ARDS who are exposed to higher driving pressures during general anesthesia face a higher risk of postoperative pulmonary complications.

Clinically, minimizing driving pressure means adjusting ventilator settings to optimize lung protection. Using lower tidal volumes—typically 6 to 8 mL per kilogram of ideal body weight—is a good starting point. Applying appropriate levels of PEEP can help prevent alveolar collapse and maintain recruitment. The goal is to keep the driving pressure below 13 to 15 cm H₂O when possible, as this threshold is associated with reduced complications. Recruitment maneuvers may be useful in some cases but should be applied with caution to avoid causing additional injury.

Excerpts

The incidence rates of postoperative pulmonary complications vary depending on the definition, ranging from 2.0% to 5.6% in the general surgical population to as high as 40% in patients having thoracic surgery

Controversy persists regarding the optimal PEEP settings during general anaesthesia. Studies in which different PEEP levels were assigned randomly without tailoring them to individual patient characteristics showed no significant difference in outcomes between higher and lower PEEP settings

Recent evidence suggests that adjusting ventilatory settings to reduce the pressure difference (driving pressure) between end-inspiratory plateau pressure and PEEP may mitigate the risk of postoperative pulmonary complications, with growing evidence suggesting that individualised PEEP may be more effective than fixed levels

We found that in healthy individuals undergoing general anaesthesia in a supine position, the PEEP required to prevent both pulmonary overdistension and alveolar collapse, as assessed with electrical impedance tomography, correlated positively with the patient's BMI and corresponded to approximately one-third of the BMI

Compared with PEEP-5, we have shown a statistically significant reduction in driving pressure with PEEP-BMI/3, accompanied by improved postoperative lung aeration. This was confirmed by a significantly lower increase in lung aeration score and a reduced incidence of newly developed atelectasis in patients allocated to group PEEP-BMI/3. These improvements may have contributed to higher SpO₂ levels on arrival in the recovery room and decreased the need for supplementary oxygen.

The selection of driving pressure as the primary outcome parameter was based on the findings of a previous meta-analysis by Neto et al., which identified this as the sole significant mediator of the effects of protective ventilation on the development of postoperative pulmonary complications

the relatively small between-group difference in driving pressure, although statistically significant, suggests a modest clinical impact.

The pragmatic approach and easy applicability of the BMI/3 PEEP adjustment render it an appealing strategy for routine clinical practice, especially for initial PEEP adjustment. The observed improvements in lung mechanics and reduction in driving pressure could potentially translate into lower rates of postoperative pulmonary complications and better outcomes, particularly in patients with a higher BMI. However, while BMI/3 could serve as an effective ‘rule of thumb’, the resulting PEEP does not represent the optimal choice for each individual patient.

Although previous analyses suggest a potential causal relationship between driving pressure and postoperative pulmonary complications, this primary endpoint remains a surrogate parameter [12]. Furthermore, by not including patients with a BMI > 60 kg.m^-2 and the narrow BMI range of the study cohort (maximum BMI 37 kg.m^-2 in Group PEEP-5 and 40 kg.m^-2 in Group PEEP-BMI/3) limits the generalisability of these findings, particularly in patients living with severely obese. However, we note that previous studies suggest that the general relationship between PEEP and BMI may be consistent even in patients with higher BMI.

Future studies might address this by including a wider BMI range, stratifying patients according to BMI and employing multicentre designs to elucidate the relationship between PEEP adjustment based on BMI and postoperative pulmonary complications across different clinical scenarios.

Although postoperative SpO₂ on arrival in the recovery room was statistically significantly higher in the intervention group, the absolute between-group difference in SpO₂ was small and of a magnitude that is not clinically relevant for most patients.

Article Link

Citation

Selpien H, Penon J, Thunecke D, Schädler D, Lautenschläger I, Ohnesorge H, Eimer C, Wolf C, Sablewski A, Becher T. Adjustment of positive end-expiratory pressure based on body mass index during general anaesthesia: a randomised controlled trial. Anaesthesia. 2025 Jun 23. doi: 10.1111/anae.16656. Epub ahead of print. PMID: 40551551.