Introduction: The Quest for Lung-Protective Ventilation in the OR
For decades, the cornerstone of lung-protective ventilation (LPV) has been the use of low tidal volumes to prevent volutrauma. However, attention has recently shifted toward the role of airway driving pressure (DP)—the difference between plateau pressure and positive end-expiratory pressure (PEEP)—as a more refined marker of lung strain. While lower driving pressures have been associated with improved survival in patients with Acute Respiratory Distress Syndrome (ARDS), its utility in the perioperative setting remains a subject of intense debate.
The logic appears sound: by individualizing PEEP to minimize driving pressure, clinicians can theoretically optimize lung recruitment and prevent cyclic atelectasis. Yet, two major recent publications—the IMPROVE-2 randomized clinical trial and a comprehensive systematic review—provide a complex, and at times contradictory, picture of whether this physiological optimization actually translates into better patient outcomes.
Background: The Physiological Rationale for Driving Pressure
Postoperative pulmonary complications (PPCs) significantly contribute to surgical morbidity and mortality, particularly in high-risk abdominal procedures. Driving pressure reflects the ratio of tidal volume to the functional compliance of the respiratory system. In simpler terms, it indicates how much pressure is required to deliver a breath into the available, aerated lung.
Setting a “fixed” PEEP (commonly 5 cmH2O) is the standard of care in many institutions, but it fails to account for individual variations in body habitus, surgical positioning (such as the Trendelenburg position), and the effects of pneumoperitoneum. Proponents of driving pressure-guided PEEP (PEEPdp) argue that titration allows for a “sweet spot” where lung recruitment is maximized without causing overdistension.
The IMPROVE-2 Trial: A Pragmatic Challenge to Physiological Assumptions
Study Design and Patient Population
The IMPROVE-2 trial was a multicenter, pragmatic, assessor-masked, randomized trial conducted across 22 hospitals in France. It focused on a high-risk cohort: adult patients undergoing emergency abdominal surgery. These patients are often hemodynamically unstable and carry a higher risk of inflammatory lung injury compared to elective surgical candidates.
A total of 679 patients were assigned 1:1 to either the intervention group or the control group.
The Intervention: Targeting Lung Compliance
In the intervention group, patients received a recruitment maneuver followed by an individualized PEEP setting. The goal was to find the highest PEEP level that maintained a driving pressure of less than 13 cmH2O. In contrast, the control group received a standard fixed PEEP of 5 cmH2O. Both groups were ventilated with a tidal volume of 8 mL/kg of predicted body weight.
Key Findings of the IMPROVE-2 Trial
Primary and Secondary Outcomes
The primary outcome was a composite of postoperative respiratory failure (including failure to wean, reintubation, or the need for curative non-invasive ventilation) or all-cause mortality within 30 days.
The results were striking. The primary outcome occurred in 25.7% of the intervention group compared to 20.2% of the control group. While this difference did not reach statistical significance for the composite (p = 0.08), the trend favored the standard, fixed PEEP approach rather than the individualized strategy.
The Safety Paradox: Increased Reintubation Risk
One of the most concerning findings in IMPROVE-2 was the significant increase in the incidence of reintubation or the need for curative non-invasive ventilation among patients in the individualized PEEP group (difference of 7.1%; 95% CI 2.5–11.9; p = 0.004). This suggests that the maneuvers used to optimize driving pressure may have had unintended consequences, potentially related to hemodynamic instability or the inflammatory response triggered by recruitment maneuvers in the context of emergency surgery.
Broadening the Lens: Insights from Systematic Review and Meta-Analysis
To provide context to the IMPROVE-2 results, a systematic review and meta-analysis by Sun et al. analyzed 19 randomized controlled trials involving 3,744 patients. This broader look provides a more nuanced view of PEEPdp.
Comparison of Individualized vs. Fixed PEEP
The meta-analysis confirmed that PEEPdp (averaging around 8.2 cmH2O) successfully improved respiratory mechanics. Patients in the PEEPdp groups were ventilated with significantly lower driving pressures (10 cmH2O vs. 11.9 cmH2O) and exhibited higher static respiratory compliance.
Crucially, unlike the IMPROVE-2 trial, the meta-analysis found that PEEPdp was associated with a reduction in the overall incidence of postoperative pulmonary complications (PPCs). However, consistent with IMPROVE-2, PEEPdp did not significantly impact hard clinical endpoints like ICU admission, mortality, or length of hospital stay.
Expert Commentary: Reconciling Conflicting Evidence
The Impact of Surgical Context: Emergency vs. Elective
Why did IMPROVE-2 show potential harm while the meta-analysis suggested benefit? The answer likely lies in the patient population. Most trials in the meta-analysis focused on elective surgeries. In elective cases, patients are optimized, and the lung is generally “healthy” before surgery. In IMPROVE-2, the emergency nature of the surgery means patients are often in a pro-inflammatory state. In these vulnerable patients, high PEEP and recruitment maneuvers might lead to right ventricular strain or bacterial translocation, outweighing the benefits of improved compliance.
Methodological Considerations: Recruitment Maneuvers and Hemodynamics
The IMPROVE-2 trial utilized recruitment maneuvers to set PEEP. While these maneuvers open collapsed alveoli, they can also cause transient hypotension and decreased cardiac output. In emergency abdominal surgery, where fluid status is often precarious, these hemodynamic shifts might contribute to secondary organ dysfunction, including respiratory failure.
Is Driving Pressure a Target or a Marker?
There is a growing consensus that while driving pressure is a powerful prognostic marker (high DP is bad), it may not be a successful therapeutic target for every patient. Reducing driving pressure by increasing PEEP only works if the lung is “recruitable.” If the lung is already fully open, adding PEEP only increases dead space and intrathoracic pressure without improving gas exchange.
Conclusion: Moving Beyond Pressure Targets
The IMPROVE-2 trial serves as a crucial reminder that physiological optimization does not always equal clinical success. In the specific context of emergency abdominal surgery, a strategy of individualized, high-PEEP titration to minimize driving pressure may be unnecessary or even detrimental.
For the clinician, the takeaway is not that driving pressure should be ignored, but rather that it must be interpreted within the clinical context. A “one-size-fits-all” approach to individualized PEEP remains elusive. Future research should focus on identifying which specific phenotypes of surgical patients—perhaps those with the highest baseline atelectasis—benefit most from PEEP titration, while avoiding aggressive maneuvers in those where the risks of hemodynamic instability outweigh the respiratory gains.
Funding and Clinical Trial Information
The IMPROVE-2 trial was supported by the French Ministry of Health. ClinicalTrials.gov Identifier: NCT03987789. The systematic review was conducted independently using data from PubMed, Cochrane Library, Web of Science, and Embase.
References
1. Futier E, et al. Personalized driving pressure-guided positive end-expiratory pressure in patients at risk of postoperative respiratory failure (IMPROVE-2): a multicenter, pragmatic, randomized clinical trial. Intensive Care Med. 2025;51(10):1797-1808.
2. Sun YH, et al. Effect of driving pressure-guided positive end-expiratory pressure on respiratory mechanics and clinical outcomes in surgical patients: a systematic review and meta-analysis of randomized controlled trials. Ann Med. 2025;57(1):2543978.
3. Amato MB, et al. Driving pressure and survival in the acute respiratory distress syndrome. N Engl J Med. 2015;372(8):747-55.
