PEEP and the Open Lung Approach
Although mechanical ventilation is a life-saving intervention, it places significant stress and strain on the lung parenchyma. Damage at the pulmonary parenchymal and muscular levels from mechanical ventilation is termed ventilator-induced lung injury (VILI). Major injuries, such as pneumothoraces, are easy to diagnose at the bedside and are relatively uncommon. By contrast, VILI is hard to diagnose at the bedside. It is insidious, manifests as worsening gas exchange and compliance, and ultimately leads to prolonged mechanical ventilation and even death. There is no straightforward, universally accepted method for preventing VILI.
There are several types of VILI. One type, atelectrauma, results from cyclic opening and closing of alveolar units. To avoid atelectrauma, clinicians set adequate positive end-expiratory pressure (PEEP) levels. They may also attempt to "recruit" areas of previously collapsed lung. Some will use airway pressure release ventilation (APRV) to prevent atelectrauma and avoid lung de-recruitment. The specific strategy of using a recruitment maneuver to optimize compliance, set PEEP, recruit lung, and avoid atelectrauma is called the open lung approach (OLA).
The OLA is backed by sound physiologic rationale. Atelectrauma and VILI are real, and patients with acute respiratory distress syndrome (ARDS) have stiff, noncompliant lungs that are susceptible to further injury. They should benefit from recruiting healthy lung, which theoretically ought to improve compliance and avoid atelectrauma. Studies of spontaneous breathing during mechanical ventilation stress optimizing PEEP, and pilot OLA studies, have shown benefit. Even APRV now has a randomized clinical trial supporting its use.
Unfortunately, the 2017 ART study found that OLA leads to worse outcomes. This large, prospective, multicenter trial included 120 ICUs in nine different countries. Patients were randomly assigned to the OLA vs the ARDSNet protocol of PEEP titration. All other mechanical ventilation interventions were essentially the same. The trial found that the OLA was associated with increased 28-day and 6-month mortality.
A more recently published phase 2 study (PHARLAP) using a similar approach was equivocal, but importantly, the ART study prohibited further recruitment because physicians were reluctant to enroll patients after the results were released. ART also pushed the Data and Safety Monitoring Committee to halt the trial early. Coupled with a negative 2019 trial using an esophageal catheter to optimize PEEP and open the lung, these results are disappointing.
Where does this leave us? The control groups in the OLA and esophageal catheter trials were all managed using the two versions of the ARDSNet PEEP table. Because the control groups did as well or better than patients receiving the more labor-intensive and invasive methods being studied, the ARDSNet PEEP tables emerge from these trials as the de facto treatment of choice. Of course, PEEP needs to be individualized, and as the authors of the OLA trials point out, using the OLA for only those who prove to be "recruitable" may have generated different results. We'll never know. The experience with the PHARLAP study is instructive; ART has eliminated the equipoise with OLA, so it will be difficult to study again. After 20 years we're still stuck with ARDSNet, and that's not a bad thing.
Aaron B. Holley, MD, is an associate professor of medicine at Uniformed Services University and program director of pulmonary and critical care medicine at Walter Reed National Military Medical Center. He covers a wide range of topics in pulmonary, critical care, and sleep medicine.
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Any views expressed above are the author's own and do not necessarily reflect the views of WebMD or Medscape.
Cite this: Aaron B. Holley. Open Lung or ARDSNet PEEP: Which Is Better for Preventing VILI? - Medscape - Jun 09, 2020.