Optimal PEEP is the level of positive end-expiratory pressure during mechanical ventilation that best balances two competing goals: keeping collapsed lung tissue open to improve oxygen exchange while avoiding overdistension that damages healthy tissue. There is no single number that qualifies as optimal for every patient. Instead, optimal PEEP depends on the individual’s lung condition, body type, and how the lungs respond to pressure changes in real time.
What PEEP Does in the Lungs
During mechanical ventilation, PEEP maintains a baseline pressure in the airways at the end of each breath, preventing the smallest airways and air sacs from collapsing between cycles. Without it, vulnerable lung units snap shut during exhalation and must be forced open again with the next breath. This repetitive opening and closing, called atelectrauma, generates shearing forces that damage delicate tissue and trigger inflammation.
PEEP increases the volume of air that stays in the lungs at rest, a measurement called end-expiratory lung volume. In injured lungs, where fluid and inflammation cause widespread collapse, applying PEEP recruits previously closed regions back into service, improving the amount of lung available for gas exchange. The challenge is that the same pressure keeping collapsed areas open can stretch already-aerated areas beyond safe limits, causing a different form of injury.
Why Too Little or Too Much PEEP Causes Harm
Setting PEEP too low allows dependent (lower) lung regions to collapse under their own weight, creating pockets of atelectasis. These collapsed zones act as stress concentrators: the mechanical forces of each breath get amplified at the boundary between open and closed tissue, accelerating damage. In animal studies, ventilation with zero PEEP produced lesions primarily in these dependent regions where atelectasis developed during exhalation.
Setting PEEP too high creates different problems. Excessive pressure ruptures the thin walls between air sacs, causing alveolar hemorrhage and air leaks such as pneumothorax. High PEEP also has significant effects on the heart. It raises pressure inside the chest, which reduces the amount of blood returning to the right side of the heart. Simultaneously, it increases the resistance the right ventricle must pump against. Together, these effects can drop cardiac output sharply and, in severe cases, cause right heart failure and cardiovascular collapse. In long-term animal experiments, high PEEP did not impair gas exchange but caused dramatic deterioration in heart function, largely from increased resistance in the lung’s blood vessels.
Optimal PEEP sits in the narrow zone between these two extremes: enough pressure to prevent cyclical collapse, not so much that it overdistends healthy tissue or compromises circulation.
The Standard Approach: FiO2/PEEP Tables
The most widely used starting point for PEEP selection comes from the ARDSNet protocol, which pairs PEEP levels with the fraction of inspired oxygen (FiO2) a patient needs. Two versions exist. The low-PEEP table starts conservatively: at 30% oxygen, PEEP is set at 5 cmH2O, rising to 10 cmH2O at 60% oxygen and reaching 18 to 24 cmH2O only when a patient needs 90 to 100% oxygen. The high-PEEP table is more aggressive, calling for PEEP of 14 cmH2O or higher even at 30% oxygen and climbing to 22 to 24 cmH2O at full oxygen support.
These tables are practical and easy to follow, but they treat all patients with the same oxygen needs identically. They don’t account for differences in lung mechanics, chest wall stiffness, or body habitus. For that reason, clinicians increasingly use physiological measurements to individualize PEEP beyond what the tables suggest.
Titrating PEEP With Driving Pressure
Driving pressure is the difference between the pressure at the peak of a breath (plateau pressure) and PEEP. It reflects how much stretch each breath inflicts on the aerated portion of the lung. A driving pressure below 15 cmH2O is considered a safe threshold associated with lower mortality in ARDS patients.
When PEEP is too low, more lung tissue is collapsed, meaning each breath inflates a smaller “baby lung,” and driving pressure rises. Increasing PEEP recruits more tissue, spreads the breath across a larger area, and brings driving pressure down. The optimal PEEP by this method is the level that minimizes driving pressure for a given tidal volume. In one trial of patients undergoing chest surgery, titrating PEEP to the lowest driving pressure cut postoperative lung complications from 12.2% to 5.5% and eliminated all cases of ARDS in the titrated group.
Titrating PEEP With Esophageal Pressure
A more direct approach uses a thin balloon catheter placed in the esophagus to estimate the pressure surrounding the lungs. Subtracting this chest wall pressure from the airway pressure gives the transpulmonary pressure, which is the actual stretching force on lung tissue. The goal is to set PEEP so that transpulmonary pressure at the end of exhalation is slightly positive, keeping the lung open without excessive stretch.
Research in lung injury models found that a transpulmonary pressure just above zero still allowed roughly 17% of the lung to remain collapsed. Preventing nearly all collapse (less than 1%) required a transpulmonary pressure of about 4.6 cmH2O, which corresponded to a PEEP of 16 cmH2O. This method also allows clinicians to monitor the transpulmonary pressure at the top of each breath separately, guarding against overdistension in the regions of lung that are already well-aerated.
Pressure-Volume Curves
Another technique involves slowly inflating the lungs while plotting pressure against volume to create a curve. Two landmarks on this curve guide PEEP selection. The lower inflection point marks the pressure at which significant numbers of air sacs begin to open. Setting PEEP at or above this point promotes recruitment. The upper inflection point marks the pressure beyond which further inflation risks overdistension and barotrauma, so the peak pressure of each breath should stay below it.
When the lower inflection point is clearly visible, it usually indicates that collapsed lung is fairly evenly distributed, and PEEP well above that point can be tested safely because recruitment will outweigh overdistension. When no clear inflection point exists, collapse tends to be concentrated in the lower lobes while upper lobes remain aerated. In that pattern, high PEEP risks blowing up already-open tissue, and a moderate level around 10 cmH2O typically offers the best compromise.
Electrical Impedance Tomography
Electrical impedance tomography (EIT) is a newer bedside tool that uses a belt of electrodes around the chest to create a real-time map of where air is moving in the lungs. It can show, region by region, which areas are collapsed and which are being overdistended at any given PEEP level. Because it is noninvasive and radiation-free, it can be used continuously or repeated as often as needed.
During a PEEP titration with EIT, clinicians typically perform a decremental trial: they start at a high PEEP, then step down while watching the images. The optimal PEEP is the level where the combined amount of collapse and overdistension across all lung regions is minimized. This approach is especially useful for distinguishing patients whose lungs are highly recruitable (and benefit from higher PEEP) from those whose lungs are not (and are harmed by it).
Why One Size Does Not Fit All
Body weight is one of the clearest examples of why optimal PEEP varies between patients. In people with obesity (BMI of 30 or higher), extra abdominal and chest wall tissue pushes on the lungs from the outside, causing more collapse at any given airway pressure. Studies of obese patients undergoing bariatric surgery found that individualized PEEP levels consistently exceeded 15 cmH2O, with a median around 15 cmH2O for patients with a BMI over 30 and as high as 25 cmH2O in patients with a BMI over 50, depending on body position and whether the abdomen was inflated for laparoscopic surgery. The standard practice of using 5 cmH2O during anesthesia for these patients leaves a large amount of lung tissue collapsed.
Lung recruitability also varies. In ARDS, some patients have widespread, potentially reversible collapse that responds dramatically to higher PEEP, while others have consolidated, fluid-filled tissue that will not reopen regardless of pressure. Applying high PEEP to non-recruitable lungs simply overdistends whatever tissue remains aerated, increasing strain and promoting injury. This distinction helps explain the results of the ART trial, a large study of over 1,000 patients with moderate to severe ARDS. Patients randomized to aggressive lung recruitment maneuvers with PEEP titrated to best compliance had higher 28-day mortality (55.3%) than those receiving a conventional low-PEEP strategy (49.3%). The aggressive group also experienced more pneumothorax and barotrauma, suggesting that a blanket high-PEEP approach harms patients whose lungs cannot be recruited.
Putting It Together
No single method for finding optimal PEEP has proven superior across all patients and settings. In practice, clinicians often combine approaches: starting with a protocol-based table, then refining PEEP using driving pressure (aiming below 15 cmH2O), oxygenation response, and hemodynamic tolerance. When available, esophageal pressure monitoring or EIT adds a layer of individualization that table-based methods cannot provide.
The consistent lesson from the research is that optimal PEEP is a moving target. It changes as a patient’s lung disease evolves, as body position shifts, and as fluid status fluctuates. The safest approach treats PEEP as a variable to be reassessed regularly rather than a number set once and left alone.