Positive end-expiratory pressure, universally called PEEP, is one of the most important tools clinicians have when ventilating patients with acute respiratory distress syndrome (ARDS). It works by maintaining a small amount of pressure in the airways even after the patient exhales, which prevents damaged, fluid-filled lung units from collapsing between breaths. Getting the right level matters enormously: too little PEEP leaves large portions of the lung collapsed and vulnerable to repeated opening-and-closing injury, while too much can overstretch the regions that are still healthy and squeeze the heart. Decades of research have tried to pin down the ideal PEEP for ARDS, and the honest answer is that the ideal number depends on the individual patient’s lungs.
What PEEP Does in Injured Lungs
In ARDS the lungs become a patchwork. Some alveoli are flooded with inflammatory fluid and completely collapsed. Others remain open but fragile. Still others sit somewhere in between, cycling between collapsed and open with each breath. PEEP targets that in-between zone. By keeping end-expiratory pressure above zero, it holds unstable alveoli open so they can participate in gas exchange on the next breath. When previously collapsed alveoli reopen, the same tidal energy spreads across a larger aerated volume, lowering the mechanical stress on each individual unit.1PubMed Central. Lung recruitability determines the impact of PEEP on mechanical power in ARDS
PEEP increases the volume of air remaining in the lungs at the end of expiration, a measurement known as end-expiratory lung volume. In one study of ARDS patients, the amount of lung tissue recruited by raising PEEP had a median of about 272 mL, though individual responses ranged widely.2PubMed. PEEP-induced changes in lung volume in acute respiratory distress syndrome. Two methods to estimate alveolar recruitment That variability matters. Some patients have a lot of recruitable lung, meaning PEEP opens substantial new territory. Others have relatively little recruitable tissue, in which case raising PEEP mainly stretches alveoli that are already open. The overall effect hinges on the balance between these two outcomes: recruitment of new lung versus overdistension of what is already aerated.3PubMed Central. Fifty Years of Research in ARDS. Setting Positive End-Expiratory Pressure in Acute Respiratory Distress Syndrome
Preventing Repeated Collapse Injury
One of the main reasons PEEP is used in ARDS is to prevent a specific type of ventilator-induced lung injury sometimes called atelectrauma. Every time an alveolus collapses during expiration and is forced back open during the next breath, the shearing forces at its walls cause damage. Over hours and days of mechanical ventilation, that cyclic collapse-and-reopen pattern can amplify inflammation and worsen the original lung injury. Simulation research has shown that this injury index climbs dramatically as PEEP drops below the level needed to keep most alveoli open. Above a certain threshold, the injury from cyclic collapse falls close to zero.4PubMed Central. Atelectrauma Versus Volutrauma: A Tale of Two Time-Constants
This protective effect is separate from the oxygenation benefit. Clinicians often think of PEEP primarily as a way to improve blood oxygen levels, and it does do that. But the lung-protective role may be equally or more important for outcomes, because reducing the mechanical insult to lung tissue can slow the cascade of inflammation that makes ARDS worse over time.
High Versus Low PEEP in Clinical Trials
Given that PEEP helps prevent collapse injury, you might assume that higher is always better. Trials have tested exactly this. The landmark ALVEOLI trial randomly assigned ARDS patients to a higher-PEEP group (averaging about 13 cmHâ‚‚O) or a lower-PEEP group (averaging about 8 cmHâ‚‚O), with both groups receiving the same low tidal volumes. Hospital mortality was almost identical: roughly 25% in the lower group and 28% in the higher group, a difference that was not statistically meaningful.5PubMed. Higher versus lower positive end-expiratory pressures in patients with the acute respiratory distress syndrome
A patient-level meta-analysis pooling data from three major trials found a similar pattern overall: hospital mortality was about 33% with higher PEEP and 35% with lower PEEP, a difference that did not reach statistical significance. But when the analysis zoomed in on the sickest patients, those who met the full criteria for ARDS at enrollment, higher PEEP was associated with lower hospital mortality (about 34% versus 39%) and patients came off the ventilator sooner. Patients with milder lung injury, on the other hand, trended toward worse outcomes with higher PEEP.6JAMA. Higher vs Lower Positive End-Expiratory Pressure in Patients With Acute Lung Injury and Acute Respiratory Distress Syndrome: Systematic Review and Meta-analysis A separate systematic review of the trial-level data reached a broadly consistent conclusion: higher PEEP did not significantly reduce mortality or barotrauma in the primary analysis, though secondary analyses including older trials that used larger tidal volumes in the low-PEEP arms did show a benefit.7Annals of the American Thoracic Society. Higher PEEP versus Lower PEEP Strategies for Patients with Acute Respiratory Distress Syndrome. A Systematic Review and Meta-Analysis
The upshot is that blanket high PEEP is not universally better. The benefit seems concentrated in patients with more severe lung injury, where there is genuinely more lung to recruit. Applying the same high pressures to patients with milder disease risks overdistending their already-open alveoli for little gain.
The Open Lung Approach and the ART Trial Warning
If moderate increases in PEEP showed modest benefit in severe ARDS, perhaps an aggressive strategy of maximal lung recruitment followed by high PEEP would do even better. The ART trial tested exactly this, using recruitment maneuvers that pushed airway pressures up to 50-60 cmHâ‚‚O before titrating PEEP downward based on lung compliance. The results were sobering: at 28 days, about 55% of patients in the recruitment group had died compared with about 49% in the control group. The experimental group also had higher rates of pneumothorax and hemodynamic instability within the first hour of recruitment.8JAMA. Effect of Lung Recruitment and Titrated Positive End-Expiratory Pressure (PEEP) vs Low PEEP on Mortality in Patients With Acute Respiratory Distress Syndrome: A Randomized Clinical Trial
The ART trial was a turning point. It demonstrated that aggressive recruitment maneuvers carry real danger, and that simply pushing PEEP as high as possible is not the path forward. The field shifted its focus toward finding the right PEEP for each patient rather than aiming for the highest tolerable level.
Tailoring PEEP to the Individual
Several approaches aim to personalize PEEP beyond the standard tables that pair PEEP levels with the fraction of inspired oxygen a patient needs.
Esophageal Pressure Monitoring
A balloon catheter placed in the esophagus can estimate the pressure surrounding the lung. By measuring the difference between airway pressure and this surrounding pressure, clinicians can calculate the transpulmonary pressure, which reflects the actual stretch on lung tissue. The goal is to set PEEP high enough that transpulmonary pressure at end-expiration stays just above zero, keeping alveoli open without overdistending them. In the original EPVent trial, this approach produced substantially better oxygenation and lung compliance compared with standard care.9PubMed Central. Mechanical ventilation guided by esophageal pressure in acute lung injury A study in traumatic ARDS patients found that esophageal-guided PEEP resulted in higher PEEP settings (about 12 versus 8 cmHâ‚‚O) and better oxygenation and compliance than standard protocol-based settings.10PubMed. A clinical study on mechanical ventilation PEEP setting for traumatic ARDS patients guided by esophageal pressure
However, a larger follow-up trial (EPVent-2) did not demonstrate a clear mortality benefit from this approach. A post-hoc analysis suggested the technique might help when the resulting end-expiratory transpulmonary pressure lands close to zero, but not when it drifts higher or more negative.11PubMed Central. Setting positive end-expiratory pressure: the use of esophageal pressure measurements The technique remains promising but has not yet proven itself in large outcome trials.
Electrical Impedance Tomography
Electrical impedance tomography (EIT) is a bedside imaging technique that maps ventilation distribution across the chest in real time. Small electrodes placed around the torso detect changes in electrical impedance as air moves in and out, creating a continuous picture of which lung regions are open, collapsed, or overdistended. When used for PEEP titration, clinicians look for the intersection between the collapse and overdistension curves: the pressure at which both are minimized.12Scientific Reports. The impact of PEEP-guided electrical impedance tomography on oxygenation and respiratory mechanics in moderate-to-severe ARDS: a randomized controlled trial Systematic reviews have examined EIT-guided PEEP as a way to optimize mechanics and reduce ventilator-induced injury, though large mortality trials are still needed.13PubMed Central. Electrical impedance tomography-guided positive end-expiratory pressure titration in ARDS: a systematic review and meta-analysis
Driving Pressure
Driving pressure is the difference between the plateau pressure (at end-inspiration) and PEEP. It reflects how much stretch the lung actually experiences with each breath and has been identified in observational studies as a strong predictor of mortality. The logic of driving pressure-guided titration is appealing: adjust PEEP and tidal volume to minimize the driving pressure, which should mean the lung is operating at its best compliance. In practice, though, randomized trials specifically targeting lower driving pressure through PEEP adjustments have not produced significant mortality reductions, although one meta-analysis found a shorter ICU stay.14PubMed Central. Effect of driving pressure-limiting strategies on outcomes of patients with ARDS: a meta-analysis of randomized controlled trials Some researchers have cautioned that when driving pressure was minimized through higher PEEP with or without recruitment maneuvers, the strategy sometimes resulted in equal or even higher mortality, suggesting that the relationship between driving pressure and outcomes may not be as straightforwardly causal as hoped.15PubMed Central. Should we titrate ventilation based on driving pressure? Maybe not in the way we would expect
How PEEP Affects the Right Side of the Heart
PEEP does not act only on the lungs. Because the heart and lungs share the same enclosed space in the chest, raising intrathoracic pressure with PEEP directly affects cardiac function, especially the right ventricle. Higher PEEP can compress the small blood vessels running through aerated lung tissue, increasing the resistance the right ventricle has to pump against. In ARDS, where some regions are collapsed and others overinflated, this effect can be unpredictable and significant.16European Respiratory Review. Right-sided heart failure in acute respiratory distress syndrome
A prospective study in moderate-to-severe ARDS patients found that stepping PEEP up from 5 to 15 cmHâ‚‚O caused the cardiac index to fall by about 14% and the stroke volume index to drop by roughly 18%. The incidence of acute cor pulmonale, a pattern of right ventricular dilation and dysfunction, more than doubled at 15 cmHâ‚‚O compared with 5 cmHâ‚‚O (about 39% versus 17%).17PubMed Central. The impact of positive end-expiratory pressure on right ventricular function in patients with moderate-to-severe ARDS: a prospective paired-design study This means that even when higher PEEP improves oxygenation, it can simultaneously compromise oxygen delivery by reducing how much blood the heart pumps each minute. Monitoring the right ventricle with bedside echocardiography has become an important part of PEEP titration in many ICUs for exactly this reason.
Barotrauma Risk
The question of whether higher PEEP increases the risk of barotrauma, particularly pneumothorax, has been debated for decades. An analysis of patients from the original ARDSNet tidal volume trial found a cumulative barotrauma incidence of about 13% over the first four study days. Higher concurrent PEEP was associated with a roughly 67% increase in barotrauma risk for each 5 cmHâ‚‚O increment, and this association held even after controlling for plateau pressure and other variables.18American Journal of Respiratory and Critical Care Medicine. Airway Pressures and Early Barotrauma in Patients with Acute Lung Injury and Acute Respiratory Distress Syndrome
However, the picture is not as simple as “more PEEP, more pneumothorax.” A systematic review stratifying patients by whether their oxygenation actually improved with higher PEEP found that in patients who responded well, higher PEEP did not increase barotrauma risk. In patients whose oxygenation did not respond, though, higher PEEP roughly doubled the incidence of clinically significant barotrauma.19PubMed Central. Higher PEEP improves outcomes in ARDS patients with clinically objective positive oxygenation response to PEEP: a systematic review and meta-analysis This makes physiological sense: in a non-recruitable lung, extra PEEP has nowhere productive to go and simply overdistends whatever tissue is already open, increasing the chance of rupture.
ARDS Subphenotypes and Differential Response
Research over the past decade has identified at least two distinct biological subtypes of ARDS: a hyperinflammatory subphenotype characterized by high levels of inflammatory markers and more organ dysfunction, and a hypoinflammatory subphenotype with lower inflammation. These subtypes appear to respond differently to ventilator strategies. In one analysis, patients with the hyperinflammatory subphenotype had substantially lower 90-day mortality when assigned to higher PEEP (about 54% versus 62%), while patients with the hypoinflammatory subphenotype showed essentially no difference between PEEP strategies.20The Lancet Respiratory Medicine. Validation and utility of routinely collected clinical data for defining acute respiratory distress syndrome subphenotypes: an observational study
This finding helps explain why large trials of higher versus lower PEEP often show no benefit across the entire ARDS population: the patients who benefit are being averaged together with patients who do not, and possibly patients who are harmed. The challenge now is developing fast, bedside-friendly ways to identify subphenotypes so that PEEP and other therapies can be targeted accordingly. Standard lab values and clinical scores can approximate the classification, but prospective trials testing subphenotype-guided ventilation strategies are still in early stages.
Combining PEEP with Prone Positioning
Prone positioning, where the patient is turned face-down, is one of the few ARDS interventions proven to reduce mortality in severe cases. When combined with PEEP, the two appear to have an additive effect on oxygenation.21PubMed. Prone position and positive end-expiratory pressure in acute respiratory distress syndrome The mechanisms complement each other: prone positioning redistributes lung perfusion and ventilation more evenly by removing the weight of the heart and abdominal organs from the dorsal lung, while PEEP prevents the newly opened regions from collapsing again. In practice, many ICUs use both simultaneously in severe ARDS, adjusting PEEP after the patient has been turned prone since the change in position can alter recruitability and the optimal PEEP level.
PEEP During ECMO Support
Patients with the most severe ARDS sometimes require extracorporeal membrane oxygenation (ECMO), where a machine takes over some or all gas exchange outside the body. During ECMO, the ventilator settings are typically dialed back to “rest” the lungs, using very low tidal volumes and modest pressures. Even in this context, PEEP still matters for preventing progressive lung collapse. EIT-guided PEEP optimization in ECMO patients has shown that the optimal level varies considerably depending on how recruitable the lung is. Patients with the highest recruitability needed a median PEEP around 14 cmHâ‚‚O, while those with the least recruitable lungs did well at around 8 cmHâ‚‚O.22Critical Care. Optimum electrical impedance tomography-based PEEP and recruitment-to-inflation ratio in patients with severe ARDS on venovenous ECMO About a quarter of these patients had a measurable airway opening pressure, a threshold below which the airways themselves are closed and PEEP essentially does nothing until that threshold is exceeded. Recognizing this phenomenon prevents clinicians from wasting PEEP below a level that cannot even reach the alveoli.
Spontaneous Breathing and Patient Self-Inflicted Lung Injury
PEEP’s role extends beyond passive mechanical ventilation. In patients breathing spontaneously, whether on non-invasive support or through a ventilator in assisted modes, strong inspiratory efforts can generate large swings in transpulmonary pressure that worsen lung injury. This phenomenon, called patient self-inflicted lung injury (P-SILI), has gained increasing attention as clinicians try to avoid intubation when possible. Excessive breathing effort can pull open fragile lung units with damaging force, and the negative pleural pressure generated during vigorous inspiration effectively adds to the transpulmonary pressure in ways that PEEP alone cannot counteract.23PubMed Central. Patient-Self Inflicted Lung Injury (P-SILI): An Insight into the Pathophysiology of Lung Injury and Management Applied PEEP through a mask or helmet interface can partially offset this by raising the baseline pressure the patient’s muscles have to overcome, reducing the overall pressure swing across the lung. However, if spontaneous efforts remain very large, the protective ceiling of non-invasive PEEP may be insufficient, and intubation with deeper sedation becomes necessary to control the injurious forces.
Body Weight and PEEP Requirements
Overweight and obese patients present a particular PEEP challenge. Excess abdominal and chest wall weight raises the pressure surrounding the lungs even before ventilation begins, meaning more PEEP is needed just to keep the lungs from being compressed by body habitus alone. A large retrospective study of over 3,200 ICU patients found that higher body mass index was correlated with mortality risk, but applying a blanket higher-PEEP strategy to overweight patients did not improve mortality or shorten hospital stays. In patients with a BMI of 25 or above, higher PEEP was actually associated with significantly longer time on the ventilator.24Respiratory Medicine. Effects of different positive end-expiratory pressure strategies on treating overweight patients with acute respiratory distress syndrome: A retrospective study based on large intensive care unit databases This does not mean these patients need less PEEP, but rather that the standard protocol-based approach to setting PEEP may be particularly inadequate for them. Individualized titration using transpulmonary pressure or imaging guidance may be especially valuable in this group, where chest wall mechanics are markedly different from the average patient the PEEP tables were designed for.
When PEEP Does Not Recruit
Not every increase in PEEP opens new lung tissue. In experimental models of both pulmonary and extrapulmonary ARDS, researchers found no significant differences in baseline lung volume between PEEP levels of 5 and 15 cmHâ‚‚O, suggesting that the recruitment effect of PEEP may be transient rather than sustained in some cases.25PubMed Central. Effect of positive end-expiratory pressure on functional residual capacity in two experimental models of acute respiratory distress syndrome This fits with the clinical observation that some patients simply do not have much recruitable lung. Their ARDS may be caused by direct lung injury, such as pneumonia, where the alveoli are filled with thick inflammatory material that pressure alone cannot clear. In these patients, pushing PEEP higher shifts the risk-benefit equation unfavorably: the overdistension and hemodynamic costs accumulate without a corresponding gain in recruited tissue. Recognizing a non-recruitable lung early, whether through poor oxygenation response, worsening compliance, or imaging tools, is one of the most important clinical judgments in ARDS management.