PEEP, short for positive end-expiratory pressure, is a baseline level of air pressure that a ventilator or breathing device holds in the lungs even after the patient breathes out. Instead of letting airway pressure drop to zero at the end of each breath, the machine maintains a small cushion of pressure that keeps the tiniest air sacs from collapsing shut. That simple concept has become one of the most consequential settings in critical care medicine, shaping outcomes for patients with severe lung injury, for newborns struggling to breathe, and even for people under general anesthesia during surgery. Getting the number right matters enormously, because too little PEEP leaves damaged lungs partially collapsed, while too much can overstretch healthy tissue and squeeze the heart.
What PEEP Actually Does Inside the Lungs
Your lungs contain hundreds of millions of tiny air sacs called alveoli. In a healthy person, these sacs stay open on their own thanks to a slippery coating called surfactant and the natural springiness of the chest wall. When the lungs are injured or inflamed, though, fluid and proteins leak into the air sacs, surfactant stops working well, and whole regions of the lung collapse. Each breath may pop some of those sacs open, only for them to slam shut again during exhalation. That repetitive opening and closing is itself damaging, creating a vicious cycle of worsening injury.
PEEP interrupts that cycle by keeping airway pressure above atmospheric even at the end of a breath. The residual pressure acts as a pneumatic splint: collapsed alveoli that have been reopened stay open, and the lung holds onto more air between breaths. In experimental lung injury models, using a strategy with higher PEEP to recruit collapsed alveoli and prevent recurrent collapse reduced the leak of blood proteins into the air spaces, which in turn preserved the function of surfactant.1PubMed. Improvement of lung mechanics by exogenous surfactant: effect of prior application of high positive end-expiratory pressure This matters because those leaked proteins poison surfactant, so PEEP can help break a self-reinforcing loop of collapse, protein leak, surfactant failure, and more collapse.
Recruitment and Overdistension
The benefit of PEEP depends on a concept clinicians call recruitability, essentially how much collapsed lung tissue can be coaxed back open with added pressure. Lungs affected by conditions like pneumonia or acute respiratory distress syndrome (ARDS) are not uniformly damaged. Some regions are flooded and heavy, some are collapsed but salvageable, and some remain aerated and relatively healthy. PEEP preferentially reopens the collapsed-but-salvageable zones, which tend to sit in the lower, gravity-dependent parts of the lung. But the same pressure that rescues those zones can overstretch the already-open tissue higher up.
Imaging studies using electrical impedance tomography, a bedside technique that maps air distribution across the chest, confirm this regional tug-of-war. When PEEP is raised, recruitment occurs mainly in the back (dorsal) regions of the lung, while overdistension appears mainly in the front (ventral) regions. Patients whose lungs responded well to PEEP, meaning they recruited more tissue than they overdistended, showed improved matching between airflow and blood flow. In contrast, patients with a high ratio of overdistension to recruitment saw no such improvement.2PubMed Central. Influence of overdistension/recruitment induced by high positive end-expiratory pressure on ventilation-perfusion matching assessed by electrical impedance tomography with saline bolus
A separate clinical study reinforced this point by measuring recruitability with a bedside index. In patients whose lungs were more recruitable, raising PEEP from 5 to 15 cm of water reduced the proportion of blood flowing past unaerated lung (shunt) without meaningfully increasing dead space, both when lying on the back and when lying face down. In patients whose lungs were less recruitable, however, the same PEEP increase in the prone position actually worsened the mismatch between ventilation and perfusion, because it failed to reduce shunt while adding dead space.3Annals of Intensive Care. Impact of PEEP on V/Q mismatch according to patient position and recruitability: a clinical prospective study The takeaway is that PEEP is not universally good or bad for the lungs; its net effect hinges on how much collapsed tissue is available to be opened versus how much healthy tissue gets stretched too far.
How PEEP Affects the Heart
The lungs and the heart share the same sealed compartment inside the chest, so anything that changes lung pressure inevitably changes cardiac function. When PEEP raises pressure inside the chest, less blood flows back toward the heart from the veins, in much the same way that squeezing a garden hose upstream reduces the flow at the nozzle. The heart receives less blood with each beat, and because heart rate usually does not change much with PEEP, the result is a drop in the total volume of blood the heart pumps per minute.4PubMed Central. Clinical review: Positive end-expiratory pressure and cardiac output
This does not always translate into a dangerous drop in blood pressure, because the body compensates. Animal studies show that mean systemic filling pressure, the baseline pressure driving blood back to the heart, actually rises with moderate PEEP levels, partially offsetting the impediment to venous return. In one study, the filling pressure increased from about 13 mmHg at a PEEP of 5 to about 14 mmHg at a PEEP of 10, and total blood flow did not fall significantly.5PubMed. Effect of PEEP, blood volume, and inspiratory hold maneuvers on venous return Still, in patients who are already low on circulating blood volume, even modest PEEP can unmask the cardiac squeeze and produce a clinically meaningful drop in blood pressure. That is why clinicians often give intravenous fluids or vasopressor drugs alongside high PEEP to maintain circulation.
PEEP and the Brain
For patients with brain injuries who also need mechanical ventilation, PEEP introduces an additional worry: intracranial pressure. Because PEEP raises pressure inside the chest, it can impede the drainage of blood from the brain back to the heart through the jugular veins, potentially raising the pressure inside the skull. In practice, this effect appears to be clinically meaningful only during severe lung injury. A study of neurocritical care patients found that for every 1 cm of water increase in PEEP, intracranial pressure rose by about 0.3 mmHg and cerebral perfusion pressure fell by about 0.85 mmHg, but only during periods when the lungs were severely injured. At milder levels of lung injury, PEEP adjustments did not significantly change brain pressures.6PubMed Central. The Effect of Positive End-Expiratory Pressure on Intracranial Pressure and Cerebral Hemodynamics Clinicians managing both a damaged brain and damaged lungs have to balance these competing demands, but the evidence suggests that moderate PEEP is usually tolerable for the brain if the lungs truly need it.
Intrinsic PEEP in Obstructive Lung Disease
Not all PEEP is deliberately set on a ventilator. Patients with severe chronic obstructive pulmonary disease (COPD) or acute asthma generate their own internal version of PEEP, known as intrinsic PEEP or auto-PEEP. Because their airways are narrowed and floppy, air gets trapped in the lungs before the next breath begins. The alveolar pressure at the end of expiration never returns to zero; it remains positive, sometimes substantially so.7PubMed Central. Intrinsic PEEP and cardiopulmonary interaction in patients with COPD and acute ventilatory failure
This trapped pressure has the same cardiac effects as externally applied PEEP: it impedes venous return and can reduce blood pressure. But it also creates a hidden hurdle for breathing. A patient trying to trigger a breath on the ventilator must first overcome the intrinsic PEEP before any new air flows in, which adds to the work of breathing and can cause the ventilator to miss the patient’s effort entirely. Clinicians sometimes apply a small amount of external PEEP, set just below the level of intrinsic PEEP, to partially counterbalance the trapped pressure and make triggering easier. The strategy is counterintuitive: adding pressure to offset pressure. But it works because the external PEEP reduces the pressure gradient the patient’s muscles must generate to initiate a breath.
Choosing the Right PEEP Level in ARDS
Decades of research have tried to answer a deceptively simple question: should patients with ARDS receive higher or lower PEEP? The answer, frustratingly, is “it depends.” Large randomized trials comparing higher PEEP strategies (averaging around 13 to 15 cm of water) to lower strategies (around 8 to 9 cm of water) have generally found no significant difference in mortality when both groups receive lung-protective ventilation with small breath volumes. The first major trial, published in 2004, compared mean PEEP levels of about 8 versus 13 cm of water and found nearly identical hospital death rates of roughly 25% and 28%.8PubMed. Higher versus lower positive end-expiratory pressures in patients with the acute respiratory distress syndrome
A systematic review pooling data from multiple such trials confirmed the pattern. Across about 2,700 patients, higher PEEP did not significantly reduce mortality, barotrauma, or ventilator-free days compared with lower PEEP.9Annals of the American Thoracic Society. Higher PEEP versus Lower PEEP Strategies for Patients with Acute Respiratory Distress Syndrome. A Systematic Review and Meta-Analysis A Bayesian meta-analysis estimated the probability that higher PEEP reduces 28-day mortality at about 65%, with a 35% chance it actually causes harm, and only a 20% probability that the benefit exceeds a 10% relative reduction.10PubMed Central. Higher PEEP for acute respiratory distress syndrome: a Bayesian meta-analysis of randomised clinical trials In other words, higher PEEP probably helps somewhat, but the effect is modest and uncertain when applied broadly to all comers.
The explanation most likely lies in the recruitability story described earlier: higher PEEP genuinely helps the subset of patients with heavily recruitable lungs, but it may harm patients whose lungs are less recruitable by overdistending whatever tissue is still functioning. Bundling both groups into one trial washes out the benefit. This realization has pushed the field toward individualized PEEP titration rather than one-size-fits-all protocols.
Guiding PEEP With Bedside Tools
Several technologies aim to help clinicians find the sweet spot for each patient. One approach uses an esophageal balloon catheter, a thin tube swallowed into the lower esophagus, to estimate the pressure surrounding the lung. By measuring the difference between airway pressure and esophageal pressure, clinicians can estimate the actual stretch across the lung wall and set PEEP to keep that stretch in a safe range. A pilot trial found that this strategy significantly improved oxygenation and lung compliance compared with standard care.11PubMed Central. Mechanical ventilation guided by esophageal pressure in acute lung injury
Electrical impedance tomography (EIT) offers a radiation-free, real-time picture of how air distributes across the chest, and several groups have used it to guide PEEP. A recent randomized trial of 108 patients with moderate-to-severe ARDS compared EIT-guided PEEP titration to a standard low-PEEP approach. On the first day, patients in the EIT group had better oxygenation, better lung compliance, and lower driving pressure. Their organ dysfunction scores also improved more quickly. Twenty-eight-day mortality trended lower at 29% versus 44%, though the difference did not quite reach conventional statistical significance.12Scientific Reports. The impact of PEEP-guided electrical impedance tomography on oxygenation and respiratory mechanics in moderate-to-severe ARDS: a randomized controlled trial Feasibility work has also shown that EIT-derived measures of ventilation homogeneity can guide PEEP selection, adding another metric to the clinician’s toolkit.13PubMed Central. PEEP titration guided by ventilation homogeneity: a feasibility study using electrical impedance tomography
PEEP During Surgery
General anesthesia causes the lungs to partially collapse even in healthy people. The combination of muscle relaxation, supine positioning, and the weight of the abdominal organs pushes the diaphragm upward and squeezes air out of the lower lung. Anesthesiologists routinely apply low levels of PEEP, typically around 5 cm of water, to counteract this. Whether cranking it higher prevents lung complications after surgery has been a major research question.
A patient-level meta-analysis pooling data from three large randomized trials of open abdominal surgery found that higher intraoperative PEEP (often guided by driving pressure and accompanied by recruitment maneuvers) produced a modest reduction in postoperative pulmonary complications compared with lower PEEP, but the overall result just missed statistical significance. Among individual complications, atelectasis was the one component that clearly occurred less often with higher PEEP.14British Journal of Anaesthesia. Intraoperative positive end-expiratory pressure and postoperative pulmonary complications: a patient-level meta-analysis of three randomised clinical trials The picture mirrors what happens in ARDS research: a broad population-level benefit is hard to prove, probably because the optimal PEEP varies from patient to patient.
Obese patients are a special case in the operating room. The extra weight on their chest wall and abdomen dramatically reduces lung compliance and predisposes them to more severe atelectasis under anesthesia. Higher PEEP levels are frequently needed in these patients just to maintain the same lung volumes that a lean patient would achieve with minimal PEEP. This is increasingly recognized as a situation where a one-size-fits-all low-PEEP approach is likely inadequate.
PEEP for Newborns
Premature babies often lack adequate surfactant, leaving their lungs stiff and prone to collapse. Bubble continuous positive airway pressure (bubble CPAP) is one of the simplest and most widely used ways to deliver PEEP to neonates: the baby breathes through short nasal prongs connected to a circuit whose expiratory limb is submerged in a water bottle. The depth of submersion sets the PEEP level, and the bubbling creates small oscillations in pressure that may help with gas exchange.
Bubble CPAP has become a frontline therapy in neonatal intensive care units around the world, including in resource-limited settings where mechanical ventilators are scarce. In one study, about 61% of newborns with respiratory distress improved on bubble CPAP without needing mechanical ventilation.15PubMed Central. Outcome of Respiratory Distress in Neonates with Bubble CPAP at Neonatal Intensive Care Unit of a Tertiary Hospital In another study of preterm infants specifically, 75% were managed successfully with CPAP alone, and pneumothorax, one of the feared complications of positive pressure in fragile neonatal lungs, occurred in only two babies. Predictors of CPAP failure included more severe initial disease, infection, and lack of prenatal steroid exposure.16PubMed. Bubble CPAP for respiratory distress syndrome in preterm infants The ability to provide effective PEEP therapy through a simple water-seal device, without electricity-dependent ventilators, has been a genuine lifesaver in low-income countries.
PEEP Before the Hospital Doors
PEEP is not confined to intensive care units and operating rooms. Paramedics and emergency medical technicians increasingly carry portable CPAP devices to deliver PEEP in the field, particularly for patients in acute heart failure with fluid-filled lungs (pulmonary edema). A study comparing prehospital CPAP plus standard treatment against standard treatment alone in patients with acute pulmonary edema found dramatic differences: the intubation rate dropped from about 25% to 9%, and mortality fell from roughly 23% to 5% in the CPAP group.17PubMed. Effectiveness of prehospital continuous positive airway pressure in the management of acute pulmonary edema
The mechanism in pulmonary edema is somewhat different from ARDS. In heart failure, PEEP does two helpful things at once: it pushes fluid out of the air sacs and back into the bloodstream, and it reduces the amount of blood returning to an already overloaded heart. Both effects improve breathing rapidly, often within minutes. A randomized trial of prehospital CPAP for broader respiratory distress confirmed that patients randomized to CPAP had roughly twice the improvement in breathing rate and dyspnea scores compared with usual care, though hospital length of stay did not differ.18PubMed. Prehospital continuous positive airway pressure (CPAP) for acute respiratory distress: a randomised controlled trial The speed and simplicity of the intervention make it well suited to the chaotic prehospital environment.
The Risk of Barotrauma
Any positive pressure applied to the lungs carries the risk of air leaking out of ruptured alveoli. Pneumothorax, where air escapes into the space between the lung and the chest wall, is the most dangerous manifestation. The risk of such barotrauma increases with excessive peak inspiratory pressure, high PEEP, overdistension of individual alveoli, and the presence of underlying lung injury.19Korean Journal of Anesthesiology. Barotrauma developed during intra-hospital transfer -A case report- Reassuringly, the large trials comparing higher and lower PEEP strategies in ARDS have not found a significant increase in barotrauma with higher PEEP, likely because the protocols also kept breath volumes and plateau pressures within safe limits. The risk is highest when PEEP is applied on top of already-dangerous inflation pressures, not when it replaces them as part of a lung-protective bundle.
Weaning Off PEEP
How to reduce PEEP as a patient improves is a less glamorous question than how to set it, but it has real clinical consequences. When a patient is being tested for readiness to breathe independently, the transition from positive pressure inside the chest to the slightly negative pressures of normal spontaneous breathing can unmask hidden cardiac problems. A study examining different spontaneous breathing trial methods found that about 15% of patients tested with a simple T-piece (which provides no PEEP at all) developed weaning-induced pulmonary edema, compared with roughly 8 to 9% of those tested with some residual pressure support or PEEP.20PubMed Central. What should be done with PEEP during weaning from mechanical ventilation? The sudden shift from positive to negative intrathoracic pressure during unassisted breathing increases blood return to the heart, and in patients with borderline cardiac function, that extra load can push fluid back into the lungs.
Gradual PEEP reduction, often in steps of 2 to 3 cm of water at a time, gives clinicians a chance to monitor for signs of cardiac decompensation. If the patient’s oxygen level drops or their breathing becomes labored after a PEEP decrease, the answer may not be “the lungs aren’t ready” but rather “the heart can’t handle the extra load.” Recognizing this distinction changes the treatment entirely: rather than simply continuing ventilation longer, the team might give diuretics to reduce fluid volume or adjust cardiac medications.
Ultra-Protective Ventilation on ECMO
At the extreme end of lung injury, when even aggressive ventilator settings cannot maintain adequate oxygen levels, patients may be placed on veno-venous extracorporeal membrane oxygenation (ECMO), a system that pulls blood out of the body, adds oxygen and removes carbon dioxide through an artificial membrane, and pumps it back. Because the machine takes over gas exchange, the ventilator can be dialed down to near-resting settings, a strategy called ultra-lung-protective ventilation. In one randomized trial, this approach combined very small tidal volumes of 1 to 2 milliliters per kilogram with a PEEP level set to keep transpulmonary pressure slightly positive at the end of expiration, along with prolonged prone positioning.21PubMed Central. Ultra-lung-protective ventilation and biotrauma in severe ARDS patients on veno-venous extracorporeal membrane oxygenation: a randomized controlled study The idea is to maintain just enough PEEP to keep the lung from collapsing entirely while giving it maximum rest from mechanical stress. The role of PEEP in this context shifts from actively recruiting the lung to simply holding it still and open while it heals.