PEEP stands for Positive End-Expiratory Pressure. It refers to a setting on a mechanical ventilator that keeps a small amount of air pressure in the lungs even after a patient finishes breathing out. This prevents the tiny air sacs in the lungs from collapsing between breaths, making it easier for oxygen to pass into the bloodstream. PEEP is one of the most important tools in critical care medicine, used daily in hospitals around the world.
How PEEP Works in the Lungs
When you breathe out naturally, a small amount of air stays in your lungs to keep them partially inflated. In people with severe lung injuries or infections, this natural system fails. The tiny air sacs (called alveoli) can collapse at the end of each breath, and the body has to work much harder to reinflate them with the next breath. Over time, more and more air sacs shut down, and oxygen levels drop dangerously.
PEEP solves this by maintaining pressure in the airways above normal atmospheric pressure, even at the very end of exhalation. Think of it like keeping a balloon slightly inflated rather than letting it deflate completely each time. This does two things: it keeps already-open air sacs from collapsing, and it can reopen collapsed ones, a process called alveolar recruitment. The result is more lung surface area available for gas exchange, which means more oxygen reaches the blood.
When PEEP Is Used
The most common reason for using PEEP is acute respiratory distress syndrome (ARDS), a life-threatening condition where the lungs become severely inflamed and fill with fluid. ARDS can be triggered by pneumonia, sepsis, trauma, or aspiration of stomach contents. The condition was first described in a landmark 1967 paper by Ashbaugh and colleagues, and positive pressure ventilation was identified as a key part of treatment from the very beginning.
PEEP is also used in other situations where the lungs struggle to stay open: pulmonary edema (fluid in the lungs from heart failure), severe pneumonia, and during surgery when general anesthesia can cause parts of the lung to collapse. In patients who have lost the natural substance that keeps air sacs open (called surfactant), PEEP is especially important. Research in lung injury models has shown that replacing surfactant only works if PEEP is also applied, because the pressure keeps the air sacs open long enough for the surfactant to reach them and do its job.
Typical PEEP Levels
PEEP is measured in centimeters of water pressure (cmH₂O), and the level used depends on how sick the patient’s lungs are. For mild lung problems or routine post-surgical ventilation, levels of 5 cmH₂O or less are common. In mild ARDS, low PEEP is usually adequate because there are few collapsed air sacs to recruit.
As lung injury worsens, higher levels are needed. Moderate ARDS typically calls for around 10 cmH₂O, while severe ARDS may require about 15 cmH₂O, which is enough to keep roughly 70% of the lung open and maintain adequate oxygen exchange. In practice, the average PEEP used worldwide in severe ARDS is around 8.5 cmH₂O, which many experts consider too conservative. Some clinicians have pushed levels as high as 25 cmH₂O in the most extreme cases, though fully opening a severely collapsed lung may require even more than that.
Risks of Too Much PEEP
PEEP is not without tradeoffs. Because it increases the pressure inside the chest, it can interfere with blood flow back to the heart. Normally, blood returns to the heart through large veins driven by a pressure difference between the body and the chest. When PEEP raises chest pressure, that gradient shrinks, slowing the return of blood. The heart receives less blood with each beat, pumps out less, and overall cardiac output drops. Heart rate typically stays the same, so the entire decrease comes from each heartbeat moving less blood. This effect is more pronounced at higher PEEP levels and in patients who are already dehydrated or have low blood volume.
Excessive PEEP can also overinflate the parts of the lung that are already healthy, stretching them beyond their normal capacity. This overdistension can damage delicate lung tissue and, in the worst cases, cause a pneumothorax, where air leaks out of the lung into the chest cavity. Balancing enough PEEP to recruit collapsed lung tissue without overinflating healthy tissue is one of the central challenges in managing patients on ventilators.
Intrinsic PEEP vs. Set PEEP
The PEEP that a clinician dials into a ventilator is called extrinsic PEEP, but there is also a version that occurs unintentionally. Intrinsic PEEP, sometimes called auto-PEEP, happens when a patient doesn’t fully exhale before the next breath begins. Air gets trapped in the lungs, creating positive pressure that stacks on top of whatever the ventilator is already providing. This is common in patients with obstructive lung diseases like COPD or severe asthma, where narrowed airways slow the flow of air out of the lungs.
Auto-PEEP is a hidden problem because standard ventilator readings don’t always detect it. It carries the same risks as extrinsic PEEP, including reduced blood return to the heart and potential lung overdistension, but because it’s unintentional, it can climb to dangerous levels before anyone notices. Recognizing and managing auto-PEEP is a routine part of caring for ventilated patients, particularly those with underlying airway disease.