What PEEP on a ventilator does, and where to start it

PEEP on a ventilator is the pressure the machine holds in the airways at the end of every breath, keeping alveoli that opened on inspiration from collapsing again before the next one arrives. The number behind the question what is PEEP on a ventilator, for most adult patients, is 5 cm H2O to start.

From there PEEP moves with the lung in front of you: higher in ARDS, kept low in a passive patient who is already trapping air, where expiratory time does the real work, and watched against blood pressure the whole way.

PEEP is a setting a formula can start, but where it lands from there is set for the lungs and the pressure in front of you, not read off a chart. This page covers PEEP alone; see where PEEP lands in the initial vent settings sequence for the rest of the sequence it’s part of.

What is PEEP on a ventilator?

PEEP, positive end-expiratory pressure, is the pressure that remains in the airways at the end of the respiratory cycle, per StatPearls’ dedicated PEEP chapter. Applied through the ventilator circuit, it prevents alveoli from collapsing during passive exhalation, which cuts atelectasis, improves alveolar ventilation, and reduces the V/Q mismatch that comes with it.

PEEP splits into two kinds. Extrinsic PEEP is what the ventilator is deliberately set to deliver, the kind this page is mostly about. Intrinsic PEEP, better known as auto-PEEP, is pressure nobody set, air trapped by incomplete exhalation, covered later on this page.

By holding the lung open at the end of exhalation, PEEP raises functional residual capacity and lowers intrapulmonary shunt, the two mechanisms every later section on this page builds from.

What is a normal PEEP on a ventilator?

Five cm H2O is where PEEP starts for most adults right after intubation, per StatPearls’ prehospital ventilation chapter, cross-checked against its invasive mechanical ventilation chapter. That routine 5 is there to hold end-expiratory lung volume rather than to treat oxygenation on its own; the next section covers why. From there, PEEP moves with the lung.

See every normal vent value on one card for how PEEP compares with pressures, EtCO2, and the rest.

What does a PEEP of 5 mean?

A PEEP of 5 means the ventilator never lets airway pressure fall back to zero between breaths; it holds 5 cm H2O in the circuit at the end of every exhalation instead. StatPearls calls this small routine amount physiologic PEEP, because it preserves the functional residual capacity zero PEEP would let collapse.

Is a PEEP of 8 high?

Not by itself. 8 sits above the 5 cm H2O starting point, but no source on this page defines a fixed cutoff for high PEEP; the ARDS Network’s PEEP trial defined higher PEEP by which paired FiO2/PEEP table a patient landed on, not a set number. Whether 8 is high for a given patient depends on the lung, the plateau pressure it produces, and how oxygenation and blood pressure respond from there.

What PEEP does to the lungs, and what it costs

Raising PEEP can buy something real: in a lung with alveoli to recruit, it holds more of them open through the end of exhalation, raises end-expiratory lung volume, cuts intrapulmonary shunt, and improves oxygenation at a lower FiO2, which is where what FiO2 does alongside PEEP comes in. None of that is guaranteed, and none of it comes free. The ATS guideline frames it as a balance between recruitment and overdistension that differs from patient to patient.

Added PEEP also raises pressure inside the chest, cutting the gradient pulling blood back toward the right atrium and dropping preload, and with it cardiac output and blood pressure can fall, per StatPearls’ PEEP chapter, by an amount that varies with the patient in front of you. Added PEEP pushes plateau pressure up as well, and the ARDS Network card’s goal is a plateau at or below 30 cm H2O, on top of the 6 mL/kg tidal volume calculation the breath itself is built from. That goal is a protocol limit that cuts the stress the lung sees, not a line below which injury is impossible; past it, barotrauma is the risk on the table.

None of this survives a circuit disconnect. Opening the circuit to suction or bag a patient costs the lung volume PEEP was holding, shown by Maggiore and colleagues in nine acute lung injury patients during endotracheal suctioning. A bag-valve-mask’s PEEP valve is the piece of kit that puts PEEP back into a hand-ventilated patient. The numbers StatPearls attaches to that valve, 5 to 15 cm H2O and not above 20 because of gastric insufflation, describe mask ventilation for preoxygenation, where no cuffed tube protects the stomach; what the valve is set to on an intubated patient is a local-protocol question.

Can PEEP drop blood pressure?

Yes. The mechanism is the same one above: less venous return, less preload, less cardiac output, and mean arterial pressure falls unless the vasculature compensates. The compensation that usually covers for it is exactly what’s missing in distributive shock, septic, neurogenic, or anaphylactic, where vascular resistance is already low, so PEEP there can drop pressure further. There’s no published PEEP number at which this becomes clinically apparent; it’s patient-dependent, and the monitor is where you find out.

Auto-PEEP: the PEEP nobody set

Auto-PEEP is the pressure nobody set. Under normal exhalation the lungs empty back to their resting volume and airway pressure falls to the set PEEP before the next breath arrives; in obstructive disease, high rates, or high tidal volumes, they don’t get there, and air stacks up breath over breath. Short expiratory time, a high respiratory rate, a high tidal volume, and airway obstruction from mucus or bronchospasm all drive it, per StatPearls’ PEEP chapter.

At the bedside the first sign is expiratory flow that never reaches zero before the next breath on the flow-time waveform. StatPearls’ PEEP chapter lists the volume-time curve not returning to baseline, a rising plateau pressure, and a falling blood pressure as the other clues. An end-expiratory hold on a passive patient measures it outright: total PEEP minus set PEEP. The fix is time, not rate: lengthen the expiratory phase rather than reflexively raising the respiratory rate, which only shortens it further.

Whether extrinsic PEEP helps depends on who is triggering the breath. In a sedated, passive patient, external PEEP does nothing to lengthen exhalation, and where the airways are not collapsing it only adds back-pressure, per StatPearls; in a small controlled-ventilation study by Caramez and colleagues, the same PEEP step lowered hyperinflation in some COPD and asthma patients and worsened it in others, with nothing in the diagnosis predicting which. In a patient triggering their own breaths through dynamic airway collapse, as in COPD, a small amount of extrinsic PEEP offsets the auto-PEEP they have to pull against to trigger; StatPearls’ PEEP chapter keeps it below 75 to 85% of the measured auto-PEEP, a figure that depends on auto-PEEP measured with an end-expiratory hold. Asthma’s obstruction is inflammation rather than a collapsing airway, which is why StatPearls describes extrinsic PEEP as ineffective there. Working the alarm this triggers is its own workflow: see working a ventilator alarm mid-transport.

Setting PEEP in ARDS: PEEP and FiO2 move together

ARDSNet doesn’t titrate PEEP by itself. Its protocol card sets a minimum PEEP of 5 cm H2O and offers FiO2/PEEP combinations from two tables, marked on the card as not required, rather than one dial someone turns until oxygenation looks better. At an FiO2 of 0.5, the lower-PEEP/higher-FiO2 table pairs PEEP at 8 or 10 cm H2O; the higher-PEEP/lower-FiO2 table pairs the same FiO2 with PEEP at 16 or 18, and its next step, 20, spans FiO2 0.5 to 0.8. Same oxygen requirement, two very different pressures, depending on which strategy the patient is on. Both tables, cell for cell, are on the ARDSNet PEEP and FiO2 ladder page.

The current ATS ARDS guideline conditionally recommends higher PEEP, without lung recruitment maneuvers, over lower PEEP in moderate-to-severe ARDS, rated low-to-moderate certainty, not a strong recommendation. That caution is deliberate. The guideline rests on two meta-analyses rather than any one trial: a network meta-analysis of 18 randomized trials and an individual-patient-data analysis of three higher-versus-lower PEEP trials, the ARDS Network’s own ALVEOLI plus LOVS and EXPRESS. ALVEOLI alone found similar outcomes whether patients received the higher or the lower table, and the panel kept the recommendation conditional because those trials set higher PEEP in very different ways, not because higher PEEP looked worse.

Whichever table PEEP is coming from, the card’s plateau-pressure goal, 30 cm H2O or less, is still the limit the whole maneuver runs inside, and the plateau pressure ceiling PEEP lives under has its own page.

Common questions about PEEP

What is the main purpose of PEEP?

To keep alveoli open at the end of exhalation so they don’t have to be reopened on every breath, which raises functional residual capacity, cuts shunt, and improves oxygenation. A small routine amount, often called physiologic PEEP, is applied even when oxygenation itself isn’t the problem, because it preserves the functional residual capacity that zero PEEP would let collapse, per StatPearls’ PEEP chapter.

What happens if PEEP is too high?

Plateau pressure rises with it, raising the risk of barotrauma, and the added intrathoracic pressure can cut venous return enough to drop cardiac output and blood pressure. In obstructive disease specifically, PEEP set too high on top of existing air trapping worsens auto-PEEP rather than fixing anything.

What happens if PEEP is too low?

Alveoli that opened on inspiration collapse again before the next breath, atelectasis builds, and the resulting V/Q mismatch worsens oxygenation. That’s the case for the 5 cm H2O most adults start at instead of zero.

PEEP is a number your patient’s pressure votes on

Rapid Vent Calculator carries a Normal Vent Values screen with a PEEP entry that expands into what to check when oxygenation stays poor and what to check when PEEP looks too high. The app carries the reference and does the tidal volume and rate arithmetic beside it. It doesn’t carry the decision: it hasn’t seen this patient’s chest, doesn’t know the local protocol, and has no vote on whether 5 is still right an hour into transport. That’s the clinician’s call, made from the plateau pressure and the blood pressure at the monitor, not off a chart. PEEP in particular is a setting the patient’s own pressure votes on before you get to read it anywhere else. The free vent calculator on your phone runs the reference and the arithmetic; the vote is still yours.

Sources

1. Levine A, Mora JI. Positive End-Expiratory Pressure. StatPearls; updated 2025 Sep 14. NBK441904. https://www.ncbi.nlm.nih.gov/books/NBK441904/
2. Kuhl EA, Perera TB. Prehospital Mechanical Ventilation. StatPearls; updated 2024 Mar 8. NBK537072. https://www.ncbi.nlm.nih.gov/books/NBK537072/
3. Hickey SM, Sankari A, Giwa AO. Invasive Mechanical Ventilation. StatPearls; updated 2024 Mar 30. NBK539742. https://www.ncbi.nlm.nih.gov/books/NBK539742/
4. National Heart, Lung, and Blood Institute ARDS Clinical Network. Mechanical Ventilation Protocol Summary (ventilator protocol card).
5. Qadir N, Sahetya S, Munshi L, et al. An Update on Management of Adult Patients with Acute Respiratory Distress Syndrome: An Official American Thoracic Society Clinical Practice Guideline. American Journal of Respiratory and Critical Care Medicine. 2024;209(1):24-36. https://doi.org/10.1164/rccm.202311-2011ST
6. Brower RG, Lanken PN, MacIntyre N, et al. Higher versus lower positive end-expiratory pressures in patients with the acute respiratory distress syndrome. New England Journal of Medicine. 2004;351(4):327-336. https://pubmed.ncbi.nlm.nih.gov/15269312/
7. Maggiore SM, Lellouche F, Pigeot J, et al. Prevention of endotracheal suctioning-induced alveolar derecruitment in acute lung injury. American Journal of Respiratory and Critical Care Medicine. 2003;167(9):1215-1224. https://pubmed.ncbi.nlm.nih.gov/12615633/
8. Bucher JT, Vashisht R, Cooper JS. Bag-Valve-Mask Ventilation. StatPearls; updated 2025 May 3. NBK441924. https://www.ncbi.nlm.nih.gov/books/NBK441924/
9. Caramez MP, Borges JB, Tucci MR, et al. Paradoxical responses to positive end-expiratory pressure in patients with airway obstruction during controlled ventilation. Critical Care Medicine. 2005;33(7):1519-1528. https://pubmed.ncbi.nlm.nih.gov/16003057/