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Ventilator glossary

Plain definitions of the terms that come up on a ventilated patient, with a link to the longer explanation where there is one.

William Owens, MD, Attending Intensivist Physician · Updated 2026-10-05

PEEP · Auto-PEEP · Peak inspiratory pressure · Plateau pressure · Driving pressure · Compliance · Airway resistance · Tidal volume · Predicted body weight · Minute ventilation · Dead space · Shunt · V/Q mismatch · FiO2 · P/F ratio · ARDS · Baby lung · Anion gap · Base excess · Permissive hypercapnia · ETCO2 · Assist control · Pressure support · SIMV · Spontaneous breathing trial · RSBI · Recruitment · Volutrauma · Barotrauma · Hypoxemia and hypoxia · I:E ratio · Equation of motion

PEEP (Positive end-expiratory pressure)

The pressure the ventilator holds in the airway at the end of expiration, so the lung never empties completely. It keeps alveoli from collapsing and reopening with each breath, which is itself a form of injury. It also raises pressure inside the chest for the whole cycle, which reduces venous return and so reduces cardiac output.
More on this: Setting PEEP in ARDS

Auto-PEEP (Intrinsic PEEP, dynamic hyperinflation)

Pressure left in the alveoli at end-expiration because the patient did not finish exhaling before the next breath arrived. The ventilator does not display it. You measure it with an end-expiratory pause: the pressure shown during the hold is total PEEP, and auto-PEEP is that minus the set PEEP.
More on this: Auto-PEEP: how to find it and what to do about it

Peak inspiratory pressure (PIP)

The highest pressure reached during a breath, measured while gas is still flowing. It therefore includes both the cost of distending the lung and the cost of pushing gas down the tube. On its own it says little, because a high value can come from either.
More on this: Reading ventilator waveforms

Plateau pressure (Pplat)

The airway pressure measured during an inspiratory pause, once flow has stopped. With no gas moving, the resistive cost disappears and what is left reflects how distended the alveoli actually are. Keep it at or below 30 cmH2O. It is the pressure that matters for lung injury, not the peak.
More on this: Driving pressure: the ventilator number that tracks mortality

Driving pressure (Plateau pressure minus PEEP)

The pressure swing the lung sees with each breath. Because it equals tidal volume divided by compliance, it indexes the breath to how much lung is actually available to receive it. Target 10 to 15 cmH2O; above 15 is associated with excess mortality.
More on this: Driving pressure: the ventilator number that tracks mortality

Compliance (Crs)

How much volume a given pressure buys: tidal volume divided by plateau pressure minus PEEP. A healthy ventilated adult is around 70 to 80 mL/cmH2O. Severe ARDS can fall to 15 to 30. Falling compliance means the lung is getting smaller or stiffer, not that the breath is getting bigger.
More on this: Reading ventilator waveforms

Airway resistance (Raw)

The pressure cost of moving gas through the tube and airways, which shows up as the gap between peak and plateau pressure. Under 5 cmH2O is normal; above 10 means something is narrowing the path, usually bronchospasm, secretions, or a kinked or bitten tube.
More on this: Reading ventilator waveforms

Tidal volume (VT)

The volume of one breath. Set it from predicted body weight, which depends on height and sex, never from actual weight. Four to 8 mL/kg is the protective range and 6 is the usual starting point.
More on this: Setting tidal volume from predicted body weight

Predicted body weight (PBW)

An estimate of what a person of a given height and sex should weigh, used because lung size tracks height rather than weight. Men: 0.91 x (height in cm minus 152.4) plus 50. Women: the same with 45.5. Two patients of the same height get the same breath whatever they weigh.
More on this: Setting tidal volume from predicted body weight

Minute ventilation (MV)

Respiratory rate multiplied by tidal volume. It governs CO2 clearance, but only the part of each breath that reaches alveoli counts: dead space ventilation moves gas without removing any CO2.
More on this: How to read an arterial blood gas

Dead space (VD)

Ventilated lung that is not perfused, so the gas in it takes no part in exchange. Anatomic dead space is roughly 1 mL per cm of height. A rising dead space fraction widens the gap between end-tidal and arterial CO2, and is a feature of pulmonary embolism, low cardiac output and alveolar overdistension.
More on this: Capnography: reading end-tidal CO2

Shunt (Qs/Qt)

Blood that passes through the lung without meeting ventilated alveoli, so it reaches the arteries still venous. It is the one cause of low oxygen that supplemental oxygen barely fixes: there is nowhere for the oxygen to meet the blood. Normal is under 3 percent.
More on this: Setting PEEP in ARDS

V/Q mismatch (Ventilation-perfusion mismatch)

Ventilation and blood flow distributed unevenly across the lung. It is the most common cause of low arterial oxygen and, unlike true shunt, it largely corrects with supplemental oxygen. That difference in response is how the two are told apart at the bedside.
More on this: Setting PEEP in ARDS

FiO2 (Fraction of inspired oxygen)

The proportion of the delivered gas that is oxygen, from 0.21 on room air to 1.0. It is paired with PEEP, usually from a table, rather than chosen alone. Sustained high values are themselves harmful, so the target is the lowest value meeting the saturation goal.
More on this: Setting PEEP in ARDS

P/F ratio (PaO2 divided by FiO2)

Arterial oxygen tension divided by the inspired fraction as a decimal. It grades the severity of ARDS: 300 or below is mild, 200 or below moderate, 100 or below severe. A value under 200 implies a shunt above 20 percent.
More on this: How to read an arterial blood gas

ARDS (Acute respiratory distress syndrome)

Acute, diffuse inflammatory lung injury with bilateral infiltrates and a P/F ratio of 300 or below, not explained by heart failure alone. The lung is not uniformly stiff so much as small: the alveoli that remain open behave fairly normally, there are simply far fewer of them.
More on this: Setting PEEP in ARDS

Baby lung

The idea that an adult with severe ARDS has the functional lung volume of a child, because so much of the lung is no longer aerated. It explains why a protocol-correct 6 mL/kg can still be too large a breath, and why driving pressure is the better safety signal.
More on this: Driving pressure: the ventilator number that tracks mortality

Anion gap

Sodium minus the sum of chloride and bicarbonate, normally 8 to 12 mmol/L. A raised gap means an unmeasured acid: lactate, ketones, uremic acids, or a toxic alcohol. Calculate it on every gas, including ones that look normal, because a coexisting alkalosis can hide it.
More on this: How to read an arterial blood gas

Base excess (BE)

How much strong acid or base would be needed to bring the sample back to a pH of 7.40 at a normal CO2. Below minus 2 indicates a metabolic acidosis or compensation for a respiratory alkalosis; above plus 2, the reverse.
More on this: How to read an arterial blood gas

Permissive hypercapnia

Deliberately accepting a high arterial CO2, and the acidosis that follows, rather than raising ventilation to a point that would injure the lung. Generally tolerated to a CO2 of 80 to 100 and a pH of about 7.15 to 7.20. Avoid it in raised intracranial pressure, pulmonary hypertension and severe right-heart failure.
More on this: Setting tidal volume from predicted body weight

ETCO2 (End-tidal carbon dioxide)

The CO2 concentration at the end of exhalation, measured continuously by infrared absorption. It is always at or below the arterial value, normally by 3 to 5 mmHg. A widening gap means rising dead space; a sudden loss means the tube, the circuit or the circulation.
More on this: Capnography: reading end-tidal CO2

Assist control (A/C)

A mode in which every breath, whether the patient triggers it or the timer does, is delivered in full by the ventilator. It does the most work of breathing of any mode, which is the point in a patient who cannot afford to spend cardiac output on their respiratory muscles.
More on this: Pressure control vs volume control

Pressure support (PSV)

The patient triggers every breath and sets their own rate and volume; the ventilator supplies a set pressure to assist. Breaths end when inspiratory flow falls to roughly a quarter of its peak. It is the usual mode for weaning, and useless in a patient with no respiratory drive.
More on this: Weaning: the spontaneous breathing trial and the RSBI

SIMV (Synchronized intermittent mandatory ventilation)

A set number of mandatory breaths per minute, synchronized to patient effort where possible, with spontaneous breaths allowed in between. Historically used for weaning, for which it has no advantage over a daily breathing trial.
More on this: Pressure control vs volume control

Spontaneous breathing trial (SBT)

A 30 to 60 minute period on minimal support, typically CPAP of 5 or pressure support of about 7, to test whether a patient still needs the ventilator. Passing it tests the lungs and the respiratory pump; it does not test whether the patient can protect an airway.
More on this: Weaning: the spontaneous breathing trial and the RSBI

RSBI (Rapid shallow breathing index)

Respiratory rate divided by tidal volume in liters. Below 80 suggests a patient will tolerate extubation, above 105 suggests they will not, and the range between is equivocal. Fast and shallow is the signature of a patient working harder than they can sustain.
More on this: Weaning: the spontaneous breathing trial and the RSBI

Recruitment

Reopening collapsed lung and keeping it open, usually with PEEP. The benefit is less about the volume regained than about removing the boundary between open and collapsed tissue, where the local stress on healthy alveoli can reach roughly twice the pressure applied globally.
More on this: Setting PEEP in ARDS

Volutrauma

Lung injury caused by overdistension. Of everything that can be set on a ventilator, tidal volume is the parameter most robustly linked to survival, and the injury is not confined to the lung: it drives the inflammatory response that damages other organs.
More on this: Setting tidal volume from predicted body weight

Barotrauma

Injury from excessive airway pressure, classically air escaping into the pleural space, the mediastinum or the tissues. Trapped gas raises the risk, which is one reason an unrecognized auto-PEEP is dangerous as well as uncomfortable.
More on this: Auto-PEEP: how to find it and what to do about it

Hypoxemia and hypoxia

Not the same thing. Hypoxemia is a low oxygen tension in arterial blood, a property of the blood. Hypoxia is inadequate oxygen delivery to or use by tissue, a property of cells. A patient can have either, both, or neither, and the treatments differ.
More on this: How to read an arterial blood gas

I:E ratio (Inspiratory to expiratory ratio)

How the breath cycle is divided between breathing in and breathing out. Around 1:2 is usual. Obstructive disease needs far longer to empty, so 1:3 to 1:5 is the target there, and getting it wrong is the most common way to create air trapping.
More on this: Auto-PEEP: how to find it and what to do about it

Equation of motion

The relationship underlying every ventilator waveform: airway pressure equals volume divided by compliance, plus resistance multiplied by flow, plus PEEP. Nearly all waveform interpretation is working out which of those three terms changed.
More on this: Reading ventilator waveforms