How PEEP Is Generated and Controlled

How ventilators maintain positive pressure at the end of expiration and why valve, flow, pressure-sensor, and leak problems can all affect it

PEEP stands for:

Published August 16, 2026 · Revised September 6, 2026

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What This Page Explains

This page covers:

The Simple Version

A ventilator creates PEEP by keeping airway pressure from returning completely to atmospheric pressure at the end of expiration. Depending on the design, an expiratory valve restricts outflow, a blower maintains baseline flow, or both work together. Pressure sensors measure the result, and the controller adjusts the valve or flow to keep the measured baseline near the set value.

That means incorrect PEEP can come from more than the expiratory valve. A circuit leak may prevent pressure from building, a restriction may raise pressure, a wet or offset sensor may report the wrong value, and an incorrect test lung or analyzer setup may change the observed waveform. First decide whether pressure is actually wrong or only displayed incorrectly.

Worked Example: Set PEEP Is 5 but Measured PEEP Is 2

Confirm the required mode, breath settings, test lung, circuit, analyzer connection, zero procedure, and stabilization time. Run the ventilator's prescribed leak test and inspect the expiratory limb, valve assembly, seals, water traps, and accessories. If both the ventilator and analyzer show low PEEP, the problem is more likely in pressure generation, the valve, flow, or leakage than in the display alone.

If the ventilator displays 5 cmH2O while the external analyzer repeatedly measures 2 cmH2O, compare the correct measurement locations and reference conditions, then investigate pressure sensing and calibration using the service procedure. Do not adjust PEEP calibration to compensate for a leaking circuit or bad test setup. Complete the required pressure, alarm, and ventilation verification before return to service.

What Is PEEP?

PEEP is positive pressure maintained at the end of expiration.

Example:

PEEP set:

5 cmH2O.

At the end of expiration, airway pressure should remain near that target under the specified test conditions.

PEEP Is Not Peak Pressure

This is important.

Peak airway pressure may be:

25 cmH2O.

PEEP may be:

5 cmH2O.

They describe different points in the respiratory cycle.

PEEP Is the Baseline

On a pressure-time waveform, PEEP is generally the pressure baseline between breaths.

Instead of returning to:

0 cmH2O,

the waveform may return to:

5 cmH2O.

How Is PEEP Created?

In simple terms:

The ventilator limits expiratory gas flow enough to maintain pressure in the circuit.

This may involve:

Expiratory Valve

The expiratory valve is one of the most important PEEP-control components on many ventilators.

During expiration, gas leaves through this valve.

The ventilator controls how open or closed the valve is.

Valve More Open

Gas leaves more easily.

Pressure falls more.

Valve More Closed

Gas leaves less freely.

Pressure remains higher.

That basic principle allows the ventilator to regulate PEEP.

Expiratory Valve Is Not Just Open or Closed

Many modern valves are continuously controlled.

The ventilator may adjust them dynamically based on:

The valve may move many times during each breath.

Pressure Feedback

The ventilator needs to know whether actual airway pressure matches the target.

A pressure sensor provides feedback.

Conceptually:

Set PEEP

↓

Pressure sensor measures actual pressure

↓

Controller compares actual vs target

↓

Valve or blower adjusted

This is a feedback loop.

Why Pressure-Sensor Accuracy Matters

Suppose actual PEEP is:

5 cmH2O.

Sensor falsely reports:

10.

The ventilator may reduce pressure because it believes PEEP is too high.

Now the pressure-sensor error creates a real PEEP-control error.

Measurement Error vs Control Error

This distinction is critical.

A wrong PEEP display does not automatically mean actual PEEP is wrong.

You need an independent analyzer.

Example

Ventilator displays:

10 cmH2O.

Analyzer:

5.

Actual PEEP is correct.

Measurement is wrong.

Now:

Ventilator displays:

10.

Analyzer:

10.

Actual PEEP is truly high.

Look at control.

Blower or Turbine Systems

Some ventilators use a blower or turbine.

The blower may maintain background circuit pressure as part of PEEP generation.

In these designs, PEEP may not depend solely on an expiratory valve.

Flow-Based PEEP

Some systems use continuous or bias flow plus controlled exhaust resistance.

Pressure is maintained by the relationship between:

Bias Flow

Bias flow is a small continuous gas flow through the circuit.

It may help with:

This is why some ventilators still show flow between breaths.

External PEEP Valve

Some breathing systems use an external mechanical PEEP valve.

The valve provides resistance until pressure reaches a set level.

Once pressure exceeds that level, gas can escape.

Mechanical PEEP Valve

A simple mechanical PEEP valve may use:

The exact mechanism varies.

Internal vs External PEEP

Know which system you are working on.

Some ventilators control PEEP internally.

Others may use accessories or circuit components.

Do not troubleshoot an internal valve when the actual PEEP source is external.

PEEP During Expiration

As the patient exhales:

Pressure falls.

The ventilator allows gas to leave.

When pressure approaches target PEEP, the control system reduces further pressure loss.

Why PEEP Can Be Too Low

Possible causes include:

Why PEEP Can Be Too High

Possible causes include:

Circuit Leak

A leak can make it difficult to maintain PEEP.

Gas escapes continuously.

The ventilator may compensate up to a point.

A large enough leak may cause PEEP to fall.

Leak Example

Set:

5 cmH2O.

Analyzer:

2.

Circuit leak test fails.

Fix leak.

PEEP returns to:

5.

The expiratory valve was fine.

Internal Leak

If external circuit is known good and PEEP remains low, investigate internal gas paths according to manufacturer documentation.

Expiratory Valve Leak

If the expiratory valve does not seal sufficiently, gas may escape too easily.

Possible symptom:

PEEP consistently lower than set.

Expiratory Valve Restriction

If the valve cannot open enough:

Gas may not leave easily.

Possible symptoms:

Valve Contamination

Moisture or contamination can affect valve movement.

Possible symptoms include:

Valve Membrane or Diaphragm

Some expiratory valve assemblies use:

Damage can cause:

Valve Assembly Installation

Incorrectly installed expiratory components may create:

Always confirm orientation and seating.

Expiratory Valve Calibration

Some ventilators require calibration after:

Follow manufacturer service procedures.

PEEP Calibration Failure

A failed PEEP or expiratory-valve calibration may be caused by:

Do not treat calibration failure as automatic proof the valve is bad.

Pressure Sensor

The ventilator may use pressure sensing to control PEEP in real time.

If the sensor has:

PEEP regulation may be affected.

Pressure Zero Error

Actual airway pressure:

0.

Ventilator reads:

+5.

Now when set PEEP is:

5,

the controller may think it has already reached target.

Actual pressure could remain near:

0.

Verify Pressure First

Before replacing an expiratory valve for wrong PEEP:

Compare:

Ventilator pressure

with:

External analyzer.

That one test can save a lot of unnecessary work.

Flow Sensor Interaction

Flow sensors may help the ventilator determine:

A bad flow signal can interfere indirectly with PEEP control.

Example

Expiratory flow sensor falsely reports:

High leak.

Ventilator changes compensation behavior.

PEEP becomes unstable.

The apparent PEEP problem originates in flow measurement.

Closed-Loop Control

Modern ventilators may constantly adjust multiple components.

That means:

PEEP problem

does not always equal:

PEEP valve problem.

Think in terms of the whole control loop.

Set PEEP vs Measured PEEP

A screen may show both:

Make sure you know which number you are reading.

Set Value

Set:

5 cmH2O.

This means:

Target PEEP is 5.

Measured Value

Measured:

3 cmH2O.

This means the ventilator believes actual PEEP is:

3.

Now investigate why.

External Analyzer

The independent analyzer tells you what pressure is actually present at the test point.

That becomes the tie-breaker.

Analyzer Placement Matters

PEEP may differ slightly at:

depending on:

Use the manufacturer-specified test point.

Test Lung

A test lung provides a controlled load.

It helps you evaluate:

without involving a patient.

Test Lung Leak

A leaking test lung can cause low PEEP.

Before opening the ventilator:

Verify the test setup.

Circuit Compliance

Very compliant circuits may behave differently during pressure changes.

Use the approved circuit and test configuration.

Resistance

Excess resistance in the expiratory path can cause pressure to remain elevated.

Possible causes include:

Expiratory Filter

A clogged expiratory filter can increase resistance.

Possible effects:

The ventilator may be functioning correctly in response to a blocked circuit.

Kinked Expiratory Limb

A simple kink can create high expiratory pressure.

Always inspect external circuit before assuming internal failure.

PEEP and Compliance

PEEP pressure itself is controlled, but the volume remaining in the lung at end expiration depends on compliance.

That is a clinical/lung mechanics issue, not equipment calibration.

PEEP in Pressure-Control Modes

PEEP remains the baseline pressure.

Inspiratory pressure may be specified:

Above PEEP

or:

As absolute airway pressure

depending on the ventilator.

Know the device's convention.

Example

PEEP:

5.

Pressure control:

15 above PEEP.

Peak target may be approximately:

20 cmH2O.

Another ventilator may describe settings differently.

Do not compare settings across devices without checking definitions.

Intrinsic PEEP

Not all end-expiratory pressure comes from the ventilator setting.

The patient or test lung may fail to fully exhale before the next breath.

This can trap pressure.

This is often called:

Intrinsic PEEP

or:

Auto-PEEP.

Auto-PEEP

Possible causes include:

The ventilator may be functioning correctly.

Why Auto-PEEP Matters in Troubleshooting

Suppose:

Set PEEP:

5.

Analyzer baseline:

10.

Before replacing the expiratory valve, check whether:

The extra pressure may be dynamic gas trapping.

Increase Expiratory Time

In a controlled bench setup, changing timing may help distinguish:

True control problem

from:

Auto-PEEP due to incomplete exhalation.

Follow approved test methods.

Pressure Waveform

The waveform is very useful.

Look at end expiration.

Does pressure:

That pattern can point toward different causes.

Flow Waveform

Expiratory flow should generally return toward baseline before the next breath in a properly configured passive test lung.

If expiratory flow is still present when the next breath begins, gas trapping may exist.

PEEP Instability

If PEEP fluctuates:

4 7 5 9

consider:

Try to reproduce the pattern.

Intermittent Valve Problem

A sticky expiratory valve may work normally most breaths and fail occasionally.

Watch:

over many breaths.

Warm-Up Effects

A component may behave differently after:

If complaint occurs after an hour, do not test only for five minutes.

PEEP Alarm

Some ventilators monitor whether measured PEEP differs too far from set value.

A PEEP alarm tells you:

Measured pressure is outside expected range.

It does not tell you why.

Low PEEP Alarm

Possible causes:

High PEEP Alarm

Possible causes:

Alarm Can Be Correct

If analyzer confirms actual PEEP is high and alarm activates:

The alarm system may be working correctly.

Find why PEEP is high.

False Alarm

Analyzer:

5.

Ventilator:

12.

High-PEEP alarm.

Measurement path is suspect.

PEEP and Disconnects

A large circuit disconnect usually prevents PEEP maintenance.

Low PEEP may therefore be part of disconnect detection.

PEEP and NIV

Noninvasive ventilation often has intentional leak.

The ventilator may use leak compensation to maintain pressure.

PEEP troubleshooting in NIV can be different from a sealed invasive circuit.

Leak Compensation

Modern ventilators may increase flow to compensate for leak.

If actual PEEP is maintained despite substantial leak, this may be normal operation.

Compensation Limit

Every system has limits.

Beyond a certain leak, PEEP may fall.

Mask Leak Is Not Device Failure

On NIV, low pressure may originate from interface leak.

Bench testing with a sealed test setup helps isolate the ventilator.

Anesthesia Ventilator PEEP

Anesthesia machines may generate PEEP using:

Do not assume ICU ventilator architecture applies exactly.

Mechanical PEEP Valve Failure

If an external mechanical PEEP valve sticks or leaks, pressure may be:

Swap with known-good approved component when appropriate.

Real-World Example: Low PEEP

Set:

5 cmH2O.

Ventilator:

2.

Analyzer:

2.

Circuit leak test:

Fails.

Loose expiratory-limb connection found.

PEEP returns to 5 after correction.

Real-World Example: False Low PEEP

Set:

5.

Ventilator:

2.

Analyzer:

5.

Pressure-sensor zero is incorrect.

Actual PEEP is fine.

Real-World Example: High PEEP

Set:

5.

Analyzer:

12.

Ventilator:

12.

Expiratory filter found obstructed.

Sensor and display were accurate.

Real-World Example: High PEEP Only at Fast Rate

Set:

5.

Slow rate:

Analyzer shows 5.

High respiratory rate:

Analyzer shows 10.

Expiratory flow does not return to baseline before next breath.

Auto-PEEP from insufficient expiratory time.

Real-World Example: PEEP Fluctuates

Set:

8.

Pressure varies from:

5 to 12.

External analyzer confirms fluctuation.

Expiratory valve movement found intermittent.

This is a real control problem.

Real-World Example: Valve Replaced but Problem Remains

Expiratory valve replaced.

PEEP still reads high.

Analyzer shows normal PEEP.

Original problem was pressure measurement, not valve control.

Common Mistakes

Replacing the Expiratory Valve Before Measuring Actual PEEP

Use an independent analyzer.

Assuming Displayed PEEP Is Actual PEEP

Verify it.

Ignoring Circuit Leaks

A leak can prevent PEEP maintenance.

Ignoring Expiratory Restrictions

High PEEP may be caused downstream.

Confusing Auto-PEEP With Ventilator-Generated PEEP

Timing and resistance matter.

Ignoring Pressure-Sensor Errors

The sensor may be the source of the problem.

Assuming PEEP Control Works the Same on Every Ventilator

Designs vary.

A Useful Troubleshooting Framework

For a PEEP problem, ask:

What PEEP is set?

Then:

What does the ventilator measure?

Then:

What does an independent analyzer measure?

If ventilator and analyzer agree:

PEEP is actually wrong.

Investigate:

If they disagree:

Investigate:

Then ask:

Is the failure constant or mode/rate dependent?

That can reveal dynamic problems.

Another Useful Question

Ask:

Is this a pressure-control problem, a pressure-measurement problem, or a breathing-circuit problem?

Those are three different repair paths.

What Did You Actually Prove?

If PEEP is set to:

5 cmH2O,

you proved:

The target is 5 cmH2O.

If the ventilator displays:

5,

you proved:

Its measurement system reports 5 cmH2O.

You still have not independently proven actual airway pressure.

If a calibrated analyzer at the correct test point also measures approximately:

5 cmH2O

within specification, you now have strong evidence the ventilator is generating and measuring the intended PEEP under that test condition.

Final Thoughts for Biomeds

PEEP is a controlled pressure baseline.

It is not usually created by one simple valve acting alone.

The ventilator may need:

So when PEEP is wrong, do not jump straight to:

Bad expiratory valve.

Start with the most important comparison:

What does the ventilator say?

versus:

What does the analyzer say?

Then work through:

Circuit → Expiratory Path → Valve/Flow Control → Pressure Sensor → Software.

And remember to consider:

Auto-PEEP.

Sometimes the ventilator is doing exactly what it was told to do, but the system has not finished exhaling.

Once you separate:

actual PEEP

from:

measured PEEP

from:

intrinsic PEEP,

the problem becomes much easier to understand.

— Jake

Important Note

PEEP-generation methods, expiratory-valve designs, pressure-feedback algorithms, leak compensation, calibration procedures, alarm limits, and test requirements vary by ventilator and anesthesia-machine manufacturer and model. Follow current manufacturer service documentation, use approved circuits and calibrated ventilator analyzers, and complete required performance and safety testing before returning equipment to clinical use.

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