How an Anesthesia Machine Breathing System Works

How fresh gas, inspiratory and expiratory valves, the breathing circuit, CO2 absorber, reservoir bag, and ventilator work together

An anesthesia machine breathing system has to do several things at once.

Published August 16, 2026 · Revised September 6, 2026

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What This Page Explains

This page covers:

The Simple Version

Fresh gas enters the circle system and joins gas already available in the breathing path. During inspiration, flow moves toward the patient through the inspiratory limb. During expiration, gas returns through the expiratory limb and passes through carbon-dioxide absorbent so usable gas can circulate again. One-way valves maintain direction, while the reservoir bag or ventilator provides volume and pressure.

Excess gas leaves through the APL or ventilator relief path and scavenging system. Leaks, reversed or stuck valves, exhausted absorbent, incorrect bag/ventilator mode, blocked limbs, misassembled canisters, and scavenging problems can all change pressure, volume, CO2 rebreathing, or checkout results.

Worked Example: Inspired CO2 Is Present

Confirm the analyzer and sample location, then inspect unidirectional-valve movement, circuit connections, absorbent condition, canister seating, fresh-gas flow, and breathing mode. A stuck inspiratory or expiratory valve and exhausted or bypassed absorbent produce different waveform patterns but can both permit rebreathing.

Use the model-specific leak and checkout procedures after any correction. Do not rely on visual absorbent color alone, and coordinate scavenging and gas safety during testing. Verify pressure, volume, valve operation, gas concentration, and alarms before clinical release.

What Is the Circle System?

Many anesthesia machines use a:

Circle breathing system.

It gets the name because gas can circulate through a loop.

That loop commonly contains:

Why Reuse Exhaled Gas?

Exhaled gas may still contain:

The major unwanted component is:

CO2.

If CO2 is removed, part of that gas can be reused.

This can reduce:

Fresh Gas Flow

Fresh gas comes from the anesthesia machine's gas-delivery system.

It may contain a mixture of:

depending on configuration.

Fresh gas enters the breathing system at a defined point.

Fresh Gas Is Not the Entire Breath

This is important.

During circle-system operation, the patient's inspired tidal volume may be much larger than the fresh-gas flow entering at that moment.

Gas already inside the breathing system contributes to the breath.

Inspiratory Limb

During inspiration, gas moves toward the patient through the inspiratory limb.

The inspiratory unidirectional valve helps ensure that flow moves in the correct direction.

Expiratory Limb

During expiration, gas returns from the patient through the expiratory limb.

The expiratory valve directs that gas away from the inspiratory path.

Unidirectional Valves

The inspiratory and expiratory valves are critical.

They help create one-way flow through the circle.

If one valve fails, gas may move backward.

Inspiratory Valve Problem

If the inspiratory valve:

possible effects include:

Expiratory Valve Problem

A faulty expiratory valve may also cause:

The symptom depends on how the valve fails.

Valve Disc Movement

Some systems use visible valve discs.

During breathing, the discs should move appropriately with gas flow.

If a disc:

it may not seal or move correctly.

One-Way Valve Failure Can Cause Rebreathing

If exhaled CO2-rich gas travels backward into the inspiratory limb, the patient may rebreathe more CO2.

That makes valve integrity important.

CO2 Absorber

The CO2 absorber removes carbon dioxide from exhaled gas.

It usually contains absorbent material such as soda lime or another approved absorbent.

Why the Absorber Matters

Without effective CO2 absorption, recirculated exhaled gas would contain too much carbon dioxide.

The absorber enables safe reuse of breathing-system gas when operated correctly.

Exhausted Absorbent

Absorbent material becomes exhausted over time.

Possible indicators may include:

But color indicators are not always perfectly reliable.

Follow manufacturer and clinical procedures.

Absorber Canister Problems

Possible issues include:

These can create:

Channeling

Gas should pass through the absorbent material effectively.

Improper packing or settling may allow gas to take a lower-resistance path.

That can reduce CO2 absorption.

Inspiratory CO2

One important clue is whether CO2 returns to baseline during inspiration.

Persistent inspired CO2 may suggest:

That is not always an equipment failure, but it is a useful system-level clue.

Reservoir Bag

During manual ventilation, the reservoir bag stores gas and allows the clinician to manually ventilate the patient.

It also provides visible and tactile information about the breathing system.

Reservoir Bag Leak

A damaged bag can cause:

Always inspect simple external components before opening the machine.

Bag/Vent Selector

Many anesthesia machines switch between:

using a bag/vent selector or similar mechanism.

This changes how the breathing system connects to:

Selector Failure

If the selector does not fully switch, possible symptoms include:

The exact mechanism varies.

Manual Ventilation

In manual mode, the clinician uses the reservoir bag.

The system generally depends on:

APL Valve

APL stands for:

Adjustable Pressure Limiting.

The APL valve helps control pressure during manual or spontaneous breathing modes.

It allows excess gas to leave the breathing system.

APL Valve More Open

More gas can escape.

Circuit pressure is lower.

APL Valve More Closed

More pressure can build before gas escapes.

That allows manual positive-pressure ventilation.

APL Valve Is Not Usually the Main Pressure Control During Mechanical Ventilation

On many machines, the APL valve is bypassed or excluded during ventilator mode.

Do not assume the APL setting controls mechanical ventilation pressure.

The exact design should be confirmed in the service manual.

APL Valve Stuck Open

Possible symptom:

Unable to build pressure in manual mode.

Gas escapes too easily.

APL Valve Stuck Closed

Possible symptom:

Excessive manual circuit pressure.

This can become a serious safety issue.

Mechanical Ventilation

In ventilator mode, the anesthesia ventilator provides breathing pressure or volume.

Depending on the machine, this may use:

Bellows Ventilator

Traditional anesthesia ventilators may use bellows.

The bellows contain breathing gas.

A separate drive-gas or mechanical system compresses them.

Ascending Bellows

Some bellows rise during expiration as they refill.

Their movement can give useful clues about:

Bellows Not Filling

Possible causes include:

The exact interpretation depends on ventilator design.

Piston Ventilator

Some anesthesia machines use a piston instead of traditional bellows.

A motor-controlled piston moves breathing gas.

Troubleshooting then involves:

Mechanical Ventilation Path

The ventilator creates pressure or volume.

The inspiratory path delivers gas toward the patient.

Exhaled gas returns through the expiratory path.

Pressure-control components manage excess gas.

Pressure in the Breathing System

Circuit pressure depends on several things:

A high-pressure alarm does not automatically mean the pressure sensor is bad.

Excess Pressure

An anesthesia breathing system needs ways to prevent dangerous pressure buildup.

Depending on operating mode and design, protection may include:

Do not defeat any pressure-limiting components during troubleshooting.

Scavenging Connection

Excess anesthetic gas eventually leaves the breathing system.

It is directed toward the:

Waste Anesthetic Gas Disposal, or scavenging, system.

The breathing circuit and scavenging system interact, but they are not the same thing.

Scavenging Should Not Directly Pull on the Patient Circuit

A properly functioning scavenging system generally uses an interface to prevent excessive suction or pressure from directly affecting the breathing circuit.

This becomes important when troubleshooting abnormal circuit pressure.

Leaks

Leaks are among the most common anesthesia breathing-system problems.

Possible leak points include:

The exact path depends on the machine.

Small Leak vs Large Leak

A small leak may cause:

A large leak may cause:

Leak Test

Anesthesia machines usually include a defined leak-test or system-checkout procedure.

Use it.

Do not rely only on:

It seems to hold pressure.

Positive-Pressure Leak Testing

Some systems are tested under positive pressure.

Pressure is applied.

Then pressure decay or flow requirement is evaluated.

Negative-Pressure Leak Testing

Certain older or specific systems may require negative-pressure leak tests, particularly for parts of the low-pressure system.

Follow manufacturer instructions.

Breathing-System Leak vs Low-Pressure System Leak

These are not automatically the same.

An anesthesia machine contains several gas sections.

Know which system the test is actually evaluating.

Obstruction

A breathing-system obstruction can cause:

Possible locations include:

Kinked Circuit

A kinked breathing hose can create a real high-pressure condition.

If the pressure sensor and analyzer agree, the machine may be detecting the problem correctly.

Valve Stuck Closed

A unidirectional valve that does not open properly can increase resistance.

The effect may appear primarily during:

depending on the valve.

Absorber Restriction

An absorber assembly with restricted gas flow can also create abnormal resistance.

Do not focus only on hoses.

Rebreathing

Rebreathing means inhaling previously exhaled gas.

Some rebreathing is intentionally allowed in circle systems after CO2 removal.

The problem is:

Rebreathing CO2.

Causes of Excessive CO2 Rebreathing

Possible causes include:

The exact system matters.

Fresh Gas Decoupling

Some modern anesthesia machines use systems that reduce the effect of fresh gas flow changes on delivered tidal volume.

This may be called:

Fresh gas decoupling

or use another manufacturer-specific design.

That means old assumptions about fresh gas and tidal volume may not apply to every machine.

Modern Machines Differ

Contemporary anesthesia machines may use:

Do not assume every machine behaves like a traditional mechanical circle system.

The underlying principles still help.

System Checkout

Modern anesthesia machines may automatically test:

A failed checkout is a clue.

It is not always the diagnosis.

Understand What the Checkout Is Testing

If the machine says:

Leak Test Failed

you still need to determine:

Where is the leak?

Possible areas include:

Remove External Variables

When troubleshooting a failed checkout:

Use:

when appropriate.

This helps determine whether the problem is:

External

or:

Internal.

Known-Good Circuit

Original circuit:

Leak test fails.

Known-good circuit:

Passes.

Original circuit fails elsewhere.

Failure follows the circuit.

No need to open the anesthesia machine.

Failure Stays With Machine

Multiple known-good circuits:

Fail.

Correct setup confirmed.

Now investigate machine-side:

Flow Sensor Interaction

The anesthesia machine may use flow sensors to measure:

A flow-sensor problem can create apparent breathing-system problems.

Use independent analyzer measurements.

Pressure Sensor Interaction

Pressure sensors monitor airway pressure.

A bad pressure sensor can create:

Again, compare actual pressure with an analyzer.

Gas Analyzer Interaction

The gas analyzer may measure:

These values can reveal breathing-system problems such as:

But sampled-gas problems can also create false measurements.

Test Lung

A test lung provides a controlled load.

It allows you to observe:

without involving a patient.

Ventilator Analyzer

An analyzer gives an independent reference for:

This helps determine whether the machine is actually delivering what it reports.

Example

Machine reports:

500 mL.

Analyzer:

495 mL.

Good agreement.

Machine:

500.

Analyzer:

320.

Now actual delivery is low.

Compliance

A breathing system and test lung have compliance.

That means some delivered volume may compress gas or expand tubing rather than entering the lung immediately.

Modern ventilators may compensate for this.

Use the manufacturer test setup.

Resistance

Circuit components create resistance.

Examples:

High resistance can cause:

Real-World Example: Cannot Build Pressure in Manual Mode

Known-good bag and circuit installed.

APL adjusted appropriately.

Pressure still will not build.

Bag/vent selector found not fully engaging manual path.

The leak was internal to the mode-selection mechanism.

Real-World Example: High Pressure During Manual Ventilation

APL valve opened.

Pressure remains abnormally high.

APL mechanism not actually opening gas-relief path.

Now the valve itself becomes suspect.

Real-World Example: CO2 Rebreathing

CO2 waveform does not return to zero during inspiration.

Gas analyzer confirmed working.

Absorbent recently replaced.

Expiratory unidirectional valve disc found stuck.

Gas was flowing in the wrong direction.

Real-World Example: Ventilator Tidal Volume Low

Set:

500 mL.

Machine reports:

500.

Analyzer:

350.

Circuit leak test fails.

Cracked reservoir/breathing-system component found.

The problem was actual gas loss, not the flow display alone.

Real-World Example: System Checkout Failure

Machine repeatedly fails breathing-system leak test.

Known-good external circuit and bag:

Same failure.

Absorber canister removed and reseated.

Damaged seal found.

Real-World Example: High Airway Pressure

Machine:

45 cmH2O.

Analyzer:

44 cmH2O.

Circuit inspected.

Expiratory hose kinked.

The pressure measurement was correct.

Common Mistakes

Treating the Breathing Circuit as Just External Tubing

Internal valves and absorber components matter too.

Calling Every Leak-Test Failure an Internal Machine Leak

Try known-good external components first.

Ignoring the Bag/Vent Selector

It changes the gas path.

Assuming the APL Valve Controls Ventilator Mode

Often it does not.

Replacing a Pressure Sensor for a Real High-Pressure Condition

Verify actual pressure.

Blaming the CO2 Absorber for Every Rebreathing Problem

Unidirectional valves matter too.

Assuming Every Anesthesia Machine Uses the Same Breathing-System Design

Modern machines differ considerably.

A Useful Troubleshooting Framework

For a breathing-system problem, ask:

What mode am I in?

Manual or ventilator?

Then:

Where should gas flow during inspiration?

Then:

Where should gas flow during expiration?

Then:

Can the system build and hold pressure?

Then:

Are the one-way valves moving correctly?

Then:

Is CO2 being removed correctly?

Then:

Does actual pressure, flow, and volume agree with the machine display?

That breaks a complicated system into understandable pieces.

Another Useful Question

Ask:

Is the problem gas delivery, gas direction, gas retention, or gas disposal?

Gas Delivery

Fresh gas and ventilator.

Gas Direction

Inspiratory/expiratory valves.

Gas Retention

Circuit integrity and leaks.

Gas Disposal

APL, pressure relief, scavenging.

That framework works surprisingly well.

What Did You Actually Prove?

If the breathing system pressurizes, you proved:

The system can build pressure under that test condition.

You did not prove:

If the complete manufacturer checkout and independent ventilator-analyzer testing pass, you have much stronger evidence across the breathing system.

Each test proves a different layer.

Final Thoughts for Biomeds

The anesthesia breathing system makes much more sense when you follow the gas.

Ask:

Where does fresh gas enter?

Where does gas go during inspiration?

Where does it go during expiration?

Where is CO2 removed?

Where can excess gas escape?

Then look at the components responsible for controlling those paths:

Do not troubleshoot the error message alone.

Follow the gas.

Once you understand where gas is supposed to go, a complicated anesthesia breathing-system failure becomes much easier to isolate.

— Jake

Important Note

Anesthesia breathing-system designs, ventilator mechanisms, valve arrangements, absorber assemblies, pressure-limiting systems, checkout procedures, and scavenging interfaces vary substantially by manufacturer and model. Follow current manufacturer documentation, use approved circuits and calibrated anesthesia/ventilator analyzers, and complete all required checkout, leak, functional, and safety testing before returning an anesthesia machine to clinical use.

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