How Sidestream CO2 Monitoring Works

How a capnography module pulls a gas sample through the sampling line, measures carbon dioxide, and turns it into an EtCO2 value and capnogram

Sidestream CO2 monitoring is common across:

Published August 16, 2026 · Revised September 6, 2026

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What This Page Explains

This page covers:

The Simple Version

A sidestream system uses a small pump to draw exhaled gas through a sampling line and moisture protection into a measurement chamber. Infrared light passes through the sample, carbon dioxide absorbs part of that light, and a detector measures the change. Software turns the changing concentration into a capnogram and identifies the end-tidal value.

Troubleshooting divides naturally into sample delivery and CO2 measurement. A kink, leak, water trap, clogged filter, weak pump, or excessive sampling delay can distort the waveform before gas reaches the chamber. Zero, calibration, optics, contamination, electronics, and software affect the measurement after it arrives.

Worked Example: EtCO2 Is Low and the Waveform Is Rounded

Use approved test gas or a simulator setup and inspect the sampling line, adapters, water trap, and sample flow. A leak can dilute the sample with room air, while partial restriction or a weak pump slows the response and rounds the waveform. Compare performance with known-good consumables before calibrating the module.

If the sample path is correct, allow warm-up and perform zero and calibration checks as specified. Verify value accuracy, waveform response, occlusion alarm, and sampling exhaust after repair. Keep test gas and scavenging requirements in mind when working around anesthetic agents.

What Is CO2 Monitoring?

Capnography measures carbon dioxide in respiratory gas over time.

The monitor can display:

The waveform is called a:

Capnogram.

What Is EtCO2?

EtCO2 means:

End-Tidal Carbon Dioxide.

It represents the measured CO2 concentration or partial pressure near the end of exhalation.

It is commonly displayed in:

depending on device configuration.

The Capnogram

The capnogram shows CO2 concentration over the respiratory cycle.

A typical waveform includes:

The shape itself contains useful information.

For a biomed, waveform shape can also help distinguish:

No gas sample

from:

Bad measurement

from:

Actual respiratory pattern.

Sidestream vs Mainstream CO2

There are two common approaches.

Sidestream

Gas is physically pulled away from the airway through tubing and measured elsewhere.

Mainstream

The CO2 sensor measures gas directly at the airway adapter.

This article focuses on sidestream.

The Sampling Line

The sampling line is one of the most important parts of the system.

It carries respiratory gas from the patient interface to the CO2 analyzer.

Possible problems include:

Because the line is external and frequently handled, it should be one of the first things checked.

Small Tubing Means Small Problems Matter

Sidestream sampling lines have relatively small internal passages.

That means a small amount of:

can significantly restrict flow.

A line can look normal externally and still be blocked internally.

Sampling Flow

The system pulls gas at a controlled flow rate.

The exact rate depends on the manufacturer and technology.

The sample flow needs to be sufficient to:

Too little flow can cause:

The Sampling Pump

A small internal pump typically creates the negative pressure needed to pull the sample through the line.

The pump may run continuously while CO2 monitoring is active.

Pump Running Does Not Mean Flow Is Good

You may hear the pump.

That proves:

The pump motor appears to be operating.

It does not prove:

Always separate pump operation from actual sample flow.

Weak Sampling Pump

A worn pump may still run but produce insufficient flow.

Possible symptoms include:

Use the manufacturer's flow or pressure test when available.

Water Trap

Many sidestream systems include a water trap.

Respiratory gas contains moisture.

The water trap helps prevent that moisture from entering the internal gas-analysis system.

Why Moisture Matters

Condensation can:

The water trap is not just an accessory.

It is part of the protective sampling system.

Water Trap Full

A full or saturated water trap may cause:

Replace or service it according to manufacturer instructions.

Water Trap Recognition

Some systems detect whether the water trap is installed correctly.

If the device reports:

Water Trap Missing

the problem may involve:

Do not assume the CO2 measurement sensor itself has failed.

Filters

The sampling system may include filters to protect internal components.

A contaminated filter can restrict flow.

This can produce symptoms similar to:

Again, think about the entire gas path.

Internal Tubing

After the external sampling line, gas may travel through internal tubing.

Possible failures include:

If known-good external accessories do not solve the problem, move inward.

Leaks

Sidestream systems rely on controlled gas flow.

A leak can allow room air into the sample.

Possible causes include:

A leak can dilute the sample.

Leak Example

Known test gas:

5% CO2.

Monitor consistently reads much lower.

If room air is entering through a leak, the sample reaching the sensor may contain less CO2 than expected.

Before calibrating the sensor, check the gas path.

Occlusion

An occlusion restricts sample flow.

Possible causes:

The device may detect abnormal pressure or flow and display an occlusion message.

“CO2 Occlusion” Is a Symptom

It tells you:

The system believes the sampling path is restricted.

It does not tell you where.

Start at the patient end and work inward.

Start With the Sampling Line

If the device reports an occlusion:

Remove or replace the external sampling line according to the manufacturer procedure.

If the alarm clears:

The restriction was likely external.

If it remains:

Investigate:

Infrared Measurement

Once the gas reaches the measurement chamber, CO2 is commonly measured using infrared absorption.

Carbon dioxide absorbs infrared light at characteristic wavelengths.

The analyzer uses this property to estimate how much CO2 is present.

The Basic Optical Path

Inside the analyzer:

Infrared source

↓

Gas sample

↓

Optical detector

The device compares how much light reaches the detector.

More CO2 changes the amount of infrared energy transmitted through the sample.

Why This Works

Different gases interact with infrared light differently.

The analyzer is designed to detect absorption associated with CO2.

The exact optical design and compensation algorithms vary by manufacturer.

Optical Chamber Contamination

If contamination reaches the measurement chamber, it can interfere with optical measurement.

Possible symptoms include:

Do not open or clean optical components unless manufacturer procedures specifically allow it.

Zeroing

CO2 analyzers need a reference for:

zero CO2.

Depending on the system, zeroing may occur:

The analyzer needs to know what the optical signal looks like without CO2 present.

Room Air and Zeroing

Room air contains a small amount of CO2.

Some systems account for this automatically.

Others use specific zeroing methods.

Follow manufacturer instructions rather than assuming:

Expose it to room air and press zero.

Zeroing Failure

Possible causes include:

A zero failure does not automatically mean the sensor must be replaced.

Calibration

Some CO2 systems require calibration using:

Calibration adjusts the relationship between the optical measurement and known CO2 concentration.

Do Not Calibrate Around a Pneumatic Problem

Suppose:

Known 5% CO2 gas.

Monitor reads:

3%.

Before calibration, verify:

If room air is leaking into the sample, calibration is not the repair.

Test Gas

Known-concentration test gas can provide a controlled reference.

Example:

5% CO2.

The monitor should display a value within the applicable manufacturer tolerance.

Use the specified gas and regulator setup.

Gas Concentration vs Partial Pressure

CO2 may be expressed as:

These are related but not interchangeable without considering pressure and conversion.

Use the same units required by the test procedure.

Atmospheric Pressure

Some CO2 measurements are affected by barometric pressure.

Systems may include compensation for:

The exact behavior varies.

If accuracy problems occur after environmental or configuration changes, check the manufacturer's compensation requirements.

Response Time

Sidestream systems have transport delay.

The gas must physically travel through the sampling line before reaching the sensor.

That creates a delay between:

Breath at the patient

and:

Waveform at the monitor.

Longer Sampling Line

A longer line may increase:

Use approved sampling accessories.

Do not assume any small tubing is equivalent.

Response Too Slow

Possible causes include:

If the waveform is delayed or sluggish, think about sample flow.

Respiratory Rate

The monitor can calculate respiratory rate from the repeating CO2 waveform.

If the capnogram is missing or distorted, the respiratory rate may also be wrong.

Do not troubleshoot the rate calculation before checking the waveform.

No Capnogram

Possible causes include:

Ask:

Is sample flow present?

Flat Zero CO2

If the monitor displays:

0 mmHg

with a flat waveform, possible explanations include:

Context matters.

CO2 Reading but No Waveform

If a numerical value appears but the waveform is absent, consider:

The measurement path may be functioning.

Waveform but Wrong Value

If the capnogram shape is stable but the numerical CO2 value is incorrect on known test gas, investigate:

Intermittent CO2

Intermittent problems commonly come from:

Try to reproduce the condition.

Water Movement

A small amount of condensation can move within the sampling line.

That can create an intermittent restriction.

The device may alternate between:

A line replacement may immediately isolate it.

Sampling Line Connection

Some connectors require proper seating to open an internal pneumatic path.

A line that looks connected may not be fully seated.

Inspect:

Known-Good Accessory Testing

Original sampling line:

Occlusion.

Known-good line:

Normal.

Original line on another compatible system:

Occlusion.

Failure follows the line.

Strong evidence.

Failure Stays With Device

Original line:

Fails.

Known-good line:

Also fails.

Both lines work elsewhere.

Now investigate:

Pump vs Restriction

Suppose flow is low.

Possible explanations:

Pump cannot pull strongly enough

or:

Something is restricting the path.

One measurement alone may not distinguish them.

Follow the service procedure for:

Pump vs Leak

A pump may run normally but a leak prevents the correct sample from reaching the analyzer.

Again:

Gas movement

and:

Gas integrity

are different.

Mainstream Confusion

If a device uses a mainstream CO2 sensor, much of the sampling-line troubleshooting does not apply.

Always identify the CO2 technology first.

Disposable vs Reusable Accessories

Some systems use:

Compatibility matters.

Using the wrong accessory can affect:

Nasal Cannula Sampling

A nasal CO2 cannula collects exhaled gas near the patient's nose.

If positioned poorly, it may sample excessive room air.

That can produce low or inconsistent EtCO2 without any monitor failure.

Oxygen Delivery Cannulas

Some cannulas combine:

Gas flow around the sampling site can affect the sample.

Use the accessory as designed.

Intubated Sampling

For intubated patients, gas may be sampled through an airway adapter.

Check for:

The sample path still matters.

Capnogram Shape

A capnogram can provide clues about:

But clinical waveform interpretation is different from equipment troubleshooting.

Biomed testing should use controlled sources when determining whether the device itself is functioning.

Slow Upstroke

A slow or distorted waveform can come from:

On the bench, if a known-good simulator or gas source produces a distorted waveform, investigate the equipment.

Baseline Does Not Return to Zero

Possible causes include:

Again, controlled testing helps separate clinical physiology from equipment behavior.

Sample Gas Exhaust

After measurement, sampled gas must go somewhere.

Some systems exhaust it:

depending on application.

On anesthesia equipment, sample-gas return and scavenging configuration may matter.

Anesthesia Applications

An anesthesia gas module may measure more than CO2.

It may also analyze:

The gas sampling system becomes even more important because one blocked line can affect multiple measured gases.

Multiple Gas Values Missing

If:

all disappear together, consider the shared sample path before assuming multiple sensors failed.

Shared Failure Points

Shared components may include:

One failure can remove several gas measurements.

Real-World Example: CO2 Occlusion

Monitor reports:

CO2 Occlusion.

Sampling line replaced.

Alarm immediately clears.

Original line contains visible moisture.

The monitor was working correctly.

Real-World Example: No Capnogram

Known-good sampling line:

No waveform.

No sample flow detected.

Pump can be heard running.

Service flow test:

Below specification.

Pump motor runs but cannot provide adequate flow.

Real-World Example: Low CO2 Reading

Known 5% test gas.

Monitor:

3%.

Sampling flow appears normal.

Inspect line:

Cracked near connector.

Room air enters the sample and dilutes the gas.

Replacing the line restores correct reading.

Real-World Example: Zero Failure

Module repeatedly fails zero.

Known-good sampling line and water trap installed.

Gas path confirmed clear.

Zero procedure still fails.

Now investigate the measurement module according to manufacturer service documentation.

Real-World Example: Intermittent Occlusion

System operates normally.

Sampling line moved.

Occlusion alarm appears.

Line repositioned.

Alarm clears.

Internal kink in disposable line.

Failure follows the accessory.

Real-World Example: Multiple Gas Measurements Lost

Anesthesia gas module loses:

at the same time.

Instead of assuming three measurement channels failed, check the shared sampling system.

Blocked water trap prevents gas from reaching the analyzer.

Common Mistakes

Replacing the CO2 Module Before Checking the Sampling Line

Start outside.

Assuming Pump Noise Means Sample Flow Is Good

Measure or verify flow.

Calibrating Around a Leak

Fix the gas path first.

Ignoring Moisture

Small amounts can block small tubing.

Treating an Occlusion Message as the Diagnosis

Find where the restriction is.

Ignoring the Water Trap

It is part of the sampling system.

Confusing Sidestream and Mainstream Troubleshooting

Identify the technology.

Troubleshooting Respiratory Rate Before Fixing the Capnogram

Rate depends on the waveform.

A Useful Troubleshooting Framework

For sidestream CO2, ask:

Is the sampling line connected and open?

Then:

Is gas actually flowing through the system?

Then:

Is the sample reaching the sensor without leaks or dilution?

Then:

Can the analyzer zero correctly?

Then:

Does known CO2 test gas measure within specification?

That separates the system into logical stages.

Another Useful Question

Ask:

Is this a gas-delivery problem or a gas-measurement problem?

That distinction can save a lot of unnecessary parts replacement.

If the sample never reaches the sensor correctly, the sensor cannot give you a correct result.

What Did You Actually Prove?

If the pump runs, you proved:

The pump motor appears to operate.

You did not prove:

If known test gas reaches the module through the complete sampling path and the monitor measures it within specification, you have much stronger evidence that:

are functioning under that test condition.

Each test proves another layer.

Final Thoughts for Biomeds

Sidestream CO2 troubleshooting becomes much easier when you remember one thing:

The gas has to physically reach the sensor.

Before worrying about calibration or replacing a CO2 module, follow the gas.

Start at the patient connection.

Check the sampling line.

Check the water trap.

Check for moisture.

Check for restriction.

Verify sample flow.

Look for leaks.

Then evaluate the CO2 measurement itself.

The system is basically:

Sample → Move → Measure → Calculate → Display.

Find where that chain breaks, and the failure becomes much easier to isolate.

— Jake

Important Note

CO2 sampling rates, water-trap designs, zeroing methods, calibration requirements, test-gas concentrations, atmospheric compensation, accessories, and acceptance limits vary by manufacturer and model. Follow current manufacturer service documentation, use approved sampling accessories and test equipment, and complete required verification before returning equipment to clinical use.

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