Mainstream vs Sidestream Capnography: How to Source a Compatible EtCO2 Sensor

Quick answer

Mainstream capnography puts an infrared sensor directly in the breathing circuit on an airway adapter, measuring CO2 inline with no sampling line. It responds fast and is typically used with intubated patients. Sidestream capnography aspirates a small gas sample through a sampling line to a sensor inside the monitor, which lets it work with both intubated and non-intubated patients (including a sampling nasal cannula), at the cost of a sampling line and moisture management. To source a compatible EtCO2 sensor, match four things to your host monitor: the method (mainstream vs sidestream), the connector, the patient population (adult / pediatric / neonatal), and the consumable it pairs with (airway adapter for mainstream, sampling line and water trap for sidestream). Confirm the fit against the OEM part number the sensor cross-references — not the monitor's brand name alone.

Who this guide is for: biomedical and clinical engineers, hospital technicians, and procurement teams sourcing replacement EtCO2 sensors and capnography consumables for existing patient monitors. It is written for the person matching a part to a fleet of monitors, not for bedside clinical decision-making.

Capnography measures the carbon dioxide in a patient's exhaled breath, reported as end-tidal CO2 (EtCO2) — the CO2 concentration at the end of exhalation. Every capnography setup does this in one of two ways: mainstream, with the sensor in the airway itself, or sidestream, which draws a gas sample back to a sensor inside the monitor. Which one a system uses decides what replacement sensor, adapter, or sampling line will actually fit it.

This guide is about the device and the sourcing decision — how the two methods differ, and how to identify and verify a compatible EtCO2 sensor for the monitor you already run. It does not cover reading or interpreting a patient's capnogram; that is a clinical question for your clinical staff and the device's own clinical documentation.

What does an EtCO2 sensor actually measure?

An EtCO2 sensor measures the concentration of carbon dioxide in breath using infrared (IR) absorption: CO2 absorbs IR light at a known wavelength, so the amount of light absorbed tells the sensor how much CO2 is present. The value the monitor reports as EtCO2 is that CO2 reading at the end of an exhaled breath.

For sourcing purposes, the important part is where the measurement happens, because that is the single biggest difference between mainstream and sidestream hardware — and it is what determines which sensor and which consumables are compatible with a given monitor.

Mainstream vs sidestream capnography: what's the difference?

Mainstream places the IR sensor at the airway, clipped onto an airway adapter that sits in the breathing circuit. The gas is measured where the patient breathes — nothing is pumped anywhere. Sidestream keeps the sensor inside the monitor and continuously draws a small sample of gas back to it through a thin sampling line.

That one design choice cascades into every practical difference a buyer cares about:

Factor	Mainstream	Sidestream
Where the sensor sits	In the airway, on an airway adapter	Inside the monitor / module
How gas reaches the sensor	Measured inline, no sampling	Aspirated through a sampling line by a pump
Sampling line	None	Required (a routine consumable)
Moisture / water handling	Little to none	Water trap or moisture-management line needed
Typical patient	Intubated	Intubated and non-intubated (sampling nasal cannula)
Response speed	Very fast (measured at the airway)	Slight delay from sample transit time
Weight at the airway	Added sensor weight at the adapter	Nothing added at the airway
Main consumables to reorder	Airway adapters	Sampling lines, water traps / filter lines
Common use setting	OR, transport, ICU on ventilated patients	Procedural sedation, PACU, ED, non-intubated monitoring

Neither method is "better" in the abstract — they suit different clinical setups, and most monitor platforms are built around one or the other. For sourcing, the job is not to choose between them but to identify which one your monitor uses and buy to match it.

How do I identify a compatible EtCO2 sensor for my monitor?

Compatibility is decided by the host monitor, not by the sensor. Work through these four checks before ordering:

1. Confirm the method the monitor uses — mainstream or sidestream. Check the monitor's capnography module or its documentation. A monitor built for a sampling line is sidestream; one that reads from an airway-adapter sensor is mainstream. Some platforms support both through different modules, so verify the specific module in service.
2. Match the connector and the module. Capnography sensors and modules use specific connectors and, often, a monitor-specific interface. The physical connector has to match, and the module has to be the one the sensor is built for.
3. Match the patient population. Airway adapters and sampling lines are supplied in adult, pediatric, and neonatal variants, with different internal volumes. Sourcing the wrong size is a common and avoidable error.
4. Match the consumable path. For mainstream, confirm the airway adapter type (and whether it is single-patient or reusable). For sidestream, confirm the sampling line and water trap / filter line — including the Luer or proprietary connector — because the sensor and its consumables are a system, not separate purchases.

What's the difference between an OEM sensor and a compatible one?

An OEM sensor is the part sold under the monitor manufacturer's own brand. A compatible (aftermarket) sensor is manufactured to work with that same monitor while being sourced independently — typically at a lower total cost of ownership across a fleet.

The one thing that matters when you buy compatible is that the part is built and specified to match the OEM component it replaces. That is why a credible compatible sensor is always cross-referenced to a specific OEM part number — the number is the compatibility claim, and it is what you verify against.

How do I verify a compatible EtCO2 sensor before I buy?

Verification is a documentation and cross-reference exercise, not a guess:

• Ask for the OEM cross-reference. A compatible sensor should list the exact OEM part number(s) and monitor model(s) it is built to work with. If a supplier can't name them, that's the signal to stop.
• Confirm the method and consumable match the four checks above — method, connector, patient size, and the airway adapter or sampling line it pairs with.
• Ask for regulatory documentation on the part you're actually buying. For EtCO2 specifically, Orantech, Inc. — Medten's manufacturing arm — holds FDA 510(k) clearance K171828 for its EtCO2 sensor, cleared 31 May 2018. Among the manufacturers cleared under that product code, the rest are original equipment manufacturers. A clearance tied to the actual product line is far more useful to a procurement file than a general marketing claim.
• Request a sample for bench check where your process allows, and confirm fit and reading against a known-good reference before committing a fleet order.

For the full regulatory picture behind that clearance, see how FDA 510(k) clearance works and what to verify for FDA and ISO 13485.

Related reading

• Browse EtCO2 sensors and capnography parts
• FDA 510(k) and CE standards for medical accessories
• Sourcing compatible parts: how to match accessories to your monitor
• How to source a compatible temperature probe
• Request a quote

Key takeaways

• Mainstream measures CO2 at the airway with an inline sensor and adapter; sidestream draws a sample back to a sensor in the monitor through a sampling line.
• The method your monitor uses dictates which sensor and consumables are compatible — sourcing starts with identifying it.
• Match four things: method, connector/module, patient size (adult/pediatric/neonatal), and the consumable path (airway adapter, or sampling line + water trap).
• A credible compatible EtCO2 sensor is always cross-referenced to a specific OEM part number — that number is the compatibility claim to verify against.
• For EtCO2 parts, Orantech, Inc. holds FDA 510(k) clearance K171828 (EtCO2 sensor, cleared 31 May 2018) — documentation tied to the actual product line, not a general claim.