What happens if I don’t replace my Capnograph Disposables?

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Airway adapters, breathing circuits, sample lines and filters are all capnograph ‘disposables’ – designed to be replaced after every patient. This is not just so the manufacturers can keep customers paying out – there are sound clinical and physical reasons why this is recommended.

Sidestream Capnographs

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VM2500 Sidestream Capnograph Pulse Oximiter

Sidestream capnographs work by drawing a gas sample away from the patient to the monitor. Drawing a constant flow of breath from a patient carries moisture (and anything else exhaled) in the same way that exhaling on a cold window causes condensation which later evaporates and leaves a residue.

This can cause a blockages and build-ups in the lines, filters become blocked and water traps need emptying and cleaning. While replacing the tubes and adapters as regularly as possible will not stop this happening, it will definitely slow the build-up and improve the general flow of your monitoring lines and circuits. A top tip to help reduce fluid build up in sidestream lines is to connect the line to the breathing circuit, so it then leads away upwards to the monitor and not like a drain from a bathtub (down).

In veterinary medicine, some practices will reuse these disposables for up to several years in the worst cases. While reusing disposables has no adverse effects for a few weeks, any extension on this could potentially jeopardise an expensive monitor for the sake of saving £10. There is also a risk of cross-contamination, although as the sample is measured away from the patient, this risk is fairly low. It’s more common that contamination of the equipment itself or outside users occurs if proper precautions are not taken. Bacteria or virus from an infected patient can become trapped in the tubes – another reason why it is important to replace all disposables regularly, rather than washing them and reusing them. This is also the reason why with highly infectious viral diseases, despite needing the data, often human ITU’s are foregoing their use in the current crisis.

Mainstream Capnographs

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VM-2500 Mainstream Capnograph Pulse Oximiter

With mainstream capnographs, the sample is measured directly at the end of the ET Tube. This means that it is just as critical to replace the measuring chamber regularly. Over time, the inside of what should be a perfectly clear measurement chamber silts up with residue from the exhaled breath. The sensor warms the chamber, so condensation is not always visible to the naked eye – which often creates the impression that your kit is cleaner than it really is. The gradual “silting up” can cause problems with the calibration of all capnographs. The sensors use infrared light to detect CO2 – and will also pick up the presence of any residual build-up (even if not visible to the naked eye) and this contributes to the absorption that the monitor sees and can thereby distort the results. Mainstream and sidestream capnographs today can self-calibrate and account for residue buildup, but the way it does this is by increasing the intensity of the infrared light.

While this may not cause any ill effect if the residue is regularly dealt with and disposables are regularly replaced, if allowed to continue this will cause two problems with the monitor. The first is that the capnograph monitor may drift so far on it’s calibration adjustment scale that it is not able to re-adjust back to the ‘true’ position once the machine has been thoroughly cleaned out. The second is that by burning the IR brighter each time, the unit will have it’s overall life reduced and could result in a sensor needing replacement much sooner than you intended. The key difference here is a sidestream measuring bench needs to be replaced by a qualified service tech, the mainstream is an off the shelf plug and play item.


To reiterate, good care of your kit is a key habit of any successful practice – this includes buying more disposables when they are needed. The more care is taken over expensive equipment, the longer it will last you and the better results you will get out of it. Many practices are more than willing to decontaminate and clean their equipment thoroughly, but not always keen to pay out for new disposables. It’s really worth making sure you include replacing them as part of your routine – £10 now is better than thousands for a new capnograph sensor later!

(We generally recommend swapping filters and lines every two weeks in Sidestream models and every month for Mainstream models.)

So, Sidestream users: don’t let that residue build up. Replace what you can, when you can – and make sure any potential contamination is dealt with effectively.

And Mainstream users: Don’t let your IR sensor get overworked. Keeping the adapter clean will keep the monitor alive for a lot longer – remember, sensors are not cheap to replace.

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