Dangers of using a Sidestream Capnograph on patients under 5kg

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Most sidestream capnographs are made for human use. While these kinds of capnograph can still be used on bigger animals such as dogs, most are not tested on patients smaller than 10kg. Capnography with smaller animals can therefore cause a few unusual (and potentially lethal) results – here’s our guide on what to watch out for and how you can avoid problems during capnography readings.

Diluted Samples

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Even adult cats weigh an average of 4-5kg – meaning sidestream capnography could cause innacuracies

A sidestream capnograph draws a gas sample away from the patient to measure it, which also causes some dilution of the sample (this can be as much as 125-160ml/min). The greater this dilution, the less CO2 is picked up by the monitor – causing an artificially lower reading on-screen. In larger patients this dilution is often negligible. For smaller patients, this could become a problem if it can read at all – for example a patient’s vitals could be reading as ‘normal’, but actual CO2 levels could be dangerously high, or the patient could be rebreathing – and the monitor will not detect it.

Sample Sizes

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Remember, small lungs mean a smaller sample!

The size of the gas sample is another issue with sidestream capnography. As patients under 5kg will have a much smaller lung capacity, the monitor may fail to register any CO2 at all during normal respiration – simply due to the very small size of the exhaled gas sample. A 5kg patient in an anatomically normal position breathes out less than about 30ml each breath (6ml/Kg). If you lie them on their back, this can be reduced by another 30-50% as the stomach impinges on the diaphragm and restricts breathing.

Gas Flows and Dead Space

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Lutech Datalys v7, Available via Thames Medical. Click image for details

With smaller patients it is imperative that the fresh gas flow is as low as possible, as higher fresh gas flows (relative to the patient) will cause even further dilution of the sample. This can also lead to exerting a positive fresh gas pressure against which the patient has to breathe, which can limit the patient’s exhalation. Another point to note is that unless the gas sample is being drawn from a specially designed low dead-space neonatal airway adapter, the dead-space in a standard connector could be as much as 100% the tidal volume of the patient. Small patients may not be able to clear their latest breath and end up re-breathing their own exhaled CO2, and the monitor in this case may not be able to detect it.

A Simple Solution To Avoid Risk

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The above combination of effects could be lethal in a worst-case scenario for our smaller patients. Fortunately, there is an easy solution. Using a mainstream capnograph with a low dead-space airway adapter will eliminate any dilution of the sample, while reducing the stress on a patient’s breathing. In addition to this, calculate your tidal volumes, minute volumes and fresh gas flow requirements as a paediatric human anaesthetist would (at 4-6ml/kg) and ensure you are delivering patient-specific fresh gas flows.

In summary, we would always recommend using a mainstream capnograph with patients under 5kg. At Thames Medical, we provide high-quality machines for veterinary practices and excellent training in the use of biotech equipment like capnographs commonly used in veterinary medicine. If you or your practice would like to know more about our CPD courses or our available products, give us a call today to see how we can support you and your patients.

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