Ok, let’s talk capnography! Despite there being tons of articles and resources available confusion around capnography remains.
First up let’s establish what capnography is. Carbon dioxide (C02) is one of two by products of cell metabolism. The second one being water.
Remember cells metabolise oxygen, and the C02is by-product of this. In order to have C02we must have metabolism and also circulation. Afterall the oxygen won’t get there by itself.
Can we see why C02 measurement is so important, already it’s giving us glimpses of why it’s important to measure. This can be done in two ways, by checking blood gases (ideally arterial) or during respiration. This is what gives us ‘End Tidal Carbon Dioxide’ or ETC02
ETC02 can be affected by three things, all of which we need to consider.
Metabolism: so the production of C02in the tissues.
Circulation: the movement of the gases around the body
Ventilation: the movement of air out of the body and in the alveolus.
Now we know where C02 come from and what it can be affected by lets look at ETC02 and what this means for us when are using it on our patients.
When we use capnography, we are getting a graphical representation of the respiratory cycle (Schauvliege 2016) Plus, as a bonus, we also get an insight into metabolism and circulation.
Most of us will be using a side stream technique. This is where a sample is collected via a port (for us it’s on the HME) and then analysed in the monitor. Your monitor will also have a water trap which will be collecting the second by product of cell metabolism, water. Main stream capnography will be quicker and is good for smaller patients but can become blocked and adds dead space (Schauvliege 2016)
Capnography: What’s the norm?
A patient’s normal ETC02 should be between 35 – 45 mmHg (Walsh 2014) but keep in mind that the graphical picture itself can also provide a lot of information. Let’s looks at a normal capnograph and its wave form.
From this wave from we can start to see not only how metabolism, ventilation and circulation can affect it but also how dead space and resistance will influence our capnograph. We will look at these in the next post but for now I hope you are happy with understanding where the shape and number originate from.
This video should help you understand the gas movement in relation to the wave.
Schauvliege, S. 2016. Patient Monitoring and Equipment. In ed. T. Duke-Novakovski, M. Vries and C. Seymour BSAVA Manual of Anaesthesia and AnalgesiaGloucester. BSAVA Ch. 7
Walsh, K. 2014 How to read a capnography trace. Companion Issue 7, p. 14 – 18Cap