Now we know what normal capnography islet’s look at when it may not be normal. Keep in mind like with all monitoring all the information needs to be analysed together. This includes your patient and the equipment you are using.
Starting with the simple bits, is it too high or too low.
Capnography: Help it’s too low
Hypocapnia, low ETC02, is often a result of hyperventilation. Think back to the factors that influence it, one being ventilation. If an animal breathes too fast, then more C02 will be expired and lead to a lower end amount. The amount of C02that needs to be ‘removed’ will be the same as the metabolism hasn’t changed. However, the respiration rate is disproportionate and therefore results in a slowly decreasing ETC02.
Trouble shooting this will include looking at your patient and asking the following questions:
Is the patient painful?
Is my anaesthesia depth adequate?
Am I over ventilating? This can be either by a ventilator or IPPV.
Once you’ve answered all of those questions it’s now time to look at your equipment. Is there a good seal around your ET Tube, are you losing C02 around the tube? Check the cuff, listen to ensure there is no leak. Worst case scenario you may even be able to smell your inhalational gas.Check the sampling line, is there a leak? Has someone run over it with the trolley….it happens!
Finally, hypothermia can also cause hypocapnia, but this tends to happen over a longer period of time (Walsh 2014)
Capnography: Help it’s too high
Hypercapnia, high ETC02, can also be looked at in the same logical way. In the context that I’ve used capnography I would say this is something I’ve seen more. We use drugs that can cause respiratory depression and low respiration rates mean the body isn’t getting rid of the metabolism by product.
This leads to a high ETC02which needs to be addressed. Often meaning we have to provide ventilation for these patients. It may also be seen with post induction apnoea, so it’s important to get your capnography on as soon as you have intubated.
If you’ve eliminated the respiratory factor, then go back to the three factors and look at metabolism. These factors can include hyperthermia and shivering, due to increased muscle activity which leads to more cell metabolism and more waste product. Finally, if you are performing laparoscopies consider the absorption of C02 (Walsh 2014) Also, equipment must be checked, what’s your absorbent looking like, is it exhausted?
Capnography: ride the wave
Once we’ve looked at the number, which should be 35-45 mmHg, we then start looking at the wave form itself. This can often give us an indication of what’s going on.
Again, yourself questions and start trouble shooting.
Does the wave form return to baseline?
Is there a smooth up and a smooth down on the curve?
Are there abnormal ‘wiggles’ on the wave.
Capnography: what about the base?
One of the first things to look at is the base line. If the wave isn’t this can indicate your patient is rebreathing. The absorbent needs to be checked and if necessary, changed. If you are unsure when it was last changed then it would be sensible to swap circuits for a fresh absorbent. Check your Fresh Gas Flow Rate (FGFR) as this may be too low. Remember this will vary with the circuit you use so calculate your patient’s FGFR before starting.
Then we start looking at the shape of the wave on the incline. Keeping in mind all the factors that can influence capnography. Resistance, either in the circuit/tube or lungs (bronchospasm) will result in a ‘shark fin’ appearance. Check the tube and change or suction if required, then eliminate the circuit and finally once all of this is eliminated consideration should be given to bronchospasm. This may be seen in asthmatic cats for example (Schauvliege 2016)
A sudden drop on the other side of the plateau can be a cause for high alarm. Either the ET tube and circuit have become disconnected or the patient has ceased having circulation, this can be an indicator for a cardiopulmonary arrest. If you lose wave form or ETC02don’t panic. Systematically check everything but equally do not be complacent, always check. Other abnormalities that can be seen on the wave form include cardiac oscillations and the ‘Curare Cleft’ but the key point to take away is to look and analyse everything in front of you.
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 – 18
Hopefully this has helped, please do let me know if I need to clarify anything!