Understanding Capnography: A Beginners Guide 1-Hr CPD

Well firstly, we need to ask ourselves, why is it important to measure the levels of CO2 in our patients?

Carbon Dioxide is a waste gas that is created during metabolic processes and is excreted from the blood, via the lungs, into the atmosphere. The presence of CO2 is vital in driving the ventilatory mechanisms in a patient and therefore a delicate balance needs to be achieved to prevent complications for the patient.

With low levels of carbon dioxide being detected in the blood, ventilation will decrease, or even cease altogether. This can create quite a rocky anaesthetic as then the patient is not breathing in anaesthetic gases, meaning they can become light and reactive to noxious stimulus.

With high levels of carbon dioxide being detected in the blood, ventilation will speed up, this can be harmful to our patients as it causes not only respiratory rate to increase, but also an increase in heart rate and blood pressure through sympathetic stimulation. The hypercapnia caused my high levels of carbon dioxide (hypercapnia) can also make the anaesthetist think that the patient is too light and reactive, therefore increasing the level of inhaled anaesthetic agent, however the reverse is actually true in this situation and increasing the volatile agent could be detrimental to the patient.

Hypercapnia causes respiratory acidosis which lowers the blood PH to below 7.35, the patient’s body then tries to correct this using other organ systems. High CO2 levels can cause cerebral vasodilation in the brain, which in a patient that already has, or is at risk of, raised inter-cranial pressure can prove fatal.

This is where the use of capnography is not only a useful tool, but could also be life-threatening in the absence of. Therefore, the use of a capnograph alongside other parameter monitoring is vital in ensuring we have the full picture to keep our anaesthetised animals as safe as possible.