It’s been a long time since I’ve posted anything instructional, and looking at the sheer size of the topic I decided to separate this into multiple posts just to over it all. This post will cover the basics of waveform capnography as well as some anatomy and respiratory physiology that is important to understand as you begin to interpret waveforms.
Simply, capnography is a measurement of exhaled CO2. We already did this previously with colorimetric CO2, however, unlike colorimetric monitoring, capnography is not affected by alcoholic beverages or other carbon dioxide producing agents that are exhaled. Also capnography does not take “a few breaths” in order to display a change in the measurement, the change is instant when being viewed on the monitor.
You should also keep in mind that capnography and oxygen saturation are two different measurements. spO2 measures “how much” oxygen is attached to the available hemoglobin. As you also know, spO2 is easily fooled by carbon monoxide as it bind with hemoglobin. The spO2 sensor will detect carboxyhemoglobin and only see that something is bound to the hemoglobin and give a false reading. For an in depth video on hemoglobin, check out this video
Capnography will also be an accurate indicator of perfusion and the effectiveness of your or their respiration or ventilation at the cellular level. This is due to several physiological processes that occur with respiration. It is important to know because it is vital to understanding exactly how capnography works, and diagnosing and treating respiratory problems and illnesses more effectively.
The first thing to understand about cellular respiration is that it takes several different actions working as one. It takes oxygen from the lungs, glucose from the liver, and in the body cells it takes insulin produced by the pancreas. Insulin acts as a transporter, carrying glucose into the cells from the bloodstream. Brain cells are the only cells in the body that use glucose directly from the bloodstream without needing insulin. This is because insulin can not cross the blood/brain barrier, which is why when the glucose level in the bloodstream drops confusion and neurological disruptions result until the glucose level is restored.
When you breathe in, oxygen and carbon dioxide exchange places at the capillary beds in the alveoli. The outgoing CO2 is waste product from the cells, which means that cellular metabolism is taking place. The membrane barrier in the alveoli is in fact so thin that it allows oxygen and carbon dioxide to diffuse at the molecular level using a pressure gradient. The oxygen is then picked up by the hemoglobin forming and carried to the cells. When it reaches a capillary, the barrier thins again allowing molecular exchange using the same pressure gradient. When measuring blood gases, there are several different terms used to express this function, PaCO2and PetCO2 are the most important.
PaCO2 is the partial pressure of CO2 in arterial blood. This number should be small and the PaCO2 actually serves many functions. Your brain regulates your breathing rate and blood pH by monitoring this. If the pH of your blood stream increases, this number will increase, which increases your respiratory rate and depth in an attempt to “blow off” the additional acid. You see this in active DKA patients, as it is a very early sign of acidosis.
PetCO2 is the partial pressure of CO2 at the end of expiration. This measures the concentration of the carbon dioxide in the alveoli as they empty. This is an important indicator of many different metabolic functions. The main ones we are interested in is cardiac output and and adequacy of ventilation. If cardiac output is low or ventilation is inadequate, the measurement will be low because carbon dioxide is not being exchanged at an adequate rate.
That’s enough physiology for now. In Part II we will begin covering the waveform, it’s parts, and how it relates to the physiology of respiration.
Kodali, Bhavani-Shankar. (June 2010). Capnography In Emergency Medicine – 1911. In Capnography. Retrieved from http://www.Capnography.com.