End-Tidal CO2 Monitoring

November, 2013

The other night I was in Bayfield and we discussed the use and potential benefits of end-tidal CO2 monitoring, often abbreviated as EtCO2. This is a topic that seems to be gaining inertia in the prehospital arena, and I think that is – generally – a good thing. However, as with all technology, there is risk for over-reliance on the technology, with potential detriment to the patient. End-tidal CO2 monitoring is just such a technology, so lets talk about it for a few minutes.

EtCO2 monitoring is often compared to SpO2 monitoring, but they are very different and making any analogies between to two is potentially dangerous. We all know that SpO2 monitors the amount of hemoglobin that is saturated with oxygen (measured as a percentage). This is the amount of oxygen available to the organs as it is carried through the blood stream. To look at it another way, it is the amount of oxygen available “on the front end”…being carried out of the lungs and pumped out of the left side of heart through the arteries and to the organs. It is this oxygen, along with sugar, that is fed into our cells as fuel.

In contrast, end-tidal CO2 is a measure of the byproducts (the exhaust) of cellular functioning (termed cellular respiration). The mitochondria in our cells take in oxygen and sugar and use it to produce ATP, which is the energy we use to survive. The byproducts of this energy production are CO2 and water. This CO2 is carried back to our lungs through the veins and exhaled with each breath. It is the measurement of this exhaled CO2 that is EtCO2. This is measured as a partial pressure of our exhaled gases (mmHg). In other words, EtCO2 is a measurement “on the back end” of cellular energy production.

So why is it beneficial to measure exhaust gases of cellular respiration? In short, because EtCO2 provides us with an earlier marker of hypoventilation that does SpO2. Why?...because CO2 diffuses across the alveolar membrane (in the lungs) about 20 times more readily then does oxygen. Consequently, when we fail to breath enough, our CO2 level in our lungs builds up and, because it diffuses so much more readily across the alveoli, the EtCO2 monitor picks up the change more quickly than does our SpO2 monitor. Data suggest that when someone stops breathing adequately, EtCO2 will start to rise 2-3 minutes before SpO2 starts to drop. It should not be overlooked that 2-3 minutes of lead-time on a decompensating patient can make a lot of difference. This allows ALS to be started towards you earlier, and allows time to prepare airway equipment, plan out a strategy for intervention, and take action BEFORE a patient decompensates, rather trying to react AFTER a patient runs into trouble.

At the paramedic and physician levels, EtCO2 is used for other things as well, including diagnosing certain pulmonary conditions, assessing the adequacy of endotracheal tube placement, and as a marker for when to consider stopping a resuscitation. But, at all levels, the value of EtCO2 to provide early warning of a hypoventilating patient is its most important asset.

As EtCO2 monitoring percolates its way down and becomes more commonplace in prehospital medicine, more and more services will obtain these devices. It has already started, with Bayfield EMS and Ashland Fire both having the capability to monitor EtCO2. As additional services purchase EtCO2 monitors, I will meet with each service to go over this material in more detail, and to discuss additional precautions to EtCO2 use, etc.

So, in summary, EtCO2 monitoring will soon be coming to an EMS service near you. It is an exciting technology that will allow each of you additional information to help manage and care for your patients.

Keep up the good work.

Dr. Shultz

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