Overall partial pressure of CO² and the constant real-time measurement of CO² present what a patient likely is experiencing physiologically.
For years, we used color-metric devices to identify for advanced airway confirmation, using basic middle-school chemistry equipment and a few medical innovations to measure carbon dioxide (CO₂) in expelled gases. Litmus paper (used to measure acid/base, or pH) was placed in a device that attached to the top of an endotracheal tube (ET). The mixture of CO₂ and condensation (H₂O) from the lungs created carbonic acid and changed the litmus paper color. If the device turned purple, you had a problem, and the ET was thought to be placed in the esophagus. If it turned gold, that meant that there was carbonic acid, which meant that CO₂ was being expelled, which meant that your ET was in the right place.
ETCO₂
Over the years, technology improved and our ability to read specific values in the field increased dramatically, particularly with CO₂ in the form of end tidal capnography, also known as end tidal CO₂ (ETCO₂).
ETCO₂ is the measurement of CO₂ that’s expelled by the body at the end of exhalation. This number
can provide different information regarding a patient but presents as two different values when you use a digital capnography device.
The first value is the overall partial pressure of CO₂, which is measured in mm/Hg. Normal is 35–45 mm/Hg. When it comes to the value on a capnography device, the higher the number, the more CO₂ that a patient is blowing out; the lower the number, the less CO₂.
The second value is the waveform, which is more of an illustration than a value. The waveform is the constant real-time measurement of CO₂ that’s being expelled in the form of ETCO₂ over time. In other words, the baseline of the wave is considered “0” when a patient is breathing in, because no CO₂ is coming out. Once a patient starts to exhale, the CO₂ increases until the patient no longer is breathing out. Your end tidal CO₂ amount is at the end of that tidal wave. Then it goes back to zero when the patient breathes in.
Let’s say that a patient is in cardiac arrest. The patient isn’t metabolizing a dang thing because of cardiac arrest. No CO₂ is being produced, because the body is switched to “off,” if you will. A small number of CO₂ might register on your device, let’s say 15; in other words, not good. If you are doing good CPR, defibrillating that person and having a return of spontaneous circulation, you’ll see an instantaneous jump in ETCO₂ from 15 to perhaps as much as 50 or even 60. This illustrates that the body switched into the “on” position and that things are firing up. The higher number is indicative of the patient trying to blow off excess CO₂ because of having been retaining CO₂ from when he or she was in cardiac arrest.
Waveform is used for a few instances, but the primary reasons are for tube confirmation on supraglottic devices and ETs, to ensure that the lungs, not the stomach, are being ventilated. The shape of the wave also can paint a picture of numerous ailments that could be occurring with a patient’s respiratory state, such as chronic obstructive pulmonary disease (COPD) and asthma.
When you combine the waveform and the numerical values you’re presented with what a patient most likely is going through physiologically.
Better insight
Two devices can detect ETCO₂. One is an adapter that goes on the end of an advanced airway; the other is a nasal cannula that provides O₂ and reads ETCO₂. You don’t have to provide the O₂ through the cannula if you just are trying to get a number. Just remember that the nasal cannula isn’t designed to flow O₂ more than 6 liters per minute on average.
Overall, I can’t speak highly enough regarding ETCO₂ for understanding disease process and confirmation of tube placement. If you aren’t measuring these numbers, you must learn how and why we use them and implement this practice into your cardiac arrest, respiratory, sepsis and diabetic calls. This will give you a more well-rounded observation as to what your patients are going through and assist in guiding your treatment paths.
