Hot Air: The Importance and Basics of ETCO2
As professionals in the medical field. I believe that we have all attended a lecture or in-service training that was less than interesting. It was during one of these lectures that the idea for this article appeared.
I am sure that everyone has heard the expression; “He is full of hot air.” Did you ever think about putting that hot air to use? It is time to harness the power of that hot air! We need to make that air work for us! In the medical field we refer to this as end-tidal CO2 (ETCO2.)
The name end-tidal refers to the tidal volume (VT) during respiration in the body. Tidal volume refers to the amount of air that is moved in and out of the body during ventilation. The average tidal volume is 400-500ml; in the average adult female and male respectively. ETCO2 is measured as capnometry and capnography. More specifically they are a partial pressure that is measured from the body during ventilation. Capnometry is the number that is measured. Capnography is that number graphed over time. Traditionally capnometry is recorded at the end of the tidal volume during expiration. This is where the “end” comes in to play. The concentration of carbon dioxide will be the highest at the end of the ventilation.
Why is carbon dioxide important? Well, if you look at the traditional uses of ETCO2 it has only been used to confirm advanced airway placement. There is so much more that ETCO2 can (and should) be used for. First, we need to understand where the carbon dioxide comes from. During cellular respiration (also known as internal respiration) the body converts glucose and oxygen to ATP. During this process carbon dioxide is formed as a waste product. While glucose itself is not directly used. Glucose is converted to acetyl-CoA. This molecule is then directly used in the Krebs Cycle. (Remember this gem from anatomy class!) The carbon dioxide is then picked up by the hemoglobin, after it offloads oxygen molecules. Approximately 20% percent of the body’s CO2 is found here. Carbon dioxide is also found dissolved in the blood plasma. This is what effects the acid base balance in the body; approximately 10% of the body’s CO2 is found here. What about the other 70 percent? It is found as HCO3 or bicarbonate and is used to assist the body as a PH buffer. Feel free to review the process of cellular metabolism. It is an important (and lengthy) process that deserves its own article.
Understanding the placement of carbon dioxide in the body is important if we are to use ETCO2 effectively. Capnometry is measured in millimeters of mercury (mmHg.) The normal range is 35-45mm/hg in ideal circumstances. To truly take advantage of this vital sign we need to display the reading over time! This is where capnography comes into play. Traditionally in prehospital and hospital-based medicine capnography has been used to monitor and confirm airway placement. However, this is just the start. To make it known as the “ventilation vital sign” (Brochard) we need to add a few things.
One of the most important things to understand is how this vital sign is physically obtained. Devices for collection are divided into 3 separate categories; main-stream; side-stream, and micro-stream. Mainstream sampling devices are inserted directly into a ventilator circuit. They are typically more accurate than side-stream devices. However, they do increase mechanical dead space in the circuit; and possibly add weight to the airway depending on placement. This must be accounted for so that the patient receives an adequate tidal volume. Side-stream devices get their name since the monitor is separate from the sampling device. Providers insert a T device into the vent circuit, and the aspirated sample is sent to the monitor via a tube. Due to the sample being moved there is a significant delay between the sample time and when the number is generated. While the sampling rate is variable it is typically between 150-200 milliliters per minute. Micro-stream monitors are the newest and most evolved of ETCO2 technology. Typically, these devices are included in modern cardiac monitors such as the ZOLL X-Series®. This sampling device requires a small device be placed near the patient with a small Micro® tube that connects the sampling device to the monitor. These micro-stream devices have more accuracy than main-stream devices because they use molecular correlation spectrography. Main-stream and side-stream devices use inferred absorption to detect the carbon dioxide. Due to the smaller profile of the monitoring devices, they can be used in both invasive monitoring; such as an advanced airway. As well as non-invasive monitoring, such as a capno mask or capno cannula. Capno cannulas are great for monitoring ETCO2 and breathing patterns for CPAP (in some agency’s it is protocol to use a cannula for CPAP).
Regarding the monitors that use the micro-stream devices. It is important to know that the waveforms that you see on the screen are compressed and not to scale. In order to see the full waveform to scale you must print it much like a 12-lead ECG. The screen can help establish your basic phases and patterns. You must remember to print it for accuracy. Capnography can be divided into four basic phases. Phase I is known as the inspiratory baseline. This is similar to the isoelectric baseline of an ECG. Your changes of the waveform will start here. Phase II commonly called the expiratory upstroke is the first positive deflection from the baseline. This includes the transition from anatomic dead space to the gases that participated in alveolar gas exchange. Phase III is commonly referred to as the alveolar plateau. This is the peak of carbon dioxide and is where the capnometry number is obtained. The last of the air that is obtained is referred to as PETCO2; or patient end-tidal CO2. The final phase is Phase 0. Phase 0 is referred to as the inspiratory downstroke. This is due to the fact that the patient has begun to inhale, and all CO2 from the patient has been drawn into the lungs. It is normal for humans to re-breathe a small portion of their tidal volume; as the lungs do not completely empty with each ventilation. This air is referred to as the functional residual capacity (FRC) and is normally around three to four liters in an average adult.
As either a prehospital or hospital base provider it is important to understand the different patterns that can present themselves during airway monitoring. I can not stress this enough. Much like a vent alarm it is important NOT to ignore any changes that you see in the capnography waveform. These changes could indicate a major or sudden change in your patient’s condition. A sudden decrease in your patient’s waveform could be as simple as a portion of your vent or CPAP circuit could have become dislodged. Or as serious as your patient going into cardiac arrest. There are various other waveforms that are possible. This subject alone is worth its own article. One of the most discussed, and taught waveforms is the classic shark fin. This waveform is indicative of a lower airway problem such as bronchoconstriction. Commonly seen in asthma patients this waveform is extremely important and can assist you in determining the aggressiveness of your treatment pathway. Should you notice your waveforms becoming consistently shorter you might want to consider a different or additional treatment path.
ETCO2 is not as simple as most providers believe it to be. It is always important to remember that every patient encounter is unique. This tool can be very useful for determining your patient’s general condition. As well as the effectiveness of your treatments. More articles will follow on the more advanced topics related to ETCO2.
As always, the content of these articles is for educational purposes only; and should not be used for clinical judgment or substitute the knowledge and experience of a good provider. We strive to push the best in evidence based medicine without bias; to help you obtain the best information possible. Please follow your agency’s patient care guidelines and procedures.
Brochard L, Martin GS, Blanch L, et al. Clinical review: Respiratory monitoring in the ICU—a consensus of 16. Crit Care. 2012 Apr 36; 16(2):219.
McSwain SD, Hamel DSS, Smith PB, et al. End-tidal and arterial carbon dioxide measurements correlate across all levels of physiologic dead space. Respir Care. 2010 Mar;55(3):288-93.
Whitaker DK. Time for capnography—everywhere. Anaesthesia. 2011 Jul;66(7):544-9.
Walsh BL, Crotwell DN, Restrepo RD, (2011) Capnography/Capnometry during mechanical ventilation: 2011. Resp Care. 2011 Apr;56(4):503-9.
Alabduladhem, Tamim O, and Bruno Bordoni. “Physiology, Krebs Cycle – StatPearls – NCBI Bookshelf.” The National Library of Medicine, The National Center for Biotechnology Information, 21 Nov. 2021, https://www.ncbi.nlm.nih.gov/books/NBK556032/.