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By Joel Koehn
What is capnography and when should we be using it? Capnography is a non-invasive method for monitoring how much CO2 patients are exhaling. It has become a widely used and very important tool to use in prehospital care. It is relatively simple to use and can be used in many different patient scenarios, including monitoring CPR effectiveness, ETT placement and diagnosing complex medical conditions.
Although capnography is not able to tell everything we need to know about all our patients, for many patients, capnography is one of the most reliable and real-time measurements that we can use in the prehospital setting to assess our patients’ status and the effectiveness of our treatment.
Pathophysiology of respiration
Oxygen is inhaled into the lungs during normal respiration and then makes its way down through the bronchioles and into the alveoli, where the exchange of oxygen and carbon dioxide takes place. Once this exchange has taken place, the oxygen is then transported to the cells in your body by the hemoglobin. At the cellular level, the oxygen is then combined with glucose and other nutrients to create energy or adenosine triphosphate (ATP). A byproduct of this process, which is called aerobic metabolism, is carbon dioxide.
This CO2 is then transported back to the alveoli by the hemoglobin to be exchanged for oxygen and then exhaled. If this process is disrupted by either a disease process or changes in ventilation status, it causes an increase in CO2, which then creates an acidic environment in the body.
Using ETCO2 to guide ventilation and resuscitation
ETCO2 is a real-time measurement of a patient’s ventilatory status, unlike SPO2, which can take a few minutes to reflect a change. Through monitoring ETCO2, we can guide both our manual and mechanical ventilation by maintaining an ETCO2 of 35-45 mmHg and adjusting our ventilation rate to change this.
Monitoring ETCO2 can also be very useful in cardiac arrest patients. A sudden spike in your end-tidal readings is an early sign of ROSC and you should be checking for a pulse. End-tidal readings can also be used to monitor the effectiveness of and correct automated CPR device or hand placement. If you have your CPR device too low, you will see very low readings of ETCO2 due to not getting an effective squeeze of the heart, which limits the amount of blood circulating in the body and the ability to distribute oxygen and blow of carbon dioxide. This would also tell you if your compressions do not have adequate depth while performing manual CPR.
One of the most common uses for monitoring ETCO2 is for confirming the placement of an ETT or supraglottic airway. In the past, a simple colorimetric device was used, which would change color at the introduction of CO2. These were noted to have some false positives as acid from the stomach could also trigger these. The current standard is continuous monitoring of capnography, which gives you a numeric reading and also a waveform, so the provider is able to better judge the placement of an airway and effectiveness of ventilation.
An absence of ETCO2 is a positive indicator of the ETT being placed into the stomach. All patients being ventilated through an ETT or supraglottic airway should always have continuous ETCO2 monitoring. Any sudden drop in your reading should immediately prompt you to re-evaluate the placement of your airway. If placement is still confirmed to be correct, you then need to start down the airway troubleshooting checklist to find out why the ventilation is no longer effective.
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Confirming your diagnosis with ETCO2
Capnography alone cannot diagnose complex medical cases, but it is an assessment tool that can give us some good clues.
We can use capnography to confirm asthma or COPD in a difficulty breathing patient by noting a waveform that appears like a shark fin instead of a normal square. This is a sign that the patient is trapping air in the lungs and is having a difficult time exhaling CO2. If this waveform improves with the use of a breathing treatment such as a DuoNeb, that is a good sign that the treatment has been effective and has opened up the airways. If the waveform does not change, you should consider another breathing treatment or more aggressive treatment measures.
If you have a patient who has a low ETCO2 despite adequate ventilation, this can be a sign of poor perfusion due to pulmonary embolism, as the embolism blocks the adequate exchange of oxygen and CO2. If you were to get an arterial sample of CO2, this would be higher due to not being able to exhale the amount of CO2 that the body needs to.
End-tidal carbon dioxide can be a measure of the severity of DKA and can guide your preferred resuscitation fluid in these patients. In early stages of DKA, the body compensates for the acidosis by hyperventilating to remove the acid in the form of CO2. This will result in a low ETCO2. As the body is unable to continue to compensate, breathing slows, which causes an increase in acid and a higher exhaled CO2.When you have identified this extreme acidosis, you should consider for resuscitation a fluid such as lactated ringers, which has a pH closer to normal and therefore will not drastically increase acidity.
In a patient where you suspect sepsis, you can use ETCO2 as another marker and early indicator of shock. Due to the poor perfusion status and inability to effectively move CO2 to the lungs for discard, you will see a drop in ETCO2 in these patients.
The last condition I will touch on here is monitoring cardiac patients for shock and heart failure. If you are caring for a patient you have identified is having an MI or severe CHF, you can help determine the severity of the condition. As cardiac cells die, the heart starts to lose its contractility, as it is not able to circulate blood as effectively, which will in turn lead to a decrease in CO2 which is being exhaled.
Making capnography common practice in EMS
We have only touched the surface for the uses of capnography in the field. I am positive that if you implement widespread use of monitoring capnography in your service and use it to guide your treatment, you will provide better patient care and see better patient outcomes. I encourage you to further explore the topic of capnography and if you have not implemented many of these practices in your service, please have a discussion with your service director and medical director about making this a common practice.
ABOUT THE AUTHOR
Joel Koehn is a career critical care paramedic and assistant chief of his local volunteer ambulance service.
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