Dude. The continuous ETCO2 (capnography) monitor is a goldmine of information but for many years was an under utilized tool in my toolkit. In large part because I lacked practical experience with its use, and in small part because I did not understand its capabilities. I saw this used by pre-hospital providers more frequently than in the hospital. Now, I love to use it in certain situations as it is an earlier marker and more sensitive monitor for badness (yes, that’s an official term coined by yours truly to encompass all sorts of near death or death harbinger like occurrences).

My favorite scenarios for use:

1. Monitoring the metabolic acidosis patient whose airway is a concern or who I am intubating

2. During cardiac arrest to guide resuscitation efforts

End tidal carbon dioxide (ETCO2) measures exhaled carbon dioxide, it therefore reflects the efficacy with which CO2 is pumped back to the lungs (indirect measurement of cardiac output) and then exhaled (ventilation). Waveform Capnography is a continuous tracing, or waveform, of ETCO2. It is very useful when assessed as a trend.

It is displayed on most ventilators (useful for confirming tube placement amongst other things) or it can be used as an independent monitoring “box” on a non-intubated patient. It is represented as a rectangular waveform, and also quantified with a numerical value: ETCO2 (nml 35-45mmHg). At first I had a hard time wrapping my brain around the normal values. I thought if a patient was hyperventilating they would be getting rid of high levels of CO2 so the number would be high. In actuality, think of it like interpreting an ABG: if the CO2 is low they have already blown it all off - they are hyperventilating. Once I started envisioning it as the downstream effects of what is going on - the CO2 is already gone and we are now measuring what’s left - it makes sense that the value would be low.

By monitoring limits; following trends; watching out for alarms you can garner data to support a diagnosis. For example, the bronchospastic pt may have a high retained ETCO2 and a shark fin appearance to the waveform tracing. See below image.

The measured ETCO2 is a far more sensitive indicator of ineffective ventilation (see hypoventilation and apnea waveforms above) than the later occurring consequence of reduced SPO2. Remember, capnography measures ventilation and the pulse oximeter measures oxygenation. If poor ventilation is the cause of hypoxia, you will see a change in ETCO2 long before you see the downstream effects of reduced SPO2.

That is why I love it for monitoring a patient with apnea, or a tenuous airway. For example, the over-sedated patient or the poorly responsive patient. Hook up an end tidal and watch the trends. Compare it to the measured respiratory rate; if you see a sudden drop in RR and increase in ETCO2 you can intervene quickly. During peri-intubation I can follow the trends to guide how quickly I bag.

Now apply this to the patient who is in metabolic acidosis who needs vigorous respiratory efforts to compensate. These patients are always marked as tenuous for intubation because they are working hard (which is a good thing) but becomes tiresome. Your job is to assess when the fatigue is tipping the scales into dangerous territory. This is even further complicated in a patient who has encephalopathy (a common occurrence in sepsis) as the GCS is reduced. Once you make the decision to intubate you must proceed with extreme caution. This person needs the heavy breathing (kussmauls), aka respiratory alkalosis, to compensate for the metabolic acidosis. When you take it away (sedate, paralyze) in order to intubate you risk losing the inherent over breathing, so it is imperative you effectively bag the patient and quickly establish an airway. If not, they begin to develop a respiratory acidosis (see the hypoventilation/apnea waveforms above) from retained CO2 which compounds the metabolic acidosis, dropping the pH and setting them up for cardiac arrest.

So when monitoring the at risk patient, or during peri-intubation:

• Look at the initial trends of ETCO2

• Monitor the values/trends over time

• If you see a sudden rise in the value, you know it’s time to intervene

• Reverse sedation, start bagging or bag more vigorously, intubate

• These are the patients you want first pass success with intubation. Not an opportunity for a novice to mess around with

• These are also the patients you should hyperventilate to help them compensate. Shoot for a lower than normal CO2

• If you are seeing numbers that are high persistently - monitor your BP and HR closely. Acidosis is brewing and a code may be imminent.

My second favorite way to use this tool is during resuscitation efforts. ETCO2 is a surrogate marker of metabolism, circulation, and ventilation so in a cardiac arrest situation this is a great value to monitor. Normal CI is 2.5-4L/min but even with effective compressions during CPR the best we can achieve is estimated to be a CI of 1.5-1.9L/m correlating with an ETCO2 of 20mmHg. AHA recommends achieving at least 1/4 of normal cardiac output so ETCO2 of 10, ideally more than 20. There are two main benefits for utility:

• Monitoring quality of compressions. If the compressor is not achieving at least an ETCO2 ~15, a gentle nudge to improve depth/rate or suggest a new compressor to alleviate fatigue

• Watch for sudden increase in the number/waveform. A jump from 15 to 35 is not related to a super human providing high quality compressions. This reflects return of inherent circulation. Pause CPR and check a pulse/blood pressure.

• A persistently low ETCO2 (less than 10) throughout the code is an indicator that the patient is more likely to die and could impact your decision to terminate resuscitation efforts.

These are just a few examples of how to use this tool. There is so much more you can do. Here is a link to the medtronic website which provides a useful review of this specific brand. And here is a link to EM article which provides a great overview of scenarios in which it can be helpful. What are your thoughts? Do you use it and what’s your favorite tip?