Capnography is a way to monitor carbon dioxide levels using waveform technology. See? I told you it was super cool! Why is this important? Think back to your gas exchange physiology…what is it that drives your need to breathe? Is it low oxygen? Nope. Is it high carbon dioxide? Yep! So, it stands to reason that monitoring the MAIN thing that spurs the respiratory drive would tell us loads about how our patient is doing in the ol’ gas exchange department.
When you use capnography to monitor your patient, you will be looking at the “end-tidal CO2” (ETCO2). Your “end-tidal” (as it is often called) is the amount of carbon dioxide present at the end of each breath. Staying with me? This handy number and its corresponding waveform can tell you a lot about your patient, and usually you’ll see it on the ETCO2 BEFORE you see it on your pulse oximetry. Now we’re beyond super-cool…we’re moving into Mega Cool territory here.
For the purposes of this blog post, we’re just going to cover the highlights of ETCO2…enough to “get you by” in your adult critical care rotation. If you want to jump down the rabbit hole, there’s a ton of great (and complex!) information at www.capnography.com, a really comprehensive and nurse-oriented website at Covidien, and a nifty article by the AACN here.
Normal end-tidal CO2 ranges from 35-45, and alterations can basically tell you quickly if your patient is breathing adequately. Let’s take a look at some of the most common causes of alterations in ETCO2 levels…to see examples of waveforms, check out the Covidien website, it’s awesome!
- A decrease in your ETCO2 with a partial loss of your waveform: consider a partial airway obstruction (airway collapse or secretions in ET tubing if intubated)
- A gradual decrease in ETCO2 to < 35 mmHg: consider hyperventilation as the cause
- A sudden drop to a very scary low number and a loss of the waveform: apnea…are they even breathing? Better go check!
- An increase in ETCO2 (above 45 mmHg): air trapping such as in bronchospasm or asthma. The waveform takes on a distinctive “shark fin” appearance when this occurs. Increased ETCO2 can also be due to rebreathing of CO2
- A gradual increase in ETCO2: consider hypoventilation due to sedation
Ok, now we get to the absolutely coolest thing you can use ETCO2 and capnography for…CPR! Adequate, excellent, hard-and-fast CPR will produce ETCO2 levels that are closer to normal (20 to 30-ish), while lack of CPR (or really bad CPR) will produce ETCO2 levels that are practically nonexistent. The point is, keeping an eye on your ETCO2 during CPR will let you know if the compressions are adequate AND will alert you when there is a return of spontaneous circulation…your ETCO2 will suddenly perk right up to normal levels. Now how amazingly cool is that?
I realize this just touches the tip of the iceberg when it comes to capnography and ETCO2…but when you see it in your critical care rotation you’ll have a basic understanding of why it’s being used and what it can tell you. If the subject interests you, check out those links and impress the heck out of everyone you see (even the respiratory therapists!).
Be safe out there!