Your 72-year-old male patient, Mr. Appleby, complains of progressive shortness of breath during your assessment. He repeatedly says, “Can’t breathe,” between difficult breaths. Chief complaint established. Now what? Through a rapid, focused exam, you note labored respirations at 22 per minute and diminished bilateral breath sounds with crackles. Or are those faint rales?
His concerned wife tells you he has a history of congestive heart failure (CHF). A quick downward scan shows you pedal edema. You announce your diagnostic achievement, “Your CHF is worsening, sir.” He gives you a blank stare. As your EMT partner applies a non-rebreather oxygen mask with 100% oxygen concentration, his wife asks you, “Will that make his COPD (chronic obstructive pulmonary disease) worse?” So much for a straightforward call.
Many EMS professionals measure their career progress by the number and type of psychomotor skills they perform. Counting airways placed, bags squeezed, chests compressed and deformed limbs splinted gives us a natural method of measuring at least a part of our EMS experience. But are these examples or other treatments rendered the most important part of our experience? Are they even the most important part of our patient care? Likely, they are not.
As the practice of EMS medicine continues to mature at an impressively fast pace, one skill becomes the foundation that all others rest on; it’s a skill heavy on “psycho” and light on “motor”—the skill of critical thinking.
How does this apply to our elderly male with CHF? Or does he have COPD? Could he have both? Or maybe his dyspnea isn’t due to either one. What else could be going on? Now we’re critically thinking and not just impulsively jumping to a quick conclusion, which is easy for us but often wrong for our patients.
We need more information about our patient. His blood pressure is 140/90, pulse is 100, and pulse oximetry reads 96% on the non-rebreather mask with high-flow oxygen (it was 92% on room air). Other medical history includes hypertension, diabetes and a heart attack. He takes multiple daily medications, including a diuretic, an angiotensin-converting enzyme inhibitor, an oral hypoglycemic, a beta-agonist inhaler and a steroid inhaler. He isn’t improving on the non-rebreather mask oxygen. So what’s the problem? CHF, COPD, both or neither?
OK critical thinking masters, you want some waveform capnography information to rapidly establish a more accurate diagnosis and related treatment? But wait, you’re on a BLS unit. Why do you want that waveform capnography information? Can you do anything with it? Assessment capabilities are clinically relevant only when your patient management can and will change based on the revealed information. Besides, waveform capnography isn’t even in the national scope of practice for EMTs. That’s one of those “paramedic things,” right? No, wrong!
It sure is a good thing we don’t all wait on national scopes of practice to define local standards of care. How would a national scope of practice get established in the first place unless someone dared to expand the “status quo?” So let’s do that for BLS professionals, at least in discussion, through a brief primer on capnography physiology, normal capnography waveform analysis and interpretation of at least one abnormal capnography waveform that could prove particularly relevant to the clinical case at hand.
Capnography is the measurement of exhaled carbon dioxide (CO2) in graphic waveform format. Technically, the isolated numerical value of the level of exhaled CO2 is so often interlinked with the waveform produced at the same time that we’re typically referring to both when using the term “capnography.” It’s believed to be particularly valid while interpreting exhaled CO2 to narrow a differential diagnosis of dyspnea.
Let’s look at an example of a normal physiology capnography waveform (see Figure 1). Moving left to right, we find the interval represented from Point A to Point B to be without measured CO2. This is the period without exhaled air, representing the period just prior to exhalation. Once exhalation starts, we would expect to see a rapid detection of CO2, as well as a rapid elevation in the CO2 levels. This is exactly what's seen in the waveform from Point B to Point C.
As exhalation continues, CO2 from the lowest airways, including alveoli, is sensed by an airway circuit detector. During this phase of respiration, represented from Point C to Point D, levels gradually approach the highest amount of CO2 expected before inhalation.
The pinnacle of respiratory cycle CO2 is at Point D, referred to as the end-tidal carbon dioxide (EtCO2). Perhaps an easier way to think of this concept is to expand the term to “end of the tidal volume of exhaled air” level of CO2. In normal human physiology, we can expect an EtCO2 level to be in the 35–45 mmHg range.
What happens to the measured CO2 level when we inhale? We should expect it would drop and typically, drop rapidly. That’s also what we see when looking at the waveform moving from point D (the end-tidal value) back to the baseline Point A.
The process (and waveform) repeats itself with every respiratory cycle of exhalation and inhalation. Reference texts exist to guide you through normal capnography waveform analysis on a far more detailed level, but the preceding will serve BLS personnel just fine in applying this assessment tool to some very sick patients.1,2
With normal waveform appearance and EtCO2 values in mind, let’s look at capnography waveform showing abnormal physiology (see Figure 2).
What’s the major difference in this example? The segment from point C to point D, which we just established is the segment that represents exhalation, is different. If the waveform’s rate of rise vs. time (also known as the “slope” of the waveform) is less than normal, then something must be slowing down the release of CO2.3
What kinds of things will slow exhalation of air? Bronchospasm and mucus are the first two things most EMS professionals correctly identify as exhalation inhibitors. Over time, as metabolism continues to produce waste products that change into CO2, levels of CO2 may rise unless exhalation becomes easier. Thus, although the slope from Point C to Point D on the capnography waveform may be slower, Point D may be much higher than normal, reflecting overall CO2 retention, also known as hypercarbia.
In this example, let’s say that Point D on Figure 2 is no longer 45 mmHg; it’s 70 mmHg. That sounds like a real problem. In fact, it sounds like it could be a real COPD-exacerbation-kind-of problem.
Let’s go back to our clinical case now that we’ve expanded your scope of practice—at least for Mr. Appleby’s treatment. You place a nasal cannula device with a sensor built in to measure exhaled CO2 (see Figure 3).
Your monitor shows capnography waveforms that look like Figure 2. Critical thinking says, “I heard a history of CHF and see pedal edema, but respiratory assessment with the additional data of waveform capnography now tells me COPD (CO2 retention) is likely the bigger problem here.”
What is your treatment plan now? Can you do something as an EMT to lower retained CO2 in a dyspnea patient? Your partner removes the non-rebreather mask as you prepare to do two things. You need to 1) assist the patient with his prescribed beta-agonist inhaler to open the lower airways; and 2) assist his ventilations with a bag-valve mask with supplemental oxygen. You purposefully hyperventilate the patient and see the capnography waveforms get shorter over time.
Consistent with shorter waveforms, EtCO2 levels fall to 55 mmHg. Mr. Appleby is breathing easier and is more responsive. We shouldn’t be surprised by his alertness because hypercarbia can be a common cause of altered mental status. Good job, and you did all that without a paramedic in sight.
OK fellow paramedics, let’s not get stingy with good assessment capabilities, right? Let’s see how BLS use of capnography can help us if we change just one vital parameter and one finding after examining Mr. Appleby.
An ALS-BLS crew arrives first to the same location. No history can be obtained from Mr. Appleby because he’s unresponsive. His EtCO2 value exceeds 100 mmHg. Although local ALS protocols may vary, most indicate endotracheal intubation (ETI) in this setting for two reasons—protection of the airway and little likelihood that bag-valve-mask ventilation will rapidly improve such profound hypercarbia.
A paramedic performs ETI and carefully confirms correct tracheal placement. Can waveform capnography help in that regard? Sure, because the stomach doesn’t rhythmically “exhale” CO2 up the esophagus. The lungs obviously do that very thing via the trachea; thus, waveform capnography is essential in advanced airway placement confirmation.4 Rhythmic rise and fall with exhalation and inhalation respectively gives all of us on scene great comfort that the endotracheal tube is correctly positioned. Therefore, waveform-capnography-educated EMTs can become expert airway placement lookouts.
Further, in most EMS systems with BLS and ALS response crews, it’s the EMTs who perform the majority of bag-valve-mask or bag-valve-airway ventilations. In those situations, who better to gauge ventilation effects than the ventilator? Waveform capnography is a great breath-by-breath (or ventilation-by-ventilation) feedback tool to guide ventilation rates and volumes.
Let’s put this last concept into practice taking care of a 22-year-old female victim of a motor-vehicle collision. She was ejected and sustained an obvious head injury. She’s confused with a Glasgow Coma Scale score of 12, which seems to worsen in short order. What could be happening? One possibility is increased intracranial pressure from the head injury. You use waveform capnography and see such waveforms as those in Figure 1 with EtCO2 levels of 45 mmHg.
Because normal EtCO2 is 35–45 mmHg, you might think that everything is fine ventilation-wise. Actually, it’s probably not. You need to intervene and work to lower suspected increased intracranial pressure (ICP).
One thing you can do to reduce ICP is to prevent cerebral vasodilation by reducing circulating CO2. How can you do that? What did you do for your COPD patient earlier? Hyperventilation will also work wonders for your head-injury patient.
As with all good things, excessive good efforts may produce bad results, specifically in this instance of hypocarbia-related cerebral vasoconstriction compromising cerebral perfusion. Waveform capnography allows you to more precisely gauge ventilation rate and volume effects.
Again, although local protocols may vary, most indicate to achieve EtCO2 values in the 30–35 mmHg range for acute head injury with increased ICP concerns. Some well-performed studies indicate some concerns that present methods of EtCO2 determination may not be accurate enough in certain trauma settings.5–7
Clinical adjustments that may be warranted from the findings in these studies are still unclear, although your medical director may be contemplating target EtCO2 values lower than historically recommended in light of these studies and hopefully further related investigations.
In many local EMS protocols, use of waveform capnography isn’t limited to the severely dyspneic or traumatized patient, and it isn’t limited to paramedic use. Here’s an example of one such protocol (see Figure 4, above).
This article introduces the concept of waveform capnography being appropriately included in the BLS scope of practice. Waveform capnography use substantially expands the assessment, diagnostic and management capabilities of EMS systems and professionals. These significant care benefits can be realized for a multitude of additional EMS patients when waveform capnography is also placed in the critical-thinking minds and important treating hands of BLS professionals.
1. Gravenstein JS, Jaffe M. Capnography. Cambridge University Press: Cambridge, UK. 2011.
2. Valente T. Capnography: King of the ABC’s. iUniverse.com. 2010.
3. Krauss B, Deykin A, Lam A, et al. Capnogram shape in obstructive lung disease. Anesth Analg. 2005;100(3):884–888.
4. Silvestri S, Ralls GA, Krauss B, et al. The effectiveness of out-of-hospital use of continuous end-tidal carbon dioxide monitoring on the rate of unrecognized misplaced intubation within a regional emergency medical services system. Ann Emerg Med. 2005;45(5):497–503.
5. Warner KJ, Cuschieri J, Garland B, et al. The utility of early end-tidal capnography in monitoring ventilation status after severe injury. J Trauma. 2009;66:26–31.
6. Jabre R, Jacob L, Auger H, et al. Capnography monitoring in nonintubated patients with respiratory distress. Am Journal of Emerg Med. 2009;27:1056–1059.
7. Delerme S, Freund Y, Renault R, et al. Concordance between capnography and capnia in adults admitted for acute dyspnea in an ED. Am Journal of Emerg Med. 2010;28:711–714.
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