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For EMS, capnography can provide a useful window into metabolism, perfusion and ventilation.
There’s good reason capnography has been called the most vital of the vital signs. Its utility goes far beyond just confirming the placement of endotracheal tubes (though of course that’s important), and it can provide EMS providers valuable early and transitioning insight into patient status and serious conditions.
Optimizing the practice of capnography was the subject of a recent EMS1 webinar, “Mastering capnography in EMS – overcoming monitoring challenges.” Speakers Chris Kroboth, MS, FP-C, and Jeff Goodloe, M.D., FACEP, overviewed the important elements of capnography, shared tips for improving accuracy and reviewed several case studies. Kroboth is a veteran paramedic and firefighter and owner of Virginia-based LifeLine EMS Training and Consulting; Goodloe is chief medical officer for Oklahoma’s EMSA (Emergency Medical Services Authority), which serves Oklahoma City, Tulsa and several smaller communities.
CO2 levels provide insight
For EMS, Kroboth began, capnography provides a useful window into three things:
- Metabolism, because carbon dioxide (CO2) is produced by metabolizing tissues.
- Perfusion, or how well nutrients are being circulated to the tissues for metabolism to occur and CO2 is moving to the lungs for exhalation.
- Ventilation, or how CO2 is being exhausted out.
“It’s literally the thing that can give you three-plus sets of status on a patient,” Kroboth said.
Carbon dioxide originates as a byproduct of metabolism through the Krebs cycle, which produces hydrogen ions. These react with bicarbonate from the kidneys in the bloodstream and convert to CO2 and water. The CO2 is transported to the lungs and exhaled; the liquid is excreted through the kidneys via urination. This bicarbonate buffering system transports about 60% of the carbon dioxide the body produces.
CO2 is highly soluble in liquid, meaning its values and waveform can be used for patients with fluid in their lungs in cases such as flash pulmonary edema or drownings.
For accurate capnography, materials matter
That’s where capnography comes in. Sampling exhaled breaths allows evaluation of end-tidal carbon dioxide (EtCO2) levels, the concentration of CO2 at the end of exhalation. This produces both a numerical value and capnogram waveform that is extremely valuable to clinicians.
Doing that accurately requires a few things. One is the right choice of sampling set.
“It’s really critical you [have] a continuous sampling line from the atmosphere to your monitor,” Kroboth said. “Because if you have anything that has breaks … and it’s not continuous, you can allow for ambient air to enter that circuit and give you false readings at your meter.”
For many in EMS, inline sampling has fallen out of favor. The sensors can be bulky and cumbersome on the ET tube, potentially difficult in transport and affected by ambient light. Conversely, sidestream sampling – done by drawing a small sample of exhaled air from the breathing circuit – is smaller and more portable, works for both intubated and nonintubated patients, and is quick and versatile in all settings, including EMS.
If you’re using inline while bagging with a bag-valve mask, Kroboth advised, use a strap around the head to ensure a good seal and pay close attention to pre- versus postintervention values, as ventilatory management is more effective when titrated to EtCO2 rather than governed by more arbitrary numbers.
Patient comfort may also be an overlooked factor. The mouth scoops on some sampling sets can be rigid and uncomfortable. Medtronic‘s Microstream sampling lines, Kroboth noted, are softer and more compliant for patients and provide functional advantages as well, sampling through both nostrils and the mouth scoop. Alternatives may deliver oxygen through one nostril and sample CO2 through the other, which can be uncomfortable and, more important, impact accuracy. Medtronic’s version delivers oxygen via small holes that saturate the environment around the patient’s face.
‘Buyer definitely beware’
Safety is an issue too. Kroboth and Goodloe briefly addressed the standard warnings that often grace capnography packaging – including one about fire risk if using oxygen within three feet of an open flame, defibrillation equipment or anything producing sparks.
“Most defib cables are at three feet,” Kroboth noted. “We don’t ever measure how far the defibrillator pads are from the nasal cannula, but for most people that’s probably … about six, eight, 10 inches, maybe 12 inches if you’re running the Jolly Green Giant.”
If you try synchronized cardioversion while doing something like monitoring capnography on a v-tach patient with a pulse, he added, “It could be a bad day pretty quick.”
Goodloe’s EMS system uses more end-tidal CO2 circuits than any system in North America, he said – a testament to how seriously it takes capnography. That naturally led EMSA leaders to look at cost. What they found was, while there are discount capnography products available, they may sacrifice performance.
“What I’ll just say is, buyer definitely beware,” Goodloe said. “Compatibility is not the same thing as equality … I would encourage you to take a deep dive on your equipment – not just capnography circuits, but anything, really – and make sure that what you think you’re buying is what you really are. Many of these devices in the capnography world do not have the dual-channel sampling. They do not have the dual-channel oxygen delivery … and that is obviously not the same.”
Know the waveform
Kroboth explored interpretation of the capnography waveform and what its variations can mean. There are three main points to remember, he said:
- The D-to-E segment of the waveform, or righthand downslope, represents inspiration and should go straight down.
- It should return, at the E point, back to zero or baseline. If it doesn’t, it suggests the patient is rebreathing exhaled gas, which may signal an equipment problem or mask over the face preventing fully clearing the previously exhaled breath.
- The ensuing B-C-D phase should be distinct and clearly defined. A to B, or the start of exhalation, clears dead space air; B to C is the bronchial phase; and C to D is the alveolar phase of exhalation – where the most gas exchange occurs.
“D-to-E straight down, back to zero, B to C, C to D tells me I’m not rebreathing, tells me I have an unobstructed inhalation phase, and tells me, from an airway perspective, my airways are intact,” Kroboth noted.
Understand the capnography waveform, and it can provide great diagnostic value.
How the waveform changes is diagnostically valuable. A C peak that’s bigger than D signals alveoli are collapsed or atelectasized, as in severe emphysema. A low EtCO2 with a high respiratory rate and good waveform may reflect a panic attack or something more serious – if the patient had a blow to the head, for instance, it could be an early compensatory sign for intracranial pressure increasing with bleeding.
Providers should also understand the difference between the capnography waveform and capnography value displayed on the monitor – and pay attention to the waveform. The waveform is instant, reflecting each breath. The value is an average of the last 20 seconds, helping to account for patients talking, sobbing or with irregular respiratory patterns.
“If they stop breathing and the capnograph stays flat for 30 seconds … the ‘no breath’ alarm kicks off saying, ‘Hey, you need to check that patient,’” Kroboth said. “It’s good to know this because there are other things, like pulse oximetry, that can have up to a 90-second lag time. The longest thing you’re working with in terms of alarms here is 30 seconds. But the waveform is instant, and it’s visual for you.”
Added Goodloe, “Some of these patients we know are truly so dynamic that we don’t want to wait 30 seconds. We certainly don’t want to wait 90 seconds to have information we can act upon and intervene to promote better oxygenation and ventilation [and] ultimately better perfusion.
“[That’s] one of the key take-homes: Be sure to know what your device is showing you.”
If it’s atypical, Kroboth urged, check your patient – then check your sampling line.
“One thing could be … the patient fell asleep and the sampling line fell off. So when you have something that’s atypical or wrong, check the patient, check the sampling line and then compare the two.”
Tips to enhance accuracy
Some further advice for accurate capnography:
- Ensure compatibility between sampling line and monitor. Make sure attachments are secure (but avoid overtwisting, which can lead to breakage and leaking).
- When beginning capnography, allow a few seconds for the device to initialize. For most devices that takes less than 20 seconds.
“No different than the gas meter you’re taking to sample a gas leak environment on a 911 incident, you want it to calibrate to atmospheric norm,” Kroboth said. “The capno system is actually very forgiving. On the in-between cycles, it will actually monitor atmospheric normal … If you have the overzealous provider who applies the circuit to the patient first and then plugs it in, it’s OK. Give it 20 to 30 seconds and it will actually calibrate the atmospheric norm and give you accurate values. Just don’t take the first thing you see right off the jump.”
Case studies
The final section of the webinar reviewed some case studies. The first involved the familiar shark fin reading as an indicator of airway obstruction, a common finding in bronchoconstriction and what capnography is most noted for.
For EMS personnel, it’s worth investing in a good stethoscope, as the back of the ambulance can be a difficult environment for discerning subtle sounds.
The constriction that causes this occurs in the bronchioles and, by reducing air passage, reduces the C phase on the capnogram. These patients will begin breathing more rapidly, but their CO2 concentration will rise over time because their bronchioles are so tight they will retain CO2 by not being able to ventilate it out.
Severe lower airway obstruction may also produce a silent chest.
“To get a lung sound, you have to have enough airflow through the tubes to make resonance,” Kroboth said. “If it’s so tight that you’re not getting good airflow in and out … look over to the left on your monitor while you’re listening and make sure you have that end-tidal wave going across. If you have the end-tidal wave going across, you know they’re breathing; they just might not have enough tidal volume to … make the sound.”
For EMS personnel, he added, it’s worth investing in a good stethoscope, as the back of the ambulance can be a difficult environment for discerning subtle sounds.
Capnography can help providers prioritize treatment. In the severe bronchospasm patient, either a bronchodilator or subcutaneous epinephrine may be appropriate, Kroboth added, but preparing a dual nebulizer takes more time. Better progress can come from starting with the epi, then chasing it with the dual neb in the bronchoconstricted patient who is tachypnic, normocarbic or hypercarbic.
Remember that even with the presence of fluid in the lungs that’s severe enough to round the C-to-D portion of the end-tidal wave, the displayed value will still be accurate due to CO2’s diffusion ability through liquid. For these patients, consider CPAP. By inhibiting full exhalation, positive end expiratory pressure (PEEP) will cause EtCO2 to rise. That’s an appropriate course for CHF patients to keep alveoli open and oxygen diffusing, along with nitrates to decrease preload and a 15-lead EKG to rule out heart attack.
Capnography is very useful in sepsis. As the body battles its infection, it becomes increasingly hypermetabolic and acidotic. Blood vessels dilate, capillaries can start to leak, and the heart can become “floppy” – “You don’t get that good cellular function [and] contractility,” Kroboth said. Perfusion will fall, and perfusion becomes primarily heart rate-driven.
This patient will be breathing fast (more than 22 beats per minute) with low carbon dioxide. That should prompt EMS personnel to consider sepsis. Researchers have found correlation, Kroboth noted, between an EtCO2 value below 24 and septic shock. “One of the things capnography can really do for us is give us accurate respiratory rates, so we can appreciate when a person is compensating or upregulating in that instance,” he said. Start these patients with fluid, then address the vasculature. And “if the patient starts to pee on your cot because you’ve started to reperfuse their kidneys,” Kroboth added, “that’s a win … It sucks to clean up, but it tells you you’ve made headway by getting that good renal perfusion.”
The real decision-maker
Another case study involved an 8-year-old with exertional dyspnea playing basketball. His vitals: heart rate 108, oxygen saturation 94%, blood pressure 129/82. That saturation is low for a healthy child, Goodloe observed, but providers aren’t getting a full picture of the respiratory situation. Here adding the capnography value – 44 with severe shark fin, the high end of normal – reveals more bronchoconstriction than first thought.
“You [may] say, ‘Well, it’s just one more channel.’ It also happens to be the real decision-maker in cases like this,” Goodloe emphasized. “Don’t forget a liberal application of capnography in patients that are experiencing and presenting with respiratory distress or even just having a little trouble breathing. That could be a huge mask of what really is underneath from a pathophysiology standpoint.”
A final case study looked at capnography’s value in deciding if to terminate resuscitation in the field or not. This posed a male in his 50s down for an unknown period following cardiac arrest. Thirty minutes of ACLS yielded an end-tidal of 9, down from 11. With an EtCO2 that low that far into a resuscitation, Goodloe said, he’d be comfortable calling it for futility. “It’s not just the absolute number, but it’s the trending of what has happened,” he said.
That can hold true even for more normal end-tidal CO2 values in the 30s and 40s – for someone in cardiac arrest, for instance, or with a COPD history and values that started in the 90s. Interpret later readings in context, not alone, and if they’ve persistently declined despite prolonged resuscitative efforts and good-quality CPR, particularly with asystole or PEA, it can justify termination.
View the webinar
View the full webinar, including a concluding Q&A session, here and find more on Medtronic’s Microstream sampling line portfolio here.
For more information, visit Medtronic.
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