It’s fall—also known as respiratory season—and you’re responding to an emergency call for a 50-year-old male patient who has severe shortness of breath. On scene, you're met by an excited woman who’s yelling, “Hurry! He’s really bad this time.” You get a déjà vu feeling.
You’re led to a man who’s sitting on the couch in a tripod position in obvious distress. His lips look dusky and slightly blue. He has on a nasal cannula, but the oxygen (O2) bottle is empty. On the end table are two inhalers: albuterol and DuoNeb.
After some questions, your EMT partner grabs the monitor and applies supplemental oxygen at 5 liters per minute (LPM) via a nasal cannula that also measures capnography. Pulse oximetry is applied, and the patient is prepped for a 12-lead ECG. The initial printout includes three parameters: heart rate, pulse oximetry and capnography, (see Figure 3, p. 16). The patient is in sinus tachycardia at 120, his pulse oximetry reads 75%, and the capnogram shows obstructed hypercapnia and an end-tidal carbon dioxide (EtCO2) reading of 100 mmHg.
Recognizing the severity of the situation, you quickly locate a nebulizer mask assembly, place albuterol and ipratroprium in it and apply it to the patient. As you auscultate the lungs, you hear coarse crackles in the upper lobes and expiratory wheezing lateral vesicular. After four minutes, the patient is able to speak in full sentences, his pulse oximetry has increased to 94%, and the capnogram shows normal waveform with an EtCO2 of 48. His heart rate has slowed a bit to 100. Your transport time of 10 minutes is uneventful, and the patient is turned over to emergency department staff.
Respiratory emergencies are a common complaint that EMS providers face daily. Over the years, we’ve been taught to assess patients based on subjective criteria, including respiratory rate, observed work of breathing, accessory muscle use, auscultation for breath sounds, skin signs and mentation. With technology, we’ve added heart-rate monitors and pulse oximetry. Even with all of these assessment tools available, the lack of a comprehensive objective tool has left much to be desired, completing a differential diagnosis has been difficult.(1,2)
Capnography represents an important clinical upgrade. It can be used as a triage tool for the severity of respiratory emergencies and be a factor in determining initial therapy.(3) It can also be used as a tool to track the effectiveness of therapy. It doesn’t replace traditional assessment techniques. Rather, it enhances the clinical assessment. It’s an objective measurement of fundamental life functions: airway patency, breathing adequacy and circulatory efficiency.
A Life Process
Breathing is a chemical thing. The essential stimulus to breathe in the healthy adult comes from CO2 levels in the brain and pH of their cerebral spinal fluid (CSF). When these levels increase, chemoreceptors report it to the medulla, and this triggers respiratory effort.
The inhalation of O2 assists in CO2 elimination. O2 transported to the cells is used for metabolism. As a byproduct of metabolism, CO2 is offloaded from the cells into the blood and carried as bicarbonate to the lungs, where it’s eliminated. We can then measure what comes back. This is the “circle of life,” so to speak. So capnography helps you measure the fundamental life process.
If someone loses their stimulus to live, they have clinical depression. If they lose their stimulus to breathe, they have respiratory depression. If they have respiratory depression, they don’t have the “desire” to eliminate EtCO2, and their levels go way up. This is one way we can use capnography as a triage tool in patients who may be under the influence of “something.” This illustrates how capnography can also be used to assess the adequacy of the patient’s breathing or your assisted breathing.
Early capnometry, as we know it in EMS, was qualitative: We used to just watch a litmus paper change from purple to yellow. This was only possible on the intubated patient. However, technological advances have now made it possible to monitor non-intubated patients.
Early sidestream technology, and now Microstream technology, have made it possible to assess all age groups, intubated or not. This is accomplished by a nasal or nasal/oral cannula that captures exhaled CO2 from the nose, mouth or switch breathers. EMS providers can even deliver oxygen via this cannula at the same time they’re reading the EtCO2. This special cannula can also be used with a non-rebreather mask and with a continuous positive airway pressure (CPAP) device.
Understanding CO2 Values
The normal value of CO2 in your body is 35–45 mmHg. In cases of normal perfusion, EtCO2 (what you exhale) should be within 5 mmHg of the CO2 blood levels. The mean difference is 2 mmHg. EtCO2 that is greater than 45 mmHg is known as hypercapnia, and EtCO2 that’s less than 35 mmHg is called hypocapnia.
Clinically speaking, if a patient has hypocapnia, there’s usually one of three conditions contributing to it: hyperventilation, hypoperfusion or hypothermia.
Hyperventilation occurs when a patient blows off more CO2 than they’re making.
The underlying condition could be hypoperfusion. Remember, if CO2 doesn’t get back to the lungs, it can’t be blown off (or detected) by EtCO2 monitoring. So consider shock, pulmonary embolism, hypotension and pulselessness. Always take capnography readings in the context of the rest of the exam.
Hypocapnia could also be caused by hypothermia because of decreased metabolism, which produces lower amounts of CO2.
All acute patients with hypercapnia are considered to be “sick.” This can indicate hypoventilation (i.e., patients make more CO2 than they blow off). High levels of exhaled EtCO2 mean high levels of CO2 in the blood. This leads to acidosis.
You’re probably thinking, “What if they’re a CO2 retainer?” Good question. A CO2 level of 50–60 could be normal for them because their medulla is accustomed to high levels over a long period of time. How can you tell? Well, if they’re indeed a hypoxic breather, giving them oxygen could depress their stimulus, causing the CO2 to go up.
A partial pressure CO2 (PaCO2) level that’s above 50 represents ventilatory failure. The patient discussed at the beginning had an EtCO2 of 100. This is critically dangerous. Add an SpO2 reading of 75%, and you have respiratory failure, which requires bold, aggressive management.
An EtCO2 value without a waveform is like a heart rate without an ECG. Capnography measures CO2 levels and draws a picture of the CO2 flow over time. CO2 comes from the alveoli. So if the capnogram is a square, obstruction to the flow of CO2 isn’t occurring. On the other hand, bronchospasm produces uneven alveolar emptying and, thus, an uneven capnogram.
This means some alveoli rapidly purge their CO2 and others may be more constricted, so it takes longer to empty their CO2. This is what produces the severe angle to the upstroke and plateau on the waveform, (see Figure 1, p. 15).
Bronchospasms respond well to bronchodilator therapies, such as albuterol and or ipratroprium. In our case, the patient had a bronchospasm with respiratory failure. The rapid triage with capnography gave the paramedic objective evidence of what the problem was, its severity and how to treat it. Further, patient observations and the capnogram demonstrated objective proof that the patient did have a bronchospasm, and it was relieved by albuterol/ipratroprium, (see Figure 2, p. 15).
This case is an outstanding, real example of the amazing benefit of capnography for patient triage, determination of severity, therapy and post-therapy monitoring. Special note: Some of the more traditional assessments were deferred because of this patient’s severe condition. For example, the crew started therapy before listening to breath sounds. Remember, capnography is a clinical upgrade. The objective findings of respiratory failure because of bronchospasm needed immediate bronchodialator therapy.
Disclosure: The author has reported no conflicts of interest with the sponsors of the supplement.
This article originally appeared in the December 2010 JEMS supplement “Measuring Life & Breath” as “A Form of Triage: Capnography use for the conscious and non-intubated patient.”
1. Ackerman R, Waldron RL. Difficulty breathing: Agreement of paramedic and emergency physician diagnoses. Prehosp Emerg Care. 2006;10:77–80.
2. Lett D, Petrie DA, Ackroyd-Stolarz S. Accuracy of prehospital assessment of acute pulmonary edema. Abstract. CJEM. 2000;3:1423.
3. Corbo J, Bijur P, Lahn M, et al. Concordance between capnography and arterial blood gas measurements of carbon dioxide in acute asthma. Ann Emerg Med. 2005;46:323–327.