BY JIM DAVIS
Defined as a graphical or numerical waveform that shows the continual presence of carbon dioxide (CO2) throughout ventilation, waveform capnography is considered the gold standard for assessing airway patency in intubated or spontaneously breathing patients. Unlike pulse oximetry, which measures how well available hemoglobin molecules are saturated with oxygen, capnography tells us how well CO2 gas is exchanging during ventilation. Originally developed in Holland, this technology became a standard of care in U.S. hospital operating rooms in the early 1990s. It is used today extensively in critical care and in procedural sedation and has evolved to field use. Capnography has been found to be easy and reliable to use and provides the earliest warning of respiratory depression, long before a decrease in pulse oximetry. It also gives objective data on the effects of treatment during the course of patient care (Figure 1).1
 Figure 1. A normal end tidal CO2 waveform. (Figures courtesy of Oridion Capnography.) |
The ventilation process requires the body to bring in oxygen and exhale CO2. For adequate gas exchange to occur, the body has to be able to produce, transport, and eliminate CO2. This depends on adequate oxygenation, adequate blood pressure, adequate circulating volume, and good ventilation. Any variance in these parameters will affect the ability to exchange gases. The amount of CO2 in exhaled breath is recorded as end tidal CO2 (EtCO2), which normally ranges between 35 and 45 mmHg and correlates very well with arterial blood gas levels.2 It is important for fire/EMS providers to recognize that although achieving a normal CO2 level is optimal, knowing why the values are out of line is much more important. In some situations, you may never get a value within normal limits.
CASE SCENARIOS
Consider the following case scenarios:
Shortness of breath.Your engine rescue arrives at a physician’s office for a report of shortness of breath. You are presented with a 49-year-old female who the doctor reports arrived in mild respiratory distress. She denies chest pain. She appears comfortable as you begin your assessment. Her vital signs are blood pressure 148/80, heart rate 100, and respiratory rate 26 with a pulse oximetry of 100 percent on room air.
During your assessment, the patient is placed on supplemental oxygen and a cardiac monitor. When placed on capnography using an EtCO2 cannula (designed for the spontaneously breathing patient), you find her EtCO2 at 14. While preparing for transport, you initiate intravenous access. You ask the physician about blood glucose; his staff opted not to check since the patient had no prior medical history. The crew packages the patient, thanks the office staff, and begins transport to a local emergency department.
En route, the medic crew reassesses the patient, concerned that she still is tachypneic, is slightly tachycardic, and continues to have an EtCO2 reading of 14 (meaning she is blowing off CO2). A finger stick blood glucose reads “high,” indicating a sugar greater than 500 mg/dl. A working diagnosis is now in focus. The crew increases the patient’s fluids to offset apparent dehydration and arrives at the emergency department without complications. This crew used waveform capnography to diagnose a new onset case of diabetes with ketoacidosis (DKA) that the family physician missed. But how did they do it?
When patients are in DKA, they mildly hyperventilate to blow off excess acid. This is referred to as Kaussmaul, or rapid and deep respirationshence, this patient’s respiratory distress. Remember, CO2 is normally between 35 and 45 mmHg. When a patient blows off CO2, as in this case, the level will decrease. Fluid shifts from the extracellular space to the intracellular space to combat dehydration caused by the elevated sugar levels. As the patient becomes more dehydrated, she will become increasingly tachycardic and her perfusion will worsen, further decreasing EtCO2.
Although this patient was a new onset diabetic, fire/EMS providers are more likely to encounter ill patients with diabetes for whom worried family or friends have called EMS. Quite frequently, these patients refuse aid. They have managed their diabetes for years and believe that they are quite capable of taking care of themselves. In these situations, waveform capnography is an excellent assessment tool to differentiate hyperglycemia from DKA. A Philadelphia study found that patients with elevated blood glucose and an EtCO2 of less than 29 are acidotic and in DKA 95 percent of the time.3 No patient with an EtCO2 greater that 36 was acidotic. These patients are beyond being able to take care of themselves at home by adjusting their insulin dosing. They need medical attention for insulin, fluids, and a determination of why they are in DKA. Infection is frequently the cause. A fire/EMS crew can use this technology to persuade a patient to accept transport.
Cardiac arrest. A rescue company responds to a 54-year-old male in cardiac arrest. As care is initiated, the medic and his field intern secure the patient’s airway. After visualization, the medic student, recognizing that waveform capnography is the gold standard for confirming placement of the endotracheal tube (ET), attaches the circuit to capnography. The medic reminds the intern that capnography does not confirm a completely secure airway (i.e., endotracheal tube through the vocal cords) but serves only to ensure that the tube is in proper position to ventilate the patient. The possibility remains, even with a good capnography waveform, that the ET tube is in the hypopharynx, allowing for adequate gas exchange, but not a completely secured airway.
The patient has an initial EtCO2 of 26. Principles of capnography used with cardiac arrest patients include the following:
- Patients in cardiac arrest will generally have low EtCO2 values because perfusion is poor (Figure 2). CPR offers roughly 30 percent of normal circulation, producing only one-third the normal exhaled carbon dioxide. In patients receiving good cardiac compressions, the value will be slightly higher. As rescuers tire, end tidal values decreasea good indication of when it is time to switch compressors. As a fresh rescuer takes over, you should see EtCO2 levels rise.
- Patients with return of spontaneous circulation (ROSC) often have a rapid rise in EtCO2 values several minutes before pulses become palpable (Figure 3).4 Metabolism is to CO2 what your thermostat is to your house. The higher you turn it up, the more production you get. As the patient’s own metabolism kicks in and excess CO2 is washed out of previously underperfused tissue, the value will rise. Noted EMS researcher Dr. Ray Fowler from Dallas, Texas, describes CO2 as “the smoke from the flame of metabolism.”(1) Note that administration of intravenous sodium bicarbonate produces CO2, causing dramatic increases in EtCO2 that mimic ROSC.
- In cardiac arrest patients with sustained end tidal CO2 values of 10 or less despite advanced cardiac life support (ACLS) therapy, resuscitation can be stopped. These patients are clinically dead, and one study found an EtCO2 cutoff of 10 made the difference between survivors and nonsurvivors both dramatic and obvious. (1) Of interest is the recent introduction of the impedance threshold device (ITD), used to improve perfusion during CPR. It remains to be seen what impact the ITD might have on using capnography to determine clinical death.
 Figure 2. Low end tidal CO2 levels seen in poor perfusion states. |
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 Figure 3. Elevated CO2 seen prior to ROSC during cardiac arrest resuscitation. |
Asthma.Your medic unit responds to a 26-year-old asthmatic in obvious distress. The patient has limited air movement and diffuse wheezing is noted. When placed on capnography, the patient’s tracing has a shark-fin appearance instead of the nice gradual upslope found on normal tracings. Patients whose waveform has a shark-fin appearance require bronchodilator therapy (Figure 4). (1)
 Figure 4. Shark-fin like appearance seen in patients with bronchospasm, asthma, and pregnancy. |
Continued observation of the waveform allows paramedics to gauge effectiveness of their treatment. In a patient responding well to treatment, the shark-fin waveform should return to a baseline normal waveform. In patients not responding to treatment, no improvement in the shark-fin waveform is seen. You may see continued increases in CO2 levels if the patient begins to hypoventilate, retains CO2, and gets tired. Shark-fin waves accompanied by rising CO2 values tell providers when to move on to other treatment options such as continuous positive airway pressure CPAP or intubation.
Patients without a shark-fin waveform and no evidence of wheezing may well be hyperventilating because of acute onset anxiety. Fire/EMS providers should remember that hyperventilation can signal serious illness as in the case of DKA or in patients with increased intracranial pressure. Also note that the capnography of pregnant patients may normally show a shark-fin waveform because the weight of the fetus does not allow the diaphragm to flatten out.
MVA with leg entrapment. As your busy day continues, you respond to a motor vehicle accident with entrapment. A female driver is alert and responsive on arrival, crying in pain. Her legs are pinned under the dash, and the vehicle has heavy front-end damage. As you assess her, you suspect multiple long bone fractures and decide that pain management and sedation are appropriate for the extrication. You know that narcotic pain medication and benzodiazapines are appropriate but have a propensity for causing respiratory depression. As the rescue crew works on extrication, space is limited and you need a means to continually monitor your patient’s respiratory status and airway.
Waveform capnography enables you to place the patient on the EtCO2 cannula used with the spontaneously breathing patient and monitor her respiratory effort while not physically watching her. Your only concern is to make sure the patient is not trapped in a position where managing her airway would be impossible if necessary. Waveform capnography is also prudent in the seizure patient given benzodiazepines to stop seizures. Since capnography has been shown to provide the earliest warning of respiratory depression, it is preferred over pulse oximetry for monitoring respiratory status.5
Multiple trauma. The next case involves a 20-year-old male involved in a motor vehicle crash. He was the restrained driver of a high-speed single vehicle that crashed into a utility pole. There is obvious mechanism of injury and the patient is slightly tachycardic at 114. His blood pressure is 100/70. You place him on capnography and find an EtCO2 of 18. You know that hyperventilation results in a low CO2, so you assess his respiratory rate and find it at 16. Why is he breathing normally and yet has a low EtCO2?
In this case, the patient has significant hypovolemia. Even though he is compensating at this point, his circulating volume is inadequate to transport CO2 back to the heart and lungs, where it can be eliminated. This concept applies to all situations involving hypoperfusion. Correcting the cause of the poor perfusion should produce a rise in the CO2 value. It is worth noting that patients with low EtCO2 values and hypotension have been found to fare worse in cases of blunt traumatic injury. In a 2004 study, only five percent of patients survived when their EtCO2 was 26 or less measured 20 minutes after intubation. (1)
Older medics probably recall when hyperventilation of head-injured patients was standard protocol. Today, we keep ventilations toward the low end of normal unless herniation is imminent. Hyperventilation decreases cerebral vessel size, which leads to decreased cerebral blood flow and increasing anoxia. Capnography assists with preventing inadvertent hyperventilation. In a study of 291 intubated head-injured patients, 5.6 percent of patients with EtCO2 monitoring were hyperventilated compared to 13.4 percent without.6
Morbid obesity. The next scenario illustrates a point that I cannot overemphasize. Your crew responds to a cardiac arrest. The intubation is difficult because of morbid obesity. The tube is visualized passing through the cords, and bilateral breath sounds are confirmed. You move the patient to the ambulance to initiate transport. On your arrival in the emergency department, the attending physician visualizes the endotracheal tube in the esophagus, and the patient is reintubated. Despite full ACLS, the patient expires. During the quality assurance process, the medical director reviews the run with the paramedics involved. They state that they really didn’t have time to get the patient on the capnography and that the tube must have dislodged when they were pulling him out of the ambulance on arrival in the emergency department because they knew it was in. Sound familiar?
There are several studies of esophageal placement of endotracheal (ET) tubes by EMS. Esophageal intubations are not the problem; every person who has ever intubated places an occasional tube in the stomach. The problem is failing to immediately detect and correct esophageal misplacements. A 2001 study in Orlando sent shock waves through EMS and led some EMS systems to stop intubating patients. Of 108 intubations, 25 percent were somewhere other than directly below the vocal cords.7 The authors determined that, with one exception, every patient with an incorrectly located endotracheal tube presented with the absence of CO2 exchange when placed on capnography. The single exception was a nasally intubated patient still spontaneously breathing with CO2 exchange.
Medics in the study system received additional training and education on airway management, and a protocol was implemented requiring the use of waveform capnography. One year later, a review demonstrated that they had completely eliminated the problem. A review of available research shows the following pattern:
- ET tubes can migrate up to five cm as a result of usual movements performed in the EMS environment. No other health care setting encounters this degree of tube migration simply because they do not move patients the way EMS providers do. (1)
- Capnography was 100-percent sensitive compared to colorimetric detectors (88-percent sensitivity) in a 2002 study by Grmec. Capnography continues to outperform other confirmation devices in every test. (1)
- The American College of Emergency Physicians has recommended the use of EtCO2 monitoring since 2001.8 Other devices are useful for confirmation of placement but do not offer the diagnostic and patient care information that waveform capnography does.
- Bad outcomes occur with esophageal intubations. In one study, 81 percent of patients with missed esophageal intubation died, 17 percent had permanent brain damage, two percent had some other permanent or partial injury, and 0 percent suffered no injury. (1)
- Missed esophageal intubations continue to plague EMS despite these recommendations, specifically in agencies that do not have waveform capnography. Other methods of confirmation are not always reliable. For example, abdominal distension was rated as normal in 90 percent of esophageal intubations, condensation of the ET tube was present 85 percent of the time, and breath sounds failed to identify 15 percent of esophageal intubations. (1)
The false positive rate for waveform capnography in detecting esophageal tube placements is extremely low and occurs generally in spontaneously breathing patients. Alcohol and antacids are both known to produce CO2. In this situation, an ET tube placed in the esophagus of a patient may briefly demonstrate an EtCO2 value until the stomach is washed out with 100-percent oxygen.9
When pulse oximetry was introduced, it took quite some time to understand its benefits and uses. It took longer to remember to use the new technology; today we wouldn’t think of going on a run without using pulse oximetry. Capnography occupies much the same place as pulse oximetry did some 20 years ago. It will become a mandatory part of advanced airway management for all practitioners and will be considered the ventilation vital sign of the future.
JIM DAVIS, RN, EMT-P, is a captain/paramedic and an EMS supervisor with the Columbus (OH) Fire Department. He is a flight nurse for Medflight of Ohio, an adjunct faculty member at Columbus State Community College, and a member of the Ohio State Board of EMS.
Endnotes
1. Jaffe, MB, DA Paulus. Capnography: Clinical Aspects. Cambridge University Press. 2004.
2. Levine RL, MA Wayne, CC Miller. “End-Tidal Carbon Dioxide and Outcome of Out-Of-Hospital Cardiac Arrest,” New Eng Journ Med. July 1997, 337:301-306.
3. Fearon, DM, DW Steele. “End-tidal Carbon Dioxide Predicts the Presence and Severity of Acidosis in Children with Diabetes.” Acad Emerg Med. December 2002; 9(12):1373-1378.
4. Deakin CD, DM Sado, TJ Coats, G Davies. “Prehospital End-Tidal Carbon Dioxide Concentration and Outcome in Major Trauma,” J Trauma. July 2004;57:65-68.
5. Krauss B, P Carroll. “Procedural Sedation and Analgesia: An Evolving Role for RCPs,” RT Magazine. June/July 2000.
6. Davis DP, JV Dunford, et al. “The use of quantitative capnometry to avoid inadvertent severe hyperventilation in patients with head injury after paramedic rapid sequence intubation,” J Trauma 2004; 56:808-814.
7. Katz SH, JL Falk. “Misplaced Endotrachael Tubes by Paramedics in an Urban Emergency Medicine System. Annal Emerg Med. 2001;37: 32-37.
8. American College of Emergency Physicians, Verification of Endotracheal Tube Placement, policy statement, October 2001.
9. Woods J., C Hsu. “The Effect of Antacid on Colorimetric End Tidal Carbon Dioxide,” Acad Emerg Med.2002; 9(5): 403.
