The goal of this supplement is to review key aspects of capnography, its powerful assessment capabilities on intubated and conscious patients, and its importance as a prehospital triage and treatment guiding tool.
Capnography provides valuable and rapid assessment information that greatly assists EMS providers and enables them to develop, monitor and modify patient care plans. This valuable assessment tool supplies immediate breath-to-breath information about the patient’s respiratory status: Are they being adequately ventilated? Are they breathing too quickly or slowly? Are they experiencing bronchospasm?
Capnography also provides you with key information about the patient’s circulatory status, such as whether they have adequate cardiac output and are perfusing well. In addition, it gives you key information about the patient’s metabolic status, such as whether they have normal metabolic activity or if it increased or decreased.
This information, when combined with the patient’s history and your physical assessment, can provide you with an accurate working diagnosis of the emergency condition. Once that clinical observation is made, you can initiate the appropriate treatment for that condition. Then you can continue to observe the capnography trend and use it to assist you in determining the effectiveness of the treatment and guide you in continuing or adjusting your treatment as required.
These outstanding assessment capabilities exhibit why capnography is required in multiple states and on every ALS unit in Europe.
Capnography offers a quantitative numerical reading and graphic waveform that measures, illustrates and documents your patient’s exhaled carbon dioxide (CO2). All living human beings produce CO2 as a byproduct of metabolism. The carbon dioxide, once produced, is diffused into the blood and transported to the lungs via the circulatory system. It’s then released by the alveoli and eliminated from the body during exhalation.
Therefore, capnography enables you to evaluate the current status of the patient’s ventilatory, circulatory and metabolic systems by measuring the exhaled CO2 and graphically depicting its path of exhalation.
Capnography not only provides you with a rapid and reliable assessment of the patient’s ventilatory, circulatory and metabolic function, it also—and more importantly—represents real-time information regarding CO2 exhalation and respiratory rates.
In the articles that follow, several actual cases will illustrate the effectiveness of capnography as an assessment and monitoring tool in the field.
The measurement of CO2 content in each exhalation reflects the CO2 produced by metabolism, transported by the circulatory system and exhaled by the respiratory system at the time of that particular breath. This allows you to make rapid adjustments in your treatment based on current information.
It’s important you understand capnography’s numerical readings and graphic waveforms to use it effectively. The numeric readings are derived from a point in the respiratory cycle known as the end-tidal CO2 (EtCO2). This is the point at the end of exhalation when the CO2 reaches its highest concentration. This concentration is generally in the range of 35–45 mmHg.
The reading closely correlates to the CO2 levels measured with arterial blood gases in patients with normal lung function and also in patients with abnormal lung function due to conditions other than a ventilation-perfusion mismatch.
In patients with a ventilation-perfusion mismatch, the gradient between the ventilation and the perfusion will widen based on the severity of the mismatch. In those cases, the EtCO2 should be used to trend the ventilatory status. An elevated EtCO2 level is typically an indication of hypoventilation or increased metabolic activity. A low exhaled CO2 level may be an indication of hyperventilation, decreased cardiac output or poor pulmonary perfusion, which can occur in shock.
The capnography waveform can be compared to an ECG because the “normal” waveform has certain rules. Each waveform represents the various phases of inhalation and exhalation and is divided into four phases, (see Figure 1 above).
Phase I (A–B) occurs at the beginning of exhalation when no CO2 is present in the upper airway, trachea, posterior pharynx, mouth and nose. No gas exchange occurs in these areas, so this “dead space” is represented as the flat baseline of the waveform.
Phase II (B–C) is the ascending phase. During this phase, CO2 from the alveoli begins to reach the upper airway and mix with the dead space air, causing a rapid rise in the amount of CO2 detected.
Phase III (C–D), the alveolar plateau, reflects a uniform concentration of CO2 from the alveoli to the mouth and nose. It culminates with the EtCO2 (D) at the end of the exhalation, which contains the highest level of CO2.
Phase IV (D–E) is the descending phase. It’s when inhalation begins. As oxygen fills the airway, the CO2 level rapidly returns to 0, or back to the baseline. This box-like appearing waveform is the classic, normal capnography waveform.
There are three basic abnormal capnography waveforms:
1. Hypoventilation waveform;
2. Hyperventilation waveform; and
3. Bronchospastic waveform.
The hypoventilation waveform, related to a decreased respiratory rate, will have fewer waveforms, with each presenting increased height due to the presence of more CO2 per breath, (see Figure 3, p. 8). There are, however, other reasons for an increased EtCO2 and increased waveform height. These include a decreased tidal volume with or without a decreased respiratory rate, an increased metabolic rate and an increased body temperature.
The hyperventilation waveform, related to an increased respiratory rate, will have a higher number of waveforms with a decreased height of the waveforms due to the presence of less CO2 per breath, (see Figure 2). As mentioned earlier, other reasons for a decreased EtCO2 and decreased waveform height include increased tidal volume, a decreased metabolic rate, a decrease in circulation and hypothermia.
The bronchospastic capnography waveform is recognized by a shark-fin shape instead of the normal box-like waveform, (see Figure 4, p. 9). This is because bronchospasm causes a slower and more erratic emptying of CO2 from the alveoli, which results in a slower rise in the expiratory upstroke.
Other, less common waveform abnormalities do occur, so read “Gotcha!” on p.18 of this supplement for more about those abnormalities.
By now it should be evident that capnography is indicated in almost every prehospital emergency. What other tool can provide you with a real-time window into your patient’s ventilation, metabolism and circulation? Capnography helps you take remedial intervention in hypoxic states before irreversible brain damage occurs, and it’s proven to be a better indicator of hypoxia than clinical observation alone.(1)
Initially, capnography was primarily used by anesthesiologists in the operating room to monitor the respiratory status of intubated or mechanically ventilated patients. Eventually, it was adopted for this same purpose by prehospital EMS systems.
Using capnography to ensure successful intubation has become the gold standard and is now required in most EMS systems. The detection of CO2 on expiration is a completely objective confirmation of tracheal intubation.(2)
Because capnography indirectly correlates with cardiac output under conditions of constant ventilation, it has many extremely beneficial uses for cardiac arrest patients (as referenced in the 2010 AHA CPR and ECC Guidelines):
1. Determine the effectiveness of cardiac compressions. For example, during CPR, providers will be able to visually see a gradually decreasing waveform height as the rescuer providing compressions tires. This allows for an early warning to change compressions;
2. Recognize the return of spontaneous circulation (ROSC) via digital and graphic waveform readings presented; and
3. Assist prehospital crews to make decisions about terminating resuscitation.(3)
Capnography can also be effectively used with the Combitube, the laryngeal mask airway or any supraglottic airway device. It can also be used with bag-valve-mask ventilation.
In addition to being an essential tool in intubated patients, capnography is quickly becoming a valued assessment method in the non-intubated patient. The noninvasive monitoring capabilities of capnography have contributed greatly to the care and survival of acutely ill patients of all age groups and conditions.
Capnography can be effectively used during the assessment and treatment of asthma and chronic obstructive pulmonary disease (COPD) patients to detect the presence and severity of bronchospasm. It can also guide treatment decisions when the shark fin denoting bronchospasm doesn't improve or even worsens, (see example, p. 6).
This capnography use can be particularly helpful in determining when, or if, to move to the next level of treatment, including intubation or continuous positive airway pressure (CPAP). The capnography level of patients receiving CPAP treatment can be continually monitored through use of a special nasal/oral cannula.
Patients who are sedated or receiving pain management can be monitored for hypoventilation, and capnography can assist in decisions regarding continued administration of sedatives or pain control.(4)
In these cases, hypoventilation will be demonstrated by gradually increasing EtCO2 and waveform height, allowing the provider to terminate the administration of the central nervous system, depressant medications and assist ventilations or intubate when indicated, (see example p. 8).
Decisions regarding the need for intubation or assisted ventilation for the overdose patient can also be guided by capnography, particularly as respiratory drive decreases.
Patients who are hyperventilating and exhibiting anxiety can be particularly difficult to diagnose. Determining whether you’re dealing with a psychological event or severe pathophysiology can be challenging in the presence of disease processes that have few clinical findings, such as the patient with a pulmonary embolus.
Capnography can assist you in determining a clinical pathway for these patients, because hyperventilation with normal or high EtCO2 levels is much more likely to reflect pathology, whereas hyperventilation with low EtCO2 levels is more likely to reflect anxiety, (see example p. 5).
Capnography waveforms can also be used as a biofeedback technique when coaching anxious patients to decrease their respiratory rate. Have the patient view the rapid waveforms on the screen and attempt to slow them. This can be quite effective.
Capnography can also be used effectively in patients with diabetes mellitus, especially to evaluate the patient for diabetic ketoacidosis.
A 2002 study demonstrated that in diabetic children presenting to the emergency department with an EtCO2 of less than 29 mmHg, 95% were in ketoacidosis, whereas if the EtCO2 was greater than 36 mmHg, no ketoacidosis was found.(5)
Capnography works effectively in determining the treatment for sympathomimetic overdoses. This includes the administration of benzodiazepines, which can be guided by the extent of the increase in metabolic rate, reflected in the amount of increase in EtCO2.
The severity of hyperthermia and hypothermia can be assessed with capnography, and it can help you adjust your treatment of a patient. It can also determine the severity of metabolic acidosis in gastroenteritis, especially in children.5
Capnography provides an indirect, real-time window to your patient’s circulatory status and is an excellent way to trend for all types of potential shock states.
Because it reflects circulatory status, and indirectly reflects cardiac output, EtCO2 may decrease before changes in systolic blood pressure occur. For this reason, capnography should be carefully monitored in patients with acute myocardial infarction, (see Figure 3, p. 8). Capnography can be particularly helpful in assessing the circulatory status of the patient experiencing a right-ventricular infarction or an inferior-wall myocardial infarction with right-ventricular involvement, because these patients often require large amounts of IV fluids to maintain adequate perfusion.
Patients in congestive heart failure (CHF) can present challenges regarding circulatory status and also treatment decisions regarding shortness of breath. Although the mainstay of treatment for difficulty breathing in many EMS systems continues to revolve around the administration of bronchodilators, patients experiencing CHF without the presence of bronchospasm have no need for this intervention. Therefore, capnography can prevent the unnecessary use of bronchodilators that may increase heart rate and blood pressure. On the other hand, if bronchospasm is noted on capnography and the patient has co-existent CHF and COPD, bronchodilators can be used appropriately.
Capnography has been proven to be an invaluable assessment tool that can detect serious patient conditions and guide prehospital and hospital treatment. So what’s next?
Ongoing research continues to suggest evidence-based uses for capnography in the prehospital setting. Recent studies have demonstrated the effectiveness of capnography as a primary assessment tool for the detection of pulmonary emboli, sepsis, thyrotoxicosis, malignant hyperthermia, respiratory status of the seizure patient and triage of patients in a bioterrorism incident. And a 2010 study in Australia has suggested the use of capnography to monitor patients for hypercapnia and hypoventilation in the intubated major trauma patient.(6) The practical uses of capnography in emergency settings are almost limitless, with new uses continually evolving.
Disclosure: The author has completed contract work for Medtronic/Physio Control Corporation, a manufacturer that utilized Oridion capnography technology in their cardiac monitors.
This article originally appeared in the December 2010 JEMS supplement “Measuring Life & Breath” as “Capnography Basics: An invaluable tool for providers & their patients.”
1. Coté CJ, Rolf N, Liu LM, et al. A single-blind study of combined pulse oximetry and capnography in children. Anesthesiology. 1991;74:980–987.
2. 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;45:497–503.
3. Levine RL, Wayne MA, Miller CC. End-tidal carbon dioxide and outcome of out-of-hospital cardiac arrest. N Engl J Med. 1997;337:301–306.
4. Krauss B, Hess D. Capnography for procedural sedation and analgesia in the emergency department. Ann Emerg Med. 2007;50:172–181.
5. Fearon DM, Steele DW. End-tidal carbon dioxide predicts the presence and severity of acidosis in children with diabetes. Acad Emerg Med. 2002;9:1373–1378.
6. Hiller J, Silvers A, McIliroy DR, et al. A retrospective observational study examining the admission arterial to end-tidal carbon dioxide gradient in intubated major trauma patients. Anaesth Intensive Care. 2010;38:302–306.