Beyond the Basics: Capnography

CEU Review Form Capnography (PDF)Valid until March 3, 2008

All levels of EMS personnel are taught that a patent airway is imperative to effective patient management. Endotracheal intubation is often used to secure and maintain a patent airway in the prehospital setting. Following endotracheal intubation, the EMS provider must assess and continually reassess tube placement, especially after any patient movement.

Initially, placement is confirmed by visualization of the tube passing through the glottic opening and between the vocal cords. Once the tube is placed, auscultation is performed over the epigastrium to listen for gastric sounds. If none are present, the lungs are auscultated for presence and equality of breath sounds.

A device that can provide invaluable information regarding initial and continuous monitoring of endotracheal tube placement is the end-tidal CO₂ monitor, which provides a numeric carbon dioxide value in addition to a continuous waveform.

RESPIRATORY PHYSIOLOGY AND GAS TRANSPORT
To understand capnography, it is important to review some basic respiratory physiology. Whether a patient is ventilating spontaneously under his or her own control or you are delivering ventilations via a bag-valve device, air travels through the respiratory tract until it reaches the alveoli--very thin-walled structures surrounded by capillaries. This allows for gas exchange between the oxygen-enriched air in the alveoli and the oxygen-depleted blood in the capillary that is also high in carbon dioxide. As a general rule, gas moves from a high to low concentration; therefore, oxygen diffuses from the alveoli into the capillary as carbon dioxide exits the capillary and enters the alveoli. The blood exiting the pulmonary capillaries has a high partial pressure of oxygen and a low partial pressure of carbon dioxide. This blood is then transported back to the left side of the heart, where it is ejected into the aorta, arteries and, eventually, arterioles. The arterioles are the terminal ends of the artery and are the entry point into the capillary bed. The cells have a lower partial pressure of oxygen and higher partial pressure of carbon dioxide. Based on the general rule of gas diffusion, oxygen leaves the capillary and moves into the cell as carbon dioxide diffuses from the cell into the capillary bed. The blood exits the capillary via a venule and enters a vein. The venous blood has high levels of carbon dioxide as a result of normal cellular metabolism.

When a cell metabolizes glucose in the mitochondria in the presence of oxygen, known as aerobic metabolism, it produces water and carbon dioxide as its byproducts. The carbon dioxide must be transported to the alveoli so it can be exhaled. Carbon dioxide is transported in the blood in three ways: 1) dissolved in plasma; 2) attached to the hemoglobin molecule forming carbaminohemoglobin (HbCO₂); and 3) in the converted form of bicarbonate (HCO₃-). Approximately 7% is transported dissolved in plasma, 23% as carbaminohemoglobin and 70% as bicarbonate.

Carbon dioxide enters the red blood cells and attaches with water. Carbonic anhydrase converts the water and carbon dioxide to carbonic acid, which is split into a hydrogen molecule and bicarbonate:

H₂O + CO₂  ¨ H₂CO₃-  ¨ H⁺ + HCO₃-

A chloride shift then occurs. As chloride enters the red blood cells, the bicarbonate leaves and is transported in the plasma.

When blood reaches the pulmonary circulation, the bicarbonate reenters the red blood cells as chloride exits. The bicarbonate combines with hydrogen and forms carbonic acid, which splits into water and carbon dioxide:

H₂O + CO₂  © H₂CO₃-  © H⁺ + HCO₃-

The carbon dioxide diffuses from the cells, leaves the capillaries and is eliminated through exhalation.

The effectiveness of the carbon dioxide transport and elimination system depends on adequate perfusion for transport of and adequate ventilation for removal of carbon dioxide. Thus, in a patient with spontaneous ventilation and perfusion, each exhaled breath will contain carbon dioxide. This CO₂ is measured in millimeters of mercury (mmHg) and is generally in the range of 35--45 mmHg, with an average of approximately 38 mmHg at sea level. A carbon dioxide level greater than 45 mmHg is typically an indication of hypoventilation. An exhaled carbon dioxide level less than 35 mmHg is typically an indication of hyperventilation. However, because blood is the transport medium of carbon dioxide, a poor perfusion state may also produce a decreased amount of exhaled carbon dioxide. Thus, a low exhaled carbon dioxide level may be a result of decreased cardiac output, poor pulmonary perfusion or a shock state, rather than ineffective ventilation.

Because gas exchange occurs in the alveoli, the first portion of exhaled breath generally has a lower concentration of CO₂, as compared to the end of the breath. This is due to the initial escape of air from dead air space in the respiratory tract and airway structures that was not involved in gas exchange. The end of exhalation would contain air from the alveoli that contains the maximum levels of carbon dioxide. When it's measured, one would expect a rise in the amount of exhaled carbon dioxide at the end of each exhaled breath, hence the term "end-tidal CO₂" or EtCO₂.

WHAT IS CAPNOGRAPHY?
Capnography is measurement of exhaled carbon dioxide in the form of a display or recording. More specifically, capnometry typically refers to a device that provides a quantitative numeric readout of the amount of exhaled carbon dioxide; capnography provides both a numeric readout and a graphic waveform display. Colorimetric capnography is a qualitative-type device that uses a pH-sensitive impregnated paper that attaches to the end of an endotracheal tube or is built into the exhalation valve of the bag-valve ventilation device. The built-in paper changes from purple to yellow in the presence of exhaled CO₂. It is a quick and easy method to determine the presence of CO₂; however, it has limitations. The colorimetric device cannot detect hypercarbia and may not be very sensitive in detecting hypocarbia. Also, acidic substances, such as gastric contents or epinephrine, that come in contact with the pH-sensitive paper may produce unreliable results.

Quantitative devices use a percentage, bar graph or waveform to display carbon dioxide levels. An infrared light is passed through the exhaled gas sample. Carbon dioxide absorbs infrared light; thus, the less light that passes through the gas, the higher the amount of CO₂ present. This method of measurement involves the use of a handheld device about the size of a pulse oximeter , or as a device that is built into the cardiac monitor. Regardless of the device, this method takes a sample of exhaled gas using mainstream or sidestream technology and electronically measures the amount of carbon dioxide present. The sample of air may be obtained directly from an endotracheal tube attachment or from a device similar to a nasal cannula for patients who are spontaneously breathing.

Reading the results of capnography varies, depending on the particular method used to collect the sample. A colorimetric device should be attached to the endotracheal tube after insertion and six ventilations delivered. Six ventilations are needed to get a true sample of the air from the alveoli, and readings prior to the six ventilations may be inaccurate. The device may display three colors: purple, yellow and tan. The initial purple color will change to yellow as it is exposed to higher levels of CO₂. With successful placement of the endotracheal tube in the trachea, one would expect the purple to change to yellow. If the device remains purple, one must suspect an unsuccessful intubation and immediately remove the endotracheal tube. A tan color indicates a problem that may result from a misplaced tube, or a correctly placed tube in the presence of very poor carbon dioxide production. Reassess placement with direct visualization of the endotracheal tube. If there is any doubt as to proper tube placement, remove the tube and resume ventilation with a bag-valve-mask device.

When reading a percentage or bar graph display device, you need to recognize normal and abnormal values. Recall that a normal reading for EtCO₂ is typically between 35--45 mmHg. The percentage and bar graph devices digitally display the current reading in a numerical format. Some models also display a waveform. Many of these devices are small, extremely portable and used only on intubated patients.

Cardiac monitor/defibrillator manufacturers have incorporated capnography into many of their devices. These devices allow for monitoring of EtCO₂ levels for both intubated and non-intubated patients. Rather than a simple numerical display of current EtCO₂ levels, a waveform is also shown.

UNDERSTANDING THE CAPNOGRAPH
This waveform is similar in concept to an ECG waveform, where the "normal" wave must follow certain rules. Deviation in the waveform can range from a critical finding to a normal condition for a particular patient.

The capnography waveform represents various phases of inhalation and exhalation and is divided into four phases (I, II, III and inspiration). Each complex has lettering similar to the PQRST labeling associated with an ECG (see Figure 1). Phase I is the period between A and B and represents late phase inspiration and the beginning of exhalation. In this period, the dead air space, which typically has no substantial amount of CO₂, is emptied. Phase II is the period between B and C and represents continued exhalation of the air from the remaining dead space and proximal alveoli. This phase should rise sharply as the CO₂ content increases. Phase III is the period between C and D, which reflects airflow from the alveoli during uniform ventilation where there is nearly a constant CO₂ level. This phase is also known as the respiratory plateau. Point D is actually considered the end-tidal point of CO₂ monitoring in which the highest concentration of carbon dioxide is typically measured. Phase IV is the last phase that identifies inspiration and is displayed from D to E. The waveform should exhibit a rapid downward slope drop as the patient inhales or is ventilated and oxygen-rich air rushes into the respiratory tract. This phase should terminate at a baseline of zero.

ABNORMAL CAPNOGRAPHS
Alterations in the "normal" waveform may be as follows:

• Phase II (B--C) rising higher than the upper limit of 45 mmHg. As a result, Phase III will also be at a higher level, and the inspiratory phase will have a longer drop to baseline. The left side of the waveform will show a scale; waves higher than 45 mmHg are indicative of an increase in CO₂ content. This can be a result of hypoventilation in the spontaneously breathing patient or inadequate ventilation when using a bag-valve-mask device (see Figure 2). Other potential causes include narcotic overdose, CNS injury and an increase in metabolic rate, as well as a rise in the body's core temperature.
• Phase II (B--C) dropping lower than 35 mmHg. As a result, Phase III will also be at a lower level, and the inspiratory phase will have a shorter drop to baseline. This can be a result of hyperventilation in the spontaneously breathing patient or ventilated patient (see Figure 3). Other causes include pulmonary embolism, decreases in metabolic rate and a decrease in the body's core temperature.
• One of the more noticeable changes is seen when Phase II takes on an appearance similar to a shark fin. Rather than the quick upward rise that Phase II should display, a very gradual rise in the CO₂ level is exhibited. This slow rise is due to a partial lower airway obstruction often found in asthma attacks, and other conditions that produce bronchoconstriction (see Figure 4). It may also be a result of plugging in the tube due to mucus, kinking or other causes. As the patient works hard to exhale against this resistance, there is no plateau (Phase III), but rather a quick return to the inspiratory phase, which is easily seen in the rapid respiratory rates of these patients.
• Phase III (C--D) has a downward slope as it approaches point D. This can be found in patients with air trapping, such as emphysema, and may indicate significant alveolar damage.
• Flat line. A flat line, usually at the baseline, can indicate a serious problem like apnea or esophageal intubation, or something as simple as a detector not plugged into the device (see Figure 5).
• Decreasing waveform size. If you find a run of waveforms that decrease in size, you must look at the shape of each waveform. If it is in the shape of a tombstone, something is going terribly wrong and must be corrected. Generally, it represents an esophageal intubation, where waveforms may be present initially due to CO₂ in the stomach, especially if there was overly aggressive ventilation with a bag-valve-mask device prior to intubation. If the waveform is not in a tombstone shape and looks like a "normal" waveform that is decreasing in size, suspect your patient has deteriorating cardiac output.

INDICATIONS FOR THE USE OF CAPNOGRAPHY
One clear indication is to confirm and continuously monitor endotracheal tube placement. Moving the patient has the potential to dislodge a properly placed tube. Capnography provides excellent continuous visual waveforms and a near immediate response to dislodgment of the tube. Immediately after placement of the endotracheal tube, employ the end-tidal CO₂ monitor as a secondary confirmation method and as a monitoring device for continuous proper tube placement.

Capnography may provide an indication of the perfusion status of the non-intubated patient and differentiate between air trapping and an obstruction. To use capnography on the non-intubated patient, attach a device similar to a nasal cannula to the capnographer and apply to the patient. This device can monitor EtCO₂ only, or deliver O₂ while, at the same time, monitoring EtCO₂. This combination device has normal O₂ delivery prongs and a single prong directed toward the mouth to detect exhaled CO₂.

CHOOSING THE RIGHT DEVICE
There are several considerations when choosing a device for capnography. Colorimetric devices are the least expensive, require no special setup and provide rapid readings. Limitations include a short life span (most work for less than 20 minutes of care); color blindness may create problems when using this device; and, exposure to vomitus, certain drugs or secretions through the endotracheal tube may greatly impede the efficacy of the device. Additionally, colorimetric devices cannot be used to monitor the non-intubated patient unless using a bag-valve-mask with the device built in. Documentation is difficult, as there is no actual numeric value to report or printout to validate successful intubation.

Handheld monitors are convenient and work well for intubated patients, but may have limited or no use for non-intubated patients. Some models have waveforms and others do not, so readings may only be based on a snapshot of information, not continuous waveform analysis.

Capnographers that are built into monitors/defibrillators can be used on intubated and non-intubated patients and provide tracings of the waveform itself, which is an invaluable tool for documentation of successful intubations. Use of these devices allows for continuous monitoring and can provide information on trends in patient assessment and care. Monitors of this type may require a warm-up period to self-calibrate and can lead to an initial delay. The devices are large, expensive and may vary among all transporting vehicles in a particular service.

The importance of capnography, especially in the intubated patient, cannot be overstated. A 2005 study that specifically addressed intubation by paramedics determined that without the use of capnography, 23% of misplaced endotracheal tubes were not recognized by EMS providers. With capnography, there were no unrecognized misplaced tubes.

Put simply, if the endotracheal tube is successfully placed, the waveform will be normal, or at the very least, close to it. If the endotracheal tube is misplaced, the waveform will not be normal. Thus, capnography results must be understood in order to use the information and the device effectively. Capnography is a tool that will help providers with a critical intervention, legal documentation, and better patient care. Its color reading, percentage indicator, and waveform are additional confirmation methods to ensure that endotracheal tubes are, and remain, properly placed.

OBJECTIVES

• Review respiratory physiology
• Discuss the process of capnography<
• Review uses of capnography in prehospital care

Mainstream vs. Sidestream
Capnography must use exhaled gas to determine the amount of CO₂ in each breath. This is accomplished through one of three different technologies: mainstream, sidestream or microstream. Mainstream samples exhalation gases directly through the endotracheal tube or a mouthpiece attachment. Sidestream and microstream are similar in that they draw a sample of gas into a small tube attached to the sampling device (endotracheal tube or nasal cannula) for analysis. One concern of sidestream is that the amount of sample withdrawn may possibly affect tidal volume in the airway circuit when using a ventilator. Additionally, microstream and sidestream methods must use filters, water condensation traps and water-permeable tubing. Mainstream works well for sampling intubated patients, while sidestream and microstream work well for both intubated and non-intubated patients.

CEU Review Form Capnography (PDF)Valid until March 3, 2008

CONTINUING EDUCATION FROM EMS
This CE activity is approved by EMS Magazine, an organization accredited by the Continuing Education Coordinating Board for Emergency Medical Services (CECBEMS), for 1.5 CEUs.

Bibliography
Anderson CT, Breen PH. Carbon dioxide kinetics and capnography during critical care. Crit Care pp. 207--215, July 12, 2000.

George S, Macnab AJ. Evaluation of a semi-quantitative CO₂ monitor with pulse oximetry for prehospital endotracheal tube placement and management. Prehosp Disas Med 17(1):38--41, 2002.

Jaffe MB. Mainstream or Sidestream Capnography? Respironics White Paper. Respironics, Inc., 2002.

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 45(5):497--503, 2005.

ZOLL E-Series Users Manual--End Tidal Carbon Dioxide (EtCO₂). ZOLL Medical Corporation, 2006.

Marc A. Minkler, NREMT-P, CCEMT-P, is a paramedic/firefighter with the Portland (ME) Fire Department. He has been learning EMS for over 18 years and is the author of several internationally published EMS instructor programs. He can be reached at pfd225@roadrunner.com.

Joseph J. Mistovich, MEd, NREMT-P, is a professor and chair of the Department of Health Professions at Youngstown (OH) State University, author of several EMS textbooks and a nationally recognized lecturer.

William S. Krost, BSAS, NREMT-P, is an operations manager and flight paramedic with the St. Vincent/Medical University of Ohio/St. Rita's Critical Care Transport Network (Life Flight) in Toledo, OH, and a nationally recognized lecturer.

Daniel D. Limmer, AS, EMT-P, is a paramedic with Kennebunk Fire-Rescue in Kennebunk, ME. He is the author of several EMS textbooks and a nationally recognized lecturer.