Vital signs comprise the cornerstone of patient assessment. They generally include blood pressure, pulse, respiratory rate, temperature, neurologic status, and color. With more sophisticated and cost-appropriate monitoring equipment and the requirement for maintaining greater patient safety, another “vital” has been added to patient assessment: ventilation and perfusion status. Pulse oximetry and capnography are two tools available for assessing ventilation and perfusion status, especially in the prehospital and emergency department arenas.

Capnography is the graphic display of airway CO2 concentration or partial pressure measurement as a function of time. The waveform produced is called a capnogram.

Analysis of CO2 concentration is performed by infrared absorption spectrophotometry utilizing mainstream or sidestream sampling. Mainstream capnograms use a sample measurement chamber that is placed in-line with the patient’s airway. Sidestream capnograms withdraw gas from the patient’s airway.

The Normal Capnogram

As the patient begins to exhale, the initial sample contains CO2-free gas from the tracheal dead space that has not been involved in gas exchange; this creates a zero baseline. As exhalation continues, CO2 concentration begins to rise as gas from the alveoli starts to empty, creating a rapid sharp uprise. As alveolar gas begins to dominate the exhaled gas volume, the change in CO2 concentration decreases significantly. This change in CO2 concentration produces the alveolar plateau. When inspiration occurs, the CO2 concentration declines rapidly as CO2-free gas enters the airway, producing a rapid sharp downstroke.

The capnogram provides a measurement of the end-tidal partial pressure of CO2 (Pet CO2), which is frequently a reliable estimate of the arterial partial pressure of CO2. Evaluation of the capnogram may be useful in the measurement of adequacy of alveolar ventilation, airway integrity, cardiopulmonary function, and ventilator function.

Indications for capnography include:

  • evaluation of maximum exhaled CO2 concentration as a reflection of the PaCO2
  • evaluation of the severity of pulmonary disease
  • monitoring the integrity of the ventilator circuit and artificial airway
  • monitoring the adequacy of pulmonary blood flow
  • confirming tracheal rather than esophageal intubation

Physicians, nurses, and paramedics have been using pulse oximetry for a long time to measure how well oxygen is being delivered to the cells and tissue. Pulse oximetry does not inform on how the body is using the oxygen that has been delivered. Now, with capnography, a patient’s level of function can be evaluated by measuring their exhaled carbon dioxide gases. These numeric measurements and graphic depictions, known as waveforms, depict how well a patient’s cells are using the oxygen in the system and how well they are eliminating the resulting carbon dioxide from the system. End-tidal CO2 (ETCO2) monitoring can reflect changes in CO2 levels within 10 seconds, whereas pulse oximetry takes several minutes to reflect a change.

Capnography has become an important tool in assessing and monitoring how a patient is responding to treatment and has been recognized by many leading associations and organizations as a standard of care. The American College of Emergency Physicians (ACEP), the National Association of EMS Physicians (NAEMSP) and the American Heart Association (AHA), through its Advanced Cardiac Life Support curriculum, have all published guidelines mandating the use of ancillary devices, eg, capnography, for verification of endotracheal tube placement. The Joint Commission has established standards of care for patient safety that require monitoring the respiratory status for patients using patient-controlled analgesia (PCA) for pain management to minimize the risk that the patient’s respiratory system becomes depressed due to overmedication. The American Society of Anesthesiologists1 is supporting use of capnography along with pulse oximetry to facilitate better detection of potentially life-threatening problems in surgical and intubated patients. Used together, capnography and pulse oximetry could help prevent up to 93% of avoidable anesthesia mishaps.2

Physiology of Respiration

The respiratory cycle begins with the body’s physiology triggering the diaphragm to contract, causing the intake of oxygen into the lungs and to the alveoli. The alveoli allow for the onloading of oxygen into the bloodstream and the offloading of carbon dioxide and other waste products from the bloodstream into the lungs for exhalation. The bloodstream carries oxygen to the muscles, organs, and cells. At the cellular level, metabolism produces CO2 as waste. CO2 diffuses from the cells back into the bloodstream where it is carried to the lungs for exhalation. Monitoring of ETCO2 provides vital information about the status of a patient’s ventilation, the movement of air in and out of the lungs; diffusion, the exchange of gases between the alveoli and the pulmonary circulation; and perfusion, the circulation of blood within the body. Measuring and interpreting the quality and rate of exhaled CO2 informs the clinician about the body’s ability or inability to create and use energy and therefore produce waste CO2.

Methods of ETCO2 Measurements

Early capnography equipment was used for monitoring patients in the operating room and was thought to be cumbersome, difficult to use, and unreliable. Recent developments in technology have resulted in equipment that is small, combinable with other monitoring equipment, portable, and much more reliable. Measurements can be taken at the patient’s nose, mouth, or the hub of the endotracheal tube. Several different carbon dioxide monitoring devices are available:

Colorimetry. A colorimeter is a small, disposable device that fits onto the end of an endotracheal tube. As exhaled gases pass through the device, a piece of litmus paper registers the gas levels and changes color indicating the CO2 levels: Purple = ETCO2 <0.5%; Tan = ETCO2 0.5%-2%; Yellow = ETCO2 >2%. Normal CO2 levels are above 4%, so a yellow color on the colorimeter indicates acceptable CO2 levels. The colorimeter is a basic device for ETCO2 evaluation and is similar to assessing a patient’s pulse for presence, strength, and rate.

A study by Ornato3 found that use of a colorimetric device was 100% sensitive in detecting correct endotracheal tube placement in nonarrest patients, but only 69% sensitive in use on intubated arrest patients.

Capnometry. A capnometer provides numeric readings of the CO2 measurements and respiratory rates. It is useful for continuous monitoring but does not allow for graphical trending. Use of a capnometer is similar to monitoring a patient’s heart rate for presence, amount, and changes over time.

Capnography. A capnograph provides both the numeric readings as well as a graphical tracing similar to an electrocardiograph (ECG) tracing. Just as an ECG tracing can provide diagnostic insight into a patient’s condition on a beat-by-beat basis, so can a capnograph tracing.

Capnography—Mainstream Versus Sidestream Monitoring

During mainstream monitoring, the ETCO2 sensor is fitted directly onto the end of the endotracheal tube. Expired gases pass directly over the sensor. Mainstream monitoring is used primarily for intubated patients, and when using mainstream monitoring with a nonintubated patient, the patient will require a mouthpiece or mask with supplemental O2 administered above 6 lpm to ensure that the patient does not rebreathe the CO2.

During sidestream monitoring, a small volume of expired gas is shunted into a monitoring unit, not directly in line with the ventilatory equipment. Older style high-flow sidestream monitoring used a larger sample of exhaled gas (150 to 200 µL of sampled gases). High-flow monitors required frequent calibration; had frequent occlusion of the sampling line due to moisture or secretions clogging the sampling line; and could be inaccurate when used with neonates, infants, and children. The newer low-flow sidestream monitoring utilizes only 50 µL of sampled gas. Low-flow monitoring does not need calibration between patients, utilizes disposable tubing and cannulas, is durable enough for prehospital use, and is especially useful for monitoring spontaneous breathing in infants, children, and adults; occlusions are uncommon.

Understanding the Capnography Waveform

A capnogram plots CO2 concentration over time and consists of four phases. The x-axis measures time, and the y-axis shows the concentration of CO2. The first phase of the capnogram starts exhalation. The first gases that pass over the capnography sensor usually do not contain carbon dioxide, because they are the gases that fill the physiologic dead spaces in the “conducting airways” (lungs, bronchii, trachea, mouth, and nose). This phase is the baseline of the capnograph. Phase II is known as the expiratory upstroke and traces steep rises in CO2 levels. This phase measures a mixture of dead space (no CO2) and alveolar air (CO2). Phase III represents the “expiratory plateau,” which represents mostly alveolar gas exhalation. This plateau is indicative of the homeostasis of the patient: a normal respiratory system, a compromised respiratory system, or a cardiac or metabolic output problem. The gases released at the end of the expiratory plateau have the highest concentration of CO2. This is also known as the end-tidal point and is what is measured with colorimetry, capnometry, or capnography. Phase IV reflects the inhalation phase, which brings oxygenated gases into the lungs, returning the gas levels and the capnograph waveform to the beginning of a new cycle and the baseline. By observing changes in the waveform, a diagnostician can evaluate changes in metabolism, circulation, ventilation, and/or equipment functioning.

Clinical Applications

Use of ETCO2 Monitoring with Intubated Patients. ETCO2 monitoring is useful in gauging the severity of hypoventilation states such as drug and ethanol intoxication, diabetic ketoacidosis, congestive heart failure, stroke, head injury, and/or sedation. Monitoring allows ongoing assessment of the patient’s perfusion status. Changes in ventilation status and changes in air movement, as measured by ETCO2, can diagnose problems with asthma, COPD, airway edema, stroke, or foreign body obstruction. By understanding changes in diffusion due to problems at the alveolar level, a clinician can look for changes in pulmonary edema, alveolar damage, carbon monoxide poisoning, or smoke inhalation. Changes in perfusion can illuminate problems such as shock, pulmonary embolus, cardiac arrest, or severe dysrhythmias.

Verification of Endotracheal Tube Placement. There are several ways to evaluate the placement of an endotracheal tube: auscultation of lungs for bilateral breath sounds and absence of sounds in the belly, visual assessment for chest movement, and clouding of the endotracheal tube from exhaled air. These evaluations are subjective and rely on the clinician’s skills and training. Incorporation of capnography now provides scientific verification for the placement of the endotracheal tube. Whether using the colorimeter or a waveform capnograph, the clinician has clinical supporting evidence for correct placement. This, in turn, translates into better patient care.

Displacement of the Endotracheal Tube. Displacement of an endotracheal tube is most likely to occur when moving a patient, especially when moving the patient in or out of an ambulance or when dealing with an awake or agitated patient postoperatively. Continuous capnographic monitoring rapidly identifies a problem. The typical waveform for a displaced endotracheal tube has the normal square-like forms that would gradually reduce and tail off to the baseline.

Adequacy of CPR. ETCO2 measurement during CPR can both assess effectiveness of the CPR and predict survivability as well. Levels of ETCO2 have a predictive correlation to cardiac output. As pulmonary bloodflow decreases during cardiac arrest, CO2 levels in the bloodstream are decreased, which can be evaluated using a capnograph waveform. Studies have shown that, without a return to ETCO2 levels of 10 mm Hg or more after 20 minutes of CPR, there is a 100% mortality rate. ETCO2 monitoring can also provide information regarding blood flow during CPR. By analysis of the waveform, a clinician can detect rescuer fatigue before the rescuer is aware of tiring.4

Return of Spontaneous Circulation. ETCO2 monitoring is sensitive enough to register a rapid rise in CO2 production (indicating metabolism), well before either pulse or blood pressure is detectable.

Evaluation During Asthma Treatment. A patient suffering bronchospasm or an asthma attack will have a characteristic “shark-fin” waveform. This is caused because the bronchospasm alters the ascending phase and the plateau phase, with a slower rise in CO2. The effectiveness of treatment can be quickly evaluated by observation of the waveform as it returns to a more normal square, or step shape.

Head Injury Patients. Hyperventilation can increase blood pressure, and, with head injury patients, increased blood pressure can exacerbate cerebral edema. Monitoring of ETCO2 can assist the clinician in maintaining stable CO2 levels, thus avoiding secondary injury from accidental increased cerebral edema.

Acute and Critical Care Uses. In addition to confirming correct ETT placement and predicting mortality following cardiac arrest, capnography can play an important role in the critical care setting. Capnography offers a rapid alarm for ventilator disconnection, identified by a flat line on the capnogram. It also assists with early identification of a dislodged tube in an awake or agitated patient. Various clinical situations, including pulmonary embolism and asthma, can be identified. Beyond the critical care environment, capnography can identify placement of orally or nasally placed gastric or small bowel tubes. The capnogram will show little or no levels of CO2, therefore decreasing complications from nasogastric tubes, such as aspiration, pneumothorax, and pneumonia.5

End-tidal carbon dioxide monitoring has become much more sophisticated, is easier to monitor and evaluate over the last decade, and is now offering a much more detailed look into the physiologic functioning of the emergent patient. Any set of vital signs should now include an analysis of the patient’s ETCO2 levels as a part of the complete picture.

Tracy Evans, RN, MS, MPH, is director of professional nursing practice; Lisa Lavorato, RN, CCRN, is cardiology clinical educator, The Stamford Hospital, Stamford, Conn; Jennifer Lord is an EMT, Norwalk Hospital, Norwalk, Conn.

References

  1. Cote CJ et al. Anesthesiology. 1991; 74:980-7.
  2. Tinker JH et al. Anesthesiology. 1989;71:541-6.
  3. Ornato JP et al. Multicenter study of a portable, hand-sized, colorimetric end-tidal carbon dioxide detector device. Annals of Emergency Medicine.1992;21:518-523.
  4. Ochoa FJ, et al. The effect of rescuer fatigue on the quality of chest compressions. Resuscitation. 1998;37:149-52.
  5. Ahrens TS, Sona C. Capnography application in acute and critical care. AACN Clinical Issues. 2003;14:123-132.