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Critical Care Nurse. 2003;23: 83-88
Copyright © 2003 by the American Association of Critical-Care Nurses.
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Protocols for Practice

End-Tidal Carbon Dioxide Monitoring

Robert E. St. John, RN, RRT, MSN

Robert E. St. John is an adjunct clinical instructor at St. Louis University School of Nursing in St. Louis, Mo. He also works for the Respiratory and Critical Care Division of Nellcor Puritan Bennett, a unit of Tyco Healthcare.

To purchase reprints, contact The InnoVision Group, 101 Columbia, Aliso Viejo, CA 92656. Phone, (800) 809-2273 or (949) 362-2050 (ext 532); fax, (949) 362-2049; e-mail, reprints{at}aacn.org.

End-tidal carbon dioxide monitoring refers to the noninvasive measurement of exhaled carbon dioxide and is most useful when applied directly to patient care.1 Although commonly used in intubated patients receiving mechanical ventilation, this technique is sometimes used in non-intubated patients.2–4 The term "capnometry" refers to the measurement and display of the concentration of exhaled carbon dioxide either as a percentage (%) or as partial pressure in millimeters of mercury (mm Hg). If the gas measuring device also includes a calibrated, visual waveform recording of the concentrations of inspired and exhaled carbon dioxide that can be examined on a breath-by-breath basis or for long-term trends, the instrument is called a capnograph.4 Although this respiratory monitoring technology is not new to the critical care environment, controversy still exists regarding potential benefits and limitations in clinical practice.5 The questions and answers presented here may enhance your understanding of end-tidal carbon dioxide monitoring.

Q: How is end-tidal carbon dioxide measured?

Depending on the type of equipment used, the concentration of inspired and expired carbon dioxide can be measured directly at the patient-ventilator interface (mainstream sensor, Figure 1Go), or a sample of gases can be collected and transported via small-bore tubing to the bedside monitor (sidestream, distal-diverting; Figure 2Go) or some intermediate connection (sidestream, proximal-diverting) for measurement.6 Concentrations of respired carbon dioxide from the patient’s airway are typically determined by using infrared light. With this method, the absorption of carbon dioxide molecules exposed to various wavelengths of light within a sample chamber or cell is measured. A photodetector then compares the relative amount of light absorbed by the sample with the amount absorbed by a gas that is free of carbon dioxide. The difference between the two represents the concentration of carbon dioxide.



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  		Figure 1 Mainstream end-tidal carbon dioxide monitor with measurement sensor and sample chamber at the patient-ventilator circuit interface

 


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  		Figure 2 Sidestream (distal-diverting) end-tidal carbon dioxide monitor with the sampling port and the tubing that transports gas to the monitor for analysis

 
Another method used to analyze exhaled gases is mass spectrometry, a technique that separates and measures respired gas on the basis of molecular weight.4 Finally, end-tidal carbon dioxide may be measured semi-quantitatively using a special colorimetric pH-sensitive indicator that changes color in response to different concentrations of carbon dioxide.4

Q: What is the best location for sampling or measuring the end-tidal carbon dioxide concentration in an intubated patient?

Regardless of the type of sampling technique, place the airway adapter or sampling port as close as possible to the patient’s airway.4,7 The closer the measuring/ sampling adapter is placed to the patient-ventilator connection, the more responsive the monitor will be when detecting a change in the level of exhaled carbon dioxide. Position the airway adapter upright according to the manufacturer’s recommendations. If the airway adapter and sampling ports are not kept upright, water and secretions will pool at the bottom of the adapter and may interfere with the measurement of carbon dioxide. Always support the positioning of the airway adapter or sampling port to minimize pull or weight on the patient’s airway.

Q: What is the clinical significance for advocating monitoring of the end-tidal carbon dioxide levels?

The ideal end-tidal carbon dioxide monitor provides both numeric and graphic waveform displays. The display on the monitor represents the highest concentration of carbon dioxide reached at the end of exhalation and is assumed to represent alveolar gas, which under normal ventilation-perfusion matching in the lungs closely parallels arterial levels of carbon dioxide.4,6 Thus, the end-tidal carbon dioxide tension (PetCO2) is thought to be a non-invasive estimate of the patient’s alveolar ventilation status by its close correlation with arterial carbon dioxide tension (PaCO2) under normal conditions.

Unfortunately, unless the patient has a stable cardiac status, stable body temperature, absence of lung disease, and normal capnogram (Figure 3Go), the PetCO2 will not accurately reflect PaCO2. Therefore, the use of capnography in patients with severe respiratory failure should be interpreted with careful attention. The increased ventilation to perfusion (V/Q) mismatch that is consistent with a increased PaCO2–PetCO2 difference or gradient, as well as worsening arterial carbon dioxide retention and increased peripheral carbon dioxide production, may lead to erroneous PetCO2 values. The normal arterial to end-tidal CO2 gradient is approximately 1 to 5 mm Hg but may be as high as 20 mm Hg or higher when an uneven V/Q pattern occurs such as when ventilation is greater than perfusion.4 Because V/Q matching in critically ill patients is often abnormal, PetCO2 values must be evaluated cautiously.8,9 However, the routine monitoring of the PaCO2–PetCO2 gradient may be valuable for determining trends.10,11 For example, a gradual narrowing of the gradient over time may represent improved ventilation-perfusion matching and pulmonary function. Much research is currently under way in evaluating the potential value that capnography brings as a tool for evaluating the effectiveness of cardiopulmonary resuscitation aimed at reestablishing circulation.12–14 There is some evidence to suggest that a persistently low PetCO2 and a widened PaCO2–PetCO2 gradient during cardiopulmonary resuscitation as well as intraoperatively during certain surgical procedures, is associated with poor outcomes.12,14,15



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  		Figure 3 Typical normal carbon dioxide waveform. A to B, exhalation of carbon dioxide-free gas from dead space. B to C, combination of dead space and alveolar gases. C to D, exhalation of mostly alveolar gas (alveolar plateau). D, end-tidal point, that is, exhalation of carbon dioxide at maximum point. D to E, inspiration begins and carbon dioxide concentration rapidly falls to baseline or zero.

 
Q: Can capnographic waveform displays provide clinically useful information independent of the numerical end-tidal carbon dioxide reading?

It is important to recognize that clinicians should never accept any PetCO2 value without first determining the quality of the capnogram or end-tidal carbon dioxide waveform. In those situations where the PetCO2 numerical value is of questionable benefit, inspection of the waveform tracings alone has the potential to provide the clinician with useful information.6,16

Capnographic waveforms can usually be visualized and recorded at several speeds. In general, the fast-speed recordings provide a real-time display of the carbon dioxide waveform on a breath-by-breath basis. Slow-speed recordings allow for trending over defined time periods.

Figure 4Go illustrates the value of using slow-speed recordings to properly identify and document abnormal breathing patterns such as Cheyne-Stokes respirations. Such tracings could also be helpful when trying to identify the time course in which events such as ventilator system air leaks occur, development of partial or acute airway obstruction, and other important clinical events that might slowly develop over time and which can be detected by capnographic trending.17 Analysis of real-time breath-by-breath tracings may be useful during mechanical ventilation to detect certain types of equipment malfunction or incorrect ventilator parameter settings that might not be immediately picked up by the ventilator system alarms. Figure 5Go illustrates the effect of respiratory efforts not being sensed by the ventilator because of an improperly set trigger sensitivity control. Real-time capnographic recordings also have the potential to provide qualitative respiratory breathing pattern information during patient weaning efforts from mechanical ventilation, which is important for overall clinical assessment purposes.16 As shown in Figure 6Go, analysis of individual tracings may yield information regarding patient response to certain types of drug therapy. There is data to suggest that the slope of the individual capnographic waveform may be helpful in the clinical assessment of patients with asthma in the acute phase of treatment.18,19



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  		Figure 4 Slow-speed capnogram shows Cheyne-Stokes respirations in a 72-year-old woman who required mechanical ventilation after a cerebral vascular accident. Note the changes in breathing pattern associated with periods of apnea that consistently repeat.

 


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  		Figure 5 Electrocardiograms and capnograms of a 57-year-old man receiving mechanical ventilation for respiratory failure associated with bacterial pneumonia. A, Respiratory efforts (arrows) are not being sensed by the ventilator because the trigger sensitivity control is set inappropriately. B, Correction of the sensitivity setting allows the patient to trigger the ventilator on demand.

 


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  		Figure 6 Real-time electrocardiogram, end-tidal carbon dioxide waveforms, and pulmonary artery pressures (PAPs) in a 67-year-old woman who was receiving mechanical ventilation because of respiratory distress and urosepsis. She was also receiving a sedative and a neuromuscular blocking agent. The end-tidal carbon dioxide waveforms show breakthrough spontaneous respiratory efforts (arrows) that are too weak to trigger a ventilator breath.

 
Utilizing researched-based, clinical practice protocols has the potential to improve the quality of nursing knowledge in the critical care environment as well as improve patient care. When appropriately applied, end-tidal carbon dioxide monitoring provides the clinician with valuable information that can assist nurses and others in patient assessment and treatment.

Note:

This article is based on the protocol "End-tidal CO2 Monitoring" by Robert St. John, RN, RRT, MSN, from the Noninvasive Monitoring Series of AACN’s Protocols for Practice. Protocols can be obtained from AACN, 101 Columbia, Aliso Viejo, CA 92656-1491, (800) 899-AACN, (949) 362-2000. $11, AACN members; $14, nonmembers.

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References

  1. Ahrens T. Technology utilization in the cardiac surgical patient:SvO2 and capnography monitoring. Crit Care Nurs Q. 1998;21:24–40.
  2. Barton CW. Correlation of end-tidal CO2 measurements to arterial PaCO2 in non-intubated patients. Ann Emerg Med. 1994;23:562–563.
  3. Liu SY, Lee TS, Bongard F. Accuracy of capnography in nonintubated surgical patients. Chest. 1992;105:1512–1515.
  4. St. John RE. Exhaled gas analysis: technical and clinical aspects of capnography and oxygen consumption. Crit Care Nurs Clin N Am. 1989;20:363–374.
  5. Ahrens T. Utilization of intensive care unit technology. New Horiz. 1998;6:41–51.[Medline]
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  7. Schena J, Thompson J, Crone RK. Mechanical influences on the capnogram. Crit Care Med. 1984;12:672–674.[Medline]
  8. Brown LH, Gough JE, Seim RH. Can quantitative capnometry differentiate between cardiac and obstructive causes of respiratory distress? Chest. 1998;113:323–326.[Abstract/Free Full Text]
  9. Morley TF, Giaimo J, Maroszan E, et al. Use of capnography for assessment of the adequacy of alveolar ventilation during weaning from mechanical ventilation. Am Rev Respir Dis. 1993;148:339–344.[Medline]
  10. Wilson RF, Tyburski JG, Kubinec SM, et al. Intraoperative end-tidal carbon dioxide levels and derived calculations correlated with outcome in trauma patients. J Trauma. 1996;41:606–611.[Medline]
  11. Hess D, Schlottag A, Levin B, Mathai J, Rexode W. An evaluation of the usefulness of end-tidal PCO2 to aid weaning from mechanical ventilation following cardiac surgery. Respir Care. 1991;36:837–843.
  12. Ward KR, Menegazzi JJ, Zelenak RR, Sullivan RJ, McSwain NE Jr. A comparison of chest compressions between mechnical and manual CPR by monitoring end-tidal PCO2 during human cardiac arrest. Ann Emerg Med. 1993;22:669–674.[Medline]
  13. Ward KR, Yearly DM. End-tidal carbon dioxide monitoring in emergency medicine, part 2: clinical applications. Acad Emerg Med. 1998;5:637–646.[Medline]
  14. White RD, Asplin BR. Out-of-hospital quantitative monitoring of end-tidal carbon dioxide pressure during CPR. Ann Emerg Med. 1994;23:25–30.[Medline]
  15. Domsky M, Wilson, RF, Heins J. Intraoperative end-tidal carbon dioxide values and derived calculations correlated with outcome: prognosis and capnography. Crit Care Med. 1995;23:1497–1503.[Medline]
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  18. Yaron M, Padyk P, Hutsinpiller M, Cairns CB. Utility of the expiratory capnogram in the assessment of bronchospasm. Ann Emerg Med. 1996;28:403–407.[Medline]
  19. You B, Peslin R, Duvivier C, Vu VD, Grilliant JP. Expiratory capnography in asthma: evaluation of various shape indices. Eur Respir J. 1994;7:318–323.[Abstract]




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