Waveform Capnography: Part of Comprehensive Vital Sign Monitoring

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Richard J. Merschen, EdS, RT(R)(CV), RCIS¹; Madalynne Ruth, RT/ICVT student^2
1Philadelphia, Pennsylvania; ²Thomas Jefferson College of Health Professions, Philadelphia, Pennsylvania

Comprehensive vital sign monitoring is an essential patient safety requirement in the cath lab and includes invasive and non-invasive blood pressure, electrocardiogram (ECG), heart rate, and pulse oximetry. Another vital sign, used by anesthesia, respiratory therapy, and other care providers, is waveform capnography. Waveform capnography is a continuous, non-invasive measurement of a patient’s ventilation effort and measures the amount of carbon dioxide (CO_²) in exhaled air.^1,2 It consists of two major elements: capnometry and waveform capnography.^1,2 Capnometry is the quantitative numerical value of CO₂ concentration, and focuses on end-tidal CO₂ (ETCO₂). ETCO₂ ranges from 35-45 mmHg, the same as CO₂ in a blood gas sample. Waveform capnography is a square-shaped graph measurement with slightly rounded corners that measures the entire respiratory cycle2 (Figure 1). On the vertical axis of the capnography waveform, ETCO₂ is captured at the top right side of the square, which represents end expiration (Figure 1). Measuring the ETCO₂ on capnography is similar to measuring a hemodynamic pressure that has respiratory variance, with the measurement taken at the same point of end expiration (Figure 2). 

Capnography also measures time along the horizontal axis, which calculates the patient’s respiratory rate. Waveform capnography is useful because it provides earlier, more proactive information than pulse oximetry when assessing and monitoring the respiratory status of a patient.^1,2 It is particularly valuable when measuring the respiratory status pf patients who are sedated and receiving oxygen.^1,2 Waveform capnography is also a critical tool for determining proper intubation, basic cardiac life support (BCLS) and advanced cardiac life support (ACLS) protocols, and the evaluation of the perfusion and metabolic states of patients. This overview will focus on the equipment used to capture waveform capnography, basic interpretation of the waveform, and its application in the cardiac cath lab.

Monitoring System
 

To perform capnography, a Sidestream or Mainstream measuring device is used. Mainstream devices are primarily used on intubated patients and directly measure ETCO₂ via a sensor on the hub of the endotracheal tube. Sidestream systems are commonly used in the cath lab and other procedural areas to measure ETCO₂ and can be used on patients receiving conscious sedation, monitored anesthesia care (MAC), or patients with advanced airways.^3,4 Sidestream systems have a sensor that measures CO₂ samples via a nasal cannula, nasal-oral cannula, or face mask. The sensor tubing is connected to an adapter that contains an infrared sensor and measures CO₂ samples throughout the respiratory cycle.³ A commonly used sensor for this system involves a three-pronged cannula. The cannula has the usual nasal prongs, and a third, elongated prong between them. This prong sits over the mouth to take CO₂ samples (Figure 3), which are measured via infrared light.^3,4 Infrared light is absorbed by CO₂ at a specific wavelength and the sensor detects the infrared light using the exhaled breath from the patient. The capnography device then plots these measurements via an ongoing waveform.^3,4 

Waveform Capnography Interpretation
 

When interpreting waveform capnography, there are 4 distinct phases for each respiratory cycle (Figure 1). The phases are:^5-7

Phase 1, which represents inhalation. This is the baseline measurement noted at the bottom of the waveform. There is no CO₂ is being eliminated when a patient is inhaling, with the baseline measurement around 0 mmHg.

Phase 2, which displays a rapid, steep upstroke on the capnography waveform as the patient begins to exhale. In phase 2, CO₂ begins to travel from the alveoli through the anatomical dead space of the airway, causing a rapid rise in the graph as the CO₂.^5-7
 

In phase 3, there is an alveolar plateau, which comes to an endpoint. The plateau should have a consistent measurement between 35-45 mmHg. The end of the plateau represents ETCO₂ (35-45 mmHg), which is the most important measurement point for waveform capnography.

In phase 0, there is a rapid change from expiration to inhalation. There is a steep decline in the ETCO2, because inhalation does not produce CO2. The waveform returns to near zero at this point. 

Waveform shape variations can indicate a wide variety of conditions such as asthma, poorly compliant lungs, displacement of an endotracheal tube, mechanical airway obstruction. Changes in the pressure indicate hypoventilation and hyperventilation (Figures 4 and 5).

3 PQRST Acronyms

When interpreting hemodynamics, PQRST is not just for ECGs. There are also three PQRST acronyms associated with waveform capnography interpretation. These acronyms interpret basic waveforms, proper intubation, and advanced airway management, and effective BCLS and ACLS care^5-10 (Table). 

1)For basic monitoring, PQRST determines the following:^7,8 

Proper ventilation: Are the numbers and waveform characteristics normal? There should be normal ranges for CO₂ quantity and breathing rate. The waveform shapes should be normal and trending of ETCO₂ should be consistent. A normal range is found in patients with no metabolism, ventilation, or perfusion problems. Although ventilation rates vary based on age, normal readings for quantity, shape, and trends are the same for men and women of all age groups, making it easy to remember.^7,8

Quantity: A normal ETCO₂ range should be 35-45 mmHg. Over 45 mmHg represents hypoventilation and under 35 mmHg represents hyperventilation. 

Rate: A normal respiratory rate should be should be 12-20 breaths per minute (bpm) for adults if the patient is breathing on their own. Ventilating too quickly will not let enough CO₂ build up in the alveoli, resulting in lower ETCO₂ readings. Ventilating too slowly will allow extra CO₂ to build up, resulting in higher readings.^7,8

Shape: The normal shape of the waaveform should normally be a squared-off waveform with rounded corners. Variant waveform shapes can indicate pathology.     

Trend: There should be a consistent pattern of activity for a waveform, just like any other vital sign. Abrupt changes can indicate problems with perfusion, breathing and metabolism. 

2)For an intubated patient, PQRST represents: 

Proper tube placement: Capnography assists in determining proper endotracheal tube placement, by identifying normal ETCO₂ and respiratory rates, and consistent trends with these measurements.^1,3,7,8

Quantity: Proper endotracheal tube placement is associated with an ETCO₂ between 35-45 mmHg.

Rate: The usual respiratory rate for an intubated patient is between 10-12 breaths per minute.

Shape: The waveform capnography should trend from zero to 35-45 mmHg, with a consistent alveolar plateau. Changes in the plateau section may be associated with displacement or leaks of the of the endotracheal tube. 

Trend: There should be consistent numbers for each breath. Abrupt changes require immediate intervention to ensure that the endotracheal tube has not been displaced, or that the patient has not become unstable. 

3)In pulseless ACLS scenarios, PQRST is used to assess the effectiveness of CPR and the return of spontaneous circulation (ROSC). Code teams use PQRST in the following ways:^2,3,7,8

Proper: Position of the endotracheal tube should be confirmed using waveform capnography. 

Quality: Effective chest compressions should have capnography of 10-20 mmHg, with closer to 20 mmHg being optimal. This indicates effective perfusion and cardiopulmonary resuscitation (CPR). Capnography can also minimize disruptions in CPR for pulseless electrical activity (PEA) arrythmia.

ROSC: When performing CPR and ACLS, an abrupt increase in waveform capnography to 35-45 mmHg indicates ROSC. 
Strategy: What treatments are indicated to preserve ROSC and stabilize the patient post resuscitation? (Treatment strategies can include medications, airway support, hypothermia protocols, etc.)

Termination: Codes are terminated due to ROSC or inability to resuscitate the patient. A carbon dioxide level of 10 mmHg or less measured 20 minutes after the initiation of ACLS accurately predicts death in patients with cardiac arrest that have electrical activity but no pulse.

Advantages of Using Capnography in the Lab

In many cath labs, pulse oximetry and visual observation are used to monitor the effects of procedural sedation and narcotics, and to prevent hypoxemia. While oxygenation is accurately assessed with pulse oximetry, capnography provides information on ventilation, perfusion, and metabolism.^9-11 Because of this capability, waveform capnography supplements pulse oximetry, and provides a critical advantage over standalone pulse oximetry, as it detects hypoventilation earlier. It also optimizes respiratory monitoring for patients in the cath who may have sleep apnea, chronic obstructive pulmonary disease (COPD), and other airway diseases.

The effectiveness, frequency, and regularity of ventilation and breathing, which is seen on waveform capnography, is detected immediately, and is more predictive than visual observation and pulse oximetry.^9-11 This is especially true in in oxygenated patients, where it may take several minutes for major respiratory changes to register due to passive oxygen movement through tissues.^1,9-11 Waveform capnography will see an immediate change in the waveform that indicates hypoventilation and possible apnea. Therefore, waveform capnography offers another tool to keep patients safe during procedures where conscious sedation and narcotics are administered. It is also valuable for ACS patients, as it can rapidly detect potentially life-threatening changes in breathing, perfusion, and metabolism.

Capnography for ACLS

Waveform capnography is highly recommended by the American Heart Association to determine the effectiveness of CPR and ROSC during a code. During CPR, ETCO₂ values mainly depend on the blood flow generated by chest compressions, on ventilation rate and tidal volume, and on the metabolic activity of the patient tissues.^3,11,12 This can be easily seen on waveform capnography. Measurement of a low ETCO₂ value (<10 mmHg) during CPR in an intubated patient means that the chest compressions are not effective. High quality chest compressions are achieved when the ETCO₂ value is at least 10-20 mmHg, and closer to 20 mmHg is optimal.^2,3,11,12 ETCO₂ is the earliest indicator of ROSC and when the heart restarts, there is a dramatic increase in cardiac output, perfusion, and a rapid increase in ETCO₂ to 35-45 mmHg.^1,2,10-12 Waveform capnography also helps to prevents unnecessary interruptions for patients in PEA, and avoids unnecessary chest compression and drug administration to patients with spontaneous circulation.^2,3,11-13

Conclusion

Waveform capnography is an important monitoring tool used throughout healthcare. It complements pulse oximetry, and detects changes in respiratory status before pulse oximetry does. It is also highly useful when intubating patients, and in the management of pulseless ACLS scenarios. Waveform capnography monitoring systems are easy to set up and waveform interpretation skills can be achieved with a minimal amount of training. Waveform capnography offers another tool to effectively monitor patients and promote a culture of safety in the cath lab. 

Reader Quiz

1.     Waveform capnography measures expired ____ 

a.     O₂ 

b.     CO₂ 

c.     NO 

2.     Normal capnometry number range

a.     10-20 mmHg 

b.     25-35 mmHg 

c.     35-45 mmHg 

d.     40-50 mmHg 

3.     Waveform capnography is able to measure respiratory depression from sedation more reliably and quicker  than pulse oximetry. 

a.     True 

b.     False 

4.     In waveform capnography, the respiratory cycle is measured in ____ distinct phases 

a.     2

b.      3 

c.      4 

d.      5 

5.     The respiratory cycle phases include:

a.     Phase 1, phase 2, phase 3, phase 4 

b.     Phase 1, phase 2, phase 3, phase 0 

c.     Phase 2, phase 3, phase 4, phase 5 

6.     Which part of the respiratory cycle represents inhalation? 

a.     Phase 0 

b.     Phase 1 

c.     Phase 2 

d.     Phase 3 

7.     Which part of the respiratory cycle represents ETCO₂?

a.     The end of phase 0 

b.      The end of phase 1 

c.     The middle of phase 2 

d.     The end of phase 3 

8.     Normal capnography waveforms should show what kind of shape? 

a.     Square with rounded corners 

b.     Triangle with rounded corners 

c.     Rounded with square corners 

d.     No distinct shape 

9.     What is the acronym that could be associated with waveform capnography interpretation?

a.     PQRST 

b.     OPQRL 

c.     PTRSQ 

d.     ABCDE 

10.  Capnography is useful in BLS and ACLS scenarios because it not only detects ROSC, but also is useful in intubation of patients and management of PEA. 

a.     True  

b.     False 

11.  High quality chest compressions are achieved when the ETCO₂ value is 

a.     0-10 mmHg 

b.     10-20 mmHg 

c.     25-35 mmHg 

d.     35-45 mmHg 

12.  ROSC is demonstrated on waveform capnography by a rapid decline in ETCO₂.

a.     True

b.     False

13.  Waveform capnography can detect changes in the position of an ET tube, including dislodgment and advancement into the mainstem bronchus.

a.     True

b.     False

14.  The normal respiratory rate for an adult is:

a.     8-12

b.     10-12

c.     12-20

d.     20-30

15. Hypoventilation is demonstrated by lowered ETCO₂ pressures ( <35-45 mmHg) and hyperventilation ids demonstrated with elevated ETCO₂ pressures ( >35-45 mmHg).

a. True

b. False

Answer Key:

1. (b) Waveform capnography is used to assess CO₂ levels, especially ETCO₂.

2. (c) Normal ETCO₂ levels range between 35-45 mmHg (Image 1).

3. True. Waveform capnography is able to measure respiratory depression from sedation more reliably and quicker than pulse oximetry.

4. (c) Waveform capnography is measured in four distinct phases (Image 1).

5. (b) The 4 phases of waveform capnography are 1,2,3,0.

6. (b) Inspiration is represented on phase 1 of the waveform capnography and should be close to zero.

7. (d) ETCO₂ is measured at the end of phase 3, which represents end expiration.

8. (a) A normal waveform capnography should be square-shaped with rounded corners.

9. (a) PQRST is the acronym used to assess breathing, intubation and CPR/ROSC (Image 6).

10. True. Waveform capnography demonstrates effective CPR, ROSC, and reduces delays in identifying arrhythmias like PEA.

11. (b) A waveform capnography reading of 10-20 mmHg demonstrates effective CPR/perfusion. The closer to 20, the more effective the chest compressions.

12. False. ROSC is associated with a rapid uptick in waveform capnography. It should rise to 35-45 mmHg.

13. True. Waveform capnography is used by anesthesia and other care providers to confirm ET tube placement and can rapidly identify malposition of an ET tube.

14. (c) The normal respiratory rate for a spontaneously breathing adult ranges between 12-20 breaths per minute.

15. False. Hypoventilation is demonstrated with elevated ETCO₂ (>45 mmHg) and hyperventilation is demonstrated with lowered ETCO₂ (<35 mmHg).

 
References
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2. Richardson M, Moulton K, Rabb D, et al. Capnography for monitoring end-tidal CO2 in hospital and pre-hospital settings: a health technology assessment [Internet]. Ottawa (ON): Canadian Agency for Drugs and Technologies in Health; 2016 Mar. (CADTH Health Technology Assessment, No. 142.) 1, Introduction. https://www.ncbi.nlm.nih.gov/books/NBK362376/

3. Kodali BS, Urman RD. Capnography during cardiopulmonary resuscitation: Current evidence and future directions. J Emerg Trauma Shock. 2014 Oct; 7(4): 332-340. doi:10.4103/0974-2700.142778

4. Puente EG, Bergese SD. Patient monitoring, equipment, and intravenous fluids. In: Urman RD, editor. Moderate and Deep Sedation in Clinical Practice. New York: Cambridge University Press; 2012. pp. 57–76. 

5. Blanch L, Romero PV, Lucangelo U. Volumetric capnography in the mechanically ventilated patient. Minerva Anestesiol. 2006 Jun; 72(6): 577-585. 

6. Kodali BS. Capnography outside the operating rooms. Anesthesiology. 2013 Jan; 118(1): 192-201. 

7. Thompson JE, Jaffe MB. Capnographic waveforms in the mechanically ventilated patient. Respir Care. 2005 Jan; 50(1): 100-108; discussion 108-109.

8. Duckworth, RL. How to read and interpret end-tidal capnography waveforms. JEMS. Aug 1, 2017. https://www.jems.com/patient-care/how-to-read-and-interpret-end-tidal-capnography-waveforms/

9. Conway A, Douglas C, Sutherland J. Capnography monitoring during procedural sedation and analgesia: a systematic review protocol. Syst Rev. 2015 Jul 14;4:92. doi:10.1186/s13643-015-0085-4

10. Wall BF, Magee K, Campbell SG, Zed PJ. Capnography versus standard monitoring for emergency department procedural sedation and analgesia (Protocol). Cochrane Database of Systematic Reviews. 2013; (8)-CD010698. doi:10.1002/14651858.CD010698

11. Green K, Brast S, Bland E, et al. Association for Radiologic & Imaging Nursing position statement: capnography. J Radiol Nurs. 2016;35(1): 63-64. https://doi.org/10.1016/j.jradnu.2016.02.001

12. Pokorna M, Necas E, Kratochvil J, Skripsky R, Andrlik M, Franek O. A sudden increase in partial pressure end-tidal carbon dioxide (P(ET)CO(2)) at the moment of return of spontaneous circulation. J Emerg Med. 2010;38:614-621. doi:10.1016/j.jemermed.2009.04.064

13. Sandroni C, De Santis P, D’Arrigo S. Capnography during cardiac arrest. Resuscitation. 2018 Nov; 132: 73-77. doi:10.1016/j.resuscitation.2018.08.018 

In CLD: Further Reading
1. Seto AH. End-tidal CO2 monitoring for respiratory adverse events during procedural sedation: an additional layer of safety. Cath Lab Digest. 2024 Feb; 32(2): 1-11. https://www.hmpgloballearningnetwork.com/site/cathlab/interview/end-tidal-co2-monitoring-respiratory-adverse-events-during-procedural