Capnography is a valuable monitoring tool used in healthcare to measure the concentration of carbon dioxide (CO₂) in exhaled air, providing real-time insights into a patient’s ventilatory status.
For students entering the field of respiratory therapy, nursing, or emergency medicine, understanding capnography is essential, not just as a technical skill, but as a critical component of patient assessment.
This article offers a clear and practical overview of capnography, including how it works, what it reveals about a patient’s respiratory function, and how to interpret the waveforms to make informed clinical decisions.
What Is Capnography?
Capnography is a critical monitoring tool in medical settings, providing data on ventilation and respiratory status. It works by measuring exhaled carbon dioxide, allowing clinicians to assess patient status both in routine and emergency situations.
Definition and Principles
Capnography is the noninvasive monitoring of the concentration or partial pressure of carbon dioxide (CO₂) in exhaled breath. It is commonly used in clinical environments such as operating rooms, intensive care units, and during sedation procedures. The technique relies on the principle that exhaled CO₂ reflects effective gas exchange and ventilation.
Most capnography systems use infrared spectroscopy to detect CO₂ levels in breath samples. The results are displayed as a waveform, known as a capnogram, and as numeric values representing end-tidal CO₂ (EtCO₂). Medical staff use this information to evaluate a patient’s respiratory rate, ventilation effectiveness, and metabolic status.
Capnography provides real-time, continuous feedback, allowing for early identification of changes in a patient’s airway, ventilation, or circulatory status. It is particularly valuable for confirming endotracheal tube placement and monitoring for respiratory depression.
Types of Capnography
Capnography can be divided into two main types: mainstream and sidestream.
Mainstream capnography measures CO₂ directly at the airway using a sensor placed between the breathing circuit and the endotracheal tube. This method provides fast, accurate readings with minimal delay but may increase dead space and is typically used for intubated patients.
Sidestream capnography samples gas through a small tube connected to the patient’s airway and transports it to an analyzer away from the breathing circuit. Sidestream devices are more versatile since they can be used with masks or nasal cannulas in both intubated and non-intubated patients but may have a slight delay and risk of sample line blockage.
The choice between types depends on factors such as patient age, ventilation method, clinical setting, and required response time.
Key Components of Capnography Devices
Every capnography system consists of several key components.
The CO₂ sensor is the core part, either placed directly at the airway (mainstream) or within an analyzer unit (sidestream). The sampling line or airway adapter ensures continuous gas flow to the sensor.
A display unit visualizes the capnogram waveform in real time, showing both the shape of the breath and numerical EtCO₂ values. User controls allow for adjustment of settings, alarm limits, and calibration of the device.
Alarms and alerts are built in to notify users if CO₂ levels fall outside safe parameters. Reliability and ease-of-use features are critical in high-stress medical environments. Device maintenance, including regular calibration and cleaning, ensures accurate and consistent readings.
How Capnography Works
Capnography measures exhaled carbon dioxide to monitor ventilation status. The method used, the waveform produced, and end-tidal carbon dioxide (EtCO₂) values each provide valuable clinical insights.
Mainstream vs. Sidestream Measurement
Capnography devices use two primary methods: mainstream and sidestream measurement. Mainstream capnography places the sensor directly between the airway and the breathing circuit, providing immediate readings. This allows for faster response times and reduces lag, which is useful for critical care or during anesthesia.
Sidestream capnography draws a small sample of gas from the breathing circuit and transports it through tubing to a sensor in the main unit. This setup is more flexible and can be used with a wider range of equipment, including non-intubated patients. However, it can introduce a slight delay and may be affected by secretions or blockages in the sample line.
Both methods are accurate but differ in setup, maintenance, and specific clinical applications. Selection depends on patient needs and clinical circumstances.
The Capnogram Waveform
A capnogram is a graphical display of CO₂ concentration over time during the respiratory cycle. It consists of several phases, each representing a part of the breath.
- Phase I (baseline): beginning of exhalation, showing gas from the airways (with little or no CO₂).
- Phase II: mixture of airway and alveolar gas, producing a sharp upstroke.
- Phase III (alveolar plateau): mostly alveolar gas, resulting in a plateau.
- Phase 0: the start of inhalation, bringing CO₂ back to zero.
Changes in the shape, height, and slope of the capnogram can indicate issues such as airway obstruction, hypoventilation, or equipment malfunction. Accurate waveform interpretation is essential for patient safety.
Interpreting EtCO₂ Values
End-tidal carbon dioxide (EtCO₂) is the maximum CO₂ concentration at the end of exhalation, measured in mmHg or kPa. Normal adult values range from 35–45 mmHg.
Decreased EtCO₂ levels may indicate hyperventilation, low cardiac output, or disconnection of the system. Elevated EtCO₂ may suggest hypoventilation, increased metabolic activity, or rebreathing. Sudden absence of an EtCO₂ reading can result from apnea, airway disconnection, or cardiac arrest.
Interpreting EtCO₂ values in context with clinical observations and the capnogram waveform allows clinicians to detect and respond to emergencies quickly. Regular calibration and maintenance of the device ensure accurate measurements.
Clinical Applications of Capnography
Capnography is widely used in modern healthcare for direct measurement and real-time analysis of exhaled carbon dioxide. Its main clinical applications include verification of airway placement and monitoring of ventilation during sedation.
Airway Management
Capnography is considered the gold standard for confirming endotracheal tube (ETT) placement. By measuring end-tidal CO₂ (EtCO₂), clinicians can immediately verify if a tube is in the trachea or mistakenly placed in the esophagus. A sustained presence of exhaled CO₂ with a characteristic waveform indicates successful tracheal intubation.
It can quickly detect accidental dislodgement or obstruction of the airway by showing a change or loss in the waveform. During cardiac arrest, the presence or absence of CO₂ can help assess the effectiveness of chest compressions and determine the likelihood of return of spontaneous circulation (ROSC). Capnography also helps with ongoing airway monitoring in ventilated patients, allowing rapid recognition of hypoventilation, apnea, or ventilator malfunction.
Monitoring During Procedural Sedation
Capnography is critical during procedural sedation because it provides early detection of respiratory depression, often before changes are seen in oxygen saturation. It enables real-time monitoring of the patient’s ventilatory status by continuously displaying EtCO₂ levels and waveforms.
Clinicians use these readings to identify hypoventilation, airway obstruction, or apnea, which can be caused by sedative medications. Intervention is possible before hypoxemia develops, improving patient safety. Guidelines from anesthesia and emergency medicine societies recommend capnography for moderate-to-deep sedation to reduce the risk of undetected respiratory compromise. Capnography also assists in adjusting sedation depth safely by providing immediate feedback on the patient’s breathing.
Capnography in Critical Care
Capnography is valuable for continuous monitoring of ventilation, providing real-time feedback on respiratory status. It is used in various critical care situations to enhance patient safety and inform clinical decisions.
Use in Anesthesia
Capnography is an essential tool during anesthesia. It confirms endotracheal tube placement by detecting exhaled CO₂, reducing the risk of unrecognized esophageal intubation. Anesthesia providers rely on capnographic waveforms to monitor ventilation throughout surgical procedures.
The EtCO₂ value helps assess the adequacy of ventilation and guides adjustments to ventilator settings. Sudden changes in readings can indicate airway obstruction, circuit disconnection, or changes in metabolic rate. Continuous monitoring also aids in the early detection of hypoventilation or apnea, allowing for immediate intervention.
Note: Capnography protects against silent respiratory events when other monitoring may not be sufficient.
Role in Emergency Medicine
In emergency medicine, capnography is frequently used during cardiopulmonary resuscitation (CPR). It helps evaluate the effectiveness of chest compressions by measuring partial pressure of CO₂ in exhaled air. Return of spontaneous circulation is often indicated by a significant rise in EtCO₂ during resuscitation.
Capnography assists with airway management, especially in confirming correct placement of advanced airways in pre-hospital and emergency department settings. It can also guide ventilation in patients with altered consciousness or respiratory compromise.
Capnography is useful for predicting outcomes in cardiac arrest and guiding decisions during advanced life support. It is standard practice in many emergency departments due to its rapid, non-invasive, and continuous feedback.
Troubleshooting and Limitations
Capnography, while valuable, can be prone to specific errors and interpretive challenges. Both equipment-related artifacts and physiological factors can limit its reliability in certain clinical scenarios.
Common Artifacts
Capnograph readings may show artifacts that mimic or obscure real respiratory changes. Moisture, secretions, or fogging in the sampling line can cause false low or erratic end-tidal CO₂ (EtCO₂) values.
Leaks in the breathing circuit or loose connections can result in unusually low or zero CO₂ readings even if the patient is ventilating. Sudden movement or agitation can introduce spikes or flatlines in the waveform. In pediatric patients, small tidal volumes can cause waveform distortion.
Cleaning and regularly checking all connections can prevent most artifacts. Regular calibration of the capnograph device also reduces technical problems. Prompt recognition of artifact patterns helps avoid misinterpretation and unnecessary interventions.
Interferences and Accuracy Issues
The accuracy of capnography may be affected by non-CO₂ gases such as oxygen, nitrous oxide, or anesthetic agents, which can dilute the sampled gas and reduce EtCO₂ readings. Sampling from sites other than the airway, like from a mask or in non-intubated patients, can also result in less reliable data.
In patients with low tidal volumes or rapid respiratory rates, the waveform may be blunted or incomplete. Conditions like severe hypotension or pulmonary embolism can alter the correlation between EtCO₂ and arterial CO₂ (PaCO₂).
Providers should be aware of these limitations, especially in settings with altered ventilation or perfusion. Consistent waveform quality and correlation with clinical findings are essential for accurate interpretation.
Capnography Practice Questions
1. What is capnography?
Capnography is a technology that provides a real-time graphic and numerical display of a patient’s ventilatory status by measuring end-tidal CO2.
2. What three physiological processes can cause changes in the capnographic waveform?
Ventilation, perfusion, and metabolism.
3. Why is capnography especially valuable for EMS responders?
It offers the fastest, non-invasive method to detect changes in a patient’s clinical condition.
4. What are the four most common capnography waveforms seen in the prehospital setting?
Normal, hypoventilation, hyperventilation, and bronchoconstriction.
5. What does the capnograph specifically measure?
End-tidal carbon dioxide (EtCO2), which is the amount of CO2 at the end of exhalation.
6. According to ILCOR Guidelines, what is a Class I recommendation for capnography?
To confirm endotracheal tube placement and detect tube dislodgement.
7. Which two phases of the capnogram are most important for interpretation?
Phases II and III.
8. What does a normal capnography waveform look like?
A square or rectangular “box-like” shape with a clear plateau phase.
9. Why is capnography considered a reliable tool for monitoring ventilation?
Because EtCO2 levels correlate with changes in alveolar ventilation and respiratory rate.
10. According to ILCOR, what is a Class IIb recommendation for capnography?
To evaluate the effectiveness of chest compressions during CPR.
11. If a patient in pain shows a small, rapid waveform, what change in the waveform is expected after pain relief?
The waveform should normalize with a slower respiratory rate and improved EtCO2.
12. What metabolic condition can capnography help detect early?
Metabolic acidosis.
13. According to ILCOR, what is another Class IIb recommendation for using capnography?
To detect return of spontaneous circulation (ROSC) during resuscitation.
14. For which patients is capnography becoming the standard of care?
Both intubated and non-intubated patients.
15. What does a capnogram look like during hypoventilation?
A square waveform with elevated EtCO2 levels due to CO2 retention.
16. What is the normal range for EtCO2?
35–45 mmHg.
17. What are some advantages of capnography?
Early detection of apnea, non-invasive monitoring, ease of use, and assessment of perfusion status.
18. Monitoring patient trends is not necessary when using capnography. True or False?
False.
19. Capnography is expensive and requires extensive training to use. True or False?
False.
20. Which of the following statements is correct regarding capnography?
E) Only b & c are correct (It uses breath-to-breath measurement and measures EtCO2).
21. Capnography is only useful during general anesthesia to assess perfusion. True or False?
False.
22. Capnography can help reduce the need for sedation reversal agents. True or False?
True.
23. In non-intubated patients, monitoring baseline capnography waveforms is essential during sedation. True or False?
True.
24. A patient’s EtCO2 waveform suddenly drops to zero during a procedure. What should you do first?
D) Both b & c (Check the patient and ensure the equipment is connected properly).
25. What is the most appropriate method for monitoring EtCO2 in a sedated patient undergoing a wrist reduction?
Sidestream ETCO2 detection.
26. What is the difference between capnography and capnometry?
Capnography provides both a numeric value and a waveform of exhaled CO2, while capnometry only provides a numeric value.
27. What does volumetric capnography compare?
It compares the amount of exhaled CO2 to the exhaled tidal volume.
28. What are five clinical applications of capnography?
Verification of ET tube placement, assessment of airway integrity, CO2 monitoring, evaluation of ventilator circuit, and validation of end-tidal CO2 values.
29. What does PETCO2 stand for and how is it measured?
Partial pressure of end-tidal CO2; measured using an infrared analyzer close to the patient’s airway.
30. What are the components used to measure PETCO2?
An infrared analyzer, a sensor in the vent circuit, and a capillary tube delivering gas to a spectrometer.
31. What is the primary method for analyzing CO2, and what are two other methods?
Primary: Infrared analysis; Others: Raman spectroscopy and mass spectrometry.
32. What are three facts about infrared CO2 analyzers?
They require a computer to calculate values, automatically correct for temperature and water vapor, and need 10–15 minutes to warm up.
33. How are capnographs calibrated and zeroed?
They are calibrated with a 5% CO2 gas mixture and zeroed using room air regularly.
34. What is the expected CO2 pressure when calibrating with 5% CO2 at body temperature?
Approximately 35.65 mmHg.
35. What are the two main sampling methods in capnography?
Mainstream (direct at airway for ventilated patients) and sidestream (aspirated through tubing for spontaneously breathing patients).
36. What are common alarm parameters in capnography?
High and low PETCO2, high and low respiratory rate, and apnea detection.
37. What are the phases of a normal capnogram?
I: End of inspiration, II: Dead space exhalation, III: Alveolar exhalation (EtCO2 peak), IV: Start of next inspiration.
38. What are the two common speeds for capnography waveform analysis?
12.5 mm/sec for breath-by-breath analysis and 25 mm/min for long-term trend analysis.
39. What five features define a normal ETCO2 waveform?
Height (EtCO2 level), frequency (RR), rhythm (CNS or ventilator control), baseline (zero), and shape (box-like).
40. What are the normal values for ETCO2?
35–45 mmHg or ~5%, with a normal PaCO2–EtCO2 gradient of 4–6 mmHg.
41. What three main variables affect ETCO2 levels?
Metabolism (↑ metabolism = ↑ ETCO2), transport/perfusion (↑ perfusion = ↑ ETCO2), and ventilation (↑ ventilation = ↓ ETCO2).
42. What is the formula for calculating PACO2?
PACO2 = (VCO2 × 0.863) / VA.
43. What factors increase CO2 production?
Fever and hypercatabolic states.
44. What factors decrease CO2 production?
Hypothermia and hypometabolic states.
45. What is deadspace in respiratory physiology?
Ventilation without adequate perfusion.
46. How does deadspace affect ETCO2?
Increased deadspace raises the PaCO2–EtCO2 gradient, making EtCO2 resemble PICO2.
47. What is a shunt in respiratory terms?
Perfusion without ventilation.
48. How does a shunt affect ETCO2?
An increased shunt lowers the PaCO2–EtCO2 gradient and makes EtCO2 resemble PvCO2.
49. What are three causes of an increased PaCO2–EtCO2 gradient?
Pulmonary embolism, cardiac arrest, and pulmonary hypoperfusion.
50. What is the formula to calculate alveolar deadspace?
VD = (PaCO2 – PECO2) / PaCO2.
51. How do PaCO2 and EtCO2 measurements differ in accuracy?
EtCO2 more closely reflects inspired CO2 (PICO2) and total CO2 production, while PaCO2 reflects arterial blood CO2.
52. What are three causes of a decreased PaCO2–EtCO2 gradient?
Increased CO2 production, decreased alveolar ventilation (e.g., COPD), and equipment malfunction.
53. What is the most likely cause of a shark-fin capnography waveform in a respiratory arrest patient?
Bronchoconstriction, commonly seen in asthma.
54. What EtCO2 value suggests a poor chance of successful resuscitation during cardiac arrest despite high-quality CPR?
Less than 10 mm Hg.
55. A ___ carbon dioxide device uses pH-sensitive paper and is typically used for a one-time confirmation of endotracheal tube placement.
Colorimetric.
56. Capnography provides information about the production of which gas at the cellular level?
Carbon dioxide.
57. What is the normal range for end-tidal CO2 (EtCO2)?
35–45 mm Hg.
58. A capnogram waveform that becomes progressively taller may indicate which condition?
Hypoventilation.
59. What is the name of the highest point on the capnogram?
End-tidal CO2 (EtCO2).
60. Which method is considered most reliable for detecting airway obstruction and accidental extubation?
Continuous EtCO2 monitoring.
61. After how many breaths can endotracheal extubation be detected using capnography?
After the first breath.
62. What condition can cause a sudden and unexpected drop in EtCO2 on a capnogram?
Cardiac arrest.
63. What does a “shark-fin” shaped capnography waveform indicate?
Bronchoconstriction or obstructive airway disease.
64. What would be the baseline (Phase I) value on a capnogram in a healthy 23-year-old patient?
0 mmHg
65. In capnography, EtCO2 can be displayed as either a numeric value or a:
Waveform
66. What is the approximate percentage of carbon dioxide in room air?
0%
67. Where does carbon dioxide transfer into exhaled air?
At the alveoli.
68. What is the source of the CO2 measured by capnography?
Cellular metabolism of glucose.
69. What change in the capnogram is expected with increased CO2 production?
A higher EtCO2 value.
70. In capnography, inspiration begins at which phase?
Phase IV
71. If an EtCO2 value suddenly drops to zero and the waveform disappears, what should be your first action?
Check the patient for apnea.
72. What does Phase II (expiratory upstroke) of the capnogram represent?
The arrival of exhaled CO2-rich alveolar gas at the sensor.
73. The presence of a waveform on the capnogram of an intubated patient confirms what?
Proper tracheal placement of the endotracheal tube.
74. What is the name of Phase I of the capnogram, and what does it represent?
The respiratory baseline, representing exhalation of CO2-free dead space gas.
75. Capnography can confirm proper placement of which airway device?
Combitube airway.
76. In a traumatic brain injury patient, when should assisted ventilation or advanced airway placement be considered?
If EtCO2 rises above 45 mm Hg.
77. What does a sudden loss of capnogram waveform during ventilation most likely indicate?
Dislodged or disconnected tracheal tube.
78. A gradually increasing Phase I baseline on a capnogram indicates what?
Rebreathing of carbon dioxide.
79. What metabolic waste product is produced in every cell of the body from glucose metabolism?
Carbon dioxide
80. What is the most common cause of a low EtCO2 reading?
Hyperventilation
81. During early expiration (Phase I), what should the EtCO2 value be?
0 mm Hg
82. What should an infant’s or child’s capnogram waveform look like?
The same as an adult’s waveform.
83. What is the most appropriate method to confirm tube placement after intubating a post-ROSC patient?
Capnography with waveform analysis.
84. How should the plateau phase (Phase III) of a normal capnogram appear?
Gradually sloping upward.
85. What does a sudden spike followed by a flatline on the capnogram suggest?
Equipment disconnection or complete airway obstruction.
86. Which capnography phase is most useful in detecting bronchospasm?
Phase III (alveolar plateau).
87. How does hypoperfusion affect EtCO2 readings?
It causes a decrease in EtCO2 levels.
88. What is the expected EtCO2 trend in a patient with increasing intracranial pressure?
EtCO2 may rise due to hypoventilation and poor perfusion.
89. What happens to the EtCO2 level during prolonged apnea?
It drops to zero.
90. What clinical use of capnography can help detect a pulmonary embolism?
A sudden and sustained drop in EtCO2.
91. What effect does excessive ventilation have on the capnography waveform?
Shorter waveform height indicating hypocapnia.
92. During CPR, what EtCO2 level suggests high-quality chest compressions?
Greater than 10 mm Hg.
93. What is a common cause of a rounded Phase III waveform?
Partial airway obstruction or poor alveolar emptying.
94. What EtCO2 pattern is commonly seen after successful return of spontaneous circulation (ROSC)?
A sudden increase in EtCO2.
95. Why might EtCO2 rise in a febrile patient?
Fever increases metabolic rate and CO2 production.
96. How does capnography help monitor sedation depth?
Decreasing respiratory rate and increasing EtCO2 can indicate oversedation.
97. What does a flattened Phase II upstroke indicate?
Delayed transition from dead space to alveolar gas due to airflow obstruction.
98. What would a sudden drop in EtCO2 during a procedure likely indicate?
A possible cardiac arrest or major circulatory collapse.
99. How does EtCO2 reflect changes in ventilation?
It increases with hypoventilation and decreases with hyperventilation.
100. What capnography change may occur with a clogged expiratory limb in a circuit?
Elevated baseline and retained CO2.
Final Thoughts
Mastering the principles of capnography can greatly enhance a student’s ability to evaluate and monitor respiratory function in real time. By recognizing normal and abnormal capnographic patterns, future healthcare professionals can quickly identify ventilation issues, guide interventions, and improve patient outcomes.
As you continue your education, keep in mind that capnography is more than just a number—it’s a powerful window into the patient’s airway, breathing, and circulation.
Written by:
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.
References
- Pandya NK, Sharma S. Capnography and Pulse Oximetry. [Updated 2022 Aug 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023.
- 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.
- Wollner EA, Nourian MM, Bertille KK, Wake PB, Lipnick MS, Whitaker DK. Capnography-An Essential Monitor, Everywhere: A Narrative Review. Anesth Analg. 2023.