What is Capnography Illustration

Capnography: A Comprehensive Overview (2024)

by | Updated: May 1, 2024

Capnography, a noninvasive diagnostic tool, is a vital component in the monitoring of ventilated patients in various clinical settings.

It provides real-time, continuous monitoring of carbon dioxide (CO2) concentration in exhaled breath, offering valuable insights into a patient’s ventilatory status, metabolic rate, and circulatory status.

The technique has evolved significantly since its inception in the 1950s, expanding its applications beyond the operating room to areas such as emergency medicine, intensive care units, and procedural sedation.

This article aims to provide a comprehensive overview of capnography, its principles, applications, and the interpretation of capnographic waveforms, along with its limitations and potential future developments.

What is Capnography?

Capnography is a noninvasive diagnostic technique that provides continuous monitoring of the concentration of carbon dioxide in exhaled breath. It offers critical insights into a patient’s ventilatory, metabolic, and circulatory status, and is widely used in various clinical settings.

Capnography waveform

Principles of Capnography

Capnography operates on the principle of infrared absorption spectroscopy. In essence, it measures the concentration of carbon dioxide in exhaled air by detecting the amount of infrared radiation absorbed by the CO2 molecules.

The capnograph, the device used in capnography, consists of a light source, a sample chamber where the exhaled air passes through, and a detector.

As the CO2 molecules in the exhaled air absorb the infrared light, the detector senses the changes in light intensity, which are then converted into electrical signals. These signals are processed and displayed as a waveform on a monitor, known as a capnogram.

The capnogram is a time-based graph that shows the partial pressure of CO2 during the respiratory cycle. It typically consists of four phases:

  1. The baseline: No CO2 is present (representing the end of inhalation and the start of exhalation)
  2. The ascending phase: CO2 concentration increases rapidly (early exhalation)
  3. The alveolar plateau: CO2 concentration is relatively stable (late exhalation)
  4. The descending phase: CO2 concentration decreases to reach the baseline again (start of inhalation)

Changes in the shape, height, and frequency of the capnogram can indicate various physiological and pathological conditions, making capnography a powerful tool for patient monitoring.

Capnography waveform phases illustration

Applications of Capnography

Capnography has a wide range of applications in healthcare, owing to its ability to provide real-time, continuous monitoring of a patient’s ventilatory status.

Here are some of the key areas where it is commonly used:

  • Anesthesia: Capnography is a standard tool in anesthesia, used to monitor the adequacy of ventilation during general anesthesia. It helps anesthesiologists to detect hypoventilation or apnea, verify the correct placement of the endotracheal tube, and monitor the patient’s response to resuscitation efforts.
  • Emergency Medicine and Critical Care: In emergency settings, capnography can provide valuable information about a patient’s condition. It can help detect life-threatening conditions such as pulmonary embolism, severe asthma, or cardiac arrest. In critical care, it is used to monitor mechanically ventilated patients and guide decisions about ventilator settings.
  • Procedural Sedation: During procedures that require sedation, capnography can help monitor the level of consciousness and detect respiratory depression early.
  • Respiratory Care: Capnography can be used to assess the effectiveness of various respiratory therapies and interventions, such as bronchodilator therapy in COPD patients or noninvasive ventilation.
  • Sleep Studies: In sleep medicine, capnography can help diagnose sleep disorders, such as sleep apnea, by identifying periods of apnea or hypoventilation during sleep.

Note: While these are some of the main applications, capnography can also be used in many other settings to improve patient outcomes.

Benefits and Limitations of Capnography

Capnography offers numerous benefits in the clinical setting, but like any medical tool, it also has its limitations.


  • Real-time Monitoring: Capnography provides immediate information about the patient’s ventilatory status, allowing for quick detection and response to changes in ventilation.
  • Noninvasive: As a noninvasive procedure, capnography is comfortable for patients and can be used for extended periods without causing discomfort or harm.
  • Quantitative Data: Capnography provides quantitative data about CO2 levels, which can be used to track changes over time and assess the effectiveness of interventions.
  • Early Detection: Capnography can detect changes in ventilation before they become apparent through other signs, such as changes in heart rate or oxygen saturation, allowing for early intervention.


  • Interpretation Challenges: The capnogram can be influenced by a variety of factors, including equipment issues, patient movement, and physiological variations. This can make interpretation challenging, particularly for less experienced clinicians.
  • Limited Information: While capnography provides valuable information about ventilation, it does not provide a complete picture of respiratory function. Other tools and assessments may be needed to fully evaluate a patient’s respiratory status.
  • Technical Issues: Issues such as moisture in the tubing, calibration errors, or equipment failure can affect the accuracy of capnography readings.

What is End-Tidal Carbon Dioxide (ETCO2)?

End-tidal carbon dioxide (ETCO2) refers to the maximum level of carbon dioxide (CO2) present at the end of an exhaled breath, or in other words, at the end of the expiration phase in the respiratory cycle.

It is often measured using capnography and is typically expressed in millimeters of mercury (mmHg) or as a percentage of CO2.

ETCO2 provides valuable information about the adequacy of ventilation and the perfusion status of the patient. It is closely related to arterial CO2 levels in healthy individuals, making it a useful non-invasive indicator of arterial CO2 levels.

However, it’s important to note that in certain conditions, such as severe lung disease or circulatory shock, the correlation between ETCO2 and arterial CO2 levels may be less reliable.

Monitoring ETCO2 is crucial in several clinical scenarios. For instance, during anesthesia, ETCO2 helps assess the patient’s ventilatory status and the correct placement of the endotracheal tube.

In emergency medicine, changes in ETCO2 can indicate serious conditions such as cardiac arrest or pulmonary embolism. In critical care, ETCO2 monitoring can guide ventilator settings for mechanically ventilated patients.

Capnography vs. Capnometry

Capnography and capnometry are both methods used to measure carbon dioxide (CO2) levels in respiratory gases, but they differ in the type of information they provide and how it is displayed.

Capnography is a technique that measures and graphically displays the CO2 concentration against time during the entire respiratory cycle. The resulting graph provides a wealth of information about the patient’s respiratory status.

It allows healthcare providers to monitor the CO2 levels in real-time and observe changes in the waveform that could indicate various physiological or pathological conditions.

Capnography is particularly useful in settings where continuous, real-time monitoring of ventilation is important, such as during anesthesia or in critical care.

Capnometry is a simpler technique that only provides a numerical readout of the CO2 concentration, specifically the end-tidal CO2 (ETCO2) level, which is the maximum level of CO2 at the end of an exhaled breath.

Capnometry does not provide a waveform display, so it offers less detailed information compared to capnography. However, it can still be useful in situations where a quick, single-point measurement of CO2 levels is sufficient.

Summary: While both capnography and capnometry measure CO2 levels in respiratory gases, capnography provides more detailed, real-time information through a graphical display, making it a more comprehensive tool for monitoring ventilation.

FAQs About Capnography

What Does Capnography Measure?

Capnography measures the concentration of carbon dioxide (CO2) in the respiratory gases during the entire respiratory cycle. This includes both inhaled and exhaled air.

The primary value derived from capnography is the End-Tidal Carbon Dioxide (ETCO2), which is the maximum level of CO2 present at the end of an exhaled breath.

What is the Capnography Waveform a Graphical Representation of?

The capnography waveform is a graphical representation of the CO2 concentration against time during the respiratory cycle.

It typically consists of four phases: the baseline, the ascending phase, the alveolar plateau, and the descending phase. Changes in the shape, height, and frequency of the capnogram can indicate various physiological and pathological conditions.

What is True of Quantitative Waveform Capnography?

Quantitative waveform capnography provides a continuous and real-time measurement of CO2 concentration throughout the respiratory cycle.

It not only measures the amount of CO2 but also displays it in a waveform that represents CO2 concentration over time.

This allows for the immediate detection of changes in ventilation and circulation, making it a valuable tool for patient monitoring in various clinical settings.

How is Capnography Measured?

Capnography is measured using a device called a capnograph, which operates on the principle of infrared absorption spectroscopy.

The capnograph consists of a light source, a sample chamber where the respiratory gases pass through, and a detector.

As the CO2 molecules in the gases absorb infrared light, the detector senses the changes in light intensity. These changes are then converted into electrical signals, processed, and displayed as a waveform on a monitor.

Final Thoughts

Capnography, with its ability to provide real-time, noninvasive monitoring of a patient’s ventilatory status, has become an indispensable tool in modern healthcare.

From its foundational principles to its wide-ranging applications, the value of capnography is evident in various clinical settings, including anesthesia, emergency medicine, and critical care.

Despite its limitations, the benefits of capnography, particularly it’s capacity for early detection of respiratory complications, underscore its critical role in patient care.

With ongoing advancements and future prospects, capnography is poised to continue its evolution, further enhancing its capabilities and applications.

As we continue to explore and understand this vital diagnostic tool, capnography will undoubtedly remain at the forefront of patient monitoring, contributing to improved patient outcomes and advancements in medical science.

John Landry, BS, RRT

Written by:

John Landry, BS, RRT

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.


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