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Capnography is the continuous, quantitative measurement of carbon dioxide (CO2) in a patient’s breath, usually shown as a number and a waveform on a monitor. It tells clinicians about ventilation, which pulse oximetry does not. The American Society of Anesthesiologists (ASA) names it as a quantitative method for continual end-tidal CO2 analysis during general anesthesia.
Short answer
Capnography monitors the carbon dioxide in exhaled gas. In anesthesia practice it is one of the quantitative methods the ASA standard names for confirming ventilation, and respiratory gas monitors that measure CO2 are covered by an international equipment standard, ISO 80601-2-55. This guide explains what the term covers, how it relates to similar terms, and where it matters for clinics and equipment decisions.
Definition and scope
Three terms appear together in professional standards: capnography, capnometry and end-tidal carbon dioxide. Capnometry is the numeric measurement of CO2 in respired gas. Capnography adds a continuous waveform of CO2 against time, so the trace itself can be watched, not only the number. End-tidal CO2 is the CO2 level at the end of an exhaled breath, and it is the value most monitors display alongside the waveform.
The ASA Standards for Basic Anesthetic Monitoring (originally approved October 21, 1986; last amended October 15, 2025) uses all three phrases. It requires “continual end-tidal carbon dioxide analysis” to be “performed using a quantitative method such as capnography, capnometry or mass spectroscopy.” So in that standard, capnography is one accepted quantitative method, not the only one.
Capnography is a ventilation measure. It is different from pulse oximetry, which reports oxygen saturation and does not measure CO2, and from a blood gas analyzer, which measures a sample rather than the continuous breath stream.
How it is used in practice
The ASA standard places capnography inside its ventilation requirement (section 2.2.2). The wording matters because it differs by type of anesthesia and sedation:
- Every patient receiving general anesthesia must have the adequacy of ventilation continually evaluated, and “every effort should be made to achieve continual monitoring of expired carbon dioxide.” Qualitative signs such as chest excursion and observation of the reservoir breathing bag are listed alongside it.
- When an endotracheal tube or supraglottic airway is in use, continual end-tidal CO2 analysis from the time of placement “shall be performed” using a quantitative method.
- When capnography or capnometry is used, the end-tidal CO2 alarm must be audible to the anesthesiologist or the anesthesia care team. The standard marks this requirement as one the responsible anesthesiologist may waive in extenuating circumstances.
The same standard sets a lighter requirement elsewhere. For local or regional anesthesia, adequacy of ventilation is assessed by continual observation of qualitative clinical signs. For moderate or deep sedation, qualitative signs are observed continually and every effort is made to monitor exhaled CO2 continually.
Related concepts at a glance
| Setting in the ASA standard | What it says about ventilation and CO2 |
|---|---|
| General anesthesia | Ventilation continually evaluated; every effort to monitor expired CO2 continually |
| General anesthesia with endotracheal tube or supraglottic airway | Continual end-tidal CO2 analysis using a quantitative method such as capnography, capnometry or mass spectroscopy; audible alarm when capnography or capnometry is used |
| Moderate or deep sedation | Qualitative signs observed continually; every effort to monitor exhaled CO2 continually |
| Local or regional anesthesia | Continual observation of qualitative clinical signs of ventilation |
This table is a comparison of the wording of one standard, not of devices. It is useful as a checklist: the monitoring a procedure room needs follows the type of anesthesia or sedation being delivered.
Where it matters for labs, clinics and supply decisions
Facilities that give sedation. A clinic or surgical center that provides sedation will usually write capnography into its monitoring policy. Facilities that give sedation can read our guide to moderate sedation policy requirements. Capnography equipment also belongs on the monitoring side of a procedure-room equipment list, next to the oximeter and the ventilator where one is used.
Equipment standards. The international standard for the monitors themselves is ISO 80601-2-55:2018, Medical electrical equipment, Part 2-55: Particular requirements for the basic safety and essential performance of respiratory gas monitors (edition 2, published 12 February 2018, with Amendment 1 published in 2023). Its scope covers respiratory gas monitors used continuously with patients, including anesthetic gas, carbon dioxide and oxygen monitoring. It does not apply to monitors for use with flammable anesthetic agents. A buyer or biomedical engineer reviewing a monitor can ask which edition of this standard the manufacturer’s documentation cites.
Animal research. Monitoring in laboratory animals does not simply copy human practice. Our guide on mouse anesthesia depth monitoring covers how anesthetic depth is monitored in mice.
Common misunderstandings
- “Capnography and pulse oximetry do the same job.” They do not. Oximetry reports oxygen saturation; capnography reports CO2 in respired gas.
- “Capnography is required in every anesthetic setting.” The ASA standard is strict for general anesthesia with an airway device and softer for sedation (“every effort”) and regional anesthesia (qualitative signs).
- “Capnography, capnometry and end-tidal CO2 are interchangeable.” They are related but distinct: a waveform method, a numeric method, and the value both report.
- “Meeting the ASA standard means a monitor meets ISO 80601-2-55.” One is a practice standard for anesthesia care; the other is a product-safety standard for equipment. They address different questions.
Frequently asked questions
What does capnography measure?
The carbon dioxide in respired gas, reported continuously. The end-tidal value is the CO2 at the end of exhalation.
Is capnography the same as capnometry?
No. The ASA standard lists them as separate methods. Capnometry gives a numeric reading; capnography adds the waveform.
Does capnography replace a pulse oximeter?
No. A pulse oximeter does not measure CO2, so the two answer different questions.
Which standard covers capnography equipment?
ISO 80601-2-55:2018 covers respiratory gas monitors, including carbon dioxide monitoring. The ASA Standards for Basic Anesthetic Monitoring covers when it is used during anesthesia.
Sources
- American Society of Anesthesiologists, Standards for Basic Anesthetic Monitoring, section 2.2.2 Ventilation; approved October 21, 1986, last amended October 15, 2025. Checked October 28, 2026. https://www.asahq.org/standards-and-practice-parameters/standards-for-basic-anesthetic-monitoring
- ISO 80601-2-55:2018, catalogue entry (SFS standards store), edition 2, published 12 February 2018, with ISO 80601-2-55:2018/AMD1:2023. Checked October 28, 2026. https://store.sfs.fi/en/iso-80601-2-55-2018
This page is general product and standards information, not medical advice.
