Carbon dioxide monitoring is especially important for patients undergoing anesthesia and for patients who have an airway management device in place.

By Regina Patrick, RPSGT

In 2015, the American Heart Association guidelines for advanced cardiovascular life support recommended the use of waveform capnography—a technology that displays changes in carbon dioxide (CO2) levels in waveform and as a percentage—in intubated patients undergoing cardiopulmonary resuscitation (CPR).[1]

Waveform capnography has spread from being primarily used by anesthesiologists during surgery to being used by emergency medical service workers outside a hospital setting. Its use is now spreading to other units within the hospital, such as the emergency room, critical care units, and general wards.

Carbon dioxide monitoring is especially important for patients undergoing anesthesia and for patients who have an airway management device in place. Signs of an increased CO2 level are confusion, feelings of paranoia or depression, muscle twitching, irregular heartbeat, hyperventilation, seizures, panic attack, fainting, headache, and blurred vision.

However, many of these symptoms will not manifest in a sedated patient or a patient who has an airway management device in place. In such patients, hypercapnia may not be noted until the patient is at serious risk of death. A capnograph can counteract this risk by providing a continuous real-time reading of changes in CO2, which allows medical staff to address improper CO2 levels before complications occur.

Capnography uses infrared spectroscopy to produce waveforms. In the sensor, an infrared beam is passed through a sample of gas (eg, exhaled air) onto a photodetector. The photodetector then produces a signal, based on the amount of infrared light that reaches it. Gases in exhaled air absorb different wavelengths of infrared light, the bandwidth of which is 1–14 µm. The wavelength absorbed by CO2 is approximately 4.3 µm; Conventional capnographs expose a gas sample to several wavelengths (ie, broadband) in the infrared spectrum, which can produce imprecise readings for CO2 because the absorption wavelengths of certain gases and CO2 are very close.

The development of Microstream technology (Medtronic, Minneapolis, MN, USA) in the late 1990s made a change in conventional capnography by directing to the photodetector an infrared beam with a wavelength of 4.3 µm rather than using a broadband infrared beam.[2] Thus, the presence of other gases such as oxygen and nitrogen or gas anesthetics, are not detected.

The correct placement of an airway management device is important to ensure proper ventilation. For example, if a laryngeal mask airway is not placed over the glottis or the cuff folds on itself during insertion, air can not pass through the airway; if an endotracheal tube is placed too high in the trachea or placed too deeply in the trachea so that the end is just above the carina or the tube enters one bronchus, the lungs will be insufficiently ventilated.

On a capnograph, the shape of the waveforms can indicate whether an airway management device is properly inserted, whether sufficient pressure is being applied when an emergency medical services (EMS) worker or hospital staff is administering chest compressions during CPR, and whether rebreathing is occurring during surgery.

The normal capnography waveform appears as a somewhat rounded rectangle: it rises vertically on exhalations as CO2 is exhaled, remains flat at the end of expiration but before the beginning of inspiration, falls vertically on inhalation, and again remains flat at the end of the inhalation until the next exhalation. Changes in the shape of the waveform and the partial pressure of end tidal CO2 (PetCO2) reading (normal level, 35%–45%) in combination with pulse oximetry and respiratory rate readings can alert a medical worker that a problem needs to be addressed. (See Sidebar below.)

For example, the waveform takes on a “shark fin shape” (the wave rises in somewhat of an arc until reaching its maximum height and then quickly falls) when a ventilation-perfusion imbalance occurs and can indicate improper placement of an airway management device, which should be checked and repositioned. Shallow waveforms with a normal shape during CPR indicates insufficient chest compressions. On seeing this waveform, a worker should increase the strength of compressions. Waveforms that suddenly increase and resume a normal shape as a medical worker is giving chest compressions during CPR indicates the return of circulation. At this point the patient should be assessed for breathing, and CPR may need to be continued, as necessary.

Capnography is the best method to monitor changes in CO2 levels. However, in some settings, a capnography monitor may not be available. A colorimetric CO2 detector can be used to help determine whether an airway is established. However, this device can not verify that an airway management device is correctly placed. The colorimetric CO2 detector has a sensor that changes from purple on inhalation (indicating 0% CO2) to yellow on exhalations (indicating 5% CO2). If such a change does not occur, the airway management device should be repositioned.

A leading cause of hypoxemia and death during surgery with general anesthesia is failure of workers to recognize the incorrect placement of an endotracheal tube.[3] However, the incorrect placement of an endotracheal tube in and out of the hospital is reduced when capnography is used. For example, Silvestri and colleagues[4] reported that, among patients who were intubated outside a hospital, all cases of unrecognized misplaced intubation on arriving to the hospital occurred in patients who were not monitored by capnography; by contrast, patients who had CO2 monitoring after intubation had an unrecognized misplaced intubation rate of 0% on arriving to the hospital.

Capnography is often used on sedated intubated patients. As the technology spreads to other units in a hospital, it is being used in more alert patients such as patients on a general care unit. Its use in more alert patients can be problematic with regard to compliance in wearing the sensor. A capnograph sensor that fits beneath the nose (much like an cannula for oxygen) may cause patients to complain of “too much under my nose” or that it “gets in the way when I’m trying to eat.” Thus, patients may remove the sensor. In addition, patients may not understand the importance of why a capnography monitor is needed for their safety (e.g., patients may ask “why isn’t this ‘oxygen thing’ on my finger enough?”). A problem with capnography sensors that fit beneath the nose is that they may not get a good sampling when a person is a mouth breather. Thus, the CO2 reading will not be as accurate. Capnograph manufacturers are working to address these problems.

To date, no contraindications exist with regard to the use of capnography in mechanically ventilated patients. However, future developments in the technology may address issues faced by different patient populations using capnography monitors.

Sidebar: Resources for Capnography Training
Medtronic (Minneapolis, Minn) has a Capno Quiz, which gives short case studies, waveform demonstration, and multiple-choice test. The samples are from patients in an emergency setting, critical care setting, and surgical/medical setting.

In addition, Medtronic also provides a capnography tutorial that explains features of the normal capnography wave and changes in the wave with apnea, hypoventilation, an obstructed airway and other respiratory effect.

A website maintained by Bhavani Shankar Kodali, MD (University of Maryland School of Medicine, Baltimore, MD) provides in-depth discussions of capnography and waveform samples.


Regina Patrick, RPSGT, is a contributing writer to RT. For further information, contact


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