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Capnography in Veterinary Anesthesia

Liz is a licensed veterinary medical technologist. She acquired a B.S. in veterinary medical technology from Lincoln Memorial University.

Capnography is an important component of veterinary anesthesia.

Capnography is an important component of veterinary anesthesia.

What Is Capnography?

Capnography is the measurement and evaluation of the levels of CO2 in a patient’s exhaled breath, or the end-title CO2 (ETCO2). This is an important component of anesthesia because it helps the anesthetist evaluate the patient’s respiratory rate, and also the quality of the respirations (breaths). Therefore, better interpretation of the patient’s anesthetic depth is allowed.

Arterial blood gas sample evaluation is the gold standard method of determining blood CO2 levels, but this is typically not feasible in most smaller practices. ETCO2 is an affordable and accurate alternative.

How Does It Work?

The capnograph measures ETCO2 by one of two methods; by a mainstream or sidestream device. Mainstream devices evaluate ETCO2 levels as the breath passes through the airway. The sensor requires that the patient be intubated, and the results correlate with the breathing pattern. Sidestream devices use a vacuum force to draw a tiny portion of the exhaled breath through a small tube to the main unit of the device for evaluation.

Because of this "dead space" between the patient and the device, there is a delay in the collection of the sample, which translates into a displayed pattern that is not in sync with the patient’s actual breathing pattern. Adequate observation and hand monitoring of the patient is always recommended in conjunction with any anesthetic monitoring device; never rely solely on the information on your monitors, as it is not always correct. No monitor can replace an actual anesthetist with good monitoring skills.

Parts and Display

Parts of a capnograph include a computerized base unit with a fitting that is placed between the endotracheal tube connector and the breathing circuit. The display on a capnograph is a digital readout in the form of a continuous wave. The incline of the wave indicates the exhaled portion of a breath, whereas the plateau at the peak of the wave indicates the ETCO2 value. As the patient starts inhaling, the wave declines because the inspired CO2 levels are near zero.

Assembled capnograph sensor.

Assembled capnograph sensor.

Using Capnography During Anesthesia

CO2 is produced in the tissues, is carried by the vasculature, and is eliminated by the lungs. Abnormal readings may be caused by disease of the lungs, cardiovascular system, tissues, or by equipment malfunction. The normal levels of expired CO2 in dogs and cats should between 35 and 45 mm Hg (millimeters of mercury). Levels that deviate from this range require quick evaluation to determine the appropriate corrective course of action.

Values below 35 mm Hg indicates hypocarbia and can be attributed to excessive artificial ventilation, increased respiratory rate, a light plane of anesthesia, lessening of ventilation, pain, or hypoxia. Administration of analgesics, lessening ventilation, increasing anesthetic depth, or treating underlying hypoxia may help resolve the hypocarbia.

Values above 45 mm Hg indicates hypercarbia may be caused by hypoventilation, decreased tidal volume, exhausted soda lime, malfunction of the anesthesia machine, or a kink in the endotracheal tube. Elevated CO2 levels may imply that some or all of the exhaled gas has been rebreathed. Ways to lower CO2 include checking the anesthesia machine and increasing ventilation.


Personal experience as a veterinary anesthetist.

Bassert, J., Thomas, J. (2014). McCurnin’s Clinical Textbook for Veterinary Technicians. (8th ed.) St. Louis, MO. Elsevier.

Lerche, P., Thomas, J. (2011). Anesthesia and Analgesia for Veterinary Technicians. (4th ed.) St. Louis, MO. Mosby-Elseveir.

Tear, M. (2012). Small Animal Surgical Nursing: Skills and Concepts. (2nd ed.) St. Louis, MO. Elsevier.

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

© 2018 Liz Hardin