Vetamac Vapors Vol. XIV Issue 3: Capnography


Vetamac Vapors Vol. XIV Issue 3 Fall 2018

By:Kelli Gill, RVT, VTS (Anesthesia & Analgesia)

There are two methods for End Tidal Carbon Dioxide (ETCO2) monitoring in an anesthetized patient. One is blood gas analysis, which is considered the gold standard, and the other is capnography. The former requires a higher skill level to obtain a sample from a peripheral artery, perform the test and interpret the results. The latter is a non-invasive method and requires an understanding of normal parameters and in some cases interpretation of waveforms. For the purposes of this newsletter, we will focus more on capnography.

End Tidal Carbon Dioxide (ETCO2) monitoring is essential in the opinion of many anesthetists. ETCO2 evaluates how the patient is ventilating under anesthesia and is also an indicator of effective gas exchange in the lungs. ETCO2 readings correlate very closely with PaCO2 (CO2 dissolved in plasma). A normal range of ETCO2 is 35-45mmHg.

There are two different types of ETCO2 monitoring equipment. A capnometer will give you a numeric value of ETCO2 and a respiratory rate (RR). A capnograph gives a numeric value, RR as well as a waveform. When you know how to interpret the waveforms, you can save yourself a lot of time troubleshooting some technical issues pertaining to equipment, as well as patient status.

There are two types of capnographs; side stream and mainstream. These terms refer to the way the gas is sampled. Side stream capnographs have a long, clear tube connected to an adapter that goes in between the endotracheal tube and the breathing circuit.  This tube draws gas samples and delivers it to a monitor. Because of the length of the tubing, it creates a delay in readings. Additionally, the tube can collect moisture and skew readings. A moisture trap on the monitor should be emptied out, and the sampling tube should be disconnected from the unit and hung up to dry when not in use. These tubes can also be easily replaced if necessary. The other type of capnograph mentioned is referred to as mainstream. This type will usually have a sensor with an adapter that takes immediate readings from in between the trach tube and breathing circuit. This type usually requires an adult and pediatric adapter that should be calibrated every time it is switched out or replaced. The purpose of having a smaller adapter for pediatrics is to reduce mechanical dead space.

As mentioned in the second paragraph, the normal range for ETCO2 is 35-45mmHg. A value less than 35mmHg indicates hypocarbia. This means the patient is blowing off too much CO2.  Reasons for this include hyperventilation which can indicate a light plane of anesthesia or possible breakthrough pain. A value greater than 45mmHg indicates hypercarbia caused by hypoventilation. Reasons for this could be a deep plane of anesthesia or ventilatory assistance needs to be strongly considered. Any patient under anesthesia, especially in dorsal recumbency, is going have impeded respiratory function and should be supplemented with assisted or mechanical ventilation (For more information on ventilation visit

Vetamac Vapors Summer 2017 and Fall 2017 issues). This also applies to patients affected by obesity or a space occupying mass either in the abdomen or thorax. Cardiac output can also be reflected on a waveform and a sharp decrease can indicate impending cardiac arrest.  Always assess the patient’s depth before addressing anything else. One lung intubation, esophageal intubation, and sticky one-way valves can also be detected on waveforms. The capnograph can be instrumental while attempting to troubleshoot mechanical or physiological issues in the anesthetized patient.  

If you have questions on how to read your capnograph waveforms, please refer to “Anesthesia for Veterinary Technicians”, Susan Bryant or you can call your local Vetamac service technician.