Vet Tips

Monitoring:End Tidal Carbon Dioxide (ET-CO2)
01/20/2009

Monitoring: End Tidal Carbon Dioxide (ET-CO2)

Determination of the amount of carbon dioxide (CO2) in the exhaled gas at the end of a breath (ET-CO2) can be used to insure the placement of the endotracheal tube in the trachea and monitor the adequacy of breathing (ventilation). The ET-CO2 is a reflection of the arterial CO2 and is used to determine whether the animal is hypoventilating or hyperventilating. Devices that monitor the amount of CO2 in the animals exhaled gas are called capnometers and the signal they produce can be displayed as a capnogram. The peak expired CO2 gas concentration can also be displayed and is ideally between 35- 25 mg Hg although values ranging from 30-55 mm Hg are acceptable. Both the digitally displayed ET-CO2 (mm Hg) and the capnogram (analog signal) provide important information that can be used to guide and modify anesthetic procedures. There are two types of capnometers, side-stream and main- stream (SHARN Veterinary, Inc.; www.SharnVet.com). Both utilize an attachment or connector that connects or is located at the junction between the endotracheal tube and the Y-piece. Side-stream capnometers draw gas from this site and determine the CO2 concentration at monitor. Main-stream capnometers connect to the monitor electrically. Clinically there is little difference between the two although main-stream capnometers are considered to be more dependable and less subject to failure.

Capnometers are excellent devices for:
1. Determining proper placement and function of the endotracheal tube.
a. Partially obstructed (mucous, blood), kinked or misplaced (esophagus) endotracheal tubes do not provide a normal capnogram or ET-CO2 value.
2. Determining the adequacy of ventilation (CO2)
3. Determining the animals pattern of breathing (regular, irregular, small breaths, large breaths)
4. Helping to insure proper breathing and delivery of inhalant anesthetic
5. Maintaining a stable level or plane of anesthesia

Note: Accurate and valid ET-CO2 values are dependent upon obtaining a normal exhaled (tidal) breath which is normally about 12-15 mg/kg. Since most anesthetic machines do not provide a means to determine a normal breath (tidal volume) a normal breath is assumed when the lungs are inflated to a pressure of 15-20 cm H20. It is critical that the attendant manually inflate the lungs periodically to determine if the ET-CO2 value obtained is similar (with 5-10 mm Hg) of the spontaneously breathing ET-CO2 value. If the manual breath ET-CO2 value is greater than 10 mm Hg different (greater) than the spontaneous breath ET-CO2 value there is probably too much mechanical or biologic (anatomic, physiologic) dead space breathing and the animals breathing should be manually assisted.

Submitted by:
William Muir, DVM, PhD, ACVA, ACVECC, VCPCS
338 West 7th Avenue
Columbus, Ohio 43201