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Anesthesia: Carbon Dioxide Monitoring in Anesthetized Animals

P.K. Hendrix, DVM, PhD
Diplomate American College of Veterinary Anesthesiologists

While pulse oximeters monitor oxygen saturation in anesthetized animals, hypercapnia occurs more commonly than hypoxemia.  Affordable, portable monitors are available to measure exhaled CO2.  These units consist of a monitor with a pump (to obtain respiratory gases), and a sampling line with an adaptor that attaches between the endotracheal tube and the breathing hose.

Capnometers are available as either mainstream or sidestream devices.  Mainstream devices sample the gas at the airway without diverting it into a monitoring unit while sidestream devices divert gas from the airway into a monitoring unit for sampling. Although mainstream devices are somewhat bulky at the ET tube connection, sidestream devices have a slight delay between the actual and displayed readouts.

Normal ETCO2 levels are 35-45 mmHg.  The most common cause of increased ETCO2 is hypercapnia secondary to anesthetic-induced hypoventilation.  Normal healthy animals can tolerate some increase in carbon dioxide levels (up to 60-70 mmHg) without suffering detrimental effects.  Extremely high carbon dioxide levels (> 100 mmHg) can cause narcosis and exacerbate deep levels of anesthesia.  The simplest way to correct high ETCO2 is to simply ventilate (“bag”) the animal more frequently.

Low ETCO2 levels are most commonly a result of hyperventilation or diluted exhaled carbon dioxide caused by high oxygen flow rates such as those used with non-rebreathing systems.  Sudden significant reductions in ETCO2 can be due to airway occlusion, ET tube dislodgement, disconnection or cardiac arrest.  Capnometers are also useful during CPR to determine ETCO2 production as an indicator of return of CO2  production and blood flow through the lungs.