Expired carbon dioxide How does it work?

Carbon dioxide absorbs infrared light with a wavelength of 4.3 m. Light at this wavelength is shone through a gas sample and the absorption is proportional to the carbon dioxide concentration. A sample of respired gas is withdrawn from the anaesthetic circuit by a pump and analyzed inside the machine. One problem with these machines in rodents is that the gas sampling rate, about l00ml/minute, is too fast and removes excessive amounts of gas from the airway but the problem can be overcome (Larach et al. 1988).

What does it tell us?

A lot of information can be obtained from the continuous measurement of carbon dioxide. At the most basic level the regular rise in carbon dioxide at the end of respiration can be used to determine respiratory rate, and the regularity of respiration can also be assessed. The shape of the capnograph (the plot of carbon dioxide against time) is a good indicator of pulmonary function. The carbon dioxide level should rise rapidly during the first part of exhalation and then flatten off - the "alveolar plateau". If there is pulmonary disease or poor lung perfusion (secondary to poor cardiac output) the alveolar plateau disappears. The level of carbon dioxide at the end of expiration (end tidal carbon dioxide, etCO₂) is normally within a few mmHg of the arterial carbon dioxide level. EtCO₂ is very useful for assessing adequacy of ventilation both during spontaneous respiration and when using a ventilator.

Volatile anesthetic concentration How does it work?

Like carbon dioxide, the volatile anesthetic agents absorb infrared light. There are several absorption peaks that can be used, 10 - 13m being a common one. Light at this wavelength is shone through a gas sample and the absorption is proportional to the anaesthetic vapor concentration. Absorption at other wavelengths may also be measured so that the agent being used can be identified automatically. One problem with these monitors for veterinary use is that methane produced by ruminants can interfere with the measurements (Moens and Gootjes 1993).

Gas and anaesthetic vapor concentrations can also be measured using a different technology, mass spectrometry. Although these machines are expensive they will measure several gases simultaneously and they have a very fast response time. They tend to be used more for research work than clinical monitoring.

What does it tell us?

The monitor displays inspired and end tidal anaesthetic concentrations. The end tidal level is very close to the arterial level and it is the latter which determines depth of anesthesia. Thus when volatile agents are used as the main anaesthetic agent the depth of anesthesia can to some extent be estimated. The drawback is that the amount of volatile anaesthetic needed to abolish the response to a noxious stimulus (defined as the minimum alveolar concentration or MAC) varies between individual animals. For example, Eger et al. (1965) found that the MAC for halothane varied from 0.7 to 1.1% in 7 dogs. Nevertheless, MAC is a very useful way of comparing the potency of volatile anesthetics.

Volatile anesthetic monitors are most useful when patients are anesthetized on rebreathing circuits such as the circle, because neither the inspired nor the expired anesthetic concentrations are known. They are particularly useful when animals are paralyzed with neuromuscular blocking agents because they provide some measure of security that the animal is adequately anesthetized. Non-rebreathing circuits are used almost exclusively in laboratory animal anesthesia and as long as a precision vaporizer is used the inspired anesthetic concentration is known.