Anesthetic circuit: overview

Functions of anesthetic circuits

• The removal of carbon dioxide from the lungs depends upon the bulk flow of gas which does not contain carbon dioxide or which contains less carbon dioxide than the pulmonary arterial blood.
• This is generally considered to be the most important function of circuits because the physiological control of ventilation is centered around the maintenance of normocapnia.
• Because of the importance of the efficient removal of carbon dioxide, circuits are designed with this function in mind. When considering the classification and function of anesthetic circuits it is often useful to ask the question 'How does this circuit deal with the removal of carbon dioxide?'.
• There are three mechamisms that are commonly used to remove CO2 from anesthetic circuits: Dilution, ie T-piece systems, non-rebreathing one-way valve systems, or carbon dioxide absorbents.

• This function is obviously as important to the patient as the removal of carbon dioxide. However, during anesthesia the inspired oxygen concentration is at least 30% and is often 100%. This means that for patients without respiratory dysfunction, oxygen is delivered to the patient in excess of requirements and so its provision is not critical to the design of the circuit.

• When inhalant anesthetic agents are used, these gases are supplied through the circuit with the oxygen and other carrier gases (some of which may be anesthetic agents in their own right) .
• The relationship of the relative concentrations of gases inspired by the patient to those from the anesthetic machine (vaporizer and rotameters) depends upon the type of circuit used and, in some cases, its mode of use.

Sources

Publications

Other sources of information

• Muir W W, Hubbell J A E, Skarda R T & Bednarski R M (2000) Handbook of Veterinary Anesthesia. 3rd edn. St Louis: Mosby Year Book. pp 210-231.
• Hartsfield S M (1996) Anesthetic machines and breathing systems. In: Lumb and Jones Veterinary Anesthesia.3rd edn. Baltimore: Williams & Wilkins. pp 366-408.
• Dorsch J A & Dorsch S E (1999) Understanding Anesthesia Equipment. 4th edn. Baltimore: Williams & Wilkins.