Bovis ISSN 2398-2993

Anesthetic circuits: overview

Contributor(s): Louise Cox-O’Shea, Vetstream Ltd , Jo Murrell

Introduction

Delivery systems
  • The anesthetic machine supplies oxygen and anesthetic gases to the patient at controlled rates via an anesthetic breathing system. 
  • The functions of an anesthetic breathing system are to deliver fresh gases, remove waste gases, eg carbon dioxide (CO2), and deliver inhalation anesthetics.
  • Large animal anesthetists tend not to use non-rebreathing systems, as these are wasteful of anesthetic gas. 
    • However, non-rebreathing systems may be used for inhalational inductions in calves, where a system with a large internal volume is undesirable.
  • An anesthetic rebreathing system consists of an absorbant canister of soda lime, a reservoir bag, connective tubing, an escape or overflow valve and a common gas inlet for the inflow of anaesthetic gases .
  • An indicator (pH-sensitive) shows when the soda lime is exhausted and ready to be replaced, however it is prudent to record hours of use so as to anticipate the need to replace soda lime.
    • Note: indicators revert back after a time and so should not be solely relied on.
  • The reaction between soda lime and CO2 is exothermic, so the canister should be warm when the circuit is in use.  
  • A rebreathing bag stores exhaled gases until the next inhalation.  
  • The bag must be compliant so as to reduce the pressure changes during inhalation or exhalation.  
  • Unless the bag is totally empty or totally full, it is very compliant.  
  • Too large a bag makes monitoring and controlling ventilation difficult and anesthetic concentrations change more slowly.  
  • Halothane can be absorbed into rubber (the most common material for reservoir bags) and can also cause the rubber to deteriorate with time.  
  • The tidal volume of the patient determines the size of the reservoir bag and of the absorbent canister. The tidal volume of cattle is 7-8ml/kg. 
  • The absorbent should equal or exceed one tidal volume.  
  • The reservoir bag should approximate to 5-6 tidal volumes.  
  • Small animal/ human circle breathing systems may be more suitable for maintenance of anesthesia in small calves, between 15-135kg, rather than a large animal circle. 

To and Fro system

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Circle system

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Closed and Semi-closed systems

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Scavenging

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Further Reading

Publications

Refereed Papers


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