Equis ISSN 2398-2977

Anesthesia: circuits - overview

Synonym(s): Anesthetic breathing systems

Contributor(s): Keith Branson, Matthew Gurney, Craig Johnson, G Mark Johnston, Mark Senior

Introduction

Delivery systems
  • The anesthetic machine   Anesthesia: machines - overview  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 CO2, and deliver inhalation anesthetics   Anesthesia: maintenance - overview  .
  • In equine anesthesia, open and semi-open, non-rebreathing systems are not often found because of the wastage of anesthetic gas. Non-rebreathing systems may however be used for inhalational inductions in foals 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 and an escape or overflow valve. 
  • Equine systems allow some rebreathing and involve the use of a carbon dioxide absorbing material, eg soda lime. The degree of rebreathing depends upon whether the breathing system is being used in a closed or semi-closed manner.
  • 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 to dictate the need to replace soda lime
  • Some indicators revert back after a time. 
  • 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   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 absorbent should equal or exceed one tidal volume (approximately 1 l in a foal and 5 l in an adult). 
  • The reservoir bag should approximate to 5 tidal volumes (5-6 l in a foal and 20-30 l in an adult). 
  • Small animal circle breathing systems may be more suitable for maintenance of anesthesia in small foals 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

  • Recent references from PubMed and VetMedResource.
  • Eicker S W & Cuvelliez S (1990) Equipment for inhalation anesthesia. Vet Clin North Am (3), 543-549 PubMed.

Other sources of information

  • Taylor P M & Clarke K W (2005) Handbook of Equine Anesthesia. 3rd edn. W B Saunders, UK. pp 55-74.
  • Riebold T W, Geiser D R & Goble D O (1995) Large Animal Anesthesia - Principles and techniques. 2nd edn. Iowa State University Press, USA. pp 83-140 & 174-204.
  • Hall L W & Clarke K W (1983) Veterinary Anesthesia. 8th edn. Bailliere Tindall, UK. pp 169-178.


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