ASE Home
Resources
Compressed gas
Regulators
Low pressure gas
Quick-flush
Flowmeters
Vaporizers
Breathing systems
Non-rebreathing
Magill
Lack
Ayre's T-piece
Humphrey ADE

Rebreathing

Endotracheal tubes

Scavenging
Anesthesia Service and Equipment

Bain Circuit

A co-axial modification of the basic T-piece system, developed to facilitate scavenging of waste anesthetic gases.

Construction

Bain Circuit

An tube carrying fresh gas (F) travels inside an outer reservoir tube (R) to the endotracheal tube connector (P).

Function

Essentially, the Bain circuit functions in the same way as the T-piece, except that the tube supplying fresh gas to the patient is located inside the reservoir tube.
Inspiration -The patient inspires fresh gas from the outer reservoir tube.
Expiration - The patient expires into the reservoir tube. Although fresh gas is still flowing into the system at this time, it is wasted as it is contaminated by expired gas.
Expiratory pause - Fresh gas from the inner tube washes the expired gas out of the reservoir tube, filling it with fresh gas for the next inspiration.

Operational requirements

  • Generally similar considerations apply as for the T-piece.
  • Fresh gas flow requirements--see separate article.
  • The Bain is more efficient at eliminating exhaled gas, since the fresh gas is directed down the endotracheal tube, which reduces dead-space.

Modifications
A bag may be added to the tail of the reservoir tube, as in the T-piece.

Alternatively, the circuit may be attached to a block assembly with a pop-off valve and mounted directly to the common gas outlet of the anesthesia machine. This arrangement facilitates scavenging and intermittent positive pressure ventilation.

Advantages

  • Compact and inexpensive.
  • Low dead-space.
  • Low resistance to breathing.
  • Facilitates scavenging of waste gases.

Disadvantages

  • High fresh gas flow requirement in larger animals.
  • High gas flow rates, e.g. if the oxygen flush valve is used, may cause lung barotrauma.
  • As with other co-axial systems, if the inner tube becomes disconnected or breaks, the entire breathing tube becomes dead-space, leading to severe alveolar hypoventilation. This may be detected in systems fitted with a bag by closing the valve and activating the oxygen quick-flush. If the inner tube is intact, the venturi effect of the rapidly-moving stream of gas leaving the inner tube will suck gas out of the bag and the bag will empty. If the inner tube is damaged, the stream of gas will be directed into the bag and it will fill.



    Alternatively, the so-called 'parallel Bain' system may be used. In this system, the inner and outer tubes are replaced by conventional circle-absorber-type tubing and Y-piece. This arrangement is also used in the Humphrey ADE system.


Uses
In small animals under 10 kg body weight.

Humphrey ADE   

Comments on this article should be addressed to Dr Guy Watney
All pages © Anesthesia Service and Equipment 1998-2003.