Waters' Canister

Construction

A carbon dioxide absorber canister (C) is connected to a breathing bag (B), the fresh gas supply (F), the patient (P) and a pressure-relief valve (V).

Function

The patient breathes to-and-fro into the circuit. Expired carbon dioxide is absorbed by the soda-lime. Excess gas is vented when necessary via the pressure-relief valve.

One problem with the original horizontal canister is that, unless it is tightly packed, the soda-lime tends to settle and allow channeling of the gas above the granules:

This can lead to substantial rebreathing and can be avoided by ensuring that the soda-lime is tightly packed using a nylon pot scrub pad. An alternative, and superior, approach usually used in large animal systems is to use a vertical canister so that channeling cannot occur:

Operational requirements

• As for the Circle absorber.

Advantages

• Inexpensive and portable.

Disadvantages

• The position of the canister close to the patient's head is a major inconvenience.
• The to-and-fro pattern of breathing causes the soda lime to become more rapidly exhausted at the patient end of the canister, leading to inefficiency in soda-lime use and a progressive increase in apparatus dead space.
• Channeling of gas can lead to rebreathing with horizontal canisters.
  

Uses
Little used in small animals, but is still useful for field anesthesia of large animals, where portability of the equipment may be a major concern.

Draw-over apparatus