Breathing systems attach between the anaesthetic machine (at the common gas outlet) and the patient (endotracheal tube). There are at least six different breathing systems, in use in veterinary practice. The functions of breathing systems are 1) to deliver oxygen (± anaesthetic vapour) to the patient, 2) to remove exhaled carbon dioxide, and 3) to allow the anaesthetist provide intermittent positive pressure ventilation (IPPV)--by squeezing the bag. They may also be used to sample inhaled and exhaled gases for levels of carbon dioxide (CO₂), airway pressure etc. The basic components of all breathing systems are similar: connections to the anaesthetic machine, patient and scavenging system; reservoir or re-breathing bag; various lengths of tubing; pop-off and other (one-way) valves; ± soda lime. However these are arranged in different ways which alter how the systems work.

The classification of breathing systems is confusing; the simplest is that which considers whether there is soda lime (rebreathing systems) or not (non-rebreathing systems). However, the accuracy of this grouping depends on how re-breathing is defined; for the purposes of this paper, re-breathing pertains to alveolar gas which is rich in CO₂. Two non-rebreathing systems allow rebreathing of exhaled dead space gas. To prevent an increase in inhaled CO₂ when using breathing systems without soda lime, it is essential that the correct fresh gas flow is used at all times. When using systems with soda lime it is necessary that the soda lime is not exhausted and that one way valves (if present) are functioning correctly. Four systems in common use do not contain soda lime: the T-piece (and its numerous modifications), Bain, Magill and (Mini) Lack. There are two systems which do contain soda lime: the To-and-Fro and Circle. Circles may be used with a vaporiser out-of-circuit or vaporiser-in-circuit (e.g., Stephens and Komesaroff). Finally, there is a hybrid system (the Humphrey ADE) which may be used with or without soda lime.

Choosing a Breathing System

This depends on:

 The body weight of the patient: some To-and-Fro and Circles may present excessive respiratory resistance for small patients. In the Mini-Lack and T-piece the diameter of the tubing is too narrow for large patients. Moreover, the gas flows required for the T-piece and Bain make their use uneconomical in larger patients.

 Whether IPPV is required: the Lack and Magill are generally not suitable for use with IPPV as, unless very high gas flows are used, alveolar gas is forced back into the patient.

 Economy of gas flows required and concerns over environmental and operating room pollution: re-breathing systems work efficiently at considerably lower gas flows than non-rebreathing systems. In the spontaneously breathing patient the (Mini) Lack and Magill require less fresh gas than the T-piece or Bain.

 Availability of disposable systems: if there is a risk of cross-infection the non-rebreathing systems are readily available (and inexpensive) for single-use.

Table 1. Suggested breathing systems for dogs and cats.

Using Non-rebreathing Systems

As stated above, it is essential that the correct fresh flow is used to prevent rebreathing of gas rich in CO₂. The gas flow is based on a multiple of the patient's minute volume (weight x tidal volume x respiratory rate). Thus, if the patient's respiratory rate increases, the fresh gas flow should be increased (Table 2). Failure to recognize this leads to a rise in levels of CO₂ (hypercapnia) with further tachypnoea. If the respiratory rate remains in the expected range for the patient, then the fresh gas flow does not need to be altered during anaesthesia. Advantages of these systems include 1) the patient will receive the same percentage of inhalant agent as the dial setting of the vaporiser, thus it is quick and simple to alter the depth of anaesthesia, 2) 66% nitrous oxide / 33% oxygen may be used safely.

Table 2. Suggested fresh gas flows (without IPPV).

*tidal volume approx: 10-15 ml kg^-1, minute volume: 200 ml kg^-1
**flow may be reduced if capnography is used.

Using Non-rebreathing Systems

The Circle and To-and-Fro systems may be used in the same manner in veterinary anaesthesia i.e., only oxygen that is required for cellular metabolism need be supplied, as exhaled gas is re-breathed. A safe estimate that also affords ease of calculation is 10 ml kg^-1 min^-1. These systems are thus economical to run and result in less atmospheric pollution. Additionally, inhaled gas is warmed and moistened.

True 'low-flow anaesthesia' involves provision of just enough fresh gas for metabolism, for the entire duration of anaesthesia. The breathing system is used in the 'closed' mode with the pressure relief valve remaining shut. This method requires considerable skill to maintain and alter depth of anaesthesia and for this reason a simpler and more reliable method is detailed below.

 Following induction of anaesthesia and endotracheal intubation, a fresh gas flow of 100 ml kg^-1 min^-1 is provided for 5 to 10 minutes. The pressure relief valve should be fully open during this period. The advantages of an initial high flow include 1) the system is purged of air (nitrogen) and filled with fresh gas (oxygen and anaesthetic agent), 2) it provides sufficient inhaled anaesthetic agent for rapid stabilisation of anaesthetic depth.

 Once anaesthetic depth is stabilized, the fresh gas flow may be reduced to approximately 10 ml kg^-1 min^-1. Alternatively, 0.5-1 litres min^-1 is adequate for all dogs weighing less than 50 kg. Moreover, many vaporisers require a fresh gas flow of at least 0.5 litres min^-1 to ensure accuracy. At this stage, the pressure relief valve should be partially closed so that the reservoir bag contains adequate gas, but there is no detectable increase in pressure within the system; the valve may require occasional adjustment during anaesthesia.

Some general points when using low-flow anaesthesia:

 The contribution of fresh gas to the overall volume of the breathing system is small when using low flow anaesthesia. Moreover, exhaled breath usually contains less anaesthetic agent than was inspired by the patient, especially at the beginning of an anaesthetic. The net result is that the exhaled breath 'dilutes' the effect of the fresh gas, and patient receives considerably less volatile agent than the vaporiser setting would suggest. The lower the fresh gas flow used and the larger the patient, the greater the discrepancy between the vaporiser setting and the inspired breath. In addition, this is more noticeable with halothane as it takes longer to equilibrate in the body than isoflurane or sevoflurane.

 The initial vaporiser setting should be maintained until depth of anaesthesia is well stabilized. When the gas flow is reduced, a higher vaporiser setting is usually required, for the reason stated above.

 During the course of an uneventful anaesthetic, the vaporiser setting may be increased or decreased by small increments (0.5 to 1.0%), without varying the gas flow.

 If it is necessary to alter the depth of anaesthesia rapidly:

 The vaporiser setting should be adjusted

 The gas flow increased to 100 ml kg^-1 min^-1 for a several minutes, and

 The pressure relief valve opened fully

 The contents of the reservoir bag may be 'dumped' into the scavenging system although this is not essential.

 This allows the new vapour setting to wash into the system rapidly. When the depth of anaesthesia has stabilised, the lower gas flow should again be used.

 During the low-flow period the pressure relief valve may be partially closed, but some gas will be vented to the scavenging system.

 If nitrous oxide is used in a re-breathing system, care should be taken to provide adequate inspired oxygen. Because oxygen is continually consumed by the patient, and nitrous oxide is not, the proportion of oxygen will decrease over time. In the absence of an inspired oxygen analyzer

 50% oxygen : 50% nitrous oxide should be used

 Oxygen flow should be set at a minimum of 20 ml kg^-1 min^-1

 If the breathing bag empties, it should be refilled by increasing the gas flow at the flowmeter. Using the oxygen flush mechanism will result in dilution of the volatile anaesthetic agent.

 At the conclusion of anaesthesia, gas flow should again be increased and/or the contents of the reservoir bag 'dumped' to flush out exhaled anaesthetic agent which will re-circulate and prevent the patient from awakening.

What System(s) Should I Choose for Small Animal Practice?

There is not one system which is suitable for use in all patients from a 1-2 kg Yorkie to an 80 kg mastiff, unless IPPV is used for every anaesthetic; in this case a Circle will suffice. However, if patients are allowed to breathe spontaneously then a minimum of two breathing systems are needed.

 A re-breathing system is the most economical for patients over 10-12 kg (check the manufacturer's guidelines for minimum size of patient). Of the two re-breathing systems the Circle has many advantages over the To-and-Fro. This system may also be used in smaller patients if IPPV is imposed.

 For patients under 10 kg a Mini-Lack is more economical to use than an Ayre's T piece, however the former is not suitable for IPPV.

 The Humphrey ADE (with soda lime attachment for dogs > 10-20 kg) is an alternative to the above systems.

References

1.  Hughes L. (2007) Breathing systems and ancillary equipment. In BSAVA Manual of Canine and Feline Anaesthesia and Analgesia, 2nd Edition. Ed. Seymour C and Duke-Novakovski T. pp 30-48. BSAVA

2.  Al-Shaikh B, Stacey S. (2001) Essentials of Anaesthetic Equipment, 2nd Edition. Churchill Livingstone, New York

3.  Davey AJ, Diba A. 2005. Ward's Anaesthetic Equipment, 5th Edition. Elsevier Saunders, Philadelphia

4.  Dorsch JA, Dorsch SE. 1999) Understanding Anesthetic Equipment, 4th Edition. Lippincott Williams and Wilkins, Pennsylvania

5.  Walsh CM, Taylor PM. 2004. A clinical evaluation of the 'mini-parallel Lack' breathing system in dogs and cats and comparison with a modified Ayre's T piece. Veterinary Anaesthesia and Analgesia 31, 207-212.