Circuits for Inhalation Anaesthesia
World Small Animal Veterinary Association World Congress Proceedings, 2014
Colin Dunlop, BVSc, DACVAA
Advanced Anaesthesia Specialists, Sydney, NSW, Australia

Components of a Typical Inhalation Anaesthesia Machine and Circle Breathing System.

Diagram 1
Diagram 1





Anaesthesia machine components


High pressure
20,000 kPa

Compressed O2 supply

O2 cylinder, valve, outlet connection (coded thread or pin-indexed system)


Medium pressure
450 kPa

Pressure regulator

Pressure regulator, cylinder contents gauge


450 kPa

O2 flow control

Flowmeter & control valve


450 kPa

O2 emergency supply

Flush valve: by-passes vaporiser


Low pressure
5 kPa


Systems to compensate for O2 flow, pressure, and temperature


Low pressure
20 cm H20

Common gas outlet

Connection to breathing system with gas lines from the vaporiser and O2 flush


Minimum pressure
0–15 cm H20

Breathing circuit

- Circle system includes 2 x 1-way valves, CO2 absorber, breathing bag & tubes
- Could be replaced by non-rebreathing circuit such as T-piece, Bain or LAC

Rebreathing or Circle Breathing Systems

In rebreathing or "circle" systems, all or part of the gases exhaled by the anaesthetized patient are returned to the system to be recycled. Rebreathing systems must contain a canister of a chemical absorbent (Diagram 1 - #10) to permit removal of all the carbon dioxide (CO2) exhaled by the patient. It is the removal of exhaled CO2 which permits the gas to be recycled. Change the CO2 absorbent when more than half the canister is exhausted or 125 ml of liquid anaesthetic (half a bottle) has been used in the vaporiser. One kg of absorbent should last around 30 hours for dogs. Carbon dioxide absorbent doesn't add resistance to the breathing circuit. Circle systems are more complex than non-rebreathing systems and typically include 2 "one-way valves" (Diagram 1 - #11) enabling one-way flow of gas in the system. Valves contribute half the total resistance of circle systems. Circle systems allow lower O2 flows to be used which results in less waste of anaesthetic and less environmental pollution. The breathing bag (15) is the compliant reservoir volume for the anaesthetic gases and permits the animal to take a spontaneous breath. The bag must be larger than the largest breath of the patient (usually about 5x tidal volume). The inspiratory breathing tube (16) delivers the fresh anaesthetic gases to the Y-piece; the expiratory tube returns the expired gases containing CO2 back to the absorber canister. Historically hoses are made of corrugated rubber or plastic tubing and are 20 mm ID. They contribute about half of the resistance of circle breathing systems. Newer small volume, low resistance tubes are now available (see below).

The pop-off or pressure relief valve (12) allows release of excess gas pressure or volume from the breathing circuit. The opening pressure of these valves can be adjusted. Gas flows out of this valve during exhalation if fresh O2 flow > metabolic O2 consumption (5 ml/kg/min dogs & cats) or circuit pressure > 2 cm H20.

Low O2 Flow Anaesthesia

The absolute minimum fresh gas flow rate for a rebreathing or circle system that has no leaks is the patient's metabolic O2 requirement:

Metabolic O2 requirement

= BW (kg)3/4 x 10 ml O2/min

= 4 to 6 ml/kg/min for dogs and cats

All of the O2 supplied is metabolized by the patient to CO2 which is removed from the circle system by the CO2 absorbent. Therefore, the breathing bag should not fill up or empty beyond the normal inspiratory/expiratory movement so no gas will flow out of the pop-off valve which can be closed (i.e., "closed circuit" anaesthesia).

Low flow anaesthesia has not been commonly used in veterinary practice because appropriate circle breathing systems and accurate low-flow vaporisers have not been available. Now such equipment is available (see below) and the economic, environmental, and body heat conservation advantages of low-flow anaesthesia in small dogs and cats should be considered.

Non-Rebreathing Systems

Non-rebreathing systems have no CO2 absorbent so all the exhaled gas must be eliminated. These circuits have minimal resistance or dead space. Non-rebreathing systems are most useful in small patients (less than 5 to 10 kg) but require higher O2 flows which increases consumption of anaesthetic and causes more waste of anaesthetic (environmental pollution). Non-rebreathing systems currently used in veterinary practice include: Ayre's T piece, Norman Elbow (Jackson-Rees modification of Ayres T piece) and the Bain's circuit.

Components: Non-rebreathing systems have a reservoir to compensate for variations in the size patients and their tidal (or breath) volume. This reservoir is the volume of the tubing on the expiratory limb of the circuit (connected to the breathing bag) and should be larger than the tidal volume of the patient. Approximately ½ to 2/3 of the volume of each new breath is obtained from this reservoir which fills with fresh gas during the passive phase of expiration.

The diagram below (Diagram 2) shows the gas flow in a Bain non-rebreathing circuit from the initial or early expiration of a patient, followed by the change that occurs as the fresh gas flow continues during the passive phase of expiration. By the end of expiration a reservoir of fresh gas (i.e., gas that doesn't contain CO2) has accumulated at the patient end of the circuit, ready for the next inspiration. If the breathing bag was squeezed in order to give the patient a breath, it is this reservoir of fresh gas that is "pushed" into the patient's airway and lungs. Having the correct fresh gas flow to a non-rebreathing circuit is critical to ensure that patients don't rebreathe CO2.

Diagram 2
Diagram 1


Comparison between breathing systems


Rebreathing or circle systems

Non-rebreathing systems

Patient size

For animals above 5–7 lb (> 2 kg)

Best for animals below 5 lb (2 kg)

Oxygen flow

30 to 50 ml/kg/min

200 to 600 ml/kg/min

Heat loss

Less - inspire warm, humidified gas

More - inspire cold, dry gas

Resistance to flow

More - have longer tubes & valves

Minimal - large tubes & no valves

Dead space CO2

Higher - rebreathe 5 to10 ml/breath

Lower - rebreathe 0 to 3 ml/breath

Equipment cost

Higher initial cost but reusable

Minimal but shorter life (disposable)


Machined components - need setup, leak testing, and troubleshooting

Simple tube - easy to setup and use but high flow so rapidly inc. press


Low O2 flows conserve anaesthetic

High O2 flows waste anaesthetic

WAG pollution



Speed of change in anaesthetic level

Slower - circuit volume dilutes vaporiser concentration

Rapid - vaporiser concentration is the inspired concentration

Vaporiser type

In-circuit or out-of-circuit vaporiser

Only out-of-circuit vaporiser

Typical out-of-circuit vaporiser settings
- Isoflurane

Start: between 2% to 3% +5 to 10 min: between 1.5% to 2%

Start: between 1.5% to 2% +3 to 5 min: between 1% to 1.5%

O2 flow rates

30 ml/kg (down to 10 ml/kg)

200 ml/kg/min

New Technology Permits Use of Circle Breathing Systems for Small Dogs and Cats

Smooth Wall Tubing (SWT) = Efficient and Responsive Breathing Systems

Smooth wall tubing reduces circuit volume by up to 70% with lower resistance compared to corrugated hose. This enables faster breathing system response to changes in anaesthetic delivery from the vaporiser, permitting circle systems to be used on smaller animals (small dogs and cats). Tubing accounts for up to ½ the resistance of circle absorber systems.1 Darvall SWT circuits use low resistance, small diameter tubes. Smooth wall tubing 16 mm ID x 1.6 M long can supporting animals up to 70 kg with less than 0.5 cm H2O pressure drop; SWT 12 mm ID x 1.6 M long can support animals up to 40 kg.2 Smooth wall tubing offers a huge efficiency advantage (volume of gas relative to the size of animal) requiring as little as 63% (SWT 16) and 32% (SWT 12) the volume of 22 mm ID corrugated tubing or Universal F tubing. Breathing circuits are easily changed and low resistance, low volume, kink resistant Darvall SWT breathing circuits permit the use of circle systems on animals as small as 2 kg* which compared to non-rebreathing systems are more economical, cause less environmental pollution, and cause less respiratory heat loss. A heating element can be imbedded into the ribbing of the tubing wall to enable the inspired gas to be heated, which recent research shows is effective at reducing heat loss especially in the "clip/prep" time, from induction to positioning for surgery where substantial heat loss occurs.

Graph 1
Graph 1

Comparative breathing hose resistance (pressure drop) flowing medical air through straightened hoses with 1.5 M patient length. Flows between 10 and 50 L/min were used to simulate peak flow rates in animals with weights in the range 7 to 80 kg.

Circle Breathing System Suitable for Use in Small Dogs and Cats

Circle breathing systems (CBS) have not been commonly used on animals smaller than 10 kg in part because of historical claims of excessive resistance from CO2 absorbent; slow response to changes in anaesthetic concentration and loss of unidirectional gas flow due to failure of 1-way valve closure at small tidal volumes. Delivery of inhalation anaesthetic is primarily a function of the ratio of CBS volume/animal's tidal volume. Circle breathing systems consist of a series of interconnected passages and volumes which do not allow complete mixing of anaesthetic gas. A purpose built CBS with a volume of 1.4 L designed to minimize resistance and maximize speed of response for animals to 2 kg. The CBS included an accumulator on the inspired limb with 12 mm ID smooth-wall breathing tubes at a 150 ml/min fresh O2 flow, demonstrated a rapid rise in isoflurane concentration, typically in 3 to 5 breaths. This was well above the predicted rate of change of concentration based on the time constant and markedly faster than other commonly used veterinary circle breathing systems. Some of these "veterinary designed" circuits performed very poorly, in one case taking more than 10 minutes for isoflurane concentration to rise. The new purpose designed circle breathing system performed similarly in clinical trials with anaesthetised small dogs or cats, proving that CBS can be used at economically low gas flows in cats and small dogs, so replacing high cost non-rebreathing circuits that exacerbate hypothermia.

Graph 2
Graph 2

Rise in isoflurane concentration in a purpose built CBS with a volume of 1.4 L designed to minimize resistance and maximize speed of response for animals to 2 kg. Note the rapid response to increasing and decreasing vaporiser delivered concentration & oxygen flow compared to the predicted time constant.


1.  Dunlop CI, Dunlop JS, Wallis T, et al. Efficiency, volume and flow resistance of anesthesia circle system breathing hose. In: Proceedings from the American College of Veterinary Anesthesiologists Annual Meeting; Sept 2012; San Antonio, TX. Abstract.

2.  Wallis T, Dunlop CI, Dunlop JS, et al. A model for analysis of flow resistance in a circle system designed for small animals to 2 kg. In: Proceedings from the World Congress of Veterinary Anesthesiology Meeting; Sept 23–27, 2012; Capetown, South Africa. Abstract.

3.  Dunlop CI, Wallis T, Dunlop JS, et al. A model for analysis of isoflurane concentration change in a circle system designed for small animals to 2 kg. In: Proceedings from Australian College of Veterinary Scientists Annual Meeting; June 2012; Surfers Paradise, QLD, Australia.

4.  Dunlop CI, Dunlop JS, Curtis RA, et al. Comparison of the dynamic response to changing anaesthetic concentration, in anaesthetic circle breathing systems used on animals from 3 to 20 Kg. In: Proceedings from the World Congress of Veterinary Anesthesiology Meeting; Sept 23–27, 2012; Capetown, South Africa.


Speaker Information
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Colin Dunlop, BVSc, DACVAA
Advanced Anaesthesia Specialists
Sydney, NSW, Australia

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