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Circle system

Created: 30/11/2004
Updated: 13/5/2010



Figure 1: Circle system

The essential features of the circle absorber are: a CO2 absorber canister (C), breathing bag (B), unidirectional inspiratory (Vi) and expiratory (Ve) valves, fresh gas supply (F) and pressure-relief valve (V). The absorber is connected to the patient via corrugated hoses and a Y-piece (not shown) attached to the inspiratory and expiratory valves (Vi and Ve).

The position of the breathing bag and pressure-relief valve may vary in relation to the absorber, but the above is a common and satisfactory arrangement.

Bag in circuit

Figure 2: Bag in circuit


Figure 3: Function of circle system

Inspiration - Inspiration causes the expiratory valve to close, and gas flows from the breathing bag to the patient via the inspiratory limb of the circuit. Anaesthetic is taken up from the in-circuit vaporiser (VIC), if fitted.

Expiration - The inspiratory valve closes and gas flows into the breathing bag via the expiratory limb. CO2 is absorbed by the soda lime canister. Excess gas is vented when necessary via the pressure-relief valve.

Closed and semi-closed

The circle absorber may be used as a closed or semi-closed system:

Closed systems: the pressure-relief valve is closed so that no gas escapes from the system. O2 flows into the system to replace that consumed by the patient, and exhaled CO2 is absorbed by the soda lime.

The advantage of closed systems is that anaesthetic and O2 consumption, and atmospheric pollution, is minimised.

The disadvantages are:

(a) The system is inherently unstable, in that if the fresh gas flow is not matched exactly to the patient's O2 consumption, the system will over-fill or empty, and the patient will be unable to breathe.
(b) The fresh gas flow rate is usually too small to allow use of a precision, out of circuit vaporiser.

Semi-closed systems: the pressure relief valve is opened, allowing excess gas to escape from the system. This allows higher fresh gas flow rates to be used.

The advantages of semi-closed systems are:

(a) The system is more stable in that, if the system fills to capacity, the excess gas is simply lost via the pressure-relief valve.
(b) The higher flow rates allow use of a precision, out of circuit vaporiser.

The disadvantage is increased anaesthetic and O2 consumption and atmospheric pollution.

Unidirectional valves

The unidirectional inspiratory and expiratory valves in most circle absorbers are of the turret type, in which the pressure generated by the patient's breathing causes the disc to rise and allows gas to pass in one direction only. Most have a transparent dome so that the operation of the valve may be observed.

Figure 4: Gas flow in unidirectional valves

The disc material may be mica, ceramic or plastic. Plastic is less expensive, but tends to warp and allow the valve to become incompetent. Incompetence may also be caused by the valve sticking in the open position, owing to condensation of water vapour. Incompetent inspiratory or expiratory valves will reduce the efficiency of gas circulation and result in rebreathing and consequent CO2 retention.

Some machines are equipped with valves made of deformable rubber:

Figure 5: Use of rubber valves in circle system

As the rubber ages, these discs tend to harden in a semi-open position, again allowing the valve to become incompetent.

Connecting tubing

The body of the absorber is connected to the patient by means of inspiratory and expiratory tubes and a Y-piece. This may be constructed of corrugated black rubber, neoprene or, more recently, plastic.

Figure 6: Connecting tubing

Recently, the so-called Universal F circuit has become popular. This is a co-axial system, the inspiratory tube running inside the expiratory limb:

Figure 7: Universal F circuit

This arrangement aids warming and humidification of the inspired gases, albeit at the expense of an increase in inspiratory resistance to breathing. One problem with this system, as with other co-axial circuits, is that, if the inner tube breaks or becomes disconnected at the absorber end (which may not be noticed on casual inspection), the entire volume of the tube becomes apparatus dead space. It should also be noted that, in all other aspects, this system is identical in function to a conventional, dual-limb system, and does not provide an economical alternative to the Bain system (although it is occasionally marketed as doing so).

Patient size

Most circle absorbers are satisfactory for use in patients weighing up to around 100 kg.
The major problem with using standard circle absorbers in smaller patients is that of dead space. Patients with very small tidal volumes may not generate enough pressure to open the valves effectively. The effective dead space of the Y-piece is larger than it appears. Inevitably, some portion of the expired gas is directed down the inspiratory limb of the circuit, and some portion of the inspired gas comes from the expiratory limb, and some mixing of inspired and expired gases occurs.

Figure 8: Apparent dead space, expired gas and effective dead space in circle absorbers

These difficulties may be reduced by the use of purpose-built infant absorbers, which are smaller than the standard models. Paediatric tubing and Y-pieces, which are simply smaller in diameter than the standard type, may be helpful.

In- and out-of-circuit vaporisers

An inhalation anaesthetic agent may be supplied from a vaporiser positioned in the circle itself (vaporiser in circuit, VIC) or in the fresh gas flow from the anaesthetic machine (vaporiser out of circuit, VOC).

Vaporiser in circuit (VIC)

These arrangements have a low-resistance vaporiser placed in the inspiratory limb of the circle, from which the anaesthetic agent is vaporised by the gases circulated around the system by the patient's breathing. 

Figure 9: Vaporiser in circuit

VIC systems are still occasionally used, since they employ an inexpensive vaporiser and provide some degree of autoregulation of the anaesthetic concentration. If the plane of anaesthesia becomes too light, respiration will be less depressed, minute volume will increase, more agent will be vaporised and the plane of anaesthesia will deepen. It is, however, found that this is not very reliable in practice.

Although low-resistance vaporisers are usually relatively inefficient (with the output of, for example, halothane limited to around 2.5% to 3%), the concentration of anaesthetic inspired by the patient may be very much higher than this because the gas entering the vaporiser also contains anaesthetic from previous circulations. Indeed, after full equilibration of the circuit and patient, the inspired concentration would equal the saturated vapour pressure of the anaesthetic, although, obviously, the patient would have expired long before this point was reached. It is therefore strongly recommended that an inhalation anaesthetic analyser be used to monitor the inspired concentration whenever such systems are used.

In-circuit vaporisers can be used with closed or semi-closed systems.

Since water vapour exhaled by the patient condenses in the vaporiser, it is necessary to drain in-circuit vaporisers regularly.

Vaporisers out of circuit (VOC)

These systems have the considerable advantage that a precision vaporiser may be used to introduce a precisely known concentration of anaesthetic into the circuit. However, since this delivered concentration is diluted by the gas already contained in the circuit, the concentration inspired by the patient is not known with certainty. The rate of change of anaesthetic concentration in the circuit depends upon the fresh gas flow rate: a high fresh gas flow rate will achieve equilibration much faster than if a low fresh gas flow rate is used: 

Figure 10: Concentration in circuit at different fresh gas flow rates 

The above graph shows anaesthetic concentrations in a typical paediatric circle system as a fraction of the out-of-circuit vaporiser concentration, using different fresh gas flow rates (0.2 to 4 L/min). When low flow rates are employed, the concentration of anaesthetic in the circuit will change very slowly, which may cause difficulty in maintaining a satisfactory plane of anaesthesia.

Out-of-circuit vaporisers are usually used in semi-closed systems; the low fresh gas flow rate required in closed systems usually makes their use impractical.

Double-canister absorbers

Many absorbers designed for use in human patients employ two canisters placed in series; the top canister is exposed to the expired gases first and removes most of the CO2. Any remaining CO2 is then removed by the bottom canister. When the top canister is exhausted, the absorbent is discarded, the bottom canister is placed in the top position and a canister with fresh absorbent is inserted underneath it. This arrangement provides optimal efficiency and economy in CO2 absorption. However, these absorbers are bulkier, heavier and more expensive than single-canister models. [Note: Some machines have the gas entering from the bottom, so the described process is in reverse].

Figure 11: Double-canister absorber

If this type of absorber is used, it is a false economy to fill only one of the two canisters. The soda lime will be exhausted at the same rate, the efficiency of absorption will be reduced and the greater volume of the circuit will delay equilibration of the gas in the circuit with the fresh gas supplied from the anaesthetic machine. This will not only slow induction and recovery, but will also tend to increase consumption of the inhalation anaesthetic.

Operational requirements

 The volume of the breathing bag must be greater than the patient's inspiratory capacity. This is usually estimated at 30 ml/kg body weight. 
 Since soda lime contains 50% - 70% air around the granules, the volume of the absorber canister should be at least double that of the tidal volume of the patient for optimal efficiency.

Fresh gas flow requirements

Closed systems

In truly closed systems, the patient consumes O2 and expires CO2, which is removed from the system by absorption. The volume of O2 flowing into the system must, therefore, equal the patient's O2 consumption.

Resting O2 consumption is approximated by the formula:

O2 consumption (ml/min) = 10 x BW 0.75 (where BW is the body weight (kg)).

Body weight (kg) O2 consumption
(ml / min)
5 33
10 56
20 95
40 160

Table 1: O2 consumption by body weight in closed systems

The use of nitrous oxide in closed systems presents the difficulty that, after equilibration, nitrous oxide will accumulate in the circuit and result in a hypoxic breathing mixture. If it is desired to use nitrous oxide in a closed system, it is mandatory to employ an inspired O2 concentration monitor.


When using a semi-closed system, the O2 flow rate must exceed the patient's O2 consumption. Any excess is simply lost via the pressure relief valve.

When using an out-of-circuit vaporiser, the fresh gas flow rates employed are a compromise between achieving a reasonable rate of change of anaesthetic concentration and economy of anaesthetic consumption.

Initially, it is necessary to use both a high flow rate and high vaporiser setting to raise the concentration of anaesthetic in the circuit. For maintenance, both the vaporiser setting and fresh gas flow rate may be reduced.

As a general rule, a flow rate of 2 to 3 L/min initially, and 500 ml to 1 L/min during maintenance of anaesthesia, will usually prove satisfactory.

The following must be present to prevent rebreathing:

 A unidirectional valve must be located between the patient and the reservoir bag on both the inspiratory and the expiratory limb
 The fresh gas inflow cannot enter the circuit between the expiratory valve and the patient
 The overflow valve cannot be located between the patient and the inspiratory valve.

Advantages of the circle system

 Economy of anaesthetic consumption
 Warming and humidification of the inspired gases
 Reduced atmospheric pollution


 Unstable if used closed
 Slow changes in the inspired anaesthetic concentration with low flows and out-of-circuit vaporiser


[i] The effects of inflow, overflow and valve placement on economy of the circle system. Eger EI 2nd, Ethans CT. Anesthesiology 1968; 29: 93-100

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