
Low-flow anaesthesia occurs when the fresh gas flow (FGF) is significantly lower than the patient's minute volume. The lower the FGF, the higher the fraction of gases recirculated around a circle system and the less the volume of gas excreted from it. Baum 1 suggested that in a low-flow system, the recirculated fraction should amount to at least 50% after carbon dioxide (CO 2 ) absorption.
In 1952, Foldes et al. Circle system structure A circle system is structurally and functionally more complex than older, high-flow, open systems. It is differentiated from other systems by the presence of unidirectional valves to ensure gas flow direction, and a means of absorbing CO 2 from the recirculated gases. The advantage of circle systems is that they enable the use of low or minimal FGF. 3 They are, however, structurally complex systems and when low FGF is used, the inspired gas concentrations do not closely relate to the FGF concentrations delivered by the anaesthetic machine. Sophisticated gas monitors, integrated within modern anaesthetic machines, are therefore essential. For the safe use of circle systems, Eger 4 suggested three rules:
(i) There must be a unidirectional valve between the reservoir bag and the patient, on both the inspiratory and expiratory sides. (ii) Fresh gas must not enter the system between the patient and the expiratory valve. (iii) The overflow valve must not be placed between the patient and the inspiratory unidirectional valve.
Within these constraints, it is possible to have different configurations of circle systems; 5, 6 the functional differences become marked as FGF is reduced.
Key points
• Circle systems enable the efficient practice of low-flow anaesthesia.
• Monitoring of gas concentrations is mandatory.
• At the start of the anaesthetic, use high flow rates to ensure denitrogenation (with N 2 O use), adequate oxygenation, and faster anaesthetic uptake. Clinical experience and pharmacokinetics show that the same principle applies for offloading anaesthetic gases at the end. In the middle, when equilibrium towards clinically desirable concentrations is approached, the fresh gas flow (FGF) can be reduced to minimum values, taking account of O 2 consumption, gas leakage from the system, and any residual uptake by body tissues or anaesthetic tubing.
• Marked changes in alveolar anaesthetic concentrations occur with changes in FGF and rate of anaesthetic uptake.
• Mathematical modelling is a useful tool for elucidating the function of circle systems. With reference to Figure 1 , the best compromise is achieved with the FGF supply downstream of the CO 2 absorber, at Position 1. Fresh gas collects retrogradely in the absorber and reservoir bag, pushing expired gas back towards the overflow valve. As the FGF increases, so does the likelihood of venting fresh gas itself and expired gas, which may already have passed through the absorber. Placing the FGF inlets upstream of the absorber, in Position 2, could help humidify the fresh gas but may cause unnecessary venting of fresh gas and passing of absorbent dust into the inspiratory limb.
During controlled ventilation, gas spills out of the overflow valve during inspiration. To preferentially allow for venting of alveolar gas before it has reached the absorber and to minimize the chance of this being fresh gas, the best location for the overflow valve is at Position 3 or 4.
Equal efficiency can be obtained during spontaneous and controlled ventilation with the reservoir bag in Positions 5 or 6. If the bag is downstream of the absorber at Position 6, then in controlled ventilation, a squeeze of the bag forces retrograde flow of gases already cleared of CO 2 back through the absorber. Placing the bag in Position 5, which is the most common position, allows retrograde filling of the bag with fresh gas. 
CO 2 absorber
The CO 2 absorber is designed to allow both low resistance to flow and avoidance of channelling, which occurs when gas flows between the crystals along paths of least resistance, thus avoiding contact with the absorbent. Soda lime, a commonly used absorbent, consists of about 4% sodium hydroxide, 1% potassium hydroxide, 14-19% water, and the rest calcium hydroxide. 8 The reactions that occur in the CO 2 absorber are:
These reactions have clinical importance for patients, as they result in the conservation of heat and humidity. 9 Monovalent hydroxides are more reactive than divalent calcium hydroxide and can lead to the production of significant amounts of carbon monoxide and formaldehyde, if the absorbent crystals are allowed to dry out. 10, 11 Absorbents also absorb volatile agents. 12 Sevoflurane has been shown to react with soda lime to produce a renally toxic agent, Compound A. This was not found to be of clinical significance to humans. 13 
Function of the circle system
The purpose of using low-flow anaesthesia is to economize on the consumption of increasingly expensive volatile agents and to minimize pollution.
To enhance oxygen and anaesthetic uptake and the excretion of nitrogen (N 2 ), a high FGF is recommended at the start of an anaesthetic.
14 When clinically desirable concentrations are reached, the FGF can be reduced, theoretically to a basal metabolic oxygen consumption rate of approximately 250 ml min À1 . This assumes there is no gas leakage and no further uptake by tissues or tubing, although these do occur. The response time of the system to a step change of gas concentration presented to the system is directly proportional to system volume and inversely proportional to the difference between FGF and tissue uptake. 15 As FGF is reduced, it becomes a significantly smaller part of the gas being delivered to the patient, the majority of which is recirculated gas. With low FGF, the gas concentrations received by the patient bear considerably less resemblance to those delivered by the anaesthetic machine. 7 Measured concentrations depend on the rate of uptake and excretion of any gas at a given moment. This demonstrates the importance of using high FGF at a time when anaesthetic uptake is high (at the beginning), and only using low FGF as anaesthetic equilibrium is approached. Although modern anaesthetic agents do not have a saturated vapour pressure high enough to make harmfully low alveolar O 2 concentrations likely, marked changes in alveolar anaesthetic concentration are likely to occur with changes in FGF and rate of anaesthetic uptake. Taken from Conway's analysis, Figure 3 shows this range of variation in relation to halothane. Note the particularly marked changes at low FGFs. Similar changes have been demonstrated with other inhalation agents. 14 
Inspiratory

Mathematical modelling of breathing systems
In a study undertaken at the University of Bath, 17 a mathematical model called Bath fp was used to calculate pressures, pressure drops, flow rates, volumes, and gas concentrations at various points throughout the circle system. The model assumed a spontaneously breathing patient and a system primed with air. High and low FGFs consisting of 50% O 2 in N 2 O were modelled, all other possible variables being kept constant for a given set of experiments.
For the standard version of the circle system, Figure 4 shows gas mass fractions (i.e. a mass fraction of O 2 of 0.42 corresponds to 50% concentration by volume) at point X in Figure 1 at high (A) and low FGF (B). (NB: Bath fp calculations use gas masses. Mass fraction in a mixture takes into account gas density and volumetric concentration.) The graphs show that at high FGF, O 2 and N 2 O mass fractions rise rapidly, whereas N 2 mass fraction falls and remains low. At low FGF, these changes do not occur reliably in the first few breaths and the gas mass fraction pattern is governed by the patient's respiratory pattern. This emphasizes that if the FGF is reduced prematurely during administration of an anaesthetic, then desired concentrations will not be reached. Figure 5 illustrates the analysis of gas mass fractions at point Y in Figure 1 . It is remarkable how much gas mass fractions vary within the system at different FGFs and at different points in the respiratory cycle. Figure 5 (A) and (B) refer to high and low FGFs, respectively. Figure 5 (A) shows that in early inspiration, there is a rise in the O 2 concentration, which dips again as maximum inspiratory flow rate occurs. This must be because when the patient's inspiratory flow is low, some fresh gas flows retrogradely towards the reservoir bag. As the patient's inspiratory flow rate then increases, O 2 mass fraction dips because some stored air mixed with this additional store of fresh gas is drawn back towards the patient. On expiration, with the inspiratory unidirectional valve closed, fresh gas is once again directed retrogradely, and O 2 mass fraction rises towards the level found in the FGF.
If this is compared with Figure 5 (B) at a low FGF, the O 2 mass fraction measured at the same point during inspiration does not change from that of air. All the FGF goes towards the patient. On expiration, with the inspiratory unidirectional valve closed, FGF flows retrogradely, so that the O 2 mass fraction rises towards that of the FGF, similar to the findings with higher FGF. Note that on the next inspiration, the O 2 mass fraction falls, since all of the FGF is directed towards the patient again. The reader may use the graphs to interpret the related mass fraction changes for the other gases in the system.
Summary of standard circle modelling results
CO 2 levels remain low throughout the system and show the expected pattern immediately downstream of the patient. Unsurprisingly, Bath fp shows that doubling the volume of the absorber improves CO 2 handling but also shows significant improvement in CO 2 handling if the path length of the CO 2 absorber is doubled isovolumetrically.
The maximum volumetric flow rate through the inspiratory tube at various FGF rates is dominated by the patient's respiratory waveform at all but the highest FGF.
If the length or diameter of the breathing tubes is halved, Bath fp shows that the rate of patient oxygenation is not markedly changed, but the rate of anaesthetic agent delivery is improved and an acceptable level of CO 2 is achieved. It was shown that the resistance to flow (pressure drop per unit volumetric flow) was no greater with small diameter tubes (5.5 mm) compared with large diameter tubes (22 mm) at low FGFs, though there was some variation at high FGFs (8.0-7.0 mbar pressure drop when the volumetric gas flow down the tube is 12 litre min À1 ). This suggests that a more compact standard circle system using shorter, narrower tubing is both safe and probably more efficient than the current models.
The overall pressure differences across the unidirectional valves were identical at both high and low FGFs.
The importance of using high FGFs at the start of an anaesthetic in order to enhance oxygenation, promote denitrogenation if N 2 O is being used, and ensure the rapid delivery of inhaled anaesthetic agent is reinforced. The software reveals that the exact fate of the FGF depends on its magnitude in relation to the patient's inspiratory flow rate, which varies throughout the inspiratory cycle.
Mathematical modelling of circle system variants
A modern development of the circle system structure is characterized by a coaxial arrangement of tubing, the inner tube being the inspiratory limb. This modification improves the response time at low FGFs to step changes in fresh gas concentrations. 18 Changing the FGFs and tubing geometry yields results similar to those of the standard circle system. Since the coaxial system is more compact than the standard system, its use for anaesthesia can be encouraged. Experiments were done on a coaxial system with a venturi shrouding the FGF nozzle in place of unidirectional valves. Volumetric entrainment ratios (entrained gas flow/FGF) were more reliably constant through the respiratory cycle at higher FGFs. The venturi was more likely to fail at lower FGFs and at higher ventilation frequencies.
Conclusion
The scientific principles of circle systems described in this article apply across the full spectrum of their use, including paediatric circle systems, although no specific clinical guidance is given on paediatric use. The main advantage of circle systems with appropriate monitoring is that they enable the safe practice of low-flow anaesthesia. Key components of the circle system are unidirectional valves and the CO 2 absorber. Conway demonstrated how alveolar anaesthetic agent concentrations are influenced by both the rate of anaesthetic uptake and FGFs. His work highlighted the importance of using high FGFs initially. Clinical experience shows that the same principle applies for offloading anaesthetic gases at the end. Mathematical modelling using Bath fp has further elucidated the function of circle systems and variants using different FGFs. It demonstrated that the use of coaxial circle systems is safe and that, currently, valveless circle systems do not work well at low FGFs.
Declaration of interest
None declared.
MCQs
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