The Mapleson A circuit is inefficient for controlled ventilation as is the Mapleson D circuit for spontaneous ventilation. David Humphrey has designed a single circuit (Figure 5) that can be changed from a Mapleson A system to a Mapleson D by moving a lever on the metallic block which connects the circuit to the fresh gas outlet on the anaesthetic machine. The reservoir bag is situated at the fresh gas inlet end of the circuit, and gas is conducted to and from the patient down the inspiratory and expiratory limbs of the circuit.

Depending on the position of the control lever at the Humphrey block, gases either pass through the expiratory valve or the ventilator port. When the lever is "up" the reservoir bag and the expiratory valve are used, creating a Mapleson A type circuit. When the lever is in the "down" position the bag and valve are by-passed and the ventilator port is opened creating a Mapleson D system for controlled ventilation. If no ventilator is attached and the port is left open the system will function like an Ayre's T piece (Mapleson E ).

 
Circle Systems

An alternative to using high flow circuits is to absorb CO₂ from the expired gases which are then recirculated to the patient. These circuits are known as circle systems, were first devised by Brian Sword in 1926 and require smaller amounts of fresh gas each minute.

Carbon dioxide is removed from the expired gas by passage through soda lime, a mixture of 94% calcium hydroxide and 5% sodium hydroxide, and 1% potassium hydroxide which reacts with CO₂ to form calcium carbonate. Soda lime also contains small amounts of silica to make the granules less likely to disintegrate into powder and a chemical dye which changes colour with pH. As more carbon dioxide is absorbed the pH decreases and the colour of the dye changes from pink to yellow/white. When around 75% of the soda lime has changed colour it should be replaced . The soda lime canister should be mounted vertically on the anaesthetic machine to prevent the gases passing only through a part of the soda lime (streaming).

Fresh soda lime contains 35% water by weight which is necessary for the reaction between carbon dioxide and soda lime to take place. This generates considerable heat. The soda lime may rise in temperature to 40° centigrade. There are therefore additional advantages of using circle systems in that the gases within the circle are warmed and humidified prior to inspiration. (Baralyme is a commercially available CO₂ absorber which contains 5% barium hydroxide instead of sodium hydroxide.)

Design of Circle Systems

A circle system (Figure 6) is composed of two one way valves (one inspiratory and one expiratory), a reservoir bag, a fresh gas inlet, a canister of soda lime and an expiratory spill valve. Although there may be slight differences in the positioning of these components, all the systems function in the same way.

Vaporiser Position. The vaporiser may be placed either outside the circle (VOC) on the anaesthetic machine in its conventional position, or rarely within the circle itself (VIC). Normal plenum vaporisers, with high internal resistance, cannot be used within the circle and a low internal resistance type vaporiser (such as the Goldman) is required. Drawover vaporisers such as the OMV are not recommended for use within the circle because of the risk of over-dosage. Since the gases are recirculated, if the vaporiser is placed in the circle, gas already containing volatile anaesthetic agent will re-enter the vaporiser and the resulting output will exceed the vaporiser setting. This is a particular danger during controlled ventilation when dangerously high concentrations can build up. Vaporisers should only be placed inside the circle (VIC) when inspired volatile anaesthetic agent monitoring is available. It is safer to use conventional plenum vaporisers mounted on the anaesthetic machine outside the circle. In this case the maximum volatile anaesthetic agent concentration achievable within the circle cannot exceed that set on the vaporiser.

Practical Use of Circle Systems. During the first 5 - 10 minutes of an inhalational anaesthetic using a volatile anaesthetic agent in oxygen and nitrous oxide, large amounts of the anaesthetic agent and nitrous oxide will be taken up by the patient, and the nitrogen contained in the patient's lungs and dissolved in their body will be washed out. If low fresh gas flows are used immediately the patient is connected to the circuit the nitrogen will not be flushed out of the circle system and will dilute the anaesthetic agent concentration. This may be prevented by using conventional fresh gas flows of 6litres/min for the first 5-10 minutes of each anaesthetic before reducing the flow rates.

Reducing the fresh gas flow rates. Inspired anaesthetic gases should contain no carbon dioxide and a minimum of 30% oxygen. Exhaled alveolar gas contains a lower concentration of oxygen and around 5% carbon dioxide which is removed from the exhaled gas on passage through the soda lime. A small amount of fresh gas is added before the next breath. At low fresh gas flow rates (<1000mls /min) unless 40-50% oxygen is supplied to the circle, the oxygen concentration within the circle can fall to unacceptably low levels due to the greater uptake of oxygen compared with nitrous oxide. Circle systems should preferably not be used at low flow rates without an oxygen analyser in the inspiratory limb. The lowest fresh gas flow rate of oxygen and nitrous oxide which can be used to ensure that the inspired oxygen concentration remains at a safe level is 1500mls/min (nitrous oxide 900mls/min and oxygen 600mls/min). Conventional flow meters and vaporisers become unreliable if flows are set lower than these levels.

These comments are less important if only oxygen and a volatile agent is being used in the circle. Under these circumstances there is no risk of oxygen dilution and the flows may be reduced to 1000mls/min.

With flows of >1500mls/min the inspired concentration of volatile agent will be similar to that set on the vaporisers. With flows <1500mls/min the volatile agent concentration may fall within the circuit and the setting on the vaporiser may need to be increased.

Halothane, isoflurane and enflurane are all safe to use in circle systems with soda lime, however trichloroethylene (no longer used in the USA or UK) produces a toxic metabolite and must not be used. When the circle system is not in use all fresh gas flows should be turned off to avoid wastage and to prevent the soda lime from drying out.

Several paediatric circle systems have been developed using smaller bore tubing and a one litre reservoir bag. The work involved in breathing through these systems is no greater than with a conventional Mapleson F system.

 
Conclusion

There are many different breathing systems available, and this review has concentrated on the most commonly used ones. It is essential for the safety of patients that an anaesthetist routinely checks the anaesthetic circuit before use and has a thorough understanding of the function and pitfalls of a particular system before using it.

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