5. Anaesthetic Circuits

fresh gas flow, together with volatile anaesthetic (if used), enters the inspiratory limb and travels through a one-way non-return valve to the patient. Expired air enters the expiratory limb and passes through a second non-return valve. Excess gas (mixed expired air) is vented through the pressure relief valve on the reservoir bag, with the remainder passing through the CO2 absorbing soda lime and back into the inspiratory limb (Figure 5.2).

The circle is very efficient in terms of conservation of gases, heat and moisture. It is therefore more economical and less polluting than semi-closed systems

: the circle is simple in outline theory but much more complex in detailed execution. The uptake and excretion of the different components within the system can vary greatly, and it is essential to monitor the FiO2, CO2 and agent concentration within the circle. The system is slow to react to changes in the inspired volatile concentration. If nitrous oxide is used in a closed system, then hypoxia is a potential risk as described earlier. Should soda lime be allowed to dry it may react with the CHF2 group of desflurane, isoflurane and enflurane to produce carbon monoxide.

circle system can be used as a genuine closed circuit in which expired gases and volatile agents recirculate more oxygen is added than is required for the patient’s metabolic demand, and CO2 is removed by passage of the gases through soda lime. Enter the system via vaporizers that can be sited either outside (VOC) the circuit or, much less commonly, within the circuit (VIC) as a drawover vaporizer truly closed system delivering only basal flows, it is important to realize that prolonged anaesthesia with nitrous oxide can cause hypoxia

circle is used with a higher than basal FGF with the excess gas being vented through the pressure relief spill valve. concentrations of gases and volatile agent in the fresh gas supply are closer to those in the circle and can be changed more quickly. This represents a compromise between economy and ease of use

total volume of the system, including the breathing hoses, the air in the absorber, reservoir bag and the patient’s FRC, is around 5 litres. This means that high flows must be used for the first 5–10 minutes. A VOC will always show a higher concentration than is being inspired (unless equilibrium has been reached), and rapid changes can only be initiated by a reversion to high flow. If a fully closed system is used, then monitors which measure inspiratory gas and volatile concentrations as close to the patient as possible must be used, and the system should be flushed at regular intervals to minimize the risk of dilutional hypoxia.

systems were classified as A to F, and they all potentially allow rebreathing. They are ‘semi-closed and supply more gas than the patient needs, with the excess being vented to the atmosphere If rebreathing of CO2 does occur, a healthy patient who is breathing spontaneously will respond by increasing alveolar ventilation which will rise, by up to 20 times if necessary, to keep the PaCO2 normal systems are defined therefore in terms of the FGF that is needed to maintain an unchanged PaCO2 in the face of unchanged ventilation.

reservoir bag into which the FGF is directed, a length of corrugated tubing (which is resistant to kinking) and, at the patient end, an adjustable pressurelimiting (APL) valve Spontaneous respiration: the system is very efficient. At the end of inspiration the valve is closed and the reservoir bag is emptying. During expiration, the FGF is filling the reservoir bag, while expired air (dead space gas and then alveolar gas) is passing into the tube. Hence the pressure in both the reservoir bag and the breathing system increase to the point at which the valve opens and vents expired air.

Mapleson C comprises an APL at the patient end with the FGF entering just proximally. short length of tubing connects this to the reservoir bag which, in the classic ‘Waters’ circuit, includes a CO2-absorbing canister is used in resuscitation and in areas such as theatre recovery allow mixing of expired air with the FGF, which must approximate three times minute ventilation to flush the system and prevent rebreathing.

Mapleson B includes a length of corrugated tubing between the FGF and the reservoir bag.

all function as T-pieces, being inefficient for spontaneous respiration but efficient for controlled ventilation they require up to three times the minute ventilation to prevent rebreathing during spontaneous respiration (150–200 ml kg1) but only 70 ml kg1 to achieve normocapnia during IPPV. The respiratory cycle is a sinusoidal waveform, and, in order to prevent rebreathing, the FGF must equal or exceed the peak inspiratory flow rate (PIFR). alveolar gas has moved into the expiratory limb where it mixes with the FGF, and, to prevent rebreathing, the FGF should approach three times the minute ventilation. Mapleson D: the Bain circuit is the co-axial version of the Mapleson D circuit

: this differs from the E only in that it has a reservoir bag, added by Jackson-Rees (who was a paediatric anaesthetist in Liverpool) to allow controlled ventilation. The system has no valves and so there is minimal expiratory resistance, hence its traditional use in paediatric anaesthesia. Effective scavenging is difficult from both the E and F systems.