Breathing systems & circuits

Overview & definition

The breathing system (the breathing circuit) is the interface between the anaesthetic machine and the patient. It delivers the fresh gas (the oxygen, the air, the nitrous oxide) and the volatile agent, it removes the carbon dioxide from the exhaled gas (by flushing or by absorption), and it provides the means for the spontaneous and the controlled ventilation (the reservoir bag or the ventilator). The breathing system is the one component the anaesthetist assembles and checks before every case, and the source of the most common equipment failures (the disconnection, the leak, the kink). [1]

The Mapleson classification: the non-rebreathing and the partial-rebreathing systems

The Mapleson classification (A to F) organises the traditional (non-circle) breathing systems by the arrangement of the components — the fresh gas inlet, the reservoir bag, the expiratory valve, and the tubing. The classification predicts the rebreathing efficiency (the amount of the exhaled gas that is rebreathed) and the fresh gas flow requirement for each system.[3]

• Mapleson A (the Magill): the fresh gas enters at the patient end; the reservoir bag and the expiratory valve are at the machine end. On expiration, the exhaled gas fills the tubing and the bag; the expiratory valve opens to vent the alveolar gas. On the next inspiration, the patient draws the fresh gas from the inlet. The most efficient system for spontaneous ventilation (the fresh gas flow equal to the minute ventilation is adequate). Inefficient for controlled ventilation (the valve behaviour changes with the positive pressure).
• Mapleson A (the Lack): a coaxial version of the Magill — the fresh gas flows through the outer tube and the exhaled gas returns through the inner tube, reducing the bulk and the dead space.
• Mapleson D (the Bain): a coaxial system — the fresh gas flows through the inner tube to the patient, and the exhaled gas returns through the outer tube to the reservoir bag and the expiratory valve. The most efficient system for controlled ventilation (the fresh gas flow of 70 mL/kg/min is adequate). Less efficient for the spontaneous ventilation. The Bain is the standard paediatric system and is used for the transport.
• Mapleson E (the Ayre's T-piece): a valveless T-piece — the fresh gas flows from one arm, the patient is at the base, and the exhaled gas escapes from the expiratory limb. Used in the paediatric anaesthesia (the low resistance, the no valve). Requires a fresh gas flow of 2 to 3 times the minute ventilation to prevent the rebreathing.
• Mapleson F (the Jackson-Rees modification of the T-piece): a reservoir bag with an open tail added to the Ayre's T-piece — allows the manual ventilation and the adjustable pressure relief. Used in the paediatric resuscitation and transport.[3]

The efficiency of each Mapleson system

The efficiency (the fresh gas flow requirement for the no-rebreathing) depends on the system and the mode of ventilation:[3]

• Mapleson A (Magill/Lack), spontaneous: the most efficient non-rebreathing system. A fresh gas flow of about 70 to 100 mL/kg/min (approximately equal to the alveolar minute ventilation) prevents the rebreathing.
• Mapleson A, controlled: very inefficient. A fresh gas flow of 2 to 3 times the minute ventilation is needed. The positive pressure changes the valve behaviour.
• Mapleson D (Bain), controlled: the most efficient. A fresh gas flow of about 70 mL/kg/min (about 5 L/min for the 70 kg adult) prevents the rebreathing.
• Mapleson D, spontaneous: less efficient. A fresh gas flow of about 2 to 3 times the minute ventilation.
• Mapleson E/F (T-piece): a fresh gas flow of 2 to 3 times the minute ventilation, or at least 4 L/min for the child. No valves (low resistance), so the preferred paediatric system.[3]

The circle system: the modern standard

The circle system is the standard adult breathing system in modern anaesthesia. It differs fundamentally from the Mapleson systems in that it rebreathes the exhaled gas (after the CO2 removal by the soda lime), rather than venting it. The components:[2]

• Two unidirectional valves (inspiratory and expiratory) — ensure the one-way flow.
• The CO2 absorber (soda lime) — removes the CO2 from the exhaled gas.
• The fresh gas inlet — from the flowmeters and the vaporiser.
• The reservoir bag — collects the gas and provides the manual ventilation.
• The APL valve — vents the excess gas to the scavenging.
• The Y-piece — connects to the patient. [1]

The circle system allows the low fresh gas flow (as low as 250 mL/min, if the soda lime is efficient), making it economical, humidified and low-pollution. The disadvantage is the complexity (the multiple components and connections) and the slow equilibration at the low flows (the large system volume dilutes the fresh gas changes).[2]

The CO2 absorption chemistry

The soda lime absorbs the CO2 by the chemical reaction: CO2 + Ca(OH)2 → CaCO3 + H2O + heat. The soda lime is a mixture of calcium hydroxide (about 75 per cent), sodium hydroxide (about 3 per cent, the catalyst), potassium hydroxide (about 1 per cent), and silica (the binder). The reaction is exothermic (the canister should be warm) and produces water (the inspired gas is humidified).[1]

The soda lime is consumed in the process and must be changed when exhausted. The exhaustion is signalled by the colour change (the indicator dye, from white/pink to purple/blue), the capnograph (the rising inspired CO2), and the canister temperature (a cold canister is not absorbing). The colour can revert on standing, so it should not be the sole criterion.[1]

The modern CO2 absorbents (the Amsorb, the lithium hydroxide) eliminate the concerns with the soda lime: the compound A production from the sevoflurane, the carbon monoxide from the desflurane, and the strong-base interactions. They are more expensive but safer.[1]

The compound A and the carbon monoxide concerns

• Compound A is a nephrotoxic degradation product of the sevoflurane, formed when the sevoflurane interacts with the strong base (the Baralyme or the desiccated soda lime). The clinical significance is debated (the animal studies at the high concentrations; the human studies at the clinical concentrations are negative), but the regulatory recommendation is a fresh gas flow of at least 2 L/min with the sevoflurane. The modern absorbents (the Amsorb, the non-Baralyme soda lime) do not produce the compound A.[1]

• Carbon monoxide is produced when the desflurane (or the other agents) interact with the desiccated soda lime or the Baralyme. The desiccated absorbent (left running overnight or over the weekend with the high gas flow) is the critical factor. The prevention is to keep the absorbent moist (change it regularly, do not leave the gas flowing through the absorber overnight) and to use the modern absorbents that do not produce the CO.[1]

The circle system at the low flows

The circle system at the low flow (below 1 L/min) has the specific considerations:[2]

• The volatile agent delivery — the vaporiser dial setting must be higher than the target alveolar concentration (the rebreathing dilutes the fresh gas). The inspired and the expired agent must be monitored.
• The oxygen delivery — the patient's own oxygen consumption (about 250 mL/min) must be supplied by the fresh gas. The inspired oxygen must be monitored at the circuit (the oxygen analyser).
• The nitrogen washout — the initial high flow (3 to 5 L/min for 5 to 10 minutes) is needed to wash out the nitrogen from the lungs and the circuit before reducing to the low flow.
• The slow equilibration — the system volume (the circuit, the absorber, the lungs) is large; the dial changes take minutes to equilibrate at the low flows.
• The monitoring — the capnograph, the agent analyser and the oxygen analyser are essential at the low flows.[2]

The closed-circuit anaesthesia

The closed-circuit anaesthesia (the fresh gas flow exactly equal to the patient's uptake — the oxygen at 250 mL/min, the volatile agent at the liquid injection rate) is the extreme form of the low flow. It is the most economical and the most humidified technique, but it requires the precise knowledge of the patient's oxygen consumption (the Denison–Watson–Smith closed-circuit technique) and the agent uptake (the square root of time model of the uptake). It is rarely practised in the routine, but it is an examined topic for the physics and the pharmacokinetics.[2]

The circle system components in detail

• The unidirectional valves: the rubber or the plastic discs that open and close with the gas flow. If a valve sticks open, the CO2 is rebreathed; if it sticks shut, the gas flow is obstructed. The valve function is checked during the pre-use check (the visual inspection through the transparent housing).
• The CO2 absorber canister: the colour indicator, the temperature, the particle size (the large particles for the low resistance, the small particles for the large surface area). The channelling (the gas finding a path of least resistance through the absorbent, bypassing some of the soda lime) is prevented by the correct packing.
• The reservoir bag: the 2 to 3 litre latex or silicone bag. The bag distends if the pressure exceeds 40 to 60 cmH2O (the safety pressure relief). The visual monitoring of the spontaneous ventilation.
• The APL (adjustable pressure-limiting) valve: the spring-loaded valve that vents the excess gas. In the fully open position, the pressure is at zero (the spontaneous ventilation). In the partially closed position (5 to 7 cmH2O), it provides the continuous positive airway pressure during the manual ventilation. In the fully closed position, the pressure rises to the dangerous level (the barotrauma risk — never leave the APL valve fully closed with the gas flowing).[3]

The pre-use check of the breathing system

The AAGBI 2012 checklist includes the specific tests of the breathing system:[3]

1. The assembly — the correct connection of the components (the inspiratory and the expiratory limbs, the absorber, the reservoir bag, the APL valve, the Y-piece).
2. The leak test — the Y-piece occluded, the APL valve closed, the system pressurised to 30 cmH2O with the oxygen flush. The pressure should hold for at least 10 seconds. If it falls, there is a leak.
3. The APL valve test — the valve opens smoothly and limits the pressure at the set level.
4. The unidirectional valve test — the visual inspection of the valve movement during the manual ventilation.[3]

The circle system failures and their capnographic signs

• The disconnection: the capnograph falls to zero (no CO2). The most common event.
• The stuck inspiratory valve: the inspired CO2 rises (the exhaled gas is rebreathed through the open inspiratory valve). The trace's baseline is no longer flat.
• The stuck expiratory valve: the inspired CO2 rises (similar). The expiratory valve sticks open.
• The exhausted soda lime: the inspired CO2 rises progressively. The canister colour changes (but may revert).
• The kinked tube: the airway pressure rises, the capnograph may fall, the saturation falls. The ventilator bellows may not move.[3]

The paediatric breathing systems

The paediatric breathing systems differ from the adult in the resistance and the dead space:[3]

• The T-piece (Mapleson E/F) is the standard for the neonate and the infant — the no-valve design (the minimal resistance), the low dead space. It requires a high fresh gas flow (2 to 3 times the minute ventilation) to prevent the rebreathing.
• The circle system can be used in the older child and the adolescent with the appropriate sized tubing (the smaller bore to reduce the volume). The circle system is increasingly used in the paediatric practice with the modern machines.[3]

[1] [1]

Red flags

[1] [1] [1] [1] [1]

References

1. [1]Feldman JM, Hendrickx J, Kennedy RR. Carbon Dioxide Absorption During Inhalation Anesthesia: A Modern Practice Anesth Analg, 2021.PMID 32947290
2. [2]Parthasarathy S, Ravishankar M. The closed circuit and the low flow systems Indian J Anaesth, 2013.PMID 24249885
3. [3]AAGBI, Hartle A, Anderson E. Checking anaesthetic equipment 2012: association of anaesthetists of Great Britain and Ireland Anaesthesia, 2012.PMID 22563957