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Mapleson A (Magill circuit)

Created: 13/5/2004
Updated: 6/12/2017
This was the original Mapleson A system, also known as Magill's circuit, (Sir Ivan Magill, London Anaesthetist)


Magill construction

Figure 1:  Mapleson A system

It consist of a three-way T-tube connected to the fresh gas outlet (F), a breathing bag (B) and a reservoir tube (R). The other end of the reservoir tube is connected to the patient (P) and a spring-loaded expiratory valve (V).



Function of the Magill Circuit
Figure 2: Function of the Mapleson A system



Spontaneous ventilation

Click here for flash animation depicting Mapleson A

Inspiration - The valve closes and the patient inspires fresh gas from the reservoir tube. Fresh gas flushes the dead space gas toward the patient.

Expiration - The patient expires into the reservoir tube. Towards the end of expiration, the bag fills and positive pressure opens the valve, allowing expired gas to escape. The patient end of of the tubing is filled with dead space gas followed by the alveolar gas. This stream travels up the tubing and meets the fresh gas flowing into the circuit. The pressure in the circuit increases and forces the expiratory valve to open, allowing the alveolar gas to escape.

Expiratory pause - Fresh gas washes the expired gas out of the reservoir tube, filling it with fresh gas for the next inspiration.

Rebreathing of alveolar gas can be prevented if the fresh gas flow = patient's minute ventilation.
However, the last gas to be washed out of the circuit is dead space gas, which consists of warmed and humidified fresh gas, and no CO2. If some rebreathing of this dead space gas is accepted, a flow approximating to around 70% of the minute volume can be used:

Fresh gas flow rate (L/min) = 175 x BW0.8

Body weight (kg) Fresh gas flow (L/min)
5 0.6
10 1.1
20 1.9
40 3.3
Table 1: Values of fresh gas flow by body weight

It should be emphasised that these values are guidelines only; if there is evidence of rebreathing (e.g. an increase in the end-tidal CO2 concentration or unexpected hyperventilation), the flow rate should be increased.

Controlled ventilation

The Mapleson A is inefficient during controlled ventilation. Venting the gas in the circuit occurs during the inspiratory phase, and the alveolar gases are retained in the tubing during the expiration phase. Hence, alveolar gas is rebreathed before the the pressure in the system increases sufficiently to force the expiratory valve open.

A fresh gas flow of >20 L/minute is required to prevent rebreathing during controlled ventilation.


[i] Fresh gas economics of the Magill circuit. Kain ML, Nunn JF. Anesthesiology 1968; 29(5): 964-74.

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