Magill Circuit

The original Mapleson A system.

Construction

A three-way T-tube is 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

Inspiration -The valve closes and the patient inspires fresh gas from the reservoir tube.
Expiration - The patient expires into the reservoir tube. Toward the end of expiration, the bag fills and positive pressure opens the valve, allowing expired 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.

Controlled ventilation

The problem with attempting to perform intermittent positive pressure ventilation with this circuit is that, when the bag is squeezed, the gas tends to escape from the circuit via the expiratory valve rather than inflating the lungs. This can be avoided by closing the expiratory valve during each inspiration, although this is highly inconvenient. It can be reduced by partially closing the expiratory valve, but relatively high fresh gas flow rates must be employed to prevent rebreathing.
    Alternatively, a Ruben non-rebreathing valve may be substituted for the expiratory valve:

On inspiration, or when the breathing bag is squeezed, the spring-loaded bobbin moves to close the expiratory limb, and on expiration it closes the inspiratory limb. When the valve is functioning correctly, the minute volume exactly equals the fresh gas flow rate. However, the bobbin can stick in the inspiratory position, particularly if the valve is wet or dirty, which directs the entire fresh gas flow into the lungs, which will rapidly lead to hyperinflation and pulmonary barotrauma. This may be avoided by adding a pressure-relief valve to the inspiratory limb, but this adds to the complexity of the arrangement. Generally, therefore, the Magill is not a suitable circuit for performing long term IPPV.

Operational requirements

• The volume of the reservoir tube must be greater than the patient's tidal volume, otherwise expired gas will enter the bag and contaminate the inspired gas.
• Fresh gas flow requirements--see separate article.
• Unlike in the T-piece and Bain, the efficiency of the circuit is unaffected by the respiratory pattern.

Advantages

• Inexpensive.
• Provides the advantages of a non-rebreathing circuit with reasonable economy of gas use.

Disadvantages

• The position of the valve close to the patient's head may be inconvenient for orofacial surgery.
• Difficult to perform intermittent positive pressure ventilation effectively.

Uses

• Small animals weighing from around 10 to 40 kg (or larger if the increased anesthetic consumption is acceptable).

Lack