Dr Q Milner,
Papworth Hospital, Cambridge, UK.

Introduction		Mapleson D, E & F
Classification		Mods to the Mapleson D system
Mods to the Mapleson A system		Humphrey A D E circuit
The Lack circuit		Circle systems
Mapleson B & C		Conclusion

The delivery systems which conduct anaesthetic gases from an anaesthetic machine to the patient are known as the breathing systems or circuits. They are designed to allow either spontaneous respiration or intermittent positive pressure ventilation (IPPV) and consist of a reservoir bag, anaesthetic tubing, and a pressure relief valve. A number of mechanical ventilators include a specific breathing system eg the Manley series. Other ventilators have been designed to operate with existing breathing systems e.g. the Penlon Nuffield 200.

The concentration of carbon dioxide in an exhaled breath varies with time; the first portion contains no carbon dioxide and comes from the upper respiratory tract where no gas exchange takes place (the anatomical dead space - 2mls/kg). The concentration of carbon dioxide then rises rapidly to a plateau of about 5% as alveolar gas is breathed out. The volume of alveolar gas expired per minute is called the alveolar minute ventilation. The anatomical dead space is 25-35% of each tidal volume. Any areas of lung that are ventilated with gas but are not perfused by blood cannot take part in gas exchange and represent the alveolar dead space. The total dead space in the patient is the physiological dead space.

The term rebreathing implies that expired alveolar gas containing 5% carbon dioxide (and less oxygen than normal) is inspired as part of the next tidal volume. Anaesthetic circuits are designed to minimise this occuring as it may lead to serious elevations in blood CO₂ levels. The amount of rebreathing that occurs with any particular anaesthetic breathing system depends on four factors; the design of the individual breathing circuit, the mode of ventilation (spontaneous or controlled), the fresh gas flow rate and the patient's respiratory pattern. Circuits may eliminate rebreathing either by ensuring an adequate flow of fresh gas which flushes the circuit clear of alveolar gas, or, in the case of a circle system by the use of sodalime which absorbs the CO₂ so that low fresh gas flows may be used. For each of the circuits described below, fresh gas flow rates that will ensure minimal rebreathing will be suggested.

 
Classification of breathing systems

A number of classifications exist and the one introduced in 1954 by Professor W W Mapleson is most commonly used in the UK (Figure 1). It does not however, include systems with carbon dioxide absorption.

The Mapleson A (Magill) system was designed by Sir Ivan Magill in the 1930's and remains an excellent system for spontaneous ventilation (Figure 2). Fresh gas enters the system at the fresh gas outlet of the anaesthesia machine. The expiratory valve (Heidbrink valve) is very close to the patient to reduce the dead space.

The respiratory cycle has three phases during spontaneous breathing; inspiration, expiration and the expiratory pause. During inspiration gas is inhaled from the 2 litre reservoir (breathing) bag which partially collapses giving a visual confirmation that breathing is occurring.

During expiration the bag and tubing are initially refilled with a combination of exhaled dead space gas (containing no carbon dioxide) and fresh gas flowing from the anaesthetic machine. Once the bag is full the pressure within the breathing system rises and the expiratory valve near the patient opens allowing the alveolar gas (containing carbon dioxide) to be vented from the system. During the expiratory pause more fresh gas enters the system driving any remaining alveolar gas back along the corrugated tubing and out through the valve. If the fresh gas flow is sufficiently high all the alveolar gas is vented from the circuit before the next inspiration and no rebreathing will take place. With careful adjustment the fresh gas flow can be reduced until there is only fresh gas and dead space gas in the breathing system at the start of inspiration. When the system is functioning correctly, without any leaks, a fresh gas flow (FGF) equal to the patients alveolar minute ventilation is sufficient to prevent rebreathing. In practice however, a FGF closer to the patients total minute ventilation (including dead space) is usually selected to provide a margin of safety. An adult's minute volume is approximately 80mls/kg /min and thus for a 75kg man a FGF of 6 litres per minute will prevent rebreathing. This is an efficient system for spontaneously breathing patients if carbon dioxide absorption is not available.

During controlled ventilation the Magill circuit works in a different way and becomes wasteful and inefficient, requiring high fresh gas flows to prevent rebreathing. The inspiratory force is provided by the anaesthetist squeezing the reservoir bag after partly closing the expiratory valve next to the patient. During lung inflation some of the gas is vented from the circuit and at the end of inspiration the reservoir bag is less than half full. During expiration, dead space and alveolar gas pass down the corrugated tubing and may reach the bag which will then contain some carbon dioxide. During the next inspiration when the bag is compressed alveolar gas re-enters the patients lungs followed by a mixture of fresh, dead space and alveolar gas. A FGF of two and a half times the patient's minute volume is required to vent enough alveolar gas to minimise rebreathing (FGF of about 12-15 litres /min) which is obviously very inefficient. In practice the Magill circuit should not be used for positive pressure ventilation except for short periods of a few minutes at a time.

(Continued...)

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