A simple modification of the Mapleson A circuit is required to make it more efficient for controlled ventilation. This is achieved by substituting a non-rebreathing valve (such as an Ambu E valve) for the Heidbrink valve at the patient end of the circuit. Not only does this arrangement prevent rebreathing, but during manual ventilation the delivered minute volume will be the same as the desired FGF which should be set at the rotameters. It is, however, a dangerous arrangement for spontaneous respiration because the valve may jam if the fresh gas flow is greater that the patient's minute volume.

 
The Lack circuit

A disadvantage of the Magill system is that the expiratory valve is attached close to the patient making it awkward to use (particularly when a scavenging circuit is added). The Lack circuit (Figure 3) is a Mapleson A system in which the exhaled gases travel down a central tube located within an outer corrugated tube towards the expiratory valve (co-axial system).

The inner tubing is wide enough to prevent an increase in the work of breathing and the expiratory valve is placed next to the reservoir bag, by the common gas outlet. The fresh gas flows required for both spontaneous and controlled ventilation are as described for the standard Mapleson A system.

 
The Mapleson B and C breathing systems (Figure 1) are similar in construction, with the fresh gas flow entry and the expiratory valves located at the patient end of the circuit. They are not commonly used in anaesthetic practice, although the C system is used on intensive care units. High flows of gases are needed to prevent rebreathing of CO₂ and this system was at one time combined with a canister of sodalime to absorb CO₂ (Waters' "To and Fro" Circuit). However the cannister proved too bulky for practical use and there was a risk of the patient inhaling soda lime dust.

 
The Mapleson D, E and F systems are all functionally similar (Figure 1).

They act as T pieces with the FGF delivered to the patient end of the circuit and differ only in the presence of valves or breathing bags at the expiratory end of the circuit. These systems are all inefficient for spontaneous respiration (Figure 4).

During expiration exhaled gas and fresh gas mix in the corrugated tubing and travel towards the reservoir bag. When the bag is full the pressure in the system rises and the expiratory valve opens venting to the atmosphere a mixture of fresh and exhaled gas. During the expiratory pause fresh gas continues to push exhaled alveolar gas down the tubing towards the valve. However, unless the FGF is at least twice the patient's minute volume, rebreathing of alveolar gas will occur. A FGF of at least 8-10 litres/min (150mls/kg/min) is required to prevent rebreathing in an adult.

When used for controlled ventilation the Mapleson D system functions more efficiently. During expiration the corrugated tubing and reservoir bag fill with a mixture of fresh and exhaled gas. Fresh gas fills the distal part of the corrugated tube during the expiratory pause prior to inspiration. When the bag is compressed this fresh gas enters the lungs and when the expiratory valve opens a mixture of fresh and exhaled gas is vented. The degree of rebreathing that occurs depends on the FGF. A FGF of 70ml/kg/min is usually adequate for controlled ventilation; 100mls/kg/min will result in a degree of hypocapnia (lowered CO₂ level in the blood).

 
Modifications of the Mapleson D system

The Bain Circuit (Figure 3) is the most commonly used form of the Mapleson D system. It is a co -axial circuit which was introduced in 1972 by Bain and Spoerel. Unlike the Lack co-axial circuit described above, fresh gas flows down the central narrow bore tubing (7mm i.d.) to the patient and exhaled gases travel in the outer corrugated tubing (22mm i.d.). The reservoir bag may be removed and replaced by a ventilator such as the Nuffield Penlon 200 for mechanical ventilation. Before use the Bain circuit should be carefully checked by the anaesthetist. The outer tubing of a Bain circuit is made of clear plastic and the inner green or black. If a leak develops in the inner tubing or it becomes detached from the fresh gas port, a huge increase in apparatus dead space occurs. In order to check for this, the lumen of the green tubing should be occluded with a finger or the plunger of a 2ml syringe when a rise in gas pressure within the anaesthetic circuit should be observed.

The degree of rebreathing that occurs suring IPPVwill depend on the FGF. In an adult, fresh gas flows of 70-80mls/kg/min (6-7litres/min) will maintain a normal arterial carbon dioxide tension (normocapnia) and a flow of 100mls/kg/min will result in mild hypocapnia.

The Mapleson E system performs in a similar way to the Mapleson D, but because there are no valves and there is very little resistance to breathing it has proved very suitable for use with children. It was originally introduced in 1937 by P Ayre and is known as the Ayre's T-piece. The version most commonly used is the Jackson-Rees modification which has an open bag attached to the expiratory limb (classified as a Mapleson F system although it was not included in the original description by Professor Mapleson). Movement of the bag can be seen during spontaneous breathing, and the bag can be compressed to provide manual ventilation. As in the Bain circuit, the bag may be replaced by a mechanical ventilator designed for use with children. This system is suitable for children under 20kg. Fresh gas flows of 2 - 3 times minute volume should be used to prevent rebreathing during spontaneous ventilation, with a minimum flow of 3 litres/minute, eg a 4 year old child weighing 20kg has a normal minute volume of 3 litres/min and would required a FGF of 6-9litres/min. During controlled ventilation in children normocapnia can be maintained with a fresh gas flow of 1000mls + 100mls/kg. e.g. a 4 year old weighing 20kg would need a total FGF of around 3litres/min. systems are all functionally similar.

(Continued...)

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