16. BREATHING SYSTEMS

An ideal breathing system should be safe and simple. It should be able to be used for spontaneous and controlled ventilation. The system would be lightweight, not bulky or complicated and efficient. It should protect the patient against barotrauma.

Breathing systems include the circle system (with carbon dioxide reabsorption) and “Mapleson” systems.

Respiratory Physiology

The volume of air inspired during normal breathing is called the tidal volume (6 to 10 ml/kg). The minute ventilation (MV) is the tidal volume (TV) times the respiratory rate (RR). The normal adult minute ventilation is 80 ml/kg/min. Some of the tidal volume air does not enter the alveoli (where it gives up oxygen and takes up carbon dioxide). It remains in the oropharynx, trachea and larger airways. This volume of air is called the anatomical dead space (DS). The normal dead space is about 30% of the tidal volume. The alveolar ventilation (AV) is the amount of air that is involved in gas exchange each minute. It is equal to the (TV – DS) x RR.

Expired air contains 5% carbon dioxide and reduced oxygen (16%). If the patient breathes in his expired air (re-breathing) he will be breathing high concentrations of carbon dioxide and low concentrations of oxygen.

Circle System

Circle systems use less gas and volatile agent, conserve heat and moisture and are suitable for spontaneous ventilation and intermittent positive pressure ventilation (controlled ventilation or IPPV).

They can be used a with very low fresh gas flow (FGF) of less than 1 litre/minute. They must only be used with a very low fresh gas flow if the anaesthetist can check the inspired oxygen concentration, there is a carbon dioxide absorber and the inspired oxygen concentration is greater than 40%.

A circle system is larger, more complex (10 connections) and requires a carbon dioxide absorber.

The circle system consists of seven parts: the fresh gas flow, inspiratory and expiratory valves, inspiratory and expiratory tubing, a Y piece connector, reservoir bag, overflow or airway pressure limiting (APL) valve and the carbon dioxide absorbent container. There are several different ways of arranging the parts. To prevent rebreathing, the fresh gas flow must not enter between the expiratory valve and the patient, the overflow valve must not be located between the patient and the inspiratory valve, and the inspiratory and expiratory valves must be located between the patient and the reservoir bag on both the inspiratory and expiratory limbs of the circuit.

The fresh gas flow enters the inspiratory limb of the circle and passes though the inspiratory valve to the patient. Exhaled gas passes along the expiratory limb though the expiratory valve to a carbon dioxide absorber and back to the patient.

There are several common carbon dioxide absorbents (e.g. soda lime). In general, they contain a hydroxide that reacts with carbon dioxide. Heat and water are produced as by-products. They contain a chemical indicator which changes colour when the soda lime is exhausted. The anaesthetist must know which chemical indicator is used. Different chemical indicators change to different colours.

A circle system can be used without soda lime but re-breathing and carbon dioxide retention can occur. The risk of re-breathing depends on the arrangement of the parts, the fresh gas flow and the ventilation. To prevent re-breathing the fresh gas flow should be 60 ml/kg/min and ventilate at thee times normal minute ventilation, or set the fresh gas flow to alveolar ventilation and ventilate at thee times normal minute ventilation.

Vaporisers can be placed in their usual position on the back bar (vaporiser out of circuit VOC) or can rarely be placed in the circle breathing system (vaporiser in circuit VIC). Vaporisers made to work with compressed gas (plenum) or drawover vaporisers must never be placed in the circuit. Gas expired from the patient will contain some volatile anaesthetic agent. If this is allowed to recirculate though the vaporiser it will continue to increase the volatile concentration above the concentration which has been selected on the vaporiser. Vaporisers should only be placed in circuit if they are made to be used in a circle breathing system and agent concentration monitoring is available.

Trichloroethylene must not be used with carbon dioxide absorbers due to production of toxic products.

Mapleson Breathing Systems

The Mapleson breathing systems have no valves to direct gases to and from the patient. There is no carbon dioxide absorber. The fresh gas flow must wash out the expired carbon dioxide in the breathing system. The parts of a Mapleson breathing system are a reservoir bag, tubing, fresh gas flow, APL valve and patient connector. The Mapleson breathing systems are simple and inexpensive. They require high fresh gas flow to prevent re-breathing and the fresh gas flow rate may need to be altered when changing from spontaneous to controlled ventilation. They do not conserve heat or moisture. The Mapleson A, B and C breathing systems have the APL valve close to the patient where it may be difficult to access. The Mapleson E and F breathing systems are difficult to scavenge. If there is a fall in fresh gas flow with the Mapleson breathing systems there is a risk of re-breathing.

There are different ways of arranging the parts.

The Mapleson A (Magill) breathing system is efficient for spontaneous ventilation. Fresh gas flow should equal minute ventilation. It is inefficient for controlled ventilation. Fresh gas flow must be 2 to 3 times minute ventilation to prevent re-breathing.

The Mapleson B and C breathing systems are rarely used for anaesthesia. They are used for resuscitation. Fresh gas flow for controlled ventilation should be 2 to 2.5 times minute ventilation.

The Mapleson D breathing system is inefficient for spontaneous ventilation. The flow rate should be 150 to 250 ml/kg/min. It is efficient for controlled ventilation. Fresh gas flow should be 70 ml/kg/min.

The Mapleson E (Ayres T piece) breathing system is used in children because it has a very low resistance and minimal dead space. The reservoir limb should be larger than the tidal volume and fresh gas flow should be 2 to 3 times minute ventilation.

The Mapleson F (Jackson Rees modification of the Ayres T piece) breathing system is a Mapleson E breathing system with an open bag attached to the expiratory limb. The bag allows easy controlled ventilation and visual assessment of spontaneous ventilation. Fresh gas flow should be 2 to 3 times minute ventilation.