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Anaesthetic apparatus


Anaesthetists must have a sound understanding and knowledge of the functioning of all the anaesthetic equipment they use. Failure to understand the use of or check equipment before use is an important recognised cause of complications and death. This is especially true of ventilators, where lack of knowledge may result in a patient being subjected to hypoxaemia, hypercapnia, pulmonary barotrauma and volutrauma. It is essential that anaesthetists check that all equipment is functioning correctly before anaesthesia (see Chapter 22 ). The routine of testing anaesthetic equipment may be compared to an aircraft pilot's checklist, an essential preliminary before every flight.

This chapter summarises the principles of anaesthetic equipment in routine use ( Box 16.1 ) with the exception of airway devices, which are discussed in Chapter 23 .

Box 16.1
Classification of anaesthetic equipment described in this chapter

  • Supply of gases:

    • From outside the operating theatre

    • From cylinders within the operating theatre, together with the connections involved

  • The anaesthetic machine:

    • Unions

    • Cylinders

    • Reducing valves

    • Flowmeters

    • Vaporisers

  • Safety features of the anaesthetic machine

  • Anaesthetic breathing systems

  • Ventilators

  • Apparatus used in scavenging waste anaesthetic gases

    • Suction apparatus

    • Infusion devices

    • Warming devices

    • Decontamination of equipment

Gas supplies

Bulk supply of anaesthetic gases

In most modern hospitals, piped medical gases and vacuum (PMGV) systems have been installed. These obviate the requirement to hold large numbers of cylinders in the operating theatre suite. Normally only a few cylinders are kept in reserve, usually attached directly to the anaesthetic machine and for patient transport.

The advantages of the PMGV system are reductions in costs, the need to transport cylinders and accidents caused by cylinder contents being exhausted. However, there have been several occurrences where critical incidents or death have resulted from incorrect connections between the components of piped medical gas supplies.

The PMGV system comprises five sections:

  • Bulk store

  • Distribution pipelines in the hospital

  • Terminal outlets, situated usually on the walls or ceilings of the operating theatre suite and other sites

  • Flexible hoses connecting the terminal outlets to the anaesthetic machine

  • Connections between flexible hoses and anaesthetic machines

Responsibility for the first three items lies with hospital engineering and pharmacy departments. Ultimately the anaesthetist is responsible for checking the correct functioning of the last two components within the operating theatre.

Bulk store

Oxygen

In small hospitals, oxygen may be supplied to the PMGV from a bank of several oxygen cylinders attached to a manifold. Oxygen cylinder manifolds consist of two groups of large cylinders (size J). The two groups alternate in supplying oxygen to the pipelines. In both groups, all cylinder valves are open so that they empty simultaneously. All cylinders have non-return valves. The supply automatically changes from one group to the other when the first group of cylinders is nearly empty. The changeover also activates an electrical signalling system, which alerts staff of the need to change the empty cylinders.

In larger hospitals, pipeline oxygen originates from a liquid oxygen store. Liquid oxygen is stored at a temperature of approximately −165°C at 10.5 bar in a vacuum-insulated evaporator (VIE) ( Fig. 16.1 ). The VIE consists of an inner stainless steel tank and an outer steel jacket, and a vacuum is maintained between the inner tank and the outer jacket. Some heat passes from the environment through the insulating layer between the two shells of the flask, increasing the tendency to evaporation and pressure increase within the chamber. Pressure is maintained constant by transfer of gaseous oxygen into the pipeline system (via a warming device). However, if the pressure increases to more than 17 bar (1700 kPa), a safety valve opens and oxygen runs to waste. During periods of high demand when the supply of oxygen from the VIE is inadequate, the pressure decreases and a valve opens to allow liquid oxygen to pass into an evaporator, from which gas vents into the pipeline system.

Fig. 16.1, Schematic diagram of a vacuum-insulated evaporator for liquid oxygen supply system.

Liquid oxygen plants are housed some distance away from hospital buildings because of the risk of fire. Even when a hospital possesses a liquid oxygen plant, reserve banks of oxygen cylinders are necessary in case of liquid oxygen supply failure.

Oxygen concentrators

Recently, oxygen concentrators have been used to supply hospitals, and the use of these devices is likely to increase. The oxygen concentrator depends upon the ability of an artificial zeolite to entrap molecules of nitrogen. These devices cannot produce pure oxygen, but the concentration usually exceeds 90%; the remainder comprises nitrogen, argon and other inert gases. Small oxygen concentrators are available for domiciliary use. Oxygen concentrators are described in Chapter 45 .

Nitrous oxide

Nitrous oxide and Entonox may be supplied from banks of cylinders connected to manifolds similar to those used for oxygen.

Medical compressed air

Compressed air is supplied from a bank of cylinders into the PMGV system. Air of medical quality is required, as industrial compressed air may contain fine particles of oil.

Piped medical vacuum

Piped medical vacuum is provided by large vacuum pumps which discharge via a filter and silencer to a suitable point, usually at roof level, where gases are vented to atmosphere. Although concern has been expressed regarding the possibility of volatile anaesthetic agents dissolving in the lubricating oil of vacuum pumps and causing malfunction, this fear has not been realised.

Terminal outlets

Terminal outlets have been standardised in the UK since 1978, but there is no universal specification. Six types of terminal outlet are found commonly in the operating theatre. The terminals are colour-coded and also have non-interchangeable connections specific to each gas:

  • Vacuum (yellow). A vacuum of at least 53 kPa (400 mmHg) should be maintained at the outlet, which should be able to accommodate a free gas flow of at least 40 L min –1 .

  • Compressed air (white/black) at 4 bar. Used for anaesthetic breathing systems and ventilators.

  • Air (white/black) at 7 bar. To be used only for powering compressed air tools used, for example during orthopaedic surgery.

  • Nitrous oxide (blue) at 4 bar.

  • Oxygen (white) at 4 bar.

  • Scavenging. There are a variety of scavenging outlets from the operating theatre. The passive systems are designed to accept a standard 30-mm connection.

Whenever a new pipeline system has been installed or servicing of an existing pipeline system undertaken, a designated member of the pharmacy staff should test the gas obtained from the sockets using an oxygen or other gas analyser. Malfunction of an oxygen/air mixing device may result in entry of compressed air into the oxygen pipeline, rendering an anaesthetic gas mixture hypoxic. Because of this and other potential mishaps, oxygen analysers should be used routinely during anaesthesia.

Gas supplies

Gas supplies to the anaesthetic machine should be checked at the beginning of each session to ensure the gas that issues from the pipeline or cylinder is the same as that passing through the appropriate flowmeter. This ensures that pipelines are not connected incorrectly. Anaesthetic machines in both the operating theatre and the anaesthetic room should be checked. Checking of anaesthetic machine and medical gas supplies is detailed in Chapter 22 .

Cylinders

Cylinders are constructed from molybdenum steel, aluminium or composites (e.g. carbon-fibre wrapped aluminium). They are checked periodically by the manufacturer to ensure that they can withstand hydraulic pressures considerably higher than those during normal use. One cylinder in every 100 is cut into strips to test the metal for tensile strength, flattening impact and bend tests. Medical gas cylinders are tested hydraulically every 10 (steel) or 5 (composite) years and the tests recorded by a mark stamped on the neck of the cylinder: this includes test pressure, date performed, chemical formula of the cylinder's content and the tare weight. Cylinders may also be inspected endoscopically or using ultrasound for cracks or defects on their inner surfaces.

The cylinders are provided in a variety of sizes (A to J) and colour coded according to the gas supplied. Modern CD (460 L) and HX (2300 L) cylinders have similar dimensions to their counterparts but are filled to 23,000 kPa rather than 13,700 kPa. Cylinders attached to the anaesthetic machine are usually size E. The cylinders comprise a body and a shoulder containing threads into which are fitted a pin index valve block, a bull-nosed valve or a hand-wheel valve.

The pin index system was devised to prevent interchangeability of cylinders of different gases. Pin index systems are provided for the smaller cylinders of oxygen and nitrous oxide (and also carbon dioxide) which may be attached to anaesthetic machines. The pegs on the inlet connection slot into corresponding holes on the cylinder valve.

Full cylinders are usually supplied with a plastic dust cover to prevent contamination by dirt. This cover should not be removed until immediately before the cylinder is fitted to the anaesthetic machine. When fitting the cylinder to a machine, the yoke is positioned and tightened with the handle of the yoke spindle. After fitting, the cylinder should be opened to make sure that it is full and that there are no leaks at the gland nut or the pin index valve junction, caused, for example, by absence of or damage to the washer. The washer used is normally a Bodok seal, which has a metal periphery designed to keep the seal in good condition for a long period.

Cylinder valves should be opened slowly to prevent sudden surges of pressure and should be closed with no more force than is necessary, otherwise the valve seating may be damaged.

The sealing material between the valve and the neck of the cylinder may be constructed from a fusible material which melts in the event of fire and allows the contents of the cylinder to escape around the threads of the joint.

The primary method of cylinder identification is the cylinder label. Colour coding is a secondary method. The colour codes used for medical gas cylinders in the UK are shown in Table 16.1 . Different colours are used for some gases in other countries. A proposal was agreed to in 2013 to harmonise cylinder colours throughout Europe. The body will be painted white and only the shoulders will be colour coded. The shoulder colours for medical gases will correspond to the current UK colours but will be horizontal rings rather than quarters. The conversion will be complete by 2025. Cylinder sizes and capacities are shown in Table 16.2 .

Table 16.1
Medical gas cylinders used in the UK
COLOUR PRESSURE AT 15°C
Body Shoulder kPa Bar
Oxygen Black White 13,700 137
Nitrous oxide Blue Blue 4400 44
CO 2 Grey Grey 5000 50
Helium Brown Brown 13,700 137
Air Grey White/black quarters 13,700 137
O 2 /helium Black White/brown quarters 13,700 137
N 2 O/O 2 (Entonox) Blue White/blue quarters 13,700 137

Table 16.2
Medical gas cylinder sizes and capacities by cylinder size (A–J) and height (inches)
CAPACITIES (L)
A/10 in B/10 in C/14 in D/18 in CD/20 in E/31 in F/34 in G/49 in J/57 in
Oxygen 170 340 460 680 1360 3400 6800
Nitrous oxide 450 900 1800 3600 9000
CO 2 450 900 1800
Helium 300 1200
Air 3200 6400
O 2 /helium 600 1200
O 2 /CO 2 1360 3400
Entonox 3200 6400

Oxygen, air and helium are stored as gases in cylinders and the cylinder contents can be estimated from the cylinder pressure. The pressure gradually decreases as the cylinder empties. According to the universal gas law, the mass of the gas is directly proportional to the pressure, and the volume of gas that would be available at atmospheric pressure can be calculated using Boyle's law.

Nitrous oxide (N 2 O) and carbon dioxide (CO 2 ) cylinders contain liquid and vapour and the cylinders are filled to a known filling ratio (see Chapter 15 ). The cylinder pressure cannot be used to estimate its contents because the pressure remains relatively constant until after all the liquid has evaporated and the cylinder is almost empty, though cylinder pressure may change slightly because of temperature changes during use. The contents of N 2 O and CO 2 cylinders can be estimated from the weight of the cylinder.

The anaesthetic machine

The anaesthetic machine comprises:

  • a means of supplying gases either from attached cylinders or from piped medical supplies via appropriate unions on the machine;

  • methods of measuring flow rate of gases;

  • apparatus for vaporising volatile anaesthetic agents;

  • breathing systems and a ventilator for delivery of gases and vapours from the machine to the patient; and

  • apparatus for scavenging anaesthetic gases to minimise environmental pollution.

Supply of gases

In the UK, gases are supplied at a pipeline pressure of 4 bar (400 kPa), and this pressure is transferred directly to the bank of flowmeters and back bar of the anaesthetic machine. Flexible colour-coded hoses connect the pipeline outlets to the anaesthetic machine. The anaesthetic machine end of the hoses should be permanently fixed using a nut and liner union where the thread is gas specific and non-interchangeable. The non-interchangeable screw thread (NIST) is the British standard.

The gas issuing from medical gas cylinders is at a much higher pressure, necessitating the interposition of a pressure regulator between the cylinder and the bank of flowmeters. In some older anaesthetic machines (and in some other countries), the pressure in the pipelines of the anaesthetic machine may be 3 bar (300 kPa).

Pressure gauges

Pressure gauges (Bourdon gauges) measure the pressure in the cylinders or pipeline. Anaesthetic machines have pressure gauges for oxygen, air and N 2 O. These are mounted usually on the front panel of the anaesthetic machine.

Pressure regulators

Pressure regulators are used on anaesthetic machines for three purposes:

  • They help to reduce the high pressure of gas in a cylinder to a safe working level.

  • They are used to prevent damage to equipment on the anaesthetic machine (e.g. flow control valves).

  • As the contents of the cylinder are used, the pressure within the cylinder decreases and the regulating mechanism maintains a constant outlet pressure, obviating the necessity to make continuous adjustments to the flowmeter controls.

The operating principles of pressure regulators are discussed in Chapter 15 .

Flow restrictors

Pressure regulators usually are omitted when anaesthetic machines are supplied directly from a pipeline at a pressure of 4 bar. Changes in pipeline pressure would cause changes in flow rate, necessitating adjustment of the flow control valves. This is prevented by the use of a flow restrictor upstream of the flowmeter (flow restrictors are simply constrictions in the low-pressure circuit).

A different type of flow restrictor may be fitted also to the downstream end of the vaporisers to prevent back-pressure effects (see Chapter 15 ).

Pressure relief valves on regulators

Pressure relief valves are often fitted on the downstream side of regulators to allow escape of gas if the regulators were to fail (thereby causing a high output pressure). Relief valves are set usually at approximately 7 bar for regulators designed to give an output pressure of 4 bar.

Flowmeters

The principles of flowmeters and some different types are described in Chapter 17 .

Problems with flowmeters

  • Non-vertical tube. This causes a change in shape of the annulus and therefore variation in flow. If the bobbin touches the side of the tube, resulting friction causes an even more inaccurate reading.

  • Static electricity. This may cause inaccuracy (by as much as 35%) and sticking of the bobbin, especially at low flows. This may be reduced by coating the inside of the tube with a transparent film of gold or tin.

  • Dirt on the bobbin may cause sticking or alteration in size of the annulus and therefore inaccuracies.

  • Back-pressure. Changes in accuracy may be produced by back-pressure. For example, the Manley ventilator may exert a back-pressure and depress the bobbin; there may be as much as 10% more gas flow than that indicated on the flowmeter. Similar problems may be produced by the insertion of any equipment which restricts flow downstream (e.g. Selectatec head, vaporiser).

  • Leakage. This results usually from defects in the top sealing washer of a flowmeter.

It is unfortunate that in the UK the standard position of the oxygen flowmeters is on the left, followed by either nitrous oxide or air (if all three gases are supplied). On several recorded occasions, patients have suffered hypoxia because of leakage from a broken flowmeter tube in this type of arrangement, as oxygen, being at the upstream end, passes out to the atmosphere through any leak. This problem is reduced if the oxygen flowmeter is placed downstream (i.e. on the right-hand side of the bank of flowmeters) as is standard practice in the United States. In the UK this problem is now avoided by designing the outlet from the oxygen flowmeter to enter the back bar downstream from the outlets of other flowmeters ( Fig. 16.2 ). Modern anaesthetic machines do not have a flowmeter for CO 2 . Some newer anaesthetic machines such as the Primus Dräger ( Fig. 16.3A ) do not have the traditional flowmeters which are used in machines such as the Blease Frontline ( Fig. 16.3B ); gas delivery is under electronic control, and there is an integrated heater within a leak-tight breathing system. The gas flow is indicated electronically by a numerical display. In the event of an electrical failure, there is a pneumatic backup which continues the delivery of fresh gas. These machines are particularly well suited to low and minimal flow anaesthesia, and they use standard vaporisers.

Fig. 16.2, Oxygen is the last gas to be added to the gas mixture being delivered to the back bar.

Fig. 16.3, (A) The Primus Dräger anaesthetic machine. (B) Blease Frontline anaesthetic machine.

The emergency oxygen flush is a non-locking button which, when pressed, delivers pure oxygen from the anaesthetic outlet. On modern anaesthetic machines, the emergency oxygen flush lever is situated downstream from the flowmeters and vaporisers. A flow of about 35–45 L min –1 at pipeline pressure is delivered. This may lead to dilution of the anaesthetic mixture with excess oxygen if the emergency oxygen tap is opened partially by mistake and may result in awareness. There is also a risk of pulmonary barotrauma if the high pressure is accidentally delivered directly to the patient's lungs.

Quantiflex

The Quantiflex mixer flowmeter ( Fig. 16.4 ) eliminates the possibility of reducing the oxygen supply inadvertently. One dial is set to the desired percentage of oxygen, and the total flow rate is adjusted independently. The oxygen passes through a flowmeter to provide evidence of correct functioning of the linked valves. Both gases arrive via linked pressure-reducing regulators. The Quantiflex is useful in particular for varying the volume of fresh gas flow (FGF) from moment to moment whilst keeping the proportions constant. In addition, the oxygen flowmeter is situated downstream of the N 2 O flowmeter.

Fig. 16.4, A Quantiflex flowmeter. The required oxygen percentage is selected using the dial, and total flow of the oxygen/nitrous oxide mixture is adjusted using the grey knob.

Hypoxic guard

The majority of modern anaesthetic machines such as that shown in Fig. 16.3B possess a mechanical linkage between the N 2 O and oxygen flowmeters. This causes the N 2 O flow to decrease if the oxygen flowmeter is adjusted to give less than 25%–30% O 2 ( Fig. 16.5 ).

Fig. 16.5, Flowmeters with mechanical linkage between nitrous oxide and oxygen.

Vaporisers

The principles of vaporisers are detailed in Chapter 15 .

Modern vaporisers may be classified as:

  • Drawover vaporisers. These have a very low resistance to gas flow and may be used for emergency use in the field (e.g. Oxford miniature vaporiser; OMV (see Chapter 45 ))

  • Plenum vaporisers. These are intended for unidirectional gas flow, have a relatively high resistance to flow and are unsuitable for use either as drawover vaporisers or within a circle system. Examples include the TEC type in which there is a variable bypass flow.

Temperature regulation in the TEC vaporisers is achieved using a bimetallic strip.

There have been several models of the TEC vaporiser. The TEC Mark 3 included several advances from the Mark 2 (now obsolete), including improved vaporisation resulting from increased area of the wicks, reduced pumping effect by having a long tube through which the vaporised gas leaves the vaporising chamber, improved accuracy at low gas flows and a bimetallic strip situated in the bypass channel and not the vaporising chamber. The Mark 4 incorporated mechanisms to prevent both spillage into the bypass channel if the vaporiser was accidentally inverted and the possibility of two vaporisers being turned on at the same time when connected to the back bar of the anaesthetic machine (see later). The TEC Mark 5 vaporiser ( Fig. 16.6 ) has improved surface area for vaporisation in the chamber, improved key-filling action and an easier mechanism for switching on the rotary valve and lock with one hand. The TEC 5 is still used but no longer produced by the manufacturer and has been superseded by the TEC 7; differences are relatively minor. Desflurane presents a particular challenge because it has a high saturated vapour pressure of 664 mmHg (89 kPa) at 20°C. A conventional vaporiser would require high FGF for useful clinical concentrations, making it uneconomical. It has a low boiling point of 23.5°C; this means that it is almost boiling at a room temperature of 20°C. As a result, small changes in ambient temperature can cause large swings in saturated vapour pressure. To combat this problem, the TEC 6 ( Figs 16.7 and 16.8 ) is heated electrically to 39°C with a pressure of 1550 mmHg (207 kPa). The vaporiser has electronic monitors of vaporiser function and alarms. The FGF does not enter the vaporisation chamber. Instead, desflurane vapour enters into the path of the FGF. A control dial regulates the flow of desflurane vapour into the FGF. The dial calibration is from 1% to 18%. The vaporiser has a backup 9-volt battery in case of mains failure.

Fig. 16.6, (A) Working principles of a vaporiser. (B) Schematic diagram of the TEC 5 vaporiser.

Fig. 16.7, Components of the TEC 6 desflurane vaporiser. Liquid in the vaporising chamber is heated and mixed with fresh gas; the pressure-regulating valve balances both fresh gas pressure and anaesthetic vapour pressure.

Fig. 16.8, A TEC 6 desflurane vaporiser. Note it is set at the ‘T’ transport setting for safe transport, and an electrical supply is required to heat the vaporising chamber.

Anaesthetic-specific connections are available to link the supply bottle (container of liquid anaesthetic agent) to the appropriate vaporiser ( Fig. 16.9 ). These connections reduce the extent of spillage (and thus atmospheric pollution) and also the likelihood of filling the vaporiser with an inappropriate liquid. In addition to being designed specifically for each liquid, the connections themselves are colour coded (e.g. purple for isoflurane, yellow for sevoflurane).

Fig. 16.9, An agent-specific connector for filling a vaporiser.

Halothane contains a stabilising agent, 0.01% thymol, to prevent breakdown of halothane by heat and ultraviolet light (see Chapter 3 ). Thymol is less volatile than halothane, and its concentration may increase during use. This can impair the vaporisation of halothane, and thymol inhalation is potentially harmful. Therefore halothane vaporisers should be emptied and refilled every 2 weeks. Longer intervals are acceptable for other volatile agents.

Safety features of modern anaesthetic machines

Safety features of modern anaesthetic machines include the following:

  • The flexible hoses are colour coded and have non-interchangeable screw-threaded connectors to the anaesthetic machine.

  • The pin index system prevents incorrect attachment of gas cylinders to an anaesthetic machine. Cylinders are colour coded, and they are labelled with the name of the gas that they contain.

  • Pressure relief valves are present on the downstream side of pressure regulators.

  • Flow restrictors are present on the upstream side of flowmeters.

  • The bank of flowmeters are arranged whereby the oxygen flowmeter is on the right (i.e. downstream) or oxygen is the last gas to be added to the gas mixture being delivered to the back bar (see Fig. 16.2 ).

  • Sometimes a single regulator and contents meter is used both for cylinders in use and for the reserve cylinder. When one cylinder runs out, the presence of a non-return valve prevents the empty cylinder from being refilled by the reserve cylinder and also enables the empty cylinder to be removed and replaced without interrupting the supply of gas to the patient.

  • Pressure gauges indicate the pressures in the pipelines and the cylinders.

  • An oxygen bypass valve (emergency oxygen) delivers oxygen directly to a point downstream of the vaporisers. When operated, the oxygen bypass should give a flow rate of at least 35 L min –1 .

  • The TEC vaporisers (Mark 4 and later) have the interlocking Selectatec system ( Fig. 16.10 ), which has locking rods to prevent more than one vaporiser being used at the same time. The locking lever must be engaged when a vaporiser is mounted on the back bar, as otherwise the control dial cannot be moved.

    Fig. 16.10, A Selectatec block on the back bar of an anaesthetic machine. This permits the vaporiser to be changed rapidly without interrupting the flow of carrier gas to the patient.

  • Modern anaesthetic machines have a mechanical linkage between the N 2 O and oxygen flowmeters that prevents the delivery of less than 25%–30% oxygen.

  • The oxygen flowmeter is always in the same position (left) on UK anaesthetic machines and is of larger dimensions and ridged to clearly differentiate it from the other flowmeters.

  • All flow meters have written labels and are usually colour coded as well.

  • A pressure relief valve may be situated downstream of the vaporiser, opening at 34 kPa to prevent damage to the flowmeters or vaporisers if the gas outlet from the anaesthetic machine is obstructed.

  • A pressure relief valve set to open at a low pressure of 5 kPa may be fitted to prevent the patient's lungs from being damaged by high pressure.

  • All anaesthetic machines should incorporate an oxygen failure device. The ideal warning device should have the following characteristics:

    • Activation depends on the pressure of oxygen alone, independent of any other gas pressures.

    • Does not use a battery or mains power.

    • Gives an audible signal of sufficient duration and distinctive character.

    • Give a warning of impending failure and a further warning that failure has occurred.

    • Interrupts the flow of all other gases when it comes into operation.

    • The breathing system should open to the atmosphere, the inspired oxygen concentration should be at least equal to that of air, and accumulation of CO 2 should not occur. In addition, it should be impossible to resume anaesthesia until the oxygen supply has been restored.

  • The reservoir bag in a breathing system is highly distensible and seldom reaches pressures exceeding 3.9 kPa or 40 cmH 2 O (see later).

In the event of power failure, there is battery backup in all modern anaesthetic machines.

Magnetic resonance imaging compatible anaesthetic machines are available such as the Prima SP Anaesthetic machine (Penlon), which is made from non-ferrous metals and can be used up to the 1000 Gauss line.

Breathing systems

The delivery system that conducts anaesthetic gases from the machine to the patient is often referred to as a circuit but is described more accurately as a breathing system. Terms such as open circuits, semi-open circuits or semi-closed circuits should be avoided. The closed circuit, or circle system, is the only true circuit, as anaesthetic gases are recycled.

Adjustable pressure-limiting valve

Most breathing systems incorporate an adjustable pressure-limiting valve (APL valve, spill valve, pop-off valve, expiratory valve), which is designed to vent gas when there is a positive pressure within the system. During spontaneous ventilation, the valve opens when the patient generates positive pressure within the system during expiration; during positive pressure ventilation, the valve is adjusted to produce a controlled leak during the inspiratory phase.

Several valves of this type are available. They comprise a lightweight disc ( Fig. 16.11 ) which rests on a knife-edge seating to minimise the area of contact and reduce the risk of adhesion resulting from surface tension of condensed water. The disc has a guiding stem to position it correctly. A light spring is incorporated in the valve so that the pressure required to open it may be adjusted. During spontaneous breathing, the tension of the spring is low so that the resistance to expiration is minimised. During controlled ventilation, the valve top is screwed down to increase the tension in the spring so that gas leaves the system at a higher pressure than during spontaneous ventilation. Modern valves, even when screwed down fully, open at a pressure of 60 cmH 2 O. Most valves are encased in a hood for scavenging.

Fig. 16.11, Diagram of a spill valve. See text for details.

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