The colour codes used for medical gas cylinders in the United Kingdom are shown in Table 15.2. Different colours are used for some gases in other countries. There is a proposal to harmonize 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. Cylinder sizes and capacities are shown in Table 15.3.

TABLE 15.2

Medical Gas Cylinders Used Currently (2013) in the UK

TABLE 15.3

Medical Gas Cylinder Sizes and Capacities by Cylinder Size (A–J) and Height (inches)

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 and carbon dioxide cylinders contain liquid and vapour and the cylinders are filled to a known filling ratio (see Ch 14). 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 due to temperature changes during use. The contents of nitrous oxide and carbon dioxide 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 vaporizing volatile anaesthetic agents

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

apparatus for scavenging anaesthetic gases in order to minimize environmental pollution.

Supply of Gases

In the UK, gases are supplied at a pipeline pressure of 4 bar (400 kPa, 60 lb in^–2) 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, 45 lb in^–2).

Pressure Gauges

Pressure gauges measure the pressure in the cylinders or pipeline. Anaesthetic machines have pressure gauges for oxygen, air and nitrous oxide. These are mounted usually on the front panel of the anaesthetic machine.

Pressure Regulators

Pressure regulators are used on anaesthetic machines for three purposes:

to reduce the high pressure of gas in a cylinder to a safe working level

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 principles underlying the operation of pressure regulators are described in detail in Chapter 14.

Flow Restrictors

Pressure regulators are omitted usually 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 vaporizers to prevent back-pressure effects (see Ch 14). The absence of such a flow restrictor may be detected if a gas-driven ventilator such as the Manley is used, as this leads to fluctuations in the positions of the flowmeter bobbins during the respiratory cycle.

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 are described in detail in Chapters 14 and 16.

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, vaporizer.

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 damage from 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 USA. 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. 15.1). Most modern anaesthetic machines do not have a flowmeter for carbon dioxide. Some new anaesthetic machines such as Primus Dräger (Fig 15.2A) do not have traditional flowmeters; 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 back-up which continues the delivery of fresh gas. These machines are particularly well suited to low and minimal flow anaesthesia and they use standard vaporizers.

FIGURE 15.1 Oxygen is the last gas to be added to the gas mixture being delivered to the back bar.

FIGURE 15.2 (A) The Primus Dräger anaesthetic machine, which does not have conventional flowmeters. (B) A 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 vaporizers. 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 barotrauma if the high pressure is accidentally delivered directly to the patient’s lungs.

Quantiflex

The Quantiflex mixer flowmeter (Fig. 15.3) 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 nitrous oxide flowmeter.

FIGURE 15.3 A Quantiflex flowmeter. The required oxygen percentage is selected using the dial, and the total flow of the oxygen/nitrous oxide mixture is adjusted using the grey knob.

Linked Flowmeters

The majority of modern anaesthetic machines such as that shown in Figure 15.2B possess a mechanical linkage between the nitrous oxide and oxygen flowmeters. This causes the nitrous oxide flow to decrease if the oxygen flowmeter is adjusted to give less than 25–30% O₂ (Fig. 15.4).

FIGURE 15.4 Flowmeters with mechanical linkage between nitrous oxide and oxygen.

Vaporizers

The principles of vaporizers are described in detail in Chapter 14.

Modern vaporizers may be classified as:

Drawover vaporizers. These have a very low resistance to gas flow and may be used for emergency use in the field (e.g. Oxford miniature vaporizer; OMV) (Fig 15.5).

FIGURE 15.5 Oxford miniature vaporizer (OMV).

Plenum vaporizers. These are intended for unidirectional gas flow, have a relatively high resistance to flow and are unsuitable for use either as drawover vaporizers or in a circle system. Examples include the ‘TEC’ type in which there is a variable bypass flow. Commonly used plenum vaporizers are shown in Figures 15.6 and 15.7A.

FIGURE 15.6 Sevoflurane vaporizer.

FIGURE 15.7 (A) Isoflurane vaporizer. (B) Schematic diagram of the TEC mark 5 vaporizer. (C) Diagram of the TEC mark 5 vaporizer.

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

There has been more than one model of the ‘TEC’ type of vaporizer. The TEC Mark 2 vaporizer is now obsolete. The TEC Mark 3 had characteristics which were an improvement on the Mark 2. These included improved vaporization as a result of increased area of the wicks, reduced pumping effect by having a long tube through which the vaporized gas leaves the vaporizing chamber, improved accuracy at low gas flows and a bimetallic strip which is situated in the bypass channel and not the vaporizing chamber. In the Mark 4, the improvements were as follows: no spillage into the bypass channel if the vaporizer was accidentally inverted, and the inability to turn two vaporizers on at the same time when on the back bar of the anaesthetic machine. The TEC Mark 5 vaporizer (Fig. 15.7A–C) has improved surface area for vaporization in the chamber, improved key-filling action and an easier mechanism for switching on the rotary valve and lock with one hand. Desflurane presents a particular challenge because it has a saturated vapour pressure of 664 mmHg (89 kPa) at 20 °C and a boiling point of 23.5 °C. In order to combat this problem, a new vaporizer, the TEC 6, was developed (Fig. 15.8). It is heated electrically to 39 °C with a pressure of 1550 mmHg (approx. 2 bar). The vaporizer has electronic monitors of vaporizer function and alarms. The FGF does not enter the vaporization chamber. Instead, desflurane vapour enters into the path of the FGF. A percentage control dial regulates the flow of desflurane vapour into the FGF. The dial calibration is from 1% to 18%. The vaporizer has a back-up 9 volt battery in case of mains failure. The functioning of the vaporizer is shown diagrammatically in Figure 15.9.

FIGURE 15.8 Tec 6 desflurane vaporizer.

FIGURE 15.9 The TEC 6 desflurane vaporizer. Liquid in the vaporizing chamber is heated and mixed with fresh gas; the pressure-regulating valve balances both fresh gas pressure and anaesthetic vapour pressure.

Anaesthetic-specific connections are available to link the supply bottle (container of liquid anaesthetic agent) to the appropriate vaporizer (Fig. 15.10). These connections reduce the extent of spillage (and thus atmospheric pollution) and also the likelihood of filling the vaporizer with an inappropriate liquid. In addition to being designed specifically for each liquid, the connections themselves may be colour-coded (e.g. purple for isoflurane, yellow for sevoflurane, orange for enflurane, red for halothane).

FIGURE 15.10 An agent-specific connector for filling a vaporizer.

Halothane contains a non-volatile stabilizing agent (0.01% thymol) to prevent breakdown of the halothane by heat and ultraviolet light. Thymol is less volatile than halothane and its concentration in the vaporizer increases as halothane is vaporized. If the vaporizer is used and refilled regularly, the concentration of thymol may become sufficiently high to impair vaporization of halothane. In addition, very high concentrations may result in a significant degree of thymol vaporization, which may be harmful to the patient. Consequently, it is recommended that halothane vaporizers are drained once every 2 weeks. Sevoflurane and isoflurane vaporizers require to be emptied at much less frequent intervals.

SAFETY FEATURES OF MODERN ANAESTHETIC MACHINES

Specificity of probes on flexible hoses between terminal outlets and connections with the anaesthetic machine. The flexible hoses are colour-coded and have non-interchangeable screw-threaded connectors to the anaesthetic machine.

Pin index system to prevent incorrect attachment of gas cylinders to anaesthetic machine. Cylinders are colour-coded and they are labelled with the name of the gas that they contain.

Pressure relief valves on the downstream side of pressure regulators.

Flow restrictors on the upstream side of flowmeters.

Arrangement of the bank of flowmeters such that the oxygen flowmeter is on the right (i.e. downstream side) or oxygen is the last gas to be added to the gas mixture being delivered to the back bar (Fig. 15.1).

Non-return valves. 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 vaporizers. When operated, the oxygen bypass should give a flow rate of at least 35 L min^–1.

Mounting of vaporizers on the back bar. There is concern about contamination of vaporizers if two vaporizers are turned on at the same time. Temperature-compensated vaporizers contain wicks and these can absorb a considerable amount of anaesthetic agent. If two vaporizers are mounted in series, the downstream vaporizer could become contaminated to a dangerous degree with the agent from the upstream vaporizer. However, the newer TEC Mark 4 and 5 vaporizers have the interlocking Selectatec system (Fig. 15.11) which has locking rods to prevent more than one vaporizer being used at the same time. When a vaporizer is mounted on the back bar, the locking lever needs to be engaged (Mark 4 and 5). If this is not done, the control dial cannot be moved.

FIGURE 15.11 A Selectatec block on the back bar of an anaesthetic machine. This permits the vaporizer to be changed rapidly without interrupting the flow of carrier gas to the patient.

Modern anaesthetic machines have a mechanical linkage between the nitrous oxide and oxygen flowmeters which prevents the delivery of less than 25–30% oxygen.

A non-return valve situated downstream of the vaporizers prevents back-pressure (e.g. when using a Manley ventilator) which might otherwise cause output of high concentrations of vapour.

A pressure relief valve may be situated downstream of the vaporizer, opening at 34 kPa to prevent damage to the flowmeters or vaporizers 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. The presence of such a valve prevents the use of gas-driven minute volume divider ventilators, such as the Manley.

Oxygen failure warning devices. Anaesthetic machines have a built in oxygen failure device. There are a variety of oxygen failure warning devices. The ideal warning device should have the following characteristics:

activation depends on the pressure of oxygen itself and does not depend on the pressure of any other gas

does not use a battery or mains power

gives a signal which is audible, of sufficient duration and of distinctive character

should give a warning of impending failure and a further warning that failure has occurred

should interrupt 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 carbon dioxide should not occur. In addition, it should be impossible to resume anaesthesia until the oxygen supply has been restored.

The reservoir bag in an anaesthetic breathing system is highly distensible and seldom reaches pressures exceeding 5 kPa.

MRI-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 which conducts anaesthetic gases from the machine to the patient is termed colloquially 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 a 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. 15.12) which rests on a ‘knife edge’ seating to minimize the area of contact and reduce the risk of adhesion resulting from surface tension of condensed water. The disc has a stem which acts as a guide 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 minimized. 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 cm H₂O. Most valves are encased in a hood for scavenging.

FIGURE 15.12 Diagram of a spill valve. See text for details.

Classification of Breathing Systems

In 1954, Mapleson classified anaesthetic breathing systems into five types (Fig. 15.13); the Mapleson E system was modified subsequently by Rees, but is classified as the Mapleson F system. The systems differ considerably in their ‘efficiency’, which is measured in terms of the fresh gas flow (FGF) rate required to prevent rebreathing of alveolar gas during ventilation.

FIGURE 15.13 Mapleson classification of anaesthetic breathing systems. The arrow indicates entry of fresh gas to the system.

Mapleson A Systems

The most commonly used version is the Magill attachment. The corrugated hose should be of adequate length (usually approximately 110 cm). It is the most efficient system during spontaneous ventilation, but one of the least efficient when ventilation is controlled.

During spontaneous ventilation (Fig. 15.14), there are three phases in the ventilatory cycle: inspiratory, expiratory and the expiratory pause. Gas is inhaled from the system during inspiration (Fig. 15.14B). During the initial part of expiration, the reservoir bag is not full and thus the pressure in the system does not increase; exhaled gas (the initial portion of which is dead space gas) passes along the corrugated tubing towards the bag (Fig. 15.14C), which is filled also by fresh gas from the anaesthetic machine. During the latter part of expiration, the bag becomes full; the pressure in the system increases and the spill valve opens, venting all subsequent exhaled gas to atmosphere. During the expiratory pause, continued flow of fresh gas from the machine pushes exhaled gas distally along the corrugated tube to be vented through the spill valve (Fig. 15.14D). Provided that the FGF rate is sufficiently high to vent all alveolar gas before the next inspiration, no rebreathing takes place from the corrugated tube. If the system is functioning correctly and no leaks are present, an FGF rate equal to the patient’s alveolar minute ventilation is sufficient to prevent rebreathing. In practice, a higher FGF is selected in order to compensate for leaks; the rate selected is usually equal to the patient’s total minute volume (approximately 6 L min^–1 for a 70-kg adult).

FIGURE 15.14 Mode of action of Magill attachment during spontaneous ventilation. See text for details. FGF, fresh gas flow.

The system increases dead space to the extent of the volume of the anaesthetic face mask and angle piece to the spill valve. The volume of this dead space may amount to 100 mL or more for an adult face mask. Paediatric face masks reduce the extent of dead space, but it remains too high to allow use of the system in infants or small children (< 4 years old).

The characteristics of the Mapleson A system are different during controlled ventilation (Fig. 15.15). At the end of inspiration (produced by the anaesthetist squeezing the reservoir bag), the bag is usually less than half full (see below). During expiration, dead space and alveolar gas pass along the corrugated tube and are likely to reach the reservoir bag, which therefore contains some carbon dioxide (Fig. 15.15A). During inspiration, the valve does not open initially because its opening pressure has been increased by the anaesthetist in order to generate a sufficient pressure within the system to inflate the lungs. Thus, alveolar gas re-enters the patient’s lungs and is followed by a mixture of fresh, dead space and alveolar gases (Fig. 15.15B). When the valve does open, it is this mixture which is vented (Fig. 15.15C). Consequently, the FGF rate must be very high (at least three times alveolar minute volume) to prevent rebreathing. The volume of gas squeezed from the reservoir bag must be sufficient both to inflate the lungs and to vent gas from the system.

FIGURE 15.15 Mode of action of Magill attachment during controlled ventilation. See text for details. FGF, fresh gas flow.

The major disadvantage of the Magill attachment during surgery is that the spill valve is attached close to the mask. This makes the system heavy, particularly when a scavenging system is used, and it is inconvenient if the valve is in this position during surgery of the head or neck. The Lack system (Fig. 15.16B) is a modification of the Mapleson A system with a coaxial arrangement of tubing. This permits positioning of the spill valve at the proximal end of the system. The inner tube must be of sufficiently wide bore to allow the patient to exhale with minimal resistance. The Lack system is not quite as efficient as the Magill attachment.

FIGURE 15.16 Coaxial anaesthetic breathing systems. (A) Bain system (Mapleson D). (B) Lack system (Mapleson A). FGF, fresh gas flow; EXP, expired gas.

Mapleson B and C Systems

These systems cause mixing of alveolar and fresh gas during spontaneous or controlled ventilation. Very high FGF rates are required to prevent rebreathing. There is no clinical role for the Mapleson B system. The Mapleson C system is used in some hospitals to ventilate the lungs with oxygen during transport, but a self-inflating bag with a non-rebreathing valve is preferable.

Mapleson D System

The Mapleson D arrangement is inefficient during spontaneous breathing (Fig. 15.17). During expiration, exhaled gas and fresh gas mix in the corrugated tube and travel towards the reservoir bag (Fig. 15.17B). When the reservoir bag is full, the pressure in the system increases, the spill valve opens and a mixture of fresh and exhaled gas is vented; this includes the dead space gas, which reaches the reservoir bag first (Fig. 15.17C). Although fresh gas pushes alveolar gas towards the valve during the expiratory pause, a mixture of alveolar and fresh gases is inhaled from the corrugated tube unless the FGF rate is at least twice as great as the patient’s minute volume (i.e. at least 12 L min^–1 in the adult); in some patients, an FGF rate of 250 mL kg^–1 min^–1 is required to prevent rebreathing.

FIGURE 15.17 Mode of action of Mapleson D breathing system during spontaneous ventilation. See text for details. FGF, fresh gas flow.

However, the Mapleson D system is more efficient than the Mapleson A during controlled ventilation (Fig. 15.18), especially if an expiratory pause is incorporated into the ventilatory cycle. During expiration, the corrugated tubing and reservoir bag fill with a mixture of fresh and alveolar gas (Fig. 15.18A). Fresh gas fills the distal part of the corrugated tube during the expiratory pause (Fig. 15.18B). When the reservoir bag is squeezed, this fresh gas enters the lungs, and when the spill valve opens a mixture of fresh and alveolar gas is vented. The degree of rebreathing may thus be controlled by adjustment of the FGF rate, but this should always exceed the patient’s minute volume.

FIGURE 15.18 Mode of action of Mapleson D breathing system during controlled ventilation. See text for details. FGF, fresh gas flow.

The Bain coaxial system (Fig. 15.16A) is the most commonly used version of the Mapleson D system. FGF is supplied through a narrow inner tube. This tube may become disconnected, resulting in hypoxaemia and hypercapnia. Before use, the system should be tested by occluding the distal end of the inner tube transiently with a finger or the plunger of a 2-mL syringe; there should be a reduction in the flowmeter bobbin reading during occlusion and an audible release of pressure when occlusion is discontinued. Movement of the reservoir bag during anaesthesia does not necessarily indicate that fresh gas is being delivered to the patient.

The Bain system may be used to ventilate the patient’s lungs with some types of automatic ventilator (e.g. Penlon Nuffield 200; Fig. 15.19). A 1-m length of corrugated tubing is interposed between the patient valve of the ventilator and the reservoir bag mount (Fig. 15.20); the spill valve must be closed completely. An appropriate tidal volume and ventilatory rate are selected on the ventilator and anaesthetic gases are supplied to the Bain system. During inspiration, the gas from the ventilator pushes a mixture of anaesthetic and alveolar gas from the corrugated outer tube into the patient’s lungs; during expiration, the ventilator gas and some of the alveolar gas are vented through the exhaust valve of the ventilator. The degree of rebreathing is regulated by the anaesthetic gas flow rate; a flow of 70–80 mL kg^–1 min^–1 should result in normocapnia and a flow of 100 mL kg^–1 min^–1 in moderate hypocapnia. A secure connection between the Bain system and the anaesthetic machine must be assured. If this connection is loose, a leak of fresh gas occurs; this causes rebreathing of ventilator gas and results in awareness, hypoxaemia and hypercapnia.

FIGURE 15.19 The Penlon Nuffield 200 ventilator.

FIGURE 15.20 The Bain system for controlled ventilation by a mechanical ventilator (e.g. Penlon Nuffield 200). A 1-m length of corrugated tubing with a capacity of at least 500 mL is required to prevent gas from the ventilator reaching the patient’s lungs. PaCO₂ is controlled by varying the fresh gas flow (FGF) rate.

Mapleson E and F Systems

The Mapleson E system, or Ayre’s T-piece, has virtually no resistance to expiration and was used extensively in paediatric anaesthesia before the advantages of continuous positive airways pressure (CPAP) were recognized. It functions in a manner similar to the Mapleson D system in that the corrugated tube fills with a mixture of exhaled and fresh gas during expiration and with fresh gas during the expiratory pause. Rebreathing is prevented if the FGF rate is 2.5–3 times the patient’s minute volume. If the volume of the corrugated tube is less than the patient’s tidal volume, some air may be inhaled at the end of inspiration; consequently, an FGF rate of at least 4 L min^–1 is recommended with a paediatric Mapleson E system.

During spontaneous ventilation, there is no indication of the presence, or the adequacy of ventilation. It is possible to attach a visual indicator, such as a piece of tissue paper or a feather, at the end of the corrugated tube, but this is not very satisfactory.

Intermittent positive pressure ventilation (IPPV) may be applied by occluding the end of the corrugated tube with a finger. However, there is no way of assessing the pressure in the system and there is a possibility of exposing the patient’s lungs to excessive volumes and pressures.

The Mapleson F system, or Rees’ modification of the Ayre’s T-piece, includes an open-ended bag attached to the end of the corrugated tube. This confers several advantages:

It provides visual evidence of breathing during spontaneous ventilation.

By occluding the open end of the bag temporarily, it is possible to confirm that fresh gas is entering the system.

It provides a degree of CPAP during spontaneous ventilation and positive end-expiratory pressure (PEEP) during IPPV.

It provides a convenient method of assisting or controlling ventilation. The open end of the reservoir bag is occluded between the fourth and fifth fingers and the bag is squeezed between the thumb and index finger; the fourth and fifth fingers are relaxed during expiration to allow gas to escape from the bag. It is possible with experience to assess (approximately) the inflation pressure and to detect changes in lung and chest wall compliance.

However, one main disadvantage of the Mapleson F system is that efficient scavenging is unsatisfactory and is non-standard.

Mapleson ADE System

This system provides the advantages of the Mapleson A, D and E systems. It can be used efficiently for spontaneous and controlled ventilation in both children and adults.

It consists of two parallel lengths of 15-mm bore tubing; one delivers fresh gas and the other carries exhaled gas. One end of the tubing connects to the patient via a Y-connection and the other end contains the Humphrey block (Fig. 15.21). The Humphrey block (Fig. 15.22) consists of an APL valve, a lever to select spontaneous or controlled ventilation, a reservoir bag, a port to connect a ventilator and a safety pressure relief valve which opens at a pressure above 6 kPa.

FIGURE 15.21 The ADE system.

FIGURE 15.22 The Humphrey block. This consists of an APL valve, a lever to select spontaneous or controlled ventilation, a reservoir bag, a port to connect to the ventilator and a safety pressure relief valve.

When the lever is in the A mode (up), the reservoir bag is connected to the breathing system as it would be in the Mapleson A system. The breathing hose connecting the bag to the patient is the inspiratory limb. The expired gases travel along the other tubing back to the APL valve, which is connected to the scavenging system.

With the lever in the D/E mode (down), the reservoir bag and the APL valve are isolated from the breathing system. What was the exspiratory limb in the A mode now delivers gas to the patient. The hose returning gas to the Humphrey block now functions as a reservoir to the T-piece. This hose would open to atmosphere via a port adjacent to the bag mount, but in practice this port is connected to a ventilator such as the Penlon Nuffield.

In adults, an appropriate FGF is 50–60 mL kg^–1 min^–1 in spontaneously breathing patients and 70 mL kg^–1 min^–1 in ventilated patients.

Drawover Systems

Occasionally, it is necessary to administer anaesthesia at the scene of a major accident. If inhalation anaesthesia is required, it is necessary to use simple, portable equipment. The Triservice apparatus has been designed by the British armed forces for use in battle conditions (Fig. 15.23). It comprises a self-inflating bag, a non-rebreathing valve (e.g. Ambu E, Rubens) which vents all expired gases to atmosphere, one or two Oxford miniature vaporizers (which have a low internal resistance), an oxygen supply and a length of corrugated tubing which serves as an oxygen reservoir. Either spontaneous or controlled ventilation may be employed using this apparatus.

FIGURE 15.23 The Triservice apparatus. (Courtesy of Dr S. Kidd.)

Rebreathing Systems

Anaesthetic breathing systems in which some gas is rebreathed by the patient were designed originally to economize in the use of cyclopropane. In addition, they reduce the risk of atmospheric pollution and increase the humidity of inspired gases, thereby reducing heat loss from the patient. Rebreathing systems may be used as ‘closed’ systems, in which fresh gas is introduced only to replace oxygen and anaesthetic agents absorbed by the patient. More commonly, the system is used with a small leak through a spill valve, and the fresh gas supply exceeds basal oxygen requirements. Because rebreathing occurs, these systems must incorporate a means of absorbing carbon dioxide from exhaled alveolar gas.

Soda Lime

Soda lime is the substance used most commonly for absorption of carbon dioxide in rebreathing systems. The composition of soda lime is shown in Table 15.4. The major constituent is calcium hydroxide, but sodium and potassium hydroxides may also be present. Absorption of carbon dioxide occurs by the following chemical reactions:

TABLE 15.4

Composition of Soda Lime

Ca(OH)₂	94%
NaOH	5%
KOH	< 1% or nil
Silica	0.2%
Moisture content	14–19%

Water is required for efficient absorption. There is some water in soda lime and more is added from the patient’s expired gas and from the chemical reaction. The reaction generates heat and the temperature in the centre of a soda lime canister may exceed 60 °C. Sevoflurane has been shown to interact with soda lime to produce substances that are toxic in animals. However, this does not appear to impose any significant risk in humans (see Ch 2). There is new evidence suggesting that the presence of strong alkalis such as sodium and potassium hydroxides could be the trigger of the interaction between volatile agents and soda lime. New carbon dioxide absorbers are now being manufactured without these hydroxides in order to reduce this interaction.

The size of soda lime granules is important. If granules are too large, the surface area for absorption is insufficient; if they are too small, the narrow space between granules results in a high resistance to breathing. Granule size is measured by a mesh number. Soda lime consists of granules in the range of 4–8 mesh. (A 4-mesh strainer has four openings per square inch and an 8-mesh strainer has eight openings.) Silica is added to soda lime to reduce the tendency of the granules to disintegrate into powder. In addition, soda lime contains an indicator which changes colour as the active constituents become exhausted. The rate at which soda lime becomes exhausted depends on the capacity of the canister, the FGF rate and the rate of carbon dioxide production. In a completely closed system, a standard 450-g canister becomes inefficient after approximately 2 h.

Baralyme

Baralyme is another carbon dioxide absorber. It is a mixture of approximately 20% barium hydroxide and 80% calcium hydroxide. It may also contain some potassium hydroxide, an indicator and moisture. Barium hydroxide contains eight molecules of water of crystallization, which help to fuse the mixture so that it retains the granular structure under various conditions of heat and moisture. The granules of Baralyme are similar to those of soda lime.

‘To-and-Fro’ (Waters’) System

This breathing system comprises a Mapleson C breathing system with a canister of soda lime interposed between the spill valve and the reservoir bag (Fig. 15.24). The soda lime granules nearest the patient become exhausted first, increasing the dead space of the system; in addition, the canister is positioned horizontally and gas may be channelled above the soda lime unless the canister is packed tightly. The system is cumbersome and there is a risk that patients may inhale soda lime dust from the canister.

FIGURE 15.24 Waters’ anaesthetic breathing system, incorporating a canister of soda lime.

Circle System

This system has replaced the ‘to-and-fro’ system. The soda lime canister is mounted on the anaesthetic machine, and inspiratory and expiratory corrugated tubing conducts gas to and from the patient (Fig. 15.25).The system incorporates a reservoir bag and spill valve and two low-resistance one-way valves to ensure unidirectional movement of gas (Fig. 15.26). These valves are normally mounted in glass domes so that they may be observed to be functioning correctly. The spill valve may be mounted close to the patient or beside the absorber; during surgery to the head or neck, it is more convenient to use a valve near the absorber. Fresh gas enters the system between the absorber and the inspiratory tubing.

FIGURE 15.25 The circle breathing system mounted on an anaesthetic machine.

FIGURE 15.26 Mechanism of the circle system. The direction of gas flow is controlled via the unidirectional valves. A lever allows the ventilation to be either spontaneous through the reservoir bag and APL valve, or controlled by a ventilator.

The soda lime canister is mounted vertically and thus channelling of gas through unfilled areas is not possible. The canister cannot contribute to dead space; consequently, a large canister may be used and the soda lime needs to be changed less often.

The major disadvantage of the circle system arises from its volume. If the system is filled with air initially, low flow rates of anaesthetic gases are diluted substantially and adequate concentrations cannot be achieved. Even if the system is primed with a mixture of anaesthetic gases, the initial rapid uptake by the patient results in a marked decrease in concentrations of anaesthetic agents in the system, resulting in light anaesthesia. Consequently, it is necessary usually to provide a total FGF rate of 3–4 L min^–1 to the system initially. This flow rate may be reduced subsequently, but it must be remembered that dilution of fresh gas continues at low flow rates and that rapid changes in depth of anaesthesia cannot be achieved.

Volatile anaesthetic agents may be delivered to a circle system in two ways:

Vaporizer outside the circle (VOC) (Fig. 15.27A). If a standard vaporizer (e.g. TEC series) is used, it must be placed on the back bar of the anaesthetic machine because of its high internal resistance. If low FGF rates (< 1 L min^–1) are used, the change in concentration of volatile anaesthetic agent achieved in the circle system is very small because of dilution, even if the vaporizer is set to deliver a high concentration (Fig. 15.28A). It may be necessary to change FGF rate rather than the vaporizer setting in order to achieve a rapid change in depth of anaesthesia. The concentration of volatile agent in the system depends on the patient’s expired concentration (which is recycled), the rate of uptake by the patient (which decreases with time and is lower with agents of low blood/gas solubility coefficient), the concentration of agent supplied and the FGF rate.

FIGURE 15.27 Diagrammatic representation of the circle system. (A) Vaporizer outside the circle (VOC). (B) Vaporizer inside the circle (VIC).

FIGURE 15.28 Variation of inspired concentration of halothane with fresh gas flow (FGF) rate. Total minute ventilation is 5 L min^–1. (A) Vaporizer outside the circle (VOC); note that dilution of the fresh gas results in much lower concentrations in the circle system than the concentration set on the vaporizer unless FGF rate approaches 3 L min^–1. (B) Vaporizer inside the circle (VIC); at low flow rates, lack of dilution of expired halothane concentration, with additional halothane vaporized during each inspiration, results in inspired concentrations much higher than those set on the vaporizer. Even at an FGF rate of 3 L min^–1, inspired concentration is approximately 50% higher than the vaporizer setting.

Vaporizer inside the circle (VIC) (Fig. 15.27B). Drawover vaporizers with a low internal resistance (e.g. Goldman) may be placed within the circle system. During each inspiration, vapour is added to the inspired gas mixture. In contrast to a VOC system, the inspired concentration is higher at low FGF rates because the expired concentration is diluted to a lesser extent (Fig. 15.28B) and the vaporizer adds to the concentration present in the expired gas. Very high concentrations of volatile agent may be inspired if minute volume is large; this risk is greatest if IPPV is employed.

If FGF rate is low, the use of the circle system by the inexperienced anaesthetist may result either in inadequate anaesthesia or in severe cardiovascular and respiratory depression. In addition, a hypoxic gas mixture may be delivered if low flow rates of a nitrous oxide/oxygen mixture are supplied, because after 10–15 min, oxygen is taken up in larger volumes than nitrous oxide. These difficulties can be overcome by monitoring the inspired concentrations of oxygen, carbon dioxide and volatile anaesthetic agent continuously (see Ch 16). The trainee anaesthetist must be aware that:

It is inadvisable to use a VIC system unless inspired concentrations of anaesthetic agents are monitored continuously.

IPPV must never be used with a VIC system unless inspired concentrations of anaesthetic agents are monitored continuously, because of the risk of generating very high concentrations of volatile agent.

Nitrous oxide must not be used in any circle system if the total FGF rate is less than 1000 mL min^–1, unless inspired oxygen concentration is measured continuously.

It is essential to monitor inspired concentrations of oxygen and inhalational anaesthetic agent and expired concentration of carbon dioxide when using the circle system.

One-way valves may stick. These should be checked both at the pre-anaesthetic check of the machine and during anaesthesia.

Because the circle system has many connections, the anaesthetist should be vigilant about checking for any leak or disconnection.

The advantages and disadvantages of the circle system are summarized in Table 15.5.

TABLE 15.5

Disadvantages and Advantages of the Circle System

Disadvantages	Advantages
Cumbersome equipment	Inspired gases are humidified and warmed
Risk of delivering hypoxic mixture	Economical
Increased resistance to breathing	Minimal pollution
Slow change in the depth of anaesthesia
Risk of awareness
Risk of a rise in end-tidal CO₂
Risk of unidirectional valves sticking
Not ideal for paediatric patients breathing spontaneously
Some inhalational agents may interact with soda lime

Manual Resuscitation Breathing Systems

Occasionally, a patient may require emergency ventilation support using a source which does not rely on pressurized gas or electricity. It is recommended that such a breathing system is readily available in all areas where anaesthetics are administered in case such an emergency arises.

There are many different types of manual resuscitation breathing systems but fundamentally they all have the following components:

a self-inflating bag

a non-rebreathing valve

a fresh gas input with or without an oxygen reservoir bag.

The self-inflating bag has a volume of approximately 1500 mL, 500 mL and 250 mL for the adult, child and infant sizes, respectively. The non-rebreathing valve has several components which ensure that, during the inspiratory phase, gas flows out of the bag into the patient and, during the expiratory phase, the valve ensures that exhaled gas escapes through the expiratory port without mixing with fresh gas. Three types of non-rebreathing systems are available, the Ruben, Ambu and Laerdal systems (Fig 15.29). Functionally, they are very similar but with some minor differences. The Ruben valve has a spring-loaded bobbin within the valve housing. The Ambu system has several series of valves which have either a single valve or double leaf valves to control unidirectional flow. The Laerdal system has three components: a duck-billed inspiratory/expiratory valve, a valve body housing inspiratory and expiratory ports and a non-return flap valve in the expiratory port.

FIGURE 15.29 The Laerdal manual resuscitation breathing system.

VENTILATORS

Mechanical ventilation of the lung may be achieved by several mechanisms, including the generation of a negative pressure around the whole of the patient’s body except the head and neck (cabinet ventilator or ‘iron lung’), a negative pressure over the thorax and abdomen (cuirass ventilators) or a positive pressure over the thorax and abdomen (inflatable cuirass ventilators). However, during anaesthesia, and in the majority of patients who require mechanical ventilation in the intensive care unit, ventilation is achieved by the application of positive pressure to the lungs through a laryngeal mask airway, tracheal tube or tracheostomy tube. Only this mode of ventilation is described here.

An enormous selection of ventilators exists and it is possible in this section to discuss only the principles involved in their use. Before using any ventilator, it is essential that the anaesthetist understands its functions fully; failure to do so may result in the delivery of a hypoxic gas mixture, rebreathing of carbon dioxide and/or delivery of a mixture that contains no anaesthetic gases. If an unfamiliar ventilator is encountered, it may be helpful to use a ‘dummy lung’ (a small reservoir bag on the patient connection) and to discuss the capabilities and limitations of the machine with a senior colleague. In addition, the manufacturer’s ‘user handbook’ may be consulted or details may be obtained from a specialist book.

Continuous clinical monitoring is essential when any ventilator is used, even those which incorporate sophisticated monitoring and warning devices. In addition to standard clinical monitoring systems attached to the patient (see Ch 16), the minimum acceptable monitoring of ventilator function includes measurement of expired tidal volume, airway pressure and inspired oxygen concentration; in addition, a ventilator disconnection alarm should be incorporated in the system. Continuous monitoring of end-tidal carbon dioxide, oxygen saturation and inspired anaesthetic gas concentrations is essential in the operating theatre.

The incorporation of a humidifier in the inspiratory limb, or of a condenser humidifier at the connection with the tracheal tube, is essential in long-term ventilation in the ICU. Bacterial filters (Fig. 15.30) are now recommended for all patients undergoing anaesthesia.

FIGURE 15.30 Bacterial filters and humidifiers which are used in breathing systems: left, a paediatric filter incorporated into an angle piece; right, an adult filter.

The principles of operation of ventilators are described best by considering each phase of the ventilatory cycle: inspiration; change from inspiration to expiration; expiration; and change from expiration to inspiration.

Inspiration

The pattern of volume change in the lung is determined by the characteristics of the ventilator. Ventilators may deliver a predetermined flow rate of gas (constant flow generators) or exert a predetermined pressure (constant pressure generators), although some machines produce a pattern which does not conform precisely to either category. Most flow generators produce a constant flow of gas during inspiration, although a few generate a sinusoidal flow pattern if the ventilator bellows is driven via a crank. The characteristics of constant flow and constant pressure generators are shown in Figure 15.31.

FIGURE 15.31 Graphs of generated pressure, mouth (or tracheal tube) and alveolar pressures, flow rate and alveolar volume changes during inspiration and subsequent expiration produced by a constant pressure generator (A) and a constant flow generator (B). A constant pressure generator exerts a low pressure (e.g. 1.5 kPa, 15 cmH₂O). At the start of inspiration, the pressure in the alveoli is zero. Gas flows rapidly into the alveoli at a rate determined by airways resistance, resulting in rapid increases in alveolar volume and pressure. The mouth-alveolar pressure gradient decreases and flow rate, and consequently the rates of increase of alveolar volume and pressure, decrease also. When the alveolar pressure equals the ventilator pressure, flow ceases. A constant flow generator generates a very high internal pressure (e.g. 400 kPa) but has a high internal resistance to limit flow rate. The pressure gradient between machine and alveoli remains virtually constant throughout inspiration and thus flow rate is constant. The increases in alveolar volume and (assuming constant compliance) pressure are linear. Because flow rate is constant, the pressure gradient between mouth and alveoli is constant throughout inspiration. (a) Mouth pressure decreases to equal alveolar pressure during the inspiratory pause when flow ceases. (b) Gas flow out of the lung during expiration (c) is passive.

Constant Pressure Generator

The ventilator produces inspiration by generating a constant, predetermined pressure. However, if airway resistance increases or if compliance decreases, these ventilators deliver a reduced tidal volume at the preset cycling pressure (Fig. 15.32). Consequently, their performance is variable.

FIGURE 15.32 Generated and alveolar pressures, and alveolar volume, during inspiration with a constant pressure generator. (A) Normal. (B) Decreased compliance. (C) Increased airway resistance. Note that both abnormalities reduce alveolar volume.

Constant Flow Generator

These ventilators produce inspiration by delivering a predetermined constant flow rate of gas during inspiration. Changes in resistance or compliance make little difference to the volume delivered (unless the ventilator is pressure-cycled; see below), although airway and alveolar pressures may change (Fig. 15.33). For example, decreased compliance results in delivery of a normal tidal volume; however, the rate of increase of alveolar pressure is greater than normal (i.e. the slope is greater) and airway pressure is correspondingly higher to maintain an appropriate pressure gradient between the tracheal tube and the alveoli. If airway resistance increases, the pressure at the tracheal tube (and the gradient between tracheal tube and alveolar pressures) is higher than normal throughout inspiration, but alveolar pressure and the slopes of both pressure curves are normal. Constant flow generators do not compensate for leaks; the tidal volume delivered to the lungs decreases.

FIGURE 15.33 Mouth and alveolar pressures during inspiration with a constant flow generator. (A) Normal. (B) Decreased compliance. (C) Increased airway resistance. Alveolar volume remains constant because flow rate is constant. Decreased compliance results in an increased rate of increase of alveolar pressure; mouth pressure also increases more steeply, but the gradient between mouth and alveolar pressures remains normal. Increased airway resistance increases the mouth-alveolar pressure gradient.

Some ventilators generate a pressure rather higher than that required to inflate the lungs but not high enough to maintain constant flow throughout inspiration. The flow, volume and pressure changes within the lung are shown in Figure 15.34.

FIGURE 15.34 Pressure, flow and alveolar volume characteristics during inspiration with a ventilator with a moderately high internal pressure. At higher bellows pressures, and in a patient with normal compliance and airway resistance, the characteristics approximate to those of a constant flow generator (Fig. 15.33). At low bellows pressures, if compliance decreases or if airway resistance increases, the pattern is similar to that of a constant pressure generator.

Change from Inspiration to Expiration

This is termed ‘cycling’ and may be achieved in one of three ways:

Volume-cycling. The ventilator cycles into expiration whenever a predetermined tidal volume has been delivered. The duration of inspiration is determined by the inspiratory flow rate.

Pressure-cycling. The ventilator cycles into expiration when a preset airway pressure is achieved. This allows compensation for small leaks but, in common with a constant pressure generator, a pressure-cycled ventilator delivers a different tidal volume if compliance or resistance change. In addition, inspiratory time varies with changes in compliance and resistance.

Time-cycling. This is the method used most commonly by modern ventilators. The duration of inspiration is predetermined. With a constant flow generator, it may be desirable to preset a tidal volume; when this has been delivered, there is a short inspiratory pause (which improves gas distribution within the lung) before the inspiratory cycle ends. The use of this ‘volume-preset’ mechanism must be differentiated from volume-cycling, in which the ventilator cycles into expiration whenever the preset tidal volume has been delivered (and therefore the respiratory rate may be variable). When a constant pressure generator is time-cycled, the tidal volume delivered depends on the compliance and resistance of the lungs and on the pressure within the bellows.

Expiration

Usually, the patient is allowed to exhale to atmospheric pressure; flow rate decreases exponentially. Subatmospheric pressure should not be used during expiration as it induces closure of small airways and air trapping. PEEP may be applied in some circumstances (see Ch 45).

Change from Expiration to Inspiration

On most ventilators, this is achieved by time-cycling. However, it may be desirable occasionally to use pressure-cycling in response to a subatmospheric pressure generated by the patient’s inspiratory effort.

Delivery of Anaesthetic Gas

Some ventilators deliver a minute volume determined by a preset tidal volume and rate. When used in anaesthesia, these machines must be supplied with a flow rate of anaesthetic gases which equals or exceeds the minute volume delivered, otherwise air, or gas used to drive the ventilator, is entrained and delivered to the patient. Ventilators such as the Blease Manley ventilator (Fig. 15.35) are driven by the anaesthetic gas supply, can deliver only that gas, and divide it into predetermined tidal volumes (minute volume dividers).

FIGURE 15.35 The Blease Manley MP5 ventilator with ventilator alarm.

Ventilators may be used to compress bellows in a separate system which contains anaesthetic gases (‘bag-in-a-bottle’); it is possible to provide IPPV in a circle system in this way. The bag-in-a-bottle ventilator (Fig. 15.36) consists of a chamber with a tidal volume range of 0–1500 mL (adult mode) or 0–400 mL (paediatric mode) and ascending bellows which accommodate FGF. The control unit has controls, displays and alarms and these may include tidal volume, respiratory rate, inspiratory to expiratory (I:E) ratio, airway pressure and an on/off/standby switch. Compressed air or oxygen is used as the driving gas. On entering the chamber, the compressed gas forces the bellows down, delivering the FGF within the bellows to the patient. The driving gas in the chamber and the FGF in the bellows remain separate.

FIGURE 15.36 Blease bag-in-a-bottle ventilator.

The Penlon Nuffield 200 ventilator (Fig. 15.19) is an intermittent blower. It is a very versatile ventilator, which may be used in different age groups and using different breathing systems. The control unit consists of an airway pressure gauge (cmH₂O), an on/off switch and controls to set inspiratory and expiratory time (seconds) and inspiratory flow rate (L s^–1). Below the control unit, there is a connection for the driving gas (oxygen or air) and the valve block. A small tube connects the valve block to an airway pressure monitor and to a ventilator alarm. The valve block consists of a port for tubing to connect to the breathing system reservoir bag mount (Bain system) or the ventilator port (ADE system), an exhaust port which can be connected to the scavenging system and a pressure relief valve which opens at 6–7 kPa. With this standard valve, the ventilator is a time-cycled flow generator. The valve block can be changed to a paediatric Newton valve and this then converts the ventilator to a time-cycled pressure generator.

Older anaesthetic machine ventilators had a minimal number of controls, usually for minute volume, tidal volume, ventilator frequency and I:E ratio. The newer models resemble the critical care ventilators in the variable settings that are available. They perform self-tests which use dual processor technology when switched on, they have volume or pressure controlled ventilation modes, assisted spontaneous ventilation and electronically adjusted PEEP. They have sophisticated spirometry which compensates for factors such as leaks and patient compliance. They are suitable for a wider range of patients’ weight, and can deliver tidal volumes as low as 20 mL. The ventilator in the Primus Dräger (Fig. 15.2A) anaesthetic machine is an electronically controlled piston ventilator, which provides several modes of ventilation including volume control, pressure control, pressure support and volume mode autoflow. All these modes can be synchronized with patient effort and can have additional pressure support.

Transport Ventilators

There are several types of ventilator available for use during the transport of critically ill patients, such as the Pneupac VR1 and the Oxylog. The Oxylog 3000 (Fig. 15.37) and 3000Plus ventilators offer sophisticated ventilation in emergency situations and during transport. They are time-cycled, constant volume and pressure-controlled ventilators and deliver tidal volumes of 50 mL or more. They incorporate various modes of ventilation suitable for critically ill patients including IPPV, synchronized intermittent mandatory ventilation (SIMV) with adjustable pressure assist during spontaneous breathing, continuous positive airways pressure (CPAP) and biphasic positive airway pressure (BIPAP), apnoea ventilation for switching over automatically to volume-controlled ventilation if breathing stops, and non-invasive ventilation (NIV). Other features include monitoring of airway pressure and expiratory minute volume, integrated capnography and enhanced data connectivity.

FIGURE 15.37 Oxylog 3000 transport ventilator.

The characteristics of several common ventilators are summarized in Table 15.6.

TABLE 15.6

Classification of Some Common Ventilators Used During Anaesthesia

High-Frequency Ventilation

High-frequency ventilation (HFV) may be defined as ventilation at a respiratory rate of greater than four times the resting respiratory rate of the subject. The different modes of HFV are shown in Table 15.7.

TABLE 15.7

Types of High-Frequency Ventilation

Type of Ventilation	Rate of Ventilation (Cycles min^–1)
High-frequency positive pressure ventilation (HFPPV)	60–100
High-frequency jet ventilation (HFJV)	100–400
High-frequency oscillation ventilation (HFOV)	400–2400

Of the three types of HFV, high-frequency jet ventilation (HFJV) is the most commonly used. The tidal volume used in HFJV is small compared with conventional ventilation. This is delivered at high pressure (up to 5 bar) through a cannula or catheter placed in the trachea. Inspiratory flow rates of up to 100 L min^–1 may be required. The inspiratory time is adjustable from 20% to 50% of the cycle. The mechanism by which HFV is able to maintain gas exchange is not clear. Typical values for adult ventilation are:

ventilation rate 100–150 cycles min^–1

driving pressure 100–200 kPa

inspiratory cycle of 20–40%.

HFJV is used during some operations on the larynx, trachea or lung and in a small number of patients in the ICU. Gases should be humidified when using HFJV. Gas exchange may be unpredictable and the technique should not be used by the trainee without supervision. Figure 15.38 illustrates a type of ventilator used for HFJV.

FIGURE 15.38 Penlon Bromsgrove jet ventilator.

Venturi Injector Device

The Venturi injector consists of a high-pressure oxygen source (at about 400 kPa from either the anaesthetic machine or direct from a pipeline), on/off trigger and connection tubing that can withstand high pressure. The Manujet (Fig. 15.39) is a newer design of a Venturi injector device. It has a dial to alter the driving pressure to suit the size of patient from a neonate to an adult. This is connected to the side of a rigid bronchoscope, to a transtracheal catheter or to a cannula inserted through the cricothyroid membrane. The injector is controlled manually. A 14 gauge cannula or a specially designed cannula (Fig. 15.40) inserted through the cricothyroid membrane may be used to ventilate using the Venturi injector device. A Venturi effect is created which entrains atmospheric air and allows intermittent insufflation of the lungs with oxygen-enriched air at airway pressures of 2.5–3.0 kPa. It is used in operations on the larynx, trachea or lung. Possible complications include barotrauma, gastric distension and awareness if inadequate quantities of intravenous anaesthetic drugs are administered. When Venturi injector devices are employed, it is essential to ensure that gas is able to leave the lungs though the upper airway during expiration.

FIGURE 15.39 Manually controlled Venturi injector.

FIGURE 15.40 Cricothyroid cannula to use with a Venturi injector device.

SCAVENGING

The possible adverse effects of pollution on staff in the operating theatre environment are discussed in Chapter 20. The principal sources of pollution by anaesthetic gases and vapours include:

discharge of anaesthetic gases from ventilators

expired gas vented from the spill valve of anaesthetic breathing systems

leaks from equipment, e.g. from an ill-fitting face mask

gas exhaled by the patient after anaesthesia. This may occur in the operating theatre, corridors and recovery room

spillage during filling of vaporizers.

Although most attention has centred on removing gas from the expiratory ports of breathing systems and ventilators, other methods of reducing pollution should also be considered:

Reduced use of anaesthetic gases and vapours. The use of the circle system reduces the potential for atmospheric pollution. The use of inhalational anaesthetics may be obviated totally by using total intravenous anaesthesia or local anaesthetic techniques.

Air conditioning. Air conditioning units which produce a rapid change of air in the operating theatre reduce pollution substantially. However, some systems recycle air, and older operating theatres, dental surgeries and obstetric delivery suites may not be equipped with air conditioning.

Care in filling vaporizers. Great care should be taken not to spill volatile anaesthetic agent when a vaporizer is filled. The use of agent-specific connections (Fig. 15.10) reduces the risk of spillage. In some countries, vaporizers must be filled only in a portable fume cupboard.

Scavenging Apparatus

Anaesthetic gases vented from the breathing system are removed by a collecting system. A variety of purpose-built scavenging spill valves is available; an example of an adjustable pressure-limiting (APL) valve is shown in Figure 15.41. Waste gases from ventilators are collected by attaching the scavenging system to the expiratory port of the ventilator. Connectors on scavenging systems have a diameter of 30 mm to ensure that inappropriate connections with anaesthetic apparatus cannot be made.

FIGURE 15.41 An APL (adjustable pressure-limiting) valve with scavenging attachment.

Disposal systems may be active, semi-active or passive.

Active Systems

These employ apparatus to generate a negative pressure within the scavenging system to propel waste gases to the outside atmosphere. The system may be powered by a vacuum pump (Fig. 15.42) or a Venturi system (Fig. 15.43). The exhaust should be capable of accommodating 75 L min^–1 continuous flow with a peak of 130 L min^–1. Usually, a reservoir system (Fig 15.42B) is used to permit high peak flow rates to be accommodated. In addition, there must be a pressure-limiting device within the system to prevent the application of negative pressure to the patient’s lungs.

FIGURE 15.42 An active scavenging system (A) and the reservoir assembly (B).

FIGURE 15.43 A Venturi system for active scavenging of anaesthetic gases.

Semi-Active Systems

The waste gases may be conducted to the extraction side of the air-conditioning system, which generates a small negative pressure within the scavenging tubing. These systems have variable performance and efficiency.

Passive Systems

These systems vent the expired gas to the outside atmosphere (Fig. 15.44). Gas movement is generated by the patient. Consequently the total length of tubing must not be excessive or resistance to expiration is high. The pressure within the system may be altered by wind conditions at the external terminal; on occasions, these may generate a negative pressure, but may also generate high positive pressures. Each scavenging location should have a separate external terminal to prevent gases being vented into adjacent locations. Relief valves must be incorporated to prevent negative or high positive pressures within the system.

FIGURE 15.44 A passive scavenging system.

Irrespective of the type of disposal system, tubing used for scavenging must not be allowed to lie on the floor of the operating theatre, as compression (e.g. by feet or by items of equipment) results in increased resistance to expiration and may generate dangerously high pressure within the patient’s lungs.

RESERVOIR BAGS

Reservoir bags are used in breathing systems. Their functions include:

serving as a reservoir of inspired gases

providing a means of manual ventilation of the lungs

serving as a visual or tactile observation to monitor the patient’s spontaneous respiration

protecting the patient from excessive pressure in the breathing system.

The reservoir bag may accommodate an increase in pressure in the breathing system to a maximum of approximately 50 cmH₂O (5 kPa).

The standard adult size is 2 L and the paediatric size is 0.5 L. However, the size of reservoir bags may vary from 0.5 to 6 L.

LARYNGOSCOPES

A laryngoscope consists of a blade which elevates the lower jaw and tongue, a light source near the tip of the blade to illuminate the larynx, and a handle to apply leverage to the blade. There are many forms of blade and handle.

Curved Blade

The most commonly used adult laryngoscope blade is the Macintosh curved blade, which is manufactured in several sizes (Figs 15.45, 15.46).

FIGURE 15.45 A laryngoscope with the Macintosh adult blade.

FIGURE 15.46 A selection of laryngoscope blades. From the top downwards: Macintosh adult blade, Miller adult blade, Soper infant blade, Wisconsin infant blade and Macintosh infant blade.

The tip of the laryngoscope blade is advanced carefully over the surface of the tongue until it reaches the vallecula (Ch 21, Fig. 21.3). The tip of the blade is rotated upwards and the laryngoscope lifted along the axis of the handle to lift the larynx; the incisor teeth must not be used as a fulcrum to lever the tip of the blade upwards. When the arytenoids and posterior part of the cords are seen, gentle pressure on the larynx using the right thumb, or provided by an assistant, may help to improve the view. Concerns have been raised about the possibility that multiple-use laryngoscopes may transfer between patients the prions thought to be responsible for causing variant Creutzfeldt–Jakob disease (vCJD), especially if used during surgery for tonsillectomy or adenoidectomy Therefore disposable laryngoscope blades are now used routinely. The quality of disposable laryngoscopes has improved enormously and is now indistinguishable from that of non-disposable devices.

Straight Blade

Straight-bladed laryngoscopes are useful adjuncts in safe airway management. However, they are not always as easy to use as curved blades. The technique of laryngoscopy is slightly different when a straight-bladed laryngoscope is used (see Fig. 21.3). Instead of placing the tip of the blade in the vallecula, it is advanced over the posterior border of the epiglottis, which is then lifted directly by the blade to provide a view of the larynx. This technique is useful particularly in babies, in whom the epiglottis is rather floppy and may obscure the view of the larynx if a curved blade is used. However, bruising of the epiglottis is more likely with a straight blade. The straight-bladed laryngoscopes available include the Miller, Magill, Soper and Wisconsin (Fig 15.46) laryngoscopes.

Light Source

Most laryngoscopes are powered by batteries contained within the handle; these must be replaced regularly to prevent failure during laryngoscopy. On many older models of laryngoscope, the light source is a bulb which screws into a socket on the blade; a tight connection should be ensured before laryngoscopy is attempted. It is usual for the electrical circuit between the batteries and the bulb to be closed by a switch which operates automatically when the blade is opened. However, the electrical contacts of the switch may become corroded, causing a reduction in power or total failure. Because of these potential problems, it is important that the function of the laryngoscope is checked carefully before use. It is also wise to have a spare functioning laryngoscope and a variety of blades available. Newer designs place the bulb in the handle and the light is transmitted to the blade by means of fibreoptics.

Laryngoscope Handle

The standard handle may result in difficult laryngoscopy in obese patients, and in women with large breasts. Short handles are available for use in these situations (Fig. 15.47). The short handle has almost replaced the use of the Polio blade (Fig. 15.48) in obstetric practice.

FIGURE 15.47 Short and standard laryngoscope handles.

FIGURE 15.48 The Polio blade.

The McCoy laryngoscope (Fig. 15.49A) is based on the standard Macintosh blade but has a hinged tip. This is operated by a lever mechanism attached to the handle. When the lever is pressed (Fig. 15.49B), the tip of the blade bends forward and this improves the view of the larynx.

FIGURE 15.49 (A) The McCoy laryngoscope. (B) The McCoy laryngoscope, demonstrating the hinged blade tip.

Video Laryngoscopes

Traditional direct laryngoscopy depends on achieving a straight line of vision to the patient’s larynx. This requires alignment of the oral, pharyngeal and laryngeal axes. Videolaryngoscopes are a new class of laryngoscopes that have high resolution video cameras incorporated in the tip of a modified laryngoscope blade. The video image is relayed by either fibreoptic bundles, lenses or via cables to a dedicated LCD video display. Videolaryngoscopy allows the operator to see around the corners as the transmitted image is the view obtained from the tip of the laryngoscope blade. Aligning the three axes (oral, pharyngeal and laryngeal) and compression or distraction of tissues to achieve a line of sight are not required. With direct laryngoscopy, poor visualization of the glottic aperture is correlated with difficult intubation. The major difference with videolaryngoscopy when compared with conventional direct laryngoscopy is that it may be difficult to intubate the trachea despite a clear view of the glottis. Examples of videolaryngoscopes are those with a guiding channel such as the Pentax Airway scope and Airtraq (Fig. 15.50) and those without a guiding channel such as the Glidescope, McGrath and C-Mac (Fig. 15.51). The Venner AP Advance videolaryngoscope is a recent addition to the videolaryngoscope market (Fig. 15.52) in which the standard blade does not have a channel but the difficult airway blade does. The Airtraq is fully disposable whereas the other videolaryngoscopes have disposable blades but a reusable main unit. The C-Mac requires sterilization between uses.

FIGURE 15.50 The Airtraq (left) and Pentax Airway scope (right).

FIGURE 15. 51 The C-Mac laryngoscope and monitor.

FIGURE 15.52 Venner AP Advance laryngoscope.

Fibreoptic Laryngo/Bronchoscope

The fibreoptic scope (Fig. 15.53) is an endoscope which is used to view the upper and lower airway through either the nose or the mouth. The principle of the fibreoptic system is that light from a powerful external light source is transmitted through a flexible instrument and an image of the area is returned to an eyepiece or camera. The fibres which transmit the light have a diameter of approximately 20 μm and are made up of a central glass core coated with a thin layer of glass material with a lower refractive index. The light passing down the fibre is repeatedly reflected down the inner glass.

FIGURE 15.53 The fibreoptic intubating laryngoscope which consists of a control unit, insertion cord and a connector for a light guide.

The fibreoptic scope consists of a light source, a universal cord and light guide connector, a control unit, an eyepiece and an insertion tube. The light source (Fig. 15.54) is usually powered by mains electrical supply and contains either a xenon or a halogen lamp. The universal cord and light guide connector contain the light guide fibre bundle and transmits light from a light source to the fibreoptic bundle in the insertion tube. In some newer models, light is powered by a battery and this enables these scopes to be easily portable and be used in places outside the operating theatres, such as the emergency department.

FIGURE 15.54 The light source and a camera system for the fibreoptic intubating laryngoscope.

The control unit consists of an angulation control lever, suction port and biopsy port. The angulation lever controls the deflection of the tip of the scope. The suction port is for connecting to a suction pump but it can also be used for insufflation of oxygen. The biopsy channel can be used for taking biopsy specimens but it can also be used to attach a syringe to instil local anaesthetic into the airway.

The eyepiece consists of the viewing lens and the dioptre adjustment ring. A camera can be attached to the eyepiece either to take photographs or to transmit the pictures to a television monitor (Figs 15.54 and 15.55A). A disposable fibreoptic scope (Ambuscope) is now available (Fig. 15.55B).

FIGURE 15.55 (A) The television monitor, camera system and light source used with the intubating fibreoptic scope. (B) The Ambuscope – a disposable fibreoptic scope.

The insertion tube contains two optical fibre bundles: the light guide and the image guide. The fibres transmitting light (light guide) are arranged in a random fashion but those returning the image (image guide) are precisely located relative to each other. The insertion tube also contains the suction channel and the biopsy channel. At the distal end of each optical fibre, there is a lens. The size of the insertion tube varies from 1.8 to 6.4 mm to fit inside tracheal tubes of internal diameter 3.0–7.0 mm.

Most anaesthetists who use the fibreoptic scope now attach a camera to the eyepiece and view the airway on a television monitor. The use of a camera and TV monitor has been demonstrated to enhance the speed with which anaesthetists can be taught fibreoptic endoscopy skills.

TRACHEAL TUBES

Most tracheal tubes are constructed of red rubber, silicone rubber or plastic. Red rubber tubes are reusable, although they may start to show signs of deterioration after 2–3 years. Today, disposable plastic tubes are the preferred choice (Fig. 15.56) as they eliminate the need to collect, clean, sterilize and check tubes after use. Plastic tubes are presented in a sterile pack and should be cut to an appropriate length before use. The plastic disposable tubes have a cuff and a pilot balloon with a self-sealing valve. The cuff may be inflated with air, nitrous oxide or saline. The internal diameter of the tube is marked on the side of the tube in millimetres and the length of the tube is marked along the length of the tube in centimetres. The tube also has a radio-opaque line running along its length. This enables the position of the tube to be determined on a chest X-ray.

FIGURE 15.56 8- and 9-mm internal diameter plastic disposable tracheal tubes.

Some tracheal tubes contain latex and must not be used in patients who have a history of latex allergy. Silicone rubber is increasingly being used in the manufacture of tracheal tubes. These are more expensive than the plastic tubes but they may be sterilized and re-used. They are softer than red rubber or plastic endotracheal tubes and they are non-irritant.

Tube Size

In adults, there is little to be gained in the way of reduced resistance to breathing by selecting a tube larger than 8.0 mm internal diameter. However, it is common to use a tube of 8.0–9.0 mm internal diameter for male and 7.0–8.0 mm for female adults. Tubes of wide diameter may exert pressure on the laryngeal cords after insertion. Appropriate sizes of tracheal tubes for children are shown in Appendix B (IX).

Plain Tubes

Uncuffed tubes are used in children (Fig. 15.57). A cuff is unnecessary to secure an airtight fit if the correct diameter of tube is selected, because the narrowest part of the airway is in the trachea at the level of the cricoid cartilage. However, the larynx is the narrowest part of the airway in the adult and a leak occurs if an uncuffed tube is used; in addition, there is a risk of aspiration of fluid from the pharynx into the trachea. Nasotracheal intubation is less traumatic if an uncuffed tube is used. The incidence of sore throat is not influenced by the presence of a cuff on the tracheal tube.

FIGURE 15.57 Paediatric uncuffed plastic tracheal tubes.

Cuffed Tubes

It is usual to use a cuffed tube whenever tracheal intubation is required in the adult. It is mandatory if IPPV is to be used or if there is a risk of blood, pus or gastric fluid entering the pharynx. Tracheal tubes with a streamlined cuff are available and are suitable for nasotracheal intubation.

Cuff Volume

Tracheal tube cuffs may be either low-volume/high-pressure or high-volume/low-pressure. A tube with a low-volume cuff may require inflation to a high pressure to effect a seal within the trachea. The pressure within a low-volume cuff does not necessarily relate to the pressure exerted by the cuff on the tracheal mucosa. However, a high pressure may be exerted on the mucosa if the cuff is overinflated. This may occur inadvertently during anaesthesia because nitrous oxide diffuses through some types of plastic. Some anaesthetists inflate the cuff with an oxygen/nitrous oxide mixture to obviate this problem. Alternatively, the cuff volume may be readjusted after 10–15 min of anaesthesia.

High-volume/low-pressure (‘floppy’) cuffs cover a larger area of tracheal wall and may effect a seal with less pressure exerted on the mucosa. However, they may cause more trauma during insertion and may become puckered in a relatively small trachea.

Herniation of an overinflated cuff may occlude the distal end of the tracheal tube and cause partial or total airway obstruction.

Shape of Tube

In most centres, a curved tracheal tube is used. These should be cut to the correct length as there is a risk of accidental intubation of a bronchus (usually the right main bronchus) if the tip is inserted too far. Some plastic tracheal tubes are preformed in shapes which either fit the pharyngeal contour or carry the proximal end of the tube away from the mouth (Fig. 15.58); the latter design is useful when surgery on the face or head is planned. The RAE (Ring, Adair and Elwyn) tubes have preformed curves and they can be used either orally or nasally (Fig. 15.59). RAE tubes may be cuffed or uncuffed. Care should be taken not to insert these preformed tubes too far as there is a risk of bronchial intubation.

FIGURE 15.58 Preformed north-facing nasotracheal tube.

FIGURE 15.59 Preformed RAE disposable plastic tracheal tubes: the uncuffed paediatric tube and the cuffed adult tube.

Specialized Tubes

An armoured latex tube is useful if there is a danger of the tube kinking during surgery; a nylon spiral is incorporated in the wall of the tube and prevents obliteration of the lumen. Alternatively, a reinforced flexometallic tube, with a metal spiral in the wall, may be used. These tubes are very floppy and a wire stilette is required for their insertion.

Reinforced flexometallic tubes are available in either uncuffed paediatric sizes or cuffed adult sizes (Fig. 15.60). These tracheal tubes are long and cannot be shortened because they have a fixed tracheal tube connector. Therefore, when inserting them into the trachea, care should be taken to avoid bronchial intubation. The adult cuffed tubes have two black rings and the paediatric uncuffed tubes have a black line at the distal end. These give guidance as to where the vocal cords should be, in order to avoid bronchial intubation.

FIGURE 15.60 The reinforced tracheal tube – the paediatric uncuffed and adult cuffed tubes.

A flexible metal tube (Fig. 15.61) may be used during procedures that require the use of lasers in the airway; plastic tubes may ignite if struck by the laser beam. Some designs have two cuffs. This ensures a tracheal seal if the upper cuff is damaged by laser. An air-filled cuff may ignite if it is hit by a laser beam. Therefore it is recommended that the cuffs are filled with saline instead of air.

FIGURE 15.61 Flexible metal tube suitable for use during laser surgery to the airway.

The Parker tube (Fig. 15.62) has a curved bevel on the inner curvature of the tube. The bevel therefore lies on the anterior aspect of the tube during insertion. The manufacturer claims that it is less likely than the conventional tube to cause trauma to the larynx or trachea, particularly during insertion over a bougie or fibreoptic laryngoscope.

FIGURE 15.62 The distal ends of a Parker (upper) and a conventional bevelled tracheal tube.

Double-lumen endobronchial tubes are used during thoracic surgery when there is a need for one lung to be deflated. They allow selective deflation of one lung whilst maintaining ventilation of the other lung. The older Robertshaw double-lumen tubes are made of red rubber and are therefore reusable. The disposable Bronchocath double-lumen tubes (Fig. 15.63), which are made of plastic, are used more commonly now.

FIGURE 15.63 Bronchocath double-lumen endobronchial tube with catheter mount.

Laryngectomy tubes (Fig. 15.64) are designed to be inserted through a tracheostomy into the trachea to maintain the airway during surgery for laryngectomy. Because of its shape, the connection to the breathing system is some distance away from the surgical field and this gives the surgeon a relatively clear field in which to operate.

FIGURE 15.64 Laryngectomy tube.

A microlaryngeal tube is a small tube (usually 5 mm internal diameter) with an adult-sized cuff which is used during surgery on the larynx. It allows better surgical access to the larynx.

Tracheostomy Tubes

There are many types of tracheostomy tube. They may be cuffed or uncuffed. The proximal end of a tracheostomy tube has a standard 15-mm connector and there are two wings with slots to which the securing tape is attached (Fig. 15.65). Tracheostomy tubes have a replaceable inner cannula which facilitates insertion and is removed once the tracheostomy tube is in place.

FIGURE 15.65 Tracheostomy tubes. Adult cuffed (7.0 mm internal diameter) and paediatric Shiley uncuffed tubes with the replaceable inner cannulae.

The fenestrated tracheostomy tube has a hole (fenestration) along its greater curvature. This allows the patient to speak by directing some of the air past the vocal cords.

Silver tracheostomy tubes are used in many patients who require long-term intubation. They have an inner tube which may be removed for cleaning. Some designs have a one-way flap valve to allow the patient to speak. Silver is non-irritant and bactericidal.

Cricothyroidotomy Devices

Cricothyroidotomy is the creation of an opening in the cricothyroid membrane in order to gain access to the airway either as an elective prophylactic cricothyroidotomy in an anticipated difficult airway or in an emergency ‘can’t intubate and can’t ventilate’ (CICV) scenario (see Ch 22). Cricothyroidotomy can be carried out using either a small cannula (Fig. 15.40), large bore cannula (4 mm or larger internal diameter) or surgically using a tracheostomy tube (6 mm or greater). There are several types of large bore cannulae available. The cuffed or uncuffed Melker kit (Fig. 15.66) uses a Seldinger technique. The cuffed or uncuffed VBM Quicktrach (Fig. 15.67) is a rigid pre-assembled cannula-over-needle device that is inserted as a single step procedure. The cuffed Portex cricothyroidotomy kit is also a rigid cannula-over-needle device, which is packaged as a pre-assembled kit with a scalpel and syringe (Fig. 15.68). It has a spring-loaded Veress needle with a blunt stilette. The Portex Minitrach II is a wire-guided kit which provides an uncuffed 4-mm airway through the cricothyroid membrane. It is designed for tracheal suction and not for an emergency situation because it is not easy to insert quickly.

FIGURE 15.66 Melker cricothyroidotomy set.

FIGURE 15.67 Quicktrach cricothyroidotomy set.

FIGURE 15.68 Portex cricothyroidotomy kit.

Connections

Catheter Mount

This is a flexible link between the breathing system and a tracheal tube, laryngeal mask airway, face mask or tracheostomy tube (Fig. 15.69). It may be made of rubber or plastic and some have a gas sampling port. The proximal end, which attaches to the breathing system, has a standard 22-mm connection and the distal end is a 15-mm connector. The length of catheter mounts varies from 45 to 170 mm.

FIGURE 15.69 A disposable catheter mount to connect the tracheal tube to the breathing system (top). A disposable angle piece to connect a face mask or tracheal tube to the breathing system (bottom).

Tracheal Tube Connectors

Disposable 15-mm diameter connectors are provided with plastic disposable tubes; the diameter of the distal end is of an appropriate size to fit the internal diameter of the tube. Several older connections (Fig. 15.70) have been used with plastic or rubber tracheal tubes.

FIGURE 15.70 A variety of new and old tracheal tube connectors. From top left in clockwise rotation: Portex with 15-mm tracheal tube connector, Nosworthy, Knight’s paediatric connector, Cobbs, Rowbotham, Magill oral, Magill nasal.

Angle Pieces

These are connectors which fit between the breathing system and the tracheal tube or mask (Fig. 15.69). They incorporate a 90° bend and have either 15- or 22-mm connectors at each end. Some angle pieces have a condenser humidifier, bacterial filter and a port for gas sampling incorporated into them (Fig. 15.30).

PROTECTING THE BREATHING SYSTEM IN ANAESTHESIA

Following recent fatal incidents in which anaesthetic tubing became blocked, the UK Department of Health has made recommendations to minimize the risk of recurrence. The recommendations include training and increasing awareness of the potential problem of anaesthetic tubing becoming blocked accidentally, and protecting vulnerable components of the breathing system by keeping the components individually wrapped until use. The Department of Health recommends the use of the Association of Anaesthetists of Great Britain and Ireland (AAGBI) document ‘Checking anaesthetic equipment’ and that all trainees should be trained in the correct procedure for checking anaesthetic equipment.

SUPRAGLOTTIC AIRWAY DEVICES (SADs)

Supraglottic airway devices (SADs) can be divided into two types. The first generation includes the classic laryngeal mask airway (LMA), flexible LMA and all the single-use LMAs. The second generation SADs include the I-gel, Pro-seal LMA and the Supreme LMA which are designed with a drainage tube to reduce aspiration, have an integral bite block and have increased airway seal to enable positive pressure ventilation to take place.

The Classic LMA (cLMA)

This device consists of a shortened conventional silicone tube with an elliptical cuff, inflated through a pilot tube, attached to the distal end (Fig. 15.71). The cuff, which resembles a miniature face mask, has been designed to form a relatively airtight seal around the posterior perimeter of the larynx (Fig. 15.72). A variety of sizes of cuff is available, ranging from size 1 which is used in the neonate to size 5 which is used in large adults. The mask is inserted and the cuff inflated until no air leak is detected. It is important to ensure that the maximum inflation volume is not exceeded (Table 15.8). The device is very effective in maintaining a patent airway in the spontaneously breathing patient. Positive pressure ventilation may be applied if necessary. The mask is not suitable for patients who are at risk from regurgitation of gastric contents (emergency surgery, hiatus hernia or history of reflux, and obesity) and should be used with caution if pharyngeal soiling is anticipated.

TABLE 15.8

Characteristics of Laryngeal Mask Airways (LMAs)

FIGURE 15.71 Laryngeal mask airways (LMAs). A paediatric size 2 mask, an adult size 4 mask and a size 3 reinforced LMA.

FIGURE 15.72 A laryngeal mask airway in situ.

The flexible LMA differs from the standard LMA in that it has a flexible, wire reinforced tube. The size of the cuff is similar to that of the standard LMA but the tube is longer and narrower and therefore offers more resistance to breathing (Fig. 15.71). Because of the wire in the tube, it is unsuitable for use in the MRI unit. The classic LMA is reusable up to 40 times. However, a large selection of disposable LMAs manufactured by different companies is available and these are now used more commonly.

Second Generation SADs

The Proseal LMA is a reusable LMA which has a drain tube, a posterior inflatable cuff, reinforced airway tube, integral bite block and an introducer (Fig.15. 73). The distal end must sit in the oesophageal inlet to ensure best performance. The drainage tube vents any gas leaking into the oesophagus as well as any fluid if regurgitation occurs and it can be used to insert an orogastric tube. The increased oesophageal and pharyngeal seal allows positive pressure ventilation to take place. The Pro-seal LMA is reusable up to 40 times and all its components are latex-free. It can be mounted on an introducer (Fig. 15.73B) which facilitates the insertion of the LMA Pro-seal into the patient’s mouth. The I-gel (Fig. 15.74) is a new single use SAD with a non-inflatable ‘cuff’ made of a soft thermoplastic elastomer. The ‘cuff’ is a preformed soft mould, which fits into the perilaryngeal structures. It has a narrow bore oesophageal drain tube, a short wide bore airway tube and an integral bite block. Both the Proseal and the I-gel come in adult and paediatric sizes. The Supreme LMA (SLMA) (Fig. 15.75) is the newest of the second generation SADs and it is sometimes described as a ‘single use Pro-seal LMA’. However, in reality, it has features of the Pro-seal LMA and the intubating LMA (ILMA). Its features include a large inflatable plastic cuff but no posterior cuff found in the Pro-seal LMA, an oesophageal drain tube, a preformed semi-rigid tube, an integral bite block and fins in the mask bowl to prevent epiglottic obstruction. It is made of PVC. Unlike a Pro-seal LMA, the SLMA does not require an introducer for insertion.

FIGURE 15.73 (A) The LMA Pro-seal. (B) The LMA Pro-seal mounted on the LMA Pro-seal introducer.

FIGURE 15.74 The I-gel supraglottic device – sizes 1–5.

FIGURE 15.75 Supreme LMA.

The Intubating LMA

The intubating LMA (ILMA) is an advanced form of the standard LMA (Fig. 15.76). It has a shorter tube and a metal handle. The handle permits single-handed insertion without moving the head and neck and without placing fingers in the mouth. It may be passed through an interdental gap as narrow as 20 mm. The mask floor has an elevating bar which replaces the two bars in the standard LMA. The caudal end of the bar is not fixed to the mask floor and this allows a tracheal tube to be passed in order to intubate the trachea. The ILMA is available in sizes 3, 4 and 5. The recommended cuff volumes are similar to the corresponding sizes of the standard LMA. The rigid curved airway has a standard 15-mm connector at the proximal end. The tube is wide enough to allow passage of a cuffed 8-mm tracheal tube. The ILMA is a reusable device which may be cleaned and sterilized up to 40 times but a single use version is now available.

FIGURE 15.76 An intubating laryngeal mask airway (ILMA) with the silicone tracheal tube and the introducer.

The C-Trach LMA (Fig. 15.77) is a variant of the ILMA with fibreoptic components integrated into the device to allow direct vision at intubation. It has a digital screen with a light source and digital camera. The lens lies behind the epiglottis elevator bar (EEB) and captures the image from the front of the mask aperture. The image is transmitted to the viewing screen.

FIGURE 15.77 LMA C-Trach with monitor attached.

OTHER APPARATUS

Face Masks

These are designed to fit the face perfectly so that no leak of gas occurs, but without applying excessive pressure to the skin. An appropriate size of face mask must be selected to ensure a proper fit, but the smallest size possible should be used to minimize dead space. Disposable masks made of transparent material are available (Fig. 15.78). These allow the detection of vomitus or secretions.

FIGURE 15.78 Sizes 1 and 5 reusable (left) and disposable (right) face masks.

A harness system (e.g. Clausen harness) is used by some anaesthetists to hold the mask on the face during surgery. However, airway obstruction may occur at any time and the excursion of the reservoir bag must be observed constantly. In many countries, the LMA is now used during maintenance of anaesthesia in almost all situations in which a face mask was formerly used.

Intubating Forceps

The most commonly used intubating forceps are those designed by Magill (Fig. 15.79A). The instrument is employed to manipulate a nasotracheal or nasogastric tube through the oropharynx and into the correct position. A laryngoscope is used to obtain a view of the oropharynx.

FIGURE 15.79 Magill intubating forceps (above). The Ferguson mouth gag (below).

Laryngeal Spray

These are used to deposit a fine mist of local anaesthetic solution (usually lidocaine 4% or 10%) on the mucosa of the larynx and upper trachea (Fig. 15.80A and B). These sprays are particularly useful in applying local anaesthetic to the upper airway during awake fibreoptic intubation. The mucosal atomization device (MAD) is a single use atomizer which comes in two shapes, the oral and the nasal atomizer (Fig. 15.80B).

FIGURE 15.80 Laryngeal sprays. (A) Forrester spray and the 10% lidocaine prefilled laryngeal spray. (B) The mucosal atomizing device (MAD).

Mouth Gag

A mouth gag (Fig. 15.79B) may be used during dental anaesthesia and is required occasionally to open the mouth in patients with trismus, or if masseter spasm is present. It is positioned between the molar teeth and must be used with great care to avoid dental trauma.

Bougie

If the larynx cannot be seen adequately during laryngoscopy, or if the tracheal tube cannot be manoeuvred into the laryngeal inlet, a bougie may be used as an aid to tracheal intubation. The lubricated bougie is inserted into the trachea to act as a guide for the tracheal tube. The tube should be rotated so that the bevel does not become lodged against the aryepiglottic fold. In a difficult intubation scenario, the correct type of bougie should be used (Fig. 15.81). The bougie with a curved tip at the end is designed to assist in this situation whereas the straight-ended bougie is intended for endotracheal tube exchange only. Disposable bougies are now available but their efficacy over the reusable ones is yet to be demonstrated. The Eschmann (gum elastic) multiple-use bougie has the highest success rate, least likelihood of causing trauma and has reliable and clinically tested signs of confirmation of tracheal placement when compared with either the Frova single-use intubation introducer or the Portex single-use introducer.

FIGURE 15.81 The straight-ended multiple-use bougie (above) and the angled-end multiple-use bougie (below).

Stilettes

A malleable metal stilette may be used to adjust the degree of curvature of a tracheal tube as an aid to its insertion. The stilette must not protrude from the distal end of the tube.

The Aintree Intubating Catheter (AIC)

This is a 56-cm hollow catheter with special adapters that connect to either a 15-mm connector for use with conventional anaesthetic circuits or a Luer lock for use with a jet ventilator (Fig. 15.82). The catheter has an internal diameter of 4.8 mm which allows it to be preloaded over a fibreoptic scope. The AIC is used for LMA-assisted orotracheal fibreoptic intubation. However, it should not be used without prior training as failure to follow the correct instructions can result in serious morbidity or mortality.

FIGURE 15.82 Aintree Intubating Catheter (AIC).

Airways

An oropharyngeal airway (Guedel airway, Fig. 15.83) may be required to prevent obstruction caused by the tongue or collapse of the pharynx in the patient without a tracheal tube. A nasopharyngeal airway (Fig. 15.83) is tolerated better during light anaesthesia and may also be used if it is difficult to insert an oropharyngeal airway, e.g. trismus. However, the use of nasopharyngeal airways may be associated with significant bleeding from the nose.

FIGURE 15.83 A nasopharyngeal airway (left) and Guedel airways.

Suction Apparatus

Suction apparatus is vital during anaesthesia and resuscitation to clear the airway of any mucus, blood or debris. It is also used during surgery to clear the operating field of either blood or fluid.

Suction apparatus consists of a source of vacuum, a suction unit and suction tubing. The source of vacuum can be either piped vacuum or electrically or manually operated units. Piped vacuum is the most commonly used source in many operating theatres.

The suction unit consists of a reservoir jar, bacterial filter, vacuum control regulator and a vacuum gauge (Fig. 15.84). The reservoir jar is graduated so that the volume of aspirate may be estimated. It contains a cut-off valve. The cut-off valve has a float that rises as the fluid level increases and shuts off the valve when the reservoir jar is full. This prevents liquid from the suction jar entering the suction system. There is a bacterial filter between the cut-off valve and the suction control unit to prevent air that has been contaminated during passage through the apparatus infecting the atmosphere when it is blown out. The filter also traps any particulate or nebulized matter. Filters should be changed at regular intervals.