Anaesthetic Apparatus
It is essential that anaesthetists check that all equipment is functioning correctly before they proceed to anaesthetize a patient (see Ch 21). In some respects, the routine of testing anaesthetic equipment resembles the airline pilot’s checklist, which is an essential preliminary to aircraft flight.
It is convenient to describe anaesthetic apparatus sequentially from the supply of gases to point of delivery to the patient. This sequence is shown in Table 15.1.
TABLE 15.1
Classification of Anaesthetic Equipment Described in this Chapter
Supply of gases:
From outside the operating theatre
From cylinders within the operating theatre, together with the connections involved
The anaesthetic machine:
Unions
Cylinders
Reducing valves
Flowmeters
Vaporizers
Safety features of the anaesthetic machine
Anaesthetic breathing systems
Ventilators
Apparatus used in scavenging waste anaesthetic gases
Apparatus used in interfacing the patient to the anaesthetic breathing system:
Laryngoscopes
Tracheal tubes
Catheter mounts and connectors
Accessory apparatus for the airway:
Anaesthetic masks and airways
Forceps
Laryngeal sprays
Bougies
Mouth gags
Stilettes
Suction apparatus
GAS SUPPLIES
Bulk Supply of Anaesthetic Gases
The PMGV services comprise five sections:
distribution pipelines in the hospital
terminal outlets, situated usually on the walls or ceilings of the operating theatre suite and other sites
flexible hoses connecting the terminal outlets to the anaesthetic machine
connections between flexible hoses and anaesthetic machines.
Bulk Store
Oxygen Concentrators: Recently, oxygen concentrators have been used to supply hospitals and it is likely that the use of these devices will increase in future. The oxygen concentrator depends upon the ability of an artificial zeolite to entrap molecules of nitrogen. These devices cannot produce pure oxygen, but the concentration usually exceeds 90%; the remainder comprises nitrogen, argon and other inert gases. Small oxygen concentrators are provided for domiciliary use.
Terminal Outlets
Vacuum (coloured yellow) – a vacuum of at least 53 kPa (400 mmHg) should be maintained at the outlet, which should be able to take a free flow of air of at least 40 L min–1.
Compressed air (coloured white/black) at 4 bar – this is used for anaesthetic breathing systems and ventilators.
Air (coloured white/black) at 7 bar – this is to be used only for powering compressed air tools and is confined usually to the orthopaedic operating theatre.
Nitrous oxide (coloured blue) at 4 bar.
Oxygen (coloured white) at 4 bar.
Scavenging – there is a variety of scavenging outlets from the operating theatre. The passive systems are designed to accept a standard 30-mm connection.
Gas Supplies
Gas supplies to the anaesthetic machine should be checked at the beginning of each session to ensure that the gas which issues from the pipeline or cylinder is the same as that which passes through the appropriate flowmeter. This ensures that pipelines are not connected incorrectly. Both the machine in the operating theatre and that in the anaesthetic room should be checked. Checking of anaesthetic machine and medical gas supplies is discussed fully in Chapter 20 (‘The operating theatre environment’).
CYLINDERS
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.
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.
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
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.
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 15.2 (A) The Primus Dräger anaesthetic machine, which does not have conventional flowmeters. (B) A Blease Frontline anaesthetic machine.
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.
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).
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.
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 15.9 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).
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.
BREATHING SYSTEMS
Adjustable Pressure-Limiting Valve
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.
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
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 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.
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