Anaesthetists must have a sound understanding and firm knowledge of the functioning of all anaesthetic equipment in common use. Although primary malfunction of equipment has not featured highly in surveys of anaesthetic-related morbidity and mortality, failure to understand the use of equipment and failure to check equipment prior to use feature in these reports as a cause of morbidity and mortality. This is true especially of ventilators, where lack of knowledge regarding the function of equipment may result in a patient being subjected to the dangers of hypoxaemia and/or hypercapnia.
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.
The purpose of this chapter is to describe briefly apparatus which is used in delivery of gases, from the sources of supply to the patient’s lungs. Clearly, it is not possible to describe in detail equivalent models produced by all manufacturers. Consequently, this chapter concentrates on principles and some equipment which is used commonly.
Supply of gases:
From outside the operating theatre
From cylinders within the operating theatre, together with the connections involved
The anaesthetic machine:
Safety features of the anaesthetic machine
Anaesthetic breathing systems
Apparatus used in scavenging waste anaesthetic gases
Apparatus used in interfacing the patient to the anaesthetic breathing system:
Catheter mounts and connectors
Accessory apparatus for the airway:
Anaesthetic masks and airways
In the majority of modern hospitals, piped medical gases and vacuum (PMGV) systems have been installed. These obviate the necessity for holding large numbers of cylinders in the operating theatre suite. Normally, only a few cylinders are kept in reserve, attached usually to the anaesthetic machine.
The advantages of the PMGV system are reductions in cost, in the necessity to transport cylinders and in accidents caused by cylinders becoming exhausted. However, there have been several well-publicized incidents in which anaesthetic morbidity or mortality has resulted from incorrect connections in piped medical gas supplies.
Responsibility for the first three items lies with the engineering and pharmacy departments. Within the operating theatre, it is partly the anaesthetist’s responsibility to check the correct functioning of the last two items.
Oxygen cylinder manifolds consist of two groups of large cylinders (size J). The two groups alternate in supplying oxygen to the pipelines. In both groups, all cylinder valves are open so that they empty simultaneously. All cylinders have non-return valves. The supply automatically changes from one group to the other when the first group of cylinders is nearly empty. The changeover also activates an electrical signalling system, which alerts staff to change the empty cylinders.
However, in larger hospitals, pipeline oxygen originates from a liquid oxygen store. Liquid oxygen is stored at a temperature of approximately −165 °C at 10.5 bar in what is in effect a giant Thermos flask – a vacuum insulated evaporator (VIE). Some heat passes from the environment through the insulating layer between the two shells of the flask, increasing the tendency to evaporation and pressure increase within the chamber. Pressure is maintained constant by transfer of gaseous oxygen into the pipeline system (via a warming device). However, if the pressure increases above 17 bar (1700 kPa), a safety valve opens and oxygen runs to waste. When the supply of oxygen resulting from the slow evaporation from the surface in the VIE is inadequate, the pressure decreases and a valve opens to allow liquid oxygen to pass into an evaporator, from which gas passes into the pipeline system.
Liquid oxygen plants are housed some distance away from hospital buildings because of the risk of fire. Even when a hospital possesses a liquid oxygen plant, it is still necessary to hold reserve banks of oxygen cylinders in case of supply failure.
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.
Piped medical vacuum is provided by large vacuum pumps which discharge via a filter and silencer to a suitable point, usually at roof level, where gases are vented to atmosphere. Although concern has been expressed regarding the possibility of volatile anaesthetic agents dissolving in the lubricating oil of vacuum pumps and causing malfunction, this fear has not been realized.
Whenever a new pipeline system has been installed or servicing of an existing pipeline system has been undertaken, a designated member of the pharmacy staff should test the gas obtained from the sockets, using an oxygen analyser. Malfunction of an oxygen/air mixing device may result in entry of compressed air into the oxygen pipeline, rendering an anaesthetic mixture hypoxic. Because of this and other potential mishaps, oxygen analysers should be used routinely during anaesthesia.
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’).
Modern cylinders are constructed from molybdenum steel. They are checked at intervals by the manufacturer to ensure that they can withstand hydraulic pressures considerably in excess of those to which they are subjected in normal use. One cylinder in every 100 is cut into strips to test the metal for tensile strength, flattening impact and bend tests. Medical gas cylinders are tested hydraulically every 5 years and the tests recorded by a mark stamped on the neck of the cylinder and this includes test pressure, dates of test performed, chemical formula of the cylinder’s content and the tare weight. Cylinders may also be inspected endoscopically or ultrasonically for cracks or defects on their inner surfaces. Light weight cylinders can be made from aluminium alloy with a fibreglass covering in an epoxy resin matrix.
The cylinders are provided in a variety of sizes (A to J), and colour-coded according to the gas supplied. Cylinders attached to the anaesthetic machine are usually size E. The cylinders comprise a body and a shoulder containing threads into which are fitted a pin index valve block, a bull-nosed valve or a hand-wheel valve.
The pin index system was devised to prevent inter-changeability of cylinders of different gases. Pin index systems are provided for the smaller cylinders of oxygen and nitrous oxide (and also carbon dioxide) which may be attached to anaesthetic machines. The pegs on the inlet connection slot into corresponding holes on the cylinder valve.
Full cylinders are supplied usually with a plastic dust cover in order to prevent contamination by dirt. This cover should not be removed until immediately before the cylinder is fitted to the anaesthetic machine. When fitting the cylinder to a machine, the yoke is positioned and tightened with the handle of the yoke spindle. After fitting, the cylinder should be opened to make sure that it is full and that there are no leaks at the gland nut or the pin index valve junction, caused, for example, by absence of or damage to the washer. The washer used is normally a Bodok seal which has a metal periphery designed to keep the seal in good condition for a long period.
The sealing material between the valve and the neck of the cylinder may be constructed from a fusable material which melts in the event of fire and allows the contents of the cylinder to escape around the threads of the joint.
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.
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.
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 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.
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.
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 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.
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.
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.
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.
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.
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.
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).
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.
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).
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.
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.
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.
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.
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:
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 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.
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.
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.
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).
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.
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.
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.
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.
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.
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