bulk store

 distribution pipelines in the hospital

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

 flexible hoses connecting the terminal outlets to the anaesthetic machine

 connections between flexible hoses and anaesthetic machines.

Oxygen

In small hospitals, oxygen may be supplied to the PMGV from a bank of several oxygen cylinders attached to a manifold.

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

Nitrous Oxide

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

Medical Compressed Air

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

Piped Medical Vacuum

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

 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.

 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.

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.

Problems with Flowmeters

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.

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.

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).

 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.

 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.

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).

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.

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.

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