The supply of anaesthetic and other medical gasses

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Chapter 1 The supply of anaesthetic and other medical gasses

Under both the Medicines Act 1968 (UK) and more recent European legislation [EC 2001/83], medical gasses are classified as medicinal products. As such, they can only be produced by companies holding a manufacturer’s licence and sold by agents holding a marketing authorization (previously known as a Product Licence). In the UK, these licences are issued by the Medicines and Healthcare products Regulatory Agency (MHRA). Similar regulatory bodies exist in most countries.

Medical gasses for use in anaesthesia and critical care are generally supplied to hospitals either in bulk (e.g. synthetic air, medical air and oxygen) or in individual cylinders collectively attached to manifolds (e.g. oxygen, nitrous oxide, medical air, Entonox and occasionally Heliox). In both cases the gasses are then delivered through a pipeline system to wall or pendant outlets. Where pipeline supplies are unavailable, for instance during patient transfers, the gasses are supplied directly from individual cylinders.

Oxygen can also be supplied by means of an oxygen concentrator, although this would be considered only where a bulk supply was either unavailable or impractical (e.g. due to geographical constraints).

Whilst the distribution of a pharmaceutical product in a highly pressurized metal cylinder, which is returned to the manufacturer for refilling, remains a unique concept in drug delivery, there have until recently been relatively few advances in the manufacturing, packaging, delivery and application of the products. The next decade will undoubtedly see the increasing clinical use of gasses such as Heliox, xenon and carbogen, but these novel agents are likely to generate new challenges – for example, the adoption of xenon, a potential neuroprotectant, in anaesthesia will bring with it problems associated with ventilating with a gas approximately five times denser than air.

Properties of medical gasses

The properties of some of the common medical gasses are summarized in Table 1.1. The newer gasses and their potential therapeutic indications are briefly discussed below.

Table 1.1 Physical properties of common pressurized gasses

N/A, not applicable. * Sublimation.
Note: all values STP. Mixed gasses, e.g. Heliox, will have different physical properties. For exact values, please contact the manufacturer.

Details of carbogen (5% CO₂ + 95% O₂), lung function gasses, medical gasses for laser surgery and other agents not listed here are available from the manufacturers. Extensive information on the properties of all gasses and data sheets containing up-to-date product information can be obtained from the BOC Medical Gasses website www.bocmedical.com.

Heliox

Heliox is a generic term for any blend of oxygen and helium. Currently only a specific premixed blend – 21% oxygen and 79% helium known in the UK as Heliox21 – has approval for distribution. Heliox21 is licensed for the treatment of respiratory obstruction and, although traditionally confined to emergency use, is increasingly being used by other hospital departments and the emergency services.

Heliox21 has different physical properties compared to air (which is sometimes called nitrox for comparison) and these cause it to behave differently when used with anaesthetic and respiratory equipment. Most importantly:

• It has a lower density than air (0.42 kg/m³ vs 1.22 kg/m³ at 15°C and 1 atm) and hence a lower Reynolds number in any given situation, resulting in a higher likelihood of laminar gas flow characteristics.

• It has a higher thermal conductivity than air, which is of significance when its gas flow measurement is based on this principle (see Chapter 2, Hot wire anemometry).

If administered via standard air or oxygen flowmeters, flow readings will be inaccurate unless a conversion factor is applied. Appropriate conversion tables are available from the manufacturer. Flow/volume measurements on ventilators are similarly affected, hence only readings from machines designed for ventilation with helium mixtures can be assumed to be accurate.

Xenon

Xenon is not a standard medical product and is provided as a ‘special’ gas. It demonstrates anaesthetic and neuroprotective activity. However, since it is five times denser than air, mechanical ventilation is only possible with specially designed equipment. Furthermore, it is supplied at a lower pressure than oxygen and Heliox21, and, consequently, should be used with compatible low-pressure regulators. Its density will slow down its flow through narrow tubes, including standard flowmeters. A conversion factor of 0.468 should be applied. Until a licence is granted for its use in anaesthesia or other applications, usage in medicine is restricted to research applications.

Nitric oxide

Although toxic at high concentrations, when given in very low dilutions (8–50 ppm) nitric oxide acts as a selective pulmonary vasodilator and has been used extensively in the paediatric intensive care setting. Because of its acidic properties in the presence of moisture, administration is restricted to specialized, compatible equipment. Research is currently focusing on its use as an immuno-modulator, in platelet function alteration and for domiciliary use.

Carbon monoxide

Carbon monoxide is being investigated in connection with a number of therapeutic areas (suppression of inflammation, vasodilatation, cytoprotection and organ transplantation). Chromium-plated equipment should never be used with CO because of the risk of generating highly toxic chromium containing compounds such as chrome carbonyl.

Medical gas cylinders

Modern cylinders are manufactured using lightweight strong chrome-molybdenum steels which, as well as conforming to stringent material standards, can be filled to pressures of up to 300 bar g (where (g) denotes gauge pressure; see Chapter 2). Cylinders were previously made of the much heavier low-carbon steels: few of these are in circulation any longer in the UK.

All-steel cylinders are perfectly adequate for applications where weight is not an issue; such as on cylinder manifolds. In situations where portability is important, lighter weight composite (hoop wrap) cylinders are used. These are constructed from two or more different materials; commonly either lightweight steel or, more commonly, an aluminium liner which is then strengthened by wrapping a filament material, such as Kevlar or carbon-fibre, coated in epoxy resin circumferentially along the parallel length of the cylinder. The cylinders combine enormous strength, with low weight and can be filled to pressures up to 300 barg.

In some locations, for example in rooms containing MRI scanners, it is not possible to use cylinders containing steel as these are ferromagnetic and can be uncontrollably accelerated into the MRI with potentially fatal consequences. Aluminium cylinders with special non-ferromagnetic pin index valves are available for this application, but it is recommended that, where possible, gasses should be piped into the unit from outside.

Cylinder sizes

Technically, cylinders are defined by their water capacity and range between 1.2 L and 47.2 L, and are identified by a size code ranging from C to J (Fig. 1.1). This notation incorporates a pressure aspect to give an indication of available gas volume. Tables 1.2–1.7 give details for oxygen, nitrous oxide, Entonox, carbon dioxide, Heliox21, xenon, nitric oxide and carbon monoxide cylinders.

Figure 1.1 Cylinder types and sizes, valves types and pin index valves.

(from BOC Medical, with permission).

Table 1.2 Relative sizes and specifications of commonly used oxygen cylinders

Table 1.3 Relative sizes and specifications of commonly used nitrous oxide cylinders

Table 1.4 Relative sizes and specifications of commonly used Entonox cylinders

Entonox, a mixture of 50% oxygen and 50% nitrous oxide, exists as a gas. The pseudo critical temperature of Entonox in pipelines at 4.1 bar is below −30°C. Nitrous oxide in an Entonox cylinder, however, begins to separate out from Entonox if the temperature falls below −6°C. A homogenous mixture is again obtained when the temperature is raised above 10°C and the cylinder is agitated.

Table 1.5 Relative sizes and specifications of commonly used carbon dioxide cylinders

Table 1.6 Relative sizes and specifications of commonly used Heliox21 cylinders

CYLINDER SIZE	HL	HX
Contents (L)	8200	1780
Nominal cylinder pressure at 15°C (bar)	200	200
Valve type	Side outlet	Integral
Dimensions (mm)	1540 × 230	940 × 140
Empty weight (kg)	50	15.5

Table 1.7 Relative sizes and specifications of commonly used xenon, nitric oxide and carbon monoxide cylinders

Cylinder filling and maintenance

Most gasses such as oxygen, medical air, helium and Heliox21 are stored in cylinders in a compressed gaseous state and are normally filled by pressure. Nitrous oxide and carbon dioxide, however, are liquefied gasses under pressure. The liquid is in equilibrium with the gas, the pressure being dependent on the temperature. These gasses are charged by weight and not by pressure. The maximum weight of gas filled into the cylinder divided by the weight of water that would completely fill the cylinder is called the maximum filling ratio and is governed by legislation. This ‘fill ratio’, termed ‘fill density’ elsewhere, is critical. In the UK this value is 0.75 at 15.5°C for nitrous oxide and carbon dioxide. Note that due to the difference in the densities of liquefied gasses and water this ratio is not the same as the proportion of the volume of the cylinder filled by the liquid phase, which is nearer 90 to 95% for nitrous oxide. If there is insufficient gas space left in the cylinder after filling, a comparatively small increase in temperature will cause a significant increase in pressure and could, in extreme circumstances, cause the cylinder to rupture.

Cylinders of nitrous oxide and carbon dioxide should always be used in the vertical position with the outlet uppermost; this prevents liquefied gas from being vented and causing cold thermal burns or equipment damage. Also when a high flow rate is drawn, the temperature of the liquefied gas will fall – this can result in a significant pressure drop which may cause poor performance when used with equipment such as a cryogenic probe. A good indicator of this phenomenon is the appearance of water vapour condensing/freezing on the outer surface of the cylinder. The effect is most common on smaller liquefied gas cylinders up to size F (9.43 litres).

Some applications require carbon dioxide in liquid form. This is supplied using cylinders fitted with a ‘dip tube’ connected to the cylinder valve, which allows liquid from the bottom of the cylinder to be drawn up through the valve (Fig. 1.2). These cylinders, supplied in F size and known as LF, have a white stripe down the length of the cylinder body. For other CO₂ applications where vapour is required, VF cylinders should be used. It is important to ensure that the correct cylinder is used to prevent damage to equipment.

Figure 1.2 LF type CO₂ cylinder with dip tube for delivering liquid CO₂.

Checking for cylinder contents is done in one of two ways: compressed gasses such as oxygen, medical air, helium, Heliox and Entonox (which, although 50% nitrous oxide, remains as a gas in the cylinder under normal ambient temperatures) are assessed using a pressure gauge, as the pressure in the cylinder is directly proportional to the volume. By contrast, the contents of cylinders containing liquefied gasses (pure nitrous oxide and carbon dioxide) can only be determined by weight, as the pressure only begins to fall once all the liquid is exhausted. Subtracting the tare weight (stamped on the neck of the cylinder) from the total weight will give an estimate of contents.

Cylinder identification

The correct method of identifying the contents of a cylinder is to read the collar identification label (rather than assuming that the colour of the cylinder or the type of valve fitted reliably indicates gas inside it). The label (Fig. 1.3) is a legal requirement and contains all the key information for the user:

Figure 1.3 Cylinder label.

• product name, chemical symbol and pharmaceutical form of the product

• product specification

• hazard warning diamond(s)

• product licence number

• cylinder contents in litres

• maximum cylinder pressure

• cylinder size code

• directions for use and information for storage and handling.

The cylinder label also has a unique batch label (Fig. 1.4), which contains: the batch number; fill and expiry date; and the size and type of gas. The label is changed every time the cylinder is filled and serves two important functions: (a) it provides vital information for a batch recall should the cylinder be involved in an incident; and (b) it provides information for proper cylinder rotation.

Figure 1.4 Batch label.

Cylinder testing

All cylinders must undergo hydraulic testing and internal inspection at regular intervals, to ensure they remain safe to use. The test is carried out every 10 years for steel cylinders and every 5 years for composite cylinders. A colour-coded plastic ring between the valve and the cylinder neck indicates when the next test date is due. In general, cylinders have a long service life and tend to be withdrawn for reasons of technical obsolescence rather than deterioration per se.

Colour coding

Medical cylinders in the UK conform to colour codes specified in ISO 32:1972 and EN 1089-3. The colour coding relates only to the shoulder of the cylinder. Figure 1.5 shows the colour codes for medical gas cylinders in the UK.

Figure 1.5 Cylinder colour codes.

Cylinder valves

Medical cylinder valves can generally be categorized as follows:

• Non-integral valves (which require manual attachment of an external pressure regulator):

Small pin index and side spindle pin index outlet valves

Bull nose outlet valves

Handwheel actuated valves

• Integral valves (which have a built in regulator and possibly flowmeter).

Pin index system

Small pin index valves (Fig. 1.6) are fitted to small cylinders (less than 5 litre capacity) which are commonly connected directly to medical equipment such as anaesthetic machines. Newer designs using a thumbwheel do away with the need for a spanner to operate the valve.

Side spindle pin index valves (Fig. 1.6) These are fitted to large cylinders of medical oxygen, medical air, Entonox used in pipeline manifolds and F size Entonox cylinders.

Figure 1.6 A, Small pin index valve on E size cylinder. B, Side spindle pin index valve.

Both types of pin index valves conform to BS EN ISO 407:2004 and adopt an indexed outlet system which incorporates a gas-specific combination of holes positioned to correspond to pins located on the receiving equipment, making it impossible to connect the cylinder to an incorrect gas connection. Fig. 1.1 shows the different pin positions. The pin index system also prevents charging with the wrong gas, as the gas suppliers use the same non-interconnectable system for their filling connections.

Pin index cylinders require a washer (seal) between the face of the cylinder valve outlet and the equipment to which it is fitted. This bonded non-combustible seal, known as a ‘Bodok’ washer (Figs 1.7 and 1.8), must be kept clean and should never become contaminated with oil or grease. If a gas tight seal cannot be achieved by moderate tightening of the screw clamp, it is recommended that the seal be renewed. Excessive force should never be used.

Figure 1.7 Bodok washer.

Figure 1.8 Bodok washer fitted to a regulator.

Bull nose outlet valve

This type of valve (Fig. 1.9) is fitted to F and G size cylinders including medical oxygen, medical air, helium, and mixed gasses such as carbogen and Heliox. The valve is spindle key operated and has a 5/8-inch female outlet thread into which a regulator is fitted. The spindle mechanism is assembled in two parts. This permits a gas tight seal to be achieved without the use of excessive force and increases its operational life.

Figure 1.9 Bull nose cylinder valve.

All bull nose valves are fitted with an RP (residual pressure) device, to ensure that a positive pressure of approximately 3 bar is retained in the cylinder (Fig. 1.10). This prevents the ingress of moisture should the valve be left open when the cylinder is empty. When connecting regulator equipment to the valve, the user should always adopt the proper connecting procedure:

Figure 1.10 Schematic showing ‘residual pressure’ device of a bull nose cylinder valve.

Redrawn using material kindly provided by Müller Gas Equipment A/S, Denmark.

• Ensure the ‘O’ ring (Fig. 1.11) fitted to the regulator is in good condition and hand tighten the regulator only.

• Once fitted, open the spindle valve slowly (to prevent a gas surge) at least one full turn and note the position of the regulator gauge needle.

• A simple test for leaks can be carried out by closing the cylinder spindle valve and noting any drop in pressure shown on the gauge. If a leak does exist, spraying the joints with a leak detection spray will identify it. If a leak is evident at the valve outlet, replace the ‘O’ ring and repeat the procedure. If the leak persists, fit a replacement regulator. (Note: It is vitally important that any leak detection spray has been approved by the cylinder supplier as being compatible with their equipment and the gas.)

• When changing an empty cylinder, close the cylinder valve and vent the regulator completely before attempting to disconnect it.

Figure 1.11 ‘O’ ring fitted to regulator.

Handwheel valves

Large nitrous oxide cylinders for use on cylinder manifolds and carbon dioxide cylinders of size F and G are fitted with handwheel valves which are surrounded by a protective guard (Fig. 1.12), and have a gas specific, male thread, side outlet.

Figure 1.12 CO₂ cylinder with handwheel valve.

Integral valves

The introduction of integral valves (Fig. 1.13), which have their own built-in regulator, has revolutionized the industry by greatly improving safety and eliminating the need for regulator maintenance by hospitals. They have a number of advantages:

Figure 1.13 An integral valve without its guard.

• The regulator assembly is manufactured in a clean environment and is much less prone to particulate contamination.

• The built-in regulator eliminates the risk of incorrect regulator attachment which could result in damage to lower-pressure rated equipment.

• The valves are surrounded by guards which reduce the risk of damage and ease manual handling of the cylinder.

• They are considerably easier to use and have live pressure gauges built in for checking cylinder contents.

• The valves possess two outlets – a variable flow outlet (via a fir tree connector) and a fixed pressure outlet to connect equipment requiring a static driving pressure.

A commonly encountered version of the integral valve has a combination of a clickstop flowmeter giving 0–15 l min⁻¹ in stepped increments and a female BS Schraeder connection at 4 bar g to provide high pressure gas. This type of valve is currently fitted to cylinders of up to 10 l water capacity. Care must be taken to ensure that the flowmeter selector dial is not ‘parked’ between click stops as this will result in a cessation of gas flow.

Cylinder manifolds

Although few hospitals rely on cylinder manifolds for their main oxygen or medical air supply, they are still used in reserve systems and as the main source of nitrous oxide and Entonox supply. Whilst there are minor differences for each gas, in general the systems are designed and operate along the same principles.

The typical configuration consists of two equal banks of gas cylinders (one demarcated duty and one stand-by). These are arranged around a central control panel and provide a nominal output pressure of 4 bar (7 bar for surgical air). The change over from the ‘duty’ to ‘stand-by’ banks is normally automatic. The installation should also contain a manually operated reserve of at least two cylinders (Fig. 1.14) also stored in the manifold room. Any additional cylinders should be held in the general medical gas store.

Figure 1.14 A. A nitrous oxide cylinder manifold. B. Schematic.

The total storage capacity of the manifold should be equivalent to 1 week’s supply; a minimum of 2 days’ supply on each bank and 3 days’ supply held in the reserve cylinders.

Table 1.8 gives nominal and usable capacities of cylinders commonly used on manifolds.

Table 1.8 Cylinder capacities

* The usable figures are based on a residual pressure of 7 bar in the cylinders (^† 15 bar residual pressure).

Cylinders are attached to the manifold via a copper tailpipe with a gas specific connection and seal. Each connection has a non-return valve fitted to enable single cylinders to be changed in the event of a leak or tailpipe rupture. The cylinders are secured by individual chains to a back bar. All cylinders on both the duty and stand-by banks should be fully open. The central control panel determines which of the banks is the active duty bank. When this bank falls to a pressure of 8 bar, it switches to the stand-by bank and indicates on the alarm panel that the duty bank is empty and the stand-by is running. The responsible person should then change the empty bank of cylinders.

If the empty bank is not changed or a manifold fault occurs, once the stand-by bank (now duty bank) pressure falls to 8 bar, a pipeline pressure fault will register. The main manifold should then be isolated via a shut-off valve and the emergency bank manually opened, until normal conditions can be restored. The following alarm states occur:

• a green ‘normal’ condition

• a yellow ‘duty bank empty, stand-by running’ condition

• a yellow ‘duty bank empty, stand-by low’ condition

• a yellow ‘emergency/reserve banks low’ condition

• a red ‘pipeline pressure fault’ condition.

Whilst it is common throughout mainland Europe for nitrous oxide to be supplied in bulk, in the UK the gas is still supplied via manifolds which have heaters fitted to the supply line to prevent freezing during periods of high demand.

Safety precautions

The manifold room should:

• be constructed from a fireproof material – either brick or concrete

• have ventilation at the top and bottom to permit the circulation of air

• ideally be located to enable a delivery vehicle access to prevent manhandling cylinders long distances

• be well lit

• be temperature controlled between 10 and 40°C. This is especially important in the case of an Entonox manifold as nitrous oxide and oxygen can separate out at temperatures below −6°C. It is advisable to let the cylinders stand for 24 hours in a temperature of more than 10°C before fitting them to the manifold

• only contain cylinders for use on the pipeline(s) located in the manifold room

• not be used as a general store

• have sufficient warning signs on the outside and inside of the building.

Only suitably trained persons should be permitted to change cylinders and an activity log should be completed when cylinders are changed.

Bulk oxygen supply systems

The first bulk medical oxygen systems were installed in the UK in the mid-1960s and have steadily increased in popularity since. Today almost all large hospitals have their piped medical oxygen supplied from an on-site, bulk oxygen supply facility. This reflects a steady rise in hospital oxygen consumption driven not only by the extension of piped oxygen from operating theatres and ICUs to most departments and wards, but also by significant changes in postoperative and ventilatory management. Currently an average 800-bed teaching hospital consumes around 500 million L of oxygen per year. It is difficult to relate to such large numbers, which is why supply companies refer to volume in hundreds of cubic metres (HCMs). Using the above example, the annual usage would be 5000 HCM or approximately 73500 J sized cylinders. Bulk oxygen systems are preferred owing to their ability to reliably deliver these volumes:

• at a lower overall gas cost

• without the labour-intensive distribution associated with compressed gas cylinders

• with greater on site storage capacity (at 15°C, one volume of liquid oxygen gives 842 times its volume compared with a manifold cylinder which delivers only 137 times its volume)

• with greater security of supply.

Whilst the volumes appear to be enormous, the actual cost per litre – around 0.0008 pence per litre in the UK – is extremely low. Unfortunately, increasing demand has necessitated the use of progressively larger bulk storage vessels, which can be difficult to site, especially in inner-city hospitals where space is at a premium and safety concerns may be difficult to mitigate.

Since the 1960s, the recommended ‘on-site’ storage capacity has increased from 6 to 14 days and is now based on a risk assessment which is referred to below.

Cryogenic liquid system (CLS)

The CLS system consists of:

• an insulated cryogenic storage vessel to store the bulk liquid oxygen

• an ambient heated vaporizer to convert the cryogenic liquid oxygen into a gas for supply to patients via a pipeline distribution system

• a control panel to control the pressure and flow of gas to the pipeline

• a telemetry system (see below).

The CLS can comprise:

• a single vessel containing operational stock with a cylinder manifold containing the secondary supply. Fig. 1.15 shows a schematic of a single-vessel installation

• a main vessel containing operational stock with a second vessel either alongside or remotely located containing reserve stock, and a cylinder manifold containing emergency stock (Fig. 1.16).

Figure 1.15 Simplified schematic of a single vessel cryogenic liquid system.

Figure 1.16 A twin vessel cryogenic liquid system installation.

A CLS should comply with recommendations in Chapter 6 of HTM 02-01 (part A) (Medical Gas Pipeline Systems) and take account of the criteria laid down in the European Standard BS EN ISO 7396, the British Compressed Gas Association (BCGA) Code of Practice CP 36 and the relevant UK legislation. HTM 02-01 also contains more detailed schematics of all types of CLS installations.

The basic function of a liquid oxygen vessel is to store cryogenic oxygen at −183°C, in what is effectively a vacuum flask; the inner vessel being made from stainless steel and the outer from carbon steel. Between the two vessels is a vacuum and the space is filled with a high-performance insulating material. The vessel and its associated controls are commonly known as a VIE (vacuum insulated evaporator).

Liquid oxygen sits in the bottom of the vessel whilst the gaseous oxygen floats above it at a pressure of 12–20 bar. Because it is impossible to maintain perfect insulation, the inner container is continually trying to draw heat from the atmosphere, though this is partially offset by the evaporation of liquid during use. If there is no demand, the pressure inside the vessel will rise, causing the safety relief valve to vent gas to atmosphere; to avoid this, the flow valves are designed to open under high pressure and permit gas to pass into the pipeline distribution line. Conversely, if demand is high, the pressure in the vessel will tend to fall. When this happens, liquid is withdrawn from the inferiorly located liquid valve and passed through a pressure-raising coil which raises the pressure to 10.5 bar.

During normal operation the liquid converts to a gas as it passes through a process vaporizer. This can be either a simple ambient vaporizer or duplex timed automatic switching vaporizers designed to allow one to operate whilst the second one defrosts. The C11 control panel (see Fig. 1.15) has duplicate regulators for security. These are designed to control the pressure at 4.1 bar for the main supply and 3.7 bar for the emergency cylinder supply. The control panel is designed to enable flows of up to 5000 L per minute from the main VIE supply and 1500 L per minute through the emergency cylinder manifold.

The control panel relays alarm conditions to a central alarm panel, usually located in the hospital telephone switchboard or other 24-h manned location, with duplicate alarm panels located in high acuity areas throughout the hospital, e.g. theatres, ITU and SCBU. The conditions can vary, depending upon the type of installation. A simple VIE with a cylinder manifold emergency would give the alarm conditions in Table 1.9.

Table 1.9 Alarm conditions for a simple VIE with a cylinder manifold backup (see text and Fig. 1.15)

STATUS/FAULT CONDITION	INDICATION	LEGEND
Normal operation	Green	Normal
Primary supply system operational stock empty Primary supply system reserve stock in use	Yellow	Liquid low Re-fill liquid
Primary supply system reserve stock empty Secondary supply system in use	Yellow	Re-fill liquid immediately
Secondary supply system low Lead secondary supply system content below 50%	Yellow	Change cylinders
Pipeline pressure fault (high, low)	Red	High pressure Low pressure

The VIE has a contents gauge which operates on differential pressure. The mass of the dependent liquid oxygen causes the pressure at the bottom of the vessel to be greater than that at the top and the gauge measures this difference and converts it into an analogue readout.

It is advisable to install a telemetry system to the CLS to provide continuous condition monitoring for both the supplier and the hospital CLS management.

Siting requirements

The installation should be located inside a fenced compound, be accessible to road tankers and be sited in accordance with the British Compressed Gasses Association (BCGA) code of practice and safety distance data provided by the installer. In general, all hazardous buildings, flammable materials, public access, vehicles and surface water drains, must be at least 5 m (and in the case of larger installations, 8 m) from the nearest point of the CLS compound. The compound floor and the hard standing area directly in front of the fill connection must be concrete and should be designed to contain any liquid spillage. Tar or asphalt should never be used near the CLS as they form an explosive mixture when in contact with liquid oxygen.

Liquid cylinder (LC) installations

Where the annual consumption of a hospital is considered too great for a compressed cylinder manifold but is insufficient for a CLS, then a liquid cylinder (LC) installation can be considered. This type of installation is not dissimilar to a cylinder manifold, having the same configuration of two cylinder banks and a control panel (Fig. 1.17), but has significant advantages in that each LC contains the equivalent gas capacity of 24 J size compressed gas cylinders. On a typical four-cylinder LC manifold, this is operationally equivalent to 72 size J cylinders (only 75% of the gas capacity is usable as the low liquid level alarm activates when the volume of contents falls to 25%). Another major advantage over compressed cylinders is that the LCs are a semi-permanent installation, and as such are charged from a remote fill point, usually on the outside wall of the compound; this removes the need for manual handling and connecting.

Figure 1.17 A liquid cylinder installation.

Any pressure build-up in the reserve manifold automatically feeds into the pipeline through the control panel.

The only disadvantage when compared to a CLS is a limitation in overall flow rate. A CLS is capable of 3000 L min⁻¹ under normal conditions whereas a LC installation peaks at 500 L min⁻¹. Whilst this is normally sufficient for a mid-range pipeline system, it should be considered when assessing supply options. In general, a LC installation will:

• usually have a 2 × 5 cylinder compressed emergency supply

• have an alarm panel linked to the control panel to provide similar warnings to those of a single CLS installation

• be sited internally or externally with protection from the elements.

Oxygen concentrators (PSA plant)

An oxygen concentrator or pressure swing adsorber (PSA) is an alternative source of oxygen utilized when a liquid oxygen supply is either unavailable or impractical, e.g. an off-shore site or where the safety criteria for liquid installations cannot be met.

Operational process

An oxygen concentrator operates on the principle of adsorbing under pressure other gasses in the atmosphere onto the surface of an adsorbent material, termed a zeolite. Oxygen is not adsorbed by the zeolite and passes freely through it into a receiver vessel, ready for later use.

The zeolite is sealed in vessels known as sieve beds which operate in pairs – one adsorbing whilst the other regenerates. The adsorbed gasses, mainly nitrogen, are removed by vacuum pump and are discharged into the atmosphere. The process is capable of producing oxygen concentrations of about 95%, the remainder being made up mainly of argon with a small percentage of nitrogen. This may be of clinical significance as it has been reported that a build-up of argon could occur during closed-circuit anaesthesia.

The major components of a hospital PSA plant are:

Fig. 1.18 shows a schematic layout for a hospital system.

Figure 1.18 A schematic layout for a hospital system.

In addition to the economic costs, a number of other issues must be considered: the process generates a great deal of heat, hence ventilation and cooling for the product and the compressors are major considerations.

Should the plant fail, the emergency cylinder manifold will feed into the pipeline at higher concentrations (99.5%) than the plant’s operating norm of 95%. This may have an effect on downstream equipment, particularly in critical care areas.

A more appropriate application for oxygen concentrators is in the home environment where a low-flow, low-pressure system can provide continuous domiciliary oxygen to COPD patients. Typical units (Fig. 1.19) operate on a mains supply and can provide up to 51 min⁻¹ at an oxygen concentration of 94%. They are extremely efficient, require little maintenance by the patient apart from periodic cleaning of the inlet filter and can pipe oxygen around the home to small wall-mounted outlets. The concentrator is usually sited in a hallway and needs no special ventilation. The typical noise level when operating normally is around 40 db.

Figure 1.19 A home oxygen concentrator: the Millennium Respironics Oxygen Concentrator.

Image courtesy of Respironics Inc. and its affiliates, Murrysville, PA, USA.

Medical compressed air

Medical compressed air (MA) is classified by the European Pharmacopoeia as a drug, and therefore warrants the same degree of care and cleanliness that any other drug requires during its manufacture, storage or distribution. MA can be provided into pipeline systems by three methods:

1. Compressors

2. Cylinders connected to a manifold

3. By mixing liquid oxygen and liquid nitrogen, also known as synthetic medical air (SMA).

This section will deal with the provision of MA by compressors.

The basic components of MA generation are the same as those found in any industrial compressed air system: a compressor, a receiver for the storage of gas and some form of regulator to monitor and control the pressure in the pipeline (Fig. 1.20). Where MA production differs is in the degree of conditioning applied to the raw compressed air before it is administered to the patient. Compressors for MA plant can be any of three types: reciprocating, Archimedean screw or rotary vane. All have their individual benefits and the ultimate choice of process is based on demand, location and initial cost. Whichever pumps are used they should be identical to each other and capable of meeting total hospital demand with one pump off line.

Figure 1.20 Main components of a typical medical compressed air plant.

MA may be contaminated by a number of substances including:

• oil – a lubricating agent used in the compressor that may become an aerosol and be carried into the air stream

• particulate debris – either present in the raw uncompressed air or picked up during compression/storage

• water – a compression process by-product which may collect in the receiver or pipeline.

Regulations mandate the removal of these contaminants before MA is administered to patients; this is achieved by a series of filters and driers installed after compression but before distribution into the pipeline system. See Table 1.10 for the specification required for MA by the European Pharmacopoeia.

Table 1.10 Maximum allowable contaminants for medical air

The reliable provision of medical air is of critical importance and there are robust mechanisms designed to maintain both the security of supply and the gas pressure. MA is normally distributed at 400 kPa, and is maintained at this pressure by a pressure-reducing valve fitted downstream of the drier/filtration assembly. Further protection is provided by a pressure relief valve that prevents over-pressurization of the system should a fault occur with the reducing valve. Of particular importance is the control system, which integrates all the disparate elements of the supply system, controls the safety back-up devices and ensures that alarms are raised if any component fails to perform in the prescribed manner.

As with all other medical gas supply units, all vital components in the design of the system are provided in at least duplex so that one component (filter/drier/compressor, etc.) can act as a standby if any other component fails or needs to be taken out of service for maintenance.

Medical air is used in patient ventilators, to power humidifiers and drug nebulizers, and in devices such as automatic tourniquets.

In some areas of the hospital, air is also distributed at a higher pressure for use as a power source for medical tools, such as orthopaedic drills and saws. Known as surgical air (SA), the gas has similar properties to MA, but is delivered at a higher pressure (700–1100 kPa compared with 400 kPa for MA).

Where overall demand for both MA and SA is small, it is acceptable to utilize one compressed air plant to provide both supplies, and to regulate the gas pressures accordingly. Under such circumstances, it is imperative that all regulation and control systems are located in the plant room with both services piped away in parallel from this point. However, some older operating theatres may have a single higher-pressure supply (7 bar) that may be regulated down to 4 bar by plugging in a reducing valve at the wall socket. The Schrader valves used for these sockets must be gas (MA or SA) specific to prevent the wrong supply being used (see below).

Although MA/SA can be provided from manifolds or as synthetic medical air, these approaches are impractical where there are high consumption rates (particularly with SA supplied from cylinder manifolds).

The terminal units will be discussed in detail under pipeline distribution, but it is important to note that those used for both MA and SA are, as with other gasses, of different dimensions, ensuring that the risk of cross connection and delivery of the wrong gas is minimized.

It is sometimes necessary to supply dental or other departments with a sterile supply of air – depending on the distances involved, it may often be more cost-effective to use small local compressors than run the pipework over long distances.

Synthetic air systems

The 1998 European Pharmaceutical monograph for medical air put greater emphasis on the control of hydrocarbons and moisture in the product and, consequently, an increased need for monitoring the performance of air systems is required. As synthetic medical air (SMA) is produced by mixing liquid oxygen and liquid nitrogen in the gas state, it does not contain either hydrocarbons or moisture and may be considered as an alternative to conventional compressed air supplies, either at the design stage of a new hospital or where purity is an issue. The installation utilizes the existing liquid oxygen supply with an adjoining liquid nitrogen vessel. Both vessels have smaller back-up vessels on the site (Fig. 1.21).

Figure 1.21 A synthetic air cryogenic liquid system installation.

The control system is very similar to the conventional CLS mentioned earlier, with the addition of duplex mixing panels and gas analyzers enabling the continuous monitoring of gas purity. The synthetic air is then stored at either at 4 bar g, or at higher pressures for surgical air and regulated down to 4 bar g for medical use.

The benefits of SMA are that it:

• is maintenance free

• is of high purity

• does not require a power supply

• provides on-site nitrogen for powering surgical power tools.

Medical vacuum systems

Although by definition a vacuum cannot be termed a gas, medical vacuums (MV) are always co-installed with true gas supplies and with the same type of valving and equipment. This, together with the fact that it is covered by the same standards as medical gasses (HTM02, C11 1999 and BS EN ISO 7396), means that piped vacuum services are invariably considered alongside the provision of medical gasses.

The purpose of MV is to enable the removal of fluids during medical or surgical procedures. The principle of fluid removal is the same in all cases: a drainage tube passes from the patient to an interceptor collection jar where any solid and liquid waste is trapped. The vacuum is then passed through a flow regulator and bacterial filter to the terminal unit in the same way as the medical gasses (Fig. 1.22; see also Chapter 20).

Figure 1.22 Typical components of a ward suction set.

The pipeline system carrying the MV back to the plant is usually of the same construction and standard as that of the medical gasses, but of larger size to prevent more significant pressure changes along the pipe. In some instances, pipes over 54 mm can be made of a plastic material to reduce installation costs.

At the vacuum plant (Fig. 1.23), there are additional bacterial traps and collection jars to collect any materials that may have by-passed the patient level filters. Although these components are designed to protect the plant and maintenance staff from contamination, it is advisable to ensure that strict infection control protocols are adhered to during any maintenance work especially if this involves changing the bacterial filters.

Figure 1.23 Main components of a typical medical vacuum plant.

From the filters, the vacuum is drawn into the receiver. The receiver is designed so that the pumps, operating between 550 and 650 mmHg sub-atmospheric, cycle no more than six times per hour. This is good engineering practice and prevents excessive wear and tear on the system.

A MV plant can be thought of as a compressor in reverse; air is taken from the MV pipeline and ‘discharged’ into the atmosphere. As with medical air, it is essential to have a degree of redundancy in the system. There should be a minimum of three identical pumps with each pump capable of delivering 100% of the design flow rate. Vacuum pump exhausts may be combined, but where this is the case, a non-return valve must be fitted to the exhaust so that it does not ‘drive’ the standby pump. The exhaust pipeline(s), however, must be vented to atmosphere at high level, normally at roof level and away from all other air intakes or openings into the building (doors, windows, etc.).

The normal pumps used for MV in the UK are of a rotary vane type, although reciprocating pumps are used in some parts of the world. Both of these types of pump have a capacity to generate a sub-atmospheric pressure of up to 650 mmHg at sea level and are perfectly adequate for the purposes of medical vacuum. At higher altitudes though, it is more difficult to achieve the negative pressures required and the settings on the plant control systems need to be adjusted to compensate for this lower operating range.

Again, as with the compressed air plant, the most important element is the plant management and control system. This operates the cut in and cut out of the pumps, cycles the pumps on duty (so that each pump experiences the same amount of use) and passes any faults back to the alarm and indication system.

In certain parts of the world (e.g. the US) medical vacuum systems are also used to deliver the negative pressure requirements for anaesthetic gas scavenging system (AGSS). In the UK we tend to utilize a totally separate vacuum source for this purpose; a less powerful vacuum but with higher flow rates (120 L min⁻¹ per terminal unit, as opposed to 40 L min⁻¹ for MV). As this vacuum source is of a lower technical specification, greater savings can be made both in capital terms and in running costs.

Performance levels and specifications for a medical vacuum service

For the UK, the specifications for a piped vacuum service are laid down in the HTM 02-01:2006. In brief the guidance states that:

• the design and operating pressure should not be less than 450 mmHg at the plant

• a pressure drop of 50 mmHg within the distribution pipework is permissible

• a minimum pressure of 400 mmHg is required at the back of the terminal unit

• a pressure drop of 100 mmHg is allowed across the terminal unit to the probe, which has to maintain a minimum pressure of at least 300 mmHg whilst delivering a flow rate of 40 1 min⁻¹.

Anaesthetic gas scavenging systems

These are considered specifically in Chapter 18. They do, however, form part of what is termed Medical Gas Piped Services, and common aspects of the piping and distribution are considered further in this chapter.

Alarm and indication systems for piped gasses

There are two different types of alarm system used within a hospital medical gas system: main plant alarms and local (or ward) alarms. The former is used to provide an indication of the condition of the plant at the source of generation or storage, the latter to provide an indication of the condition of the gas at the point of use.

The main plant alarm (Table 1.11) consists of a series of panels placed in strategic locations throughout the hospital. These will usually give the indication that everything is normal; their main function though is to give advance warning of a potential system failure. For example, if the duty bank on a manifold runs out, the standby bank will automatically come on-stream. As soon as this happens, the first condition alarm will be triggered indicating that cylinders need changing on that manifold. The service is not in danger, as the manifold is designed to act in this way. If no one attends to the manifold and the standby bank also runs out, the second condition alarm will be activated: at this point the system is about to run out of gas. If the pressure does fall below the minimum required then the final condition – pressure fault – will commence. At this stage, patients will need to be provided with alternative supplies.

Table 1.11 Main plant alarm

The third condition on the system is used to monitor the failsafe emergency supply source; although this should not be used as a main supply, it may provide the hospital with enough time to rectify matters.

As well as the indications on the main alarm panels, additional indications will appear on each plant control panel or manifold. These provide a more detailed visual indication of the nature of the fault or emergency.

A local or area alarm panel fulfils a very different function. Here the alarm condition is used to indicate that something has already gone wrong. Each gas supplied to a ward or department is monitored for faults by a pressure switch mounted in the pipeline, downstream of the final area valved service unit (AVSU). Typically this is set at ±20% of the line pressure specified for a particular gas such that, if a high- or low-pressure condition occurs within the area, the alarm will indicate the fact (Table 1.12).

Table 1.12 Typical local alarm panel legends

On both types of alarm panel the indication is both audible and visual. A two-tone sounder and a flashing legend indicates what the fault is and on what service. The audible alarm can be muted but will reinstate itself after 15 minutes if the fault has not been corrected. On clearance of the fault, the alarm panel will automatically reset itself to ‘Normal’.

Distribution systems

Medical gasses (other than surgical air) are distributed throughout the hospital at a nominal 400 kPa through pipelines designed to minimize the pressure drop from source to point of use. This is achieved by means of calculations based on the initial pressure, the specified flow rate and the dimensions of the pipework. In simple terms, the higher the required flow rate, the larger the diameter of the pipe needed to carry it. So, for example, a high flow pipeline in a plant room can be 54 or even 76 mm diameter, whereas by the time the pipes enter the ward they normally do not exceed 22 mm and those supplying the patient’s bedside, are usually 12 mm.

The ‘gasses’ normally distributed by pipeline in hospitals within the UK are:

• Oxygen O₂ (400 kPa)

• Nitrous oxide N₂O (400 kPa)

• Entonox 50% O₂/50% N₂O (400 kPa)

• Medical air MA (400 kPa)

• Surgical air SA (700 kPa)

• Vacuum vac

• Anaesthetic gas scavenging AGSS.

They all carry an individual colour code, as shown in Figure 1.24.

Figure 1.24 Colour codes for medical gas pipework.

Other gasses, such as helium and hydrogen, are usually only supplied as piped services to pathology laboratories and so are not considered further.

Detailed information concerning the regulations and standards required for fixed distribution pipework can be obtained from the appropriate Government or Health Ministries. In the UK, the ordinances covering the service, maintenance, repair or alterations of fixed distribution pipework systems are detailed in HTM 02-01.

In this chapter, only a brief description will be given of the fixed pipework, as it resides ‘behind the wall’ and is more appropriately the concern of the hospital engineer. The anaesthetist or designated medical officer should, however, be aware of the nature of the installation and should always be informed and consulted before any alterations to it are made.

The pipes used are half hard and manufactured from phosphorous de-oxidized non-arsenical copper to BS EN 1412:1996 grade CW024A to prevent degradation of gasses. ‘Half hard’ refers to the heat treatment of copper pipes, which allows them to have a higher pressure rating. Pipes are degreased, purged, filled with nitrogen and capped to maintain cleanliness prior to delivery and installation. Pipefittings used for jointing these pipes are made from the same materials.

Valves need to be installed at various points along the network: on exiting the plant room, entering buildings, at the branch of each riser and on entry to each department or ward.

Valves within plant rooms should be left unlocked but all other valves should be locked and only unlocked under a permit to work order and the supervision of the ‘authorized person’.

Valves at ward entrances or departmental isolation valves are normally termed AVSUs (Area Valved Service Units) or ZSU (Zone Service Units) and are specifically designed to provide not only gas isolation but also other functions as described below. Essentially, an AVSU comprises a lockable box that has a glass-fronted panel, which can be broken by the ward staff to allow isolation of gas flow to areas in case of fire, pipeline fracture or other emergency (Fig. 1.25).

Figure 1.25 A typical area valved service unit (see text), note blanked off non-interchangeable screw thread connectors on either side of valve.

AVSUs also permit additional connections into the gas stream by the use of a non-interchangeable screw thread (NIST) system. The NIST union contains a self-sealing valve so that when the blanking nut is removed and the appropriate NIST connection is made, the self-sealing valve is automatically reopened. These branches may be used to purge pipelines or to introduce a local supply during alterations or breakdowns. The AVSU junction also allows the insertion of a ‘spade’, which is used by service engineers to ensure absolute closure of the pipeline, irrespective of the action of the valves.

AVSUs are installed in such positions as to protect each department or ward. They should be installed into the natural route that would be passed on an emergency exit from the department unit.

The pipework itself should be identified by labels placed upon it at regular intervals (Fig. 1.24) in accordance with the identification code described in Section 13 of HTM 02-01 and further marked as to the direction of flow.

Pipework is normally always concealed in modern-day installations, though in the past it was mounted on the surface. The older arrangement was not only unattractive, but also less satisfactory from the standpoint of general hygiene and cleanliness.

Terminal outlets

The distribution pipework terminates in wall- or pendant-mounted self-sealing socket outlets (Fig. 1.26).

Figure 1.26 Terminal outlets. Note the different diameter recesses (collar indexing system) that match the collar on the relevant probe (see Fig. 1.27).

The specification for the design of terminal units and their probes was originally set in 1978 (BS 5682) and upgraded in 1998 (BS EN 739:1998 ‘Low pressure hose assemblies for use with medical gasses’) – this is now in the process of being superseded by a new standard: BS EN ISO 9170–1:2008. Current legislation specifies that the terminal unit should consist of two sections:

1. A termination assembly permanently attached to the appropriate pipeline

2. A socket assembly containing the quick connect probe socket, commonly known as a Schrader socket in the UK. This can be removed by a service engineer, but must be designed so that it cannot be accidentally connected to a different gas service.

A termination assembly for a pressurized gas (but not a vacuum) must also have a check valve so that work can be carried out on any terminal unit without shutting down all the terminal units for that gas in that area. The valve should operate automatically as soon as the socket assembly is removed.

It is essential that the assembly design ensures gas specificity for each component, such that no terminal unit can be assembled and include parts from different gas termination outlets. The identity of the gas for each terminal unit should be permanently displayed on all individual components. The socket assembly, when assembled will only accept a probe with the same gas identity, by utilizing a collar indexing system that is unique to that gas service.

Flexible pipeline

This connects the terminal outlet to the medical equipment; it has three components:

1. A quick connect (Schrader) probe that fits the terminal outlet

2. A flexible hosepipe

3. A NIST connection that fits the equipment, e.g. the anaesthetic machine.

Quick connect probe

The male part of the probe for the terminal outlet is the same size for all gasses, but in order to prevent connection to the wrong gas service it has a protruding (gas specific) indexing collar (Fig. 1.27). This collar has a unique diameter that only fits the matching recess of the socket assembly for that gas.

Figure 1.27 Flexible hose probes. Note the difference in size of the indexing collar.

The British Standard also stipulates the following:

• It must not be possible to twist the probe while it is connected to the unit (unless it is connected vertically to a pendant). To this end, the collar is provided with a notch that fits over a rigid pin in the socket assembly.

• It must be possible to insert or remove the probe simply and quickly using only one hand. In order to achieve this, the socket assembly has a spring-loaded outer ring, which when depressed releases the locking mechanism holding the probe, and causes the probe to be ejected.

• The unit must seal off the flow of gas when the probe is withdrawn.

The flexible hosepipe

This is the section of the system in which damage and wear are most likely to occur. Originally, the hoses for each gas were constructed from the same black reinforced rubber or neoprene tubing, and only identifiable by a short length of coloured sheath at each end. This resulted in several fatal incidents when the probe for one gas was fitted to the upstream end of the hose, but a socket or union for a different gas was attached to the downstream end. The incidence of such accidents has been minimized by the current practice of most manufacturers, which is:

• to produce complete hose assemblies (with the appropriate connections pre-attached) for the different gas services, under strict quality-control systems and with necessary checks to ensure correct application. Furthermore, it is now recommended practice that a damaged hose should not be repaired on-site but returned in its entirety to the manufacturer in exchange for a factory-made service replacement

• to use characteristically coloured tubing for each gas (Fig. 1.28).

Figure 1.28 Colour-coded hoses.

The non-interchangeable screw thread (NIST) connector

To ensure that the gas supply is attached correctly to the relevant piece of medical equipment, each hose is fitted with an unique downstream connector. This takes the form of a probe and nut (Fig. 1.29). The probe has a unique profile for each gas supply and fits only the union on the receiving equipment. This profile consists of two cylindrical shapes which together form a unique combination. For example, in Fig. 1.29 the outer part of the nitrous oxide probe is smaller than that for oxygen, but the inner part is larger. Thus, although the outer tip of the nitrous probe fits into the oxygen receiver, the larger inner part will prevent full engagement. Conversely, the outer part of the oxygen probe is too large to engage with the nitrous oxide receiver.