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: