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.

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.

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 1.21.7 give details for oxygen, nitrous oxide, Entonox, carbon dioxide, Heliox21, xenon, nitric oxide and carbon monoxide cylinders.

Table 1.4 Relative sizes and specifications of commonly used Entonox cylinders

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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 1.6 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

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.

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 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.

Cylinder valves

Medical cylinder valves can generally be categorized as follows:

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.

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.

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.

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:

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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.