Steam remains preferred method of sterilisation

Ease of production and efficiency in destroying micro-organisms are the two principle reasons why steam continues to be the most prominent sterilant in use in healthcare institutions. Despite availability of low temperature processes of increasing complexity for temperature-sensitive items, steam remains a powerful weapon in the sterilisation arsenal.
Since 1880, when the first steam steriliser was built by a pupil of Louis Pasteur, steam has remained the sterilant of choice unless factors such as temperature sensitivity deem it inappropriate.

Originally, the advantages of using steam were accepted as its rapid heating and rapid penetration of textiles or fabrics; its destruction of the most resistant bacterial spores in brief intervals of exposure; the easy control of quality and lethality for various materials and supplies; the absence of toxic residue on materials following the sterilisation process; and its economy.

Many of these statements are now open to question or, at least, qualification — rapid penetration, for example, can only occur in a vacuum. It is also easy to assume that steam is an easy medium to use and that it requires no special management.

In reality, high standards and good practice are neces-sary in generating and distributing steam of a quality to ensure that its sterilant efficacy is maintained.

The generation of steam must be carried out in a manner that is safe and efficient and produces a high-quality product. The European standards such as EN 554 specify that the environment in contact with the medical devices shall not impair their safety. The initial requirement is thus to define the end product.

Sterilisation by steam is performed by exposure to a ombination of temperature and moisture for a set time. As the temperature relates to the steam’s energy content, the higher the temperature the shorter the exposure time required. The time/temperature relationships for moist heat sterilisation are well known. The moisture content, however, is a matter which needs careful thought.

If the moisture content of the steam is too small, then the combination of parameters required (time, tempera-ture and moisture) is not achieved, and sterilisation may not occur. If the moisture content is too high then sterilisation will occur but there may be more moisture than the steriliser is designed to cope with and wet loads may result. The end product requires just the right amount of moisture.

Two other factors can hinder the efficacy of the sterilisation process. Steam sterilisation will only occur if the steam - and moisture - come into contact with every surface area of the load item. For this to occur all the air from within the steriliser chamber or load wrapping must be removed. In practice it is impossible to remove all the air but it is necessary to remove sufficient air so that the very small amount of air remaining will not impair the sterilisation process.

It is unfortunate that all steam supplies will contain a certain amount of air, or gases, that will have been generated from the boiler. These gases will have been dissolved in the water from a variety of sources and, if not controlled, can reach the appreciable amounts necessary to impair sterilisation.

There is, however, yet another source for gases. Feed water treatment may give rise to the generation of gases via the chemical reaction between treatment and hardness chemicals. These gases can be detrimental to the distribution system as they may recombine with moisture to form acidic condensate responsible for shortening the lifetime of the pipework.

Dissolved gas is not the only impurity in raw water. There will always be some particulate or physical contamination, the amount and definition of which will be determined by the source of the water. Some of this contamination, if allowed to remain on the surface of medical devices will be harmful to patients because of its toxic or pyrogenic nature. It is necessary to remove this contamination in order for sterilisation to take place so that the health of the patient upon whom the devices will be used is not put in danger. The maximum level of contamination given in EN 285 is 1.0 mg/kg.

To ensure that the end product is as pure as possible, acceptable levels for moisture and non-condensable gases have been specified in addition to the maximum level for particulate contamination. Regular monitoring of the levels of these elements of steam quality will be required to demonstrate continuing compliance with these requirements. The qualities of non-condensable gases and physical contamination will largely depend upon the quality or source of the raw water supply. The purer the supply, the purer the steam will be. Very pure water is aggressive to surfaces so that if it were used as raw water then it would strip off the surface coating of boiler or pipework to increase its particulate matter. As there are many different qualities of water clean, pure, distilled, purified, water for injection - knowledge of the precise quality of raw water is essential so that corrective action can be taken.

Chemical treatment for hardness must precisely match both the eventual quality required as well as the hard-ness of the raw water available. If the source of water changes then water treatment must change immediately, otherwise control of steam quality is lost.

As chemical treatment is expensive, costs may be reduced by regaining as much condensate from the distribution system as possible. This condensate will require no further chemical treatment and requires less thermal input. This condensate will, however, carry contamination from the pipework system and steps should be taken to ensure that this contamination is as low as possible.

While modern technology might provide the means of automatically modulating steam production and raw make-up, such technology is incapable of continually moni-toring the levels of physical contamination. Testing and monitoring of these values needs to be carried out at a frequency determined by their results and the constancy of the raw water quality.

Ideally the distribution system will send steam to its point of use with no loss of energy and with no reaction on, or with, the distribution pipework. Both of these factors are impractical.

Steam at high pressure and temperature will always lose energy to the pipework’s surroundings whatever the efficiency of its thermal insu-lation. This energy loss will always create condensate within the pipework. Good insulation will always reduce but not eliminate this. The design of the pipework must accommodate this condensate and include points at which this condensate can be removed or drained.

Whatever methods are used to generate and distribute steam it is likely that some impurities will be transmitted from the boiler to the steriliser chamber. The steriliser must include some means of either monitoring or removing some, if not all, of these impurities.

Whatever the wetness of the steam delivered, as soon as it comes into contact with load items or loading furniture at lower temperatures, energy will be transferred and condensate will be produced. The amount of water unavoidably produced in this way is likely to exceed the extra moisture content of steam that is too wet. Experience of wrap-ping and loading items to handle this naturally-produced condensate include deflector plates, positioning of heavy load items and the use of porous material in combined trays. If these techniques are successful they will almost certainly handle the problem of steam wetter than desired.

Non-condensable gases are more difficult to remove, but it is possible to monitor its existence. The longer the sterilisation hold period the more accurate an air detector will be in this function. If the steriliser has a prion based cycle, eg. with an exposure to 134-137C for 18 minutes as recommended in the UK, then an air-detector temperature which remains constant over this period will suggest that the level of non-condensable gases is acceptably low.

The only method of removing physical and particulate matter is by filtration. At the quality of removal and the capacity required, filtration equipment may be expensive but is an option.

Choice of design and materials of construction must be made with the quality of steam in mind. Although the steriliser does not determine the quality of steam, its design can help to maintain a high quality or monitor unacceptably high levels of quality aspects.
Tool design can hinder proper cleaning

The design and construction of modern surgical instru-ments has been cited in observations about cleaning and sterilisation. Sufficient concern was expressed to prompt an Australian authority to issue the following guideline:

‘The importance of thorough cleaning prior to any disin-fection or sterilisation regimen should be emphasised in infection control education programs. Failure to achieve adequate cleaning may result in ineffective disinfection or sterilisation of instruments or equipment.’

Doctors had noted the difficulties encountered in cleaning certain surgical instruments. The particular device in question was a cutter/nibbler used for dissecting bone from around an operating site.

On disassembling a number of these devices, it was noted that at least 20 per cent of the instrument was
contaminated with residual blood inside the shaft of the instrument where the upper segment slides over the shaft. The design of many such instrument is such that disas-sembly by theatre staff is not possible because of the use of screws, and, in some cases, rivets.

“Normal cleaning methods, including ultrasonics, will frequently not remove all organic material, such as blood or bone residue, because of this construction,” commented an observer.

“Disassembly of an instrument to ensure all surfaces can be cleaned is essential. If the design of an instrument is such that it cannot be appropriately cleaned, the user cannot be confident that sterilisation has been achieved.”

It was recommended that purchasers of all surgical instruments ensure the instruments are able to be
adequately cleaned and dismantled prior to disinfection or sterilisation.


Experience counts for European suppliers

European companies Tuttnauer and BAG serve the cleaning and sterilisation market in very different ways.

For more than 75 years, the Tuttnauer Company has been designing and manufacturing sterilisation equipment for the medical, dental, pharmaceutical, bio-technology, and surgical markets. Drawing on this experience, as well as input from its international network of offices, Tuttnauer incorporates the best design and engineering features into its standard, commercially available, infection control products.

Tuttnauer, which maintains a representative office in Kuwait, is one of the top three manufacturers in the world in the infection control market. Its products have developed alongside scientific research in a variety of fields. They offer safe and environmental friendly sterilisation.

A large variety of Tuttnauer sterilisers are available, from table tops units as small as 7.5 litres to Steam and Gas Bulk Sterilisers from over 17,000 litres with regular and highly technical special cycles. The company’s autoclaves comply with the leading international standards of FDA, CE Medical Device, TUV, ISO 9002, as well as ASME, UL, EN601 and others.

German company BAG specialises in the development, production and distribution of indicators for monitoring of sterilisation and disinfection control.

As a leading manufacturer of sterilisation and disinfection monitors, BAG provides a large range of monitoring systems for steam, ethylene oxide gas, dry heat and radiation sterilisation that meet today’s strict and demanding requirements.

Reliability and safety in the production of sterile articles are imperative in modern quality management of hospitals. BAG complies with directives issued by national and international authorities to ensure that the control systems enable the customer to realise state-of-the-art sterilisation.
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