Decontamination
Understanding steam sterilisation failures
Dr. Tim Sandle discusses some of the common issues encountered in the sterilisation of surgical instruments, using autoclave technologies. He argues that to make a thorough evaluation of sterilisation, reliance cannot be restricted to chemical or biological indicators, and a complete understanding of hazards and physical operating parameters is required.
Staff working in decontamination services will ensure that reusable medical devices, such as endoscopes and other surgical instruments and equipment are cleaned, sterilised, and repackaged to high standards. With sterilisation (or sometimes decontamination), one of the commonly used items is the autoclave. The autoclave is one of the longest-established sterilisation technologies using a method of sterilisation involving moist heat (achieved using steam).1 Well-designed and suitably qualified
autoclaves operate well and consistently. However, autoclaves can periodically go wrong, such as when improperly maintained, or they can fail at the point of revalidation. This article presents an overview of autoclave operations and considers those things that can go wrong and affect autoclave operations.
Inside the box: how an autoclave works An autoclave is, crudely, a pressure-cooker, based on the manipulation of pressure to raise the boiling point of water. Water boils at 100°C, at atmospheric pressure; whereas at lower pressures it boils at lower temperatures and at higher pressure it will boil at a higher temperature. For example, at a steam over- pressure of one bar (or 100,000 Pascals) water boils at approximately 121°C. This allows the autoclave to produce temperatures above those that can ordinarily be achieved. Then, if operated for sufficient time, and with the correct conditions, such temperatures can destroy bacterial endospores. The standard sterilising temperature in steam autoclaves is 121°C, but lower (such as 116°C) and higher (for example. 134°C) temperatures are also used for certain cycles. For the sterilisation of microbiological culture media, for example, a lower temperature is required to avoid physical damage. There are two common types of autoclaves:
l Gravity displacement autoclave: Steam is admitted at the top or the sides of the sterilising chamber and because the steam is lighter than air, this forces air out the bottom of the chamber through the drain vent. The gravity displacement autoclaves are primarily used to process laboratory media, water, pharmaceutical products, regulated medical waste, and non-porous articles whose surfaces have direct steam contact.
l High-speed pre-vacuum steriliser: These autoclaves are fitted with a vacuum pump (or ejector) to ensure air removal from the sterilising chamber and load before the steam is admitted into the chamber.
For autoclaves fitted with a drain at the bottom of the chamber, the temperature should be recorded at this position throughout the sterilisation period, to verify a successful operation.
Prerequisites for success sterilisation There are some basic requirements that need to be observed in order for an autoclave to operate satisfactorily. When operating an autoclave, it is important to ensure that all the trapped air is removed before commencement of the sterilisation cycle because hot air is an insulator and prevents sufficient heat from reaching the microbial population. There is also the necessity to achieve uniformity of heat transfer, so defining materials to be sterilised and their load patterns are important. Furthermore, the steam also needs to be dry, with a dryness factor of above 97%. This is because moisture can lead to wet loads, and wet loads means non-sterility. Wetness can also affect packaging integrity.2 Some common areas to assess to ensure successful autoclave operation are:3 l The feed water supplied to the pure steam (clean steam) generator should be
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