INSTRUMENT STERILISATION AND DECONTAMINATION Sterilisation process Any sterilisation process
(non-specific generic standard) Moist heat (steam)
Dry heat Irradiation Vaporised hydrogen peroxide Ethylene oxide gas
Low temp steam with formaldehyde
*In preparation – to be published as prEN Q3/22 **Undergoing revision
Table 1: The British Standards for sterilisation processes and sterilisers.
implemented as British Standards for sterilisation processes and sterilisers.
Sterilisation processes There are certain variables associated with a given sterilisation process which are needed to achieve microbial inactivation – termed the process variables – and other variables which are used to control and monitor sterilisation processes – termed cycle variables. The following sections identify each for each process.
n Physical methods: High temperature – Moist heat
Moist heat or steam sterilisation is carried out using saturated steam which comes into direct contact with the surfaces which need to be sterilised, creating a moist heat environment which, at the right temperature and for sufficient time, rapidly inactivates microorganisms. The process variables for moist heat sterilisation are time and temperature in the presence of moisture. Pressure is a cycle variable, and allows a carefully controlled process to be delivered.
A typical saturated steam sterilisation
process consists of three basic phases. In phase one, air remaining in the chamber once the door is closed is removed by a
Temperature (°C) 134
132 126 121 115
Time (mins) 3*
4** 10* 15*
30***
*Recommended by Medical Research Council Lancet. 1959 Feb 28;1(7070):425-35 ** Used in USA
*** Cited in British Pharmaceutical Codex, rarely used today
Table 2: Temperature and time combinations used in moist heat (saturated steam) sterilisation processes.
36 Health Estate Journal October 2022
series of evacuation and steam injection pulses. The application of such pulses gradually replaces the air within the chamber, and, most importantly, the load, by steam. At the end of the air removal stage steam is injected into the chamber to a pre-defined pressure, giving an equivalent temperature, and at this point the next phase commences – the sterilisation stage. During the sterilisation stage any microorganisms present on or in the load will be rapidly inactivated. After a suitable exposure period, the third and final stage commences. The chamber is evacuated to a deep vacuum (typically <100mB), and this allows the condensate which has accumulated in the chamber and load to evaporate (with the aid of the residual heat present in the load), and be removed, so that at the end of sterilisation the load is warm (not hot), dry, and sterile. Table 2 shows some typical temperature and time combinations which are used in moist heat sterilisation. Moist heat sterilisation using saturated
steam is typically used to sterilise the reusable surgical instruments and other medical devices commonly used in a hospital surgical department.
Aqueous fluids In some healthcare settings, aqueous fluids contained in flexible or rigid containers may also be sterilised by moist heat (in, for example, a pharmaceutical production unit). In these types of processes, the steam entering the chamber behaves as a heat transfer medium, transferring energy through the container walls and into the aqueous fluid contained within the container. Such processes also have three phases – in which the load is heated to a predetermined temperature, held for a predetermined time, and then cooled back to ambient temperature. It is also possible to include air within the chamber to act
Sterilisation standard BS EN ISO 14937
BS EN ISO 17665 BS EN ISO 20857 BS EN ISO 11137 BS EN ISO 22441 BS EN ISO 11135 BS EN ISO 25424
Steriliser standard BS ISO TS 22421
BS EN 13060 (small) BS EN 285 (large)
None None BS EN 17180* BS EN 1422** BS EN 14180**
as a ballasting overpressure to prevent flexible containers bursting as a result of the high pressures which can develop during sterilisation. It is vital that any such air-steam mixtures are intimately mixed, so as to avoid separation and stratification in the chamber which could give rise to dangerous cold spots.
Sterilisers Saturated steam sterilisers can fall into one of two categories. There are small steam sterilisers often used in dental or GP surgeries, and large steam sterilisers found in hospital settings. The difference between the two is related to chamber size. A steriliser of less than 60 litres, unable to accommodate a sterilisation module measuring 30 x 30 x 60 cms, can be categorised as a small steriliser. Any steriliser with a chamber larger than 60 l, meanwhile, is a large steam steriliser, and these can be of any size up to several cubic metres. In a hospital, large sterilisers of between 750 and 1000 l in size are typically found. While fundamentally the same, i.e. they use saturated steam at high temperature, and pressure in a sealed pressure vessel to effect sterilisation, the requirements for small and large sterilisers differ, as prescribed in published standards (see Table 1).
n Physical methods: High temperature – dry heat
Dry heat sterilisation is carried out in the absence of moisture. The process stages involve heating the load to the sterilisation temperature (typically 160 °C), and holding it there (e.g. for two hours), followed by a cooling period. Heating and cooling the load can take a considerable time if bulky items are being sterilised. Dry heat is used for the sterilisation of non-aqueous pharmaceutical powders, oils and waxes, and glassware, but rarely used in hospital Sterile Services Departments. Great care must be taken to ensure homogenous thermal distribution in loads. Hot and cold spots can easily occur, and this is why hot air fan-assisted ovens are common.
n Physical Methods: Low temperature sterilisation
The sterilisation processes fitting into this category are almost exclusively used by the medical device industry. This would include processes which use ionising radiation to inactivate microorganisms. The process variable for all radiation processes is ‘absorbed dose’, which is a function of radiation intensity and time of exposure. Gamma sterilisation employs radiation emanating from the radioisotope, Cobalt 60 e-beam sterilisation employs a highly energetic electron beam produced by a cathode ray array. The advantage of this type of process is that it is on/off, and so no specialist storage facilities are required for the source. A
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