INSTRUMENT STERILISATION AND DECONTAMINATION
Sterilisers Generally, VH2O2 sterilisers for use in hospitals are free- standing, and do not require complex services to operate. Generally, sterilisers are between 200 and 500 litre chamber size. The sterilant is usually supplied as a concentrated solution of H2
O2 (ca 59%) in water, with
added stabilisers. The solution is usually in a single or multi- shot disposable container, or in a cassette-based system with sufficient VH2O2 for one sterilisation cycle per pod, and several pods per cassette.
n Chemical methods: low temperature – alkylating agents
Figure 1 shows a number of sterilisation methods which use chemical agents, which bring about microbial inactivation by alkylation of the vital molecules found within the cell. The mechanism of alkylation involves a short chain organic chemical binding to the cell molecules, or the microbicidal agent binding two of the cell molecules together so they cannot function.
be sterilised by EO. Unlike the oxidising agents – which can be chemically aggressive and corrosive towards certain materials – EO is a ‘gentle’ process that causes little damage to the materials that make up the medical devices. EO is also capable of penetrating into complex medical devices by penetration through the materials of construction, or into complex shapes, cavities, and lumen, by diffusion given sufficient time.
Figure 4: A low temperature steam with formaldehyde steriliser.
Glutaraldehyde While not a sterilisation process in the sense commonly used (i.e. use of a microbicidal vapour in a sealed chamber, with medical device loads wrapped in a sterile barrier system), glutaraldehyde has been used in liquid form for use in very specific situations. It is a toxic chemical which – if mishandled – can lead to acute and chronic health conditions – such as respiratory sensitisation – in operators, Once used in open baths for decontamination of endoscopes, it is rarely seen today.
Ethylene oxide Ethylene oxide (EO) is termed an oxygen bridge molecule. The internal chemical bonds are strained, making it highly reactive, and able to form alkyl hydroxy bridges between biologically active cellular molecules. The process variables for an EO process are temperature, time, and EO gas concentration in the presence of moisture as humidity. A simple way of remembering the process parameters is ‘all the sixes’ – 60 °C, 60 % rh, 600 mg/L chamber gas concentration, 60 minutes exposure, although in fact EO processes might operate at as low as 35 °C, but often operate at 55 °C. A typical EO sterilisation cycle involves
drawing a deep vacuum in the chamber, so as to remove residual air. This is followed by a conditioning stage involving pre-
38 Health Estate Journal October 2022
heating and humidification of the load by introduction of pulses of steam into the chamber. Sometimes a pre-conditioning stage takes place external to the steriliser chamber in a pre-conditioning chamber, or room held at 50-60% rh and 50-55 °C. Gas exposure follows conditioning, where EO gas is introduced into the chamber to a pre-specified gas concentration, which is often much higher than the minimum required to effect microbial kill in order to hasten gas penetration into the load. The gas can be supplied from a large cylinder, or from a single shot cannister containing sufficient for one cycle. Gas can either be supplied pure, or mixed with an inert diluent gas. The gas is then held at the required concentration for a specified period.
Chamber again evacuated After gas exposure the chamber is again evacuated to remove the EO from the chamber. After this ‘flushing’ stage an aeration stage follows, where the EO is eluted from the load, often using a series of air injection and evacuation stages. Aeration can take several hours depending on the size and materials present in the load. Some plastic materials absorb large quantities of EO, and it is these which take much longer to aerate at the end of the gas exposure stage. Medical devices which are sensitive to heat and damaged by irradiation can
Sterilisers Industrial EO sterilisers can be very large, and capable of holding several pallets of medical devices such as plastic syringes. In healthcare settings, much smaller, often table-top, EO sterilisers can be bought with chamber capacities of 100 to 300 litres. These latter units tend to require minimal services, e.g. mains power and compressed air. EO is a toxic gas, and
so sterilisers are normally located in their own specially constructed
rooms, which ensure at least 10 air changes per hour, ventilated using special spark-proof fan systems designed to prevent fires. The ventilation exit points must be located away from public areas, usually on a roof, so that free air flow dilutes any emissions to very low levels. Similarly, the effluent from the steriliser at the end of the sterilisation cycle must be vented to a safe place to allow free air flow dilution, or, preferably, passed through an abatement system such as a catalytic
converter which will convert the EO to CO2 and water.
Low temperature steam with formaldehyde (LTSF) Once very common in the UK, but rarely found today, the LTSF process takes place in a sealed chamber, and uses saturated steam at circa 60 to 80 °C under low pressure (ca 200 to 500 mB absolute), mixed with formaldehyde vapour at a concentration of ca 15mg/litre. The process variables are temperature, time, and vapour concentration in the presence of moisture (as saturated steam). The process is complex. After evacuation of the chamber, steam is admitted to heat and humidify the load. This is followed by introduction of formaldehyde vapour, vaporised from formalin, which is contained within a reservoir tank. A series of formaldehyde and steam pulses are
Image used courtesy of Matachana
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