The Principles of Medical Device Decontamination

A simple way of remembering the process parameters is “all the sixes”; 60 o C, 60% rh, 600mg/L chamber gas concentration, 60 minutes exposure although in fact EO processes might operate as low as 35 o C but often operate at 55 o C.

A typical EO sterilization cycle involves drawing a deep vacuum in the chamber so as to remove residual air. This is followed by a conditioning stage involving pre-heating and humidification of the load by introduction of pulses of steam introduced into the chamber. Sometimes a pre-conditioning stage takes place external to the sterilizer chamber in a preconditioning chamber or room held at 50-60% rh and 50-55 o 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. 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 be sterilized by EO. Unlike the oxidising agents which can be chemically aggressive and corrosive towards certain materials, EO is a ”gentle” process causing little damage to the materials making up the medical devices. EO is also capable of penetrating into complex medical devices by penetration through the materials of construction or penetration into complex shapes, cavities and lumen, by diffusion given sufficient time. Sterilizers Industrial EO sterilizers can be very large, capable of holding several pallets of medical devices such as plastic syringes. In healthcare settings much smaller, often tabletop, EO sterilizers can be bought with chamber capacities of 100 to 300L. These latter units tend to require minimal services e.g. mains power and compressed air.

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