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The need for sterilization by steam under increased pressure arises from the fact that the spores of many common and dangerous bacteria are strongly resistant to dry heat. The most effective way of killing these spores is by means of steam at high temperature and high humidity which will condense into their substance and moisten them thoroughly.To bring about this affect all parts of the load to be sterilized must be free of air, so that it may be permeated by the steam and the steam must be not only hot 121OC or 134OC but nearly saturated that is, it must be at a pressure so high that in spite of its high temperature it is close to the point of condensing to liquid water. Steam in this condition is termed Just Dry. Departures from this condition result in inefficient sterilizing operation!

Autoclave Pours Cycle Requirements:Air Removal:The adequate removal of air is the first essential in the sterilisation of textiles; air is removed by a vacuum pump which reduces the absolute pressure of air in the chamber to a value of a few millimetres of mercury column before steam is admitted. This is often described as drawing an almost perfect vacuum, and it means removing more than 98 percentof the air initially present. It must be emphasised that during the preconditioning stage it is imperative that the set vacuum points are met via actual chamber vacuum and not via a timing source as vacuum set points are load dependant and load variable. Departures from the recommended means of measuring vacuum set points can cause inadequate air removal.

Timing:When the steam has reached the correct set sterilizing temperate, sufficient time must be allowed for the physical changes that kill the bacteria and their spores to take place throughout the load; it is these changes that constitute the sterilizing process. Times corresponding to different temperatures have been given by Perkins as follows: 2min s at 132OC and 12min s at 121 OC.To allow for deviations in steam quality a further (safety) period is added. Adapting Perkins figures (above) Medi-Clave recommend the following times and temperatures as a minimum:4 min s at 134OC and 15 mins at 121OC.

Drying:At the end of the sterilizing phase it is necessary to dry the load without re-contaminating it.The state of dryness of the load will be fixed by the equilibrium established between the vapour pressure over the load and the pressure in the chamber the lower the chamber pressure the more rapid will be the boiling off of the water. At this stage practically no air should be present in the chamber. If dressings have not been made grossly wet by inadequately dried steam, the mere attaining of the vacuum should be sufficient to achieve drying without a need to maintain it! Medi-Clave found that with a high-vacuum sterilizer operated with dry steam, the load was back at its starting condition as regards to temperature and humidity, at the end of the sterilizing cycle.Wet Steam is a frequent cause of incomplete sterilization. It causes the dressings in the sterilizer to become soaked with moisture, usually early on in the sterilizing process. Moreover, the subsequent drying of the wet dressings is unaccomplished and it carries a serious risk of recontamination by airborne organisms that can re penetrate in to the pack.

Vacuum-Break:Air must be admitted to break the vacuum before the sterilizer is opened. This air must be drawn from a clean source through a bacteriological filter. More detail on autoclaves can be found at Medi-clave website

By: Johann Hoffman Medi-Clave

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