Porous loads

Great care is needed in the design of autoclaves and sterilization cycles because of the requirement for the presence of moisture. The autoclave must be loaded to allow complete steam penetration to occur in all parts of the load before timing of the sterilization cycle commences. The time required for complete penetration, the so-called heat-up time, varies with different autoclave constmction and different types of loads and packaging materials. The time may not exceed specific limits in order to guarantee reproducibility and, for porous loads, saturated steam. The volume of each container has a considerable effect on the heatup time whenever fluids are sterilized. Thermocouples led into the chamber through a special connector are often employed to determine heatup times and peak temperatures. The pressure is refleved at the end of each sterilization cycle. Either vented containers must be used or... 

By far the most commonly employed standard temperatuie/time cycles for bottled fluids and porous loads (e.g. surgical dressings) are 121 °C for 15 minutes and 134°C... 

To act as an efficient sterilizing agent, steam should be able to provide moisture and heat efficiently to the article to be sterilized. This is most effectively done using saturated steam, which is steam in thermal equilibrium with the water from which it is derived, i.e. steam on the phase boundary (Fig. 20.5). Under these circumstances, contact with a cooler surface causes condensation and contraction drawing in fresh steam and leading to the immediate release ofthe latent heat, which represents approximately 80% ofthe heat energy. In this way heat and moisture are imparted rapidly to articles being sterilized and dry porous loads are quickly penetrated by the steam. 

Steam for sterilization can either be generated within the sterilizer, as with portable bench or instrument and utensil sterilizers, in which case it is constantly in contact with water and is known as wef steam, or can be supplied underpressure (350-400kPa) from a separate boiler as dry saturated steam with no entrained water droplets. The killing potential of wef steam is the same as that of dry saturated steam at the same temperature, but it is more likely to soak a porous load creating physical difficulties for further steam penetration. Thus, major industrial and hospital sterilizers are usually supplied with dry saturated steam and attention is paid to the removal of entrained water droplets within the supply line to prevent introduction of a water fog into the sterilizer. 

In prevacuum autoclaves, problems are created by the removal of air and the air-insulation systems. A specific test called the Bowie Dick test (12), was developed to evaluate the abflity of prevacuum sterilizers to provide rapid and even steam penetration which includes the abflity to eliminate air from the chambers, and prevent air from lealdng back during the prevacuum phase. The test utilizes a pack of specific constmction (or its proven equivalent) placed in the empty chamber and exposed to specific test conditions (9,12). The pack contains a chemical indicator sheet. A correctly fimctioning sterilizer produces a uniform color change. A nonuniform color change indicates poor steam penetration and the possible presence of air which requires the attention of a qualified mechanic. The daily testing of all porous load sterilizers is recommended (1,9,10). 

In consideration of the extent, thoroughness, and history of the research evidence that micro-organisms are inactivated in a regular fashion in response to temperature and time, it is periodically suggested that biovalidation should not be necessary where there is evidence of adequate heat penetration. In practice, however, the expected lethality may not always be achieved. Most frequently, such deviations from ideality occur in equipment and porous load sterilization... 

Fig. 4 Recommended substrates for Bis used in biovalidation of porous loads.

With modern autoclaves, the major impediment of concern to successful sterilization is air in porous loads. Air may be present as a contaminant of the steam supply such that the temperatures theoretically achievable at particular steam pressures are depressed, or air may be present within the load, insulating it from the contact with the steam required for predictable lethality. 

Drying or cooling. Dressings packs and other porous loads may become dampened during the sterilization process and must be dried before removal from the chamber. This is achieved by steam exhaust and application of a vacuum, often assisted by heat from the steam-filled j acket if fitted. After drying, atmospheric pressure within the... 

Rubber closures for vials or other containers being filled aseptically on an industrial scale are nonporous but share some of the problems of porous loads. They are most frequently steam sterilized by passage through double-ended rotary washer autoclaves, which by agitation of the product ensure effective steam penetration to even the potentially occluded parts of the closures. Steam comes into direct contact with these bulk items they are not usually wrapped nor packed into hermetically sealed containers. Special precautions must be taken for unloading to avoid compromising their sterility. Static autoclaves may also be used for prew ashed versions of these types of components. They should then be loaded into the autoclave in shallow layers in perforated trays or boxes. 

I. Porous Loads—Pulsed Cycles Of all types of autoclave load, porous materials present the most serious problems of air removal. Air acts as an insulator and therefore impedes the condensation of steam on microorganisms and reduces process lethality. A further problem is that it is necessary that porous dressing packs and other cellulosic materials be completely dry at cycle completion. 

Up any air remaining in the chamber or in the load. Cycle exposure is timed from when a floating load temperature sensor and the air detector probe reach the specified set temperature. The importance of the air detector probe in porous load cycles cannot be overemphasised. 

In the case of porous loads it may be valuable to perform heat distribution studies on fully loaded autoclaves to provide additional assurance with regard to air removal and thermal uniformity. 

For porous loads, two other monitoring techniques are in use to give assurance of sterility where there is a risk of air entrainment, superheating, etc. These are the Bowie-Dick test pack and the Lantor Cube. It should be emphasized that these are used in rather special applications and would be of little significance to fluid loads or nonporous solid loads. 

---