Gravity Displacement Autoclave


Most medical and surgical devices used in healthcare facilities are made of materials that are heat-stable and therefore undergo heat (primarily steam) sterilization. However since 1950, an increase in medical devices and instruments made of materials that require low-temperature sterilization (i.e. plastics); therefore ethylene oxide gas had since been introduced for heat- and moisture- sensitive medical devices. Within the last two decades, new and improved low-temperature sterilization systems have been developed and utilized.

Sterilization is defined as the destruction of all microorganisms, as pathogenic or saprophytic bacteria, vegetative forms, and spores on the surface of an article or in a fluid to prevent disease transmission associated with the use of that item. Medical devices considered as critical items and should be sterile when used because any microbial contamination could result in disease transmission.

Of all sterilization methods, saturated steam under pressure (steam sterilization) is the most widely used and most dependable as it is nontoxic, inexpensive, rapidly microbicidal, sporicidal, and penetrates fabric. The basic principle of steam sterilization is to expose each item to direct steam contact at the required temperature and pressure for the specified time in an autoclave. There are four parameters of steam sterilization: steam, pressure, temperature, and time. The ideal steam for sterilization is dry saturated steam and entrained water (dryness fraction ≥97%). The ideal pressure is in relation with specific temperature, maintained for a minimal time, of 121°C (250°F) or 132°C (270°F). The required minimum exposure period for sterilization of wrapped healthcare supplies are 30 minutes at 121°C (250°F) in a gravity displacement sterilizer or 4 minutes at 132°C (270°F) in a pre-vacuum sterilizer.

There are two basic types of steam sterilizers (autoclaves): gravity displacement autoclave and high-speed pre-vacuum autoclave. In a gravity displacement autoclave, steam is admitted at the top or the sides of the sterilizing chamber and forces air out the bottom of the chamber through the drain vent. The penetration time into porous items is prolonged because of incomplete air elimination in the sterilizing chamber (i.e. the entrapped air remaining in a load of waste greatly retards steam permeation and heating efficiency). In a high-speed pre-vacuum autoclave however, steam penetration is instantaneous and regardless of porous loads. High speed pre-vacuum autoclave differs from gravity displacement autoclave that it ensures air removal from the sterilizing chamber and load before the steam is admitted.

(Video on Autoclave Process)

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The mode of microbicidal is moist heat destroys microorganisms by the irreversible coagulation and denaturation of enzymes and structural proteins. Steam sterilization should be used whenever possible on all critical and semicritical items that are heat and moisture resistant, even when not essential to prevent pathogen transmission; another use is decontamination of microbiological waste and sharp containers.

Another type of sterilization is flash steam sterilization. This method is applied in a gravity displacement autoclave at 132°C (270°F) for 3 minutes at 27-28 lbs of pressure with unwrapped object. Flash steam sterilization is a modification of conventional steam sterilization as equipments are placed in an open tray or placed in a specially designed, covered, rigid container to allow for rapid penetration of steam. The use of flash sterilization is considered acceptable for processing cleaned patient-care items that cannot be packaged, sterilised and stored before use. It is also used when there is insufficient time to sterilise an item by the preferred package method. Despite the short sterilization period, it is by no means for reasons of convenience or time saving. Several restrictions on the application of this method are: 1) implantable devices 2) orthopaedic screws and 3) plates as they need to be tracked for sterility and evaluated.

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Low temperature sterilization technologies also has been applied in different situations. Ethylene oxide (ETO) has been widely used as a low temperature sterilant as it is used to sterilise temperature- and moisture- sensitive medical devices and supplies in healthcare institutions. There are two types of ETO sterilizers available: mixed gas (ETO with CFC stabilizing agent of a ratio of 12% ETO and 88% CFC) and 100% ETO. However, as part of the Clean Air Act CFC were classified as a Class I substance and were prohibited for use. Alternative technologies have become available include: 100% ETO, ETO with a different stabilizing gas (carbon dioxide or hydrochlorofluorocarbons), immersion in peracetic acid, hydrogen peroxide gas plasma, and ozone.

One new technology is hydrogen peroxide gas plasma. Gas plasmas are generated in an enclosed chamber under deep vacuum using radio frequency or microwave energy to excite the gas molecules and produce charged particles. The free radicals produced within the plasma field with interact with essential cell components and disrupt metabolism of microorganisms. This method inactivates a broad range of microorganisms, including resistant bacterial spores, on materials and devices that cannot tolerate high temperature, humidity, and corrosion-susceptible metal alloys.

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Packing, loading and storing of instruments are necessary after sterilization. There are several packaging methods to maintain sterility of surgical instruments: 1) rigid containers, 2) peel-open pouches, 3) roll stock, 4) reels, and 5) sterilization wraps. The packaging material must allow penetration of the sterilant, provide protection against contact contamination during handling, provide an effective barrier to microbial penetration, and maintain sterility of the processed items after sterilization. An example of ideal sterilization wrap will successfully address barrier effectiveness, penetrability, aeration, ease of use, drape-ability, flexibility, puncture resistance, tear strength, toxicity, odour, waste disposal, linting, cost and transparency.

Loading items into the package must be arranged on all surfaces that will be directly exposed on sterilizing agents. A free circulation of steam (or sterilant) around each item. Several key principles in loading are: allow for proper sterilant circulation, perforated trays should be placed so the tray is parallel to the shelf, non-perforated containers should be placed on their edge, small items should be loosely placed in wire baskets, and peel packs should be placed on edge in perforated or mesh bottom racks or baskets.

Storage time can vary due to various environment and conditions. There are two types of practice currently applied in hospitals: time-related shelf-life practice and event-related shelf-life practice. The latter practice recognizes that the product should remain sterile until some event causes the item to become contaminated. Factors include bioburden, air movement, traffic, location, humidity, insects, vermin, flooding, storage area space, open/close shelves, temperature, and the properties of packages. In hospitals, sterile supplies should be stored far enough from the floor (8 to 10 inches), the ceiling (5 inches and 18 inches from sprinkler head), and the outside wall (2 inches) to allow adequate air circulation, ease of cleaning and compliance with local fire codes. Medical and surgical supplies should not be stored under sinks or in other locations where moist is present. Sterile items that become wet are considered contaminated because moisture brings with it microorganisms from the air and surfaces. Any package that has fallen or been dropped on the floor must be inspected for damage to the packaging and contents.

The sterilization procedure should be monitored routinely by applying a combination of mechanical, chemical and biological indicators to evaluate the sterilizing conditions and microbiologic status of the processed items. Physical monitoring includes checking the pressure gauge, temperature, and cycle time. Chemical monitoring are convenient and inexpensive but indicate that the item has been exposed to the sterilization process. Biological monitoring are recognised by most authorities as being closest to the ideal monitors of the sterilization process as they measure the sterilization process directly by using the most resistant microorganisms. New spore-strip biological indicators are self-contained in plastic vials and can produce a rapid-readout.

New rapid-readout ETO biological indicator has been designed for rapid and reliable monitoring of ETO sterilization processes. It detects the presence of B. atrophaeus by detecting a fluorescent signal.

If a false-positive biological indicator occur from improper testing or faulty indicators, it may be caused by improper storage processing, product contamination, material failure, or variation in resistance of spores. These results should be considered when applying sterilization processes.

Healthcare providers should be aware during all procedures of sterilization as any opportunistic infection can happen in all states. However, if standards and protocols are followed correctly, chances of infections can be greatly reduced and further guarantee a safer environment for patients and healthcare providers.

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(Video on General Overview of Sterilization and Disinfection Process)


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