Autoclave Steam Sterilization is a vital measure in infection control and is said to be the most efficient sterilization method available.
Moist heat in the form of saturated steam under pressure is the most widely used and the most dependable method of sterilization according to the Center for Disease Control (CDC).
Steam sterilization using autoclaves is nontoxic, inexpensive, rapidly microbicidal, sporicidal, and rapidly heats and penetrates fabrics easily making it most preferred for medical facilities.
Basic Principles of Steam Sterilization
The basic principle of steam sterilization, as accomplished in an autoclave, is to expose each item to direct steam contact at the required temperature and pressure for the specified time.
As such, four basic factors or parameters are involved in the steam sterilization process namely:
- temperature, and
The ideal steam for sterilization is dry saturated steam and entrained water (dryness fraction ≥97%).
Pressure serves as a means to obtain the high temperatures necessary to quickly kill microorganisms.
Specific temperatures must be obtained to ensure the microbicidal activity.
The two common steam-sterilizing temperatures are 121°C (250°F) and 132°C (270°F). These temperatures (and other high temperatures) must be maintained for a minimal time to kill microorganisms.
Recognized minimum exposure periods 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 (Table 7).
At constant temperatures, sterilization times vary depending on the type of item (e.g., metal versus rubber, plastic, items with lumens), whether the item is wrapped or unwrapped, and the sterilizer type.
Basic Steam Autoclave Types
There are basically two types of autoclave steam sterilizers:
- the gravity displacement autoclave, and
- the high-speed pre-vacuum sterilizer
Gravity Displacement Autoclave:
In the gravity displacement autoclave, steam is admitted at the top or the sides of the sterilizing chamber and, because the steam is lighter than air, it forces air out the bottom of the chamber through the drain vent.
The gravity displacement autoclaves are primarily used to process laboratory media, water, pharmaceutical products, regulated medical waste, and nonporous articles whose surfaces have direct steam contact.
For gravity displacement sterilizers the penetration time into porous items is prolonged because of incomplete air elimination.
High-speed Pre-vacuum Autoclave:
The high-speed pre-vacuum sterilizers are similar to the gravity displacement sterilizers except they are fitted with a vacuum pump (or ejector) to ensure air removal from the sterilizing chamber and load before the steam is admitted.
The advantage of using a vacuum pump is that there is nearly instantaneous steam penetration even into porous loads.
Typical sterilization temperatures and times are 132°C to 135°C with 3 to 4 minutes exposure time for porous loads and instruments.
Autoclave Steam Sterilization Process
Like other sterilization systems, the steam cycle is monitored by mechanical, chemical, and biological monitors.
Steam sterilizers usually are monitored using a printout (or graphically) by measuring temperature, the time at the temperature, and pressure.
Typically, chemical indicators are affixed to the outside and incorporated into the pack to monitor the temperature or time and temperature.
The effectiveness of autoclave steam sterilization is monitored with a biological indicator containing spores of Geobacillus stearothermophilus (formerly Bacillus stearothermophilus).
Positive spore test results are a relatively rare event 838 and can be attributed to operator error, inadequate steam delivery,839 or equipment malfunction.
Portable (table-top) steam sterilizers are used in outpatient, dental, and small clinics. These sterilizers are designed for small instruments, such as hypodermic syringes and needles and dental instruments.
The ability of the sterilizer to reach physical parameters necessary to achieve sterilization should be monitored by mechanical, chemical, and biological indicators.
Autoclave steam sterilization should be used whenever possible on all critical and semi-critical items that are heat and moisture resistant (e.g., steam sterilizable respiratory therapy and anesthesia equipment), even when not essential to prevent pathogen transmission.
Autoclave steam sterilization is also used in healthcare facilities to decontaminate microbiological waste and sharps containers but additional exposure time is required in the gravity displacement sterilizer for these items.
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