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Several sterilization methods can be used for microsurgical instruments, although steam remains the most widely used method.
Three essential conditions, according to Harmer, must be met before an item can be considered sterile:
Among the various methods available, steam is the oldest, most reliable, most used, and least expensive method used by health-care organizations in the United States, Harmer said. One way to sterilize instruments with steam is through air dilution in tabletop sterilizers. Such devices generate their own steam through the addition of distilled or deionized water.
Site visits have shown that many healthcare workers who use tabletop sterilizers are unaware that the unit itself must be cleaned according to the manufacturer's guidelines. Sediment and dissolved solids may be left in the unit's reservoir if it has not been cleaned properly.
Gravity displacement and vacuum are other types of steam sterilization. Gravity displacement units rely on an injection of heat and steam under pressure into the autoclave; cool air is forced into the bottom and through a vent into the atmosphere. Vacuum steam sterilizers evacuate all of the air inside the chamber before introducing steam. Those units usually are faster than gravity displacement sterilizers.
Flash steam autoclaving is a high-speed sterilization method typically used when there is not enough time for preferred wrapped approaches. Sterilization takes place at 270° F or higher for shorter periods that are required for wrapped instruments.
"There is no shelf life with a flashed item," Harmer said. "It must be used immediately."
Sterilization with ethylene oxide gas is an older method that is slower and more complex than newer approaches. It is lethal to bacterial spores, molds, yeast, fungi, and viruses, which can be killed only through direct contact. In this method, effective sterilization depends on a combination of gas concentration, relative humidity, temperature, and exposure time.
Ethylene oxide is highly toxic and requires aeration cycles. The units still are available despite the advent of newer methods of sterilization and now are designed with improved mechanisms for aeration, according to Harmer. Ethylene oxide gas is mainly used to sterilize items that cannot withstand the high temperatures used in steam-based systems.
Gas plasma sterilization, using hydrogen peroxide vapor, is one of the newer methods. Only one FDA-approved gas plasma device is available in the United States. It works at a low temperature, is toxin-free and noncorrosive, and does not require an aeration cycle. Its byproducts-oxygen and water-are harmless. The device, however, will not work with all lumen sizes. It is not effective on metallic or nonmetallic instruments with lumens smaller than 6 mm or larger than 310 mm or on stainless steel lumens smaller than 3 mm or larger than 400 mm.
Cold sterilization is an alternative approach to instrument cleaning. Glutaraldehyde is the most commonly used sterilant in cold systems, although this chemical is more widely used as a high-level disinfectant.
"It can also be considered a sterilant if in fact it is used at room temperature, usually for 10 to 12 hours," Harmer said. "You can decrease that time if you're using an automated system with elevated temperatures; that usually takes about 8 hours.
"Glutaraldehyde is capable of killing highly resistant bacterial spores, but once you use it, it starts to lose its effectiveness," she said. "It needs to be marked with an expiration date, and you will need to check it for its effectiveness."
Glutaraldahyde evaporates quickly, has a strong odor, and can irritate the eyes, nose, nasal passages, throat, and respiratory tract. Staff members should wear eye protection when working around this chemical, and it must be thoroughly rinsed off the instruments. Any residual solution could be extremely toxic to intraocular or extraocular tissues. It is not a recommended method of sterilization for ophthalmology instrumentation, Harmer said.
Liquid chemical sterilization rarely is used in health-care settings. The exception is peracetic acid, a pH-neutral chemical with an extra oxygen atom; it is a strong oxidizing agent with an anti-corrosive additive. This low-temperature sterilization method is appropriate for heat-sensitive surgical items that can be immersed. Effectiveness depends on the concentration of the active chemical and length of exposure time. Peracetic acid does not require aeration or special monitoring, has a relatively short cycle, leaves no residue when rinsed properly, and is not carcinogenic or mutagenic.
Dry heat sterilization destroys organisms by oxidation. It requires high temperatures and long exposure times, but there is less potential for dulling and corrosion of the instrument. Extreme heat, however, can remove the finish on stainless steel.