Infection Control Today - 05/2001: Sterilization

Low-Temperature Sterilization

By Cynthia Spry, RN

Increasingly, operating rooms (ORs) are pressured to carry out heavier workloads, turn rooms over with less downtime, and reduce costs with little or no increase in instrument inventory. In such situations, maximal use of costly, heat-and moisture-sensitive medical, and surgical devices, such as cameras, fiber optic cables, and rigid endoscopes, becomes paramount. Advanced sterilization systems that enable more rapid availability of wrapped, sterile devices and instruments to improve throughput of inventory can contribute to quality patient care and ease the pressure on OR managers and staff. One such advance is a low-temperature hydrogen peroxide gas plasma sterilization system that provides terminal sterilization of sophisticated instruments in 55 minutes.

Low-Temperature Sterilization and Throughput

Many of today's sophisticated, delicate medical devices withstand sterilization only in low-temperature, low-moisture environments. Arthroscopes, laparoscopes, cystoscopes, other rigid endoscopes, and light cables--are examples of devices with optics and electrical connections that limit the type of sterilization procedures that can successfully be used without damaging the device.

Commonly used sterilization processes have a variety of advantages and disadvantages. For example, the steam autoclave, a 200-year-old sterilization technology, is an effective sterilization process, but its high temperature and moisture make it unusable for many of today's devices. Likewise, dry-heat sterilization has process temperatures that cannot be tolerated by most devices. As a result, these sterilization processes cannot be used with endoscopes and similar devices. Instead, low-temperature, low-moisture processes such as sterilization by ethylene oxide (EO) gas or hydrogen peroxide gas plasma must be used.

The gold standard for items used in surgery is a sterilized item in a wrapped package ready for use at any time. Ideally, production of this terminally sterilized, patient-ready item would be quick, safe, and easy.

EO, a sterilization technology now more than half a century old, is an example of a low-temperature, low-moisture process that allows terminal sterilization and spares sharp edges that are degraded by steam. However, EO produces noxious fumes that require special venting, and its concentration must be monitored in work areas to prevent injury to staff. Because it leaves toxic residues, the process requires extensive aeration of sterilized products. Every item must be out of service for approximately 14 hours or more while undergoing EO sterilization. As a result of this lengthy process, items sterilized with ethylene oxide are typically used only one time in a 24-hour period. This lengthy cycle time limits the throughput of surgical instruments.

Other methods are typically used to improve throughput when the primary sterilization process of a facility requires many hours for instrument processing, as is the case with EO. But these supplementary methods may have marked limitations. For example, flash sterilization is a high-temperature, high-moisture process used to sterilize instruments quickly in the operating room. However, it cannot be used for heat-and moisture-sensitive devices, such as most endoscopes.

Some institutions faced with a small inventory of heat-and moisture-sensitive items will employ high-level disinfection to optimize instrument utilization and move the OR schedule forward. High-level disinfection is compatible with endoscopes and other delicate instruments. The process takes well under an hour and is commonly used to improve throughput of instruments. However, the most obvious drawback to utilization of a high-level disinfection process is that it does not achieve sterilization. The result is an undesirable approach to patient care. For example, an instrument processed using EO and aerated overnight might be used as a sterile instrument in the morning. After use, and to improve throughput, that same instrument might then undergo high-level disinfection in glutaraldehyde and subsequently be used as a disinfected not sterile instrument later the same day.

Another instrument processing technology used to improve throughput is liquid peracetic acid, which sterilizes instruments in less than an hour. However, the major drawback to this system is that the final product is not wrapped nor dried and cannot be stored. Peracetic acid sterilization is intended for point-of-use sterilization only and is used to process instruments between procedures during the day. In many cases, however, these same instruments are sterilized in EO overnight. In theory, any sterilization process lengthy enough to reduce throughput could put a facility at risk of operating under a dual system whose driving factor depends on time of day rather than desired optimum practice. A better solution is a sterilization process that is rapid enough to improve throughput without increasing inventory.

Hydrogen peroxide gas plasma sterilization is a low-temperature, low-moisture sterilization process that is rapid enough to provide high throughput. There are no toxic residuals; therefore, no aeration is required. The primary byproducts of the process are water vapor and oxygen. As a consequence, the cycle time for processing can be relatively short. Several new improvements in hydrogen peroxide gas plasma sterilization technology have reduced cycle time from 74 minutes to 55 minutes, allowing more instruments to be processed.

Hydrogen peroxide gas plasma is used worldwide for terminal sterilization of medical equipment. Sterilization occurs in a low-moisture environment at a temperature less than 50°C. It is suited for sterilizing heat-and moisture-sensitive items, delicate instruments, and instruments with sharp edges. Recent technological advances have resulted in a hydrogen peroxide gas plasma sterilization system that has improved robustness and a shorter cycle. The STERRAD® 100S Sterilization System from Advanced Sterilization Products, Irvine, Calif, represents the next generation of low-temperature hydrogen peroxide gas plasma sterilization. Independent studies have demonstrated an improved load tolerance and faster kill in diffusion-restricted areas as opposed to previous-generation hydrogen peroxide gas plasma sterilizers. The system provides terminal sterilization in 55 minutes, shorter than the typical 14 hours required to produce sterile instruments using EO. The short cycle time also means that a single unit can sterilize, not merely disinfect, numerous loads per day.

The sterilization process in the new system consists of two consecutive and equal sterilization phases. The process begins when clean, dry items in porous, noncellulosic wraps, such as nonwoven polypropylene CSR wraps or Tyvek® pouches, are placed in the sterilization chamber. After the chamber door is closed, air and residual moisture are pumped out, creating a vacuum. A low-temperature air plasma is generated to help remove residual moisture from the chamber. Filtered air is then introduced into the chamber, bringing it up to atmospheric pressure and completing the 20-minute pretreatment drying phase.

After pressure is again reduced in the chamber, sterilization begins with the automatic injection into the chamber of an aqueous solution of hydrogen peroxide from a self-contained cassette. Hydrogen peroxide vaporizes in the vacuum and diffuses throughout the chamber, surrounding items to be sterilized and initiating inactivation of microorganism encountered in the sterilization chamber.

Radio-frequency (RF) energy is then applied to create an electric field, which in turn transforms the hydrogen peroxide into a low-temperature gas plasma. In this physical state, hydrogen peroxide vapor breaks apart into reactive species known as free radicals. Unreacted free radicals are ultimately converted into nontoxic byproducts, primarily water and oxygen. This activity completes the first half of the sterilization cycle. The second half is an identical 17-minute sterilization phase that includes a second injection of hydrogen peroxide. After the second phase, the RF energy automatically shuts off, and filtered air is introduced into the chamber. Sterilized items are ready for use as soon as the chamber reaches atmospheric pressure (one minute). Total cycle time is 55 minutes. Because items are wrapped and dry, they may be stored for future use.

This sterilization process is designed for safe use. There are no toxic fumes, byproducts, or residues. No aeration is needed, and there are no associated safety regulations from OSHA, as there are with EO. The active ingredient, hydrogen peroxide, is contained in sealed cassettes--no mixing of chemicals or connecting of tanks is required. When a cassette is empty, it automatically ejects into a collection box. When the collection box is full, the system alerts the user to discard the collection box. There is no handling of opened cassettes. Once the chamber has been loaded and the start button pressed, everything is automatic. After each run, the sterilization system generates a printout that can serve as part of a permanent performance record.

The hydrogen peroxide gas plasma sterilization system can safely and effectively sterilize most surgical instruments, except for powders, liquids, devices with long, narrow lumens, linens, and other cellulosic materials.

Integration into the Existing Plant

This advanced hydrogen peroxide gas plasma sterilization system requires no water hook up, and only a dedicated 208V electrical supply is needed. No toxic residues or emissions are created, no aeration is needed, and no specialized venting is required. Finally, the sterilizer's compact footprint (less than 3 ft. x 4 ft.) makes placement easy and its built-in wheels allow it to be moved.

Compared with other low-temperature, low-moisture sterilization processes, the short cycle time of this advanced hydrogen peroxide system means that instruments and devices will spend less time in the sterilizer and more time in use. The consequence is improved productivity and an uninterrupted OR schedule. This becomes of greater importance as hospitals and surgical centers increasingly take on heavier workloads, often without budgets sufficient for expanding instrument inventory.

In addition, the availability of terminally sterilized, not merely disinfected patient-ready items that may be used immediately or stored for future use will greatly benefit hospitals and surgical centers.

Conclusion

Technological advances in hydrogen peroxide gas plasma sterilization have produced systems that address the worldwide trend requiring greater productivity from medical device inventory. An advanced hydrogen peroxide gas plasma sterilization system uses a two-phase, low-temperature, low-moisture hydrogen peroxide gas plasma process to produce wrapped, sterile items, including rigid surgical endoscopes.

Cynthia Spry, RN, is International Clinical Consultant for Advanced Sterilization Products.
Tyvek® is a registered trademark of DuPont
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