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This series of self-study lessons on Central Service topics was developed by the International Association of Healthcare Central Service Materiel Management (IAHCSMM). The lessons are administered by Purdue University’s Continuing Education Division.

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Lesson Plan CRCST 107
Basics of Plasma Sterilization
[Reprinted from Communiqué: July/August 2009]

The use of hydrogen peroxide plasma vapor (plasma) sterilization has become more popular in the healthcare industry since its introduction in the 1990s by Advanced Sterilization Products (ASP), a Johnson & Johnson company. Its Sterrad® sterilization process is designed for devices that cannot be sterilized by high heat (steam) and/or for those instruments affected by the lengthy turn-around time, safety concerns, and expenses incurred when Ethylene Oxide (EtO) sterilization is used. This lesson explains the plasma sterilization process using the ASP Sterrad® sterilizer.

Objective 1 – Define the term “plasma” and review the types of ASP Sterrad® sterilizers used in healthcare facilities

Plasma is the fourth state of matter in addition to solids, liquids and gases. While difficult to envision, it is a collection of charged particles that contain positive ions and electrons that exhibit some properties of a gas, but that differ from a gas because they are good conductors of electricity and are affected by a magnetic field.¹ Lightning is a common example of plasma occurring in nature.

Plasma sterilization is used for moisture-and heat-sensitive devices, such as cameras, scopes and light cords. Five different models of Sterrad® sterilizers are available, and they operate at temperatures varying from 104°F-131°F (40°C-55°C). Primarily, differences between the available models relate to their size and time required to complete the sterilization cycle.

Advantages of plasma sterilization include speed and safety of use, and the process does not require aeration. Instruments may be used immediately after cycle completion, and it is less expensive (with a much faster time) than EtO sterilization. Faster turn-around improves instrument utilization because it allows for more efficiency in utilizing limited instrument resources.

The Sterrad® 50 is no longer manufactured; however, it is still used in some facilities. This sterilizer has a 1.55-ft rectangular chamber with one shelf, and it takes 45 minutes to run a full cycle. The chamber’s height is only seven inches. The Sterrad® 100S has a 3.53-ft circular chamber with two shelves and a run time of 55 minutes, and the Sterrad® 200 uses a 75-minute run time in a 5.3-ft rectangular chamber with two shelves. The newest plasma sterilizers are the Sterrad® NX and Sterrad®100NX models that allow sterilization of single-channel flexible endoscopes and semi-rigid ureteroscopes.

The NX and the 100NX plasma sterilizers are rectangular, and their volumes are, respectively, 1.1 ft and 2.9 ft. The NX sterilizer uses two different cycles: the standard cycle (28 minutes) is similar to other models and is used to sterilize cameras, scopes and light cords. The advanced cycle (38 minutes) is designed to process one single lumen flexible scope. This cycle removes more water during the cycle and allows more hydrogen peroxide to move through the lumen. In contrast, the 100NX has a standard cycle of 47 minutes and a flex cycle of 42 minutes.2 The advanced cycle in the NX is now called the “flex” cycle to indicate that a flexible scope can be processed in this cycle.

Objective 2 – Explain basic procedures to prepare instruments for plasma sterilization

Decontamination is the first step in any sterilization process, and the manufacturers’ processing instructions for all instruments and devices should be carefully followed. All items must be absolutely dry before being assembled and wrapped for plasma sterilization. Excessive moisture left in the instrument trays will lengthen the evacuation phase and can cause the load to cancel.

Compatibility of devices and materials to plasma sterilization must be verified prior to sterilization. Paper, cellulose materials such as muslin, powders, liquids, and devices with dead-end lumens are not indicated for sterilization in the Sterrad® process. There are restrictions relating to dimensions of cannulated items that can be sterilized with plasma because the time required for the vapor to travel through long and narrow lumens can exceed the length of the diffusion cycle. Note: refer to the manufacturer’s recommendations regarding cannulated instruments.

Sterile processing personnel must understand that absorbable materials including towels, gauze, paper, and other cellulose-based products cannot be present in a Sterrad® load. If hydrogen peroxide is absorbed during the sterilization process, the chamber will not rise to the appropriate pressure, and the load will cancel (stop). Note: if instrument count sheets are needed for operating room staff, they must be sterilized separately using an alternative method, and paper lot control stickers should be added to packages only after cycle completion. Also, if trays must be reinforced to prevent rips and tears, non-absorbent “Plasma-taz” material (not towels) should be used.

Non-woven polypropylene wrapping material can be used in plasma sterilization because it absorbs a minimal amount of hydrogen peroxide. However, the appropriate-sized wrapper is required to prevent cycle problems. Only pouches made with Tyvek® may be used for plasma sterilization because this material is permeable to hydrogen peroxide vapor, and it provides a good microbial barrier. Paper/plastic peel pouches are not permitted. The Sterrad® wrapping tape should not be overlapped on itself or across the chemical indicator as this will prevent color change beneath the tape.

Objective 3 – Provide an overview of the plasma sterilization process

There are five phases to the Sterrad® sterilization cycle: vacuum, injection, diffusion, plasma, and vent, and there are four stages to the process: pre-exposure, exposure 1, diffusion, and exposure 2. The pre-exposure stage includes vacuum and vent phases. First, air is pumped out of the chamber, and a vacuum is drawn. This phase also helps to remove moisture in the chamber. Pressure is controlled and measured by sensors in the sterilizer.

During the exposure 1 stage, a concentrated dose of hydrogen peroxide and water is injected into the deep vacuum chamber of the sterilizer. This aqueous (watery) solution of hydrogen peroxide is vaporized in the chamber so all items are covered by the hydrogen peroxide vapor. The lowering of pressure in the chamber generates the low temperature gas plasma. “Plasma is created when energy is applied to atoms with enough force to free the electrons. The resulting mixture of ions, electrons, and neutral atomic particles is known as plasma.” The plasma cloud interferes with the enzymes, cell membranes and nucleic acids of microorganisms and kills them.

Radio frequency waves create an electromagnetic field in the sterilizer chamber. As radio frequency energy is applied, the electrons are stripped from some of the molecules and, when they recombine, a visible glow is produced. Collisions between ions and molecules create free radicals (highly reactive molecules that contain an unpaired electron and from which the name “Ster-rad” is derived). Plasma and free radicals exist only while the radio frequency is turned on. When it is turned off, the free radicals recombine to form water vapor and oxygen. There are no odors or hazardous emissions during the plasma sterilization process.

Pressure rises because of the increase of molecules in the chamber, and this increased pressure diffuses the hydrogen peroxide sterilant into instrument lumens, trays, and peel pouches. Pressure is monitored during the diffusion phase to ensure that the items within the load do not contain materials that absorb an excessive amount of the vapor.

During the Exposure 2 stage, there is a second injection of hydrogen peroxide that repeats the exposure 1 process. The last phase of the plasma sterilization process is the vent phase in which filtered air enters the chamber to return it to atmospheric pressure. The combined use of hydrogen peroxide vapor and plasma sterilizes the items in the chamber in 45–75 minutes, depending on the chamber size and model. The vent phase concludes the sterilization process, and the instruments are ready to be removed for immediate use or storage.

Objective 4 – Discuss basic procedures to initiate the plasma sterilization cycle

Liquid hydrogen peroxide used for plasma sterilization is packaged in a sealed cassette with a clear plastic wrap. A visible chemical indicator is integrated into the package label to alert Central Service technicians about possible leaks. If personnel make contact with concentrated hydrogen peroxide, they should immediately flush the exposed area with large amounts of water. Information in applicable Material Safety Data Sheets (MSDSs) and a physician should be consulted immediately. Cassettes should be stored away from other sterilizers and from light. Each cassette’s expiration date is printed on the side of its box and; if it is past this expiration date, the cartridge should not be used. Plasma sterilizers “recognize” when the correct cassette has been inserted to avoid the use of incorrect cassettes.

Area monitoring can be done, but is not required. The Occupational Safety and Health Administration (OSHA) Permissible Exposure Limit (PEL) is 1ppm in an 8-hour Time Weighted Average (TWA). Testing has shown that hydrogen peroxide emission levels are significantly less than the OSHA limit just cited. Therefore, routine monitoring of hydrogen peroxide is not required.

To initiate a cycle, all trays should be placed flat on the sterilizer’s shelf without stacking to allow the hydrogen peroxide to circulate around all packages. Pouches should be placed on edge in an open tray, and they should be positioned so that the plastic of one pouch faces the opaque side of the next pouch to allow the sterilant to easily enter the pouches. Do not allow packages to touch the walls, door, or electrode of the chamber, and there should be at least one inch of space between the chamber’s ceiling and the top of the load.

Each Sterrad® cassette contains enough hydrogen peroxide ampules to run more than one load. Model 200 can operate for two cycles, and cassettes for the other four models have a five cycle capacity. Sterilizer units alert the operator when a new cassette must be inserted in the unit. At the completion of the cycle, the operator should verify that the chemical indicator tape has changed and that the printout tape indicates all parameters were met.

Objective 5 – Describe basic procedures to monitor the plasma sterilization process

The Sterrad® sterilizer automatically monitors and controls the sterilization process in several ways. The message screen indicates the unit’s status at all times. The status indicator lights indicate the current phase of the cycle: vacuum, injection, diffusion, plasma, and/or vent, and there are a series of beeps that occur when the cycle is completed or cancelled. The paper printout records the phases, pressures, and times of each stage in black, and the cancelled cycles are printed in red to alert the operator about an error.

A Chemical Indicator (CI) strip and appropriate sterilization wrapping tape should be used on the inside and the outside of the wrapped tray. Chemical indicators react to the presence of hydrogen peroxide, and their color changes from red to a golden yellow. Note: a golden yellow color means that the wrapping tape or indicator strip was exposed to hydrogen peroxide and, if the chemical indicator remains red, it was not exposed to hydrogen peroxide. Any color darker than that on the strip should be rejected, and the contents should be reprocessed. Color change of the CI should be noted at the completion of the sterilization cycle.

The CIs should be protected from sun and room light because long-term exposure to light can cause the indicator to change color before sterilization. The indicators should be stored in and used directly from their original box.

Biological (BI) testing should occur at the beginning of each day. ASP’s Sterrad® Cyclesure biologicals contain Geobacillus Stearothermophilus to challenge the sterilization process. The BI should be labeled with the date, sterilizer, and load number, and it should be placed inside a Tyvek® pouch before placing in the load on the back of the bottom shelf with the white Tyvek® side facing up. Pouches may be placed on top of wrapped trays, but do not place trays or pouches on top of the BI pouch.

The BI should be incubated within five minutes of the cycle’s completion. While the BI is still in its pouch, the cap should be pressed down until it is firmly seated to ensure that BI contamination does not occur after the cycle. The BI can then be removed from the pouch while keeping it in the upright position at all times. The BI tube crushing tool should be used to gently squeeze the BI until the media has been crushed, and the BI should then be placed upright in the incubator.

An unprocessed Cyclesure BI should be labeled with the date and a “C” to indicate it is a positive control. While holding the vial upright, its cap should be pressed closed. The vial should be crushed using the crusher tool, and placed in the incubator next to the processed BI. Biologicals should be checked at 48 hours for color change, and results should be recorded. The processed BI should have a golden yellow top with purple solution inside. The unprocessed Cyclesure control BI should have a red top with yellow solution to indicate internal growth that is expected for the positive control BI. Biologicals should be discarded in a biohazard container after recording the results in the BI log book.

Effective documentation ensures that the sterilization process is monitored as it occurs, that required cycle parameters have been met, and that accountability is established. Each sterilizer should have complete documentation for each load that includes the date, sterilizer number, sterilizer load number, sterilizer operator, contents of the load, and initialed printout tape of cycle parameters. Sterilization records are considered legal documents and, since the length of time they must be kept varies, each healthcare facility must develop record-retention policies based upon state and local regulations and legal considerations.

In Conclusion

Central Service technicians must understand the procedures required for the safe and efficient use of plasma sterilization. As with all sterilization methods, it is always important to follow the requirements established by manufacturers of the equipment and the reusable instruments and devices to be processed within them. As this is done, some of the ever-increasing challenges associated with the processing of today’s complicated equipment can be better addressed to best assure the safety of the patients being served.

Endnotes

1. To learn more about plasma at a website written for the general public, go to: www.plasma.org.

2. Cathy Rocco. Clinical Education Consultant, Advanced Sterilization Products. Hydrogen Peroxide Low Temperature Plasma Sterilization Technology. (PowerPoint Presentation)

3. Adapted from reference #2 above.

4. AAMI Standards and Recommended Practices. Sterilization Part I. Sterilization in Health Care Facilities. Arlington, VA. 2006. (Page 669)

5. AAMI Standards and Recommended Practices. Sterilization Part I. Sterilization in Health Care Facilities. Arlington, VA. 2006. (Page 671)

6. Sterrad® 200 Sterilization System Education Manual. LC-521 65-0-001. Rev. A. 2004.

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