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Infection Prevention
Inside Tabletop Steam Sterilizers
Janet Schultz
Publish Date: June 9, 2008   |  Tags:   Infection Prevention

Four Steps of Steam Sterilization

Most medical offices and some ambulatory surgery settings use tabletop steam sterilizers to process medical devices. Here's advice on operating and monitoring these space-saving units.

Properly loading your packages
During a steam sterilization cycle, the combination of moisture and heat denature protein, killing the microorganisms. To achieve the temperatures needed for sterilization (between 250-270? F for most tabletop units), steam is introduced into a closed chamber. As more steam enters, the pressure in the chamber rises. As the pressure increases, so does the temperature of the steam. Because air prevents steam from contacting surfaces, air must be eliminated from the chamber at the beginning of the sterilization cycle. Here's how the two types of tabletop steam sterilizers eliminate air.

  • Gravity displacement sterilizers. They rely on the force of gravity to remove air. Because air is heavier than steam, it sinks to the bottom of the chamber when steam is introduced. The sterilizer chamber drain remains open until the temperature in the drain is the same as the temperature of the steam entering the chamber, indicating that the air has been removed.

  • Pre-vacuum steam sterilizers. They pull air from the chamber by creating a vacuum in the chamber. This process rapidly and efficiently removes air. Pre-vacuum tabletop steam sterilizers are much more expensive than gravity displacement units and, as a result, are not as widely used.

    With both gravity displacement and pre-vacuum sterilizers, properly position items in the sterilizer so that air cannot be trapped. Never crowd or stuff items into the sterilizer. Crowding may interfere with both air elimination and drying. If the shape of an item would allow water to collect in any part, position that part on its side or upside down so the water can run out. If water can pool, then air could be trapped against that surface.

    Monitoring your sterilizer
    Once you've properly loaded your packages, you'll want to assure quality control during the cycle. Here's how to do that.

  • Distinguish processed from unprocessed. It's not always possible to tell if an item has gone through a sterilization cycle by its appearance. Seal each wrapped pack with a striped sterilization indicator tape; the stripes turn black when exposed to heat. This simple throughput type of chemical indicator does not indicate sterility, just that the item has been through a process of high heat.

  • Monitor wrapped items with a chemical indicator. Include in each package a chemical indicator that you can examine once the package is opened onto the sterile field. Make sure you know not only how to determine if the indicator has reacted, but what that reaction is telling you about conditions in the sterilizer.

  • Monitor sterilizers with a biological indicator. You should do this at least weekly with a biological indicator (BI) to verify that sterilization conditions are present during the cycle. BIs use live dried spores of the organism Geobacillus stearothermophilis, formerly known as Bacillus stearothermophilis, because they are very resistant to being killed by high heat and moisture. If these spores are killed, then all forms of disease producing bacteria and viruses are also going to be inactivated. There are several types and brands of biological indicators: spore strips in glassine envelops that require careful handling to prevent inadvertent contamination after sterilization, self-contained biological indicators that contain not only spores but also the culture media to allow growth if there are survivors and disposable test packs that contain a biological indicator and material that provides an air removal challenge. These traditional biological indicators require two to seven days of incubation at 131? F (the test organism does not grow at normal body temperature, so routine laboratory incubators cannot be used).

    Whenever you use a test biological indicator, you should also incubate an unexposed control to demonstrate that the spores used in the test were still alive. Make certain to read and follow the indicator manufacturer's instructions for handling, storage, incubation and interpretation. Commercial mail-in biological monitoring services eliminate the need for on-site incubation. Biological indicators with early enzyme readout let you determine cycle effectiveness on-site in about 3 hours, but they require use of a special reader.

  • Run an air removal test. If you're using a pre-vacuum steam sterilizer, you'll also need to run an air removal test or Bowie-Dick type test to verify that the air-removal system is functioning. Perform this test every day that the sterilizer is in use. If your sterilizer fails the Bowie-Dick type test, you should not use it until it is repaired.

  • Monitor time, temperature and pressure. The most important way to monitor your tabletop sterilizer is to pay attention to such physical conditions during the cycle as time, temperature and pressure. Some newer tabletop sterilizers include a recorder or a printer that documents the cycle. Examine the chart or strip at the conclusion of the cycle to make certain that conditions were within manufacturer's specifications. If there is no recorder, you must take the time to check the temperature and pressure gauges during the exposure phase to make certain that proper conditions have been attained.

    Attention to detail
    With careful attention to detail in loading and cycle monitoring, steam sterilization will remain the primary method of sterilization for reusable surgical devices and instrumentation well into the future.

    Janet K. Schultz, RN, MSN, ([email protected]) is a consultant on aseptic practice and instrument processing in the perioperative care setting.