Sterilization Cycles: An Overview for Hospitals and Industry

There are four general types of sterilization cycles or "standard" cycles. 

 Sterilization Cycles At-A-Glance

Sterilizer performance can be effectively understood through a graphical expression of cycle function. The frequency and duration of vacuum and purge (conditioning), ramp-up, dwell and cool-down functions is not literally expressed herein, and depends on sterilizer configuration, control selection and programming. 

  Sterilization Cycle Phases

• Purge
  • Steam enters the chamber and displaces ambient air by expelling it through the chamber drain. This displacement can occur by gravity or with use of a vacuum system.
  • All outlet valves are OPEN during this phase.
  • Discussion
    • Air in the chamber acts as a barrier to adequate steam penetration and will negatively affect the sterilization process. A heavily loaded sterilizer will contain many "pockets" where air may become trapped. "Conditioning" aids in the removal of this air, helping to prevent stratification of the functional steam/air mixture.
    • Depending on what controller is selected, conditioning times can be programmed by the operator and can last from 30 seconds to several minutes.
    • The overall length of the Conditioning phase will depend on the following factors:
      • Quantity and pressure of the supplied steam.
      • Density of the goods in the chamber.
      • Chamber size.
• Pre-Vacuum (Vacuum System Required)
  • Ambient air is efficiently removed from the chamber and the load. This allows steam to completely penetrate the products being sterilized.
  • This phase is useful for porous instruments or wrapped goods.
  • Alternative steam and vacuum phases creates pressure/vacuum "pulses".
• Sterilization (Dwell)
  • Interior temperature and pressure are maintained at setpoint for a time specified by the controller or manual protocol.
• Drying (Cool Down)
  • The interior load returns to ambient temperature and pressure in any of three typical methods.
    • Gravity (Fast Exhaust)
      • This method allows the steam and condensate to exit through the chamber drain with no mechanical assistance. As the effluent discharges, the pressure and temperature in the chamber decrease. When the chamber pressure is near zero, the door can be opened and the goods may be removed. This method is simple, but not the fastest.
    • Gravity (Slow Exhaust)
      • For liquids. The cool down is gradual to permit dissolved gases in the liquid media to reach equilibrium; this avoids boilover until ambient temperature and pressure are restored.
      • Liquid, air-over-pressure. The cool down is gradual as pressurized air is injected to replace steam, thereby lowering the interior temperature under pressure and preventing smaller media samples from evaporating. Once temperature is reduced, air pressure is gradually equalized and ambient temperature and pressure are achieved.
    • Vacuum Drying (Post Vacuum)
      • A vacuum system pulls the steam and condensate through the drain port. The longer the vacuum system runs during the dry phase, the cooler and dryer the goods will be when removed from the chamber.
      • There are three different methods that this system may be set to run:
        • Programmed by time
        • Programmed until a set vacuum pressure is achieved.
        • Programmed to start when a set vacuum pressure is achieved.
      • Vacuum exhaust should not be used on sterilized liquid because a quick reduction in pressure will cause the liquid to boil over.

 Sterilization Cycle Parameters

Many first-time sterilizer users want to know how to select the best cycle parameters for their applications. The answer depends on at least four factors:

• Size of load
• Type of load (waste, media, glassware, or other)
• Time and throughput constraints
• Steam source (house steam, integral steam generation or other)

Guidelines for optimum cycle types are suggested in the chart below. 

 Minimum Recommended Settings for Various Liquid Volumes 

The following chart suggests minimum exposure times. Additional time may be required if flasks are placed in a plastic container. Best practice suggests using smaller volumes whenever possible. Larger liquid volumes will take longer to achieve temperature. 

 Disclaimer

The content of this issue is presented for general information only as perceived by the writer of the article. Reader should independently verify the correctness and accuracy of the material before using the same for any purpose whatsoever. The Company or Management or Staff of Puneet Industries or the Writer shall not be responsible for any inaccuracy or error in the article.

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