7.3 Autoclave Operation and Sterilization Quality Control

Autoclave Operation and Sterilization Quality Control

The autoclave is the most common and reliable method of sterilization used in veterinary clinics. It utilizes saturated steam under pressure to destroy all microbial life. The veterinary assistant must understand how to operate the autoclave correctly and perform quality control testing to ensure sterilization is achieved.

Principles of Steam Autoclaving

Steam sterilization works by denaturing and coagulating microbial proteins. Water boiled at normal atmospheric pressure reaches 212°F (100°C), which is insufficient to kill highly resistant bacterial endospores. By sealing water inside a chamber (the autoclave) and heating it, pressure increases. This pressure allows the steam to reach much higher temperatures.

• Standard Gravity-Displacement Parameters: For wrapped packs, the standard sterilization cycle parameters are 250°F (121°C) at 15 psi of pressure for 15 to 30 minutes.
• High-Vacuum (Pre-Vacuum) Parameters: These autoclaves actively vacuum air out of the chamber before injecting steam, allowing faster steam penetration. They run at 270°F (132°C) at 30 psi of pressure for 3 to 4 minutes (often used for flash sterilization of single, unwrapped instruments).
• The Drying Cycle: After the sterilization cycle, the autoclave chamber must vent its steam. The packs are wet from condensation. The autoclave must run a drying cycle for 15 to 20 minutes (often with the door cracked open 1/2 inch). Safety Hazard: Touching or moving wet packs will cause moisture to wick bacteria through the wrap (strike-through contamination). Packs must be completely dry and cool to the touch before they are handled or stored.

Autoclave Sterilization Cycles

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Loading the Autoclave

How packs are loaded inside the autoclave chamber directly affects whether steam can penetrate them:

• Vertical Placement: All packs must be loaded vertically (on edge) on the racks, parallel to the flow of steam. Do not stack packs flat or on top of one another, as this traps air pockets and prevents steam circulation.
• Spacing: Leave 1 to 2 inches of space between packs and between packs and the chamber walls to allow steam to circulate freely.
• Peel Pouches: Paper-plastic pouches should be placed in specialized racks on edge, or flat with the paper side facing down (to allow steam to penetrate and condensate to drain out).
• Overloading: Overloading the chamber is the most common cause of autoclave sterilization failure because it prevents steam from reaching the center of the load.

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Quality Control Indicators

The veterinary assistant must monitor and document autoclave runs using three types of quality control indicators:

1. Physical/Mechanical Indicators: This involves watching and logging the temperature, pressure, and time gauges during the cycle. While physical readings show the machine is functioning, they do not prove that steam actually penetrated the center of the packs.
2. Chemical Indicators:
   • Class 1 (External): Autoclave Tape: Placed on the outside of wrapped packs. It features chemical lines that turn black when exposed to high temperatures. Crucial Exam Fact: Autoclave tape only indicates that the pack has been exposed to heat (i.e., it went through the autoclave). It does NOT prove sterility, that the target temperature was held for the required time, or that steam penetrated to the inside of the pack.
   • Class 5/6 (Internal): Chemical Integrators/Strips: Placed inside the center of each pack prior to wrapping. These strips contain chemical reagents that change color only when they have been exposed to steam, temperature, and time. If the strip does not show the correct color change, the pack is considered contaminated and must be re-wrapped and re-sterilized.
3. Biological Indicators (Spore Tests):
   • This is the gold standard of sterilization verification because it tests the actual destruction of living microbial life.
   • It uses vials of Geobacillus stearothermophilus bacterial endospores, which are highly resistant to moist heat.
   • Process: A vial containing the spores is placed in the center of a pack and positioned in the coldest or most difficult-to-sterilize part of the autoclave (typically on the bottom shelf near the door). After the cycle is complete, the vial is retrieved and incubated at 55°C to 60°C for 24 to 48 hours. A control vial (from the same batch, which was not autoclaved) must also be incubated alongside it.
   • Interpretation:
     • No Growth (Pass): The autoclaved vial remains its original color (typically purple) and is clear. The control vial must change color (typically yellow) and turn turbid, proving that the spores were viable before autoclaving.
     • Growth (Fail): If the autoclaved vial changes color to yellow and turns cloudy, the sterilization cycle failed. This means the spores survived. The autoclave must be taken out of service, and all packs processed since the last successful spore test must be recalled and re-sterilized.
   • Frequency: Spore tests should be conducted at least weekly, and in every load containing implantable items (like orthopedic pins or plates).

Comparison of Sterilization Indicators

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Pack Storage and Expiration

• Time-Related Expiration: Historically, clinics assigned strict expiration dates to packs based on the wrap material. For example, double-wrapped muslin is considered sterile for 3 weeks on an open shelf or 7 weeks in a closed cabinet. Paper-wrapped packs are sterile for 8 weeks, and paper-plastic peel pouches are sterile for up to 1 year.
• Event-Related Expiration: The current standard of care in modern veterinary medicine. This policy states that a sterile pack remains sterile indefinitely unless some event occurs that compromises the integrity of the wrapper. Compromising events include:
  • Tears, punctures, or rips in the wrap.
  • Moisture exposure or water stains (strike-through).
  • Packs being dropped on the floor.
  • Accumulation of dust or dirt on the package.
• Storage Environment: Sterile packs must be stored in clean, dry, dust-free, and moisture-free areas. Closed cabinets are highly preferred over open shelving to minimize dust accumulation. Storage rooms should be temperature-controlled (68°F to 72°F or 20°C to 22°C) with relative humidity between 30% and 60%. Practice a "First-In, First-Out" (FIFO) system to ensure older packs are used first.