Autoclave

Steam sterilization involves heating water to generate steam in a closed chamber (Autoclave- in which drums and trays with the materials to be sterilized can be kept inside) producing moist heat that rapidly kills microorganisms. There is a mass heat transfer as the steam condenses. Use of saturated steam under pressure is the most practical , quickest, safest, effective method of sterilization, known for the destruction of all forms of microbial life, because of its 1.high penetrating capacity, and 2.it gives up a large amount of heat (latent heat) to the surface with which it comes into contact, and on which it condenses as water.

The advantages are: 1.the results are consistently good, and reliable, 2.the instruments can be wrapped prior to sterilization. 3.time efficient. 4.good penetration.

The main disadvantages are: 1.blunting and corrosion of sharp instruments, and 2.damage to certain rubber goods.

There are two types of autoclaves or steam sterilizers available: 1.Downward (gravitation) displacement sterilizer: This is non-vacuum type autoclave. 2.Steam sterilizers (autoclave) with pre and post vacuum processes.

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  Rectangular-medical-autoclave-cutaway

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  Cylindrical-research-autoclave-illustration

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  Dry sterilizer , Autoclave

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  Autoclave stove top

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  Systec H-Series Autoclaves

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  Pass-through-autoclaves

Steam Sterilizers (Autoclave) with Pre and Post Vacuum Processes

The sterilization process is composed of 3 main phases: 1. Pretreatment Phase/Heat-up cycle All air is virtually expelled by a number of pulses (atleast 3) of vacuum and the introduction of steam, so that the saturated steam can affect the goods during second phase. 2. Sterilizing phase/sterilization cycle: The temperature increases sufficiently upto the degree at which sterilization is to take place. Actual sterilizing period, is also called “Holding Time”, starts when the temperature in all parts of the autoclave chamber and its contents (The Load) have reached the sterilizing temperature. This should remain constant within specified temperature band, throughout the whole sterilization phase. (Plateau/ Holding time). 3. Post treatement phase/depressurisation cycle and drying cycle: In this phase either the steam or the revaporized condensed water is removed by vacuum to ensure that the goods are dried rapidly.

Precautions

Trapped air pockets in the load prevent steam penetration during sterilization of porous material such as textiles and hollow items with long narrow lumina. A single vacuum pulse is inadequate for wrapped/ hollow/porous loads. Several,( at least 3) prevacuum pulses are needed to define preset vacuum level. There are three major factors required for effective autoclaving: pressure, temperature and time.

Pressure It is expressed in terms of psi (pounds per square inch) or kpa (kilopascals; kpa = 0.145 psi).

Temperature To achieve required pressure, the temperature must be reached and maintained at 121°C (250°F). With the increase in temperature and pressure, superheated steam is formed. The exposure to this superheated steam brings about the sterilization. This steam which is lighter than air rises to the upper portion of the autoclave as more steam is formed, it eliminates air from autoclave. The purpose of complete elimination of air is to help superheated steam penetrate the entire load in the autoclave and remain in contact for the appropriate length of time.

Time Wrapped loads (paper wraps or a thin cloth) require a minimum of 20 to 30 minutes after reaching full temperature and pressure. By selecting the correct temperature and time cycle, a wide variety of materials can be sterilized by this method: dressing packs, surgical instruments, laboratory equipment, culture media and pharmaceutical products. Liquids can be sterilized by autoclave, provided that the contents do not get inactivated by the high temperatures. Higher temperatures and greater pressures shorten the time required for sterilization. At 15 psi pressure, the temperature of steam can reach 121°C. The time required to kill all living organisms at this temperature is 15 minutes. At 126°C for 10 minutes, and at 134°C for 3 minutes sterilization can be completed. The time required for sterilization for a particular item also varies with the amount of material and the thickness of the wrap. Perkins (1969) has discussed the instrument wrapping techniques and the placement of the packs in the autoclave. The superiority of steam over hot air as a method of sterilization is due to the following

• Moist heat acts by denaturation and coagulation of enzymes and proteins; whereas dry heat acts by destructive oxidation of cell constituents. Moist heat kills more rapidly and at much lower temperature than dry heat.
• With the condensation of a large amount of latent heat to the surface. With steam condensation, besides emission of large amount of heat, there is a great contraction in it‘s volume. This results in production of negative pressure, and brings more steam to the site. Soon the temperature of the surface is raised to that of the surrounding steam. Thus, steam acts more quickly in heating exposed articles than hot air.
• Steam penetrates better than hot air. This is, partly due to its density, which is half that of air; and partly due to negative pressure.

Presently, two types of autoclaves are available:

1. Porous load autoclaves: To achieve complete sterilization, 121°C at 20 psi are maintained for 20 minutes. The articles which can be sterilized by this method include: towels used for draping, suture materials, cotton rolls rubber gloves, etc.
2. Small portable bench model type autoclaves: For complete sterilization, 136°C at 32 psi are maintained for 5 minutes. The steam entering the top of the chamber penetrates the material in a top to bottom flow.

Sterilization cycles that operate at a high temperature for shorter times and are indicated for unwrapped and solid instruments which need to be used immediately. Standard sterilizing conditions- 3-10 minutes at 273 degree F. It invariably results in residual air in the autoclave chamber To avoid or minimize corrosive action of steam on metals, the use of following agents have been reported. Crawford and Oldenburg (1967) recommended the addition of ammonia to the autoclave. Accepted Dental Therapeutics (1977) recommends use of dicyclohexylammonium nitrate or cyclohexylamine and decylamine. Bertolotti and Hurst (1978) recommend 2 percent sodium nitrate.

Packaging OR Wrapping Instruments for Autoclaving

Packing instruments before sterilizing prevents them from becoming contaminated after sterilization till it is opened and used. Instruments must be clean, but not necessarily dry. Closed (non-perforated) containers (closed metal trays, capped glass vials) and aluminium foils cannot be used, because they prevent the steam from reaching the inner sections of the packs. Cassettes, drums,trays with opening on all sides may be used. Packaging used for autoclaving must be porous, to permit steam to penetrate through ; and reach the instruments. The materials used for packaging could be fabric or sealed biofilm/paper/muslin cloth pouches, nylon bags and tubings, sterilization wrap, and paperwrapped cassettes. The bag or wrap is heat-sealed or sealed with tape. Pins, staples, and paper clips are not advisable; as they make holes in the wrap; and may allow microorganisms to pass through subsequently. In case, the pack of instruments is to be stored and not used shortly after sterilization, the autoclave cycle should end with a drying phase. The instruments stored in damp conditions result in tarnish, corrosion or rusting.

Marking of Packs

Packs should have some external indication, showing that the pack has been processed. Autoclave tapes on packs change colour after exposure to sterilization cycle. That does not prove sterilization, hence, it should be used in conjunction with the test for cycle sterilization. Each pack must be labeled with the contents, date of sterilization, autoclave number, and load number. This will help in locating processed items in case of recall.

Tests for Efficiency for Heat Sterilization

Spore testing Should be done once a week to verify proper functioning of the autoclave with the help of Geobacillus Stearothermophilus strips or vials.

Thermocouple This is a thermometric testing, and a reliable gauge of efficiency. One recording is taken from a thermocouple placed inside a test pack of towels and a second one from the chamber drain. Comparison between the two recordings gives a good guide regarding the speed at which the steam penetrates the load.

Brown’s test These are ampoules that contain a chemical indicator; which changes it’s colour; from red through amber to green at a specific temperature.

Autoclave tape This is a tape printed with sensitive ink that undergoes a colour change at a specific temperature. This test forms the basis of the Bowie-Dick test for high-vacuum autoclaves. Two pieces of strips are stuck onto a piece of square paper and placed in the middle of the test pack. With the application of temperature of 134°C for 3.5 minutes, there is a uniform development of bars throughout the length of the strips. This shows that the steam has passed freely and rapidly to the center of the load.

Storage of Sterile Goods

Crump (1966) reported that the storage of instruments is also a problem. The pattern of storage varies from place to place. They are either stored in drawers, or in containers, in packs or sterile trays. The maintenance of sterility during transportation and storage is of utmost importance. Packs should be stored with the following considerations:

1. Instruments are kept wrapped until ready for use.
2. To reduce the risk of contamination, sterile packs must be handled as little as possible.
3. Sterilized packs should be allowed to cool before storage; otherwise condensation will occur inside the packs.
4. To prevent contamination from rodents, ants, and cockroaches, the store must be subjected to adequate pest control.v. Materials should be stored at least 8” off the floor and 18” from the ceiling.
5. Sterile packs must be stored and issued in correct date order. The packs, preferably, are stored in drums which can be locked. Preset trays and cassettes are useful, as the instruments can be organized as per the procedure.