Use of saturated steam under pressure is the safest and dependable method of sterilisation, in universal use for destruction of all forms of microorganisms.
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The higher the temperature the more rapidly is sterilisation accomplished. The minimum time and pressure for sterilisation by autoclaving process is 121 °C at 15 lbs psi in 15 minutes.
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If the temperature is raised to 126°C, the time can be reduced to 10 minutes. At 134°C, it is 3 minutes, and only a minute at 150°C.
The last process is utilised in “flash sterilisers” installed in some operation theatres for spot sterilisation of urgently required instruments during operation.
Incomplete penetration of steam can occur because of overloading the autoclave or incorrect positioning of packages inside the autoclave. There is no such thing as “almost sterile”. An object is either sterile or not sterile.
1. Quality Control: Sterilization Checks:
In spite of the process instruments fitted on autoclaves which give a graphic record of temperature and pressure, routine methods of check for sterilisation must be incorporated. Colour index strips available for this purpose should be kept in each pack being autoclaved. Manufacturer’s instructions must be followed scrupulously regarding loading of autoclaves, temperature and pressure levels and timings.
2. Sterility Indicators:
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Mechanical indicators which are monitoring instruments record time, temperature, humidity and pressure during the sterilisation cycle
Chemical indicators are devices with a sensitive chemical or dye to monitor one or more parameters of a sterilisation cycle.
Biological indicators employ the principle of inhibition of growth of microorganism of high resistance to the mode of sterilisation. Subsequent failure of growth of microorganisms indicates adequacy of sterilisation.
Articles should not be considered properly treated unless the time, temperature and pressure indicators indicate that the required time, temperature and pressure were reached during the autoclaving process.
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If not, the entire load of articles must be autoclaved again, until the required temperature pressure and residual time are achieved again.
No single method is capable of monitoring completely all parameters necessary for a fool-proof sterilisation.
Recording instruments on the autoclave which give a progressive graphic record of temperature, pressure, etc. can be inspected immediately after a load has been autoclaved.
In addition, chemical indicators change colour when conditions necessary for sterilisation have been met.
These are available as tapes and strips, and are attached or implanted inside the packing material.
This should be supplemented with periodical use of biological indicators to detect (which may take several days) failure of the autoclave.
To achieve a high degree of certainty that the autoclave is functioning properly, biological indicators should be placed in the most inaccessible location in the load and then cultured.
They provide positive assurance, albeit retrospectively, that each package has been subjected to proper sterilising conditions.
3. Shelf-life:
The shelf-life of sterilised packs, i.e. the time for which it can be stored safely before use, should be determined by a committee of experts including a bacteriologist.
If the pack is not used, during its shelf-life, it should be again put through the autoclaving process without being opened.
Although a pack can remain sterile for up to four weeks on the CSSD shelf, experts are of the opinion that it should be reautoclaved without opening after one week.
4. Chemical Sterilisation by Ethylene Oxide:
Ethylene oxide (EO) is a gas which is now commonly used as sterilising agent for heat-sensitive and moisture-sensitive materials like rubber, plastics and fiberglass. Effective sterilisation by EO depends upon the following.
Concentration of gas: 450 mg/liter or higher is essential for complete sterilisation.
Temperature: Exposure time can be reduced by increasing the temperature. Two temperature ranges are generally provided in the EO sterilisers 49° to 63°C and 30° to 37.8°C. Humidity
In automatic sterilisers, steam is injected under vacuum before admitting the gas. In some a wet gauze or sponge is required to be placed.
Other sterilizers depend on ambient humidity, and compensate for lower moisture by a higher exposure time.
Packing: The type of wrapping used should be penetrable by ethylene oxide and water vapour. Polyethylene is commonly used for loose wrapping.
Period of Exposure:
In automatic sterilisers, the time ranges from 110 to 260 minutes. Up to 12 hours may be required in others.
Manufacturer’s instructions must be followed meticulously as regards above parameters for effective sterilisation of different types of equipment.
Ethylene oxide sterilisers generally fall into two categories-manually operated and automatic.
The source of the gas is provided by large compressed gas cylinders or by a unit dose cartridge sufficient for one cycle.
Simple sterilizers carry out sterilisation at room temperature and ambient humidity, with single-use glass ampoule of ethylene oxide seated inside a gas-release bag.
On breaking, liquid ethylene oxide vaporizes and diffuses out of the gas-release bag into a larger bag in which the material to be sterilized has been placed.
This bag performs the function of diffusing chamber and allows the gas to retain long enough to achieve sterilisation.
5. Sterilization Process:
After the steriliser chamber is sealed and the controls set, sterilisation process goes through the following phases:
1. Warming the chamber
2. Evacuating residual air to partial vacuum
3. Introduction of moisture to ensure that it penetrates wrappings and material
4. Introduction of EO
5. Raising the temperature (if required)
6. Exposure for the required time
7. Release of chamber pressure
8. Removal of the gas under vacuum (called the “Purge cycle”)
9. Reestablishment of atmospheric pressure by introduction of filtered air into chamber.
6. CSSD Distribution System:
Distribution system for sterile article should also be decided at the planning stage itself. The following four systems are in use. The last two are door deliveries. Clean for dirty-exchange: A sterilised articles is issued at the CSSD issue counter on return of a used one at any time.
Requisition system:
Articles needed are requisitioned by users on a daily or regular basis and collected by them at CSSD counter.
Grocery system:
Each user’s requirements for all items for a specific period are delivered at suitable interval irrespective of whether the contents have been used or not and the previous basket or container withdrawn.
Quota system:
Predetermined stock levels (quota) of various items for each user unit are maintained by delivery personnel from CSSD through regular deliveries.
The choice of system will ultimately be dependent on local circumstances. A combination of the systems may be more appropriate in most hospitals.
Depending upon the size and requirement of each hospital, the department may have to work in more than one shift.
If the department works on only one full shift, then provision for issues of required packs outside of normal working hour will have to be made.
In smaller hospitals, this task may be assigned to the casualty or emergency department who would then be stocked at levels over and above its own requirement.