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Methods of sterilisation of surgical instruments (Dry heat - Hot air oven,…
Methods of sterilisation of surgical instruments
Dry heat - Hot air oven
Less effective at killing micro-organisms than when heat is produced with moisture. For it to be effective, far higher temperatures are required.
The hot air oven is a small unit, heated by electrical elements which is relatively inexpensive. It has a safety device which prevents the oven being opened until cooled
Sterilisation time starts once the correct operating temperature has been reached. High temperatures are required usually between 150-180 degrees centigrade. The cycle consists of
Heating the chamber to the selected sterilising temperature
Sterilising the load
Cooling the load
Advantages of the hot air oven:
Excellent sterilisation for the following:
Metal needles
Cutting/ ophthalmic instruments
Powders/oils
Waxes
Disadvantages of the hot air oven:
Cycle takes a long time
Instruments need to cool before use
Fabrics burn
Rubbers scorch
Plastics melt
Steam - The autoclave
The principle of the autoclave is the use of steam under pressure. A given high temperature is achieved for a specified time enabling sterilisation to take place. As a result of the presence of moisture (steam), the temperatures required are not as high as those in the hot air oven
The autoclave process consists of:
Removing air in the chamber
Sterilising the load with steam under pressure
Removing the steam
Drying the load (very important as if the packages are wet at the end of the cycle, microorganisms are able to contaminate the package and corrosion of instruments may occur)
Restoring the chamber to normal atmospheric pressure
Vertical pressure cooker type autoclave
Operates by boiling water in a closed container
It has an air vent at the top which is closed once the air is evacuated and pressure builds up
The main disadvantages are that air will be trapped underneath the steam. The subsequent temperature in this area remains low, and therefore sterility is not guaranteed. Secondly, it is manually operated and is therefore subject to human error
Horizontal or vertical downward displacement type autoclave
A type of autoclave that is larger and fully automatic. It uses an electrically operated boiler that is incorporated within the autoclave and provides a source of steam
These machines are designed for loose instruments only as they have insufficient penetrating ability
The disadvantage with these machines is that packs appear dry, but frequently they are damp, encouraging the entry of micro-organisms during storage
Vacuum assisted autoclave
Works on the same principle of other two systems but uses a high vacuum pump to evacuate air rapidly from the chamber at the start of the cycle
Steam penetration is almost instantaneous once air is evacuated, and sterilisation occurs quickly. A second vacuum cycle withdraws moisture after the sterilisation process and dries the load
An advantage of this type of autoclave is that it has a fail safe system, if sterilisation is not completed. It is suitable for instruments and drapes
The holding time for sterilisation is stated when the autoclave chamber reaches a given temperature and is maintained for a given time
Maintenance
All autoclaves should be serviced by a qualified engineer. Vacuum assisted autoclaves should be serviced every 3 months to comply with health and safety regulations
Thermocouple testing is recommended at least annually to ensure effective sterilisation takes place
Ethylene oxide
This method is classed as
disinfection
and not sterilisation. Ethylene oxide is released from a liquid
anprolene
. It acts by inactivating the DNA of a cell and therefore preventing cell reproduction. Bacterial spores are not killed.
A number of factors influence the efficacy of ethylene oxide including:
Temperature
Pressure
Concentration
Humidity
Exposure time: It is suggested that the room temperature is maintained at 20 degrees C whilst the cycle is running. Ethylene oxide has some limiting factors and is subject to COSHH regulations:
It is extremely toxic to skin and MM
It is volatile and extremely flammable
It can be explosive when mixed with air
Advantages:
Used for delicate equipment that is damaged by heat
Can be used on fibre optics
Can be used on high speed drills
Can be used on plastic tubing
Disadvantages:
Carcinogenic and toxic
Subject to many COSHH regulations
Need ventilation pump and special equipment
Cycle takes 12 hours to run and 12 hours to air
Cold chemical sterilisation
Should only be used in an emergency. Manufacturers will guarantee sterilisation following prolonged immersion (usually 24h).
Should only be considered as a form of disinfection. The items to be sterilised are placed in a sterile bowl, the cold chemical solution is then added so the entire object is immersed and a lid is placed on the top to prevent evaporation
Examples are:
Glutaraldehyde - acts against the protein of the cells. It is subject to COSHH regulations and is an irritant to MM - most endoscopes are sterilised in this way
Alcohol based solutions act by denaturation and coagulation of proteins. They also have bactericidal properties. Formaldehyde is limited in its use due to COSHH regulations imposed
Sterilisation Indicators
Bowie dick tape - has an indicator impregnated onto it. After being exposed to steam, 121 degrees it changes in colour to produce black stripes. It does not measure time at a given temperature and therefore does not confirm that sterilisation has taken place
Brownes tubes - there are 4 types of tube, each selected for the specific temperature involved. The tube contains a red dye that changes green once sterilisation is complete. They can be used in autoclaves and hot air ovens but are not commonly used anymore
TST (time, pressure, temperature) strips - paper strips that has an indicator at one end. The indicator changes from yellow to purple once sterilisation has taken place. Used in autoclaves and is very reliable. Different TST strips are available for varying time, temperature and pressure combinations
Spore test strips - are put through a sterilisation cycle and are then sent away and incubated for a minimum of 72 hours. The check is carried out bi-monthly. The disadvantage is that the equipment will have already been used on a patient once the results are back.
Thermocouples are the only method of measuring heat penetration time for steam and dry heat sterilisation. The test is completed annually by the manufacturers' service team