Sterilisation by steam
Instruments can be sterilised by steam under pressure using autoclaves. Vegetative bacteria, including tuberculosis, and viruses such as hepatitis B, hepatitis C and human immunodeficiency virus (HIV) and heat-resistant spores, including Clostridium tetani and Clostridium perfringens, are killed. The combination of pressure, temperature and time with the moist heat is important:
• 1340C (30 lb/in.2) for a hold time of 3 minutes;
• 1210C (15 lb/in.2) for a hold time of 15 minutes;
• prepacked materials and instruments are processed through a porous load autoclave which incorporates a prevacuum cycle necessary to extract air. If this is not achieved then the dried saturated steam cannot penetrate efficiently. Unwrapped instruments can be sterilised in a small autoclave within the theatre precinct, which is convenient when instruments are dropped.
All autoclaves must be regularly maintained according to the manufacturer’s instructions, and a record should be kept of the cycle time, the prevacuum phase, the pressure and temperature. In addition, the steam penetration test (Bowie—Dick test) and chemical indicators, for example Brownes tubes or impregnated tapes, are used to ensure that such errors as poor packing do not prejudice the efficiency of the process. Biological indicators are not appropriate.
Sterilisation by ethylene oxide
Ethylene oxide is a highly penetrative noncorrosive gas which has a broad-spectrum cidal action. It is utilised for heat-sensitive materials including electrical equipment. It is not recommended for ventilator respiratory equipment or soiled instruments because organic debris, including serum, has a marked adverse effect.
Sterilisation by hot air
This is inefficient compared with moist steam sterilisation, but it has the advantage in the ability to treat solid nonaqueous liquids grease/ointments and to process closed (airtight) containers. Lack of corrosion may be important, particularly with instruments with fine cutting edges such as ophthalmic instruments. It cannot be used for substances such as rubber, plastics and intravenous fluids which are denatured.
Sterilisation by low-temperature steam and formaldehyde
This uses a combination of dried saturated steam and formaldehyde, with the main advantage being that sterilisation is achieved at a low temperature (730C) and the method is therefore suitable for heat-sensitive materials and items of equipment with integral plastic components. It is not recommended for sealed, oily items or those with retained air. Some plastics and fabrics absorb formaldehyde, releasing this in a delayed manner which may cause hypersensitivity to the users.
Sterilisation by irradiation
This technique employs gamma rays or accelerated electrons. It is an industrial process and is particularly appropriate to the sterilisation of large batches of similar products, such as syringes, catheters and intravenous cannulas. The delivery of an irradiation dose in excess of 25 kGy is accepted as providing adequate sterility assurance.