Hot-air ovens produce a dry heat. Microorganisms are more resistant to dry heat so high working temperatures are required for a long period of time (Table 7.1). Long cooling periods are also required and the very high temperatures may damage metal items. A safety device should be fitted to the door to prevent accidental opening before the oven is cool. Care should be taken not to overload the oven, as air will be unable to circulate freely.

Table 7.1

Hot-air ovens: working temperature and time ratios

Item	Temperature (°C)	Time (min)
Glassware and non-cutting instruments	180	60
Powders and oils	160	120
Sharp cutting instruments	150	180

Use is limited by the long period of time required for sterilization and cooling. However, hot-air ovens are useful for items damaged by moist heat such as glassware, powders, oils and sharp cutting instruments.

AUTOCLAVE

This is the most common method of sterilization used in veterinary practice. In normal circumstances, water cannot reach temperatures greater than 100 °C (boiling point) before producing steam. If water is boiled under pressure, the boiling point is raised so the temperature of the steam is greater. This steam produces heat which penetrates to the innermost layer of the packs. The moisture increases the permeability to the heat. Care should be taken to avoid overloading or blocking the inlet and outlet valves. Items to be autoclaved should be free from grease and protein in order to achieve effective penetration of steam.

The majority of autoclaves designed for modern veterinary practice incorporate a drying cycle. Steam is exhausted and replaced by filtered air which dries the packs (Table 7.2).

Table 7.2

Autoclaves: working temperature, time and pressure ratios

Pressure (kg/cm²)	Pressure (psi)	Temperature (°C)	Time (min)
1.2	15	121	12
1.4	20	126	10
2	30	134	3.5

Autoclaves are used for sterilizing instruments, drapes, gowns, swabs and some rubber or plastic items.

ETHYLENE OXIDE

Ethylene oxide gas sterilizes by inactivating the DNA in the cells of the pathogen, thus preventing their replication. It is, however, toxic, irritant to tissues and very inflammable. In order to comply with Control of Substances Hazardous to Health (COSHH) regulations manufacturer’s instructions must be followed.

The sterilizer, a plastic container fitted with a ventilation system, should be located in a well-ventilated area, such as a fume cupboard away from working areas. Room temperature must be kept at a minimum of 20 °C during the cycle.

Procedure: Use of the ethylene oxide sterilizer

1. Action: Place individually packed items into a polythene liner bag.

Rationale: Liner bags are supplied with the sterilizer.

2. Action: Place a scored ampoule containing ethylene oxide liquid inside the liner bag and seal the bag with a metal twist tie.

Rationale: The bag must be sealed in order to keep the gas circulating around the contents.

3. Action: Put the liner bag into the sterilizer unit.

4. Action: Snap the ampoule from outside the bag to release the gas.

Rationale: To minimize exposure to gas.

5. Action: Close and lock the door to the sterilizer unit and turn the ventilator on.

Rationale: Accidental opening of the unit can be prevented if the unit is locked.

6. Action: After 12 hours turn on the pump.

Rationale: This aerates the unit before it is safe for the operator to open.

7. Action: Two hours after aerating, remove sterilized items.

Rationale: This will ensure that any toxic gas has been removed.

8. Action: Store items for a further 24 hours in a well-ventilated room.

Rationale: This makes sure that all the ethylene oxide has dissipated.

Sterilization by ethylene oxide is suitable for anaesthetic tubing, endotracheal tubes, fibreoptic equipment, optical instruments, plastic items such as catheters and syringes, high-speed drills and battery-operated drills.

Everyday items such as instruments, gowns and drapes may also be sterilized in this manner but the length of the cycle restricts its use.

RADIATION

Sterilization is achieved using gamma irradiation. It can only be carried out under controlled conditions within industry. Many prepackaged items used in practice, such as needles, syringes and catheters, are sterilized in this way.

MONITORING THE EFFICACY OF STERILIZATION

It is essential that the effectiveness of any sterilization method be constantly monitored to ensure that all microorganisms, including bacterial spores, are destroyed. Different sterilization methods require different working conditions in terms of time and temperature. It is also important to choose the correct method of monitoring the efficacy of sterilization (Table 7.3).

Table 7.3

Methods of monitoring the efficacy of sterilization

Method	Description	Use
Chemical indicator strips	Paper strips which change colour when the correct temperature and time have been reached. They are placed in the centre of the pack prior to sterilization	Autoclave – select the correct strip for the cycle Ethylene oxide
Browne’s tubes	Small glass tubes filled with an orange liquid which turns green when the correct temperature is reached and maintained for the correct time	Autoclave Hot-air oven
Bowie-Dick indicator tape	A beige tape impregnated with chemical stripes that change to black when the correct temperature has been reached (121 °C). It does not indicate that the pack has been exposed for the correct time; therefore it is not a reliable method	Autoclave
Ethylene oxide tape	As above, only the tape is green with lines that change to red on exposure to ethylene oxide	Ethylene oxide
Spore strips	Strips of paper impregnated with spores (usually Bacillus stearothermophilius) are placed in the load. After sterilization, they are cultured for 72 hours. Provided that sterilization has been achieved, no growth will be visible. This is an accurate method, although the delay in obtaining the results is a major disadvantage	Autoclave Ethylene oxide Hot-air oven
Thermocouples	Electrical leads with temperature-sensitive tips which are placed in the autoclave with the leads passed out and attached to a recording device. The temperature is checked throughout the cycle and the results are recorded	Autoclave

PACKING MATERIALS FOR STERILIZATION

There are a number of different packing materials available for the preparation of items to be sterilized. Selection will depend largely on the method of sterilization but factors such as cost and personal preference may also be taken into account (Table 7.4).

Table 7.4

Packing materials for sterilization

Packing material	Advantage	Disadvantage	Method of sterilization
Self-seal pouches	Easy to pack Clear front to view contents Paper back with sterilization indicator Ideal for individual instruments	Puncture by heavy or sharp instruments (double packing will prevent puncturing but increase the cost)	Autoclave Ethylene oxide
Nylon film	Cheap May be reused Sealed with Bowie-Dick tape	Repeated use leads to brittleness and can cause tiny holes which can go unnoticed	Autoclave
Polythene bags supplied by ethylene oxide manufacturers	Easy to pack Strong	Overpacking can lead to poor gas circulation	Ethylene oxide
Linen drapes	Conforming Used to pack surgical equipment Strong Reusable	Permeable to moisture Require laundering Liable to wear	Autoclave (with a drying cycle) Ethylene oxide (if not too tightly packed)
Paper drapes	Water-repellent Disposable Used to pack surgical equipment	Non-conforming Can tear easily	Autoclave (with a drying cycle) Ethylene oxide (if not too tightly packed)
Metal tins	Long-lasting Useful for gowns, drapes, swabs, instruments Cannot be punctured	Expensive to buy Require a large autoclave Often multi-use, which may lead to contamination of contents	Autoclave (with a drying cycle) Hot-air oven
Cardboard cartons	Reusable Cannot be punctured easily Sturdy Useful for specialized kits	Expensive to buy Bulky to store	Autoclave (with a drying cycle)

Procedure: Packing an item for sterilization

1. Action: Select the appropriate packaging material for the method of sterilization to be used.

Rationale: A packing material that is non-permeable to steam would not be suitable for an autoclave.

2. Action: Select the correct size for the item to be sterilized.

Rationale: Some methods of packing, such as self-seal pouches, can be costly to use so do not choose too large a pouch.

3. Action: Label the pack with the contents.

Rationale: This will save opening incorrect packs, which would then require re-sterilization.

4. Action: Write the date on the pack.

Rationale: Sterilized items should be repackaged and sterilized again if not used within 3 months.

5. Action: Write the name of the person preparing the pack on the pack label.

Rationale: This allows any problems with the packing to be traced.

MAINTAINING THE THEATRE ENVIRONMENT

It is vital to have a strict cleaning regime in the operating theatre and preparation room to maintain a high standard of asepsis. Both daily and weekly cleaning procedures are essential. In addition to this, there are some general rules for maintaining asepsis in the theatre (Table 7.5).

Table 7.5

Maintaining an aseptic theatre

Action	Rationale
1. The least number of people should be present and movement kept to a minimum	1. Any movement will increase the risk of wound contamination by airborne particles
2. Personnel must wear the correct theatre attire at all times	2. This will avoid contamination from clothing, skin and hair
3. Clip and disinfect the patient away from the theatre	3. Hair and debris would contaminate the theatre
4. Use a new set of instruments for each surgical procedure	4. Cross-contamination between patients must be avoided
5. Carry out ‘clean’ operations before contaminated procedures	5. Contamination from high-risk procedures should then not occur
6. Discard any instrument that becomes contaminated	6. The remaining surgical instruments must remain aseptic
7. If asepsis is broken by any member of the surgical team it must be rectified	7. Further contamination can then be avoided