Preparing for surgery – equipment

Unlike the animal and the surgical team, which are disinfected, most equipment used during surgery can be sterilised to remove all organisms. Sterilisation or disinfection can be achieved in a number of ways depending on the degree of cleaning required and the type of equipment to be cleaned. The methods fall into two broad categories:

1) Physical sterilisation

• This is generally more reliable than chemical methods
• Three methods exist
  • thermal energy
  • radiation energy
  • filtration

2) Chemical sterilisation

• This is usually accomplished using ethylene oxide (gas)
• Various liquid disinfectants can be used and are often the only rapid method available in field situations (where there is no option of using of a more reliable method)
• This tends to be less efficient than physical means
• Two methods exist
  • gas
  • liquid

Before sterilisation is undertaken several factors need to be considered:

• Chemicals or steam do not penetrate, and may be inactivated by, contaminating debris (e.g. dried on blood) so items to be sterilised must be very clean
• The concentration of chemical solutions must be correct, it doesn’t necessarily follow that a high concentration will increase the effectiveness
• All methods take time, some minutes, others hours, some days (e.g. ethylene oxide) so sufficient time must be allowed before surgery for instruments to be sterilised
• Many chemical methods employ irritant substances that must be allowed to disperse or be completely rinsed off before use. If they are not, damage to either the operator or the animal may occur.

Methods of physical sterilisation

a) Thermal energy e.g. boiling water, steam sterilisation in an autoclave (wet heat) or 160˚C for 1 hour in a hot air oven (dry heat)

Wet heat

• Boiling water: at atmospheric pressure water boils at 100˚C but this temperature is not sufficiently high to destroy certain viruses and spores. This technique is best considered disinfection rather than sterilisation. If it must be used all items should be completely immersed in at least 2cm of water and boiled for 20 minutes. If possible a sodium bicarbonate (2%) or sodium hydroxide (0.1%) solution should be used as this significantly increases the effectiveness of sterilisation and decreases the corrosive effect of boiling water on metal instruments. These solutions are not appropriate for use when boiling glassware or rubber items, as they are corrosive. Boiling is unsuitable for fabrics.
• Steam sterilisation: the steam in an autoclave is maintained under pressure in order to achieve a higher temperature. Autoclaving with moist heat in the form of steam, under pressure, is the most reliable way known for destroying all contaminating microbes and is the preferred means of sterilizing instruments and other apparatus not affected by the steam and high temperature, such as drapes, gowns, swabs and some glassware. Surgical kits are placed in commercially produced packing material or wrapped in cloth drapes and placed inside the autoclave. Items should be positioned in the packs to allow complete penetration by steam, and fabrics should not be too tightly folded. Steam displaces air in the autoclave, or a vacuum is used to evacuate the air from the autoclave before the sterilisation cycle begins. This is because air inhibits the diffusion of steam and reduces the temperature that can be achieved. The steam penetrates each pack, heating the contents through condensation and leaving the contents slightly moistened. The timing and temperature of a cycle is key to achieving sterilisation. Instruments are usually subject to a 20 minute cycle at 121˚C (1.03kg / cm3, 15 psi) but this will depend on the autoclave used. Steam must be allowed to escape from the packaging at the end of the cycle, so the contents cool and are able to dry out. A number of indicator systems are available to enable checking of the effectiveness of the autoclave cycle.
  • Autoclave tape is a chemical indicator system and consists of paper impregnated with a chemical that changes colour at a certain temperature. This system does not gauge exposure time to the temperature.
  • Browns tubes – these are small glass vials that change colour when exposed to prolonged high heat. This system is able to show that all sterilizing conditions have been met.
• After sterilisation, the sterilised packs have a limited shelf life, which is prolonged by storage in closed cabinets rather than open shelves. The date of sterilisation should be recorded on the outer packing, and packs must be re-sterilised at regular intervals.

Dry heat

• Hot air ovens sterilise items that can’t be exposed to moisture or may be damaged by repeated autoclaving. A temperature of 160˚C for an hour or more is required for sterilisation, so this is a relatively slow method.
• Using a hot bead steriliser is particularly useful when batch surgery involving rodents is undertaken (see Compromising on full asepsis – batch surgery). The tips of the instruments that have been used are placed in a heated beaker of plastic beads at 250˚C for 10-15 seconds. This effectively sterilises the tips of the instruments, (although it does tend to bake on any organic debris making the instruments hard to clean at the end of surgery). Instruments must be allowed to cool after sterilisation.

b) Radiation energy – This method is suitable for materials that are sensitive to heat or chemicals e.g. catheters, syringes, suture material etc.

The radiation ionises the organisms and kills them. This method is very efficient and gamma radiation is often used by commercial companies to sterilize pre-packaged goods.

Methods of chemical sterilisation

These methods were developed for materials that are damaged by heat or for use in situations where it is not possible to utilize a heating method or there is not time to perform sterilisation by that technique. Very few “sterilising solutions” are truly capable of sterilisation and instructions for their use must be followed carefully to achieve good results. Two ways in which to achieve chemical sterilisation are to soak the items in liquid or to use a liquid that vaporizes and the resultant gas sterilizes. Many of the liquid solutions merely perform disinfection, the gaseous agent ethylene oxide is capable of sterilisation.

Ethylene oxide

Instructions for use of ethylene oxide must be followed carefully if it is to be used effectively and safely. Some points to note regarding the use of ethylene oxide are as follows:

• Effectiveness of the technique depends on four variables:
  • Gas concentration
  • Exposure time
  • Temperature
  • Humidity
• The gas diffuses and penetrates pre-wrapped objects and following sterilisation diffuses out from the object and its packaging.
• The aeration period required to facilitate diffusion of ethylene oxide gas from sterilised objects is as follows:
  • Natural / passive aeration = five days for all materials
  • Mechanical / active aeration = minimum of 24 hours for all materials.
• There are health and safety issues associated with the use of ethylene oxide: the gas is explosive, flammable and highly toxic causing irritation to skin and mucous membranes. This may affect personnel or any animal tissues that are exposed to it during surgery.
• Storage life of items sterilised using this method varies according to the wrapping used to package objects

Cold chemical ‘sterilisation’

This involves soaking of instruments in disinfectant solutions, usually over a long period of time (hours/days). Bacterial spores and viruses are often not destroyed when these techniques are used. Instruments or equipment treated in this way require thorough rinsing in sterile water or saline to avoid producing tissue irritation and damage.

Factors influencing the efficacy of sterilisation

• Number of organisms present at the start of the procedure
• The resistance of the organism to the agent being used (e.g. bacterial spores are a highly resistant form of agent)
• The ability of the sterilizing agent to come into contact with the organisms (e.g. there may be a “protective” covering of organic matter)
• The exposure time of the organisms to the sterilizing agent
• Reliability of the mechanical means used to undertake the sterilizing procedure
• Reliability of the personnel undertaking the procedure and their degree of adherence to standard operating procedures

Next Article : Peri-operative antibiotics and post-operative infection