STERILISATION

Ethylene oxide

Ionising radiation

Steam

Dry heat

Chemical

Indicators, shelf life & precautions for each method of sterilisation

Properties of ethylene oxide (EO); requirements of EO including: concentration, relative humidity, temperature, time

Gas Sterilisation

• Sterilisation by process of alkylation in which a hydrogen atom is replaced by a hydroxy ethyl radical within a protein molecule thereby killing microbes

Advantages:

• Colourless gas

• Effective against all microorganisms

• Excellent penetrative power-allows items to be prepackaged

• Leaves most materials undamaged

• Useful for sterilising heat sensitive and moisture sensitive materials and ventilator circuits

Disadvantages:

• Toxic to inhale

• Explosive

• Slow — up to 8 hours

• Requires considerable skill for correct use

• Difficult to remove traces of EO from rubber & plastics

Aeration

• Ethylene oxide causes irritation of mucous membranes, contact dermatitis and blisters

Concentration of gas

• For effective sterilisation 400 — 1000 mg of EO is required for every liter of space

Humidity

• Moisture enhances the diffusion of EO

• Dry or desiccated cells are much more resistant to desiccation than moist cells

Temperature

• Required exposure time can be decreased by increasing the temp

• Effective against both bacteria and bacterial spores

• Most automatic sterilises operated at 40-60°C

Time

• The time needed for sterilisation is dependent on the concentration of the gas, temperature, humidity, type of packaging and items to be sterilised

• Generally from 1-12 hours

Sterilisation by ionising radiation and its biocidal action including: ionisation, radical formation, biochemical changes

Irradiation Sterilisation

• Gamma rays from a radiative source, usually cobalt-60 in a dose of 2.5 megarads

Advantages:

• Effective against viruses, bacteria and bacterial spores (if dose high enough)

• Good penetration of closed packs; items can be prepackaged before sterilisation

• Heat labile materials can be safely sterilised as no heat is involved

• Commonly used for disposable equipment

• No residual radiation; sterilised items can be immediately used

Disadvantages:

• Expensive process

• Glass tends to go brown and may be damaged

• Damages some types of rubber

• Damages PVC, nylon, paper, wood, cotton and metals if irradiated more than once

• When PVC is initially irradiated, chloride ions are released which does not cause any problems when in contact with tissues; However on repeat irradiations, the gas forms ethylene chlorohydrin which is extremely toxic to tissues

Sterilisation by steam and its biocidal action including: moisture content, heat content, penetration

Note: Pasteurisation—moist heat < 100°C

Autoclaving—moist heat > 100°C

Heat Sterilisation - Moist heat: autoclaving

• Autoclave is a sterilising container from which the air is first removed and replace by steam under pressure

• As the pressure is increased, the temperature of the steam rises and the length of the exposure required for sterilisation is reduced

—> eg: 134°C for 3 minutes ∫ 147°C for 30 sec

• Heat causes coagulation & denaturation of cellular proteins resulting in irreversible changes within a cell

• Air is evacuated from the chamber as it impairs sterilisation; air is denser than steam thereby gravitating to the bottom of the chamber and remain cooler

• In addition to residual air, incomplete steam penetration can occur because of overloaded container & incorrect positioning of containers

Advantages:

• Effective against all viruses, bacteria & bacteria spores

• Penetrates paper, cardboard & fabrics

• Absence of toxic products or residues

• Always the first choice for items that can tolerate heat & steam

Disadvantages:

• Items to be sterilised must be packed in such a way that the steam can gain access to them

• Steam blunts sharp edges

• Steam speeds deterioration of rubber and plastic and other heat sensitive items

• Does not penetrate oils, greases or powders

Dry heat

This is basically like an oven and cooks the bacteria by convection. This requires temperature in the range of 160 degrees Celsius. It is suitable for items which can withstand the temperature extremes. Approx 5 mins

Chemical (Glutaraldehyde)

This chemical is basically a fixing agent that fixes all proteins including that of the infectious agents.

Indicators, shelf life & precautions for each method of sterilisation

	Indicators	Shelf Life	Precautions	Time Required
Ethylene oxide	at SVH masking tape with markings that turn from yellow to red when EO processed. - ATTEST (biological indicator) placed in the most challenging part of the steriliser the vials are incubated post sterilisation if growth occurs then an indicator dye is activated indicating that the sterilisation procedure was inadequate.	6 mths	Carcinogenic , toxic	3hrs -12 hrs aeration
Steam sterilisation	1. at SVH masking tape with markings that change colour. - ATTEST (biological indicator) placed in the most challenging part of the steriliser the vials are incubated post sterilisation if growth occurs then an indicator dye is activated indicating that the sterilisation procedure was inadequate.	28 days	correct placement of goods in steriliser	20-45 mins
Dry Heat	dye indicators	usually used immediately	be aware of the materials placed in the dry heat steriliser.	5 mins
Chemical (gluteraldehyde)	A test kit is available to test the concentration of glutaraldehyde in the solution this is similar to a glucose test strip.	Used for bronchoscopes usually for immediate use	Very dangerous chemical requires safety cabinet ,gloves ,facial protection , Soaking then rinsing in sterile water.	30mins soak then rinse in sterile water

KCPotgerÓ