Cleaning, disinfection and sterilization of equipment

EQUIPMENT AND CLINICAL PHYSICS

© 2004 The Medicine Publishing Company Ltd360ANAESTHESIA AND INTENSIVE CARE MEDICINE 5:11

Hospital-acquired infections have significant ramifications for patient morbidity and mortality. It is estimated that they cost the NHS about £1 billion/year. The causative agent of infection can be a bacterium, virus, fungus, parasite or prion. In order for disease transmission to occur, the pathogen must be virulent, invasive, infective and have the appropriate reservoir to survive. Medical equipment can act as reservoirs for microorganisms. The protection of patients and healthcare personnel from medical equipment that has come into contact with patients or their body fluids necessitates the adoption of various processes enforceable by law. Guidelines from the Microbiology Advisory Committee (MAC) to the Medical Devices Agency (MDA) provide guidance on decontamination, disinfection and sterilization practices for application in the health service.

Contamination

Contamination describes the soiling or pollution of inanimate objects with harmful, potentially infectious or other unwanted matter. In the clinical environment this is most likely to be organic matter and microorganisms, but may also involve undesirable inorganic substances (e.g. dust, chemical residues). Contamination can expose the patient to risk and adversely affect the functional capacity of equipment. The degree of risk to the patient depends on:• the nature of the procedure to which the patient is exposed

(i.e. how invasive)• patient susceptibility (i.e. how immunocompromised)• the nature, extent and density of microbial contamination

(the bioburden).

Decontamination

Decontamination is the use of physical or chemical means to remove, inactivate or destroy pathogens on an item, to the point

where they are no longer capable of transmitting infectious par-ticles and are rendered safe for handling, use or disposal. The three methods of decontamination commonly used are cleaning, disinfection and sterilization. The process of decontamination chosen should take account of:• the infection risk associated with the intended use of the

equipment (Figure 1)• the heat, pressure, moisture or chemical tolerance of the

equipment• the availability of processing equipment• the risks associated with the decontamination process• the time available. Manufacturers should provide instructions for decontamination processes that are effective without compromising the perform-ance of the equipment. It is essential to maintain adequate records for each item to document the frequency and effectiveness of de contamination.

CleaningCleaning physically removes infectious agents and the organic matter on which they thrive but does not necessarily destroy the microorganisms. It is an essential prerequisite to disinfection and sterilization and can be undertaken manually, mechanically (automated) or using a combination of both methods.

Manual cleaning should be undertaken only when automated methods are inappropriate or unavailable. Two processes are used.

Immersion – the item is completely immersed in a warm (< 35oC)compatible solution of water and detergent at the correct dilution, ensuring that all surfaces are exposed to the cleaning solution. Brushing, wiping, irrigation and agitation are used to remove all

Cleaning, disinfection andsterilization of equipmentSimon Lewis

Andrew K McIndoe

Simon Lewis is Specialist Registrar in Anaesthesia on the Bristol

Rotation. His interests include vascular and cardiac anaesthesia.

Andrew K McIndoe is Consultant Anaesthetist and Senior Clinical Lecturer

in the Sir Humphry Davy Department of Anaesthesia, Bristol Royal

Infirmary. He is also the Director of Research and Education at the Bristol

Medical Simulation Centre where he has developed a specific interest in

human factors and crisis management.

Classification of infection risk associated with medical equipment and suggested decontamination process

Application of item Recommendation

High risk• In close contact with a break in

the skin or mucous membrane

• Introduced into sterile body

areas

Sterilization

Intermediate risk• In contact with mucous

membranes

• Contaminated with particularly

virulent or readily transmissible

organisms

• Before use on immuno-

compromised patients

Sterilization or disinfection

Cleaning may be acceptable

in certain agreed situations

Low risk• In contact with healthy skin

• No contact with patient

Cleaning

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visible contaminants. The device is then completely rinsed and drained with clean water.

Non-immersion is used if items would be compromised by soaking in aqueous solutions (e.g. electrical or electronic devices).A cloth is soaked in cleaning solution, wrung out and then used to wipe the surfaces of the device. The surfaces are then hand-dried using a cloth, hot-air dryer or drying cabinet.

Mechanical cleaning is commonly accomplished using automated cleaning and rinsing processes and often incorporates disinfection.

Thermal washer-disinfectors – cleaning is achieved by the continual spraying of the equipment with water and detergent as part of a timed cycle. The initial cleaning is performed at 35oC or below. This is followed by hot water disinfection at a minimum of 71oC for 3 minutes, 80oC for 1 minute, or 90oC for 1 second.

Chemical washer-disinfectors – cleaning is achieved as in the thermal washer. Disinfection is carried out by exposure to an approved chemical disinfectant for a predetermined time at less than 60oC. This is followed by water rinsing to remove disinfectant residues. In addition, an instrument lubrication treatment can be used (e.g. in scopes, cleaning will have removed the lubricant used to reduce friction in use, so it will have to be reapplied). This is followed by hot-air drying.

Ultrasonic cleaners are designed for fine cleaning of medical devices to remove contaminants from areas that are hard to reach (e.g. crevices). They work through sonic waves that create tiny bubbles on the surfaces of instruments. These bubbles expand until they become unstable and collapse or implode (cavitation). This produces localized vacuum areas responsible for dislodging contaminants. Disinfection is not part of the process.

DisinfectionDisinfection is the destruction, removal or reduction in numbers of pathogens to an acceptable level. Certain viruses and bacterial spores may be resistant to disinfection. Disinfection is a relative state compared with sterilization.

Thermal washer-disinfectors combine cleaning and disinfection, as described above. They inactivate all microorganisms except bacterial spores, some heat-resistant viruses and cryptosporidia. They are suitable only for devices that can withstand repeated exposure to wet heat at temperatures of up to 80oC.

Low-temperature steam (pasteurization) kills vegetative micro-organisms and some heat-sensitive viruses by exposure of the equipment to saturated steam below atmospheric pressure at 73oC for at least 10 minutes. The process is unsuitable for oily or greasy items or those with sealed cavities.

Boiling water disinfection: disinfection can be achieved by sub-merging a device in soft water and boiling for at least 5 minutes. This inactivates most non-sporing microorganisms, fungi, viruses and some heat-sensitive spores. This method is unsuitable for heat-labile equipment and hollow or porous items into which the water will not penetrate. However, the process is non-toxic and inexpensive.

Chemical disinfection: a chemical disinfectant is a compound or mixture that is capable of destroying microorganisms by chemical

or physicochemical means. It is usually presented in liquid form. Most disinfectants are capable of eliminating Gram-positive and Gram-negative bacteria and enveloped viruses. They are less effec-tive against non-enveloped viruses, protozoan cysts and bacterial endospores. The microbicidal activity of various disinfectants is shown in Figure 2. The main advantages of chemical disinfection are its suitability for heat-sensitive equipment and relatively low cost. Disadvantages of chemical disinfectants include:• they are potentially toxic, corrosive or volatile, necessitating

staff protection and training• many are inactivated by organic matter• inability to penetrate crevices• concentration and pH are critical• slow activity.

SterilizationSterilization is the complete destruction, inactivation and removal of all microbes including bacterial spores and viruses. Sterilization is an absolute state.

Steam: when steam is pressurized to above atmospheric pres-sure it can reach temperatures greater than 100oC. In order for sterilization to occur, the pure dry saturated steam must be in direct contact with the material at the required temperature for the required time in the absence of air. Steam is dry and saturated when introduced above the condensation point. The temperature reached by the object determines the time required for sterilization. The minimum is 121oC for 15 minutes; the maximum is 134oC for3 minutes. Steam sterilization is the established method of choice for sterilization in health services. In the simplest steam autoclaves, air is removed by displacement with steam. This limits their use to sterilization of unwrapped, nonporous clean items only. To overcome the problem of trapped air in wrapped items a porous load sterilizer is used. This incor-porates a vacuum pump, which removes air before the addition of steam. The advantages of this method are that steam is non-toxic, non-corrosive, highly effective and inexpensive. Disadvantages are that it is unsuitable for heat-labile items and the operator must wear protective clothing.

Dry hot air: sterilization occurs by exposing materials to dry heat in hot-air ovens. Typical temperatures are 160oC for 120 minutes, 170 oCfor 60 minutes, or 180oC for 30 minutes. Dry heat can be used to sterilize heat-stable powders, waxes and non-aqueous liquids (e.g. paraffin gauze dressings, silicone lubricant). The procedure is not recommended for use in hospital. Disadvantages of the process are:• that it is inefficient compared with autoclaves• heat-up time can be long thereby lengthening the duration of

the procedure• many devices cannot withstand the high temperatures

involved.

Ethylene oxide was introduced in the 1950s. Ethylene oxide gas is a very effective low-temperature sterilant. A boiling point of 10.4oCmeans that it vaporizes easily at ambient pressure and tempera-ture. This makes it highly penetrative with a wide spectrum of activity against bacteria, spores, fungi and viruses. Sterilization is usually undertaken at 20–60oC for 2–24 hours. Pure ethylene




oxide is a flammable ether so it is often diluted with nitrogen or carbon dioxide to reduce the risk of explosion. Disadvantages of ethylene oxide are that it is:• highly toxic, necessitating a long period of aeration after steri-

lization to remove residues• teratogenic, mutagenic and carcinogenic • expensive and requires carefully controlled conditions.

Low-temperature steam and formaldehyde uses a combination of dry saturated steam and formaldehyde at subatmospheric pressure and at 60–70oC. Devices are placed in the sterilizer and formal-dehyde entrained with the steam. The process is fully automated and can be used to sterilize heat-sensitive instruments and plastic items. Disadvantages of the process are that:• formaldehyde vapours are potentially carcinogenic and are

irritating to skin, eyes and the respiratory tract• it is expensive• it requires complex equipment, making it unsuitable for hospital

use.

Ionizing (γ) radiationγγIrradiation uses γ rays from cobalt-60 at over 25,000 Gray to pro-γduce sterility. It is ideal for pre-packed heat-labile single-use items (e.g. needles, syringes, face masks) and is widely used in industry. Some materials undergo degradative changes making radiation unsuitable for resterilization of hospital equipment.

Monitoring sterilization procedures can be carried out routinely

using a combination of biological, chemical and mechanical indicators.

Biological indicators – monitoring the sterilization process with reliable biological indicators at regular intervals is strongly recommended. Bacterial spores of known resistance are used as indicators:• steam sterilizers: Bacillus stearothermophilus• dry-heat sterilizers: Bacillus subtilis.The MDA advises that biological indicators should not be used for the routine monitoring of steam sterilization.

Chemical indicators include indicator tape or labels, which monitor time, temperature and pressure for steam and dry-heat sterilization. They show a defined colour change when specified conditions have been attained.

Anaesthetic equipment

Anaesthetic equipment can become contaminated by direct and indirect patient contact. The contamination is not always macroscopic and all used equipment must be assumed to be con-taminated. If the item is reusable it should undergo a process of decontamination appropriate to its infection risk (Figure 1).

Single-use disposable equipment removes the problems of reprocessing and decontamination. The Working Party of the Association of Anaesthetists of Great Britain and Ireland (AAGBI) is currently investigating methods to reduce the risk of cross-infection in patients undergoing anaesthesia and encourages

Chemical disinfectants: their spectrum of activity and physical properties

Disinfectant Microbiological activity Inactivation by organic matter

Corrosive/damaging matter

Irritant Other examples

Spores Mycobacteria Bacteria VirusEnveloped Non-

enveloped

• Glutaraldehyde

2%

+++

(slow)

+++ +++ +++ +++ No No Yes Cidex

• Peracetic acid

0.2–0.35%

+++ +++ +++ +++ ++ No Slight Slight

• Alcoholic (ethyl

alcohol) 60–80%

None ++ +++ +++ ++ Yes

(fixative)

Slight No, but

flammable

Methanol,

ethanol

• Peroxygen com-

pounds

None + +++ +++ ++ Yes Slight No

• Chlorine-

releasing agents

>1000 ppm

average Cl2

+++ +++ +++ +++ ++ Yes Yes Yes Domestic

bleach

• Clear soluble

phenolics

0.6–2%

None +++ +++ + None No Slight Yes Chlorhexidine

• Quarternary

ammonium

compounds

None Variable Moderate + +++ Yes No No

2




the use of single-use equipment. However, there are difficulties with cost, storage and disposal of single-use items. Concern has also been raised about the performance of single-use devices when compared with their reusable counterparts (e.g. disposablelaryngoscopes). Devices labelled ‘single patient use’ may be used for an extended period of time or intermittently on the same patient. Users who disregard the manufacturer’s guidelines by reprocessing single-use items or reprocessing them for more than the recommended number of times may be transferring legal liabil-ity for the safe performance of that product from the manufacturer to themselves or their employer.

Anaesthetic masks are often contaminated by patient secretions and it is recommended that they are disposable and used for single patients only or are sterilized.

Airways and tubes are categorized as intermediate risk and it is recommended that oral, nasopharyngeal and tracheal tubes are for single patient use. Reusable laryngeal mask airways (LMA) can be sterilized up to a maximum of 40 times. Studies have demonstrated that whatever decontamination process is used pro-teinaceous deposits may remain adherent to the laryngeal mask. With the recognition that prions are resistant to the sterilization process, the development and use of disposable laryngeal airways is likely to increase.

Anaesthetic breathing systems: many anaesthetic departments routinely reuse disposable anaesthetic breathing circuits marked for single patient use. The MDA has re-emphasized the clinical and legal implications of this practice. Since then a number of manufacturers have demonstrated that anaesthetic circuits may be safely reused for up to 1 week if a fresh bacterial/viral filter is used for every patient. The Working Party accepts the manufacturers’ recommendations but advises that circuits are disposed of when the anaesthetic machine and monitors are cleaned. No attempt should be made to reprocess these items.

Laryngoscope blades are regularly contaminated with blood, plac-ing them in the high-risk category. Repeated autoclaving can affect the function of laryngoscopes; but the Working Party recommends that reusable laryngoscope blades are resterilized by the Sterile Supplies Department between patients. Single-use laryngoscope blades are available.

Prion diseases

Prions are small proteinaceous particles that are recognized as the causative agents of the transmissible spongiform encepha-lopathies (TSEs). These are fatal degenerative brain diseases that are characterized by rapidly progressive dementia and a spongy macroscopic appearance of brain tissue. New variant Creutzfeldt-Jakob disease (vCJD) is a member of the TSE family manifesting in humans and is caused by the same prion that results in BSE in cattle. In the UK, vCJD was first reported in 1996 and by 2002 a total of 125 definite or probable cases had been identified. A number of national committees including the Spongiform Encepha-lopathy Advisory Committee (SEAC) and the Advisory Committee on Dangerous Pathogens (ACDP) have made recommendations concerning prevention and management.

In vCJD, and the originally identified sporadic CJD, prion pro-tein accumulates in the brain, spinal cord and CNS. Post-mortem examinations on patients with vCJD have also detected prion protein in lymphoreticular tissue (e.g. appendix, tonsil, spleen). Prions are small enough to reside in the microscopic crypts on stainless steel instruments and are resistant to the methods of steri-lization currently used, including autoclaving, ionizing radiation and ultraviolet radiation. However, standard washing techniques reduce the concentration of prions exponentially so that after 10–20 cycles of washing and decontamination it is thought that infectivity is negligible. The only completely safe way to prevent transmission of vCJD via surgical instruments is to use disposable (single-use) items.

Tonsillectomy and adenoidectomyPrions have been detected in lymphoid tissue. In 2001, the DoH issued circulars implying that tonsillectomy was a particular risk for spreading vCJD. They advised that disposable surgical and anaesthetic equipment should be introduced for use in the summer of 2001. Shortly after the introduction of this equipment the morbidity from tonsillectomy surgery increased. In December 2001, this led the DoH to reverse its directive and recommend the return to using reusable surgical instruments for routine tonsil-lectomy and adenoidectomy. Rather confusingly, however, the DoH decided that this did not apply to anaesthetic equipment. The current recommendations of the Royal College of Anaesthetists and the AAGBI concerning tonsillectomies and adenoidectomies are as follows.• An LMA should not be reused. If an LMA is used it should be a single-use disposable LMA. Alternatively if tracking of equipment can be guaranteed, an LMA that has previously been resterilized on a number of occasions (> 35 times) can be used and thendiscarded. This minimizes the financial burden.• An alternative technique is to intubate the trachea using a single-use tracheal tube. The laryngoscope blade must be either a disposable blade or a standard metal blade covered by a disposable transparent plastic sheath.• If bougies or stylettes are required they should be for singleuse only.

FURTHER READINGAssociation of Anaesthetists of Great Britain and Ireland (AAGBI)

Infection control in anaesthesia. London: AAGBI, November 2002.

Blunt M C, Burchett K R. Variant Creutzfeldt–Jakob disease and

disposable anaesthetic equipment – balancing the risks. Br J Anaesth

2003; 90: 1–3.

Microbiology Advisory Committee to the Department of Health Medical

Devices Agency. Sterilization, disinfection and cleaning of medical

equipment. London: Medical Devices Agency, 1996–2002.


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Cleaning, disinfection and sterilization of equipment