CAN STERILE REALLY BE STERILE?: EFFECTIVE AUTOCLAVING METHODS

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1 Vet Times The website for the veterinary profession CAN STERILE REALLY BE STERILE?: EFFECTIVE AUTOCLAVING METHODS Author : James Gasson Categories : Vets Date : February 2, 2009 James Gasson examines how advances in sterilisation technology are reducing the risk of failure and human error, before comparing machine types THE continued emergence of diffi cult infections within the veterinary community has led to an increased awareness and appreciation of the basic hygiene, disinfection and sterilisation techniques used to minimise patient complications. To provide the highest level of assurance that infective agents have not been introduced, all items placed into the body of the patient below the skin or mucous membranes must be sterile. This may also be applied to other items where the risk of infection is considered high, such as food bowls. Sterility is an absolute term either an object is sterile or it is not. Vegetative organisms are relatively easy to kill, but spores are resistant and, therefore, referenced as a benchmark for achieving sterility. Sterility may be achieved by a number of processes, but steam under pressure (autoclave) is the most common form. Others include ethylene oxide at temperatures between 20 C and 55 C, hot air with temperatures above 160 C, and gas plasmas, such as ionised hydrogen peroxide vapour. Gamma irradiation sterilisation is a commonly employed industrial process. Boiling instruments in water is not considered a suitable process, as the maximum achievable temperature does not inactivate spores. Since autoclaving remains the most common and suitable process for heat- 1 / 13

2 stable items, this article will focus on this method. The autoclave was introduced at the end of the 19th century. The basic concept of heating water in a closed vessel to produce steam above atmospheric pressure, and thus at a higher temperature, has changed very little. Sterilisation science, however, has moved on considerably, and a sterilisation process must be able to demonstrate a minimum sterility assurance level (SAL) of This means that the probability of a non-sterile unit (PNSU) is one in one million, which is universally considered to be an acceptable level of risk. Sterilisation has essentially become a mathematical science and theoretical concept. The thermal death of microbes essentially follows a first-order reaction, and thus it is possible to extrapolate, using logarithms at the required time and at certain temperatures, to produce a SAL of 10-6, since it is not possible to measure fractions of microbes. The decimal reduction time (D value) of a microbe is described as the time interval at a specified temperature whereby a 90 per cent reduction in its population is achieved a one-logarithm reduction. Spores of Geobacillus stearothermophilus are considered to be moist, heat-resistant organisms, and are normally used in the validation and testing of steam sterilisation. The actual mechanism of destruction for microbes using steam under pressure remains a debatable topic. It is generally considered that the release of energy from saturated, phase boundary steam s change of state (enthalpy) to water (as it condenses on the cooler surfaces of the objects to be sterilised) is responsible. As steam condenses, a partial vacuum is created due to volume reduction, and more steam is drawn in. Saturated steam with a temperature of 134 C occurs at two bar, with an enthalpy of 2,725kJ/kg. For steam at 121 C and 1.1 bar, 6D (six-log reduction) of G stearothermophilus is achieved in six minutes, assuming there is an initial population (bioburden) of one million spores; this is described as the half cycle. Employing a method known as overkill, it is possible to achieve a 12D or full cycle in 12 minutes. That is the 12-log reduction necessary to produce a sterility assurance level of 10-6 (Figure 1). The term FO describes the lethality factor, equating the equivalent time in minutes at the various temperatures required to produce a given sterilisation effect in saturated steam at 121 C. For its calculation, a z value of 10 C is assumed the term z value means the slope of the thermal death curve, which may be expressed as the number of degrees Celsius required to bring about a 10-fold change in the death rate. FO =?t?10(t-121)/z. 2 / 13

3 ?t = time interval between measurements of T. T = temperature of sterilised product at time (t). z = temperature coeffi cient, which is assumed to be 10 C. Taking the three minutes at 134 C standard sterilisation holding time as an example: 3 10 ( )/10 = = Therefore, three minutes at 134 C is equivalent, in terms of lethality, to minutes at 121 C. At 136 C for 3.5 minutes, this value rises to , demonstrating how only a slight adjustment in parameters has a massive effect on lethality. The rise in heat up to the sterilisation temperature and the cooling down sequence also contributes to lethality, and is an extra margin of safety in terms of SAL, although this isn t measured. In practice, assuming an item has been cleaned to an acceptable level, the typical bioburden of reprocessed items to be autoclaved is nowhere near as high or as resistant as has been allowed for in the validation process. It is impossible to sterilise a dirty item, as coagulated blood and proteins inhibit steam penetration. Prions responsible for transmissible spongiform encephalopathies (TSE) are resistant to many processes, including steam sterilisation. A minimum of 18 minutes at 134 C, or 60 minutes at 121 C, is recommended for inactivation of the prion protein. TSE infections have resulted from the reuse of surgical supplies in humans and, to date, France is the only country to extend its standard sterilisation cycles to accommodate this. Autoclaves and loads Not all autoclaves are the same. The type of autoclave used dictates the type of load that may be safely processed within it. The overall holding time at a specifi ed temperature is only a small part of the overall cycle. To achieve sterilisation, the sterilant, in this case saturated steam, must be able to penetrate the load. This includes any wrapping materials used to achieve terminal sterilisation (items for use at a later date). For successful sterilisation in the autoclave, air must be completely replaced by steam. Residual air within the sterilisation chamber inhibits the condensation of steam on to the surfaces of the articles to be sterilised. The air would then be compressed into the centre of the load, creating cold spots. Porous loads are those that are, or comprise of, non-solid items, including wrappers. Textile packs are one of the most diffi cult items to deal with, in terms of air removal, and the use of disposable 3 / 13

4 drapes and/or gowns greatly reduces this problem. Hollow load type A This describes an object where the ratio of length of cavity to diameter is greater than one and greater than five. Hollow load type B This describes an object where the ratio of length of cavity to diameter is greater than one and less than five. Post-sterilisation, the steam must be completely removed from the chamber and the load dried. A wet load is non-sterile, as strike through of packaging materials allows for the passage of microbes to the items within. Drying items after removal from the autoclave is not acceptable. Class-N autoclaves are the most basic. These machines use a passive air removal system and are commonly known as gravity displacement autoclaves. Because steam is less dense than air, it displaces the air downwards as the pressure increases and out of a valve in the bottom of the chamber. Having the valve situated at the top of the machine disadvantages vertical pressurecooker- type autoclaves. Bench-top gravity displacement autoclaves with preset parameters are designed only for sterilising unwrapped, non-porous (solid) or nonlumened supplies that are intended for immediate use. The cycles are not validated for use with wrapping materials; this is commonly known as flash sterilisation, which (for practical reasons) should be avoided in all but emergency situations transfer and presentation to the sterile field are further considerations for this method. Cycle times from as little as 12 minutes are possible with a warm chamber, sterilising at 134 C at three to four minutes. Drying is achieved by residual heat driving off moisture from the load. Increasing the sterilisation holding time can improve steam penetration for wrapped and porous loads if they are sterilised in this type of machine. Class-S autoclaves incorporate active partial air removal and are suitable for the sterilisation of a specified load as per the manufacturers instructions. These loads usually include type A hollow devices and small porous loads and/or single-wrapped supplies. A common method is to produce a single vacuum within the chamber prior to sterilisation. While overcoming some of the diffi culties of the class-n autoclaves, air removal is not complete or suffi cient enough to sterilise type-b devices or larger porous loads, such as wrapped instrument sets. Load drying is often achieved by creating a further vacuum post-sterilisation while the chamber is still hot, with steam occurring at sub-atmospheric pressures with reduced temperature. Cycle times 4 / 13

5 for bench-top machines are usually between 45 and 60 minutes. Class-B autoclaves are suitable for all wrapped, porous and type A and type B hollow devices. These autoclaves incorporate a pulsed-vacuum, fractionated steam air removal system. A deep vaccum is applied to the chamber, followed by alternate pulses of vacuum and steam admission, first under negative pressure and subsequently while above atmospheric pressure (Figure 2). Near-instantaneous steam penetration is achieved and sterilisation of the pre-conditioned load occurs, which is just below sterilising temperature. Post-sterilisation, a further vacuum facilitates drying. Benchtop machines of this design are available, but their main limitation is the small cylindrical section of the chamber and long cycle times (Figure 3). Large, rectangular section autoclaves with a separate steam supply can achieve total cycle times of 15 to 30 minutes, including drying. Stand-alone units with integral steam generators are also available, taking up minimal floor space. Most large class-b autoclaves have a steam jacket, which is a second chamber surrounding the load chamber. This is heated to just above sterilisation temperature on steam admission, encouraging the steam to condense on the items to be sterilised, rather than the chamber walls. Large porous-load autoclaves are dynamic and respond to the characteristics of the load and, therefore, no two cycles are identical, apart from the critical parameters attained (Figure 4). Monitoring The margin of safety to achieve sterilisation using an autoclave should be huge, but failures do occur. Modern, fully automated autoclaves should be able to detect a fault within a cycle and abort accordingly, such as when temperatures fall during hold time. Most new autoclaves have the facility to print out the parameters achieved as documented evidence of a successful cycle. Even if all critical parameters of steam sterilisation have been achieved, it does not guarantee a sterile load indeed, it is impossible to guarantee all items within a load are sterile. A push-and-go approach to operating modern autoclaves ensures reproducibility of the cycle. Human errors are frequently associated with sterilisation failure, and these may include but are not limited to the wrong type of autoclave being used (such as fabrics within class N), an improper loading pattern and overloading. Biological indicators (BI) have previously been described as the gold standard, and it has been implied they can guarantee sterility. BI consist of a population of moist, heat-resistant spores, with approximately one million per test strip (Table 1). These are placed alongside the load to be sterilised and then subsequently incubated for between 24 and 48 hours. Any lack of growth is taken to imply the load is sterile. For the reasons stated earlier in the article, the BI can only demonstrate a six-log 5 / 13

6 reduction. To demonstrate a SAL of 10-6, approximately one million test strips would be required which is impractical. It must be remembered that no indicator can guarantee sterility. A pass merely indicates the conditions at the test strip location were suffi cient to bring about a change. For this reason, to mean anything useful, test strips should be positioned in the most inaccessible part of the load. This is generally considered to be the centre of the pack, where any residual air compresses during steam admission. A process challenge device may be used, which represents the worst-case scenario. The indicators are retrieved and the result applied to the rest of the load. Class-five and class-six chemical indicators should be considered for routine use within all packs. Apart from detecting all critical parameters, some can also indicate the presence of noncondensable gases and the steam quality, such as superheated or supersaturated. As class-six indicators are cycle specific and respond to the whole holding time, it is possible to imply a SAL of 10-6 has been achieved (Figure 5). Maintenance Autoclaves should have a written schedule of maintenance, in accordance to the manufacturers instructions. If routine cleaning of the chamber is required, only mildly acidic cleaners should be used. The use of distilled water within benchtop machines will greatly improve the quality of the steam, and preserve heating elements and/or chambers from the build-up of hardwater deposits. Tap water is heavily contaminated with many dissolved substances that can damage not only the autoclave itself, but also be deposited on to the articles being sterilised. Dissolved gases contribute to residual air upon heating. Many bench-top sterilisers recycle water. This is undesirable, due to a build up of endotoxins and a biofilm within the reservoir. These can be deposited on instruments and lead to an inflammatory process, especially when used intraocularly. Implants should not be processed in these machines. Autoclaves that condense steam into a separate reservoir and only intake fresh, clean water are desirable. Autoclaves are pressure vessels and, as such, require regular servicing and inspection to comply with The Pressure Systems Safety Regulations A written scheme of examination should be available, together with documented evidence of inspection and maintenance. A service is not necessarily an inspection under the regulations. Periodic thermocouple testing is recommended for large porousload autoclaves, in addition to a quarterly service. Operating at full steam? It s a fact of life that you get what you pay for, and autoclaves are no exception. The decision to purchase an autoclave on cost alone is a false economy. 6 / 13

7 The range and complexity of surgical procedures carried out in veterinary practice has expanded considerably, which is paralleled by client expectations and awareness. It is easy to take sterilisation for granted. Next time you are handed a pack of sterile supplies, how confi dent can you be that it is? Figure 1 (left). An illustration of the SAL concept. 7 / 13

8 Figure 2. Air removal and steam admission sequence, class-b. A sterilisation process must be able to demonstrate a minimum SAL of / 13

9 Figure 3 (left). An example of a small bench-top class-b autoclave. In this type of autoclave, a deep vacuum is applied to the chamber, followed by alternate pulses of vacuum and steam admission, first under negative pressure and subsequently while above atmospheric pressure. 9 / 13

10 Figure 4 (centre). Large porous-load autoclaves are dynamic and respond to the characteristics of the load and, therefore, no two cycles are identical apart from the critical parameters attained. 10 / 13

11 Figure 5 (above). Class-six indicators and a class-one process label. Some indicators, including class-six types, can show the presence of non-condensable gases and the steam quality. 11 / 13

12 12 / 13

13 TABLE 1. Different types of indicator compared 13 / 13 Powered by TCPDF (