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• W2064453428 abstract "In its publication ‘Checking Anaesthetic Equipment’, the Association of Anaesthetists of Great Britain and Ireland (AAGBI) stated that ‘A new, single-use bacterial/viral filter … must be used for each patient’[1]. This reiterated the advice of the AAGBI given in its booklet ‘Infection Control in Anaesthesia’[2]. The Medicines and Healthcare products Regulatory Agency (MHRA) published a report in March 2004 on breathing system filters available on the market in the United Kingdom [3]. This report included results from tests on 104 different filters intended for use with both adult and paediatric patients. The filtration performance, expressed as the penetration of a mass of particles passing through the filter under test as a percentage of the mass of particles in the challenge to the filter, varied from 0.006% to 74%. This implied that, under the conditions of this challenge loading, one particular filter allowed over 10 000 times more particles, by mass, to pass through than another filter intended for the same purpose. In other words, the filtration efficiency, a measure of the effectiveness of the filter in preventing particles from passing through, varied from 99.994% to 26% for the various filters tested. The minimum performance required for breathing system filters to be effective has yet to be established (a minimum is not stated in the European standard for breathing system filters [4]). However, filters are intended to be used so that the breathing system between the filter and the anaesthetic machine can be used for more than one patient, thus reducing cost and waste. The manufacturer of a breathing system can recommend that the breathing system may be used for up to 1 week, during which time dozens of patients may have used it, provided it is protected by an appropriate filter. Therefore, when using a filter, the anaesthetist has to be confident that it will prevent the transmission of infective droplets expelled by the patient from passing into the breathing system. Three recent events have highlighted the potential risks to patients of cross-infection from droplets of blood containing prions during anaesthesia. Prions are considered to be responsible for diseases such as variant Creutzfeldt-Jakob disease (vCJD). As filters are intended to reduce the risk of cross-infection occurring during anaesthesia, it is pertinent to discuss these events in relation to the wide range in performance found for the filters available on the market and demonstrated in the MHRA report. The first event was the second reported case of prions being transferred during blood transfusion [5], following the first case reported earlier in 2004 [6]. In the first case, a patient received blood from a patient who subsequently died from vCJD. The recipient then developed vCJD and also died. In the second case, the patient, who had received blood from the same index patient, died from a non-neurological disorder, but prions were found in the patient's spleen and cervical lymph node. Transferring blood infected with prions between patients therefore poses a considerable risk to the recipient. Measures have already been taken by the UK Government to reduce this risk. In the second event, three samples of appendicectomy specimens of 12 674 samples of tonsillectomy and appendicectomy specimens were found to contain prion proteins [7]. If these numbers are verified in further studies, this implies that there is a much greater prevalence of prions in the UK population than previously thought. The third event concerned the case of Billie-Jo Jenkins. The original conviction of the father of Billie-Jo, Siôn Jenkins, for murder, was quashed after an expert stated that the microscopic traces of Billie-Jo's blood found on Siôn Jenkins' clothes were from a fine spray of droplets of blood resulting from a forcible expiration after blood had drained into Billie-Jo's airways [8]. Trauma to the airway, resulting in bleeding, occurs frequently during anaesthesia [9, 10]. Forcible expirations, such as coughing, also occur frequently, particularly when smokers are anaesthetised [11]. Although the evidence for the air-borne spread of blood-borne viruses, such as HIV, is yet to be established [12, 13], there is clearly a mechanism by which prions could be expelled by a patient into the breathing system in droplets. However, once in the breathing system, what then happens to any infective droplets? Langevin and colleagues demonstrated that droplet nuclei containing Mycobacterium tuberculosis can remain suspended in the gas within a breathing system for up to 1 h [14]. Any droplets containing prions expelled by a patient may also remain suspended in the breathing system. These droplets could then be inhaled by a subsequent patient. Prions are robust and are resistant to most commonly used techniques of sterilisation [15]. They would be able to withstand being suspended in the gas in breathing systems for short periods of time. A filter placed between the patient and the breathing system should therefore reliably prevent the transmission of any infective droplets expelled by the patient. The data in the MHRA report suggest that this might be not the case for all filters [3]. A recent study demonstrated that pleated resin-bonded ceramic filter material prevented the gas-borne transmission of Mycobacterium chelonae, used by the authors as a (non-pathogenic) model for M. tuberculosis[16]. Two other types of filter included in the study did not completely prevent the transmission of M. chelonae. The filtration performance of filters depends on the size of the droplet or particle passing through the filter. Large particles are readily intercepted by the fibres in the filter material. Smaller particles undergo Brownian motion through random collisions with gas molecules and therefore the effective surface area for interception is larger than the diameter of the particle. Thus, there is a size of particle that can pass through the fibres most easily and the size of these particles is known as the most penetrating particle size. This size is typically 0.05–0.5 μm. The European standard for breathing system filters specifies that filters are challenged with an aerosol of particles with a mass median aerodynamic diameter of 0.3 μm, within the range of the most penetrating particle size. This standard formed the basis of the test protocol used to obtain the data for the report published by the MHRA. Prions, being proteins, could easily be carried in droplets of the size specified in the standard to be used to challenge filters. Hence, the use of filters that allow a large fraction of such droplets to pass from the patient to the breathing system might result in contamination and, subsequently, cross-infection. The upper limit of the 95% confidence interval for the prevalence of prions in the UK population from the work of Hilton and colleagues was 692 per million [7]. Cross-infection with prions during anaesthesia would only result from a patient expelling prion-containing droplets with a forcible expiration or cough into the breathing system. The droplets would then remain suspended and subsequently be inhaled by a second patient. The risk of cross-infection is thus clearly low, but should not be discounted. In addition, the prevalence of microbial infections in the UK population may be much greater than that for prions. For example, the Chief Medical Officer recently stated that the current estimate of the prevalence of people chronically infected with hepatitis C in England was 200 000 [17] and the increase in the incidence of tuberculosis is causing concern [18]. The risk of patients presenting for surgery harbouring some sort of infection is clearly not negligible, as the majority of people infected with these particular microbes would not be aware of their infection. However, using a filter with a small penetration value (a high filtration efficiency) would substantially reduce any risk of cross-infection between patients. Such filters are available which do not excessively increase either the deadspace (by having too large an internal volume) or the resistance to gas flow (by having too high a pressure drop) [3, 19]. The smallest penetration value measured for any filter in the MHRA report was 0.006%. This means that only six in every 100 000 droplets of the most penetrating particle size would pass through the filter. Larger or smaller particles would be intercepted more easily. The surface properties of sodium chloride particles specified in the standard as the challenge aerosol are likely to be different to that of droplet nuclei expelled by patients which contain bacteria and viruses. The different levels of electrostatic charge on the particles and droplets will affect penetration to some extent [20]. However, the main effect on penetration is the particle size, in particular the aerodynamic particle size. The rank order of various filters remains very similar [20] whether the filter is challenged with microbes [21] or sodium chloride particles [22], although the absolute values will be very different due to the difference in the size of the particles or droplets. This means that challenging a filter with sodium chloride particles, as used for the MHRA report, allows a rapid, easy technique of ranking that filter compared to other filters available on the market. A second potential route of transmission for prions could be via blood-stained sputum expelled by a patient. Therefore, breathing system filters should also adequately prevent the transmission of liquid from the patient to the breathing system during anaesthesia. Previous work has demonstrated that pleated resin-bonded ceramic filter material present in some breathing system filters prevents the transmission of liquid under pressures found in normal clinical practice [23] and work by one manufacturer has demonstrated that prions contained in liquid will not pass through this type of filter material either [24]. Unfortunately, the MHRA no longer funds evaluations of medical devices used in anaesthesia, and the report on breathing system filters was one of the last published by the MHRA in this area. A minimum level of performance is not stated in the standard for breathing system filters. Furthermore, compliance with standards is not a mandatory requirement for devices placed on the market in the European Union and manufacturers tend to use different test protocols to describe the performance of their devices [25]. The report published by the MHRA on breathing system filters is a salutary reminder of the wide range in performance of devices available to the anaesthetist. The anaesthetist needs objective, independent assessments of medical devices on which to base decisions about which device(s) to use [26]– assessments that are now, unfortunately, even more difficult to find. The possibility of litigation against manufacturers and users from patients who have been infected during anaesthetic procedures could not be discounted if a filter were used with a known poor comparative performance. The difference in cost between filters with various levels of performance is small compared to the overall cost of the surgical procedure. Merely using any filter during anaesthesia may not be an adequate defence in court if cross-infection did occur. The poor performance of some filters intended for use with paediatric patients, highlighted in the MHRA report, has resulted in the Safety Committee of the Association of Anaesthetists of Great Britain and Ireland recommending that this type of filter is not used; rather, that a new breathing system is used for each patient [27, 28]. In the light of these comments, it would clearly be difficult to justify using a filter with an adult patient which has a similar performance to a filter intended for paediatric patients. However, a contrary view is that no evidence has yet been published demonstrating that the incidence of postoperative infections is reduced if a filter is used or varies with the filtration performance of the filter, or even that cross-infection has occurred when any filter has been used. A reduction in postoperative infection rates by using a filter is presently at best, only theoretical. The effect of the use of filters on postoperative infection rates might be insignificant compared to the effect of ensuring adequate standards of hygiene, including handwashing. Nevertheless, the true incidence of postoperative infections is not known; some diseases might not become apparent until months after surgery. The next stage must surely be a study to establish whether the variation in filtration performance demonstrated in the MHRA report has any effect on patient outcome and, if so, to be able to set an effective minimum level of filtration performance for these devices. The author was funded by the Medicines and Healthcare products Regulatory Agency to evaluate medical devices used in anaesthesia and intensive care until the end of March 2004. He has received funds from various manufacturers of breathing system filters to enable him to visit their manufacturing sites, to participate at European and International Standards meetings, and to visit Trade Exhibitions." @default.
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• W2064453428 title "Reducing the risk of prion transmission in anaesthesia" @default.
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