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• W2127421057 abstract "Current airway dust filter technology emanates from the USA mining industry during the 1930s. It was further developed and improved by the USA military during World War II to prevent workers from inhaling very fine radioactive particles in the nuclear industry. These later filters we now recognize as high efficiency particulate air filters (HEPA). The efficiency of filters varies according to the number and size of particles they capture. Filters may be classified commonly as 95, 99.95, and 99.97% efficient (95, 99 and 100, respectively). The higher the efficiency the higher the classification number used. The efficiency of filters may also be affected by volatile chemicals which may be inhaled with the particles. Industrial filters which are categorized by the above system may also have a prefix of N, R, or P; N for Not resistant to oil, R for Resistant to oil, and P for oil Proof; thus, a filter would be classified as for example a P100. Requirements for military and environmental biohazard filters are based on these classifications and surprisingly may only achieve 95% efficiency. A generally perceived increased threat from biological hazards that may be inhaled, including bioterrorism threats, especially in hospital environments, has increased interest in respiratory filters.94Smith SM Chemical and biological weapons. Implications for anaesthesia and intensive care.Br J Anaesth. 2002; 89: 306-324Crossref PubMed Google Scholar Protecting personnel from inhaled biological hazards requires a filter efficiency of several orders of magnitude greater than the industrial dust filters used in respirators alluded to above. As few as 10 inhaled smallpox viruses may be sufficient to infect patients with the disease.94Smith SM Chemical and biological weapons. Implications for anaesthesia and intensive care.Br J Anaesth. 2002; 89: 306-324Crossref PubMed Google Scholar This has resulted in the sub-classification of HEPA filters into true HEPA filters and HEPA type filters. A further sub group has also developed. ULPA filters (Ultra Low Penetration Air) and ‘absolute’ filters are designed for industrial applications such as microelectronic clean rooms, but have too high a resistance for medical breathing apparatus. All HEPA filters are manufactured from glass fibre materials supported on a rigid frame. In order to reduce resistance to air flow and increase efficiency, the surface area is increased by pleating. Filtration is achieved for larger particles (>0.3 μ) by inertial impaction and interception; smaller particles are captured by Brownian diffusion. The size of particles used to test filters is measured in microns. Microns are units used for particles that can be seen with a light microscope. A micron is a thousandth of a millimeter, which is in turn a thousandth of a meter (1 μ=1000 nm or 0.001 mm). The variation in filtration efficiency is tested by the British BS3928 Sodium Flame method and the USA Hot DOP method. The most difficult particle size to capture by filtration is a particle of 0.3 μ as at this size the effects of inertial impaction, interception and Brownian motion are least effective. Particles of 0.3 μ are also most likely to be deposited in the lungs if inhaled. The di-octyl-phthalate (DOP) test, used for testing the efficiency of filters, takes advantage of the properties of DOP. In particulate form, DOP has a constant mean diameter of 0.3 μ. By passing DOP through the filter, the capturing efficiency can be classified. Bacterial size generally equates to 0.3 μ; virus sizes are considerably smaller (Table 1). Wilkes has recently used the sodium chloride test for measuring filter efficiency on 33 breathing system filters (nine pleated hydrophobic and 24 electrostatic filters). The particles in this test are considerably smaller than those of the DOP test and have a size distribution with a median diameter of 0.07 μ and a geometric standard deviation not exceeding 1.83.110Wilkes AR Measuring the filtration performance of breathing system filters using sodium chloride particles.Anaesthesia. 2002; 57: 162-168Crossref PubMed Scopus (30) Google ScholarTable 1Target organisms which filters are designed to capture can vary in size. A comparative table illustrates this point. Light microscopes resolve images down to about 200 nm = 0.2 μ. 0.2 μ = 200 nm, μ = 0.001 mm = 1000 nmBacteriaOrganismSize (μ)ConditionStaphylococcus aureus1.0 μPneumonia, sepsis, renal failurePseudomonas diminuta0.62 μSepticaemiaPseudomonas aeruginosa0.5 μSepticaemia, endocarditis, osteomyelitis, pneumoniaMycobacterium tuberculosis0.3 × 1.0 μTuberculosisVirusesSmallpox0.3 × 0.2 μSmallpoxOrthomyxovirus0.12 μInfluenzaCytomegalovirus0.1 μIntrauterine birth defects, hepatitisHIV0.08 μAcquired Immunodeficiency SyndromeAdenovirus0.07 μRespiratory infectionsHepatitis C virus0.03 μHepatitis, cirrhosis, liver failure, hepatocellular carcinomaHepatitis A virus0.02 μHepatitisT1-Coliphage0.017 μTest virus for filtration efficiency testingCellsRed blood cell5 μLymphocyte5 to 8 μParticlesSmog (moisture droplet)0.5 mm to <2 μAsthma Open table in a new tab A filtration efficiency of 99.999% indicates that only one particle of 100 000 challenging the filter has the potential for penetration beyond the filter. Even if a filter package states that it is 99.97% efficient it is not necessarily a true HEPA filter unless this efficiency was achieved using particles of 0.3 μ. HEPA type filters are made of the same materials as true HEPA filters but they may achieve as little as 25% efficiency. A third type of filter known as an electrostatic filter was, until recently, unable to achieve this level of filtration, but some are now capable of achieving efficiencies of 99.999%. Aside from presenting a barrier to inhalation of organisms and latex particles, filters may be modified to perform additional functions. These are to conserve patient's body heat and to ensure an adequate moisture content of inhaled anaesthetic and respiratory gases in order to protect airways from drying out. With these modifications, filters function as heat and moisture exchanger filters (HMEF). The filters in industrial HEPA respirators are usually colour coded, for example, magenta (reddish-purple), in order to quickly identify the purpose for which the filter is designed, that is to protect against dust, volatile solvents, spray paints, etc. The different functions of medical filters are also identified by at least one manufacturer (Intersurgical®) by colour coding their products in a way similar to industrial filters thus enabling the most appropriate use of the device (Table 2). The manufactures of all heat and moisture exchangers (HME) are expected to adhere to voluntary standard ISO 9360 ‘Anaesthetic and Respiratory Equipment—Heat and Moisture Exchangers for Use in Humidifying Respired Gases in Humans’.59ISO 9360-1:2000 Anaesthetic and respiratory equipment. Heat and moisture exchangers (HMEs) for humidifying respired gases in humans Part 1: HMEs for use with minimum tidal volumes of 250 mlGoogle Scholar This standard provides advice concerning the construction of the filter housing, connections suitable for anaesthetic circuits, labelling, and packaging. The most important section of this standard is Section 6, ‘Test Methods’. This section ensures all devices are tested by the same methods, under the same conditions of tidal volume, breathing frequency, and duration of test (24 h), thus facilitating objective appraisal of similar devices. The efficiency of a HME is tested by adding a precise level of moisture at a precise temperature setting (34°C and 37.6 mg H2O litre−1 of inspired air) and is maintained during the 24 h test period. A detailed review of the structure of HMEs in relation to their function is given by Wilkes.109Wilkes AR Heat and moisture exchangers. Structure and function. In: Branson RD, Pererson BD, Carson KD, eds. Humidification: Current Therapy and Controversy.Respiratory Care Clinics of North America. 1998; 4: 261-279PubMed Google Scholar The idea that the ‘wrong filter’ can lead to disaster will come as no surprise to observers of modern history or investigators of civil aviation disasters. Spectacular examples are the abortive Iranian hostage rescue in the Middle East in the early 1980s, during which the USA marines’ helicopters were grounded by sand in their engines, and the failure of Britain's main battle tank in the Gulf and more recently in Oman. All were because of the inappropriate use of the engine air intake filters that were unsuited to the prevailing environment and, in the latter cases, as the result of cost cutting. Similar problems have repeated themselves in the current Afghan crisis. Recently, anaesthetic departments have been asking whether they should carry the cost of single use, disposable anaesthetic circuits, discarded between cases, in order to avoid cross contamination between patients. Alternatively, should we rely on the properties of single use disposable filters, whilst retaining the circuits? Disposing of filters is cheaper than disposing of circuits, but which filters are the right ones and which are the wrong ones? Are we changing one set of problems for another? The debate is neither new nor resolved but it would be advisable to examine these options closely. Christopher and colleagues, and Wille highlight the risks of cross infection that are presumed to exist following long-term mechanical ventilation and short-term anaesthesia.29Christopher K Saravolatz LD Bush TL Conway WA The potential role of respiratory therapy equipment in cross-infection.Am Rev Resp Dis. 1983; 128: 271-275PubMed Google Scholar 115Wille B Hygiene measures for anaesthesia and ventilator equipment.Krankenhaus-HygieneInfektionsverhutung. 1989; 11: 17-21Google Scholar Numerous articles and reviews attest to the advantages of breathing system filters, circuit filters, HME, and HMEF, with or without bactericidal properties, introduced to avoid the risk of cross contamination. The properties and performance characteristics of these devices have been investigated by Hedley and Allt-Graham in the laboratory under simulated clinical conditions.52Hedley RM Allt-Graham J Comparison of the filtration properties of heat and moisture exchangers.Anaesthesia. 1992; 47: 414-420Crossref PubMed Scopus (54) Google Scholar As a result of this, and similar investigations that demonstrate the beneficial properties of filters in the laboratory, there is a danger of implementing poorly thought out guidelines requiring the use of filters in clinical settings without considering all of the consequences. Filters are being used in the interests of economy and legislation, aside from clinical indications, and without regard to the properties of these devices. The potential detrimental consequences of making such a simple change to clinical practice as to include one of these devices in an anaesthetic circuit are rarely highlighted. There is some overlap between the requirements for heat and moisture conservation and biological filtration for long-term mechanical ventilation in intensive care, and short-term ventilation using circle absorbers in operating theatres, but there are also marked differences. It is instructive to consider the evolution of filters introduced in intensive care units (ITU) following laboratory investigations. Their introduction was not without mishap. Numerous studies highlight the increased resistance to breathing associated with filters. HME with bacterial filtering capabilities were investigated in an intensive care environment by Cohen and colleagues.30Cohen IL Weinberg PF Fein IL Rowinski GS Endotracheal tube occlusion associated with the use of heat and moisture exchangers in the intensive care unit.Crit Care Med. 1988; 16: 277-279Crossref PubMed Scopus (120) Google Scholar They concluded that not only did this early model of a HME not provide sufficient airway humidification but also their use was associated with an increase in tracheal tube occlusion, atelectasis, and an increased incidence of pneumonia. Not all technical problems had been resolved 6 yr later and it was noted that increased resistance could interfere with patient monitoring. Manthous and Schmidt were one of the first to criticize the indiscriminate use of humidifiers in ITU.70Manthous CA Schmidt GA Resistive pressure of a condenser humidifier in mechanically ventilated patients.Crit Care Med. 1994; 22: 1792-1795Crossref PubMed Scopus (25) Google Scholar They concluded that the humidifier added a significant resistance to the ventilator circuit that may lead to incorrect assessment of respiratory system mechanics; to inappropriate therapy (e.g. bronchodilators); or to difficulty in weaning from mechanical ventilation. Le Bourdelles and colleagues also reported difficulties weaning patients from mechanical ventilation when using HMEs in an ITU.63Le Bourdelles G Mier L Fiquet B et al.Comparison of the effects of heat and moisture exchangers and heated humidifiers on ventilation and gas exchange during weaning trials from mechanical ventilation.Chest. 1996; 110: 1294-1298Abstract Full Text Full Text PDF PubMed Scopus (72) Google Scholar These authors’ findings and their concerns were similar to those of others dealing with weaning from mechanical ventilators. Problems primarily related to airways resistance persisted. Pelosi and colleagues warned that as HME substantially increase minute ventilation, ventilatory drive, and work of breathing, these devices should be used carefully in patients with acute respiratory failure during pressure-support ventilation.83Pelosi P Solca M Ravagnan I et al.Effects of heat and moisture exchangers on minute ventilation, ventilatory drive, and work of breathing during pressure-support ventilation in acute respiratory failure.Crit Care Med. 1996; 24: 1184-1188Crossref PubMed Scopus (90) Google Scholar However, they pointed out that an increase in pressure-support ventilation (5–10 cm H2O) might compensate for the increased work of breathing. Iotti and colleagues concluded that HMEs cause unfavourable mechanical effects by increasing inspiratory resistance, ventilation requirements, and dynamic intrinsic PEEP.58Iotti GA Olivei MC Palo A et al.Unfavorable mechanical effects of heat and moisture exchangers in ventilated patients.Intensive Care Med. 1997; 23: 399-405Crossref PubMed Scopus (84) Google Scholar They warn clinicians to consider these effects when setting mechanical ventilation and when assessing patients’ ability to breathe spontaneously. In contrast, Subayi and colleagues reviewed the literature following a Medline search. They concluded that despite reservations such as those above and others like them, that the evidence in the literature not only supported using an HMEF but that they were highly recommended in intensive care and are essential during anaesthesia.99Subayi L Chergui K Beydon L Filtres echangeurs de chaleur et d'humidite pour le conditionnement des gaz inspires en anesthesie-reanimation de l'adulte.Ann Francaises Anesth Reanimation. 1998; 17: 699-708Crossref PubMed Google Scholar There is no doubt that circuit colonization occurs in unfiltered long-term ventilated patients in ITU. No one would risk cross contamination between patients by reusing such circuits on consecutive patients. The significance of circuit colonization in a patient's own breathing system is, however, largely unknown. Circuit colonization is not necessarily associated with an increased incidence of pneumonia. Dreyfuss and colleagues were able to demonstrate that ventilator tubing contamination was considerably reduced with the use of a heat and moisture exchanger.37Dreyfuss D Djedaini K Gros I et al.Mechanical ventilation with heated humidifiers or heat and moisture exchangers: effects on patient colonization and incidence of nosocomial pneumonia.Am J Respir Crit Care Med. 1995; 151: 986-992PubMed Google Scholar In contrast, bacterial colonization of the pharynx and trachea was identical to that found in the control group without a HME. The authors suggest that provided the usual hygiene and maintenance precautions are applied, circuit colonization plays little or no role in the occurrence of ventilator-associated pneumonia. The way in which HMEs are used in ITU is changing subtly with doubts being cast on the necessity to change filters frequently. The need to replace more recent designs of HMEs on a daily basis during mechanical ventilation has been investigated by Davis and colleagues.36Davis K Evans SL Campbell RS et al.Prolonged use of heat and moisture exchangers does not affect device efficiency or frequency rate of nosocomial pneumonia.Crit Care Med. 2000; 28: 1412-1418Crossref PubMed Scopus (79) Google Scholar The authors investigated recently introduced hydrophobic or hygroscopic HMEs and found that using the device for 3 days between changes does not diminish their efficiency, increase resistance, or alter bacterial colonization. They noted that with this regime there was also no increase in the frequency of nosocomial pneumonia. They concluded that it is safe and cost effective to use HMEs for >24 h, and certainly up to 72 h. Ricard and colleagues extended the change interval and concluded that mechanical ventilation can be safely conducted in non-COPD patients using an HME changed only once a week, and that this is safe, efficient, and cost-effective.88Ricard JD Le Miere E Markowicz P et al.Efficiency and safety of mechanical ventilation with a heat and moisture exchanger changed only once a week.Am J Respir Crit Care Med. 2000; 161: 104-109Crossref PubMed Scopus (91) Google Scholar This interval between the need to change used filters for fresh ones has been confirmed by Han and colleagues who investigated the possibility of retaining the HME for 7 days and also found the devices to be equally effective after this interval.48Han JN Liu YP Ma S et al.Effects of decreasing the frequency of ventilator circuit changes to every 7 days on the rate of ventilator-associated pneumonia in a Beijing hospital.Resp Care. 2001; 46: 891-896PubMed Google Scholar Littlewood and Durbin take the argument a stage further and contend that there is little evidence to support using HME at all, and that there is substantial evidence to support the changing of ventilator circuits no more frequently than once every 7 days.67Littlewood K Durbin jr, CG Evidenced-based airway management.Resp Care. 2001; 46: 1392-1405PubMed Google Scholar However, until the change interval has been investigated further, it is important that anaesthetists understand the significance and responsibilities of using devices outside the limits of their product licences. Atkinson and colleagues found that the terms for filters, HMEFs and HMEs are commonly used interchangeably without reference to their efficiency in respect of either bacteriological function or heat and moisture conservation.6Atkinson MC Girgis Y Broome IJ Extent and practicalities of filter use in anaesthetic breathing circuits and attitudes towards their use: a postal survey of UK hospitals.Anaesthesia. 1999; 54: 37-41Crossref PubMed Scopus (17) Google Scholar A consequence of the interchangeable use of the terms for filters and HMEs is that anaesthetist's attitudes towards filters are varied and inconsistent as to both their cost effectiveness and their ability to prevent cross contamination between patients. Wilkes and colleagues remind us that the use of HMEs may be inappropriate for some patient groups and the significance of their placement in anaesthetic circuits is poorly understood.106Wilkes AR Benbough JE Speight SE Harmer M The bacterial and viral filtration performance of breathing system filters.Anaesthesia. 2000; 55: 458-465Crossref PubMed Scopus (50) Google Scholar This lack of differentiation between different devices may have an unexpected impact on the more complex circumstances of an anaesthetic circuit compared with an intensive care ventilator, especially in conjunction with circle absorbers. Elucidating the problems associated with filters in these circumstances depends more on laboratory investigations, case reports and correspondence rather than prospective clinical trials. Current product licences require that single use anaesthetic circuits be used once and replaced between patients to eliminate the potential, but unclear risk, of infection between consecutive patients. The financial implications are enormous. The strategy of incorporating a fresh disposable filter in the circuit and changing this, whilst retaining the circuit between cases, has been suggested by Berry and Nolte as a cheaper alternative to discarding the circuit.15Berry A Nolte F An alternative strategy for infection control of anesthesia breathing circuits: a laboratory assessment of the Pall HME Filter.Anesth Analg. 1991; 72: 651-655Crossref PubMed Google Scholar The consequence of this recommendation is that filters are now commonly placed on every anaesthetic circuit without reference to pertinent arguments regarding the wisdom of such action. Laboratory studies point to the potential of bacterial cross contamination between patients using unprotected anaesthetic circuits. The magnitude of potential cross contamination between patients during routine and short procedures in fit, well patients, whose immune system is not compromised is, however, largely unknown. Contamination of circuits is not the same as contamination occurring between two consecutive patients on an operating list. The Association of Anaesthetists (GBI) have published recommendations regarding the use of bacteriological filters and their use in preventing cross infection.5Association of Anaesthesists of Great Britain and Ireland. Blood Borne Viruses and Anaesthesia: an update, January 1996Google Scholar Wilkes and Stevens provide additional guidance;112Wilkes AR Stevens AJ Guidance on the use of breathing system filters.Anaesthesia. 2000; 55: 187Crossref PubMed Scopus (1) Google Scholar however, Stevens warns of inconsistencies in performance between filter manufacturers, the consequence of which is that not all filters are equally effective.98Stevens J Breathing system filters.Anaesthesia. 1999; 54: 90Crossref PubMed Scopus (3) Google Scholar This difference in filter efficiency between manufacturers may explain the contradictory nature of articles investigating the beneficial effects on contamination of filtration of anaesthetic circuits. The original report by Chant and colleagues that reawakened us to the possible role that liquid from an unfiltered breathing circuit played in transmission of Hepatitis C virus came from Australia.28Chant K Kociuba K Munro R et al.Investigation of possible patient-to-patient transmission of hepatitis C in a hospital.NSW Public Health Bull. 1994; 5: 47-51Crossref Google Scholar In this report, consecutive use of the anaesthetic circuit was thought to be the link between the patients. Hepatitis transmission through a reusable part of an anaesthetic system was thought to be a possible mode of transmission in a further incident reported by Heinsen and colleagues.53Heinsen A Brendtsen F Fomsgaard A A phylogenetic analysis elucidating a case of patient-to-patient transmission of hepatitis C virus during surgery.J Hosp Infect. 2000; 46: 309-313Abstract Full Text PDF PubMed Scopus (21) Google Scholar Although the virus involved was clearly genetically identical in the two patients, this specific route of transmission could not be established conclusively. If cross contamination with hepatitis did occur in the manner suggested, what would have been the effect of using a filter? If a filter is used, which one should it be? It is by no means clear that the presence of a filter would have prevented such an unusual means of transmission of Hepatitis C, or even whether we can trust all filters to protect our patients against this or any other organism if filters are used. The barrier to organism transmission varies in effectiveness between different filter manufacturers’ products. Lloyd and colleagues investigated this problem by examining the transfer of viruses through filters and concluded that not all filters are equally suited to preventing the passage of viruses.68Lloyd G Howells J Liddle C Klineberg PL Barriers to hepatitis C transmission within breathing systems: efficacy of a pleated hydrophobic filter.Anaesth Int Care. 1997; 25: 235-238PubMed Google Scholar Leijten and colleagues contested the old idea that halothane and soda lime had bactericidal properties and demonstrated, in a laboratory experiment, that without a HMEF the whole interior of the anaesthetic circuits becomes contaminated with bacteria thus supporting the use of filters.64Leijten D Rejger V Mouton R Bacterial contamination and the effect of filters in anaesthetic circuits in a simulated patient model.J Hosp Infect. 1992; 21: 51-60Abstract Full Text PDF PubMed Scopus (29) Google Scholar The protection of the anaesthetic circuits by a filter is also tentatively supported by Vezina and colleagues.103Vezina DP Trepanier CA Lessard MR Gourdeau M Tremblay C Anesthesia breathing circuits protected by the DAR Barrierbac S breathing filter have a low bacterial contamination rate.Can J Anaesth. 2001; 48: 748-754Crossref PubMed Scopus (15) Google Scholar These authors demonstrated that using a sterile DAR Barrierbac S breathing filter for every patient was highly effective at preventing, but not eliminating contamination of the anaesthesia breathing circuit. They calculated that the use of this filter would result in a cross contamination rate of the breathing circuit less than once in every 250 cases. du Moulin and Sauberman investigated this problem many years ago and concluded that anaesthetic machines were unlikely sources of contamination and that basic hygienic management of anaesthesia machines ensured patient safety.38du Moulin CG Saubermann AJ The anaesthesia machine and circle system are not likely to be sources of bacterial contamination.Anesthesiology. 1977; 47: 353-358Crossref PubMed Scopus (47) Google Scholar Garibaldi and colleagues, in a blinded prospective study of 520 patients undergoing anaesthesia, investigated the use of filters during anaesthesia and concluded that circuit filters played no part in the incidence of postoperative chest infection.43Garibaldi RA Britt MR Webster C Pace NL Failure of bacterial filters to reduce the incidence of pneumonia after inhalation anaesthesia.Anesthesiology. 1981; 54: 364-368Crossref PubMed Scopus (52) Google Scholar Hogarth goes one step further, and points out that although filters can be shown to decrease circuit colonization in laboratory studies, they have not been proven to decrease ventilator-pneumonia infection rates.57Hogarth I Anaesthetic machine and breathing system contamination and the efficacy of bacterial/viral filters.Anaesth Intens Care. 1996; 24: 154-163PubMed Google Scholar He contends that the use of filters is based less on science than on defensive medical practice. In a clinical study, Rathgeber and colleagues found that only 13% of anaesthetic circuits become contaminated with bacteria if used without filters.87Rathgeber J Kietzmann D Mergeryan H et al.Prevention of patient bacterial contamination of anaesthesia circle systems: a clinical study of the contamination risk and performance of different heat and moisture exchangers with electret filter (HMEF).Eur J Anaesthesiol. 1997; 14: 368-373Crossref PubMed Scopus (27) Google Scholar In this study the author concedes that circuit contamination occurs, but despite this he was unable to confirm either an increase or a decrease in the incidence of patient infections. More recently, Body and Philip came to the conclusion that the use of filters may not influence the incidence of cross contamination and expressed their reservations in using circuit filters.21Body SC Philip JH Gram-negative rod contamination of an ohmeda anesthesia machine.Anesthesiology. 2000; 92: 911Crossref PubMed Scopus (3) Google Scholar Following their investigations of water trap contamination on their anaesthetic machines despite using filters, they have elected to use disposable circle anaesthesia circuits and a regular (daily) cleaning programme without the use of filters. The inconsistency between the results of these studies lies in the failure of manufacturers, until recently, to state the efficiency of their filters. Many manufacturers claim 99.95% efficiency for their filters, whilst the early recommendations from Lumley and colleagues were that filters should be at least 99.9977% efficient (i.e. only 23 per 1×106 organisms to pass through). 99.99999% efficient is 10 times more efficient than 99.9999%, 100 times more efficient than 99.999% and 1000 times more efficient than 99.99%.69Lumley J Gaya H Holdcroft A Darlow HM Adams DJ Expiratory bacterial filters.Lancet. 1976; 2: 22-23Abstract PubMed Scopus (7) Google Scholar Even if filter efficiency is published by a manufacturer, a confounding factor is the microenvironment of the circuit and the filter that develops in a clinical setting. Not all filters perform in the same manner when presented with either wet or dry challenges. Hedley and Allt-Graham published two papers revealing their findings that some filters are better at filtering dry particles, and some at wet particles or fluid.51Hedley RM Allt-Graham J Heat and moisture exchangers and breathing systems.Br J Anaesth. 1994; 73: 227-236Crossref PubMed Scopus (37) Google Scholar 52Hedley RM Allt-Graham J Comparison of the filtration properties of heat and moisture exchangers.Anaesthesia. 1992; 47: 414-420Crossref PubMed Scopus (54) Google Scholar Mebius investigated, in the laboratory, the physical characteristics, humidification efficiency, filtration capability, and resistance to flow of six commercially available HMEs" @default.
• W2127421057 created "2016-06-24" @default.
• W2127421057 creator A5076858262 @default.
• W2127421057 date "2003-08-01" @default.
• W2127421057 modified "2023-10-14" @default.
• W2127421057 title "Hidden hazards and dangers associated with the use of HME/filters in breathing circuits. Their effect on toxic metabolite production, pulse oximetry and airway resistance" @default.
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