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BACTERIAL/VIRAL FILTERS IN PULMONARY FUNCTION DEPARTMENTS 10 144 THE BUYERS’ GUIDE TO RESPIRATORY CARE PRODUCTS

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Routine pulmonary function testsmay require patients to performmaximal inspiratory and expiratorybreathing manoeuvres. They maybe asked to rebreathe via thebreathing circuits and equipment,which may be difficult to disinfectbetween patients. Infection can betransmitted by direct or indirectcontact via mouthpieces andimmediate proximal surfaces of thevalves or tubing within theequipment, and possibly by aerosoldroplet formation.

Patients performing pulmonaryfunction tests may generate flows(peak expiratory flows) as high as12 L·s-1 (720 L·min-1). Infectivedroplets may be expelled duringthese forced expirations, whichcould contaminate the pulmonaryfunction equipment. A subsequentpatient carrying out the samemanoeuvre on this equipment could

inhale these infective dropletsduring the forced inspiratory phase.Very few bacteria are required tofacilitate the infectious process ofsome diseases, such as tuberculosis,and therefore the potential risk ofcross-infection using this particulartype of equipment may becomparatively high [1].

In order to essentially eliminate anypotential risk of cross-infection, abacterial/viral filter can be placedbetween the patient’s mouth andthe test equipment. It is commonpractice to use bacterial/viral filterswhen patients are known to beinfectious or immunocompromised.However, as the infectious status ofmany patients is unknown, it maybe prudent to presuppose that anypatient performing the tests couldbe infectious. Using a single-usebacterial/viral filter for all patientsovercomes this issue.

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BACTERIAL/VIRAL FILTERS IN PULMONARY FUNCTIONDEPARTMENTSD. Cramer and S. Ward

CorrespondenceD. Cramer, S. WardLung Function UnitRoyal Brompton HospitalFulham RoadLondonSW3 6HPUK

E-mail: [email protected];[email protected]

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What are bacterial/viralfilters? And how do theywork?

Filters used in pulmonary functionlaboratories are consumable itemsused for trapping bacteria andviruses, ensuring prevention of anycross-contamination. The filtersgenerally consist of a flat wad ofelectrostatically charged fibres orpleats formed from a wad of fibres.These filters may/should havehydrophobic properties.

The electrostatically charged fibresare of two types: fibrillated ortribocharged. Fibrillated fibres aremade by splitting sheets ofelectrostatically chargedpolypropylene. Tribocharged fibresare created by rubbing two types offibre together (polypropylene andmodified acrylic) [1]. In the firstinstance, bacteria and viruses aretrapped as they pass through theinterlocking fibres of the filtermaterial. They are further attractedto these fibres by the positive andnegative electrostatic charges on thefibres.

The pleated filters, also known asmechanical filters, achieve theirefficiency by making use of tightlypacked layers of mixed strands offibres that physically preventbacteria and viruses from passingthrough. These fibres have ahydrophobic coating that repels anywater droplets containing bacteriaor viruses thus preventing thepassage of these organisms.

The efficiency of filtration dependson the density of the fibres, thedepth of the filter layer and thevelocity of the gas to be filtered.Direct interception or filtrationremoves large particles (>1 µm)whose diameter is greater than thatof the pores of the filter membrane.Inertial impaction results in theremoval of smaller particles (0.5–1.0µm in diameter) by collision withinthe filter material. Diffusionalinterception removes very smallparticles (<0.5 µm) due to theirBrownian motion, which increases

the likelihood that they will collidewith the filter material [2].

Issues for considerationwhen making a choice offilters

There are several factors to considerprior to purchasing bacterial/viralfilters.

Bacterial removal efficiency (BRE)

A system has been developed at theCentre for Emergency Preparednessand Response (Porton Down, UK)enabling the efficiencies of manytypes of microbial filters to beassessed, including filters used withpulmonary function equipment.

An apparatus developed originallyby HENDERSON [3] and DRUETT [4] tostudy experimental airborneinfection is used. A suspension ofmicro-organisms in aqueoussolution is nebulised by a three-jetcollision spray forming a fineaerosol containing viable microbes.It is designed to deliver a challengeof >107 Bacillus subtilis spores inaerosol at a relative humidity of≥96% at 30 L·min-1.

The efficiencies of the filters arecalculated by determining theairborne concentration of viablemicro-organisms upstream anddownstream of the filter usingsuitable aerosol samplingtechniques and microbial assaymethods.

More recently, an alternative systemfor assessing BRE was developed atthe National Institute forOccupational Safety and Health(Atlanta, GA, USA) [5]. An aerosolof sodium chloride particles with amedian diameter of 0.07 µm isgenerated from a 2% sodiumchloride solution. Samples of air aredrawn from the upstream(challenge) and downstream(penetration) sides of the filterthrough two laser photometers.These photometers measure theconcentration (mass of sodium

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chloride particles per unit volumeof air) in the challenge and in theair that passes through the filter [6].Manufacturers can currently offerfilters with efficiency levels of>99.9%.

Resistance

A filter must have maximalefficiency in trapping andremoving bacteria and viruses, butit should also have a low resistanceto airflow. Depending on the typeof breathing manoeuvre beingperformed, resistance must bechecked at the appropriate flowrates (peaking >12 L·s-1, 720 L·min-1

in some subjects). The AmericanThoracic Society criteria forpulmonary function testing statethat the resistance of therespiratory circuit (including thebacterial/viral filter) must notexceed 1.5 cmH2O·L-1·s-1 for flowsup to 14 L·s-1 [7]. During

measurement of airway resistanceusing a plethysmograph, theresistance of the filter in the circuitshould be established. With mostmodern software, the additionalresistance offered by the filtershould be taken into account whencarrying out the calculations. Iffilters are used during pulmonaryfunction tests, when calibratingpneumotachograph devices forvolume it is essential to place afilter between the calibrationsyringe and the instrument, as theadded resistance of the filter willaffect pressure changes across thepneumotachograph.

It appears that the filters with thelowest resistances to flow are thosewith the poorest BRE, implying thatBRE may be sacrificed for a lowresistance. If the resistance of therespiratory circuit is too high, theremay be clinically significant effectson the pulmonary function results

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Micro Medical LtdPO Box 6, Rochester,

Kent ME1 2AZ,United KingdomTel +44 (0) 1634 893500

Fax +44 (0) 1634 893600

Infection Control

• 99.99% efficient bacterial / viral filter

• Designed to stop moisture droplets passing to and from the spirometer

• Protects both patient and spirometer

• Should be used when an inspiratory manoeuvre is performed or when testing patients who pose a health risk to others

'SpiroSafe' filter shown with Paediatric adaptor and mouthpiece

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obtained, particularly peak flowrates [8, 9].

Dead space

Generally the dead space of thebacterial/viral filter should be assmall as possible in order that nodetriment to the work of breathingis experienced by the patient. Forsome patients with small lungvolumes (young children orpatients with severe pulmonarydisease), it is even more importantthat the dead space is reduced to itsminimum, otherwise rebreathingissues may occur. Currentlymanufacturers supplyingbacterial/viral filters for pulmonaryfunction equipment can offer deadspaces on their equipment ofbetween 50–75 mL.

Single use

Clearly, the bacterial/viral filters aresolely intended for single-patientuse. This is absolutely necessary toprevent any cross-contamination.

However, the filter can be used forthe same patient for severalmanoeuvres. In reality, thisgenerally means that a patient canuse the same bacterial/viral filterfor the duration of the pulmonaryfunction tests. After the tests arecompleted, the filter must bedisposed of according to localinfection-control procedures. Somemanufacturers will supply a filterwith a reusable filter housing and adisposable pad. The filter housingmust be disinfected betweenpatients in accordance with localinfection-control procedures and thepad must be disposed ofaccordingly.

Multiple equipment use

It is appropriate that the filters canbe adapted to fit the multiple typesof pulmonary function equipmentthat is currently available. Also,many of the filters now available,allow the patient to use the filteritself as a mouthpiece, consequentlyreducing dead space and cost.

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Name KoKo Moe Spirobac Microguard BVF ErgoFilter SP1 All_flow Protec 30s

Company nSpire Tyco Viasys Vitalograph Pulmolink Clement Clarke Pall

Bacterial filtration % >99.99 >99.9 >99.9 99.5 >99.98 99.99 -

Viral filtration % >99.9 >99.9 >99.0 99.4 >99.98 99.99 -

Resistance kPa·L-1·s-1 0.038–0.067 0.09 @12 L·s-1 <0.70 0.08 @12 L·s-1 0.06 @12 L·s-1 0.023 <0.15

Dead space mL 50 56 50 65 65 35 40

Material 3M Filtrete Electrostatic - Electrostatic 3M Electrostatic Electrostatic

Weight g - 14 - - - - -

Single patient use Yes Yes Yes Yes Yes Yes Yes

Dimensions mm Depends on 30F–33M. 30 ID; 30 OD Depends on 30 ID; 33 OD Depends on 30 IDsystem to be Adaptors system to be system to be used with, available used with, used with,i.e. Jaeger, i.e. Jaeger, i.e. Jaeger, KoKo etc. KoKo etc. KoKo etc.

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Why use bacterial/viralfilters?

All patients are susceptible to therisk of infection after performingpulmonary function tests. Pre-pulmonary function test screeningfor infection by request form,although helpful, cannot be asubstitute for more effective controlmeasures. Most outpatients visitingpulmonary function departmentsare not routinely screened forinfectious diseases prior toperforming tests. Even whenpatients are screened, there may bea significant time interval betweenobtaining culture results andperforming the tests. It is verydifficult to identify all the patientswith infectious diseases or who areimmunocompromised. A recentstudy showed as many as 40% ofpatients with chronic obstructivepulmonary disease (COPD) hadpositive sputum cultures topotentially pathogenic micro-organisms [10]. Therefore, universalstringent precautions for everyoneneeding pulmonary function tests

are necessary [11]. A previous paperhas shown that ultra-cleantechniques can be used whenperforming most routine pulmonaryfunction tests [12]. However, themost practical and cost-effectiveway to ensure that there is no risk ofcross-infection between patients isto use bacterial/viral filters.

Other advantages offered by usingbacterial/viral filters are as follows:protection of breathing circuits,especially flow sensors, fromcontamination with droplets ofsaliva and mucus that mayintroduce errors in testmeasurement and contain micro-organisms [13, 14]; and protectionfor patients and staff from inhalingpathogens from the breathingcircuitry. (Many centres now usestaff to perform pulmonary functiontests on the equipment (biologicalcontrols), which can be used as partof the quality-assuranceprogramme.)

It is widely recognised thatrespiratory equipment is not

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Name Spirosafe Spiroguard 2800/22 Spiroguard 2800/R Spiroguard 2800/21

Company Micro Medical Air Safety Air Safety Air Safety

Bacterial filtration % 99.99 99.999 99.999 99.999

Viral filtration % 99.99 99.999 99.999 99.999

Resistance kPa·L-1·s-1 - 0.07 @12 L·s-1 0.07 @12 L·s-1 0.07 @12 L·s-1

Dead space mL - 75 75 75

Material - Electrostatic Electrostatic Electrostatic

Weight g - - - -

Single patient use Yes Yes Re-usable Yes

Dimensions (ID; OD mm) 30 Depends on system to Depends on system to Depends on system tobe used with, i.e. Jaeger, be used with, i.e. Jaeger, be used with, i.e. Jaeger,KoKo etc. KoKo etc. KoKo etc. Includes

integral mouthpiece

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sterile [15] and that exposure tonormal levels of environmentalorganisms during testing poses nogreater risk than being in publicareas [16]. However, as alreadysuggested, some patients arecolonised with potentiallydangerous levels of pathogenicorganisms and in order tominimise any potential risks topatients it is prudent to usebacterial/viral filters whenperforming pulmonary functiontests. A recent study assessed theefficacy of a single usebacterial/viral filter (Spiroguard2800, Air Safety Ltd, Morecambe,UK) for the prevention ofequipment contamination duringpulmonary function assessment[17]. The outcome of the studywhich included two groups ofpatients (infectious andnoninfectious) showed that it wasvery important to use filters whenperforming pulmonary functiontests as bacteria, includingpathogenic organisms, can freelybe transmitted to the equipment.The study results indicated asignificantly greater bacterialgrowth on the proximal side of the

filter compared with the distalside.

Where and when canbacterial/viral filters beused?

It appears that many pulmonaryfunction departments in hospitalsare now using bacterial/viral filtersand awareness of the necessity forsuitable infection controlprocedures is well promoted. Thereare still, however, many hospitalswhere bacterial/viral filters are notbeing used, either because of cost orbecause there is insufficientawareness or knowledge regardinginfection control. Nowadays,infection-control nurses play animportant role in educating staffabout the importance of reducingrisk for patients undergoingpulmonary function tests.

In the general practitioner setting,however, it would seem thatrelatively few patients are givenbacterial/viral filters whenperforming pulmonary functiontests. This is an area where very

little screening of the patient’sinfectious status has been carriedout prior to testing and the use ofuniversal stringent precautionswould seem appropriate. Again,cost appears to be the reason thatbacterial/viral filters are not morereadily used, even though mostdisposable filters cost <£1 (<€1.5)each. It may also be a question ofeducating doctors and practicenurses.

As more spirometry is performed inthe community, it is clearly veryimportant that the practice of usingbacterial/viral filters on all patientsis adopted universally. Patientbenefit, while paramount, is not theonly consideration: we live in avery litigious society andconsequently hospitals are onoccasion sued for large sums ofmoney. By using bacterial/viralfilters, the risk of any cross-infectionbetween patients during pulmonaryfunction testing is reduced andtherefore at the same time, so arethe chances of a patient trying tosue the hospital trust over poorinfection control in the pulmonaryfunction laboratory. ■

Filter in use

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PRODUCT LISTINGS

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www.micromedical.co.ukPhone: +44 (0) 1634 893500

Micro Medical Consumables

� Nose Clips

� Disposable One-way Safety Mouthpieces

� Paediatric Disposable Mouthpieces

� Paediatric Adaptor

� 'SpiroSafe' Pulmonary Filters

� SpiroSafe Paediatric Adaptor

� Adult Disposable Mouthpieces

� Branded Mouthpieces

� Paediatric Mouthpiece Dispenser Pack

� Adult Mouthpiece Dispenser Pack

� Thermal Printer Paper

� Archive Printer Paper

� PeraSafe Sterilising Powder

� Universal Mouthpiece

� Lung Function Calculator

� Touch-screen stylus pens

� Micro CO and Smoke Check Disposable Mouthpieces

� Mouthpiece Adaptor with One-way Valve

� Nasal Probes, 4 sizes

� Rubber Flanged Mouthpiece

� Mouth Pressure Bacterial Filters

� MicroRint Filters

� Face Mask for Rint Transducer

Micro Medical supplies a wide range of low costPulmonary Function consumables including itemsthat are essential in the day to day use of aSpirometer, including SpiroSafe Filters andPeraSafe sterilising powder which will ensurecross infection control for patients during lungfunction testing.

1. Wilkes AR. Pulmonary function testequipment filter facts. Medical DeviceDevelopments. Filtration systemsUniversity of Wales College of Medicine.

2. Demers RR. Bacterial/viral filtration.Chest 2001; 120: 1377–1389.

3. Henderson DW. An apparatus for thestudy of airborne infections. J Hyg Camb1952; 50: 53–67.

4. Druett HA. A mobile form of theHenderson apparatus. J Hyg Camb 1969;67: 437–448.

5. National Institute for Occupational Safetyand Health (NIOSH). Respiratoryprotective devices. Code of federalregulations, title 42, part 84. Morgantown,West Virginia

6. Wilkes AR. Measuring the filtrationperformance of breathing system filtersusing sodium chloride particles.Anaesthesia 2002; 57: 162–168.

7. American Thoracic SocietyStandardization of Spirometry OfficialStatement 1994 update. Am J Respir CritCare Med 1995; 152: 1107–1136.

8. Guimond VJ, Gibson NN. Effect of in-linefilters on spirometry. Can J Respir Ther1990; 26: 9–12.

9. Johns DP, Ingram C, Booth H, WilliamsTJ, Walters EH. Effect of a micro aerosolbarrier filter on the measurement of lungfunction. Chest 1995 Apr; 107:1045–1048.

10. Banerjee D, Khair OA, Honeybourne D.Impact of sputum bacteria on airwayinflammation and health status in clinicalstable COPD. Eur Respir J 2004; 23:685–691.

11. Zhang Y. High justification for universalstringent precautions in lung functiontesting. Letter to editor. Resp Med 2005;99: 1064–1066.

12. Denison DM, Cramer DS, Hanson PJV.Lung function testing and AIDS. RespirMed 1989; 83: 133–138.

13. Townsend MC, Iiankinson JL, LindesmithLA, et al. Is my lung function reallygood? Flow-type spirometer problemsthat elevate test results. Chest 2004;125: 1902–1909.

14. Zhang Y. Using barrier filters to protectspirometer sensors from dropletdeposition. Letter to editor. Chest 2005;127: 2294.

15. Burgos F, Torres A, González J, Puig-de-la-Bellacasa J, Rodrigez-Roisin R, RocaJ. Bacterial colonisation as a potentialsource of nosocomial respiratoryinfections in two types of spirometer. EurRespir J 1997; 9: 2612–2617.

16. Kendrick AH, Johns DP, Leeming JP.Infection control of lung functionequipment: a practical approach. RespirMed 2003; 97: 1163–1179.

17. Unstead M, Stearn MD, Cramer DChadwick MV, Wilson R. An audit intothe efficiency of single use bacterial/viralfilters for the prevention of equipmentcontamination during lung functionassessment. Respir Med 2006; 100:946–950.

REFERENCES

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