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* From Demers Consulting Services, Carmel, CA.
Correspondence to: Robert R. Demers, BS, RRT, 225 Crossroads Blvd, PMB 415, Carmel, CA 93923; e-mail: BobDemers{at}AOL.com
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Abstract |
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 TOP Abstract The Accidental PuristFiltration of Inspired GasesReferences |
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Key Words: bacterial/viral filter • endotoxin • heat/moisture exchanging filter • high-efficiency particulate aerosol • tuberculosis
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The Accidental Purist |
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TOPAbstractThe Accidental PuristFiltration of Inspired GasesReferences |
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Like all aerosols, ribavirin droplets will spontaneously "age" as soon as the particles are generated. Aging results in evaporation of water from the droplets into any carrier gas that is less than saturated. As the aqueous solvent leaves the droplet, (1) the relative humidity of the carrier gas progressively increases, and (2) the concentration of the solution remaining behind in the droplet steadily rises. We tested the circuitry that we had configured for this application by means of a lung model that incorporated the heated-wire tubing circuit (Isothermal; Rancho Cucamonga, CA) that was currently in use in the pediatric ICU at Stanford University Hospital. Under the conditions of use simulating those that would prevail during actual mechanical ventilation of pediatric patients, we observed the formation of crystalline precipitate in the inspiratory limb. This was not altogether unanticipated, because we had observed identical precipitation of this agent during its delivery to babies via a head box. The presence of these crystals was worrisome, though, insofar as their mobilization into the valve box of the ventilator could be expected to wreak havoc with the delicate sensors and transducers of the machine. Consequently, we were determined to filter any and all ribavirin particles that might access the expiratory limb of the ventilator circuit. We initially thought that this would be easy to accomplish by mounting a so-called "bacterial/viral filter" at the point where gases re-enter the valve box of the ventilator. The specific filter that we initially employed was the one that our department already stocked, the MQ-303 (Marquest Medical; Englewood, CO) breathing circuit filter. The specifications for this device, as cited in the package insert, stated that it was "... > 99.9% efficient vs bacteria and viruses." Much to our surprise, however, we discovered that ribavirin aerosol freely traversed the filter.
On the basis of the solubility figures previously cited, it is possible to determine the size of the ribavirin crystals present in the airborne precipitate. Because a sevenfold concentration of the aqueous solution will predictably trigger precipitation, the requisite shrinkage in droplet volume must be sevenfold. The volume of the original droplet is given as the volume of a sphere having a radius of 0.65 µm: 1.15 µm3. The volume of the crystals formed from these droplets is, therefore, one seventh of this figure, or 0.164 µm3. The MMAD of a spherical droplet having this volume is 0.68 µm. This analysis, performed long after our bench testing of the prototype circuit, revealed that we had unwittingly exposed the breathing circuit filter to a fairly rigorous challenge. Aerosol physicists3 point out that the single most penetrable particle known is one having an MMAD of approximately 0.3 µm. Particles larger than this follow straight-line paths when traversing a filter medium having depth, as opposed to a thin, planar medium. The presence of pores or channels, the diameter of which does not exceed this value, will succeed in trapping the particles by simple physical interception. Particles having diameters < 0.3 µm possess so little mass that they are observed to pursue Brownian movement. As they randomly collide with molecules of the carrier gas, they are displaced in a direction opposite to the angle of incidence of the incoming gas molecule. This causes them to follow extremely convoluted paths as they are carried along by their carrier gas. Hence, their effective diameters, in terms of filtration, are appreciably larger than the physical dimensions of the particles themselves, and a depth filter succeeds in trapping such particles by their adherence to the walls of the pores incorporated within the filter matrix. Our bench testing had thus succeeded in accidentally creating an experimental challenge that was equivalent to that which a rigorous purist might design for the purpose of a fairly stringent test of a breathing circuit filter.
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Filtration of Inspired Gases |
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TOPAbstractThe Accidental PuristFiltration of Inspired GasesReferences |
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When we verified that the breathing circuit filter that we had initially employed had failed to block the passage of ribavirin precipitate, we telephoned the manufacturer of the device to inquire about the claim of "99.9% efficiency" contained in the package insert. We were informed that the efficiency data cited had been derived using a test to which the manufacturer referred as the "military spec." This testing methodology was originally developed by the US Army Chemical Corps, and consisted of exposing the filter medium to aerosol particles having a diameter of 3.0 µm. Various commercial laboratories continue to employ tests using 3-µm diameter particles as a challenge aerosol to this day. If the filter medium being tested is capable of blocking 99.9% of this aerosol, this is the efficiency figure that is subsequently cited. But an individual specimen of an airborne bacterium or virus is likely to be far smaller than the particulates used in these tests, and caregivers are entitled to be concerned about the level of protection that they are actually affording to patients whenever they employ a device that has been tested in this fashion.
The water being nebulized during tests of this type is commonly inoculated with various species of organisms. Manufacturers of the media thus tested routinely claim that their filters possess an efficiency of, say, 99.9% vs the organism suspended in the aqueous solution being nebulized, despite the fact that the organism itself is vastly smaller than the droplet within which it is transported. When we were made aware of this practice, our naiveté in assuming that bacterial/viral filters must, by definition, be capable of filtering bacteria and viruses became painfully obvious to us. In point of fact, the design of the tests described herein is so far removed from the conditions that prevail during actual clinical use that the test results derived using this methodology are essentially meaningless.
Separate and distinct from this issue, however, was our immediate goal of identifying a breathing circuit filter that was competent against ribavirin. We proceeded to test several other brands of breathing circuit filter without success. Finally, we tested a resin-bonded ceramic fiber (RBCF) filter (model BB50T; Pall Medical Corporation; East Hills, NY) and found it to be an absolute filter for ribavirin aerosol. This prompted us to specify that this device be used to protect the ventilator that clinicians employ when administering ribavirin aerosol to infants receiving mechanical ventilation.7 8 9 Caregivers are well advised to routinely utilize this type of breathing circuit filter in the inspiratory limb of a mechanical ventilator circuit whenever they intend to protect the patient from inspired contaminants.
We were curious whether the manufacturer of the RBCF filter used the military spec methodology to test that medium, and learned that such was not the case. When the RBCF medium is tested, it is challenged with an aerosol comprised, not of aqueous droplets, but rather of individual bacteria of the species Pseudomonas diminuta.10 "Naked" bacteria (organisms that have been stripped of any enclosing aqueous envelope) are obtained by first nebulizing an inoculum and then using a large volume of dry air as the carrier gas. This succeeds in desiccating the water envelope initially enclosing the bacterium, leaving the bacterial "kernel" behind to be borne by the carrier gas and to impact the filter medium. P diminuta was chosen (1) because its small size, 0.3 µm, renders it the most difficult particle to trap; and (2) because it is nearly identical in size to the tubercle bacillus, the single organism that manufacturers (and users!) of a breathing circuit filter are most anxious to trap. Using this single-bacterium aerosol challenge, the RBCF filter is observed to exhibit an efficiency > 99.999%.
Filtration of Expired Gases
Another potential application of a breathing circuit filter,
certainly, is to trap particles borne by gases traversing the
expiratory limb of a ventilator circuit. This was obviously desirable
in the context already mentioned, wherein the penetration of
particulates into the internal components of a mechanical ventilator
could prove to be harmful to the machine. To the extent that at least
some of those particles might be viable bacteria and/or viruses, an
additional concern exists for those patients and caregivers downstream
from the valve box who could be exposed to a bacterial/viral
microaerosol. Figure 1
depicts various and sundry particles that might be recovered from the
expirate of a patient receiving mechanical ventilation. Note that not
only organisms, but also aerosolized droplets of nebulized drugs such
as pentamidine isethionate, can exhibit a diameter that is submicronic.
Corkery et al11
performed experiments with various brands
of commercially available nebulizers in their studies of pentamidine
prophylaxis of Pneumocystis carinii pneumonia. They found
that the nebulizer (Acorn II; Marquest Medical) that is an integral
component of the apparatus (Respirgard; Marquest Medical) ultimately
recommended for use in this application generates pentamidine in the
form of a monodisperse aerosol having an MMAD of 0.92 µm. Most
caregivers wish to avoid incidental exposure to pentamidine in the
course of supervising the administration of that nebulized drug to
P carinii pneumonia-susceptible patients. Indeed, Marquest
Medical incorporated an MQ-303 filter in the Respirgard circuitry for
that express purpose. Furthermore, it must be recognized that a host of
aerosolized drugs are commonly administered to patients receiving
mechanical ventilation: adrenergic and cholinergic bronchodilators,
steroids, antibiotics, antiviral agents, mucolytics, antiprotozoan
agents, etc. These are all prescription drugs intended for the patient,
and not the caregiver. Clinicians are wise to avoid incidental exposure
to even trace amounts of these drugs. Here again, the placement of a
competent breathing circuit filter in the expiratory circuit represents
a prudent step in accomplishing such a goal.
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Our initial enthusiasm for the RBCF filter that had proven its mettle in our ribavirin circuit prompted us to contemplate using that same device as an HMEF. But, as efficient as that device is as a filter, its performance as an HME leaves much to be desired. Quantitative studies of the humidification capabilities of the BB50T revealed that its water vapor output hovers around 25 mg H2O/L of inspired air,17 whereas the minimum level of humidification recommended by the American Society for Testing and Materials is 30 mg/L.18 In a study of normal subjects breathing through an intact upper airway, Primiano and coworkers19 have shown that, at the level of the carina, the vicinity at which the tip of an indwelling endotracheal tube commonly resides, inspired gas exhibits an absolute humidity of about 32 mg/L. An HMEF incorporating a modified RBCF filter has recently been introduced20 that exceeds this physiologic level (Ultipor; Pall Medical), while preserving the ultraefficient filtration capabilities of the RBCF medium.
Certain clinicians prefer to routinely use active (heated water-bath) humidifiers during mechanical ventilation. And, even those who favor the use of the HMEF in lieu of the active humidifier must recognize that certain contraindications mandate that HMEFs cannot be used for all patients.21 Table 1 enumerates these situations. Clinicians are well advised, for their own protection, to employ a competent bacterial/viral filter in the expiratory limb whenever an active humidifier is in use. The algorithm shown in Figure 2 outlines a strategy that can be used to promote patient/caregiver safety whenever an HMEF is contraindicated.
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Two physicians, Kern and Frumkin,32 noticed that many of their respiratory care practitioner (RCP) colleagues were afflicted with RAD. This prompted them to design a controlled study in order to determine if their own personal impressions could be empirically verified. A questionnaire, eliciting details of symptomatology, was completed by two groups of hospital workers: (1) a study group, consisting of practicing RCPs who frequently visited the bedsides of patients receiving mechanical ventilation; and (2) a control group, comprised of physical therapists and radiologic technicians who only occasionally visited the ICU. At the outset, Kern and Frumkin32 considered it likely that at least some of the RCPs would have had long-standing RAD even before entry into their profession. Therefore, any respondents reporting symptoms of RAD antecedent to initiating their respiratory therapy training were excluded, in order to eliminate the confounding variable of autoselection. These researchers also considered the possibility that RCPs might be hyperaware of pulmonary symptoms, so they randomly subjected therapists reporting RAD symptoms to methacholine challenge testing. The RCPs’ self-grading of symptoms was found to coincide closely with the results of methacholine testing. Using the methods described, the RCPs were found to be more than four times as likely to be afflicted with RAD than the members of the cohort group. A follow-up study33 performed in another medical center generated similar results. One can only speculate what might be eliciting RAD among the therapists, but it is distinctly possible that recurrent exposure to microaerosols emanating from mechanical ventilators is an important contributing factor. Presumably, ICU physicians and nurses will also be vulnerable to microaerosol-induced RAD.
One article34 that appeared in the medical literature failed to show the benefit of bacterial/viral filters, with specific regard to a decrease in the incidence of ventilator-associated pneumonia (VAP). In light of our previous discussion with respect to the incompetence of many filters, this lack of efficacy is hardly surprising. In 1997, however, an article written by Kirton et al35 forcefully demonstrated that a competent HMEF can be spectacularly successful in this context. The incidence of VAP in a randomly selected cohort group managed with heated water-bath humidifiers was 16%, a figure comparable to the rates of VAP reported in ICUs nationally. However, the rate of VAP in the study group (6%), composed of patients fitted with the competent HMEF, was strikingly lower, a difference that was statistically significant to the 95% level of confidence.
It is important that a breathing circuit filter be hydrophobic (water repellant) if it is going to succeed in preventing mobilization of water, watery secretions, bacteria, viruses, and endotoxic suspensions across its filter element. The matrices of filter elements are comprised of water-repellant material. But it must be recognized that various degrees of hydrophobicity exist, attributable to (1) how intrinsically water repellant the fibers actually are, and (2) how tightly or loosely the filter element is woven (Fig 4 , 5 ). Lee and colleagues36 subjected various brands of HMEFs to a hydrostatic head of pressure, and observed the pressure head required to elicit breaching of the media. Four of the five units tested were observed to pass water at transfilter pressures ranging between 10 cm H2O and 14 cm H2O. When a bacterial inoculum was used in lieu of sterile water, suspended bacteria were also observed to be carried through the media of the same filters. Although these four devices might justifiably be billed as HMEs, the label of "filter" certainly seems to represent a triumph of imagination over reality. In contradistinction, the RBCF medium required a pressure head of > 100 cm H2O before breaching (by water, but not by suspended bacteria) was observed to occur. At first blush, it might be suspected that a breaching pressure of 14 cm H2O would pose a risk only to the patient interfaced to the leaky device, because the peak inspiratory pressure during positive-pressure ventilation commonly exceeds this figure by a wide margin in various ventilatory modes. But, even though the absolute pressure upstream of the filter is high during the inspiratory phase, the pressure immediately downstream from it is nearly as high, so that the trans-HME pressure throughout inflation is far lower than 14 cm H2O. Such is not the case during early expiration, however. As soon as the exhalation valve of the ventilator opens at the onset of expiratory flow, the pressure at the downstream (or machine) side of the HME rapidly falls to zero. The end-inspiratory pressure, frequently far > 14 cm H2O, that persists at the proximal (patient) side of the HME will create a trans-HME pressure that is more than sufficient to elicit breaching of the filter element. This situation represents a hazard to those who attend the patient, and potentially to the other patients residing in the ICU in proximity to the patient receiving ventilation, rather than to that patient himself or herself.
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Relative Efficiencies of Filter Media
We have already alluded to the practice of some filter
manufacturers to quote efficiency figures derived by challenging their
media with droplets having diameters of 
3 µm. This might prompt
us to wonder about the efficiency of those same media when bombarded
with the 0.3-µm diameter particles that are more comparable to the
actual bacteria that inhabit the ICU. As it happens,
the performance of some of these media appears, on initial
inspection, to be fairly acceptable. Minnesota Mining and Manufacturing
Company (3M Corporation; St. Paul, MN) manufactures filter media
in various grades under the Filtrete brand designation. The grade of
Filtrete incorporated in many commercially-available HMEFs is
designated G-200. Figure 6
, adapted from data38
supplied to us by 3M Corporation,
depicts the efficiency of G-200 at two face velocities, 10 cm/s and 20
cm/s. The efficiencies of filter media vary with the velocity of the
carrier gas traversing the face of the filter, the so-called "face
velocity." The data shown in Figure 6
reveal that the efficiency of
G-200 at face velocities of 10 cm/s and 20 cm/s is 99.0% and 98.4%,
respectively, when bombarded with an aerosol comprised of droplets
having a diameter of 0.3 µm.
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Numbers of Particles Generated by a Nebulizer
Because the droplets elaborated by a nebulizer, such as the Acorn
II, are infinitesimally small, the sheer numbers of such particles that
the device is capable of producing are astronomically large. Although
Corkery and colleagues11
cite an MMAD of 0.92 µm for
pentamidine particles elaborated by the Acorn II, it is unclear what
degree of aging had occurred at the point in time when the MMAD was
measured. Let us assume that the diameter of the pentamidine particles
generated by the Acorn II was 1.0 µm at the instant that they were
created. The volume of such a spherical particle is exceedingly small:
approximately 0.5 x 10-12
cm3. The number of such particles that will be
produced from a single milliliter of solution is mind-boggling:
approximately 2 trillion (where a trillion is a thousand billion)!
Hence, a filter that is, say, 99.9% efficient when bombarded with
1-µm diameter particles will allow > 2 billion
[0.001 x (2 x 1012)] pentamidine
particles to pass when challenged by the effluent from an Acorn II.
Nevertheless, filtration is certainly a worthwhile exercise; this still
constitutes a miniscule amount of the agent as compared to the dose
delivered to the patient.
If a bacterium is suspended in the solution to be nebulized, the number of bacteria will be far smaller than the number of particles produced. In other words, most of the droplets generated from an inoculum will be comprised of sterile water, devoid of any bacterial "stowaways." Suppose, for example, that the solution residing in the nebulizer cup of the Acorn II were to contain 105 cfu/mL of a bacterium. Suppose further that the aerosol proceeding from the nebulizer was directed onto Filtrete G-200. As mentioned earlier, about 2 billion 1-µm diameter droplets arriving at the face of the medium will pass through it, but only 100 of these droplets will harbor an organism. To be sure, if we are considering the clinical environment and not the aerosol research laboratory, and if the organism in question is a virulent pathogen, even this level of protection is less than comforting to clinicians who practice therein! And, if the particle arriving at the face of the medium is a tubercle bacillus presenting a profile measuring 0.3 µm, caregivers would be prudent to be concerned about their own safety, as well as the welfare of the patients to whom they are rendering care.
The Centers for Disease Control and Prevention Guidelines and
Tuberculosis: a New Standard for Filtration?
In 1994, the Centers for Disease Control and Prevention (CDC)
promulgated guidelines for the treatment of tuberculosis
(TB).39
These guidelines mandate that caregivers don
high-efficiency particulate aerosol (HEPA) masks whenever they care for
nonintubated patients suspected of having active TB. A HEPA-grade
device is defined as any filter capable of trapping at least 99.97% of
particles having a diameter of 0.3 µm. An alternative view of this
definition is the expression of the maximum inefficiency of a HEPA
device, which is simply 0.03%. Once a TB patient is intubated and the
airway isolated by means of an endotracheal or tracheostomy tube, we
would certainly strive to furnish caregivers at least the same level of
protection stipulated in the CDC guidelines. As noted previously,
however, the inefficiency of Filtrete G-200 vs a 0.3-µm aerosol is
1.4%. The relative efficiency of this device can be compared to
HEPA-grade filtration simply by dividing their respective inefficiency
values. This reveals that commercially available HMEFs that incorporate
this medium exhibit an efficiency that is only 1/47th (0.03/1.4) that
of a HEPA-grade filter.40
41
An analogous mathematical
exercise reveals that the RBCF filter medium exceeds the HEPA-grade
specification by > 30-fold (0.03/0.001). These calculations were also
performed for two other HMEFs, the Enternet 9001 (Enternet; Las Vegas,
NV) and the Humid-Vent (Gibeck; Indianapolis, IN); these units
incorporate Electrostat 200/415 and 300/415 (All Felt; Ingleside, IL),
respectively. The results of the calculations revealed them to be
1/43rd and 1/20th as efficient as the HEPA standard.
TB-positive patients are usually identified as such only after being hospitalized for several days, and it would certainly appear to be prudent to mount safeguards against TB for all intubated/tracheostomized patients. This approach is reminiscent of the universal precautions that are currently mandated for caregivers who are exposed to secretions that might harbor the HIV virus. Hence, a persuasive argument could be mounted for routinely employing a filter that incorporates the RBCF medium within the tubing circuit of all patients receiving mechanical ventilation. To the extent that this medium not merely meets, but considerably exceeds, the CDC guidelines, caregivers can enjoy a measure of protection that is appreciably superior to that which is mandated by regulation.
We need to comment on the performance characteristics of the multiple-patient-use nondisposable filters that are incorporated within the expiratory circuit of various commercial ventilators. A sales representative for the Omni filter (Puritan-Bennett Corporation; Carlsbad, CA) advised us that the performance of this device was poised "... on the edge of HEPA... " This renders it far superior to most disposable HMEFs, but still short of the standard enunciated by the CDC guidelines, and a far cry from the level of protection furnished by the RBCF filter.
This writer was elated when recently informed of the availability of two single-patient-use disposable filters, the HEPA and the HEPA Compact (Mallinckrodt Corporation; St. Louis, MO). Because the branding of these units incorporates the acronym for "high-efficiency particulate aerosol," it was natural to assume that each was a HEPA-grade filter. Unfortunately, this cannot be verified. References were provided to us by the manufacturer42 43 purporting to document the superior performance of these filters. Those references disclosed that the filter medium embedded in these units was challenged with an aerosol elaborated from a solution in which the organism Staphylococcus rosaceus was suspended. Although the authors failed to mention the size of the aerosol particles produced by the nebulizer used in these tests, they did specify the size of the organism itself: 0.7 to 1.2 µm! This being the case, there is absolutely no way that these investigators can ascertain whether or not the filter medium that they tested adheres to the HEPA criterion. The diameter of the aerosol particles produced by the nebulizer used by Borghi and coworkers42 43 might have been 3 µm, or even larger. But, irrespective of the size of the aerosol particles produced by their nebulizer, the organisms used in these studies are far too large to undertake a test of any device that aspires to HEPA-grade capabilities. The inescapable conclusion is that the HEPA and HEPA Compact products might be HEPA-grade filters, but, then again, they might not be. Apparently, it is no more advisable to assume that filters branded "HEPA" are necessarily capable of HEPA-grade filtration than to presume that a "bacterial/viral filter" is necessarily capable of filtering bacteria and viruses.
Practical Clinical Ramifications of This Review
What steps might be useful for practicing clinicians to adopt in
light of the foregoing discussion? Several studies have demonstrated
that the placement of a filter between the patient and the ventilator
tubing circuit can succeed in keeping the circuit clean. Gallagher et
al44
demonstrated that ventilator circuits could be left
in place for the entire duration of the patient’s ventilator course
(which was > 3 weeks for several of the patients of Gallagher et al)
without becoming contaminated. Thus, it would appear to be justified to
keep a circuit in place for the duration of a patient’s ventilator
course, rather than to change the circuit at arbitrary intervals,
provided that the HMEF being used is competent. Routine bacteriologic
surveillance at periodic intervals is a wise precaution to employ in
order to verify the ongoing advisability of this practice.
Caregivers might initially be reluctant to leave tubing circuits in place indefinitely, as suggested herein. But it must be remembered that, when Craven and colleagues45 46 first suggested that tubing circuits be replaced less often than once per day, many clinicians vocalized alarm that this would invite an epidemic of VAP.
It seems reasonable to assume that endotoxins will be absent from the expiratory limb if live organisms are not present. It must be recognized that disposable ventilator tubing circuits are supplied in clean, and not sterile, condition. Therefore, one should not be unduly alarmed to document the occasional presence of nonpathogenic organisms, such as Staphylococcus epidermidis, during surveillance programs. If the patient is to be disconnected from the ventilator for any reason (for transport to the CT scanning suite, for example), caregivers need to be mindful to keep the HMEF attached to the tubing circuit in order to keep it clean until the patient returns and is reconnected. Furthermore, it is important to routinely mount a competent filter to the manual resuscitator, in order to prevent the expulsion of a bacterial microaerosol into the air during the episode of manual ventilation that ensues. This practice will also serve to prevent the gross contamination of the bag that has been shown to occur soon after the use of the resuscitator is initiated.47 Generally, a single filter mated to a resuscitator will suffice for the duration of the patient’s intubation, because the cumulative duration of ventilation with this device tends to be quite short. This stands in sharp contradistinction to an HMEF used in the ventilator circuit, which must be routinely replaced at 1-day or 2-day intervals. Of course, if a breathing circuit filter or HMEF becomes saturated with condensate, water in bulk, or secretions, that device must be replaced promptly in order to avoid excessive back pressures from building up in the patient circuit. The gradual increase in the peak inspiratory pressure that occurs secondary to accumulation of liquid within a filter medium will serve to alert the caregiver team that the filter is due to be replaced.
A larger question is at issue with respect to the concept of bacterial/viral filtration, however. A veritable cottage industry has arisen over the last few decades, wherein numerous purveyors of filters have managed to persuade clinicians that filtration, in general, is necessary, and that their filter, in particular, is competent. These claims need to be scrutinized carefully. One of three mutually exclusive scenarios is consistent with reality: (1) the whole concept of bacterial/viral filtration has been oversold, and patient/caregiver hazards traceable to the expulsion of a toxic microaerosol from a mechanical ventilator are seldom, if ever, encountered; (2) pathogenic microaerosols can be expected to access the bedside environment in the absence of a filter, but the deployment of any of the host of filters that are commercially available will prevent this from occurring; or (3) bacterial/viral microaerosols will predictably be expelled into the ambient air of the ICU unless one employs a specific brand/model of filter or HMEF. Which of these applies to the institution in which you practice? The virulence of the pathogenic strains of organisms invading ICUs across the country and around the world appears to be escalating.48 49 50 51 52 The medical director or technical director of any respiratory care department that has expended tens of thousands of dollars for the procurement of filters/HMEFs over successive years would be well advised to undertake the bacteriologic surveillance that is necessary to justify continued use of these units. To persist in purchasing these devices in the absence of a clear clinical hazard is expensive in monetary terms. But to fail to employ a competent filtration strategy in the presence of a documented risk will surely be expensive in terms of morbidity and mortality. Performing such tests is especially important to ascertain the microbiosis of the ICU at hand, rather than to rely on figures reported from surveys, case studies, or even literature reviews such as this one. In this age of evidence-based medicine, this is an eminently reasonable approach. If this empiric approach comes into widespread practice, a clearer picture will emerge with respect to the routine advisability of bacterial/viral filter usage. Hence, it is entirely possible that the collective experience of multiple medical centers with respect to this practice will be reported in the pages of this, or other, journals within a few years’ time. Those of us who practice within the ICU will then be able, literally and figuratively, to breathe a little easier.
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Acknowledgements |
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Footnotes |
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Received for publication January 31, 2001. Accepted for publication April 19, 2001.
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References |
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TOPAbstractThe Accidental PuristFiltration of Inspired GasesReferences |
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berg, B (1986) Bacterial contamination of compressed air for medical use. Anesthesia 41,148-150[ISI][Medline]
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