Humidification Performance of 48 Passive Airway … de Evaluation 48...manufacturers (indication for...

DOI 10.1378/chest.08-0679 2009;135;276-286Chest

Bruno Fumagalli and Laurent BrochardSalvatore Maurizio Maggiore, Philippe Jouvet, Jean-Damien Ricard, François Lellouche, Solenne Taillé, Frédéric Lefrançois, Nicolas Deye, Airway HumidifiersHumidification Performance of 48 Passive

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Humidification Performance of 48Passive Airway Humidifiers*Comparison With Manufacturer Data

Francois Lellouche, MD, PhD; Solenne Taille, Eng; Frederic Lefrancois, Eng;Nicolas Deye, MD, MSc; Salvatore Maurizio Maggiore, MD, PhD;Philippe Jouvet, MD, PhD; Jean-Damien Ricard, MD, PhD; Bruno Fumagalli, Eng;and Laurent Brochard, MD; and Groupe de travail sur les Respirateurs de l’AP-HP

Introduction: Heat and moisture exchangers (HMEs) are increasingly used in the ICU for gasconditioning during mechanical ventilation. Independent assessments of the humidificationperformance of HMEs are scarce. The aim of the present study was thus to assess thehumidification performance of a large number of adult HMEs.Method: We assessed 48 devices using a bench test apparatus that simulated real-life physiologicventilation conditions. Thirty-two devices were described by the manufacturers as HMEs, and 16were described as antibacterial filters. The test apparatus provided expiratory gases with anabsolute humidity (AH) of 35 mg H2O/L. The AH of inspired gases was measured after steadystate using the psychrometric method. We performed three hygrometric measurements for eachdevice, measured their resistance, and compared our results with the manufacturer data.Results: Of the 32 HMEs tested, only 37.5% performed well (> 30 mg H2O/L), while 25%performed poorly (< 25 mg H2O/L). The mean difference (� SD) between our measurementsand the manufacturer data was 3.0 � 2.7 mg H2O/L for devices described as HMEs (maximum,8.9 mg H2O/L) [p � 0.0001], while the mean difference for 36% of the HMEs was > 4 mg H2O/L.The mean difference for the antibacterial filters was 0.2 � 1.4 mg H2O/L. The mean resistance ofall the tested devices was 2.17 � 0.70 cm H2O/L/s.Conclusions: Several HMEs performed poorly and should not be used as HMEs. The valuesdetermined by independent assessments may be lower than the manufacturer data. Describing adevice as an HME does not guarantee that it provides adequate humidification. The performanceof HMEs must be verified by independent assessment. (CHEST 2009; 135:276–286)

Key words: absolute humidity; endotracheal tube; heat and moisture exchangers; mechanical ventilation; occlusions;psychrometry

Abbreviations: AH � absolute humidity; HME � heat and moisture exchanger; HMEF � heat and moisture ex-changer with an antibacterial filter; ISO � International Standard Organization

D uring invasive mechanical ventilation, gases de-livered to the patient must be heated and

humidified.1,2 Heat and moisture exchangers(HMEs) are the most commonly used humidificationdevices in Europe3 and are being increasingly usedin North America.3,4 While some hygrometric stud-ies of passive humidification systems have beenconducted, few have included a large number ofdevices, and clinicians largely rely on data from themanufacturer. Given this conflict of interest, inde-pendent performance assessments are needed.5

In the past, strictly hydrophobic HMEs often ledto endotracheal tube occlusions.6–10 Nowadays, mostHMEs have good hydrophobic and hygroscopicproperties, and many devices have both humidifica-tion and antibacterial-filtering properties. Some,however, should only be used for short-term venti-lation, such as during anesthesia, because they aremainly antibacterial filters with poor humidificationperformance. Despite this, some manufacturers de-scribe them as humidification devices that can beused for prolonged mechanical ventilation. Also, very

Original ResearchCRITICAL CARE MEDICINE

276 Original Research



similar aspects of devices with and without humidi-fication properties can be a source of confusion forclinicians.

There are many commercially available devices,but few large-scale, independent studies havebeen conducted to assess their humidificationperformance.11,12 The goal of the present studywas to assess a large number of such devices withand without humidification proprieties. An invita-tion to tender by the Assistance Publique-Hopi-taux de Paris was used to collect the devices. Wedeveloped a bench test apparatus that reproducedreal-life saturated expired gas conditions in orderto assess the hygrometric performance of thesedevices. We also measured the resistance of thedevices, classified into two categories: HMEs withan antibacterial filter (HMEF) or without anantibacterial filter (HME), and those described asantibacterial filters. The measurements obtainedwith the bench test apparatus were compared tomanufacturer data.

Materials and Methods

We received 47 devices for adults from the manufacturersfollowing the invitation to tender for the HMEs and antibacterialfilters. Five of each type of device were provided by themanufacturer: three were assessed for their humidification per-formances, one was used for resistance measurement, and oneadditional device was available if required. Also, the hydrophobicPall BB2215 filter (Pall Biomedical; Saint-Germaine-en-Laye,

France), which has a poor humidification performance and hasbeen associated with a high rate of endotracheal intubations, wasused as a reference device.6–10 The goal of the present study wasto assess the performance of the devices using a bench testapparatus and to select the most efficient ones for a subsequentclinical trial.

The bench test apparatus was a stable, reproducible model andused the same ventilation and physiologic conditions (wet expi-ratory gases, dry motor gases, and constant ambient temperature)for assessing the HMEs and antibacterial filters. The apparatus isshown in Figure 1. The relative humidity of real-life expired gasesis 100% while the temperature at the T-piece is 32 to 33°C,13–15

which corresponds to an absolute humidity (AH) of 34 to 36 mgH2O/L. We generated expiratory gases with an AH of 35 mgH2O/L using a heated humidifier (MR 730; Fisher&Paykel;Auckland, New Zealand, with a heated wire circuit) connected tothe expiratory limb of a ventilator to simulate real-life expiratoryparameters. The humidity of simulated expired gas was measuredwith the psychrometric method (see below) until the target valuewas reached and then regularly during the study days to ensurethe maintain of a stable value. A Servo 900C (Maquet; Lund,Sweden) ventilator was used in assist-control mode with arespiratory rate of 20 breaths/min, a tidal volume of 500 mL, apositive expiratory pressure at 5 cm H2O, and an fraction ofinspired oxygen of 21%. With this ventilator, motor gases usedare dry medical gases (� 5 mg H2O/L). The room temperaturewas kept constant between 24.5°C and 25.5°C.

Hygrometric Measurements

The parameters of the test apparatus were stabilized during 3 hfor each device on the bench, and then the hygrometry of theinspiratory gases was measured using the psychrometric method.We used a variant of the previously described method: weconsidered the maximal temperature value and not the meantemperature.16,17 This method was used in previous publishedstudies15,18 with hygrometric measurements. Although, this mayslightly overestimate the values of hygrometric measurements,comparison with published data shows very good agreement forsimilar devices tested.17,19 Three measurements for each devicewere recorded on 3 separate days in a randomized order.Forty-eight devices (HMEs and filters) for adult patients wereassessed (47 from the tender and the Pall BB2215). We alsocalculated the humidity efficiency of the devices using thefollowing formula: humidity efficiency � (inspired AH/expiredAH) � 100. This provided information on the intrinsic perfor-mances of the devices. Also we displayed another index ofefficiency: AH delivered/dead space (volume of the device) aspreviously described.11

Resistance Measurements

The resistance of 34 devices was also measured on aseparate bench. Resistance measurements were performed byplacing a pressure sensor (MP45 � 100 cm H2O; Validyne;Northridge, CA) before the filter and by progressively increas-ing the gas flow to 1 L/s. Flow was measured using a Fleisch#1 pneumotachograph (Fleisch; Zurich, Switzerland) con-nected to a differential pressure transducer (MP45 � 2 cmH2O; Validyne) placed before the pressure sensor and thetested device. The pressure after the filter was consideredequal to 0 (atmospheric pressure). The differential pressurethus corresponded to the pressure measured before the filter.The flow and pressure signals were recorded using software(Acknowledge; Biopac Systems; Santa Barbara, CA) with anacquisition frequency of 200 Hz. The analog signals wereconverted into digital signals (MP100WS; Biopac Systems).

*From the Service de Reanimation Medicale (Mrs. Taille,Mr. Lefrancois, and Dr. Brochard), Assistance Publique-Hopitaux de Paris, Centre Hospitalier Albert Chenevier-Henri-Mondor, Creteil, France; Service de Soins Intensifs de ChirurgieCardiaque (Dr. Lellouche), Centre de Recherche Hopital Laval,Universite Laval, Quebec City, QC, Canada; Service de Reani-mation Medicale (Dr. Deye), Hopital Lariboisiere, Paris, France;Istituto di Anestesiologia e Rianimazione-Universita CattolicaPoliclinico A. Gemelli (Dr. Maggiore), Rome, Italy; Service deReanimation Medicale Pediatrique (Dr. Jouvet), Hopital Necker-Enfants Malades, Paris, France; Service de Reanimation Medi-cale (Dr. Ricard), Hopital Louis Mourier, Hopitaux de Paris,Colombes, Paris, France; and Pharmacie Centrale des Hopitaux-Assistance Publique-Hopitaux de Paris (Mr. Fumagalli), Paris,France.Presented in part at the 14th Annual Congress of the EuropeanSociety of Intensive Care Medicine, Geneva, Switzerland, Sep-tember 30 to October 3, 2001.Dr. Brochard, as director of the research laboratory, received agrant from the Hudson Company of $10,000 in 2002, and a grantfrom Fisher & Paykel Company of $8,700 in 2001, with $10,000in 2002 to the same laboratory. The other authors have noconflict of interest to declare.Manuscript received March 11, 2008; revision accepted August13, 2008.Reproduction of this article is prohibited without written permissionfrom the American College of Chest Physicians (www.chestjournal.org/misc/reprints.shtml).Correspondence to: Francois Lellouche, Unite de soins intensifsde chirurgie cardiaque, Centre de Recherche Hopital Laval,Universite Laval, 2725, Chemin Sainte-Foy, QC, Canada, G1V4G5; e-mail: [email protected]: 10.1378/chest.08-0679

www.chestjournal.org CHEST / 135 / 2 / FEBRUARY, 2009 277



Dead space values (devices volume) as well as bacterial andviral filtration efficiency provided by manufacturers are also pre-sented. Lastly, we compared our hygrometric and resistance mea-surements with the manufacturer data when they were available.

Statistical Analysis

Results are expressed as mean � SD. Comparisons betweenour measured data and manufacturer data for AH and resistancewere obtained by Wilcoxon signed-rank test; p � 0.05 was con-sidered significant.

Results

Hygrometric and resistance values obtained on thebench are presented in Table 1. Data coming frommanufacturers (indication for use, hygrometry, resis-tance, dead space, antimicrobial filtration efficiency)are presented in Table 2.

Hygrometry

We assessed 48 devices (47 from tender and theBB 2215) using the bench test apparatus and per-formed three measurements for each device. Thirty-two devices were described as HME or HMEF bythe manufacturers, but some were proposed forhumidification during anesthesia only. The average

AH of the devices was 27.5 � 4.4 mg H2O/L. Twelvedevices performed well (� 30 mg H2O/L), 12 wereintermediate (� 30 to � 25 mg H2O/L), and 8performed poorly (� 25 mg H2O/L). Sixteen deviceswere described as antibacterial filters only by themanufacturers. The average AH of these devices was17.3 � 3.6 mg H2O/L. The hygrometric measure-ments are presented in Table 1 and Figure 2.Humidity efficiency of the devices ranged from 91.1to 37.8% (Fig 3).

We could compare our results with manufac-turer data for 29 devices, 25 of the HMEs and 4 ofthe antibacterial filters. There were no hygromet-ric measurements provided by the manufacturerfor four HMEs, while for two others the hygro-metric values were obtained with tidal volumes of250 mL and 1,500 mL and were thus not used.Among the devices presented as HME or HMEF,55% were evaluated by the manufacturers accord-ing to the International Standards Organization(ISO) 9360 standard (gravimetric method). Themethod was not specified for the other devices.

Of the 29 devices that could be compared,manufacturer data were higher than the valuesdetermined using the test apparatus for 23 devices(79%) [Fig 2]. The mean difference between ourvalues and those of the manufacturers was

One-way valves

Tdry 32.1°C

Hygrometer(PSYCHROMETRIC METHOD)

Bacterial filter

Simulated lung

Device being tested

Heated humidifier (MR 730) simulating expiratory gases

Ventilator

Twet 30.9°C

Figure 1. Hygrometric bench test apparatus used to measure the humidification performances ofHMEs and antibacterial filters. A ventilator delivered controlled cycles (respiratory rate of 20breaths/min with a tidal volume of 500 mL and a positive end-expiratory pressure of 5 cm H2O).A heated humidifier (MR 730) was connected to the expiratory limb of the model and was set todeliver gases with a water content of 35 mg H2O/L at the Y-piece. A circuit with heated wire wasused after the humidification chamber. The room temperature was kept constant between 24.5°Cand 25.5°C.




Table 1—Measured AH Obtained Using the Bench Test Apparatus for All Devices and Measured Resistances for 34Devices*

DeviceNo. Device Make†

Manufacturer(Location)

MeasuredAH, mgH2O/L

� AHManufacturer-Measured

Values, mg H2O/LAH/mL of

Dead Space

MeasuredResistance

at 60 L/min,cm H2O

� ResistancesManufacturer-Measured

Values, cm H2O/L/s

1 Hygrovent Peters 31.9 � 0.6 2.7 0.34 1.8 �0.72 Hygrobac Mallinckrodt 31.7 � 0.7 1.0 0.33 2.1 0.03 Hygrovent S Peters 31.7 � 0.5 2.8 0.58 2.8 �1.24 Hygrobac S Mallinckrodt 31.2 � 0.2 �0.5 0.69 2.3 0.25 HMEF 1000 Datex 31.2 � 0.7 1.8 0.41 NA NA6 9000/100 Allégiance 31.2 � 1.4 2.6 0.35 2.7 �0.77 Servo Humidifier 172

(173)Siemens 30.9 � 0.3 NA 0.56 NA NA

8 Humid Vent FilterCompact

Hudson 30.8 � 0.3 0.2 0.88 2.3 �0.5

9 All round Filter MAP05 Peters 30.7 � 0.2 �0.7 0.34 NA NA10 Hygroster Mallinckrodt 30.7 � 0.6 2.1 0.32 2.3 �0.411 Slimeline HMEF

9040/01Sodis 30.3 � 0.3 2.7 0.67 NA NA

12 BACT HME Ansell 30.1 � 0.5 4.2 0.43 2.4 �1.013 Filtraflux Vygon 29.9 � 0.9 1.1 0.54 NA NA14 HME 12 BASIC Ansell 29.8 � 0.4 3.2 0.37 0.4 0.115 Humid Vent 2 Hudson 29.7 � 0.4 �1.7 1.03 NA NA16 Servo Humidifier 162

(163)Siemens 29.7 � 0.8 NA 0.78 NA NA

17 FE52A Rüsch 29.3 � 0.4 NA 0.35 2.4 NA18 Humid Vent 2S Hudson 29.2 � 0.4 �1.2 1.01 NA NA19 9040/01 Allégiance 28.6 � 1.1 NA 0.61 2.4 NA20 9000/01 Allégiance 28.5 � 0.8 2.9 0.32 3.9 �1.921 Thermovent 1200 Sims 27.8 � 0.0 NA 0.86 NA NA22 BB100E Pall 27.2 � 0.7 3.8 0.32 1.4 0.623 BB100 Pall 26.8 � 0.5 4.2 0.30 2.0 0.524 Clear Therm � Intersurgical 26.2 � 2.1 5.8 0.50 2.1 0.325 Filta Therm Intersurgical 24.9 � 1.1 5.1 0.51 2.9 �0.426 Stérivent Mallinckrodt 23.8 � 0.9 �0.7 0.26 1.9 �0.127 Iso Gard Hepa Light Hudson 23.6 � 0.3 2.2 0.47 2.4 �0.428 Filtraflux Vygon 22.4 � 0.4 NA 0.41 NA NA29 Stérivent S Mallinckrodt 22.2 � 0.2 �1.0 0.36 1.7 0.230 Pall BB 2,215‡ Pall 21.8 � 1.5 NA NA NA NA31 8222/01 Allégiance 20.5 � 0.2 NA 0.28 NA NA32 Maxipleat Sodis 20.1 � 0.6 4.8 0.27 NA NA33 HME 10 Ansell 20.0 � 0.6 8.9 0.74 NA NA34 BB25 Pall 19.6 � 1.4 6.4 0.56 2.6 0.935 All Round Filter

MAP01Peters 19.0 � 0.3 NA 0.27 1.7 �0.1

36 BB2000AP Pall 18.9 � 0.4 7.1 0.54 3.1 0.437 BACT TRAP HEPA Ansell 18.4 � 0.7 NA NA 1.3 NA38 Clear Therm Midi Intersurgical 17.3 � 0.3 5.7 0.72 2.1 0.439 Barr Vent Peters 16.8 � 1.1 NA 0.17 1.4 0.140 Stérivent Mini Mallinckrodt 16.6 � 1.0 0.4 0.47 2.2 0.841 4444/66 Allégiance 16.4 � 0.6 NA 0.35 3.4 NA42 Filta Guard Intersurgical 16.2 � 1.0 NA 0.29 2.3 0.043 4000/01 Allégiance 15.1 � 0.9 NA 0.40 2.2 NA44 Clear Guard II Intersurgical 14.9 � 0.6 NA 0.35 2.1 0.245 Clear Guard Midi Intersurgical 13.7 � 0.8 NA 0.57 1.6 0.546 ISO GUARD

FILTER SHudson 13.4 � 0.6 NA 0.52 2.1 �0.5

47 Barr Vent S Peters 13.3 � 0.8 NA 0.36 1.4 0.348 Barrierbac S Mallinckrodt 13.2 � 0.2 NA 0.38 2.1 �0.3

*The difference between our hygrometric data and those of the manufacturers was 3.0 � 2.7 mg H2O/L for the HMEs (p � 0.0001) and 0.2 � 1.4mgH2O/L for the antibacterial filters (p � 0.99). Mean difference between our measured resistances and those of the manufacturers was– 0.1 � 1.6 cm H2O/L (p � 0.63). NA � not available.

†The numbering is based on the measured hygrometric results of the tested devices.‡Filter BB2215 (not part of the tender) was also assessed.




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Yes

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Yes

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9995

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Tab

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Con

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Indi

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Yes

Yes

Yes

Yes

NA

270.

928

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BB

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Yes

Yes

Yes

(�10

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3.0 � 2.7 mg H2O/L for the HMEs (p � 0.0001).The maximum difference was 8.9 mg H2O/L,while for 9 of the 25 HMEs the difference was � 4mg H2O/L. The mean difference for the antibac-terial filters was 0.2 � 1.4 mg H2O/L (p � 0.99).

Resistance

We performed 34 resistance measurements of the48 tested devices. Once again, we were able tocompare the values for 29 devices. The averageresistance of these devices was 2.17 � 0.70 cm H2O/L/s at 1 L/s, with a maximum of 3.9 cm H2O/L/s anda minimum of 0.4 cm H2O/L/s. The mean resis-tances at 1 L/s were 2.32 � 0.70 cm H2O/L/s and1.98 � 0.53 cm H2O/L/s for the HMEs and antibac-terial filters, respectively. The mean difference be-tween our values and those of the manufacturers was– 0.1 � 1.6 cm H2O/L (p � 0.63). Values are pre-sented in Table 1.

Dead Space

The dead space of the HMEs ranged from 22 to 95mL, with a mean of 60 � 26 mL. The dead space ofthe antibacterial filters ranged from 24 to 101 mL,with a mean of 55 � 24 mL.

Discussion

In the present study, we assessed the humidifica-tion performances and resistance of 48 HMEs and

antibacterial filters, and compared our results tomanufacturer data. The most important result of thestudy was the heterogeneity of the humidificationperformance of the devices. This highlights that thechoice of a suitable HME used for prolonged me-chanical ventilation is very difficult and may have areal clinical impact. In addition, the humidificationperformance of a significant number of the HMEstested was much lower than recommended, whichwould put patients at high risk for endotracheal tubeocclusions.6–10 These HMEs should be avoided forpatients receiving mechanical ventilation. We ob-served a marked difference between the AH valuesof the HMEs obtained using the bench test appara-tus and manufacturer data, with a mean difference of3.0 � 2.7 mg H2O/L. The difference was � 4 mgH2O/L for over one third of the HMEs, while thelargest was 8.9 mg H2O/L. The difference was only0.2 � 1.4 mg H2O/L for the antibacterial filters. Weonly observed small differences for the resistancemeasurements. Because of potentially fatal conse-quences resulting from inadequate humidification,independent assessments of the performance of hu-midification devices should be conducted to enableclinicians to select the most suitable devices.6,7,20

The present study is the largest evaluation ofHMEs and antibacterial filters ever conducted.Branson and Davis11 compared the performances of21 devices according to the ISO 9360 standard usingthe gravimetric method, Cigada et al12 assessed 12devices, and other studies12,16,21–34 assessed smallnumbers of humidification devices using various

15

20

25

30

35

Manufacturers’ data

Measured valuesBB2215 (measured)

1 2 3 4 5 6 8 9 10 11 26 27 29 30 3220 22 23 24 2512 13 14 15 18 33 34 36 38 40* * * *

Abs

olut

eH

umid

ity(m

gH2O

/L)

Figure 2. Inspired AH values obtained using the bench test apparatus compared to manufacturer data when available (29 devices).Results of the hydrophobic HME Pall BB2215 are also displayed. The asterisks indicate the devices that are proposed as antimicrobialfilters. The numbering corresponds to Tables 1 and 2.




hygrometric measurement methods. To our knowl-edge, only two other bench studies16,34 used thepsychrometric method.

In the present study, 12 of the 48 HMEs testeddelivered inspiratory gases with an AH exceeding 30mg H2O/L, compared to 18 according to the manu-facturer (Table 1). This level of AH is recommendedfor humidification devices used during mechanicalventilation.35,36 Twelve other devices delivered gaseswith an AH ranging from 25 to 30 mg H2O/L, whichis usually considered a “gray zone.” The remainingdevices delivered gases with an AH � 25 mg H2O/Land should, in our opinion, be avoided during me-chanical ventilation, apart from limited use for theirfiltration properties. We also assessed the PallBB2215 hydrophobic HME, which has been associ-ated with a high rate of endotracheal tube occlu-sions.6–10 In the present study, this device deliveredgases with an AH of 21.8 � 1.5 mg H2O/L (Table 1).Our results were consistent with clinical resultsobtained using the psychrometric method.19,37 Byextrapolation, our data suggest that using an HMEwith a hygrometric performance close to 20 mgH2O/L carries a high risk of endotracheal tubeocclusion (Fig 4). Clinical results indicate that de-vices that deliver gases with an AH � 30 mg H2O/Lhave a low risk of endotracheal tube occlusions,38–43

even during prolonged use.17,44 However, slightlylower levels may also be safe44–48 (Fig 4).

The humidity efficiency of the most efficientHME exceeded 90% within the experimental condi-tions of our bench test (Fig 3). The efficiency valuesmust be interpreted cautiously because changing thedenominator value (ie, AH of the expiratory gas)could improve or worsen the calculated efficiency ofa given HME. We found it useful, however, topresent these data (Fig 2) because it allows compar-ing any filter with another, and also allows compar-isons with published studies.11,15 In addition, thevalue of AH used in this study represents a veryrealistic value of the normal water content of expiredgases.14,15 This means that all devices tested receivedadequate humidification to meet or exceed theirindividual AH capacity.

Resistance measurements was not the primaryobjective of the study, and therefore we performedresistance measurements only on 34 of the 48 de-vices tested for humidification properties. However,this evaluation was large enough to demonstrate thatthere was no significant difference when comparedwith manufacturer measurements, in opposition withhygrometric comparisons. This may underline thespecific issue of the hygrometric measurements.

Given that the performance of some devices maydecrease with prolonged use, one limitation of thepresent study was that the performances of thedevices were not assessed after prolonged use.17,49

The impact of prolonged use on HME performance

Figure 3. Humidity efficiency of the 48 devices tested calculated using the following formula: humidity efficiency (%) � (inspiredAH/expired AH) � 100. Minimum and maximum efficiencies are indicated.




and clinical consequences remains poorly docu-mented and should be assessed, especially if passivehumidifiers are used as first-line humidification de-vices. In addition, given that the performances ofsome humidification devices are reduced by highambient air and ventilator output temperatures, asecond limitation was that we did not assess theimpact of external factors such as temperature.18

However, it is likely that ambient air and ventilatortemperature have little influence on HME perfor-mance,50 and the room temperature was kept con-stant during the study days between 24.5°C and25.5°C to reduce the influence of this parameter.

The bench test apparatus may also be considered asa limitation; however, it allowed us to compare a largenumber of devices under the same conditions. It hasbeen shown that the efficacy of HMEs depends on theAH of expired gases.15 Studies11,12,22–27,30,31 usingmodels to simulate saturated expired gases used gastemperatures ranging from 32 to 37°C, making themdifficult to compare. In the present study, however,we assessed a large number of devices using a gaswith 35 mg H2O/L (saturated gas at 32.6°C), likeprevious studies13–15 that assessed expiratory gases inpatients receiving ventilation. In clinical studies, thetemperature of expiratory gas depends on the coretemperature of patients and is difficult to control.

Minute ventilation can also have an impact onhydrophobic HMEs.37,49,51 In the present study,minute ventilation was maintained at 10 L/min for allthe devices. It is likely that humidification perfor-mances may be influenced by the tidal volume used,mainly for hydrophobic HMEs.

The psychrometric method used in the presentstudy may also be considered as a limitation sincesome researchers may consider the ISO 9360 as the“reference method.”52 The ISO 9360 using the gravi-metric method is an attempt to homogenize thepractices for HME evaluation and is used by amajority of manufacturers. However, the gravimetricmethod is difficult to set up, and weight-in as well asspirometric measurement errors make this tech-nique difficult to use.11,22 Another limitation of thistechnique is that it was not described for evaluationof heated humidifiers. In addition, the gravimetricmethod cannot be used with patients, and the resultsthus cannot be directly compared with clinical data,unlike the psychrometric method.15,17,49 To date,there are no data that allow to state that one methodis more accurate than another, and there is no “goldstandard” for hygrometric measurements. The ISO9360 standard was used by 55% of the manufacturersfor HME or HMEF evaluation, and this techniquedoes not seem to overestimate humidity measure-

0

5

10

15

20

25

30

35

40

20 25 30 35

Absolute Humidity (mgH2O/L)

% E

ndot

rach

eal t

ube

occl

usio

ns

Figure 4. Frequency of endotracheal tube occlusions reported in the literature6–10,17,38–43,45–48,53–55

compared to the AH delivered by the HMEs measured using the bench test apparatus (Table 1). Onestudy (circle) is an outlier56 with no endotracheal occlusions despite very poor humidification. However,this study56 only included 12 patients, which is not enough to reach a conclusion about the safety of thedevices tested. A hydrophobic HME (Pall BB2215), which delivered a measured AH of 21.8 � 1.5 mgH2O/L in the present study, has been associated with high rates of endotracheal tube occlusions in fivepreviously published studies6–10 (Table 1). A number of studies are represented by gray dots to makethem stand out. The sizes of the dots are proportional to the number of patients in the studies.




ments in comparison with the psychrometricmethod. Indeed, when comparing the performancesof the same HMEs evaluated by the hygrometric orthe gravimetric methods, the differences are small.In the study by Branson and Davis,11 Hygrobac,Hygrobac S, Hygroster, and Sims Portex 1200 pro-vided 32.5, 29.6, 33.2, and 27.2 mg H2O/L, respec-tively, when tested with the gravimetric method. Inthe present study, with the psychrometric method,the same devices provided very close results: 31.7,31.2, 30.7, and 27.8 mg H2O/L (Table 1). Thus, thedifferences we found in the present study betweenour measurements and manufacturer data are likelynot related to the measurement technique.

In conclusion, we conducted a large-scale assess-ment of passive airway humidifiers and showed thatthe humidification performances of these devicesvaried widely. Independent assessments of HMEperformances are required to guide clinicians intheir choice of a suitable device. The distinctionbetween devices with adequate humidification per-formances and those with predominantly antibacte-rial filtering properties should be made muchclearer, and the external appearance of devices withand without humidification proprieties might differ.Lastly, the use of poorly performing HMEs duringinvasive mechanical ventilation can lead to poten-tially fatal complications.

References1 Burton JDK. Effect of dry anaesthetic gases on the respira-

tory mucus membrane. Lancet 1962; 1:235–2382 Ricard JD. Humidification. In: Tobin M, ed. Mechanical

ventilation. New York, NY: McGraw-Hill, 2006; 1109–11203 Ricard JD, Cook D, Griffith L, et al. Physicians’ attitude to

use heat and moisture exchangers or heated humidifiers: aFranco-Canadian survey. Intensive Care Med 2002; 28:719–725

4 Burns KEA, Lellouche F, Loisel F, et al. Weaning critically illadults from invasive mechanical ventilation: a national survey[abstract]. Am J Respir Crit Care Med 2007; 175:A963

5 Thiery G, Boyer A, Pigne E, et al. Heat and moistureexchangers in mechanically ventilated intensive care unitpatients: a plea for an independent assessment of theirperformance. Crit Care Med 2003; 31:699–704

6 Cohen IL, Weinberg PF, Fein IA, et al. Endotracheal tubeocclusion associated with the use of heat and moistureexchangers in the intensive care unit. Crit Care Med 1988;16:277–279

7 Martin C, Perrin G, Gevaudan MJ, et al. Heat and moistureexchangers and vaporizing humidifiers in the intensive careunit. Chest 1990; 97:144–149

8 Misset B, Escudier B, Rivara D, et al. Heat and moistureexchanger vs heated humidifier during long-term mechanicalventilation: a prospective randomized study. Chest 1991;100:160–163

9 Roustan JP, Kienlen J, Aubas P, et al. Comparison ofhydrophobic heat and moisture exchanger with heated hu-midifier during prolonged mechanical ventilation. IntensiveCare Med 1992; 18:97–100

10 Villafane MC, Cinnella G, Lofaso F, et al. Gradual reductionof endotracheal tube diameter during mechanical ventilationvia different humidification devices. Anesthesiology 1996;85:1341–1349

11 Branson R, Davis J. Evaluation of 21 passive humidifiersaccording to the ISO 9360 standard: moisture output, deadspace, and flow resistance. Respir Care 1996; 41:736–743

12 Cigada M, Elena A, Solca M, et al. The efficiency of twelveheat and moisture exchangers: an in vitro evaluation. Int CareWorld 1990; 7:98–101

13 Dery R. Humidity in anesthesiology: III. Heat and moisturepatterns in the respiratory tract during anesthesia with thesemi-closed system. Can Anaesth Soc J 1967; 14:287–298

14 Dery R. The evolution of heat and moisture in the respiratorytract during anesthesia with a nonrebreathing system. CanAnaesth Soc J 1973; 20:296–309

15 Lellouche F, Qader S, Taille S, et al. Under-humidificationand over-humidification during moderate induced hypother-mia with usual devices. Intensive Care Med 2006; 32:1014–1021

16 Jackson C, Webb AR. An evaluation of the heat and moistureexchange performance of four ventilator circuit filters. Inten-sive Care Med 1992; 18:264–268

17 Ricard JD, Le Miere E, Markowicz P, et al. Efficiency andsafety of mechanical ventilation with a heat and moistureexchanger changed only once a week. Am J Respir Crit CareMed 2000; 161:104–109

18 Lellouche L, Taille S, Maggiore M, et al. Influence ofambient air and ventilator output temperature on perfor-mances of heated-wire humidifiers. Am J Respir Crit CareMed 2004; 170:1073–1079

19 Ricard JD, Markowicz P, Djedaini K, et al. Bedside evalua-tion of efficient airway humidification during mechanicalventilation of the critically ill. Chest 1999; 115:1646–1652

20 Kapadia FN. Factors associated with blocked tracheal tubes.Intensive Care Med 2001; 27:1679–1681

21 Chalon J, Markham JP, Ali MM, et al. The Pall Ultiporbreathing circuit filter: an efficient heat and moisture ex-changer. Anesth Analg 1984; 63:566–570

22 Eckerbom B, Lindholm CE. Performance evaluation of sixheat and moisture exchangers according to the Draft Inter-national Standard (ISO/DIS 9360). Acta Anaesthesiol Scand1990; 34:404–409

23 Gedeon A, Mebius C. The hygroscopic condenser humidifier:a new device for general use in anaesthesia and intensive care.Anaesthesia 1979; 34:1043–1047

24 Mebius C. A comparative evaluation of disposable humidifi-ers. Acta Anaesthesiol Scand 1983; 27:403–409

25 Mebius C. Heat and moisture exchangers with bacterialfilters: a laboratory evaluation. Acta Anaesthesiol Scand 1992;36:572–576

26 Ogino M, Kopotic R, Mannino FL. Moisture-conservingefficiency of condenser humidifiers. Anaesthesia 1985; 40:990–995

27 Shelly M, Bethune BW, Latimer RD. A comparison of fiveheat and moisture exchangers. Anaesthesia 1986; 41:527–532

28 Turtle MJ, Ilsley AH, Rutten AJ, et al. An evaluation of sixdisposable heat and moisture exchangers. Anaesth IntensiveCare 1987; 15:317–322

29 Unal N, Kanhai JK, Buijk SL, et al. A novel method ofevaluation of three heat-moisture exchangers in six differentventilator settings. Intensive Care Med 1998; 24:138–146

30 Unal N, Pompe JC, Holland WP, et al. An experimentalset-up to test heat-moisture exchangers. Intensive Care Med1995; 21:142–148

31 Walker AK, Bethune DW. A comparative study of condenserhumidifiers. Anaesthesia 1976; 31:1086–1093




32 Branson R, Hurst JM. Laboratory evaluation of of moistureoutput of seven airway heat and moisture exchangers. RespirCare 1987; 32:741–747

33 Weeks DB, Ramsay FM. A laboratory investigation of sixartificial nose for use during endotracheal anesthesia. AnesthAnalg 1981; 62:758–763

34 Vanderbroucke-Grauls CM, Teeuw KB, Ballemans K, et al.Bacterial and viral removal efficiency, heat and moistureexchange properties of four filtration devices. J Hosp Infect1995; 29:45–56

35 International Organization of Standards. Humidifiers formedical use. ISO 1988; 60:14

36 AARC clinical practice guideline: humidification during me-chanical ventilation; American Association for RespiratoryCare. Respir Care 1992; 37:887–890

37 Martin C, Papazian L, Perrin G, et al. Performance evaluationof three vaporizing humidifiers and two heat and moistureexchangers in patients with minute ventilation � 10 L/min.Chest 1992; 102:1347–1350

38 Boots RJ, George N, Faoagali JL, et al. Double-heater-wirecircuits and heat-and-moisture exchangers and the risk ofventilator-associated pneumonia. Crit Care Med 2006; 34:687–693

39 Boots RJ, Howe S, George N, et al. Clinical utility ofhygroscopic heat and moisture exchangers in intensive carepatients. Crit Care Med 1997; 25:1707–1712

40 Dreyfuss D, Djedaini K, Gros I, et al. Mechanical ventilationwith heated humidifiers or heat and moisture exchangers:effects on patient colonization and incidence of nosocomialpneumonia. Am J Respir Crit Care Med 1995; 151:986–992

41 Hurni JM, Feihl F, Lazor R, et al. Safety of combined heatand moisture exchanger filters in long-term mechanical ven-tilation. Chest 1997; 111:686–691

42 Lacherade JC, Auburtin M, Cerf C, et al. Impact of humid-ification systems on ventilator-associated pneumonia: a ran-domized multicenter trial. Am J Respir Crit Care Med 2005;172:1276–1282

43 Thomachot L, Vialet R, Viguier JM, et al. Efficacy of heat andmoisture exchangers after changing every 48 hours ratherthan 24 hours. Crit Care Med 1998; 26:477–481

44 Markowicz P, Ricard JD, Dreyfuss D, et al. Safety, efficacy,and cost-effectiveness of mechanical ventilation with humid-ifying filters changed every 48 hours: a prospective, random-ized study. Crit Care Med 2000; 28:665–671

45 Kirton OC, DeHaven B, Morgan J, et al. A prospective,randomized comparison of an in-line heat moisture exchange

filter and heated wire humidifiers: rates of ventilator-associatedearly-onset (community-acquired) or late-onset (hospital-acquired)pneumonia and incidence of endotracheal tube occlusion. Chest1997; 112:1055–1059

46 Thomachot L, Leone M, Razzouk K, et al. Randomizedclinical trial of extended use of a hydrophobic condenserhumidifier: 1 vs. 7 days. Crit Care Med 2002; 30:232–237

47 Thomachot L, Vialet R, Arnaud S, et al. Do the componentsof heat and moisture exchanger filters affect their humidifyingefficacy and the incidence of nosocomial pneumonia? CritCare Med 1999; 27:923–928

48 Thomachot L, Viviand X, Arnaud S, et al. Comparing twoheat and moisture exchangers, one hydrophobic and onehygroscopic, on humidifying efficacy and the rate of nosoco-mial pneumonia. Chest 1998; 114:1383–1389

49 Martin C, Thomachot L, Quinio B, et al. Comparing two heatand moisture exchangers with one vaporizing humidifier inpatients with minute ventilation greater than 10 L/min. Chest1995; 107:1411–1415

50 Lellouche F, Qader S, Taille S, et al. Impact of ambient airtemperature on a new active HME and on standard HMES:bench evaluation. Intensive Care Med 2003; 29:S169

51 Martin C, Papazian L, Perrin G, et al. Preservation ofhumidity and heat of respiratory gases in patients with aminute ventilation greater than 10 L/min. Crit Care Med1994; 22:1871–1876

52 Anesthetic and respiratory equipment: heat and moistureexchangers for use in humidifying gases in humans, 1st ed;ISO International Standard 9360. Geneva, Switzerland: In-ternational Organization for Standardization, 1992

53 Branson RD, Davis K, Campbell RS, et al. Humidification inthe intensive care unit: prospective study of a new protocolutilizing heated humidification and a hygroscopic condenserhumidifier. Chest 1993; 104:1800–1805

54 Jaber S, Pigeot J, Fodil R, et al. Long-term effects of differenthumidification systems on endotracheal tube patency: evalu-ation by the accoustic reflection method. Anesthesiology2004; 100:782–788

55 Kollef MH, Shapiro SD, Boyd V, et al. A randomized clinicaltrial comparing an extended-use hygroscopic condenser hu-midifier with heated-water humidification in mechanicallyventilated patients. Chest 1998; 113:759–767

56 Boisson C, Viviand X, Arnaud S, et al. Changing a hydropho-bic heat and moisture exchanger after 48 hours rather than 24hours: a clinical and microbiological evaluation. IntensiveCare Med 1999; 25:1237–1243




DOI 10.1378/chest.08-0679 2009;135; 276-286Chest

Fumagalli and Laurent BrochardSalvatore Maurizio Maggiore, Philippe Jouvet, Jean-Damien Ricard, Bruno

François Lellouche, Solenne Taillé, Frédéric Lefrançois, Nicolas Deye,Humidification Performance of 48 Passive Airway Humidifiers

February 10, 2009This information is current as of

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