Content uploaded by Jamie Gross
Author content
All content in this area was uploaded by Jamie Gross on Sep 13, 2016
Content may be subject to copyright.
496 MINERVA ANESTESIOLOGICA April 2012
R E V I E W
Anno: 2012
Mese: April
Volume: 78
No: 4
Rivista: MINERVA ANESTESIOLOGICA
Cod Rivista: Minerva Anestesiol
Lavoro:
titolo breve: Humidication of inspired gases
primo autore: GROSS
pagine: 496-502
Mechanical ventilation uses dry, piped gas
instead of room air to ventilate the lungs
which bypass the body’s normal warming and
humidifying mechanisms. Dry gas has detri-
mental eects on the respiratory tract in intubat-
ed patients, damaging epithelium and causing
secretions to become more viscous. To prevent
this occurring, several types of humidiers have
been produced for clinical use and each has its
own advantages and disadvantages.
Dening humidity
Humidity refers to the amount of water vapor
in a gaseous environment. It can be expressed in
two ways. Absolute humidity (AH) is the mass
of water in a given volume of gas, usually ex-
pressed in mg H2O/L or Kg H2O/m3.
Relative humidity (RH) is the amount of wa-
ter vapor present in a volume of gas as a percent-
age of the amount of water vapor that is required
to fully saturate the same volume of gas at the
same temperature and pressure.
e temperature must be specied, as the
maximum amount of water vapor that a volume
of gas can hold increases as temperature increas-
es. Conversely, as gas that is fully saturated with
water is cooled, the gas can no longer accommo-
date all the water vapor present and water con-
denses out onto the surroundings. A common
example of this occurs within the expiratory
limb of breathing circuits.
Humidity and heat within
the respiratory tract
Indoor atmospheric air at 20 °C has a AH of
around 10 mg H2O/L water and a RH of 55-
60%. As this air passes through the nose and up-
per airways it is warmed and moistened. In the al-
veoli, the air is at 37 °C and is fully saturated with
44 mg H2O/L water (RH=100%). is provides
optimal conditions for gas exchange at the alveo-
lar-capillary interface and is referred to as “body
temperature and pressure saturated with water va-
por” (BTPS). e airway provides both heat and
moisture to inspired air to meet these conditions.
Heat losses may result as heat energy is ex-
Humidication of inspired gases
during mechanical ventilation
J. L. GROSS, G. R. PARK
Intensive Care Unit, North Middlesex, University Hospital, Sterling Way, London, UK
ABSTRACT
Humidication of inspired gas is mandatory for all mechanically ventilated patients to prevent secretion retention,
tracheal tube blockage and adverse changes occurring to the respiratory tract epithelium. However, the debate over
“ideal” humidication continues. Several devices are available that include active and passive heat and moisture ex-
changers and hot water humidiers Each have their advantages and disadvantages in mechanically ventilated patients.
is review explores each device in turn and denes their role in clinical practice. (Minerva Anestesiol 2012;78:496-502)
Key words: Respiration, articial - Humidity - Tracheostomy.
HUMIDIFICATION OF INSPIRED GASES GROSS
Vol. 78 - No. 4 MINERVA ANESTESIOLOGICA 497
pended to convert liquid moisture within the
airway epithelium to vapor (latent heat of va-
porisation) and by direct transfer of heat from
mucosa to incoming air.
Eects of inadequate humidication
Inadequate humidication causes increased
mucus viscosity and inspissation, depressed cili-
ary function, tracheal inammation and mucosal
ulceration. Later changes include epithelial cell
necrosis and squamous metaplasia. ese changes
may result in an increased incidence of respira-
tory tract infection and airway obstruction or at-
electasis due to retained secretions. Additionally,
as the body tries to humidify dry gas, there is an
increased loss of body water and heat. Although
this may be of insignicant consequence in adults,
it may have important clinical implications in
children, particularly neonates who have a higher
minute ventilation to body surface area ratio. Bis-
sonnette et al.1 showed a reduction in core body
temperature of 0.75 °C in anesthetised children
after 90 minutes in those not receiveing any form
of humidication compared with those who did.
Hypothermia has many adverse eects. Amongst
these, ventilation with cold dry gases may cause
deterioration in FEV1 of up to 21% in asthmatic
patients and a less favorable improvement in FEV1
in non-asthmatic patients that would normally
occur in warm humid conditions.2 is may re-
sult in greater work of breathing and impair wean-
ing from mechanical ventilation.
Eects of over humidication
Currently, there are no criteria for over-hu-
midication. In some reports, it has been sug-
gested that over humidication occurs when tra-
cheal gas is fully saturated above around 32 °C,
a point where water vapor content is higher dur-
ing inspiration than expiration and exchanges of
heat and water are reversed during a respiratory
cycle.3 Others suggest over-humidication oc-
curs when delivered inspired gas is above BTPS.4
Overhumidication reduces mucus viscosity,
increases the pericillary layer, dilutes surfactant
and causes neutrophilic inltration of lungs and
bronchioles. e resulting eect of these changes
causes secretion retention, atelectasis, worsening
lung compliance, increased pulmonary shunt
fraction with an increased alveolar-arterial oxy-
gen gradient.4 ese changes may result in pul-
monary and generalized edema, weight gain,
hyponatremia and increased local susceptibility
to bacterial invasion leading to bronchopneu-
monia. Delivering inspired over-humidied gas
may cause water to condense on the walls of the
breathing circuit resulting in increased airow re-
sistance with further potential for infection from
retained droplets. Problems with under and over
humidication are summarised in Table I.
Eects of excess heat
Overheating the respiratory tract causes mu-
cosal sloughing, impairment of mucociliary
clearance and deposition of brin casts in small
airways. is results in mechanical obstruction
leading to carbon dioxide retention and impaired
oxygenation with ventilation-perfusion mis-
match. e temperature at which these changes
occur is also dependent on humidity and dura-
tion of exposure, but it has been recommended
that respiratory gases that arrive at the tracheal
end of the endotracheal tube should average less
T I.—Summary of the adverse eects of under and over humidication.
Ia
Eects of dry gas inhalation
Ib
Eects of excess pulmonary water delivery
–Mucosal ulceration
–Destruction of cilia
–Hyperaemia and inammation
–Desquamation of cells
–Disorganisation of basement membrane and epithelial layer
–Cytoplasmic and nuclear degeneration
–Microatelectasis from obstruction of small airways and
reduced surfactant leading to reduced lung compliance
–Excessive pulmonary secretions
–Edema (pulmonary and generalized)
–Weight gain
–Hyponatremia
–Decreased pulmonary compliance
–Reduced vital capacity
–Lowered alveolar-arterial oxygen gradient
GROSS HUMIDIFICATION OF INSPIRED GASES
498 MINERVA ANESTESIOLOGICA April 2012
H20/L. is nding has been replicated else-
where.10 While some manufacturers use the
gravimetric method to test performance (as
used by the international standard ISO 9360 11)
which involves weighing the humidier before
and after the period of operation under strictly
controlled conditions, others use the psychro-
metric method.
Although there is little discrepency between
both methods in vitro,6, 7 only a psychrometric
test can be used in patients and future devices
should be benchmarked against this technique
in vivo.
Current evidence suggests that p-HMEs that
can deliver gases with an AH >30 mg H20/L
have a low risk of tracheal tube occlusions while
those providing AH of <25 mg H20/L are associ-
ated with a signicant increase in tracheal tube
occlusion and should be avoided.6 Providing AH
between 25 and 30 mg H20/L is considered a
“grey zone” and those using a device providing
this range of AH, should consider to the risk of
tracheal tube occlusion. In terms of durability,
manufacturers recommend that p-HMEs are
changed every 24 hours, but providing an AH
>30 mgH20/L is achieved and maintained, the
life-span of these devices could be extended to
48 hours 12 or even as long as one week in certain
patients without any increase in the risk of tra-
cheal tube occlusion or bacterial colonisation.9
However, further testing of devices that consist-
ently achieve AH levels >30 mg H20/L in vivo
and determining the durability of each device
are needed.
Active heat and moisture exchangers
Active heat and moisture exchangers (a-
HMEs) include a regular HME, but place a
small heater between the HME and the patient
that vaporises added water.
e Humid-Heat® device (Gilbeck AB, Swe-
den) allows water to drip onto a heated paper
element that acts as a wick. e HME Booster®
(Medisize, Belgium) features a heater covered
with a Gore-Tex® membrane. Water is added to
the surface of the heater and vaporised, allowing
passage through the membrane, that regulates
the amount of water vaporised. Both devices
than 42 °C to prevent the adverse eects associ-
ated with thermal injury.5
Methods of humidication
Passive heat and moisture exchangers
Passive heat and moisture exchangers (p-
HME) sit between the tracheal tube and ventila-
tor tubing and work by trapping heat and mois-
ture as a patient expires and returning them to
the patient in the next inspiration. Because all
heat and moisture is derived entirely from the
patient and no energy is added to the system this
is a passive system. Under optimal conditions
they can provide up to 30-32 mgH2O/L AH
at 27-30 °C 6 but their ultimate performance is
dependent on a number of factors such as ambi-
ent temperature, inspiratory and expiratory ow
rates, surface area and water vapour content of
the medium.
ey are composed of material of high ther-
mal capacity and conductivity that is arranged in
a spun and pleated fashion to allow gas to cool
and condense on expiration and warm and evap-
orate on inspiration. Some have a hygroscopic
element, usually calcium or lithium chloride,
which improves water retention following expi-
ration and hence improveseciency.7
Despite the theoretical advantages of hygro-
scopic p-HMEs compared with hydrophobic de-
vices no dierences were shown in the quantity
of tracheal aspirates, mucus viscosity, atalectasis,
tracheal tube occlusion, bacterial colonization
and ventilator associated pneumonia (VAP).8
Studies looking at dierent types of p-HMEs,
have been shown to vary considerably in terms
of performance and durability.6, 7, 9 Some devices
perform sub-optimally leading to increased air-
way resistance and tracheal tube occlusion from
retained secretions. Lellouche et al.6 independ-
ently tested the performance of 48 p-HMEs
and showed only 37.5% performed well (AH
≥30 mgH20/L) with 25% performing poorly,
providing AH <25 mgH20/L, a level associated
with tracheal tube occlusion. Furthermore, there
was a signicant discrepancy between the meas-
ured AH and the manufacturers data, where in
36% of devices the dierence of AH was >4 mg
HUMIDIFICATION OF INSPIRED GASES GROSS
Vol. 78 - No. 4 MINERVA ANESTESIOLOGICA 499
wires within the walls of the tubing. e risk of
colonisation may also be reduced by increasing
the temperature of the water bath up to 45-60
°C (continuous Pasteurisation),17 adding anti-
bacterial agents to the water or breathing circuit
tubing 18 or maintaining a closed sterile system.
Increasing temperature poses an increased risk of
inhalational thermal injury. Antibacterial agents
are rarely used due to the risk of ingestion and
maintaining a closed sterile system is dicult to
achieve. Unless visibly soiled, breathing circuits
need not be replaced routinely 19 and unneces-
sary manipulations and breaks in circuit tubing
should be avoided.
Choice of humidier
Although providing some sort of humidica-
tion is essential in mechanically ventilated pa-
tients, it is still unknown what device is most
benecial. Table II summarises each device.
e ideal humidier should provide optimal
temperature and humidication with low risk
of adverse events, be simple to use and inexpen-
sive. One reason for lack of such a device is that
the optimal level of humidity and temperature
is still unknown. Standards for humidiers for
medical use state that p-HMEs should provide
at least 30 mgH2O/L at 30 °C when tested at
tidal volumes greater that 250 mL (ISO 936011)
and that HWHs should be able to provide at
least 33 mgH2O/L with maximum respirato-
ry gas temperature not exceeding 42 °C (ISO
81855). However, these standards are based on
in-vitro testing and set a minimum perform-
ance for humidiers, but do not dene a level of
humidication that provides maximum clinical
benet.
While p-HMEs provide AH of up to 32 mg
H2O/L at 27-30 °C, HWHs and the newer a-
HMEs can provide AH close to 44 mg H2O/L
at 37 °C. Providing such conditions with HWH
and a-HMEs may not be necessary and can re-
sult in over-humidication and heat related
airway injury.20 Both p-HMEs and a-HMEs
can become occluded with blood, secretions or
condensate resulting in an increased resistance
to airow and work of breathing. In hypother-
mic patients p-HMEs and possibly a-HMEs
have the advantage that should they run dry, the
HME functions normally.
In vitro studies comparing a-HMEs with pas-
sive devices have consistently shown increased
inspired AH with airway temperature ranging
from 31.9 to 37 °C and AH from 34.3 to 44 mg
H2O/L.13-15 Similar results have been produced
in-vivo, where the same device (the Performer)
provided signicantly higher levels of humidi-
cation (AH range 30-36 mg H20/L) when used
as an active device compared with its use as a
passive device.14 In the same study, whilst the
ecacy of p-HMEs worsened at minute venti-
lation both above or below 10 L/min, a-HMEs
retained their function suggesting these devices
may be more suitable for extremes of minute
ventilation. Others have shown that the aHME
retains its eciency between 3 and 25 L/min.15
In hypothermic conditions, Pelosi et al. dem-
onstrated that a-HMEs perform better, provid-
ing AH of 27.1 mg H2O/L compared with 24.6
mg H20/L from the best performing passive de-
vice when expired airway temperature was 28
°C.13 Overall, data for a-HMEs looked promis-
ing at rst but has not translated into widespread
clinical use.
Hot water humidiers
Hot water humidiers (HWH) have tradi-
tionally been considered to be the gold standard
in providing humidication as they deliver gas at
37 °C with an AH of 44 mg H2O/L, but in clini-
cal use may only deliverAH levels between 35 and
40 mg H2O/L.16 ey have a heating element,
which heats the water within a chamber. Dry gas
is then passed through this chamber, over the
hot liquid surface or bubbled through the water
to become humidied. e temperature with-
in the chamber is thermostatically controlled
which allows fully saturated gas to be produced
at a variety of temperatures. It is more ecient in
providing humidication when compared to p-
HMEs, but the risks and costs are greater. Risks
include overheating, causing inhalational burns
and the possibility of water condensing within
the inspiratory limb of ventilator tubing as gas
cools, which may lead to bacterial colonisation.
is may be reduced by incorporating heated
GROSS HUMIDIFICATION OF INSPIRED GASES
500 MINERVA ANESTESIOLOGICA April 2012
HMEs daily in accordance to manufacture rec-
ommendations. Studies showing safe use of p-
HMEs beyond the 24 hour period 12 should be
repeated and if p-HMEs can be used safely for
more prolonged periods, such as up to 48 hours,
it would represent a major cost advantage for p-
HMEs over HWHs.
Several meta-analyses have shown no dier-
ence in VAP rates or airway occlusion between
HMEs and HWHs.26, 27 ere were also no dif-
ferences seen with respect to atelectasis, PaCO2,
work of breathing, secretion clearance and
length of ICU stay.27 e only dierences ob-
served were a lower body temperature and lower
cost in the p-HME group.
ere is not one method of humidication
that is universal for every patient in every situa-
tion, so the choice of device should be tailored to
the individual patient. A number of algorithms
have been developed to aid choice of humidier
in each situation. One suggested example was
developed by Branson et al. at the University of
Cincinnati 28 (Figure 1).
In this study, the quality of pulmonary secre-
tions aspirated by suction catheter was used as
a measure of adequacy of humidication, based
on the scale described by Suzukawa et al.29 (Ta-
ble III). e algorithm was evaluated in 120 pa-
tients and was shown to be cost eective and safe
in one surgical ICU.
have limited performance and should be used
with caution, while HWHs can potentially
lead to over-humidication.20 e American
Association of Respiratory Care guidelines sug-
gest p-HMEs should be avoided for patients
with body temperature less than 32 °C.21 Both
active and passive HMEs should also be used
cautiously with patients undergoing low tidal
volume ventilation (such as in acute respiratory
distress syndrome) as HMEs increase dead space
by up to 90 mL, which may increase the risk of
hypercapnia.22 Similarly, HMEs should not be
used for patients with an expired tidal volume
less than 70% the delivered tidal volume (e.g.
those with large bronchopleurocutaneous stu-
las or incompetent or absent tracheal tube cus)
21 nor should they be used in dicult to wean
patients with chronic respiratory failure, such
as chronic obstructive pulmonary disease.23 P-
HMEs may also be contraindicated in patients
with high minute volumes exceeding 10 L/min
as they have reduced ecacy.23, 24
In some situations the choice of humidica-
tion method may be inuenced by cost eective-
ness. Boots et al. in 2006 evaluated daily cost
of p-HME compared with HWHs, taking into
account purchasing and maintenance costs.25
e daily cost of p-HMEs were comparable to
HWHs (AUS $ 8.62 vs. AUS $ 8.98, respec-
tively). ese costings were based on changing
T II.—Comparison of dierent types of humidiers.
Advantages Disadvantages
Cold water humidier Simple
Cheap
Inadequate humidity
Infection risk
Nebuliser device Suitable for high frequency jet ventilation Risk of over humidication, hypothermia and infection
Passive HME device Simple
Cheap
Provides adequate humidity for many patients
Increased dead space
Increased circuit resistance
Risk of obstruction
Inadequate in some cases
Active HME device Relatively simple
Relatively cheap
Boosted humidity output compared to HME.
Still performs as pHME if allowed to run dry
Increased dead space
Increased circuit resistance
Risk of obstruction
Hot water humidier Delivers maximal humidication at 37 °C Complex to setup and run
Expensive to acquire and maintain
Risk of infection
Risk of aspiration of water
Risk of burns/electric sock
Over-humidication possible
Large number of connection to become disconnected
HUMIDIFICATION OF INSPIRED GASES GROSS
Vol. 78 - No. 4 MINERVA ANESTESIOLOGICA 501
nose and mouth dryness) which may contribute
to NIV failure.30 Secondly, If NIV fails as a re-
sult of insucient humidication, subsequent
tracheal intubation may be more dicult ow-
ing to mucosal drying and secretion retention.31
Lellouche et al.30 studied the impact of HMEs
and HWHs in normal volunteers exposed to
NIV in the presence and absence of mask leak
for one hour. No humidication provided an
AH of around 5 mg H2O/L, while use of either
an HME or HWH provided AH of inspired
gases to between 25-30 mg H2O/L. In the pres-
ence of mask leaks the AH of delivered dry gas
decreased to 15 mgH2O/L with HMEs but was
maintained at 30 mg H2O/L with HWHs. Be-
cause of inevitable air leaks which occur around
the mask, the gas ow associated with NIV is
mostly unidirectional and therefore the amount
of heat and moisture that can be exchanged
with HMEs is reduced. However, using HMEs
during NIV increases work of breathing be-
cause of the additional dead space added to the
circuit.32, 33 Although humidication should be
provided, HWHs should be used in preference
to HMEs. ese factors may decrease patient
adherence to therapy and ultimately cause NIV
failure.32, 33
Conclusions
e devices most suited for humidication
include the HWH, the p-HME and more re-
cently the a-HME. e HWH and a-HME
undoubtedly provide higher humidity levels for
inspired gas but in some patients, p-HMEs may
have advantages. ere is no one method of hu-
midication that suits all patients and clinicians
need to tailor the method used to the patient’s
needs.
Humidication during non-
invasive ventilation
Gas delivered when using non-invasive venti-
lation (NIV) passes through the upper airways
and is exposed to the body’s normal humidify-
ing system. Despite this there is increasing evi-
dence that additional humidication is needed.
Firstly, patient comfort is key to NIV success
and not providing additional humdication sig-
nicantly reduces patient comfort levels (mostly
Figure 1.—Algorithm to aid choice of humidication (devel-
oped by Branson et al.28). HCH: hygroscopic condenser hu-
midier (equivalent to p-HME).
T III.—Grading of secretions aspirated with suction catheter.
Type of secretion Description
in Suction catheter clean after use
Moderate Secretions adhere to inside of catheter after suctioning but are easily cleared by aspirating water
ick Secretions adhere to inside of catheter after suctioning, but cannot be cleared by aspirating water
Reproduced from Suzukawa et al.29
GROSS HUMIDIFICATION OF INSPIRED GASES
502 MINERVA ANESTESIOLOGICA April 2012
18. Yousefshahi F, Khajavi MR, Anbarafshan M, Khashayar P,
Naja A. Sanosil, a more eective agent for preventing the
hospital-acquired ventilator associated pneumonia. Int J
Health Care Qual Assur 2010;23:583-90.
19. Long MN, Wickstrom G, Grimes A, Benton CF, Belcher
B, Stamm AM. Prospective, randomized study of ventila-
tor-associated pneumonia in patients with one versus three
ventilator circuit changes per week. Infect Control Hosp
Epidemiol 1996;17:14-9.
20. Lellouche F, Qader S, Taille S, Lyazidi A, Brochard L. Un-
der humidication and overhumidication during moder-
ate induced hypothermia with usual devices. Intens Care
Med 2006;32:1014-21.
21. AARC Clinical Practice Guideline: Humidication during
mechanical ventilation. Respir Care 1992;37:887-90.
22. Prat G, Renault A, Tonnelier J-M, Goetghebeur D, Oger E,
Boles JM et al. Inuence of the humidication device dur-
ing acute respiratory distress syndrome. Intens Care Med
2003;29:2211-5.
23. Girault C, Breton L, Richard JC, Tamion F, Vandelet P,
Aboab J et al. Mechanical eects of airway humidica-
tion devices in dicult to wean patients. Crit Care Med
2003;31:1306-11.
24. Martin C, Ppazian L, Perrin G, Bantz P, Gouin F. Perform-
ance evaluation of three vaporizing humidiers and two
heat and moisture exchangers in patients with minute ven-
tilation >10 L/min. Chest 1992;102:1347-50.
25. Boots RJ, George N, Faoagali JL, Druery J, Dean K, Hel-
ler RF. Double-heater-wire circuits and heat-and-moisture
exchangers and the risk of ventilator associated pneumonia.
Crit Care Med 2006;34:687-93.
26. Siempos II, Vardakas KZ, Kopterides P, Falagas M. Impact
of passive humidication on clinical outcomes of mechani-
cally ventilated patients: A meta-analysis of randomized
controlled trials. Crit Care Med 2007;35:2843-51.
27. Kelly M, Gillies D, Todd DA, Lockwood C. Heated hu-
midication versus heat and moisture exchangers for venti-
lated adults and children. Cochrane Database of Systematic
Reviews 2010, issue 4.
28. Branson RD, Davis K, Jr, Campbell RS, Johnson DJ, Po-
rembka DT. Humidication in the intensive care unit.
Prospective study of a new protocol utilizing heated hu-
midication and a hygroscopic condenser humidier. Chest
1993;104:1800-5.
29. Suzukawa M, Usada Y, Numata K. e eect of sputum
characteristics of combining an unheated humidier with a
heat-moisture exchanging lter. Resp Care 1989;34:976-84.
30. Lellouche F, Maggiore SM, Lyazidi A, Deye N, Taille S,
Brochard L. Water content of delivered gases during non-
invasive ventilation in healthy subjects. Intens Care Med
2009;35:987-95.
31. Esquinas A, Nava S, Scala R, Carrillo A, Gonzalez Diaz G,
Artacho R et al. Humidication and dicult endotracheal
intubation in failure of noninvasive mechanical ventilation.
Preliminary results (abstract). Am J Respir Crit Care Med
177:A644.
32. Lellouche F, Maggiore SM, Deye N, Taille S, Pigeot J,
Harf A et al. Eect of humidication device on the work of
breathing during noninvasive ventilation. Intens Care Med
2002;28:1582-9.
33. Jaber S, Chanques G, Matecki S, Ramonatxo M, Souche B,
Perrigault PF et al. Comparison of the eects of heat and
moisture exchangers and heated humidiers on ventilation
and gas exchange during non-invasive ventilation. Intens
Care Med 2002;28:1590-4.
References
1. Bissonnette B, Sessler DI, LaFlamme P. Passive and active
inspired gas humidication in infants and children. An-
esthesiology 1989;71:350-4.
2. Eschenbacher WL, Moore TB, Lorenzen TJ. Pulmonary
response of asthmatic and normal subjects to dierent
temperature and humidity conditions in an environmental
chamber. Lung 1992;170:51-62.
3. Sottiaux TM. Consequences of under- and over humidica-
tion. Respir Care Clin 2006;12:233-52.
4. Williams RB, Rankin N, Smith T, Galler D, Seakins P.
Relationship between the humidity and temperature of in-
spired gas and the function of the airway mucosa. Crit Care
Med 1996;24:1920-9.
5. International Organization for Standardization. Hu-
midiers for Medical Use – Safety Requirements (ISO
8185:1988). Geneva: International Organization for Stand-
ardization, 1988.
6. Lellouche F, Taille S, Lefrancois F, Deye N, Maggiore SM,
Jouvet P et al. Humidication performance of 48 passive
airway humidiers. Comparison with manufacture data.
Chest 2009;135:276-86.
7. Branson RD, Davis K. Evaluation of 21 passive humidiers
according to the ISO 9360 standard: Moisture output, dead
space and ow resistance. Respir Care 1996;41:736-43.
8. omachot L, Viviand X, Arnaud S, Boisson C, Mar-
tin CD. Comparing two heat and moisture exchangers,
one hydrophobic and one hygroscopic, on humidifying
ecacy and the rate of nosocomial pneumonia. Chest
1998;114:1412-8.
9. Ricard JD, Le Miere E, Morkowicz P, Lasry S, Saumon G,
Djedaini K et al. Eciency and safety of mechanical venti-
lation with heat and moisture exchanger changed only once
a week. Am J Respir Crit Care Med 2000;161:104-9.
10. Guillaume T, Boyer A, Etienne P, Salah, A, de Lassence
A, Dreyfuss D et al. Heat and moisture exchangers in me-
chanically ventilated intensive care unit patients: A plea for
an independent assessment of their performance. Crit Care
Med 2003;31:699-704.
11. International Organisation for Standardisation. Anaesthetic
and Respiratory Equipment – Heat and Moisture Exchang-
ers for Use in Humidifying Respired Gases in Humans. Ge-
neva: International Organisation for Standardisation Tech-
nical Committee 1992 International Standard ISO 9360,2.
12. omachot L, Vialet R, Viguier JM, Benjamin S, Roulier
P, Claude M. Ecacy of heat and moisture exchangers after
changing every 48 hours rather than 24 hours. Crit Care
Med 1998;26:477-81.
13. Pelosi P, Severgnini P, Selmo G, Corradini M, Chiaranda
M, Novario R et al. In vitro evaluation of an active heat-
and-moisture exchanger: the Hygrovent Gold. Respir Care
2010;55:460-6.
14. Chiumello D, Pelosi P, Park G, Candiani A, Bottino N,
Strorelli E. In vitro and in vivo evaluation of a new active
heat moisture exchanger. Crit Care 2004;8:R281-R8.
15. Larsson A, Gustafsson A, Svanborg L. A new device for 100 per
cent humidication of inspired air. Crit Care 2000;4:54-60.
16. Lellouche F, Taille S, Maggiore SM, Qader S, L’Her E, Dey
N et al. Inuence of ambient and ventilator output tem-
peratures on performance of heated wire humidiers. Am J
Respir Crit Care Med2004;170:1073-9.
17. Redding PJ, McWalter PW. Pseudomonas uorescence
cross infection due to contaminated humidier water. Br
Med J 1980;281:275.
Conicts of interests.—GRP Consults to Inspired Technologies Ltd, who design novel active heat and moisture exchanging humidiers.
Received on March 31, 2011. - Accepted for publication on January 11, 2012.
Corresponding author: G. R. Park, North Middlesex Hospital, Sterling Way, London, UK. E-mail: gilbertpark@me.com
is article is freely available at www.minervamedica.it