HME Booster in AARC Times March 2009 - Medisize

Clinical Perspectives
New Developments in Mechanical Ventilation
and Humidification
While controversy continues around the issue of
how much humidity is optimal and studies report what
may appear to be conflicting results, clinicians at the
bedside choose daily which device is appropriate for patients
under their care. This article reviews current literature
and new devices in an effort to determine what
impact new information may or may not have on present
standards of care and the AARC Clinical Practice Guideline,
“Humidification During Mechanical Ventilation.” 1
ing fully saturated gas at core temperature offers significant
benefit to intubated patients.
Heat and moisture exchangers
Heat and moisture exchangers (HMEs) store heat
and moisture from the patient’s exhaled gas, which is
released to the patient during the next inspiration. The
AARC Clinical Practice Guideline recommends that an
HME provide a minimum of 30 mg H 2 O/L of delivered
gas at 30°C. 1 There is variability in the performance of
Why we do what we do
During normal breathing, inspired gas is heated and
humidified while traveling through the upper airways.
The isothermic saturation boundary
(ISB) is the point at which inspired gas
reaches body temperature and is fully
saturated (44 mg H 2 O/L at 37°C). This
HMEs; but even the most efficient will return only a
limited amount of heat and moisture, and HMEs must
be removed from the circuit for aerosolized medication
administration. Performance may also
be affected by changes in minute ventilation
or tidal volume. 3 HMEs may
increase resistance and work of
normally occurs around 5 cm below the
breath ing. 3,4 In patients ventilated
carina. Endotracheal intubation bypasses
the upper airway’s efficient
with lung protective strategies, the increased
deadspace may contribute to
mechanisms for heating and humidifying
hypercapnea. 4 Contraindications for
inspired gas, creating a humidity
use include hypothermia, bron-
deficit and shifting the ISB further into
the respiratory tract. This may lead to
underhumidification and mucociliary
transport dysfunction with resultant
loss of airway patency, mucous plugging,
chopleural fistula, hemoptysis, thick
or copious secretions, and spontaneous
minute volume greater than 10
L/minute. 1 Key advantages of the HME
include lower cost, ease of use, and
atelectasis, and infection. Williams et al
portability.
About the Author
reviewed 200 articles on respiratory tract
A recent meta-analysis of 13 randomized
controlled trials compared
physiology and proposed a model for Martha Lemin, RRT, is the ICU
supervisor in the Respiratory
the relationship between airway mucosal
dysfunction, the humidity of in-
Cleveland, OH.
and found no difference between
heated humidifiers (HH) with HMEs
Institute at Cleveland Clinic in
spired gas, and duration of exposure. 2
The model indicates that mucosal damage can occur
after 24 hours of exposure to humidity levels below 30
groups regarding mortality or morbidity,
duration of mechanical ventilation, length of intensive
care stay, episodes of airway occlusion, or
mg H 2 O/L and that increasing exposure time to sub-optimal
ventilator-associated pneumonia (VAP). 5 HMEs were
humidity levels leads to increasing mucosal dys-
function. Both underhydration and overhydration affect
mucociliary transport. According to the authors, deliver-
more cost effective. While no direct impact on clinical
outcomes was noted, patients with contraindications
for HME use were excluded from some of the random-
32 AARC Times March 2009

Clinical Perspectives Mechanical Ventilation
ized controlled trials. Heated wire circuits (HWCs) were
used, making it unlikely that any patients received boluses
of contaminated water that might overwhelm
pulmonary defenses and contribute to VAP.
Interestingly, in another randomized controlled
trial, patients mechanically ventilated for more than
five days with a HWC developed a lower incidence of
VAP than the HME group. 6 Mean duration of mechanical
ventilation was higher than in previous studies.
The authors suggest that the lower incidence of VAP
may be attributed to higher levels of humidification
with the HWCs, leading to improved mucociliary clearance.
The authors believe that mucociliary dysfunction
can occur later than 24–48 hours with absolute humidity
(AH) levels less than
32 mg/L. They refer to the
model proposed by Williams 2
and the trend in the data of
AH versus exposure time.
Another issue of importance
is endotracheal tube
patency. Intraluminal volume
loss is directly associated
with duration of tube placement.
7 A study that used an
acoustic reflection method
found that prolonged use of any humidification system
resulted in decreased endotracheal tube patency
and this occurred to a greater degree with HMEs. 8 Increased
resistance and work of breathing may lead to
weaning failure and prolonged mechanical ventilation.
4,6,7-10
The Booster ® (Medisize, Netherlands), an example
of an active HME, is a T-shaped element equipped with
a hydrophobic membrane constructed of layers of
Gore-Tex and aluminum, water infusion system, and
heating element. The Booster improves the heat and
water preservation of inspiratory gas. 11 It also adds 9
mL of deadspace and additional weight to the HME.
Heated humidifiers
Theoretically able to deliver gas at physiologic values,
HHs are recommended for patients with COPD,
thick secretions, increased work of breathing, higher
minute ventilation requirements, increased risk of airway
occlusion, and for patients receiving lung-protective
ventilation. 4,7 HHs aren’t cost effective when used
for less than 48 hours. A HH may be used with a standard
circuit or with a HWC. Using a HH with a standard
circuit leads to condensation with associated increase
in workload and may lead to inadvertent lung lavage.
Heated humidifiers are
generally able to deliver greater
humidity than HMEs, but their
function may be degraded by
ambient conditions, patient
conditions, or operator errors.
Some HWCs allow the clinician independent control of
temperature and gradient but may also allow an operator
to enter inappropriate settings, leading to underhumidification.
Setting the airway temperature higher
than the chamber temperature reduces condensation
and relative humidity (RH). Setting the airway temperature
lower than the chamber temperature assures
100% RH but leads to condensation in the circuit. Other
humidifiers with HWC are equipped with an automatic
compensation system.
Comparing three HWCs with a HH and standard circuit,
Pelosi et al found the only humidifier that provided
the theoretical value of fully saturated gas for a set temperature
was the HH with the standard circuit. 12 Of the
HWCs, the humidifier with
the hydrophobic membrane
and higher surface area between
the vapor and inspiratory
gas showed levels of
temperature and humidity
that were always in the acceptable
range. The HWC
with the automatic compensation
system presented significantly
lower levels of
humidity, although performance
improved with an increase in minute ventilation.
The third HWC was unable to deliver an absolute humidity
level higher than 30 mg/L for higher minute ventilation
ranges. The authors concluded that HWCs
eliminate water condensation but differ markedly in
gas conditioning efficiency, with temperatures and humidity
levels lower than expected. Another study found
a relatively high rate of endotracheal tube occlusion in
the HWC group, as compared to an HME group, raising
more questions about HWC performance. 13
As demonstrated by Lellouche et al, performance of
new generation HWCs is significantly influenced by ambient
and ventilator output temperatures. 14 When
chamber inlet temperatures are high, the heater plate
stops heating and absolute humidity may be well below
the recommended 30 mg/L. When ambient air temperature
is normal, there is a strong correlation between
condensation on the chamber wall of a HH and inspired
gas hygrometry. However, condensation is not a reliable
marker for adequate humidification when ambient air
temperature is high.
In view of the complexities of humidification, several
new products are of considerable interest. One is the
Hydrate Omni with C-Force Technology (Hydrate Inc.,
Midlothian, VA). Per manufacturer, the water feed and
34 AARC Times March 2009

Clinical Perspectives Mechanical Ventilation
heater are controlled by software and the amount of
vapor and heat added to a gas can be set independently
within a range. Changes in patient minute ventilation
trigger automatic changes in water pump rate. The humidifier
may be suspended for aerosol delivery with automatic
resumption by setting the time. The weight of
the C-Force unit is 52.4 gm, necessitating adequate support.
Maximum duration of use is 14 days. The mechanical
ventilator unit has been approved by the U.S. Food
& Drug Administration (FDA). This technology differs
from other HHs because it allows the clinician to control
more than temperature (and temperature gradient).
Another product of interest is the HumiCare ® 200
(Gründler Medical, Germany), which uses a large
gas/water interface and the counter-flow principle to
humidify air. The counter-flow type humidifier displays
improved gas conditioning when compared with a conventional
humidifier. Resistance is lower at the lower
flow rates, which are characteristic of triggering the
ventilator and the initiation of inspiration. 15 This product
is not FDA approved at this time, and further studies
are indicated.
Continuing research needed
Strong assessment skills and education about humidification
devices are needed at the bedside. Heated
humidifiers are generally able to deliver greater humidity
than HMEs, but their function may be degraded
by ambient conditions, patient conditions, or operator
errors. It is not clear that the increase in delivered humidity
with HHs has any impact on patient outcomes
for the majority of mechanically ventilated patients.
It is difficult to draw conclusions, as patients with
contraindications for HME use were excluded from
many comparison studies. VAP should not be the primary
consideration when choosing a humidification
device. The use of HHs in patients requiring mechanical
ventilation for periods longer than 96 hours concurs
with established guidelines. Continuing research
on the subject of humidification and the development
of superior products are needed. However, there is no
clear indication at this time for changing standards of
practice as delineated in the AARC Clinical Practice
Guideline. ■
EDITORS’ NOTE
The author reports no conflict of interest related to the content of this
article.
REFERENCES
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6. Lorente L, Lecuona M, Jimenez A, et al. Ventilator-associated pneumonia
using a heated humidifier or a heat and moisture exchanger: a
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8. Jaber S, Pigeot J, Fodil R, et al. Long-term effects of different humidification
systems on endotracheal tube patency: evaluation by the
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9. Branson, RD. Secretion management in the mechanically ventilated
patient. Respir Care 2007; 52(10):1328-1342.
10. Boots RJ, George N, Faoagali JL, et al. Double-heater-wire circuits
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11. Thomachot L, Viviand X, Boyadjiev I, et al. The combination of a
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12. Pelosi P, Chiumello D, Severgnini P, et al. Performance of heated
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13. Lacherade JC, Auburtin M, Cerf C, et al. Impact of humidification
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ADDITIONAL READING
Kola A, Eckmanns T, Gastmeier P. Efficacy
of heat and moisture exchangers in preventing
ventilator-associated pneumonia:
meta-analysis of randomized controlled
trials. Intensive Care Med 2005; 31(1):5-11.
Ryan SN, Rankin N, Meyer E, Williams R.
Energy balance in the intubated human
airway is an indicator of optimal gas conditioning.
Crit Care Med 2002; 30(2):355-
361.
36 AARC Times March 2009