Transcript Gas Conditioning During Mechanical Ventilation

Gas Conditioning During
Mechanical Ventilation
BY
AHMAD YOUNES
PROFESSOR OF THORACIC MEDICINE
Mansoura Faculty of Medicine
Introduction
• In 1871, Friedrich Trendelenburg described the first
endo-tracheal intubation for administration of general
anesthesia . Since then, there has been a growing body
of literature addressing the effect of dry gases on
respiratory tract of intubated patients.
• After three hours of exposure to dry anesthetic gas,
respiratory epithelial cells had 39% cilliary damage, 39%
cytoplasmic changes, and 48% nuclear changes In the
group exposed to dry gas, mucous flow had reduced
clearance velocity compared to the group that inhaled
completely humidified gas .
• Due to unfavorable effects of inadequate humidification
of exposure to dry anesthetic gas on the respiratory
tract ,humidification during invasive mechanical
ventilation is currently an accepted standard of care
Physiological Airway Control of Heat and Humidity
• Humidity is the amount of water in vaporous state
contained in a gas.
• Absolute humidity (AH) is the weight of water present in a
given volume of gas and it is usually expressed in mg/L.
• Relative humidity (RH) is the ratio of the actual weight of
water vapor (AH) relative to the gas capacity to keep water
vapour at a specific temperature.
• Whenever the amount of gas contained in a sample is
equal to its water vapor capacity, the RH is 100% and the
gas is completely saturated.
• It is important to understand that water vapor capacity of a
sample will increase exponentially to the temperature
Therefore, if the absolute humidity remains constant, RH
will decrease whenever the temperature increases
(because the denominator increases), and RH will increase
when the temperature decreases (because the capacity to
hold water vapor decreases).
Physiological Airway Control of Heat and Humidity
• The connective tissue of the nose is
characterized by a rich vascular system of
numerous and thin walled veins. This system is
responsible for warming the inspired air to
increase its humidity carrying capacity.
• As the inspired air goes down the respiratory
tract, it reaches a point at which its temperature
is 37 ∘C and its relative humidity is 100%. This
point is known as the isothermic saturation
boundary (ISB), and it is usually located 5 cm
below the carina .
Physiological Airway Control of Heat and Humidity
• The respiratory mucosa is lined by pseudostratified
columnar ciliated epithelium and with numerous
goblet cells.
• These cells, as well as submucosal glands
underneath the epithelium, are responsible for
maintaining the mucous layer that serves as a trap
for pathogens and as an interface for humidity
exchange.
• At the level of the terminal bronchioles, the
epithelium turns into a simple cuboidal type with
minimal goblet cells and scarce submucosal glands
Hence, the capacity of these airways to carry on the
same level of humidification is limited .
Pseudo-stratified columnar epithelium
• It consists of columnar cells crowded very closely
together.
• In cross section, the nucIei appear to be on two or more
levels and because of the crowding, the cells appear
distorted, and they all do not reach the surface.
• But in fact every cell rests on the basement membrane,
so the epithelium is technically "simple", in spite of
appearances.
• Cells that reach the surface are either ciliated or goblet
cells
• The goblet cells secrete mucous, which forms a film on
the luminal surface of the respiratory passages and
prevents dust from being inhaled into the lungs.
• The cillia move the mucous containing the dust particles
up and out of the respiratory passages.
• The cells that do not reach the surface probably , serves
as progenitor cells producing new cells.
Physiological Airway Control of Heat and Humidity
• After endotracheal intubation, as the upper airway loses
its capacity to heat and moisture inhaled gas, the ISB is
shifted down the respiratory tract.
• This imposes a burden on the lower respiratory tract, as
it is not well prepared for the humidification process.
Consequently, delivery of partially cold and dry medical
gases brings about potential damage to the respiratory
epithelium, manifested by increased work of breathing,
atelectasis, thick and dehydrated secretions, and cough
and / or bronchospasm .
• Notably, there are other factors that may shift the ISB
distally producing the same effects, such as mouth
breathing, cold and dry air breathing, and/or high minute
ventilation.
• inhalation of large volumes of cold air during exercise is
thought to be the inciting event of exercise-induced
asthma .
Physiological Airway Control of Heat and Humidity
• During the exhalation process, the expired gas transfers
heat back to the upper airway mucosa.
• As the airway temperature decreases, the capacity to hold
water also decreases. Therefore, condensed water is
reabsorbed by the mucosa, recovering its hydration.
• Importantly, in periods of cold weather, the amount of
water condensation may exceed the mucosal capacity to
accept water. Therefore, the remaining water accumulates
in the upper airway with consequent rhinorrhea.
• In order to avoid the aforementioned consequences
associated with lack of humidification in mechanically
ventilated patients, a variety of devices (humidifiers) have
been introduced in clinical practice.
• The human airway must provide gas at core temperature
and 100% RH at the alveolar surface in order to optimize
gas exchange and protect lung tissue.
Physiological Airway Control of Heat and Humidity
• At the level of the sea ,the capacity of gas to hold water at
body temperature and pressure saturated (BTPS) is 44 mg
of water per liter of gas.
• As the content of water in the gas exceeds its holding
capacity, water will condensate into liquid droplets. This
situation becomes particularly relevant to mechanically
ventilated patients, as liquid water has a tendency to
accumulate in the lower point of the tubing, increasing
resistance to gas delivery.
Effects of inefficient gas conditioning
during non-invasive ventilation
• Non-invasive ventilation (NIV) is a mechanical ventilation
modality that does not utilize an invasive artificial airway
(endotracheal tube or tracheostomy tube) .
• NIV is usually delivered through a nasal or oro-nasal mask
so the inspired gas passes through the upper airway
where it is conditioned.
• Like during spontaneous breathing, patients under NIV
require adequate humidification and heating of the
inspired air (that is, gas conditioning) .
• NIV delivers inspired air at high flow rates, which may
overwhelm the usual airway humidification mechanisms .
• Inadequate gas conditioning has been associated with
negative effects on tolerance to NIV when a patient
breathes inadequately humidified air .
Effects of inefficient gas conditioning
during non-invasive ventilation
Effects of inefficient gas conditioning during noninvasive ventilation
• Metaplastic changes and keratinization of the nasal
epithelium and submucosa have been reported in
patients on home-NIV when the level of
humidification was inadequate for long periods.
• Similar structural changes of the nasal mucosa in
four patients with acute respiratory failure treated
for 7 days with NIV without a humidification system
added . This suggests that changes in the nasal
mucosa occur relatively early after starting NIV in
an acute setting and that humidification should be
considered even when only short-term use of NIV is
expected.
Biopsy of nasal mucosa. Metaplasia (1) and keratinization (2) in
nasal respiratory mucosa in one patient without humidification
during non-invasive ventilation.
Metaplasia versus Dysplasia
• Dysplasia comes from the root Greek term meaning ‘bad
formation’. It is a pathological term used to refer to an
irregularity that hinders cell maturation within a
particular tissue;
• Dysplasia in general comprises of the increased growth
of immature cells with a simultaneous reduction in the
growth of mature cells, their numbers and their site of
growth.
• Dysplasia is the indication of a premature neo-plastic
progression. It directly indicates a state when the cellular
defect is constrained within the tissue origin, for instance
in case of an in-situ neoplasm.
• Dysplasia basically comprises of four distinct stages of
pathological change. These are, Anisocytosis or the
growth of cells of disproportionate size, Poikilocytosis or
the growth of unusually shaped cells, Hyperchromatism
and lastly the presence of mitotic lumps of cells that
continuously keep on dividing
Metaplasia versus Dysplasia
• Metaplasia derives from the original Greek term
denoting ‘change in form’. It is the process of the
reversible substitution of a distinct kind of cell with
another mature cell of another differentiated kind.
• The transformation from one cell type to another in
Metaplasia is often the result of the initiation caused by
an unusual stimulus.
• The original cells in this case are not strong enough to
survive in a new environment comprising of unknown
and abnormal stimuli.
• A condition of Dysplasia where the growth and
differentiation of cells are delayed are compared with
Metaplasia where a mature cell of a distinct kind is
replaced by another mature cell of yet another
distinguished type.
Metaplasia versus Dysplasia
• Dysplasia is a pathological term used to refer to
an irregularity that hinders cell maturation
within a particular tissue whereas Metaplasia is
the process of the reversible substitution of a
distinct kind of cell with another mature cell of
the similar distinct kind.
• Dysplasia is cancerous whereas Metaplasia is
non-cancerous.
• Metaplasia can be stopped by removing the
abnormal stimulus, but Dysplasia is a nonreversible process.
Keratinized versus Non-keratinized Epithelium
• Keratinized tissue like the skin has lots of
vessicles containing keratohyaline, a protein
that eventually turns into keratine when the cell
dies, providing that epithelium with protection
and resistance to traction and friction, and it
makes it impermeable.
• The only epithelium that keratinizes is stratified
squamous, and it only happens in the skin (in
normal conditions).
• Non keratinized tissue does not offer as much
protection and allows the diffusion of materials
through the cell junctions and the cell's body.
Keratinized versus Non-keratinized Epithelium
Kertinized Suamous Epithelium:
1. The cells of few outer layers of stratified squamous
epithetium replace their cytoplasm with a hard water
proof protein
2. The layers of flat dead cells-stratum corneum or horney
layers are present.
3. It forms epidermis of the skin of land vertebrates.
4. It prevents loss of water and mechanical injury.
Non-kertinized Epithelium:
1. It has living squamous cells at the surface.
2. Strtum corneum absent as non-kertinization or
cornification occurs.
3. It is formed in lining of cornea, mouth, pharynx,
oesophagus, vocal cords, vigina, cervix etc.
4. It provides mechanical protection from injury.
Effects of inefficient gas conditioning
during non-invasive ventilation
• Inadequate airway gas conditioning may have serious
consequences in critically ill patients using NIV .
• Difficulties were recently reported with intubation in patients
failing a trial of NIV delivered at high inspiratory oxygen
fraction with a low level of humidification.
• One case study showed inspissated secretions, causing lifethreatening airway obstruction in a patient using NIV for
hypoxemic respiratory failure .
• A high fraction of inspired oxygen was also entrained into
the NIV circuit and this extra anhydrous gas may have
contributed to the airway obstruction.
• Inadequate humidification can also cause significant
discomfort for chronic NIV users.
Effects of inefficient gas conditioning during noninvasive ventilation
• Dryness elated symptoms started to appear
when AH was lower than 15 mg H2O/L.
• This was associated with increased nasal
airway resistance (NAWR).
• Heated Humidification improved comfort and
compliance to NIV.
• In healthy subjects under CPAP delivered by a
oro-nasal mask, when a humidification device
was applied and AH was higher than 10 to 12
mgH2O/L , comfort was significantly better .
Effects of inefficient gas conditioning
during non-invasive ventilation
• In home-NIV, comparing two humidification systems, HH
and a heat and moisture exchange filter (HME);
compliance was much better (75% of patients) with the
former. However, other symptoms, such as dry throat,
the number of hospital admissions and the rate of
complications caused by infection (mainly pneumonia),
were similar with the two systems .
• Similar results were published stressed the importance
of ‘early’ humidification from the very start of ventilator
treatment, so as to ensure the best possible compliance
in home-NIV patients.
Contributors to gas conditioning
during non-invasive ventilation
•
Analysis of the need for gas conditioning
during NIV must clearly take account of the
following parameters:
1 ) Air leaks;
2) Interface for NIV delivery;
3) Type of ventilator;
4) Room temperature;
5) Temperatures of inhaled gas and the
vaporization chamber;
6) Airflow and pressure at the entrance of the
humidification system; and
7) Type of humidification system.
Factors which must be considered when choosing a
gas conditioning device for non-invasive ventilation.
How to convert Fahrenheit to Celsius
• The temperature T in degrees Celsius (°C) is
equal to the temperature T in degrees
Fahrenheit (°F) minus 32, times 5/9:
• T(°C) = (T(°F) - 32) × 5/9
Air leaks
• Mouth and / or peripheral mask air leaks, especially in
tachypneic patients under NIV, cause unidirectional nasal
airflow, so the mucosa recovers less heat and moisture
during expiration. This may cause a continuous drop in AH.
• An increase in NAWR is the typical consequence of large
mouth air leaks during NIV from a nasal mask. This reflects
the nasal vaso-constrictive response to prolonged
inhalation of dry air .
• The flow of cold air through the nose dries the mucosa,
resulting in the release of vaso-active and pro-inflammatory
mediators. These boost superficial mucosal blood flow and
cause engorgement of deeper capacitance vessels, leading
to increased nasal resistance. This in turn promotes mouth
breathing, setting up a vicious circle.
• Increased NAWR is likely to lead to unsuccessful
acclimatization to NIV.
Air leaks
• Most current CPAP devices come with an
integrated HH system. As these deliver more
moisture than cold pass-over humidifiers, they
may be more effective in patients with mouth
leak and nasal congestion .
• HH reduces nasal symptoms and nasal
resistance , consequently attenuating
inflammatory cell infiltration and fibrosis of the
nasal mucosa. Therefore, the American
Academy of Sleep Medicine has recommended
the use of HH to improve CPAP compliance and
adherence as a standard of practice .
Patho-physiological effects of air leak on gas
conditioning of the upper airways and its impact on
mucosa vasoconstriction and nasal airway resistance.
Air leaks
• Air leaks affect the performance of HMEs because the
HME recovers less moisture when the expired air is drier.
• The site of the leak, either unintentional (around the mask)
or intentional (mask or circuit) also affects humidity.
• While the unintentional leaks are likely to have a
substantial impact on AH because of the compensatory
increase in inspiratory flow, the intentional leaks are not
known to contribute to insufficient humidification of air
during NIV.
• We can only speculate that inefficient washout of the
exhaled air throughout an unintentional leak system (that
is, plateau valve, anti-rebreathing valve) is likely to involve
a substantial degree of humidity, especially in interfaces
with high dead space, such as a total face mask.
Air leaks
• In a study in adult volunteers, nasal CPAP with a
mouth leak resulted in a three-fold increase in
NAWR that was substantially attenuated by
effective humidification .
• This high NAWR would result in substantially
lower effective pressure being transmitted to the
nasopharynx and subsequently to the distal
airway .
Interfaces
• The most common interfaces used to deliver NIV are
nasal and facial masks, the former being tolerated
better in the chronic than the acute setting.
• Nasal masks tend to have more leaks than face
masks, and this can result in inadequate gas
conditioning of inspired air.
• The use of an oro-nasal mask avoids the changes in
RH related to mouth leaks . This choice is likely to be
crucial to the success of NIV in selected categories
of acute patients (for example, need for prolonged
ventilator support, difficulty in spontaneously
clearing secretions , mouth-breathers).
Left: Nasal mask.
Right: Oronasal mask.
(Courtesy of Respironics, Murrysville, Pennsylvania.)
Left: Nasal pillows. (Courtesy of Puritan-Bennett.)
Center. Total face mask. (Courtesy of Respironics,
Murrysville, Pennsylvania.) Right: Helmet.
Interfaces
•
•
Compared to the other more popular
interfaces (nasal, oro-nasal, total face masks),
the helmet has a much larger inner space
(more than 10 liters), which may act as a
‘reservoir’ of humidity because of the amount
of exhaled gas that remains there .
The clinician must therefore carefully adjust
the humidification of the inhaled gas
depending on the type of interface and the
resulting leak pattern.
Ventilator types
• Intensive care ventilators, home-care mechanical
ventilators, and high-flow CPAP systems operate by
providing a very high inspiratory flow to compensate for the
inspiratory demand of a patient with acute respiratory
failure and the air leaks when applied in a NIV mode.
• Single-circuit home ventilators and/or dedicated NIV
ventilators equipped with a turbine or piston differ from
double-circuit ICU ventilators, which are pneumatic and
supplied with high-pressure sources of gas.
• Home models - that is, blower-based devices - compress
room air and have higher humidity than ICU ventilators,
which obtain dry gas directly from a mains hospital outlet.
• So ,ICU ventilators provided a lower level of AH during NIV
than turbine mechanical ventilators (5 versus 13 mgH2O/L) .
Ventilator types
• we have to bear in mind that AH lower than 5
mgH2O/L is ‘critical’ for the likelihood of
complications related to inadequate gas
conditioning of inspired air .
• Physicians have also to remember that the
higher the oxygen fraction delivered during NIV,
the greater the risk of inadequate gas
conditioning without the addition of HH or a
HME.
Ambient temperature
• Even though insufficient humidification during NIV can
be caused by specific climatic or environmental
conditions in the place where the treatment is
implemented (for example, an excessively cold sleeping
area), the majority of cases suffering from excessive
airway dryness are due to technical factors related to
the ventilation process itself and its interaction with the
patient (interface, leaks, inspired oxygen fraction,
respiratory rate, use of humidifiers).
• The effects of ambient temperature probably only need
be considered for patients who sleep in very cold
premises.
Res Med Climate Line™
• Unique to ResMed, the Climate Line
heated tube works with our exclusive
Climate Control system* to deliver
a constant comfortable temperature
at the mask throughout the night,
even as temperature and humidity levels
change.
• The Climate Line heated tube is smaller, slimmer and
sleeker than your average mask tubing, resulting in
increased flexibility and less mask drag for a more
comfortable night’s sleep.
• *The Climate Control system is available in ResMed’s
S9™ Series of devices
ResMed Climate Line Air
• Climate Line Air is our most
advanced heated tubing and it’s
the key that unlocks ResMed's
Climate Control solution for the
AirSense™ 10 and Air Curve™ 10
series of devices.
• Climate Line Air Oxy is a variant
of Climate Line Air that comes
with a built‐in oxygen connector
for patients who require
supplemental oxygen.
Climate Line™ Max Oxy Air Tubing
• The Climate Line™ Max Oxy Air Tubing with Supplemental
Oxygen Port is a 19mm inner diameter, heated tubing with a
port to add oxygen to the air stream. It is for use ONLY with
a Res Med S9 Series machine using an H5i Heated
Humidifier.
• The Climate Line tubing uses a sensor that measures
humidity to maintain a uniform temperature from the
machine all the way to the mask.
• The temperature in the tube itself is warmed by a thin
copper wire on the outside of the tube.
• The Climate Line™ Max Oxy Air Tubing can easily support
higher pressures, such as though frequently found in
BiLevel therapy.
• The Climate Line™ Max Oxy Air Tubing with Supplemental
Oxygen Port is a latex free product.
Temperature of the inhaled gas
• Conditioning inspired gas involves both heating
and humidification.
• To facilitate gas exchange and protect lung
tissue, inspired gas must reach body
temperature by the time it arrives at the alveolar
surface . How easily this can be achieved
depends on the temperature of the inhaled gas.
• The ventilator power source affects gas
temperature, as turbine-driven systems create
more heat than piston-driven ventilators.
Temperature of the inhaled gas
• Increasing levels of IPAP have also been
reported to raise the gas temperature with a
turbine-driven NIV device . This tended to
preserve AH at high IPAP levels, but it was not
sufficient to maintain adequate humidity when
ambient RH was low.
• HH set at its highest temperature was most
effective in countering the deleterious effects of
NIV on delivered humidity. In clinical practice
the temperature setting for HH may be based on
the patient’s tolerance.
Airflow at the entrance to the humidifier
• The impact of the airflow entering the HH
humidification chamber is one of the most
important physical phenomena affecting airway
humidity.
• By analyzing the factors underlying
humidification capacity over a variable range of
airflows at the entrance to the humidification
chamber (20, 55, and 90 L/minute). At high flow
rates, many commercial humidifiers were
unable to generate adequate RH.
• This suggests that a humidifier alone may not
be enough to ensure adequate humidification of
inspired gases, particularly at high flow rates .
Types of Humidifiers
• Humidifiers are devices that add molecules of water to gas.
They are classified as active or passive based on the
presence of external sources of heat and water (active
humidifiers), or the utilization of patients’ own temperature
and hydration to achieve humidification in successive
breaths (passive humidifiers).
• Active Humidifiers act by allowing air passage inside a
heated water reservoir. These devices are placed in the
inspiratory limb of the ventilator circuit, proximal to the
ventilator. After the air is loaded with water vapor in the
reservoir, it travels along the inspiratory limb to the
patient’s airway.
• As condensation of water vapor may accumulate as the
surrounding temperature of the inspiratory limb decreases,
these systems are used with the addition of water traps,
which require frequent evacuation to avoid risk of
contamination of the circuit.
Heated humidifier and condensation
Types of Humidifiers
• Heated humidifiers are usually supplied with heated
wires (HWH) along the inspiratory limb to minimize this
problem.
• These humidifiers have sensors at the outlet of the
humidifier and at the Y-piece, near the patient. These
sensors work in a closed-loop fashion, providing
continuous feedback to a central regulator to maintain
the desired temperature at the distal level (Y-piece).
• When the actual temperature exceeds or decreases
beyond certain extreme level, the alarm system is
triggered. Even though the ideal system should permit
auto-corrections based on humidity levels,
commercially available sensors provide feed back based
on changes in temperature .
An active humidifier with a heated wire in the inspiratory limb;
both temperature sensors, one at the side of the patient and the
other at the outlet of the heated reservoir, are shown
,
Types of Humidifiers
• Usual temperature setting for the current heated
humidifiers is 37 ∘C.
• The performance of humidifiers may be affected by
room temperature, as well as patient minute ventilation.
In the last situation, an increase in minute ventilation
preserving the same temperature of the heated reservoir
may not be adequate to deliver appropriate AH to the
patient.
• Some humidifiers are supplemented with automatic
compensation systems, which compute the amount of
thermal energy needed to humidify certain volume of
gas and change the temperature of the water reservoir
accordingly.
Heated humidifiers are classified as
• (1) Bubble . Gas is forced down a tube into the bottom of a
water container . The gas escapes from the distal end of the
tube under water surface forming bubbles, which gain
humidity as they rise to the water surface.
• Some of these humidifiers have a diffuser at the distal end of
the tube that breaks gas into smaller bubbles.
• The smaller the bubbles, the larger the gas-water interface
Allowing for higher water vapor content .
• Other factors that influence water vapor content of the
produced gas are the amount of water in the container and
the flow rate. Simply , the higher the water column in the
container, the more gas-water interface will ensue, so water
levels should be checked on a frequent basis.
• In terms of flow rate, when slow flows are delivered, there is
more time for gas humidification.
Bubble humidifiers may be unheated or heated
.
• Typically, unheated bubble humidifiers are used with lowflow oral-nasal oxygen delivery systems.
• Heated bubble humidifiers provide higher absolute humidity.
They are designed to work with flow rates as high as 100
L/min. These humidifiers usually use diffusers to increase the
liquid-air interface.
• A problem with heated bubble humidifiers is that they exhibit
high resistance to Air flow imposing higher work of breathing
than passover ones .Furthermore, they may generate
microaerosol . Nevertheless, the amount of aerosol produced
by these types of humidifiers may not be clinically significant
• Despite this, the use of bubble humidifiers during mechanical
ventilation has fallen in favor of passover ones.
(2) Passover
• In passover humidifiers , gas passes over a heated
water reservoir carrying water vapor to the patient.
These are typically used for the purpose of invasive and
noninvasive mechanical ventilation.
• Another variant of passover humidifiers is the wick one .
In this type of device, the gas enters a reservoir and
passes over a wick that acts as a sponge that has its
distal end immersed in water.
• The wick pores provide more gas-water interface
allowing for more humidification compared to simple
passover humidifiers.
(2) Passover
• The water reservoir is fed through a closed
system. This system can be supplied with water
either manually through a port or float feed
system that ensures the water level remains
constant all the time.
• As dry gas enters the chamber and travels
through the wick, heat and moisture increase.
Due to the fact that gas does not emerge
underneath the water surface, no bubbles are
generated.
(2) Passover
• A third type of passover humidifier involves a
hydrophobic membrane .As with the wick
device, dry gas passes through a membrane.
• Nevertheless, its hydrophobic characteristic
only allows passage of water vapor, precluding
liquid water to travel through it.
• Similarly to the wick humidifier, bubbles and
aerosols are not generated.
• These humidifiers are more commonly used
during mechanical ventilation than bubble ones
due to their lower flow resistance and absence
of micro-aerosols.
In all cases
• A temperature probe is placed near the Y piece of
the ventilator circuit to ensure delivery of gas
with optimal temperature.
• The presence of condensate in the tubing may
increase resistance, which can decrease volume
delivered in pressure controlled, or increase peak
pressure in volume controlled modes .
• The use of these wires does not come without
thermal risks.
• The AARC clinical practice guidelines
recommend gas delivery with a maximum
temperature of 37∘C and 100% RH (44mgH2O/L) .
Currently there are 6 types of humidifier heating
systems devices
• The hot plate element , which sits at the bottom
of the humidifier, is one of the most commonly
used. Other devices include the wrap around
element, which surrounds the humidifier
chamber; a collar element, which sits between
the reservoir and the outlet; the immersion
heater, which is placed directly inside the water
reservoir; and the heated wire, which is placed
in the inspiratory limb of the ventilator.
Passive Humidifiers
• Heat and Moisture Exchangers (HMEs) are also
called artificial noses because they mimic the
action of nasal cavity in gas humidification.
• They operate on the same physical principle , as
they contain a condenser element, which retains
moisture from every exhaled breath and returns it
back to the next inspired breath .
• Unlike heat humidifiers, which are placed in the
inspiratory limb of the circuit, these devices are
placed between the Y Piece and the patient .
• This may increase resistance to airflow not only
during inspiration, but also during the expiratory
phase.
Passive Humidifiers
Passive Humidifiers
• In situations in which administration of aerosolized
medications is needed, HMEs need to be removed
from the circuit to avoid aerosol deposition in HME
filters. Otherwise, HMEs with capability to change
from “HME function” to “aerosol function” should
be used.
• Initial designs of HMEs used condensers made of
metallic elements that had high thermal
conductivity. Thus, they were able to recapture only
50% of the patient’s exhaled moisture. Hence, they
provided humidification of 10–14mgH2O/L, at tidal
volumes (VT) ranging between 500mL and1000mL.
These devices were known as simple HMEs .
Passive Humidifiers
• Newer designs of HMEs include:
1- Hydrophobic HMEs, the condenser is made of a water
repelling element with low thermal conductivity that
maintains higher temperature gradients than in the case of
simple HMEs.
2- Combined hydrophobic hygroscopic HMEs, a hygroscopic
salt (calcium or lithium chloride) is added inside the
hydrophobic HME. These salts have a chemical affinity to
attract water particles and thus increase the humidification
capacity of the HME.
3- Pure hygroscopic HMEs have only the hygroscopic
compartment.
• During exhalation, vapor condenses in the element as well
as in the hygroscopic salts. During inspiration, water vapor
is obtained from the salts, obtaining an absolute humidity
ranging between 22 and 34 mgH2O/L.
Passive Humidifiers
• Hydrophobic HMEs were found to cause more
narrowing in ETT diameter compared to hygroscopic
ones. Therefore, the aforementioned HMEs are not
frequently used.
• Filters can be added to either hydrophobic or
hygroscopic HMEs resulting in a heat and moisture
exchanging filter (HMEF). These filters operate based on
electrostatic or mechanical filtration.
• Based on the predominant mechanism applied, these
filters may be classified into pleated or electrostatic
filters.
• The pleated filters have more dense fibers and less
electrostatic charges, whereas the electro-static filters
have more electrostatic charges and less dense fibers.
Passive Humidifiers
• Pleated filters function better as barriers to bacterial and
viral pathogens than electrostatic filters. However, they
confer higher airflow resistance .
• The pleated nature of the membrane causes a turbulent air
flow, which increases the pathogen’s deposition onto the
inside of the filter.
• The electrostatic filters are subjected to an electric field.
Since bacteria and viruses carry electric charges, they get
trapped within the electric field of these filters. These filters
usually have larger pores than the pleated membranes, and
they rely mainly on the electrostatic mechanism.
• The previously described filter confers little to the
humidification process and increases resistance. Therefore,
they are mainly used as barriers to pathogens .
HMEs design and performance standards are
defined by the International Organization for
Standardization (ISO).
• According to these standards, The appropriate
HME should have at least 70% efficiency,
providing at least 30mg/L of water vapor.
• In a recent study, assessment of the
humidification capacity of 32 HMEs. Strikingly,
36% of tested HMEs had an AH of 4mgH2O/L
lower than what was listed by the manufacturer.
In fact, in some of them the difference was
higher than 8mgH2O/L .
Passive Humidifiers
• Intuitively, as HMEs eliminate the problem of tubing
condensation, it may be considered as “elements of
choice” to prevent ventilator-associated pneumonia
(VAP). Nevertheless, whether the presence of tubing
condensate represents an important factor for the
development of VAP in well-maintained circuits remains
controversial.
• HMEs present some shortcomings. Specifically,
impaction of secretions or blood within the device may
increase airway resistance and work of breathing. In
extreme circumstances, complete airway obstruction has
been reported. Therefore, patient selection becomes an
essential component in the use of HMEs.
Contraindications for heat and moisture exchangers
(i) Patients with thick or copious secretions.
(ii) When there is loss in expired tidal volume
(e.g., large broncho-pleuro-cutaneous fistulas or
presence of endotracheal tube cuff leak).
(iii) In patients managed with low tidal volumes
like those with ARDS.
(iv) In difficult to wean patients and those with
limited respiratory reserve.
(v) Hypothermic patients with body temperature
of<32 ∘C.
(vi) In patients with high minute ventilations
volumes (>10L/min).
Passive Humidifiers
• In certain devices , an active heated water
source can be added to HMEs converting them
from passive to active, increasing their
humidification capacity.
• If the external source of water runs out, these
devices will still work as passive HMEs.
• Several models exist, including the Booster, the
Performer, the Humid Heat, and the Hygrovent
Gold.
In the Booster model
• The heating unit is incorporated between the HME and
the patient. During inspiration the gas passes through
the HME carrying water vapor based on the passive
operation of the HME and then the heating unit adds to
the humidity content of the gas before it reaches the
patient.
• As water enters the HME-Booster, it saturates the
hydrophobic membrane contained in it. The moisture in
the saturated membrane is then heated by the positive
temperature control element connected to it .
• It is thought that the utilization of this device may
increase AH by 2-3mg/L of H2O more than passive HMEs
The performer
• The Performer device is characterized by a metal
plate in the middle of the HME, in between two
hydrophobic and hygroscopic membranes.
• This metal plate is heated by an external source
that has three sets of temperature to deliver 40 ∘C,
50∘C, and 60 ∘C.
• A water source provides it to one end of the
humidifier.
• The water reaches the two membranes and the
metal plate heats it. Then, the water evaporates
augmenting vapor content in the inspired gas.
• The performer is able to deliver AH of 31.9 to 34.3
under normo-thermic conditions .
The performer
The Humid Heat
• The Humid Heat is a hygroscopic HME that has
an external heating source with the water being
added at the patient side .
• In one bench study, it was found to provide an
absolute humidity of 34.5 mgH2O/L .
• Humid Heat has preset values for temperature
and humidity .
• The only parameter that needs to be set is the
value of minute volume of the ventilator, making
its use very simple.
The Hygrovent Gold heat-and-moisture exchanger has
an adapter into which a heating element is inserted, and
a line that continuously supplies water.
Monitoring of Humidification Systems
• American Association of Respiratory Care (AARC)
recommend a temperature of 33 ± 2 ∘C with RH of
100% and a water vapor level of 44mg/L.
• The clinician commonly faces the issue of relying
on different humidifiers without being certain
about device accuracy. Independent assessments
raise concerns about the validity of data included
by the manufacturer .
• The most reliable mean to measure humidity is by
using a hygrometer-thermometer system . But
these devices are not always available at the
bedside for every patient.
Different surrogate markers have been
suggested to monitor humidification levels.
• The most popular surrogates are secretion
characteristics, visual observation of
condensate in tubing system, and requirement
for saline instillation.
• Excessive humidification will increase secretion
volume, and suboptimal humidification will lead
to crusting, inspissation of secretions, and a
decrease in their volume .
• As a matter of fact, secretion volume may be
altered by administered aerosolized medications,
frequency of suctioning, and saline instillation.
Different surrogate markers have been
suggested to monitor humidification levels.
• Frequency of saline instillation has been
proposed by some as a surrogate of gas humidity.
• There was a significant correlation between the
visual observation method and the hygroscopic
measurements .
• Despite the previously described data, there is
still no clear consensus about a universal way to
assess for humidity adequacy at the bedside
Types of humidification system
• HH and HME technically produce similar AH levels (25 to
30 mg H2O/L), which are adequate for the physiological
functioning of the upper airway.
• The gas conditioning performance of each system may
vary over a range of respiratory rates, especially at the
ventilator airflow that enters the system through the
humidification chamber .
• The choice of active HH or HME may have repercussions
on respiratory mechanics - such as tidal volume, minute
volume, and work of breathing – and gas exchange .
• HMEs have been associated with greater dead space and
possibly also CO2 retention. This was shown by studies,
who reported higher partial pressure of PaCO2 during
NIV with HME than with HH.
Types of humidification system
• Inspiratory effort was greater in patients with
hyper capnia during HME than HH. This has also
been associated with increased work of
breathing.
• On the basis of these studies, therefore, it
would seem that HH is superior to HME during
acute NIV but this advantage is probably limited
to acute rather than chronic settings.
Thermovent® T - Heat and Moisture Exchanger (HME) with
15mm Female Connector. Humidifying Tracheostomy
Types of humidification system
• In the absence of substantial leaks, the AH was no
different with HH or HME when using a face mask during
NIV . However, when there were excessive leaks, the AH
significantly dropped when using HME (around 40%).
• The leak affects the HME’s performance by changing the
difference between inspired volume (cool air) and expired
volume (warm, moist air).
• Increasing the IPAP led to a significant decrease in RH
which returned to normal when HH was applied.
• Changing the respiratory rate or inspiratory / expiratory
ratio had no significant effect.
• RH and AH both rose with the addition of humidification,
and the air was fully saturated at the maximum heater
setting . A key conclusion was that the incorporation of
HH systems in NIV ventilators increased the RH.
Types of humidification system
• By analyzing AH values in a series of patients with
hypoxemic acute respiratory failure, with NIV administered
by a turbine-driven ventilator and a face mask, in four
different NIV environments: 1)without humidification; 2)
with a HH-MR850; 3) with a HH730; 4) with a HME booster.
The main findings were that the increase in inspired
oxygen fraction led to a proportional decrease in AH and
this effect was greater in an environment without
humidification than during NIV delivered with HH and HMEbooster systems, and that AH levels were ‘critical’ when
the inspired oxygen fraction was higher than 60%.AH was
higher with a humidification system.
• When HH and HME booster systems were compared, AH
was higher with the latter; however, the HME booster
caused more patient-ventilator asynchrony and
hypercapnia.
Types of humidification system
• The usual practice was to use HH more often
than HME for acute NIV applications (53% versus
6.6%).
• Surprisingly, despite the importance of gas
conditioning during NIV, there were relatively few
hospital protocols referring to humidification
practice in the participating centers (55%) .
Selecting the Appropriate Humidifier
• According to AARC guidelines , HHs should provide an
absolute humidity level between 33 and 44mgH2O/L ,
whereas HMEs should provides minimum of 30 mg
H2O/L .
• Combined hydrophobic hygroscopic HMEs should be
the first choice if passive humidification is selected, as
they have better humidification capacity than the
hydrophobic ones.
• In terms of HME length of use, some concerns of
decreased performance with their prolonged duration
have been expressed. Hence, most manufacturers
recommend exchanging HMEs every 24 hours.
Conclusion
• Airway humidification represents a key intervention in
mechanically ventilated patients.
• Inappropriate humidifier settings or selection of devices
may negatively impact clinical outcomes by damaging
airway mucosa, prolonging mechanical ventilation, or
increasing work of breathing.
• Humidifier devices may function passively or actively,
depending on the source of heat and humidity.
• Depending on the clinical scenario, humidifier selection
may change over time. Therefore , knowledge of the
advantages and disadvantages of each of these devices
is essential for respiratory care practitioners.
Conclusion
• During NIV, adequate gas conditioning is essential
because of the deleterious effects of inhalation of dry air ,
which may then negatively influence adherence to and
the success of the ventilatory treatment.
• Several parameters, mostly involving the technical
aspects of NIV, are determinants of inefficient
humidification.
• The correct application of an appropriate humidification
system may help prevent NIV-induced airway dryness.
However ,there are still open questions about when
exactly to apply a humidifier in acute or chronic settings,
the best type of humidification device in each situation,
the interaction between the humidifier and the underlying
disease and the effects of individual ventilators on
delivered humidity.
PFT WorkShop- Dr.Khaled Hussein