Humidification: its importance
and delivery
A R Wilkes MSc BSc
Key points
Much more energy is
required to evaporate
water than to warm gas
Humidifying and warming
inspired gas can account
for 20% of the body’s
total basal heat loss
Over-humidification may
be as harmful as underhumidification
The range of optimal
humidification narrows as
the duration of intubation increases
A sputum score can be
used as a basis for
choosing which humidification device is most
appropriate
The physics of humidification
Humidification involves adding water vapour
to a gas. Water consists of molecules which
have varying energies. The temperature of a
substance is determined by the mean kinetic
energy of its molecules. In a container partly
filled with water, some of the water molecules
have sufficient energy, and hence velocity, to
escape from the surface of the water and enter
the air above. Some of these molecules get
attracted back into the water, but some have sufficient energy to leave the zone of intermolecular attraction above the liquid water and remain
in the air. As these molecules have the greatest
energy, the mean energy of the liquid water, and
hence its temperature, falls as evaporation
occurs. Under equilibrium conditions in a
sealed container, when the temperature remains
constant and when the number of molecules
falling back into the water through random
movement balances the number of molecules
escaping from the liquid into the air, the air
becomes saturated with water vapour.
The molecules of water vapour in the air
exert a pressure. When the air is saturated with
water vapour, the water molecules are said to
exert a saturated vapour pressure. This saturated vapour pressure depends only on the temperature of the liquid water. If the temperature
increases, the mean energy of the molecules in
the water increases, more molecules escape
from the surface of the water, and the saturated
vapour pressure increases.
Definitions
A R Wilkes
MSc BSc
Department of Anaesthetics and
Intensive Care Medicine, University
of Wales College of Medicine,
Heath Park, Cardiff CF14 4XN
40
The water vapour pressure (expressed in kPa) is
the pressure that would be exerted by the water
vapour if it alone occupied the space. Normal
atmospheric pressure is about 101 kPa.
Therefore, there is an approximate equivalence
between water vapour pressure expressed in
kPa and a fractional quantity expressed as a
percentage.
The saturated water vapour pressure is the
maximum attainable water vapour pressure at a
particular temperature. Relative humidity (RH)
is the ratio of the actual water vapour pressure
in the air at a particular temperature to the maximum attainable water vapour pressure at that
temperature, expressed as a percentage.
Absolute humidity is the mass of water vapour
per unit volume of gas, expressed in g m–3
(numerically equal to mg l–1). The dew-point
(expressed in °C) is the temperature at which
condensation occurs when the gas is cooled.
The specific heat capacity of a substance is
the heat required to raise the temperature of 1 g
of that substance by 1 Kelvin (K), expressed in
J g–1 K–1, where temperature expressed in
Kelvin is equal to the temperature in degrees
Celsius (°C) plus 273.15.
The massic enthalpy of evaporation (previously termed the latent heat of vaporization) is
the heat required to convert 1 g of a substance
from the liquid phase to the gaseous phase at a
given temperature, expressed in J g–1.
Examples
At a normal atmospheric pressure of about 100
kPa, air saturated with water vapour at 20°C
consists of 2% water vapour, and at 37°C consists of 6% water vapour (Fig. 1). Room air at
approximately 22°C and 50% RH has a moisture content of 10 g m–3. If the temperature of
this air decreases, the relative humidity
increases until a temperature is reached
(approximately 11°C) when the air is saturated
with water vapour (100% RH). Below this
temperature, condensation occurs, so that the
room air has a dew-point of 11°C.
If the temperature of the room air is
increased, the relative humidity decreases,
British Journal of Anaesthesia | CEPD Reviews | Volume 1 Number 2 2001
© The Board of Management and Trustees of the British Journal of Anaesthesia 2001
Humidification: its importance and delivery
Fig. 1 Saturated vapour pressure against temperature (dew-point) for water.
(A) Typical room air with an RH of 50%; (B) room air cooled to its dew-point
temperature 11°C (100% RH); (C) room air warmed to body temperature,
37°C, RH now at 23%; (D) room air saturated with water vapour at 22°C;
(E) room air saturated with water vapour at 22°C warmed to 37°C, RH now
at 42%; (F) air at BTPS conditions (body temperature and pressure, saturated), i.e. 37°C, 100% RH at ambient pressure.
until at 37°C (i.e. body temperature), the RH is 23%. If condensation does not occur, as the temperature of the room air
changes, the absolute humidity remains at about the same
level (approximately 10 g m–3), although there will be a small
change due to the expansion or contraction of the air as it
warms or cools, respectively.
Physiology of the airways related to
humidification
Gas in the alveoli is saturated with water vapour at 37°C and
has an absolute humidity of 44 g m–3. This alveolar gas is said
to be at BTPS conditions (body temperature and pressure, saturated with water vapour). The moisture deficit is the difference
between 44 g m-3 and the absolute humidity of the inspired air,
i.e. 34 g m–3, if typical room air with an absolute humidity of 10
g m–3 is inspired (Fig. 1). The moisture deficit is the humidity
that must be added by the upper airways to condition the
inspired air for optimal gas exchange in the alveoli. The level in
the airways where the gas reaches BTPS conditions is called the
isothermic saturation boundary.
When a patient is intubated, the upper airways are by-passed.
Gas delivered from either a pipeline or cylinder has a very low
moisture content. During artificial ventilation, this dry, cool gas
can, therefore, be delivered directly to the trachea. Water and
energy are required to humidify and warm this gas.
For example, a 70 kg adult is ventilated using dry air with a
tidal volume of 0.7 l (10 ml kg–1 x 70 kg) and a frequency of
12 min–1, giving a ventilation of 8.4 l min–1. The moisture
content of the air in the lungs is 0.044 g l–1. The total amount
of water required to humidify the air is therefore 8.4 x 0.044
= 0.37 g min–1. The massic enthalpy of evaporation of water
is 2.4 kJ g–1 at 37°C. The heat required is, therefore, 2.4 x
1000 x 0.37 = 887 J min–1 (i.e. 14.8 W). This represents 18%
of the total basal heat loss of about 80 W.
The specific heat capacity of air is 1.0 J g–1 K–1. The density of air is 1.2 g l–1. To raise the temperature of 8.4 l min–1 of
inspired gas from 22°C to 37°C (295 to 310 K), therefore,
requires 8.4 x 1.2 x 1.0 x (310 – 295) = 150 J min–1 (i.e. 2.5
W). The total heat required to both humidify and warm the dry
gas at room temperature is therefore 17.3 W, of which 85% is
required for humidification and 15% for warming the gas.
The specific heat capacity of water is comparatively high,
i.e. 4.2 J g–1 K–1. The energy required to warm 0.7 l of air by
1°C (or 1 K) is 0.7 x 1.2 x 1.0 x 1.0 = 0.84 J, and is equal to
the energy released by the cooling of 1 ml (or 1 g) of mucus
by only 0.84/4.2 = 0.2°C assuming that the mucus is composed mainly of water. Therefore, the mucus in the airways
acts as a thermal buffer.
The trachea is lined with ciliated columnar epithelium. The
cilia move a layer of mucus produced by goblet cells towards
the larynx. This mechanism is known as the mucociliary elevator. Particles are trapped on the mucus, so that the airways
are continually cleared of microbes and debris. The layer of
mucus in the trachea is exposed to the inspired gas. If the
humidity of the inspired gas is low, water evaporates from the
mucus so that it becomes increasingly viscous. The cilia are
then unable to move the mucus. Evaporation removes heat
from the trachea, which then cools. The isothermic saturation
boundary falls to a lower level within the airways and cell
damage and infection can result. In addition, if the patient is
intubated, the viscous secretions gradually occlude the tracheal tube.
Alternatively, if the temperature and humidity of the gas is
high, condensation occurs in the airways. The viscosity of the
mucus reduces, the volume of mucus increases, the cilia again
become ineffective, and the mucociliary elevator stops. The
actual humidity level and the duration of exposure at that
humidity level are therefore both important parameters.
Humidification equipment
Gas can be humidified by using either an active or a passive
device or system. Active devices, such as heated humidifiers,
add water vapour to a flow of gas independent of the patient.
Passive devices, such as heat and moisture exchangers
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41
Humidification: its importance and delivery
as it passes through the delivery tube, some condensation
occurs, but there is the re-assurance that the delivered gas is
fully saturated with water vapour.
A water trap collects condensation in the expiratory limb.
Alternatively, a heater wire can be used to reduce condensation in the expiratory limb, but it may cause excessive condensation within the expiratory valve of the ventilator,
adversely affecting the performance of the expiratory flow
sensor that may be sited there.
Nebulizers
Fig. 2 Typical arrangement for a heated humidifier. (A) Water reservoir; (B)
humidification chamber; (C) wick; (D) delivery tube; (E) patient connection
port; (F & G) temperature sensors; (H) heater wire; (I) temperature control;
(J) relative humidity control.
(HME), return a portion of the exhaled moisture, or rely on a
chemical reaction with exhaled carbon dioxide, to humidify
the inspired gas.
Active humidification systems
Heated humidifiers
An example is shown in Figure 2. A water reservoir delivers
water to a humidification chamber where the water is heated.
The water evaporates and is added to the gas to be delivered
to the patient. A wick in the chamber increases the surface
area for evaporation. A delivery tube connects the humidification chamber to the patient connection port of the breathing
system.
One temperature sensor monitors the temperature of the gas
at the patient connection port. A second sensor measures the
temperature of the gas at the outlet of the humidification
chamber. Heater wires in the delivery tube are controlled by
feedback from the two sensors. The temperature of the gas
required at the patient end of the delivery tube is set using a
control. A second control is used to alter the relative humidity
by varying the difference in temperature between the two temperature sensors. If the temperature of the gas to be delivered
to the patient connection port is set to be higher than at the
humidifier end of the delivery tube, the gas is warmed as it
passes through the delivery tube. Condensation is therefore
reduced, but the relative humidity of the gas also decreases.
Alternatively, if the settings are made such that the gas cools
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There are two types of nebulizer: gas-driven or ultrasonic. In
both devices, droplets of water are produced, ideally with a
diameter of about 1 µm. Some of these droplets evaporate in
the gas delivered to the patient so that the gas is likely to be
fully saturated with water vapour. However, heat is required
for evaporation, so that the temperature of the gas falls. A
heater can maintain the desired temperature of the gas.
However, with these devices, it is relatively easy to add excessive moisture to the delivered gas, as some of the droplets do
not evaporate, leading to excessive loading of the lungs with
water and hypoxia due to blockage of alveoli. In addition, the
size of the droplets produced may be effective for the transmission of microbes, so care must be taken to ensure that the
water is sterile.
Passive humidification systems
Heat and moisture exchangers
These devices consist of a layer of either foam or paper that is
generally coated with a hygroscopic salt such as calcium chloride. The expired gas cools as it passes through the device and
condensation occurs, releasing the massic enthalpy of vaporization to the HME layer. The hygroscopic salt reduces the
relative humidity of the gas to below saturation level by chemically combining with the water molecules, although some
water vapour is always lost into the breathing system. On
inspiration, the absorbed heat evaporates the condensate and
warms the gas. The hygroscopic salt, to which the water molecules are loosely bound, releases the water molecules when
the water vapour pressure is low. The inspired gas is, therefore, warmed and humidified to an extent that depends on the
moisture content of the expired gas, and hence on the patient’s
core temperature, and the condition of the airways and lungs.
A filter layer may also be added to the device. Two types of
filter layer are commonly used, either a wad of electrostatically-
British Journal of Anaesthesia | CEPD Reviews | Volume 1 Number 2 2001
Humidification: its importance and delivery
Use humidifier with
manufacturer’s
recommended setting
Admission to ICU
Examine patient
Examine patient
Bloody or copious secretions?
Tenacious sputum?
Hypothermia?
(Core temp. < 32°C)
Copious secretions?
Tenacious sputum?
yes
no
yes
Check temperature
settings and water
level in humidifier
no
Use HME
Copious secretions?
Tenacious sputum?
33 g m–3, or an absolute humidity 75% of that at BTPS conditions. This humidity level is equivalent to that measured in
the subglottic space during normal nasal breathing. Few
HMEs have a moisture output of this level. Data on HMEs
and humidifiers are available from the Medical Devices
Agency in London, UK. A minimum level for moisture output
is not specified in the International Standard for HMEs (BS
EN ISO 9360-1:2000), although, as with humidifiers, the
manufacturer must declare the performance of the device.
However, HMEs are cheaper, easier to use and can keep the
breathing system dry.
Protocols have been devised to assist clinicians choose
between heated humidifiers or HMEs (Fig. 3). The protocols
are based on the patient’s history and sputum score, and
assume that an HME can be used, unless indicated otherwise.
no
Problems with humidification devices
no
> 4 HMEs used in 24 h?
(Due to blockage with
secretions)
yes
yes
Consider other factors
e.g. patient fluid
balance
Fig. 3 Example of a strategy for deciding between the two main humidification techniques.
charged material, or a pleated layer of hydrophobic material.
An electrostatic filter layer used on its own has a very low
moisture-conserving performance, but a pleated hydrophobic
filter layer can return some moisture as a temperature gradient
builds up within the pleats, allowing condensation and evaporation to occur.
Circle absorbers
Soda lime is used in circle breathing systems to absorb carbon
dioxide. The reaction generates water, as follows:
CO2 + H2O → H2CO3
2H2CO3 + 2NaOH + Ca(OH)2 →
Na2CO3 + CaCO3 + 4H2O + heat
Scalding and burning of patients has been reported due to
humidifiers overheating, causing delivery tubes to melt or
even combustion of the tubing material. It has also been
reported that gas with a high temperature was delivered to a
neonate when the temperature sensor of a humidifier became
encrusted with ribavirin during ribavirin therapy.
HMEs add dead-space and increase the resistance to gas
flow of the breathing system. The additional dead-space will
increase the level of PaCO2 unless corrected for by increasing
total ventilation to maintain alveolar ventilation. The increase
in resistance to gas flow may affect the patient’s ability to be
weaned from artificial ventilation, and may affect the triggering of some ventilators. The increase in resistance may be particularly marked if liquid, such as condensation or secretions,
or nebulized drugs, enters the HME.
Eq. 1
Key references
Eq. 2
Branson RD, Davis Jr K, Campbell RS, Johnson DJ, Porembka DT.
Humidification in the Intensive Care Unit. Prospective study of a new
protocol utilizing heated humidification and a hygroscopic condenser
humidifier. Chest 1993; 104: 1800–5
During anaesthesia, the humidity of the gas in the circle system therefore increases, although it may take up to an hour or
more to reach a maximum level.
Choosing between different humidification devices
The optimal humidity level has not been determined conclusively. The International Standard for humidifiers (BS EN
ISO 8185:1998) specifies that humidifiers intended for use
with intubated patients must have a moisture output of at least
Branson RD, Peterson BD, Carson KD. (eds) Humidification: current
therapy and controversy. Respir Care Clin North Am 1998; 4: 189–344
Chalon J, Ali M, Turndorf H, Fischgrund GK. Humidification of anesthetic
gases. Springfield: Charles C Thomas, 1981
Shelly MP. Humidification. In: Intensive Care Rounds. Abingdon: The
Medicine Group (Education) Ltd, 1993
Williams R, Rankin N, Smith T, Galler D, Seakins P. Relationship between
the humidity and temperature of inspired gas and the function of the
airway mucosa. Crit Care Med 1996; 24: 1920–9
See multiple choice questions 22–24.
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