Humidification - Respiratory Therapy Files

RT Physics Week 4
Review of Pressure Conversions
Humidity and Aerosol
– PAO2
– P(A-a)
– CaO2
– DO2
Pressure Conversions
• Respiratory therapists use several different
pressure measurement scales. We have
already discussed conversion from gauge to
absolute pressure. It is also important to be
able to convert from one scale to another, ie:
cmH2O to torr or psig to cmH2O.
Pressure Conversions
• One approach to converting from one
pressure scale to another would be to
memorize a conversion factor for each
pressure scale. An easier approach is to
remember the value for 1 atmosphere in each
of the commonly used pressure scales.
• Remember: 1 ATM = 15 psi = 760 torr = 1034
Pressure Conversion
• Then use the following equation to convert:
• New Pressure= 1 ATM in new scale
------------------------- x Given pressure Value
1 ATM given scale
Pressure Conversion
• Example: Convert 590 torr to cmH2O
• New Pressure value= 1034 cmH2O x 590 torr
----------------760 torr
Torr units cancel
New pressure value= 1.36 x 590 = 802 cmH2O
Remember: 1 ATM = 15 psi = 760 torr = 1034 cmH2O
Humidity and Aerosol
• The body’s systems require a certain amount
of hydration in order to maintain homeostasis.
The best way is through drinking plenty of
fluids and administration of IV fluids. Signs of
dehydration are chapped lips, flaky skin, dry
cracked elbows and heels. The respiratory
signs include crusty nasal cavities, nosebleeds,
dry mouth, scratchy throat and a dry hacking
Humidity and Aerosol
• Many of our patients have bypassed normal
airway and an important part of our job is to
provide for their special needs.
• Under normal conditions, body water loss
through respiration is approximately 200 –
500 mL / day. Respiratory therapists deliver
oxygen that needs to humidified and often
heated to adequately meet the needs of their
Humidity and Aerosol
• Vapor pressure – Pressure water as a vapor or gas exerts and
is part of the total atmospheric pressure. Water vapor
pressure in the lungs exert 47 mmHg
• Absolute Humidity – the actual amount (in mg./l) of water
vapor in the atmosphere
• Relative Humidity – the percent of water vapor in the air as
compared to the amount necessary to cause saturation at the
same temperature.
• % Body Humidity – the relative humidity at 37 degrees Celsius
• Humidity Deficit – the amount of water vapor needed to
achieve full saturation at body temperature (44 mg/l - A.H)
Vapor pressure
 Vaporization: the change of matter from a
liquid to a gaseous form
 Water vapor pressure – the direct measure of
the kinetic activity of water vapor molecules
 Reducing the pressure above a liquid lowers its
boiling point. Ex. water boiling in mountains
Water Vapor Pressure
• When a gas is in contact with a liquid, and is in equilibrium (saturated)
with the liquid, the partial pressure of the gas is a function of
temperature. The one gas to which this applies in a normal respiration is
water. The lungs and airways are always moist, and inspired gas is rapidly
saturated with water vapor in the upper segments of the respiratory
system. The temperature in the airways and lungs is almost identical with
deep body temperature (approximately 37°C); at this temperature water
vapor has a partial pressure of 47 mmHg. (Note that the gaseous form of a
liquid frequently is termed a "vapor").
• Using the value of 47 mmHg, we can calculate partial pressure of oxygen
and nitrogen in inspired air, after the gas mixture becomes saturated with
water vapor in the upper airway (so-called tracheal air):
• Ptotal = 760 mmHg
PH20 = 47 mmHg
--713 mmHg for remaining inspired gases (21% O2 and 79% N2)
• PO2 = 0.21 · 713 = 150 mmHg
PN2 = 0.79 · 713 = 563 mmHg
Water Vapor Pressure
• That is, since water vapor partial pressure must
be 47 mmHg in a saturated gas mixture at 37°C,
the total pressure remaining for the inspired
gases is only 760-47 or 713 mmHg. The
composition of this remaining gas is 21% O2 and
79% N2, giving the partial pressures indicated
above which is then substrated by the partial
pressure of PaCO2 (PACO2, is a product of the
amount of CO2 diffused into the lung)
• PAO2 = FIO2 (Pb-PH2O) – (PaCO2/0.8)
• Absolute humidity: the actual content or water vapor present in a
given volume of air
• Relative humidity: the actual water vapor present in a gas
compared with the capacity of that gas to hold the vapor at a given
– If the water vapor content of a volume of gas equals its capacity,
the relative humidity of the gas equals 100%
Both are essential in effective ventilation.
Prevents drying of airway mucosa and
Various respiratory care devices are used to
ensure adequate humidification of inspired
• The NOSE is the bodies natural humidifier and
filter, when bypassed we must use a artificial
Humidity Terms
Vapor pressure – Pressure water as a vapor or gas exerts and is part of the total
atmospheric pressure. Water vapor pressure in the lungs exert 47 mmHg
Absolute Humidity – the actual amount (in mg./l) of water vapor in the
Relative Humidity – the percent of water vapor in the air as compared to the
amount necessary to cause saturation at the same temperature.
% Body Humidity – the relative humidity at 37 degrees Celsius
Humidity Deficit – the amount of water vapor needed to achieve full saturation at
body temperature (44 mg/l - A.H)
Isothermic Saturation Boundary – At or just below carina (end of trachea) The
point at which inspired gases are fully 100% saturated and warmed to body
temperature (44 mg/L at 37oC)
Uses of Humidity therapy
Humidification of inspired gases
Thinning of bronchial secretions
Sputum induction
Solutions Used
• Sterile water used in humidifiers and continuous nebulizers
• (Normal) Isotonic saline (.9% Na) with (Aerosol / Medicine)
• Hypertonic saline (10%) (for sputum induction)
• A gas is flowing thru a ventilator circuit at 50 C with a relative
humidity of 100%. As it flows thru the tubing it is cooled to 37
C by the surrounding ambient temperature0 of the room.
What effects will occur within the tubing? What will occur to
the ambient temperature of the air surrounding the tubing?
1. Condensation will occur on the inside surface of the tubing
as the water vapor reaches its dew point
2. There will be visible droplet formation when dew point is
3. There will be warming of the adjacent air due to convection
Humidity and Aerosol
• Solutions Used
• Sterile water used in humidifiers and continuous
nebulizers (Hypotonic)
• (Normal) Isotonic saline (.9% Na) with (Aerosol /
Medicine) Treatments
• Hypertonic saline (10%) (for sputum induction)
Consequences of Inadequate
• Inspissated secretions
• Damage to tracheal epithelium
• Decrease in ciliary activity
• Retention of secretions
Consequences of Inadequate
• Hypothermia
• Infection
• Blockage of airway
• Atelectasis
Indications For Humidification
• Provide humidity for dry gases
• Correct humidity deficit in intubated or
tracheostomized patients
• Treatment of hypothermia
• Correct bronchospasm induced by
inspiration of cold air
Factors Affecting Efficiency of Humidifiers
• Temperature
• Surface area of fluid exposed to water
• Time of contact with water
Basic Concepts
• As gas travels through
the lungs it achieves
– Body temp ~ 37C
– Barometric pressure
– Saturation with water
vapor (100% relative
humidity @ 37C)
Basic Concepts
• The point at which this occurs is
called the isothermic saturation
boundary (ISB)
– Usually occurs ~ 5 cm below the
– If the upper airway is bypassed or
VE is significantly higher than
• The ISB will be deeper into the
lungs and HUMIDITY therapy may
be indicated
Basic Physical Principles of Humidity
• Humidity is essentially the water vapor in a gas.
• This water vapor can be described in several ways,
• 1. Absolute humidity - The actual content of water vapor in a gas
measured in milligrams per liter.
• 2. Potential humidity - The maximum amount of water vapor that a gas
can hold at a given temperature.
• 3. Relative humidity - The amount of water vapor in a gas as compared to
the maximum amount possible, expressed as a percentage
• 4. Body humidity - The absolute humidity in a volume of gas saturated
at body temperature of 37 C; equivalent to 43.8 mg/L
Formulas Used When Calculating
• %RH=(absolute humidity/saturated capacity) x 100
• %BH = (absolute humidity/43.8mg/L) x 100
Primary Humidity Deficit
• If the atmosphere's relative humidity is less than 100%, the air of the
atmosphere has what is referred to as a humidity deficit.
If outside air at 20°C has 14 mg/l of water vapor, and needs to have 17.3
mg/l to be fully saturated, it is said to have a primary humidity deficit of
3.3 mg/l.
– 17.3 mg/L (potential) – 14 mg/L (absolute) = 3.3 mg/L (primary deficit)
• Remember that the potential is based temp
• The primary humidity deficit occurs in the atmosphere and represents
the difference between what humidity there is and what there could be.
• Primary Humidity Deficit = Potential Water Vapor Content - Actual Water
Vapor Content
Secondary Humidity Deficit
• This is the moisture deficit in the inspired air that the nose and upper
airway need to compensate for.
– The amount of water vapor the body needs to add to inspired air to achieve
saturation at body temperature.
• When air is breathed into the nasal cavity and heated to body
temperature, its potential water vapor rises to 44 mg/l, which is the
potential water vapor content of air at 37°C.
• Therefore, unless the air of the atmosphere is at least 37°C and
fully saturated, there exists a moisture deficit.
• Secondary Humidity Deficit = 44 mg/l - Absolute Humidity.
Water Losses
• Insensible: skin and lungs
• Sensible: urine, GI tract, sweat
• Additive: vomiting, diarrhea,
suction from intestines, severe
burns, and fever
• For each degree of temperature
above 99F for over 24 hours,
1000m of fluid is required for
Water Vapor Correction
• Water vapor acts in most ways like any
other gas, it creates a partial pressure
when it’s in a mixture of gases.
• That partial pressure depends
– The amount of water vapor present
• Which in turn depends on the temperature.
• Unlike other gases in the air, changes in
the barometric pressure of the
atmosphere under normal conditions do
not have much impact on the partial
pressure of water.
Water Vapor Correction
• As a result, it is best to calculate the partial
pressures of the other gases in the air after
the partial pressure of water vapor has been
determined--especially when measuring the
air within the lungs.
• Inside the lungs, the partial pressure of
water vapor is approximately 47 mm Hg.
• This value is relatively constant because the
air entering the lungs is normally saturated
and at 37°C.
• By subtracting the partial pressure of the
water vapor from the total atmospheric
pressure, you will find what is referred to as
the dry gas pressure
Importance of Humidity
• It is needed to maintain normal
bronchial hygiene
• It promotes functions of the normal
mucociliary escalator
• It maintains the body's vital
• Without humidity:
– the nearly 100 ml of mucus
secreted daily would become quite
thick and tenacious.
– actual lung parenchyma would dry
up, causing a loss of normal
compliance which would restrict
lung movement and reduce
Importance of Humidity
If the upper airway were bypassed or dry
gases were inhaled, a series of adverse
reactions could occur, including:
– Slowing of mucus movement
– Inflammatory changes and possible
necrosis of pulmonary epithelium
– Retention of thick secretions and
– Bacterial infiltration of mucosa
– Atelectasis
– Pneumonia
– Impairment of ciliary activity
Importance of Humidity
The general goals of humidity and
aerosol therapy are to:
1. Promote bronchial hygiene
2. Loosen dried and/or thick
3. Promote a effective coughs to
clear secretions
4. Provide adequate humidity in
the presence of an artificial
5. Deliver adequate humidity
when administering dry gases
6. Delivering prescribed
Clinical Evaluation of the Need for Humidity
and/or Aerosol Use
• Patient's age and ability to move normal secretions
Neuromuscular status
Recent or planned surgeries
Disease conditions
The presence of any of these may impair the patient's ability to cough and
move secretions.
Another problem may occur when patients develop very thick and
abundant amounts of secretions which cannot be moved with normal
muscle activity--making humidity or aerosol therapy necessary.
Indications for delivery of humidified gases
and aerosols
• Primary indications for
humidifying inspired
gases include:
• Administration of medical
• Delivery of gas to the
bypassed upper airway
• Thick secretions in
nonintubated patients
Indications for delivery of humidified gases
and aerosols
• Additional indications
for warming inspired
– Hypothermia
– Reactive airway
response to cold
inspired gas
Sign/Symptoms of Inadequate Airway
Dry, nonproductive cough
Increased airway resistance
Increased in incidence of
• Increased work of
• Substernal pain
• Thick, dehydrated
Humidification Devices
• The purpose of humidifiers is to deliver a gas with
a maximum amount of water vapor content.
• May be heated or unheated, and the factors
affecting the efficiency of humidification devices
– temperature
– time of exposure between gas and
– surface area involved in the gas/water
Humidification Devices
• As temperature rises, the force exerted by the water
molecules increases, enabling their escape into the gas,
adding to the humidity.
– So the higher the tempthe more humidity
• Longer exposure of a gas to the water increases the
opportunity for the water molecules to evaporate during the
humidifier's operation.
• The greater the area of contact between water and gas, the
more opportunity for evaporation to occur.
Aerosol Therapy
Basic Concepts
Aerosol Therapy
• It is important to remember that
an aerosol is not the same as
Humidity is water in a gas in
molecular form, while an aerosol
is liquid or solid particles
suspended in a gas.
Examples of aerosol particles
can be seen everywhere: as
pollen, spores, dust, smoke,
smog, fog, mists, and viruses.
Aerosol Therapy
• Aerosol therapy is designed
to increase the water
content delivered while
delivering drugs to the
pulmonary tree
• Deposition location is of
vital concern
• Some factors that affect
aerosol deposition are
aerosol particle size and
particle number.
Aerosol Output
• The actual weight or mass
of aerosol that is produced
by nebulization.
• Usually measured as
mg/L/min also called
aerosol density
• Aerosol output does not
predict aerosol delivery to
desired site of action.
Particle Size
The particle size of an aerosol depends on the device used
to generate it and the substance being aerosolized.
Particles of this nature, between 0.005 and 50 microns, are
considered an aerosol.
The smaller the particle, the greater the chance it will be
deposited in the tracheobronchial tree.
 Particles between 2 and 5 microns are optimal in size for
depositing in the bronchi, trachea and pharynx.
Particle Size
• Heterodisperse:
– aerosol with a wide
range of particle sizes
(medical aerosols)
• Monodisperse:
– aerosol consisting of
particles similar in size
(laboratory, industry)
• The aerosol particles are
retained in the mucosa of
the respiratory tract. They
get stuck!
• The site of deposition
depends on size, shape,
motion and physical
characteristics of the
Mechanism resulting in Deposition: Inertial
• Moving particles collide with airway surface.
– Large particles (>5micros), upper and large
• Physics: the larger the particle, the more
likely it will remain moving in a straight line
even when the direction of flow changes.
• Physics: greater velocity and turbulence
results in greater tendency for deposition
Mechanism resulting in Deposition
Table: Particle size and area of deposition.
Particle Size in Microns
1 to 0.25
1 to 2
2 to 5
5 to 100
Area of Deposition
Minimal settling
Enter alveoli with 95% deposition
Deposit proximal to alveoli
Trapped in nose and mouth
Mechanism resulting in Deposition:
• Particles settle out of aerosol
suspension due to gravity.
• The bigger it is the faster it
• Medium particles: 1-5
microns, central airways
• Directly proportional to time.
• The longer you hold your
breath the greater the
Mechanism resulting in Deposition:
• Actual diffusion particles
via the alveolar-capillary
membrane and to a lesser
extent tissue-capillary
membranes of respiratory
• Lower airways: 2-5
• Alveoli: 1-3 microns
• These values are from
your book
Deposition of Particles is also affected by:
• Gravity –
– Gravity affects large particles more than small
particles, causing them to rain-out.
• Viscosity - The viscosity of the carrier gas plays an
important role in deposition.
• For example, if a gas like helium, which has a low
viscosity and molecular weight, is used as a carrier
gas, gravity will have more of an effect upon the
Helium is very light and hence can't carry these
particles well, leading to rain-out and early
Deposition of Particles is also affected by:
• Kinetic activity - As aerosolized particles
become smaller, they begin to exhibit the
properties of a gas, including the
phenomenon of "Brownian movement."
• This random movement of these small
(below lmm) particles causes them to collide
with each other and the surfaces of the
surrounding structures, causing their
• As particle size drops below 0.1m, they
become more stable with less deposition and
are exhaled.
Deposition of Particles is also affected by:
• Particle inertia (repeated)
- Affects larger particles
which are less likely to
follow a course or pattern of
flow that is not in a straight
• As the tracheobronchial tree
bifurcates, the course of gas
flow is constantly changing,
causing deposition of these
large particles at the
Deposition of Particles is also affected by:
• Composition or nature of the aerosol particles Some particles absorb water, become large and rainout, while others evaporate, become smaller and are
conducted further into the respiratory tree.
• Hypertonic solutions absorb water from the
respiratory tract, become larger and rain-out sooner.
• Hypotonic solutions tends to lose water through
evaporation and are carried deeper into the respiratory
tract for deposition.
• Isotonic solutions (0.9% NaCl) will remain fairly
stable in size until they are deposited.
Deposition of Particles is also affected by:
• Heating and humidifying - As aerosols enter a
warm humidified gas stream, the particle size of
these aerosols will increase due to the cooling of
the gas in transit to the patient.
• This occurs because of the warm humidified gas
cooling and depositing liquid (humidity) upon the
aerosol particles through condensation.
Deposition of Particles is also affected by:
• Ventilatory pattern - RCPs easily control this by
simple observation and instruction.
• For maximum deposition, the patient must be
instructed to:
– Take a slow, deep breath.
– Inhale through an open mouth (not through the nose).
– At the end of inspiration, use an inspiratory pause, if
possible, to provide maximum deposition.
– Follow with a slow, complete exhalation through the
Aerosol vs. Systemic
• In many cases, aerosols are superior in terms of
efficacy and safety to the same systemically
administered drugs used to treat pulmonary
• Aerosols deliver a high concentration of the drugs
with a minimum of systemic side effects.
• As a result, aerosol drug delivery has a high
therapeutic index; especially since they can be
delivered using small, large volume, and metered
dose nebulizers.
Aerosol delivery is accomplished in a variety of
nasal spray pump
metered-dose inhaler (MDI)
dry powder inhaler (DPI)
jet nebulizer
small volume nebulizer (SVN)
large volume nebulizer
small-particle aerosol generator (SPAG)
mainstream nebulizers
ultrasonic nebulizer (USN)
intermittent positive pressure breathing
(IPPB) devices
Formula Review
• PAO2 (page 55)
• FIO2(PB-PH2O) – PaCO2/0.8
• This formula represents the amount of partial
pressure of Oxygen that is in a patients lung
• Normal value is approximately 100 given
normal conditions
FIO2 is the inspired fractional oxygen
concentration. When breathing room air this is
Formula Review
• PAO2 (page 55)
• FIO2(PB-PH2O) – PaCO2/0.8
• PB is the atmospheric pressure, at sea level this is
760 mmhg
• PH2O is the water vapor pressure in the lung. Under
normal conditions this is 47 mmHg
• PaCO2 is the partial pressure of CO2 in a patients
arterial blood. Obtained through a blood draw from
the artery
• 0.8 is a factor which represents the amount of O2 vs
Co2 is produced by the body
Formula Review
• PAO2 (page 55)
• FIO2(PB-PH2O) – PaCO2/0.8
Normal values will vary as a patient is:
• On supplemental FIO2 greater than 21%
• In environment where the atmospheric
pressure is higher or lower
• In a state where they are inhaling dry air
• Have an increase in their PaCo2
Formula Review
Formula Review
• CaO2 (Total Oxygen Content)
• This formula is used to determine the amount
of oxygen that is available to the tissues
• CaO2 = (Hb x 1.34 x SaO2) + (PaO2 x 0.003)
• Hb= Hemoglobin (carries O2)
• 1.34 (sometimes 1.36 is used) carrying
capacity of Hb for O2 is 1.34ml O2 / Gram Hb
• SaO2= Saturation of Hb with Oxygen, obtained
from a ABG. HBO2/Total Hb
Formula Review
• CaO2 (Total Oxygen Content)
• (Hb x 1.34 x SaO2) = the amount of O2 binded
with Hb
• (PaO2 x 0.003)= amount of O2 dissolved in
• PaO2 = partial pressure of Oxygen found in
arterial blood, obtained with a ABG
• Normal values: (14 x 1.34 x .97) + (90 x 0.003)
O2 Content
Formula Review
• DO2
• The delivery of oxygen to the tissues per
minute is calculated from: DO2 = [1.39 x Hb x
SaO2 + (0.003 x PaO2)] x Q
• Q = Cardiac Output
Formula Review
• The amount of oxygen in the blood: the
oxygen binding capacity of hemoglobin x the
concentration of hemoglobin x the saturation
of hemoglobin + the amount of dissolved
oxygen all Multiplied by the Cardiac Output
• The cardiac output is determined by preload,
afterload and contractility.
• The hemoglobin concentration is determined
by production, destruction and loss.
Formula Review
• CvO2
• The amount of O2 in the venous blood (left
over after cellular metabolism)
• CvO2 used to estimate Oxygen consumption
when compared to CaO2
• CvO2 = (Hb x 1.34 x SVO2) + (PvO2 x 0.003)
• Oxygen Consumption (VO2)= C(a-v)QT
• *QT= Cardiac Output

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