Presented By
Dr. Mohamed Sayed Shorbagy
Lecturer of Anesthesia and Intensive care
Faculty of Medicine
Ain-Shams University
2012
http://telemed.shams.edu.eg/moodle
Why to
care ?
Humidity= safety?
Importance of humidity of inspired gases:
 Humidity and warming of inspired air occurs by
nose and upper respiratory tract.
By-passing
the nose
↑ viscosity (mucus plug)
+
Ciliary dysfunction
Dry
inspired
gases
+
hypothermia
3
Particularly in
Mechanically
ventilated
I.C.U patient
Prolonged
anesthesia in
pediatrics &
geriatrics
Infant
incubator
4
Importance of Humidity in OR
Keep the humidity of the O.R > 50% but NOT
> 60% ?
 To dissipate the static electricity to  the
risk of explosion.
 As higher humidity  discomfort to the
personnel.
5
Importance of humidity of the
environment around the body
In order to ↑ the heat loss from the body
as in burns and infants in incubator
6
Is: The amount of water vapor
in the air
7
Humidity and temperature
  T   water vapor in the gas and  relative humidity.
 At 20 C = the air is fully saturated with 17 g/m3
 At 37 C = the air is fully saturated with 44 g/m3
So:
 At 20o C the absolute humidity=17 & the relative =100%.
 At 37o C the absolute = 44 & the relative = 17/44=40%
(when water content is constant)
 The relative = 100% (when AIR IS fully Saturated)
Absolute humidity as a function of temperature
8
Measurement of humidity
(hygrometers)
The idea:
 Measurement devices relay on measurement of
some other quantity such as temperature,
pressure, mass, mechanical or electrical change in
a substance as moisture is absorbed
Relative
- Wet and dry bulb.
- Hair tension
Absolute
- Transducers.
- Mass spectrometry.
- Weighing
Both
- DEW point “Regnault’s”
9
Regnault’s hygrometer (Dew point)
The idea:
 Cooling effect of evaporating ether on the temperature
of a silver tube.
 Evaporation of ether is associated with a decrease in
the temperature of the tube till reaching the Dew point
causing small droplet to condense on the tube.
Dew-point:
Regnault’s
hygrometer
 Temperature at which the
ambient air around tube is fully saturated
10
Absolute humidity is obtained by special
tables and graphics containing the
relationship between S.V.P. And H₂O
content of gas.
Relative humidity actual vapor pressure/
S.V.P At that temperature= S.V.P at dew
point/ S.V.P at ambient temperature.
11
Hair hygrometer
The idea:
 Increase the hair length when the relative humidity of the
surroundings increases.
 The end of the hair is attached to pointer on a scale
 It measures relative humidity in the operating room.
Hair hygrometer
Transducers
 The electrical resistance or capacitance of a
substance change when the substance absorb
water vapor from the surrounding (absolute).
Mass spectrometry
 For absolute humidity
13
Wet and Dry bulb
(psychrometer)
 Dry thermometer = ambient temperature.
 Wet thermometer = lower temperature
Why??
 The cooling effect from the
evaporating water from the wick
surrounding the wet bulb (latent heat
of vaporization)
 The temperature difference is related
to the rate of evaporation which is
related to relative humidity.
(controlled airflow over the
thermometer why??)
Controlled
Air
Amb. Air
temp.
Flow
Wet and dry bulb hygrometer
14
Weighing
 Cooling of air will condense water vapor
and its quantity = absolute humidity.
 Weighing substances before and after the
absorption of vapor (as sulphuric acid).
15
Humidifiers
Passive
Conserve heat
and water vapor
(H.M.E.)
Active
Add water vapor
(humidifiers) or
droplets
(nebulizers)
16
Passive humidifiers
 HME= artificial nose =condenser humidifier
The form= capsule containing filter with hygroscopic material
The idea = expiration= water deposited and heat retained.
inspiration= heat and warming of inspired dry gas
The efficiency =70% If high difference in humidity &
temperature between cap. and fresh gas flow.
Material= plastic (low thermal conductivity) to preserve temp.
inside the cap.
Additives= port for EtCO2 & bacterial filter (HMEF)
Advantage = cheap, light and easy to use.
17
Disadvantages
 ↑the dead space (60ml) thus ↑ the rebreathing
in pediatric patient.
 Excess water and secretions  obstruction &
↑ in airway resistance
 ↓ efficiency in large tidal volumes.
 Infection (disposable)
18
Direct installation of water directly
into trachea:
 Not recommended- simplest- least efficient.
 Disad.  rapid and excessive installation 
aspiration syndrome
19
Heated  H2O
Heated  element
Idea: electric heater  ↑ temp  H2O dripping on electric heated
↑ capacity of gas to hold H2O element up to 100-250◦C 
vapor.
vaporization.
We need also to ↑ temp as 
latent heat of vaporaization 
cooling
of
H2O

↓
Humidification
Efficacy and advantage 
Efficacy and Adv.
Thermostatistically
controlled The same efficacy
electric heater is most effective ~ -High temp  sterility.
80%.
-Used in anaestheisa and ICU as
allow inspired gas saturation at
body temp.
-Antibacterial if temp > 60◦C
20
Disadvantages of both
- Heating cause:
Thermal injury of the trach. And Lung so use:
 Thermistor in heater "prevent over heating".
 Thermometer in humidifier
 Thermometer for inhaled gas temp.
 Hyperthermia esp. in pediatrics.
-Infection but ……
H2O Condensation
 flow obstruction
so use H2O trap.
for heated H2O
and……
Chemical change to
volatile aneth. gas
d. to high heat for
heated element
21
Types of heated - H2O  4 types
 Passover (large surface area of heated H2O:
 Add (wick)
 Bubble through:
 Sintered glass with tiny holes
 Tiny holes at bottom of wide tube (cascade)
 Vapor-phase (dry gas mixed with vaporized H2O
22
Hot water bath humidifier
23
Nebulizers
Gas driven
Ultrasonic
Idea  bernouli effect:
• High speed flow of gas  pressure drop 
H2O to be drawn up (entrained)
• Anivl  breaks H2O into droplets.
• Heater  why?!
• Efficacy  more efficient than heated – H2O
Vibrating surface within H2O bath  very small
droplets.
Adv. Humidity as normally found in trachea
Adv. ↑ freq. of vibrating surface  H2O droplet
size adjustment.
NB. (Idea size ~ 1 micron droplet):
 H. of inspired gas
 Adm. Inhaled drugs (as bronchodilators)
Disad. Entrainment ratio affected by (back
Expensive.
Supersaturation of inspired gas:
H2O over- loading  Pulmoy edema (in ped.?)
↑ density of gas  ↑ resistance to turbulent flow
pressure) as in ventilator
eff  most efficient
NB risk of infection: with all types, so allow: sterile and freq.
changed solutions of humidifiers
Gas-driven
nebulizer
Plate vibrating at
ulrasonic frequency
(e.g.2MHz)
Relative Efficiency of Humidifiers
Approximate efficiency of humidifiers; the exact
values depend on the model of humidifier in use
MCQ
All are active humidifiers except:
 Passover H.
 Cascade H.
 H.M.E.
 Nebulizers.
Dew point is a:
 Pressure at which ambient air is fully saturated.
 Temp at which ambient air is partially saturated.
 Temp at which ambient air is fully saturated.
Hygrometer used in the O.R is:
 Mass spectrometry.
 Wet and dry hygrometer.
 Hair hygrometer.
Plz review:
 H. and relation to aneasthesia
 Dew point and its applications.
 Hygrometers
 Humdifiers.
 Nebulizers