Humidification is important when the nose or mouth are bypassed to prevent drying of the trachea mucosa. Several methods are used to measure humidity including electrical resistance/capacitance transducers, mass spectrometers, and ultraviolet light absorption. Humidifiers aim to saturate inspired gases to 34g/m3 which is normal humidity levels in the upper trachea. Common humidifiers include heat and moisture exchangers (HME), water baths, cascade, and heated element types. Precise control of humidity levels and droplet size is needed to avoid issues like burns, water intoxication, hypoxia, or increased breathing resistance.

1. Humidification
MOHAMED ANWER RIFKY
**Absolute humidity can be measured by transducers based on
electrical resistance or capacitance as they absorbing water.
The mass spectrometer is also used in humidity,which is also
measured by ultraviolet light absorption.

2. **The nose and mouth are been bypassed by tracheostomy or endotracheal intubation>> drying of the mucosa of the trachea>> humidification reduces heat loss
,reduces coughing ,and breath holding .High humidity is unpleasant while low humidity >> build-up of static charges >>explosion with flammable agents .
UNITS OF HUMIDITY:
Absolute humidity: is the mass of water vapour present in a given volume of air. Expressed as mg litre-1 or g m-3>>measurements are numerically the same. The
maximum amount of water when fully saturated of air at 20°C contains about 17 g m-3, whereas, at 37°C it contains 44 g m-3 when fully saturated.
(Expressed in two ways).
Relative humidity: is the ratio of the mass of water vapour in a given volume of air to the mass required to saturate that given volume of air at the same
temperature. Expressed as
a percentage.1 M3 of air at 20°C, 100% saturated, contains about 17 g water. If warmed to 37°C, the mass of water vapour or absolute humidity is still the same but the
relative
humidity is only 39% (at 37°C 1 m-3 of air contains 44 g water vapour).
1-Relative humidity = Mass PRESENT Mp/Mass to saturate Ms 2-n =PV/RT 3-M is proportional to
n.
If V, R and T are constant
then:
4-Relative
humidity=Pp/Ps
Can also be expressed as a ratio of vapour
pressure.
MEASUREMENT OF HUMIDITY:
1-The hair hygrometer. (mounted on a wall of the operating theatre >>reading of relative humidity-hair gets longer as the humidity rises >>a pointer moving
over a scale-advantage of simplicity, and is most accurate between (30and 90%)- Or special membrane in place of the hair ).
Most instruments measure humidity as relative
humidity.
2-The wet and dry bulb hygrometer :(It consists of two thermometers-One temperature reads the true ambient temperature and, the left thermometer
reads a lower temperature (cooling effect from the evaporation of water from the wicks) –difference ΔT, is related to the rate of evaporation of water which, in
turn, depends on the
ambient humidity- tables >>obtain relative humidity from this temperature difference - Accurate instruments supply a controlled air flow over the
thermometers.
3-Regnault's hygrometer: A more accurate instrument for measurement >> (a silver tube containing ether-air is blown through the
ether>>cooling >>condensation on the shiny outside surface of the tube- The temperature at condensation is noted(the dew point)>>the
temperature at which the ambient air is fully saturated) - Relative humidity is deduced as follows: Relative humidity=actual vapour pressure/s.v.p at
that temp.= s.v.p at dew point/s.v.p at ambient temp.
-
CLINICAL ASPECTS OF HUMIDITY:
If the nose is bypassed by an endotracheal tube or tracheostomy, dry air enters the trachea>>secretions present in the trachea >>dried and tenacious and
mucous plugs >>block the respiratory tract.>>difficult for the patient to cough and defecult to aspirate. The cilia >>inhibited or damaged by the dry gases,and to
the latent heat lost from the mucosa as moisture evaporates to humidify the dry inspired gases. If cilia are exposed to dry gases for a long period, they disappear
and the epitheliumof the trachea becomes keratinized. (patients on in long-term intensive care) >>Need humidification.
Normally, the air entering the upper trachea is saturated with water vapour to a level of 34 g m-3, i.e. it is almost fully saturated at a temperature of 34°C,but if it
is bypassed, e.g. by a tracheostomy!!. Two methods of increasing the inspired humidity artificially. 1-humidifying the environment 2-by humidifying
the inspired gases alone(first occurs in oxygen tents and infant incubators,and the second in anaesthesia practice (humidifers).
HUMIDIFIERS:
1-The heat and moisture exchanger (HME):It has an inlet and an outlet which can be screwed together >>enclosed unit -a disposable
element of foam or fibre material impregnated with a hygroscopic substance such as silica gel -When the warm moist expired gas passes
through this element the gas is cooled and water condenses while the element is simultaneously warmed by the latent heat of the water
condensing on it .During inspiration >> the moisture evaporates humidifying the inspired gas and sine the element has been warmed. as
the nose) so,called the artificial nose. The efficiency of the humidifier will dependon the ambient temperature >>at hot climate >>little
**Humidifying the inspired gases by instillation into the trachea>>if the drip is run too rapidly >> acid-aspiration like syndrome (simplest
method) ).

3. At high minute volumes the HME is less efficient that secretions may be deposited on the mesh, with increasing the resistance to breathing and the
risk of infection >>Frequent change. A variant of the HME is available which includes a bacterial filter (HMEF).
2-The water bath: Dry gases bubbling through water at room temperature, with two disadvantages>>a-the large bubbles>>the sintered >>reduce
the bubble size.b-the loss of heat (latent heat of vaporization )>>cooling reducing humidity>>an electric djusted heater, The operating temperature
is normally about 40 to 45°C, up to 60°C>> (bacteria!!) ,but care of scaling in patients on ventilators, and Hyperthermia may occur in young
children>>a thermostat,a second thermostat in (modern humidifiers)-Condensing of water in the loop tubing>>obstruct the flow or blown into a
patient's tracheostomy,so,ensure that the water goes ack to the humidifier.
3-The cascade humidifier: In which gas bubbles through a perforated screen at the bottom of a wide tube >> foam of
water
and gas to pass up the tube>>large surface area >>water is fully humidified. The thermistor at the patient>>controls the
heater element in the water bath to maintain an optimal delivery >>risk of burns .
4-The heated element humidifier:Water is vaporized by dripping it on to an electric element heated to
100°C(sterility).-water trap which collects excess water.
5-An alternative device:A small pulsed volumes of water are delivered to the heated element (250°C is used)-
less suited to use with anaesthetic vapours or volatile drugs as the heat >>chemical changes. The amount of water vapour
delivered from these humidifiers must be controlled according to the minute volume and humidity required, - A thermistor
at the patient (risk of burns).
NEBULIZERS: Is another form of humidifier. Generates an erosol pattern.
A-The gas-driven nebulizer: (Bernoulli effect)-A high speed flow >>drop in pressure at the end of a tube which allows water
to be
entrained from the tube -Entrained water is then broken up into a spray of droplets of fast moving gas stream. If the gas
stream hits an anvil>e larger droplets are divided into smaller droplets. (Latent heat)>>here is a fall of temperature in the
humidifer, so its effectiveness>>(a heater). A further problem with a back pressure that may alter the entrainment ratio.
B- The ultrasonic form of nebulizer:The droplets of water are formed by a vibrating surface. The water may be dropped on
a vibrating surface or formed from a pool lying on such a surface. The frequency f vibration is usually a few megahertz which produces
extremely small droplets.
Size of Droplets:
Droplets of over 20 microns (μιη), if not baffled out>> are a nuisance as they fall out to >>the tubing or in the upper respiratory tract.
Those of 5 μιη fall out in the region of the trachea, while any particles of 1 μιη tend to pass right through to the alveoli and be deposited
there. Droplets below 1 μπι in size tend to be extremely stable and can be inspired and then expired again. One micron is ideal and
ultrasonic nebulizers are the most effecient ones.
In addition to this danger of water intoxication, problems may arise from hypoxia if too high a humidity is provided .Hypoxia may also be
caused by an increased resistance to breathing. The inspired droplets also increase the density of the gases and raise the resistance to
turbulent flow.
Small droplets are stable and can be a source of cross-infection (solutions must be sterile).
RELATIVE EFFICIENCY OF HUMIDIFIERS:
There are vertical dotted lines is at 34 gm m -3 representing the normal level of humidity in the upper trachea and at 44 gm -3