Humidification Flashcards
Define absolute humdity
The mass of water vapour in a given volume of air
mg/L
g/m^3
What determines the maximum amount of water that can be present in air
The temperature (HOT in Durban by the sea and high humidity)
When temperature increases the amount of water which can be present as vapour also increases
Fully saturated air at 20 deg C = 17 g/m^3
Fully saturated air at 37 deg C = 44 g/m^3
Define relative humidity
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 (%)
Relative humidity = Mass present
__________
Mass to saturate
Using PV = nRT and noting that n proportional to Mass
Relative humidity = Actual vapour pressure
___________________
Saturated vapour pressure
Explain how a hair hygrometer works
As humidity increases hair length increases. The hair is connected to a pointer which moves with changes of hair length over a scale
Works best relative humidity 30 - 90%
Explain how a wet and dry bulb hygrometer works
Two thermometers
One surrounded by a wick connected to water
One surrounded by ambient air
Water surrounds the ‘wet’ thermometer and evaporates leading to reduced temperature. The rate of evaporation is related to the change in temperature and also the relative humidity.
Constant air flow over wick required for accuracy to prevent localized enhancement of humidity
What is a Regnaults hydrometer
Silver tube containing ether
Air is bubbled through –> cooling the air and leading to condensation on the shiny outside surface of the silver tube. The temperature at which condensation or misting occurs can then be noted as the dew point.
The dew point is the temperature at which ambient air is fully saturated
Relative humidity = Actual vapour pressure
___________________
Saturated vapour pressure
Relative humidity = SVP at dew point
______________________
SVP at ambient temperature
How can absolute humidity be measured
- Transducers: change in electrical resistance/capcitance with absorption of water
- Mass spectrometer
- Light absorption technique
Why is it important for breathed air to be humidified
- Thickened mucus –> blockages –> stasis –> infection/atalectasis
- Damage to cilia by dry gases (keratinization of tracheal epithelia –> loss of cilia)
What is a suitable level of humidity for the air entering the trachea
34 g/m^3 (this is almost fully saturated at 34 deg C)
What are the methods for artificially increasing the inspired humidity artificially
- Humidify the environment
- Infant incubators - Humidify the inspired gas
- humidifiers (HME)
How does a Heat Moisture exchanger work?
- Inlet + outlet + enclosed disposable sponge/foam/fiber material impregnated iwth a HYGROSCOPIC substance e.g. Silica gel.
Combination of humidification and the the latent heat of vaporisation/condensation during exhalation and inspiration on the hygroscopic substance
EXHALATION
Gas cools + water condenses onto the hygroscopic substance + the substance is simultaneously warmed by the latent heat of condensation
INSPIRATION
Moisture evaporates –> humidifying the inspired gas
Hygroscopic substance warmed during exhalation now warms the inhaled gas
When do HME’s become less effective
Under optimum conditions, what level of inspired humidity is achieved by HMEs
- High ambient temperature
- as little temperature change may lead to minimal condensation - High minute volumes
- Secretion deposited on moisture exchanging unit with coughing (increased resistance to breathing + risk of infection e.g. pseudomonas)
Under Optimum conditions humidification of 24 g/m^3
What is an HMEF
Heat Moisture Exchanging Filter.
- A bacterial filter has been added to the HME device
What is the problem with bubbling gas through water to humidify it
Ineffective especially with large bubbles
Passage through sintered metal is better
Also, heat loss through latent heat of vaporization occurs cooling the water and worsening the efficiency
Describe the function of a hot water bath humidifier. Whats are the risks associated with its use
Water is heated to 40 - 60 deg C of which the fresh gas flows to become 100% humdified.
- Temp controlled by thermostat
Temps of 60 deg C to kill bacteria may scald patient especially with IPPV
What is a cascade humidifier
Its a hot water bath humidifier with the as flow bubbled through a perforated screen at the bottom of a wide tube (cascade tower).
- large surface area ensures 100% saturated
- thermostat at patient outlet controls the water bath heater
What is a heated element humidifier
Water vaporized by dripping it onto a heating element 100 deg C. High temps ensure sterility.
Water trap included below to prevent water accumulation in circuit
thermistor at patient connector prevent burns and monitor humidity supplied
List the types of humidifier
HME HMEF Hot water bath humidifier Cascade humidifier Heated element humidifier with water trap
What are the different types of Nebulizers
Gas-driven
Ultrasonic
How does a gas-driven nebulizer work?
Bernoulli effect + jet entrainment
High flow of gas into chamber with narrowing at the end. The narrowing of the tubing causes a drop in pressure at the end of the tube which draws water up the tube and entrains it into the flow of gas at high velocity. The entrained water is broken into a spray of droplets by the fast-moving gas stream and upon collision with the anvil which baffles and breaks up the droplets even further.
What are the problems with a gas-driven nebulizer
- Fall of temperature from latent heat of vaporization (add heater)
- Back pressure from ventilator may alter entrainment ratio.
How does an ultrasonic nebulizer work?
Water droplets dropped onto or water bathing around a fast vibrating surface (frequency –> a few megahertz)
Describe the relevant nebulizer particle sizes
> 20 micron –> nuisance as fall to form pools of water
5 microns –> fall out in trachea and loosen secretions here
1 micron –> pass into alveoli and are deposited there
< 1 micron –> very stable and are inhaled and exhaled again
How do nebulized substances affect resistance to flow
They increase the density of gases and hence increase the resistance to turbulent flow.
