Humidification

Question 1

Define absolute humdity

Answer 1

The mass of water vapour in a given volume of air

mg/L

g/m^3

Question 2

What determines the maximum amount of water that can be present in air

Answer 2

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

Question 3

Define relative humidity

Answer 3

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

Question 4

Explain how a hair hygrometer works

Answer 4

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%

Question 5

Explain how a wet and dry bulb hygrometer works

Answer 5

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

Question 6

What is a Regnaults hydrometer

Answer 6

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

Question 7

How can absolute humidity be measured

Answer 7

1. Transducers: change in electrical resistance/capcitance with absorption of water
2. Mass spectrometer
3. Light absorption technique

Question 8

Why is it important for breathed air to be humidified

Answer 8

1. Thickened mucus –> blockages –> stasis –> infection/atalectasis
2. Damage to cilia by dry gases (keratinization of tracheal epithelia –> loss of cilia)

Question 9

What is a suitable level of humidity for the air entering the trachea

Answer 9

34 g/m^3 (this is almost fully saturated at 34 deg C)

Question 10

What are the methods for artificially increasing the inspired humidity artificially

Answer 10

1. Humidify the environment
   - Infant incubators
2. Humidify the inspired gas
   - humidifiers (HME)

Question 11

How does a Heat Moisture exchanger work?

Answer 11

1. 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

Question 12

When do HME’s become less effective

Under optimum conditions, what level of inspired humidity is achieved by HMEs

Answer 12

1. High ambient temperature
   - as little temperature change may lead to minimal condensation
2. High minute volumes
3. 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

Question 13

What is an HMEF

Answer 13

Heat Moisture Exchanging Filter.

• A bacterial filter has been added to the HME device

Question 14

What is the problem with bubbling gas through water to humidify it

Answer 14

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

Question 15

Describe the function of a hot water bath humidifier. Whats are the risks associated with its use

Answer 15

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

Question 16

What is a cascade humidifier

Answer 16

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

Question 17

What is a heated element humidifier

Answer 17

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

Question 18

List the types of humidifier

Answer 18

HME
HMEF
Hot water bath humidifier
Cascade humidifier
Heated element humidifier with water trap

Question 19

What are the different types of Nebulizers

Answer 19

Gas-driven

Ultrasonic

Question 20

How does a gas-driven nebulizer work?

Answer 20

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.

Question 21

What are the problems with a gas-driven nebulizer

Answer 21

1. Fall of temperature from latent heat of vaporization (add heater)
2. Back pressure from ventilator may alter entrainment ratio.

Question 22

How does an ultrasonic nebulizer work?

Answer 22

Water droplets dropped onto or water bathing around a fast vibrating surface (frequency –> a few megahertz)

Question 23

Describe the relevant nebulizer particle sizes

Answer 23

> 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

Question 24

How do nebulized substances affect resistance to flow

Answer 24

They increase the density of gases and hence increase the resistance to turbulent flow.

Question 25

What is the Poiuselle equation

What is the Reynolds number equation

Answer 25

Q = pi . P . d^4
_________
128. nl

R = vpd
____
n

Question 26

State the efficiency of the different humdifiers in terms of the absolute humidity achieved relative to the normal absolute humidity in the upper airway

Answer 26

Parameter (Absolute humidity g/m^3)

Normal level in upper trachea 34

100% saturation at 37 deg C 44

Air bubbled through water 10

HMEF 25

Heated water bath humidifier 40

Heated bernoulli neb and anvil 60

Ultrasonic neb (adjustable) 99

Therefore the nebulizer devices have the risk of flooding the lungs and causing pulmonary oedema if not monitored and adjusted appropriately