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Anaesthetic FRCA Primary - eLFH Physics > eLFH - Humidity > Flashcards

eLFH - Humidity Flashcards

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1
Q

Humidity definition

A

Amount of water contained within a defined volume of gas

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2
Q

Why does humid climate feel more hot

A

High levels of water in atmosphere inhibit the evaporation of sweat

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3
Q

Absolute humidity definition

A

Mass of water vapour present in a given volume of gas at a given temperature and pressureU

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4
Q

Units of absolute humidity

A

g/m^3

or

mg/L

Both are numerically the same

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5
Q

Absolute humidity equation

A

Absolute humidity = Mass of water vapour present / Mass or Volume of gas

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6
Q

Relative humidity definition

A

Ratio of actual mass of water vapour in a gas compared to the maximum amount of water vapour that a gas could contain at the same temperature and pressure

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7
Q

Units of relative humidity

A

Percentage

or

Vapour pressure

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8
Q

Relative humidity equation and working out

A

Mass is proportional to number of moles n
n = PV / RT
Therefore is temp and pressure are constant, number of moles is directly proportional to pressure

Therefore:
Relative humidity = Actual vapour pressure / Saturated vapour pressure

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9
Q

Dew point definition

A

100% relative humidity
At this temperature, air is fully saturated with water vapour so condensation occurs

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10
Q

Frost point definition

A

When dew point falls below freezing, called frost point

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11
Q

Relationship between temperature and absolute humidity

A

Increasing temp does not affect absolute humidity as mass of water vapour is the same

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12
Q

Relationship between temperature and relative humidity

A

Relative humidity decreases with increasing temperature as the maximum possible absolute humidity to cause saturation increases with increased temp

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13
Q

Approximate maximum absolute humidity at 20 degrees Celsius at sea level air saturated with water vapour

A

17 g//m^3

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14
Q

Approximate maximum absolute humidity at 37 degrees Celsius (body temperature) at sea level air saturated with water vapour

A

44 g//m^3

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15
Q

Effect of increased humidity on temperature

A

The greater the atmospheric humidity, the less likely water is to evaporate and therefore less likely to cause drop in temperature reading as a result of latent heat of vaporisation

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16
Q

Effect of pressure on humidity in a closed system

A

At full saturation, partial pressure of water vapour = SVP
SVP is unaffected by changes in pressure

When not in full saturation, ideal gas law applies in a closed system
PV = nRT
As volume of air is compressed, water vapour increases as per Dalton’s law so relative humidity increases as SVP is unchanged

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17
Q

Effect of pressure on humidity in an open space

A

Relative humidity is directly proportional to barometric pressure until relative humidity reaches 100%, then condensation occurs

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18
Q

How to measure dew point

A

Regnault’s hygrometer

Silver tube containing Ether
Air bubbled through causing cooling
Temperature at which condensation forms on tube is the Dew point

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19
Q

Benefit of measuring Dew point

A

Relative humidity can be derived from Dew point as SVP is known

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20
Q

Methods of measuring humidity

A

Mass measurement
Temp change from latent heat of vaporisation
Pressure changes affecting partial pressure of water vapour
Moisture effect on electrical circuit
Physical change in a porous material
Formation of Dew point

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21
Q

Physical change in a porous material example

A

Hair hygrometer

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22
Q

Functions of airway humidification - what does it provide?

A

Moist pulmonary secretions
Increased ciliary activity
Improved gas exchange

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23
Q

Functions of airway humidification - what does it prevent?

A

Inflammatory change in pulmonary epithelium
Mucus plugging
Heat loss
Impaired mucociliary escalator
Atelectasis

24
Q

Ideal droplet size for humidification of airways

A

1 micron - these can reach alveoli

25
Q

Most common method of airway humidification in intubated patients

A

HME filter

Heat and moisture exchange

26
Q

Mechanism of HME

A

Internal paper, sponge or foam is impregnated with a hygroscopic substance (absorbs water from air)

Water vapour passing through condenses and provides latent heat to the HME

Next breath is warmed and absorbs moisture collected by HME

27
Q

Advantages of HME

A

Disposable
Cheap
Can incorporate a bacterial filter

28
Q

Disadvantages of HME

A

Secretions collected in mesh can increase resistance in breathing circuit

29
Q

Efficiency of HME

A

~70%

30
Q

Most commonly used humidification system on wards

A

Cold water bath / Bubble humidifier

31
Q

Mechanism of cold water bath / bubble humidifier

A

Dry gas is bubbled through water at room temperature

32
Q

Advantages of cold water bath / bubble humidifier

A

Cheap
Easy to run
Gas driven - no power source required

33
Q

Disadvantages of cold water bath / bubble humidifier

A

Loss of heat through latent heat of vaporisation reduces maximum obtainable humidity

34
Q

Efficiency of cold water bath / bubble humidifier and why

A

~30% due to large bubbles and heat loss

35
Q

Commonly used humidification system on ITU

A

Hot water bath

36
Q

Mechanism of hot water bath

A

Dry gas bubbled through water but water is heated

Requires thermostatic control at both the humidifier and the patient to prevent scalding

37
Q

Temperature of hot water bath

A

40 - 45 degrees Celsius

38
Q

Advantages of hot water bath

A

Greatly improved efficiency

No heat loss via respiratory system

39
Q

Disadvantages of hot water bath

A

Risk of scalding

Hyperthermia possible in young children

Water vapour can condense in ventilation tubing increasing resistance in breathing circuit

40
Q

Efficiency of hot water bath

A

~90%

41
Q

Mechanisms of Nebulisers

A

Gas driven

Ultrasonic

42
Q

Mechanism of gas driven nebuliser

A

High flow gas ejected close to a tube filled with nebuliser fluid

Dorp in pressure created by gas (Bernoulli effect) leads to entrainment of the fluid

Fluid stream hits an ‘anvil’ causing droplets to divide

43
Q

Mechanism of ultrasonic nebulisers

A

Fluid dropped onto vibrating plate at ultrasonic frequency, producing tiny droplets

44
Q

Advantages of nebulisers

A

Full humidification

45
Q

Disadvantages of nebulisers

A

Some droplets vaporise using latent heat so heater should be added

46
Q

Efficiency of nebulisers

A

100%

Highest with ultrasonic nebulisers

47
Q

Two other humidifiers rarely used now

A

heated element humidifiers

Cascade humidifier

48
Q

Mechanism of heated element humidifier

A

Water vaporised by dripping onto heated element at 100 degrees Celsius

High risk of burns

49
Q

Advantages of heated element humidifier

A

High temp ensures sterility

50
Q

Disadvantages of heated element humidifier

A

High risk of burns

heat may alter chemical properties of anaesthetic / medicinal gases

51
Q

Efficiency of heated element humidifier

A

100%

52
Q

Mechanism of cascade humidifier

A

Gas bubbled through a perforated screen at the base of a tube within a heated water bath causing mixing within the tube

Refined hot water bath system

53
Q

Advantages of cascade humidifier

A

Large surface area ensure full humidification
Avoids risk of burns

54
Q

Disadvantages of cascade humidifier

A

Bulky equipment - reduces practical use

55
Q

Efficiency of cascade humidifier

A

100%

Anaesthetic FRCA Primary - eLFH Physics (33 decks)

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