Gases and Vapours

Question 1

Why is it not possible to make a really good cup of tea in the high Himalayas?

A. SVP is lower at high altitude
B. Boiling point decreases with ambient pressure so the teabag will have to stew for longer to get the same flavour out Missed answer
C. SVP is higher at high altitude Wrong answer
D. SVP cannot increase to atmospheric pressure
E. Boiling point is higher at high altitude so the temperature required cannot be attained with normal heating equipment

Answer 1

B. The boiling point occurs when the vapour pressure of the liquid equals the ambient pressure above the liquid. Therefore, the boiling point varies with ambient pressure and is lower at high altitude. Molecules will escape as the liquid boils rather than the temperature rising further.

Question 2

Latent heat is important in anaesthesia in which of the following areas?

A. Liquid oxygen storage systems
B. Vaporizers
C. Heat and moisture exchange filters
D. Nitrous oxide cylinders
E. Wet drapes and head injured patients
F. Ethyl chloride spray

Answer 2

All of them

A. Vacuum insulated systems retain heat, cooling the remaining liquid as latent heat is expended.

B. Volatile agents lose heat as they are vaporized. The temperature of the remaining fluid falls, lowering its SVP, which reduces vaporizer output, requiring temperature compensation.

C. Heat and moisture exchange filters reduce water loss in the respiratory tract, reducing the requirement for further vaporization.

D. Latent heat is taken from the stored nitrous oxide as the gas is used and more liquid vaporizes.

E. Evaporation from wet drapes creates effective cooling. This cooling can improve outcome following head injury.

F. Ethyl chloride is stored as a liquid. When it is sprayed on skin it vaporizes, cooling the skin and sensory nerves providing brief but intense periods of local analgesia.

Question 3

What would be the danger of putting halothane in a sevoflurane vaporizer?

A. Overdosing
B. Underdosing

Answer 3

Overdosing.

Halothane has a higher SVP and a lower MAC and for both reasons potentially fatal overdosage may occur. Each vaporizer is agent specific in order to deliver the dialled percentage. There are many safety precautions taken to ensure that the only inhalational agent that can be put into the chamber is the one for which the vaporizer is calibrated.

Question 4

Regarding saturated vapour pressure (true or false):

SVP is temperature dependent

Answer 4

True

Question 5

Regarding saturated vapour pressure (true or false):

SVP leads to an increase in boiling point at high altitude

Answer 5

False. Liquid boils when SVP reaches ambient pressure, which is lower at high altitude. The SVP is unchanged but the boiling point is reduced.

Question 5

Regarding saturated vapour pressure (true or false):

SVP is pressure dependent

Answer 5

False. SVP is unchanged with ambient pressure, only the proportion of total pressure occupied by the given vapour changes with ambient pressure.

Question 5

Regarding saturated vapour pressure (true or false):

SVP is 23 kPa for Isoflurane at 20 oC

Answer 5

False. SVP for Isoflurane at 20 oC is 32 kPa.

Question 5

Regarding saturated vapour pressure (true or false):

SVP leads to an increase in boiling point at high altitude

Answer 5

False. Liquid boils when SVP reaches ambient pressure, which is lower at high altitude. The SVP is unchanged but the boiling point is reduced.

Question 6

Regarding saturated vapour pressure (true or false):

SVP determines the splitting ratio of an anaesthetic vaporizer

Answer 6

True

Question 7

Regarding latent heat (true or false):

Latent heat of fusion explains the energy transfer when there is a change of state from liquid to solid, without a change in temperature

Answer 7

False. Latent heat of fusion applies to the change of state from solid to liquid

Question 8

Regarding latent heat (true or false):

The ambient temperature must always be specified

Answer 8

True

Question 9

Regarding latent heat (true or false):

Specific latent heat is measured in J/Kg/min

Answer 9

False. Specific latent heat is measured in J/Kg and is the heat required to change a substance from one state into another.

Question 10

Regarding latent heat (true or false):

Latent heat of vaporization increases with increasing temperature of the liquid

Answer 10

False. Latent heat of vaporization decreases with increasing temperature because less energy is then required to convert the liquid into a gas.

Question 11

Regarding latent heat (true or false):

Latent heat of vaporization is the temperature increase required to turn a liquid into a gas

Answer 11

False. Latent heat of vaporization describes the energy required rather than the increase in temperature and is measured in Joules.

Question 12

Which one of the following safety features on the modern Tec vaporizer prevents the variation in concentration delivery from latent heat of vaporization?

A. The heat sink can be filled with water
B. Dial linked to the splitting ratio to determine concentration provided
C. The entrance to the vaporization chamber is controlled by a bimetallic strip
D. The entrance to the vaporizing chamber is partially occluded by bellows
E. A non-spill mechanism prevents excess agent entering the bypass channel

Answer 12

C.

A. Incorrect. The heat sink in this vaporizer is metal only.

B. Incorrect. This is true but does not relate to the issue of latent heat.

D. Incorrect. TEC vaporizers do not have bellows. These are features on the EMO and Ohio vaporizers.

E. Incorrect. This is true but does not relate to the issue of latent heat.

Question 13

Assuming fixed temperature, a change in altitude will affect delivery of anaesthetic agent in the following way (true or false):

The saturated vapour pressure changes

Answer 13

False. Saturated vapour pressure is solely a function of temperature.

Question 14

Assuming fixed temperature, a change in altitude will affect delivery of anaesthetic agent in the following way (true or false):

The proportion of total pressure occupied by a set concentration of vapour changes

Answer 14

True

Question 15

Assuming fixed temperature, a change in altitude will affect delivery of anaesthetic agent in the following way (true or false):

The setting on the vaporizer should be adjusted in order to provide sufficient anaesthetic action

Answer 15

False. The anaesthetic action relies on the partial pressure of anaesthetic vapour.

Question 16

Assuming fixed temperature, a change in altitude will affect delivery of anaesthetic agent in the following way (true or false):

A vaporizer calibrated to give 1% isoflurane will give 2% when used at 5500 m above sea level

Answer 16

True. 5500 m above sea level corresponds to about half normal atmospheric pressure (101.325 kPa ?50.66 kPa). The vaporizer will give 2%, but the partial pressure will remain the same (1% of 101.325 = 2% of 50.66), and therefore the effect of the agent will be the same.

Question 17

Assuming fixed temperature, a change in altitude will affect delivery of anaesthetic agent in the following way (true or false):

A vaporizer must be recalibrated for use at high altitude

Answer 17

False. Dialling 1% isoflurane at sea level will give 1% isoflurane and the expected effects. Dialling 1% isoflurane at 5500 m will give 2%, but the partial pressure will remain the same and the effect the same. Therefore, the vaporizer should be used in the same way at any altitude.

Question 18

Regarding the measurement of humidity (true or false):

Dew point varies with ambient temperature

Answer 18

False. Dew point is dependent on humidity: it is achieved when the ambient temperature falls sufficiently for water to precipitate.

Question 19

Regarding the measurement of humidity (true or false):

Hair length elongates with increasing partial pressure of water vapour

Answer 19

True. Hair length does increase with humidity and hence partial pressure of water vapour.

Question 20

Regarding the measurement of humidity (true or false):

Ether is bubbled through water to obtain dew point on the silver casing of a Regnault’s hygrometer

Answer 20

False. The Regnault’s hygrometer uses air bubbled through ether to find the dew point.

Question 21

Regarding the measurement of humidity (true or false):

Humidity is calculated from the temperature shown on the wet wick thermometer

Answer 21

False. With wet and dry wicks, the humidity is calculated by the temperature difference between that shown on the wet and the dry wick.

Question 22

Regarding the measurement of humidity (true or false):

Risk of sparking increases with increasing humidity

Answer 22

False. Sparking risk is reduced with increasing humidity.

Question 23

Regarding relative humidity (true or false):

It is measured in g/m3

Answer 23

False. Relative humidity is expressed as a percentage, absolute humidity is measured as g/m3.

Question 24

Regarding relative humidity (true or false):

It can be described as actual vapour pressure/saturated vapour pressure

Answer 24

True

Question 25

Regarding relative humidity (true or false):

It is increased with decreasing temperature

Answer 25

True

Question 26

Regarding relative humidity (true or false):

It is inversely proportional to pressure in an open space

Answer 26

False. Humidity is directly proportional to pressure in an open space.

Question 27

Regarding relative humidity (true or false):

It is always 100% at the dew point

Answer 27

True. Relative humidity must be 100% at the dew point for precipitation to occur.

Question 28

Regarding the clinical implications of humidification:

Absolute humidity in the upper trachea is approximately 34 g/m3

Answer 28

True

Question 29

Regarding the clinical implications of humidification:

Invasive ventilation increases respiratory tract mucus production

Answer 29

False. Intubation reduces humidification of the respiratory tract and so decreases mucus production and increases its viscosity.

Question 30

Regarding the clinical implications of humidification:

Low levels of humidification in the invasively ventilated patient are associated with increased risk of respiratory infection

Answer 30

True

Question 31

Regarding the clinical implications of humidification:

A patient with acute asthma should not have humidified oxygen because of the risk of water droplets causing further obstruction to the airway

Answer 31

False. Though a patient with acute asthma should always have high flow oxygen initially, this is to enable as much oxygen delivery as possible. There is no justification to withhold humidification, which actually prevents further mucosal damage.

Question 32

Regarding the clinical implications of humidification:

High humidity increases the risks of sparks in the theatre environment

Answer 32

False. Humidification reduces sparking.

Question 33

Which one of the following best describes how temperature change affects relative humidity?

A. An increase in temperature does not affect the amount of water vapour measured in g/m3
B. A decrease in temperature of a gas causes formation of dew
C. Condensation occurs at 100% humidity
D. In an open space, relative humidity is directly proportional to barometric pressure
E. Increasing temperature increases the maximum amount of water vapour that can be contained within the atmosphere, so results in a decrease in relative humidity

Answer 33

E. This is the best fit because it explains how relative humidity decreases with increasing temperature. Relative humidity is a comparison of the mass of water vapour present with the maximum amount that gas could contain, expressed as a percentage.

A. Incorrect. Absolute humidity is measured in g/m3 and is not affected by temperature.

B. Incorrect. A decrease in temperature leads to formation of dew when saturation point is reached. However, this statement relates dew point to temperature rather than directly describing the change in relative humidity.

C. Incorrect. Condensation occurs when saturation is reached at a relative humidity of 100%. However, this statement does directly link temperature change to relative humidity.

D. Incorrect. This statement relates pressure change to humidity rather than temperature.