Temperature and Humidity

Question 1

The mechanism by which most heat is lost during anaesthesia is radiation

Answer 1

True

Question 2

NICE guidelines suggest the use of forced air warming for all surgery

Answer 2

False. NICE recommends the use of forced air warming if a patient is at higher risk or surgery is >30 min.

Question 3

Hypothermia shifts the oxyhaemoglobin curve to the left, reducing tissue O2 delivery

Answer 3

True

Question 4

The triple point of water is at 273.16°C

Answer 4

False. The triple point of water is at 273.16K (0.01°C).

Question 5

Critical temperature is the temperature below which a vapour cannot be liquefied by any amount of pressure

Answer 5

False. Critical temperature is the temperature ABOVE which a vapour cannot be liquefied by any amount of pressure.

Question 6

Thermocouples rely on the Seebeck effect

Answer 6

True. The Seebeck effect is the production of a voltage at the junction of 2 difference conductors. The magnitude of this voltage is dependant on the temperature of the junction.

Question 7

1K is equal in magnitude to 1°C

Answer 7

True

Question 8

If the oesophagus is used for measuring temperature, the upper third should be used for greatest accuracy

Answer 8

False. The lower third should be used.

Question 9

Liquid crystal may be used in thermometers

Answer 9

True. They are used in chemical thermometers.

Question 10

Resistance thermometers can only reliably be used for measuring large temperature changes

Answer 10

False. Resistance thermometers are extremely accurate and can be used for measuring small temperature differences.

Question 11

Over one third of heat loss during anaesthesia is via conduction

Answer 11

False. 40-50% of heat loss is via radiation, 30% by convection, 20% by evaporation and 3% by conduction.

Question 12

Maintaining the theatre environment at approximately 30% humidity is the ideal compromise between staff comfort and minimizing the patient’s heat loss

Answer 12

False. Relative humidity in theatres should be kept at around 50-60%. Higher humidity is uncomfortable for staff, lower increases the risk of sparks.

Question 13

Hypothermia increases MAC

Answer 13

False. Hypothermia reduces MAC, i.e. less anaesthetic is required.

Question 14

Infrared ear thermometers detect electromagnetic radioation emitted by the ear drum

Answer 14

True

Question 15

Rectal temperaure tends to be lower than core temperature

Answer 15

False. Rectal temperature is usually 0.5-1°C higher than core temperature due to bacterial fermentation.

Question 16

Thermistors Have a slow response time

Answer 16

False. They have a fast response time.

Question 17

Thermistors Utilize the resistance properties of metal oxides

Answer 17

True. The resistance of metal oxide semiconductors fall exponentially with temperature.

Question 18

Thermistors Are used in PA catheters

Answer 18

True. Thermistors can be made very small.

Question 19

Thermistors Are often used with Wheatstone bridges

Answer 19

True. Wheatstone bridges are often used to improve accuracy.

Question 20

Thermistors Depend on the Seebeck effect

Answer 20

False. Thermocouples rely on the Seebeck effect.

Question 21

Mercury is more suitable than alcohol for measuring low temperatures

Answer 21

False. Alcohol is more suitable for measuring low temperatures as mercury solidifies at -39°C. Mercury is more suitable for high temperature as alcohol boils at 78.5°C

Question 22

The Seebeck effect describes the non-linear fall in resistance seen when the temperature of a metal oxide rises

Answer 22

False. The Seebeck effect describes the production of voltage at the junction of two different conductors. The magnitude of this voltage is proportional to temperature. This principle is used in thermocouples. The non linear fall in resistance seen when the temperature of a metal oxide rises occurs in thermistors.

Question 23

Bimetallic strips may be used in dial thermometers

Answer 23

True. Bimetallic strips containing two dissimilar metals with different coefficients of expansions are fixed together in a coil to make dial thermometers. (Volatile liquids can also be used). Bimetallic strips are also used to control the splitting ratios in Plenum vapourizers.

Question 24

If the temperature (in K) of a volume of gas is doubled, the volume of the gas will double

Answer 24

True. This is Charles’s Law

Question 25

The resistance of a metal increases exponentially with temperature

Answer 25

False. The resistance of a metal increases linearly with temperature. This is the principle behind resistance thermometers.

Question 26

A hair hygrometer measures relative humidity

Answer 26

True

Question 27

Relative humidity is equal to the absolute humidity divided by the saturated vapour pressure of water at that temperature

Answer 27

False. HME have one 15 mm and one 22 mm connector.

Question 28

Relative humidity is equal to the absolute humidity divided by the saturated vapour pressure of water at that temperature

Answer 28

True. Relative humidity is the ratio of the mass of water vapour in a given volume of water (i.e. its absolute humidity) to the mass required to saturate that given voume of air at the same temperature (i.e. the SVP of water at that temperature).

Question 29

HME (if not blocked) increase resistance to air flow by approximately 5cm H2O

Answer 29

False. HME increase reisitance to flow by 0.1-2 cm H2O.

Question 30

The use of sodalime can achieve absolute humidity of 34 mg/L

Answer 30

False. The reaction of CO2 with soda lime can achieve an absolute humidity of 29 mg/L: CO2 + H2O -> H2CO3 2NaOH + H2CO3 -> NaCO3 +2H2O + heat Na2CO3 +Ca(OH)2 -> CaCO3 +2NaOH + heat (34 mg/L is the absolute humidity in the upper airways )

Question 31

Wet and dry bulb hygrometers are more accurate than hair or Reganult's hygrometers

Answer 31

False. Reganult's hygrometers are more accurate than wet and dry bulb hygrometers or hair hygrometers.

Question 32

The same volume of water will produce a lower relative humidity in warmer air than in cooler air

Answer 32

True. Warmer air can hold more water when fully saturated; e.g. 17 g/m3 would produce a relative humidity of 100% at 20°C but only 38% at 37°C as air at this temperature can hold up to 44 g/m3.

Question 33

Absolute humidity is the volume of water vapour present per unit volume of gas

Answer 33

False. Absolute humidity is the MASS of water vapour present per unit volume of gas (at a given temperature).

Question 34

Fully saturated air at body temperature can hold 44 g/m3 of water

Answer 34

True

Question 35

HME can achieve at best a relative humidity of 40-50%

Answer 35

False. HME can achieve relative humidity of 60-70%

Question 36

A hair hygrometer is most accurate at relative humidities of 80-100%

Answer 36

False. Hair hygrometers are most accurate between relative humidities of 30-90%

Question 37

Tables are required to interpret the reading of hair hygrometer

Answer 37

False.The change in humidity moves the hair directly over a scale. Tables are required for the interpretation of wet and dry bulb hygrometers and Reganult's hygrometers.

Question 38

Wet and dry bulb hygrometers utilize the cooling effect of ether to measure relative humidity

Answer 38

False. Regnault’s hygrometers utilise the cooling effect of bubbling air through ether to measure relative humidity.

Question 39

Reganult’s hygrometer can measure absolute and relative humidities

Answer 39

True

Question 40

Wet and dry bulb hygrometers should only be used when there is minimal air movement around the thermometers

Answer 40

False. Wet and dry bulb hygrometers require air movement to allow the evaporation of water which cools the wet thermometer.

Question 41

Bourdon Gauge can be used to measure humidity T/F

Answer 41

False. Bourdon gauges are used to measure pressure or temperature but not humidity.

Question 42

Mass Spectrometry can be used to measure humidity T/F

Answer 42

True

Question 43

UV Light can be used to measure humidity T/F

Answer 43

True. The absorption of UV light by water vapour can be used to measure humidity.

Question 44

Transducers can be used to measure humidity T/F

Answer 44

True. The change in the resistance of capacitance of a substance when it absorbs water vapour can be used to measure humidity.

Question 45

A Pitot Tube can be used to measure humidity T/F

Answer 45

False. Pitot tubes are used to improve the accuracy of pneumotachographs which are used to measure gas flow.

Question 46

HME are most efficient with small tidal volumes

Answer 46

True. The humidifying efficiency of HMEs decreases with large tidal volumes.

Question 47

The amount of water droplets that can be delivered by a nebulizer in a given volume of air is limited by temperature

Answer 47

True. The humidifying efficiency of HMEs decreases with large tidal volumes.

Question 48

Gas driven (Jet) nebulisers rely on the Coanda effect

Answer 48

False. Gas driven nebulizers rely on the Venturi/Bernoulli effect.

Question 49

Nebulized droplets of size 2-5 μm are deposited in the tracheo-bronchial tree

Answer 49

True

Question 50

The upper airways humidify inhaled gases by the addition of up to 17 g/m3 of water

Answer 50

False. The upper airways increase the absolute humidity of inhaled air from 17 g/m3 at 20°C to 44 g/m3 at 37°C, an increase of 27 g/m3.