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Exam prep tutorials (Physics and Equipment) > Temperature and Humidity > Flashcards

Temperature and Humidity Flashcards

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1
Q

By which mechanism is most heat lost during anaesthesia?

A

Radiation

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2
Q

Which way does hypothermia shift the oxyhaemoglobin curve?

A

To the left, reducing tissue O2 delivery

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3
Q

What principles do thermocouples rely on?

A

The Seebeck effect

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4
Q

What is 1 Kelvin equal in magnitude to?

A

1 K is equal in magnitude to 1 degree Celcius

To get the temp in Kelvin from Celcius, add 273.

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5
Q

What is heat?

A

The form of energy that passes between 2 substances due to the difference in their temperature. The energy is in the form of the vibrational energy within the molecules of the substance.

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6
Q

What is temperature?

A

The property of a substance that determines whether it will transfer or receive heat to/from another substance. Heat energy will flow from an object of high temperature to one of low temperature.

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7
Q

How can an object with high temperature contain less heat than a large object at a lower temperature?

A

Because the amount of heat contained by an object is dependent on it’s mass (heat is a measure of kinetic energy, proportional to mass x velocity2) and specific heat capacity.

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8
Q

What is the specific heat capacity?

A

The amount of heat required to raise the temperature of a unit of mass by 1K (J/kg/K)

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9
Q

What is the triple point?

A

The temperature and pressure at which the solid, liquid and gas phases of a substance exist in equilibrium

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10
Q

What is the triple point for H2O?

A
  1. 16 K
    - 0.01 degrees C
  2. 2 Pa
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11
Q

What is STP?

A

Standard temperature and pressure.

Used to standardize gas volume measurements.

= 273.15K, 101.3 kPa (760 mmHg)

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12
Q

What is the critical temp?

A

The temp above which a vapour cannot be liquefied by any amount of pressure

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13
Q

What is latent heat?

A

The energy required to change the state of a substance without changing it’s temperature. It provides the energy needed to break the bonds holding the substance together.

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14
Q

What is the latent heat of vaporization?

A

The heat required to change a substance from liquid to vapour without changing it’s temperature. Therefore in an isolated system as liquid evaporates - the temperature of the remaining liquid falls as it gives up the energy needed for the vaporization.

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15
Q

What is the latent heat of fusion?

A

The heat required to change a substance from solid to liquid state without changing it’s temperature.

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16
Q

What is a Kelvin?

A

It is the SI unit for measuring temperature.

It is defined as 1/273.16 of the thermodynamic triple point of water.

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17
Q

How does doubling the temperature in kelvin affect the volume of gas?

A

The volume of gas will be doubled

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18
Q

How does a resistance thermometer work?

A

The resistance of metal (eg platinum) increases linearly with temperature.

Wheatstone bridge is used to increase sensitivity.

Very accurate but fragile and slow response time.

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19
Q

How does a thermistor work to measure temperature?

A

Small bead of metal oxide (semiconductor). Resistance falls exponentially with temperature.

Often used with a Wheatstone bridge.

Used in PA catheter and oesophageal stethoscope.

Small, cheap and fast response time.

BUT deteriorate over time, and calibration errors especially if subjected to large changes in temperature (eg autoclaving).

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20
Q

How does a thermocouple work to measure temperature?

A

Utilizes the Seebeck effect.

The circuit requires 2 junctions - the reference junction kept at a constant temp, a near-linear curve of voltage against temp is produced at the other, measuring junction.

Can be made very small. Cheap and tough.

BUT needs signal amplification.

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21
Q

What are the non-electrical methods of temperature measurement?

A
  • liquid expansion
  • dial thermometers
  • tympanic and infrared ear thermometers
  • chemical thermometers
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22
Q

How do liquid expansion thermometers work?

A

Mercury/alcohol can be used, these change in volume dependent on temp.

Mercury (-39 C to 250C), solidifies below -39C.

Alcohol boils at 78.5C but is cheap, less toxic if breaks and more suitable for low temps (-117C to 78C)

Adv: simple, reliable, no power needed.

Disadv: 2-3 min needed for equalibration, risk of cross infection (non-disposable), rigid - risk breakage/trauma

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23
Q

What are dial thermometers?

A
  • bimetallic strip (usually copper/constantan)
    • 2 dissimilar metals with different coefficients of expansion fixed together in a coil, as temp rises the metals expand - coil tightens - moves a pointer on a scale
    • used in thermostat switches
  • bourdon (aneroid) gauge
    • same principle used when measuring pressure but the hollow tube is filled with a volatile liquid which expands as the temp rises
    • the tube unwinds which moves a pointer
  • cheap
  • robust
  • not very accurate
  • prone to calibration erros
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24
Q

How do tympanic and infrared ear thermometers work?

A
  • the tympanic membrane emits electromagnetic (primarily infrared) radiation - the intensity and wavelength for which the intensity of radiation is maximum varies with body temp
  • infrared ear thermometers detect radiation emitted by the ear drum and ear canal
  • tympanic membrane thermometers are inserted further into the ear and receive radiation from the tympanic membrane only - more representative of core temp
  • require 2 sensors
    • pytoelectric sensor
    • thermopile sensor (made of thermocouples)
  • adv - quick
  • disadv - inaccurate if ear wax excessive, not suitable for kids < 1 yr
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25
Q

How does a chemical thermometer work?

A

Plastic strip containing cells which contains liquid crystals which change colour according to temperature.

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26
Q

Which sites in the body can be used to measure core temperature?

A

Core temperature is the temp of blood perfusing the vital organs.

  • oesophageal (lower 1/3 as cooler higher up due to air)
  • rectal (1 degree higher than core due to bacterial fermentation + insulation of faeces)
  • blood via PA catheter (very invasive but best estimate)
  • skin (diff between skin/core can be used to assess degree of shock/peripheral perfusion)
  • bladder - needs high flow rate for accuracy
  • tympanic membrane - correlates best with hypothalamic temperature
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27
Q

What methods of heat loss can occur during anaesthesia?

A
  • radiation (40%)
  • convection (30%)
  • evaporation (20%)
  • conduction (<5%)
  • 10% is lost via respiration (heating of air plus latent heat of vaporization to humidify)
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28
Q

What is radiation?

A

The emission of energy in the form of waves or particles (ie objects don’t need to be in contact). Esp from bright shiny objects to dark matt bodies.

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29
Q

What is convection?

A

Transfer of heat by movement of liquid or gas

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30
Q

What is evaporation?

A

Conversion of a liquid to a vapour state by the addition of heat which provides the energy to the liquid molecules to break the bonds between them.

(eg loss of moisture on skin causes cooling)

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31
Q

What is conduction heat loss?

A

Transfer of heat energy from one substance to another via the transfer of energy from molecule to adjacent molecules. No movement of the substance itself.

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32
Q

How is heat production reduced during anaesthesia?

A
  • peripheral mechanisms
    • reduced movement
    • reduced ability to shiver
  • central mechanisms
    • effects of drugs, depress hypothalamic function so thermoregulatory responses not initiated until much lower temp
33
Q

What are the effects of hypothermia on the CVS?

A
  • vasoconstriction
  • increased blood viscosity
  • reduced CO
  • below 30 degrees - arrhythmias
34
Q

What are the RS effects of hypothermia?

A
  • oxyHb shifts to the left reducing O2 delivery to tissues
  • reduced O2 demand
  • reduced CO2 production
35
Q

What neurological effects does hypothermia have?

A

Confusion followed by unconsciousness

36
Q

What effect does hypothermia have on anaesthetic agents?

A
  • Less volatile required (reduces MAC)
  • Prolongs effects of neuromuscular blocking agents
37
Q

What other effects does hypothermia have on the body?

A
  • reduces metabolic rate
  • diuresis (2ndry to inability to reabsorb Na and H2O)
  • shivering causes postop O2 consumption to increase up to 10x
  • coagulopathy (clotting cascade is enzymatic and platelet function is temp dependent)
  • increased incidence of wound breakdown and infection
  • metabolic acidosis
38
Q

What are the NICE guidelines for perioperative management of temperature?

A
  • need forced air warming if higher risk or surgery >30mins
  • temp should be measured every 30mins
  • IV fluids >500ml should be warmed
    • high risk if they have 2 or more of:
      • ASA2 or above
      • preop temp <36
      • combined RA and GA
      • intermediate or major surgery
      • at risk of CVS complications
39
Q

What relative humidity should theatres be kept at?

A

50-60%. Higher humidity is uncomfortable for staff, lower increases the risk of sparks.

40
Q

What is the Seebeck effect?

A

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.

41
Q

What is Charles’ Law?

A

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

42
Q

As the temperature of air increases, can it hold more or less water? What does this mean for humidity?

A

Warmer air can hold more water, so will be less humid for a given amount of vapour.

Air at body temperature (37°C) is fully saturated with 44 g/m3 of vapour but this same amount of vapour would provide only 50% humidity at just over 50°C

43
Q

What is absolute humidity?

A

The mass of water vapour present per unit volume of gas at a given temp and pressure (mg/L or kg/m3).

44
Q

What is relative humidity?

A

The ratio of the amount of water vapour in the air compared with the amount that would be present at the same temperature if the air was fully saturated.

OR

The ratio of the vapour pressure of water in the air compared with the saturated vapour pressure of water at that temperature

45
Q

What is the Dew point?

A

The temperature at which the relative humidity of the air exceeds 100% and water condenses out of the vapour phase to form liquid (dew).

46
Q

What is a hygrometer?

A

An instrument used for measuring the humidity of a gas

47
Q

What is hygroscopic material?

A

Material that attracts moisture from the atmosphere (eg found in heat and moisture exchange filters).

48
Q

What is a hair hygrometer?

A
  • measures relative humidity
  • as relative humidity increases, the length of the hair increases which moves a pointer (human or animal hair or paper)
  • simple, most accurate between relative humidities of 30-90%
  • time lag of 5mins between change in humidity and reading
  • unreliable when ambient temperature below freezing
49
Q

What is a wet and dry bulb hygrometer?

A
  • relative humidity can be calculated
  • consists of 2 mercury thermometers
    • 1 reads ambient temperature
    • 1 sits in a container of water which cools as the water evaporates due to loss of latent heat of vaporization
  • the rate of evaporation varies with the humidity of the surrounding air
  • so the difference in temperature measured by the 2 thermometers varies as humidity varies
  • requires adequate air movement fo be accurate
50
Q

What is Regnault’s hygrometer?

A
  • relative and absolute humidities can be calculated
  • consists of a silver tube containing ether
  • air is bubbled through the ether, cooling it
  • when condensation occurs outside of the tube, this is the dew point (the temp which ambient air is fully saturated)
  • graphs of water content of saturated air against temperature can then be used to determine relative humidity
  • more accurate than either the hair hygrometer/wet + dry bulb hygrometer
51
Q

What other methods are there of measuring humidity?

A
  • mass spectrometer
  • UV light absorption (reduction in transmission of UV light when water vapour is present)
  • transducer - use the change in either the resistance or the capacitance of a substance when it absorbs water vapour from the atmosphere *used to measure absolute humidity*
52
Q

How much water can air at room temperature (20C) hold?

A

17 g/m3 of water

53
Q

How much water can air in the upper trachea hold?

A

Air in the upper trachea (34C) can hold 34g/m3

54
Q

How much water can air at body temperature (37C) hold?

A

44 g/m3 of water

55
Q

How much water is added to the air by the upper airways during normal respiration?

A

An extra 27g/m3 (air at room temp - 17 g/m3 of water, air at body temp in lungs 44g/m3) is added by the upper airways during normal respiration, intubation bypasses this method of humidification

56
Q

If air in the airways is dry, what will this cause?

A
  • thickening of respiratory mucosa
  • thickening of mucous causing airway plugging
  • reduced ciliary activity
  • keratinization and ulceration of airways
57
Q

What humidity should theatres be maintained at?

A
  • 50 - 60% humidity because:
    • higher values are uncomfortable for staff
    • lower values will increase the patient’s heat loss via evaporation
    • lower values increase risk of sparks due to build up of static
58
Q

What are the various methods for humidifying inspired gases? Rank them in order of least efficient to most efficient.

A
  1. Cold water bath
  2. HME filter
  3. Hot water bath
  4. Bournoulli (gas driven) nebulizer
  5. Ultrasonic nebulizer
59
Q

What are the passive methods of humidification?

A

Passive (not requiring energy).

  • cold water bath
  • soda lime
  • heat and moisture exchanged (HME)
60
Q

What are the active methods of humidification?

A
  • hot water bath
  • nebulizers
61
Q

What is a cold water bath for humidification?

A
  • bubbling gas through cold water
  • simple
  • inefficient
  • vaporization cools the water, decreasing efficiency further
62
Q

What humidity can soda lime achieve?

A

Absolute humidity of 26 mg/L

63
Q

How does the HME filter work to humidify?

A
  • 2 ports - (15mm and 22mm) +/- sampling port for gas monitoring
  • main body contains a hygrophobic or hygroscopic (readily absorbs moisture) medium (ceramic fibre, paper, aluminium)
  • as the patient expires warm, moist gas, it condenses on the cooler HME and warms it, during inspiration gas flows through the HME the other way, the condensed water evaporates and is warmed
  • heat is conserved during expiration and inspired air is heated and humidified
  • mostly consist of hygroscopic material (foam or chemically coated paper)
64
Q

How much resistance do HME filters add?

A

0.1 - 2cm H2O

65
Q

What humidity can HMEs achieve?

A

Relative humidity of 60 - 70%

66
Q

What are the advantages of the HME filters?

A
  • can incorparate a bacterial/viral filter
  • simple
  • cheap
  • small dead space
67
Q

What are the disadvantages of the HME filters?

A
  • can become obstructed by foreign bodies or mucus
  • less efficient with large tidal volumes
68
Q

How does a hot water bath work to humidify air?

A

Gas is passed through hot water.

The gas is not saturated at the temperature of the bath, but as it cools along the tubing to the patient it becomes near saturated ie 100% humidity.

69
Q

What are the potential risks of a hot water bath and how are these minimized?

A
  • condensation of water in tubing obstructing air flow
    • heating the tubing
    • use of a water trap
  • excessively hot gases
    • use of a thermostat
  • spillage of water into patient airway
    • keep water level below that of the patient
  • infection risk eg pseudomonas
    • use containers prefilled with sterile water
    • high temp of water
70
Q

How do nebulizers work to humidify gases?

A
  • they produce microdroplets of water/drug suspended in a gas
  • the quantity of water droplets delivered is not limited by gas temperature as it would be with vapours
  • smaller droplets are more stable
  • the size of the droplets affects where they are deposited
71
Q

Where are droplets deposited of 20μm, 2-5 μm, 1 μm and < 1μm?

A

20μm

  • deposited in tubing/upper resp tract

2-5 μm

  • tracheo-bronchial tree

1 μm

  • alveoli - may impair gas exchange

<1 μm

  • no significant deposition - get inhaled and then exhaled again
72
Q

What are the 3 types of nebulizer?

A
  • gas-driven
  • spinning disc
  • ultrasonic
73
Q

How do gas driven (jet) nebs work?

A
  • capillary tube with bottom end immersed in water and gas driven through a venturi constriction near it’s top
  • as gas emerges from the Venturi it creates negative pressure which draws water up into the capillary tube
  • this is then broken into a fine spray by the gas
    • water droplets can then be driven onto an anvil to break them up further
  • produces droplets of 2-5 μm
  • compact
74
Q

How does a spinning disc nebulizer work?

A

Water is drawn up a tube onto a spinning disc and droplets are then thrown out due to centrifugal force.

75
Q

How do ultrasonic nebulizers work?

A
  • water dropped onto a plate which is vibrating at ultrasonic freq
  • produces droplets <2 μm
  • risk of overhydration (can produce gas with 200% relative humidity at 37C)
76
Q

What is a bourdon gauge used to measure?

A

Pressure or temperature but NOT humidity

77
Q

What is a pitot tube?

A

Used to improve accuracy of pneumotachographs which are used to measure gas flow

78
Q
A

Exam prep tutorials (Physics and Equipment) (8 decks)

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