Temperature

Question 1

What is heat

Answer 1

It is a form of energy that can be transferred from a hotter substance to a colder substance, the energy being a form of kinetic energy of the molecules of a substance

Question 2

Define temperature

Answer 2

The thermal state of a substance that determines whether it will give heat to another substance or receive heat from another substance. Heat moves from high to low temperature

the temperature of a gas is the speed at which the molecules are moving

Question 3

Define the SI unit of temperature

Answer 3

The Kelvin

The fraction 1/273.16 of the thermodynamic temperature of the triple point of water

Question 4

Describe the relationship between degrees Celsius and Kelvin

Answer 4

Temperature (K) = Temperature (deg C) + 273.15

Question 5

What is the triple point of water

Answer 5

The triple point of water is the temperature at which ice, liquid water and water vapour are in equilibrium

0. 01 deg C
1. 16 K

Relevance: The triple point of water is the basis for the starting point of the Celsius temperature scale

Question 6

What are the advantages and disadvantages of mercury thermometers

Answer 6

ADV
1. Maximum reading form (narrow/angulated bases or metal index above)

DISADV

1. Take long to equilibrate with surrounding temp
2. Difficult insertion orifices: rigid + break risk

Question 7

What thermometers are more suited than mercury thermometers for lower temperatures and why

Answer 7

Mercury solidifies at -39 deg C (Boils at 356.7 deg c)

Alcohol (freezes at -114.1 deg C) is better but bad at higher temps as it boils at 78.5 deg C

Question 8

What two mechanisms to dial thermometers use?

Answer 8

1. Bimetallic strip –> change in coil tension with temp moves a dial
2. Bourdon gauge –> Sensing element containing mercury or a volatile fluid in a closed system. Temperature change leads to change in volume or pressure of the sensing fluid and this is recorded on a calibrated bourdon gauge (Uses 3rd perfect gas law - Gay Lussac)

Question 9

Name the three electrical techniques for measuring temperature

Answer 9

1. Resistance thermometer
2. Thermistor
3. Thermocouple

Question 10

How does the resistance thermometer work?

Answer 10

Works on the principle that electrical resistance increases linearly with increasing temperature.

Battery + platinum wire resistor + ammeter to measure current –> not very sensitive
(much more sensitive if a wheatstone bridge is used)

Question 11

What is a thermistor? How does a thermistor differ from a resistance thermometer

Answer 11

A thermistor is a small bead of metal oxide

Unlike a platinum resistance thermometer, the electrical resistance within a thermistor DECREASES EXPONENTIALLY with increasing temperature

There are, however, some thermistors whose resistance increases with increasing temperature

Question 12

What are the advantages of thermistors over the platinum resistance thermometer

What are the disadvantages of thermistor vs. thermometer

Answer 12

ADV

1. More sensitive (undergo greater RESISTANCE CHANGE in the clinical temperature range)
2. Metal bead can be made very SMALL
3. Can be manufactured more CHEAP

DISADV
1. Calibration is liable to change at extremes of temperature

Question 13

What is the Seebeck effect

Answer 13

At any junction of two dissimilar metals a small voltage is produced, the magnitude of which depends on the temperature of the junction

Question 14

What is a thermocouple and on what principle is this based

Answer 14

A thermocouple is a device used to measure temperature. It is based on the Seebeck effect (two different metals in close proximity generate a voltage the magnitude of which depends on the temperature). A second junction is required to complete the electric circuit. There is a reference junction and a measuring junction. Reference junction must be kept at constant temperature.

Question 15

What are the advantages and disadvantages of the thermocouple

Answer 15

Advantages

1. Can be in the form of a needle

Question 16

How do infrared ear and tympanic membrane thermometers work

Answer 16

1. Objects (body) emits electromagnetic radiation over a range of wavelengths.
2. Intensity of radiation and intensity of emitted radiation depend on the temperature of the object.
3. Objects at body temperature emit infrared radiation
4. Ear drum more reflective of cor body temp than skin in ear canal.

There are two types of sensors onto which the infrared radiation is measured and converted to electrical signal.
1. Pyroelectric sensor (Polarization of material = electrical signal proportional to the magnitude of the change of incident radiation (shutter mechanism) –> allow snap shot readings

2. Thermopile sensor (many thermocouples connected in prallel –> allows continuous readings

Question 17

What are the names of the two different infrared sensors used in infrared and tympanic membrane thermometers

Answer 17

1. Pyroelectric sensor (change in polarization/capacitor –> electric signal) –> snap shot readings (shutter mechanism)
2. Thermopile –> many thermocouples connected in parallel –> continuous readings

Question 18

Describe the special mechanism used to measure skin temperature to determine trends in shocked patients

Answer 18

Thermography

- Infrared sensitive camera provides coloured display the relative warmth of different regions of the body.

Question 19

Describe the body’s temperature zones

Answer 19

Core (kept constant - 37 ± 0.5 deg C)
- brain, thoracic, abdominal organs

Intermediate (variable core size)

Shell (32 - 35 deg C)
- variable depth: around 2.5 cm deep

Temp in body core is balance between temp production in the core and loss from the shell

Question 20

What is total heat production of the body at rest

Answer 20

50W/m^2 or total of 80W

Question 21

Can heat production be reduced or increased in response to changes in core temp

Answer 21

Shivering can increase heat production

Heat production cannot be reduced

Question 22

How significantly can shivering increase heat production

Answer 22

2 fold and up to 5 fold in special circumstances

Question 23

Does the onset of shivering occur prior to body’s mechanisms to reduce heat loss?

Answer 23

No

Question 24

Summarise the contributions to heat loss from a patient in theatre

Answer 24

Radiation ± 40%
Convection ± 30%
Evaporation ± 20%
Respiration ± 10% (*% evaporation, 2% heating of air)

(Conductive heat loss not prominent in this context)

Question 25

Describe radiation heat loss

Answer 25

The body emits radiation over a spectrum of wavelengths (predominantly in the infrared region). The radiation carries heat energy away from the hot object causes it to cool down, conversely if this is absorbed by another object, that object will become hotter. Thus radiation can transfer heat energy between two objects

Question 26

Describe heat loss by convection

Answer 26

Air layer adjacent to surface of body is warmed by conduction. As it is heated it expands and becomes less dense so rises and carries heat energy away from the body

Question 27

Describe heat loss by evaporation

Answer 27

Surface evaporative heat loss is due to the loss of the latent heat of vapourization of moisture on the skins surface. Depends on 1. Water vapour pressure gradient from skin to air 2. Surface area of moist skin exposed to air

Question 28

Now much may sweating increase heat loss through the evaporative mechanism

Answer 28

up to 10 fold

Question 29

Describe the body's responses to changes in environmental temperature

Answer 29

Peripheral (skin) and central (hypothalamus) thermoreceptors. Both signal thermoregulatory centre in hypothalamus: Excessive cold --> Vasoconstriction and shivering Excessive heat --> Vasodilatation and sweating Hypothalamus to cortex: Appropriate behavioural responses

Question 30

Explain induced hypothermia

Answer 30

Decreases metabolism and so reduces O2 demand | Reduce Tb to 30 degrees and double the safe time for circulator occlusion

Question 31

Below what temperature in induced hypothermia is there risk for ventricular fibrillation

Answer 31

Below 30 deg C

Question 32

In severe pyrexia, what occurs at temperatures above 42 deg C and at above 45 deg C

Answer 32

Above 42 deg C --> Cerebral impairment and disturbance of physiological control mechanisms of heat loss by sweating Above 45 deg C --> Rise in temp induces rise in metabolism which worsens heat production and precipitates a vicious cycle --> fatal.

Question 33

At what skin temperature is there a risk of thermal burns

Answer 33

Above 45 deg C but depends on the rate at which heat arrives from the heat source and the rate at which it is removed by the blood flow. Therefore burns are more likely in patients who are vasoconstricted or shocked.