temperature and heat

Question 1

what is the difference between heat and temperature?

Answer 1

heat is the added kinetic energy of molecules within a substance. i.e. how much thermal energy the substance holds.

temperature is the measurement of heat emitted by an object i.e. ability of an object to transfer heat energy.

thermal energy travels from object of higher temp to colder temp

Question 2

what is normal core body temp?

Answer 2

36.5 to 37.5

Question 2

what is absolute 0?

Answer 2

Kelvin = 0
equivalent to -273.15 degrees C
no colder temperature exists.

Question 3

what is the target ambient temp and humidity in theatre?

Answer 3

22 to 24 degrees C
50-60 % humidity

Question 4

describe the differences between solid, liquid and gas.

Answer 4

solid:
- organised molecules
- strong intermolecular bonds
- fixed shape
- lowest amount of kinetic energy of the states

liquid:
- weak intermolecular bonds
- can break and reform such that can change shape to occupy container
- more kinetic energy than solids.

gas:
- no bonds between molecules
- molecules move freely and randomly
- expand to fill container
- space between molecules is larger than molecules
- highest kinetic energy

BONDS, SHAPE, ENERGY

Question 5

what is meant by latent heat of fusion and vaporisation? and specific heat of these?

Answer 5

latent heat of fusion = energy required to break bonds of solid to it change state to a liquid without changing its temperature (temp of surroundings will change)

specific latent heat of fusion = the heat required to convert 1kg of solid at its melting point into a liquid at same temperature. (J/ kg)

same for latent heat of vaporisation except from liquid to gas

Question 6

draw a graph to demonstrate the latent heat of fusion and vaporisation..

Answer 6

As heat is supplied to a solid, its kinetic energy will increase and temperature will increase.

When its melting point is reached, the heat energy will be used to break the bonds and thus there will be no change in temperature as it is converted to a liquid.

the same happens again when liquid is vapourised to gas

Question 7

give example of when latent heat of vapourisation is seen in threatre and how this is minimised?

Answer 7

Vaporisers - as sevo vapourisers, heat is taken from surrounding for latent heat. needs temp compensation mechanism

N20 canister - as N20 evapourates with use out of canister, the canister cools because heat has been taken from surroundings.

heat loss from laparotomy / skin preps - open abdomen leads to evaporative losses - uses patients body heat hence can lead to hypothermia. Can put exposed bowel in plastic coverings.

HME filters - reduce evaporative losses from humidying air. air is already supplied with humidified water so no energy is needed to evaporate water.

Question 8

what is meant by specific heat capacity?

Answer 8

heat capacity = the energy required to raise the temp of a substance by 1 degree C

specific heat capacity = the energy required to raise the temp of 1kg of a substance by 1 degree C = Joules/Kg/Kelvin

(doesnt matter if in K or degrees as the difference of 1 degree C is same as 1 K i.e. just shifted by 273.15)

Question 9

what is the specific heat capacity of water?

Answer 9

4.18 joule/kg/kelvin

Question 10

how is specific heat capacity calculated?

Answer 10

c = joules of energy needed / (temp change x mass)

C = J / (ΔT × m)

Question 11

what is molar heat capacity?

Answer 11

the amount of energy required to increase temp of 1 mole of a substance by 1 degree

Question 12

what is the zeroth law of thermodynamics?

Answer 12

If 2 systems are in thermal equilibrium with a 3rd they are also in equilbrium with eachother.

hence energy will be transferred until all 3 are in equilibrium i.e. the middle object acts as a conduit and the other 2 objects dont need to be directly touching.

Question 13

what is the first law of thermodynamics ?

Answer 13

energy is not created or destroyed
only converted from one form to another

Question 14

what is the second law of thermodynamics?

Answer 14

entropy of the universe is always increasing with time

Question 15

what is the third law of thermodynamics?

Answer 15

entropy of a pure crystalline substance at absolute 0 is 0.
i.e at absolute 0 all motions theoretically stop

theoretical only and can never be reached (as soon as its reached, it will absorb heat from surroundings)

Question 16

what is thermal equilibrium?

Answer 16

when temperature and pressure of a system are not changing with time.

Question 17

what is the triple point of water?
(draw a graph)

Answer 17

when all 3 phases of water are in equilibrium

0.01 degrees C = 273.16 kelvin
611.73 Pa = 0.006 atm

Question 18

what are the different methods of heat loss in theatre?

Answer 18

radiation = 50%
- heat loss via infrared radiation emitted by object to cooler surroundings.

Conduction = 5%
- loss of heat by direct contact and transfer (may be bed or air)

Convection = 30%
- by circulation of air
- heat rises, colder air replaces this and then more heat can be lost by conduction.
- and again - convection current

evaporation = 5-50%
- heat loss from latent heat of vaporisation e.g. open abdomen, cold spray , sweating

Question 19

what is Stefan Boltzmann law?

Answer 19

the amount of heat emitted per m2 by radiation is related to the 4th power of temp

E = ST4
(s = constant)
(T = temp in kelvin)
( E - radiation emitted per m2)

Question 20

describe the pattern of heat loss from induction to maintenance anaesthesia (draw a graph)

Answer 20

phase 1 = redistribution
- at induction
- vasodilation
- warm core temp mixes with colder peripheral blood
- causes reduction in core body temp
- also increases heat loss by radiation

Phase 2 = linear drop
- levels out but still gradual drop
- depends on radiation (cold theatre room), conduction (cold bed, cold fluids), convection (laminar air flow in theatre), evaporation (cold antiseptic wash)

phase 3 = equilibrium
- plateaus out
- thermoregulatory systems stabilise at 34 degrees

graph
phase 1 steepest drop - from 37 to 35.5 over 1 hr
phase 2 = gradual decline to 3.5 hours
phase 3 = plateau from 3.5 hours around 34 degrees

Question 21

how can phase 1 of heat loss seen during anaesthesia be minimised?

Answer 21

warm patient before starting
minimises the difference between core and peripheral temp so less effect of redistribution

Question 22

why is it important to maintain body temp in anaesthesia?

Answer 22

enzyme function - drug metabolism, clotting, Hb dissociation curve

prevents energy wastage e.g. shivering

prevents vasoconstriction - increases SVR and reduces perfusion for healing.

linked to arrhythmias, MIs and infection

better patient experience.

Question 23

what factors contribute to hypothermia in theatre?

Answer 23

body temp is maintained by
HEAT PRODUCTION - HEAT LOSS

reduction in Heat production
- patient factors: elderly, low muscle mass/ low BMR, hypothyroid
- anaesthetic factors: NMBA (reduce shivering), anaesthetic agents reduce BMR

increase in heat loss
- can be categorised by mechanism - radiation, conduction, convection and evaporation.

PLUS loss of homeostatic control - effect of anaesthesia - agents suppress hypothalamus and unconsciousness inhibits behaviour responses e.g. putting jumper on.

Question 24

what are the methods of minimising heat loss in theatre?

Answer 24

heat production - heat loss

heat production
- reduce use of NMBA or anaeshetic agents if possible

heat loss -
- radiation - warm theatre room
- conduction - warm fluids
- convection - warm blanket, avoid unnecessary exposure
- evaporation - cover bowel loops in plastic , minimise exposure of body cavities, HME filters

Question 25

how do forced air warmers reduce heat loss?

Answer 25

powered air flow with hot air pumped into blanket
create insulation layer such that not in direct contact with surrounding cold air
reduces conduction and convection

Question 26

how do fluid warmers reduce heat loss?

Answer 26

reduce heat loss by conduction
reducing cold fluid contacting warm blood

Question 27

what are the 2 subtypes of fluid warming devices?

Answer 27

low flow rates - need to maintain the temp for longer

high flow rates - need to heat quick enough and reduce resistance to flow

Question 28

what is the mechanism of warming fluid in an IV fluid warmer?

Answer 28

heating unit
countercurrent systems
water bath
convective air systems

Question 29

what temperature are fluid warmers limited too?

Answer 29

41 degrees
any higher would cause haemolysis

Question 30

what are the features of an ideal fluid warmer?

Answer 30

cheap and efficient
easy to use
compact
safe - removes air bubbles, no risk of burns i.e. alarms when too hot.

Question 31

why are bubbles formed in a fluid warmer?

Answer 31

gas becomes less soluble in solution at higher temp
therefore comes out of solution as gas

Question 32

what are the causes of hyperthermia in theatre?

Answer 32

environment
- accidentally over heating with bear hugger
- room too hot

anaesthetic
- MH

patient
- sepsis, hyperthyroid

Question 33

what are the risks of hyperthermia in theatre?

Answer 33

fluid loss
electrolyte disturbances
enzyme function

Question 34

how does a fall in temperature influence pH?

Answer 34

0.06 increase in pH with 1 degree fall in temp

(inverse proportional)