temperature and heat Flashcards
what is the difference between heat and temperature?
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
what is normal core body temp?
36.5 to 37.5
what is absolute 0?
Kelvin = 0
equivalent to -273.15 degrees C
no colder temperature exists.
what is the target ambient temp and humidity in theatre?
22 to 24 degrees C
50-60 % humidity
describe the differences between solid, liquid and gas.
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
what is meant by latent heat of fusion and vaporisation? and specific heat of these?
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
draw a graph to demonstrate the latent heat of fusion and vaporisation..
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
give example of when latent heat of vapourisation is seen in threatre and how this is minimised?
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.
what is meant by specific heat capacity?
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)
what is the specific heat capacity of water?
4.18 joule/kg/kelvin
how is specific heat capacity calculated?
c = joules of energy needed / (temp change x mass)
C = J / (ΔT × m)
what is molar heat capacity?
the amount of energy required to increase temp of 1 mole of a substance by 1 degree
what is the zeroth law of thermodynamics?
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.
what is the first law of thermodynamics ?
energy is not created or destroyed
only converted from one form to another
what is the second law of thermodynamics?
entropy of the universe is always increasing with time
what is the third law of thermodynamics?
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)
what is thermal equilibrium?
when temperature and pressure of a system are not changing with time.
what is the triple point of water?
(draw a graph)
when all 3 phases of water are in equilibrium
0.01 degrees C = 273.16 kelvin
611.73 Pa = 0.006 atm
what are the different methods of heat loss in theatre?
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
what is Stefan Boltzmann law?
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)
describe the pattern of heat loss from induction to maintenance anaesthesia (draw a graph)
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
how can phase 1 of heat loss seen during anaesthesia be minimised?
warm patient before starting
minimises the difference between core and peripheral temp so less effect of redistribution
why is it important to maintain body temp in anaesthesia?
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.
what factors contribute to hypothermia in theatre?
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.
what are the methods of minimising heat loss in theatre?
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
how do forced air warmers reduce heat loss?
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
how do fluid warmers reduce heat loss?
reduce heat loss by conduction
reducing cold fluid contacting warm blood
what are the 2 subtypes of fluid warming devices?
low flow rates - need to maintain the temp for longer
high flow rates - need to heat quick enough and reduce resistance to flow
what is the mechanism of warming fluid in an IV fluid warmer?
heating unit
countercurrent systems
water bath
convective air systems
what temperature are fluid warmers limited too?
41 degrees
any higher would cause haemolysis
what are the features of an ideal fluid warmer?
cheap and efficient
easy to use
compact
safe - removes air bubbles, no risk of burns i.e. alarms when too hot.
why are bubbles formed in a fluid warmer?
gas becomes less soluble in solution at higher temp
therefore comes out of solution as gas
what are the causes of hyperthermia in theatre?
environment
- accidentally over heating with bear hugger
- room too hot
anaesthetic
- MH
patient
- sepsis, hyperthyroid
what are the risks of hyperthermia in theatre?
fluid loss
electrolyte disturbances
enzyme function
how does a fall in temperature influence pH?
0.06 increase in pH with 1 degree fall in temp
(inverse proportional)
