Temperature and Heat

Heat is the kinetic vibrational energy of the molecules within a substance, and it can be transferred from one substance to another. Temperature is the quantitive measure of the manifestation of heat energy.

It measures the ability of an object to transfer heat to another object. For example a hot frying pan has less overall heat energy than an iceberg, however the higher temperature means the frying pan will transfer heat energy to the iceberg.

Specific heat capacity

  • The amount of energy needed to increase the temperature of one kilogram of a substance by one kelvin

What are the four laws of thermodynamics?

  • Zeroth law
    • If two systems X and Y are both in thermal equilibrium with a third system, Z, then X and Y must be in equilibrium with each other
  • First Law
    • In a closed, perfect system, the total amount of internal energy remains constant
  • Second Law
    • In a natural thermodynamic process, entropy increases
  • Third Law
    • Entropy tends to a constant as temperature decreases to zero

What are the units of temperature?

  • SI unit is Kelvin (K)
    • This is 1/273.16 of the thermodynamic triple point of water
    • This means it is -273.15 degrees celsius
    • Celsius and Kelvin have the same interval unit
      • Farenheit is based on properties of mercury and is used far less frequently in scientific measurement

How can temperature be measured?

Temperature can be measured using electrical and non-electrical methods

  • Electrical
    • Platinum resistance thermometer
      • Linear increase in temperature as temperature increases
      • Slow response time
    • Thermistor
      • Metal oxide semiconductor (used in PICCOs, Pulmonary Artery Floatation Catheter)
        • As temperature increases, resistance decreases exponentially
    • Thermocouple
      • Utilises the Seebeck Effect
  • Non-electrical
    • Thermometer (expansion of fluid is proportional to temperature)
      • Mercury – solidifies at 39 degrees
      • Alcohol – boils at 78.5 degrees
      • Gas expansion thermometer
    • Bimetallic strip
      • 2 metals with different coefficients of expansion
    • Bourdon Gauge
      • Charles’ law (volume of gas increases with temperature)
    • Infra-red (e.g. Tympanic)
      • pyroelectric sensor can detect infrared
    • Liquid crystals

What body sites can be used to measure temperature?

  • Tympanic
    • Fast, accurate
      • risk of tympanic perforation and earwax can give false readings
  • Oesophageal
    • reliable and not overly invasive
  • Skin
    • not accurate for core temperature, but comparing core and skin temperature gives a measure of perfusion
  • Bladder
    • Accurate but invasive
  • Rectal
    • Slow
    • usually up to one degree celsius higher due to bacterial fermentation

How does anaesthesia lead to hypothermia?

  • Reduced sensitivity to hypothermia
    • Anaesthetic agents reset the hypothalamic set point to 34 degrees celsius
  • Reduced muscle tone
    • Respiratory muscles
    • Skeletal muscles
    • Lack of shivering
  • Behavioural
    • Lack of heat-seeking behaviour
  • Increased heat loss
    • vasodilatation from anaesthetic agent
    • conductive loss to IV fluids

What are the main routes of heat loss in the anaesthetised patient?

  • Radiation 40%
    • Heat loss by infrared radiation to the environment, mainly determined by the difference in temperature between patient and environment
  • Convection 30%
    • Heat loss to the air in contact with the skin which is then carried away and replaced by cold air
  • Evaporation 15%
    • Latent heat of vaporisation from mucous membranes and exposed body surfaces
  • Respiration 10%
    • 8% due to the energy required to humidify the air
    • 2% to warm the air
  • Conduction 5%
    • Loss of heat by direct contact
      • IV fluids
      • Operating table

What are the effects of Hypothermia?

  • Impaired cognition and delirium
  • Arrhythmias and reduced cardiac myocyte function
  • Vasoconstriction, increased systemic vascular resistance and hypoperfusion to skin
  • Delayed drug clearance
  • Wound infection
  • Oxygen dissociation curve shifts left
  • Reduced MAC requirement for volatile agents
  • Bleeding and coagulopathy
  • Increased blood viscosity and DVT risk
  • Discomfort
  • Hyperkalaemia and hyperglycaemia
  • Shivering and increased energy expenditure

How does a fall in temperature influence blood gas solubility and acid-base values?

  • pH increases by around 0.02 for each 1°C drop in temperature
  • Gas solubility increases at temperature drops
  • If a gas dissolves into the fluid, the partial pressure of the gas drops
  • Blood gas machines are calibrated to 37°C, so will give a falsely low reading for pH and CO2 in a hypothermic patient, unless adjusted for temperature

What methods are there of keeping patients warm?

  • Ambient temperature
    • Target ambient temperature is 22°C–24°C and humidity 50-60%
  • Patient warming devices
    • Blankets and mattresses
    • Forced air warmers (FAWs)
      • Use convection to prevent and treat hypothermia
      • In children and neonates it is best done by placing the patient on the blanket and forcing the warm air up around the baby
      • Complications include burns and pressure ulcers
    • Electrical blankets
      • Can interfere with other monitoring equipment
    • Water-filled mattresses/blankets
    • Resistive polymer blankets
    • Electric heating pads
  • Fluid warming devices
    • These should be used if more than 500ml of fluid are to be given, and can be divided into pre-warming and warming during administration
    • Counter-current heat exchange systems (good for low-flow, e.g. in neonates)
    • Water baths
    • Convective air systems
    • Insulators
    • Invasive methods
    • Warmed irrigation of cavities (e.g. peritoneal) and organs (e.g. stomach, bladder)
    • ECMO and CPB (cardiopulmonary bypass)

What are the definitions of boiling point, freezing point, and latent heat?

  • Boiling point
    • The temperature at which a substance transitions from liquid phase to gaseous phase
    • It is the temperature at which the saturated vapour pressure exceeds ambient pressure
    • Boiling point therefore depends on ambient pressure, and increases as ambient pressure increases
      • This is why water boils at around 8°C on Mars
  • Freezing point
    • The temperature at which a substance transitions from liquid phase to solid phase
    • This also correlates with ambient pressure
  • Latent heat
    • Transitioning from solid to liquid or liquid to gas is an endothermic process, and so requires energy input
    • This is the amount of energy required to disrupt molecular bonds, measured in Joules/kilogram
    • Latent heat of fusion is the energy required to convert a solid to a liquid without changing the temperature
    • Latent heat of vaporisation is the energy required to convert a liquid to a gas, without changing the temperature

FRCA Primary Toolkit

All of these questions and more are available in our FRCA Primary Toolkit, available here