2018A14 Describe the mechanisms by which heat is transferred between the body and its environment (30%).
Describe methods by which body heat may be conserved under anaesthesia (70%).

 

List:

·       Definitions

·       Radiation

·       Convection

·       Evaporation

·       Respiration

·       Conduction

·       Heat production

 

Definitions:

Heat

·  Form of energy

·  Transferred from higher temp object to lower temp object

·  Unit Joule = kgm2s-2

Temperature

·  Measure of the average kinetic energy of particles per degree of freedom

·  Unit Kelvin

Specific heat capacity

·  Ratio ↑heat:↑temp

·  Unit kJ/K/kg)

 

Radiation 40%:

Mechanism

·  Transfer of heat between bodies by EMR

·  Bidirectional

·  Contact not required

·  ∝ (T1)4 – (T2)4   (T in Kelvin)

·  e.g. patient <-> wall

Reduce loss

·  Vasoconstriction -> ↓skin temp (↓T1) (impaired under GA)

·  Insulation e.g. blanket

·  Pre-warming of operating theatre (↑T2)

Increase gain

·  Radiator above the neonatal resuscitation trolley -> invert temperature gradient

 

Convection 30%:

Mechanism

·  Transfer of heat away from a body by motion of a fluid

·  ∝T1-T2)

·  e.g. patient -> surrounding air

Reduce loss

·  Vasoconstriction -> ↓skin temp (T1) (impaired under GA)

·  Insulation e.g. blanket traps a layer of air next to patient

Increase gain

·  Forced air warmer

 

Evaporation 15%:

Mechanism

·  Due to latent heat of vaporization, 2.24MJ/kg for water

·  Both sweating and transepidermal diffusion = insensitive loss

·  ∝ skin temp / ambient relative humidity

·  Does not require temperature gradient

Reduce loss

·  Vasoconstriction -> ↓skin temp (impaired under GA)

·  ↑Relative ambient humidity

Increase gain

·  Cannot!

 

Respiration 10%:

Mechanism

·  Cool, dry air inspired, warm moist air expired

·  ∝ minute ventilation

Reduce loss

·  Heat and moisture exchanger (HME)

o  Expired air cooled by HME, water vapour condenses

o  Inspired air warmed by HME, water evaporates

o  Extends gradients for temp and humidity: lung > HME > circuit

o  Hence extends countercurrent heat and moisture exchange

·  CO2 absorber:

o  CO2 + sodalime reactions produce heat and water

o  Hence ↓temperature gradient, ↓humidity gradient

o  Hence ↓latent heat loss

 

Conduction 5%

Mechanism

·  Transfer of heat between two objects in direct contact

·  ∝(T1-T2) x surface area x thermal conductivity

·  e.g. patient -> cold operating table

Reduce loss

·  Vasoconstriction -> ↓skin temp (T1) (impaired under GA)

·  Heated OT table -> reverse temp gradient

·  Insulated material on table -> ↓thermal conductivity

 

Heat production:

Brown fat metabolism

·  Uncoupling protein inserted into inner membrane

·  Free passage H+ into matrix without ATP synthesis

·  ↑BMR 2-3x

·  Only significant in neonates

Calorigenic hormones

·  Effects of catecholamine etc. on liver, skeletal muscle

Shivering

·  Oscillating contraction/relaxation

·  Especially truncal muscles

·  ↑BMR 2-3x

·  Ablated under GA

·  Ablated by neuraxial anaesthesia below block level