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%).

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)

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

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

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!

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

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

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