16B20: Exam Report

Describe the mechanisms by which heat is lost from the body (40% of marks).Discuss the importance of each of these in a sedated and intubated adult patient (60% of marks).

59% of candidates passed this question.

A satisfactory answer required use of terms radiation, convection, conduction etc. in the manner defined in the texts, rather than the layman’s use of the terms. Better answers displayed understanding of the meaning, relative importance and mechanism of methods of heat loss.

The second part of the question required application of these concepts in patients with artificial airway and sedatives, particularly change in control of vascular tone and voluntary behavioural control.

R1ii / 16B20: Describe the mechanisms by which heat is lost from the body (40 marks). Discuss the importance of each of these in a sedated and intubated adult patients (60 marks)

Define:  Heat = a form of energy determined by how active the molecules of a substance are

Heat Loss

Occurs when body is in contact with external environment

For heat loss to occur:

  1. Transfer heat from core → skin
  2. Transfer heat from skin → environment

Core → Skin

  • Depends on insulation and blood flow
  • Insulation
    • Fat conducts heat 1/3 the rate of other tissue
    • Fat and subcutaneous tissue acts as a heat insulation
    • Body heat is not evenly distributed and peripheral temps are 2-3°C less than core temperature
  • Blood Flow
    • Skin has rich blood vessel network
    • Venous plexus is supplied by skin capillaries and a-v anastomoses
    • Allow heat to bypass the insulator system
    • Therefore blood flow can vary depending on VC/VD
    • Increase blood flow = increase heat transfer to skin = promotes heat loss

Skin → Environment

  1. Radiation 60% heat loss

Heat loss in the form of infrared waves

All objects not 0°C emit infrared waves

VD = increased blood flow to skin

Skin becomes an effective radiator

  1. Conduction 3% heat loss

Skin in direct contact w cooler object transfers its kinetic energy molecules to molecules it is in contact with

  1. Convection 15% heat loss

Removal of heat from body by convesction of air currents

Heat must first be conducted to air then removed by convection

Increase wind = increase convection = increase heat loss

Clothing traps air next to skin thus reducing conduction and heat loss

  1. Evaporation 20% heat loss

Energy is required to change H2O from liquid to vapour

Heat is lost due to the latent heat of water

Water evaporates from skin and airways

Insensible losses  approx. 600ml/day, due to continuous diffusion of water through skin and respiratory surface despite body temperature

Sweating = loss of heat by evaporation of sweat

  1. Behaviour

Less clothing = increase heat loss by radiation

Increase activity – increase heat loss by radiation and evaporation

Touching cool objects – increase heat loss by conduction

Sedated & Intubated Patient

Humans are homeothermic = actively maintain core body temp w/in narrow range 36.5 – 37.3°C

0.5°C outside of this range activates thermoregulatory mechanisms to restore body temperature

Posterior hypothalamus compares sensory thermal input and initiates response depending on temperature.  Temperature where response is triggered KA Threshold Temperature

Intubation requires GA +/- paralysis

  • GA alters interthreshold range; 2.5 degrees lower & 1.3 degrees higher
  • Therefore, harder to maintain core normothermia
  • GA will result in mild hypothermia with a drop in core body temp 1-3 degrees pver 3 phases:

Phase I

  • Rapid 1-2 degree fall in first hour of sedation
  • VD by sedatives → heat redistributed from core → mass heat loss by Radiation
  • GA → widens interthreshold range and delays activation of compensatory VC

Phase II

  • Gradual further decline by 1 degrees over 2-3hrs by
    • Radiation: Loss of VC = increased heat loss by radiation
    • Conduction: heat loss by direct contact of patient w bed, probes, soiling
    • Convection: Loss of errrector pili increases heat loss by convection, no wind currents in ICU therefore heat conservation by hospital gowns, but frequent exposure for examination, procedures & nursing checks
    • Evaporation: insensible heat losses from skin & lungs, but loss of sweating mechanism
    • Beahviour: inability to shiver and increase BME, seek warmth, clothing etc
    • Equipment: indwelling catheters, ECMO, CRRT all increase heat loss by conduction, convection and radiation

Phase III

  • Plateau phase
  • Heat loss matched by heat production
  • Patient becomes sufficiently hypothermic to activate compensatory VC