1. Body Temperature and Thermoregulation
Dr Hannah Rose
EDGH 2005

2. Kelvin to Centigrade K = °C + 273
INTRODUCTION
Temperature – thermal state of a substance which is determined by its ability to transfer or gain heat energy from another body
SI unit = Kelvin
Kelvin to Centigrade K = °C + 273
Fahrenheit to Centigrade °F = (°C x 9/5) + 32
LORD KELVIN
‘Turn of the century’ Scottish physicist / mathematician,
now buried next to Isaac Newton in Westminster Abbey

3. NORMOTHERMIA Homeothermic Core T = 37°C +/- 0.2°C
Normal variations up to 0.7°C
Ovulation, circadian, exercise, feeding
Infection, thyroid disease, drugs, anaesthesia can affect thermoregulation
Deviations can cause range of problems affecting ALL organ systems

4. Thermoregulation CORE COMPARTMENT INNER 2/3RD 37°C +/- 0.2 °C
PERIPHERAL
COMPARTMENT
OUTER 1/3RD
28°C - 36°C
Small in neonates
Relative decrease with anaesthesia

5. THERMOREGULATION HEAT LOSS Behavioural Vasodilation Panting
Decreased activity
Sweating (Ach)
HEAT PRODUCTION
Behavioural
Vasoconstriction
Piloerection
Shivering
Eating
Non-shivering thermogenesis

6. HEAT TRANSFER MECHANISMS
RADIATION %
CONVECTION 30%
EVAPORATION 15%
RESPIRATION 10%
CONDUCTION 5%

7. CONTROL OF TEMPERATURE
Central
Controller
Sensors
Effectors

8. Central Controller
Posterior Hypothalamus
Integrates sensory input and compares to a ‘set point’
Implements appropriate thermoregulatory responses
in a graded fashion
Maintenance of core T in interthreshold range

9. Thermoregulatory thresholds
Non-shivering thermogenesis
Vasoconstriction
Piloerection
Shivering
No physiological
response
Interthreshold
range
Vasodilatation
Sweating

10. Anterior Hypothalamus
Brain
Spinal Cord
Thoracic and Abdominal
tissue
Skin
SENSORS
Warm Cold C-fibres Ad-fibres Normally quiet Always firing T = firing T= firing

11. SKELETAL MUSCLE Shivering
BROWN ADIPOSE TISSUE
Non shivering thermogenesis
EFFECTORS
CORTEX
Behaviour
SKIN ARTERIOLES
VC/VD
ADRENAL MEDULLA
Catecholamines
SWEAT GLANDS
SWEATING
ANTERIOR PITUITARY
Thyroid hormone production

12. Effectors NON SHIVERING THERMOGENESIS A b3 response Brown fat high
SHIVERING 10x Increase O2 consumption 17 min for 1°C 2-5x increase in heat production Fine (local) vs coarse (CNS) Not in kids
NON SHIVERING
THERMOGENESIS
A b3 response
Brown fat high
in mitochondria
Oxidative uncoupling
Doubles heat
production
Infants/neonates
SWEATING
Ach SNS
Evaporation
VASOCONSTRICT/VASODILATE
An a response
Countercurrent exchange
Radiation and convection affected

13. Effect of General Anaesthesia on Thermoregulation
No behavioural response available
Obtund hypothalamic control-widen interthreshold range
Peripheral vasodilatation
Cool anaesthetic gasses
Cool environment and exposure
Cool cleaning fluids
Open wounds and body cavities
Decreased metabolism

14. Triphasic Hypothermia of General Anaesthesia
REDISTRIBUTION- initial fall of 1C over 1 Hour as core
heat distributes to periphery
LINEAR DECLINE- Core temp below thermoregulatory
threshold for vasoconstriction (34-35C). Secondary to
radiation, convection, evaporation, respiration…..
PLATEAU- reestablishment of core-periphery gradient
and thermal equilibrium reestablished at lower
temperature

15. Effect on Regional Anaesthesia
Decreased afferent signals from below block
No vasoconstriction or shivering below
level of block
No establishment of plateau phase – blocked VC
Shivering and VC in unblocked dermatomes
Can be AS severe as with GA
RA + GA together…………CHILLING!!!!!

16. Adverse effects of Hypothermia
CVS – dysrhythmias coagulopathy CO viscous blood
RESP – O2Hb to the left, MV decreases, bronchial dilatation
RENAL – GFR and tubular fxn decrease, diuresis, poor clearance
CNS – poor memory, AP and neuromuscular transmission prolonged, impaired consciousness
Reduced cellular metabolism and enzyme function and immunity
Poor wound healing, longer hospital stays, increased blood TXF, increased…..

17. MEASUREMENT OF TEMPERATURE
Mercury / Alcohol thermometer
Bourdon Gauge
Bimetallic Strip
Platinum resistance wire
Thermistor
Thermocouple
Infra red thermometers and thermography

18. Thermometers Based on Charles’ law
Mercury or alcohol (lower temperatures, FP Hg –39C, BP ETOH 79C)
Angle constriction
2-3 mins
Rectal / Oral / Axilla

19. Bourdon Gauge Sensing element attached to p gauge Volatile / Hg
Gas laws

20. Bimetallic Strip
2 dissimilar metals with differing expansion coefficients

21. Platinum resistance wire
Increase T causes a linear increase in resistance causing a reduced current flow
R = V/I

22. THERMISTOR Metal oxide bead
Exponential DECREASE in resistance with increasing temperature
Small, cheap,versatile
Sensitive to T extremes
Regular calibration
necessary

23. THERMOCOUPLE
Seebeck Effect – Generation of a voltage in a circuit containing two different metals, or semiconductors, by keeping the junctions between them at different temperatures.
Copper / Constantin
Reference jxn and measuring jxn
Rapid response time on smaller probes

24. Infrared thermometers
Measure infrared radiant heat emitted from tissues
Quick, reproducible, easy to use, socially acceptable

25. Questions?