Transcript Body Temperature and Thermoregulation

Body Temperature and
Thermoregulation
Dr Hannah Rose
EDGH 2005
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
NORMOTHERMIA
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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
Thermoregulation
CORE
COMPARTMENT
• INNER 2/3RD
• 37°C +/- 0.2 °C
PERIPHERAL
COMPARTMENT
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OUTER 1/3RD
28°C - 36°C
Small in neonates
Relative decrease with
anaesthesia
THERMOREGULATION
HEAT LOSS
• Behavioural
• Vasodilation
• Panting
• Decreased activity
• Sweating (Ach)
HEAT PRODUCTION
• Behavioural
• Vasoconstriction
• Piloerection
• Shivering
• Eating
• Non-shivering
thermogenesis
HEAT TRANSFER MECHANISMS
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RADIATION
CONVECTION
EVAPORATION
RESPIRATION
CONDUCTION
40%
30%
15%
10%
5%
CONTROL OF TEMPERATURE
Central
Controller
Sensors
Effectors
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
Piloerection
Vasoconstriction
Non-shivering
thermogenesis
Shivering
Thermoregulatory thresholds
No physiological
response
Interthreshold
range
36.8-37.2
SENSORS
Warm
C-fibres
Normally quiet
T=
firing
• Anterior Hypothalamus
• Brain
• Spinal Cord
• Thoracic and Abdominal
tissue
• Skin
Cold
Ad-fibres
Always firing
T= firing
SKELETAL MUSCLE
Shivering
BROWN ADIPOSE TISSUE
Non shivering thermogenesis
CORTEX
Behaviour
SKIN ARTERIOLES
VC/VD
EFFECTORS
ADRENAL MEDULLA
Catecholamines
SWEAT GLANDS
SWEATING
ANTERIOR PITUITARY
Thyroid hormone production
SHIVERING
10x Increase O2
consumption
17 min for 1°C
2-5x increase in
heat production
Fine (local) vs
coarse (CNS)
Not in kids
Effectors
NON SHIVERING
THERMOGENESIS
SWEATING
•Ach SNS
• A b3 response
•Evaporation
• Brown fat high
in mitochondria
VASOCONSTRICT/VASODILATE
• Oxidative uncoupling
•An a response
• Doubles heat
•Countercurrent exchange
•Radiation and convection affected
production
• Infants/neonates
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
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
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!!!!!
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…..
MEASUREMENT OF TEMPERATURE
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Mercury / Alcohol thermometer
Bourdon Gauge
Bimetallic Strip
Platinum resistance wire
Thermistor
Thermocouple
Infra red thermometers and
thermography
Thermometers
• Based on Charles’ law
• Mercury or alcohol (lower temperatures,
FP Hg –39C, BP ETOH 79C)
• Angle constriction
• 2-3 mins
• Rectal / Oral / Axilla
Bourdon Gauge
• Sensing element attached to p gauge
• Volatile / Hg
• Gas laws
Bimetallic Strip
• 2 dissimilar metals with differing
expansion coefficients
Platinum resistance wire
• Increase T causes a linear increase in
resistance causing a reduced current
flow
• R = V/I
THERMISTOR
• Metal oxide bead
• Exponential DECREASE in resistance
with increasing temperature
• Small, cheap,versatile
• Sensitive to T extremes
• Regular calibration
necessary
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
Infrared thermometers
• Measure infrared radiant heat emitted
from tissues
• Quick, reproducible, easy to use,
socially acceptable
Questions?