1. Heat, Thermal Conductivity In
The Body, Sources of Heat Loss
During Anesthesia, Methods Of
Measuring Temperature,
Thermistors, Thermocouples
Dr Bikash Subedi
Moderator: Dr Binod Gautam
2. Heat Vs Temperature
Heat Temperature
• Energy form,total sum of
kinetic energy of molecules
• Calorimeter
• joules
• temperature is a numerical
measure of how hot or cold!
• measure of the average
kinetic energy of molecules
• Thermometer
• Celsius, Fahrenheit, Kelvin
7. Normal body temperature
• 37+/- 0.5 0 C
• regular circadian fluctuation of 0.5–0.7 °C
• lowest early morning/ highest evening
• Mid-cycle fluctuation in women
• Less precise in children may normally have a
temp. 0.50C above the normal for adults
8. • the anus (rectum/rectal), vagina, or in the ear
(otic) is about 37+/- 0.5 °C
• Temperature in the mouth (oral) is about
36.8 °C (98.2 °F)
• Axillary temp is about 36.5 °C (97.7 °F)
9. Core Vs Periphery
CORE PERIPHERY
• major thoracic and
abdominal organs & Brain
• holds two-thirds of the
body heat content
• Maintained within a
narrow temp. range (36.6
to 37.4°C)
• limbs and skin and
subcutaneous tissue
• About one-third
• varies widely.depending
on the environment
• usually 30-32°C.
This 5-7°C difference between core and peripheral
body temperature is maintained by vasoconstriction in the blood
vessels
15. HEAT GAIN
• Vasoconstriction (sympathetic activity) <36.5 0C
- enhances insulation
• Chemical thermogenesis
- Non shivering in neonates
- effect of thyroid hormone on BMR
• Shivering (<36.0 0C)
4-6 fold increase in heat production
• Muscular exercise
• Piloerection
16. Nonshivering Thermogenesis
• metabolic process located primarily in brown
adipose tissue and controlled by the activity
of the sympathetic nervous supply
• all the energy of metabolism is dispersed as
heat without any ATP!
• Adrenaline and Noradrenaline stimulate
uncoupled oxidative phosphorylation
• Conversion of T4 to T3 inside brown fat cells. T3
↑cellular metabolic rate
17. Thermal conductivity
• property of a material to conduct heat
• watts per meter kelvin
• Thermal Conductivity = heat × distance / (area ×
temperature gradient
• Thermal capacity: Necessary amount of energy
needed to increase temperature by 1 kelvin
• Fat: very low thermal conductivity and thermal
capacity
• Muscles and skin: higher thermal conductivity
19. Radiation
• Major type of heat loss (40%)
• Heat transfer is proportional to the difference of
4th power of the absolute temperature difference
between the surfaces
20. Convection
• 2nd most important factor (30%)
• Heat loss in proportional to square root of the
air speed
21. Evaporation
• accounts for 10% of heat loss
• Evaporation of 1g water removes about 0.6
Kcal. Of heat
• Insensible loss (600ml/d even without sweating)
• Increased by sweating
• In infants higher fraction of heat loss by
transpiration of water through thin skin( in
preterms heat loss can be 1/5th of metabolic heat
production)
• Heat loss from respiratory system trivial
• Exposure of surgical wounds potentiates heat loss
22. Conduction
• Directly proportional to the temperature
difference 2 adjacent surfaces and the strength
of the thermal insulation separating them.
• <5%
23. Sweating
• Sympathetic cholinergic fibers
• prevented by nerve block or administration of
atropine
• only mechanism by which the body can dissipate
heat in an environment exceeding core
temperature
• 0.6 kcal dissipated per gram of
evaporated sweat
24. Intra-operative heat loss
(i) Exposure
- cold OT
- shed clothes, cold table, cold skin prep
(ii) Cold IV fluids
(iii) Cold irrigating fluids
(iv) Evaporative heat losses
- dry anesthetic gases
- body cavity exposure
26. General Anesthesia
• decline in body temp. occurs in three phases
Phase
I
Upto 1st hour. 1-
1.5 0C
Rapid fall
Core to periphery
redistribution
due to
vasodilation
Phase
II
2-3 hours.
Slower decline
↓BMR
Loss > production
Phase
III
After 3-5 hrs
Plateau
Loss =
production
vasoconstriction
29. Hypothermia Considerations in
anesthesia
• Widens interthreshold range from 0.4 - 3°C
• ↓ Drug metabolism
DOA vecuronium is more than doubled in patients with a core temp.
<35°C.
• ↑Solubility of inhaled anesthetic agents
↓ MAC of 5% for every 10 decrease in core temp.
• delayed awakening
• Left shift of the hemoglobin–oxygen saturation curve
• ↓ BMR
• Reversible coagulopathy (platelet dysfunction)
• Poor wound healing
• Cardiac arrhythmias and ischemia
• Impaired renal function
• Postoperative protein catabolism and stress response
• CNS dysfunction
31. MEDICAL CONDITIONS PREDISPOSING
TO HYPOTHERMIA
•
1. Skin loss - burns, severe exfoliating
dermatoses;
2. Autonomic neuropathy -
para/quadriplegia, diabetes, uraemia;
3. Acute alcohol intoxication;
4. Endocrinopathies - myxoedema, adrenal
insufficiency;
5. General debilitation;
(6. Sickle cell diseases - hypothermia can
precipitate a crisis).
32. SPECIAL CONSIDERATIONS
Neonates and infants
• increased surface area to volume ratio
• thin skin with minimal insulating fat
• less effective efferent responses to cold
temperatures.
• Infants <3 months cannot shiver
34. Monitoring recommendations
• Core body temperature should be measured in
GA >30 minutes.
• During regional anesthesia when changes in
body temperature are intended, anticipated, or
suspected. 3.
• Unless hypothermia is specifically indicated
(e.g., for protection against ischemia), effort
should be made to maintain intraoperative core
temperature higher than 36°C.
35. ASA recommendation
• “every patient receiving anesthesia shall have
temperature monitored when clinically
significant changes in body temperature are
intended, anticipated or suspected.”
• For office-based sedation, regional anesthesia,
or general anesthesia, the ASA also requires that
“the body temperature of pediatric patient
shall be measured continuously
36. Methods of measuring temperature
• Non – electrical technique
• Electrical technique
• Infrared ear and tympanic membrane
thermometers
37. Non electric thermometers
• Mercury , alcohol
• Based on “ change in volume with temperature”
• Mercury thermometer - German physicist Daniel
Gabriel Fahrenheit in 1714
• Mercury for higher temperature and alcohol for
lower temperatures
38. Dial thermometers :
Bimetallic stripTwo dissimilar metals fixed
together in a coil
Unequal Metal expansion
leads to coil tightening and
movement of level clock-wise
39. Bourdon gauge
•Actually a device to
measure pressure which is
attached to sensing element
containing a small tube of
mercury or a volatile fluid
•With temperature , volume
or pressure changes in the
sensing element which is
recorded on bourdon gauge
, and is calibrated in units of
temperature.
41. Resistance thermometer
• Based on the fact that the electrical resistance of
a metal increases linearly with temperature
• Consists of of : platinum wire resistor , a battery
, ammeter
• Insensitive
42. Thermistor
•a type of resistor whose
resistance varies significantly
with temp. more so than in
standard resistors
•Is a small bead of metal oxide ,
the resistance of which falls
exponentially as the
temperature rises
•Used clinically in PA catheters
, esophageal stethescopes , can
be kept in body cavities
•Cheaper
44. Thermocouple
•Based on Seebeck’s effect
•At junction of 2 different
metal ( copper and constantan)
small voltage is developed
,magnitude of which is
dependent on the temp. of the
junction
•Reference junction should be
kept at constant temperature
•Used in clinical hyperthermia
45.
46. Infrared ear and tympanic membrane
thermometer
• Based on emission of electromagnetic radiation by
objects and the intensity of radiation and the
wavelength depend on the temperature of the object.
• At body temperature object primarily emits Infrared
radiation
• The sensor is kept in ear canal or near ear drum , which
senses the radiation and converts it into electric signal
