explaining from a physic book,with a summary,and voice,for anaesthesia specialests

1. TEMPERATURE
MOHAMED ANWER RIFKY
THERMISTOR
Pyroelectricity can be
described as the ability of
certain materials to generate
a temporary voltage when
they are heated or cooled.
Pyroelectric
sensor

2. 1-THE CONCEPT OF TEMPERATURE AND
HEAT:
Relationship of heat to temperature =the quantity of a solute to its concentration
(concentration rises as a solute is added).
2-TEMPERATURE SCALES: Heat energy added to a substance>>temperature rise, and also changes in its physical
properties
.
In the SI system the unit of temperature is the
kelvin
(i.e mercury which
expands).
=(1/273.16 of the thermodynamic temperature of the triple point of
water).
Temperature (K) = Temperature (°C) +
273.15
The triple point of water occurs is at a temperature of 0.01°C.
(N.B The Celsius scale is also
approved).
3-NON-ELECTRICAL TECHNIQUES OF TEMPERATUREMEASUREMENT:
1-The mercury
thermometer:
Angulated constriction-metal index (which is left behind)-used as thermostats(switch)-disadvantages>>
takes two
to three minutes b-risk of it breaking -Alcohol instead of mercury>>a-If used at very low temperatures>>solidifies at -
39°C b-unsuitable. for high temperatures because alcohol boils at 78.5°C c-scale of the alcohol>> A less linear scale
than mercury.
.
2-Dial thermometers: a-Bimetallic strip (two dissimilar metals fixed together in a coil- metals expand by different amounts_
coil tightens to move the lever over the scale b-Bourdon gauge.(sensing element containing a small tube of mercury or a
volatile fluid).
Bourdon
gauge
Bimetalik strip
4-ELECTRICAL TECHNIQUES OF TEMPERATURE
MEASUREMENT:
A-The resistance
thermometer:
Based >>electrical resistance of a metal increases linearly with temperature-
platinum wire
resistor, a battery for electrical potential ,and an ammeter-Wheatstone bridge.
B-A thermistor (little bead of metal oxide-falls exponentially as the temperature rises but also available in which
resistance rises with temperature-Wheatstone bridge-metal oxide bead can be made very small-cheap-but liable
to change if the thermistor
is subjected to severe changes of heat ( sterilization).
5-CLINICAL APPLICATIONS:
C-The thermocouple: based on the Seebeck effect>>at any junction of two dissimilar metals a small
voltage is produced-copper and constantan are used-a second junction is needed (the reference junction,
must be kept at a constant temperature-can be used in the form of a needle-The response time >> depends
mostly on size from( 0.1 s to 15 s).
Seebeck effect. Principle of the thermocouple.
Resistance with temperature
Maximum reading thermometers.
1-Mercury thermometer (Used in the mouth,axilla or rectum-limitation to its use owing >>rigidity.
2-Rectal temperature (risk in small babies of perforation - in adults rectal temperature measurements may be inaccurate owing to the slowness of temperature
equilibrates .
3-The most popular site is the lower oesophagus ,but not too high, >>proximity of the lower trachea >>cooled by inspired air.
4-The ear has also been used>> risk of perforation of the drum.
5-Nasal temperature fluctuates>> utilized in the design of respiratory monitors.The pulses from
fluctuation of temperature over the thermistor >>electronically processed >>respiration rate
6-The nose may be used as an alternative to the oesophagus to measure body temperature in an anaesthetized patien (no respiratory cooling).
7-Skin temperature measurements (fall of skin temperature in the periphery can be correlated with the degree of shock) .
8-A special system for measuring skin temperature is called Thermography. Radiant heat from patient's skin is imaged by means of an infrared sensitive
camera>>warmth of the skin with different colours >>indication of the underlying vascularity>> detect tumours and vascular abnormalities.
N.B constantan(cupper and nickle )

4. 6-BODY TEMPERATURE:
1-Humans, like all mammals and birds, are homeothermic ( control
their
central body temperature within a narrow range despite a wide range of environmental
temp. of
37 ±
0.5°C).
2-There is a circadian or diurnal rhythm of around 0.4°C,
3-There is a slight variation in body temperature during the menstrual cycle in women..
4-Exercise can also cause a rise in body temperature,as much as 2°C.
5-The central core of the body which is maintained at the constant temperature, while the surface layer or shell of the
body is at a lower, (from 32 to 35°C),
6-The central core includes the brain, thoracic and abdominal organs and also some of the deep tissues of the limbs.
7-The shell is a layer with a variable depth of around 2.5 cm. The temperature in the body core >> on the balance
between heat production in the core and heat loss through the shell .
7-HEAT PRODUCTION: Is of the order of 50 W m-2 body
surface area or 80 W total.
Minor shivering is not apparent to the naked eye ,detected by>>Electromyogram. Major shivering is needed only when a large increase in heat
production is required.
8-HEAT LOSS:
1-It is seen that conduction per se is not important(energy is transmitted through a substance).
2-Heat loss from radiation (A hot object emits radiation over a spectrum of wavelengths predominantly in the infrared region)- Carry energy from the hot
object>>cooling.Conversely if radiation >>object will become hotter-No contact- the rate of transfer >>the relative temperatures and the surface characteristics of the
objects-Radiant heat losses are increased if the body is surrounded by cold objects and reduced by radiation received from warm surroundings Space blankets of shiny
reflective metallized plastic foil >>radiant heat loss, but>>increased risks of burns and of electric shock,
3-In convection, the air layer adjacent to the surface of the body is warmed by conduction >>
heated it expands (less dense) and so rises>>heat moves away from the subject.
4-Surface evaporative heat loss is due to the loss of latent heat of vaporization
of moisture on the skin's surface>>water vapour pressure gradient-increases 10 times by sweetimg.
5-The body tissues lying between the core and the skin surface consist mainly of
fat >>avascular insulating layer. Clothing, too,act as an insulator.
6-Eight per cent of this loss occurs through increasing the humidity of the inspired air from about 50% to 100% and 2% is due to warming the air-inspiration of dry
gases >>risk of hypothermia.
A-Physiological control over temperature is mediated by temperature receptors
in the skin and by a temperature receptor in the hypothalamus >> (temperature of the blood perfusing it).
B-The thermoregulatory centre is depressed by anaesthesia and many anaesthetics
also have an effect on vasomotor tone >> peripheral vasodilation
>>heat loss. During surgery there is a greater area of the patient exposed to the atmosphere !!,humidifying the inspired air ,and by warming the
transfused blood..
C-Hypothermia may be induced in anaesthetized patients
(neurosurgery or cardiac surgery). The reduced body temperature
decreases metabolism and thus the tissue demand for oxygen>>
the time for safe circulatory occlusion is double that at 37°C( below 30°C are avoided >>
(ventricular fibrillation).
D-Temperatures over 42°C cause cerebral impairment and disturbance of the physiological control (over 45°C, the rise of temperature induces a rise in metabolism which leads to a vicious circle>>fatal. A rare anaesthetic cause of
such hyperpyrexia is the malignant hyperpyrexia. (have an abnormal response to halothane or suxamethonium).
E-There is a risk of burns and tissue damage if the temperature of the surface skin or tissues approaches 45°C from an external heat source and are more likely in patients who are vasoconstricted or shocked..
9-RESPONSES TO CHANGES IN
ENVIRONMENTAL
TEMPERATURE.HYPOTHERMIA,P
YREXI,AND THERMAL BURNS.
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