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TEMPERATURE FOR ANAESTHESIA PHYSCIANS

TEMPERATURE MEASUREMENT

mohamed397
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TEMPERATURE FOR ANAESTHESIA PHYSCIANS

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  1. TEMPERATURE IN ANAESTHESIA MOHAMED ANWER RIFKY

  2. Temperature rises as heat energy is added to a substance, so concentration rises as a solute is added to a solution. Temperature 1-THE CONCEPT OF TEMPERATURE AND HEAT **Temperature of a gas is a measure of its speed. 2-TEMPERATURE SCALES Mercury which expands and contracts by temperature changes>>Fahrenheit scale. constructed the first temperature scale triple point of water (all phases are in… equlibriumequlibrium equilibrium). In the SI system the unit of temperature is the kelvin, which is defined as the fraction 1/273.16 of the thermodynamic temperature of the of 0.01°C. Temperature (K) = Temperature (°C) + 273.15 .- The triple point of water occurs at a temperature The mercury thermometer>>change in volume of mercury with temperature, An- angulated constriction splits the mercury column afterit has reached its maximum 3-NON-ELECTRICAL TECHNIQUES OF TEMPERATURE MEASUREMENT reading>>prevent contracting into the bulb until the thermometer is shaken ,or use a smallmetal index (left behind)-Mercury thermometers may also be used as thermostats-Has two main disadvantages: Two to three minutes are necessary for complete thermal equilibrium between the mercury and its surroundingssecondly, it may be difficult to introduce the thermometer into some orifices >>risk of it breaking-Alcohol is sometimes used ( solidifies at — 39°C.& boils at 78.5°C)-Disadvantage: The scale of the alcohol thermometer tends to be less linear than that of the mercury type. Dial thermometers may use either a bimetallic strip or a Bourdon gauge: 1-Two dissimilar metals fixed together in a coil >> expand by different amounts - 2-Sensing element>> the coil tightens to move the lever clockwise over the scale-Variation in temperature causes a volume or pressure change in the sensing fluid. 1-Resistance thermometer>> electrical resistance of a metal increases linearly **FREEZIN POINT OF MERCURY IS -39 DEGREE C 4-ELECTRICAL TECHNIQUES OF TEMPERATURE MEASUREMENT with temperature. As the platinum wire resistor ( a battery as a source of electrical potential and an ammeter to measure current which could then be calibrated to indicate temperature or useWheatstone bridgecircuit containing an array of resistors). 2-The thermistor is a little bead of metal oxide, the resistance of whichunlike the platinum resistance thermometerfalls exponentially as the temperature riseshowever, are also available in which resistance rises with temperature. Often used in a Wheatstone bridge circuit-Advantages: that the metal oxide bead can be made very small and is more cheaply than the platinumthermometer. A disadvantage is that the calibration is liable to change when subjected to severe changes of temperature (during sterilization). 3-A systemsystem based on the Seebeckeffect.At any junction of two dissimilar metals a small voltage is produced, the magnitude of which depends on the temperature of the junction. (Thermocouple), and metals such as copper and constantan(alloy of copper and nickel) may be used. A second junction is needed >>complete electric circuit, one called the reference junction, >> kept at a constant temperature while the measuring junction acts as a temperature probe- Electrical compensation for changes in reference junction temperature. An advantage>> form of a needle. The response time (depends mostly on size with smaller heat capacity) >>0.1 s to 15 s. Mercury thermometer: Risk insmall babies >> perforation of the rectum -In adults rectal temperature>> may be inaccurate >>slowness in equilibrates with body temperature. Oesophageal: Tip of the thermometer is not too . 5-CLINICAL APPLICATIONS high>>cooled by inspired air. The Ear site: Risk of perforation of thedrum-Nasal temperature fluctuates >>respiratorymonitors (electronically processed).The Nose may be used in intubated patients.Skin temperature:>>fallof skin temperature >> degree of shock -The radiant heat from the patient's skin is imaged by means of an infrared sensitive camera (thermography); give an indication of the underlying vascularity>>tumours and vascular abnormalities. Human like all mammals and birds (homeothermic) >> control their central body temperature within a narrow range despite a wide range of environmental temperature. In man, this range is normally 37 ± 0.5°C. >> for proper enzyme reaction . 6-BODY TEMPERATURE Slight variation in body temperature>>occur naturally in man. The circadian or diurnal rhythm is around 0.4°C,also in menstrual cycle ,and during exercise more than 2°C.The central core includes thebrain, thoracic and abdominal organs and also some of the deep tissuesof the limbs, and the shell is a layer with a variabledepth of around 2.5 cm.>>32to 35°C. The temperature in the body core depends on the balance between heat production in the core and heat loss throughthe shell or surface layer. Average patient temperature = 0.66 x Core temperature + 0.34 x Average skin temperature. In (Two-thirds of the heat is in the core andone-third in the shell area).

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  4. 7-HEAT PRODUCTION Heat production is of the order of 50 W m-2body surface area or 80 W total, but increases of metabolic rate occur because of food consumption, exercise and other factors. There is no mechanism by which heat production can be reduced to compensate ,but an increase of heat production >> shivering. Minor shivering is not apparent to the naked eye >> electromyogram. Shivering can be more thandouble the metabolic heat production and a five-fold increase is possible in special circumstances. 8-HEAT LOSS A-Radiation 40% B-Convection 30% C-Evaporation 20% D-Respiration 10% (8% evaporation of water;2% heating of air). It is seen that conduction per se is not important molecules ( Metals and some crystals are good conductors). 1-A hot object emits radiation over a spectrum of wavelengths predominantly in the infrared region >>carry energy from the hot object, >>cooling. Two objects which are not in contact, the rate of transfer depending !!. . Radiant heat losses are increased if the body is surrounded by cold objects , and reduced if !!.Space blankets of shiny reflective metallized plastic foil >>restrict radiant heat loss, but >> risks of burns and of electric shock. 2-Convection :The air layer adjacent to the surface of the body is warmed by conduction and as it is heated it expands and becomes less dense and so rises>>carries heat away from the subject. 3-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 from the skin to the air) and to the total area. Sweating!!.The rate of heat loss is also influenced by insulation. Clothing (reducing the amount of skin surface area exposed &also trapping air!!).Fat forms a relatively avascular insulating. 4-Respiration accounts for only a small amount of the total heat loss, typically perhaps 10%. 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. Dry gases>>hypothermia!!. 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. In.mildchanges of environmental temperature, reflex changes of vascular tone occur and these alter skin temperature and so alter heat loss. In major cooling of the body, reflexes elicit shivering while excessive heat causes reflex sweating and vasodilation. Signals from the thermoregulatory centre to the cerebral cortex >>behavioural responses, >>amount of clothing or his level of activity. 10-HYPOTHERMIA The thermoregulatory centre is depressed byanaesthesia-anaesthetics >>vasomotor tone giving peripheral vasodilation>>heat loss. During surgery there is a greater area of the patient exposed to the atmosphere, in addition to which the loss of heat may be increased by rapid air movement from modern theatre ventilation. In prolonged surgery, therefore, there is a risk of hypothermia and the patient's core temperature must be monitored and appropriate measures taken to maintain itby raising theatre temperature, for example, by humidifying the inspired air , and by warming the transfused blood. 9-RESPONSES TO CHANGES IN ENVIRONMENTAL TEMPERATURE

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