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Basic Physics in Anesthesia
Davis & Kenny 5th edition Ch9 Temperature
48
Medical
02/24/2009

Term
 The Concept of Temperature and Heat
Definition
 Heat is kinetic energy that can be transferred from a hotter to colder substance  Temperature is the thermal state of a substance which determines whether it will give or receive heat.  Temp rises as heat energy is added
Term
 Temperature Scale
Definition
 Quantifies physical changes that occur in substances when heat energy is added  Scales have arbitrarily fixed points
Term
 Triple point of water
Definition
 (ice, water, and water vapor in equilibrium)  = 0.01 degrees C
Term
 SI system for temperature measurement
Definition
 Uses Kelvin  K = C + 273.15  no degree symbol for Kelvin
Term
 Dial thermometers
Definition
 Bimetallic strip and Bourdon guage
Term
 Mercury Thermometer
Definition
 contracts and expands with temp changes.  Advantages: Can be made into a maximum reading thermometer  Disadvantages: Response time = 2-3 min, rigid and may break in body orifice
Term
 Alcohol Thermometers
Definition
 Uses alcohol rather than mercury  Advantage: Can be used at low temps (solidifies at -39 C)  Disadvantage: Unsuitable for high temps ( boils at 78.5 C)
Term
 Three types of Electrical Thermometers
Definition
 1. Resistance Thermometers  2. Thermistor  3. Thermocouple
Term
Definition
 Response time 0.1 to 15 seconds.  Small heat probes have smaller heat capacity (Chapter 10), therefore a quick response time.
Term
 Resistance Thermometer
Definition
 Based on electrical resistance of a metal increases linearly with temperature.  Uses platinum (expensive)  Used in a Wheatstone bridge circuit (Chapter 14) Not very sensitive
Term
 Thermistor
Definition
 Uses small, inexpensive bead of medal oxide.  Most common model: Resistance falls as the temperature rises  Special model: Resistance rises as the temp rises  Used in a Wheatstone bridge circuit (Chapter 14) & in cardiac output  Advantages: small, inexpensive Disadvantage: Calibration changes if subjected to severe temp (i.e., heat sterilization)
Term
 Thermocouple
Definition
 Uses copper metal alloy and copper.  Based on Seeback effect—at a junction of two dissimilar metals a voltage is produced, the magnitude of which depends upon the temperature at the junction. Uses two temperature-dependent junctions to complete an electrical circuit.  First junction is the reference junction, kept a constant temp.  Second junction is the measuring junction.  Advantage: This technology can be manufactured and formed into the shape of a needle.
Term
 TWO TYPES OF SENSORS
Definition
 (1) Pyroelectric --Employs shutter mechanism to provide a period of exposure.  A snap shot. (2) Thermopile uses thermocouples—allows for continuous readings
Term
 Thermography
Definition
 Measures skin temperature by using radiant heat imaging connected to camera.  Different colors = different temps & underlying vascularity  Has been used to detect tumors and vascular abnormalities
Term
 Homeothermic
Definition
 Humans are homeothermic (core, central temp w/i narrow range despite environment. T = 37 +/- 0.5 degrees C)
Term
 Core temperature
Definition
 temperature of brain, thoracic, abdominal organs, deep tissues of limbs Depends on balance between heat production in core VS heat loss at surface layer
Term
 Effect of temperature variations
Definition
 Causes changes in enzyme reaction rates.  Metabolic process cannot occur when cooled.
Term
 Natural Variations in Body Temp
Definition
 Circadian 0.4 C (lowest in early AM, highest in early evening) Menstrual cycle (increase after ovulation for the second half of the cycle)  Exercise (can be +2C if high energy expenditure)
Term
 Measuring Heat Production
Definition
 Determined by the metabolic rate.  Measured in watts.
Term
 Average heat produced by a man at rest
Definition
 50 W m-2 body surface  or  80 W total
Term
 Principles of Heat Loss
Definition
 Radiation ** Convection ** Evaporation of water **Conduction** Heat loss through respiration  (**) Depend on amount of skin exposed to environment and can be controlled by covering the skin with clothes/blankets
Term
 Fat and Heat Loss
Definition
 Fat is an avascular insulating layer
Term
Definition
 Accounts for 50% of normal heat loss from the body  Can transfer heat between two objects not in contact, the rate depends on the relative temps and surface characteristics of the objects.  Radiant heat losses increase if body surrounded by cool objects and reduced by warm objects  Metallic blankets should NOT be used in OR due to risk of burns/electrical shocks  Example: fireplace across the room
Term
 CONVECTION
Definition
 Occurs when air or water with a lower temperature than the body comes into contact with the skin and then moves away  Examples of convection: Blowing on hot food to cool it down Wind blowing across bare skin  Amount of heat loss depends on the temperature difference between the body and environment plus the speed with which air or water is moving
Term
 EVAPORATION
Definition
 Loss of latent heat   Rate depends upon: * water pressure gradient from skin to air,  * amount of skin exposed to environment, and * skin wetness (10x greater during sweating)
Term
 Heat Loss Through Respiration
Definition
 Respiration combines the processes of evaporation (of moisture in the lungs) and convection (displacement of warm air in the lungs by cold air from outside environment) Accounts for only 10% of total heat loss  * 8% by increasing the humidity of inspired air from 50% to 100% * 2% by warming the air This heat loss route is unimportant--EXCEPT during anesthesia. Dry gases increase heat loss thru respiration and increase risk of hypothermia.
Term
 CONDUCTION
Definition
 Transfer of heat to objects or substances the body comes into direct contact withExample: Pt on cold OR tableMetal and stones are good heat conductors, which is why they feel cold to touch, even at room temp.Air conducts heat poorly, which is why still air is an excellent insulator.
Term
 Physiological Control of Temp
Definition
 is mediated by temperature receptors in the skin and hypothalamus (receptors in hypothalamus are sensitive to blood temp perfusing it)Both signal to the thermoregulatory center in the hypothalamus
Term
 Thermoregulatory Center in Hypothalamus
Definition
 Mild changes in environmental temps cause reflexive changes of vascular tone, these alter skin temp and so alter heat loss.  If major temp changes, reflexes elicit shivering. If hot, reflexes elicit sweating and vasodilatation.  Thermoregulatory center signals the cerebral cortex to alter behavior (i.e., change clothing or activity level)
Term
 Hypothermia in the OR
Definition
 Occurs in OR by: * Anesthesia depresses the thermoregulatory center * Many anesthetics increase peripheral vasodilatation * More skin exposed * Rapid ambient air movement in theater * IV fluid infusion
Term
 Measures to minimize risks for hypothermia
Definition
 * Raise ambient temp * Use forced air warmer * Heating and humidifying inspired gases * Warming infused fluids
Term
 Induced hypothermia for cardiac cases
Definition
 * Decreases metabolism and oxygen tissue demand, so that blood flow to vital areas may be interrupted for a time without the risk of hypoxic damage * At 30 degrees C, the time for safe circulatory occlusion is doubled * Less than 30 degrees is avoided to prevent Vfib * Low temps sometime used on extracorporeal circulation
Term
 42 degrees C
Definition
 cerebral impairment & disturbance of the physiological control mechanisms--lose sweating capabilities
Term
 44 degrees C
Definition
 death by irreversible protein "denaturation", or unfolding (once their shape changes, they cease to function properly.).  Viscous cycle of increased temp causing increased metabolic rate, which increases the temperature…  Heat production rises exponentially with temperature, while heat loss rises at a slower rate.  Heat production > heat loss = death
Term
 Malignant hyperthermia
Definition
 abnormal genetic response to volatile gases, causes massive increase in heat production, fatal if not corrected
Term
Definition
 Changes temp as well as physcial properties* Substances can expand*Electrical resistance may change
Term
 Response time of probes using electrical techniqes
Definition
 Based on size * Small probes – smaller heat capacity therefore shorter response time * Faster response time are found in thermistors used for cardiac output measurements( thermal dilution technique)
Term
 TYMPANIC MEMBRANE THERMOMETERS
Definition
 Based on fact that objects emit electromagnetic radiation over a range of wavelengthsObjects at body temperature primarily emit infrared radiation Sensor which converts radiation into an electrical signalTympanic membrane temp - Represents core temp
Term
 CLINICAL APPLICATIONS/CHOICES OF SITES FOR MERCURY THERMOMETER
Definition
 ◦Limited by rigidity ◦Mouth ◦Axilla ◦Rectal –Infants – risks of perforation –Adults – slow to equilibrate with core temp
Term
 CHOICE OF SITES WITH ELECTRICAL THERMOMETERS
Definition
 ◦Nose Fluctuates due to inspired and expired air Can be used with an anesthetized patient with an ETT --No respiratory air movement to cool nasal cavity ◦Ear Risk of perforation◦Esophagususe lower 1/3cool distal tracheal temp will lower temp◦SkinNot a true indication of core temp
Term
 HEAT PRODUCTION
Definition
 Determined by metabolic rate --Increased with food consumption, exercise , and ↑ body temp--Measured in Watts No mechanism to compensate for overheating Shivering ↑ heat production --Can more than double heat production --Occurs after body’s mechanism for control of heat loss have been activated
Term
 HEAT LOSS
Definition
 Heat is generated in the muscles by metabolic chemical reactions, mainly in the liver Some heat is lost through the lungs, although 90-95% is lost through the skin Heat is transferred from the core to the skin by blood passing through peripheral blood vesselsRate of heat loss is determined by extent of blood vessel dilationFully dilated they will allow blood to travel 100 times faster than when constricted, thus losing body heat fasterHeat loss rates are also greatly increased by sweating, especially in dry environments
Term
 HOW THE BODY CONTROLS HEAT LOSS
Definition
 Vasoconstriction Allows a cooler, outer 'shell' to form an insulating barrier that slows heat loss from the body's core Hands and feet have fewer large blood vessels When the flow of blood is restricted it is harder for the blood to keep flowing to these areas which quickly become cold }
Term
 HEAT TRANSFER FROM THE SKIN TO ENVIRONMENT
Definition
 In comfortable environments 65% is lost through radiation--the rest through evaporation In cold environments, most heat lost is via convection and conduction.
Term
 TWO WAYS HEAT LOSS OCCURS IN EVAPORATION
Definition
 SENSIBLE/ACTIVE PERSPIRATION--caused by the formation of liquid sweat droplets at the skin surface in response to excess heat.Sensible perspiration is most significant during periods of activity.INSENSIBLE/PASSIVE PERSPIRATION--direct emission of sweat vapor from the skin in response to humidity gradient (i.e., your skin is drying out).Insensible perspiration most significant at rest or while sleeping.Example: Open abdomen with exposed bowel.
Term
 TEMPERATURE CHANGES IN THE OR
Definition
 ANESTHESIA--* Depresses thermoregulatory center *Effects vasomotor tone Vasodilation = increased heat lossDURING SURGERY* More body surface exposed* Constant air exchanges* Cold OR table* Cold IV fluids* Cold OR
Term
 HYPOTHERMIA
Definition
 Risk is higher for longer surgery
Term
 TERMAL BURNS
Definition
 More likely seen with pts who are in shock or vasoconstricted  Can occur with * Heat lamps * Forced warm air blankets * Electric blankets * Heated water filled mattress
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