Basic Physics in Anesthesia Flashcards

Terms Definitions
The Concept of Temperature and Heat
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
Temperature Scale
Quantifies physical changes that occur in substances when heat energy is added
 
Scales have arbitrarily fixed points
Triple point of water
(ice, water, and water vapor in equilibrium)
 
= 0.01 degrees C
SI system for temperature measurement
Uses Kelvin
 
K = C + 273.15
 
no degree symbol for Kelvin
Dial thermometers
Bimetallic strip and Bourdon guage
Mercury Thermometer
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
Alcohol Thermometers
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)
Three types of Electrical Thermometers
1. Resistance Thermometers
 
2. Thermistor
 
3. Thermocouple
Advantages of Electrical Thermometers
Response time 0.1 to 15 seconds.
 
Small heat probes have smaller heat capacity (Chapter 10), therefore a quick response time.
Resistance Thermometer
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
Thermistor
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)
Thermocouple
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.
TWO TYPES OF SENSORS
 
(1) Pyroelectric --Employs shutter mechanism to provide a period of exposure.  A snap shot.
 
(2) Thermopile uses thermocouples—allows for continuous readings
Thermography
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
Homeothermic
Humans are homeothermic (core, central temp w/i narrow range despite environment. T = 37 +/- 0.5 degrees C)
Core temperature
temperature of brain, thoracic, abdominal organs, deep tissues of limbs
 
Depends on balance between heat production in core VS heat loss at surface layer
Effect of temperature variations
Causes changes in enzyme reaction rates.
 
Metabolic process cannot occur when cooled.
Natural Variations in Body Temp
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)
Measuring Heat Production
Determined by the metabolic rate.
 
Measured in watts.
Average heat produced by a man at rest
50 W m-2 body surface
 
or
 
80 W total
Principles of Heat Loss
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
Fat and Heat Loss
Fat is an avascular insulating layer
RADIATION
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
CONVECTION
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
EVAPORATION
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)
Heat Loss Through Respiration
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.
CONDUCTION
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.
Physiological Control of Temp
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
Thermoregulatory Center in Hypothalamus
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)
Hypothermia in the OR
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
Measures to minimize risks for hypothermia
* Raise ambient temp
* Use forced air warmer
* Heating and humidifying inspired gases
* Warming infused fluids
Induced hypothermia for cardiac cases
* 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
42 degrees C
cerebral impairment & disturbance of the physiological control mechanisms--
lose sweating capabilities
44 degrees C
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
Malignant hyperthermia
abnormal genetic response to volatile gases, causes massive increase in heat production, fatal if not corrected
Effects of adding Heat
Changes temp as well as physcial properties
* Substances can expand
*Electrical resistance may change
Response time of probes using electrical techniqes
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)
TYMPANIC MEMBRANE THERMOMETERS
Based on fact that objects emit electromagnetic radiation over a range of wavelengths
Objects at body temperature primarily emit infrared radiation Sensor which converts radiation into an electrical signal

Tympanic membrane temp - Represents core temp
CLINICAL APPLICATIONS/CHOICES OF SITES FOR MERCURY THERMOMETER
◦Limited by rigidity

◦Mouth

◦Axilla

◦Rectal

–Infants – risks of perforation

–Adults – slow to equilibrate with core temp
CHOICE OF SITES WITH ELECTRICAL THERMOMETERS
◦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



◦Esophagus

use lower 1/3
cool distal tracheal temp will lower temp


◦Skin

Not a true indication of core temp
 
HEAT PRODUCTION
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
HEAT LOSS
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 vessels
Rate of heat loss is determined by extent of blood vessel dilation
Fully dilated they will allow blood to travel 100 times faster than when constricted, thus losing body heat faster
Heat loss rates are also greatly increased by sweating, especially in dry environments



 
HOW THE BODY CONTROLS HEAT LOSS
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
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HEAT TRANSFER FROM THE SKIN TO ENVIRONMENT
In comfortable environments 65% is lost through radiation--the rest through evaporation
 
In cold environments, most heat lost is via convection and conduction.
TWO WAYS HEAT LOSS OCCURS IN EVAPORATION
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.
TEMPERATURE CHANGES IN THE OR
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
HYPOTHERMIA
Risk is higher for longer surgery
TERMAL BURNS
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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