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

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Cards

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
Advantages of Electrical Thermometers
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
RADIATION
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 with
Example: Pt on cold OR table
Metal 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
Effects of adding Heat
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 wavelengths
Objects at body temperature primarily emit infrared radiation
Sensor which converts radiation into an electrical signal
Tympanic 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
Esophagus
use lower 1/3
cool distal tracheal temp will lower temp
Skin
Not 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 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

 

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

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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 loss
DURING 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