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

Quantifies physical changes that occur in substances when heat energy is added

Scales have arbitrarily fixed points

(ice, water, and water vapor in equilibrium)

= 0.01 degrees C

Uses Kelvin

K = C + 273.15

no degree symbol for Kelvin

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

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)

1. Resistance Thermometers

2. Thermistor

3. Thermocouple

Response time 0.1 to 15 seconds.

Small heat probes have smaller heat capacity (Chapter 10), therefore a quick response time.

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

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)

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.

(1) Pyroelectric --Employs shutter mechanism to provide a period of exposure.  A snap shot.

(2) Thermopile uses thermocouples—allows for continuous readings

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

temperature of brain, thoracic, abdominal organs, deep tissues of limbs

Depends on balance between heat production in core VS heat loss at surface layer

Causes changes in enzyme reaction rates.

Metabolic process cannot occur when cooled.

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)

Determined by the metabolic rate.

Measured in watts.

50 W m-2 body surface

or

80 W total

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

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

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

Rate depends upon:

* water pressure gradient from skin to air, 

* amount of skin exposed to environment, and

* skin wetness (10x greater during sweating)

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.

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)

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

* Raise ambient temp

* Use forced air warmer

* Heating and humidifying inspired gases

* Warming infused fluids

* 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

cerebral impairment & disturbance of the physiological control mechanisms--

lose sweating capabilities

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

Changes temp as well as physcial properties

* Substances can expand

*Electrical resistance may change

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 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

In comfortable environments 65% is lost through radiation--the rest through evaporation

In cold environments, most heat lost is via convection and conduction.

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