Physiological Properties of Thermal Modalities (1)

Heat is a form of energy which is interchangeable with other forms of energy such as electrical or mechanical energy

When the body is heated rise in temperature due to the increased energy of motion of molecules in the body

Physiological Properties of Thermal Modalities

Heat and Temperature

Various forms of energy converted into heat. There is a constant ratio between the energy

lost and the heat produced

The First Law of Thermodynamics In all processes occurring in an isolated

system, the energy of the system remains constant

Or simply, energy is neither created nor destroyed

Electrical, chemical and magnetic energy can be converted into heat

One form of energy converted into another form and some energy is always converted into heat

Heat absorbed by small area more than by big area

Temperature is the expression of all status of matter, solid, liquid and gas

Temperature measured by Celsius (C) and Fahrenheit scales

0°C = 32°F

100°C = 212°F

Heat unit is Joules (J)

Specific Heat

Specific heat is the amount of heat energy required to raise a unit mass of material by 1°C

S.H. of water = 4.185 J/g/C° S.H. of water = 1.01 J/g/C° S.H. of skin > muscle > fat > bone

Physical Effects of Heat

When heat added to matter it increases the kinetic energy of its microstructure

1. When rising the temperature, the average kinetic energy of molecules increases

2. Expansion of the material Kinetic energy vibration of molecules

expand the material Gas > liquid > solid

Physical Effects of Heat

3. Change in physical state: changing substance from one physical state to another requires a specific amount of heat energy

Latent heat: the energy required to convert 1g of water at 100°C to 1g of steam at 100°C

Latent heat of water [2268J]

4. Production of an electrical potential difference

5. Acceleration of chemical reactions

Physical Effects of Heat

Van Hoff’s Law

Any chemical reaction capable of being accelerated is accelerated by rise of temperature

6. Production of electromagnetic waves

7. Thermionic emission

8. Reduction in viscosity of fluids

Methods of Heat Transfer

1. Conduction

2. Convection

3. Conversion

4. Radiation

5. Evaporation

Conduction

Heat transfer by direct contact (e.g. hot pack, cold pack)

Heat conducted from high temperature material to low temperature material

Guidelines of heat transfer by conduction

1. The greater the temperature difference between the heating or cooling agents and the body part it is applied to, the faster the rate of heat transfer

Guidelines of heat transfer by conduction

2. Materials with high thermal conductivity transfer heat more rapidly than those with low thermal conductivity

Thermal conductivity: is the rate at which a material transfers heat by conduction

Metal > water > air

3. The larger the area of contact between a thermal agent and the patient, the greater the total heat transfer

4. The rate of temperature rise decreases in proportion to tissue thickness

Convection

Heat transfer by circulation of a medium of a different temperature (e.g. fluidotherapy. Whirlpool)

Due to direct contact between a circulating medium and another material of a different temperature

Heat transfer more than if by conduction

Blood transfers heat to reduce local change in tissue temperature reduce tissue damage

Conversion

Conversion from one type of energy to another (e.g. ultrasound, diathermy, metabolism)

Conversion of a non thermal form of energy into heat

Not affected by the temperature of thermal agent Rate of heat transfer depends on the power of

energy source Not require direct contact. Require good transmitter

Radiation

Exchange directly without an intervening medium or contact

Rate of temperature increase depends on intensity of radiation, size of radiation source, treated are, distance and angle of radiation

Evaporation

Absorption of energy as a result of conversion of a material from a liquid to vapor state (e.g. vapocoolant spray, sweating)

Body Heat Transfer Heat exchanged by conductive processes between body

surface and the environment

The body core temperature is constant

Equilibrium is maintained between internal heat production and heat loss or gain at skin surface

Heat transfer within tissue is by conduction and convection

Heating modalities are subdivided according to their primary mode of heat

In Thermotherapy The properties concerned with heat conduction are

thermal conductivity, tissue density and specific heat Convection involves these properties in addition to fluid

viscosity

Thermal homeostasis:

homeothermy is the pattern of temperature in which cyclic variation in deep body teperature (core) is maintained within the limits of -2/+2°C

Hyperthermia +2 °C 39 °C Hypothermia -2 °C 37 °C

Body Temperature

It consists of two compartments:

1. The core (central compartment)

Controlled by physiological mechanisms at a constant level, it is around 37 °C, more in organs.

2. The shell or superficial layer

In subject to much greater variations in temperature Varies according to the core temperatue and external

environment

Body Temperature Measurement

It is measured by different types of thermometers

Thermal balance

Core temperature is constant, there is equilibrium between internal heat production and external heat loss

Metabolic Heat Production (M)

Maximum values of heat production occur during sever physical work

Heat production can increase at rest in cold conditions by involuntary muscle contractions that produce shivering

Control of body temperature

Thermoregulation integrated by a controlling in the CNS respond to heat content of tissues signaled by thermoreceptors

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