Thermal Modalities

General Principles

Physical Laws

Cosine Law

Inverse Square Law

Arndth-Schultz Principle

Law of Grotthus-Draper

Cosine Law

• Angle of incidence: The angle at which radiant energy strikes the body.

• As the angle of incidence changes from 90º, the less effective the transmission.

• Based on the cosine of the angle of incidence:– Effective energy = Energy * Cosine (angle)

• Radiant energy should be ±90º

Inverse Square Law

• Intensity of radiant energy depends on the distance between the source and the target.

• Changing the distance changes the intensity

• Change is proportional to the square of the distance.

Inverse Square Law

• Formula:E = Es/D2

E – energy received by the tissue

Es – energy produced by the source

D2 – Square of the distance between the target and the source

• Doubling the distance between the tissues and the target decreases the intensity by a factor of four.

Arndth-Schultz Principle

• Energy must be absorbed by the tissues

• Must be sufficient to stimulate a physiological response– Too little stimulus: no effect– Too much stimulus: injury

Grotthus-Draper

• Inverse relationship between absorption and penetration of energy.

• Energy absorbed by one tissue layer is not passed along to deeper layers.

• The more energy absorbed in superficial layers, the less available for deeper layers.

General Physiology

Metabolic Changes

• Heat increases metabolism

• Cold decreases metabolism

• A 1.8ºF (1ºC) change in tissue temperature = 13% change in metabolism

Tissue Properties

• Deeper tissues have higher temperatures• Different tissues have different

conductivity properties

Thermal Conductivity

• Skinc 0.96• Adipose Tissuei 0.19• Musclec 0.64

c – conductori - insulator

Thermoreceptors

• Cold-responsive receptors

• Heat-responsive receptors

• More cold receptors than heat receptors

Physics

General Principles

• Exchange of kinetic energy (heat)• Transfer of energy is based on a gradient

between two points• The greater the gradient, the more energy

that is transferred• Energy always moves from a high

concentration to a low concentration– Moist heat pack to the skin– Skin to an ice pack

Transfer of Thermal Energy

ConductionConvectionRadiation

EvaporationConversion

Conduction

• Objects are touching each other

• Conductors– Skin– Muscle

• Insulators– Adipose tissue– Terrycloth towels

Convection

• Involves the circulation of air or water

• One object is cooled

• Another object is heated

• Example:– Whirlpool

Radiation

• No medium is required

• Examples:– LASER– Infrared light– Ultraviolet light

• Thermal modalities provide radiant energy– But is not the primary form of heat exchange

Evaporation

• Change from liquid to gaseous state

• Draws heat from the body

• Cools superficial tissues

• Examples:– Sweating– Vapocoolant sprays

Conversion

• Change of one form of energy to another

• Electromagnetic energy to heat

• Acoustical energy to heat

• Examples:– Short wave diathermy– Ultrasound