© 2004 Thermal Modalities General Principles

© 2004 Physical Laws Cosine Law Inverse Square Law Arndth-Schultz Principle Law of Grotthus-Draper

© 2004 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

© 2004 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.

© 2004 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.

© 2004 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

© 2004 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.

© 2004 General Physiology

© 2004 Metabolic Changes Heat increases metabolism Cold decreases metabolism A 1.8ºF (1ºC) change in tissue temperature = 13% change in metabolism

© 2004 Tissue Properties Deeper tissues have higher temperatures Different tissues have different conductivity properties Thermal Conductivity Skin c 0.96 Adipose Tissue i 0.19 Muscle c 0.64 c – conductor i - insulator

© 2004 Thermoreceptors Cold-responsive receptors Heat-responsive receptors More cold receptors than heat receptors

© 2004 Physics

© 2004 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

© 2004 Transfer of Thermal Energy Conduction Convection Radiation Evaporation Conversion

© 2004 Conduction Objects are touching each other Conductors –Skin –Muscle Insulators –Adipose tissue –Terrycloth towels

© 2004 Convection Involves the circulation of air or water One object is cooled Another object is heated Example: –Whirlpool

© 2004 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

© 2004 Evaporation Change from liquid to gaseous state Draws heat from the body Cools superficial tissues Examples: –Sweating –Vapocoolant sprays

© 2004 Conversion Change of one form of energy to another Electromagnetic energy to heat Acoustical energy to heat Examples: –Short wave diathermy –Ultrasound