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PHYSICAL PRINCIPLES

THERMAL AGENTS

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Specific Heat

….is the amount of energy required to raise thetemperature of a given weight of a material bya given number of degrees

Latent Heat

…. is the heat energy required to change asubstance from one state to another.

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Mode of Heat Transfer

� Conduction

� Convection

� Conversion/Radiation

� Evaporation

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Conduction

� Heat loss or gain throughdirect contact betweenmaterials with differenttemperatures

Guidelines for heat transfer by

conduction

� Greater the temperaturedifference faster the heattransfer

� Materials with high thermalconductivity

� Larger area of contact� Rate of temperature rise

decreases in proportion to tissuethickness

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Convection

�Transference of heat to a body by the movement of air, matter, or liquid around or past the body

�During heating or cooling by convection the thermal agent is in motion, so new parts of the agent at the initial treatment temperature keep coming into

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Radiation

�This is conversion of non thermal form of energy into Heat

�Transfer of energy from a material with high temperature to one with lower temperature without the need for an intervening medium or contact

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Evaporation

� Evaporation is a

cooling process.

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Temperature has an effect on:

� Viscosity

� Nerve conduction—heat increases nerve

conduction velocity; cold decreases it

� Blood flow—heat increases arterial and capillary

blood flow; cold decreases blood flow

� Collagen extensibility—heat increases tendon

extensibility, collagenase activity is increased; cold

decreases enzyme activity

� Temperatures > 45–50°C or < 0°C can injure tissue

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THERMOTHERAPY

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Therapeutic Heat

Superficial Heat

Convective agents Conductive agents Conversive agents

Fluidotherapy Hydrotherapy

Hot packs Heating pads Paraffin wax bath

IR

Deep heat

Ultrasound SWD MWD

Contrast Bath

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Physiological Effects of heat

• Hemodynamic effect

• Neuromuscular effect

• Metabolic effect

• Tissue extensibility

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Physiological Effects of heat

↑ Temperature

Inflammation

↑ Vasodilator

(Histamine &

Prostaglandin)

release

Vasodilatiation

Cutaneous

thermoreceptors

Spinal cord dorsal

root ganglion

Smooth muscle

relaxation

Hemodynamic effect

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Physiological Effects of heat

Neuromuscular effect

• Increased NCV

• Increased pain threshold

• Changes in muscle strength

Metabolic effect

• Increased metabolic rate

Tissue extensibility

• Increased tissue extensibility

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Applications for heat therapy

Generally used for chronic process

• Decrease muscle spasms

• Decrease pain

• Reduction in joint stiffness, contractures

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Contraindications

• Ischemia. e.g., arterial insufficiency

• Haemorrhage. there is an increased arterial and capillary blood flow with heat

• Impaired sensation. e.g., spinal cord injury (SCI) may predispose to burns

• Inability to communicate or respond to

pain. e.g., dementia

• Malignancy. May increase tumour growth

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Contraindications

• Acute trauma or inflammation. Diffusion across membranes is increased

• Scar tissue. Elevation of temperature increases the metabolic demand of the tissue. Scar tissue has inadequate vascular supply, and is not able to provide an adequate vascular response when heated, which can lead to ischemic necrosis.

• Poor thermal regulation