HEAT – THERMAL ENERGY
PHT 221 – SECTION - 1099
Lecture Outline
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
This lecture deals about the topic of heat in following sub-categories;

1. Basics of heat & Modes of heat transfer

2. Physiological effects of heat

3. Therapeutic effects of heat
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Lecture Objective
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
At the end of this lecture, the students will be able to;

1. Differentiate between the modes of thermal transfer.


2. List down the different physiological changes which occurs as a
result of application of heating agent to the patient.
3. Explain the different therapeutic effects of thermal application.
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HEAT - INTRODUCTION
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Every living organism produces heat.


In man’s natural environment the predominant form of
radiation is THERMAL.
Heat is continually produced in the body as a byproduct of METABOLISM.
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BALANCE OF HEAT
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HEAT PRODUCTION
1. Basal metabolism
2.
Muscular activity
3.
Food intake
4.
Temperature effect on cells
HEAT LOSS
1. Radiation
 2. Conduction
 3. Evaporation of sweat,
Respiration
4. Urination & Defecation

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Physical Effects of Heat
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1. EXPANSION: It is the result of increased kinetic energy producing
a greater vibration of molecules, which thus move further apart.
2. CHANGE OF STATE: One form of state (Solid) to the other form of
state (Liquid)
3. ACCELERATION OF CHEMICAL ACTION: Van’t Hoff’s Law states
that “Any chemical action capable of being accelerated is accelerated
by a rise in temperature”.
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Physical Effects of Heat
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4. THERMOCOUPLE PRINCIPLE: If the junction of two dissimilar
metals is heated, a potential difference is produced between their
free ends.
5. PRODUCTION OF ELECTROMAGNETIC WAVES: If energy is added
to an atom by heat, this can cause electron to move out to a
HIGHER-ENERGY electron shell. It is then said to be in a EXCITED
STATE. When the electron returns to its normal level, energy is
released as a PULSE of electromagnetic energy (A PHOTON).
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Physical Effects of Heat
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6. THERMIONIC EMISSION: The heating of molecules of some
materials (e.g.) TUNGSTEN may cause such molecular AGITATION
that some electrons leave their atoms.
7. REDUCED VISCOSITY OF FLUIDS: The molecules in viscous fluids
are fairly strongly attracted to one another. Heating increases the
KINETIC MOVEMENT of these molecules & reduces their
COHESIVE MUTUAL ATTRACTION; this makes the fluid less
viscous.
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THERAPEUTIC HEAT 9


In physical therapy, Several THERMAL AGENTS are available for
heat application to tissues.
Types of therapeutic heating agents are classified as follows;
SUPERFICIAL HEATING
DEEP HEATING AGENT AGENTS
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1. SUPERFICIAL HEATING AGENTS:
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Heat from outside or external source transferred to skin by
conduction, convection or radiation – DOES NOT PASS THROUGH
the THERMAL BARRIER.
Note:Adipose tissue acts as insulation (THERMAL BARRIER) to the
underlying tissues, thus limiting the degree of temperature
change.
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EXAMPLES OF SUPERFICIAL HEATING AGENTS/
MODALITIES PRODUCING HEAT
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1.
Hot water bottle
6.
Wax Bath
2.
Hot packs
7.
Electric heat pad
Hydro collator unit
8.
Fluido therapy
4.
Hot water bath
9.
Hydrotherapy
5.
Hot mud's
3.
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I.R.R.
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2. DEEP HEATING AGENTS
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
One form of energy is converted to heat in the tissues – can PASS
THE THERMAL BARRIER.
EXAMPLES OF DEEP HEATING AGENTS ARE:
1.
Ultrasound therapy (U.S.T)
2.
Shortwave Diathermy (S.W.D)
3.
Microwave Diathermy (M.W.D)

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MECHANISM/MODES OF HEAT EXCHANGE
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
1.
2.
3.
4.
5.
The means by which therapeutic heat is delivered to the target
tissues is attributed to the following physical mechanisms;
CONDUCTION
CONVECTION
RADIATION
CONVERSION
EVAPORATION
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1. CONDUCTION
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Heat gain or loss through DIRECT CONTACT between materials with
DIFFERENT TEMPERATURE is called conduction.
E.g. Heat is absorbed by the body tissues when using a HEATING
PAD – heat exchange by conduction.
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2. CONVECTION:15

It is defined as the transference of heat to a body by the
MOVEMENT OF AIR, MATTER OR LIQUID around or past the body.
The heat is carried by the particles themselves moving
Convection currents
e.g. Warm or cool whirlpool in which movement of the water
around a body part results in a temperature change.
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3. RADIATION:16




Transfer of heat directly form the source to the object by a wave, travelling
as rays. It transfers heat (usually through air) from a warmer source to a
cooler source.
Heat radiation is also known as INFRARED RADIATION.
All objects that are hotter than their surroundings give out heat as infrared radiation
e.g. Infra red radiation lamp (I.R.R.)
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4. CONVERSION:17
It refers to the temperature change that results when energy is
transformed from one form to another such as the conversion from
MECHANICAL or ELECTRICAL ENERGY to THERMAL ENERGY.
e.g. Ultrasound Therapy (U.S.T.)
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5. EVAPORATION:18
It is defined as the transformation from a liquid state to a gas state.
Heat is given off when liquids transform to gases.
e.g. Sweating results from heat production within the body.
Cooling occurs as the perspiration evaporates from the surface of
the skin.
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Physiological
effect of heat
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Blood vessels
effects
Metabolic effect
Neuromuscular
effects
Tissue effects
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local body tissue heating
complicated set of physiological changes
complex responses.


Heating is MILD when less than 40˚C.
VIGOROUS heating occurs when tissue temperature reaches 40˚ - 45˚C
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Vascular effects on Heat Application
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

At these temperatures HYPEREMIA or READNESS is noted which indicates an INCREASE BLOOD FLOW.
Vasodilatation occurs not only to distribute the additional heat around the body – compensatory heat loss,
but also to protect the heated skin.
Temperature increase greater than 45˚C may potentially result in THERMAL(HEAT) PAIN & IRREVESIBLE
TISSUE DAMAGE – BURNS.
1. HYPEREMIA or READNESS indicates

INCREASE BLOOD FLOW.
2. Temperature increase > 45˚C


THERMAL(HEAT) PAIN &
IRREVESIBLE TISSUE DAMAGE – BURNS.
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1. Metabolism
series of chemical reactions, will INCREASE with a
RISE of temperature & DECREASE with a FALL of temperature.
(Van’t Hoff’s Law)
2. Increase in temperature

LIVING TISSUES

DENATURE PROTEINS
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3.
Temperatures > 45˚C
protein damage occurs
destruction of cells & tissues.
4.



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With appropriate rise in temperature
all CELL ACTIVITY INCREASE, INCLUDING CELL MOTILITY &
the synthesis & release of CHEMICAL MEDIATORS.
The rate of cellular interactions such as PHAGOCYTOSIS or GROWTH is ACCELERATED.
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The resistance to flow in a blood vessel depends directly on the
viscosity of the fluid & inversely on the 4th power of the radius of
the vessels.


It is temperature dependent, so raising the temperature in the
liquids lowers the viscosity.
Viscosity changes affect not only the fluids in narrow vessels
(Blood & Lymph), but also fluid movement with in & throughout
the tissue spaces.
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1.
2.
It is the most abundant protein in mammals
the whole-body protein content.
making up about 25% to 35% of
Collagen constitutes
1% to 2% of muscle tissue, and accounts for 6% of the
weight of strong, tendinous muscles.
3.
At normal tissue temperatures, collagen primarily exhibits
only minimal viscous flow. E.g. Cooking of meat.
4.
At temperatures within the range (40 – 45˚C), the extensibility of collagen tissue INCREASE.
5.
Collagen melts at temperature above 50˚C.
PHT 221 – SECTION - 1099
elastic properties &
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Heat
stimulate the sensory receptor of the skin
heat regulating centers
control of body temperatures
ANALGESIC EFFECT
GATE CONTROL MECHANISM.
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Heating the SECONDARY afferent NERVES
i.e. MUSCLE SPINDLE NERVE ENDINGS & GOLGI TENDON ENDINGS
MUSCLE SPASM DECREASE.
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1.
2.
3.
4.
5.
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Increasing sweating - owing to the stimulation of ANTERIOR
HYPOTHALAMUS.
Increased PULSE RATE.
Lowering of BLOOD PRESSURE due to decrease in sodium
concentration, loss of urea & other nitrogenous substance.
Increased rate of BREATHING.
Increased elimination through KIDNEYS.
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It is evident that any condition in which increased metabolic rate, cell
activity & local blood flow were beneficial could be appropriately
treated by MILD HEATING.
Note:The application of heat to INFLAMMATORY injuries on the EARLY
STAGES is NOT BENEFICIAL.
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

Chronic inflammatory stages, the stages of repair & regeneration
are all appropriately treated with MILD HEATING.
All forms of therapeutic heating are applied to a wide range of
CHRONIC & POST-TRAUMATIC CONDITIONS including the
ARTHROSES, SOFT TISSUE LESIONS & POST SURGICAL HEATING.
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Therapeutic heat is widely used for the RELIEF OF PAIN. It was
found that heat is the most effective NON-ANALGESIC method of
pain control.

Much therapeutic heating is of the skin. It is therefore reasonable
to assume that the major pain relieving effects are mainly
REFLEX as far as SUBCUTANEOUS structures are concerned.
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

Stimulation of SENSORY HEAT RECEPTORS may activate PAIN
GATE MECHANISM.
Vascular changes could also decrease LOCAL PAIN.(The
increased blood flow that has been observed could WASH OUT
some of the pain provoking metabolites resulting from tissue
injury.) (PROSTAGLANDINS & BRADYKININ)
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It has been suggested that heating the secondary afferent muscle
spindle nerve endings & Golgi tendon organs could be a way in
which an INHIBITORY INFLUENCE is applied to the MOTOR
NEURON POOL to DIMNISH MUSCLE EXCITATION.

The pain & muscle spasm are interdependent – A reduction in one
will cause a reduction in the other.
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During & after heat treatments patients have been found to sleep
more readily.


This might be simply a consequence of pain relief.
This sedative effect of superficial heat could be a REFLEX
PHENOMENON.
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(A)
The analgesic effect of heat allows greater tolerance of stretching.
(B)The viscosity of tissues will be decreased which partly account for
the REDUCTION OF JOINT STIFFNESS that occurs with heating.
©Increased COLLAGEN EXTENSIBILITY occurs at higher temperatures.
Note: Heat is used PRIOR to passive stretching & exercise to increase
joint movement or lengthen scars or contractures.
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Heat applied to areas of skin subjected to prolonged pressure or
friction has been suggested in order to promote a greater BLOOD
FLOW in the skin & thus decrease the risk of skin breakdown.
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Heat has been recommended for the treatment of CHRONIC
OEDEMA of the hand & foot.

This must be given with the part in ELEVATION since the
application of superficial heating will tend to increase oedema if
the part is dependent.
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

Vessel dilatation induced by heating will allow increased rates of
fluid exchange & thus may help to increase the reabsorption of
exudates.
To be successful, the modality used would need to encompass the
whole swollen region. Such heating arrangements coupled with
active ex’s are valuable in the treatment of hand injuries in
general.
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Fungal infections which are difficult to control & thrive in moist
conditions are sometimes treated with regular I.R.R. therapy.
(PARONYCHIA)
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1.
Due to heat there is Increased capillary permeability & Increased
capillary dilatation causes – REDNESS + OEDEMA.
2. The skin temperatures over 45˚C causes tissue damage. Further
rise in temperature will lead to DENATURATION & DEATH OF
CELLS & TISSUES.
3. The dosage of heat treatment can only be guided by the FEELING
OF WARMTH on the part of the patient.
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a)
MINIMAL WARMTH:- Threshold value, Gentle comforting warmth.
b)
MEDIUM WARMTH:- Distinct feeling of agreeable warmth.
c)
d)
MAXIMUM WARMTH:- Intense feeling of heat, maximum heat
tolerance is exercised.
DANGER LEVEL:- Intolerable heat, Burning sensation.
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1.
2.
3.
4.
Apply 2 test tubes of water at 40 – 45˚C & 15 -20˚C randomly to
the treatment area.
Ask the patient to identify which is hot & cold with the eyes
closed.
The patient should actually discriminate between only a few
degrees of Celsius.
Temperature over 45˚C or much below 15˚C should not be used.
(Because they may test PAIN instead of THERMAL SENSATION)
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