Thermal agents: PHYSICAL PRINCIPLES Introduction to Thermotherapy

1. Sreeraj S R
Dr Sreeraj S R, Ph.D.
THERMAL AGENTS:
PHYSICAL PRINCIPLES

2. Sreeraj S R
States of Matter
o Matter can be solid, liquid or gaseous. E.g., water may exist as ice water
or steam.
o The molecules of a substance are influenced by two forces:
1. Cohesive force, which attracts the molecules of the substance to one
another and a
2. Kinetic force of movement of the molecules
o Kinetic force depends on the thermal energy contained by the mass of
molecules.
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3. Sreeraj S R
Specific Heat
o It is the amount of energy required to raise the temperature of a given
weight of a material by a given number of degrees.
o Materials with high SH require more energy to achieve the same
temperature increase than materials with low SH.
o They hold more energy at a given temperature than materials with low
specific heat.
o SH of Water 4.19 J/g/°C
o SH of Air 1.01J/g/°C
o
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Specific Heat of Various Tissues
Body Tissue Specific Heat in J/g/°C
Skin 3.77
Muscle 3.75
Fat 2.30
Bone 1.59
Water has the second highest
SH capacity of any known chemical
compound.
Source: https://www.physicsforums.com/threads/properties of-air-vs water.293733/

4. Sreeraj S R
Mode of Heat Transfer
o Heat can be transferred by;
1. Conduction
2. Convection
3. Conversion
4. Radiation
5. Evaporation
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5. Sreeraj S R
Conduction
o Heat loss or gain through direct
contact between materials with
different temperatures.
o The thermal conductivity of a
material is a measure of its ability
to conduct heat.
o Hot Packs and Cryotherapy transfer
heat by conduction.
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6. Sreeraj S R
Conduction
o The rate at which heat transfers by conduction between two materials
depends on
1. The temperature difference between the materials,
2. Their thermal conductivity, and
3. Their area of contact.
o Water’s thermal conductivity is higher than air's (0.6 vs 0.025
W/(m.K)Source: https://www.physicsforums.com/threads/properties-of-air-vs-
water.293733/
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7. Sreeraj S R
Conduction
o Thermal Conductivity of Various Tissues
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Body Tissue Thermal Conductivity (W/m-K)
Bone 2.28
Muscle 0.42
Fat 0.16
Skin 0.21
Blood 0.52
Rugh JP, Bharathan D. Predicting Human Thermal Comfort in Automobiles. SAE Technical Papers.
2005;2508–16. DOI: 10.2172/15016823

8. Sreeraj S R
Conduction
o Heat may also transfer to or from a patient by conduction.
o If the physical agent used has a higher temperature than the patient's skin,
heat will transfer from the agent to the patient, and the temperature of
superficial tissues in contact with the heating agent will rise.
o If the physical agent used is colder than the patient’s skin, the opposite will
happen.
o Rate of temperature rise decreases in proportion to tissue thickness.
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9. Sreeraj S R
Convection
o Heat transfer by convection occurs as the
result of direct contact between a circulating
medium and another material of a different
temperature.
o Since the thermal agent is in motion, a
constant temperature is maintained with the
patient's body part.
o Example: Whirlpools and fluidotherapy
transfer heat by convection.
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10. Sreeraj S R
Convection
Heat transfer by blood circulation;
o Vasodilation increases the rate of circulation, increasing the rate at which the tissue
temperature returns to normal.
o The circulation constantly moves the heated blood out of the area and moves cooler
blood into the area to return the local tissue temperature to a normal level.
o This local cooling by convection reduces the impact of superficial heating agents on
the local tissue temperature.
o To protects the tissues by reducing the risk of burning.
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11. Sreeraj S R
Conduction vs Convection
o Heat transfer by stationary
medium
o Transfers less heat
o Heat transfer by circulating
medium
o Transfers more heat
Conduction Convection
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12. Sreeraj S R
Conversion
o Heat transfer by conversion involves
the conversion of a nonthermal form
of energy, such as mechanical,
electrical, or chemical energy, into
heat.
o Example: Diathermy and ultrasound
heat patients by conversion.
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13. Sreeraj S R
Conversion
o When heat is transferred by conversion, the rate of heat transfer depends on
the power of the energy source.
o Heat transfer by conversion does not require direct contact between the
thermal agent and the body;
o But the intervening material should be a good transmitter of that type of
energy.
o For example, transmission gel, lotion, or water must be used between an
ultrasound transducer and the patient.
o Air transmits ultrasound poorly.
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14. Sreeraj S R
Radiation
o This involves the transfer of energy
from a material with a higher
temperature to one with a lower
temperature without an intervening
medium or contact.
o This is conversion of non thermal form
of energy into Heat
o Example: Infrared lamps transfer heat
by radiation.
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15. Sreeraj S R
Radiation
o The rate of temperature rise caused by radiation depends on
1. The intensity of the radiation,
2. The distance of the source from the treatment area, and
3. The angle between the radiation and the tissue.
4. The relative sizes of the radiation source,
5. The area being treated,
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16. Sreeraj S R
Evaporation
o A material must absorb energy to evaporate or
change from a liquid to a gas (or vapor).
o This energy is absorbed as heat derived from
the material itself decreasing its temperature.
o So, evaporation is a cooling process.
o Example: A vapocoolant spray evaporates at
an even lower temperature than water.
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17. Sreeraj S R
Latent Heat
o Latent heat is defined as the heat or
energy that is absorbed or released
during a phase change of a
substance.
o Energy is required to change from
1. solid to liquid ,
2. liquid to gas (evaporation), or
3. solid to gas (sublimation).
o Energy will be released to change
from
1. liquid to solid (solidification),
2. gas to liquid (condensation), or
3. gas to solid.
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• Latent Heat of Fusion
• Latent Heat of Vaporization

18. Sreeraj S R
Temperature influences:
o Viscosity
o Nerve conduction—heat increases nerve conduction velocity; cold
decreases it
o Blood flow—heat increases arterial and capillary blood flow; cold
decreases blood flow
o Collagen extensibility—heat increases tendon extensibility;
collagenase activity is increased; cold decreases enzyme activity
o Temperatures > 45–50°C or < 0°C can injure tissue
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19. Sreeraj S R
Physical Principles of Heat
o The greatest temperature elevation with heating modalities occurs in the skin
and the subcutaneous tissues within 0.5 to 2 cm of the skin surface.
o In areas of adequate blood supply, temperature will increase to a maximum
within 6 to 8 minutes of exposure.
o Muscle temperature will require a longer duration of exposure of 15 to 30
minutes to reach peak values.
o At a depth of 3 cm, using clinically tolerable intensities, muscle temperature
elevation can be expected to be about 1°C or less.
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20. Sreeraj S R
Physiological Effects of Heat
o Therapeutic levels of heating are categorized as
1. Mild, less than 40°C and
2. Vigorous 40°C to 45°C
o At vigorous temperatures, hyperemia or erythema or redness of the skin
is noted, caused by an increase in the blood flow in the capillaries
(hyperemia) in the lower layers of the skin.
o Temperature greater than 45° results in thermal pain and irreversible
tissue damage.
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21. Sreeraj S R
References
1. Cameron MH. Physical Agents in Rehabilitation: An Evidence-Based Approach to Practice. In:
5th ed. St. Louis, Missouri: Elsevier, Inc; 2018. p. 398–411.
2. Bellew JW, Michlovitz SL, Nolan T. Modalities for therapeutic intervention. In: 6th ed.
Philadelphia, Pa: F.A. Davis Company; 2016. p. 21–6.
3. Admin. Latent Heat - Definition, Types, Formula, Fusion and Vaporization [Internet]. BYJUS.
BYJU’S; 2019 [cited 2021 Mar 14]. Available from: https://tinyurl.com/xukpjmsc
4. Heat Transfer [Internet]. Mesonet.org. 2021 [cited 2021 Mar 14]. Available from:
https://tinyurl.com/efy5y62z
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