Physiological & Therapeutic Effects of heat therapy for UG physiotherapy students.

1. THERPEUTIC HEAT
PHYSIOLOGICAL EFFECTS
DR. SREERAJ S R, PHD.

2. Sreeraj S R 2
Therapeutic Heat
Superficial Heat
Convective agents Conductive agents Conversive agents
Fluidotherapy Hydrotherapy
Hot packs Heating pads Paraffin wax bath
IR/UVR/LASER
Deep heat
Ultrasound SWD MWD
Contrast Bath

3. Sreeraj S R
Therapeutic Heat
• The greatest temperature elevation with superficial heating
modalities occurs in the skin and the subcutaneous tissues within 0.5
to 2 cm of the skin surface.
• Deep heating agents, such as continuous TUS and continuous SWD,
can increase the temperature of tissues at depths of 3 to 5 cm.
• Muscle temperature will require a longer duration of exposure of 15
to 30 minutes to reach peak values.
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4. Sreeraj S R
Therapeutic Heat
• Degrees of heat sensation can be categorized as follows;
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MINIMAL WARMTH Threshold value, Gentle comforting warmth.
MEDIUM WARMTH Distinct feeling of agreeable warmth.
MAXIMUM WARMTH Intense feeling of heat, close to tolerance.
DANGER LEVEL Intolerable heat, Burning sensation.

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Therapeutic Heat
• Therapeutic levels of heating are categorized as
1.Mild, less than 40°C and
2.Vigorous 40°C to 45°C
• Hyperemia or Erythema or redness of the skin is noted, caused by an
increase in the blood flow in the capillaries in the lower layers of the
skin.
• Temperature greater than 45° results in thermal pain and irreversible
tissue damage.
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6. Sreeraj S R
Physiological Effects of Heat
• Physiological changes depend on several factors:
1. Extent of the temperature increase
2. Rate at which energy is being added to the tissue
3. Volume of tissue exposed
4. Composition of the absorbing tissue
5. Capacity of the tissue to dissipate heat (largely a factor of blood
supply)
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Physiological Effects of Heat
• Hemodynamic effect
• Neuromuscular effect
• Metabolic effect
• Connective Tissue Effects / Tissue extensibility
• Pulse Rate & Blood Pressure
• Rate of Breathing
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Hemodynamic effect
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↑ Temparature
Inflammation
↑ Vasodilator
(Histamine &
Prostaglandin)
release
Vasodilatiation
Cutaneous
thermoreceptors
Spinal cord dorsal
root ganglion
Smooth muscle
relaxation
Direct
Reflex
Activation
Indirect
Activation

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Hemodynamic effect:
Inflammation
• The stimulation of cutaneous
thermoreceptors causes the
release of nitrous oxide, causing
relaxation of the smooth muscles
of the vessel walls > vasodilation
at the area.
• Temperature elevation causes
the release of enzyme kallikrein
from sweat glands leads to
increase in vascular permeability.
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10. Sreeraj S R
Hemodynamic effect:
Cutaneous Thermoreceptors
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• The sympathetic innervation of the blood vessels regulates vasomotor
tone.
• These nerve fibers leave the spinal cord through thoracolumbar spinal
nerves.
• Cutaneous thermoreceptors also synapse with sympathetic neurons in the
lateral gray horn of the thoracolumbar segments in the spinal cord.
• When there is local heating, the firing of cutaneous thermoreceptors inhibits
this firing and thus decreasing sympathetic output.
• This reduces vasomotor tone, resulting in vasodilation at the site of heat

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Neuromuscular Effects:
Pain
• The increased firing rate of thermoreceptors in cutaneous tissue
may block input from the primary nociceptive afferent to the
dorsal horn (the “thermal gate theory”).
• Heat is used therapeutically to provide analgesia and to help
resolve pain and muscle-guarding spasms.
• Heat can cause a conduction block on demyelinated peripheral
nerves because it can shorten the duration of sodium channel
opening during neuronal depolarization.
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Neuromuscular Effects:
Spasm
• Muscle-guarding spasms can result from the injury and overuse
of a muscle.
• Pain can be a trigger beginning the pain-spasm-pain cycle.
• Elevating muscle temperature to 42°C has been shown to
1.Decreasing the firing rate of muscle spindles (alpha and
gamma motor neurons) but
2. Increase the firing rate from GTOs.
• These changes in nerve firing rates are thought to contribute to a
reduction in muscle spasm.
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13. Sreeraj S R
Neuromuscular Effects:
Muscle Strength
• Muscle strength and endurance have been found to decrease
during the initial 30 minutes after applying either deep or
superficial heating agents.
• This is because of heat on
1.Decreasing the firing rate of muscle spindles (alpha and
gamma motor neurons) but
2. Increasing the firing rate from GTOs.
• Beyond 30 minutes and for the next 2 hours after the heat is
applied, muscle strength gradually recovers.
• Heat is not used prior to strengthening or strength measurement.
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14. Sreeraj S R
Tissue Extensibility
• Temperature elevation in combination with a stretch can alter the
viscoelastic properties of connective tissues.
• The effects of heat on connective tissue include:
– Elasticity
– Viscosity
– Joint stiffness
– Muscle flexibility
• A maximum increase in residual length is achieved when the
tissue is maintained at 40°C to 45°C for 5 to 10 minutes.
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15. Sreeraj S R
Metabolic Effects
• Metabolic rate will increase 2 to 3-fold for each 10°C rise in
temperature.
• With every rise in tissue temperature, the oxygen-hemoglobin
dissociation curve shifts to the right, making more oxygen
available for tissue repair.
• The effects of heat on metabolism increases;
1.Enzymatic activity
2.Cellular biochemical reactions
3.Oxygen uptake and speed up healing
• Energy expenditure will increase with increasing temperature.
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Metabolic Effects
• Heat is not advised in arthritic conditions like rheumatoid arthritis,
where heat can increase the activity of collagenase and thus
speed up the destruction of articular cartilage.
• Therefore, thermotherapy should be used with caution in patients
with acute inflammatory disorders.
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Pulse Rate & Blood Pressure
• The blood vessel dilation lowers the blood pressure because the
blood distribution has increased to new areas.
• The body now must compensate by increasing the cardiac
output by increasing the heart rate.
• Because of the change in pressure, less blood is pumped to the
vital organs of the body.
• Lowering of blood pressure also due to decrease in sodium
concentration, loss of urea & other nitrogenous substance due to
increased metabolism because of heating.
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Rate of Breathing
• As the temperature increases, there is an increase in metabolism
and more oxyhemoglobin dissociation.
• Metabolically active cells require more O2.
• Because of these changes in O2 demand, the rate of respiration
too increases to meet the demand.
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Therapeutic Effects
• Encouragement of healing
• Relief of Pain
• Reduction of Muscle Spasm
• Increase in Range of Joint Motion
• Prophylaxis (Prevention) of Pressure Sores
• Reduction of Oedema in Extremities
• Resolution of Some Skin Diseases
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Contraindications
1. Ischemia—e.g., arterial insufficiency
2. Haemorrhage—there is an increased arterial and capillary blood flow with
heat
3. Impaired sensation—e.g., spinal cord injury (SCI) may predispose to burns
4. Inability to communicate or respond to pain—e.g., dementia
5. Malignancy—May increase tumour growth
6. Acute trauma or inflammation—Diffusion across membranes is increased
7. Scar tissue—Elevation of temperature increases the metabolic demand of
the tissue. Scar tissue has inadequate vascular supply, and cannot provide
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Hot packs
• Hydrocollator: canvas bags filled with
silicon dioxide immersed in tanks of
heated water 75° C
• Applied over several layers of
insulating towels
• Heat treatment lasts 10 to 30 minutes
• Advantages: low cost, minimal
maintenance, long life, patient
acceptance, ease of use
• Disadvantages: Prolonged superficial
heat can produce telangiectasia.
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Paraffin wax bath
• Paraffin wax and mineral oil in a
7:1 or 6:1 ratio heated to 40 to
45° C
• Commonly used in irregular
surfaces such as distal
extremities
• Patients have good compliance,
and home units are available
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Hydrotherapy
• External use of water to treat a
physical condition. Water can
produce convective heating or
cooling, massage, and gentle
debridement.
• Unit size, water temperature,
and agitation intensity can be
adjusted to meet treatment goals
• Whirlpool baths for partial body
immersion
• Hubbard tanks used for total
body immersion
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Contrast Bath
• Distal limbs receive alternating
heat and cold in a whirlpool tank
to produce reflex hyperemia.
• Temperatures range from hot
35–45° C and cold 15–20° C
Technique:
• Begin with warm soaks to the
extremity, then follow with four
cycles of alternating 1–4 minute
cold soaks and 4–6 minutes
warm soaks
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35–45° C 15–20° C

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Heating pads
• Available as electric pads and pads
with circulating heated fluid such as
water.
• Peak temperature is 52° C
• The temperature is maintained at a
constant level.
• If used with moist towels there is a
potential risk for electrical shock
• If the patient lies on the pad there is a
potential for burns. This is common in
patients with decreased adipose tissue
• Generally used for periods of 20
minutes
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Fluidotherapy
Hot air is blown through a
container
holding fine cellulose particles
(bed
of beads or corn husks), which
produces a warm air-fluid mixture
with properties similar to liquid
• Advantages: massage action of
the turbulent solid-gas mixture;
freedom to perform ROM
activities
• Good for hands and feet
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