Physiologic and therapeutic effects of heat

Physiologic and Therapeutic Effects of Heat 1

Sreeraj S R

PHYSIOLOGIC AND THERAPEUTIC EFFECTS OF HEAT

PHYSIOLOGICAL EFFECTS OF HEAT

1. Vasodilatiation

2. Viscosity

3. Increased nerve stimulation

4. Increased pain threshold

5. Changes in muscle strength

6. Increased metabolic rate

7. Increased tissue extensibility/ effects on Collagenous tissue

8. Increased sweating

9. Effect on pulse rate & blood pressure

10. Increased rate of breathing

1. Vasodilatiation

Heat causes vasodilatation and thus an increase in the rate of blood flow. With heating the skin

surface erythema or redness is produced due to Vasodilatation.Vasodilatation occurs not only to

distribute the additional heat around the body i.e. compensatory heat loss, but also to protect the

heated skin. Superficial therapeutic heating agents produce more pronounced vasodilation in the

local cutaneous blood vessels where they cause the greatest change in temperature and less

pronounced dilation in the deeper vessels such as those running through muscles.

Thermotherapy causes vasodilatation by variety of mechanisms such as;

• Direct reflex activation of the smooth muscles of the blood vessels by cutaneous thermo

receptors,

• Indirect activation of local spinal cord reflexes by cutaneous thermo receptors and

• Increasing the local release of chemical mediators of inflammation.

Superficial heating agents stimulate the cutaneous thermo receptors via their axons directly to

nearby cutaneous blood vessels causes the release of bradykinin and nitrous oxide. Bradykinin

and nitrous oxide act as vasoactive mediators stimulating relaxation of the smooth muscles of the

vessel walls to cause vasodilatation. This vasodilatation occurs locally, in the area where the

heat is applied.

Cutaneous thermo receptors project via the dorsal root ganglion to synapse with interneurons in

the dorsal horn of the gray matter of the spinal cord. These interneurons synapse with

sympathetic neurons in the lateral gray horn of the thoracolumbar segments of the spinal cord to

inhibit their firing and thus decrease sympathetic output. This decrease in sympathetic activity

causes a reduction in smooth muscle contraction, resulting in vasodilation both at the site of heat


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application and in the cutaneous vessels of the distal extremities. This distant vasodilative effect

of thermotherapy may be used to increase cutaneous blood flow to an area where it is difficult or

unsafe to apply a heating agent directly.

Increase in blood flow that occurs in response to increased tissue temperature, acts to protect the

body from excessive heating and tissue damage. The increased rate of blood flow increases the

rate at which an area is cooled by convection.

2. Viscosity

The resistance to flow in a blood vessel depends directly on the viscosity of the fluid & inversely

on 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 within & throughout the tissue spaces.

• Viscosity is a measure of the resistance of a fluid. It describes a fluid's internal resistance

to flow.

↑ Temperature

Inflammation

Cutaneous

Thermo receptors

Smooth Muscle

relaxation

Release of ↑Vasodilators

like histamine &

prostaglandin

Spinal Dorsal

Root Ganglia

↓ Sympathetic

adrenergic

activation

Vasodilatation


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• Increasing the temperature in the liquids lowers the viscosity not only the fluids in narrow

vessels (Blood & Lymph, but also fluid movement within & throughout the tissue spaces.

3. Increased nerve stimulation

Increased temperature changes nerve conduction velocity and firing rate i.e decrease the

conduction latency of both sensory and motor nerves. Nerve conduction velocity has been

reported to increase by approximately 2 meters/second for every 1 °C increase in temperature.

Elevation of muscle temperature has been shown to result in a;

• decreased firing rate of type II muscle spindle efferents and gamma efferents

• an increased firing rate of type lb fibers from Golgi tendon organs

• a reduction in the firing rate of alpha motor neurons

• decrease in gamma neuron activity which causes the stretch on the muscle spindles to

decrease

• decreased alpha motor neuron activity

All these leads to relaxation of muscle contraction and thus to a reduction in muscle spasm

4. Increased pain threshold

Heat causes more blood to flow into the area. When blood flow increases to an area, it brings

along oxygen and nutrients that can help to speed healing and relax muscles, which can decrease

pain sensations. The sensation of heat also alters the perception of pain. Heat increases the

activity of the cutaneous thermoreceptors which can have an immediate inhibitory gating effect

on the transmission of the sensation of pain at the spinal cord level.

5. Changes in muscle strength

Muscle strength and endurance have been found to decrease during the initial 30 minutes after

the application of heating agents. It is proposed that this decrease in muscle strength is the result

of decreased firing rate of alpha motor neurons due to the changes in the firing rates of type II

muscle spindle efferent, gamma efferent, and type Ib fibers from Golgi tendon organs caused by

heating of the motor nerves. This also thought to result in augmentation of the individual's pain

threshold.

6. Increased metabolic rate

According to Van’t Hoff’s law the rate of chemical reaction increases two to three folds or each

100 rise in temperature.

Metabolism being a series of chemical reactions will increase with a rise & decrease with a fall

of temperature. Any increase in enzymatic activity will result in increase in the rate of cellular

biochemical reactions. This can increase oxygen uptake and accelerate healing but may also

increase the rate of destructive processes. This is the reason heat is not advised in arthritic

conditions like rheumatoid arthritis where heat can increase the activity of collagenase and thus


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accelerate the destruction of articular cartilage. Therefore thermotherapy should be used with

caution in patients with acute inflammatory disorders.

Increasing tissue temperature increases the oxygen-hemoglobin dissociation, resulting in more

oxygen available for tissue repair. It has been shown that hemoglobin releases twice as much

oxygen at 410C which in conjunction with the increased rate of blood flow and the increased

enzymatic reaction rate, contribute to the acceleration of tissue healing by thermotherapy.

7. Increased tissue extensibility/ effects on Collagenous tissue

At normal tissue temperatures, collagen primarily exhibits elastic properties & only minimal

viscous flow. At temperatures within the range of 40 – 45˚C, the extensibility of collagen tissue

increase. Joint stiffness is often associated with changes in the visco-elastic properties of joints.

So heat can be used prior to treating joint stiffness.

8. Increased sweating

Sweating is mechanism of thermo regulation. Raised temperature increases sweating owing to

the stimulation of Anterior Hypothalamus. The heat-regulatory function of the hypothalamus is

affected by inputs from temperature receptors in the skin.

9. Effect on pulse rate & blood pressure

The blood vessels of the body, such as peripheral blood vessel of skin dilates or contracts

depending on body temperature. When the temperature of the body drops the peripheral blood

vessels contracts to conserve heat. When body temperature increases above normal, the

peripheral blood vessels dilate to allow more heat to escape. The blood vessel dilation lowers the

blood pressure because the blood distribution has increased to areas which previously had less

distribution. The body now has to compensate. Because of the change in pressure, less blood is

pumped to the vital organs of the body. It does this by increasing the cardiac output by increasing

the heart rate. Thus an increase in temperature through vasodilatation of blood vessels causes a

drop in BP which is compensated for by increasing the cardiac output, which can be achieved

through increase in heart rate. Therefore heart rate increases when the temperature increases.

Lowering of blood pressure also due to decrease in sodium concentration, loss of urea & other

nitrogenous substance due to increased metabolism due to heating.

10. Increased rate of breathing

As the temperature increases there is more oxyhemoglobin dissociation. Heat also increases

metabolism. Metabolically active cells require more O2 and liberate more acids and heat. The

acids and heat, in turn, promote release of O2 from oxyhemoglobin. Due to these change in O2

demand the rate of respiration too increases to meet the demand.

1. Increased elimination through KIDNEYS.


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NOTE: - Heat therapy is most commonly used for rehabilitation purposes. The therapeutic

effects of heat include increasing the extensibility of collagen tissues; decreasing joint stiffness;

reducing pain; relieving muscle spasms; reducing inflammation, oedema, and aids in the post-

acute phase of healing; and increasing blood flow. The increased blood flow to the affected area

provides proteins, nutrients, and oxygen for better healing.

THERAPEUTIC EFFECTS OF HEAT

1. Encouragement of healing

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. But the application of heat to

inflammatory injuries on the early stages is not beneficial.

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.

Because superficial heating agents increase the temperature of only the superficial few

millimeters of tissue, they are most likely to accelerate the healing of only superficial structures

such as the skin, or deeper tissue layers exposed due to skin ulceration. Deeper effects may also

occur as the result of consensual vasodilatation in areas distant from or deep to the area of

increased temperature. It is advised to use deep heating modalities to achieve deeper effects.

2. Relief of Pain

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. 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 like Prostaglandins & Bradykinin resulting from tissue injury.

3. Reduction of Muscle Spasm

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 diminish muscle excitation. Also the pain & muscle spasm are interdependent – A

reduction in one will cause a reduction in the other.

4. Sedative Effect

This might be simply a consequence of pain relief. The sedative effects are evident in milder

form of heat therapy.


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5. Increase in Range of Joint Motion

The analgesic effect of heat allows greater tolerance of stretching. 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. Thus heat can used prior to

passive stretching & exercise to increase joint movement or lengthen scars or contractures.

6. Prophylaxis (Prevention) of Pressure Sores

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.

7. Reduction of Oedema in Extremities

Heat has been recommended for the treatment of chronic oedema of the hand & foot. Vessel

dilatation induced by heating will allow increased rates of fluid exchange & thus may help to

increase the reabsorption of exudates. 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. Such

heating arrangements coupled with exercises are valuable.

8. Resolution of Some Skin Diseases

Fungal infections which are difficult to control & thrive in moist conditions are sometimes

treated with regular infrared therapy.

Important Facts to Note in Thermotherapy Treatment Techniques

1. Due to heat there is increased capillary permeability & Increased capillary dilatation causes –

Erythema or redness.

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.

4. No patient with oedema should be treated with any heat modality until the reasons for the

oedema are determined.

Degrees of heat sensation can be categorized as follows;

a) MINIMAL WARMTH: - Threshold value, Gentle comforting warmth.

b) MEDIUM WARMTH: - Distinct feeling of agreeable warmth.

c) MAXIMUM WARMTH: - Intense feeling of heat, maximum heat tolerance is exercised.

d) DANGER LEVEL: - Intolerable heat, Burning sensation.

References


Sreeraj S R

1. Introduction to Physiotherapy & Electrotherapy – Unit – 1.

http://faculty.mu.edu.sa/download.php?fid=8915

2. Cameron MH. Physical Agents in Rehabilitation: From Research to Practice. Second

Edition. 2003, Elsevier.

3. Singh J. Textbook of Electrotherapy. 2 edition. Jaypee. 2012.

4. Forster A, Palastanga N. Clayton’s Electotherapy, Theory and Practic. 9th Ed. W B

Saunders; 2006.

5. http://en.wikipedia.org/wiki/Perspiration

6. http://wiki.answers.com/Q/How_does_temperature_increase_heart_rate?#slide=14

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Physiologic and therapeutic effects of heat