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Effect of Heat

The therapeutic application of heat is called thermotherapy. Outside of the rehabilitation setting, thermotherapy is used primarily to destroy malignant tissue or to treat cold-related injuries. Within rehabilitation, thermotherapy is used primarily to control pain, increase soft tissue extensibility and circulation, and accelerate healing. Heat has these therapeutic effects because of its influence on hemodynamic, neuromuscular, and metabolic processes, the mechanisms of which are explained in detail in the following section.

EFFECTS OF HEAT

HEMODYNAMIC EFFECTS

Vasodilation

Heat causes vasodilation and thus an increase in the rate of blood flow.92
When heat is applied to one area of the body, there is vasodilation where the heat is applied and to a lesser degree, systemically, in areas distant from the site of heat application. Superficial heating agents produce more pronounced vasodilation in local cutaneous blood vessels, where they cause the greatest change in temperature, and less pronounced dilation in the deeper vessels that run through muscles, where they cause little if any change in temperature. Thermotherapy applied to the whole body can cause generalized vasodilation and may improve vascular endothelial function in the setting of cardiac risk factors and in chronic heart failure.93-95
In rats, whole-body hyperthermia was associated with the growth of new blood vessels in the heart.96 Thermotherapy may cause vasodilation by a variety of mechanisms, including direct reflex activation of the smooth muscles of the blood vessels by cutaneous thermoreceptors, indirect activation of local spinal cord reflexes by cutaneous thermoreceptors, or local release of chemical mediators of inflammation (Fig. 8-19).97,98
One study demonstrated that at least two independent mechanisms contribute to the rise in skin blood flow during local heating: a fast-responding vasodilator system mediated by axon reflexes, and a more slowly responding vasodilator system that relies on local production of nitrous oxide.99 Superficial heating agents stimulate the activity of cutaneous thermoreceptors. It is proposed that transmission from these cutaneous thermoreceptors via their axons directly to nearby cutaneous blood vessels causes the release of bradykinin and nitrous oxide, and that bradykinin and nitrous oxide then stimulate relaxation of the smooth muscles of the vessel walls to cause vasodilation in the area where the heat is applied.98-100 However, the role of bradykinin in heat-mediated vasodilation was recently called into question when it was found that blocking bradykinin receptors during whole-body heating did not alter the amount of cutaneous vasodilation.101 This finding suggests that nitrous oxide is the primary chemical mediator of heat-induced vasodilation. Cutaneous thermoreceptors also project via the dorsal root ganglion to synapse with interneurons in the dorsal horn of the grey matter of the spinal cord. These interneurons synapse with sympathetic neurons in the lateral grey horn of the thoracolumbar segments of the spinal cord to inhibit their firing and thus decrease sympathetic output.102 This decrease in sympathetic activity causes a reduction in smooth muscle contraction, resulting in vasodilation at the site of heat application, as well as in the cutaneous vessels of the distal extremities.103 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.104
For example, if a patient has an ulcer on his leg as the result of arterial insufficiency in the extremity, thermotherapy may be applied to his lower back to increase the circulation to his lower extremity, thereby facilitating wound healing. This would be most appropriate if the ulcer was bandaged or did not tolerate pressure, or if the area lacked sufficient circulation or sensation to safely tolerate the direct application of heat. Because blood flow within the skeletal muscles is primarily influenced by metabolic factors rather than by changes in sympathetic activity, and because superficial heating agents do not increase the temperature to the depth of most muscles, skeletal muscle blood flow is much less affected by superficial heating modalities than is skin blood flow.105,106 The use of exercise or deep-heating modalities, such as ultrasound or diathermy, or a combination of these interventions, is therefore recommended when the goal of treatment is to increase skeletal muscle blood flow.
Cutaneous vasodilation and the increase in blood flow that occurs in response to increased tissue temperature
act 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. Thus, when an area is heated with a thermal agent, it is simultaneously cooled by circulating blood, and as the temperature of the area increases, the rates of circulation and cooling increase to reduce the impact of the thermal agent on tissue temperature, thereby reducing the risk of burning.

NEUROMUSCULAR EFFECTS

Changes in Nerve Conduction Velocity and Firing Rate

Increased temperature increases nerve conduction velocity and decreases the conduction latency of sensory and motor nerves.107-109 Nerve conduction velocity increases by approximately 2 m/second for every 1° C (1.8° F) increase in temperature. Although the clinical implications of these effects are not well understood, they may contribute to the reduced pain perception or improved circulation that occurs in response to increasing tissue temperature. Although conduction velocity in normal nerves increases with heat, demyelinated peripheral nerves treated with heat can undergo conduction block.110,111 This occurs because heat shortens the duration of sodium channel opening at the nodes of Ranvier during neuronal depolarization112. In demyelinated nerves, less current reaches the nodes of Ranvier. If heat is added, the shortened opening time of the sodium channel can prevent the node from depolarizing, leading to conduction block. Therefore, heat should be applied with caution to patients with demyelinating conditions such as carpal tunnel syndrome or multiple sclerosis. Nerve firing rate (frequency) has also been found to change in response to changes in temperature. Elevation of muscle temperature to 42° C (108° F) has been shown to result in a decreased firing rate of type II muscle spindle efferents and gamma efferents and an increased firing rate of type Ib fibers from GTOs.113,114 These changes in nerve firing rates are thought to contribute to a reduction in the firing rate of alpha motor neurons, and thus to a reduction in muscle spasm.115 The decrease in gamma neuron activity causes the stretch on the muscle spindles to decrease, reducing afferent firing from the spindles.116 The decreased spindle afferent activity results in decreased alpha motor neuron activity, and thus in relaxation of muscle contraction.

Increased Pain Threshold

Several studies demonstrate that the application of local heat can increase the pain threshold.117,118 Proposed mechanisms of this effect include a direct and immediate reduction of pain by activation of the spinal gating mechanism and an indirect, later, and more prolonged reduction of pain by reduction of ischemia and muscle spasm or by facilitation of tissue healing. Heat increases the activity of the cutaneous thermoreceptors; this can have an immediate inhibitory gating effect on the transmission of the sensation of pain at the spinal cord level. Stimulation of the thermoreceptors can also result in vasodilation, as described previously, causing an increase in blood flow and thus potentially reducing the pain caused by ischemia. Ischemia may also be decreased as a result of reduction of spasm in muscles that compress blood vessels. The vasodilation produced by thermotherapy may accelerate recovery of the local pain threshold to a normal level by speeding tissue healing.

Changes in Muscle Strength

Muscle strength and endurance have been found to decrease during the initial 30 minutes after the application of deep or superficial heating agents.119-121 It is proposed that this initial decrease in muscle strength is the result of 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. In turn, this decreases the firing rate of alpha motor neurons. Beyond 30 minutes after the application of heat and for the next 2 hours, muscle strength gradually recovers and then increases to above pretreatment levels. This delayed increase in strength is thought to be caused by an increase in pain threshold. Because the changes in muscle strength produced by heating are temporary, heat is not used for strengthening. However, it is important to be aware of the effects of heat on strength when muscle strength is being used as a measure of patient progress. Because comparing preheating strength with postheating strength from the same session or another session can provide misleading information, it is recommended that muscle strength and endurance always be measured before and not after a heating modality is applied.

METABOLIC EFFECTS

Increased Metabolic Rate

Heat increases the rate of endothermic chemical reactions, including the rate of enzymatic biological reactions. Increased enzymatic activity has been observed in tissues at 39° C to 43° C (102° F to 109° F), with the reaction rate increasing by approximately 13% for every 1.0°C (1.8°F) increase in temperature and doubling for every 10° C (18° F) increase in temperature.33 Enzymatic and metabolic activity rates continue to increase up to a temperature of 45° C (113° F). Beyond this temperature, the protein constituents of enzymes begin to denature and enzyme activity rates decrease, ceasing completely at about 50° C (122° F).122 Any increase in enzymatic activity will result in an 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. For example, heat may accelerate the healing of a chronic wound; however, it has also been shown to increase the activity of collagenase and thus may accelerate the destruction of articular cartilage in patients with rheumatoid arthritis.31 Therefore, thermotherapy should be used with caution in patients with acute inflammatory disorders.

Increasing tissue temperature shifts the oxygenhemoglobin dissociation curve to the right, making more oxygen available for tissue repair (see Fig. 8-4). It has been shown that hemoglobin releases twice as much oxygen at 41° C (106° F) as it does at 36° C (97° F).123 In conjunction with the increased rate of blood flow stimulated by increased temperature and the increased enzymatic reaction rate, this increased oxygen availability may contribute to acceleration of tissue healing by thermotherapy.

ALTERED TISSUE EXTENSIBILITY

Increased Collagen Extensibility

Increasing the temperature of soft tissue increases its extensibility.124
When soft tissue is heated before stretching, it maintains a greater increase in length after the stretching force is applied, less force is required to achieve the increase in length, and the risk of tissue tearing is reduced.125,126 If heat is applied to collagenous soft tissue, such as tendon, ligament, scar tissue, or joint capsule, before prolonged stretching, plastic deformation, in which the tissue increases in length and maintains most of the increase after cooling, can be achieved.127,128 In contrast, if collagenous tissue is stretched without prior heating, elastic deformation, in which the tissue increases in length while the force is applied but loses most of the increase when the force is removed, generally occurs. The maintained elongation of collagenous tissue that occurs after heating and stretching is caused by changes in the organization of the collagen fibers and by changes in the viscoelasticity of the fibers. For heat to increase the extensibility of soft tissue, the appropriate temperature range and structures must be reached. A maximum increase in residual length is achieved when the tissue temperature is maintained at 40° C to 45° C (104° F to 113° F) for 5 to 10 minutes.113,128 The superficial heating agents described in the next sections can cause this level of temperature increase in superficial structures such as cutaneous scar tissue or superficial tendons. However, to adequately heat deeper structures, such as the joint capsules of large joints or deep tendons, deep-heating agents, such as ultrasound or diathermy, must be used.

USES OF SUPERFICIAL HEAT

PAIN CONTROL

Thermotherapy can be used clinically to control pain. This therapeutic effect may be mediated by gating of pain transmission through activation of cutaneous thermoreceptors, or may indirectly result from improved healing, decreased muscle spasm, or reduced ischemia.129 Increasing skin temperature may reduce the sensation of pain by altering nerve conduction or transmission.130 For example, it is likely that the analgesia produced in the sensory distribution of the ulnar nerve (the volar and medial forearm), when infrared radiation is applied over the ulnar nerve at the elbow, is caused by altered nerve conduction.117 The indirect effects of thermotherapy on tissue healing and ischemia are primarily attributable to vasodilation and increased blood flow. It has been proposed that the psychological experience of heat as comfortable and relaxing may also influence the patient’s perception of pain. Although thermotherapy may reduce pain of any origin, it is generally not recommended as an intervention for pain caused by acute inflammation because an increase in tissue temperature may aggravate other signs and symptoms of inflammation, including heat, redness, and edema.131 However, recent studies have found that heat can reduce the pain associated with acute low back pain, pelvic pain, and renal colic (the pain associated with kidney stones). A systematic review found moderate evidence that continuous low-level local heat (using a commercially available disposable pack inside a Velcro closure belt that heats up to 40° C [104° F] when exposed to air and maintains this heat for 8 hours) reduces pain and disability for patients with back pain lasting less than 3 months.132 However, the relief lasts for a short time, and the effect is relatively small. Adding exercise to heat therapy appears to provide additional benefit, based on this review. In two trials with a total of 258 participants with acute or subacute low back pain, application of a heated back wrap for 8 hours a day for 3 consecutive days was associated with significantly reduced pain at 5 days compared with oral placebo.133,134 One trial with 90 subjects with acute low back pain found that a heated blanket significantly decreased acute (,6 hours’ duration) low back pain 25 minutes after application when compared with a nonheated blanket.135 Another trial of 100 participants with back pain of less than 3 months’ duration combined a heated back wrap with exercise and compared this with heat alone, exercise alone, or providing subjects with an educational booklet, and found that heat plus exercise provided significantly better pain relief and improvement in function than heat or exercise alone.136 When a blanket heated to 42° C (106° F) is used during emergency transport for patients with acute pelvic pain, low back pain, or renal colic, patients had less pain than with an unheated blanket.135,137,138 Additionally, warming with an electric blanket decreased anxiety and nausea in patients with acute pelvic pain or renal colic when compared with an unheated blanket during emergency transport.137,138 The application of at least 8 hours of continuous lowlevel heat has been shown to decrease pain in various other conditions, including DOMS when compared with a cold pack, acute low back pain when compared with placebo, and wrist pain when compared with placebo.134,139,140
Submersion of the affected body part in water at 45° C (113° F) for 20 minutes was more effective than ice for the reduction of pain from jelly fish–type stings.141 Given these findings, current evidence suggests that heat may be used to control pain in patients with certain acute conditions. However, heat should be discontinued if signs of worsening inflammation, including increased pain, edema, or erythema, are noted.

INCREASED RANGE OF MOTION AND DECREASED JOINT STIFFNESS

Thermotherapy can be used clinically when the goals are to increase joint ROM and decrease joint stiffness.142-144 Both of these effects are thought to be the result of the increase in soft tissue extensibility that occurs with increasing soft tissue temperature. Increasing soft tissue extensibility contributes to increasing joint ROM because it results in greater increases in soft tissue length and less injury when a passive stretch is applied. A maximum increase in length with the lowest risk of injury is obtained if the tissue temperature is maintained at 40° C to 45° C (104° F to 113° F) for 5 to 10 minutes, and if a low-load, prolonged stretch is applied during the heating period and while the tissue is cooling (Fig. 8-20).113,128 Therefore, it is recommended that stretching be performed during and immediately after the application of thermotherapy, because if the tissues are allowed to cool before being stretched, the effects of prior heating on tissue extensibility will be lost. Thermotherapy can decrease joint stiffness, which is a quality related to the amount of force and the time required to move a joint; as joint stiffness decreases, less force and time are required to produce joint motion.145-147 For example, increasing tissue temperature by placing the hands in a warm water bath or warm paraffin or heating the surface with an infrared (IR) lamp has been shown to decrease finger joint stiffness.148 Proposed mechanisms of this effect include the increased extensibility and viscoelasticity of periarticular structures, including the joint capsule and surrounding ligaments. When a heating agent is used to increase soft tissue extensibility before stretching, an agent that can reach the shortened tissue must be used. Thus superficial agents, such as hot packs, paraffin, or infrared lamps, are appropriate for use before stretching of skin, superficial muscle, joints, or fascia, whereas deep-heating agents, such as ultrasound or diathermy, should be used before stretching of deeper joint capsules, muscles, or tendons.

ACCELERATED HEALING

Thermotherapy can accelerate tissue healing by increasing circulation and the enzymatic activity rate and by increasing the availability of oxygen to the tissues. Increasing the rate of circulation accelerates the delivery of blood to the tissues, bringing in oxygen and other nutrients and removing waste products. The application of any physical agent that increases circulation can be beneficial during the proliferative or remodeling stage of healing, or when chronic inflammation is present. However, because increasing circulation can increase edema, thermotherapy should be applied with caution during the acute inflammation phase to avoid prolonging this phase and delaying healing. By increasing the enzymatic activity rate, thermotherapy increases the rate of metabolic reactions, thus allowing the processes of inflammation and healing to proceed more rapidly. Increasing the temperature of the blood also increases the dissociation of oxygen from hemoglobin, making more oxygen available for the processes of tissue repair. 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 because of skin ulceration. Deeper effects may occur as the result of consensual vasodilation in areas distant from or deep to the area of increased temperature.

INFRARED RADIATION FOR PSORIASIS

Although the ultraviolet (UV) frequency range of electromagnetic radiation is used most commonly in the treatment of psoriasis (see Chapter 16), the IR range is occasionally used for this application.149,150 The increased temperature of the upper epidermis and the dermis in the region of psoriatic plaques produced by IR radiation has been proposed as the mechanism for the reduction in psoriatic plaques that occurs in some individuals exposed to IR radiation.150 Other applications of IR not related to heat are covered in Chapter 15.

CONTRAINDICATIONS AND PRECAUTIONS FORTHERMOTHERAPY

Although thermotherapy is a relatively safe intervention, its use is contraindicated in some circumstances, and it should be applied with caution in others. Thermotherapy may be applied by a qualified clinician or by a properly instructed patient. Clinicians may use all forms of thermotherapy, and patients may be instructed to use hot packs, paraffin, or IR lamps at home to treat themselves. When patients are taught to use these modalities at home, they should be instructed on how to use the modality, including the location at which it should be applied, the temperature to be used, safety precautions, and the duration and frequency of treatment. Patients must also be taught how to identify possible adverse effects and must be told to discontinue treatment should any of these occur. Even when thermotherapy is not contraindicated, as with all interventions, if the patient’s condition is worsening or is not improving after two to three treatments, the treatment approach should be reevaluated, or the patient should be referred to a physician for reevaluation.

CONTRAINDICATIONS FOR THE USE OF THERMOTHERAPY

CONTRAINDICATIONS
for the Use of Thermotherapy

  • Recent or potential hemorrhage
  • Thrombophlebitis
  • Impaired sensation
  • Impaired mentation
  • Malignant tumor
  • IR irradiation of the eyes151

Recent or Potential Hemorrhage

Heat causes vasodilation and an increased rate of blood flow. Because vasodilation may cause reopening of a vascular lesion, increasing the rate of blood flow in an area of recent hemorrhage can restart or worsen the bleeding. In addition, increasing blood flow in an area of potential hemorrhage can cause hemorrhage to start. Therefore, it is recommended that heat not be applied to areas of recent or potential hemorrhage.
Thermotherapy should not be applied if the patient reports bruising or bleeding in the previous 48 to 72 hours, or if recently formed red, purple, or blue ecchymosis is present.

Thrombophlebitis

The vasodilation and increased rate of circulation caused by increased tissue temperature may cause a thrombus or a blood clot to become dislodged from the area being treated and to be moved to the vessels of vital organs, resulting in morbidity or even death.
Thermotherapy should not be applied if the patient says that there is a blood clot in the area. Thermotherapy to the leg should not be applied if there is tenderness and swelling of the calf until the presence of a thrombus in the lower extremity has been ruled out.

Impaired Sensation or Impaired Mentation

A patient’s sensation and a report of heat or pain are used as the primary indicators of the maximum safe temperature for thermotherapy; thus a patient who cannot feel or report the sensation of heat can easily be burned before the clinician realizes that there is a problem. Therefore, heat should not be applied to areas where sensation is impaired or to patients who may have any other difficulty letting the therapist know when they are too hot.

Malignant Tissue

Thermotherapy may increase the growth rate or rate of metastasis of malignant tissue by increasing circulation to the area or by increasing the metabolic rate. Because a patient may not know that he or she has cancer or may be uncomfortable discussing this diagnosis directly, the therapist first should check the chart for a diagnosis of cancer, and then ask the patient the following questions.

Infrared Irradiation of the Eyes

IR irradiation of the eyes should be avoided because such treatment may cause optical damage. To avoid irradiation of the eyes, IR opaque goggles should be worn by the patient throughout treatment using an IR lamp and by the therapist when near the lamp, as occurs when setting up the treatment.

PRECAUTIONS FOR THE USE OF THERMOTHERAPY

PRECAUTIONS

  • Acute injury or inflammation
  • Pregnancy
  • Impaired circulation
  • Poor thermal regulation
  • Edema
  • Cardiac insufficiency
  • Metal in the area
  • Over an open wound
  • Over areas where topical counterirritants have recently been applied
  • Demyelinated nerves

Acute Injury or Inflammation

Heat should be applied with caution to the area of an acute injury or acute inflammation because increasing tissue temperature can increase edema and bleeding as a result of vasodilation and increased blood flow.152 This may aggravate the injury, increase pain, and delay recovery.
Heat should not be applied within the first 48 to 72 hours after an injury. Elevation of skin temperature, rubor, and local edema demonstrate the presence of acute inflammation and indicate that heat should not be applied to the area.

Pregnancy

A fetus may be damaged by maternal hyperthermia. Because this is unlikely to occur with superficial heating of the limbs, thermotherapy may be applied to such areas, but full body heating, as occurs with immersion of most of the body in a whirlpool, should be avoided during pregnancy. Although maternal hyperthermia has not been demonstrated with application of hot packs to the low back or abdomen, such application generally is not recommended.
If the patient is or may be pregnant, heat should not be applied to the abdomen or low back, and the patient should not be immersed in a warm or hot whirlpool.

Impaired Circulation or Poor Thermal Regulation

Areas with impaired circulation and patients with poor thermal regulation may not vasodilate to a normal degree in response to an increase in tissue temperature and therefore may not have a sufficient increase in blood flow when tissue temperature increases to protect the tissues from burning. In general, poor thermal regulation is encountered in the elderly and the very young.
Decreased skin temperature, thin skin, poor nails, tissue swelling, and ulceration are all signs of impaired circulation. Milder superficial heat should be used in areas with poor circulation or in elderly or very young patients. Heat should be applied at a lower temperature or with more insulation, and patients should be checked frequently for any discomfort or signs of burning.

Edema

The application of thermotherapy to a dependent extremity has been shown to increase edema.131 This effect is thought to be the result of the vasodilation and enhanced circulation that occur with raised tissue temperature and the increase in inflammation caused by increased metabolic rate.
Heat may be applied with caution with the area elevated if edema is present and is thought to be a result of impaired venous circulation.

Cardiac Insufficiency

Heat can cause both local and generalized vasodilation, which can contribute to increased cardiac demand. Because this may not be well tolerated by patients with cardiac insufficiency, such patients should be monitored closely if heat is applied, particularly if heat is applied to a large area.
A slight decrease in blood pressure and an increase in heart rate are normal consensual responses to the application of heat. Heat treatment should be discontinued in a patient with cardiac insufficiency if the patient’s heart rate falls, or if the patient complains of feeling faint.

Metal in the Area

Metal has higher thermal conductivity and higher specific heat than body tissue and therefore may become very hot with the application of conductive heating modalities. For this reason, jewelry should be removed before superficial heating modalities are applied, and caution should be taken when metal, such as staples or bullet fragments, is present in superficial tissues of the area being treated.

Over an Open Wound

Paraffin should not be used over an open wound because it may contaminate the wound and is difficult to remove. All other forms of thermotherapy should be applied over open wounds with caution because loss of epidermis reduces the insulation of subcutaneous tissues. If forms of thermotherapy other than paraffin are used in the area of an open wound, they should be applied at a lower temperature or intensity or with more insulation than would be used when areas with intact skin are treated. One should check frequently during treatment for any signs of burning. When a heating agent is applied with the goal of increasing circulation and accelerating the healing of an open wound, hydrotherapy with clean, warm water may be applied directly to the wound, or other superficial heating agents may be applied close to but not directly over the wound to provide a therapeutic effect while reducing the risk of cross-contamination and burns.

Over Areas Where Topical Counterirritants Have Recently BeenApplied

Topical counterirritants are ointments or creams that cause a sensation of heat when applied to the skin. These preparations generally contain substances such as menthol that stimulate the sensation of heat by causing a mild inflammatory reaction in the skin. These preparations also cause local superficial vasodilation. If a thermal agent is applied to an area that is already vasodilated as the result of application of a topical counterirritant, vessels in the area may not be able to vasodilate further to dissipate heat from the thermal agent, and a burn may result.
If the patient has recently applied a topical counterirritant to an area, a superficial heating agent should not be applied. The patient should be told not to use this type of preparation before future treatment sessions and not to apply a superficial heating agent at home after using this type of preparation.

Demyelinated Nerves

Conditions that are associated with demyelination of peripheral nerves include carpal tunnel syndrome and ulnar nerve entrapment. Apply heat with caution to areas with demyelinated nerves because superficial heat, including fluidotherapy, heat lamp, and water bath, has been shown to cause conduction block when applied to peripheral nerves.109-111
If the patient has a peripheral demyelinating condition, heat should be applied with caution to affected areas.

ADVERSE EFFECTS OF THERMOTHERAPY

BURNS

Excessive heating can cause protein denaturation and cell death. These effects may occur when heat is applied for too long, when the heating agent is too hot, or when heat is applied to a patient who does not have the appropriate protective vasodilation response to increased tissue temperature. The effects of heat on cell viability are exploited in the medical treatment of malignancies, in which heat is applied with the goal of killing the malignant cells; however, during application of heat in rehabilitation, cell death is to be avoided. Because protein begins to denature at 45° C (113° F) and cell death has been observed when cells were maintained at 43° C (109° F) for 60 minutes or at 46° C (115° F) for only 71⁄2 minutes, when applying heat in rehabilitation, duration and tissue temperature should be kept below these levels.153,154 Overheating and tissue damage can be avoided by using superficial heating agents that get cooler during their application, by limiting the initial temperature of the agent, or by using insulation between the agent and the patient’s skin (Box 8-1). For example, hot packs that are warmed in hot water before being placed on the patient start to cool as soon as they are removed from the hot water and applied, and therefore are unlikely to cause burns. In contrast, superficial heating agents, such as plugin electrical hot packs or IR lamps that do not cool with use, are more likely to cause burns. The higher the temperature of a conductive superficial heating agent, the greater the rate of heat transfer to the patient, and thus the greater the risk of burns; therefore, it is important not to overheat a conductive superficial heating agent and to always use adequate insulation. To avoid burns, heating agents should be applied in the manner recommended here. They should not be applied for longer periods or at higher temperatures, and the treatment time and temperature of the heating agent should be reduced if the patient has impaired circulation. Heating agents should not be applied where contraindicated, and all patients should be provided with a means of calling for assistance, such as a bell, if the clinician or another staff member is not in the immediate treatment area. During the intervention, the clinician should check to make sure that the patient has not fallen asleep and should instruct the patient to use a timer that rings loudly at the end of the treatment time, if the patient uses a superficial heating agent at home. A superficial heating agent used at home should be the type that cools over time, such as a microwavable hot pack or a hot water bottle. If an electrical heating pad is used by a patient at home, it should be the type that requires the patient to hold down a switch at all times for it to stay on. This safety feature ensures that the heating pad will turn off if the patient falls asleep and stops holding down the switch. It is recommended that the patient’s skin be inspected for burns before treatment initiation because the patient may have been burned previously. The skin should also be inspected during and after thermotherapy. A recent superficial burn will appear red and may have blistering. As the burn heals, the skin will appear pale and scarred.

FAINTING

Occasionally, a patient may feel faint when heat is applied. Fainting, which is a sudden, transient loss of consciousness, is generally the result of inadequate cerebral blood flow and is most commonly caused by peripheral vasodilation and decreased blood pressure, generally in association with a decreased heart rate.155 Heating an area of the body generally causes vasodilation locally and, to a lesser extent, in areas distant from the site of application. This distant, or consensual, response can result in a decrease in cerebral blood flow sufficient to cause a patient to faint during the application of thermotherapy. If a patient feels faint while heat is being applied, lowering the head and raising the feet will bring more blood to the brain to help the patient recover. Heating as small an area as clinically beneficial and removing excessive heavy clothing that insulates the whole body may help limit this consensual decrease in blood pressure, thus reducing the probability of fainting. Patients may also feel faint when getting up after thermotherapy. This feeling is caused by the additive hypotensive effects of postural (orthostatic) hypotension and the hypotensive effect of the heat, as described previously. The patient’s head should be kept elevated with a pillow during heat application; this can help to decrease posttreatment postural hypotension by reducing the extent of positional change at the completion of the intervention. It is recommended that the patient remain in the position used during treatment for a few minutes after the thermal agent has been removed to allow blood pressure to normalize before rising.

BLEEDING

The vasodilation and increased blood flow caused by increasing tissue temperature may cause or aggravate bleeding in areas of acute trauma or in patients with hemophilia. Vasodilation may also cause reopening of any recent vascular lesion.

SKIN AND EYE DAMAGE FROM INFRARED RADIATION

Infrared (IR) radiation can produce adverse effects that are not produced by other superficial thermal agents. These include permanent damage to the eyes and permanent changes in skin pigmentation. Injury to the eyes, including corneal burning and retinal and lenticular damage, is considered to be the most likely and most severe hazard of IR radiation application.151 Prolonged exposure to IR radiation may also cause epidermal hyperplasia.156

Depending on the agent and the amount of insulation, warmth may not be felt for the first few minutes of treatment. The patient should not feel excessively hot and should not feel any sensation of increased pain or burning. If the patient reports any of these sensations, discontinue the treatment or reduce the intensity of the heat.

APPLICATION TECHNIQUES

GENERAL THERMOTHERAPY

Thermotherapy may be applied using a variety of materials, including hot packs, paraffin, fluidotherapy, IR lamp, whirlpool, or contrast baths. Different materials heat at different rates and to different degrees and depths. Hot packs heat the skin more, and more quickly, than does paraffin because the water in the hot packs has higher specific heat and thermal conductivity than paraffin wax. When at the same temperature as a hot pack, fluidotherapy also heats more slowly because it uses air, which has a low thermal conductivity and specific heat as its heating medium. However, fluidotherapy heats faster than stationary air at the same temperature because the movement of the air allows for heating by convection and constant replacement of hot air adjacent to the patient’s skin. Furthermore, with fluidotherapy there is a constant input of energy maintaining the air at a constant temperature, in contrast to hot packs which generally cool over time. Heating with a whirlpool offers the advantages of heating by convection using a medium with high specific heat and thermal conductivity. However, whirlpools are rarely used for superficial heating because they are difficult to keep clean.
During the application of thermotherapy by any means, the patient will usually experience a sensation of gentle warmth. If at any time the patient feels burning or discomfort remove the heating agent.

HOT PACKS

Commercially available hot packs are usually made of bentonite, a hydrophilic silicate gel, covered with canvas. Bentonite is used for this application because it can hold a large quantity of water for efficient delivery of heat. These types of hot packs are made in various sizes and shapes designed to fit different areas of the body (Fig. 8-21). They are stored in hot water kept at about 70° C to 75° C (158° F to 167° F) inside a purpose-designed, thermostatically controlled water cabinet (Fig. 8-22) that stays on at all times. This type of hot pack initially takes 2 hours to heat and 30 minutes to reheat between uses. Although bentonite-filled moist hot packs are generally recommended for clinical use, a variety of other types of hot or warm packs are available. These include chemical heating pads that are activated by mixing and contact of their contents with air and electrical plug-in heating pads. Chemical heating pads are made from a variety of materials that warm up and maintain a therapeutic temperature range for 1 to 8 hours when exposed to air by opening the package or breaking open an inner sealed bag, or when mechanically agitated. Different chemicals are activated by different means, heat to slightly different temperatures, have different specific heats, and maintain their temperature for different lengths of time. Although most chemical packs cannot be reused, some can, and although none produces moist heat directly, most can be wrapped in a moist towel or cover to produce moist heat. These chemical packs come in a variety of shapes and sizes for application to different body areas; some are designed to be placed in a wrap, allowing them to be worn during activity. Recent studies have found that the low-level prolonged heating produced by wearing this type of heating pad during activity can reduce low back and wrist pain and the sensation of stiffness, can increase flexibility,132,134,139,140
and may reduce acute low back pain more effectively than ibuprofen or acetaminophen.157 Electrical plug-in heating pads are not recommended for clinical use because they do not cool during application and therefore may more easily burn a patient. If patients are using an electrical plug-in heating pad at home, advise them to use a pad that requires the “on” switch to be held down for the pad to heat, to use only the medium or low setting, to limit application at the medium setting to 20 minutes, and to discontinue use if any sensation of pain, overheating, or burning occurs. Patients should also be advised to inspect the skin for any signs of burns directly after the use of a hot pack and for the following 24 hours.

PARAFFIN

Warm, melted paraffin can be used for thermotherapy. For this application, paraffin is mixed with mineral oil in a 6:1 or 7:1 ratio of paraffin to oil to reduce the melting temperature of the paraffin from 54° C (129° F) to between 45° C and 50° C (113° F to 122° F). Paraffin can be safely applied directly to the skin at this temperature because of its low specific heat and thermal conductivity. To minimize heat loss, insulating mitts should be applied to the hands or feet (Fig. 8-25). For this application, paraffin is heated and stored in a thermostatically controlled container that generally can heat the paraffin to 52° C to 57° C (126° F to 134° F).159 Such containers are available in small portable sizes for home or clinic use and in larger sizes designed primarily for clinic use (Fig. 8-26). The manufacturer’s usage and safety instructions for proper setting and adjustment of these devices and for selection of appropriate paraffin wax products should be followed because some units are preset to the correct temperature for a specific product. Paraffin usually is used for heating the distal extremities because it can maintain good contact with these irregularly contoured areas. Paraffin may also be applied to more proximal areas, such as the elbows and knees, or even the low back, by using the paint method described in Application Technique 8-8.

FLUIDOTHERAPY

Fluidotherapy is a dry heating agent that transfers heat by convection.160 It consists of a cabinet containing finely ground cellulose particles made from corn cobs (Fig. 8-29). Heated air is circulated through the particles, suspending and moving them so that they act like a liquid. The patient extends a body part into the cabinet, where it floats, as if in water. Portals in the cabinet allow the therapist to access the patient’s body part while it is being heated. Fluidotherapy units come in a variety of sizes suitable for treating different body parts. Both the temperature and the amount of particle agitation can be controlled by the clinician (Fig. 8-30).

INFRARED LAMPS

IR lamps emit electromagnetic radiation within the frequency range that gives rise to heat when absorbed by matter (Fig. 8-31). IR radiation has a wavelength of 770 to 106 nm, lying between visible light and microwaves on the electromagnetic spectrum (see Fig. 15-3), and is emitted by many sources that emit visible light or ultraviolet radiation such as the sun. IR radiation is divided into three bands with different wavelength ranges: IR-A, with wavelengths of 770 to 1400 nm; IR-B, with wavelengths of 1400 to 3000 nm; and IR-C, with wavelengths of 3000 to 106 nm. IR sources used in rehabilitation include sunlight, IR lamps, IR light-emitting diodes (LEDs), supraluminous diodes (SLDs), and low-intensity lasers. IR lamps currently available for clinical use all emit IR-A, generally with mixed wavelengths of approximately 780 to 1500 nm with peak intensity at around 1000 nm. The tissue temperature increase produced by IR radiation is directly proportional to the amount of radiation that penetrates the tissue. This is related to the power and wavelength of the radiation, the distance of the radiation source from the tissue, the angle of incidence of the radiation to the tissue, and the absorption coefficient of the tissue. Higher-power IR will deliver more radiation to the skin. Most lamps deliver IR radiation with power in the range of 50 to 1500 watts. Most of the IR radiation produced by today’s lamps (780 to 1500 nm wavelength) is absorbed within the first few millimeters of human tissue. It has been shown that at least 50% of IR radiation of 1200 nm wavelength penetrates beyond 0.8 mm and therefore is able to pass through the skin to interact with subcutaneous capillaries and cutaneous nerve endings.161 Human skin allows maximum penetration of radiation with a wavelength of 1200 nm while being virtually opaque to IR radiation with a wavelength of 2000 nm or greater.151 The amount of energy reaching the patient from an IR radiation source is also related to the distance between the source and the tissue. As the distance of the source from the target increases, the intensity of radiation reaching the target changes in proportion to the inverse square of the distance. For example, if the source is moved from a position 5 cm from the target to a position 10 cm from the target, increasing by a factor of 2, the intensity of radiation reaching the target will fall to onefourth of its prior level. The amount of energy reaching the target is also related to the angle of incidence of the radiation. As the angle of incidence of the radiation changes, the intensity of the energy reaching the target changes in proportion to the cosine of the angle of incidence of the radiation. For example, if the angle of incidence changes from 0 degrees (i.e., perpendicular to the surface of the skin), with a cosine of 1, to 45 degrees, with a cosine of 1 2, the intensity of radiation will fall by a factor of 1 2. Thus the intensity reaching the skin is greatest when the radiation source is close to the patient’s skin and the radiation beam is perpendicular to the skin surface, and as the distance or the angle of incidence increases, the intensity of radiation reaching the skin will diminish. IR radiation is absorbed most by tissues with high IR absorption coefficients. IR absorption coefficients are affected primarily by color, with darker tissue and skin absorbing more radiation than lighter tissue and skin. Therefore, with the same radiation and lamp positioning, dark skin will absorb more IR and therefore will increase more in temperature than light skin. A number of authors have provided formulae for calculating the exact amount of heat being delivered to a patient by IR radiation149,162 or methods for measuring the exact tissue temperature increase150; however, in clinical practice, as with other thermal agents, the sensory report of the patient is usually used to gauge the skin temperature. The amount of heat transfer is adjusted by changing the power output of the lamp and/or the distance of the lamp from the patient, so the patient feels a comfortable level of warmth. Although clinical use of IR lamps for heating superficial tissues was popular during the 1940s and 1950s, this practice has waned in recent years. The fall in popularity appears to be the result of changes in practice style preferences and concern about overheating patients if the are placed or move too close to the lamp, rather than reflecting any evidence of excessive adverse effects or lack of therapeutic efficacy. Recent studies continue to show that IR produces expected effects of heat, including reducing pain in patients with chronic low back pain163 and increasing joint flexibility and thus the increase in ROM produced by stretching in joints with contractures.143 Most current uses and literature regarding IR in therapy relate to low-intensity IR lasers with nonthermal effects, as discussed in detail in Chapter 15.

CONTRAST BATH

Contrast baths are applied by alternately immersing an area, generally a distal extremity, first in warm or hot water and then in cool or cold water (Fig. 8-32). Contrast baths have been shown to cause fluctuations in blood flow over a 20-minute treatment.164 A 2009 systematic review of 28 studies from 1938 to 2009 found evidence that contrast baths may increase superficial blood flow and skin temperature.165 This form of hydrotherapy is frequently used clinically when a goal of treatment is to achieve the benefits of heat, including decreased pain and increased flexibility, while avoiding the risk of increased edema. The varying sensory stimulus is thought to promote pain relief and desensitization. Thus treatment with a contrast bath may be considered when patients present with chronic edema; subacute trauma; inflammatory conditions such as sprains, strains, or tendinitis; or hyperalgesia or hypersensitivity caused by reflex sympathetic dystrophy or other conditions. The use of contrast baths for edema is based on the rationale that the alternating vasodilation and vasoconstriction produced by alternating immersion in hot and cold water may help to train or condition the smooth muscles of the blood vessels. However, because no research data on the efficacy or mechanisms of this effect are available, it is recommended that clinicians carefully assess the effects of such treatment on the individual patient when considering using this form of hydrotherapy treatment.

CHOOSING BETWEEN CRYOTHERAPY AND THERMOTHERAPY

Because some of the effects and clinical indications for the use of cryotherapy and thermotherapy are the same and others are different, there are some situations in which either may be used and others in which only one or the other would be appropriate. Table 8-1 provides a summary of the effects of cryotherapy and thermotherapy to assist the clinician in choosing between these options. Although both heat and cold can decrease pain and muscle spasm, they produce opposite effects on blood flow, edema formation, nerve conduction velocity, tissue metabolism, and collagen extensibility. Cryotherapy decreases these effects, and thermotherapy increases them.

REVIEW

  1. Cryotherapy is the transfer of heat from a patient with the use of a cooling agent. Cryotherapy has been shown to decrease blood flow, decrease nerve conduction velocity, increase the pain threshold, alter muscle strength, decrease the enzymatic activity rate, temporarily decrease spasticity, and facilitate muscle contraction. These effects of cryotherapy are used clinically to control inflammation, pain, edema, and muscle spasm; to reduce spasticity temporarily; and to facilitate muscle contraction. Examples of physical agents used for cryotherapy include ice pack, cold pack, ice massage, and vapocoolant spray.
  2. Thermotherapy is the transfer of heat to a patient with a heating agent. Thermotherapy has been shown to increase blood flow, increase nerve conduction velocity, increase pain threshold, alter muscle strength, and increase the enzymatic activity rate. These effects of thermotherapy are used clinically to control pain, increase soft tissue extensibility, and accelerate healing. Examples of physical agents used for thermotherapy include hot pack, paraffin, fluidotherapy, IR lamp, and contrast baths.
  3. Thermal agents should not be applied in situations in which they may aggravate an existing pathology, such as a malignancy, or may cause damage, such as frostbite or burns.
  4. The reader is referred to the Evolve web site for further exercises and links to resources and references.

GLOSSARY

  • Angle of incidence : The angle at which a beam (e.g., from an infrared lamp) contacts the skin.
  • Cold-induced vasodilation (CIVD) : The dilation of blood vessels that occurs after cold is applied fora prolonged time or after tissue temperature reaches less than 10° C. Also known as the hunting response.
  • Contrast bath : Alternating immersion in hot and cold water.
  • Controlled cold compression : Alternate pumping of cold water and air into a sleeve wrapped around
    a patient’s limb; used most commonly to control pain and edema immediately after surgery.
  • Cryokinetics : A technique that combines the use of cold and exercise.
  • Cryostretch : The application of a cooling agent before stretching.
  • Cryotherapy : The therapeutic use of cold.
  • Delayed-onset muscle soreness (DOMS) : Soreness that often occurs 24 to 72 hours after eccentric
    exercise or unaccustomed training levels. DOMS probably is caused by inflammation as a result of tiny muscle tears.
  • Edema : Swelling resulting from accumulation of fluid in the interstitial space.
  • Fluidotherapy : A dry heating agent that transfers heat by convection. It consists of a cabinet containing finely ground particles of cellulose through which heated air is circulated.
  • Infrared (IR) lamp : A lamp that emits electromagnetic radiation in the infrared range (wavelength approximately 750 to 1300 nm). IR radiation of sufficient intensity can cause an increase in superficial tissue temperature.
  • Paraffin : A waxy substance that can be warmed and used to coat the extremities for thermotherapy.
  • Protein denaturation : Breakdown of proteins that permanently alters their biological activity; it can be caused by excessive heat.
  • Quick icing : The rapid application of ice as a stimulus to elicit desired motor patterns in patients with reduced muscle tone or impaired muscle control.
  • RICE : An acronym for rest, ice, compression, and elevation. RICE is used to decrease edema formation and inflammation after an acute injury.
  • Spasticity : Muscle hypertonicity and increased deep tendon reflexes.
  • Thermotherapy : The therapeutic application of heat.
  • Vapocoolant spray : A liquid that evaporates quickly when sprayed on the skin, causing quick superficial cooling of the skin.
  • Vasoconstriction : A decrease in blood vessel diameter. Cold generally causes vasoconstriction.

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Effect of Heat
IR Effect as the Light
References

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