The electric heating pouch is compared with the conventional electric heating pad. The standard flat electric heating pad has been in use for the past eight decades to treat muscle aches, sprains and joint pains. The basic flaw in the design of the heating pad is that, by definition, it is flat. This makes it inconvenient to use over most body parts. Also, the heat is not uniform and much of the heat generated is lost to the surrounding atmosphere, significant enough to decrease the efficiency.
Against this backdrop, the electric heating pouch was developed. The electric heating pouch is contoured to fit all major joints of the body, which ensures that most of the heat generated is transferred to the joint and not lost to the external environment. This allows for a lower temperature setting than the standard electric heating pad, making it safer. Also the flexible design and innovatively packed heating element provides deep penetrating heat necessary for faster tissue healing and better pain relief.
Key words: Superficial heat; electric heating pad; electric heating pouch.
Heat and cold therapies have been used in the treatment of muscle strains, ligament sprains and arthritis of the joints for ages. Raising the temperature of the injured tissue through the application of local heat has been used for centuries to provide symptomatic relief of pain and muscle spasm. Local heat therapy has also been used to promote tissue healing by increasing the local blood flow, and to relax stiff joints and tight muscles for exercise.1-5 Heat therapy acts on the basic principle that healing of any tissue injury requires adequate blood supply which carries oxygen and other healing nutrients while removing the toxic by-products of the damaged tissue.
The employment of heating agents in physical therapy is based not just on historic and observational data, but on scientific rationale as well. For instance, raising the temperature of collagen tissue alters its viscoelastic properties, enhancing the results of passive stretching to improve the range of movements of a joint.6,7
Various types of thermal agents are available for tissue heating, which can be broadly classified as superficial and deep heating agents.
Superficial heating agents, as the term suggests, primarily only raise the temperature of the skin and the superficial layers of the sub-cutaneous tissue. The insulating effect of the sub-cutaneous layer of fat prevents further penetration of the heat. Superficial heating agents come in various forms such as hot packs, paraffin wax, and fluidotherapy. In addition various home-based methods of superficial heating are available, of which the electric heating pad has been an important part.
Superficial heating agents find their application primarily in applying heat to superficial joints, such as the small joints of the hand, which are entirely sub-cutaneous on the dorsum of the hand. They may also raise the temperature of the deeper structures such as muscles, tendons and ligaments through reflex mechanisms. By increasing the temperature of the collagen tissue, they not only provide pain relief but increase its extensibility as well.
Deep heating agents such as continuous-wave ultrasound and short wave diathermy are used to raise the temperature of tissues 3 - 5 cms deep without burning the skin and sub-cutaneous tissues.
FACTORS AFFECTING PHYSIOLOGIC RESPONSE TO TISSUE HEATING
Elevation of tissue temperature results in many physiologic changes both locally and in other parts of the body. The magnitude of these physiologic responses depends upon (1) The range of the subsequent temperature, (2) the rate of thermal energy transfer to the tissue, and (3) the volume of tissue directly exposed to the heating agent.
(1) Range of the subsequent temperature
To achieve a therapeutic response in the tissues, tissue temperature must be elevated to between 40Ëšc and 45Ëšc (104ËšF to 113ËšF).1 Within this temperature range, vasodilation and hyperemia occur which aid in tissue healing. Higher temperatures can cause tissue damage. Temperatures below 40Ëšc are considered too mild to be of therapeutic significance.
(2) Rate of temperature increase
Tissue response to heat therapy depends upon the rate of increase of thermal energy of the tissue. Too slow a temperature rise in the tissue could be ineffective due to dissipation of the heat by the increased blood flow in the tissue. On the other hand, too fast a temperature rise could lead to excessive build-up of heat leading to stimulation of pain receptors and causing tissue damage.
(3) Volume of tissue exposed
Physiologic responses to temperature elevation in tissues are not confined to the area in contact with the heating agent alone, but also occur in areas remote from the site of heat application. Generally, the greater the volume of tissue involved in thermal energy uptake, the greater is the occurrence of reflex, or consensual responses in remote areas and for systemic changes. Systemic responses may include decrease in mean blood pressure, increase in heart rate, and an increase in pulmonary minute ventilation.1
PHYSIOLOGIC RESPONSES TO ELEVATION OF TISSUE TEMPERATURE
Several physiologic changes occur in response to the transfer of thermal energy to the tissue from a heating agent. The most relevant of these affect cellular metabolic activity, hemodynamic function, skeletal muscle activity, neural activity, and collagen tissue physical characteristics. A basic understanding of these responses and how they may be adversely affected by excessive temperature elevation is elementary to ensure safe treatment delivery.
Cellular chemical reactions are influenced by body temperature. Roughly, a 10Ëšc (50ËšF) rise in temperature will cause a twofold to threefold increase in cellular chemical activity and metabolic rate.8, 9 This increases energy expenditure. An increase in the cellular chemical reaction rate increases the oxygen uptake by the tissues.10 Therefore, more oxygen and nutrients will be available to accelerate tissue healing.5 Temperature rise beyond 45Ëšc to 50Ëšc (113ËšF to 122ËšF) causes tissue burning due to thermally induced protein denaturation.
It is a well-known fact that the application of heat to any tissue causes local vasodilation, an increase in the caliber of the blood vessels supplying the tissue. This causes an increase in blood flow to the affected tissue, a process known as hyperemia.1,2,8,10-17 This response is not uniform and may vary in direction and magnitude between different tissues.
Vasodilatation of cutaneous blood vessels may be mediated by an axon reflex, release of chemical mediators in response to heat application, and local spinal reflexes. The release of chemical mediators such as bradykinin and histamine also increases the capillary and post-capillary venule permeability causing tissue edema. Permeability refers to the property of the blood vessel wall by virtue of which it allows fluid containing inflammatory cells, oxygen and nutrients to seep into the injured tissue while clearing away the toxic by-products of tissue injury in the opposite direction. Thus tissue healing is promoted.
The vasodilatory response is not limited to the area in contact with the heating agent. A consensual reflex response occurs at areas removed from the site of heat application. For instance heat applied to the low back may cause vasodilation in the peripheries. This has important implications in patients with peripheral vascular disease.18
It is unlikely that skeletal muscle blood flow is significantly affected by the application of superficial heating agents.11,19,20 But it has an additive effect in increasing skeletal muscle blood flow when combined with exercise.13
The mechanism of action by which the application of superficial heat relieves pain and muscle spasm is not entirely clear. But it is thought to be related to the ability of superficial heat to raise pain threshold17,21, alter nerve conduction velocity22-24, and change muscle spindle firing rates.
Pain threshold may be elevated by applying heat over the area of a peripheral nerve21 or directly over the painful area. This response is related to the ability of superficial heat to alter sensory nerve conduction velocity.23,24
Muscle spasm may begin as a protective reflex to guard against movement of painful joints, but with the onset of the pain-spasm-pain cycle, it requires treatment. Elevation of muscle temperature to about 42Ëšc (107.6ËšF) will alter the firing of muscle spindle afferents, thus relieving spasm.1,25 But superficial heating agents cannot elevate muscle temperature to this desired level. The muscle spasm relieving effect of superficial heat has therefore been explained by an indirect action through reduction in gamma efferent activity.8
Connective tissue response
Heat therapy is used to treat joint stiffness and contracture. Temperature elevation combined with stretching the contracted tissue alters the viscoelastic properties of connective tissues.8,26-28 Pre-treatment with heat followed by application of stretch to the contracted connective tissue causes residual elongation of the tissue taking advantage of its viscous properties. This residual lengthening of connective tissue is called plastic deformation.29
Raising the tissue temperature to therapeutic levels between 40Ëšc and 45Ëšc (104ËšF to 113ËšF) results in greater residual connective tissue elongation with less tissue damage.30,31 Similarly, application of stretch with lower loads for longer duration produces greater increases in joint range of motion with less tissue damage.7,29,31,32
PHYSICAL PRINCIPLES GOVERNING HEAT THERAPY
Flow of heat through tissue depends upon thermal conductivity of the tissue. The intensity of the applied heat, duration of exposure, and the thermal conductivity, density and specific heat of the tissue are all factors, which influence the temperature alteration in the tissue.33 With superficial heat application, the greatest temperature raise occurs in the skin and subcutaneous tissues within 0.5cm from the skin surface. Muscle temperature at a depth of 1 to 2 cms is raised to a lesser degree, and at greater depths the rise is negligible. The maximum temperature gain is obtained within 6 to 8 minutes of heat application. After reaching the peak, tissue temperature gain plateaus. 10, 13, 21
Superficial heat may be employed to raise intra-articular temperatures in joints of the hand and wrist, or foot and ankle, as they have relatively little soft tissue cover on their dorsum.3, 34, 35
Superficial heating agents transfer heat to tissues by conduction, convection or radiation. Conduction describes the method of heat transfer between two static mediums, usually solids, in contact, where thermal energy flows from the medium with higher temperature to the medium with the lower temperature. Hot packs, paraffin wax, and electric heating pads are examples of conductive heating agents.
Convection describes the transfer of heat by the bulk movement of liquid or gaseous molecules. This is the principle used to heat tissues in fluidotherapy.
Radiation is the conversion of thermal energy into electromagnetic radiation. All objects at temperatures above absolute zero (- 273ËšC) emit and absorb radiant energy. This principle is utilized in the infrared heating lamp.
THE ELECTRIC HEATING PAD
The electric heating pad is a superficial heating agent that operates on the principle of conductive heating. Electrically heated pads vary in size from small pads to blankets. Small heating pads are used for providing superficial heat therapy to treat various musculoskeletal aches and pains. Electric blankets are used to prevent perioperative hypothermia.
The heating pad basically consists of an electrical resistance wire, which generates the heat. In order to generate the required therapeutic levels of heat, the wire is, of necessity, thick thus limiting the flexibility and drapeability of the heating element. The resistance wire is covered with an insulating material to enable it to withstand the temperatures produced and to prevent electrical shock and shorting hazards.
Limitations of the electric heating pad
Many commercial heating pads today consist of a heating element loosely encased in a packaging envelope. Often, the heating element is packaged in highly flammable material. Furthermore, the resulting pockets of air decrease the efficiency of heat transfer from the pad to the object over which it is placed. Separation of the heating element from the enclosure increases the bulkiness of the heating pad. Therefore, these pads are not drapeable.
It is especially difficult to use the heating pad over a joint such as the shoulder, elbow, hip, knee, ankle or the wrist because the pad does not fit over any of these major joints and the resultant area of contact is only fractional. This leads to loss of heat to the environment, decreasing its efficacy.
Prolonged exposure of a body part to the electric heating pad may cause thermal burns, even if the pad is set at a low temperature. Hence patients should be cautioned against falling asleep with the heating pad on.
FDA and CPSC recommend the following precautions be taken while using electric heating pads 36:
- Inspect the heating pad before each use to ensure it is in proper working order; discard it if it looks worn or cracked or if the electrical cord is frayed.
- Place the heating pad on top of the body part to be treated and never underneath it as this will trap heat and increase the temperature of the heating pad.
- Unplug heating pad when not in use.
- Use on an infant.
- Use on a sleeping or unconscious patient.
- Use on a paralyzed person or one with temperature insensitive skin.
- Use in an oxygen-enriched environment.
- Sit on or against a heating pad.
- Crush or fold a heating pad during use or storage.
- Unplug heating pad by pulling its connecting cord.
- Use pins or other metallic fasteners to hold heating pad in place.
THE ELECTRIC HEATING POUCH
It is now clear that relief from joint pain by heat treatment needs a device in which the heating elements are uniformly distributed throughout the entire unit that will envelope the entire affected body area to ensure steady and significant penetration of heat to the deeper tissues and to the entire joint for effective relief.
As discussed earlier, this enhanced response to deep penetrating heat is due to better and more uniform vasodilatation. The relief from pain is directly proportional to the extent of vasodilatation in the involved area, due to the improved blood flow and optimal healing process stimulated by uniform vasodilatation.
Incorporating the above concepts, the electric heating pouch is a recent development, which is an improvement over the deficiencies of the electric heating pad. It is a heating agent based on the principles of conductive heating and due to a double-thick waterproof vinyl envelope can be used to deliver dry or moist heat. The major advantage of the electric heating pouch is that it can be contoured to fit snugly over all major joints of the body.
The electric heating pouch is not a heating pad because, by definition, a pad is flat. However, the heating pouch is actually three electric heating panels, formed together to form a sculptured pocket that silhouettes the curvature of the relaxed shoulder, the crooked elbow, knee or ankle.
The heating pouch has a webbed cloth strap, which is used to hold the pouch in place.
The temperature of the heating unit is governed by a heat controller with multiple heat settings.
Application of the electric heating pouch over 6 major joints of the body:
Shoulder: The heating pouch fits snugly over either shoulder. The Velcro fastener on either end of the webbed strap is attached to the Velcro receiver on the rear panel, the pouch is fitted on the shoulder with the electric cord hanging down by the elbow, and the strap is brought around the chest wall and fixed to the Velcro receiver on the front panel.
Elbow: The strap is fixed to one of the heating panels. The elbow is then flexed to fit into the pouch of the heating device. The strap is brought around the neck, adjusted for length, and fixed to the Velcro receiver on the front panel.
Hand/Wrist: The heating pouch is wrapped around the hand/wrist. The pouch design seals the ends, thereby retaining almost all of the heat. A moist hand towel wrapped around the hand/wrist underneath the pouch facilitates deeper, more penetrating heat.
Hip: The heating unit fits snugly around either hip and can be held in place by placing the strap around the waist.
Knee: In the sitting position, one of the side panels is wrapped around the knee. The top panel is then wrapped around the other panel. The webbed strap is attached to top panel, wrapped around the heating unit and then secured underneath it.
Ankle: The heel is placed in the crook of the heating unit; the panels are wrapped snugly around the ankle and held with the strap.
Advantages over the standard heating pad
- The one-size-fits all design of the heating pouch fits snugly over all the major joints of the body.
- Heat is delivered uniformly to all parts of the joint because of the uniform distribution of heating elements throughout the heating unit. This prevents hotspots.
- Can be used to deliver dry or moist heat.
- Very little heat escapes around the edges of the heating unit, thus ensuring most of the heat is delivered to the area.
- As most of the heat is delivered to the tissues, there is no need to raise the temperature of the heating unit.
- The risk of electrical and thermal injuries from the heating pouch is significantly less due to its superior design.
The electric heating pouch successfully fulfills all the requirements for better pain relief and tissue healing in muscle aches, sprains and joint pains. It overcomes all the shortcomings of the standard heating pad. By virtue of its superior design, it ensures uniform delivery of therapeutic heat to any of the major joints of the body without posing any of the risks associated with the heating pad. It is slowly supplanting the standard heating pad as the heating agent of choice for ambulatory and household use.
1) Dr.G.Balamurugan, MBBS. Completed DNB General Medicine and Postgraduate Diploma in Maternal and Child Health training periods. Formerly a Family Physician, Registrar in Department of Kidney Diseases and Institute of Organ Transplantation.
2) Dr.N.P.Veenavani, BHMS, Homoeopathic Consultant, Masters Degree in Psychology.
3) Dr.G.T.Prasad, MBBS, DNB (Orthopaedics), specializing in Arthroscopic and Joint Replacement Surgeries in addition to routine orthopaedic and traumatology commitments.
4) Dr.N.Paarthipan, MBBS, MD (Radiodiagnosis).
5) Dr.R.Radhika Kiran, MBBS, Industrial Health.
1. Lehmann JF and deLateur BJ. Therapeutic heat. In Lehmann JF (ed): Therapeutic Heat and Cold, 4th ed. Williams & Wilkins, Baltimore, 1990.
2. Baker RJ and Bell GW. The effect of therapeutic modalities on blood flow in the human calf. Journal of Orthopedics Sports and Physical Therapy 13:23; 1991.
3. Borrell RM et al. Comparison of in vivo temperatures produced by hydrotherapy, paraffin wax treatment and Fluidotherapy. Physical Therapy 60:1273; 1980.
4. Levi SJ and Maihafer GC. Traditional approaches to pain (Chap. 3). In Echternach J (ed): Pain. Churchill Livingstone, New York; 1987.
5. Halvorsen GA. Therapeutic heat and cold for athletic injuries. Physiotherapy and Sports Medicine 18:87; 1990.
6. Warren CG. The use of heat and cold in the treatment of common musculoskeletal disorders. In Hertling D and Kessler RM. Management of Common Musculoskeletal Disorders, 2nd ed. Harper & Row, Philadelphia; 1989.
7. Lentell G, et al. The use of thermal agents to influence the effectiveness of a low-load prolonged stretch. Journal of Orthopedics, Sports and Physical Therapy 16:20; 1992.
8. Fischer E and Solomon S. Physiological responses to heat and cold. In Licht S (ed): Therapeutic Heat and Cold, 2nd ed. Waverly Press, Baltimore; 1965.
9. Handy JD and Bard P. Body temperature regulation. In Mountcastle VB (ed): Medical Physiology, Vol. 2, 14th ed. CV Mosby, St Louis; 1979.
10. Abramson D et al. Changes in blood flow, oxygen uptake and tissue temperatures produced by the topical application of wet heat. Archives of Physical Medicine and Rehabilitation, 42:305; 1961.
11. Crockford GW and Hellon RF. Vascular responses in human skin to infra-red radiation. Journal of Physiology. 149:424, 1959.
12. Crockford GW, Hellon RF and Parkhouse J. Thermal vasomotor response in human skin mediated by local mechanisms. Journal of Physiology 1962; 161:10.
13. Greenberg RS. The effects of hot packs and exercise on local blood flow. Physical Therapy 1972; 52:273.
14. Krusen EM, et al. Effects of hot packs on peripheral circulation. Archives of Physical Medicine 195; 31:145.
15. Randall BF, Imig CJ, and Hines HM. Effects of some physical therapies on blood flow. Archives of Physical Medicine 1952; 33:73.
16. Fyfe M. Skin temperature, colour, and warmth felt, in hydrocollator pack applications to the lumbar region. Australian Journal of Physiology 1982; 28:12.
17. Wadsworth H, and Chanmugam APP. Electrophysical Agents in Physiotherapy. 2nd ed. Science Press, Marrickville, NSW, 1988.
18. Abramson DI, et al. Changes in blood flow, oxygen uptake and tissue temperatures produced by therapeutic agents. III. Effect of indirect or reflex vasodilation. American Journal of Physical Medicine 1961; 404:5.
19. Wyper DJ, and McNiven DR. Effects of some physiotherapeutic agents on musculoskeletal blood flow. Physiotherapy 1976; 62:83.
20. Cobbold AF, and Lewis OJ. Blood flow to the knee joint of the dog: Effect of heating, cooling and adrenaline. Journal of Physiology 1956; 132:379.
21. Lehmann JD, Brunner GD, and Stow RW. Pain threshold measurement after application of ultrasound, microwaves and infrared. Archives of Physical Medicine and Rehabilitation 1958; 39:560.
22. Abramson DL, et al. Effect of tissue temperatures and blood flow on motor nerve conduction velocity. Journal of American Medical Association 1966; 198:1082.
23. Currier DP, and Kramer JF. Sensory nerve conduction: Heating effects of ultrasound and infrared. Physiotherapy Canada 1982; 34:241.
24. Halle JS, Scoville CR, and Greathouse DG. Ultrasoundï¿½s effect on the conduction latency of the superficial radial nerve in man. Physical Therapy 1981; 61:345.
25. Mense S. Effects of temperature on the discharges of muscle spindles and tendon organs. Pflugers Archives 1978; 374:159.
26. Le Ban MM. Collagen tissue: Implications of its response to stress in vitro. Archives of Physical Medicine and Rehabilitation 1962; 43:461.
27. Wright V, and Johns RJ. Quantitative and qualitative analysis of joint stiffness in normal subjects and in patients with connective tissue diseases. Annals of Rheumatological Diseases 1961; 20:36.
28. Backlund L, and Tiselius P. Objective measurements of joint stiffness in rheumatoid arthritis. Acta Rheumatica Scandinavia 1967; 13:275.
29. Reid DC. Sports Injury Assessment and Rehabilitation. Churchill Livingstone, New York, 1992.
30. Lehmann JF. Effect of therapeutic temperatures on tendon extensibility. Archives of Physical Medicine and Rehabilitation 1970; 51:481.
31. Warren GC, Lehmann JF, and Koblanski JN. Heat and stretch procedures: An evaluation using rat-tail tendon. Archives of Physical Medicine and Rehabilitation 1976; 57: 122.
32. Sapega AA, et al. Biophysical factors in range-of-motion exercise. Physiotherapy and Sports Medicine 1981; 9:57.
33.Hendler E, Crosbie R, and Hardy JD. Measurement of heating of the skin during exposure to infrared radiation. Journal of Applied Physiology 1958; 12:177.
34. Mainardi CL, et al. Rheumatoid Arthritis: Failure of daily heat therapy to affect its progression. Archives of Physical Medicine and Rehabilitation 1979; 60:390.
35. Wakim KG, Porter AN, and Krusen KH. Influence of physical agents and of certain drugs on intra-articular temperature. Archives of Physical Medicine and Rehabilitation 1951; 32:714.
36. U.S. Food and Drug Administration/ Consumer Product Safety Commission Public Health Advisory: Hazards Associated with the Use of Electric Heating Pads. 1995.
37. Consumer Product Safety Commission, Death Certificate File and Injury or Potential Injury File.
38. Consumer Product Safety Commission, National Electronic Injury Surveillance System (NEISS).