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In medicine, diathermy [di´ah-ther″me] is electrically induced heat or the use of high-frequency electromagnetic currents as a form of physical or occupational therapy and in surgical procedures. The field was pioneered in 1907 by German physician Karl Franz Nagelschmidt, who coined the term diathermy from the Greek words dia and θέρμη therma, literally meaning "heating through" (adj., diather´mal, diather´mic).
Diathermy is commonly used for muscle relaxation, and as a method of heating tissue electromagnetically or ultrasonically for therapeutic purposes in medicine. It is used in physical therapy and occupational therapy to deliver moderate heat directly to pathologic lesions in the deeper tissues of the body.
Diathermy is produced by three techniques: ultrasound (ultrasonic diathermy), short-wave radio frequencies in the range 1–100 MHz (shortwave diathermy) or microwaves typically in the 915 MHz or 2.45 GHz bands (microwave diathermy), the methods differing mainly in their penetration capability. It exerts physical effects and elicits a spectrum of physiological responses.
Surgically, the extreme heat that can be produced by diathermy may be used to destroy neoplasms, warts, and infected tissues, and to cauterize blood vessels to prevent excessive bleeding. The technique is particularly valuable in neurosurgery and surgery of the eye.
Serbian American engineer Nikola Tesla first noted around 1891 the ability of high frequency currents to produce heat in the body and suggested its use in medicine. At about the same time French physician and biophysicist Jacques Arsene d'Arsonval performed the first systematic studies of the effect of alternating current on the body, and discovered that frequencies above 10 kHz did not cause the physiological reaction of electric shock, but warming. He also developed the three methods that have been used to apply high frequency current to the body: contact electrodes, capacitive plates, and inductive coils. In 1899 Austrian chemist von Zaynek determined the rate of heat production in tissue as a function of frequency and current density, and first proposed using high frequency currents for deep heating therapy. In 1908 German physician Karl Franz Nagelschmidt coined the term diathermy, and performed the first extensive experiments on patients. Nagelschmidt is considered the founder of the field. He wrote the first textbook on diathermy in 1913, which revolutionized the field.
Until the 1920s noisy spark-discharge Tesla coil machines were used. These were limited to frequencies of 0.1 - 2 MHz, called "longwave" diathermy. In the 1920s the development of vacuum tube machines allowed frequencies to be increased to 10 - 300 MHz, called "shortwave" diathermy.
The three forms of diathermy employed by physical therapists are ultrasound, short wave and microwave. The application of moderate heat by diathermy increases blood flow and speeds up metabolism and the rate of ion diffusion across cellular membranes. The fibrous tissues in tendons, joint capsules, and scars are more easily stretched when subjected to heat, thus facilitating the relief of stiffness of joints and promoting relaxation of the muscles and decrease of muscle spasms.
Ultrasound diathermy employs high-frequency acoustic vibrations which, when propelled through the tissues, are converted into heat. This type of diathermy is especially useful in the delivery of heat to selected musculatures and structures because there is a difference in the sensitivity of various fibers to the acoustic vibrations; some are more absorptive and some are more reflective. For example, in subcutaneous fat, relatively little energy is converted into heat, but in muscle tissues there is a much higher rate of conversion to heat.
The therapeutic ultrasound apparatus generates a high-frequency alternating current, which is then converted into acoustic vibrations. The apparatus is moved slowly across the surface of the part being treated. Ultrasound is a very effective agent for the application of heat, but it should be used only by a therapist who is fully aware of its potential hazards and the contraindications for its use.
Short wave diathermy machines use two condenser plates that are placed on either side of the body part to be treated. Another mode of application is by induction coils that are pliable and can be molded to fit the part of the body under treatment. As the high-frequency waves travel through the body tissues between the condensers or the coils, they are converted into heat. The degree of heat and depth of penetration depend in part on the absorptive and resistance properties of the tissues that the waves encounter.
Short wave diathermy operations use the ISM band frequencies of 13.56, 27.12, and 40.68 megahertz. Most commercial machines operate at a frequency of 27.12 MHz, a wavelength of approximately 11 meters.
Short wave diathermy usually is prescribed for treatment of deep muscles and joints that are covered with a heavy soft-tissue mass, for example, the hip. In some instances short wave diathermy may be applied to localize deep inflammatory processes, as in pelvic inflammatory disease.
Microwave diathermy uses microwaves, radio waves which are higher in frequency and shorter in wavelength than the short waves above. Microwaves, which are also used in radar, have a frequency above 300 MHz and a wavelength less than one meter. Most, if not all, of the therapeutic effects of microwave therapy are related to the conversion of energy into heat and its distribution throughout the body tissues. This mode of diathermy is considered to be the easiest to use, but the microwaves have a relatively poor depth of penetration.
Microwaves cannot be used in high dosage on edematous tissue, over wet dressings, or near metallic implants in the body because of the danger of local burns. Microwaves and short waves cannot be used on or near persons with implanted electronic cardiac pacemakers.
Hyperthermia induced by microwave diathermy raises the temperature of deep tissues from 41 °C to 45 °C using electromagnetic power. The biological mechanism that regulates the relationship between the thermal dose and the healing process of soft tissues with low or high water content or with low or high blood perfusion is still under study. Microwave diathermy treatment at 434 and 915 MHz can be effective in the short-term management of musculo-skeletal injuries.
Hyperthermia is safe if the temperature is kept under 45 °C or 113 °F. The absolute temperature is, however, not sufficient to predict the damage that it may produce.
Microwave diathermy-induced hyperthermia produced short-term pain relief in established supraspinatus tendinopathy.
The physical characteristics of most of the devices used clinically to heat tissues have been proved to be inefficient to reach the necessary therapeutic heating patterns in the range of depth of the damage tissue. The preliminary studies performed with new microwave devices working at 434 MHz have demonstrated encouraging results. Nevertheless, adequately designed prospective-controlled clinical studies need to be completed to confirm the therapeutic effectiveness of hyperthermia with large number of patients, longer-term follow-up and mixed populations.
Microwave diathermy is used in the management of superficial tumours with conventional radiotherapy and chemotherapy. Hyperthermia has been used in oncology for more than 35 years, in addition to radiotherapy, in the management of different tumours. In 1994, hyperthermia was introduced in several countries of the European Union as a modality for use in physical medicine and sports traumatology. its use has been successfully extended to physical medicine and sports traumatology in Central and Southern Europe.
Surgical diathermy is usually better known as "electrosurgery". (It is also referred to occasionally as "electrocautery", but see disambiguation below.) Electrosurgery and surgical diathermy involve the use of high frequency A.C. electric current in surgery as either a cutting modality, or else to cauterize small blood vessels to stop bleeding. This technique induces localized tissue burning and damage, the zone of which is controlled by the frequency and power of the device.
Some sources insist that electrosurgery be applied to surgery accomplished by high-frequency alternating current (AC) cutting, and that "electrocautery" be used only for the practice of cauterization with heated nichrome wires powered by direct current (DC), as in the handheld battery-operated portable cautery tools.
Diathermy used in surgery is of typically two types.
- Monopolar, where electric current passes from one electrode near the tissue to be treated to other fixed electrode (indifferent electrode) elsewhere in the body. Usually this type of electrode is placed in contact with buttocks or around the leg.
- Bipolar, where both electrodes are mounted on same pen-like device and electric current passes only through the tissue being treated. Advantage of bipolar electrosurgery is that it prevents the flow of current through other tissues of the body and focuses only on the tissue in contact. This is useful in microsurgery and in patients with cardiac pacemaker.
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Burns from electrocautery generally arise from a faulty grounding pad or from an outbreak of a fire. Monopolar electrocautery works because radio frequency energy is concentrated by the surgical instrument's small surface area. The electrical circuit is completed by passing current through the patient's body to a conductive pad that is connectd to the radio frequency generator. Because the pad's surface area is large relative to the instrument's tip, energy density across the pad is reliably low enough that no tissue injury occurs at the pad site. Electrical shocks and burns are possible, however, if the circuit is interrupted or energy is concentrated in some way. This can happen if the pad surface in contact is small, e.g. if the pad's electrolytic gel is dry, if the pad becomes disconnected from the radio frequency generator, or via a metal implant. Modern electrocautery systems are equipped with sensors to detect high resistance in the circuit that can prevent some injuries.
As with all forms of heat applications, care must be taken to avoid burns during diathermy treatments, especially to patients with decreased sensitivity to heat and cold. With electrocautery there have been reported cases of flash fires in the operating theatre related to heat generation meeting chemical flash points, especially in the presence of increased oxygen concentrations associated with anaesthetic.
Concerns have also been raised regarding the toxicity of surgical smoke produced by electrocautery. This has been shown to contain chemicals which may cause harm by inhalation by the patients, surgeon or operating theatre staff.
For patients that have a surgically implanted Spinal Cord Stimulator (SCS) system, diathermy can cause tissue damage through energy that is transferred into the implanted SCS components resulting in severe injury or death.
Medical diathermy devices were used to cause interference to German radio beams used for targeting nighttime bombing raids in World War II during the Battle of the Beams.
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