Production of Short-Wave Diathermy and its methods, Physiological effects and dangers of Short-Wave Diathermy.
The question should be answered in four parts,
Short Wave Diathermy: –
- Production,
- Method of application,
- Physiological effects,
- Dangers
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Definition:
Short wave diathermy is the use of high frequency electromagnetic waves of the frequency between 107 and 108 Hz, and a wavelength between 30 and 3 m to generate heat in the body tissues. It provides the deepest form of heat available to the physiotherapist. The therapeutically commonly used frequencies and wavelengths are 27.12 MHz and 11 m.
Production of Short-Wave Diathermy:
Construction
The system consists of two circuits:
- The machine circuit
- The patient circuit.
The machine circuit
It consists of two transformers, whose primary coils are connected to source of AC. One is a step-down transformer and its secondary coil supplies current to the filament heating circuit of triode valve. The other is step-up transformer and connected to Anode Circuit. Anode circuit carries the current produced by valve. It consists of triode valve and oscillator circuit. Oscillator circuit consists of condenser (XY) and inductor or oscillator coil (CD). Current of different frequencies are obtained by selecting suitable condensers and inductances. To produce a current of high frequency the capacitance and inductance used must be small and is made to charge and discharge repeatedly and for obtaining this an oscillator is incorporated in to machine circuit along with valve circuit. Another coil AB lie close to oscillator coil (CD) and has one end connected to the grid of the valve and other through grid leak (GL) resistance to the filament.
The patient circuit
The patient or resonator circuit is coupled to machine circuit by inductor coil (EF) lying close to oscillator coil (CD) and also consist variable condenser (HK) which is usually in parallel to patient terminal. A matching high frequency current is produced in the resonator circuit by electromagnetic induction. For this to happen the oscillator and resonator circuits must be in resonance with each other, which requires that the product of inductance and capacitance must be the same for both circuits.
Working
The AC from main passes through primary coils of the transformers and EMF is induced in secondary coils. An EMF of 20–25 volt is set-up in secondary coil of step-down transformer and produces current through filament of the valve. The filament is heated and thermionic emission takes place and current flows through valve. The EMF of about 4000 volts is induced in the secondary coil of step-up transformer and provided that anode of valve is positive and filament is negative, current flows in anode circuit. The electrons flows from filament to anode through valve, through oscillator coil in direction C to D and to transformer back to filament.
The electron form in CD will induce EMF in coil AB in direction that electrons will move to grid of valve making it negative thus blocking the flow of electrons from filament. This will lead to dying of current in anode circuit. This reduction in current will lead to self-induced EMF. According to Lenz law, this EMF will try to prevent fall in current by offering resistance to flow of current. This will charge condenser X (positive) and Y (negative) polarity opposite to earlier one. Now when self-induced EMF totally dies away, condensers again discharges through oscillator coil, but in opposite direction (D to C).
Flow of current from D to C induce an EMF in AB such that electrons move from A to B and grid loses its negative charge and anode current flows again. This sequence continues and each time condenser charges and discharges through oscillator circuit leading to production of high frequency current (SWD).
Method of application of SWD
- Inductive coil
An inductive diathermy applicator is made up of a coil through which an alternating electrical current flow. The alternating current in the coil produces a magnetic field perpendicular to the coil, which in turn induces electrical eddy currents in the tissues. These induced electrical currents cause charged particles in the tissue to oscillate. The friction produced by this oscillation produces an elevation in tissue temperature.
Inductive coil applicators have traditionally been produced in two basic forms—cables and drums—and have recently also become available in garments.
The cables are bundles of plastic-coated wires that are applied by wrapping them around the patient’s limb. When an alternating electrical current flow through these wires, eddy currents are induced inside the limb. The garments, in the form of sleeves, have cables inside them that wrap around the patient’s limb when the garment is worn.
A drum applicator is made of a flat spiral coil contained within plastic housing. Diathermy devices with drum applicators may have one or two drums or a single drum that can be bent to conform to the area being treated. The drum is placed directly over the area being treated, and the flow of alternating electrical current in the coil produces a magnetic field, which in turn induces eddy currents within the tissues directly in front of it.
- Capacitive plates
Capacitive plate diathermy applicators are made of metal encased in plastic housing, or transmissive carbon rubber electrodes that are placed between felt pads. A high frequency alternating electrical current flows from one plate to the other through the patient, producing an electrical field and a flow of current in body tissue that is between the plates. As current flows through the tissue, it causes oscillation of charged particles and thus an increase in tissue temperature. Heating with capacitive plate diathermy applicators is known as heating by the electrical field method, because the electrical current that generates the heat is produced directly by an electrical field. As with inductive coils, the amount of heat generated in an area of tissue depends on the strength and density of the current, with most heating occurring in tissues with highest conductivity.
Physiological effects of SWD
The thermal effects:
The physiological effects of increasing tissue temperature are include vasodilation, increased rate of nerve conduction, elevation of pain threshold, alteration of muscle strength, acceleration of enzymatic activity, and increased soft tissue extensibility. All of these effects have been observed in response to the application of diathermy.
The non-thermal effects
Increased Microvascular Perfusion
Increasing microvascular perfusion, and thus local circulation, can increase local tissue oxygenation, nutrient availability, and phagocytosis.
Altered Cell Membrane Function and Cellular Activity
It has been reported that electromagnetic fields can affect ion binding to the cell membrane, and that this can trigger a cascade of biological processes, including growth factor activation in fibroblasts, chondrocytes, and nerve cells; macrophage activation; and changes in myosin phosphorylation.
Dangers of Short-Wave Diathermy
Burns: Short wave diathermy can cause burn, therefore the word ‘burn’ must be used to warn the patient of this possible danger. In milder cases tissue is not destroyed but a bright red patch, i.e. erythema is seen and blistering is liable to occur. In severe cases, there is coagulation and therefore destruction the tissues, and then burn appears as a white patch surrounded by a reddened area.
Scalds: A scald is caused by moist heat. It may occur if the area being treated is damp or moist, e.g. due to perspiration, or if damp towels are used for treatment. If the moisture is not localized it does not cause concentration of the field. But if it is localized, it may become overheated and may cause scalding of the skin.
Electric shock: An electric shock can occur if contact is made with the apparatus circuit with the current switched on. It is less possible in modern systems to come in contact with the apparatus circuit. An electric shock could result from contact with the casing of the apparatus if casing is not proper or plastic coating is not made on the apparatus.
Overdose: Overdose of application of treatment may cause an increase in symptoms, especially pain and is most liable to occur when there is acute inflammation within a confined space.
Precipitation of gangrene: Heat accelerates chemical changes, including metabolic processes in the tissues, so increasing the demand for oxygen. Normally, this is supplied by the increased blood flow, but should there be some impedance of the flow of arterial blood to the tissues the demand of oxygen is not met and gangrene is liable to develop. Consequently heat should never be applied directly to an area with an impaired arterial blood supply.
Faintness: Faintness is produced by hypoxia of the brain following a fall in blood pressure. It is particularly liable to occur if, after an extensive treatment, the patient rises suddenly from the reclining to the erect position from the bed. So, patient should not be allowed to rise up suddenly from the bed after the treatment. Patient should be allowed to drink water after treatment.
Giddiness: Any electrical current applied to the head may cause giddiness due to its effects on the contents of the semicircular canals. All diathermic treatments to the head should be given with the patient fully supported and, if possible, with the head in a horizontal or an erect position. Also, it is wiser to avoid concentration of diathermy currents to the eyes because of poor dissipation of heat from the eyes.
Dangers to hearing aids or cardiac pacemakers: As the short wave diathermy produces substantial amount of radiofrequency energy, it may cause interference with the electrical implants such as hearing aids or cardiac pacemakers. Such patients those who are using hearing aids or cardiac pacemakers should not be treated with short wave diathermy and should not be allowed to come in close proximity of the apparatus for at least two meters.
Dangers to other equipment: Low frequency stimulators or interferential therapy apparatus are also at risk with the short wave diathermy. There are also chances of interference and damage to these low frequency stimulators or Interferential therapy apparatus. Therefore, these apparatus must not be kept in the close proximity of the short wave diathermy and at least a distance of two meters must be maintained.
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