Eberhart's Manual of High Frequency Currents
Noble M. Eberhart
Chapter Eleven
Diathermic Currents.
    Characteristics and Synonyms. As Long ago as 1899, Prof. D'Arsonval noticed that passing the D'Arsonval current through certain tissues of the body produced a higher temperature in these tissues. Nagelschmidt, Doyon and others confirmed these early observations, and the name diathermy was applied to this method of increasing the temperature of the internal tissues of the body.
    The direct D'Arsonval current is used and diathermy is synonymous also with the terms thermo-penetration, electro-coagulation and electro-thermic coagulation. The application is always bi-polar.
    It really amounts to the application of electrically generated heat, which may be made to cover certain definite areas between the poles of the apparatus, and may be used to raise the temperature of the tissues or even carried to the point of coagulation with the formation and separation of a slough. In the latter case it is the equivalent of a surgical procedure.
    We are familiar with the action of the galvanic current. If we pass this current through water, owing to electrolysis, the water is decomposed into hydrogen and oxygen which collect at the positive and negative poles respectively, or, if the current is passed through a solution of potassium iodide and starch, a dark color is immediately noticed at the negative pole, due to the decomposition of the potassium iodide and consequent action of the iodine on the starch solution. If we dip our hand into the water while the current is passing, there is a distinct tingling sensation.
    If, now, instead of using the galvanic current, we pass the direct D'Arsonval current through the water or solution, no electrolytic action is apparent and the only effect is an increase of temperature in the solution, as is evidenced by the introduction of a thermometer which shows the steady rise in temperature until the water boils. It is estimated that with the diathermic current the internal temperature of the body, located between the points of application can be raised to a temperature of 110 to 140 degrees Fahrenheit, though many believe 110 degrees to be about the limit of skin and tissue resistance.
    Effects. On account of there being practically no skin resistance to the passage of the diathermic current, the current passes directly from one electrode to the other, heating the tissues that lie between the electrodes. The current can be concentrated in this manner, and is always under absolute control. This is of great value, as a little experience will enable the operator to gauge the amount of current required to produce any given temperature in the tissues and reduces the application of this current to exact measurement, so difficult to obtain when applying an ordinary high frequency current.
    If electrodes of the same amount of surface are used on the opposite sides of a part, the same effect would be produced at each electrode or a uniform effect produced between the two, making allowance for the fact that fatty tissues heat quickly and that bones are slow to heat, but retain their heat longer, while muscular tissues are slower to heat than either fat or bones.
    If we decrease the size of one electrode, since the same amount of current is passing, there will be a more intense heat over the area of the smaller electrode. If sufficient current is used the heat acts like the cautery, coagulating and destroying the tissue.
    By regulating the size of the electrodes the heat may be delivered wherever desired within the tissues, thus the value of this current in all inflammatory conditions of joints, etc.
    When the cauterizing effect is desired, the active electrode is usually a metal point or a wire. In this form it represents one form of fulguration. It is employed in papilloma of the bladder. (The technique is given under this heading in Chapter VI.)
    Diathermic currents are measured with the hot wire meter.
    Range of Usefulness. Outside of their use in one form of fulguration as above referred to, they nave been very successfully used in connection with deep Roentgen therapy in malignant growths, and in the form of increased heat without carrying it to tissue coagulation, in arteriosclerosis, local or peripheral disturbances, sciatica, neuralgia, lumbago, asthma neuritis, headaches and rheumatic or other forms of arthritis. Many other conditions naturally suggest themselves where these currents may be employed.
    Techniques. For increased internal heat, the method of the use of this current is as follows: Connecting cords are attached to electrode of suitable size or shape for the part that is to be treated. These electrodes are made either of solid discs, or a thin brass sheet covered with absorbent material. In order to make better contact they are first covered with gauze saturated with a normal salt solution, and the part to be treated is also moistened with this solution,. The control of the current is easily maintained by using a meter and the exact amount that is applied can be measured at all times. In general treatment, the current is slowly turned on until the meter shows from 700 to 800 milliamperes passing, and then it is gradually increased up to the tolerance of the patient. The operator should remember that it is better to commence with a small current and gradually and uniformly heat the part between the electrodes rather than to start with a heavier current where the greatest effect is nearer the two electrodes, for if too much current is used, the tissues near the electrodes will be heated beyond the pont of tolerance before the tissues that lie deeply are heated as much as they should be, and burns will be produced.
Fig. 64b - A New Electrode for Modified Diathermy.

    In the second method, where coagulation and cauterization are desired, the active electrode must be metallic, and if extensive work is to be done anaesthesia is required.
    Dr. G. E. Pfahler (Journal Advanced Therapeutics, Dec., 1914) has accomplished remarkable results by combining this method with deep Roentgen therapy in malignant growths. The work is done under an anaesthetic, as the pain is severe. In most of his cases ether was employed, but in mouth cases hypodermic injections of one or one and a half HMC tablets (hyoseine hydrobromide gr. 1/100; morphine hydrobromide gr. 1; and cactoid gr. 1/64) were used. These in some cases had to be supplemented by ether.

Fig. 64c - Diathermy Electrodes.

    He uses an instrument capable of generating 1,000 to 2,000 milliamperes. In removing a portion of a lip he uses a ball electrode inside, about three-eights of an inch in diameter, and a needle-point electrode externally. He says; "I would outline the area of the diseased tissue to be removed by allowing the current to flow from this point toward the ball electrode on the inside, and then coagulate the entire diseased tissue. At first I depended upon the needle electrode to actually carve out the diseased tissues, but now I find it simpler to cut this away with a pair of curved scissors after coagulation, always cutting within the coagulated tissues. In this way there is no bleeding and the edges are completedly sealed off.
    "In destroying an extensive lesion in the cheek I used a flat electrode, one inch in diameter, on the inside and a point electrode on the outside. In destroying a portion of the tongue I have used two point electrodes. The electrodes are held in contact with the tissue."