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Thermal Effects of Diathermy

Thermal Effects of Diathermy Thermal Effects of Diathermy Allan Hemingway , Ph.D. Departments of Physics and Biophysics and Physiological Chemistry, University of Minnesota, Minneapolis Excerpt IN diathermy the tissues of the human body are traversed by alternating electric currents of such high frequency that no faradaic stimulation of the tissue takes place, the non-stimulation being due, according to Nernst (1908) and Hill (1910), to the relatively small motion of the electrolytic ions in the rapidly alternating electric field. All investigators in the field of diathermy attribute the physiological effects of the high frequency current to the production of heat within the tissues. Sellheim (1910), using a mercury thermometer, and Furstenberg and Schemel (1912), a platinum resistance thermometer, in body cavities between the electrodes, have shown that the heating effect takes place in the deeper tissues. Lonergan (1927), and Binger and Christie, using thermocouple needles with apparatus similar to that described by Clark (1922), and using anesthetized dogs, have demonstrated the effect of the blood circulation on the temperature distribution in the deep-lying tissues in diathermy. From the experiments of Deesen (1913) and Maccleod (1920) it is seen that diathermy is more efficient in heating the deeper tissues than surface-heating methods. The heating of the tissue is attributed to the joule effect of the electric current, when the heat energy given to any conductor per second is I 2 R, I being the heating current and R the resistance of the conductor. In the diathermy machine used in clinical practice the ammeter in series with the patient is the sole method of determining the diathermy dosage. This will not enable one to determine the heat energy given to the patient if the resistance of the patient is not known, but it is useful in setting an upper limit to the current to prevent burning of the skin at the electrodes. In many cases the electrode is small in comparison with the section of the body to which it is applied. Thus there will be a converging of the current stream lines at the electrode, and, the current density being greatest at that place, the greatest heating will occur there. This is especially the case if there is a thick layer of subcutaneous fat of high resistance beneath the skin at the electrode, as Wildermuth (1911) has shown that fat has a higher specific resistance than other tissue. For many years it was assumed that the current as registered by the milliamperemeter gave the true heating current. This conclusion was perhaps based on the experiments of Nesper (1910), who measured the current and voltage when dead animal tissue and a non-inductive resistance of the same value as the dead animal tissue were in the circuit. He found that the tissue behaved as a pure resistance and concluded from this that the capacitative effect was small. However, the resistance used was small, the frequency of the machine used was low (300 kilocycles), and the measurements were not calorimetric. Copyrighted by the Radiological Society of North America http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiology Radiological Society of North America, Inc.

Thermal Effects of Diathermy

Hemingway, Allan
Radiology , Volume 14 (2): 99 – Feb 1, 1930
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Publisher
Radiological Society of North America, Inc.
Copyright
Copyright © 1930 by Radiological Society of North America
ISSN
1527-1315
eISSN
0033-8419
DOI
10.1148/14.2.99
Publisher site
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Abstract

Thermal Effects of Diathermy Allan Hemingway , Ph.D. Departments of Physics and Biophysics and Physiological Chemistry, University of Minnesota, Minneapolis Excerpt IN diathermy the tissues of the human body are traversed by alternating electric currents of such high frequency that no faradaic stimulation of the tissue takes place, the non-stimulation being due, according to Nernst (1908) and Hill (1910), to the relatively small motion of the electrolytic ions in the rapidly alternating electric field. All investigators in the field of diathermy attribute the physiological effects of the high frequency current to the production of heat within the tissues. Sellheim (1910), using a mercury thermometer, and Furstenberg and Schemel (1912), a platinum resistance thermometer, in body cavities between the electrodes, have shown that the heating effect takes place in the deeper tissues. Lonergan (1927), and Binger and Christie, using thermocouple needles with apparatus similar to that described by Clark (1922), and using anesthetized dogs, have demonstrated the effect of the blood circulation on the temperature distribution in the deep-lying tissues in diathermy. From the experiments of Deesen (1913) and Maccleod (1920) it is seen that diathermy is more efficient in heating the deeper tissues than surface-heating methods. The heating of the tissue is attributed to the joule effect of the electric current, when the heat energy given to any conductor per second is I 2 R, I being the heating current and R the resistance of the conductor. In the diathermy machine used in clinical practice the ammeter in series with the patient is the sole method of determining the diathermy dosage. This will not enable one to determine the heat energy given to the patient if the resistance of the patient is not known, but it is useful in setting an upper limit to the current to prevent burning of the skin at the electrodes. In many cases the electrode is small in comparison with the section of the body to which it is applied. Thus there will be a converging of the current stream lines at the electrode, and, the current density being greatest at that place, the greatest heating will occur there. This is especially the case if there is a thick layer of subcutaneous fat of high resistance beneath the skin at the electrode, as Wildermuth (1911) has shown that fat has a higher specific resistance than other tissue. For many years it was assumed that the current as registered by the milliamperemeter gave the true heating current. This conclusion was perhaps based on the experiments of Nesper (1910), who measured the current and voltage when dead animal tissue and a non-inductive resistance of the same value as the dead animal tissue were in the circuit. He found that the tissue behaved as a pure resistance and concluded from this that the capacitative effect was small. However, the resistance used was small, the frequency of the machine used was low (300 kilocycles), and the measurements were not calorimetric. Copyrighted by the Radiological Society of North America

Journal

Radiology – Radiological Society of North America, Inc.

Published: Feb 1, 1930

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