Therapeutic Microwave and Shortwave Diathermy
. ., - ' ~·.- :·..:.;:: ___ ,,.,: ··"'·. ' A Review of Thermal Effect-i-veness, Safe Use, and State of the Art: 1984
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(Continued on inside back cover) HHS Publication FDA 85-8237
Therapeutic Microwave and Shortwave Diathermy
A Review of Thermal Effectiveness, Safe Use, and State of the Art: 1984
Luther Kloth Mary Ann Morrison Marquette University, Milwaukee, Wisc. • Barbara H. Ferguson Project Officer Office of Training and Assistance
WHO Collaborating Centers for:.~ • Standardizatio_n of Protection ' . ,~ Against Nonionizing Radiations Ill • Training and General Tasks in · Radiation Medicine ~ • Nuclear Medicine S.~ ~;;;;;;.-.r.,.,.. a_;;.,.;;::;
December 1984
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES Public Health Service Food and Drug Administration Center for Devices and Radiological Health Rockville, Maryland 20857
For sale by the Superintendent of Documents, U.S. Government Printing Office Washlni:ton, D.C. 20402 FOREWORD
In October 1982, the Food and Drug Administration established the Center for Devices and Radiological Health (CDRH) by merging the Bureau of Medical Devices and the Bureau of Radiological Health.
The Center develops and implements national programs to protect the public health in the fields of medical devices and radiological health. These programs are intended to assure the safety, effectiveness and proper labeling of medical devices, to control unnecessary human exposure to potentially hazardous ionizing and nonionizing radiation, and to ensure the safe, efficacious use of such radiation.
The Center publishes the results of its work in scientific journals and in its own technical reports. These reports provide a mechanism for disseminating results of CDRH and contractor projects. They are sold by the Government Printing Office and/or the National Technical Information Service.
Also, CDRH technical reports in radiological health are made available to the World Health Organization (WHO) under a memorandum of agreement between WHO and the Department of Health and Human Services. Three WHO Collaborating Centers, established under the Bureau of Radiological Health, continue to function under CDRH:
WHO Collaborating Center for Standardization of Protection Against Nonionizing Radiations;
WHO Collaborating Center for Training and General Tasks in Radiation Medicine; and
WHO Collaborating Center for Nuclear Medicine.
We welcome your comments and requests for further information.
Director Center for Devices and Radiological Health
ii PREFACE
Within the Center for Devices and Radiological Health (CDRH), the Office of Training and Assistance (OTA) is responsible for developing programs to assist health practitioners in the use of radiation in the healing arts. An important aspect of this work is the development and distribution of educational materials, both for students considering the health care profession and for the continuing education of individuals already in practice. Recent surveys conducted by the Food and Drug Administration (FDA) have strongly suggested that many shortwave and microwave diathermy operators may be inadequately trained on the use of their equipment. These surveys also revealed a large number of operator practices that could result in unnecessary radiation exposure to both patients and operators. A subsequent General Accounting Office (GAO) study made similar findings. This led to a recommendation by that Agency that FDA develop materials for use in training diathermy equipment operators that would help them to minimize unnecessary radiation exposure to themselves and to their patients. This publication was developed in response to the GAO and other recommenda tions. This state-of-the-art review of the literature on the therapeutic use of microwave and shortwave diathermy discusses key issues regarding the safe use of these modalities and it identifies gaps in our knowledge about the proper use of this equipment. This publication is the product of a variety of professional interest groups. Initially, the OTA/CDRH engaged a contractor to develop a draft report. Next, it assembled an Advisory Group which was comprised of clinicians, educators, researchers, and other representatives of the various user groups. Finally, the Advisory Group was convened for two days to review, discuss and revise the draft report in order to assure that the final document would be as comprehensive as possible and state-of-the-art in its coverage. We recognize that in an endeavor such as this, the specific interests of all groups will not be fully met. However, the contents of this publication is a compromise made necessary by our desire to make its coverage as broadly applicable as possible.
Joseph S. Arcarese Acting Director · Office of Training and Assistance Center for Devices and Radiological Health
iii Contents
Page
Foreword ...... ii.
Preface ••••.•.••••••••••.••••••••••.•.•..••••.••••.••.•••••• iii
Abstract ...... vi
Acknowledgments ...... vii
Definitions ...... viii
History of Microwave and Shortwave Diathermy ••••••••••••••••••• 1
Fundamentals of Electromagnetic Fields ••••••••••••••••••••••••• 2
Electromagnetic Fields ...... 3 Biophysical Principles of Shortwave and Microwave Diathermy ...... •...... •...... •. , ..•.•. 4 Control of Clinical Diathermy by the Federal Communications Commission •••••••••••••••••••••••••••••••• • 6
Shortwave Diathermy ...... 7
Pulsed Shortwave Diathermy •••••••••••••••••••••••••••••••••• 9
Shortwave Diathermy Electrodes •••••••••••••••••••••••••••••• 9
Microwave Diathermy ...... 11
Microwave Diathermy Applicators •••••••••••••••••••••••••••• 11
Physiological Effects of Diathermy ••••••••••••••••••••••••••••• 14
Use of Electromagnetic Energy in Hyperthermia •••••••••••••••••• 16
Hazards of Shortwave and Microwave Diathermy ••••••••••••••••• 16
Metallic Objects and Electromagnetic Radiation •••••••••••••••• 17
Eye ...••.•.•...... •...... •.•...... •.....• 18
Joints ...... 19
Ischemic Tissue ...... 19
Other Hazards and Concerns for Users •••••••••••••••••••••••• 20
iv Radiofrequency/Microwave Radiation Safety Considerations for the Users (Operators) •••••••••••••••••••••••••••••••••••• 21
Radiation Safety Standards and Guidelines ••••••••••••••••••••• 21
State of the Art ...... 22
Considerations for Applying SWD/MWD ••••••••••••••••••••••• 23
Application of SWD and MWD •••••••••••••••••••••••••••••••• 24
S WD Techniques ...... 28
M WD Techniques ••••••••••••••••••••.•••••••••••••••••••••• 28
Diathermy Treatment Outcomes ••••••••••••••••••••••••••••• 29
Surveys of Diathermy Use in Clinical Practice ••• ~ ••••••••••••• 29
Diathermy in Physical Therapy Education •••••••••••••••••••••• 29
Diathermy and Cancer ..••....•.....•.•...•.•••••.••.••••••. 30
Recommendations ••.••..•.•.•.••.•.•••••..••• ~ ••••••••••••••• 30 Diathermy and Research •••••••••••••••••••••••••••••••••••• 30
Reporting Injuries ...... 31
References . .•...... tt ••••••••••••••••••••• 32
V ABSTRACT
Luther Kloth and Mary Ann Morrison, Marquette University and Barbara H. Ferguson, Project Officer, Office of Training and Assistance. Therapeutic Microwave and Shortwave Diathermy - A Review of Thermal Effectiveness, Safe Use, and State of the Art: 1984. HHS Publication FDA 85-8237 (December 1984) (pp. 37).
This state-of-the-art review of the literature on the therapeutic use of microwave and shortwave diathermy discusses key issues regarding the safe use of these modalities and it identifies gaps in our knowledge about the proper use of this equipment. This publication is the product of input from a variety of health professional groups.
The mention of commercial products, their sources, or their use in connection with material reported herein is not to be construed as either an actual or implied endorsement of"such products by the Department of Health and Human Services (HHS) or the World Health Organization (WHO).
vi ACKNOWLEDGMENTS
This report represents the combined effort of many individuals. Luther Kloth M.S.,P.T. and Mary Morrison M.S.,P.T. prepared the original working draft that was used by the Diathermy State-of-the-Art Group. The group consisted of:
Mary Jo Bamford, P.T., A.T.C. Mike Long, M.D. John J. Bush, A.T.C. Edward L. Maurer, D.C. Dean Currier, P.T., Ph.D. Susan Michlovitz, P.T. John Echternach, P.T., Ed.D. James F. Newton, D.P.M. Jane Elderidge, P.T. Lynda Woodruff, P.T., Ph.D. Joe Gieck, Ed.D., P.T., A.T.C. Normal Yucel, P.T. Marvin Ziskin, M.D.
The following individuals have also researched, revised and/or written portions of the material:
Howard Bassen Ho.ward Waniga, P.T. Gideon Kantor, Ph.D. Paul Ruggera, P .E. James Griffin, P.T., Ph.D. Barbara H. Ferguson, M.A., P. T.
In addition, many researchers and experts in the field have reviewed and commented on this document during its development. The final document represents many changes and additions based on these comments. We wish to express our appreciation to all individuals for their review and comments on this document during its development. Barbara H. Ferguson served as Project Officer and coordinator on the development of this report.
vii DEFINITIONS
Related to Therapeutic Microwave and Shortwave Diathermy
Applicator - (or Director) consists of an antenna from which microwaves are radiated, and a reflector which focuses and directs the microwave radiation toward the tissues.
Capacitance - that property of a system of two or more conductors (usually flat plates) separated by a dielectric which permits the storage of electric charges. Capacitor/Condenser - a device capable of storing electrical charges.
Conduction - a therapeutic modality for heat to be applied to the body by contact with warmed substances.
Convection - a therapeutic modality for applying heat to the body surface by changes in environmental (air) temperature.
Current - the movement or flow of electric charges.
Diathermy - a therapeutic modality for heating of body tissues involving the use of high frequency electromagnetic radiation and eddy currents. Dielectric - a nonconducting substance; an insulator. Director (MW) - See Applicator
Direct Contact Applicator (MW) - modern design applicators which are spaced l cm or less from the body.
Eddy Currents - currents induced inside the body by a magnetic (H) field applied from outside the body. The energy of the eddy current is in turn converted into heat. ffiectric (E) Field - is a region in which a force is exerted on an electric charge. In diathermy, electric fields are often produced between two metallic electrodes. mectrode/Applicator - the metallic structure to which the current and voltage are applied in shortwave diathermy to induce heating in the patient.
ffiectromagnetic Radiation - energy transported through space as electromagnetic waves.
ffiectromagnetic Waves - energy moving at the speed of light in the form of electric and magnetic fields oscillating at right angles to each other and to the direction in which the waves are traveling.
Field - a region of space under the influence of some agent, as electricity or magnetism.
Frequency- the number of cycles or completed oscillations per unit time of a wave.
viii Generator - a device that converts mechanical energy into electrical energy.
Hertz - the frequency unit representing one cycle per second.
Induction - the process of creating an electromotive force and current in a conductor by variation of a magnetic field in that conductor.
Insulator - any substance or appliance with nonconducting properties, that is, a poor conductor of electricity or heat.
Ionizing Radiation - radiation, such as x ray, with high enough energy to remove electrons from atoms or molecules. An atom is ionized when it loses or gains an electron.
Longwave Diathermy - an obsolete modality which used energy produced via a spark gap generator with frequencies between 0.5 and 10 MHz and a wavelengt_h of 600 to 30 meters.
Magnetic (H) Field - any region in which a for,ce is exerted on a magnetic pole. Units are expressed in amperes/meter (A/m). In diathermy a magnetic field is produced in the area around eddy currents.
Magnetron - a vacuum tube with a cathode and multisplit anode used to produce microwave.
Microwave Diathermy (MWD) - a modality using electromagnetic radiation with a frequency above 300 MHz and a wavelength shorter than 1 meter.
Near Field- a region that is within three wavelengths of the radiation source.
Nonionizing Radiation - radiation, such as radiofrequency, ultrasound and microwave, possessing insufficient energy for removing electrons from atoms.
Oscillating Current - flow of electrons in a conductor that is rapidly changing in amplitude and polarity.
Patient Circuit - the part of a diathermy device circuit that transfers the electrical energy to the patient via electromagnetic fields.
Power - work done or energy transferred per unit of time. Units are expressed in watts (W).
Power Density - the term frequently used to describe the porer per surface area. Units are expressed in milliwatts per square centimeter (mW/cm ).
Radiofrequency Radiation - the term generally applied to propagating electromagnetic waves between 10 kilohertz (kHz or 1000 Hz) and 300 gigahertz (GHz or 1 billion Hz) in frequency.
Refraction - the change in direction of electromagnetic waves as they pass from,one medium to another with different dielectric properties.
ix Reflection - the rebounding of waves from the surface of a new medium or barrier.
Shortwave Diathermy (SWD) - the therapeutic heating of body tissues by means of an oscillating electromagnetic field with a frequency between 10 to 100 MHz and a wave length of 30 to 3 meters, respectively.
Spaced Applicators (MWD) - conventional applicators which are spaced 3 to 6 cm from the body. Generally they have nonuniform heating patterns.
Specific Absorption Rate (SAR) - the rate expressed in units of watts per kilogram which quantifies the rate of energy absorbed per unit mass in tissues exposed to electromagnetic fields.
Specific Heat - the number of calories required to raise the temperature of 1 gram of a substance 1 °c.
Standing Waves - stationary wave patterns found in a medium when two sets of waves of equal wavelength and amplitude pass through the medium in opposite directions. User/Operator - the individual applying the diathermy is, or is under the supervision of, the following licensed or certified health professionals: medical and osteopathic physician, dentist, physical therapist, chiropractor, podiatrist, or athletic trainer. Voltage - electromotive force which causes current to flow in a conductor. Units are expressed in volts (V).
Watt - a unit of electric power (energy/time).
Wavelength - the distance between peaks of any two successive waves of electromagnetic field or the distance over which the wave repeats itself.
X THERAPEUTIC MICROWAVE AND SHORTWAVE DIATHERMY A REVIEW OF THERMAL EFFECTIVENESS, SAFE USE, AND ST ATE OF THE ART: 1984
IDSTORY OF MICRO WA VE AND SHORTWAVE DIATHERMY
In the late 19th century, the French physi.cian-physiologist d'Arsonval found that electric currents with a frequency of 10,000 cycles per second (0.01 MHz) or more produced perceptible warming of the skin (1). Using a current of 3 amperes, he discovered that a sensation of warmth occurred without the painful muscular contractions that were caused by lower frequency currents. As early as 1900, physicians began using radiofrequency energy to heat muscles and joints (2). A diathermy unit is a device which induces heat in subcutaneous tissue and muscle for medical purposes through the absorption of electromagnetic energy. The early diathermy delivered unstable frequencies between .05 and 10 MHz (longwave diathermy) and required direct electrode contact with the skin. Use of longwave diathermy for therapeutic heating diminished in the 1930's and 1940's when shortwave diathermy (SWD) generators were introduced. Longwave diathermy was eventually prohibited by international agreement because of interference with radio transmission and reception. In the late 1940's the Federal Communications Commission (FCC) designated specific frequencies and bandwidths for industrial, scientific, and medical purposes. (ISM). Frequencies of 13.56, 27 .12, and 40.68 MHz, with respective bandwidths of 0.014, 0.32 and 0.04 MHz, were assigned for shortwave diathermy. Wavelengths for these frequencies are 22m, llm, and 7m, respectively. Because the frequency of 27.12 MHz had the widest bandwidth tolerance (0.32 MHz), and was easier and less expensive to manufacture, it rapidly became the most common frequency used in commercial shortwave diathermy devices (2). The advent of radar during World War II provided the impetus for considerable electromagnetic (EM) research at frequencies between 100 to 10,000 MHz. Krusen studied microwave exposure on animals and found that deep tissues could be heated with microwaves (3). Microwave diathermy (MWD) for therapeutic purposes became a reality in the postwar 1940's. It was used in various therapeutic settings, such as physical therapy clinics and chiropractor's · offices, to treat a variety of musculoskeletal disorders. Originally, 2450 MHz was the only frequency permitted by the FCC for therapeutic MWD units. However, studies by Schwan in the 1950's on the dielectric properties of biological tissues and wave propagation and absorption by various tissues demonstrated that 2450 MHz was too high a frequency to use for optimum therapeutic effect (2,4). Based on these studies, he advocated lowering the frequency to 900 MHz or less. Lehmann, DeLateur, and associates have published several articles on heating patterns and depth of penetration at frequencies of 2450 and 915 MHz (5-7). Their results indicate that the lower frequency is to be preferred because of its ability to achieve greater depth of penetration and increased blood flow. Although the FCC presently allows frequencies of 915 and 2450 MHz (wavelengths= 33 and 12m, respectively) for MWD (and 13.56 MHz and 27.12 MHz for SWD), the 915 MHz frequency is used primarily in MWD research settings. No commercial MWD device is available at 915 MHz. Modern research has done much to ensure a better understanding of the effects of high frequency electromagnetic energy for the user.
1 The therapeutic use of diathermy in the treatment of various medical conditions has experienced high and low periods of popularity during the past century. Some of the factors that have caused this fluctuating popularity include: (1) overly enthusiastic claims by practitioners, (2) society's quest for a panacea for relief of pain and dysfunction, and, (3) the public concern about the hazards of high frequency radiation. It is difficult to determine the exact number of diathermy treatments given in the United States per year. This is due primarily to the multiple and varied settings in which diathermy treatments are given and to the difference in the backgrounds and therapeutic philosophies of the practitioners who operate the equipment. Treatments are usually provided in the offices of chiropractors, physical therapists, physicians, and various clinics in hospitals or outpatient medical centers.
FUNDAMENTALS OF ELECTROMAGNETIC FIBLDS
Electromagnetic radiation includes light, x rays and radio. Electromagnetic radiation propagates through space as waves and possesses a definite particulate nature since a discrete energy and momentum are associated with them. The wavelength of electromagnetic radiation is inversely proportional to the frequency of the radiation. Radiofrequency radiation wavelength in air ranges from 1.8 miles (3,000 meters) at 100 kHz, to one twenty-fifth of an inch (one millimeter at 300 GHz) (8). Radiowaves (radiofrequency (RF) and microwave radiation) exist in that portion of the electromagnetic spectrum that includes frequencies ranging from about 100 kHz to 300 GHz (300 billion Hz). This form of nonionizing radiation lies below the light and x-radiation (ionizing) portions of the spectrum (Fig. 1). Some of the more common devices that emit relatively significant levels of RF /microwave radiation are shown in Figure 2.
Electromagnetic Spectrum
Light X-ray !Radio Frequ~ncy ) 1 M,crowave1 8 0 30x10 8 9 300 X 10 300 X 10 1014 ' 1018' Frequency (Hz)
Meter Millimeter Micron Angstrom
10 1 10-3 10-6 10-10
Wavelength (meters)
Figure l.
2 Radiofrequency Spectrum Utiltization
RF Radar Sealers.. TV AM CB - .. ... Microwave Ovens Radio • • .. - Shortwave Microwave Diathermy Diathermy • Intrusion • • Alarms 27 2450 ·
1 10 100 1,000 10,000 100,000 Frequency (MHz)
Figure 2.
ELECTROMAGNETIC FIELDS
. Electromagnetic radiation can be broken down into two components: electric and magnetic fields. The electric field strength (E) is expressed in units of volts/meter, while the magnetic field strength (H) is expressed in units of amps/meter. Both fields must be present in any "emission" associated with a radiating device. At distances close to the source of radiation (less than one wavelength), the relationship between the E field and the H field is not easily predicted. The term "near field" is used to describe that region which is close to the radiation source. In this region (closer than one wavelength to the source) both E and H field strengths must be measured to fully assess the potential health hazard. In the near field, both the electric and magnetic field strengths generally fall off quickly as distance from the source increases. This may not always be the case, however, so careful measurements of both E and H fields must be performed at a number of points in the close proximity of a device (8).
In addition to the electric (E) and magnetic (H) field strength, another term, "Power Density," often is used to describe the strength of microwave or radiofrequency radiation. This term is not correct when used to describe radiofrequency or microwave radiation in the near field of an emitter. However, because power density is a term that is in widespread use, and is specified in various safety s;andards, it is mentioned here. Units of milliwatts per square centimeter (mW/cm ) are used to express the level of power density existing at a point (8).
3 BIOPHYSICAL PRINCIPL~ OP SHORTWAVE AND MICROWAVE DIATHERMY
Diathermy generators produce high frequency (10 MHz to 10,000 MHz) electromagnetic (EM) radiation. Frequencies generated by both SWD and MWD generators are much higher than is required to elicit a neuromuscular contractile response. However, these wavelengths and frequencies do permit absorption of EM energy by the cutaneous, subcutaneous, and the more deeply seated muscle tissue. Anytime there is current flow (electron movement) within a metallic conductor there is always simultaneous generation of an electric (E) field parallel to the direction of current flow within the conductor as well as generation of a magnetic (H) field "loop" surrounding the conductors (9). The increased tissue temperature, as a result of diathermy application to the patient muscle tissue, may be due to the E or H field. The E field associated with capacitive applicators allows penetration through the body, from one electrode to the other. Guy has given an excellent technical summary of biophysical investigation of these different reactions (10).
Other agents considered to be thermal energy reservoirs (hot packs, infrared, paraffin and warm water) are intended to alter the temperature of superficial tissues only. Some of these superficial heating agents are capable of providing sufficient thermal energy to increase tissue temperature up to 2.0 cm below the skin surface (11). Several heating agents, when applieavigorously, can elevate superficial muscle temperatures. Only SWD, MWD and ultrasound have been shown to significantly elevate temperatures in deep muscle (11).
In SWD the patient circuit (Fig. 3) produces either a strong electric (E) field (Fig. 4) or strong magnetic (H) field (Fig. 5) or both fields may be nearly equal depending upon the type of applicator/electrode used. It is the absorption of E and/or H field energy in tissue that leads to increased temperature when a patient is exposed to SWD. Regardless of the type of electrode used, the power absorption or specific heating is proportional to the square of the induced E field and the electrical conductivity of the tissue (12).
Patient Circuit
Power Meter
Stable Radio• Power Patient frequency Amplifier Tuning Generator Circuit
Power Timing and Supply Control Circuitry
Figure 3 ..
4 Electric (E) Field
Capacitive Electrodes
+ ...... - ... ---:::-, r:::.:::. ✓ -Cable Electric Field Lines
Figure 4.
Magnetic (H) Field
Single Conductor ,,,, \ t/;.\\ + + ""'..... -'f;;o.,,JO'J.7'J/$1~,,,.,.;."".J:.7\4.c.o.o,'-"' - ""\ \\'-"I/ \\'::,// \\'::I/ , ...... , '.,, '/ I ------◄------✓ -Cable
F1gure 5.
In contrast to SWD, the frequency and wavelength of MWD are such that the E field predominates and little H field is generated and directed at the area to be treated. Unlike transmission of SWD energy, which requires the patient to become part of the patient output circuit, the higher frequency and shorter wavelength energy emitted by MWD gives it certain qualities resembling visible light. Therefore,
5 therapeutic microwaves can be radiated, reflected, and/or focused and do not require close contact between the applicator and the patient (13). A recently designed direct contact MWD applicator minimizes stray radiation, allowing for greater absorption of energy.
CONTROL OF CLINICAL DIATHERMY BY THE FEDERAL COMMUNICATIONS COMMISSION
All diathermy generators are analogous to radio transmitters with the electrode functioning as a broadcast antenna. Energy is delivered to the patient within the frequency ranges specified by the Federal Communications Commission (FCC). Frequencies must be closely controlled to prevent the transmitted energy from interfering with output and/or operation of nearby electrically powered medical and non medical devices. The FCC is responsible for assigning industrial, scientific, and medical (ISM) frequencies and for monitoring the frequency spectrum to ensure that stray energy does not interfere with communications application (Table 1). The FCC has the authority to locate and remove the source of illegal transmission from use or insist on adequate redesign to bring it into conformity with their specified emission limits.
Table 1. Shortwave and microwave diathermy frequencies (FCC approved)
Frequency (MHz) Wavelength Diathermy
13.56 22 meters SWD *27.12 11 meters SWD
40.68 7.5 meters SWD
915 33 cm MWD
*2450 12 cm MWD
Each operating frequency is allowed some fluctuation. A frequency tolerance of + 0.05 percent is allowed when the primary frequency is 13.56 MHz, which equals + 0.007 MHz. When the operating frequency is 27 .12 MHz, + 0.6 percent fluctuation is allowed, equaling±. 0.16 MHz. When the primary frequency-is 2450 MHz the tolerance must be substantially higher. Maintaining the frequency within this range requires crystal control of the oscillating circuit. Since crystal controls at higher powers are much too expensive at microwave frequencies, the FCC allows a fluctuation of ±. 50 MHz which is±. 2 percent.
6 SHORTWAVE DIATHERMY
Two components are common to all SWD devices: (1) radiofrequency generating circuit and (2) the applicator (director) or electrode. The method of application utilizes treatment electrodes that are classified as capacitive or inductive. The capacitive (electric field) method places the patient between two capacitive plates (Fig. 6). The inductive (magnetic field) method utilizes the application of an induction coil (Fig. 7 and 8). Capacitive Applicators .. .•...... ~ ·~--~· -~- i
Depth Effect Surface Effect
Figure 6.
Inductive (Drum) Electrodes
!I•~ ...... • .• I± ~II! : : Condenser : ••••••••••••••••
Monode Figure 7. (Reprinted wtth per mission from Dr. J.F. Lehmann's chapter "Therapeutic Heat," of the 1982 edition of Therapeutic Heat and Cold. l
7 Inductive Shortwave Diathermy (Drum) Electrodes
Each Flap "=:----Contains Coil
Dlplode Diplode Ftgure 8. (RepriMted with permission: from Dr. J. F. Lehmann's chapter "Therapeutic Heat, 11 of the 1982 edi tion of Therapeutic Heat and Cold.)
Different types of SWD electrodes/applicators cause predominantly E field or H field production. For the most effective treatment in a given patient situation it is the responsibility of the operator to know whether the applicator to be used provides predominantly an E or an H field. It would be helpful if the manufacturer could indicate on the equipment the ratio of E to H field energy available for each electrode (14).
Subcutaneous fat is an insulator. Clinically this causes tissue with a high fat content to absorb the E field output, particularly if the direction of the E field is perpendicular to the fat layer as with capacitive electrodes. For the perpendicular direction, this significantly reduces the presence of the E field in muscle tissue because of electromagnetic tissue boundary conditions (15). SWD treatments can produce deep tissue temperature rise without much risk of overheating the intervening subcutaneous fat if the electrodes are an inductive (coil) design and have predominantly an H field output.
The H field output produces negligible absorption within subcutaneous fat. Hence, it readily penetrates to those tissues which have a high electrolyte content. Blood has the highest electrolyte content of any normal human tissue, approximately 0.9 percent. However, blood supply to ligament, fascia, tendon, fat and cartilage is poor. This may limit the ability of these tissues to absorb electromagnetic energy or dissipate thermal energy which accumulates from a significant increase in tissue temperature. An excellent nontechnical review of these fundamental facts about tissue reactions to E versus H fields produced by the diathermies is that published by Hall in 1952 (16).
Measurement of the quantity of E and H field emission from a SWD electrode is difficult. However, probes which will permit quantitative measurement (+ 25 to 50 percent accuracy) of E and H field from unloaded applicators are now commercially available. These probes must be positioned at least 10 cm from the
8 electrodes or the patient. This distance is also recommended for MWD measurement, however, it should be noted that the nonionizing radiation emission from the therapeutic microwaves consist primarily of the E field and can be measured at 5 cm from the applicator.
PULSED SHORTWAVE DIATHERMY
Pulsed S WD was originally. developed to produce non thermal effects in contrast to the thermal effects of continuous diathermy. Pulsed high frequency power is produced by a device that allows control of the duration and pulse rate as well as intensity of the current. This power is then delivered to the applicator in the same manner as standard SWD. Despite many scientific attempts to demonstrate the efficacy of pulsed SWD for therapeutic purposes there is insufficient evidence to support the clinical use of pulsed SWD. There is no specific therapeutic indication for the use of pulsed SWD devices (17).
SHORTWAVE DIATHERMY ELECTRODES
There are two fundamentally different types of applicators in use with SWD, namely capacitive and inductive as previously mentioned. Capacitive applicators deliver only E field to tissues while inductive applicators deliver primarily H field plus a residual E field. The capacitor consists of two plates with an electrical potential (voltage) difference between them and an intervening dielectric (insulator). The patient is an integral part of the circuit in the capacitive electrode design. The two electrode plates constitute the capacitor, and the patient's skin, subcutaneous fat, muscle, bone, etc. serve as the dielectric. The capacitive electrode system consists of two bare metal plates, each usually surrounded by a "plate guard." The guard prevents direct contact of the electrode to the patient and excessive skin and subcutaneous fat heating. Most plates are manually movable through a distance· of about 3 cm within the guard. For those models where the plate is rigid within the plate guard, the guard can be manually adjusted so that the plate is positioned 2 to 3 cm distant to the skin. However, with newer guard design the bare metal plate cannot be touched unless the plate guard is removed. Severe electric burns may occur when it is possible for the operator or patient to touch the bare metal conductor. In the capacitive applicator system each plate is physicaily connected by wire or cable to the oscillating circuit. Once set up for treatment, the plates should always be positioned so the distance between any part of the two plates is at least as great as the diameter of the plate. The majority of plates range in diameter from 7.5 to 17 .5 cm. The high frequency alternating current allows the energy to flow from one · plate to another. For optimum heating, the guard should be as close to the skin as possible and the plate as far away from the skin as the plate guard allows. This positioning of the plate and guard with reference to the skin provides for increased relative depth and heating effects of the tissues absorbing the energy (13). Figure 4 shows the placement of capacitive electrodes with resulting heating patterns. An electric field is created between the plates which heat that area of the body. This results in a patient circuit which wl)en correctly adjusted produces heat from the diathermy energy. (13).
9 Most conventional SWD units deliver high power to the patient when capacitive electrodes are spaced close to the body and low power when the distance between electrode and skin increases. However, some SWD units* are engineered to automatically reduce power output when the electrodes are close to the body to avoid the possibility of power producing superficial burns. In these devices, when the electrode-skin distance is 2.5 cm or more, a deep field efficient circuitry automatically provides full power output for greater depth and uniformity of heating. This is dependent on variations in the loading of the electrodes and generator by the patient. SWD units that generate deep field heating through capacitive electrodes permit heating of deep structures. With capacitive electrodes the E field is oriented between the plates and is perpendicular to the body surface between them. The electric field created by capacitive electrodes may selectively heat fatty tissues because fat has a higher electrical resistance compared to muscle and moist tissues (10).
The patient circuit is different for the inductive shortwave technique. When using an inductive applicator, a magnetic field is established. This magnetic field in the conductive body tissues serves to create a current flow (eddy currents) and produce heat (10,18). Inductive applicators do not provide the same depth of heating produced by the capactive electrodes and they do not selectively heat subcutaneous fat as much as the capacitive electrodes. The induction applicator is available in two forms-the more widely used "drum" and the "cable." The drum (Figs. 7 & 8) may be a single or multiple coil unit. The single coil unit (monode or minode) is intended to treat only one surface. The multiple coil unit (diplode) is hinged so that more than one surface can be treated simultaneously. (Fig. 8). The induction coil in the drum electrode is housed within a rigid electrical insulator which allows propagation of E and H field radiation only from the patient treatment surface. The treatment surface of the housing serves the same function as the plate guard in the capacitor electrode system to space the conductor {coil) away from the skin.
The cable electrode {Fig. 9) may vary in length from 2 to 5 meters and may be wound in a planar or {pancake) fashion to be placed upon the patient part. It can also be wound in a helical (tubular) fashion around the surface of a body part such as a limb. Hence, the cable electrode can be used to treat one or more surfaces simultaneously. However, its greater flexibility compared to the drum requires more time to position the cable properly for each treatment. Because there is no rigid housing as with the drum or a capacitive electrode, the user must always add a dielectric material, such as dry toweling, between the cable insulation and the patient's skin. This dielectric is needed to absorb perspiration and to space the conductor away from the patient which could cause a superficial skin burn. For ideal energy transfer to the patient, the intervening dielectric material shoulctbe 1 to 2 cm thick, except when the body weight is resting on the cable which requires a dielectric 2 to 3 cm thick. Energy can be undesirably coupled from coil-turn to coil-turn along the surface of the skin. Therefore, when the cable turns are less than 3 cm apart, inefficient operation results.
*The only commercially available SWD device that utilizes this principle is Siemens, Ultratherm 708: ELMED INC., Addison, IL 60101.
10 Shortwave Diathermy Cable Electrode
./2 .. . ---~-~ .... ~~ ~ '. ' i
(a) (b)
Figure 9. (Reprinted wttH permtsstoni from Dr. J.F. Lehmann's chapter "Therapeutic Heat, 11 of the 1982 editfon of Therapeu tic Heat and Cold.) All diathermy devices have cables which conduct the EM energy from the oscillating circuit to the electrode. Many SWD units do not use coaxial (shielded) cables to transmit energy from the generator to the electrode. When SWD electrodes are energized, these unshielded cables emit high levels of stray radiation in all directions from the cable. A further discussion of this is presented later in this report.
MICROWAVE DIATHERMY
The basic components of microwave diathermy are: (1) power supply and control circuitry; (2) magnetron oscillator (microwave generator); and (3) various treatment applicators (Fig. 10). The standard frequency in clinical MWD equipment use is 2450 MHz. This high frequency allows microwave energy to be directed or beamed from the applicator toward the patient. This is in contrast to any shortwave frequency where focusing or beaming is not possible at practical cost. A therapeutic 2450 MHz MWD unit consists of a magnetron generator activated by a variable power control. The E field of the MWD should be mainly oriented parallel to the body surf aces to reduce fat heating while providing deep heating. Several types of applicators can be used with this modality (Fig. 11, 12).
MICROWAVE DIATHERMY APPLICATORS
Over the years, numerous MWD applicators have been developed with the intent of increasing the size of the skin area toward which the energy could be beamed. One popular applicator (Type E) is enclosed in a rectangular metallic reflector about 17 cm long and 14 cm wide and can be positioned parallel to the skin surface (Fig. 11). When spacing is approximately 3 cm, a tissue temperature increase the applicator to skin 2 can be generated over an area of about 150 cm . The heating pattern of the
11 rectangular applicator is uneven, doughnut-shaped and produces minimal heat at its periphery. This MWD applicator is recommended for flat or When concave body surfaces. the applicator is further from the skin, up to about 15 cm, the can be treated increases area size which proportionately. However, as the applicator to skin increases, stray radiation distance also increases, requiring an increase in the power output to provide adequate patient heating.
Microwave Diathermy Components
Applicator
Power Magnatron Supply Tube
Antenna Reflector Control & Timing Circuit
Figure l O. Spaced MW Diathermy Applicators
Coaxial Cable Coaxial Cable \ J
A applicator B applicator
Coaxial Cable "
E applicator Figure 11 . 12 Another spaced MWD applicator design (Type A or B) in clinical use is the hemispherical antenna (Fig. 11) (19). This antenna provides an uneven, doughnut shaped heating pattern which produces minimal heat at the center. The size of this area will vary with the diameter of the metallic reflector which houses the applicator. There will be a strong area of stray radiation output 3 to 6 cm wide lateral to the central area with a gradual decline in output toward the periphery.
A relatively new direct contact applicator is commercially available from TAG MED.* (Fig. 12). The applicator allows the treatment to be applied at lower forward power levels with minimal stray radiation and highly uniform patient heating. A dual power meter displays the forward power to the applicator as well as the reflected power from the applicator from which the net power delivered to the patient can be determined. The TAG MED unit can be purchased as a complete new system or as a conversion kit with the direct contact applicator which can be adapted to an existing MWD generator. ELMED Inc.** also has a small 1.5 cm diameter direct contact applicator available for therapeutic MWD (20).
Direct Contact (Microwave Diathermy) Applicator
Figure 12.
An investigation with E field mapping of various MWD applicators by Witters and Kantor provides information to users concerning radiation leakage and applicator efficiency (19). Their experiments included Types B, C, and E applicators, the direct contact prototype applicator which has become part of the TAG MED device (a direct contact applicator with a choke around the center aperture to reduce leakage). In comparing the thermal effectiveness of the applicators, the direct contact applicators were far superior to the Types B, C and E. Furthermore, the radiation leakage from the direct contact applicators was well below the 5 mW/ cm 2 maximum leakage recommended by the Food and Drug Administration at 5 cm distance in its proposed Microwave Diathermy Performance Standard. The leakage for Types C and E were variable arout the applicator but as with Type B, exceeded the recommended 10 mW/cm at a distance of 5 cm. Lehmann and associates, using phantom models,
*TAG MED: Medical Electronics Corporation of America, Boulder, Colorado, 80301. **ELMED Inc: 60 W. Fay Ave., Addison, IL 60101
13 .. also found that direct contact MWD applicators produced much less stray radiation than Type C and E directors (14).
Direct contact applicators have been evaluated that operate at frequencies of 915 MHz and 2450 MHz using a planar phantom (19,21,22). The specific absorption rate (SAR), or rate of heating remains more constant across the surf ace of the body with the 2450 MHz applicator than with the 915 MHz applicator when each was spaced 1 cm from the phantom. However, the depth of penetration was less for 2450 MHz than for 915 MHz. More power was required and leakage was considerably higher for the 915 MHz applicator than the 2450 MHz applicator when both were used at power levels that produced identical tissue heating rates. An external choke with a 2.5. cm absorbing material was added to the small 915 MHz applicator 2 which reduced the leakage below 10 mW/cm as measured 5 cm from the applicator. Depth of penetration desired should be a criterion in deciding which frequency to use (22).
PHYSIOLOGICAL EFFECTS OF DIATHERMY
Basically, the physiological effects of heat on the human body are to increase the extensibility of collagen tissue, decrease joint stiffness, relieve pain, help reduce muscle spasm, increase blood flow, and assist in the resolution of inflammatory infiltrates, edema, and exudates.
Application of thermal energy to a specific area of the body for a sustained period of time will result in an elevated temperature of the absorbing tissue. Localized heating of the tissue produces an increase in metabolic activity and dilitation of the blood vessels in the affected area. Van'tHoff's law states that there is roughly a 10 percent increase in metabolic heat generation in tissues per degree Celsius increase in temperature (10). The circulating blood has the effect of equalizing wide ranges of temperature between various parts of the body and assists in minimizing excessive rises in temperature in any local area. Application of thermal energy produces local reactions in cellular function and distal reactions by means of reflex heating. Local effects are more intense than distal reactions.
The rise in tissue temperature and the rate of that rise are critical for determining the extent of the physiological reactions to the absorption of energy. To be cli11ically · effective, deep tissue temperature needs to reach between 40° and 45°C during three to 30 minutes of application (11).
In comparison to other known methods of inducing an increase in tissue temperature with superficial heating devices, diathermy penetrates ~to a greater depth, producing effects from within, after the energy has been absorbed. However, the absorption patterns of energy can vary resulting in differences in the distribution of heat produced. Much has been written about the selective heating of tissues by these currents and their abilities to produce a temperature rise within specific body tissues (11,22-24).
Variations in the electrical conductivity of tissues will affect their ability to be heated (25). Energy distribution patterns within tissues of high water content, e.g., muscle, have been shown to be fairly uniform. Fat has different dielectric properties and heats very differently from muscle. Any type of fat has a significantly lower specific heat as compared to muscle. With its lower electrical conductivity and lower
, 14 specific heat, fat can act as a thermal barrier to heat induced in underlying muscle which results in a greater thermal energy buildup in the overlying fat (4). For any quantity of energy absorbed, fat temperature will increase more than muscle temperature, therefore, it is difficult to raise muscle temperature to above 45 °c (113 °F) without causing damage to the fat. When any tissue has its temperature raised above 45 °c for more than a few minutes, thermal necrosis (tissue death) will occur (12).
The site of highest temperature rise using SWD can vary, depending upon whether capacitive or inductive electrodes are used. The highest temperature rise with capacitive electrodes is in fat and superficial musculature (12,25), while inductive application produces peak temperature in the superficial to deep muscle layers.
Currently the most widely used SWD electrodes are air spaced (capacitive) and the drum (inductive). Schwan (4) and Guy (24) have presented extensive evidence that the E field strength is substantially greater as compared with H field output with any form of capacitive electrode. Since the E field. is usually perpendicular to tissue, the intensity is significantly reduced in the muscle tissue after traversing the fat layer. This reduced intensity in muscle is due to the increased dielectric constant of the muscle tissue itself. Thus, more power is needed to adquately heat the muscle (15). With capacitive electrodes there is a greater risk of overheating subcutaneous fat without obtaining a clinically-useful temperature increase in superficial muscle. This risk can be somewhat ameliorated by placing capacitive plates as far away from the skin (up to 3 cm) as is practical and using a SWD device with deep field efficient circuitry for greater power output and depth and uniformity of heating.
MWD energy is absorbed in greater quantity than SWD for a fixed thickness of high water content tissues such as muscle and blood. Since microwave energy may be reflected from tissues (at boundaries between tissues with dissimilar dielectric properties) increased thermal concentration may occur at interfaces between bone and adjacent soft tissues. Reflection from bone surfaces may cause concentration from standing waves. In studies on the effects of microwaves on bone, muscle, and nearby tissue (26), temperature increases of 4.3 °c in subcutaneous fat, 4.6 °c in muscle overlying bone, 3.8 °c in bone itself, and 3.6 °c in bone marrow were recorded.
Temperature differences in muscle and fat have been reported for the various frequencies (5,27). Studies by Schwan and associates led other researchers to compare heating patterns of various microwave frequencies (10,16,25-27). These studies clearly indicate the superiority of frequencies lower then 2450 MHz for deep heating (greater than 1 cm). With MWD, time to achieve a maximum level of heating was found to be 20 minutes, with a drop in tissue temperature observed after 30 minutes (28). Worden and others (29) demonstrated a significant increase in blood flow from MWD especially toward the end of the treatment period. Richardson (30) reported that the magnitude of heating played the most significant role in the alteration of blood flow and this was supported by Leden's studies (31) which revealed a significant increase in blood flow when deep muscle temperature was raised to 41 °c or more. Comparison of the 915 and 2450 MHz frequencies shows. that for an average thickness of subcutaneous fat, more heat occurs in the subcutaneous tissues (outer 2.1 cm) at 2450 MHz because of reflection at the fat-muscle interface using the higher frequency. Furthermore, as was measured with phantoms at 915 MHz, the depth of penetration is better at the lower frequency (32,33).
15 USE OP ELECTROMAGNETIC ENERGY IN HYPERTHERMIA
Users of therapeutic heating devices (including SWD, MWD and ultrasound) have routinely been advised to avoid application of thermal energy to tissues in which malignant cells are present. The potential deleterious effect to the patient has been based primarily on the presumption that an increase in tissue temperature may cause increased perfusion of blood to the neoplastic cells resulting in metastases (12,24). However, the user should distinguish between microwave diathermy for therapeutic purposes and electromagnetic energy for cell destruction (hyperthermia) in oncology.
Moderate elevation of tissue temperature surrounding a tumor has been shown to accelerate the rate of growth of the tumor while still higher tissue temperatures (above 42 °c) slow the rate of tumor growth (18). More recently researchers have demonstrated that special, high power radiofrequency and microwave heating devices were effective in treating certain human neoplasms (hyperthermia) either as single agents or synergistically with ionizing radiation (34-36). In 1983 the FDA approved the use of certain specialized radiofrequency and microwave equipment for cancer hyperthermia treatment of superficial tumors. Contact applicators with skin-cooling accessories and inserted nonperturbating thermometer probes must be used for controlled tumor heating. The treatment of cancer with high power MWD or SWD units is not without iatrogenic side effects which may be manifested as blistering of the skin, fat layer burns, ulceration and sloughing of tissue consequent to tumor necrosis. Fat-layer burns were reported by Luk and associates who maintained increases in tissue temperature to more than 44 °c for longer than 10 minutes (34). The development of complications such as these require coordinated health team care and careful planning in selecting special equipment in the treatment of the cancer patient. Commercial therapeutic diathermy devices used in physical therapy are entirely unsatisfactory for cancer therapy, primarily because they provide no internal temperature monitoring capability.
HAZARDS OP SHORTWAVE AND MICRO WA VE DIATHERMY
The literature cites numerous potential hazards of nonionizing radiation, such as used in SWD or MWD. Hazards may result either from high levels of unintended exposures to tissues or materials in or adjacent to the patient or to the user of the device. This section reviews the hazards and discusses the possible deleterious effects which may occur in specific tissues and anatomical structures. Also included are radiofrequency/microwave radiation effects on electronic devices, such as cardiac pacemakers, that are implanted, externally worn, or within close proximity of the SWD and/or MWD device. The well-established contraindications to heat therapy apply to SWD and MWD. In addition, specific tissues or anatomic structures that may be adversely affected by SWD or MWD include the eyes, testes, fluid-filled joints, space occupying lesions, ischemic, hemorrhagic, malignant, and acutely inflamed tissues as well as sensory-impaired tissues.
16 METALLIC OBJECTS AND ELECTROMAGNETIC RADIATION
Foremost among the potential hazards to be considered for both SWD and MWD are internally and externally worn metallic objects and electromedical devices. Neither SWD nor MWD should be applied over surgical metal implants or tissues containing other foreign metallic objects because the energy absorbed or reflected by the metal may transfer excessive heat to adjacent tissues (13,37,38). Also, externally worn metallic objects (jewelry, zippers, etc.) should be removed and placed out of the electromagnetic fields during SWD and/or MWD treatment (38). Users should also take caution to avoid any. metal objects around the treatment area.
In addition to biological hazards, electromagnetic interference (EMI) from diathermy radiation may cause disturbances in the function of nearby electromedical devices. Diagnostic medical equipment, such as electroencephalograms, electrocardiograms and electromyograms, cardiac pacemakers, transcutaneous electrical nerve/muscle stimulators, electrophrenic pacers and cerebellar and urinary bladder stimulators (39,40) may malfunction when the patient is undergoing either SWD or MWD in the immediate vicinity (1 to 20 feet). Unpleasant paresthesias or burns may occur beneath percutaneous electrodes that connect the device to the patient whether the device is in operation or not. Care should be taken by users of SWD and MWD to avoid applying electromagnetic fields over or near externally worn or implanted electronic devices or their lead wires or electrodes. Other materials and objects that may create hazardous situations during application of SWD include certain synthetic substances such as nylon, foam rubber, and plastics (41). Objects like pillows, pillow cases, treatment tables, rmdbag coverings, and clothing often contain these nonconductive materials. It has been reported that a pillow being used by a patient receiving a SWD treatment was charred at areas of contact with or directly adjacent to the unshielded RF cables of a SWD unit (41). This is due to high stray fields adjacent to unshielded SWD cables. Users of SWD equipment should avoid potential fire hazards such as this by correct placement of equipment and by keeping cables and electrodes well away from synthetic materials (see section on SWD Electrodes). All microwave cables and certain shortwave cables are shielded and cannot cause this problem.
Neither SWD nor MWD should be applied over moist wound dressings, ace bandages, clothing, or areas where excessive perspiration tends to accumulate without being absorbed by intervening absorbent material. The presence of moisture may create a high conductivity layer that has an affinity for being heated by the fields of shortwave and MWD. Such preferential heating of moisture could result in overheating or burning of superficial tissues. Users of SWD equipment should minimize the latter risk by using wide spacing between skin, air spaced electrodes, and inserting absorbent toweling between skin and induction electrodes (42). A great concern regarding EMI is that diathermy units (SWD more than MWD) generate high electric and magnetic field energy in the vicinity of the patient. These fields through EMI have caused older models of cardiac pacemakers to stop pacing, revert to a prefixed rate or pace rapidly and erratically if the wearer of the pacemaker comes within 15 feet of a SW device that is in operation (39). Secondary to the induced malfunction, the patient's cardiac rhythm may revert to asystole or ventricular fibrillation. Improvements in pacemaker engineering and design have reduced their susceptibility to electromagnetic fields and eliminated this problem in some pacemakers (40). 17 EYE
The predominant permanent effect on an eye exposed to microwave radiation is the development of opacities in the lens. Studies performed with rabbits showed that single doses can induce opacities; the higher the power density, the shorter the exposure required to produce the opacities. Most exposures were with 2450 MHz continuous wave MWD from a dipole antenna. The animals were positioned close to the dipole, and anesthesized or restrained. In addition, repetitive exposures of rabbits under conditions of power density and time have shown not to have apparent effects (43).
Other ocular reactions occur after cataractogenic exposures, but are transient. Within 24-48 hours after exposure, a translucent or milky band appears in the posterior cortex of the lens, just under the capsule. Swelling and chemosis of bulbar and palpebral conjunctiva may be observed. Other changes include pupillary constriction, hyperemia of iris and limbal vessels, and vitreous floaters and filaments.
One study has examined the histopathology of the microwave-exposed lens in the time interval between exposure and time of appearance of opacities (44). Carpenter "observed changes in the lens fibers and epithelial cells at the lens equator as early as 18 hours post-irradiation. The progressive deterioration in equatorial epithelial cells apparently resemble the changes caused by exposure of the eye to ionizing radiation rather than those caused by heat (43).
Scott (44) suggests that SWD may cause localized heating in the ciliary body of the eye especially if the patient is wearing contact lenses and treatment is being applied to or near the head. In addition, the presence of contact lenses may interfere with evaporative cooling of the cornea (10). Therefore, the user should check to make certain that the patient's contact lenses are removed before SWD treatment is applied in the vicinity of the eye.
Although cataracts were not produced following the short-term exposure of the eyes of a monkey to SWD (45), Daily and associates (45-48) and Richardson and associates (49,50) have demonstrated that cataracts can develop in the eyes of experimental animals exposed to microwaves. Various other authors (51-54) have confirmed, in animal models, that short duration eye exposures to MWD required very high power densities tofroduce cataracts and that long duration exposure to power densities of 150 mW/cm or more were required to cause cataract development. Very long MWD exposures below this level did not produce cataract formation. Guy and associates determined that the power density of 150 mW/cm2 applied for 100 minutes produced a maximum absorbed power density of 138 W/kg in the vitreous humor and a temperature of approximately 41 °c in the eye (52). The high fluid volume of the eye provides it with susceptibility to selectively absorb unintended or accidental doses of energy from either SWD (55) or MWD (11). Lehmann notes that these cataractogenic dosages of MWD are painful and that animals in these studies required anesthesia. He suggests that similar doses of MWD applied to the human eye would cause pain and immediate movement away from the exposure (37). Consequently, exposure of the eye should be avoided.
18 JOINTS
Hollander and associates (56) have reported that intra-articular temperature of normal synovial joints ranges between 30 °c and 31 °c. Harris and McCroskery (57,58) have demonstrated in vitro that an increase of 5 °c produced an increase in enzymatic lysis of human cartilage by rheumatoid synovial collagenase that was four times normal~ Feibel and Fast (59) point out that the intra-articular temperature in knees with rheumatoid arthritis is about 36.5 °c and that this increased temperature probably accelerates destruction of cartilage. Based on this assumption, they encourage individuals who operate SWD and MWD units to use caution when screening patients for treatment of subacute and chronic joint conditions with diathermy. Lehmann et al. suggest that exacerbation of acutely inflamed, fluid filled joint cavities may occur as a result of selective heating by vigorous treatment using SWD or MWD when the joint is covered by a thin soft tissue layer (11). In contrast, the effect of SWD application on normal knees resulted in a 100 percent increase in circulation according to Harris in a study of radio-sodium clearance from the knee joint (60). Similar SWD treatment of chronic (quiescent) rheumatoid knees showed a circulation increase of 60 percent, whereas with most acute rheumatoid knees treated there was a resultant decrease in circulation. This decrease was comparable to decreases found with intra-articular hydrocortisone. Harris suggests that this provides some rationale for using local heat therapy in rheumatoid arthritis (58).
ISCHEMIC TISSUE
Local application of heat from any source, but especially from SWD or MWD, to ischemic tissue is hazardous because the compromised blood flow may not meet the increased metabolic demand placed on the tissues by the thermal energy buildup (61). Lehmann and colleagues (11) have indicated that the time-rate of change of temperature for vascularized tissues with SWD should range between 0.8 °c-2. 7 °c per minute, achieving a critical temperature of 44 °c within 10 minutes. This would correspond to absorbed power densities of 50-170 W/kg. To avoid rapid temperature increases in tissues with compromised blood flow, they suggest that power density and time-rate of temperature increase be adjusted to avoid possible tissue necrosis. An alternative method using SWD that avoids potential damage to tissues with arterial insufficiency is to apply the treatment proximal to the area of occluded circulation. This may evoke a consensual increase in distal blood flow due to reflex vasodilation in the involved area (62.,63). The magnitude of the consensual effect is dependent upon the size of the local area heated. The greater the area heated, the greater the consensual response (61). An increase in tissue temperature increases blood flow due to dilation of arterioles and capillaries. Therefore, neither SWD nor MWD should be used to treat individuals who have a predisposition for hemorrhage, such as hemophilia, since the increase in blood flow secondary to the induced vasodilation would potentiate the tendency to bleed (11,37). Mild heating from SWD may be used in recurring inflammatory conditions to improve blood flow and facilitate diffusion of oxygen and metabolite clearance (34).
19 Lehmann et al. advocate the use of SWD to induce mild heating (that which produces mild physiologic responses) in the later stages of traumatic arthritis, in chronic pelvic inflammatory disease, epicondylitis, degenerative joint disease, ankylosing spondylitis and other chronic arthritic conditions (ll,3'1,64). Lehmann emphasizes that vigorous heating (that which produces vigorous physiologic responses) should be avoided in acute inflammatory processes because of the potential for causing tissue necrosis by imposing an inflammatory reaction on an existing acute inflammatory process (11). Likewise, vigorous heating of tissue adjacent to a space occupying lesion, such as a protruded nucleus pulposis, may exacerbate symptoms by increasing swelling and congestion of tissues surrounding the disc lesion (11,3'1).
OTHER HAZARDS AND CONCERNS FOR USERS
Other tissues of the body that may be adversely affected by a temperature rise elicited by SWD or MWD are the gonads, epiphyseal growth zones in children, and the fetus, especially during the first trimester of pregnancy (10). Animal studies have demonstrated deleterious effects of MWD on gonad structure and function (65). While the testicles and ovaries in humans are generally considered to be sensitive to temperature rise, the testicles (because of their superficial location) are more susceptible to stray radiation than the ovaries. Therefore, unnecessary exposure of the testes to SWD and MWD should be avoided (68).
Other studies on the effect of SWD and MWD on fetal and embryonic growth and development have demonstrated that anomalies occurred in rat fetuses exposed to 27.12 MHz diathermy (69) and that 2450 MHz microwave exposures of chick embryos resulted in inhibition of growth and development ('10). In both studies, the observed effects were attributed to hyperthermia. Similar effects induced by MWD and SWD on human subjects have not been documented. Case study reports by Rubin and Erdman ('11) indicate that MWD exposures of 2450 MHz at 100 W output were given to four women being treated for chronic pelvic inflammatory disease; all four women were either pregnant or became pregnant during the treatment period and had no interference with ovulation, conception or pregnancy. Daels ('12) reported no adverse effects in a 1-year followup of children whose mothers received MWD treatments during pregnancy. While exposure of pregnant laboratory animals to MWD ('13) and SWD (69) does cause fetal abnormalities, it is not known whether the fetus in the human can be reached with significant microwave radiation at clinically used intensities. Despite the possibility that amniotic fluid may selectively absorb microwave or shortwave energy, there are no clinical reports in the literature to substantiate that selective heating of the pregnant human uterus does or does not occur during exposure. Based on the finding that temperatures of 39.8°C or more are damaging to the human fetus ('14), Lehmann (l'l) advises that MWD may be hazardous if aciministered in a way that would allow "a significant amount of energy" to reach the pregnant uterus, and SWD should not be administered with vaginal electrodes for the same reason. In addition, until clinical studies prove otherwise, women with metallic intrauterine devices should not receive SWD or MWD to the lumbar, pelvic, or abdominal regions ('15, '16).
An experiment, conducted at the U.S. Public Health Service's Center for Devices and Radiological Health, concluded that MWD applied at power levels well below
20 recommended safety standards can cause chromosomal damage to sperm cells of mice. They further found that at slightly higher power levels (2.5 times the American National Standards Institute's (ANSI) current standard) the rate of spontaneous abortion in exposed pregnant mice quadrupled (77).
One study states that there is a potential for disturbing bone growth in children when SWD creates a significant rise in temperature of the epiphyses (56). A few animal studies have demonstrated that SWD may either enhance (78) or inhibit (79) bone growth. Device operators applying SWD to children should be aware that bone is not effectively heated when covered by an adequate thickness of soft tissue (57) and that a disturbance in bone growth will occur only at intensities that produce pain (10). However, the size of the SWD applicator relative to the size of the child may prevent the device from being applied in a way that avoids superficial bone growth centers. Because MWD has reflection properties of radiant energy it should be applied with caution over bony prominences to avoid burning of tissues overlying the bone (80).
RADIOFREQUENCY/MICROWA VE RADIATION SAFETY CONSIDERATIONS FOR THE USERS (OPERATORS)
The majority of SWD units in clinical use do P.~t have shielded (coaxial) leads to transmit the high frequency energy generated to the applicator. Most SWD and MWD units have no provision to minimize radiation los11 from the applicator in directions away from the patient. Hence, if the user stays near the energized SWD of MWD unit and treats several patients daily, he or she could absorb significant electric and magnetic field radiation.
Mosely and Davidson measured the radiation exposure to MWD users. They concluded that the exposure would not b-? harmful if reasonable care is taken (81). Safety for the user includes the need to carefully observe that the equipment is working consistently, and that routine maintenance of diathermy devices is performed (82). Additionally the manufacturer's recommended power output should not be exceeded and the applicator should be applied perpendicular to the part of the body to be heated. A study by Stuchly and colleagues on SWD found that overexposure to the user is possible when the user is 20 cm or less from the applicator (83).
RADIATION SAFETY STANDARDS AND GUIDELIN~
Many safety standards and guidelines have been proposed in the United States and in other countries in an attempt to restrict potentially harmful human exposure to RF and MW radiation. In 1968, the U.S. Congress adopted the Radiation Control for Health and Safety Act of 1968 (PL 90-602) to protect the public from unnecessary · exposure to radiation from electronic products. The American National StandardS Institute issued a series of voluntary exposure fields standards (ANSI C95.l) in 1974 and 1982. The latest voluntary guides for human exposure is based on the observation that the effects of RF and MW radiation on biological tissues are related to the level of the absorbed energy in animals and humans. The standard sets a maximum permissible power density of 1 to 5 mW/cm 2 (depending on frequency) for continuous wave (RF or MW) radiation having an exposure duration of 6 minutes or longer (Figure 13). Six minutes was selected to be compatible with intermittent exposure
21 intermittent RF/MW energy. For high levels above 100 mW/cm 2, no exposure time is permissable. The standards apply to whole body and partial body exposure conditions.
American National Standard
• ~ 'iii C Cl) C 100 Cl).. a.c-r~ C E 10 0 U 'jg i 9 ·-e .....E Cl) Cl. ,.o
E:, E 'ii( ftl 0.1 :E 3 30 300 3,000 100,000 Frequency (MHz) *Exposure averaged over any six-minute period Figure 13.
In 1979, Ruggera (84) measured stray emission levels (E and H fields) during MWD and SWD treatment of patients in the immediate proximity of the diathermy units, at locations adjacent to portions of the patient's body which were not intended to be exposed and at locations which the operator might have occupied during a treatment procedure. His study demonstrated that a variety of MWD units gperating at 2450 MHz produced stray exposure field strengths exceeding 15 mW/cm equivalent power density at a 15 cm distance from the surface of the MWD appli]ator. Stray E and H field emissions recorded for SWD measured up to 16 mW/cm equivalent power density at the operator location (about 1 meter from the patient and electrode). E and H field strengths exceeding 265 and 97 mW/cm 2 equivalent power densities were measured in the proximity of the unshielded cables connecting the SWD electrode to the device console. The SWD levels would represent a violation of the ANSI standard for exposures of any period of time. Therefore, the operator should stay at least 1 meter (39 inches) from any part of unshielded "twin-loads" of a SWD units, even at the generator (machine console).
STATE OP THE ART
CONSIDERATIONS FOR APPL YING SWD/MWD
Because it is clinically impractical to measure the quantity of electromagnetic energy actually transferred from a SWD and MWD device to the tissues of the body without invasive temperature probes, users of diathermy equipment must continue to observe, as a guideline for dosage, the patient's subjective heat sensation response. A
22 ------
significant problem often encountered with patients who do not have normal heat sensation is the difficulty they may have in making even moderately reliable verbal responses about what they feel. Patients with cutaneous (sensory) nerve deficiency should be tested for pain/temperature sensation prior to beginning treatment.
The user should evaluate the patient and determine whether the stage of his/her condition is acute, subacute or chronic. Having identified the stage of the condition, the user should then determine the extent of involvement, that is, the volume of tissue involved and then select a SWD or MWD unit with electrodes that are appropriate for delivering energy to the pathology.
Users of SWD and MWD should always keep in mind that the extent of biologic reactions elicited depen~ on the tissue temperature reached at the end of treatment. Since the total therapeutic range for increasing tissue temperature is between 40 °c and 45 °c, it behooves the user to know how intense, how long and how frequent the treatment should be applied based on the stage of the condition (acute, subacute or chronic). According to Thom, the therapeutic effect of a lower dose (intensity) administered over a somewhat longer period of time is better than the effect of a more intense dose given over a shorter time (85). Lehmann classified therapeutic heating as either mild or vigorous.
"Mild heating is defined as follows: a relatively low temperature is obtained in the tissues at the site of the pathologic lesion or the highest temperature is produced in a superficial tissue distant from the site of the lesion; effective tissue temperature is usually maintained for a relatively short period of time; the rate of the rise of temperature in the tissues is often slow. Mild heating is often used in more subacute disease processes.
Vigorous heating is defined as follows: the highest temperature is produced at the site of the pathologic lesion or where the therapeutic response is desired; the tissue temperature is elevated close to the tolerance level; the effective elevation of the tissue temperature is maintained for a relatively long period of time; the rate of increase of the tissue temperature is rapid. Vigorous heating is most often used in chronic disease processes" (86). Dose I lowest - just below the point of any sensation of heat (acute inflammatory conditions).
Dose II low - heat sensation barely felt (subacute resolving inflammatory conditions). Dose III medium - distinct but pleasant heat sensation (subacute resolving inflammatory conditions). Dose IV heavy - heat sensation which is well tolerated (chronic conditions).
23 Acute conditions should be treated with a low dose and treatment periods of 2 to 5 minutes. The acute condition may not require the deep tissue temperature rise that results from longer, more intense applications (vigorous heating). Chronic conditions call for a higher dose, longer (5-30 minutes) treatment periods, and greater intervals between treatments.
APPLICATION OF SWD AND MWD
The patient should be in a comfortable, relaxed position. For safety always provide patient with a bell and/or turn-off switch cord. No metal should be on the patient or in the treatment area. Users should carefully read and follow instructions in the operator's manual and comply with recommended equipment maintenance procedures.
Mild Heating
When mild heating is desired (usually acute conditions)
Duration of tr{;'!atment - 2 to 5 minutes
Intensity of treatment - patient should feel a minimal sensation of heat Frequency of treatment - usually daily for 1 to 2 weeks Vigorous Heating When vigorous heating is desired (usually chronic conditions) Duration of treatment - 20-30 minutes
Intensity of treatment - patient should feel a comfortable level of warmth
Frequency of treatment - varies from daily to twice weekly for 1 week to a month
The patient should be evaluated prior to receiving treatment and reevaluated at appropriate intervals. Keep in mind that diathermy is, in most cases, an adjunct to a total plan of care for the patient. A brief description of the diathermy equipment commercially available (in 1983) is listed in Tables 2 and 3.
24 TABLE 2. Microwave Diathermy Equipment
Company Model Output Power Frequency Assessories
HE/WA Electronic Microtron 250 Watts 2450 MHz Horn-Type Antenna Indust. Co.,Inc. MT-12
Elmed Inc. Microwave 250 Watts 2450 MHz 1 1/2" Circular Contact Applicator (25 W) 150 ± 50 MHz Ear Applicator (l0W) Pelvic Applicator Rectal Tubus Vaginal Tubus N 3/4" Circular Contact Applicator (10 W) u, Elmed Inc. Radarmed 250 Watte;; 2450 MH7. Longfield Director (200 W) 202 1 1/2" Circular Contact Applicator Ear Applicator (10 W) Pelvic Applicator Rectal Tubus Vaginal Tubus 3/4" Contact Applicator, Circular (10 W) Pyrostat 200 Watt Contour Applicator
Tag-Med TDS-2450S Low 0-15 Watts 2450 MHz 16.25 Diameter Contact Applicator Adjustable 7 cm Diameter Focus Applicator High -0150 Watts Adjustable Table 3. Shortwave Diathermy Equipment
Company Model Output Power Frequency Assessories
Birtcher Corp. 802-1 150 Watts 13.56 MHz+ 4. 76 Adjustable Induction Drum Crystal Minimum KHz Crystal 14' Induction Cable Bandmaster Controlled 8 11 X 10" & 611 X 8" Condensor Pad Electrodes
Birtcher Corp. 875 Crusader 100 Watts 27.12 MHz Adjustable Induction III Drum
Enraf Nonius Curomed 401 450 Watts 27.12 MHz 10 cm Capacitor Delft Maximum + 0.6% Electrodes N 0-, Adjustable Induction Drum 7 Meter Induction Cable
Enraf Nonius Curapuls 400 Watts 27.12 MHz 13 cm Capacitor Delft 419 Continuous + 0.6% Electrodes 1000 Watts Pulsed 110 cm Induction Cable
International Magnetherm 1000 Watts 27.12 MHz Induction Coil Drum Medical Peak Power Applicator Electronics Pulse Duration 65 Microseconds Adjustable From 100-5200 P.P.S.
Mettler ME 300 250 Watts 27.12 MHz Induction Coil Circular Electronics Auto-Therm Maximum + 18kHz Drum Corp. Table 3. Shortwave Diathermy Equpment (con't).
Company Model Output Power Frequency Assessories
J.A. Preston Deluxe 500 Watts 27.12 MHz 13 cm Air Spaced Electrodes Corp. Preston + 0.6% 13 x 19 cm Condensor Electrodes Erbotherm Diplode Applicator Monode Applicator 3.5 m Induction Cable
Rich-Mar Thermowave 325 Watts 27.12 MHz 15 cm Diameter Air-Spaced Electrodes Corp. 350 Maximum 110 cm Induction Cable
Siemens Ultratherm 400 Watts 27.12 MHz Air-Spaced Electrodes 708 Maximum + 0.5% Adjustable Induction Drum 1.5 m Induction Cable SWD TECHNIQUES
Choose an applicator that is approximately the same size as the area to be treated. Use capacitive applicators to irradiate two symmetrical body surfaces. With inductive electrodes do not allow the cable turns to touch one another or the patient.
Use capacitive electrodes (predominantly electric field) for heating superficial tissues. Although SWD devices with capacitive electrodes are capable of deep-field heating, they may cause superficial (fat) burns when appropriate application techniques are not observed. It is important for the user to assess the relative thickness of the patient's fat layer when using capacitive electrodes to prevent superficial burns.
Use inductive electrodes (predominantly magnetic field) to provide more uniform heating of superficial and deep musculature without as much risk of overheating superficial tissues and subcutaneous fat. Electrodes available for inductive heating include the cable, diplode, monode, and minode. Place towels between applicator and skin when using inductive electrodes.
There is higher H field leakage for inductive applicators and higher E field leakage for capacitive applicators. Always keep unshielded "twin" cables away from the patient, pillows, and other items that might overheat from high stray fields. The operator should stay away from any part of the unshielded twin cables during treatment.
MWD TECHNIQUES
Spaced conventional MWD applicators have been described elsewhere in this text. A recent Food and Drug Administration publication reviews the safety consideration for applying microwave diathermy (87). Contemporary research has singled out direct contact applicators as being more effective and safer than the older spaced applicators. The modern design MWD applicators are either direct contact or are spaced within 1 cm of the tissues. Generally, direct contact applicators have uniform heating patterns. Even the older spaced applicators should be used at their closest prescribed distance in most cases to minimize stray radiation.
For vigorous heating effects the forward power of the direct contac\ applicator should be 25 watts with an average intensity of approximately 500 mW/cm. A feeling of warmth is the best available guide for providing mild heating (10)
DIATHERMY TREATMENT OUTCOMES Diathermy is used therapeutically to increase the temperature of tissues. It is generally accepted that heat produces the following desirable therapeutic effects: (1) increases the extensibility of collagen tissues (2) decreases joint stiffness (3) relieves pain (4) relieves muscle spasm (5) assists in resolution of inflammatory infiltrates, edema and exudates, and,
28 (6) increases blood flow (10)
Indications for diathermy include:
(1) Disorders of the musculoskeletal system: ligamentous strains and sprains muscle spasm joint stiffness, contractures scarred synovium, capsule lesions degenerative joint disease myof ascial pain syndromes
(2) Chronic inflammatory or infective conditions: tenosynovitis, bursitis, synovitis carbuncles, abscesses, f uruncles* chronic inflammatory pelvic diseases
SURVEYS OF DIATHERMY USE IN CLINICAL PRACTICE
A questionnaire with closed format responses was designed by the FDA and its contractor, to obtain information on the current use of diathermy in physical therapy clinics. In the spring of 1983, the surveys were distributed and responses were obtained from 49 physical therapy clinics in eight States (Wisconsin, California, Indiana, Illinois, Virginia, Minnesota, Missouri and Ohio).
Responses indicated that SWD is being used in 38 of the 49 (79 percent) facilities surveyed. The use of SWD diathermy in each clinic ranged from O to 38 times a week with an average of 5.92 times per week. MWD is not being used often in the clinics surveyed. Only 10 facilities (20 percent) indicated they used MWD and averaged less than one treatment per week.
The American Chiropractic Association suggests much higher utilization of diathermy by its members. In a 1982 survey of over 11,000 chiropractors, 70 percent reported having a diathermy unit in their practice (88). ·
DIATHERMY IN PHYSICAL THERAPY EDUCATION
Faculty of 11 physical therapy schools were asked specific questions about the teaching of diathermy to their students. More than twice as much emphasis is given to SWD than to MWD in lecture and classroom laboratory practice. A mean of 6.8 hours was reported for SWD whereas 3 hours was devoted to MWD. These schools listed the equipment used for teaching diathermy which included 34 shortwave and 11 microwave devices. ·
*Capactive electrodes recommended since they do not demand skin contact with the applicator.
29 One physical therapy school reported the number of diathermy experiences their students had during full-time clinical education. Forty-seven students had 20 weeks of clinical education and 16 (33 percent) had an opportunity to demonstrate competence in performing a SWD treatment and only 1 (2 percent) used MWD. The affiliation sites used by this school are in 16 States.
DIATHERMY AND CANCER
Use of radiofrequency radiation and microwave heating devices in treating cancer patients (hyperthermia) is restricted by the FDA to specific hyperthermia equipment used by persons with authorization from their hospital's or research facility's Institutional Review Board and the FDA. Standard SWD and MWD units used primarily in therapeutic settings are not allowed. If a person is found to have a malignant tumor, an oncologist should be contacted immediately. That person should be refused standard diathermy treatment since heat can increase the spread or growth of the cancer cells.
Occasionally, a patient with cancer diagnosis may be referred to physical therapy for heat to decrease pain. The decision to use diathermy to relieve pain in the presence of cancer should only be done on the advice of the referring physician or after consultation with the patient's physician~
RECOMMENDATIONS
DIATHERMY AND RESEARCH
Reference has been made to the various clinical applications (physiological effects) of MWD and SWD. Additional clinical studies are needed to substantiate the efficacy of diathermy treatments administered for the purposes of augmenting collagen tissue extensibility, ameliorating pain, reducing muscle spasm, and assisting in the resolution of chronic inflammatory processes.
There appears to be a need for clinical research comparing diathermy to other thermal applications as well as to the more recent modes of pain relief. The 1970's and '80's brought about several alternatives to diathermy in relieving pain. Some of these are: transcutaneous electrical nerve stimulation (electroanalgesia), biofeedback for relief of pain, and more recently, experimental laser therapy. These innovations in pain relief were orginally accepted by users without much clinical evidence that they were more beneficial than the older methods of pain relief.
The direct contact MWD applicators have recently been shown to be more effective in providing uniform heating than conventional spaced MWD applicators. Stray radiation from the direct contact applicators is considerably less than with the spaced MWD applicators and SWD electrodes. The result of studies on direct contact MWD applicators may lead to a resurgence of MWD in clinical practice as the user's confidence is restored that MWD can be both thermally effective and safe.
There is a need for diathermy videotapes to update the users knowledge of current safety concerns. Several user training programs mentioned this as a. great need. There appears to be a need for more diathermy continuing education courses. Additionally, there is a need for users to be more involved in the design and labeling of diathermy equipment.
30 REPORTING INJURIF.S
The Food and Drug Administration maintains a reporting system for the tabulation and analysis of injuries occurring from medical devices. The information obtained in this data base provides the Agency with an opportunity to become alerted to the types and frequency of injuries that are occurring with the use of various medical devices. Patients, manufacturers and practitioners are all encouraged to report information into this system which is called the "Practitioner Reporting System." Under the regulations, manufacturers who become aware of injuries involved in the use of their products are required to report these to the Food and Drug Administration. Any incident involving therapeutic mi~rowave and shortwave diathermy or any other medical device related problems can be reported simply by calling one of the following numbers: 800-638-6725 outside of Maryland and in Maryland call collect: 301-881-0256.
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