CHAPTER 11
APPLICATION OF ELECTRICITY AND MAGNETISM IN MEDICINE
These applications include :
1. The uncontrolled application of the electricity ( electrical shock) ,and shock hazards in a clinic or hospital .
2. Controlled applications of electricity ( diathermy , electrosurgery, the measurement of skin resistance, magnetic methods of monitoring blood flow, electroanesthesia, Kirlian photography, and acupuncture.
Physiatrist: is the physician specialist in physical medicine that associated with diathermy.
Physical therapist: is a paramedical specialist, his work in rehabilitation ,but he cannot prescribe treatment.
ELECTRICAL SHOCK :
When 60 Hz currents of different levels are passed through the body, various reactions are produced:
1. The perception level ( at which we can just feel the current ) is 1 mA in men and less by one-third in women.
2. Above perception level ,the current causes a tingling sensation in the hands or body.
3. At currents of 10 to 20 mA , sustained muscular contraction (many subjects cannot let go of electrodes).
4. At currents near 100 mA , pain and in some cases fainting occur, and if it pass through the heart causes ventricular fibrillation.
?? Ventricular fibrillation ( rapid irregular ,and ineffectual contraction of the ventricles).
The curve of current versus frequencies (Fig. 11.1 ) , shows the current above which 50% of adult men do not have the ability to control their muscle actions . The effective current is higher at both low and high frequencies.
For a 60 Hz shock, the maximum current (Imax ) that will not induce fibrillation , is given by the following equation:
Imax = 116 / ? t ------- (Imax) in mA , and (t) in sec.
( if t =1 sec. ,the safe current is 116 mA).
Current level of 6 A causes sustained muscular contraction of the heart ( this level is used in defibrillator for treatment of ventricular fibrillation and atrial fibrillation , but in atrial fibrillation the shock must be given after R wave (depolarization) and before the upswing of the T wave (repolarization).
Continuous currents above 6 A can cause temporary respiratory paralysis and serious burns. There are two kinds of shock :
1.Macroshock : which occurs when electrical contact is on the surface of the body.
2.Microshock : which occurs when the current is applied inside the body.
In the hospital (in ICU and operating room ), it is possible for microshock to occur , due to the using of internal blood pressure measurements , pacemaker catheters, and catheters contain conducting fluids. It has been estimated that about 30 µA through the human heart cause ventricular fibrillation.
Leakage current:
Modern power - cords have three wires , two that supply the ac power and one that serves as ground wire. The main source of the leakage current is the capacitance between the ac power wires and ground (Fig. 11.2 ).The impedence (Xc) of a capacitance (C) for an applied voltage of frequency (f ) is :
Xc = 1 / 2? fC ...... atypical leakage capacitance is 2x10?² µF, V=110 , f = 60 Hz
5
Xc = 1.3x 10 ?
5 -4
I = V / Xc = 110 / .3x 10 = 8.5x 10 A ( 850 µA).
( if the patient with pacemaker is connected to an ECG instrument with broken ground wire ,the leakage current (I) will flow through pacemaker to ground and results in ventricular fibrillation and death due to this microshock current ( Fig. 11.3).
Shock hazards in hospital
There are a number of ways to reduce shock hazards in hospital:
1. By operating electrical equipment with direct current
( Xc= ¥ , f=0 for direct current).
2. By operating electrical equipment at high frequencies ( much higher than 60 Hz).
3. By using rechargeable battery –powered instruments in diagnostic ,therapeutic , and monitoring purposes.
HIGH-FREQUENCY ELECTRICITY IN MEDICINE
The heating effects of high frequency current are produced by the interaction of the electricity with body tissue. The applications in this field are:
1.Electrical diathermy: The use of frequencies near 30MHz
a. Short-wave diathermy:
Two methods are used ,the capacitance method and inductance method . In both methods the parts to be treated becomes a part of a resonant electrical circuit. Short-wave diathermy is used in the treatment of busitis,arthritis, traumatic-injuries ,strains, and sprains.
· Capacitance method
The tissue to be heated is placed between capacitor plates ,that they are supplied with an oscillating electric field. The ions in the tissue are move back and forth ,these ions lost part of their kinetic energy (dissipated as heat ) when collide with the molecules in tissue . The heat depend on the current times a constant which is related to the properties of the tissue. This type of heating is called joule (resistive) heating ( Fig. 11.4).
· Inductance method
The part of the body to be heated s placed near the inductor. A 30 MHz current in the coil produces an alternating magnetic field in the tissue that produces eddy currents in it ,which appears as heat in the tissue.
b. Microwave diathermy
In microwave diathermy the tissue absorbs electro-magnetic waves ( 2450 MHz with wavelength ~ 12 cm) ,that are incident upon it and produce heat. The radiation is produced in a tube called (magnetron). Microwave energy is absorbed by muscle tissue more than by fatty tissue ( which have less water).
?? Microwave diathermy is used to heat joints , tendon , and muscles. Long-term radiation exposure level is 10mW /cm². The testes and eyes are more sensitive to high temperature than other parts. Overheating of the testes can cause temporary sterility , and cataracts for eyes.
?? Fig. 11.7 shows the penetration versus the frequency of the microwaves ,for fatty tissue and tissue with high water content. The absorption can be described by the following equation: -x/D
I = Io e
(I= radiation intensity at a depth x in the tissue, Io = radiation intensity at the surface , and D is the tissue thickness needed to reduce Io to 37%).
As the absorption increases D gets smaller ( as in the equation above).
2. Elctrocautery and Electrosugery:
Are done with a high frequency (> 2 MHz, of £ 15 kV) . Fig.11.8 shows butt plate electrode provides an electrical contact with a large area. The current density at the butt plate is small compared to the current density at the probe elctrode (cause rapid temperature rises of about 800° C.
?? The probe can be used to either coagulate small to moderate size blood vessels ( electrocautery),or cut through tissue (electrosurgery).
?? The "cutting" of electrosurgery is thought to be the physical rupturing of tissue due to rapid boiling of the fluids from the local heating ( high power densities). It is used in operations on the brain, spleen, bladder, prostate, and cervix.
LOW-FREQUENCY ELECTRICITY AND MAGNETISM IN MEDICINE:
The measurement of blood flow by electromagnetic methods and the measurement of skin resistance due to psychological changes.
1. Magnetic blood flow meters
Blood is a conducting fluid in blood vessel with a diameter (d). If it passes with mean velocity (v) through a magnetic filed (B) ,a voltage (V) is induced between the electrodes on the blood vessel (Fig. 11.9) :
V = B d v
Q = cross-section area x mean velocity
Q = (? d² /4) (V/Bd) ......V,B and d can all be measured
?? This technique is used as research tool and it is not practical for clinical use (the blood vessel must be exposed surgically).
2. Measurement of skin resistance
Elevated psychological activity can change the activity of sweat gland and cause perspiration , which is relate to the galvanic skin response (GSR).
?? GSR : is the change in the basal skin resistance due to psychological changes or external stimuli.
?? a decrease in GSR indicates increased sweat gland activity ,while an increase in GSR indicates reduced sweat gland activity.
?? To measure GSR an active electrode is placed on the palm of the hand and a second neutral electrode is placed on the wrist . A constant tiny direct current ( ~ 10 µA/cm²) is passed through the electrodes ,the resulting voltage indicate GSR ( voltage is proportional to resistance).
?? GSR is used in :
1. Lie detector – to detect changes in subject s emotional state resulting from the change in anxiety level during questioning (Fig. 11.11) .
2. some neurological diseases affect the function of sweat glands on one side of the face ,GSR helps in the diagnosis of these diseases.
Current research:
1. Kirlian photography (capturing the "aura" or corona from a person exposed to a high frequency( ~ 1 MHz ) electromagnetic voltage.
2. Electroanesthesia : electrodes are applied to the skull as in Fig. 11.12 . It is used when chemical anesthetics cannot be used, also for sleeping in psychiatry.
3. Acupunture: is used to reduce or prevent pain by inserting stainless steel needles in certain acupundture sites (1000 sites).
Example: calculate the time above which the current ( 50 mA and 60 Hz) will induce left ventricular fibrillation due to macroshock
Imax = 116/?t
50 = 116 /?t ........ t = (116/50)² ---- t = 5.4 sec
Example: Find (D) of a microwave beam , if half of this beam is absorbed by a tissue at depth 2 cm .
-x/D -x/D
I = Io e --- I/ Io =1/2 ---- 1/2 = e (lin for both sides)
-0.693=-2/D --- D = 2.88 cm
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .