Dr.Rajaa CH.11. أ.م.د. رجاء سهيل جنم طب االسنان Applications of Electricity and Magnetism In Medicine Electrical Shock: When an electrode is connected to each hand & 60 Hz currets of different levels are passed through the body, Various reactions are produced. 1. About 50% of adult men feel a 60 Hz currets about 1.0 m A. 2. Women feel lower levels, about 0.67 m A. The perception levels rise as the frequency in creases above 100 Hz. 1. As a 60 Hz current is increased above the perception level, it causes a tingling sensation in the hauds or body. 2. At currents of 10 to 20 ma, a sustoined muscular contraction takes place in the hands & many subjects do not have the a bility to control their mascle a ctions. 3. As the current is increased still further, pain & in some cases fainting occur. 4. Near the 100 ma level, the portion of the 60 Hz current passing throuh the heart is sufficient to cause ventricular fibrillation ( rapid irregular & ineffectual, contraction of the ventricles ). The heart is es pecially vulnerable to fibillation during one portion of its cycle. This portion is beginning repolarization of the ventricle ( the upswing of the T Wave ) 1
Note: أ.م.د. رجاء سهيل جنم طب as the االسنان The current level that will induce fibrillation decreases duration of the shock increases. * Maximum current that will not induce fibrillation in man is given by: Imax = 16/ T ma where ( t ) is the time in sec. Ex: If t = 1sec., the safe current is 116 ma. If t = 4sec., the safe current is 58 ma. * Current levels of (6A) & above cause sustained muscular contracction of the heart similar to the " Can not let go " behavier of the hands. * If a patient has ventricuar fibrillation, a brief shock from a defibrllator usually restores normal coordinated pumping in the heart. The defibrllator uses a brief pulse of up 10 KV. * To treat a patient with atrial fibrillation. Jn this case, the electrical pulse is applied after the Rwave (depolarization of the ventricles ) but befor the upswing of the Twave ( repolarization the ventricles ). ñ Continuous current above (6A) can cause temporary paralysis & serious burns. The damage depends upon: 1. The individual the dampness of the skin. 2. The contact of the skin with the conductor. 2
طب االسنان The 60 Hz current level at which 99.5% of an can let go of the electrods is ~ 9mA. At current 10 20 ma a sustained muscular contraction take place in the hand and can not let go the electrod. Note : This current is higher at both low and high frequencies. Marco shock : when the electrical contact is made on the surface of the body and the level of current that cause damage and fibrillation is in ma. Micro shock : it is the shock which occurs when the current is applied inside the body. it does not move to pass through the high resistance of the skin. *ventricular fibrillation can be induced with very much smaller than the current level of macro shock. it has been estimated 30 µa through human heart would cause ventricular fibrillation. A patient in tensive care units ( ICU ) may have apace -maker catheter running through a major vassel & touching the heart muscle to stimulate the heart if its own timing mechanism should fail, some others catheters contain wires or electrically conducting fluids, therefore ; provide low resistance electrical paths greatly increase the possibility of microshock significant factor in fatal accidents in operating rooms & ICU. 3
elecrical hazard جامعة تكريت cords كلية *to protect patients from microshock, modern power طب one that االسنانand have three wires ; two that supply the AC power serves as a ground. * leakage current : in all electronical equipment, there is some current flow from the AC power parts to the metal case of the instrument or appliance. this leackage current usually flowes to ground through the ground wire in the power cord. the main source of the leakage current is the capacitance between the AC power wires & ground or between the power transformer & its case. The impedance X C of the capacitance C for an applied voltage of frequency f is Xc = 1/2π ƒ c A typical leakage capacitance is : 2*10-2 µf ( f= farad ) Ex. 1. what are the current that induce fibrillation if the exposure time was 1,4,16 seconds? 2. calculate the capacitor that must be connected to ( AC ) power supply to make the instrument safe for microshock in a country supplied by 220v & 50 Hz frequency. 4
ways to reduce shock hazards in hospitals أ.م.د. رجاء سهيل جنم 1. By using DC current to operate the equipment because طب االسنان. current human body is less sensitivety to this Ie Xc = 1/2π ƒ c & f = 0 in D.c.. Xc = ω So there is no leakage current ie I ( LEAKGE ) = v/χc = v/ω = 0 2. By operated electrical equipment of frequency much higher than 60 Hz, where the sensitive of the heart to ventricular fibrillation is much less. 3. By using rechargeable battry powered instrument in diagnostic therapeutic and monitoring sitaation. 4. By using AC current with connecting safe ground wire with high frequency. Types of elecricity : I. high frequency electricity ( HFE ) in Medicine used in : A. heat therapy : the heating effects of ( HFE ) is done by using : 1. short wave & iathermy. 2. long wave & iathermy. 3. microwave & iatherrmy. B. Electro surgery : uses of ( HFE ) to : 1. control hemorrhage during suring surgery. 2. sear the wounds. 3. Electro surgery = cut through tissues. 4. Electro cautrizing = coagulate small or moderate- size blood vassels that are too tie. 5
II. Low frequency electricity ( LFE ) & magnetism in medicine it is used to : طب االسنان A. measure the blood flow by electromagnetic methods. When an electrical conductor is moved perpendicular to a magnetic field a voltage is induced. Blood act as a conducting fluid. if it passed with a mean velocity v through a magnetic field B. A voltage V is induced between the electrodes such that V= Bdv d= diameter of blood vessel. The volume flow of blood (Q) = mean velocity x the area of the vessel Q = ( V/Bd )* π d 2 /4 (cm 3 / s) 6
2. Measure the skin resistance by LFE to monitor psychological change : طب mostly االسنان * perspiration depends on sweat gland activity that are concentrated in the palm of the hand and sole of the foot. * change perspiration ( sweat gland activity ) are related to skin resistance. a decrease in skin resistance indicates increased sweat gland activity, while an increased in skin resistance indicates reduced sweat gland. * The variation from the basal skin resistance due to pschological changes or external stimuli is called " GSR" ( galvanic skin response ). * the " GSR " can be easily measure where there is a concentration of sweat gland, such as the palm of the hand or the sole of the foot. : " GSR " depends on the activity of sweat gland, only & not upon the amount of visible perspiration. * an active electrode is placed on the palm of the hard & a second neutral electrode is placed on the wrist or the back of the hand. Usually a constant direct current ( ~ 10ma/cm 2 ) is passed through the electrodes. the resulting voltage indcate the " GSR ", since the voltage isproportional to resistance. 7
* the problem with using a " DC" is that a polarizing voltage. skin is developed between the electrodes & the To reduce this problem, we use a : طب االسنان 1. silver silver chloride electrode 2. with an electrodee jelly between the electrodes & the skin. example : a. find the mean velocity in the vessei V= Bdv v = V/Bd = 1.5*10-5 / (3*10 2 )(5*10 3 ) = 0.1 m/sec b. Assuming all the blood travels at the mean velocity. what is the volume flow rate? Q = v/bd. π d 2 / 4 Q = π ( 5*10-3 ) 2 /4 *( 0.1 ) = 1.9 * 10-6 m 3 / sec = 1.9 cm 3 /sec Absorption of Microwave beam depend on : 1. The a mount of water in the tissue. The energy is deposited more effectively in tissue with high water content,microwave energy is a bsorbed better in musle tissue which have less water. 2. The frequency of microwaves: The energy is absorbed % is very high at frequency 20 GHz (GHz = 10 9 Hz). It's poorly absorbed at lower frequency nearly 100 MHz & at very high frequency >1000 GHz. *2450 MHz for kitchen uses but best frequency for medical treatment uses is 900 MHz. Ex: If the rediation intensity of the surface is 10 & tissue for treatmen half of intensity absorbed at depth 3cm. Calculate the intensity under 2cm in tissue. 8