Lecture #6 Electrical Currents and Electromagnetic Fields Effects on Biological Objects

1. Lecture #6Electrical Currents and Electromagnetic Fields Effects on Biological Objects

2. Plan of the lecture • Electrical properties of biological tissues • Direct current effect on biological tissues • Pulse current effect on biological tissues • Alternating current effect on biological tissues • Alternating electromagnetic field effect on biological tissues • Direct electric and magnetic fields effect on biological tissues th

3. A great number of diagnostic and therapeutic methods of modern medicine are based on effects developing in human body tissues under the influence of external electrical currents and electromagnetic fields.

4. Electrical currents and fields effect on biological objects depends on: • Properties of External electrical current or fields 2) Own Electrical activity of biological tissue

5. 1. Electrical properties of Biological Tissues All biological tissue can be divided into conductors (conduct electric current well) and dielectrics (conduct electric current poorly) : conductors are cerebro-spinal fluid, blood, lymph, dielectrics are bone without periosteum,nervous, fatty tissue, dried-up skin.

6. Electrical specific conductivity ( ) for various biological tissues :

7. The best conductor of electrical current is cerebrospinal fluid. • The best dielectric is bone without periosteum.

8. . Two classes of conductors: first - fine metals (free charge carriers are electrons), second – electrolytes (free charge carriers are ions). Biological tissues aresecond class conductors, or ionic conductors(it means, current carriers in biological tissues are ions).

9. Electrical properties of biological tissue depend on its Impedance (Z). Impedance (Z) is a Total Resistance, it has two components: • Active resistance (R)Resistor Symbol • Reactive resistance (X): - capacitive (XC) Capacitor Symbol - inductive (XL ) Inductor Symbol

11. Biological tissueshave only: • active resistance (R) • capacitive resistance (XC ) • Inductive resistance XL of biological tissue is zero XL ≈ 0 !!!

13. Due to capacity resistance (XC ) electrical propertiesof the biological tissue are different for direct and alternating currents: Direct Current (DC) doesn’t pass through structures with the capacity properties, and Alternating Current (AC) passes (the bigger frequency of alternating current, the better it passes).

14. Human body hascapacity properties (XC ) due to capacitor analogs in the body : • an intact cell Cell membrane is a dielectric; cell content and intercellular fluid are conductors. Membrane is located between two conductors, like a dielectric layer between two conductors in a capacitor. 2) tissues-dielectrics and tissues-conductors

15. solid line – for alive tissue, dotted line – for dead tissue • Dependence of tissue impedance Z • on alternating current frequency ω: • When frequency ω increases, the tissue impedance modulus decreases (solid line), • but not to zero, up to constant value (dotted line).

16. Coefficient of dispersion(К) is calculated for evaluation of tissues functional state, widely used in transplantology : • Zlfis impedance at low frequencies (102-104Hz); • Zhf is impedance at high frequencies (106-108Hz) • If К=1,the biological tissue is dead; • IfК>1,the biological tissue is alive.

17. Electrical equivalent of a biological tissue is an electrical circuit having impedance dependence on alternating current frequency the same as the biological tissue; it consists of two resistors (R1 and R2) and one capacitor (C) resistor R1 - corresponds to intercellular fluid, resistor R2 - corresponds to intracellular contents, capacitor C - corresponds to cell membranes.

18. 2. Electrical currents effects on biological tissues Electrical current effect on tissues depends on current type. Basic types of current : • direct current • alternating current • pulse current

19. 1. Direct current 2.Alternating current

20. 3. Pulse currentElectrical pulses can have different shape – rectangular, triangular, trapezoid etc.

21. Currents that can change in time are divided into pulse and alternating one. • Alternating current - changes in time according to the harmonic low (a sine or a cosine law). • Pulse current - depends on time periodically but not harmonically. Single electrical pulses are also used in medicine.

22. Direct current (DC) Under the influence of DC the positive and negative ions in a tissue move in the opposite sides: positive ions (cations) move to the cathode (-) and accumulate under it; negative ions (anions) move to the anode (+). • The main mechanism of direct current effect on biological tissuesis change of usual ion concentrations in different parts of tissues.

23. Medical methods based on application of Direct Current : • Galvanization 2. Medical electrophoresis

24. Galvanization : Voltage is U=60-80 V; current density is j ≤ 1 A·m-2 • Device for galvanization is an alternative current rectifier. • Electrodes used to get current to a patient are made of sheet lead or foil. • Hydrophilic layers wet with water or physiological solution are placed between a patient’s skin and the electrodes.

25. Response reactions of the human organism on DC: 1. Stimulation of blood circulation, lymph circulation, metabolism 2. Irritation of the tissues under cathode (“-” electrode) and decrease of the tissues sensitivity under anode (“+” electrode) - the effect of local analgesia (pain relief).

26. 2. Medical electrophoresis is a therapeutic method of introducing of medical substances through the skin or mucous membranes under the influence of DC. • Medical electrophoresis is performed similar to galvanization but one of hydrophilic layers is wet not with water but with a special medicine solution.

27. Introducing ofmedicalsubstances by means of electrophoresis is possible if a medicine, dissolving in water, forms ions. • Anions are introduced to patient from under cathode, • cations are introduced to patient from under anode. Electrodes Medical substance

28. The basic mechanism of pulse currents effect on biological objects is the irritation of excitable tissues. • Excitable tissues: • muscular • nervous • glandular 2. Pulse Current

29. Front is a pulse section that corresponds to voltage or current growth as well as their diminution (back front or cut). • Amplitude is modulus of voltage or current maximum value (Umax or Imax correspondingly). • Pulse durationis a time interval during which voltage (or current) exceeds 0,1·Umax (or 0,1·Imax forcurrent).

30. Front steepness (S) is determined as follows where is a duration of growth (or diminution) of pulse front when voltage ranges between 0,1·Umax and 0,9·Umax. Pulse current is characterized by a period of pulse repetition (T)and pulse repetition frequency , at that .

31. Irritation action is a result of generation of an action potential in tissue cells. Irritation action depends on • pulse amplitude • pulse steepness • pulse frequency • pulse duration 1) The tissue irritation is possible if pulse amplitude exceeds certain minimum value. Minimum current value when response reaction (irritation) starts is called threshold current (Ithr).

32. 2) The more pulse front steepness is, the more current irritation action. • Du Bois-Reymond law describes this phenomenon: an electric current irritation action is directly proportional to the rate of current increasing (or decreasing) that is proportional to a derivative of current with respect to time.

33. 3) As pulse duration increases its irritation action on excitable tissues increases, i.e. threshold current decreases. • The method of electrodiagnostics is based on this conclusion- a method of examining of tissue excitability properties by means of determination of dependence of a threshold current upon pulse duration (τ) when single rectangular pulses irritate tissue. • Hoorweg-Weiss-Lapicque equation : where a and b are constants.

34. From the diagram the more pulse duration, the less the threshold current depends on  and tends to a certain limiting value that is called arheobase (Re) in electrophysiology. • Rheobase is equal to a limit to which threshold current tends when pulse duration tends to infinity ithr Other words, rheobase is determined as the value of threshold current when it does not depend upon pulse duration.

35. Chronaxie (Chr) is one more feature that describes tissue exciting properties as well as rheobase. Chronaxie is the pulse duration at which threshold current is equal to doubled rheobase. • Values of parameters a and b in Hoorweg-Weiss-Lapicque equation are determined by rheobase and chronaxie values. Hence, Hoorweq-Weiss-Lapicque equation can be presented like • Rheobase and chronaxie certain values are characteristic for the exciting tissues state. At different pathological states these parameters change.

36. Medical methods based on use of Pulse Current : • Pacing • Defibrillation • Electrogymnastics • Electrosleeping

37. Pacing • In normal state pulses causing heart systoles are produced by a sine node (nodus sinuatrialis) called a rhythm driver. • If sine node does not execute its function, external rhythm driver (or heart pacemaker) is used. • Heart pacemaker is carried or implanted cardiological electrical stimulator. Heart pacemaker generates electrical pulses with repetition frequency of 1-1.2 Hz and pulse duration of 0.8 –3 ms.

38. 2. Defibrillation Defibrillator is used during cardiac arrest or heart ventricle fibrillation, i.e. when separate muscle fibers are contracted irregularly as a result of their irritation by each other. Defibrillator produces single high voltage electrical pulses (discharges) that cause great contractions of heart muscle and restoration of regular heart rhythm. Voltage equal to 8 kV is commonly used. In case of unsuccessful attempt to start the heart a greater voltage is used for the following attempts.

39. 3. Electrogymnastics (electrical exercises for muscles) support muscle tonus, improve circulation of the blood and metabolism in weak muscles or in muscles with bad innervations, support their ability for contraction. • Pulse current with pulses of a triangle shape with pulse duration of 1-1.5 ms and repetition frequency of 100 Hz as well as pulses of an exponential shape with pulse duration of 3-60 ms and repetition frequency of 8-80 Hz are used for electrical exercises.

40. 4. Electrosleeping (electrical sleep) • is a method of inhibition of central nervous system by a pulse current of a rectangular shape with pulse duration of 0.1-1 ms and repetition frequency of 5-150 Hz.

41. 3. Alternating Current (AC) AC effect on tissues can be different and depends on its frequency : • at low frequencies AC causes irritation of excitable tissues (similar to pulse current); • at high frequencies AC causes thermal effect (tissue heating).

42. Dependence of threshold current (ithr) upon frequency () is defined byNernst law: • in frequency range from 100 to 300 Hz threshold current is proportional to square root from current frequency • in frequency range from 50 to 300 kHz • threshold current is proportional to current frequency wherek1andk2are certain constants. In case of alternating current threshold current is also called perception current.

43. Medical methods based on use of Alternating Current : • Rheography • Diathermy (diathermocoagulation, diathermotomy) • Local D’Arsonvalisation

44. 1. Rheography (or impedance-plethysmography, or rheoplethysmo-graphy) is diagnostic method based on registration of changing of tissue impedance Z that takes place due to change in tissue filling with the blood (that was caused by heart functioning) At frequency 30 Hz current does not irritate excitable tissues (currents strength is less then threshold current).

45. When cerebrum is examined with the help of rheography we get a rheoencephalogram, • when heart is examined with the help of rheography, we get a rheocardiogram. • Arterial vessels of lungs, liver and extremities can be examined with the help rheography. In stomatology vessels of paradont, mouth mucous membrane, salivary glands etc. can be examined with the help of rheography.

46. Rheodentography - a method similar to rheography, is used in stomatology : tooth pulp is examined. • An alternating current of 0.5-1 MHz frequencies is used.

47. 2. Diathermy • is based on heating effect when alternating current flows through tissues. • There are therapeutic diathermy and surgical diathermy (electrical surgery), which, in its turn, is divided into diathermotomy and diathermocoagulation. • When diathermy is used aspecific heat power(an amount of heat evolved per 1 second in 1 m3 of tissue) of currentflowing through the tissue is defined by the formula : whereqis specific heat power ; jis current density; ρis resistivity of tissue through which current flows.

48. In electrical surgery - the current of 10 MHz frequency is used. Two electrodes are applied for it: one electrode have a great area of contact with patient’s body and the other electrode is sharp. • During diathermocoagulation (j = 6…10 mА/mm2) there is possibility to ‘seal’ blood vessels due to the effect of coagulation. • During diathermotomy (j ≈ 40 mА/mm2) a sharp electrode acts as an electrical knife, which cuts tissue by burning it. Diathermotomy is good for a surgeon, as this method is practically bloodless because the vessels are cut and sealed simultaneously.

49. Local D’Arsonvalisation. When certain methods used currents are applied, the current affecting the patient is an alternating and a pulse at the same time. • So, at local D’Arsonvalisation the alternating current of 100-400 kHz frequency acts at a patient, but the current is applied to him like pulses with repetition frequency of 50 Hz. In this case the voltage is 10 kV.

50. ALTERNATING ELECTROMAGNETIC FIELD EFFECT ON BIOLOGICAL TISSUES • Electrical currents can be induced in tissues without electrodes. If tissues (some parts of human body) are placed into an alternating electromagnetic field, alternating currents are induced in them. Heating of tissues with the help of currents induced by an alternating field is the base of the following methods, such as inductothermy, UHF-therapy (UHF – ultra-high frequencies), and microwave therapy.