1. © 2008 LWW Chapter 9. Principles of Electricity for Electrotherapy (Part B)

2. © 2008 LWW Electrical Currents Input and Output In and out of what? –The box; the modality Input currents: DC and AC –What is the difference? –Where do they come from? Output currents –Numerous forms –Numerous responses Important to understanding these processes: –Current flow –Therapeutic use of electrical currents

3. © 2008 LWW Electrical Generation/Conversion Process of converting another form of energy into electrical energy Most electricity is converted from thermal, chemical, mechanical, or solar energy. Look at only two: –Chemical: DC –Mechanical: AC

4. © 2008 LWW Chemical Generation of Electricity Most common form is a battery Two different metal plates are put into a solution of H 2 SO 4. H 2 SO 4 dissociates into 2H + and SO 4 2−.

5. © 2008 LWW Chemical Generation of Electricity (cont.) SO 4 2− attracts Zn 2+ from the zinc plate, leaving it negatively charged. SO 4 2− and Zn 2+ combine to form ZnSO 4, which then precipitates to the bottom of the battery.

6. © 2008 LWW Chemical Generation of Electricity (cont.) H + ions pull an electron from a copper molecule and becomes free hydrogen. –Dissolves as gas The copper plate becomes positively charged (Cu 2+ ).

7. © 2008 LWW Chemical Generation of Electricity (cont.) As the process continues, charges accumulate and a difference in potential (voltage) develops between the negatively charged Zn 2− plate and the positively charged Cu 2+ plate. - - + Zn Cu SO -- 4 H SO 4 2 ZnSO 4 2H + + SO -- 4 Zn ++ 2H + + ++ + - - + Zn Cu SO -- 4 H SO 4 2 ZnSO 4 2H + + SO -- 4 Zn ++ 2H + + ++ + - - +

8. © 2008 LWW Chemical Generation of Electricity (cont.) If a wire is attached between the two plates, electrons flow from the plate with the extra electrons to the plate that lost electrons. –Which way do electrons flow? –Which way does current flow? Zn Cu SO -- 4 H SO 4 2 ZnSO 4 2H + + SO -- 4 Zn ++ 2H + + - - - - + +

9. © 2008 LWW Chemical Generation of Electricity (cont.) Because electrons always flow from one pole to the other, it is called direct current. Remember: Although electrons flow from the Zn − pole to the Cu + pole, we say that current flows from the Cu + pole to the Zn − pole.

10. © 2008 LWW Two types of batteries –Galvanic or wet cells –Dry cells A wet cell consists of two metals and an electrolyte solution (earlier example) –Car battery Types of Batteries

11. © 2008 LWW Types of Batteries (cont.) Dry cell –Electropaste rather than solution –Zinc-carbon battery Zinc tube filled with electropaste and a carbon rod inserted into the middle –Example: flashlight battery

12. © 2008 LWW Types of Batteries (cont.) Storage batteries –Rechargeable battery –An electric current passes through it, causing a reverse chemical reaction. Restores the H 2 SO 4 –Reaction can go again.

13. © 2008 LWW Mechanical Power: Generation/Conversion Based on the relationship between electricity and magnetism Magnetic field –Force that develops when a critical number of a substance's ionized molecules polarize –The substance is said to have poles. –A force field develops between the two poles and is called a magnetic field.

14. © 2008 LWW Generating AC Current, Simplified Electromagnetic induction When a coil of insulated wire is moved toward or away from a magnet, electricity flows in the wire.

15. © 2008 LWW Generating AC Current, Simplified (cont.) Conversely, when electricity passes through a wire, a magnetic field is created.

16. © 2008 LWW Generating AC Current, Simplified (cont.) An electrical generator consists of: –A bar magnet mounted on a rotating pedestal –Two metal plates positioned at the end of the magnet and connected with a large loop of wire (or a metal core with a coil of wire around) –Source of mechanical energy to keep the bar magnet spinning in a circle.

17. © 2008 LWW Generating AC Current, Simplified (cont.) Magnet in starting position –Its positive pole attracts electrons. –Its negative pole repels them. Electrons flow through the core, inducing electron flow in the wire coil, Rotate the magnet 180°. The poles are now reversed, so electrons move in the opposite direction.

18. © 2008 LWW Generating AC Current, Simplified (cont.) Continue rotating, and AC flows through wire coil.

19. © 2008 LWW Electrical Motor vs. Electrical Generator Electric motor: conceptually the same as but opposite of generator –Consist of the same basic components, except the processes are opposite Generator converts mechanical energy to electrical energy Electrical motor converts electrical energy to mechanical energy

20. © 2008 LWW AC Terms Impulse Current flow in a single direction Appears as a half circle (or egg) Portion of graph representing current flowing from baseline to maximum in one direction and back to the baseline When generating AC current, represents electron flow during time magnet rotates 180°

21. © 2008 LWW AC Terms (cont.) Cycle –Two impulses Portion of graph representing current flow from baseline to maximum in one direction, back across baseline to maximum in opposite direction, and back to baseline Electron flow as magnet rotates 360°

22. © 2008 LWW AC Terms (cont.) Frequency –Cycles/sec (cps): the number of cycles completed each second. –Low-frequency current: <1000 cps –High-frequency current: >1,000,000 cps

23. © 2008 LWW Devices for Measuring and Regulating Electricity Based on electromagnetic effects of current –Permanent magnet and electromagnet that can rotate –When charged, magnets repel each other, causing the electromagnet to rotate away. –Repulsion is proportional to the strength of the electromagnet (proportional to the amount of current).

24. © 2008 LWW Devices for Measuring and Regulating Electricity (cont.) Ampmeter (ampere meter) –Measures rate of flow of current –Milliampmeters Voltmeter –Measures voltage Ohmmeter –Measures resistance to current flow

25. © 2008 LWW Output Current Characteristics Input current (AC or DC) is manipulated, regulated, and adjusted to create different output current forms. Sends (outputs) to tissue: –Pure AC –Pure DC –Modulated (manipulated) pulsed current

26. © 2008 LWW Output Current Characteristics (cont.) Output to tissue: –Pure DC –Modulated (manipulated) pulsed current –Pure AC

27. © 2008 LWW Output Current Characteristics (cont.) DC current –Continuous flow of electrons in a single direction AC current –Continuous back-and-forth flow of electrons –Defined by frequency or cycles per second –Can be turned off and on to create bursts

28. © 2008 LWW Output Current Characteristics (cont.) Pulsed current –Interrupted electron flow –The simplest form of interruption is to turn the switch on and off

29. © 2008 LWW Current Modulation Includes all manipulating, regulating, and adjusting to create a variety of specific output wave forms Most output pulsed or as AC trains Factors modulated –Shape –Charge –Timing –Amplitude –Stimulation pattern

30. © 2008 LWW Pulse and Cycle Characteristics Phase shape –Sinusoidal –Rectangular –Spike

31. © 2008 LWW Pulse and Cycle Characteristics (cont.) Pulse: finite period of charged particle movement, separated from other pulses by a finite time during which no current flows Made up of one or more phases

32. © 2008 LWW Pulse and Cycle Characteristics (cont.) Pulse named by number of phases –Monophasic One phase Current flows in one direction only. –Biphasic Two phases Current flows in both directions. –Polyphasic Many phases

33. © 2008 LWW Pulse and Cycle Characteristics (cont.) Phase charge –Electrical charge of a single phase, expressed as coulombs –Time integral; result of both amplitude and width

34. © 2008 LWW Phase and Pulse Charge Pulse charge –Electrical charge of a single pulse –Sum of phase charges

35. © 2008 LWW Phase and Pulse Charge (cont.) Pulse symmetry Applies only to biphasic pulse Relationship between shapes of the two phases Symmetrical: phases identical Asymmetrical: phases different

36. © 2008 LWW Phase and Pulse Charge (cont.) Pulse charge balance Applies only to biphasic pulses Charges of two phases equal (balanced) or different Independent of whether the phases are symmetrical

37. © 2008 LWW Phase and Pulse Charge (cont.) Train –A continuous repetitive series of pulses at a fixed frequency –Polyphasic –Pure AC

38. © 2008 LWW Train and Burst Characteristics Burst –Finite series of pulses flowing for a finite time period followed by no current flow Think of it as turning a pulse train or AC current on and off. –Burst interval Time during which burst occurs –Interburst interval Time between bursts, usually in milliseconds

39. © 2008 LWW Train and Burst Characteristics (cont.) Duty Cycle –Ratio of time on vs. total time Thus current with an on time of 10 msec and an off time of 40 msec would have a 20% duty cycle

40. © 2008 LWW Current Timing Modulation Phase duration –Time during which current flows in a single direction Rise time –Time from beginning of a phase until maximum amplitude Decay time –Time from maximal amplitude to end of a phase

41. © 2008 LWW Current Timing Modulation (cont.) Pulse width (pulse duration) –Time required for each pulse to complete its cycle –Reported in microseconds or milliseconds Short pulse duration: <150 µsec Long pulse duration: >200 µsec Interpulse interval –Time between successive pulses

42. © 2008 LWW Current Timing Modulation (cont.) Period –Beginning of the pulse to the beginning of the subsequent pulse Pulse rate (frequency) –Rate at which pulses are repeated –Pulses per second Similar to cycles per second for AC

43. © 2008 LWW Current Amplitude Modulation (cont.) Amplitude (intensity, output) –Measured in two ways Voltage delivered to the electrodes Current flowing through the circuit Peak current –Highest magnitude of the pulse

44. © 2008 LWW Current Amplitude Modulation (cont.) Average current –Average magnitude of a pulse –Computed in two ways Average current during the pulse Average current during the period Includes the off time between pulses

45. © 2008 LWW Stimulation Pattern Constant stimulation –Amplitude of successive pulses (or cycles) is the same Surged stimulation –Individual pulses gradually increase from zero to a maximum preset intensity Surge characteristics

46. © 2008 LWW Surge Characteristics Ramp up –Time during which the intensity increases Plateau –Time during which pulses remain at maximum preset intensity Ramp down –Time during which the intensity decreases

47. © 2008 LWW Surge Characteristics (cont.) Time on –Time during which current flows; from the beginning to the end of a surge Time off –Time during which current does not flow; time between surges

48. © 2008 LWW Modulation of DC and AC Currents Produce a Variety of Output Forms (Reprinted with permission from Robinson AJ, Snyder-Mackler L. Clinical Electrophysiology; Electrotherapy and Electrophysiologic Testing. Baltimore: Williams & Wilkins; 1995. )

49. © 2008 LWW Commonly Used Wave Forms Modulation of DC and AC currents produces a variety of output forms. The most common output wave forms are described here.

50. © 2008 LWW Commonly Used Wave Forms (cont.) Direct (galvanic) wave form –Pure DC current, used for iontophoresis

51. © 2008 LWW Commonly Used Wave Forms Interrupted DC wave form –Unidirectional flow caused by rapid and repeated turning on and off of the current –Similar to modified square wave

52. © 2008 LWW Commonly Used Wave Forms (cont.) –Monophasic, rectangular, pulsed Also called a modified square wave Similar to DC but modulated from AC input current On and off times are not necessarily equal

53. © 2008 LWW Commonly Used Wave Forms Sinusoidal wave form –Pure AC current

54. © 2008 LWW Commonly Used Wave Forms (cont.) –Polyphasic, symmetrical, balanced, sinusoidal Wave form generated and sold by utility companies

55. © 2008 LWW Commonly Used Wave Forms (cont.) Faradic wave form –Induced asymmetrical AC current –Biphasic, asymmetric, unbalanced, spiked –Positive portion: short duration, high amplitude, and spiked –Negative portion: long duration, low amplitude, and curved

56. © 2008 LWW Commonly Used Wave Forms (cont.) –Faradic has a double meaning Specific wave form (previous slide) Any AC current stimulation Similar to galvanic as a synonym for DC current Be careful not to confuse the two

57. © 2008 LWW Commonly Used Wave Forms (cont.) Biphasic wave form –Symmetrical, balanced, rectangular, pulsed

58. © 2008 LWW Commonly Used Wave Forms (cont.) Twin pulse wave form –Monophasic, pulsed, twin spiked –Common wave form of high-volt muscle simulators –Has been called high-volt galvanic and pulsed direct current –However, not direct or galvanic current –Result of misunderstanding physiology

59. © 2008 LWW Commonly Used Wave Forms (cont.) Russian wave form –Polyphasic, symmetrical, sinusoidal, burst –Developed by Russian scientist Kots; thus the name –Initially a 2500 Hz AC current burst, modulated every 10 msec, now many frequency choices

60. © 2008 LWW Commonly Used Wave Forms (cont.) Interferential wave form –Symmetrical, sinusoidal, high frequency (2000–5000 Hz) AC –Two channels, with different frequencies, used simultaneously –Two currents cause a tissue current amplitude modulation

61. © 2008 LWW Commonly Used Wave Forms (cont.) Interferential wave form: current amplitude modulation Two identical currents Two opposite currents Two offset currents Usually accomplished with two different frequency currents