Chapter 9. Principles of Electricity for Electrotherapy (Part B)

1

• Chapter 9. Principles of Electricity for
  Electrotherapy (Part B)

2
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

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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

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Chemical Generation of Electricity

• Most common form is a battery
• Two different metal plates are put into a
  solution of H2SO4.
• H2SO4 dissociates into 2H and SO42-.

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Chemical Generation of Electricity (cont.)

• SO42- attracts Zn2 from the zinc plate, leaving
  it negatively charged.
• SO42- and Zn2 combine to form ZnSO4, which then
  precipitates to the bottom of the battery.

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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
  (Cu2).

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Chemical Generation of Electricity (cont.)

• As the process continues, charges accumulate and
  a difference in potential (voltage) develops
  between the negatively charged Zn2- plate and the
  positively charged Cu2 plate.

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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?

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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.

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Types of Batteries

• 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

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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

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Types of Batteries (cont.)

• Storage batteries
• Rechargeable battery
• An electric current passes through it, causing a
  reverse chemical reaction.
• Restores the H2SO4
• Reaction can go again.

13
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.

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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.

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Generating AC Current, Simplified (cont.)

• Conversely, when electricity passes through a
  wire, a magnetic field is created.

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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.

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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.

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Generating AC Current, Simplified (cont.)

• Continue rotating, and AC flows through wire coil.

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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

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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

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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

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AC Terms (cont.)

• Frequency
• Cycles/sec (cps) the number of cycles completed
  each second.
• Low-frequency current lt1000 cps
• High-frequency current gt1,000,000 cps

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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).

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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

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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

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Output Current Characteristics (cont.)

• Output to tissue
• Pure DC
• Modulated (manipulated) pulsed current
• Pure AC

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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

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Output Current Characteristics (cont.)

• Pulsed current
• Interrupted electron flow
• The simplest form of interruption is to turn the
  switch on and off

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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

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Pulse and Cycle Characteristics

• Phase shape
• Sinusoidal
• Rectangular
• Spike

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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

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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

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Pulse and Cycle Characteristics (cont.)

• Phase charge
• Electrical charge of a single phase, expressed as
  coulombs
• Time integral result of both amplitude and width

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Phase and Pulse Charge

• Pulse charge
• Electrical charge of a single pulse
• Sum of phase charges

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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

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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

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Phase and Pulse Charge (cont.)

• Train
• A continuous repetitive series of pulses at a
  fixed frequency
• Polyphasic
• Pure AC

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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

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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

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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

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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 lt150 µsec
• Long pulse duration gt200 µsec
• Interpulse interval
• Time between successive pulses

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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

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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

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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

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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

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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

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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

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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. )

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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.

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Commonly Used Wave Forms (cont.)

• Direct (galvanic) wave form
• Pure DC current, used for iontophoresis

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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

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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

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Commonly Used Wave Forms

• Sinusoidal wave form
• Pure AC current

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Commonly Used Wave Forms (cont.)

• Polyphasic, symmetrical, balanced, sinusoidal
• Wave form generated and sold by utility companies

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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

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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

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Commonly Used Wave Forms (cont.)

• Biphasic wave form
• Symmetrical, balanced, rectangular, pulsed

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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

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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

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Commonly Used Wave Forms (cont.)

• Interferential wave form
• Symmetrical, sinusoidal, high frequency
  (20005000 Hz) AC
• Two channels, with different frequencies, used
  simultaneously
• Two currents cause a tissue current amplitude
  modulation

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Commonly Used Wave Forms (cont.)

• Interferential wave form current amplitude
  modulation

Two identical currents
Two offset currents
Two opposite currents
Usually accomplished with two different frequency
currents