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

© 2008 LWW

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

© 2008 LWW

Electrical Currents Input and Output 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/ConversionElectrical 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 ElectricityChemical Generation of Electricity

Zn Cu

H SO42H SO42

H SO42

H SO42

H SO42H SO42

• Most common form is a battery

• Two different metal plates are put into a solution of H2SO4.

• H2SO4 dissociates into 2H+ and SO4

2−.

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Chemical Generation of Electricity (cont.)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.

Zn Cu

SO--4 H SO42H SO422H+ + SO-- 4

Zn++

-

ZnSO4

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• H+ ions pull an electron

from a copper molecule and becomes free hydrogen.

– Dissolves as gas

The copper plate becomes positively charged (Cu2+).

Zn Cu

H SO42H SO42

2H+ + SO-- 4

- +

ZnSO4

2Hoo

oooo

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• 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.

- - +

Zn Cu

SO--4

H SO42ZnSO4 2H+ + SO-- 4

Zn++ 2H+

+ + - -

+Zn Cu

SO--4


Zn++ 2H+

+ + - -

+

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• 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


Zn++ 2H+

+ + - - - - +

+

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• 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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• 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 BatteriesTypes of Batteries

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Types of Batteries (cont.)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.)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.

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Mechanical Power: Generation/Conversion 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 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.) 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.)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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• 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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• Continue rotating, and AC flows through wire coil.

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Electrical Motor vs. Electrical Generator 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 TermsAC TermsImpulse• 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.)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.)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

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Devices for Measuring and Regulating ElectricityDevices 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.)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 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 Current Characteristics (cont.)

• Output to tissue:– Pure DC– Modulated

(manipulated) pulsed current

– Pure AC

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Output Current Characteristics (cont.) 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.)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 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 Pulse and Cycle Characteristics

• Phase shape– Sinusoidal

– Rectangular

– Spike

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Pulse and Cycle Characteristics (cont.)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 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.)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 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.)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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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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• Train– A continuous repetitive series of pulses at

a fixed frequency – Polyphasic– Pure AC

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Train and Burst Characteristics 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.)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 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.)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

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Current Timing Modulation (cont.)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.) 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.)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 PatternStimulation 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 CharacteristicsSurge 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.)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 FormsModulation 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. )

© 2008 LWW

Commonly Used Wave FormsCommonly 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.)Commonly Used Wave Forms (cont.)• Direct (galvanic)

wave form– Pure DC current,

used for iontophoresis

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Commonly Used Wave FormsCommonly 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.)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 FormsCommonly Used Wave Forms• Sinusoidal wave

form– Pure AC current

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

utility companies

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

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• 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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• 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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• 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

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Commonly Used Wave Forms (cont.)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

Documents

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