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© 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
© 2008 LWW
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
© 2008 LWW
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−.
© 2008 LWW
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
© 2008 LWW
Chemical Generation of Electricity (cont.)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+).
Zn Cu
H SO42H SO42
2H+ + SO-- 4
- +
ZnSO4
2Hoo
oooo
© 2008 LWW
Chemical Generation of Electricity (cont.)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.
- - +
Zn Cu
SO--4
H SO42ZnSO4 2H+ + SO-- 4
Zn++ 2H+
+ + - -
+Zn Cu
SO--4
H SO42ZnSO4 2H+ + SO-- 4
Zn++ 2H+
+ + - -
+
© 2008 LWW
Chemical Generation of Electricity (cont.)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 SO42ZnSO4 2H+ + SO-- 4
Zn++ 2H+
+ + - - - - +
+
© 2008 LWW
Chemical Generation of Electricity (cont.)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.
© 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 BatteriesTypes of Batteries
© 2008 LWW
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
© 2008 LWW
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.
© 2008 LWW
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.
© 2008 LWW
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.
© 2008 LWW
Generating AC Current, Simplified (cont.) Generating AC Current, Simplified (cont.)
• Conversely, when electricity passes through a wire, a magnetic field is created.
© 2008 LWW
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.
© 2008 LWW
Generating AC Current, Simplified (cont.) 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.
© 2008 LWW
Generating AC Current, Simplified (cont.) Generating AC Current, Simplified (cont.)
• Continue rotating, and AC flows through wire coil.
© 2008 LWW
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
© 2008 LWW
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°
© 2008 LWW
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°
© 2008 LWW
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
© 2008 LWW
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).
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
Output Current Characteristics (cont.) Output Current Characteristics (cont.)
• Output to tissue:– Pure DC– Modulated
(manipulated) pulsed current
– Pure AC
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
Pulse and Cycle Characteristics Pulse and Cycle Characteristics
• Phase shape– Sinusoidal
– Rectangular
– Spike
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
Phase and Pulse Charge Phase and Pulse Charge
• Pulse charge– Electrical charge of a single pulse– Sum of phase charges
© 2008 LWW
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
© 2008 LWW
Phase and Pulse Charge (cont.)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
© 2008 LWW
Phase and Pulse Charge (cont.)Phase and Pulse Charge (cont.)
• Train– A continuous repetitive series of pulses at
a fixed frequency – Polyphasic– Pure AC
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
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.
© 2008 LWW
Commonly Used Wave Forms (cont.)Commonly Used Wave Forms (cont.)• Direct (galvanic)
wave form– Pure DC current,
used for iontophoresis
© 2008 LWW
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
© 2008 LWW
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
© 2008 LWW
Commonly Used Wave FormsCommonly Used Wave Forms• Sinusoidal wave
form– Pure AC current
© 2008 LWW
Commonly Used Wave Forms (cont.)Commonly Used Wave Forms (cont.)
– Polyphasic, symmetrical, balanced, sinusoidal• Wave form generated and sold by
utility companies
© 2008 LWW
Commonly Used Wave Forms (cont.)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
© 2008 LWW
Commonly Used Wave Forms (cont.)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
© 2008 LWW
Commonly Used Wave Forms (cont.)Commonly Used Wave Forms (cont.)
• Biphasic wave form– Symmetrical, balanced, rectangular, pulsed
© 2008 LWW
Commonly Used Wave Forms (cont.)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
© 2008 LWW
Commonly Used Wave Forms (cont.)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
© 2008 LWW
Commonly Used Wave Forms (cont.)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
© 2008 LWW
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