Transcript Electrical Currents in Rehabilitation: II

Chapter 9. Principles of Electricity for
Electrotherapy (Part B)
© 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
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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
Cu
Zn
a battery
• Two different metal
plates are put into a
solution of H2SO4.
H2 SO4 H2 SO4
• H2SO4 dissociates into
H2 SO4
H2 SO4
2H+ and SO42−.
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Chemical Generation of
Electricity (cont.)
• SO42− attracts Zn2+ from Zn Cu
the zinc plate, leaving it
negatively charged.
• SO42− and Zn2+
-SO
combine to form ZnSO4,
4 H SO
++
Zn H SO 2 4
which then precipitates
4
2
to the bottom of the
ZnSO 4 2H+ + SO-- 4
battery.
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Chemical Generation of
Electricity (cont.)
• H+ ions pull an electron
from a copper
molecule and becomes
free hydrogen.
Zn -
o
+ Cu
o o o
o
2H
– Dissolves as gas
The copper plate
becomes positively
charged (Cu2+).
H2 SO4
H2 SO 4
ZnSO 4 2H+ + SO-- 4
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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.
Zn ---
++
++
Cu
+
2H
++ SO4
Zn
H2 SO4
ZnSO 4 2H+ + SO-- 4
--
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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?
+++ Cu
Zn - --
+
+
2H
++ SO4
Zn
H2 SO4
ZnSO 4 2H+ + SO-- 4
--
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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.
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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: <1000 cps
– High-frequency current: >1,000,000 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: <150
µsec
• Long pulse duration: >200
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. )
© 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.
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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 (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.)
• Interferential wave form: current amplitude modulation
Two identical currents
Two offset currents
Two opposite currents
Usually accomplished with two
different frequency currents
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