Uses for e-stim
Reduction of pain and inflammation, stimulate bone/tendon growth and wound healing, muscle re-education and strengthening, replace or augment muscle function during functional activities, and evaluate nerve and muscle injury/pathology
Current
Represented with "I". The rate of...
Electrotherapy Exam 1
Uses for e-stim - Answer Reduction of pain and inflammation, stimulate bone/tendon
growth and wound healing, muscle re-education and strengthening, replace or augment
muscle function during functional activities, and evaluate nerve and muscle
injury/pathology
Current - Answer Represented with "I". The rate of flow of electrons through a
conduction medium in response to an applied electric field. Unit is the ampere. Size of
the current determines whether or not you feel a shock (AKA determines the effects of
e-stim), NOT voltage. I = change in charge (measured in coulombs) / time in seconds. 1
ampere = 1 coulomb / sec or 6.2 x 10^18 electrons/sec. Producing current requires the
presence of freely movable charged particles (e.g. ions) and the application of a driving
force to move them. Therefore current is directly proportional to voltage. Three types:
Direct, alternating, and pulsed.
Resistance - Answer Opposition to current flow, represented with "R". One of the factors
that regulates the flow of an electrical current through biological tissue. Unit is ohms. R
= resistivity x (length / cross-sectional area). However in the case of leads, length isn't
important because resistivity of wire is so low that length is negligible. Length IS
important for something like skin and subcutaneous fat thickness (AKA length of the
conducting pathway; more fat will increase length) because tissues have thousands of
ohms of resistivity. Cross-sectional area is the effective size of the electrodes.
Resistivity - Answer The property of a substance that makes it quick or slow to move,
e.g. honey moves more slowly than water. Represented by the Greek letter p (rho). For
biological tissue it is inversely proportional to the water content, which means that
virtually all resistance occurs at the electrode to skin interface.
Conductance - Answer The opposite/inverse of resistance. Unit is siemens. Proportional
to the current flow; the higher the conductance, the greater the current flow, and the
lower the resistance.
Series vs. parallel circuits - Answer Series: The current has only one path it can follow,
so the resistors summate. Identical current has to go through each resistor.
Parallel: Resistance is reduced because the current is able to "choose" the path that
has the least resistance (the path with the resistor that has the fewest ohms). The total
resistance can never be greater than the lowest resistor. Therefore if one resistor has 0
ohms of resistance, the total resistance will also be 0.
Resistance in electrotherapy - Answer We have a combo of series and parallel
components to take into account. This is because all of the current must go into one
electrode (anode, positively charged), through the skin under the electrode, through the
body, and out through the skin under the other electrode (cathode, negatively charged,
ALL of the original current MUST leave at the cathode). Parallel resistors include blood,
,muscle, fat, bone, etc, with more current moving through more watery substances (e.g.
blood because water has high conductance). Increasing resistance of the anode will
increase resistance of the cathode and vice versa because they are in series with each
other.
Factors influencing ability to feel resistance - Answer Worsened by: Dry skin/electrodes
(won't get good contact), some soaps or lotions (insulators), non-uniform conductivity
(decreased cross-sectional area increases resistance), air trapped under the electrode.
All of the current will be forced to flow through the parts you have good contact, so it
causes a burning sensation to these areas (if the contact area is small).
Minimized by: Rubbing off dry skin, conductive gel, moistening pre-gelled electrodes,
sweat (electrodes are like little blankets which causes sweating, making it more
tolerable over the course of the treatment session).
Voltage - Answer AKA potential difference or electromotive force (EMF). The change in
electrical potential energy between two points in an electrical field per unit of charge. In
other words, it represents the driving force that makes charge particles move. Produced
when oppositely charge substances are separated, when like-charge substances are
approximated, or when charged particles in a system aren't evenly distributed. 1 volt = 1
ampere of current in a circuit containing 1 ohm of resistance. If the circuit is incomplete,
you will get a reading of 0 volts.
Ohm's law and its role in the clinic - Answer I = V/R, so current is proportional to
voltage. All clinical devices are either voltage or current regulated. If voltage regulated
(you set the amount of volts), the current flow is determined by the resistance (is the
proportionality constant because the electrode size, skin thickness, and resistivity won't
change throughout treatment). Current is what produces clinical effects, so this can be
dangerous because you are not directly in control of current. Current regulated is
therefore safer (you set the current), and in this case its the voltage that is determined
by the resistance.
Impedance - Answer Opposition to alternating currents (current moving back and forth),
represented by Z. Z = sqrt(R^2 + Xc^2), where Xc is the capacitance in the circuit.
Analogous to resistance (for direct currents). The higher the frequency, the more current
is able to flow through (the lower the impedance), so impedance is inversely
proportional to the frequency of the AC.
Capacitance - Answer Only relevant with AC. The property of a system of conductors
and insulators to store charge (like with a stretchy rubber band that can store more
energy compared to a stiff one). Represented with C and has units of farads (F). C =
q/V. The bigger the plates and the closer they are together, the higher the capacitance.
Capacitors block low frequencies and direct currents and allow high frequencies and
alternating currents (nerve and muscle phospholipid bilayer membranes are leaky
capacitors; a fatty insulator between two conductors).
, Time constant - Answer Represented with the Greek letter tau (T). T = R x C, where R
is resistance and C is capacitance. The time it takes to charge or discharge up to 2/3 of
an applied voltage. Helps determine which tissue is being excited (nerve or muscle).
Muscle membrane has a high capacitance (more surface area due to T-tubules) and
therefore a high time constant, and the nerve has a lower capacitance and therefore a
more rapid rise of discharge across its membrane/a low time constant. If an applied
voltage is low, it will take less time to reach that applied voltage.
Capacitor voltage formula - Answer Vc = Vs x (1- 1/e^[t/T]) where Vs is the limiting
value of capacitor voltage as t approaches infinity. The capacitor voltage will reach 99%
of this value during a time lapse of five time constants. Vc is the capacitor voltage at any
instant of time after the switch closes, t is the lapsed time in seconds that the circuit
voltages and currents have been changing, and T is the time constant.
Frequency of square vs. sine waves - Answer Square: Frequency is 0 when the line is
"flat" (during the middle of the pulse) and infinite right at the instant the pulse beings
(because it is completely vertical with no horizontal component)
Sine: Frequency depends on pulse duration (frequency is inversely proportional to
period). If it has a pulse duration of 100 ms (aka 0.1 sec), then the frequency is 10 Hz
(1/0.1 sec)
Thermal effects of e-stim - Answer Heat generated is proportional to the power
generated; Power = I x V = R x I^2. Heat = 0.24 x power x T, so also H = 0.24 x R x T x
I^2, and H = 0.24 x R x T x (V/R)^2, so therefore H = 0.24 x (V^2/R) x T where current is
the average current flow during time T (treatment time; the longer you apply a current
the hotter it gets). Infinite resistance means no heat (e.g. wall socket). The lower the R,
the higher the I (because of Ohm's law), and therefore the more heat produced; current
is a greater factor in heat generation because it is squared in the equation. In the clinic
we want to minimize heat (and therefore burns, although usually not a concern in clinic
because good circulation washes away heat), which is done by reducing power by
reducing current and increasing resistance. The average current is a function of the
waveform and duty cycle within and across trains, so changing the waveform and duty
cycle (Russian stim has a high duty cycle so more risk of burns) will also indirectly affect
heat production.
Electrical short - Answer Usually seen as a big spark with smoke; what is happening is
the resistance is going toward 0, so the current goes toward infinity. In other words
there is a massive amount of current. This is how people are able to melt metal when
welding.
Hot spot - Answer Area of high current density due to a small electrode or wrinkles or
something on the skin that breaks up the contact area. Feels more intense under a hot
spot and can lead to burns.
Chemical effects of e-stim - Answer A function of the net current flow. Unbalanced
phase charge yields net current flow. At the cathode (negative, attracts cations which
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