Electrical Stimulation Terminology

Physiologic Effects of Electrical Stimulation

1.) decrease pain

2.) decrease muscle spasm

3.) reduce edema

4.) simulate exercise by muscle contraction: increase muscle fiber recruitment, retard atrophy

5.) stimulate healing

Electricity = flow of electrons

requires: source of electrons, driving force (electron imbalance), path (conductor)

Comparison to fluid flow

1.) electrons are drops of water in a river

2.) current flow (flow of electrons) is like the flow of the river ampere (amp A)

3.) voltage is like a waterfall (as height of the waterfall increases, the potential energy increases)

4.) resistance: length of conductor, cross sectional area and temperature dictate the amount of resistance. Least resistance with short smooth large diameter hose (wire)

Capacitance

ability of a material to store a charge

Ohms law

V=IxR

R = V/I

I=V/R

Monophasic current

1.) current flow in one direction

2.) unique positive and negative electrodes

3.) direct current (DC), AKA galvanic

4.) net charge: positive and negative electrode, charge builds up in the tissue

Biphasic current (alternating current)

1.) flow of electrons changes direction regularly (change polarity)

2.) wave form: symmetrical → same shape in both phases, asymmetrical —> different positive and negative

3.) net charge: balance → equal electrical change in both phases (minimize or eliminate polarity effect in the tissue); unbalanced → unequal charge

4.) shape: sinusoidal, square, rectangular, triangular

Phase duration

1.) time it takes current to leave the isoelectric line to when it returns to this line

-biphasic: two phase durations for each pulse; monophasic: phase duration and pulse duration are the same

2.) tissue respond to phase duration not pulse duration, must be long enough to overcome capacitance and cause an action potential → large diameter nerves have low capacitance and reach threshold quickly

Amplitude

1.) intensity or magnitude of the current

2.) must be high enough to reach threshold of muscle or nerve; A beta is close to skin and has lower threshold so it will be stimulated first, gives sensory response before motor

3.) high peak current is associated with greater depth of penetration → the deeper the penetration the more neuron recruitment and therefore muscle fiber recruitment possible

peak current

maximum amplitude of the current regardless of duration

Average current

1.) amount of current supplied over a period of time

2.) takes into consideration peak amplitude and the phase duration

-higher average current needed for some physiologic responses, too high average current can cause tissue damage, depending on wave form, can have high peak but low average current

Strength duration curve

1.) Describes the relationship between amplitude (strength) of the electrical current and the duration (phase duration)

-if charge is sufficient to overcome the capacitance of a nerve fiber it will depolarize

-if the charge does not exceed the capacitance then no depolarization will take place

-likewise if the amplitude is too low no depolarization will occur no matter how long the duration

Targeting capacitance

1.) alter the phase duration and the amplitude; do not need to know the precise phase duration or amplitude (use patient feedback)

2.) pt reports tingling but no muscle twitch (we have exceeded the a-beta fibers but not A-alpha motor neurons)

3.) muscle contraction: a-alpha motor neurons have been exceeded

4.) burning, needling sensation signals you have exceeded the a-delta fiber capacitance

Rheobase

1.) minimum amplitude needed to depolarize a nerve fiber when phase duration is infinite

2.) if peak amplitude fails to exceed rheobase the nerve will not depolarize regardless of phase duration

Chronaxie

1.) the time (or phase duration) required to depolarize a nerve fiber when the peak current is twice rheobase

2.) when amplitude is twice rheobase and the phase duration is slightly greater than chronaxie the result will be greatest comfort for the patient

Frequency

1.) number of pulses or cycles generated per second (PPs or Hz)

2.) affects number of action potentials elicited during the stimulation, higher frequency leads to summation (motor neuron: tetany)

3.) the absolute refractory period is the rate-limiting factor of the number of impulses that can be generated by a nerve

Temporal summation

1.) single twitch: contraction and then relaxation

2.) summation: Force from two twitches, no relaxation after 1st twitch

Wedensky’s Inhibition

1.) stimulation at high frequency near refractory period of the sensory nerve causes inhibition

-wedenksi’s inhibition (>1000 Hz sensory nerves)

-action potential failure

-results in anesthesia between the electrodes

Low frequency generations

up to 1000 Hz, produce action potentials

Medium frequency generations

1.) 1000-100,000 hz

2.) Interferential current generators use 4,000-5,000hz

3/) this carrier frequency is interfered with to give a treatment frequency or beat frequency

4.) 1-10hz or 60-100 hz are common treatment frequencies

5.) Russian stimulation uses carrier frequency of 2,500hz. intrinsic duty cycle of 10ms on and 10ms off creates a burst frequency of 50 hz

High frequency generators

1.) greater than 100,000hz

2.) used for thermal purposes

3.) diathermy uses high frequency and creates minimal sensory effects

electrode considerations

1.) electrodes are attached to the current generator by wires called leads, there must be two leads to complete a circuit

2.) leads can be split or bifurcated leads usually are wired into pairs that plug into a single channel

3.) unequal size will concentrate the current in the smaller electrode and it will give the perception of increased intensity

4.) when sizes vary greatly one may not be able to perceive current under the larger electrode —> this become the dispersal electrode

Current density

1.) if pads are placed close together the current is most concentrated in superficial tissues

2.) when pads are far apart the current has the potential to take a deeper path through the nerve and blood vessels that have less resistance

Reduce the skin-electrode resistance

1.) minimize air-electrode interface

2.) keep electrode clean of oils, dirt, etc

3.) use adequate moisture on pads, use largest electrical practical

4.) use shortest pathway for energy flow

5.) if resistance increases, more voltage will be needed to get the same current flow

Monopolar electrode configuration

1.) two or more unequal sized electrodes are used, can be used with either biphasic (AC) or monophasic (DC) current)

2.) one is the active and one is the dispersive electrode, active at the target site and dispersive away from target site

3.) 3 reasons for this placement:

-leads placed far apart: deeper penetration

-greater comfort at dispersive pad when using a point stimulator

-to create an electrical field with specific polarity (polarity effect)

Bipolar electrode configuration

1.) can be sued with either monophasic (DC) or biphasic (AC) currents

2.) 2 equal sized electrodes are placed over the treatment site, most common for TENS (transcutaneous electrical nerve stimulation)

Quadripolar configuration

1.) often used with interferential current

2.) two separate medium frequency currents are used with electrodes placed as cross currents

-current is interfered with in the center of the two currents, can change this location of interference where you feel the beat frequency