intro to electrotherapy: electrical currents, activation of excitable tissues and instrumentation

what is electrotherapy

he induction of electrical currents in biological tissue to achieve or facilitate a therapeutic outcome (the resolution of impairment and the restoration of function)

– Electrical stimulation for pain control
– Electrical stimulation for muscle weakness (strengthening)
– Electrical stimulation for wound healing

Impairment oriented interventions

what is electrical current?

the movement of electrically charged particles (mainly ions in humans)

How do electrodes become polarized?

plug it in the wall and stick a battery

What are the ions in human tissues?

POS.: sodium

NEG.: hydrogen and hydroxide

what produces electrical current?

• voltage (V)

• Current (I)

• he driving force for moving charged particles
  – a.k.a.: electromotive force (EMF) or electrical potential difference
  – EMF is created by electrodes of opposite charge or polarity

Voltage (V)

is proportional to the applied voltage

current (I)

What produces the voltages that separate electrically charged particles?

– Batteries or stimulators and subsequently the surface electrodes

Current is limited by Resistance of tissue to current flow

V = I x R

• direct current

• alternating current

• pulsed current

types of therapeutic currents

• – Continuous (uninterrupted) unidirectional movement of charged particles

• – For biological tissues: Anions in one direction, cations in the other

• pH changes

• moving in one direction and back stop

direct current

• – Uninterrupted (continuous) bidirectional movement of charged particles

• – Oscillation of anions and cations back and forth

• DC plug it in the wall its AC

• pH does not change and ions move back and fourth

alternating current

is alternating current pulse monophasic or biphasic

biphasic

• Interrupted, uni- or bidirectional movement of charged particles... ions move very briefly then stop and start

• it would be stop, move, stop, move, etc.

• often use pulse current

pulse current

is pulse current biphasic or monophasic

can be monophasic and biphasic

why does pulse current not alternate

because there is a pause/time in between

what is the common current PT’s use?

• flow, flow, pause

• the flow flow rate is frequency

what is the use of therapeutic currents direct current (DC)

• – Delivery of medication...iontophoresis

• – Enhance wound healing

• – Denervated muscle

• can use to stimulate and activate a denervated axon

what is the use of therapeutic currents pulsed current (PC) and alternating current (AC)

• – Control pain

• – Produce muscle contraction....

  • – Strengthen muscle

  • – Produce movement (FES)

  • – Improve blood flow

• – Enhance wound healing

what are we stimulating?

we are stimulating the A-MN going to the muscle not the muscle itself

what type of current do we need to stimulate the muscle itself?

direct current (DC), skeletal muscle fibers have to be a DC

What tissues are directly affected by electrical currents in electrotherapy?

electrically excitable tissues:

• peripheral nerve fibers

  • motor axons

  • sensory axons

  • autonomic axons

• skeletal muscle fibers

• cells tissues and bloods

to skeletal muscle fibers (A-alpha)

motor axons

from touch receptors and pain endings A-beta, A-delta, C

Sensory axons

to vascular smooth muscle

Autonomic axons

types of axons in peripheral nerves

How does electrical stimulation activate peripheral nerve axons?

• recruiting more axons to make it feel stronger

• recruitment = treat bigger area (amplitude/duration)

Axon Membrane at rest

what does stimulation open?

Stimulation opens sodium and potassium channels..........channels are “voltage gated”

where does depolarization spread to?

Depolarization spreads to adjacent membrane

What happens when AP’s are produced in peripheral axons with e-stimulation?

A-alpha motoneuron AP’s activate skeletal muscle and cause muscle contraction (motor response)

What happens when AP’s are produced in peripheral axons with e-stim?

• A beta cutaneous touch axon AP’s pass into the CNS and produce a “tactile” or touch sensation

• A Beta = sensory response (touch pressure)

• A-delta or C fibers = pain

  • A-alpha is the biggest and more myelinated than A-delta

What happens when AP’s are produced in peripheral axons with e-stim?

A-delta and C sensory axon AP’s pass into the CNS and produce the sensations of sharp, fast pain and slow, achy pain respectively

What factors determine the order of activation of peripheral axons with ES?

• Size (diameter) of peripheral axon

• Location of axon

• Stimulus amplitude and duration

– larger diameter axons activated before smaller with ES

Size (diameter) of peripheral axon

• – Axons closer to the electrodes activated before those further away

• – Why?..... Higher current density close to the
  electrodes

Location of axon

– As amplitude or duration are increased, more axons are activated in response to each stimulus .... called “recruitment

Stimulus amplitude and duration

Type and size of peripheral axons

Axons to skeletal muscle fibers Axons from muscle sensory receptors

A alpha

Axons from touch receptors, joint receptors

A beta

Axons to muscle spindle muscle fibers

A gamma

fast, sharp pain

A delta

slow, achy pain

C fibers

Axon size determines the ___ of peripheral axons

“inherent excitability”

as the stimulus is increased

• larger axons are activated before smaller

• muscle contraction 1st, cutaneous sensation 2nd and pain response 3 rd

Implications of axon location on the pattern of activation with ES

A beta touch axons, and A delta and C pain axons in the __

skin

bundle of axons of different sizes and functions

peripheral nerve

Skeletal muscle fibers innervated by

alpha motoneurons

The closer the nerve fibers to the electrodes the more likely

they will be activated

__ afferents beneath electrodes first to be activated followed by superficial pain afferents and last motor neurons to muscle fibers... not always the case!

cutaneous touch

Pattern of activation depends on what specific tissues are actually near the electrodes

– e.g. electrode over a superficial nerve bundle vs. over a bony area

By your choice of electrode placement, you may determine which nerve fibers may be activated

– True sometimes but not in every stimulation application
– You must know the anatomy of the area where you are placing electrodes!

The SD curve is a plot of all of the stimulus amplitude and duration combinations that activate a particular axon

Strength-duration curve

How does the amplitude/or pulse duration of stimulus effect the pattern of axon activation with ES?

Increasing the amplitude or pulse duration increases the number of nerve fibers activated (increases the “recruitment” of nerve axons)

S-D curve based on axon size alone

• sensory level stimulation

• motor level stimulation

• noxious level stimulation

Clinical Levels of Stimulation

• This is the graph that you need to keep in mind whenever you are using ES in a clinical application

• when you get to motor, sensory does not stop

Typical S-D curves in electrotherapy based on both axon size and proximity of axons (location) to surface electrodes

• Activates (produces AP’s in) superficial A beta cutaneous touch-pressure axons

• Produces a tapping sensation with low frequency stimulation (< 5 pps)

• Produces a tingling or “pins-and-needles” sensation with higher frequency stimulation (>20 pps)

• Tingling may diminish with fixed amplitude stimulation......this is called “accommodation

Sensory level stimulation

• Activates (produces AP’s in) A alpha motor neuron axons to skeletal muscle

• – Be prepared to describe physiologic sequence of steps leading to muscle contraction

• Produces weak,“twitch” contractions with low frequency (< 5 pps) of stimulation and a “tapping” sensation

• Produces “unfused” tetanic contractions (“tremor”) at frequencies of 5 15 pps

• Produces stronger, “smooth (fused) tetanic” contractions with higher frequency (>20 pps) stimulation with “tingling” sensation

Motor level stimulation

what is being activated at motor level?

A-alphas

if you want fused titanic you want?

frequency (rate coding)

• Produces a “pain” response

• – due to A delta and/or C fiber axon activation

• Cutaneous sensations and contraction persist as stimulation “intensity” is increased from motor to noxious level – Why?

• A level of stimulation to be avoided in some clinical applications...Why?

Noxious level stimulation

• Traditional designations

• Commercial designations

• Contemporary designations

Types of currents in electrotherapy

How are electrical currents displayed?...in graphs

Such graphs are often used by manufacturer’s to illustrate the waveform produced by stimulators

• Direct Current

• Alternating Current

• Pulsed Current

• Differences between various types of AC and PC clarified by identifying defined quantitative characteristics (parameters) and qualitative characteristics using diagrams of the current or voltage waveforms

Contemporary Designations for Therapeutic Currents

What is a “Waveform”

• A visual representation in a graph of the changes in amplitude of current or voltage over time

• both are pulsed current

• both are biphasic

• For individual waveforms or pulses

  • – Amplitude

  • – Pulse or phase duration

• For a series of waveforms

  • – Frequency

    • Pulses/second (pps) for PC

    • Cycles/second (cps) or Hertz for AC

  • – On times/off times

    • seconds on/seconds off

Stimulation parameters that you select, adjust or set

• – Amplitude

• – Pulse or phase duration

For individual waveforms or pulses

• – Frequency

  • Pulses/second (pps) for PC

  • Cycles/second (cps) or Hertz for AC

• – On times/off times

  • seconds on/seconds off

For a series of waveforms

• On Time/Off Time (secs)

• Burst duration/interburst interval

Timing modulations

Automatic, systematic variations in one or more waveform parameters

Modulations:

• electrodes

• stimulators

Instrumentation in Electrotherapy

• – Interface between the client and the “stimulator”

• – 2 needed for each “channel” of
  stimulator

electrodes

• – Clinical models: line-powered

• – Portable models: battery operated

• – Produce the driving forces to induce currents in tissues

Stimulators

• Electrodes size and shapes selected based upon the specific type of ES intervention selected and the requirements of that intervention

• You do not use the same type and size of electrodes for all ES applications

Electrodes Sizes and Shapes

• Electrodes are connected to the stimulator by electrode leads

• An electrode lead is a wire with connectors that attached securely to both the electrode and the stimulator

Electrodes and Leads

• Always securely attached to client

• Always securely attached to the “leads” of the stimulator

• Placement in a manner consistent with the particular impairment managed

• Electrodes are not “forever”... all types need to
  be regularly replaced

  • – “hot spots” during use indicates electrode must be
    discarded and replaced

• Specify size and location of electrodes for each
  clinical application using anatomical references

Electrode best practices

Electrode Size and Current Density

Effect of size and location

• one electrode in target area is called a monopolar set up

• two in the area is a bipolar set up

• 4 in the area is quadripolar

• more placements = recruitment

  • use amplitude to increase

monopolar electrode placements

bipolar electrode orientation

quadripolar electrode orientation

“Selectrode System”

• Clinical and portable styles

• Constant current vs. constant voltage

  • – Constant current may be safer

• Analog design (rotating dials for controls) vs. digital design (pressure sensitive switches with electronic displays)

• Commercial designations (not recommended)
  – High volt
  – Interferential
  – TENS
  – NMES
  – MENS
  – Russian

Types of Therapeutic Stimulators