Electrotherapy: Ions and Electrical Charge

• Electrotherapy's earliest known use was AD/CE 46
• Luigi Galvani is associated with electrotherapy
• Electrotherapy involves applying electrical current to the body for therapeutic purposes
• Applications of electrotherapy include the management of pain, muscle strengthening, edema reduction, inflammation and wound control

Electrical Phenomena: Ion

• Atoms are electrically neutral
• An ION is an atom that is no longer in its neutral state
• Ions exist in electrolytic solutions, including biological solutions
• Ionization refers to the acquisition of a negative or positive charge
• A negative ion has gained electrons
• Anions are negative ions attracted to the anode
• A positive ion has given up electrons
• Cations are positive ions attracted to the cathode

Electrical Phenomena: Charge

• Charge is the fundamental concept of electrical phenomena
• Charge involves the taking away or adding electrons
• Types of charge includes negative and positive charges
• A negative charge contains more electrons than normal
• A positive charge contains less electrons than normal
• Charge (q) is measured in coulombs (C)
• One coulomb is the quantity of electricity transported in 1 second by a current of 1 ampere
• One coulomb of charge equals 6.25 x 10^18 electrons
• The charge of a single electron equals -e = 1.6 x 10^-19 C

Electrical Phenomena: Polarity

• Polarity describes the property of having two oppositely charged conductors
• Cathode is the negative pole
• Anode is the positive pole
• Free electrons flow from an area of excess electrons (negative polarity) to an area deficient in electrons (positive polarity)

Electrical Phenomena: Voltage

• Voltage is the potential energy difference, or electromotive force
• Volt (V) is the unit of potential difference
• 1(V) is the force required to move 1(C) of charge through 1(Ω) of resistance

Electrical Phenomena: Current

• Current refers to the directed flow of charge within matter
• Charge can be free electrons or ions
• Ampere (A/amp) represents the unit of current (I)
• (I) measures the rate at which charge flows
• 1(A) equals 1(C)/sec
• Typical units in Electrotherapy includes milliamps (mA) eqalling (1/1000) and microamps (μA) eqalling (1/1000,000)

Electrical Phenomena: Resistance

• Resistance is the property of a material to resist current flow
• Resistance (R) is measured in ohms (Ω)
• Ohm's Law states if one volt of potential difference causes a current of one ampere to flow in an electrical circuit, the limiting resistance equals to one ohm

Electrical Phenomena: Ohm's Law

• Ohm's Law relates current, resistance, and voltage in an electrical circuit
• The flow of current (I) is in direct proportion to the electromotive force (V) and inversely proportional to the resistance (R)
• I=V/R

Therapeutic Currents/ Waveforms

• Direct Currents
• Alternating Currents
• Pulsatile (pulsed) Currents

Direct Current (DC)

• Direct Current is historically called Galvanic
• DC is the CONTINUOUS unidirectional flow of charged particles to the positive pole
• In modern DC devices, the polarity, and thus the direction of current flow, can be reversed
• Uninterrupted flow of ions/electrons lasts for at least ONE second
• Has no pulses or wave form

Types of Direct Current

• Convention: A deflection up signifies flow in a positive direction
• Convention: A deflection down signifies flow in a negative direction
• Interrupted DC ceases for 1 second, then resumes for at least 1 second in the same direction
• Reversed DC flow ceases after 1 second and resumes in the opposite direction for at least 1 second
• Interrupted reversed DC refers to a combination of interrupted and reversed direct current

Alternating Current (AC)

• AC refers to the continuous/uninterrupted BI-DIRECTIONAL flow of charged particles
• Change in direction occurs at least once every second
• Continuous bi-directional flow of charges must last at least ONE second and cross the isoelectric line at least twice
• Waveform is biphasic
• The rate of change is measured in hertz (Hz) or cycles/second

Shapes of Alternating Current Waveforms

• Different waveforms include sine wave, triangle wave, square wave and sawtooth wave

Pulsatile Current (PC)

• PC is the BRIEF unidirectional or bidirectional flow of charged particles separated by a brief period of no flow
• Electrophysiological effects are better suited than DC and AC current for electrotherapeutic applications
• Waveforms can be monophasic, biphasic, or polyphasic
• It is measured in milliseconds (ms) or microseconds (µs)

Pulsatile Current Waveforms

• The waveform of "Russian current" is polyphasic
• The waveform of interferential current is polyphasic

Waveform Characteristics

• Peak amplitude
• Peak-to-peak amp
• Phase duration
• Pulse duration
• Phase charge
• Pulse charge
• Rise time
• Decay time
• Interphase interval (intrapulse interval)
• Interpulse interval
• Average Current
• Pulse rate/Frequency

Amplitude

• Amplitude measures the magnitude of current (I) or voltage (V) with reference to the isoelectric line
• To avoid confusion, "intensity" should not be used to describe amplitude
• "depth of penetration" relies on the higher the peak amplitude the deeper the penetration
• Amplitude determines the ability to discriminate between the clinical levels of stimulation

Phase & Pulse Duration

• Phase and pulse duration is the elapsed time from the initiation of the phase/pulse until its termination and is expressed in (µs) or (ms)
• Avoid the term "pulse width" as this parameter is time-dependent and not distance-dependent
• In monophasic waveforms, phase duration and pulse duration are equal

Pulse Duration

• Pulse, not phase, is the traditional basic descriptive unit
• Pulse is associated with comfort of stimulation, factor in determining pulse charge, and ability to discriminate between the clinical levels of stimulation

Phase & Pulse Charge

• Phase charge is the charge per phase
• Pulse charge is the charge per pulse
• Integrates the sum of current amplitude multiplied by time under the area of the waveform
• In monophasic waveforms, phase charge equals pulse charge
• Measured in (µQ)
• 12-15 μc is weak
• 20-25 µc is moderate
• 30-40 μc is strong
• It is responsible for tissue excitation, tissue regeneration (monophasic) and tissue damage if the charge is too great

Rise & Decay Time

• Rise time is the time it takes for the leading edge of a phase to increase from the baseline to peak amplitude and may occur with physiological "accommodation" that occurs with repeated stimulation
• Decay time refers to the time it takes the trailing edge of the phase to return to baseline from the peak amplitude

Interphase & Interpulse Interval

• The interphase interval is the period of no electrical activity between two successive phases of a pulse and decreases the pulse charge and makes the waveform more comfortable
• The interpulse interval is the period of no electrical activity between two successive pulses and a longer interval decreases the average current

Average Current

• Average current is the amount of current flowing per unit time
• DC’s average (I) equals peak amplitude
• AC’s average (I) equals 65-70% peak amplitude (peak amplitude/v2)
• PC’s average (I) is dependent on the shape of pulse duration of pulse and length of the interpulse interval and however, always much lower than AC or DC
• Recommended average current is 1.5 to 4ma/cm2/ on a surface electrode and 10 ma/cm2/ on a surface electrode
• 30-40 ma is the maximum irrespective of electrode size
• 70 ma will cause cardiac fibrillation
• 100 ma will cause death
• Ideally, you want to minimize average (I)
• Use the lowest average (I) that will produce the desired physiologic response
• Excess, unnecessary current increases patient discomfort, and may cause tissue damage

Pulse Rate (Frequency)

• Pulse rate refers to the number of times per second the waveform (pulses, cycles, burst or beats) repeats itself
• DC has no frequency
• AC is measured in cycles per second (cps) or Hertz (Hz)
• PC is measured in pulses per second (pps)

Pulse Rate (Frequency): Alternating Current

• In AC, frequency and pulse duration are inversely proportional
• An increase in pulse rate will decrease the phase/pulse duration
• The average current remains the same

Pulse Rate (Frequency): Pulsatile Current

• In PC, frequency and pulse duration are independent
• Increasing the rate will not affect the phase/pulse duration.
• Average current is rate dependent.

Pulse Rate (Frequency) and Physiological Effect

• For a muscle contraction twitch use 1-10pps
• For a muscle contraction Tetanic effect, use nonfatiguing 20-50pps and fatiguing 80-100pps
• Analgesic effect from enkephalin 40-150, serotonin 2-5, 15-100 and beta endorphins 2-5

Modulation

• Modulation involves any pattern of variation in one or more of the stimulation parameters
• Modulation is used to limit neural adaptation to an electrical current
• Modulation may be cyclical or random
• Modulation, for example, can alter the amplitude, duration, or frequency of the current during a series of pulses or cycles

Modulation Types

• Burst
• Beat
• Amplitude (ramping)

Modulation: Burst

• Burst involves two or more successive pulses separated by interburst intervals and delivered at an identified frequency

Modulation: Beat

• A beat modulation will be produced when two interfering currents with differing frequencies are delivered

Modulation: Amplitude (ramping modulation)

• A gradual increase (ramp-up) or decrease (ramp-down) of phase charge occurs over a predetermined period and usually lasts 1 to 5 seconds
• Modulation is associated with the ON portion of stimulation
• It should be specified independently of the ON time

Physiological Effects

• The direct effects include electrothermal, electrochemical and electrophysical

Physiological Effects: Electrothermal

• The mobility of charged particles causes microvibration leading to heat production
• An increased heat effect results from high average current, high skin impedance, and prolonged treatment time

Physiological Effects: Electrochemical

• DC breaks up certain molecules into component atoms or ions which then form new chemical compounds
• An alkaline reaction occurs under the cathode which is 2Na+2H2O ->2NaOH +H2
• An acid reaction occurs under the anode which is 2CL+2H2O ->4HCL+O2

Physiological Effects: Electrophysical

• Electrophysical effects can fall on excitable cells or non-excitable cells
• Excitable cells refers to PNS (sensory and motor function), ANS (blood vessels, internal organs, heart) and skeletal muscle
• Non-excitable cells refer to skin, bone, protein synthesis, extracellular fluid and blood cell concentration

Excitatory Responses

• The pattern of activation of tissues in clinical applications is determined by the inherent excitability of the fiber (size) and its location in relation to the electrode

Excitatory Responses: Fiber Size

• The size of the fibers determines the internal resistance
• Larger fibers have less internal resistance and are more excitable than smaller fibers
• Fiber size alone does not predict recruitment pattern
• The muscle spindle primary afferent, Golgi tendon organ uses fiber type Aα with a fiber diameter (µm) range of 12-20 and are the most excitable
• Touch and pressure use fiber type Aβ with a fiber diameter (µm) range of 5-12
• To muscle spindles (motor and sensory) use fiber type Aγ with a fiber diameter (µm) range of 3-6
• Pain and cold touch use fiber type Aδ with a fiber diameter (µm) range of 2-5
• Preganglionic autonomic uses fiber type B with a fiber diameter (µm) of <3
• Lastly, pain (slow) uses fiber type C with a fiber diameter (µm) range of 0.5-1.2 and are the least excitable

Excitable Tissues: Resting Membrane Potential (RMP)

• The Sodium-Potassium pump plays a role in creating the Resting Membrane Potential
• There are 3Na+ out and 2K+ ions in the cell
• Resting Membrane Potential (RMP) should be -60 to 90 mV

Excitable Tissues: Action Potential (AP)

• An action potential is the basic unit of nerve communication (1-5 ms)

• The process is achieved by rapid sequential nerve depolarization and repolarization with last anode hyperpolarization

• Under the cathode (primarily) a reversal of charge (depolarization) occurs

• If the reversal of charge is sufficient (-55 mV) an AP is generated

• The return of charge is the resting stage after an initial hyperpolarization (past resting charge -60 - 90 mV)

Excitable Tissues: Action Potential (AP) Absolute & Relative Refractory Period

• Absolute refractory period: During an action potential the nerve cannot be stimulated regardless of intensity and lasts 0.5 microseconds
• During the relative refractory period of hyperpolarization and a stimulus of higher than usual intensity is needed to generate an AP

Excitable Tissues: Propagation of Nerve Impulse

• A small electrical circuit has an electrical potential difference between the depolarized region and the neighboring inactive region, therefore, the depolarization self-propagates along the fiber in each direction of the depolarization site

Excitable Tissues: Strength Duration Curve

• Amplitude and duration of electrical current required to produce an AP depends on the type of nerve being stimulated
• Short duration pulses of 50 to 100 µs stimulate sensory nerves
• 150 to 350 µs stimulate muscle
• 1 ms stimulate pain fibers
• Longer duration pulses of longer than 10 ms produce contractions of denervated muscle

Excitability: Stimulation Strength and Duration

• Rheobase is the minimum amplitude of current to cause an Action potential given a long pulse duration
• Chronaxie is the minimum duration needed to stimulate tissue at twice rheobase intensity

Excitatory Responses: Current Density

• The closer an excitable fiber is to an electrode, its more likely to be stimulated by induced currents
• Current density is greatest closer to the electrode
• Current density is greatest under smaller electrodes
• Current density is greater in deeper tissues if electrodes are further apart
• This density (current density) is measured in [amps/m^2], the formula for this is I(Current [Amps]) / A(Area [m^2])

Electrical Effects On Non-Excitable Cells

• Tissue healing occurs in response to the electrical field
• Bone remodeling
• Frequency window selectivity
• Activation of genes for protein molecules