Electrical basis of electrotherapy

Electrical current is the flow of electrons

Measured in amperes (A) – one ampere = the flow of 6.25 x 1018 electrons/sec

Charge is measured in coulombs (C). 1A = 1C per second

Voltage (V) is a measure of electromotive force

If electrons are to flow, there needs to be a difference in the number of electrons between two points

Electrons have a negative charge flow from negative to positive electrical current

The magnitude of the difference between the positive and negative poles is the electromotive force that drives the current.

Impedance = resistance + inductance + capacitance

Resistance (R) = the amount of resistance a material has to electrical current; = volts/current

Inductance = the opposition to electrical current created by electromagnetic eddy currents created when a current passes through a wire

Capacitance – the ability of a material to store an electrical charge

Ohm’s law – voltage = current x resistance

Impedance:

• measure of the opposition to current flow

Types of current:

Two types of continuous current – alternating (AC) and direct (DC)

AC – eg household supply; the polarity at each end of the circuit is constantly being reversed creates a sinusoidal pattern of positive and negative phases

DC – eg car battery; polarity remains constant flow in only one direction

In most electrotherapy’s, the current is modulated into a pulsed form.

Three waveforms of pulsed current:

1) Monophasic – similar to DC, but is noncontinuous due to interpulse intervals

2) Biphasic – similar to AC, but non-continuous due to interpulse intervals

3) Polyphasic – created by interference of two AC currents; current has high peaks when two currents are in phase and low peaks when out of phase; small peaks are too small to affect tissues

Parameters of electrical stimulation:

Phase duration:

• phase charge is a measure of the number of electrons delivered in each phase of a pulsatile current – determined by phase duration and amplitude

Amplitude:

• intensity

• mA

• Rheobase – minimum amplitude to depolarise a nerve fibre

• Chronaxie – the time required to depolarise a nerve fibre when the peak current is twice rheobase

Frequency:

• pulses per second (Hz)

• different types of muscle contraction occur at different frequencies: 1-10Hz twitch contraction; above 30Hz tetanic contraction, if between 30-70Hz nonfatiguing tetanic contraction; if over 100Hz fatiguing tetanic contraction

Waveform:

• variety of wave forms used in electrotherapy’s

Polarity:

•

Duty cycle:

• the pattern of ‘on/of’ sequencing

Ramp:

• ramp time is the period of time during which the amplitude of the stimulus is gradually increase or decreased (ramp up or down)

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