Electrical basis of electrotherapy

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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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