Principles of Electrical Stimulation

Current Types  Direct Current  Alternating Current  Pulsed Current

Direct Current Description:  One-directional flow of electrons  Constant positive and negative poles Use:  Iontophoresis  Low-voltage stimulation

Alternating Current Description:  Bidirectional flow of electrons  No true positive and negative poles Use:  Interferential stimulation  Premodulated currents

Pulsed Currents MONOPHASIC CURRENT Description:  One-directional flow marked by periods of non- current flow  Electrons stay on one side of the baseline or the other Use:  High voltage pulsed stimulation BIPHASIC CURRENT Description:  Bidirectional flow of electrons marked by periods of non-current flow  Electrons flow on both sides of the baseline (positive and negative) Use:  Neuromuscular electrical stimulation  Three types of biphasic currents

Biphasic Current Types Symmetrical  Mirror images on each side of the baseline  No net positive or negative charges under the electrodes Balanced Asymmetrical  The shape of the pulse allows for anodal (positive) or cathodal (negative) effects  No net positive or negative charge Unbalanced Asymmetrical  Positive or negative effects  The imbalance in positive and negative charges results in a net change over time. Can cause skin irritation if used for long durations

Pulse Attributes

Time-dependent Attributes  Pulse duration  Phase duration  Interpulse interval  Intrapulse interval  Pulse period  Pulse frequency  Pulse trains (bursts) Note: These attributes do not apply to direct and alternating currents

Pulse Duration  The time (horizontal distance) from when the pulse rises to the baseline to the point where it terminates on the baseline.  [instructor note: click to start animation] Monophasic PulseBiphasic Pulse

Phase Duration  Phases are individual portions of the pulse that appear on one side of the baseline  For monophasic currents, pulse duration and phase duration are synonymous (only 1 phase).  Biphasic pulses have two phase durations  The phase duration determines which nerve type is affected.  [instructor note: click to start animation] 1 1 Monophasic PulseBiphasic Pulse 2

Interpulse Interval  The time between the end of one pulse and the start of the next pulse  Allows for mechanical changes in the tissues, such as when eliciting muscle contractions  Increasing the pulse frequency decreases the interpulse interval and vice-versa  [instructor note: click to start animation] Two Monophasic PulsesTwo Biphasic Pulses

Intrapulse Interval  Intrapulse intervals are brief interruptions of current flow.  Are always shorter than the interpulse interval.  They allow for physiologic adaptations to the current and/or to decrease the total charge delivered by the pulse.  Are normally not adjustable on the unit.  Intrapulse intervals can also apply to monophasic currents.  [instructor note: click to start animation] Biphasic Pulse

Pulse Period  The pulse period is the amount of time from the start of one pulse to the start of the next pulse.  Includes the phase durations, intrapulse interval,and interpulse interval.  Inversely proportional to pulse frequency. As the pulse frequency increases, the pulse period decreases and vice-versa.  [instructor note: click to start animation] Two Monophasic PulsesTwo Biphasic Pulses

Pulse Frequency  The number of times a pulse occurs per second  With alternating currents this measure is described as cycles per second

Pulse Trains (Bursts)  Trains contain individual pulses  Pulses in the train still have time-dependent characteristics: pulse duration, interpulse interval, etc.  Each train is separated by “off” times – the intertrain (or interburst) interval

Generator Attributes

Current Density  The amount of current per unit of area  The higher the current density, the more intense the effects 10 in V 30 V/in 2 5 in V 60 V/in 2

Duty Cycle  The amount (percentage of time) that the current is flowing relative to the time it is not flowing  Duty cycle = “ON”/(“ON + OFF”) * 100  Example:  Current is on for 20 seconds and is off for 40 seconds  DC = 20/(20+40)*100  DC = 20/60 * 100  DC = 33.3%

Pulse Ramp  Used with a duty cycle  Gradually increases the current  Produces a more natural contraction  More comfortable

Electrical Currents

Measures of Electrical Current  Charge:  Microcoulomb, the charge delivered per pulse  Voltage:  The potential for electrical flow to occur.  The difference in charges between the positive (anode) and negative (cathode) poles  Current:  Amperage: The rate of electron flow  Wattage:  Measure of the ability to perform work  Calculated as W = Amperage * Voltage  Resistance:  Those structures (electrodes, wires, tissues) that do not transmit electrical energy

Average Current  The amount of charge delivered by one-half of a pulse or a cycle  Considers the amount of time required to deliver the charge

Circuit Types  Series Circuit  Electrons only have one path to flow  Parallel Circuit  Electrons have multiple paths to travel  The less the resistance within the path, the more flow that occurs  In the body, different tissues have different resistances  Nerves have relatively little resistance  Bone has high resistance