© 2008 LWW Chapter 9. Principles of Electricity for Electrotherapy (Part C)

© 2008 LWW Tissue Responses to Electrical Stimulation Four basic responses in the tissue: –Chemical –Thermal –Magnetic –Kinetic

© 2008 LWW Chemical Effects Driving ions (of medication) into the body –Positively charged ions move to the negative pole (or cathode). –Negatively charged ions move to the positive pole (or anode).

© 2008 LWW LeDuc’s Potato Experiment Cut a hole in a potato Filled it with a potassium iodine solution, which was partially dissociated into positive potassium ions and negative iodine ions Stuck wires into either end and attached them to a battery As the DC current passed through the potato, ions were attracted to the two poles. –The positive potassium ion to the negative pole and the negative iodine ion to the positive pole

© 2008 LWW LeDuc’s Potato Experiment (cont.) Illustrates chemical effect –The iodine ions of the potassium iodine solution migrated to the end of the potato with the positive pole. –The iodine interacted with the starch near the positive pole to form blue starch iodine.

© 2008 LWW LeDuc’s Rabbit Experiment Placed two rabbits in series of electrical circuits so current passed through both rabbits

© 2008 LWW LeDuc’s Rabbit Experiment (cont.) Electrode A attached to first rabbit, with electrode pad soaked in strychnine sulfate solution Electrode B attached to second rabbit with electrode pad soaked in potassium cyanide solution Electrical line between rabbits attached to electrode pads soaked in water only

© 2008 LWW LeDuc’s Rabbit Experiment (cont.) When electrode A was negative and electrode B was positive, the rabbits felt the current but were fine. When the electrodes were reversed, both rabbits died. Why?

© 2008 LWW LeDuc’s Rabbit Experiment (cont.) When A was negative and B was positive –A attracted the positive strychnine and repelled the negative sulfate. –B attracted the negative cyanide and repelled the positive potassium. –Thus current passed through the rabbits but the poisons did not.

© 2008 LWW LeDuc’s Rabbit Experiment (cont.) When A was positive and B was negative –A attracted the negative sulfate and repelled the positive strychnine. –B attracted the positive potassium and repelled the negative cyanide. –Thus current passed through rabbits and drove the poisons into them.

© 2008 LWW Requirements for Ion Migration: Chemical Effects Must have continuous monophasic DC electron flow to cause ion migration. Moving electrons against gradient –Like pushing a car uphill –When you pause, it rolls back down. Why does a twin-pulse high-volt current not produce a chemical effect?

© 2008 LWW Requirements for Ion Migration: Chemical Effects (cont.) Iontophoresis –Driving ions of medication into the body –Requires long-term flow of a DC current Medication must ionize in solution and be placed under appropriate electrode

© 2008 LWW Thermal Effects Minimal during muscle and nerve stimulation Discussed in Chapter 16

© 2008 LWW Magnetic Effects Magnetic field created with electrical current flow through a conductor (body part through which therapeutic electrical stimulation applied)

© 2008 LWW Magnetic Effects (cont.) Specific responses of tissue are poorly understood. Seem to be related to bone and wound healing

© 2008 LWW Kinetic Effects Sensation and muscle contraction, which result from stimulation of sensory and motor nerves Muscle contraction can be either –Single contraction –Twitch –Multiple contractions fused together into a tetanic contraction

© 2008 LWW Polarization and Action Potentials Stimulation requires a polarized membrane (between inside and outside of nerve membrane). –More positive ions than negative ions outside nerve and more negative ions than positive ions inside membrane –When polarized, membranes have a potential of −70 to −90 mV between inside and outside of membrane

© 2008 LWW Polarization and Action Potentials (cont.) Membrane stimulation causes depolarization at point of stimulation. Positive ions rush into the nerve and negative ones move out.

© 2008 LWW Polarization and Action Potentials (cont.) Change in potential causes adjacent membrane tissue to depolarize, which causes adjacent tissue depolarization. Depolarization down the nerve is called an action potential.

© 2008 LWW Polarization and Action Potentials (cont.) Nerve action potential eventually causes –An ascending sensory impulse to the brain Or –A muscle action potential Muscle action potential causes muscle contraction.

© 2008 LWW Polarization and Action Potentials (cont.) Twigs of a single motor nerve synapse with multiple muscle fibers. –A motor nerve and all the muscle fibers it synapses with are known as a motor unit.

© 2008 LWW Action Potential vs. Muscle Contraction Action potential is much quicker than muscle contraction. –Think of a boulder on the side of a mountain: One quick push (the action potential) causes the boulder to tumble down the mountain for minutes (muscle contraction). Muscle twitch Nerve AP

© 2008 LWW Polarization and Action Potentials (cont.) All-or-none law –All muscle fibers of a motor unit contract in response to a single action potential in its nerve.

© 2008 LWW Polarization and Action Potentials (cont.) All-or-none law –All muscle fibers of a motor unit contract in response to a single action potential in its nerve. Gradation of contraction comes from the number of motor units stimulated. –Not by the strength of stimulation of individual motor units Action potential will occur if the stimulation is great enough.

© 2008 LWW Polarization and Action Potentials (cont.) After the action potential passes, the nerve must repolarize before another action potential can be generated. –Time during which membrane repolarizes known as refractory period Absolute refractory period: Nothing can be done to make the membrane fire. Relative refractory period follows the absolute refractory period. Membrane can fire but requires a much greater stimulus.

© 2008 LWW Polarization and Action Potentials (cont.) Nerve repolarizes quickly. Absolute refractory periods vary from 0.4 to 2 msec –Depends on specific nerve

© 2008 LWW Nerve Excitability Differentiate between stimulating –An individual, isolated nerve fiber –Part or all of a mixed nerve within other tissue –Nerve threshold vs. nerve excitability Action potential is generated when the stimulus meets or exceeds a nerve’s threshold. Nerve excitability –Amount of current applied to the surface necessary to elicit an action potential in a specific nerve

© 2008 LWW Nerve Excitability (cont.) Nerve excitability –Amount of current applied to the surface necessary to elicit an action potential in a specific nerve Factors that influence nerve excitability follow.

© 2008 LWW Excitability: Stimulation Parameters Therapeutic goal –Stimulate numerous fibers of a mixed nerve –Current must pass through the skin, subcutaneous tissue, and usually muscle. Amount of current applied to the skin to evoke action potentials in the target nerves –Depends on numerous factors

© 2008 LWW Excitability: Nerve Size and Depth The larger the nerve, the easier it can be stimulated. The more superficial the nerve, the easier it can be stimulated. In a practical sense: –Large sensory nerves are more excitable than motor nerves. –Motor nerves are more excitable than pain fibers.

© 2008 LWW Excitability: Tissue Resistance The nerve stimulated depends on tissue resistance. The horny layer of skin is a good insulator. Hair and oils add to the insulation. Coupling medium is used to decrease resistance.

© 2008 LWW Excitability: Current Density A measure of the quantity of charged ions moving through an electrode and the tissue immediately beneath it. = current flowing/area of the electrode If electrode pair are the same size, the current density is equal in and beneath the two electrodes (a) If electrodes of a pair are unequal size, the current density will be greater in the smaller electrode (b)

© 2008 LWW Excitability: Current Density Pulse rise time –Time to go from 0 to maximal amplitude –Slow rise time allows for accommodation –Accommodation: gradual increase of threshold –Sodium pump may work to reverse depolarization.

© 2008 LWW Excitability: Current Amplitude and Pulse Duration Current strength is a product of amplitude and duration.

© 2008 LWW Excitability: Rate of Stimulation Relationship between action potential and muscle contraction –Very brief nerve stimulation causes much longer muscle contraction Muscle twitch Nerve AP

© 2008 LWW Excitability: Rate or Frequency Doesn’t effect individual threshold –Effects torque as it approaches tetany –Increased rate means increased fatigue rate. fatigue tetany 5/sec

© 2008 LWW Excitability: Motor Point Point at which given amount of current will elicit greatest muscular contraction Point at which motor nerve enters muscle Locate motor points by trial and error. Do not confuse motor points with trigger points. –Trigger points are localized areas that are extremely sensitive to palpation, electrical stimulation, and ultrasound.

© 2008 LWW Excitability: Other Duty cycle (on–off cycle) Muscle conducts four times better longitudinally than transversely.

© 2008 LWW Electrodes Devices attached to the terminals of a generator or electrical stimulator Come in a variety of sizes, shapes, and materials

© 2008 LWW Electrodes (cont.) Cross-contamination of reusable electrodes

© 2008 LWW Electrodes (cont.) One-use electrodes –Come on a roll like postage stamps

© 2008 LWW Electrodes: Physical Dimensions Shape is unimportant –Most are round or square or rectangular. Size and placement determine the number of motor units stimulated.

© 2008 LWW Electrodes: Physical Dimensions (cont.) Small electrode placed over single muscle stimulates only that muscle Size has a bearing on current density under electrode –The smaller the electrode, the greater the current density. As long as the current output is the same

© 2008 LWW Electrodes: Material Conductivity effects amount of current flow Carbon or silicon electrodes offer better conductivity than metal-sponge electrodes –The carbon or silicon will leach out with use, reducing the electrodes’ conductivity.

© 2008 LWW Electrode Function Active electrode –Electrode under which the current density is great enough to elicit the desired response Indifferent (dispersive) electrode –Electrode under which the current density is not great enough to elicit the desired response

© 2008 LWW Electrode Function (cont.) –When electrode is much larger than electrode(s) from opposite terminal –Used to complete the circuit

© 2008 LWW Electrode Placement Technique Three techniques –Bipolar –Unipolar –Quadrapolar

© 2008 LWW Electrode Placement Technique (cont.) Bipolar technique –Electrodes from terminals are of equal size, resulting in nearly equal current density under them –Both active –Applied to treatment area in relative proximity to each other

© 2008 LWW Electrode Placement Technique (cont.) Unipolar technique –Electrodes of unequal size, creating active and indifferent electrodes –May have multiple active electrodes –Active electrode(s) applied to treatment area –Indifferent electrode applied to remote location

© 2008 LWW Electrode Placement Technique (cont.) Quadrapolar technique –Four electrodes of equal size –Pair from each of two channels. –Generally crisscross the target tissue –Most popular use is with interferential stimulation Two currents of differing frequencies are applied.

© 2008 LWW Polarity Voltage (positive or negative) on active electrode compared to voltage on indifferent electrode Different from unipolar and bipolar techniques Applies only with unipolar technique Affects excitability of nerves –Some people respond better to negative polarity and others to positive polarity

© 2008 LWW Tissue Responses to Electrical Stimulation Five general tissue responses –Ion migration –Sensory twitch –Sensory fused –Twitch contraction –Tetanic contraction

© 2008 LWW Tissue Responses to Electrical Stimulation (cont.) DC –Continuous DC stimulation drives ions into tissue.

© 2008 LWW Tissue Responses to Electrical Stimulation (cont.) Sensory twitch –Single sensation followed by nothing –Caused by moderate-amplitude, low- frequency pulsed stimulation –Not used therapeutically

© 2008 LWW Tissue Responses to Electrical Stimulation (cont.) Sensory fused –Sensory response that feels like pins and needles –Caused by moderate-amplitude, high- frequency pulsed or AC stimulation

© 2008 LWW Tissue Responses to Electrical Stimulation (cont.) Twitch contraction –Isolated brief muscular contraction followed by relaxation –Can occur in an individual muscle fiber or in an entire muscle group –Caused by low-frequency, high-amplitude pulsed stimulation

© 2008 LWW Tissue Responses to Electrical Stimulation (cont.) Tetanic contraction –Sustained muscular contraction that occurs in response to repetitive stimulation of 20–30 or more pulses per second –Caused by high-frequency, high-amplitude pulsed or AC stimulation

© 2008 LWW Tissue Responses to Electrical Stimulation (cont.) Response Amplitude Frequency Ion migration?None Sensory twitchLowLow Sensory fusedLowHigh Twitch contraction HighLow Tetanic contraction HighHigh

© 2008 LWW Tissue Responses to Electrical Stimulation (cont.) Response Amplitude Frequency Current Ion migration?NoneDC Sensory twitchLowLow? Sensory fusedLowHighHV, IF Twitch contraction HighLowHV, LV Tetanic contraction HighHighLV

© 2008 LWW Tissue Responses to Electrical Stimulation (cont.) Most Commonly Used Wave Forms DC: DC IF: Polyphasic HV: Twin pulse LV: Biphasic and polyphasic burst (Russian) TENS: Biphasic

© 2008 LWW Tissue Responses to Electrical Stimulation (cont.) Response Amplitude Frequency Current Use Ion migration?NoneDCIontophoresis Sensory twitchLowLowHV, LV? Sensory fusedLowHighHV, IFWound healing, pain Twitch contraction HighLowHV, LVMuscle with US Tetanic contraction HighHighLVSpasm, strength HV, twin pulse IF, interferential LV, biphasic, Russian (polyphasic, burst) TENS, biphasic