ELECTRICAL STIMULATIONAVANIANBAN CHAKKARAPANI

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Outline Diagnosis Therapeutic

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Review: Polarization & 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

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Review: Polarization and Action Potentials

Nerve action potential eventually causes An ascending sensory impulse to the brainOr A descending muscle action potential

Muscle action potential causes muscle contraction.

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Review: Polarization and Action Potentials

Nerve repolarizes quickly.

Absolute refractory periods vary from 0.4 to 2 msec

Depends on specific nerve

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Diagnosis Faradic Galvanic Test Measurement of Rheobase and

Chronaxie Strength Duration Curve Nerve Conduction Velocity Studies

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Faradic Galvanic Test Faradic stimulus evoked no response in

denervated muscle. Galvanic stimulus produce sluggish

response. Based on various researches it has been

shown that the reaction to FG test applied to muscle are correctly interpreted only in 50% of cases.

This test is inaccurate and unreliable.

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Measurement of Rheobase and Chronaxie

Rheobase: minimum current for infinite duration(in practice 100msec or more) will cause contraction.

Chronaxie: minimum time for which a current of intensity twice rheobase will cause contraction.

Both are increased in denervated muscle. These values are greatly varies with few

variables like temp, blood supply, electrode size and skin resistance.

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NORMAL VALUE OF RHEOBASE OF DIFFERENT MUSCLEDeltoid 14 volts, 5mA

Triceps 18 volts, 5mA

Abductor digiti minimi 30volts, 8mA

Frontalis 14volts,4mA

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FACTORS AFFECTING RHEOBASE Resistance of skin and subcutaneous tissue Edema and inflammation Ischemia and underlying pain Temperature variation Position of electrode Amount of subcutaneous tissue Degeneration Deneravtion Partial denervation generally produce no

changes in rheobase. Re-innervation can show a sharp rise in

rheobase which indicates clinical recovery.

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NORMAL VALUE OF CHRONAXIE OF DIFFERENT MUSCLEMuscle Constant voltage Constant current

Deltoid 0.01ms 0.1ms

Abductor digiti minimi 0.04ms 0.2ms

Tibialis anterior 0.04ms 0.1ms

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FACTORS AFFECTING CHRONAXIE Texture of skin Ischemia Oedema Fatigue Position of stimulating electrode Denervation Partial denervation Re-inervation Nerve root lesion Peripheral neuropathy Myopathy (No significant change)

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Strength duration curve is a graph between electrical stimuli of different intensities and recording the time needed by each stimulus to start the response.

S-D curve should be plotted after 20th day of injury/lesion.

After 21st/22nd day, regeneration of nerve will start, generally it take about 270 days to regenerate.

The purpose of S-D curve plotting is to know whether the stimulated muscle is innervated, denervated or partially denervated.

There are also other method for this purpose like EMG and NCV.

Strength Duration Curve13

APPARATUS The apparatus with rectangular impulses of

different duration. Impulse with duration of 0.01, 0.03, 0.1, 0.3,

10, 30, 100, 300 ms are required. The stimulator may be of either the constant

current or constant voltage type. The constant current stimulator was thought

to produce the more accurate result but constant voltage stimulator is rather more comfortable for patient.

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STRENGTH DURATION CURVE15

Normal innervation The S-D Curve is of this typical shape

because the impulses of longer duration all produce a response with same strength of stimulus, irrespective of their duration, while those of shorter duration, require an increase in the strength of the stimulus each time the duration is reduced.

The point at which the curve begin to rise is variable, but is usually at a duration of impulse of 1 ms with constant current and 0.1 ms with constant voltage stimulator.

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Complete Denervation S-D Curve of complete denervation is

when duration of impulse is 100 ms or less, the strength of the stimulus must be increased each time the duration the duration is reduced and no response is obtained to the impulse of very short duration.

So the curve rises steeply and is further to the right than of normally innervated muscle.

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Partial Deneravation S-D Curve of partial denervation is the impulses of longer

duration can stimulate both innervated and denervated muscle fibers, so a contraction is obtained with a stimulus of low intensity.

As impulse are shortened, the denervation fibers responds less readily, a stronger stimulus is required to produce a perceptible contraction and the curve rises steeply like that of denervated muscle.

With the impulses of shorter durations, the innervated fibers responds to a weaker stimulus than that required for the denervated fibers.

Kink in S-D Curve is seen at the point where two section meet.

The shape of curve indicates the proportion of denervation. A kink appears in the curve and as reinnervation

progresses. Progressive denervation is indicated by the appearance of a

kink, increase in the slope and shift of the curve to the right.

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EQUIPEMENT REQUIRED FOR S-D CURVE Low frequency generator with varying

pulses from 0.02 to 1000ms. Moist saline pad Electrodes Leads Bandage Plastic protactors

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ADVANTAGES OF S-D CURVE It is simple, reliable and cheaper. Indicate proportion of denervation. Less time consuming.

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DISADVANTEGES OF S-D CURVE In large muscles, only proportion of

fibers may respond hence picture is not clearly shown.

It’s a qualitative rather than quantitative method of testing innervation.

It won’t point out the site of lesion.

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Nerve Conduction Velocity Studies Nerve conduction velocity (NCV) is a test

to see how fast electrical signals move through a nerve.

Surface electrodes are placed on the skin over nerves at different spots. Each patch gives off a very mild electrical impulse. This stimulates the nerve.

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Nerve Conduction Velocity Studies The nerve's resulting electrical activity is

recorded by the other electrodes. The distance between electrodes and

the time it takes for electrical impulses to travel between electrodes are used to measure the speed of the nerve signals.

Electromyography (recording from needles placed into the muscles) is often done at the same time as this test.

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THERAPEUTIC/REHABILITATIVE

Neuro Muscular Electrical Stimulation

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Muscle Fibre Types MOTOR UNIT - AHC + α motor

neurone + muscle fibres.

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Muscle Fibre Types29

Muscle Fibre Types30

Neuro Muscular Electrical Stimulation NMES is used for

Muscle re-education and prevention of disuse atrophy

Decreasing muscle spasm Decreasing edema

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Why NMES? Used on patients who cannot perform a

voluntary muscle contraction Peripheral nerve innervation is intact, yet

muscle is too weak to contract from atrophy, pain, immobilization, etc.

Promotes early AROM in postsurgical and immobilized limbs

Break pain-spasm-pain cycle of muscle spasms

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Don’t Replace Strength Training with NMES NMES recruits fibers in the opposite

order than that of a voluntary contraction.

Machine = large fibers followed by small Voluntary = small fibers followed by

large Patient needs to move on to more

traditional weight training ASAP.

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Physiological Sequence in Contraction Asynchronous motor unit pattern -------->

smooth graded contraction Relates to : No of motor units firing (spatial summation)

Rate of motor unit firing (temporal summation)

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Normal Contraction Increase no of motor units in early

contraction (to force) then increase firing rate to increase

force further. Type I MU fire first, then Type II. Type IIb

brought in last of all

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Electrical Stimulation Pattern SYNCHRONOUS firing pattern (all MU’s

fire together) Type II neurons are LARGER (therefore

have a lower threshold, therefore fire first - reverse of the natural sequence)

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Effects of Electrical Stimulation Short Term

Contraction & altered (local) blood flow.

Longer Term (‘chronic’) strengthening structural changesbiochemical changes

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Mechanisms

Most likely NEURAL (due to speed of response & lack of volume changes)

?spinal motor pool activation ?synaptic facilitation ?muscle motor unit firing pattern (change

SO to FOG or FG?)

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Best effects for weak muscles (Gibson et al 1988) 30Hz @ 300μs, 2 sec ON 9 sec OFF 1

hr/day Knee immobilisation. Treatment group no strength loss, Non

treatment group17% reduced Xsect Area

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Waveforms Kramer et al (1984), Walmsley et al (1984), Snyder-Mackler et al 1989) have all published evidence which supports the asymmetric over the symmetric waveform (max quadriceps force production).40

INTENSITY AND FORCE OF CONTRACTION Approximately linear relationship

between CURRENT INTENSITY and FORCE OF CONTRACTION (Ferguson et al 1989, Underwood et al 1990)

The greatest effects with least current intensity by using BIPHASIC PULSED or BURST AC currents.

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FORCE OF CONTRACTION Stronger muscle contractions with 300-400μs

pulses, BUT these will also produce significant stimulation of sensory fibres.

Stimulation frequency affects FORCE GENERATION.

Higher forces produced with tetanic contractions, but also more discomfort and potential for muscle damage, more especially with patients (the tetanic stim is widely researched with athletes/fit individuals rather than those with muscle dysfunction)

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Force Generation Vs Fatigue Maximum at 60 - 100Hz (Binder et al

1990), BUT also get higher fatigue. 20Hz stimulation will achieve about 65%

force, BUT also much less fatigue

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Stimulation Parameters Duty Cycle : (ON : OFF ratio) Minimum is to use equal cycles (1:1) but only for

the stronger / end rehab / fit patients Use higher ratios for the weaker to allow stim

with minimal chance of fatigue Weaker / poorer state the muscles, larger rest

time proportion Might start at 1:9 for v weak patients and

progressively reduce (towards 1:1) For example, if using stim for quads in a very

weak patient (post TKR) might use a 1:9 ratio, so 10 sec stim would be followed by 90 sec rest.

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Ramp45

Review Electrodes: Physical Dimensions

Shape is unimportant Most are round or square or rectangular.

Size and placement determine the number of motor units stimulated.

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Review 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

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Electrodes Best if both electrodes on muscle belly Best if one is at or near motor point Larger electrodes better (less current

density, therefore less discomfort) ?advantage if electrodes placed in

LONGITUDINAL orientation (Brooks et al 1990) - stronger contraction with less discomfort

Special electrodes are available for pelvic floor stimulation

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Strengthening Protocols Athletes + Non Injured Subjects 2500Hz burst AC [Kramer et al 1984,

Snyder-Mackler 1989, Walmsley et al 1984] Symmetric and asymmetric biphasic pulsed

[Alon et al 1987, Grimb et al 1989] Frequency usually at around 60Hz + Stim

intensity at max tollerance BUT can get an effect at 25-50% MVC

(ISOMETRIC) PULSE WIDTH 300-400μS may be best

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Strengthening Protocols Athletes + Non Injured Subjects Duty cycle relates to fatigue If less fatigue resistant 1:8 - 1:5 Once less likely to fatigue drop to 1:3 -

1:2 - 1:1

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Strengthening Protocols Athletes + Non Injured Subjects Ramp - no definitive rules, BUT with

stronger stimulation use longer ramp. Usually 2-4 sec ramp up and 1-2 sec

ramp down 8 - 15 max contractions / session ; 3 - 5

sessions / week ; 3 - 6 weeks for significant effect

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Strengthening Protocols : Rehabilitation Programmes Similar ideas BUT tend to use LOWER

frequencies - (minimum required to get tetany - 20 - 35 Hz).

Continue for longer (per session) and use a Duty Cycle which minimises fatigue (at least 1:4 or more).

The most effective treatment approach (??) may employ 100 - 200 contractions per session, usually over 1 - 2 hours

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Suggested Clinical Treatment ParametersMuscle Strengthening 30 - 35Hz @ 400 μs 4 sec ON / 4 sec OFF (minimum) but

usually 10 sec ON / OFF at least 15 mins alt days, but usually 30 min / day

Need strong contraction (not just mild twitch) + voluntary as well

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Suggested Clinical Treatment ParametersMuscle Endurance 20Hz @ 400 μs 2 sec ON / 2 sec OFF (minimum) at least

1 hr day Minimal contractions

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Suggested Clinical Treatment ParametersVery Weak Muscles / Marked Atrophy 10Hz @ 400 μs 2 sec ON / 2 sec OFF (minimum) minimum 1 hr day Minimal contraction

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Tetanic Contraction to break Muscle Spasm

Goals Increase local circulation Remove metabolic wastes Mechanically stimulate muscle fibers Induce some muscle spasm fatigue

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NMES for Decreasing Edema Produce cyclic muscle contractions to

help pump chronic edema 5–10 sec on; 5–10 sec off

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NMES EffectsEffects

1. Muscle contractiona. Increase blood flowb. Retard atrophy developmentc. Decrease and retard neuromuscular inhibitionsd. Increase muscle relaxation; decrease spasm

2. Decrease paina. Possibly by decreasing muscle spasm

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NMES Advantages & DisadvantagesC. Advantages

1. Can be applied to immobilized body part

D. Disadvantages1. Sometimes

becomes a panacea

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NMES Indications & Contraindications Indications

1. Residual or chronic muscle spasm

2. Any time normal neuromuscular function is not possible

3. Muscle strains4. During cast

immobilization or disuse atrophy

5. Pain owing to muscle spasm

Contraindications1. Do not use:

a. On a person with a pacemaker

b. Over the heart or brain

c. Over recent or non-union fractures

d. Over potential malignancies

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NMES PrecautionsG. Precautions

1. Be cautious over an area with:a. Impaired sensationb. Skin lesions (cuts, abrasions, new skin, recent scar tissue)c. Decreased range of motiond. Extensive torn tissue

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Technique of Application Group muscle stimulation; and Motor Point stimulation.

Group Muscle Stimulation Stationary stimulation Active electrode & Passive electrode will

be kept stationary

Motor Point Stimulation

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Reference

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