PRINCIPLES OF THERAUPEUTIC CURRENTS AND NEUROMUSCULAR ELECTRICAL STIMULATION

1. PRINCIPLES OF THERAUPEUTIC CURRENTS AND NEUROMUSCULAR ELECTRICAL STIMULATION

2. OBJECTIVES • Describe the electrophysical principles related to therapeutic currents; • Given a therapeutic current, describe the characteristics of the current; • List and ratify the factors that affect current conductivity; • Identify the essential features of each NMES, apply in a safe and correct manner

3. OBJECTIVES • Describe the physiological effect of electrical stimulation; • Given a clinical condition, justify the choice of stimulation parameter and protocol based on the physiological and clinical effects

4. CONTENTS • Basic concepts of electrophysics and therapeutic currents; • Description and characteristics of therapeutic currents; • Basic features of the commonly used NMES • Review of basic physiology of muscle and nerve; • Application principles and selection criteria; • Clinical indication and precaution

5. THERAPEUTIC CURRENTS • Therapeutic currents can broadly be defined as electrical currents, produced by electrotherapeutic devices, induced to the body tissues to elicit certain physiological /clinical effects. Neuromuscular electrical stimulation (NMES) is the application of electrical current to elicit a muscle contraction

6. CLASSIFICATION OF THERAPEUTIC CURRENTS

7. A continuous unidirectional flow of charged particles (current) Direct Current Uninterrupted bi-direction flow of current Alternating Current cps or Hz Unidirectional or bi-directional flow of current that periodically ceases for a finite period of time Pulsed Current pps

8. DESCRIPTIVE CHARACTERISTICS • PHASES • Monophasic • Biphasic • Triphasic • Polyphasic

9. SYMMETRY OF PHASES • Symmetric • Asymmetric

10. BALANCE OF PHASE CHARGE • Balanced • Unbalanced

11. QUANTITATIVE DESCRIPTIONS OF PULSED AND AC • Amplitude-Dependent Characteristics • Peak amplitude • Peak-peak amplitude • rms amplitude • Average amplitude • Time Dependent Characteristics • Phase and pulse duration • Rise and decay time • Interpulse and intrapulse • period and frequency

12. QUANTITATIVE DESCRIPTIONS OF PULSED AND AC • Amplitude and time-dependent characteristics • Phase charge • Pulse charge • µC

13. 20s 5s Duty cycle 20% CURRENT MODULATION • Amplitude modulation • pulse duration modulation • frequency modulation • Timing Modulations • burst mode • duty cycle

15. TERMINOLOGY OF THERAPEUTIC CURRENTS Shape DC Shape Monophasic Shape Therapeutic currents Pulsed Current Sym Biphasic Shape Balanced Asym Shape Unbal Shape AC Sym Balanced Shape Asym Shape Unbal

16. TYPES OF THERAPEUTIC CURRENTS STIMULATORS http://www.rs.polyu.edu.hk/rssyeung/ept2machine.html

17. TYPES OF THERAPEUTIC CURRENTS STIMULATORS • Galvanic stimulators • Produce continuous direct current • e.g PTU, Ten Pulses Stimulators

18. Produces the effect of muscle nerve stimulation “Faradic” type stimulators “Russian” stimulators High Voltage stimulators NEUROMUSCULAR ELECTRICAL STIMULATOR

19. Faradic current an asymmetrical biphasic current with a pulse duration of 1 ms 1ms FARADIC TYPE STIMULATORS

22. A 2500 Hz AC modulated every 10ms to provide 50 bursts per sec. 10ms interburst 10 ms Burst RUSSIAN CURRENT

23. A twin-spike Waveform: monophasic pulsed current pusle duration: 5-65s Freq: 1-120 pps Peak amp: 500 V (2000-2500 mA) HIGH VOLTAGE STIMULATORS 5-65 µs

25. FUNCTIONAL ELECTRICAL STIMULATOR (FES)

26. INTERFERENCE CURRENT • Two AC with slightly different frequency from two independent low voltage AC superimposes (interferes) on the same time axis; • The result is a unique pattern of amplitude modulation “beat” with a beat frequency of 1-100 bps.

27. ELECTRODES • A conductive materials that serves as the interface between a stimulator and the patient’s tissues

28. TYPES OF SURFACE ELECTRODES • Metal electrode • durable • reusable • inexpensive • inflexible

29. Carbonised Rubber • relatively inexpensive • fairly durable • gel or water required • may cause skin irritation

31. SPECIALISED PROBE • Allow point stimulation • location of motor point

32. EFFECTIVE ELECTRODES • With low skin-impedance • Uniform in current conductivity • avoid skin irritation • with uniform contact on treatment surface • cost effective

33. SIZE OF ELECTRODES • Current density inversely proportional to the electrode contact area

34. ELECTRODE PLACEMENT • Motor point: a point on the skin overlying a concentration of terminal motor nerve branches

35. METHOD OF APPLICATION • Polarity - relative charge (positive or negative) of the terminals (electrodes) of an electrical circuit at any one moment of time • Cathode-gains electrons and becomes negatively charged • Anode-loss electrons and becomes positively charged • Active electrode-cathode

36. MONOPOLAR TECHNIQUE • The active electrode (cathode) to the muscle motor point • An indifferent electrode to complete the circuit

37. BIPOLAR TECHNIQUE • Both electrodes on the muscle • usually for large muscle

38. BIFURCATED TECHNIQUE • Common in HVG machine • electrode from same polarity bifurcated into two • muscle with multiple motor points • composite muscle action with different muscles

39. ACTIVATION OF EXCITABLE CELLS WITH THERAPEUTIC CURRENTS

40. EXCITABILITY OF NERVE CELLS • Resting potential • -70 mv • Intracellular or extracellular stimulation, offset the resting potential • when membrane potential reaches threshold, AP generated Na+ + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - K+

41. Action potential of muscle Action potential of nerve -90 -80 -70 -60 -50 -40 -30 -20 -10 0 +10 +20 +30 +40 mv Threshold of muscle and nerve

42. APPLICATION PRINCIPLES • In electrical stimulation, current induced must be of sufficient amplitude and duration to bring excitable cells to the threshold of deplorization • SD curve

43. 2x mA Rheobase x mA Chronaxie STRENGTH DURATION CURVE Pulse amplitude mA Sub threshold Suprathreshold Duration ms

44. A C fiber Motor fiber Mixed peripheral nerve inherent excitability of the nerve fibre Site and location

45. Prickling sensation Sensory Nociception Peripheral nerve Skeletal mm Motor Therapeutic current Visceral mm Denervated Muscle fibre Muscle contraction

46. Sizes dependent! Selective activation S-D CURVE OF DIFFERENT NERVE FIBRES    C Pulse amplitude mA Duration ms

47. FACTORS THAT AFFECT CURRENT CONDUCTIVITY • Electrode-skin interface • Ohm’s law V=IR • Resistance=the ease or difficulty of a DC passes through a material. • the number of free electrons • viscosity of the material • temperature • Impedance= the ease or difficulty of AC passes through a material.

48. HUMAN SKIN • Poor conductor • Dry skin = 1 M ohm • Wet skin = 1 K ohm • The conductivity of human tissue depends on the proportion of water content • skin 5% • bone 5% • fat 15% • muscle 75% Clean Wet Warm Site

49. 0 30 60 90 120 ms MUSCLE FORCE GENERATION • Types of muscle fibres • Type I • twitch duration 120 ms • Type II • twitch duration 30-50 ms

50. FACTORS DETERMINE MUSCLE FORCE GENERATION • Spatial and order of recruitment • (size principles) • Temporal summation • Frequency • Asynchronisation vs Synchronisation