Principles of Electrical Currents HuP 272

Electricity is an element of PT modalities most frightening and least understood. Understanding the basis principles will later aid you in establishing treatment protocols.

General Therapeutic Uses of Electricity Controlling acute and chronic pain Edema reduction Muscle spasm reduction Reducing joint contractures Minimizing disuse/ atrophy Facilitating tissue healing Strengthening muscle Facilitating fracture healing

Contraindications of Electrotherapy Cardiac disability Pacemakers Pregnancy Menstruation (over abdomen, lumbar or pelvic region) Cancerous lesions Site of infection Exposed metal implants Nerve Sensitivity

Terms of electricity Electrical current: the flow of energy between two points Needs A driving force (voltage) some material which will conduct the electricity Amper: unit of measurement, the amount of current (amp) Conductors: Materials and tissues which allow free flow of energy

Fundamentals of Electricity Electricity is the force created by an imbalance in the number of electrons at two points Negative pole an area of high electron concentration (Cathode) Positive pole and area of low electron concentration (Anode)

Charge An imbalance in energy. The charge of a solution has significance when attempting to “drive” medicinal drugs topically via inotophoresis and in attempting to artificially fires a denervated muscle

Charge: Factors to understand Coulomb’s Law: Like charges repel, unlike charges attract Like charges repel allow the drug to be “driven” Reduce edema/blood

Charge: Factors Membranes rest at a “resting potential” which is an electrical balance of charges. This balance must be disrupted to achieve muscle firing Muscle depolarization is difficult to achieve with physical therapy modalities Nerve depolarization occurs very easily with PT modalities

Terms of electricity Insulators: materials and tissues which deter the passage of energy Semiconductors: both insulators and conductors. These materials will conduct better in one direction than the other Rate: How fast the energy travels. This depends on two factors: the voltage (the driving force) and the resistance.

Terms of electricity Voltage: electromotive force or potential difference between the two poles Voltage: an electromotive force, a driving force. Two modality classification are: Hi Volt: greater than 100-150 V Lo Volt: less than 100-150 V

Terms of electricity Resistance: the opposition to flow of current. Factors affecting resistance: Material composition Length (greater length yields greater resistance) Temperature (increased temperature, increase resistance)

Clinical application of Electricity: minimizing the resistance Reduce the skin-electrode resistance Minimize air-electrode interface Keep electrode clean of oils, etc. Clean the skill on oils, etc. Use the shortest pathway for energy flow Use the largest electrode that will selectively stimulate the target tissues If resistance increases, more voltage will be needed to get the same current flow

Clinical application of Electricity: Temperature Relationship An increase in temperature increases resistance to current flow Applicability Preheating the tx area may increase the comfort of the tx but also increases resistance and need for higher output intensities

Clinical Application of Electricity: Length of Circuit Relationship: Greater the cross-sectional area of a path the less resistance to current flow Application: Nerves having a larger diameter are depolarized before nerves having smaller diameters

Clinical Application of Electricity: Material of Circuit Not all of the body’s tissues conduct electrical current the same Excitable Tissues Nerves Muscle fibers blood cells cell membranes Non-excitable tissues Bone Cartilage Tendons Ligaments Current prefers to travel along excitable tissues

Laws and Principles of Electricity Ohm’s Law: V-IR (V is voltage, a measure of the driving force which is equal to the IxR where I is the Ampere (the amount of current flow) and R is the resistance. Or, expressed differently: The Ampere is equal to the Voltage divided by the resistance. If you know the inter-relationship you can understand if one increased what happens to the other Watt= electrical power=volt x amps- ohms

Stimulation Parameter: Amplitude: the intensity of the current, the magnitude of the charge. The amplitude is associated with the depth of penetration. The deeper the penetration the more muscle fiber recruitment possible remember the all or none response and the Arndt-Schultz Principle

Simulation Parameter Pulse duration: the length of time the electrical flow is “on” also known as the pulse width. It is the time of 1 cycle to take place (will be both phases in a biphasic current) phase duration important factor in determining which tissue stimulated: if too short there will be no action potential

Stimulation Parameter: Pulse rise time: the time to peak intensity of the pulse (ramp) rapid rising pulses cause nerve depolarization Slow rise: the nerve accommodates to stimulus and a action potential is not elicited Good for muscle re-education with assisted contraction - ramping (shock of current is reduced)

Stimulation Parameters Pulse Frequency: (PPS=Hertz) How many pulses occur in a unit of time Do not assume the lower the frequency the longer the pulse duration Low Frequency: 1K Hz and below (MENS .1-1K Hz), muscle stim units) Medium frequency: 1K to 100K Hz (Interferential, Russian stim LVGS) High Frequency: above 100K Hz (TENS, HVGS, diathermies)

Stimulation Parameter: Current types: alternating or Direct Current (AC or DC) AC indicates that the energy travels in a positive and negative direction. The wave form which occurs will be replicated on both sides of the isoelectric line DC indicated that the energy travels only in the positive or on in the negative direction DC AC

Stimulation Parameter: Waveforms; the path of the energy. May be smooth (sine) spiked, square,, continuous etc. Method to direct current Peaked - sharper Sign - smoother

Stimulation Parameter: Duty cycles: on-off time. May also be called inter-pulse interval which is the time between pulses. The more rest of “off” time, the less muscle fatigue will occur 1:1 Raito fatigues muscle rapidly 1:5 ratio less fatigue 1:7 no fatigue (passive muscle exercise)

Stimulation Parameter: Average current (also called Root Mean Square) the “average” intensity Factors effective the average current: pulse amplitude pulse duration waveform (DC has more net charge over time thus causing a thermal effect. AC has a zero net charge (ZNC). The DC may have long term adverse physiological effects)

Stimulation Parameter: Current Density The amount of charge per unit area. This is usually relative to the size of the electrode. Density will be greater with a small electrode, but also the small electrode offers more resistance.

Capacitance: The ability of tissue (or other material) to store electricity. For a given current intensity and pulse duration The higher the capacitance the longer before a response. Body tissues have different capacitance. From least to most: Nerve (will fire first, if healthy) Muscle fiber Muscle tissue

Capacitance: Increase intensity (with decrease pulse duration) is needed to stimulate tissues with a higher capacitance. Muscle membrane has 10x the capacitance of nerve

Factors effecting the clinical application of electricity Factors effecting the clinical application of electricity Rise Time: the time to peak intensity The onset of stimulation must be rapid enough that tissue accommodation is prevented The lower the capacitance the less the charge can be stored If a stimulus is applied too slowly, it is dispersed

Factors effecting the clinical application of electricity An increase in the diameter of a nerve decreased it’s capacitance and it will respond more quickly. Thus, large nerves will respond more quickly than small nerves. Denervated muscles will require a long rise time to allow accommodation of sensory nerves. Best source for denervated muscle stimulation is continuous current DC

Factors effecting the clinical application of electricity: Ramp: A group of waveforms may be ramped (surge function) which is an increase of intensity over time. The rise time is of the specific waveform and is intrinsic to the machine.

Law of DuBois Reymond: The amplitude of the individual stimulus must be high enough so that depolarization of the membrane will occur. The rate of change of voltage must be sufficiently rapid so that accommodation does not occur The duration of the individual stimulus must be long enough so that the time course of the latent period (capacitance), action potential, and recovery can take place

Muscle Contractions Are described according to the pulse width 1 pps = twitch 10 pps = summation 25-30 pps = tetanus (most fibers will reach tetany by 50 pps)

Frequency selection: 100Hz - pain relief 50-60 Hz = muscle contraction 1-50 Hz = increased circulation The higher the frequency (Hz) the more quickly the muscle will fatigue

Electrodes used in clinical application of current: Electrodes used in clinical application of current: At least two electrodes are required to complete the circuit The body becomes the conductor Monophasic application requires one negative electrode and one positive electrode The strongest stimulation is where the current exists the body Electrodes placed close together will give a superficial stimulation and be of high density

Electrodes used in clinical application of current: Electrodes spaced far apart will penetrate more deeply with less current density Generally the larger the electrode the less density. If a large “dispersive” pad is creating muscle contractions there may be areas of high current concentration and other areas relatively inactive, thus functionally reducing the total size of the electrode A multitude of placement techniques may be used to create the clinical and physiological effects you desire

General E-Stim Parameters

E-Stim for Pain Control: typical Settings