Low Frequency Electrical Stimulating Currents (PST 410) - Lecture Notes PDF

Low Frequency Electrical Stimulating Currents (PST 410) Mrs. J.O Adeleke Physiotherapy Programme Bowen University, Iwo Course Outline Introduction Physical principles and procedures governing the use of low frequency elect...

Low Frequency Electrical Stimulating Currents (PST 410) Mrs. J.O Adeleke Physiotherapy Programme Bowen University, Iwo Course Outline Introduction Physical principles and procedures governing the use of low frequency electrical stimulating currents. Electro-diagnosis. Electro-analgesia. Functional Electrical Stimulating Currents Current innovations in electrotherapy Physical principles and procedures governing the use of low frequency electrical stimulating currents. Electrical Conductivity Ohms’ Law Electrode Placement Waveform Dose/Duration Physiological effects Therapeutic effects Polarity Safety considerations Electric Shock Introduction: Electrical muscle stimulation Electrical muscle stimulation (EMS) involves the stimulation of muscles to maintain muscle viability and to restore muscle function May include muscle strengthening, reduction of muscle guarding and spasticity reduction, atrophy prevention, enhancement of range of motion (ROM), and muscle reeducation Neuromuscular electrical stimulation. Electrical stimulation for tissue repair (ESTR) Functional electrical stimulation (FES) Trancutaneous electrical nerve stimulation (TENS) Introduction Introduction Electricity is most often described by its strength (charge), rate of flow (current), driving force (voltage), and opposition (resistance/impedance) Introduction Biological tissues possess an inherent resistance. Excitable tissues and non-excitable tissues Excitable tissues include nerve, skeletal muscle, smooth muscle, and cardiac muscle: in which there is the ability of neurons and neurotransmitters to conduct signals Non-excitable Tissues: Bone Opposition to current flow the body “impedance” rather than “resistance.” Introduction Resistance and Impedance in the body results from the combination of resistive and capacitive reactance properties of tissue. Capacitance is the ability to store charge in an electric field and oppose change in current flow. Nerve and muscle membranes are examples of capacitors Introduction Tissue impedance varies throughout the body and conductivity depends on the water content of tissue. High water content decreases impedance and improves conductance. Healthy skin offers one of the highest impedances to current flow because the outer layer of the skin, the epidermis, contains little fluid. The amount of moisture in the deeper layers is determined by age and the number of sweat glands. Skin resistance is also inversely proportional to its temperature. Heat increases moisture and surface salt content, which promotes conductivity. Introduction Impedance can dramatically influence the ability to electrically generate an adequate response in underlying muscle. A greater intensity of current would be necessary to obtain a motor response in an area covered by adipose tissue (e.g. gluteus maximus muscle) compared with an area with little fat (e.g. anterior tibialis muscle). Increasing the current intensity to a level enough to drive current through the adipose tissue to the nerve may make the sensation of the stimulation unbearable for the patient. This may rule out stimulation as a treatment option, or limit its effectiveness Introduction So what can be done to reduce impedance? Minimizing impedance is important for all applications of electrical stimulation because this allows current intensity to be reduced and so increase patient comfort. Cleaning the skin surface with alcohol/ methylated spirit prior to electrode application Wash the skin surface with soap ad rinse with water remove excess body hair beneath electrodes Warming the region to be stimulated (how?) or warming the electrode gel. Introduction Impedance changes in the presence of injury and disease. It increases with edema, ischemia, atherosclerosis, scarring, and denervation It decreases in open wounds and abrasions. Introduction Positive ions (cations) are repelled from the positive electrode and migrate toward the negative electrode (cathode), Negative ions (anions) migrate toward the positive electrode (anode). The cathode is often referred to as the active electrode because nerve activation (excitation) takes place more easily under this electrode Types of current. All therapeutic electrical stimulation units use one of three forms of current: Direct Current (DC), Alternating Current (AC), Pulsed (Pulsatile) Current Types of current. (A) Direct current. (B) Alternating current. (C) Pulsatile current. (D) Common waveform shapes for pulsed current Direct current (DC) Direct current (DC) continuous unidirectional flow of charged particles with a duration of at least 1 second. One electrode is always the anode (positive) and one electrode is always the cathode (negative) for the period of stimulation. Direct current (DC) has a strong chemical effect on the tissues and can be delivered continuously to promote absorption of medication through the skin (iontophoresis), or it can be interrupted to stimulate denervated muscle (Galvanic type current ). Alternating current (AC) Alternating current (AC) an uninterrupted bidirectional flow of charged particles changing direction at leas once a second. AC can also be delivered in an interrupted form, sometimes referred to as bursts. Each electrode becomes positive for one phase of the cycle and then negative as the current reverses. AC no longer directly used to stimulate tissue. Some commercial stimulators, including interferential and Russian, use AC as their base or carrier current, which is then modified and delivered to the patient in the form of beats or bursts, respectively Pulsed or pulsatile current Pulsed or pulsatile current can take on the directionality characteristics of AC or DC current. The unidirectional (like DC) or bidirectional (like AC) flow of charged particles periodically ceasing for less than 1 second (milliseconds or microseconds) before the next electrical event. This small interruption in current, or charge movement, between successive pulses differentiates pulsed current from AC and DC current forms. Pulsed current is composed of individual pulses of short duration delivered in a continuous series called a pulse train. The pulse train can be delivered continuously or interrupted as in the AC and DC current forms. Each individual pulse is composed of one or more phases Pulsed or pulsatile current Pulsed current has a negligible chemical effect in the tissues the amount of effect depends on whether the pulse is unidirectional or bidirectional Manipulating the characteristics of both the single pulse and the pulse train are important for customizing treatment protocols. Pulsed current Characteristics of Pulsed Current The single pulse for electrical stimulation is an event and it can be described in terms of the characteristics of how it occurs. For example, the characteristics relate to how long it takes for it to occur or “by time,” how strong the event is when it is measured or “by amplitude,” and a relationship between the two or a “time/amplitude dependent characteristic.” Ohms’ Law Ohm's law “ the size of an electric current varies directly with the voltage and inversely with the resistance within the circuit.” An increase in resistance when voltage is constant will decrease current. The magnitude of current therefore increases when voltage increases or resistance decreases. High resistance requires high voltages to produce necessary current flow in the tissues V=IR Voltage= Current*Resistance Wave Form Waveform is a visual representation of the pulse or event. It is a spatial drawing depicting the shape of the pulse Reflects amplitude (strength) and duration (length of time) that the pulse or event takes place within. Pulses are classified by the number of phases they have for example, there are monophasic, biphasic, and polyphasic waveform Waveforms Monophasic waveform the entire event takes place either above or below isoelectric zero (the pulse is either positive or negative). Isoelectric zero is the demarcation between positive and negative where there is no net charge/ baseline Biphasic waveforms Two phases with one above and one below the isoelectric zero demarcation. Biphasic waveforms can either be balanced, where there would be no net charge, or unbalanced, where there would be either a positive or negative charge remaining. Polyphasic waveforms Multiple phases occurring above and below isoelectric zero Waveforms Electric Shock A painful stimulation of sensory and motor nerves, caused by a sudden flow, ceasation of flow or variation of intensity of current passing through the body resulting in mild discomfort and fear for loss of consciousness and death in a few cases Intensity of shock is measured by the amount of energy passing through the body that causes the damage Person becomes part of the electrical circuit Shock may result from poorly designed or faulty equipment/ appliances Electric Shock Type of electric shock (Severity) Mild Severe Type of electric shock (Severity) Mild Shock Intesity of shock up to 20mA Painful sensory stimulation Laboured/ irregular breathing No loss of consciousness Type of electric shock (Severity) Severe Shock Current flow is more than 20mA Difficulty in letting go Muscular paralysis Fall in blood pressure Ceasation of respiration Ventricular firillation] Death Causes of electric shock Sudden change in current flow Improper earth connection Two pin connections Faulty electrical components Non-insulated floorings Faulty switch and fuse connection Treatment of electric Shock Disconnect the victim from the contact with the current source. Do not get shocked If shock is mild, reassure the victim and allow him/her to rest Give water…not hot…as it can cause vasodilation, sweating and blood pressure If shock is severe, lay victim flat, ensure respiratory passages are clear. Loosen tight clothing If breathing has ceased, perform CPR and call for medical help Electro diagnosis Electrodiagnosis Electrodiagnosis is the study of electrical activity in motor units when stimulated by electrical pulses Physiological basis is the mechanism underlying normal activity of muscle, nerve when stimulated by electrical impulses Types of nerve injury Strength Duration Curve Strength Duration Curve (SDC) A curve that shows the relationship between the strength of electrical impulses applied to a nerve and the minimal time required to produce a muscle contraction As the strength of the stimulus increases, the duration decreases Strength Duration Curve A strength duration curve can be plotted by gradually reducing the duration below 300 milliseconds while recording the minimum intensity of stimulation required to generate a response. Obtained by joining points that represent the various values (intensities) along the ordinate for the various duration of the stimulus Strength Duration Curve Normal curve has a characteristic shape If muscle is denervated, the curve shifts to the right If muscle is partially denervated, a kink is present in the curve Normal Curve Complete denervation Partial denervation Rheobase The intensity of the current required to produce a minimal perceptible and palpable contraction at infinite duration Pulses of 100- 300 ms are used to record rheobase Utilization time….Time/ duration that coincides with the rheobase Factors affecting chronaxie and rheobase Chronaxie: The duration required to produce a muscle contraction with a stimulus that is double the Rheobase Advantages and Disadvantages of SDC Advantages Simple Reliable Indicates the proportion of denervation May show changes in innervation / denervation if plotted over time Disadvantages In a large muscle, only a proportion of fibres may respond (You may not get the full picture) It does not tell about the site of the lesion References Basanta K. N. (2014)Electrotherapy Simplified. Jaypee Brothers Medical Publishers Pvt. Limited Strength duration curve images. Physiopedia https://www.physio- pedia.com/index.php?curid=32912 Strength duration curve images. Physiopedia https://www.physio- pedia.com/index.php?curid=32912