Foundations of Electrical Stimulation
PTA 101 Intro to Clinical Practice 1

Instructional Use Statement

The following information is used for instructional purposes for students enrolled in the Physical Therapist Assistant Program at Lane Community College. It is not intended for commercial use or distribution for commercial purposes. It is not intended to serve as or take the place of medial advice or treatment.

Contact howardc@lanecc.edu for permission

Introduction

Electrical stimulation use dates back to ancient time when electric eels were used to treat painful spines and limbs. In this lesson, we will study characteristics of electricity, electricity effects in biological tissue, and its use in rehabilitation. By understanding how electrical stimulation effects muscle and nerve function, we can safely and effectively select the most appropriate "estim" to maximize patient progress toward goals in the plan of care.

Lesson Objectives

After completing this lesson, the successful student will be able to:

Describe the key terms, principles, and theory of electrical stimulation.
Describe the physiologic effects of electrical stimulation.

Describe the three major forms of current used in electrical stimulation Describe common electrical stimulation devices and their clinical indications.

Describe expected goals and outcomes for application of electrical stimulation.
Recognize precautions and contraindications to electrical stimulation.
Define electrical stimulation parameters (i.e., frequency, duty cycle, ramp time, waveform, pulse rate, pulse duration, and treatment duration).
Describe the properties and configuration of electrodes used in electrical stimulation Identify the role of the PTA during application of electrical stimulation.

Overview of Electricity and Physiology

All use of electricity to produce a change in body tissue is directly related to the properties of the cell membrane. Electricity can stimulate ionic flow, and therefore action potentials, which result in activation of nerve pathways, muscle tissue, and chemical changes

Review the process of an action potential at the cell membrane.

(Approximately 7 minutes)

This will refresh your understanding of cellular physiology and help make connections between physiology and electrical stimulation as a rehabilitation modality

Key Terms and Law's of Electricity

Approximately 9 minutes

charge - Addition or removal of electrons. A gain of electrons is a net negative (-) charge, and removal of electrons is a net positive (+) charge.

recall from A&P that cells depolarize to initiate or transmit signals to other cells by the movement of ions; ionic flow is the movement of ions across the membrane
the separation of charges (ions) across the resting cell membrane is maintained by a greater concentration of sodium (Na+) the outside of the cell, and a greater concentration of potassium (K+) inside the cell
the charge imbalance at the cell membrane is maintained by the sodium-potassium pump
cathode - The side of the cell membrane with excess electrons (net negative charge)
anode - The side of the cell membrane with a deficiency of electrons (net positive charge)
when electricity is introduced into tissue, there is a localized effect in the extracellular space in response to a sustained charge

When we introduce electrical stimulation, the salt water in our bodies (NaCl) dissociates into its respective ions: Na+ and Cl-, which then migrate toward the cathode (-) and anode (+) respectively. This creates a chemical reaction in the extracellular space that effects the protein density in the tissue.
This response supports the use of electrical stimulation in wound and tissue healing

Voltage

driving force that moves electrons; the force that pushes charge
like charges repel, opposite charges attract; energy from attraction and repulsion creates an electrical field (Coulomb's Law)

High Volt = 100-150V
Low Volt <100 V

Conductors - a material that permits the movement of ions

metals
water
biological tissue example: nerve

Insulators - a material that restricts the movement of ions.

rubber
plastic
biological tissue example: fat (recall that the myelin sheath that surrounds nerve axons has high lipid content)

Current - measured in amperes (A) of electron flow per second

movement of ions in response in a conductor in response to a voltage force
determines depth of penetration in biological tissue

Watt - measure of power or work (W)
Frequency - number of pulses or wave forms per unit of time

Hz = 1 cycle per second

Resistance (Impedance) - opposing the flow of current; The following are biological sources of high resistance:

skin, adipose (fat), hair, fascia, ligament, callus, bone, tendon, scar
more that 99% of the resistance to current flow in the body is from the epidermis (top layer of skin)
adipose (fat) is a resistor, causing increased impedance to current flow. A body part covered with a thick layer of adipose tissue may require an increase in current to elicit the desired response, which can lead to discomfort and pain due to activation of nociceptive nerve endings in the skin.
High water content decreases impedance and increases conductivity. Tissues that have high water content are muscle and nerve
Low water content increases impedance. Bone, fat, tendon, and fascia are poor conductors due to low water content

Accommodation - decreased sensitivity to tissue excitability
Capacitance and Impedence ability to store a charge

capacitance is the "capacity" for a system, like the human body, to store an electrical charge
impedence is a form of resistance; it is a frequency-dependent resistance to flow of current

gels and adhesive material found on electrodes decrease impedence and increase conductivity between the electrode and skin

Ohm's Law - relationship between electrical current, voltage, and resistance

Current = Voltage/Resistance

INCREASE Voltage = INCREASE Current, DECREASE Voltage = DECREASE Current
INCREASE Resistance = DECREASE Current, DECREASE Resistance = INCREASE Current

Ohm's law is important because the amount of current that flows through biological tissues determines changes in the physiological effects

More about Ionic Flow

Approximately 3 minutes

This excerpt from the American Society of Neurological Monitoring has one of the best explanations I've seen to help understand ionic flow in biological tissue:

"How an Electrical Stimulator Works

In an electrical stimulator, the flow of anions (-) and cations (+) is controlled by the mechanics of the circuitry within the stimulator. The stimulator is unique in that the cathode is the negative pole (-) because it discharges anions (-), and the anode is the positive pole (+) because it discharges cations (+). At the end of the day, that's the fundamental difference between a battery and a stimulator.

Depending on how we configure the polarity, the stimulator will discharge either cations or anions into the body part being stimulated.

In cathodal stimulation, anions (-) are discharged into the body as current flows from the cathode (-), through the tissue, and back to the anode (+).

In anodal stimulation, cations (+) are discharged into the body as current flows from the anode (+), through the tissue, and back to the cathode (-).

Now, let's imagine that we place an electrical stimulator on the surface of the skin with a nerve bundle running underneath (Figure 2). Within the nerve bundle is a single nerve fibre (axon) upon which we will focus.

At rest, the inside of a cell is more negative than the outside of a cell.

This occurs because there is a slightly greater number of negative charges than positive charges inside of the cell (intracellular space), and a slightly greater number of positive charges than negative charge outside of the cell (extracellular space). Because of the electrical difference, the cell is said to be polarized- just like a magnet, one side is more positive and the other side is more negative. If the electrical gradient were suddenly reversed, the cell would be depolarized, and we might see an action potential."

(Reference: Vogel, R. W. (2017, December). Understanding Anodal and Cathodal Stimulation [Blog post]. American Society of Neurophysiological Monitoring. Retrieved from

https://www.asnm.org/blogpost/1635804/290597/Understanding-Anodal-and-Cathodal-Stimulation

Below is a table view of the difference between a contraction elicited through normal central nervous system function as compared to that via electrical stimulation.

Motor Unit Recruitment - Central Nervous System	Motor Unit Recruitment and Contraction - Electrical Stimulation
Active	Passive
Small type I motor units are recruited first then larger type II motor units for smooth and gradual tension	Large superficial fatigable type II motor units are recruited first, then smaller motor units
Asynchronous firing in off and on pattern - energy efficient and slower onset of fatigue	Synchronous firing - motor units stimulated continue to fire until stimulus removed, causing quick onset of fatigue
Action potential moved away from the nerve cell body	Action potential generated in two directions, away from the cell body and back toward the cell body

Three Major Types of Current Used in Electrical Stimulation (estim)

Approximately 6 minutes

There are three basic waveforms used in commercial therapeutic electrical stimulation units: direct current, alternating current, and pulsed current.

Direct Current (DC) - Galvanic

Continuous unidirectional flow of charged particles with a duration of at least 1 second.
One electrode is always the anode (+) and one is always the cathode (-) for the entire event.
There is a build-up of charge since it is moving in one direction causing a strong chemical effect on the tissue under the electrode
Most commonly used for wound care and with iontophoresis
Iontophoresis and microcurrent are clinical examples of direct current interventions
Note : Monophasic also refers to direct current, but it is interrupted and not continuous (i.e., pulsed), so the chemical effect is minimal

Alternating Current (AC) - Biphasic

Uninterrupted, bidirectional flow of ions; direction changes at least once per second.
Frequency - the rate at which the current switches direction
Electrodes continuously alternate their polarity each cycle, therefore no build-up of charge under the electrodes
Types of modulated AC current used on biological tissue

Burst-modulated - known as "Russian" current
Amplitude-modulated - known as interferential current

Pulsed Current (pulsed - AC and DC)

Flow of charged particles stops periodically for less than 1 second before the next event
Pulses can occur individually or in a series
Monophasic pulses do not alternate; pulsed monophasic (travels in one direction) current allows for a charge to accumulate in biological tissue

Electrical Stimulation Names and Indications

Approximately 9 minutes

There are four common clinical names that describe electrical stimulation devices:

Electrical Stimulation Device	Type of Current	Summary of Action
Russian	AC - Burst modulation	uses current to depolarize muscle and stimulate a muscle contraction
High Volt Pulsed Current (HVPC)	monophasic, pulsed	uses high intensity, short duration, pulsed current to promote charge accumulation in the tissue
Interferential Current (IFC)	amplitude- modulated AC	asynchronous waves interfere and produce a pain modulating effect; uses four electrodes
Low intensity direct current (LIDC; Microcurrent)	DC or monophasic pulsed	uses unidirectional flow of current to accumulate a charge in tissue

In addition to the common practice of referring to electrical devices when talking about electrical stimulation (ES or "estim), there are three common acronyms that describe types of electrical stimulation:

Electrical Stimulation Acronyms

Acronym

Description

Effect

Sample conditions

NMES

(motor effect)

neuromuscular electrical stimulation

activation of skeletal muscle for strengthening

Measure: manual muscle test or dynamometry

Post-operative joint repair or replacement

Muscle is innervated

FES

(motor effect)

Functional electrical stimulation

activation of skeletal muscle for reactivation or movement training and recovery of purposeful movement

Test or measure: functional task performance

Post-stroke, spinal cord injury or other paralytic conditions

Peripheral nerve innervation is intact

TENS

(sensory and circulation effect)

Transcutaneous nerve stimulation

applied over skin for pain modulation

referred to as "conventional", "sensory" or "high frequency"

Low back pain; neck pain

TENS

(motor and circulation effect)

Transcutaneous nerve stimulation

Twitch motor response applied to affected areas related to the pain

referred to as "acupuncture" or "low frequency"

Generalized pain

HVPC

(edema reduction effect)

High Volt Pulsed Current

Applied to joints and limbs to reduce acute swelling. The active electrode is the cathode (-) in acute stages

Acute ligament sprain

HVPC

(tissue repair)

High Volt Pulsed Current

Applied to wound in the inflammation and proliferation stages; changes in cell membrane and influx of calcium ions promotes tissue healing

early stages of healing: cathode (-) is treatment electrode

debridement stages (e.g. remodeling): anode (+) is the treatment electrode

Decubitus ulcers; pressure injury

Naming can be confusing! To recap, you may hear or read any of the following terms about electrical stimulation applications:

Russian, HVPC, IFC, Microcurrent, NMES, FES, or TENS

In order for therapists to reproduce the effect of electrical stimulation, best practice is to make decisions based on the waveform (e.g., DC, AC, burst or modulated AC, low intensity direct current), pulse frequency, and pulse duration

Treatment - Clinical Application

Approximately 6 minutes

When selecting electrical stimulation, a therapist must select a device and an application that will support achieving the desired effect. Common goals that may be achieved with including electrical stimulation into the treatment include:

improving muscle performance and strength
decreasing or alleviating pain
improving blood flow
decreasing or controlling edema
facilitating tissue healing

Application Decision-Making

The physical therapist will screen each patient for contraindications. Precautions are conditions to consider when selecting and applying electrical stimulation

Contraindications

Pregnancy - except during labor for pain control
Pacemaker or other cardiac implants for interference
Cancer - risk metastasis
Location of thrombophlebitis or phlebothrombosis for risk of embolism
Active Tuberculosis
Over carotid sinus
Area of active hemorrhage
Cognitive Impairments - unable to follow directions or provide feedback

Precautions

Obesity - insulator
Absent or diminished sensation
Skin conditions - eczema, psoriasis, acne, dermatitis, infection, allergies to gel or tape
Diabetes - fragile thin skin
Peripheral neuropathies or areas of denervation
Metal - internal or external
Range of Motion or exercise limitations
Spinal Cord Injury - dysreflexia

There are times when a Physical Therapist may decide to use a modality that is normally contraindicated after discussion with the patient's physician. For example, a patient with a history of cancer in the distant past or application of the modality at an extremity away from the cancer site may not pose more risk than benefit.

Estim Parameters

Electrical stimulation parameters are intentionally selected to produce the desired effect. Electrical stimulation parameters are frequency, duty cycle, ramp time, and duration. For direct current and monophasic pulsed current, the active electrode is set for a positive or negative tissue charge.

Frequency - number of electrical pulses delivered to the body in one second

Also called pulses per second (pps), Hertz (Hz) with AC
Higher frequencies cause higher levels of muscle fatigue due to less time between bursts/pulses
Impedance decreases when frequency increases
Frequencies of 1-120pps meets most therapeutic goals
Stimulation at 50pps tends to be more comfortable than 35pps
Classified as low, medium, and high in clinical units:

low

medium

high

<= 1 kHz

"microcurrent"

1-100kHz

IFC, Russian, LIDC

>100kHz

TENS, HVPS

Duty Cycle - percentage of on time to total time between pulses

Current flows during 'on time' and ceases during 'off time'
Percentage of on time divided by the sum of the on and off time
Example, on time is 5 seconds, off time is 20 seconds = 1:4 ratio or 5 sec/(5+20sec) = 20%
Clinically speaking, muscle contractions elicited through electrical stimulation are more fatiguing, so longer off time allows for recovery/rest and fends off fatigue.

Ramp Time - the time it takes to increase wave amplitude to its peak amplitude

Fixed on some commercial machines, ranging from 1 - 8 seconds
2 second ramp up is often adequate for comfort
Slow rise times can allow a tissue to accommodate to a stimulus
Ramp down or off can increase patient comfort and provides opportunity to actively hold a contraction after the stimuli has ended.

F-08-20 Ramp.jpg

Duration - the total time providing electrical stimulation. Treatment times can range from 10 to 45+ minutes

This table provides a summary of conditions that may be treated effectively with electrical stimulation.

Key point: You will find variations in the literature about parameters with respect to pulse frequency, pulse duration, and duration. For this reason, you will not be teste on pulse frequency or pulse duration or duration. In clinical practice, you will consult the literature and use observation, patient feedback, and measures to inform your effectiveness.

Indication	Type	Waveform	Pulse Frequency (pps)	Pulse Duration (µS)	Intensity	Duration
Muscle strengthening	NMES	AC, Russian	50-80	200-800	to 60&-70& max voluntary contraction	10-20 strong contractions
Contraction for function	FES	AC, Russian	20-60	200-800	to the desired effect	task specific
Pain Modulation	High frequency TENS	pulsed AC or DC	>50; generally 80-110	50-100	to perception of stimulus; no motor response	20-30+ minutes
	Low frequency TENS	pulsed AC or DC	<10	> 150	to point of visible muscle twitch	20-45 minutes
	Hyperstimulation	DC or pulsed DC	(High) 100 (Low) 1-5	250. up to 1 second	to highest tolerated painful stimulus	30-60 seconds each area
Edema Management (acute)	Sensory stim	pulsed DC	100-125	2-100	to comfortable sensory threshold; just below motor response	20-45+ minutes
Edema Management (subacute/chronic)	NMES	AC, Russian	20-80	100-600	to tetanic muscle contraction	3 seconds on: 3 seconds off x 10-20 minutes

Electrodes - Material and Care

There are many choices of electrode shape, size, and configuration to fit the need of the patient and therapeutic goal for electrical stimulation.

Types of Electrodes

Metal Plate Electrodes - early version, limited sizes, required wet sponge conduction medium, difficult to secure in place

Carbon - Impregnated Rubber Electrodes - degrade over time and become non-uniform with "hot spots", many shapes and sizes, rinse and dry after each use and replaced every 12 months to ensure conductivity.

Self-Adhering or Single use Electrodes - flexible conductors, convenient application, no strapping or taping to keep in place, resealable bag for multiple uses, often high impedance, possibility of cross-contamination, used most frequently these days. Single use electrodes MAY NOT be shared between patients.

Electrode Size and Current Density

Current density is the concentration of current under an electrodes.
Electrode surface area is inversely proportional to current flow. (Larger electrode = current is less dense as it is distributed over a larger area; the smaller the electrode, the more intense the same current becomes over a smaller area.
Keep the electrode in proportion with size of body area being treated. If the electrode is too large for the area, there could be unwanted carryover to other surrounding structures; if too small, the current is too dense and may not be tolerated to elicit the desired response.

Completing the Circuit - an electrical stimulation treatment must include a full circuit. To complete the circuit, there must be:

A source of energy creating an electrical potential difference
A conductive pathway - including electrodes, leads, and a conductive surface with good contact

Cleaning the patient's skin with alcohol to remove oil and dirt before electrode application
Clipping excess body hair under electrodes
Minimize warming the treatment area of the body prior to stimulation so there is minimal increased resistance

Electrode Configuration

Monopolar - always two poles: cathode (-) is active and produces cell depolarization. Produces a net charge

Bipolar - two electrodes in a single circuit; each electrode alternates between positive and negative; electrodes are of equal size; no net charge is produced

Patient will feel excitatory response under both electrodes, eliciting motor response or electrode placed over motor point, other electrode over muscle belly and may be larger

Quadripolar - four electrodes in two circuits that operate independently, yet interact

Interferential, large area, pain management, sensory stimulation of larger fiber

Note: electrical stimulation units will call the circuits "channels"

Techniques for Application of Electrical Stimulation

Treatment Goal	# Leads and Electrodes	Monopolar	Bipolar	Quadpolar
Muscle (motor) Stimulation	One channel per muscle with both electrodes on the same muscle, two leads if it Is a larger muscle or if the device has more than one head		X
Sensory Stimulation	One or two channels depending on the size of the area; use as many electrodes as possible to achieve the desired sensory stimulation One channel only if two electrodes fit in the treatment area		X	X
Tissue repair	One channel: active electrode in the treatment area and the dispersive is placed proximally to target area >12-18" away	X

Evidence

Evidence is emerging constantly and it is important to consider both the theoretical effects and the strength of the evidence, while keeping PICO in mind. It is a best practice to consider the patient population (P), intervention (I), comparison group (C) and outcomes (O) when making decisions about evidence-guided practice.

TENS as part of a PT plan of care for patients post stroke can help reduce post-stroke spasticity (a condition associated with changes in neuromuscular function) https://www.ncbi.nlm.nih.gov/pubmed/30326752
In a study of subjects with low back pain from disc herniation, TENS as a stand alone treatment for low back pain and abdominal muscle activation has been shown to be less effective than mobility training https://www.ncbi.nlm.nih.gov/pubmed/30247159
TENS as a component of standard wound care in older adults with pressure injury has been shown to reduce pain and improve tissue healing rates https://www.ncbi.nlm.nih.gov/pubmed/30234576
There is mixed-to-low evidence on the effectiveness of TENS in treating neuropathic pain: https://www.ncbi.nlm.nih.gov/pubmed/28905362
NMES has shown some benefit on function in strength in persons with advanced disease:https://www.ncbi.nlm.nih.gov/pubmed/27748503
A recent studies show that use of electrical stimulation in various forms did not result in adverse heating of medical implants: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5315240/
A recent study supports the use of functional electrical stimulation for improving gait post-stroke: https://www.ncbi.nlm.nih.gov/pubmed/29944167

Clinical Decision Making and Role of the PTA

The PTA is given a Physical Therapy evaluation with patient medical history, diagnosis, goals, and plan of care (POC). Often, simply 'modalities' or 'electrical stimulation' is listed but no specific treatment parameters. Based on this information and subjective information gathered from the patient at the time of treatment, the PTA determines the most appropriate modality and parameters for that specific treatment as it falls within the POC. PTAs should also consider evidence and make evidence-guided decisions.

Patient response to treatment is always monitored and reported back to the PT. It is the responsibility of the PTA to make modifications within the plan of care and consult with the PT as needed. Since there is an object or substance being applied to the body, remember skin checks before and after application of any modality.

Communication with the patient is crucial, including informed consent, and checking for changes in the patient's medical history since the last visit. There may have been a medical test, appointment with a physician, or changes in physician's orders since the previous therapy appointment. The PTA must decide if and how the changes may affect the impending treatment and communicate with appropriate staff.

Refer to Table 10-8 in your Dutton text "Key Points to Remember When Using Electrotherapeutic Modalities"