Intro to E-Stim 2 (10/17a) [Biomedical] Flashcards
Alternating Current (AC)
Uninterrupted bidirectional flow of charged particles
Descriptive characteristics
- Biphasic (bipolar)
- Symmetrical (usually) or asymmetrical
- Balanced (usually) or unbalanced
- Varied shapes — sine wave, rectangular, triangular
Direction of current flow — back and forth between - and +
Zero net current flow — no chemical effects
Burst modulated alternating current (BMAC)
AC current delivered in bursts (with a break), can be more comfortable
EX: Russian Current
Pulsed Current (PC)
Interrupted uni or bidirectional flow of charged particles
Flow ceases for a finite period
Descriptive characteristics of pulses
- Mono or biphasic pulses
- Symmetrical or asymmetrical
- Balanced or unbalanced
- Various shapes (waveforms) rectangular, twin peak, etc
Direction of current flow — back and forth between - and +
Wave form
Mono/Bi phasic
Symmetrical/Asymmetrical — symmetric has same shape, duration, and intensity on both sides
Balanced/Unbalanced — same duration and intensity on both sides
Shape — sinusoidal, rectangle, triangle, spike
Duration (pulse/phase)
Both measured in microseconds
Most stimulators talk about pulse duration
Phase duration — time elapsed from beginning to end of one phase (cross ‘0’)
Pulse duration — time elapsed from beginning to end of all phases
Amplitude
Peak — each phase
Peak to peak — entire pulse
RMS — 70% of peak
Average — 64% of peak
Pulse/Phase Charge
Pulse charge — area under the curve of all phases
Phase charge — area under the curve of one phase
2 pulses with different amplitudes and durations can have the same pulse/phase charge
Pulse duration could be too short to reach threshold
As pulse duration increases, lower amplitudes are needed to excite the tissue
RMP for nerves is ___ mV, for muscles is ___ mV
Nerves = -70 mV Muscles = -90 mV
Frequency - Related Definitions
Interpulse interval (IPI) — time between successive pulses, can be mono/biphasic
Period — time elapsed from one point in waveform to identical point in next waveform
- Period = pulse duration + interpulse interval
- Have to convert to seconds (he will likely give in milliseconds)
Frequency = number of pulses per second (pps, Hz)
- Freq = 1 / Period
Frequency - Usage
When using NMES for strengthening, we want high freq (30-50 pps)
- Has to do with having pulse stay on and create tetanic contraction
Frequency Modulation — we regulate frequency to achieve different stimulations
Carrier Frequency Modulation — used only for BMAC (Russian waveforms, interferential waveforms), you can put bursts at diff frequencies
Ion response around the nerve membrane
More Na+ outside of the cell, anions and K+ more inside the cell
When you have a negative electrode, cations will be attracted to it
You can change the polarity at both electrodes, and if you have a large enough amplitude you can depolarize
For AC or pulse current, this goes back and forth constantly
Resistive/Capacitive Model
Capacitance — property of a system of conductors and insulators to store charge
Resistance built into cell membrane that can block flow of ions/direct current
Allows alternating current to pass
C = q / V , with C measured in Farads (F)
Neuron Membrane
Stores energy because extracellular more +, intracellular more -
Neuron Activation - Successful Initiation
Local current produces increase in Na conductance which depolarizes the membrane which further depolarizes the membrane
When sodium influx greater than Potassium efflux, threshold is reached
When initiations are successful, depolarization occurs to threshold, more sodium channels open and creates action potential, sodium channels close and potassium channels, repolarization, potassium channels slow to close, brought back to resting potential
Neuron Activation - Failed Initiation
when you have depolarization but not enough to reach threshold
Membrane Time Constant (t)
When voltage is applied to a membrane
t= time it takes for a membrane to reach 63% of the applied voltage
The rate of rise of the membrane depolarization depends on the time constant and pulse amplitude
Nerves with small diameter and/or high internal resistance are harder to excite
Diameter vs Resistance vs Excitement
As diameter increases, excitement is easier
As resistance increases, excitement is harder
The larger the nerve, the smaller the internal resistance and easier to excite
Fiber Activation with EStim
Fibers below go from larger diameter/least resistance to smaller diameter/most resistance:
Aα—muscle spindle primary afferent; GTO afferent; skeletal muscle efferent
Aβ—touch pressure receptor afferent; muscle spindle secondary afferent
Aδ—mechanical and thermal afferents
C—mechanical and thermal afferents
The recruitment patterns of nerve types also depends on the location of the excitable tissue relative to the ___ ___
electric field
aka the skin
Recruitment Order (generally)
sensory → motor → pain stimulation
Why do you feel sensory stimulation first even though motor fibers are larger?
the touch/pressure sensory fibers are closer to the electric field
Why don’t you feel pain stimulation first even though their fibers are close to the electric field?
They have such a high internal resistance that they typically get recruited last
Pick electrical stimulation devices that
Allow control of ON:OFF times
Allow control of pulse frequency (2-150pps)
Allow control of pulse duration (50- at least 400µs)
Has adequate power (up to 100mA through a 1000Ω resistor)
Possible mechanisms of edema reduction
Reduction in microvascular permeability
Motor level contractions → improve venous and lymphatic drainage
Pain reduction → improve/increase limb use that will accelerate venous and lymphatic drainage and return to function
Edema Management - Evidence Summary
ANIMALS
- research on rats and hamsters have shown HVPC can reduce edema
HUMANS
- estim had no definitive treatment effect on limb volume, pain, and function after ankle sprain
- no evidence that ES is better than PRICE
- poorly designed studies
To excite tissue, ___ and ___ are most important
intensity and duration
