Week 6--Impedance measures Flashcards
voltage (V)
the amount of work applied to move electrons from point a to b
current (I)
the flow of electrons through a medium
impedance (R)
the opposition of current flow
capacitance (C)
the ability to store charge
Ohm’s law
V=IR which means voltage= currentimpedance
—if you have a set voltage, then as the impedance increases the current will decrease
electrical circuit
a network that required a closed pathway for current to go through
- –smaller resistance means the more current will go through
- –the CI circuit is monopolar, bipolar, or common ground electrode coupling to use for stimulation and diagnostics
monopolar CI circuit
- with monopolar stimulation, the current flows through an electrode and returns through the extracochlear electrode
- –with cochlear for example the return could be MP1 which is the extracochlear electrode or MP2 which is the electrode on the casing
- —–will be one or the other, however MP1+2 means both are activated and one is the default
bipolar and BP+1 circuit
the return in bipolar is the electrode right next to the active one on the apical side
- –Bp+1 means the electrode 2 down from the active electrode on the apical side is the return path (and so on how many+)
- —–only use bipolar +x in times when some electrodes are turned off or damaged
- –can do a mashup of bipolar and bipolar+x for situations when needed, for when the next apical electrode is turned off, this is called variable bipolar
- **note pseudomonopolar= when no extracochlear electrode is not working so always use the most basal electrode as the return
common ground circuit
1 active electrode and all other electrodes are the return path
- –the amount of current that comes back through each electrode is dependent on the electrode’s impedance
- this type of stimulation is only available through cochlear because it cannot be done when all electrodes have separate power sources
the importance of impedance measures (2 main)
1) will tell you if there are any issues with electrodes or a problem with the whole array or internal device
2) will tel you about the tissue interface/quality of the contact between the tissue and the electrode to tell about the stimulation of the cochlea
* significant changes in impedance are driven by the tissue interface or internal device
- — it is important to know normal impedance/ change in impedance to tell if there is an abnormal change
resistance vs impedance
- resistance is scalar and impedance is vector
- –therefore impedance is for CI because CI is AC, resistance would be for DC because it would only be in 1 direction=scalar
impedance components
- access resistance (Ra)= lead, contact, and medium act as resisters
- reactance (c)= is what determines the direction and is determined by the capacitance
- –also know as polarization component
what happens to a biphasic stimulus?
the biphasic pulse coming in would make electrons keep building up until fully charged, the charging process between the electrode contact and tissue will end up like a capacitor
- how much of the voltage is used to get the electrons in and how much is being used to charge the capacitor, want to use large voltage to get electrons in and a small amount to charge the capacitor
- in CI electrons are not released inside the cochlea, they are used to charge the contact which uses the fluids to charge the tissue which makes the neurons fire
what type of stimulus must be used with a CI?
- biphasic or other charge balanced stimulus because whatever electrons go in to charge the place need to be brought back to not damage the electrode contact or tissue
- –basically the plate is where electrons build up, the negative ions in the cochlea are pushed away from the negative charge which then stimulates the neurons by creating action potential through the nerve
- —–the electrons dont flow through the plate to the tissue, it simply causes the ions in the tissue to want to move away because of the negativity of the plate
what is the relationship between electrode area and impedance
the larger the plate of the electrode contact, the smaller the impedance which allows fro easier charging
—the electrodes on the lateral wall are generally larger and rougher to increase surface area because further away and the ones closer to the modiolus are smaller but dont need as much current to stimulate because they are closer
impedance changes over time
- lowest impedance during surgery,then increases over time due to body’s natural attempt to remove foreign body/protect self against it by forming fibrous tissue around it in an attempt to engulf it
- with electric stimulation on activation, impedance drops then slowly stabilizes
factors affecting impedance (7)
- no stimulation for a period of time (build up of proteins and microphages)
- electrode migration
- re-implantation
- middle ear infection, labyrinthitis
- fast-rate strategy causing platinum dissolution due to stimulating out of compliance
- hormones (puberty, pregnancy)
- early in the day, after long night or n stimulation
- –was the old way for data-logging
4 internal device issues identified by impedance measures
- open circuit
- short circuit
- partially short circuit
- voltage compliance
- –identifying these issues is dependent on the electrode coupling method used for measuring impedance