Week 6--Impedance measures Flashcards
(46 cards)
1
Q
voltage (V)
A
the amount of work applied to move electrons from point a to b
2
Q
current (I)
A
the flow of electrons through a medium
3
Q
impedance (R)
A
the opposition of current flow
4
Q
capacitance (C)
A
the ability to store charge
5
Q
Ohm’s law
A
V=IR which means voltage= currentimpedance
—if you have a set voltage, then as the impedance increases the current will decrease
6
Q
electrical circuit
A
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
7
Q
monopolar CI circuit
A
- 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
8
Q
bipolar and BP+1 circuit
A
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
9
Q
common ground circuit
A
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
10
Q
the importance of impedance measures (2 main)
A
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
11
Q
resistance vs impedance
A
- 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
12
Q
impedance components
A
- 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
13
Q
what happens to a biphasic stimulus?
A
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
14
Q
what type of stimulus must be used with a CI?
A
- 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
15
Q
what is the relationship between electrode area and impedance
A
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
16
Q
impedance changes over time
A
- 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
17
Q
factors affecting impedance (7)
A
- 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
18
Q
4 internal device issues identified by impedance measures
A
- open circuit
- short circuit
- partially short circuit
- voltage compliance
- –identifying these issues is dependent on the electrode coupling method used for measuring impedance
19
Q
typical impedance
A
typically 1-15 kilo ohms
20
Q
open circuit
A
- open circuit means that there is not a complete lop, this could be from damage to the internal device
- causes:
- –damages electrode
- –damaged lead wire
- –non-conductive medium (could be air bubble)
- impedance would be really high: greater than 20-30 kilo ohms
- disable the electrode and reevaluate later
21
Q
stimulation types that can be used to detect open circuit
A
- monopolar
- common ground
- bipolar
22
Q
short circuit
A
- results in low resistance which is actually a problem because there is an additional contact between electrodes which is not supposed to happen
- –current follows the path of least resistance so it will not go where it needs to go, can mess up the path of the current going through
- possible causes:
- –array curled on itself so the electrodes are shorted together and thus when you stimulate 1 you will stimulate the other as well
- –damage to the lead to the electrode contact where leads touch and then you accidentally stimulate more electrodes than desired
- impedance will be less than 1 kilo ohm
- should permanently disable shorted electrodes
23
Q
electrode stimulation types that can detect short circuit
A
- common ground is good at detecting short circuit because if the active electrode is shorted with one you would see the current coming back
- bipolar is also good at detecting short circuit because it would end up sending current to both and bringing it back from both shorted electrodes
- monopolar is not good at detecting short circuit because even if electrodes are shorted together the active will still fire and the extracochlear return will still work
- –basically monopolar is the only one that wont work well because it is only looking at 1 electrode at a time
24
Q
partial short circuit
A
- when the insulation to the electrode leads is damaged, the fluid goes into the tears and indirectly connects the electrodes together which causes the current to lead into the electrodes indirectly in contact with the active electrode
- –overtime the device will start failing
- –impedance will drop and drop overtime–this is why it is so important to test impedance every time the pt comes in
25
identifying voltage compliance issues with impedance
* voltage is constant so:
- --V=I*R
- --supply voltage (between 0 and 1.4V*batteries)
- --electrode area
- --battery life is affected by voltage compliance because if you push a battery to its max, it will drain faster
- --an electrode out of compliance could cause platinum dissolution
* voltage compliance is evaluating the interface between electrodes and tissue
- --impedance will control the amount of current sent in because voltage is constant
- ----to increase loudness perception: increase amplitude or increase pulse width
26
how does Cochlear measure impedance in software
* you must click on "measure impedance" and then it will send a low level biphasic pulse to the electrodes to measure impedance and a diagram of the cochlea and electrodes will pop up
- --green means good, and red means there is a problem with that electrode
- --to know what is going on, click "details"
27
stimulation methods cochlear uses to measure impedance
* 4 ways:
- --common ground
- --monopolar 1
- --monopolar 2
- --monopolar 1+2
- ------note that cochlear doesnt need to test bipolar because common ground does the same thing
* on the scale shown of the electrodes, they are #ed by 2s
28
impedance numbers with cochlear
* contour device will be between 0.565-30 kilo ohms
* strait device will be between 0.7-20 kilo ohms
- --short circuit will see a decrease in impedance
- --open circuit will see an increase in impedance
* short circuit will be most seen with common ground measurements
29
partial short circuit in cochlear devices
* have an interesting pattern because all odd numbered electrodes and all even numbered electrodes are bundled together
- --this will give a zig zag pattern because 1 bundle will be affected
- --be careful because there is natural variability to impedance so need to know how to tel if it is a real zig zag pattern:
- -----there needs to be at lease a 1.5 Kilo ohm difference between 1 electrode and the next
- -----and it must affect at least 3 electrodes in a bundle (3 evens or 3 odds)
* low impedance flat across all electrodes can indicate a problem with short of the internal device
- --must be more than 5 electrodes in a row and a drop of at least 30% if baseline
- -----measure this based off of all impedance measurements, but common ground will drop away from all the monopolar testing
30
impedance testing with AB software
* low-level biphasic stimulus
* will run this test automatically as soon as the CI are connected to software
* can also do conditioning
31
type of stimulus used for impedance testing with AB
*can only test using monopolar, so can only show open circuits (kinda...)
32
impedance numbers with AB
* will show green if valid which would be between 0.5-30 kilo ohms
* short circuit will show purple and be between 0.5-1 kilo ohms
* open circuit will be orange and will have high impedance ( over 30 ko)
* invalid will be gray and will be under 1.5 kilo ohms which means you need to contact manufacturer and figure out what is going on
33
how does AB know if there is a short circuit?
use the electric field imaging and modeling which is a test the software runs behind the scenes
---remember it is hard to see short circuit with monopolar which is what AB uses
34
impedance testing with Med-El software
* low level biphasic stimulus
* no automatic impedance testing so must open "impedance field telemetry (IFT)" and hit measure
* will measure the integrity of the device which will say okay or give you a dash
* will also measure the impedance of the ground electrode which is typically around 1 Kilo ohm
* will tell about coupling which is how the processor and internal device are communicating
35
impedance numbers with Med-El
typically between 5-15 kilo ohms
* will tell you if the status of the electrode is okay or if the impedance is high (HI) which is over 21 kilo ohms
* will also day SC fro short circuit
36
package test with Med-El
* requires you to do an impedance test while the device is still in the package before implanting to make sure there is no problem causing short circuit
- --during package test, expect to see all impedances to be high because they are not in contact with anything, but it will say HSC-X if there is a short between electrodes which is for high but impaired
37
voltage distribution profile (VDP)
* how med-el detects short circuit because they are monopolar stimulation only
* what this is sis sending current to one electrode and the internal device runs with multiple monopolars at the same time, while measuring from 1 electrode, you shouldnt really see anything going on in the other electrodes on the diagonal boxes of the graph, if there is a short circuit, you will see something (more than 50% of what you are presenting to the active electrode) in another electrode
- --take away, no electrode should have more the half of what the active electrode has
38
what to do if there is electrode failure for a single electrode and for multiple electrodes
* 1-2 electrode with issue= turn off
* multiple electrodes= evaluate for device failure and consider removal
* if the electrodes are intermittent, then just turn off
39
what to do if problem with voltage compliance
* with AB and Med-El they automatically increase pulse width to make louder without reaching voltage compliance
* cochlear gives compliance limits and you can change the pulse width manually if needed to not reach compliance limits
40
what to monitor for in terms of unexpected changes in impedance over time
* baseline for impedance is measured during surgery for normal or abnormal function
- --if impedance are normal intraopertively and then abnormal: there is incomplete insertion or delayed malfunction
- --if impedance is high intraoperatively then improve: this is relatively normal and often impacts multiple electrodes, not just one
* non-use will result in higher impedances
* middle ear infections, labrynthitis, colds will cause temporary increase in impedance
* fast rate of stimulation can cause impedance to increase if functioning out of compliance so need to use wider pulse width
* if impedance drops over time this is a red flag that can indicate short circuit
41
9 signs of partial short circuit
1) reduction of impedance over time
2) lower impedance compared to other electrodes
3) increased map levels
4) reduction in speech perception
5) poor sound quality (buzzing/hum)
6) reduced/reversed pitch perception
7) reduced loudness growth
8) non-auditory stimulation (pain/facial twitch)
9) refusal to wear
42
solutions to sign of partial short circuit
* troubleshoot processor
* all or every other electrode
* re-implant
- --will need map levels higher because current is leaking out
43
basic what are electric field potentials
the method AB and Mel-El use to identify short circuit
- --does monopolar while monitoring the other electrodes for activity
- --unlike a regular impedance test which is MP, this is 3D modeling
44
what does AB call electric field potentials
electrical field imaging and modeling (EFIM)
45
what does Med-El call electric field potentials
voltage matrix
46
AB criteria for short circuit
1) 2 electrodes have similar impedance
| 2) impedance is roughly half of that of a normal functioning electrode
