Electrode Impedance

Question 1

What is telemetry?

Answer 1

Bi-directional communication of data between the programming hardware and the implant, using radio-frequency code
Data transmission via radio frequency from a source to a receiving station
Implant can also give information to the programming hardware

Question 2

Is telemetry an objective measure?

Answer 2

Yes, doesn’t require the patient to respond
Impedance as a whole is an objective
Impedance is a nonphysiological measure
Physiological measures are also objective (electrically evoked compound action potentials (ECAP)) - when the nerve is stimulated, an action potential is generated

Question 3

What are objective measures used for?

Answer 3

Verify the device function
Verify the integrity and function of the auditory pathway
Obtain a baseline of neural function for tracking potential changes over time
Assist in programming the cochlear implant sound processor
Measure discrimination of different stimuli
Measure the plasticity of the auditory system

Question 4

What are the principles of verification of device function?

Answer 4

Stimulate an electrode
Measure voltage at same or other electrodes
Return via programming interface
Evaluate data in the programming software

Question 5

Why do we verify device function?

Answer 5

Verify coupling
Monitor integrity of the implant electronics (within receiver stimulator package)
Monitor the integrity of the active electrode array (depend on these to do the work)
*Can check these by measuring impedance

Question 6

What could be wrong with the coupling?

Answer 6

External magnet not aligned
Weak magnet

Question 7

How can you look at the integrity of the active electrode array?

Answer 7

Electrode impedances
Ground path impedance
Voltage field along the cochlea
Automatic and manual detection of short-circuits

Question 8

What is electrode impedance?

Answer 8

Refers to the resistance to the flow of energy through any medium; measured in ohms
Electrode impedance is a measure of the opposition to electrical current flow across an electrode when a certain voltage is applied
*Small voltage is presented and impedance is measured off of that

Question 9

What are the properties of an electrical circuit?

Answer 9

Current
Voltage
Resistance

Question 10

What is a current?

Answer 10

The rate at which electric charge (ions or electrons) passes a given point (the passage of electrons in a conducting material)
It is measured is amperes
The actual substance that is flowing through the wires of the circuit

Question 11

What is voltage?

Answer 11

Potential difference between two points in an electric circuit
The energy that are required to accelerate electrons from one point to another
Measured in volts (V)
A force that pushes the current through the circuit (gravity on a waterfall)

Question 12

What is resistance?

Answer 12

Occurs when electrons collide with atoms and lose energy
Measured in Ohms
Rocks in a river which will slow down the water
Impedance is resistance
Tissue acts as resistance in the case of cochlear implants

Question 13

What is OHMs law?

Answer 13

Intensity = Voltage/Resistance

Question 14

What is the maximum voltage provided by a cochlear implant determined by?

Answer 14

The battery of the sound processor and the radiofrequency link between the processor and the implant
Because the voltage of the power source is known, Ohm’s law may be used to measure the impedance present at each electrode contact

Question 15

What is electric impedance?

Answer 15

The opposition of matter to electric changes is called the electric impedance (Z)

Question 16

Does a higher impedance require a higher voltage?

Answer 16

Yes
But the voltage has a limit in CIs
When the limit is reached then that is called out of compliance

Question 17

How is electric impedance measured?

Answer 17

A small amount of electrical current, which is typically inaudible to the recipient, is delivered sequentially to each active intracochlear electrode
Impedance at each electrode contact can be determined using Ohm’s law

Question 18

How is electrode impedance measured clinically?

Answer 18

All three manufacturers have incorporated impedance measures into their clinical software
A fixed current is applied to a given electrode, and the voltage is measured between the active and return electrodes at the output of the current source
The known values for current and voltage then allow for impedance to be calculated
The stimulus used for the impedance measures is typically a low-level biphasic current pulse; however, it may be audible for some recipients

Question 19

What is the purpose of measuring electrode impedance?

Answer 19

To confirm that both intracochlear and extracochlear electrodes are functioning properly
Provide information about the properties of the tissue environment surrounding the electrodes (electrode-tissue interface)

Question 20

What are the clinical uses for impedance measures?

Answer 20

Identification of electrode failures for purposes of programming the sound processor
Verification of voltage compliance
Monitoring of electrode function over time so that appropriate decisions can be made for clinical management
Evaluate intraoperative to postoperative changes
Monitor changes across follow-up visits

Question 21

What are the types of abnormal impedance that are seen clinically?

Answer 21

Open circuits
Short circuits
Partial short circuits
Voltage compliance (not really abnormal but very challenging)

Question 22

What is an open circuit?

Answer 22

An incomplete path for current to flow, or a discontinuous circuit
An infinite resistance prohibits the flow of current
An electrode with an excessively high impedance (>30 kohms)
True open circuits are characterized by near-infinite impedance values (megohms)

Question 23

What are some causes for an open circuit?

Answer 23

Broken electrode contact
Broken lead wire
Anomalies (e.g., ossification) or by an air bubble or protein buildup in the electrode-tissue interface
Nonconductive medium surrounding the electrode contact (such as air)

Question 24

What is good impedance?

Answer 24

1-15 kohms

Question 25

How are open circuits identified?

Answer 25

Using an of the usual electrode coupling modes that are clinically available Recent version of each manufacturer’s software identifies the presence of abnormal electrodes and “flags” them for management by the programming clinician

Question 26

What is a short circuit?

Answer 26

Low resistance between two points in a circuit that differ in potential which typically are separated by higher resistance Close enough where they are treated as one Results in an increase in current flow (for a fixed voltage) Term short refers to unintended contact between conductive electrical elements

Question 27

What are the causes of a short circuit?

Answer 27

Electrode contacts touching due to a kinked or curled array Electrode lead wires that are touching within the electrode carrier due to damaged insulation Excessive distortion or tension on the electrode array

Question 28

How are short circuits identified?

Answer 28

Excessively low impedance values (1 kohm or less) Two electrodes that are electrically connected and consequently possess an identical voltage that is distributed to each electrode when only one is stimulated Short circuit between two intracochlear electrodes can be identified with a common/ground or various bipolar coupling modes (but not reliably identified with monopolar coupling) - due to normal impedances when comparing the intracochlear contact to the extracochlear common

Question 29

How do you manage an open or short circuit?

Answer 29

Electrodes should be disabled but reevaluated after a period of implant use - could go back to normal if air-bubble disappears If a short or open circuit resolves, reactivation may be considered if the impedance remains normal over several visits Electrodes with intermittent short or open circuits should remain disabled, as the issue may recur while the device is in use and the clinic is not immediately accessible Repeated intermittencies may be indicative of impending device failure Electrode impedances are often high at initial activation but usually decrease with electrical stimulation Electrodes with moderately high impedance (e.g., 20–30 kΩ) should not be disabled, as values will likely decrease with continued implant use - likely due to an air bubble

Question 30

What are partial short circuits?

Answer 30

A relatively low resistance resulting in increased current flow, but less so than for a true short circuit Also referred to as "atypical impedances" or "insulation damage" Present impedances that are lower than that of nonaffected electrodes, but not low enough to be flagged by the software as a true short circuit

Question 31

What can cause a partial short circuit?

Answer 31

Small tears or fractures in the silicone surrounding the electrode fantail or lead allowing fluid ingress into the silicone carrier altering the impedance (resistance to current flow decreases and impedance measures drop)

Question 32

How are partial short circuits identified?

Answer 32

Can be difficult while implant is still in use The problem is typically only confirmed upon explant

Question 33

What are the two key components for identifying suspected partial short circuits?

Answer 33

Impedances (common ground is the preferred mode) will decrease over time Impedances for only certain electrodes systematically decrease over time, whereas others remain stable

Question 34

Can you increase the voltage of a single open electrode or an electrode with high impedance?

Answer 34

No Limited by the power of the implant Will also give them decreased sound quality Will compromise the stimulation rate for the other electrodes

Question 35

For certain models of Cochlear Americas devices, are the even and odd numbered electrode lead wires bundled separately from each side of the internal receiver-stimulator? How does this help with identifying partial short circuits?

Answer 35

Yes Depending on where the fracture in the silicone occurs, the result can present as an alternating or zigzag pattern in the impedance measures

Question 36

How are partial short circuits managed?

Answer 36

May or may not be disabled Depending on the extent to which performance is affected and whether or not non-auditory percepts are experienced

Question 37

Are most cochlear implants constant current devices?

Answer 37

Yes, where a fixed amount of current is specified in the software, and the amount of supply voltage (from the battery) is varied depending on the impedance of the electrode and surrounding tissue Voltage is determined by the capacity of the battery Current will be variable across electrodes and determine by the impedance at each electrode The max amount of electrical current available to stimulate an electrode contact is determined by Ohm's law

Question 38

What is voltage compliance?

Answer 38

Using the voltage available without exceeding the limit Amount of power available to operate the implant and the current delivered to each electrode depend on the efficiency of signal delivery, battery voltage capacity, and electrode impedance For electrodes with low impedance, higher current levels can be delivered before reaching the supply voltage limit For electrodes with high impedance, the device is limited to lower current levels before reaching voltage compliance Clinically, high-impedance electrodes may require more voltage to deliver the same current, potentially reducing battery life Once stimulation reaches the voltage compliance limit, additional current cannot be delivered. As a result, increasing the current level in the programming software will not result in loudness growth

Question 39

Are short circuits also permanent?

Answer 39

Yes, just like open circuits

Question 40

What can help avoid reaching the compliance limit?

Answer 40

Using a battery with a higher voltage capacity

Question 41

What will happen if a patient uses a map with electrodes that are out of voltage compliance?

Answer 41

Speech recognition and sound quality may be reduced Loudness may be unbalanced across the electrode array The user may experience nonauditory effects, such as facial nerve stimulation, due to maximum current amplitude

Question 42

Does normal electrode impedance mean all is well always?

Answer 42

No A “normal” electrode impedance value does not necessarily imply that all is well An absolute electrode impedance value that is within normal limits does not even indicate that the electrode contact is within the cochlea Electrode impedance indicates how electrical current flows across an electrode contact to surrounding tissues or fluid A “normal” electrode impedance value may be obtained when an electrode is in contact with middle ear tissue or fluids or body tissue remote from the cochlea

Question 43

Does impedance change over time?

Answer 43

Yes Impedance is usually lowest at the time if surgery because the medium surrounding the array most likely consists of perilymph Over time, changes in impedance may reflect changes in electrode-tissue interface or electrode function (these may result from post-surgical changes or other clinical factors) In the 2 to 3 weeks following surgery, impedance typically increases due to fibrous tissue encapsulation of the electrode array— a by-product of the body’s immune response to a foreign body

Question 44

How does steroid use during insertion of the electrode array affect impedance?

Answer 44

Reduces fibrous tissue growth and lowers impedance Keeps inflammation in the cochlea to a minimum

Question 45

What happens to impedance post-surgery?

Answer 45

Once the implant is stimulated, impedance typically decreases over time - higher impedance means that the current needs to be high to overcome barrier (higher stimulation levels) Impedances should remain stable with no fluctuations by approximately one to three months post-activation - may be due to reorganization of fibrous tissue matrix (less dense)

Question 46

What are impedance measures influenced by?

Answer 46

The electrode contact The electrode lead that is coupled to the contact Surrounding medium, including: Cochlear fluids Surrounding tissues Electrolytes Macrophages Proteins….. and so forth

Question 47

What is electrode conditioning?

Answer 47

A feature available in some implant systems that allows for the presentation of low-level current to each electrode to remove air bubbles, protein buildup, and so on Only done during first stimulation - because some time has passed from implantation

Question 48

When is electrode conditioning typically used?

Answer 48

Prior to testing in the operating room At initial activation At a programming session preceded by a prolonged period of nonuse When activating electrodes were previously disabled and reactivate

Question 49

Can impedance change over time beyond initial activation?

Answer 49

Yes, due to multiple factors that need to be managed clinically Impedance can increase for electrodes that are not stimulated for a period of time Re-implantation often results in higher impedances Impedance may temporarily increase due to middle ear issues (e.g., infection, fluid, cold), often resolving on their own after the episode is resolved

Question 50

What does abnormal impedance result in?

Answer 50

Compromised sound quality Produce non-auditory sensations, such as facial nerve stimulation Poor speech recognition performance Inadequate loudness growth Sudden changes in loudness Potential discomfort