Cochlear implants Flashcards
Team of developers
engineers, otologists, audiologists, psychoacousticians, and neurophsysicits
how does it work?
Cochlear implants bypass damaged peripheral hearing system and directly stimulate the CNVIII
indicaters a mapping appointment is needed
changes in auditory discrim increased repetition needed adition/omission of syllables prolongation of vowels change in vocal quality
telemetry
confirms proper communication of processor and electrodes
neural response assessment
electrophysiologic response from the nerve in response to electrode stim
especially useful in children
t levels
measure of threshold for sound on each electrode (cochlear)
C or M levels
comfort levels using a scale
speech strategies
different methods of stimulation can produce different perceptions from the patient
ex simultaneous sequential or both
condition
AB only - sends stim to all cannels at the same time. it can push away any buildup around array
activation?
impedance
opposition to electrical current flow across an electrode
voltage/current measured in kOhms
influenced by electrode and lead, but also the medium
fibrous tissue, electrolytes, macrophages, proteins
voltage
current x impedance
current stays constant, increases in impedance will cause an increase in voltage.
high impedance
open
may be due to air bubbles, send stim then measure again?
hopfully its temporary
short circuit
low impedance, usually a phsyical breakdown of two channels permanently
usually only occurs at implantation
compare over time, note dips in impedance
NRI (ab)
corresponds well to M levels
NRT (cochlear)
not always correlated to C levels, can sometimes be more in the middle
Uses of NRT/NRI
relatively stable over time
used with impedances to tell if change in performance is due to device function or neural responsiveness
get within first few months of stimulation to use as baseline
assess for internal device failure, if you had them before and nwo you dont it could be a soft failure
can also use this so determine what pitch abilities.
highly influences by neural survival, synchronous neural activity
threshold levels - (cochlear)
at or jsut above threshold, ascending bracketing loudness growth chart
med el threshold levels
highest stim where no sound is perceived
sound detection thresholds
15-20 for cochlear
30-40 others
c levels
set below maximum confort level (summation)
m levels - med el
highest stim level at which sound is loud but comfortable
m level - AB
most comfortable level
soundwave 2.0 uses speech bursts for m levels
methods of loudness balancing
balancing and sweeping
may say “sounds good but something is weird”
doesnt always do this at activation,
usually at about 80% of MCL? at or near upper stim level
sweeping
start at one end and start just below of every channel as itmoves through the array going low to high, “pitch should get higher, loudness should not.”
pitch ranking
if the electrode array is rolled over on itself, pitch does not increase from low to high on sequential electrodes
reorder channels if necessary to match the perception
compliance
the amount of voltage allowed for each electrode
“out of compliance”
max voltage available from the implant is not suffiecient to generate the desired current level
insuffient loudness growth, variable loudness, sound may be distorted, poor battery life, decrease performance in general
dont want your c level to be just below compliance
if you cant you may need to increase pulse width
some days it sounds good and some days not.
automatic pulse width (APW) - ab
calculates and adjusts pulse width and rate pased on compliance and M-level requirements
it can do this when the patient is not in front of you so if you have fluctuating impedance you get more compliance headroom so you may want to turn on
when to use manual pulse width?
poor sound quality
cannot obtain adequate loudness due to facial nerve stim
fluttering or choppy
maxima
takes incoming signal to spectogram and pulls our where the most energy is, usually 8-10, transmitted via T levels of the map
loudness growth - amp
as long as there is sufficient voltage available, current amp can be increased and the precept of loudness growth will result
if insufficient voltage is not available to meet the current requirement, the channel will be out of compliance. (no further loudness growth)
loudness growth
amplitude, pulse width, rate
loudness growth - PW
short (narrow) pulse widths produce higher thresholds and steeper loudness growth curves.
longer pulse durations (wider) produce lower thresholds and larger dynamic ranges, due to slower loudness growth near threshold.
loudness growth - rate
pitch changes due to rate saturate at 500 PPS
higher stim rates require narrow PW which consumes sig power.
eCAP
electrically evoked compound action potential
potential that reflects fynchronious firing of large # of electrically stimulated nVIII fibers
started by putting needle electrode, cochlear discovvered with Nucleus to use intracochlear to stimualte instead
N1 ~.2-.5 ms followed by P1 ~ 1 ms
amp 20-1500 uV
utility of eCAP
integirty of system, how are the devices talking and how is the nerve responding.
at activation, once you get all channels you dont really have to do it that much, more useful in children
series of sounds, get louder and try to tolerate it
these are the highest levels so if you dont get facial nerve stim that you probably wont, also can start to get pitch information
NRT/NRI high influenced by
neural survival
synchronous neural activity
nerve survival with respect to the measuring electrode.
eSRT
electrically evoked stapedial reflex threshold
SRT occurs at or near the M/C levels levels used by the processor
bilateral contraction so stim the implant and measure non implanted ear
need to consider middle ear history especially for bilateral
how to administer eSRT
probe in contra ear,
continue to record acoustic admittance with 226 probe tone,
present programming stimulus for upper limit of DR
change in admittance occurs time=locked with stimulus when presentation level is of ideal intensity of C/M level
measure as many as you can, then interpolate
reduce c/m for live mode then increase to eSRT level
speech encoding strategy
method which the implant translates the incoming acoustic signal into patterns of electrical pulses
provide spectral and envelope info
stimulation mode
location of the reference electrode to the active electrode
-monopolar vs bipolar
med el monopolar only
threshold levels
ensure soft sounds are audible
sound field responses are in line with specs
perception of soft softs
if soft sounds are too loud bring down T
T-Tail
limited change in loudness perception across a wide range of stimulation ltevels in the lower part of the EDR (electrical dynamic range)
T levels for tinnitus
Set t levels over level of tinnitus, then reduce globally, loudness balancing at 50% over t levels
interpolation
estimation of stimulation levels on the basis of measure values from neighboring channels.
programming method
behavioral, objective preset (using NRT or NRT to set up map)
objective w/ behavioral offset
help set configuration measure on T then interpolate
influences on levels
speech processing strategy
bipolar/monopolar
stim rate
C and M levels (current)
proximity of electrode array to modiolus
non auditory stimulation?
current can reach neighboring nerves, run through measurements and summate to make sure, test loudness clapping and jingling keys
reduce levels or deactivate electrode if causing coughing or favial tiwtch. if you are way out of your DR then just turn off, otherwise drop 5 CU
the need for power
sufficiant voltage available to the implant to ensure that all electrodes are in compliance sufficient power (voltage and current) for the demands of he chosen MAP
power optimization
can the battery provide sifficient voltage to deliver the requested amount of current
calculates amount of power needed in worst case condition (ambient noise >65) for all electrodes to be within compliance
problems with battery life
increase pw
decrease rate/maxima if high stim levels or compliance conditions are not an issue
live voice modifications —–
tilting
decrease or increase levels on all channels
gain
frequency adjustments
tilting
increase high 2 and decrease low 2 rather than decreasing low 4 steps
hearing low level humming?
take down the c levels for the lows, but they are still hearing if then it could be their own voice so bring down T levels.
sensitivity
distance of hearing, perception in noise
processing strategy
set of rules and processes for converting the acoustic input signal into electrical stimulation waveforms
paired or sequential
paired or sequential
paired is faster so you may do sequential for older patients
intensity domain
influenced by how sound is captured and processed directional mics mic placement AGC input dynamic range
spectral domain
nromal hearing 20-20kHz, implants 250-8000 Hz
music is different, notes that are close in frequency can processed through the same ci filter bank so perception is the same, larger difference to differentiate
the 5 processing strategies
continuous interleaved sampling (CIS) HiResolution (HiRes) - AB Advances combination encoder (ACE) - cochlear Spectral peak (SPEAK) Fine structure processing (FSP) -mel el
CIS
filters sounds using band pass filters
envelope compressed non linearly
1000 pps
pulses sent sequentially
HiRes
high stim rate and up to 16 channels
up to 5000 pps
uses half-wave rectifier for envelope detection
current steering, using multiple electrodes in different patterns for different frequencies, more sophisticated AGC algorithm
new versions - increase virtual channels
improve spectral res and speech recognition in noise, soud quality and music appreciation
designed to reduce battery consumption
n of m
acoustic energy in each m channel is determined and stimulation is administered to only the n channels with the highest amplitude inputs. reduces number of active electrodes = faster stim rates, increase batt life
SPEAK ACE
channel selection scheme
envelope signals for different channels re scanned prior to each from of stiulation, the highet amplitudes are identified and stimulus is delievered to those electrodes. fixed parameter ACE 1000 pps and SPEAK 250 pps
FSP
new form of CIS offered by med el
intermetiate pitches generate via bell shaped overlapping filters diesigned to improve perception of high frequency phonemes based on place cues
selecting speech processor strategy
high rate = ace FSP HiRes
sig research indicates that performance is improved with high rate strategies
preference is strongly correlated with performance
voltage compliance
each system has a finite voltage capacity (size and type of battery) which determines the maximum amount of current it can deliver.
The amount of power available to operate the implant and the current that will be delivered to an individual electrode is determined by the efficiency of the signal delivery from the external coil to the internal device, the voltage capacity of the battery, and the electrode impedance.
The maximum amount of electrical current available to stimulate an electrode is determined by Ohm’s law
The max amount of current that can be delivered to an electrode by given power source. The voltage is determined by capacity of the battery so its fixed. the current available at each electrode is variable across recipients and electrodes by the impedance of each electrode
high impedance will result in lower amount of current available to stimulate.
lower electrode impedance results in a higher amount of current available o stimulate the electrode
