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Impediance
- the opposition to the flow of energy through a system
- -sound strikes system and a portion is reflected back depending on the stiffness of the system.
- -reflected wave carries info about the status of the middle ear
- -harder surface has higher impediance
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Middle ear is stiff or immobile
- greater amount of energy reflected back
- -otis media or otosclerosis
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Flaccid system (middle ear)
- will reflect less energy
- -ossicular chain disruption
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admittance
- how much energy is going into the system/how easy energy flows through
- -ear with high impedance will have a low admittance
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Acoustic Immittance
- a measure of impedance or admittance in order to get info about middle ear status
- -fluid no fluid in middle ear?
- -evaluate eustachian tube
- -facial nerve function
- -determine nature of hearing loss
- -predict audiometric findings
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No drum movement
no compliance or impedance measure
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Measuring intensity vs Pressure
Compliance of the middle ear system
- -if compliance is high impedance is low
- -compliance is highest in centre of eardrum and lowest at edges
- -negative and positive pressure push the eardrum in or out
- -different pressures change the compliance of the eardrum and the impedance
- -you push air in or out of the system to see when the eardrum is maximally compliant by monitoring tone changes
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Immittance test: Tympanometry
- measurement of the mobility of the middle ear when pressure in the canal is varied from +200 to -400 daPa
- -tells us 2 things
- 1.middle ear drum peak pressure (normal is -150 to 50 daPa)
- 2.where the eardrum moves best (normal is 0.3 to 1.6) lower than 0.3 is stiff and higher than 1.6 is flaccid
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Immittance test: Static acoustic immittance
- -can infer volume of ear canal in cubic cm
- -determine immittance ad +200daPa then immittance at most compliant point. The difference between the two = the equivalent ear canal volume(air space in canal)
- -range of normal= 0.9 to 2.0 adults and 0.25 to 0.9 kids
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Type A Tympanogram
peak pressure within normal range, height of peak tells us flaccid (Ad) if peak is too high and stiff if the peak is too low (As)
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Type B tympanogram
- flat low volume, with no peak pressure meaning no drum movement
- -lower than normal compliance
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Type C tympanogram
peak pressure out of normal range, ear infection possibly
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Immittance test: Acoustic reflex
- -They help you make sense of info you already know
- -normal ears will have AR's at 500,1000,2000Hz at 65 to 95 db. higher than this is disordered
- -elevated AR thresholds could mean conductive loss
- -AR thresholds can help differentiate cochlear from retrocochlear pathology.
- -with cochlear loss ARs are severe but present-with retrocochlear lesions AR thresholds are not present
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Stapedious reflex
- The lowest possible intensity required to elicit a middle ear muscle contraction
- -reflex triggered in both ears
- -contraction of the muscle results in temporary increase in impedance of middle ear
- -if your drum is too stiff to move there is no reflex-if no reflex:inner ear, nerve, brain stem problems
- -when reflex occurs impedance and sound pressure increase
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