Welch Allyn TM286 Auto Tymp - User Manual - Manual (Page 9)

Directions for Use Introduction 5
Acoustic energy, commonly referred to as the probe tone (226 Hz) is introduced into a
hermetically sealed ear canal by means of a loudspeaker located within the probe. The
intensity of this tone is monitored via a microphone, also located within the probe box.
Measurements are taken at fixed time intervals.
As pressure within the ear canal is varied, the eardrum is subjected to varying degrees of
stress which alters the mobility of the eardrum. Maximum mobility will occur when the
pressure on both sides of the eardrum are equal. Changes in mobility of the eardrum tend
to produce changes in the probe tone level within the ear canal. Probe tone intensity
changes indicate the amount of sound energy entering the middle ear.
Compliance is calculated based on these measurements. Since the sound pressure level
of the probe tone within the ear canal varies as a function of mobility, it is possible to
record these changes in mobility as a function of pressure. While the recording is
visualized in the horizontal direction (X-axis) as a function of differential pressure across
the eardrum, the tracing also moves in the vertical direction (Y-axis) as a function of
mobility or admittance of the middle ear system. A graphic presentation of this
information is known as a tympanogram.
The point of the tympanogram which represents the point of maximum compliance is the
compliance peak of the tympanogram. The air pressure (pressure at the peak) where this
compliance peak occurs approximates the pressure within the middle-ear system, since
maximum mobility is only possible when there is little or no pressure difference between
the ear canal and the middle-ear space. Compliance using a 226 Hz probe tone is
measured with respect to the ability of an equivalent volume of air to conduct sound and
the scientific quantity used is cm
3
.
The presence of a pathological condition which interferes with the mobility of the
tympanic membrane, the ossicular chain, or the air pressure within the middle-ear space
can be detected during tympanometry.
• If the air pressure within the middle-ear space becomes negative due to a blocked
eustachian tube, tympanometry measures this negative pressure and its effect on
middle-ear compliance.
• If fluid builds up within the middle-ear space, this fluid will restrict the ability of the
ossicular chain to conduct sound to the cochlea. If small air pockets exist within the
fluid, the tympanogram will indicate the negative pressure where the restricted
mobility occurs. With a totally fluid-filled middle-ear space, no mobility will be
measured during tympanometry at any pressure value.
• In the case of a “glue-ear”, the ossicular chain is restricted in mobility. This
tympanogram would depict a flat line with no identifiable pressure peak.
Gradient
Gradient (width) measurements are used to describe the shape of a tympanogram near
the peak. Often, the presence or absence of fluid in the middle ear is not clearly indicated
by otoscopy and tympanometry alone. This evaluation is especially difficult when the peak
pressure is within the normal range.
The presence of fluid within the middle-ear space alters the shape of a tympanogram (i.e.,
makes the tympanogram wider near its peak). A larger-than-normal gradient can indicate
the presence of fluid in the middle ear when other parameters are within normal limits. In
this way, the gradient acts as an adjunct to the tympanogram and ear canal volume
Note 1.02 mmH
2
O = 1.0 daPa