A transducer system was developed for measuring the closing pressure profile along the female urethra, which provides up to five sensors within the functional length of the urethra. This new development is an improvement over an earlier measurement method that has a smaller sensor area and was unable to respond to transient events. Three sensors were constructed; one of them was subjected to approximately eight hours of use in a clinical setting during which 576 data points were obtained. The complete instrument system, including the signal conditioning electronics, data acquisition unit, and the computer with its display and printer is described and illustrated

Moore, Thomas

English

NASA Technical Reports Server

N91-23066
URODYNAMIC PRESSURE SENSOR
Thomas Moore
Aerospace Engineering Technician
NASA Langley Research Center
Introduction
A widespread method for measuring the closing pressure profile along the female urethra is to insert a
sensor into the bladder and withdraw it at a controlled rate such that time translates into distance and sensor
output or pressure is the dependent variable. A second sensor element, located distally from the first,
remains in the bladder to provide a reference pressure. By synchronizing the speed of an analog strip chart
recorder with the withdrawal rate of the sensor, one obtains a measure of the pressure profile along the
urethral length. Two aspects of this method are controversial, first, because the sensor area is quite small, it
includes only a relatively small portion of the total circumference giving rise to questions regarding
circumferential sensitivity, and, second, as the sensor is being withdrawn, transient events, coughs, muscular
impulses, etc., cannot be duplicated over time in order to examine the response of the total organ to a single
event. With these concerns in mind, a transducer system has been developed which provides up to five
sensors within the functional length of the urethra, addressing the simultaneity issue, and where each sensor
has an axial symmetry, thereby addressing the circumferential sensitivity issue. Three sensors have been
constructed and one of them has been subjected to approximately eight hours of use in a clinical setting
during which 576 dynamic data points were obtained. The complete instrument system including the signal
conditioning electronics, data acquisition unit, and the computer with its display and printer is shown in
Figure 1. The technical details of the catheter sensor system itself is described in this paper.
Transducer System Design
Requirements:
The requirement for the measurement system, as originally stated, was to be able to measure the closing
forces along the continuous female urethra, to include an internal lumen for infusion or venting of the
bladder, be very flexible and less than three-French in outside diameter, sense pressures in the range of 10 to
200 centimeters of water, and to be integrated into a computer system to facilitate the data reduction
process. Continuous sensing of the closing forces within the functional length of the urethra was deemed
impractical and was changed to "as many discrete measurement points as is practical." This translated into a
cylindrical transducer structure having five sensing areas, each area approximately 0.2 inches long and within
a length of one and one-half inches, approximately one-eighth inch in diameter, hollow, in order to contain
an internal lumen, and a sixth sensing element located approximately one and one-half inches from the
others. The resulting transducer system, without its protective sheath, is shown in Figure 2. Here one sees
the five closely spaced sensor elements and a sixth element located some distance from the others to be
always in the bladder and thus provide a reference. With this arrangement, discrete but simultaneous
measurements are obtained throughout the urethra. Figure 3 is an enlarged view of one sensor element and
is seen to be shaped like a "C'. As pressure is applied around the circumference of this element
displacement of the two halves (tending to close the longitudinal gap) results causing bending stresses to
appear in the spine portion. These stresses cause minute deformations in the outer fibers of the metal which
are then sensed with strain gages.
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Sensor Design
Each sensor dement is cylindrical, nominally 0.120" diameter by 0.118" long by 0.004" thick; has a
slot approximately 0.010" wide cut in the axial direction; and has slots machined transverse to the axis,
leaving approximately 0.040" of circumferential area to act as a spine. To obtain a quantitative
relationship for the sensor, first convert pressure in centimeters of water to pounds per square inch:
Ppsi ---0.014215 x Pcm-H20
Therefore:
Pmin = 0.142psi and Pmax = 2.843 psi
This distributed pressure acting over the movable portion of the circumference can be represented as an
equivalent force acting at the midpoint of each segment which then produces a bending moment at the
root or spine. Young's modulus is then used to relate stress to swain as follows:
F=PxA
M=FxL/2
_ hlrdI
_--s/E
P = Pressure and A = Area
L -- Length and M = Root Bending Moment
s = stress, c = distance to outer fiber
I = Moment of area of sensor segment
e = strain, E = Young's Modulus
Combining the applicable constants and values gives the following expression for strain as a function
of pressure tending to close or compress the sensor:
e = 2.378 x 10-6 x Pcm
This being the expression for mechanical strain at the root of the movable section, strain gages are
placed in this area to convert the minute mechanical deformations into electrical signals proportional to
strain. At the minimum pressures anticipated, the swains are very small, and every effort must be made
to minimize extraneous effects and to enhance the signal at the point of inception. This is
accomplished by placing a full, active four-arm Wheatstone bridge at the measurement site. In this
manner, two gages are mounted in each half of the "C" section, one mounted in the circumferential
direction or maximum strain direction, and the other mounted normal to the fast. Connected in this
fashion, two gages sense the compressive strains present on the inner surface; and two gages, mounted
at 90 degrees to the others, measure the Poisson's ratio strains. Significantly, all gages see the same
environment which minimizes the effects of extraneous inputs.
The above analysis must be qualified somewhat, acknowledging the finite size of the strain gages and
the presence of an elastomeric sheath surrounding the final sensor. Finite size of the gages means that
the entire gage does not see the level of strain as shown above, but rather an average over its sensitive
area; and the sheath is a non-linear elastic member spanning the measuring section which effectively
presents some load-carrying capacity in parallel with the measuring "C" sections. The non-linear
effects are assumed small at this point, and the gage installation effects are accommodated through
calibration.
The usable output voltage or signal from the strain gage bridge is dependent upon the gage factor, the
number of active bridge arms, and the excitation or bridge voltage. The present configuration consists
of two active arms (plus two Poisson's ratio arms) with a nominal gage factor of two. Semiconductor
strain gages exhibit gage factors on the order of 120, sixty times that of the foil gages used, which
would significantly improve the signal-to-noise ratio; however, semiconductor gages require a relatively
flat surface because of their brittleness. In this application, the tight radius of curvature precludes their
use. Utilizing a large, bridge excitation voltage will also maximize the output and signal-to-noise-
ratio; however, one must always be cognizant of the self heating of the gage caused by 12Rlosses
which introduces long-term drift of the system. Combining all the applicable factors, one arrives at the
following expression for sensor output voltage:
364
Output Voltage -'4 x Bridge Voltage x e
which for a bridge excitation voltage equal to 2 Volts,
Output Voltage -' 19 x 10 -6 Pcm
From this it is seen that the range of output voltages from the transducer is from approximately 190 to
3800 lxvolts. Most acquisition or display devices require signals in the +2.5 volt range. Thus it is
obvious that a significant amount of amplification or gain in the system is required.
Signal Conditioning
Signal conditioning consists of individual channel signal isolation, offset zero and gain adjustments,
and low pass f'dtering of the data. The data is passed through isolation amplifiers and filtered to pass
frequencies below 150 Hertz. The overall gain is adjustable and typically on the order of 250 to 300
making full scale signals approximately 1 Volt. The data is then passed to a sampling and analog-to-
digital conversion subsystem where the data is quantized to 12-bit precision, and 250 samples from
each channel are accumulated at a 100 samples-per-second rate. These records are then stored on disk for
post test analysis.
Transducer Calibration
A hydrostatic calibrator was constructed which consisted of a sealed chamber in which the catheter could
be horizontally placed and water allowed to enter, expelling any air from the chamber. The chamber is
connected via a flexible hose to an accumulator which can be raised or lowered to change the pressure in
the chamber. Once the air is completely expelled from the system, the pressure is allowed to
equilibrate with the level of water in the accumulator adjusted to the level of the catheter. At this
point, the channels are adjusted for zero output after which the accumulator is raised to 100 centimeters
and the gain adjusted to some convenient value. Iteration between the zero level and the 100cm level a
few times and adjusting the zero and gain controls at each end will remove any interactions, and a
convenient conversion factor, say 200 cm/volt, is established. By adjusting the height of the water
column in a definite sequence, one can determine the drift, hysteresis, and linearity. Because pressure is
a scaler quantity, it exerts the same influence in all directions making a good calibration of the
instrument to measure circumferential closing forces.
System Output
As stated previously, the data is accumulated on a hard disk in the computer making it possible to
perform any number of analysis procedures. Figures 4a and 4b illustrate two different formats that were
used up to this point. Figure 4a includes test parameter and system information as well as a plot of the
actual time history values, whereas figure 4b is merely a full-page representation of the particular time
history. The two figures are from two different subjects, and one can see distinct differences in the
shape of the time history.
365
CONCLUDINGREMARKS
Based on the needs expressed at the outset, it is concluded that a sensor system has been developed
which captm_s the essence of the requirements. Simultaneous measurement, at several discrete
locations within the urethra, and insensitive, though not yet proven absolutely, to circumferential
positioning of the catheter are features of the design. As a result of the clinical testing, a new catheter
has been fabricated which is physically shorter and has the segments positioned closer together. This
allows more sensor elements to be positioned within the effective length of the urethra, potentially four
of the five as opposed to three for functionally shorter urethral lengths. Previously, the medical
specialist was interested primarily in the amplitude at the respective element location and its ratio to
the bladder pressure. With the new sensor and the associated data acquisition and display capability, not
only amplitudes but also shape, rise and fall rimes, and muscular response propagation times are new
parameters upon which diagnostic evaluations can be made.
366
ORIGINAL PAGE
BLACK AND WHITE PHOTOGI_APH
Figure 1: Signal Conditioning and Data Acquisition
Instrumentation.
367
ORIGtr'JAL Pr_,Gc.
BLACK AND WHITE PHOTOGI_AP;._
Figure 2: Six Element Catheter without Sheath
Figure 3: Enlarged View of Single Element
368
kfaf i Is Nmm: Unslmc i_ ie_!
Urn.siGn _: 8
Signal Nmm: UnslM_if isd
hie Acquired: 11-23-1988
Tim _:quir,ed: 9:89:58._
Units: Peessu_
Hm.iz t_its: gecs
Itm. kspIN: 258
,_np 1o Pate: 188
J%ximm: 192.7%882
Minlmm: 89.987793
Cmawnts: Signal Gnm.at_ in
COUGH L|-I CHNLq/ |tNOV$$
: '• i i i • •...............................iO0,0 .....................................
,,o,oi........: .....i........: ....: ../ .....i_ ........i ...._ ...._ ...._ ...._
.o,o j ........ :........ .........:........ , ...... ,_"! ...... :.....
.o ,o ._........ :........ !........ i........ i' ........ :........ :........ :........ " ........ :........ :...... " ........ "
i _' ; i I" i ; t' : i i i
0,0 0.1 O,q O,t O,e l,O 1.1 t,q t,¢ t,O 2,0 t,| |,_
Figure 4a: Output Display, Format No. 1
COUGH 3-3 CHHL½/ 09J,_N$_I
i i ! i i/ ! i ! : :
_oo.o..........i ...: .....i ....: ......i ._i/""li'......i ......: ...: ......: ....i '
,,oo.................................i........i (: :...: !,,,::.i..... ..........i i .
•.,...-..,%. . , . : ' .
,oo.o./ :"....'_'_*_-_-_,_,_:.......................... :.........i ......
I i _ i i , i i ," :' i i
_,o ,:J,_ 0,_' 0,6 0,'3 t,0 i,_ t,Lt L,_ t,_ 2,0 :>,_ Z, tt
Figure 4b: Output Display, Format No. 2
369


Urodynamic pressure sensor

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