Pressure measurement device

A device measures pressures in animals and humans and includes a pressure transmission catheter (PTC) filled with a pressure transmitting medium and implantable in an area in having a physiological pressure. A transducer conmmunicates with the pressure transmitting medium to provide a pressure signal representing variations in the physiologic pressure on electrical wires. A connecting catheter carries the electrical wires to signal processing and telemetry circuitry, which transmits a telemetry signal representing the pressure signal to a receiver external to the animal or human. A housing holds the signal processing and telemetry circuitry, but the transducer is remote from the housing. The device is particularly useful in measuring venous pressure, pulmonary pressure, bladder pressure, or intracranial pressure without significant head pressure artifact and with a sufficient dynamic response. One embodiment of the PTC includes a multi-durometer stem.

THE FIELD OF THE INVENTION 
The present invention relates generally to implantable devices, and in 
particular to implantable devices for measuring various physiological 
pressures in humans or animals, such as blood pressure, intracranial 
pressure, bladder pressure, and pulmonary pressure. 
BACKGROUND OF THE INVENTION 
Measurement of physiological pressures is of interest to both clinicians 
and researchers. Physiological pressure measurements obtained from 
laboratory animals provide researchers with valuable information regarding 
the safety and efficiency of pharmaceutical agents, and the toxicity of 
chemicals, and leads to better understanding of human physiology. 
Physiological pressure measurements also have human clinical values, such 
as providing diagnostic information, assessing the safety and efficiency 
of drugs in clinical trials, and controlling implantable medical devices, 
such as pacemakers. 
Arterial blood pressure is of particular interest to the researcher or 
clinician, because arterial blood pressure fluctuates over time in 
response to various conditions, such as an injection of pharmaceutical 
agent or chemical, or the activity level of an animal being observed. 
Arterial blood pressure fluctuations, however, often make it necessary to 
obtain chronic, frequent measurements to identify the effect of the 
injected pharmaceutical agent or chemical, or to properly control an 
implantable medical device. 
In addition to arterial blood pressure, other pressure measurements are 
also of interest, such as venous pressure, pulmonary pressure, 
intracranial pressure, bladder pressure, intrauterine pressure, 
gastro-intestinal pressure, and other physiological pressures. For 
example, intrapleural or blood pressure can be used to determine the rate 
of respiration in addition to providing general information related to 
respiratory function. Measurements of intracranial pressure from 
laboratory animals are often used to project which methods of treatment 
and management are most effective in humans. 
Chronic measurement of physiological pressures provides vital information 
for clinical care of humans. Patients with high blood pressure could 
benefit from an implantable device which could chronically monitor 
pressure as a means of determining optimal dosage for a drug or 
biofeedback therapy. Such a device could also be used as a means of 
providing feedback to a closed-loop drug delivery system for controlling 
blood pressure, or to a cardiac pacemaker as a means of optimizing pacing 
control parameters. 
Infants who have been identified as being at risk for sudden infant death 
syndrome could also benefit. It is desirable to monitor changes in 
intrapleural pressure as a reliable measurement of respiratory rate in 
these infants by means which would allow the infant to roll and move 
freely about its crib without being restrained by wires extending from a 
vest. 
Chronic monitoring of intracranial pressure is also important for infants 
with hydrocephalitis and patients with head injury. Hydrocephalitis and 
head injuries can cause excessive pressure buildup within the brain, 
resulting in death or serious brain damage. In most cases, corrective 
action can be taken if the buildup of pressure can be quickly detected. 
This need to obtain accurate and ongoing physiological pressure 
measurements within various parts of animals and humans is discussed in 
detail in the Brockway U.S. Pat. No. 4,846,191 assigned the assignee of 
the present application, and which is herein incorporated by reference. 
The Brockway et al. `191 patent discloses a pressure measurement device 
for monitoring physiological pressures, such as blood pressure, in various 
locations in an animal or human. The pressure measurement device utilizes 
a fluid-filled pressure transmission catheter (PTC) with a gel membrane 
located at a tip of the PTC. The tip of the PTC is positioned in an area 
where physiological pressure is to be measured. The PTC extends from a 
small implantable housing that contains a transducer, signal-processing 
and telemetry circuitry, and a battery. The fluid-filled PTC communicates 
the pressure from the area where pressure is to be measured to the 
transducer within the housing, which generates an electrical pressure 
signal representing the communicated pressure. The signal-processing and 
telemetry circuitry in the housing receives the pressure signal generated 
by the transducer and provides a telemetry signal representing the 
pressure signal. The signal-processing and telemetry circuitry transmits 
the telemetry signal to a receiver which is external to the animal or 
human. 
In some applications of the pressure measurement device disclosed in the 
Brockway et al. `191 patent, the housing cannot be implanted within close 
proximity to the area where pressure is to be measured due to physical 
limitations and practical considerations of surgical procedures. When the 
housing is not within close proximity to the area where pressure is to be 
measured, the length of the catheter that is required may be too long to 
assure that errors, resulting from decreased dynamic response or changes 
in posture, be within acceptable limits for the given application. For 
example, if the vertical distance from the PTC tip to the transducer 
changes due to posture, an error in the pressure measurement occurs. Every 
one centimeter change in vertical distance creates approximately one 
millimeter Hg error in the pressure measurement for one preferred 
low-viscosity fluid used in the catheter. This pressure measurement error 
is known as "head pressure artifact" and is very significant in certain 
applications. Furthermore, as the length of the PTC increases, the dynamic 
response of the pressure measurement device is reduced. In certain 
applications, the required length of the PTC is so long that a sufficient 
dynamic response cannot be obtained. 
In addition, the Brockway et al. `191 patent does not disclose a 
pre-compensated, disposable, and easily replaceable transducer. Rather, 
since the transducer is inside the housing, if the transducer disclosed in 
the Brockway et al. `191 patent fails, the entire pressure-sensing device 
must be returned to the manufacturer for replacement to ensure proper 
compensation, mounting of the transducer, and sealing of the implant body. 
For reasons stated above and for other reasons presented in greater detail 
in the Description of the Preferred Embodiments section of the present 
specification, there is a need for a pressure measurement device that is 
capable of measuring pressures in more animal and human applications, with 
better dynamic response, and with more accurate pressure measurements than 
currently possible with present pressure measurement devices. In addition, 
it is desired that the transducer and catheter of the pressure measurement 
device be more easily replaceable than currently possible with present 
pressure measurement devices. 
SUMMARY OF THE INVENTION 
The present invention provides a pressure measurement device which measures 
physiological pressures in animals and humans. The pressure measurement 
device includes a pressure transmission catheter filled with a pressure 
transmitting medium and implantable in an area having a physiological 
pressure. A transducer is in communication with the pressure transmitting 
medium to provide a pressure signal representing variations in the 
physiologic pressure on electrical wires. A connecting catheter carries 
the electrical wires to signal processing and telemetry circuitry, which 
receives the pressure signal and provides a telemetry signal representing 
the pressure signal. A housing holds the signal processing and telemetry 
circuitry. The transducer is remote from the housing. 
The pressure transmission catheter preferably has a length short enough to 
avoid significant head pressure artifact and to provide sufficient dynamic 
response, but long enough to accommodate surgical limitations and 
tolerance concerns. For example, depending on the particular application 
of the pressure measurement device, the pressure transmission catheter 
typically has a length somewhere in the range from approximately five 
millimeters to approximately four centimeters. In most applications, the 
pressure transmitting medium comprises a gel and a liquid. Nevertheless, 
because the present invention permits the pressure transmission catheter 
to be significantly shorter than previously possible, in certain 
applications, the pressure transmitting medium includes only a gel. In one 
embodiment, the transducer is integral with the pressure measurement 
catheter to form a transducer-tipped catheter. 
The pressure measurement device according to the present invention can be 
employed to accurately measure low pressure where head pressure artifact 
can constitute a significant percentage of the pressure being measured. 
These pressures include: venous pressure; pulmonary pressure; intracranial 
pressure; bladder pressure; and other pressures. The pressure measurement 
device measures these pressures without significant head pressure artifact 
and with a sufficient dynamic response. 
The transducer is preferably pre-temperature compensated and disposable. In 
this way, the transducer, which is external to the housing, can be easily 
replaced without replacing the entire pressure measurement device. In many 
applications of the pressure measurement device, the housing is 
implantable remote from the area having the physiological pressure. 
In one form of the invention, the pressure transmitting catheter includes a 
lumen filled with the pressure transmitting medium. An inner layer 
material surrounds the lumen and an outer layer material surrounds the 
inner layer material. The outer layer material is of a different hardness 
than the inner layer material. In a preferred embodiment, the inner layer 
material is harder then the outer layer material. Preferably, the harder 
layer material essentially determines the frequency response of the 
pressure transmitting catheter so that compared to a catheter fabricated 
of only softer material, the catheter of the present invention provides 
improved frequency response. Preferably, the softer layer material makes 
the pressure transmitting catheter more flexible and kink resistant 
compared to a catheter fabricated of only harder material. A transition 
between the inner layer material and the outer layer material can be a 
sharp transition or a gradient transition. In one embodiment, the inner 
layer material comprises 72 D urethane and the outer layer material 
comprises 80 A urethane. 
The pressure measurement device according to the present invention achieves 
more accurate measurement of physiological pressure and can be employed in 
many new applications for pressure measurement in animals and humans. The 
pressure measurement device according to the present invention obtains 
high-fidelity measurements with negligible head pressure error in 
applications where the distance from the distal tip of the pressure 
transmission catheter to the transmitter is such that significant head 
pressure errors could occur with conventional devices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following detailed description of the preferred embodiments, 
reference is made to the accompanying drawings which form a part hereof, 
and in which is shown by way of illustration specific embodiments in which 
the invention may be practiced. It is to be understood that other 
embodiments may be utilized and structural or logical changes may be made 
without departing from the scope of the present invention. The following 
detailed description, therefore, is not to be taken in a limiting sense, 
and the scope of the present invention is defined by the appended claims. 
A pressure measurement device according to the present invention is 
illustrated generally at 20 in FIG. 1. Pressure measurement device 20 is a 
miniature implantable device capable of measuring internal physiological 
body pressure in humans or animals. The fundamental principles by which 
pressure measurement device 20 measures pressure are described in detail 
in the Brockway et al. `191 patent, which was incorporated by reference in 
the Background of the Invention section of the present specification. 
Therefore, for clarity, many of the features of pressure measurement 
device 20 which are similar to the pressure measurement device described 
in the Brockway et al. `191 patent are not discussed herein. However, the 
features which differ from the pressure measurement device described in 
the Brockway et al. `191 patent are discussed in detail below. 
Pressure measurement device 20 includes a pressure transmission catheter 
(PTC) 22 having a distal lumen tip 24 which is positioned within the body 
of a human or animal at the site where pressure is to be measured. One 
embodiment of PTC 22 is flexible, while another embodiment of PTC 22 is 
rigid, and the particular embodiment of PTC 22 selected depends on the 
given application of pressure measurement device 20. PTC 22 is filled with 
a pressure-transmitting medium 26 which communicates the pressure at the 
distal tip 24 of PTC 22 to a proximal lumen end 28 of PTC 22. Thus, a 
portion of pressure-transmitting medium 26 at distal tip 24 interfaces 
with the substance in the body area where pressure is to be measured, such 
as with blood in an artery. 
A transducer 30 is in communication with pressure-transmitting medium 26 at 
the proximal end 28 of PTC 22. Transducer 30 is contained in a transducer 
housing 32. Transducer 30 responds to variations in the 
pressure-transmitting medium at proximal end 28 to provide an electrical 
pressure signal representing variations in the physiological pressure at 
distal tip 24 on electrical lead wires 34. Electrical leads wires 34 are 
carried within a connecting catheter 36. By separating PTC 22 from 
connecting catheter 36, the length of connecting catheter 36, indicated by 
arrows 48, can be independently determined from the length of PTC 22, 
indicated by arrows 50. The length of connecting catheter 36, indicated by 
arrows 48, varies from zero to very long, depending on the application of 
pressure measurement device 20. 
Electrical lead wires 34 are coupled to an electronics module 38 of a 
transmitter 40. Electronics module 38 is powered by a battery 42. Battery 
42 and electronics module 38 are contained within a transmitter housing 
44. The electronics module 38 includes signal-processing and telemetry 
circuitry and a transmitting antenna for generating and transmitting a 
telemetry signal representing the pressure signal from transducer 30 to an 
external receiver (not shown) disposed outside of the human or animal. The 
electrical pressure signal produced by transducer 30 is amplified and 
filtered with the signal-processing circuitry in electronics module 38 and 
is then modulated onto a radio-frequency carrier by the telemetry 
circuitry in electronics module 38 for transmission to the external 
receiver. A suitable telemetry system is described in the Brockway et al. 
`191 patent and the patent application entitled "Respiration Monitoring 
System Based on Sensed Blood pressure Variations," Ser. No. 08/535,656, 
filed Sep. 28, 1995, assigned to the assignee of the present application, 
and which is herein incorporated by reference. 
In one embodiment of pressure measurement device 20, connecting catheter 36 
is coupled to transmitter 40 with a water-tight or non-water-tight 
connector 46. In this embodiment, the electrical lead wires 34 contained 
connecting catheter 36 interface with electronics module 38 via connector 
46. Water-tight or non-water-tight connector 46 permits an assembly 47 
including connecting catheter 36, transducer 30, and PTC 22 to be 
manufactured and sold separately from transmitter 40. Connector 46 also 
provides more flexibility for manufacturing assembly 47 and transmitter 40 
and additional flexibility for customers in selecting lengths of 
connecting catheter 36 and PTC 22 independent from transmitter 40. 
One embodiment of PTC 22 is illustrated in more detail in FIG. 2A. In this 
embodiment of PTC 22, a viscous gel membrane 52 is disposed at the distal 
tip 24 of PTC 22. A thin-walled section 54 defines an open cavity 56. A 
stem 55 of PTC 22 runs from the thin-walled section 54 to proximal end 28 
of PTC 22, as illustrated in FIG. 1. As illustrated in FIG. 2A, the gel 
membrane 52 is contained in a distal portion of open cavity 56. Open 
cavity 56 is connected to a lumen 58 of PTC 22. The portion of open cavity 
56 not filled with viscous gel 52 and lumen 58 are filled with a 
low-viscosity fluid 60. In this way, physiological pressure is transmitted 
from distal tip 24 of PTC 22 through the walls of the PTC and via viscous 
gel 52 contained within thin-walled section 54 to the low-viscosity fluid 
60 which communicates pressure directly to transducer 30 at the proximal 
end 28 of lumen 58. The low frequency components of the physiological 
pressure are essentially transmitted via viscous gel 52 while the high 
frequency components of the physiological pressure are essentially 
transmitted through the walls of the PTC. 
In one embodiment, PTC 22 is fabricated of a urethane material or other 
suitable biocompatible material. Viscous gel membrane 52 is a 
biocompatible and blood-compatible gel or other gel-like material that 
provides a direct interface with the tissue or fluid from which pressure 
is to be measured, such as blood in an artery. Viscous gel 52 provides a 
means of retaining fluid within lumen 58 and is of a viscosity much higher 
than that of low-viscosity fluid 60. Viscous gel 52 can be comprised of 
any material which is capable of flowing or moving within PTC 22 as does a 
viscous fluid or a plug that can slide or deform easily and contains 
intramolecular forces which make it very unlikely that any portion of this 
material will dissolve, break apart, slough off, or wash away when 
measuring physiological pressure within a human or animal. Viscous gel 52 
must be viscous enough not to wash out of PTC 22, but also must be low 
enough in viscosity that it can "flow" without significant pressure 
differential. In one embodiment of the invention, viscous gel 52 is a 
silicone gel which contains cross-linked molecular entities. 
Low-viscosity fluid 60 preferably has a minimal biological activity (in 
case of failure of a seal), has a low thermal coefficient of expansion, is 
insoluble in gel 52, has a low specific gravity, has a negligible rate of 
migration through the walls of PTC 22, and has a low viscosity at body 
temperature. In one embodiment, low-viscosity fluid 60 is an inert 
perfluorocarbon. 
In other embodiments of the pressure measurement device according to the 
present invention, PTC 22, which can be rigid or flexible, is very short, 
and can be as short as approximately 2 mm long. One such embodiment is 
illustrated in FIG. 2B. In the embodiment illustrated in FIG. 2B, since 
the length of PTC 22 is very short, PTC 22 is typically filled entirely 
with viscous gel 52 (i.e, the low-viscosity fluid 60 is not used), but 
still provides a sufficient dynamic response. 
The thin-walled section 54 reduces movement of viscous gel 52 during events 
that change either the volume of low-viscosity fluid 60 or the internal 
volume of lumen 58 of PTC 22, such as occurs during thermal expansion and 
contraction, bending, and hydration of the catheter material of PTC 22. 
Reducing the degree of displacement of gel 52 during bending of PTC 22 has 
the effect of reducing measurement artifact that can occur during normal 
movement of the human or animal into which pressure measurement device 20 
is implanted. Reducing the degree of displacement of gel 52 during bending 
of PTC 22 reduces the amount of dead space within PTC 22 and beyond gel 
52, and therefore, contributes to improved patency in blood. Thin-walled 
section 54 also improves the frequency response of PTC 22 by providing a 
means by which to transfer high-frequency components of the pressure 
signal into lumen 58 through the compliant thin walls of the tip. 
Two additional embodiments of a PTC 22 are illustrated in FIGS. 2C and 2D. 
In these embodiments, PTC 22 does not include an open cavity 56 defined by 
a thin-walled section 54, but instead, the small diameter portion of lumen 
58 runs all the way to the distal tip 24. This embodiment can be used in 
certain applications where the above advantages of having such a 
thin-walled section are not as significant to obtaining satisfactory 
pressure measurements. In the embodiment of PTC 22 illustrated in FIG. 2C, 
a viscous gel membrane 52 is disposed at the distal tip 24 of PTC 22 with 
the remainder of lumen 58 being filled with a low-viscosity fluid 60. In 
the embodiment of PTC 22 illustrated in FIG. 2D, the PTC is filled 
entirely with viscous gel 52 (i.e., the low-viscosity fluid 60 is not 
used). 
A more detailed diagram of one embodiment of transducer 30 and the coupling 
of transducer 30 to PTC 22 and to connecting catheter 36 is illustrated in 
FIG. 3. As illustrated, PTC 22 is attached to transducer 30 via a nipple 
62. In one embodiment of the invention, transducer housing 32 comprises a 
hermetic titanium housing. Transducer 30 is contained within a sealed 
chamber 64. Sealed chamber 64 protects transducer 30 from body fluids. 
Electrical connections from transducer 30 are coupled to a circuit board 
68. Circuit board 68 includes circuitry 67 employed for 
temperature-compensating transducer 30. In an alternate embodiment, 
temperature of transducer 30 is measured by a sensor 69 on circuit board 
68 and a remote computing device (not shown) employs these temperature 
measurements to temperature-compensate transducer 30. Electrical 
connections from circuit board 68 pass out of sealed chamber 64 via 
glass-metal seals 70 to thereby connect to electrical lead wires 34 
contained within connecting catheter 36. 
Although transducer 30 is typically smaller than the transducer employed in 
the pressure measurement device described in the Brockway et al. `191 
patent to permit transducer 30 to be disposed remote from the transmitter 
housing 44, the general operation and construction of a suitable 
transducer 30 is described in detail in the Brockway `191 patent. 
An alternative embodiment pressure measurement device 120 is partially 
illustrated in FIG. 4. Pressure measurement device 120 is similar to 
pressure measurement device 20 illustrated in FIG. 1. However, PTC 22 
couples to transducer housing 32 at a right angle in pressure measurement 
device 120. This is made possible with a nipple 162 which is L-shaped to 
receive the proximal end 28 of lumen 58 of PTC 22 to couple the 
low-viscosity fluid 60 to transducer 30. This right-angle embodiment is 
only one of many examples of the great flexibility provided by having PTC 
22 separated from connecting catheter 36 and having transducer 30 being 
remote from transmitter housing 44. 
In the preferred embodiment of the pressure measurement device described in 
the Brockway et al. `191 patent, a transmitter housing houses an 
electronics module, a battery to power the electronics module, and a 
transducer. For some applications, locating the transducer within the 
transmitter housing creates certain disadvantages to sensing accurate 
pressure. For example, for one preferred low-viscosity fluid 60, a head 
pressure error is created approximately equal to one mm Hg for every one 
cm of vertical distance between the tip of the PTC and the transducer. 
This head pressure error can be very significant relative to the pressures 
being measured in some applications. Another disadvantage is that the 
dynamic response of the PTC is inversely proportional to its length. 
Therefore, when the required length of the PTC becomes too long in some 
applications, the dynamic response is reduced to a level which is not 
sufficient to reproduce a high-fidelity waveform. Still another 
disadvantage of the preferred embodiment described in the Brockway et al. 
`191 patent is that for some combinations of material employed in the PTC, 
a greater volume of low-viscosity fluid contained in the lumen of the PTC 
results in a greater degree of thermal expansion and contraction, and 
greater degree of movement of the gel membrane. If movement of the gel 
membrane is too great, a void can develop within the tip of the PTC 
resulting in dead space thrombosis. Consequently, keeping the PTC short 
reduces the volume of low-viscosity fluid contained in the lumen of the 
PTC which contributes to improved patency in blood in some applications. 
The pressure measurement device according to the present invention, such as 
pressure measurement device 20, overcomes all of the above disadvantages 
by providing a means of shortening the required length of PTC 22 in many 
applications. This length of PTC 22 is from the distal tip 24 to the 
proximal end 28 and is indicated by arrows 50. The reduced length 50 can 
greatly reduce head pressure error and improve the dynamic response to a 
degree which is acceptable to the researcher and clinician using the 
pressure measurement device. 
A much shorter PTC is achievable because pressure measurement device 20 
disposes transducer 30 remote from transmitter housing 44. In addition, 
pressure measurement device 20 employs a connecting catheter 36 which is 
separated from PTC 22 to carry the electrical leads 34 which couple the 
pressure signal from transducer 30 to electronics module 38 of transmitter 
40. This permits the length of connecting catheter 36 and PTC 22 to be 
independently determined. 
Head pressure error is significantly reduced as the distal tip of PTC 22 
and transducer 30 can be brought much closer together in many 
applications. Since the dynamic response is inversely proportional to the 
length 50 of PTC 22, a shorter PTC 22 increases the dynamic response to 
permit reproduction of high-fidelity waveforms in many applications where 
previous pressure measurement devices, having too long of PTC, cannot 
reproduce high-fidelity waveforms. In addition, the volume of 
low-viscosity fluid 60 contained in lumen 58 of PTC 22 is reduced with the 
significantly shorter PTC 22 in some applications. The reduced volume 
results in significantly less thermal expansion and contraction and less 
degree of movement of viscous gel membrane 52. With less movement of gel 
membrane 52, voids are avoided at distal tip 24 of PTC 22 to prevent dead 
space thrombosis. The shorter PTC 22, therefore, reduces the volume of 
low-viscosity fluid 60 to thereby improve patency in blood. Moreover, as 
discussed above with reference to FIG. 2B, because PTC 22 can be very 
short in many applications, PTC 22 is alternatively completely filled with 
viscous gel 52 in certain applications and still performs acceptably. 
Since transducer housing 32 is much smaller than transmitter housing 44, 
it is possible to locate transducer 30 much closer to the pressure source 
with much less physiologic impact and better convenience from a surgical 
perspective. 
The present invention also eliminates certain logistical problems for 
pressure measurement devices in the animal market. Presently, if a 
catheter is damaged or if a customer accidentally applies too much 
pressure and bursts the transducer, it is necessary to send the entire 
pressure measurement device back to the factory. With transducer 30 
located remote from transmitter housing 44 and connected electrically to 
transmitter 40 via water-tight or non-water-tight connector 46 and the 
electrical lead wires 34 in connecting catheter 36, transducer 30 can 
easily be detached. In this way, pre-compensated transducers 30 can be 
sold to the customer as a disposable product and be attached by the 
customer without the need to return transmitter 40 for repair. 
Multi-durometer Catheter 
As discussed above, the present invention permits a significant reduction 
in the required length of PTC in certain applications. Nevertheless, there 
are many characteristics of the fluid/gel-filled PTC which are critical in 
order to assure that the pressure communicated to transducer 30 is an 
accurate representation of the physiological pressure present at the 
distal tip 24 of PTC 22. In particular, if PTC 22 is not capable of 
transmitting high-frequency components of the physiological pressure at 
distal tip 24 to transducer 30, pressure information which is not 
transmitted causes an inaccurate representation of the physiological 
pressure to be produced by transducer 30. 
Physical characteristics which affect the ability of PTC 22 to accurately 
transmit the physiological pressure include: the viscosity of the fluid 
within the PTC; the surface area of thin-walled section 54 that is exposed 
to low-viscosity fluid 60; the compliance of the walls of the PTC; the 
inner diameter of the PTC; and the length of the PTC. As to the viscosity 
of the fluid, viscosity of the fluid is dependent upon available materials 
and is to a large extent out of the control of the designer. As discussed 
above, the present application describes a pressure measurement device 20 
having transducer 30 disposed between PTC 22 and transmitter 40 where 
connecting catheter 36 carries wires to couple the pressure signal from 
the transducer to the transmitter. This significantly reduces the required 
length of the PTC in certain applications. The inner diameter of the PTC 
and the compliance of the walls of the PTC are two factors that greatly 
affect the fidelity of the measured pressure. Once a critical inner 
diameter is reached, further reduction results in a rapid drop in 
frequency response of the PTC. The frequency response of the PTC improves 
as the compliance of the PTC walls is lowered (i.e., the stiffness of the 
walls is increased). The compliance of the walls is a function of the 
materials employed to fabricate the PTC, the construction of the PTC, and 
the thickness of the PTC walls. 
In current commercially available pressure measurement devices having 
fluid-filled PTCs to refer pressure from a point of interest to a 
transducer, the outer diameter of the PTC is sufficiently large to permit 
the inner diameter to be sufficiently large and the PTC walls to be 
sufficiently thick to provide a sufficient frequency response. However, 
when a physiological pressure is measured in very small vessels, such as 
those in mice or in human coronary arteries, the required PTC outer 
diameter typically ranges from approximately 0.014 to 0.022 inches. In 
such a PTC, the inner diameter of PTC stem 55 must be very small (e.g., 
less than approximately 0.008 inches) and the walls of the PTC stem must 
be very thin (e.g., less than approximately 0.005 inches). The very small 
inner diameter of PTC stem 55 and the thin walls of the PTC stem result in 
reduced frequency response when a flexible thermoplastic is used to 
fabricate the PTC. 
One approach to solve the frequency response problem caused by thin PTC 
stem walls and a very small diameter PTC stem is to fabricate PTC stem 55 
of a hard material, such as 75 Shore D urethane. However, a PTC stem 
fabricated of this hard material is too stiff to be handled easily during 
surgery and kinks too easily. Another approach to improve the frequency 
response of a PTC stem having a very small inner diameter and thin walls 
is to reduce the compliance of the walls by winding a wire in a helix 
around the PTC stem. Nevertheless, this wire-around approach is expensive 
to manufacture and is very difficult to do in a reliable manner when the 
wall thickness of the PTC is less than 0.004 inches. 
A cross-section of a multi-durometer catheter according to the present 
invention is illustrated generally at 200 in FIG. 5. Multi-durometer 
catheter 200 forms the stem portion of a fluid-filled catheter (PTC), such 
as the stem 55 of PTC 22 described above. This stem portion transfers 
pressure from the tip of the PTC to the transducer. 
Multi-durometer catheter 200 includes a lumen 202. Lumen 202 is surrounded 
by an inner layer of harder material 204. The inner layer of harder 
material 204 is surrounded by an outer layer of softer (more compliant) 
material 206. The transition between the inner layer of harder material 
204 and the outer layer of softer material 206 is defined by an interface 
208. The interface 208 can be a gradient interface to gradually transition 
from the inner layer to the outer layer or can alternatively be a sharp 
transition interface between the inner layer and the outer layer. 
Multi-durometer catheter 200 has an outer diameter indicated by arrows 210 
and defined by an outer surface 212. The outer diameter indicated by 
arrows 210 is typically in the range from approximately 0.014 to 0.022 
inches. A wall 214 of catheter 200 is formed by the inner layer of harder 
material 204 and the outer layer of softer material 206. With the outer 
catheter diameter being in the range of approximately 0.014 to 0.022 
inches, the inner diameter of lumen 202, as indicated by arrows 216, is 
typically less than approximately 0.008 inches and the thickness of wall 
214, indicated by arrows 218, is typically less than approximately 0.005 
inches. In one embodiment of a multi-durometer catheter 200 having such 
dimensions, the thickness of the inner layer of harder material 204, 
indicated by arrows 220, is less than approximately 0.002 inches. Thus, in 
this embodiment the thickness of the outer layer of softer material 206, 
indicated by arrows 222, is approximately less than 0.003 inches. 
The type of satisfactory harder material 204 and softer material 206 which 
can be employed to fabricate catheter 200 vary depending the particular 
implementation and application of the PTC. In one embodiment, the inner 
layer of harder material 204 comprises 72 D urethane with the outer layer 
of softer material 206 comprising 80 A urethane. With this embodiment of a 
5 cm long multi-durometer catheter 200, having the above dimensions, for a 
hydrated catheter at 37.degree. C., the drop in frequency response is only 
approximately 1 dB at 100 Hz. In some applications, there can possibly be 
delamination between the two durometers of urethane represented by inner 
layer of harder material 204 and outer layer of softer material 206. This 
delamination problem is solved by creating a chemical bond between inner 
layer of harder material 204 and outer layer of softer material 206 during 
a real co-extrusion. 
Multi-durometer catheter 200 is inexpensive and easy to fabricate using a 
true co-extrusion technique or by extruding the outer layer over the inner 
layer by making a second pass through the extruder. The true co-extrusion 
technique typically obtains a gradient transition between the inner layer 
and outer layer and the two pass extrusion technique typically obtains a 
sharp transition between the inner layer and outer layer. 
The multi-durometer catheter 200 according to the present invention 
provides a means of producing a good frequency response in a thin-walled, 
very small-diameter catheter while maintaining excellent flexibility. The 
compliance of catheter 200 is primarily derived by the properties of the 
harder material 204 to produce sufficient frequency response. 
Nevertheless, the layer of softer material 206 makes catheter 200 kink 
resistant and flexible. Thus, catheter 200 is easily handled and kink 
resistant during surgery. In applications where a flexible catheter 200 is 
not required, a rigid multi-durometer catheter 200 still provides better 
frequency response than conventional rigid single-durometer catheters. 
In one embodiment of multi-durometer catheter 200, inner layer of harder 
material 204 includes a radiopaque material. For example, in one 
embodiment, the inner layer of harder material 204 includes an 
approximately 20-30% barium filled 72 D urethane. One problem with 
introducing a radiopaque material into a single-durometer catheter is that 
radiopaque materials are often thrombogenic. This problem is solved with 
this embodiment of multi-durometer catheter 200, because outer layer of 
softer material 206 comprises only non-thrombogenic material. In this way, 
this embodiment of multi-durometer 200 is radiopaque and non-thrombogenic. 
Alternatively, a softer inner layer having a radiopaque material could be 
disposed between the inner layer of harder material 204 and the outer 
layer of softer material 206. 
One alternative embodiment of a multi-durometer catheter according to the 
present invention comprises an outer layer of harder material and an inner 
layer of softer material. In this alternative embodiment, the compliance 
of the multi-durometer catheter is still primarily derived by the 
properties of the harder material to produce sufficient frequency response 
and the layer of softer material makes the catheter kink resistant. 
Example Applications of the Pressure Measurement Device According to the 
Present Invention 
The pressure measurement device can be advantageously used to obtain 
pressures in animals and humans for all of the example uses disclosed in 
the Brockway et al. `191 patent, such as: for measuring blood pressure in 
an artery of an animal; for measuring intrapleural pressure in animals; 
and for measuring intracranial pressure in animals. However, the following 
three example applications of the pressure measurement device of the 
present invention illustrate three particularly useful applications, which 
take advantage of the features of the present invention, such as having a 
reduced length PTC 22, having a separate PTC 22 and connecting catheter 
36, and having the transducer 30 being remote from transmitter housing 44. 
Application of Pressure Measurement Device for Measuring Venous Pressure 
FIG. 6 illustrates an embodiment of the pressure measurement device 20 
according to the present invention for measuring venous pressure, such as 
venous pressure in a laboratory rat. Venous pressure is relatively low and 
head pressure error can thus be significant and highly undesirable. In 
this embodiment, the length of PTC 22 (indicated by arrows 50 in FIG. 1) 
is typically approximately 4 centimeters long and the length of connecting 
catheter 36 (indicated by arrows 48 in FIG. 1) is typically approximately 
25 centimeters long. In this application, PTC 22 typically comprises an 
approximately 2-3 mm long gel plug 52 at the distal tip 24, with the 
remainder of PTC 22 filled with low-viscosity fluid 60. 
To surgically implant a pressure measurement device according to the 
present invention in a vein of a rat or other laboratory animal, a vein 80 
is exposed, such as an abdominal vein. PTC 22 is inserted into vein 80 to 
sense pressure of blood 81 and is secured at a point where PTC 22 exits 
vein 80 using medical grade tissue adhesive or a purse-string suture. As 
illustrated in FIG. 6, about one-half of the approximately 4 centimeter 
long PTC 22 is inserted into vein 80. Transducer housing 32 containing 
transducer 30 is disposed outside of vein 80 and is secured to tissues at 
a point immediately adjacent to vein 80 and as near to distal tip 24 of 
PTC 22 as possible. Transmitter 40 is secured to a muscle or within a 
subcutaneous pocket at a site which is convenient to the surgeon 
performing the procedure. For example, when PTC 22 is inserted into the 
abdominal vein of a laboratory rat, transducer housing 32 is typically 
sutured to the muscle next to the abdominal vein. In this application, 
transmitter housing 44 is typically sutured to a ventral abdominal muscle 
at the incision made to access the abdomen. 
Application of Pressure Measurement Device for Monitoring Pulmonary 
Pressure 
FIG. 7 illustrates an embodiment of the pressure measurement device 20 
according to the present invention employed to monitor pulmonary pressure 
in a human. In this embodiment, PTC 22 is typically approximately 1-2 cm 
long and connecting catheter 36 is typically approximately 50 cm long. In 
this application, connecting catheter 36 must be quite long (e.g., 
approximately 50 cm). Since the monitored pulmonary pressure is low, head 
pressure artifact is a problem overcome by using the short PTC 22 (e.g., 
approximately 1-2 cm). In this application PTC 22 typically contains both 
viscous gel membrane 52 and low-viscosity fluid 60. Nevertheless, in this 
application, PTC 22 can be completely filled with viscous gel 52, such as 
illustrated in FIG. 2B, and still perform acceptably, because of the very 
short length of PTC 22. In addition, depending on the thermal 
characteristics of PTC 22, transducer 30, and low-viscosity fluid 60, this 
application optionally employs a PTC 22 without a larger diameter 
thin-walled section 54, such as illustrated in FIG. 2C. 
To surgically implant the pressure measurement device 20 according to the 
present invention in this application, PTC 22, transducer housing 32, and 
connecting catheter 36 are inserted into subclavian vein 82, passed into 
right ventricle 84 of heart 86, and guided out of heart 86 through 
pulmonary semilunar valve 88 into pulmonary artery 90. Following the above 
procedure to position PTC 22 and connecting catheter 36, transmitter 
housing 44 is disposed in a subcutaneous pocket 92 near the site of entry 
to subclavian vein 82. As illustrated in FIG. 7, in this application of 
the pressure measurement device 20 according to the present invention, the 
complete length of PTC 22 and a large portion of connecting catheter 36 
reside within the circulatory system. 
Application of Pressure Measurement Device for Monitoring Intracranial 
Pressure 
An embodiment of the pressure measurement device 120 according to the 
present invention for monitoring intracranial pressure is illustrated in 
FIG. 8. Intracranial pressures are relatively low and thus head pressure 
errors can be significant. In this application, it is typically 
undesirable for transmitter housing 44 to be placed beneath the scalp 
because it may be uncomfortable due to its size. Therefore, transmitter 
housing 44 containing transmitter 40 is typically placed subcutaneously, 
as indicated at 94, in a convenient location on the neck of the patient or 
on the upper ventral thorax, based on surgeons preference. PTC 22 is 
typically approximately 1.5 cm long and preferably exits transducer 
housing 32 at an approximately 90.degree. angle. Thus, the right angle 
pressure measurement device 120 illustrated in FIG. 4 is preferable used 
in this application. The right angle provides a more convenient surgical 
placement. In this application, connecting catheter 36 is typically 
approximately 70 cm long. In this application, PTC 22 typically includes 
viscous gel membrane 52 at distal tip 24 with the remainder of PTC 22 
filled with low-viscosity fluid 60. 
Surgical implantation in this application involves making a subcutaneous 
pocket 96 at the location 94 where transmitter 40 is to be placed. PTC 22 
and connecting catheter 36 are directed under the skin from location 94 to 
a location 97 within cranium 98 where pressure is to be monitored. 
Following exposure of cranium 98 at the location 97 where pressure is to 
be monitored, a hole is drilled through cranium 98. PTC 22 is then 
directed through the hole into a subarachnoid space 100. To prevent 
transducer housing 32 from extending above the normal plane of the scalp, 
a shallow cavity 102 is formed in cranium 98. Transducer housing 32 is 
placed in shallow cavity 102. In this embodiment, transducer housing 32 is 
constructed with a flat profile to inhibit migration under the skin and to 
improve tolerance by the patient. 
Alternative Pressure Measurement Device 
An alternative pressure measurement device is illustrated generally at 320 
in FIG. 9. Pressure measurement device 320 includes a pressure 
transmission catheter (PTC) 322 having a distal lumen tip 324 which is 
positioned within the body of a human or animal at the site where pressure 
is to be measured. One embodiment of PTC 322 is flexible, while another 
embodiment of PTC 322 is rigid, and the particular embodiment of PTC 322 
selected depends on the given application of pressure measurement device 
320. PTC 322 is filled entirely with a pressure-transmitting gel 352 which 
communicates the pressure at the distal tip 324 of PTC 322 to a proximal 
lumen end 328 of PTC 322. Thus, a portion of pressure-transmitting gel 352 
at distal tip 324 interfaces with the substance in the body area where 
pressure is to be measured, such as with blood in an artery. 
A transducer 330 is in communication with pressure-transmitting gel 352 at 
the proximal end 328 of PTC 322. As illustrated, PTC 322 is attached to 
transducer 330 via a nipple 362. Transducer 330 is contained in a 
transducer housing 332. Transducer 330 responds to variations in the 
pressure-transmitting gel at proximal end 328 to provide an electrical 
pressure signal representing variations in the physiological pressure at 
distal tip 324 on electrical lead wires, which are coupled to an 
electronics module 338 of a transmitter 340. Electronics module 338 is 
powered by a battery 342. Battery 342, electronics module 338, and 
transducer housing 332 are contained within a transmitter housing 344. The 
electronics module 338 includes signal-processing and telemetry circuitry 
and a transmitting antenna for generating and transmitting a telemetry 
signal representing the pressure signal from transducer 330 to an external 
receiver (not shown) disposed outside of the human or animal. The 
electrical pressure signal produced by transducer 330 is amplified and 
filtered with the signal-processing circuitry in electronics module 338 
and is then modulated onto a radio-frequency carrier by the telemetry 
circuitry in electronics module 338 for transmission to the external 
receiver. 
In alternative pressure measurement device 320, the transducer 330 is 
contained within transmitter housing 344, and PTC 322 is filled entirely 
with pressure-transmitting gel 352 (i.e., a low viscosity fluid is not 
used). This embodiment can be employed in applications where PTC 322 is 
very short and the transmitter housing 344 is sufficiently small to enable 
it to be located at the pressure source. 
Pressure transmitting gel 352 is a biocompatible and blood-compatible gel 
or other gel-like material that provides a direct interface with the 
tissue or fluid from which pressure is to be measured, such as blood in an 
artery. Gel 352 provides a means of retaining fluid within PTC 322 and can 
be comprised of any material which is capable of flowing as does a viscous 
fluid and contains intramolecular forces which make it very unlikely that 
any portion of this material will dissolve, break apart, slough off, or 
wash away when measuring physiological pressure within a human or animal. 
Gel 352 must be viscous enough not to wash out of PTC 322, but also must 
be low enough in viscosity that it can "flow" without significant pressure 
differential. In one embodiment of the invention, pressure transmitting 
gel 352 is a silicone gel which contains cross-linked molecular entities. 
CONCLUSION 
The pressure measurement device according to the present invention can be 
employed to sense numerous internal body pressures in humans and animals 
including pulmonary pressure, venous pressure, left ventricle pressure, 
intracranial pressure, bladder pressure, and other physiological 
pressures. Pressure information sensed with the pressure measurement 
device according to the present invention is available for diagnostic 
purposes, research, or feedback for closed-loop control of infusion pumps 
capable of administering pharmaceutical agents. 
The pressure measurement device according to the present invention, such as 
pressure measurement device 20, overcomes the above-discussed 
disadvantages of the previous pressure measurement devices, offers 
significant new opportunities for more accurate measurement of pressure, 
and opens new applications for pressure measurement in animals and humans. 
The pressure measurement device according to the present invention obtains 
high-fidelity measurements with negligible head pressure error in 
applications where the distance from the distal tip of the PTC to the 
transmitter is large. 
Although specific embodiments have been illustrated and described herein 
for purposes of description of the preferred embodiment, it will be 
appreciated by those of ordinary skill in the art that a wide variety of 
alternate and/or equivalent implementations calculated to achieve the same 
purposes may be substituted for the specific embodiments shown and 
described without departing from the scope of the present invention. Those 
with skill in the mechanical, electro-mechanical, electrical, and computer 
arts will readily appreciate that the present invention may be implemented 
in a very wide variety of embodiments. This application is intended to 
cover any adaptations or variations of the preferred embodiments discussed 
herein. Therefore, it is manifestly intended that this invention be 
limited only by the claims and the equivalents thereof.