U.S. patent application number 09/804936 was filed with the patent office on 2001-12-20 for low disturbance pulsatile flow system.
Invention is credited to Edelman, Elazer, Kolandaivelu, Kumaran.
Application Number | 20010053928 09/804936 |
Document ID | / |
Family ID | 26884410 |
Filed Date | 2001-12-20 |
United States Patent
Application |
20010053928 |
Kind Code |
A1 |
Edelman, Elazer ; et
al. |
December 20, 2001 |
Low disturbance pulsatile flow system
Abstract
A flow system device used for testing/creating fluid flow. The
system comprises at least one fluid filled loop and a rotor stage
for maintaining at least one rotor. The loop is positioned on the
rotor. The device also includes a driving motor for rotating the
rotor stage and a motion controller for controlling the speed and
directional motion of the motor.
Inventors: |
Edelman, Elazer; (Brookline,
MA) ; Kolandaivelu, Kumaran; (Boston, MA) |
Correspondence
Address: |
Samuels, Gauthier & Stevens LLP
Suite 3300
225 Franklin Street
Boston
MA
02110
US
|
Family ID: |
26884410 |
Appl. No.: |
09/804936 |
Filed: |
March 13, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60188723 |
Mar 13, 2000 |
|
|
|
Current U.S.
Class: |
623/1.1 ;
623/1.3 |
Current CPC
Class: |
A61F 2/06 20130101 |
Class at
Publication: |
623/1.1 ;
623/1.3 |
International
Class: |
A61F 002/06 |
Claims
What is claimed is:
1. A flow system device used for creating fluid flow, said system
comprising: at least one fluid filled loop; a rotor stage for
maintaining at least one rotor, said loop positioned on said rotor;
a driving motor for rotating said rotor stage; and a motion
controller for controlling the speed and directional motion of said
motor.
2. The flow system device of claim 1 further comprising a
measurement system to record and calculate desired properties of
the fluid within said at least one loop.
3. The flow system device of claim 1 wherein a vascular prosthesis
is placed within the tube.
4. The flow system device of claim 3, wherein said vascular
prosthesis is a stent or graft.
5. The flow system device of claim 1 wherein the created fluid flow
is bidirectional.
6. The flow system device of claim 1 wherein the loop includes a
one way valve.
7. The flow system device of claim 1 wherein the system included
six rotors with six corresponding fluid filled loops.
8. The flow system device of claim 1 wherein the fluid is
blood.
9. The flow system device of claim 1 wherein the stents are coated
with gold or stainless steel.
10. The flow system device of claim 1 wherein the fluid flow within
the loop is controllable such that thrombotic signal is
created.
11. The flow system device of claim 1 wherein the fluid flow within
the loop is controllable such that the effects of background noise
is minimized.
12. A method of creating fluid flow, said method comprises:
providing a fluid flow system including at least one loop, a rotor
stage for maintaining at least one rotor, the loop positioned on
the rotor, a driving motor for rotating the rotor stage and, a
motion controller for controlling the speed and directional motion
of the motor; filling the at least one loop with fluid which is to
be tested; controlling the motor to obtain the desired motion of
the fluid within the tube; measuring the desired effects of the
fluid flow.
13. The method of claim 12 wherein the fluid flow system further
includes a measurement system to record and calculate desired
properties of the fluid flow within the loop.
14. The method of claim 12 wherein the fluid is blood.
15. The method of claim 12 wherein a vascular prosthesis is
maintained within the tube.
16. The method of claim 15 wherein the vascular prosthesis is a
stent or graft.
17. The method of claim 15 wherein the thrombotic effect of the
vascular prosthesis on the blood is measured.
18. The method of claim 12 wherein the fluid flow is controlled
such that the fluid flow begins, stops and begins to mimic the flow
of blood due to the pumping of a heart.
19. A connector for connecting opposing ends of a tube, said
connector comprises: a section of tubing to be positioned over the
two opposing ends of a tube, and an elastic sleeve to be placed
over said section of tubing such that the two ends of the tube are
in axial alignment.
20. The connector of claim 15 wherein the inside diameter of the
section of tubing is approximately the same as the outer diameter
of the tube.
21. The connector of claim 15 wherein the elastic sleeve provides
radial compression on the section of tubing.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to the field of coronary implants, and
in particular to a low disturbance, pulsatile, in vitro flow
circuit for modeling coronary implant thrombosis.
[0002] Biocompatibility has been a major issue in the ability to
use prosthetic implants in clinical settings. One such set of
applications includes vascular prosthesis such as endoluminal
stents or grafts to allow blood to flow either through or past a
previously stenosed vascular segment. When such a foreign structure
comes into contact with tissue and blood, a variety of biological
consequences ensue. These reactions, ranging from thrombosis, to
inflammation, to restenosis, can result in acute or long-term
device failure. Not only is coagulation responsible for the obvious
occurrences of acute thrombotic events, but sub-clinical levels
have also been implicated as a player in the pathophysiology of
restenosis through the release of chemical mediators and by
providing a scaffold for the ingrowth of migrating and
proliferating cells.
[0003] The thrombotic reaction is one of the earliest responses to
implantation and by virtue of its potential for rapid acceleration
and complete luminal occlusion, one of the most devastating.
Forming clot not only serves as a scaffold for the ingrowth of
migrating and proliferating cells, but as a source and reservoir
for chemical mediators of these cellular events, such as platelet
derived growth factor and thrombin. Elucidation and control of the
thrombotic process is especially important for the continued use
and development of vascular implants.
[0004] Vascular patency relies on a careful balance of chemical
mediators and local fluid dynamics. With vascular injury, even as
simple as the insertion of a small intravascular wire, profound
micro-environmental changes ensue, altering blood flow and
coaguability. A thrombus develops and propagates when the
stimulatory forces cannot be balanced by the negative regulatory
measures. Platelets adhere and activate at a given implantation
site, potentiating the coagulation reactions by acting as an
enzymatic surface and sequestering reactants both from flow and
other inhibitory influences. These coagulation processes then
potentiate further platelet activation directly via the production
of mediators such as thrombin and indirectly by stabilizing the
adherent platelets via a fibrin meshwork. Physiologically, these
cellular and molecular systems interact in a highly inter-dependent
manner to make thrombosis possible in the face of arterial flow
conditions.
[0005] One difficulty that has limited the extensive examination of
bioprosthetic thrombosis is the highly flow-dependent nature of
thrombosis and lack of widely applicable flow models. Flow can
affect the components of thrombosis either through physical shear
dependent mechanism, such as von Willebrand's Factor dependent
platelet activation, or through mass transport of cellular and
molecular substances into and out of a given region. Thus, control
and documentation of reproducible flows are essential to the study
of the dynamically coupled cellular and protein pathways leading to
implant thrombosis. Also, doing so in a controllable in vitro
setting is desirable as individually and controllably perturbing
the various thrombotic components is essential to studying the
dynamically coupled cellular and protein pathways.
[0006] Various prior art flow systems have been developed in order
to study the thrombotic process. One such method includes placing a
loop partially filled with blood on a tilted turntable. As the
table spins, gravity keeps the fluid at the bottom of the tube,
creating flow. This method is known as the Chandler loop technique.
It is not ideal as a large air/blood interface can cause protein
aggregation and denaturation, creating a significant departure from
the physiological situation. Furthermore, this method does not
allow for arterial flow profiles to be obtained.
[0007] Another method for the investigation of flowing blood was
the development of parallel-plate flow chambers. This apparatus is
particularly useful in studying cellular interactions with a
surface as the chambers are microscopically viewed in real time.
However this is not helpful when studying actual coronary
prosthetic configurations as the chambers and flow rates are not
arterial in nature.
[0008] When studying coronary prosthesis, and in particular stents,
one prior art method includes the use of a roller or peristaltic
pump to drive flow through a length of tubing. The described setup
utilizes a 3 mm ID, 82 cm long peristaltic tubing (PVC or silicon)
filled with 6 ml of platelet rich plasma. A 3-way valve is used for
the placement of fluid. The stent is expanded in a discontinuous
connecting 4 mm ID segment. This methodology has recently been used
to show variations in platelet activation, via Flow cytometry
methodology and the clotting times for stents of different lengths
and with heparin coatings, though it could not distinguish between
tantilum and stainless steel stents. However, there are several
factors that reduce the potential of this system to study stent
thrombosis. One is the level of background noise that is created
with the large surface area of peristaltic tubing and the roller
pump's action. In order to keep the pump's background effects to a
minimum, a low 8 ml/min flow rate was used, while actual mean flow
rates of 50 ml/min are achieved in the coronary arteries with peak
values normally reaching 100 ml/min. Furthermore, placing the stent
in a discontinuous 4 mm region not only increases system background
noise, but substantially perturbs the flow over the stent. Both the
flow rate and stent placement create a dramatic misrepresentation
of the dynamics of flow dependent thrombosis.
[0009] Another method that has recently been described as an in
vitro evaluation of stent thrombosis includes a simple setup
wherein blood is drained directly from a volunteer into a funnel
connected to a length of tubing into which the stent is placed. The
blood is directly collected into a tube and then analyzed for
variations in platelet activation. This system reduces the
background noise by using a shorter tubing length and no
peristaltic pump. On the other hand, the signal is also reduced due
to the one pass methodology rather than recirculation. Although
some differences could be noted with certain stents, others were
not significantly different than control runs, thus indicating the
lack of sensitivity and that the flow rate was not controlled.
Additionally, bleeding a volunteer requires a substantially greater
amount of blood than recirculant setups.
[0010] Some animal in-vivo and ex-vivo models have been used.
Although these have the ability to create physiological flows, they
have a drawback in that there is a limit on the amount of control
that is attainable in the system as parameter variation must be
within life-sustaining margins. Therefore, studying the coupled
nature of thrombus formation is difficult because the components
cannot be varied to the extent that they may in an in-vitro setup.
Many extraneous variables exist in in-vivo systems that could
complicate the process being observed rendering unanalyzable
results. Also interspecimen variation can create noise, which if
large enough, could obscure potential findings. Another concern is
that although observations may be made in one species, they may not
be robust enough to occur in humans due to relative functional
component differences. Practically, there are other issues, from
the expense to the ethics, that must also be taken into account
when using such systems. Though these issues limit what can be
gained from in-vivo models, some studies have nonetheless been
performed which are of relevance. For instance, Makkar et al.,
1998, "Effects of lopidogrel, asprin, and combined therapy in a
porcine ex-vivo model of high-shear induced stent thrombosis,"
European Heart Journal. 19(10), 1538-1546 show in an ex-vivo pig
model that polishing or polyethylene oxide modified nitinol
surfaces were less thrombogenic than nitinol surfaces.
[0011] Other types of studies have included clinical trials. These
carry with them many of the same problems as the animal studies.
Additionally, there is even less controllability as the welfare of
the patient is the primary concern, with many observations being
taken retrospectively. Although in the end, these trials must be
performed to validate findings from other models, the preliminary
use of models can be used to investigate processes in a more
scientifically rigorous fashion, while decreasing patient risk in
clinical trials. Therefore, it is desired to develop a more
suitable in-vitro model of the coronary situation to aid in the
study of vascular phenomenon such as thrombosis.
SUMMARY OF THE INVENTION
[0012] A model has been created to observe the physiological,
controllable flows in a manner to create a large thrombotic signal,
while minimizing the effects of background noise. This is
accomplished by minimizing the length and discontinuities of a
tubing loop into which a prosthetic, such as a stent or a graft, is
placed. The loop is then filled with the desired blood constituents
and spun about its axis in a prescribed fashion. This spinning is
controlled in such a way as to modulate the inertial flow of the
contained fluid through transmitted shear forces from the tubing
wall, thereby creating a low disturbance flow.
[0013] An object of the present invention is to provide a low
disturbance, pulsatile flow system used for testing/creating fluid
flow.
[0014] Another object is to provide a system for testing the
thrombotic effects of blood when a stent is positioned within the
system.
[0015] A further object is to provide a method of using a low
disturbance pulsatile flow system to study fluid flow.
[0016] An additional object is to provide a method to test for
thrombotic effects.
[0017] Another object is to provide an improved connecting device
such that two opposing ends of a tube are held in near perfect
axial alignment, minimizing luminal discontinuity.
[0018] These and other objects, features and advantages of the
present invention will become apparent in light of the following
detailed description of preferred embodiments thereof, as
illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a diagrammatic illustration of a low-disturbance,
pulsatile system device in accordance with the present
invention;
[0020] FIG. 2 is sectional view of a fluid fill torus;
[0021] FIG. 3 is a sectional view of cylindrical pipe with linearly
accelerating walls;
[0022] FIG. 4A is a sectional view of a loop including the
connectors;
[0023] FIG. 4B is a sectional view of FIG. 4A taken along line
4B;
[0024] FIG. 5A is a sectional view of the rotors mounted on a
shaft;
[0025] FIG. 5B is a sectional view of the shaft and cap;
[0026] FIG. 6 is a sectional view of the loop before being
connected including a stent;
[0027] FIG. 7 is a sectional view of the couplings of the measuring
system;
[0028] FIG. 8 is a schematic view of the system including the
measuring system;
[0029] FIG. 9 is a graph of a sample test;
[0030] FIG. 10 is a graph of a coronary blood phase;
[0031] FIG. 11 is a graph of an impulse profile;
[0032] FIGS. 12 A, B & C are graphs illustrating square,
triangular and sine waves; and
[0033] FIG. 13 is a graph illustrating background noise.
DETAILED DESCRIPTION OF THE INVENTION
[0034] As shown initially in FIG. 1, is a low-disturbance,
pulsatile, in vitro flow device is generally
[0035] shown at 10. The device includes a fluid torus 12,
rotor-stage 14, driving motor 16, motion controller 16, and a
measurement system 20 utilized to observe the physiological,
controllable flows in a manner to create a large thrombotic signal.
The system is usually utilized in an incubator, not shown, to keep
the samples at a stable temperature. As described in detail below,
this includes placing a stent 24 or a graft in a torus or loop 12,
as seen in the Figures. The loop 12 is then filled with the desired
blood constituents and spun about its axis in a prescribed fashion.
This spinning is controlled in such a way as to modulate the
inertial flow of the contained fluid through transmitted shear
forces from the tubing wall, thereby creating a low disturbance
flow.
[0036] To create the desired flow profiles, the fluid-filled torus
12 is rotated about its axis. When impulsively started, there is
inertial fluid motion relative to the toroid wall as shown in FIG.
2. As time passes, the fluid is accelerated due to momentum
transfer into the fluid bulk via shear forces. If the driving torus
is spinning at a constant angular velocity, the fluid eventually
achieves solid body rotation coincident with the torus and relative
motion ceases. However, should the loop maintain an acceleration,
there continues to be relative motion, and hence, flow. The radial
profile of this type of fluid motion is found by the Navier-Stokes
equation. A critical simplifying assumption is that the fluid tube
radius is much smaller than the torus radius, allowing streamline
curvature affects to be neglected. Therefore, the system can be
modeled as a straight, cylindrical pipe with linearly accelerating
walls as seen in FIG. 3, and only the axial component of the
Navier-Stokes equations need be considered.
[0037] In the case of flow in a circular pipe of constant cross
sectional area, several terms can be eliminated, as there is an
axial component of the velocity vector that changes with the radial
dimension and with time. Assuming a reference frame that
accelerates with the tube wall, the Navier-Stokes equations can be
simplified to: 1 ( V z t ) = a + ( 2 V z r 2 + 1 r ( V z r ) ) ( 1
)
[0038] where Vz is the axial velocity, t is time, r is the radius,
v is the kinematic viscosity, and a is the tube acceleration.
[0039] The steady state solution resembles that of pressure driven
Pousille flow with the driving force given by the acceleration
rather than an axial pressure gradient. Therefore, the method of
flow creation not only drives flow in an undisturbed fashion, but
also in a manner identical to the pressure driven case if the
accelerations are scaled to appropriately match the would-be
imposed pressure gradients (divided by the density).
[0040] A solution to this partial differential equation with the
customary pipe boundary conditions (Vz=0 at r=R, dVz/dr=0 at r=0)
and an initial condition of Vz=0 with a constant acceleration, a,
is given by: 2 V z = c 2 a 4 [ 1 - r 2 c 2 - 8 n = 1 .infin. J 0 (
n r / c ) n 3 J 1 ( n ) - n 2 t / c 2 ] ( 2 )
[0041] The time dependence of this equation is governed by a time
constant dependent on the inverse of the kinematic viscosity, v,
and the tube radius, c, squared. Furthermore, as t approaches
infinity, the solution approaches the Pousille-like steady state
solution.
[0042] Equation 2 illustrates that by controlling the wall
accelerations, the flow within the tube can be modulated with the
steady state flow rates being linearly related to the tube
accelerations. However, since the system time constant (0.1 sec) is
of a similar order of magnitude as the heart rate (1 sec), it must
be noted that the actual developed flow rates may not be the steady
(or quasi-steady) state values for a given acceleration profile.
For a true flow pattern, a numerical simulation with the imposed
acceleration profile can be obtained. Still, the analytical results
give sufficient insight into the suitability of the methodology, as
well as several factors such as accelerations, time constants, and
parameter dependence which must be considered when designing a
modeling system.
[0043] Although a relative flow can be created through some pattern
of wall accelerations, one issue is that keeping a constant flow
requires a constant acceleration. Moreover, a net positive flow
requires a net positive acceleration resulting in infinite (or at
least impractically large) angular velocities. Since the coronary
arteries run through the myocardial tissue of the heart, the
intramural pressure rises during contraction (systole), blocking
off flow in the coronary arteries. Conversely, during relaxation
(diastole), the wall pressure is reduced and flow is driven via the
higher arterial pressure. Thus, unlike other flows in the body,
most coronary flow takes place during diastole.
[0044] As seen in FIG. 10, the left anterior descending coronary
flow actually comes to a halt. This sets up a situation where the
acceleration of the loop can be reduced to zero allowing solid body
rotation. However, at this point, the absolute velocity of the loop
in the inertial reference frame will be greater than before the
cycle had begun having just gone through a period of accelerations
to achieve the desired flow profile. To begin another cycle in the
same direction would mean again adding to the net uni-directional
loop velocity, quickly reaching the maximal velocity limits of the
motor. Instead, two alternative options exist: 1) introducing a
one-way valve or 2) accepting bi-directional flow.
[0045] In a one-way valve system, when the fluid within the torus
has reached a state of zero flow, the loop can be rapidly stopped
from its constant angular velocity. This creates a negative
impulsive wall velocity that creates flow in the opposite
direction. However, by virtue of the directional valve, the fluid
can be kept in solid body rotation and brought to rest in the
inertial reference frame along with the torus. From this point, the
acceleration pattern required for a desired flow profile can begin
again and cycle indefinitely without a compounding net angular
velocity.
[0046] Although this technique could be employed for certain
applications, it is undesirable for the purposes of studying
thrombosis. Such a valve would increase the thrombotic background
potential of the system, both from its physical presence, and the
imposed water-hammer effect when the fluid is jerked to a halt.
[0047] The second possibility is to allow bi-directional flow by
following each loop acceleration profile with a symmetric
deceleration, thereby bounding the angular velocity. The reversal
of flow creates some concerns at macroscopic, microscopic, and
molecular levels. Alternating flow direction means an alternating
embolic shear force. This acts from the level of initial platelet
adherence to that of macroscopic thrombi. Platelet adherence to a
surface is generally characterized by a rolling and sticking phase.
Since the duration of this process, during which an oscillating
shear force might be imagined to cause a difference, is much less
than that of a heart beat (system oscillatory period), the
reversing shear is assumed to have a small effect. However, as the
thrombus and imposed embolizing force grow, the oscillating shear
may invoke a fatigue type failure response in the fibrillar
connections, thereby increasing the probability of detachment. This
is an important in-vivo occurrence, however it is of little
consequence in the in vitro model since by the time macroscopic
emboli form, much of the highly amplified thrombotic phenomenon
under study would have already been determined. The changes in
embolizing probability would simply affect the final stage of
luminal blockage.
[0048] Oscillating flow affects mass transport. One factor is the
change in convective flow patterns created by uni vs bi-directional
flow. This is more of an issue at higher Reynolds's numbers,
however, the small values i.e. <10 considered means that the
flow and shear around obstacles (such as stent struts) is
essentially symmetric. Therefore, regardless of flow direction,
instantaneous species flux phenomenon should be governed by similar
processes in the vicinity of wall protrusions as these are
dependent on a shear dependent mass transport coefficient and an
independent reaction rate coefficient.
[0049] A schematic of the fluid torus 12 is show in FIGS. 4A and
4B. In the current embodiment, the toroids are made of a 24 cm
circumferential loop of 1/8" ID::{fraction (5/32)}" OD S-50-HL
Tygon tubing. The connecting ends 26a, 26b of the loop have been
squarely cut with the axial dimension to ensure a matching
end-to-end fit. This connection is held via a 1.75 cm overlapping
segment 28 of S-50-HL Tygon tubing of {fraction (3/16)}"
ID::{fraction (5/16)}" OD. The close OD/ID match provides a good
compression fit and axial alignment. Further support is provided by
a 1 cm elastic band 30 made from a silicon tube (Silastic) of 1/4
"ID::3/8" OD placed over the joiner segment of tygon. The elastic
radial compression provided by the segment's smaller ID assures a
suitable joint connection.
[0050] The connectors 27 allow the tubing circuit to be free of
geometry and luminal surface discontinuities. The application of
the internal pressures tends to strengthen the joint, rather than
weaken it. The connector is such that the tube has an inner
diameter equal to the outer diameter of the circuit's tube
diameter, and an elastic outer sleeve that slightly compresses the
formed joint to provide a tightly sealed connection.
[0051] Two similar structures 32, 34 are slid onto the 1/8" loop at
equally spaced 120 degree intervals in order to cause the least
deviation in toroid curvature possible as the structures provide
some rigidity to the underlying fluid loop segment. The two
additional sleeves 32, 34 serve as outlet and inlet ports for the
replacement of the loop's contained air by the desired fluid (ie.
blood/plasma/buffer). This is accomplished by sliding a needle
under the outer most elastic sleeve, and then, pushing the needle
through the middle sleeve and inner loop layers at approximately a
45 degree angle. Small-bore needles are used to create the smallest
possible disturbance to the loop's inner, fluid-contact surface.
The elastic outer sleeve 30 provides a final seal to the escape of
loop contents. A 26-gauge needle is used at one of the ports which
serves as an outlet for the evacuation of air. A larger bore is
used for the injection of cellular fluid to limit the handling
trauma. Generally, a 19-gauge phlebotomy needle is used for the
transfer of blood products as a compromise between the need for a
small injection port and an untraumatic injection.
[0052] An estimate of the diameter of a normal adult left anterior
descending coronary branch is 3-3.5 mm. The 1/8" ID tube falls
within this range at 3.175 mm. The OD of {fraction (5/32)}". It is
important to have as little extraneous surface contact as possible
to reduce the circuit's background thrombotic potential. The small,
contained recirculating volume allows the thrombotic process to
proceed in an amplified fashion. The tube 12 also has to be long
enough so that the assumptions of linear flow would remain a valid
approximation. As a 3" diameter is nearly 2.5 orders of magnitude
larger than the 1/8 ID of the loop and the loop length 12 is 24 cm.
However, depending on the relative need to reduce surface
area/recirculating volume while keeping secondary, curvature
related flow effects to a minimum, the length can be modified to
other values.
[0053] The Tygon tubing has low protein absorption and
bioreactivity. Though this tubing was chosen for its
low-reactivity, the inertial mechanism which drives the flow is not
limited to any type of tubing (compliant tubing is used in
peristaltic pumps). If desired, the tubing is replaceable with one
of a given surface quality whose properties are to be study.
Furthermore, the tubing can be completely lined or coated with a
substance. This is of particular value when investigating processes
such as thrombosis, where the endothelium and underlying
composition plays an important role. The tubing can be coated with
a type-I collagen surface as a rough approximation of the
subendothelium. Confluent endothelialization is also possible since
there is no disturbance (structural or dynamic) of the inner loop
surface once the torus 12 is formed.
[0054] The loop 12 is then fit onto a rotor platform or stage 14
and placed in axial alignment with other loops to be tested under
the same flow conditions as seen in FIGS. 5A and 5B. Although any
number of loops 12 may be selected, the embodied system
accommodates six simultaneous runs via six modular rotor platforms
14. The entire rotor system is then driven through a desired
angular motion profile via the motor 16 and controller system 18.
This motion creates the bi-directional flows which are measured via
onboard flow transducers built into the rotor stages. Each
transducer sends the flow from a particular fluid loop to the
measuring/recording system, which can be used to instantaneously
monitor the flow profiles and fluidity of the blood.
[0055] The prototypical implants include 7-9 NIR stents 36, as seen
in FIG. 6, and have a diameter of 3.5 mm and are 9 mm in length.
They are obtained from Medinol Ltd. (Jerusalem, ISRAEL). Stainless
steel and gold coated surfaces were selected to offer a variable
thrombotic potential. The stents 36 were expanded 1 cm from the end
of a given sample tube (9.5" long 1/8" ID {fraction (5/32)}" od
3350 Tygon tubing). The ID dimension was between 3-3.5 mm and an OD
that was imposed by the wall thickness requirements of the flow
transducers. The length was determined by a balance between
minimizing the extraneous surface area while keeping the loop
curvature considerably larger than the tube diameter.
[0056] After the tubes were closed into their loop format they were
filled with the desired blood components. The loops were fit onto a
rotor platform 14 and placed in axial alignment with the other
loops to be tested under the same flow conditions. The entire rotor
system is driven through a desired angular motion profile via the
motor/controller system 18, and held at a constant 37.degree. C. in
the incubator. The motion created pulsatile type flows which are
measured via onboard Transonic flow transducers built into the
rotor stages. Each transducer sent the flow from a particular fluid
loop, through the rotary electric coupling 38 of FIG. 7 to the
recording system, (LAB-PC/LABTECH software v8.2 manufactured by
Laboratory Technologies Corporation), which was used to
instantaneously monitor the flow profiles and fluidity of the blood
in a given loop.
[0057] The rotors are the discoid platforms upon which the fluid
loops are held. In the system that was built, there are six such
rotors, accommodating six loops. FIG. 5A shows the design of an
individual rotor (x3 orientation A/x3 orientation B). Each was
manufactured out of a stock of 3" diameter delrin plastic. The
rotors include a grooved, resting stage for the fluid loop, a keyed
axial hole for rotor stacking and alignment of the rotors, a chiral
notch for the placement of the flow transducer, and a shaft or slot
44 through which the transducer connections may be passed. The
notch chirality allows sequential rotors to be stacked with the
probes facing opposite directions in order to minimize asymmetrical
loading. With the current rotor system's shaft, six rotor stages
can be stacked on a shaft 42, along with a cap structure that
serves as a location for on-board instrumentation. A centered hole
allows for axial coupling to the motor.
[0058] The rotors were placed at a 180 degree shift for the A and B
orientations. This chirality allows the sequential rotors to be
placed one on top of the other, with the probes facing opposite
directions. In doing so, the forces produced on the motor axis do
to asymmetry are minimized.
[0059] The rotor system's shaft is shown in FIG. 8 along with a
diagram depicting six stacked, alternating rotor stages. The shaft
was machined from a 1" diameter stock of delrin and is in two
sections. The top section has been reduced to a diameter of 1.59 cm
to accommodate the stackable rotors. The entire rotor system was
driven through a desired angular motion profile via the
motor/controller system, and held at a constant 37.degree. C. in
the incubator. The bottom section remained at the initial stock
diameter of 1". A centered 0.5" diameter hole was drilled into it
to allow for an axial coupling to the motor.
[0060] The shaft 42 extends past the length of six combined rotor
heights. This allows for the placement of a cap structure 40 that
could serve as a location for on-board instrumentation for the
transducers.
[0061] The torque, T, needed to drive the system is simply the peak
acceleration times the moment of inertia of the rotor/motor setup,
assuming negligible friction effects. The peak torque is determined
from the maximum angular acceleration required to drive the fluid,
which in turn, is given by the maximum flow rates required. Peak
physiological flow rates are around 100 ml/min in the coronary
arteries (averaging .about.33 ml/min). To achieve this flow at
steady-state, equation 2 yields a wall acceleration of 2.67 m/sec
2, or an angular acceleration of 70 rad/sec 2 for a 3" diameter
loop. To allow for a broader range of possibilities, the maximal
acceleration was taken to be 175 rad/sec 2. The moment of inertia
is estimated assuming the rotor was a solid delrin cylinder 3" in
diameter and 27" in length with a corresponding mass of 1.2 kg to
allow for an overestimate of the moment. The values result in a
moment of inertia of 8.5e-4 Nms 2, and a corresponding peak torque
of 0.15 N/m (670 oz/in).
[0062] In order to determine the maximal operating speed, an
estimate of the speed required to accommodate one pulse is
determined. A high estimate is found by taking the required
acceleration for peak steady state flow (70 rad/sec 2) and
multiplying this by the duration of one pulse. The pulse, which
represents a heart beat, is approximately 1 sec (60 beats/min). To
allot for changes in rate for a wider range of possible
experiments, a 2 sec duration was used (30 beats/min). This yields
a maximum angular velocity of 140 rad/sec (1340 rpm).
[0063] The driving motor is an Electrocraft NEMA 42C DC servo-brush
motor. The NEMA 42C model provides a peak torque of 720 oz-in and a
maximum, absolute operating speed of 4800 rpm, allowing for 6
unidirectional beats if desired.
[0064] The components of the motor control system integrate readily
and allow for the generation of specific flow profiles. These
components include a Renco RM15 Encoder, an Electro-Craft IQ-550
Position Control Module, an Electro-Craft Max-100 PWM Servo Drive,
and a Windows compatible PC terminal running IQ Master software.
The motor used to drive the system was an Electrocraft to NEMA 42C
DC servo-brush motor.
[0065] The components are interchangeable and are easily adjusted
via the programmable controller through software rather than
hardware means. Therefore, various flow profiles can be readily
made and modified according to the desired experiment.
[0066] To measure the loop flow rates, Transonic 3CA flow probe
leads are connected to a specially constructed junction on each
rotor stage as illustrated in FIG. 8. Upon stacking, the male
connector junctions on a given rotor stage allow communication with
the female junctions on the stage immediately below it. Thus, each
stacked rotor is hardwired to all of the probes. This design allows
the stages to be modular for loading and possible future expansion,
with the top most stage relaying all probe signals to the on-board
probe multiplexer. The signals were passed sequentially to a
Transonic T106 Flowmeter which outputs a voltage signal, recordable
on a computer via a National Instruments LAB-PC A/D interface and
LABTECH Version 8.1 software package. The trigger to sequentially
switch probes is provided by the high to low or low to high state
change of a digital output pin on the IQ 550 controller. This
switch was programmed to occur after each flow cycle or beat. In
this method, all of the probes' signals were merged into a
continuous waveform. A final signal is sent from the multiplexer to
the computer encoding a specific probe label. Therefore, with the
waveform and corresponding probe label information, an individual
fluid loop can be monitored throughout the time course of an
experiment.
[0067] An eight lead rotary electrical coupling interfaces the
rotating loop reference frame with the inertial frame. The onboard
multiplexer probe output is wired to four of the rotary couplings
as seen in FIG. 8. Two additional couplings provide power
(+10V,GND) to the multiplexer. The last two lines provide contacts
for the probe switch trigger and the probe label. Although the
system can monitor and record the full flow profiles in the fluid
loops, only the peak flow values were stored to disk in actual
thrombosis experiments to reduce the amount of data storage. These
peaks effectively convey information on change in the fluidity of
the blood and luminal potency.
[0068] On each rotor stage, the four probe leads are passed to the
connector shaft and soldered onto a given pin on a specially
constructed 24 lead connector. Once the proper connections are
made, the pieces are press fit into the connector shaft on the
corresponding rotor stage. Upon stacking, the male junctions on a
given rotor stage allowed 24 pin communication with the female
junctions on the stage immediately below it. In such a manner, each
stacked rotor stage was essentially hardwired to all of the probes
(up to 6 in the current embodiment) via the 24 pin connections.
This design allowed the stages to be modular for loading and
possible future expansion purposes, with the top most stage
relaying all probe signals to the probe multiplexer.
[0069] The multiplexer (powered by a TENMA 30V/3A adjustable power
supply set at 10 V) is used to relay the probe signals in a
sequential order to the flow meter which is only equipped to handle
a single probe. To do this, a BASIC Stamp II Microcontroller
(Parallax, Inc.) is used as a switcher to send a binary signal to a
given lead (1-6) corresponding to the desired probe. This signal is
then sent through a power amplifying stage to provide the current
needed to trigger a single pole/quadruple throw telecommunications
relay. The four poles of the relay are normally 5 open. Upon
activation, a connection is closed between the four leads of the
selected probe and four common, non-specific output leads.
[0070] The output of the multiplexer is then sent to the T106 Flow
meter, which is used to power the transducers and convert the probe
output into a voltage signal representing the bulk flow (1V=50
ml/min). This is directly recorded on the LABVIEW software via the
A/D interface. However, each of the probes' signals is merged into
a continuous waveform. To know which probe was being recorded from
at a particular time, a final signal was sent from the multiplexer
to the PC encoding a probe label. Therefore, with the waveform and
corresponding probe label information, an individual fluid loop
could be monitored throughout the course of an experiment.
[0071] Although the system could monitor and record the full flow
profiles in the fluid loops, only the peak flow values were stored
to disk in actual thrombosis experiments to reduce the amount of
data. These peaks would effectively convey information on change in
the fluidity of the blood. The compression was performed in
real-time using the LABVIEW software capabilities. To do this the
IQ550 was programmed to send a brief 5V pulse to the PC during the
peak acceleration signaling a consistent time point in each flow
profile to be sampled, corresponding to the peak flow. To ensure a
good sample value, 5 samples were taken at a rate of 50 Hz and
averaged into a single peak value.
[0072] To test the mechanical capabilities of the system, first an
impulse profile was generated, as seen in FIG. 11. This profile
reveals that the up phase indicates the maximal rate of flow onset.
With a true velocity step, this onset is also instantaneous.
However, due to the realistic limitations of rotor inertia,
friction, and peak torque, there is a deviation from the ideal
impulse. The example above shows this deviation, where it takes 0.1
sec to achieve the peak flow. Another aspect that can be observed
is the maximal rate of flow decay. Theoretically, a time constant
of 0.1 seconds was determined, meaning that after 4 intervals, the
flow would essentially drop to 0 (98% of original value). In
reality, a similar time constant of 0.1 seconds is obtained, with
the flow dropping to a 98% level in approximately 0.4 sec.
[0073] The impulse is an important function in that any other
function can be broken down into a summed set of weighted impulses
(Green's Functions). Thus, the realistic impulse-like function
gives the limiting building block from which other functions can be
composed. Some examples are square, triangular, sine waves, as seen
in FIG. 12 and still other flows are possible.
[0074] Each pattern has a frequency of 1.11 Hz. However, the
amplitudes vary from 50 ml/min for the square wave, to 100 ml/min
for the sine wave, to 160 ml/min for the triangular wave. This
variation is the result of the rotor acceleration profiles, which
are bounded at equal peak angular velocities for each case.
Therefore, since the square wave had its peak acceleration through
out most of its cycle, this acceleration had to be smaller in
magnitude than that of the sine or triangular wave (where the
acceleration was varying through out the cycle) in order to keep
similar limits on the loop angular velocity.
[0075] The flow is periodic and bidirectional in nature. This type
of oscillation is necessary in the methodology used to create flow,
as it was deemed more important to eliminate the high thrombotic
background levels that would have been created through the use of a
unidirectional valve setup.
[0076] The pliability of the system allows wave characteristics
such as frequency and amplitude to be readily varied according to
the experimental protocol. Furthermore the system allows for
variation of more detailed parameters such as the
systolic:diastolic ratio if desired.
[0077] The system is utilized to study prosthetic thrombosis. In
order to do this, a source of blood and prototypical implants are
required. Blood was obtained from the American Red Cross. The
quantities of blood available made it possible to run several
experiments on the same batch of blood, further limiting external
variability. Secondly, the blood was obtained in pre-separated
components. This allowed a mixing of components in any desired
ratio (to assess, for example, the influence of a small recirculant
volume on the experimental findings). For the following
experiments, fresh frozen plasma and fresh platelets concentrates
(both anticoagulated with 10 mmol citrate) were utilized as these
contained the key ingredients of classical thrombosis, neglecting
the red and white blood cells in the first level of study.
[0078] Type AB+ fresh frozen plasma (FFP) with a prescribed storage
life of 6 months post-collection was stored at -20.degree. C. The
plasma was thawed in a 37.degree. C. water bath for 45 minutes and
then spun down at 10000 G's to eliminate any debris (particularly
cellular matter such as preformed platelet microvesicles). The
supernatant (upper 80%) was then filtered 4 times threw a 0.2 um
filter to further ensure clean FFP. The platelets (type AB+PRP)
were obtained within one day of collection and stored on a 70 RPM
rocker at 22.degree. C. These were used within the first two days
post-collection as was justified from the robustness of findings
when compared with freshly drawn volunteer platelets.
[0079] One hour before a planned experiment, the platelets were
added to the FFP in the desired ratio and returned to the rocker.
This equilibration in fresh plasma has been shown to revive the
platelets from some of the shock they experience during the storage
process. For all of the following preliminary experiments, a
constant ratio of 1:4 PRP to FFP was used. Each loop required 2.5
ml (0.6 ml allotted for leeway) of the FFP/platelet mix. To reduce
experimental error from mixing and handling variation, the total
volume of the suspension for a given run is pooled and prepared in
a single tube.
[0080] The prototypical implant chosen was a polished 7-9 stainless
steel NIR stent obtained from Medinol. While the platelets were
equilibrating in the filtered FFP, the stents were expanded in the
1/8 ID tygon tubing via a 36 mm Maxxum 3.5 SCIMED balloon catheter
to a pressure of 12 atm. For consistency, they were all placed 1 cm
from one end as shown in FIG. 6. Since the tube was symmetric,
either end was acceptable with the middle third being avoided, as
this was the site of flow measurement.
[0081] Once all of the stents were placed in the tubes, the tubes
were closed into their loop format ensuring a gapless fit. When the
plasma/platelet mix was ready, 5M Ca2+ was added to bring the
sample to an additional 10 mmol Ca2+ concentration, negating the
citrate's anticoagulant chelating effect. The fluid loops were then
filled with the plasma/platelet mix as previously described via the
19-gauge needle and placed onto the rotors. This process was
sequentially performed as rapidly as possible while ensuring safe
handling of the blood components and proper filling of the tubes
(i.e. no air bubbles). Once complete, the rotors were placed onto
the rotor shaft and spun according to the desired motion profile.
For the maximum six loops, the filling procedure took approximately
five minutes from start to spin. The time could be further reduced,
either through multiple participation or a novel filling
method.
[0082] As an initial test, six stents were positioned in their
respective fluid loops and run through the described protocol. The
results obtained after parsing the data according to the probe
label and running a 5-point moving average are shown in FIG. 9.
[0083] In this figure, each line represents an individual fluid
loop. The two predominant characteristics of these lines are an
initial constant flow rate followed by a fairly rapid drop off to
zero flow.
[0084] The initial flows in each loop are identical since they all
have the same dimensions, fluid properties, and driving motion
profiles. However, a discrepancy can be seen in the graph as there
is some spread in the start-up flow rates. This variation is due to
the fact that the meter is hard calibrated for a specific flow
probes (#1's) signal while the multiplexer passes six different
signals to it. To correct this, each signal can be re-calibrated
according to these initial deviations where identical fluid
conditions are known to exist.
[0085] The drop off point indicates when the thrombus has blocked
of the flow. If a zero flow condition is taken as the end point, an
average clotting time of 43.1 min with a standard deviation of 6.8
min is obtained for the given run. From it, we can say that with a
sample size of 10, two comparative stents of equal pool variance
would have to have an average clotting time difference of at least
5.3 min in order to have a 95% confidence in their differential
response (two-tailed p-value <0.05). By decreasing the standard
deviation, the system gains power by being able to statistically
distinguish smaller inter-stent differences in a given sample
size.
[0086] Another test of the system's validity and circuit noise
levels is to compare a trial of three stents to three empty control
tubes, as seen in FIG. 13. The initial flow period followed by a
drop off is witnessed in the stented samples, with an average
clotting time of 39.1+/-1.7 min. The stentless controls, however,
remain unthrombosed for the 2.5 hour duration of the test
indicating the sufficiently low levels of extraneous thombotic
potential. The variation is stented loop clotting times between
this run and the previous example (39.1 min vs 43.1 min) could be
due to variations is blood component batches.
[0087] The proposed design is an in-vitro method that minimizes the
background noise in a flow circuit to allow controllable, sensitive
studies to be performed in a pliable coronary vascular setting.
Furthermore, theoretical analysis revealed that these flows are
identical in nature to that of pressure-driven flows. As a
biological test of the system's capabilities, the thrombotic
potential of a stent was assessed by performing trials on stented
loops. In these trials, the peak flow data show an essentially
binary phenomenon, characterized by an initial flow rate which
quickly dropped off to zero as the thrombus occluded the circuit.
Stentless control loops remained unoccluded for the duration of the
trials, indicating the sufficiently low levels of background
thrombosis.
[0088] While the current application of the described flow system
is in studying coronary thrombosis, its use can be generalized to
other situations where a carefully controlled pulsatile flow is
required with adjustments (loop characteristics, uni/bi directional
flow issue, etc) being made to suit the different requirements.
[0089] Although the present invention has been shown and described
with respect to several preferred embodiments thereof, various
changes, omissions and additions to the form and detail thereof,
may be made therein, without departing from the spirit and scope of
the invention.
* * * * *