U.S. patent application number 09/862207 was filed with the patent office on 2001-09-13 for methods, systems, and kits for the extracorporeal processing of blood.
This patent application is currently assigned to VascA, Inc.. Invention is credited to Brugger, James M., Burbank, Jeffrey H., Finch, Charles David.
Application Number | 20010021817 09/862207 |
Document ID | / |
Family ID | 21742376 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010021817 |
Kind Code |
A1 |
Brugger, James M. ; et
al. |
September 13, 2001 |
Methods, systems, and kits for the extracorporeal processing of
blood
Abstract
Methods, systems, and kits for extracorporeally circulating and
processing blood are described. The systems include a pump, a
processing unit, and blood drawn return lines for accessing a
patient's vasculature. Blood flow through the return line is
measured and pump speed controlled to maintain a desired blood flow
rate. Alarm conditions can be initiated when expected pump
performance differs from that needed to maintain the control point
flow rate. By using a ultrasonic flow detector, gas bubbles in the
blood flow can be detected.
Inventors: |
Brugger, James M.;
(Newburyport, MA) ; Finch, Charles David;
(Clinton, MS) ; Burbank, Jeffrey H.; (Boxford,
MA) |
Correspondence
Address: |
LYON & LYON LLP
SUITE 4700
633 WEST FIFTH STREET
LOS ANGELES
CA
90071-2066
US
|
Assignee: |
VascA, Inc.
Tewksbury
MA
|
Family ID: |
21742376 |
Appl. No.: |
09/862207 |
Filed: |
May 21, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09862207 |
May 21, 2001 |
|
|
|
09174721 |
Oct 19, 1998 |
|
|
|
60074387 |
Feb 11, 1998 |
|
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Current U.S.
Class: |
604/6.11 ;
604/67 |
Current CPC
Class: |
A61M 60/50 20210101;
A61M 60/546 20210101; A61M 60/438 20210101; A61M 60/523 20210101;
A61M 1/3663 20130101; A61M 60/113 20210101; A61M 60/268 20210101;
A61M 60/279 20210101; A61M 2205/3334 20130101; A61M 1/3626
20130101 |
Class at
Publication: |
604/6.11 ;
604/67 |
International
Class: |
A61M 037/00; A61M
031/00 |
Claims
What is claimed is:
1. A method for extracorporeally processing blood, said method
comprising: drawing blood from a patient; pumping the blood with a
pump at a predetermined blood flow rate; measuring an actual blood
flow rate delivered by the pump; comparing the actual blood flow
rate with the predetermined blood flow rate; and adjusting the
blood flow rate delivered by the pump.
2. The method of claim 1, further comprising the step of signaling
an alarm condition if a difference of a predetermined minimum value
exists.
3. The method of claim 1, wherein the actual blood flow rate is
directly measured.
4. The method of claim 1, wherein the step of pumping the drawn
blood with a pump at a predetermined blood flow rate includes
pumping the drawn blood with a pump at a predetermined stroke
volume and blood flow rate.
5. The method of claim 1, wherein the drawn blood is pumped at a
blood flow rate that corresponds to a theoretical pumped blood flow
rate.
6. The method of claim 1, wherein the pump is a peristaltic
pump.
7. The method of claim 1, further comprising the step of slowing or
stopping pump operation when said difference has said predetermined
minimum value.
8. The method of claim 1 wherein the measuring step comprises
ultrasonically measuring the flow rate using a sensor disposed
externally to the blood flow.
Description
[0001] This is a division of U.S. application Ser. No. 09/174,721,
filed on Oct. 19, 1998, which is a continuation-in-part of
Provisional Application No. 60/074,387, filed on Feb. 11, 1998, the
full disclosures of which are incorporated herein by reference in
their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to medical methods,
apparatus, and kits. More particularly, the present invention
relates to methods, systems, and kits for pumping blood through
extracorporeal processing units and returning the processed blood
to patients.
[0004] A variety of extracorporeal blood therapies exist which
require blood withdrawal, passage through processing equipment, and
return of the processed blood to the patient. Examples of such
extracorporeal blood therapies include hemodialysis,
hemofiltration, hemodiafiltration, apheresis, and the like. Access
to a patient's vasculature may be provided through implanted ports,
transcutaneous catheters, direct needle access into blood vessels,
and other approaches. Once blood withdrawal and blood return lines
have been established, the blood is pumped through an appropriate
processing unit, such as a dialysis unit, filtration unit,
apheresis unit, or the like and the treated blood returned to the
patient.
[0005] It is easy to appreciate that careful control and monitoring
of the extracorporeal blood circulation is important to both
successful blood treatment and patient safety. Important parameters
and conditions to be monitored and controlled include blood flow
rate, line pressures upstream and downstream of the pump, blockages
in the blood draw line, blockages in the blood return line, air
leakage into the recirculation blood stream, and the like. Previous
extracorporeal blood circulation systems have often relied on
setting the speed of a peristaltic pump to control the blood flow
rate. Since peristaltic pumps operate by the positive displacement
of blood, it has been assumed that the flow rate will be fixed by
the pump speed.
[0006] As recognized by the inventors herein, however, that
assumption is not warranted. Peristaltic pumps, also referred to
tube or roller pumps, rely on moving rollers to progressively
"pinch" a tube to advance a series of small blood volumes through
the tube and out of the pump. So long as the inlet pressure to the
pump tube is generally constant, the pump output will be a
predictable function of pump speed. In the case of extracorporeal
blood circulation, however, where blood is being drawn through a
relatively small needle or other access tube, the inlet pressure of
blood to the pump can vary significantly. Moreover, the flow
characteristics of a peristaltic pump may vary over time so that
the volumetric output will change even if the inlet pressure
remains generally constant. While use of a peristaltic pump does
have a number of advantages, e.g. there are much less likely to
apply a deleterious negative pressure to the blood being
circulated, calculating the flow rate based on pump speed alone is
nonetheless problematic.
[0007] To help monitor whether the pump is starved of inlet blood
flow (which can alter the flow rate as discussed above), some prior
art systems have employed pressure monitors on the blood draw
and/or return lines. A fall in pressure in the draw line indicates
that a blockage or other failure has occurred in the draw line,
that the access needle is too small and/or that the access vessel
has undergone a partial or total collapse. In contrast, a rise in
pressure in the return line indicates the occurrence of an
occlusion or other problem in the return line and/or the occurrence
of a blockage in the vessel, access device, or fistula. In order to
help assure sterility, pressure measurement has usually been
performed using drip chambers where the pressure is transmitted via
an isolated air line and a transducer protector to the appropriate
transducer. Such drip chambers, however, increase the cost of the
catheters (blood lines) used for the draw and return lines and the
air interface can cause clotting, air entrapment, and other flow
problems in the blood recirculation.
[0008] For these reasons, it would be desirable to provide improved
methods, systems, and kits for the extracorporeal recirculation and
processing of blood. In particular, it would be desirable to
provide extracorporeal blood flow systems having improved blood
flow rate control as well as improved capability for monitoring
proper operation of the blood circulation circuit. Such systems
should permit monitoring with a reduced risk of contaminating the
blood or causing clotting, air entrapment, or other degradation of
the blood. Preferably, such improved systems and system components
will permit relatively low cost operation, and specifically will
permit implementation without the use of drip chambers as required
by certain prior art systems. At least some of these objectives
will be met by the invention described hereinafter.
[0009] 2. Description of the Background Art
[0010] U.S. Pat. No. 5,562,617 assigned to the assignee of the
present application, describes a system of implantable ports and
catheters for accessing a patient's vasculature, which system could
be used together with the extracorporeal blood recirculation
systems of the present invention. U.S. Pat. No. 4,181,132,
describing an extracorporeal processing and blood circulation unit
which is attached to a patient's vasculature through an implanted
port. Co-pending applications assigned to the assignee of the
present invention and including related subject matter include:
These patents and pending applications are incorporated herein by
reference.
SUMMARY OF THE INVENTION
[0011] The present invention provides improved methods, systems,
and kits for the extracorporeal circulation and processing of blood
for a variety of purposes including but not limited to
hemodialysis, hemofiltration, hemodifiltration, apheresis and the
like. Particular improvements provided by the present invention
include non-contact measurement of the actual blood flow rate in
the circuit, preferably at a location close or adjacent to the
return access site on the patient. Based on such actual blood flow
measurement, the speed of the blood pump in the flow system can be
adjusted to maintain the measured blood flow rate at a control
point. Thus, operation of the of the system does not rely on an
inferred flow rate based on the operational speed of the pump.
Moreover, by monitoring the pump operation characteristics, system
failures can be detected. For example, a measured blood flow rate
which is significantly below an expected blood flow rate calculated
from the speed at which the pump is being driven and known pump
characteristics indicates a system failure, most likely loss of
blood flow in the return line. Actual power consumption by the pump
which is significantly above an expected power consumption based on
the measured blood flow rate indicates a failure in the blood
circulation system, most likely a blockage in the return line or
elsewhere distal to the pump. The non-contact flow measurement is
preferably performed using an ultrasonic flow detector. Output of
the ultrasonic flow detector is also useful for indicating the
presence of air in the blood flow which can result from a leak
anywhere proximal to (upstream of) the flow detector. Air leaks may
also be detected by an actual pump speed which is higher than
expected for the measured blood rate. The methods and systems for
implementing these safety and monitoring features are described in
more detail below.
[0012] Methods according to the present invention for
extracorporeally processing blood comprise pumping blood with a
pump having a nominal relationship between pump speed and flow
rate, i.e., pump output may be approximated based on pump speed but
will be variable due to the factors discussed above. Such pumps
will usually be positive displacement pumps, typically being
peristaltic pumps which are often preferred since they permit
complete isolation of the blood and reduced risk of blood
contamination. It would be possible, however, to utilize
centrifical and other nonpositive displacement pumps so long as the
pumps permit monitoring of the pump speed and prediction of an
expected flow rate based on the pump speed.
[0013] The blood flow rate delivered by the pump is measured, and
the pump speed is controlled to maintain the measured blood flow at
a control point, typically in the range from 100 ml/min to 1000
ml/min, preferably from 250 min to 500 m/min. Pumped blood is
processed in any desired manner, including dialysis,
hemofiltration, hemodifiltration, apheresis, and the like, and then
returned to the patient. Usually, the blood will be withdrawn from
an artery and returned to a vein or will be withdrawn from a vein
and returned to a vein. It is also possible, although generally
less preferred, to both draw the blood from and return the blood to
an artery.
[0014] The blood flow measuring step is preferably performed with a
non-contact flow sensing device, such as an ultrasonic flow sensor.
By "non-contact," it is meant that no component of the measuring
device need be immersed in or otherwise in contact with the flowing
blood. Preferably, the flow sensors will be mounted or attached
over the blood return line or other conduit of the system. Suitable
ultrasonic flow sensors are commercially available from suppliers,
such as Transonics, Ithaca, N.Y. Other suitable non-contact flow
sensing devices include magnetic flow meters, optical flow
detectors, electrical conductance flow detectors, and the like. The
ultrasonic or other non-contact flow measuring device is preferably
mounted over an exterior surface of a blood return line to the
patient, more preferably being close to the blood return site on
the patient so that the blood is monitored immediately prior to its
return to the patient. Use of the ultrasonic flow sensing device
also permits the detection of entrained air or other gases in the
blood since the ultrasonic signal generated by air passing through
the sensor will be immediately detectable i.e. the air will disrupt
reflectance of the ultrasound signal which can be readily
detected.
[0015] In a preferred aspect of the methods of the present
invention, a failure in the extracorporeal blood flood flow circuit
will be detected by calculating or otherwise determining an
expected blood flow rate value based on the pump speed. Usually,
such a determination can be made by a microprocessor or other
controller which is controlling operation of the system as
described in more detail below. The expected blood flow rate value
is compared with the measured blood flow rate value (i.e. the value
measured by the blood flow measurement device), and a difference is
determined. If the difference exceeds a threshold value, typically
about 5% of the measured flow rate, usually about 10% of the
measured flow rate, then an alarm condition will be initiated. An
alarm condition may comprise an audible, visual, or other signal
being initiated to alert the system user, and/or may include system
shut down, or preferably both.
[0016] In a still further preferred aspect of the method of the
present invention, the blood flow status through the system may be
monitored by measuring or otherwise determining the actual power
being consumed by the pump while it is operating to establish
extracorporeal blood flow. An expected value of the power
consumption level can be determined by the system based on the pump
speed and measured blood flow rate. Any differences between the
actual power level being consumed and the expected power level can
then be determined. If such a difference exceeds a threshold value,
typically above 5% of the measured power consumption, usually above
10% of the measured power consumption, then an alarm condition can
be initiated. The alarm conditions may be any of those set forth
above.
[0017] As a further safety measure, pressure of the blood flow in
the return line from the processor to the patient may also be
detected and monitored. Preferably, the pressure is monitored
externally on the blood return line, e.g. by placing a radially
inward constriction on the return flow line. Radially outward
forces on the constriction can then be monitored and will increase
as the pressure within the flow line increases. Such a system can
be calibrated to provide a rough estimation of pressure within the
blood return line and alarm conditions can be initiated when
threshold values are exceeded.
[0018] Optionally, a safety valve can be placed externally on the
blood return line to positively stop blood flow from the system
should an alarm condition occur.
[0019] Systems according to the present invention comprise a pump,
a processing unit, a blood draw line, a blood return line, an
external flow detector which may be positioned over an exterior
surface of the blood return line, and a control unit. The pump is
of a type generally described above, preferably being a positive
displacement pump, and more preferably being a peristaltic pump.
The processing unit may be a convention hemodialysis,
hemofiltration, hemodifiltration, or apheresis unit. The blood draw
and return lines will typically comprise catheters which are
connectable in the system. In particular, the blood draw line will
be connectable between the patient and the pump, while the blood
return line will be connectable between the processing unit and the
patient. The control unit is preferably a microprocessor and is
connectable to both the pump and the flow detector so that the
control unit can monitor flow and control pump speed according to
the methods described above.
[0020] In particular aspects of the system, the control unit will
be programmed to perform other functions as described in connection
with the methods above. In particular, the control unit can monitor
the actual pump speed and actual blood flow rate to determine if
the expected blood flow rate based on pump speed is being achieved.
Further, the control unit can monitor power consumption by the pump
to determine if it is higher or lower than the expected value of
power consumption based on the measured blood flow rate. Still
further, the control unit can monitor the output of an ultrasonic
flow detector to determine if there are air or other gas bubbles
entrained in the flowing blood. Still further, the control unit may
be programmed to monitor pressure in the blood return line from an
external pressure detector.
[0021] The present invention will still further comprise kits
including system components together with instructions for use. In
a specific embodiment, the kit may comprise a blood draw catheter,
a blood return catheter, and instructions for use setting forth any
of the methods described above. System components will typically be
packaged in a conventional medical device package, such as a pouch,
tray, box, tube, or the like. Instructions may be printed on a
separate sheet of paper or may be printed in whole or in part on
part of the package materials. Usually, the system components will
be maintained in a sterile condition within the packaging.
[0022] In an additional aspect of the present invention, a method
for extracorporeally processing blood comprises drawing blood from
the patient and pumping the drawn blood with a peristaltic pump at
a predetermined stroke volume and rate corresponding to a
theoretical pumped blood flow rate, i.e. a theoretical or expected
value of blood flow rate that can be calculated based upon the
known stroke volume and actual rate at which the peristaltic pump
is being driven. An actual blood flow rate delivered by the pump is
directly measured using any of the techniques described above, and
the measured actual blood flow rate is compared with the
theoretical or calculated blood flow rate. In a first instance, an
alarm condition is signaled if the difference between the actual
blood flow rate and the theoretical blood flow rate exceeds a
predetermined minimum or threshold value. In a second instance, the
rate at which the peristaltic pump is being driven is altered or
varied in order to change the pumped blood flow rate to a desired
value, e.g. one that more closely matches the theoretical pumped
flow rate. It will be appreciated, of course, that the theoretical
blood flow rate will vary over time as the pump speed is varied so
that the theoretical flow rate and a target or control point flow
rate will not always be precisely the same. It will further be
appreciated that both the alarm and control aspects of this method
may be employed or together.
[0023] In yet another aspect of the present invention, apparatus
for extracorporeally processing blood comprises tubing having
connectors for drawing blood from a patient and returning blood to
the patient, where the tubing is connected to or comprises a
section of peristaltic pump tubing. A controller operates the
peristaltic pump at a desired stroke volume and rate which, at
least at the outset, corresponds to a theoretical pumped blood flow
rate. Apparatus directly measures the actual blood flow rate
delivered by the pump through the tubing, and further apparatus
compares the measured actual blood flow rate with the theoretical
pumped blood flow rate. A signal is generated corresponding to a
difference between the theoretical and actual blood flow rates. A
signal may be used to initiate an alarm condition and/or control
the actual pump speed in order to return the actual blood flow rate
to a desired level or control point. Preferably, all tubing in the
apparatus will be free of air-containing chambers, such as drip
chambers.
[0024] In a still further aspect of the present invention, a blood
processing system comprises a pump operable at different speeds to
convey blood through a path. The system further includes a sensor
which monitors blood flow rate and optionally detects the presence
of air in the blood flow. A controller is coupled to both the
sensor and the pump in order to control pump speed (and thus blood
flow rate) as a function of monitored flow rate. Usually, the
controller adjusts the flow rate as a function of deviation between
the monitored blood flow rate and a desired (set point) flow rate.
The control algorithm can be proportional, integral, derivative, or
virtually any other known control algorithm. Usually, the sensor
will be an ultrasonic sensor as described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a schematic illustration of the system
constructing in accordance with the principles of the present
invention performing extracorporeal blood circulation and
processing on a patient.
[0026] FIG. 2 illustrates an exemplary kit constructed in
accordance with the principles of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0027] An exemplary system 10 for performing the methods of the
present invention is illustrated in FIG. 1. The system 10 comprises
a pump 12 coupled to a pump driver 14, a processing unit 16, a
blood inlet line 18, and a blood return line 20. The pump 12 is
illustrated as a peristaltic pump having a pair of opposed rollers
22 which are rotatably driven on an armature 24 to engage a
resilient flow tube 26 (which may optionally be part of the
replaceable inlet line 18). The driver 14 causes the armature 24 to
rotate at a preselected rotational rate, typically comprising a
digitally or servo controlled drive motor. The volumetric flow rate
through the pump 12 may thus be approximated in the first instance
by the internal diameter of the flow tube 26, stroke length of the
pump (i.e., the length of tubing between the engagements points of
the rollers 22), and the rotational rate of the armature 24. For
the purposes of the present invention, it is important that there
be a theoretical relationship between the pump speed, i.e.
rotational rate of the armature 24, and the flow rate. In the case
of the peristaltic pump, the theoretical relationship is linear. It
will be appreciate that other types of pumps could also be
utilized. Preferred pumps include other positive displacement pumps
such as piston pumps, and the like, where the flow rate will have a
linear relationship with the speed at which the pump is driven. It
will also be possible to use centrifical pumps which have a
non-linear, but predictable relationship between the pump speed and
flow rate. The use of peristaltic pumps, however, is most preferred
since in addition to providing a known, theoretically linear
relationship between pump speed and flow, they also provide for
complete isolation of the blood passing thorough the pump.
[0028] The processing unit 16 may be any device or apparatus
intended for the extracorporeal treatment of blood. Most commonly,
the processing unit 16 will be a hemodialysis unit, a
hemofiltration, a hemodifiltration, a apheresis, or the like. Such
processing units will typically have other associated components
which are not shown in FIG. 1. For example, hemodialysis units will
have the components necessary for continuously flowing a dialysate
solution past an internal membrane to perform the desired dialysis
function. Hemofiltration and diafiltration may have components for
regenerating and controlling the filtering operation.
[0029] The blood draw line 18 will typically comprise a flexible
tube or catheter having a distal end 30 adapted to access a
patient's vasculature, e.g. a percutaneous access device adapted to
connect to a subcutaneous port, and a proximal end 32 adapted to
connect to an inlet port 34 of the pump 12. The distal end 30 can
be adapted in a variety of ways. As illustrated, an access needle
36 is provided for percutaneous access to an implanted port, as
generally described in a co-pending application Ser. No.
08/942,990, filed on Oct. 2, 1997, assigned to the assignee of the
present application, the full disclosure of which is incorporated
herein by reference. The access port will be subcutaneously
connected to an artery or a vein to provide a source of blood for
processing as more completely described in the co-pending
application. The blood draw line could also be configured for
connection to transcutaneous catheters, other implanted ports, or
other blood access systems as described in the medical and patent
literature. For use in the present invention, tubes or catheters
comprising the blood draw line will typically have inner lumen
diameters in the range from 2 mm to 8 mm, preferably from 4 mm to 6
mm, and lengths in the range from 50 cm to 300 cm, typically from
120 cm to 180 cm. The tubes or catheters may be composed of
conventional materials, such as polyvinylchloride, silicone
elastomer, polyurethane, and the like.
[0030] The processing unit 16 will receive blood from an outlet
port 40 of the pump 12 via a connector 43. After passing through
the processing unit 16 (typically a dialysis membrane or
hemofiltration filter), the blood will flow outwardly through the
port 42 and into the blood return line 20. The blood return line 20
usually comprises a tube or catheter having a distal end 44 adapted
for accessing the patient vasculature typically through an
implanted port or other conventional access device as described
above. The proximal end 46 of the tube or catheter is preferably
connectable directly to the outlet 42 of the processing unit 16.
Thus, the extracorporeal circuit which is established comprises the
blood draw line 18, the flow tube 26 of the pump 12, the processing
unit 16, and the return line 20. Preferably, at least the draw line
18, flow tube 26, and return line 20 will be disposable and
replaceable with new, sterile components to lower the risk of
patient infection. Usually, at least the internal components of the
processing unit 16 will also be disposable and replaceable for the
same reason. In this way, all system components which contact the
circulating blood will be initially sterile and used only once.
[0031] As described thus far, the extracorporeal circuit is
generally conventional. One significant difference, however, with
many previous systems is that neither the blood draw line nor the
blood return line 20 need include drip chamber(s) to facilitate
pressure monitoring (although the present invention does not
preclude the use of drip chambers). It is a particular advantage of
the present system that the use of such drip chambers is not
necessary.
[0032] The system 10 is monitored and controlled by a control unit
50 which is typically a microprocessor based programmable
controller integrated with the processing unit 16 but which may
also be a separate personal computer or work station. The control
unit 50 will have appropriate input/output devices 52, such as
knobs, dials, a display screen, keyboard, hardisk, floppy disk, CD
drive, and the like, for permitting control, monitoring, and data
acquisition in a generally conventional manner.
[0033] In particular, the control unit 50 will be connected to the
pump driver 14 in order to permit the user to set the desired blood
flow rate, typically in the ranges set forth above. The user will
usually input a value of flow rate, typically in ml/min, and the
control unit 50 will determine the corresponding pump speed which
is expected to provide such a full rate based on the known pump
characteristics. This selected flow rate will be the "expected"
flow rate which is considered in a number of contexts below in
connection with operation of the system. This user-selected
"expected" flow rate will typically be a fixed value throughout the
entire treatment protocol. The flow rate, however, could also be
varied over time in which case the "expected" value for the flow
rate will also vary as the treatment protocol progresses.
[0034] The actual blood flow rate is measured by a flow sensor 60
which is positioned to measure the output of the pump 12 after it
passes through the processing unit 16. The sensor 60 is preferably
a "non-contact" sensor which can be placed over an exterior surface
of the blood return line 20 to measure the blood flow without any
contact between the sensor and the blood itself. In this way, the
flow sensor 60 can be reused without contamination from any
individual patient. Preferably, the flow sensor 60 will be an
ultrasonic flow sensor, such as model HT109, available from
Transonics, Ithaca, N.Y. The ultrasonic sensor is particularly
preferred, however, since it also permits monitoring of gas bubbles
within the return line 20, as described in more detail below.
Output of the flow sensor 60 is directed back to the control unit
50 where it is used for several purposes.
[0035] In particular, real-time determination of the blood flow
rate through the return line 20 can be used for feedback control of
the blood flow rate. While the blood flow rate may be nominally
selected based on the pump speed, feedback of the actual flow from
flow sensor 60 to the control unit 50 permits the control unit to
adjust the 20 pump speed to more precisely achieve the actual blood
flow rate. The control unit 50 can be programmed to implement a
variety of suitable control algorithms, including proportional
control, derivative control, integral control, and combinations
thereof.
[0036] In addition to real-time control of the blood flow rate,
monitoring of the actual blood flow rate with flow sensor 60
permits the system 10 to monitoring for malfunctions. In the first
instance, the control unit 50 can compare the actual flow rate as
measured by the sensor 60 with the flow rate which would be
expected for the pump 12 based on its known relationship between
pump speed and flow output. If the pump speed is significantly
higher than the speed which would be expected for achieving the
actual flow rate, it is likely that the system is malfunctioning.
For example, there may be a blockage between the patient and the
pump 12 which starves the pump of blood. The pump 12 will then turn
faster in response to the control algorithm which is attempting to
maintain the flow control point.
[0037] Alternatively, there may be a leak between the output of the
pump 12 and the flow detector 60, e.g. in the processing unit 16,
which may also cause the pump to turn faster in an attempt to
achieve the control point flow through the flow sensor. In either
case, the control unit 50 can initiate an alarm condition when the
pump speed is greater than the expected speed or the control point
flow rate by some threshold amount, usually at least 1%, more
usually at least 5%, and often 10%, or more, based on the preselect
flow rate. The alarm condition may comprise shutting down the pump
12, initiating a visual or audible alarm and/or closing a safety
valve 70 on the blood return line 20. Usually, all three actions
will be taken.
[0038] When using an ultrasonic flow sensor 60, the system 10 can
also detect the presence of air or other gas bubbles in the return
line 20 to the patient. The ultrasonic reflective characteristics
of blood and gas vary considerably, permitting the control unit 50
to detect the presence of the gas based on a very significant
disruption in the detected ultrasonic signal. The presence of air
or other gases in the blood return in the patient can result from a
leak in the system anywhere upstream of the flow sensor 60.
Regardless of the cause, the system 10 will initiate an alarm
condition generally as described above in the case of pump
overspeed. Since the controller 50 will know the cause, the alarm
condition can indicate that it results from the presence of gas
bubbles in the blood return line.
[0039] Knowledge of the actual flow rate provided by flow sensor 60
to the control unit 50 can also be used to detect a blockage in the
downstream of the pump 12, usually in the return line 20. Any
blockage downstream or distal from the pump 12 discharge port 40
will cause a greater pressure drop across the pump in order to
maintain a given flow rate. Thus, by monitoring the power or
current being consumed by the pump driver 14 through signal line
80, the actual power needed to drive the pump 12 can be compared
with the expected power based on the actual flow rate. When the
actual power consumed by the pump 12 exceeds the expected value by
a threshold amount, typically at least 1%, usually at least 5%, and
often 10% or more, based on the expected power consumption, then an
alarm condition can be initiated generally as described above. An
alarm condition can particularly indicated that there is a blockage
in that portion of the system which is downstream or proximal from
the pump 12.
[0040] Optionally, an external pressure sensor 90 can also be
provided on the blood return line 20. The pressure sensor 90 can be
a collar or other restriction which applies a small radially inward
force on the resilient body of the return line 20. As blood flows
through the return line, the corresponding radially outward
pressure will be applied against the collar or other constriction.
By monitoring this radially outward force, excess pressures through
the return line 20 can be detected. Such secondary pressure
monitoring is desirable for detecting significant overpressures,
typically above 400 mmHg, and can be used to immediately shut down
the system and initiate an alarm condition as described above.
[0041] In operation, the vasculature of a patient P is accessed by
connecting the blood draw line 18 to an arterial or venous source
within the patient. The blood draw line is then connected to the
inlet port 34 of the pump 12. The blood return line 20 is then
connected to a venous return location within the patient P and, at
its other end, to an output port 42 of the processing unit 16. The
flow sensor 60 will then be connected typically about the exterior
of the blood return line 20. Optionally, the stop valve 70 and the
overpressure detector 90 will also be connected to the exterior of
the blood return line 20. The operation in the pump 12 will then be
initiated to begin blood circulation through the draw line 18, pump
12, processing unit 16, and back to the patient to the return line
20. The blood flow rate will be controlled using the active control
scheme described above, while system operation and malfunction will
be monitored, also as described above.
[0042] The present invention will also provide kits 100 including
some or all of the disposable components which can be used with the
system 10 for performing the methods of the present invention. For
example, the kit 100 can include tubes or catheters comprising the
draw line 18 and return line 20 as well as instructions for use 102
setting forth methods for extracorporeally circulating blood as
described above. The catheters 18 and 20 and instructions for use
102 will typically be sterilely packaged within a conventional
medical device package 104, such as a pouch, tray, tube, box, or
the line. Instructions for use 102 will usually be printed on a
separate sheet of paper, but may also be printed in whole or in
part on a portion of the packing materials.
[0043] While the above is a complete description of the preferred
embodiments of the invention, various alternatives, modifications,
and equivalents may be used. Therefore, the above description
should not be taken as limiting the scope of the invention which is
defined by the appended claims.
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