U.S. patent application number 15/670983 was filed with the patent office on 2017-11-23 for control of circulatory assist systems.
The applicant listed for this patent is TC1 LLC. Invention is credited to Kevin Bourque, David J. Burke.
Application Number | 20170333611 15/670983 |
Document ID | / |
Family ID | 45871302 |
Filed Date | 2017-11-23 |
United States Patent
Application |
20170333611 |
Kind Code |
A1 |
Burke; David J. ; et
al. |
November 23, 2017 |
CONTROL OF CIRCULATORY ASSIST SYSTEMS
Abstract
In one general aspect, a method includes measuring blood flow
through a right rotary blood pump, measuring blood flow through a
left rotary blood pump, and controlling a speed of one of the
rotary blood pumps using a controller that calculates the speed of
one of the rotary blood pumps based on the measured blood flow
through the other rotary blood pump.
Inventors: |
Burke; David J.; (Concord,
MA) ; Bourque; Kevin; (Reading, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TC1 LLC |
St. Paul |
MN |
US |
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|
Family ID: |
45871302 |
Appl. No.: |
15/670983 |
Filed: |
August 7, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13242248 |
Sep 23, 2011 |
9757502 |
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15670983 |
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61386018 |
Sep 24, 2010 |
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61472241 |
Apr 6, 2011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61M 1/1086 20130101;
A61M 1/122 20140204; A61M 2205/3365 20130101; A61M 1/101 20130101;
A61M 1/1005 20140204; A61M 2205/3334 20130101 |
International
Class: |
A61M 1/10 20060101
A61M001/10 |
Claims
1. A controller for a heart assist system, comprising: a processing
unit configured to generate a pulsatility index for a right blood
pump, set a speed of the right blood pump based on the pulsatility
index for the right blood pump, generate a pulsatility index for a
left blood pump, and set a speed of the left blood pump based on
the pulsatility index for the left blood pump.
2. A method of controlling a heart assist system, comprising:
calculating a pulsatility index for a right blood pump, the right
blood pump supporting a right ventricle; controlling the speed of
the right blood pump based on the pulsatility index for the right
blood pump; calculating a pulsatility index for a left blood pump,
the left blood pump supporting a left ventricle; and controlling
the speed of the left blood pump based on the pulsatility index for
the left blood pump.
3. The method of claim 2, wherein the right blood pump is a rotary
pump, and wherein the left blood pump is a rotary pump.
4. The method of claim 2, wherein the pulsatility index for the
right blood pump indicates a load on the right ventricle
experienced during contraction of the right ventricle, and wherein
the pulsatility index for the left blood pump indicates a load on
the left ventricle experienced during contraction of the left
ventricle.
5. The method of claim 2, wherein each pulsatility index (PI) is
calculated over a control interval according to the following
equation: PI=(Q.sub.max-Q.sub.min)/Q.sub.ave, where Q.sub.max is a
maximum flow rate through the pump in the control interval,
Q.sub.min is a minimum flow rate through the pump in the control
interval, and Q.sub.ave is an average flow rate through the pump
over the control interval.
6. The method of claim 2, further comprising: measuring blood flow
through the right blood pump; measuring blood flow through the left
blood pump; and controlling a speed of one of the rotary blood
pumps based on the measured blood flow through the other blood
pump.
7. The method of claim 6, further comprising determining whether
the blood flow through one of the blood pumps exceeds a flow
threshold, wherein controlling the speed of the right blood pump
and controlling the speed of the left blood pump comprise, when the
pulsatility index for the right blood pump is below a first target
level and the pulsatility index for the left blood pump is below a
second target level: when the blood flow through the one of the
blood pumps does not exceed the flow threshold, decreasing the
speed of the right blood pump and decreasing the speed of the left
blood pump, and when the blood flow through the one of the blood
pumps exceeds the flow threshold, maintaining the speed of the
right blood pump and maintaining the speed of the left blood
pump.
8. The method of claim 7, wherein controlling the speed of one of
the rotary blood pumps based on the measured blood flow through the
other blood pump comprises: determining that a relationship between
the measured blood flow through the right blood pump and the
measured blood flow through the left blood pump is not satisfied;
and in response to determining that the relationship is not
satisfied, adjusting the speed of the one of the blood pumps such
that the relationship is achieved.
9. The method of claim 2, wherein controlling the speed of the
right blood pump is further based on the pulsatility index for the
left blood pump.
10. The method of claim 2, wherein controlling the speed of the
left blood pump is further based on the pulsatility index for the
right blood pump.
11. The method of claim 2, further comprising detecting a heart
rate, wherein controlling the speed of the right blood pump is
further based on the heart rate, and wherein controlling the speed
of the left blood pump is further based on the heart rate.
12. The method of claim 11, further comprising determining whether
the heart rate exceeds a threshold heart rate, wherein controlling
the speed of the right blood pump and controlling the speed of the
left blood pump comprise, when the pulsatility index for the right
blood pump is below a first target level and the pulsatility index
for the left blood pump is below a second target level: when the
heart rate does not exceed the threshold heart rate, decreasing the
speed of the right blood pump and decreasing the speed of the left
blood pump, and when the heart rate exceeds the threshold heart
rate, maintaining the speed of the right blood pump and maintaining
the speed of the left blood pump.
13. The method of claim 2, further comprising operating the left
blood pump to produce an artificially induced pulsatile blood flow;
and calculating the left pulsatility index such that data
influenced by artificial blood flow variations of the artificially
induced pulsatile blood flow are excluded from calculating the left
pulsatility index.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 13/242,248, filed Sep. 23, 2011, which application claims the
benefit of U.S. Provisional Application No. 61/386,018, filed Sep.
24, 2010 and claims the benefit of U.S. Provisional Application No.
61/472,241, filed Apr. 6, 2011, the entire contents of which are
hereby incorporated by reference in their entirety for all
purposes.
TECHNICAL FIELD
[0002] This disclosure relates to control of circulatory assist
systems.
BACKGROUND
[0003] Heart assist devices or pumps can be inserted in the
circulatory system to pump blood from either ventricle or atrium of
a heart to the vasculature. A pump supplementing a ventricle is
known as a ventricular assist device, or VAD. A VAD is useful when
the ventricle alone is incapable of providing adequate blood flow.
A pump can also completely replace the function of a ventricle. It
is known to use two blood pumps, one assisting or replacing the
right ventricle and one assisting or replacing the left
ventricle.
BRIEF SUMMARY
[0004] In one general aspect, a blood pump controller can set the
motor speed of a blood pump based on blood flow through another
blood pump. One blood pump can support a left ventricle and the
other blood pump can support a right ventricle.
[0005] In another general aspect, a blood pump controller for
controlling right and left rotary blood pumps includes an input
interface configured to receive a signal indicating blood flow
through a right rotary blood pump and a signal indicating blood
flow through a left rotary blood pump. The blood pump controller
includes a processing unit configured to calculate a speed of one
of the rotary blood pumps based on the blood flow through the other
blood pump, and to control one of the rotary blood pumps to operate
at the calculated speed.
[0006] In another general aspect, a controller for a heart assist
system includes a processing unit configured to generate a
pulsatility index for a right blood pump, set a speed of the right
blood pump based on the pulsatility index for the right blood pump,
generate a pulsatility index for a left blood pump, and set a speed
of the left blood pump based on the pulsatility index for the left
blood pump.
[0007] In another general aspect, a method of controlling blood
flow includes measuring blood flow through a right rotary blood
pump, measuring blood flow through a left rotary blood pump, and
controlling a speed of one of the rotary blood pumps using a
controller that calculates the speed of one of the rotary blood
pumps based on the measured blood flow through the other rotary
blood pump.
[0008] Implementations can include one or more of the following
features. For example, the left blood pump supplies blood to a
vasculature; the right blood pump supplies blood to a pulmonary
system; and controlling a speed of one of the blood pumps using a
controller that calculates the speed of one of the blood pumps
based on the measured blood flow of the other blood pump includes
controlling a speed of one of the blood pumps such that the blood
flow through the right rotary blood pump is less than the blood
flow through the left rotary blood pump. Controlling a speed of one
of the blood pumps such that the blood flow through the right
rotary blood pump is less than the blood flow through the left
rotary blood pump includes controlling a speed of one of the blood
pumps such that the blood flow through the right rotary blood pump
is less than the blood flow through the left rotary blood pump by a
minimum percentage of blood flow.
[0009] Implementations can also include one or more of the
following features. For example, controlling a speed of one of the
blood pumps using a controller that calculates the speed of one of
the blood pumps based on the measured blood flow of the other blood
pump includes determining that the measured blood flow through the
right rotary blood pump has changed or that the measured blood flow
through the left rotary blood pump has changed; and in response to
determining that the measured blood flow through the right rotary
blood pump has changed or that the measured blood flow through the
left rotary blood pump has changed, adjusting the speed of the one
of the blood pumps based on the measured blood flow through the
other blood pump.
[0010] Implementations can also include one or more of the
following features. For example, controlling a speed of one of the
rotary blood pumps using a controller that calculates the speed of
one of the rotary blood pumps based on the measured blood flow
through the other rotary blood pump includes determining that a
predetermined relationship between the measured blood flow through
the right rotary blood pump and the measured blood flow through the
left rotary blood pump is not satisfied; and in response to
determining that the predetermined relationship is not satisfied,
adjusting the speed of one of the rotary blood pumps such that the
predetermined relationship is achieved. Controlling a speed of one
of the rotary blood pumps using a controller that calculates the
speed of one of the rotary blood pumps based on the measured blood
flow through the other rotary blood pump includes determining that
the measured blood flow through one of the rotary blood pumps
exceeds a threshold; and reducing the speed of one of the blood
pumps such that the measured blood flow is reduced below the
threshold.
[0011] Implementations can also include one or more of the
following features. For example, while controlling the speed of one
of the rotary blood pumps using the controller that calculates the
speed of one of the rotary blood pumps based on the measured blood
flow through the other rotary blood pump, the speed of the other
rotary blood pump can be controlled to generate a pulsatile flow.
Operating a selected blood pump of the rotary blood pumps at a
first speed for a first period of time; reducing the speed of the
selected blood pump from the first speed to a second speed;
operating the selected blood pump at the second speed for a second
period of time; reducing the speed of the selected blood pump from
the second speed to a third speed; operating the selected blood
pump at the third speed for a third period of time; and increasing
the speed of the selected blood pump from the third speed to the
first speed. Controlling one of the rotary blood pumps to generate
a rate of pressure change that simulates a natural physiologic
pulse. Controlling one of the rotary blood pumps to generate a rate
of pressure change that simulates a natural physiologic pulse
includes changing the operating speed of one of the rotary blood
pumps from a first speed to a second speed higher than the first
speed such that the operating speed overshoots the second speed to
produce the rate of pressure change that simulates a pressure
change of a natural physiologic pulse.
[0012] In another general aspect, a method of controlling a heart
assist system includes calculating a pulsatility index for a right
blood pump, the right blood pump supporting a right ventricle,
controlling the speed of the right blood pump based on the
pulsatility index for the right blood pump, calculating a
pulsatility index for a left blood pump, the left blood pump
supporting a left ventricle, and controlling the speed of the left
blood pump based on the pulsatility index for the left blood
pump.
[0013] Implementations can include one or more of the following
features. For example, the right blood pump is a rotary pump, and
the left blood pump is a rotary pump. The pulsatility index for the
right blood pump indicates a load on the right ventricle
experienced during contraction of the right ventricle, and the
pulsatility index for the left blood pump indicates a load on the
left ventricle experienced during contraction of the left
ventricle. Each pulsatility index (PI) is calculated over a control
interval according to the following equation:
PI=(Q.sub.max-Q.sub.min)/Q.sub.ave, where Q.sub.max is a maximum
flow rate through the pump in the control interval, Q.sub.min is a
minimum flow rate through the pump in the control interval, and
Q.sub.ave is an average flow rate through the pump over the control
interval. Measuring blood flow through the right blood pump,
measuring blood flow through the left blood pump, and controlling a
speed of one of the rotary blood pumps based on the measured blood
flow through the other blood pump.
[0014] Implementations can also include one or more of the
following features. For example, determining whether the blood flow
through one of the blood pumps exceeds a flow threshold, and
controlling the speed of the right blood pump and controlling the
speed of the left blood pump include, when the pulsatility index
for the right blood pump is below a first target level and the
pulsatility index for the left blood pump is below a second target
level: when the blood flow through the one of the blood pumps does
not exceed the flow threshold, decreasing the speed of the right
blood pump and decreasing the speed of the left blood pump, and
when the blood flow through the one of the blood pumps exceeds the
flow threshold, maintaining the speed of the right blood pump and
maintaining the speed of the left blood pump. Controlling a speed
of one of the rotary blood pumps based on the measured blood flow
through the other blood pump includes determining that a
relationship between the measured blood flow through the right
blood pump and the measured blood flow through the left blood pump
is not satisfied, and in response to determining that the
relationship is not satisfied, adjusting the speed of the one of
the blood pumps such that the relationship is achieved.
[0015] Implementations can also include one or more of the
following features. For example, controlling the speed of the right
blood pump is further based on the pulsatility index for the left
blood pump. Controlling the speed of the left blood pump is further
based on the pulsatility index for the right blood pump. Detecting
a heart rate, and controlling the speed of the right blood pump is
further based on the heart rate, and controlling the speed of the
left blood pump is further based on the heart rate. Determining
whether the heart rate exceeds a threshold heart rate, and
controlling the speed of the right blood pump and controlling the
speed of the left blood pump include, when the pulsatility index
for the right blood pump is below a first target level and the
pulsatility index for the left blood pump is below a second target
level: when the heart rate does not exceed the threshold heart
rate, decreasing the speed of the right blood pump and decreasing
the speed of the left blood pump, and when the heart rate exceeds
the threshold heart rate, maintaining the speed of the right blood
pump and maintaining the speed of the left blood pump.
[0016] Implementations can also include one or more of the
following features. For example, operating one of the blood pumps
to produce an artificially induced pulsatile blood flow and
calculating the corresponding pulsatility index for the blood pump
that produces the artificially induced pulsatile flow such that
data influenced by artificial blood flow variations of the
artificially induced pulsatile blood flow are excluded from the
calculating the corresponding pulsatility index. Operating the left
blood pump to produce an artificially induced pulsatile blood flow,
and calculating the left pulsatility index such that data
influenced by artificial blood flow variations of the artificially
induced pulsatile blood flow are excluded from calculating the left
pulsatility index.
[0017] Implementations can also include one or more of the
following features. For example, after controlling the speed of the
right blood pump based on the pulsatility index for the right blood
pump for a first period of time, controlling the speed of the right
blood pump to generate a rate of pressure change that simulates a
natural physiologic pulse. After controlling the speed of the left
blood pump based on the pulsatility index for the left blood pump,
controlling the speed of the left blood pump to generate a rate of
pressure change that simulates a natural physiologic pulse.
Alternating control of the left blood pump or the right blood pump
between (i) control based on a pulsatility index and (ii) control
to generate a rate of pressure change that simulates a natural
physiologic pulse. Repeating a cycle that includes: controlling a
selected blood pump of the blood pumps based on the corresponding
pulsatility index for a first period of time; and controlling the
selected blood pump to generate a pulsatile flow during a second
period of time. Controlling the selected blood pump to generate a
pulsatile flow during a second period of time includes controlling
the selected blood pump to generate a rate of pressure change that
simulates a natural physiologic pulse during the second period of
time. Controlling the selected blood pump to generate a rate of
pressure change that simulates a natural physiologic pulse for a
second period of time includes generating the rate of pressure
change that simulates the natural physiologic pulse by changing an
operating speed of the selected pump from a first speed to a second
speed higher than the first speed such that the operating speed
overshoots the second speed.
[0018] Implementations can also include one or more of the
following features. For example, controlling the selected pump to
generate a pulsatile flow during a second period of time includes:
operating a selected blood pump of the rotary blood pumps at a
first speed for a first period of time; reducing the speed of the
selected blood pump from the first speed to a second speed;
operating the selected blood pump at the second speed for a second
period of time; reducing the speed of the selected blood pump from
the second speed to a third speed; operating the selected blood
pump at the third speed for a third period of time; and increasing
the speed of the selected blood pump from the third speed to the
first speed. Operating a selected pump of the blood pumps to
generate a pulsatile flow, including: operating the selected blood
pump to produce a first blood flow rate through the selected blood
pump associated with the relatively low pressure portion of the
pulsatile blood flow, operating the selected blood pump to produce
a second blood flow rate through the selected blood pump associated
with the relatively high pressure portion of the pulsatile blood
flow, and controlling the selected blood pump to increase a blood
flow rate through the selected blood pump from the first flow rate
to the second flow rate to produce the rate of pressure change that
mimics the rate of pressure change of the natural physiologic
pulse.
[0019] Implementations can also include one or more of the
following features. For example, increasing the speed of the
selected blood pump from the third speed to the first speed
includes increasing the speed of the selected blood pump from the
third speed to a fourth speed, operating the selected blood pump at
the fourth speed for a fourth period of time, and increasing the
speed of the selected blood pump from the fourth speed to the first
speed. The second period of time is longer than a sum of the first
period of time and the third period of time. Operating the selected
blood pump at the first speed, reducing the speed of the selected
blood pump from the first speed to the second speed, operating the
selected blood pump at the second speed, reducing the speed of the
selected blood pump from the second speed to the third speed,
operating the selected blood pump at the third speed, and
increasing the speed of the selected blood pump from the third
speed to the first speed comprise a cycle, and pumping blood in a
pulsatile manner further includes repeating the cycle. The duration
of the second period of time is greater than half of the duration
of the cycle. Operating the selected blood pump at the second speed
for the second period of time includes operating the selected blood
pump to produce a blood flow rate that has a predetermined
relationship relative to an average blood flow rate for the cycle.
Operating the selected blood pump at the second speed for the
second period of time includes operating the selected blood pump to
produce a blood flow substantially the same as the average blood
flow rate for the cycle.
[0020] Implementations can also include one or more of the
following features. For example, one or more of reducing the speed
of the selected blood pump from the first speed to a second speed,
reducing the speed of the selected blood pump from the second speed
to a third speed, and increasing the speed of the selected blood
pump from the third speed to the first speed includes one or more
of a step-wise reduction in speed and a curvilinear reduction in
speed. Operating the selected blood pump at the second speed
includes operating the selected blood pump at the second speed
during at least a portion of a contraction of a ventricle of human
heart that is in blood flow communication with the selected blood
pump. Pumping blood in a pulsatile manner also includes
determining, based on a relationship between a speed of the
selected blood pump and a power consumption of the selected blood
pump, a synchronization between operating the impeller at the
second speed and contraction of a ventricle of a human heart that
is in blood flow communication with the selected blood pump. A
generated pulsatile blood flow includes a temporal rate of change
of blood pressure that approximates a temporal rate of change of
blood pressure of a physiologic pulse. One or more of reducing the
speed of the selected blood pump from the first speed to a second
speed, reducing the speed of the selected blood pump from the
second speed to a third speed, and increasing the speed of the
selected blood pump from the third speed to the first speed
includes generating a drive signal at a first time to produce a
corresponding change in operating speed at a desired time. The
second period of time is greater than the first period of time.
[0021] The details of one or more implementations are set forth in
the accompanying drawings and the description below. Other
features, objects, and advantages will be apparent from the
description and drawings, and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is an illustration of a biventricular assist system
including two blood pumps.
[0023] FIG. 2 is a block diagram of the biventricular assist
system.
[0024] FIG. 3 is a flow diagram of a process for controlling one of
the blood pumps based on blood flow.
[0025] FIG. 4 is a flow diagram of a process for controlling one of
the blood pumps based on a pulsatility index.
[0026] FIG. 5 is a flow diagram of a process for controlling both
blood pumps based on two pulsatility indices and a heart rate.
[0027] FIG. 6 is a flow diagram of a process for controlling both
blood pumps based on two pulsatility indices and blood flow.
[0028] FIGS. 7 to 11 are diagrams illustrating pump speed patterns
for generating an artificial pulse.
[0029] FIG. 12 is a diagram of a computer system.
DETAILED DESCRIPTION OF THE INVENTION
[0030] In a system with two blood pumps, a controller dynamically
adjusts the speed of at least one of the pumps to maintain a
relationship between the blood flow through the pumps. As
physiological conditions of the patient change, the speed of at
least one of the pumps is automatically adjusted to maintain the
relationship. Additionally, the speed of the blood pumps can be
adjusted to maintain a target load on one or both ventricles
supported by the blood pumps.
[0031] Referring to FIG. 1, a biventricular assist system 10 for
treating, for example, a patient with a weakened left ventricle 12
and a weakened right ventricle 14, includes a left blood pump 16
and a right blood pump 18. The left blood pump 16 receives blood
from the left ventricle 12 and supplies blood to the patient's
vasculature. The right blood pump 18 receives blood from the right
ventricle 14 and supplies blood to the patient's pulmonary system.
The pumps 16, 18 are operated by independent control signals and
can be independent units capable of being implanted separately.
[0032] As an alternative to the configuration of FIG. 1, rather
than support weakened ventricles, the pumps 16, 18 can entirely
replace the function of the left and right ventricles,
respectively. For example, one or both of the ventricles can be
removed, and the pump(s) can take over the function of the
ventricle(s).
[0033] The pumps 16, 18 can be non-pulsatile pumps, for example,
rotary pumps such as axial flow pumps or centrifugal pumps. In some
implementations, one of the pumps is a centrifugal pump and the
other pump is an axial flow pump. Each pump 16, 18 includes a
motor. The motor speed of each pump 16, 18, which corresponds to
the pump speed, is the dominant factor that affects blood flow
through the pumps 16, 18. Thus the pump speed determines the level
of support provided to the ventricles 12, 14 by the system 10.
Also, as described further below, the pumps 16, 18 can be
non-pulsatile pumps that are operated in an artificial pulse mode.
In such case, the nature of the blood flow is a factor that affects
the nature of support provided to the patient.
[0034] Referring to FIG. 2, the biventricular assist system 10
includes a controller 20 that controls the operation of the left
blood pump 16 and the right blood pump 18. The controller 20 is
implanted, for example, in the patient's abdomen near the pumps 16,
18. Alternatively, the controller 20 can reside outside of the
patient's body. The controller 20 coordinates operation of the
pumps 16, 18 and ensures that the circulatory needs of the patient
are met. For example, the controller 20 sets the speed of each pump
16, 18 to provide a desired level of circulatory support. As
physiological conditions of the patient change, the controller 20
varies the speed of the pumps 16, 18 to adjust the level of support
provided. For example, the controller 20 increases the speed of the
pumps 16, 18 to increase circulatory support when needed, and
decreases the speed of the pumps 16, 18 to avoid dangerous
conditions, such as inducing suction in one of the ventricles 12,
14.
[0035] The controller 20 can be implemented as a single device
separate from the pumps 16, 18, can be integrated into one of the
pumps 16, 18, or the functions performed by the controller 20 can
be distributed among several different devices.
[0036] The controller 20 includes a processing unit 22 that
calculates the appropriate speed for each pump 16, 18. The
controller 20 includes memory 24 that stores target operating
parameters for the pumps 16, 18 and results of calculations by the
processing unit 22. The processing unit 22 can include one or more
processing devices. The memory 24 also stores executable
instructions that, when executed by the processing unit 22, cause
the controller 20 to perform the operations described below,
including calculating speeds for the pumps 16, 18 in response to
changing conditions. Alternatively, the processing unit 22 can
include fixed-function logic that performs control operations.
[0037] Input to the controller 20 can be received through an input
interface (not shown) which can provide an interface to receive
data from sensors, the blood pumps 16, 18, and other devices.
Output from the controller 20 can be provided through an output
interface (not shown) to, for example, a display or a computer
system.
[0038] The controller 20 includes a speed control unit 26 that
outputs control signals causing the pumps 16, 18 to operate at the
speeds calculated by the processing unit 22. The speed control unit
26 communicates with the pumps 16, 18 over communication links 32,
34, which carry power and control signals. The speed control unit
26 varies a voltage or current supplied to the pumps 16, 18 to
change the speed of the pumps 16, 18, which changes the flow of
blood through the pumps 16, 18. The speed control unit 26 also
measures operating conditions of the pumps 16, 18, such as current
speed, power consumption, electrical current draw, and back
electromotive force (BEMF) of the pumps 16, 18, which the
processing unit 22 uses to calculate blood flow through the pumps
16, 18 and other operating parameters of the pumps 16, 18. The
controller 20 sets the speed of the pumps 16, 18 independently, for
example, using a different control signal to set the speed of each
pump 16, 18.
[0039] The power consumed by the pumps 16, 18 is proportional to
the speed of the motor of the pumps 16, 18, and thus proportional
to the blood flow through the pumps 16, 18. The processing unit 22
calculates blood flow through the pumps 16, 18 using the current
draw, rotational speed, and empirical constants known for a
particular pump. Changes in power consumption or current draw by
the pumps 16, 18 indicate changes in blood flow through the pumps
16, 18.
[0040] The system 10 includes a heart rate sensor 40 to measure the
heart rate of the patient, a left blood flow sensor 36 to measure
blood flow through the left blood pump 16, and a right blood flow
sensor 38 to measure blood flow through the right blood pump 18.
The controller 20, in addition to, or instead of measuring blood
flow using pump operating data, measures blood flow through the
pumps 16, 18 using outputs of the blood flow sensors 36, 38. In
some implementations, pressure sensors can be included in addition
to, or as an alternative to, the blood flow sensors 36, 38. Blood
flow through the pumps 16, 18 can also be calculated based on the
input of the pressure sensors.
[0041] The system 10 receives power from a power source 28, such as
a battery or power conversion unit. The power source 28 is located
outside the patient, and electrical power is transmitted to the
system 10 through a percutaneous driveline 30 or through inductive
coupling.
[0042] The controller 20 communicates with a clinical device 42
external to the patient. The controller 20 and the clinical device
communicate via a telemetric interface 44, which may be wired or
wireless. In some implementations, the telemetric interface 44 is
integrated with the percutaneous driveline 30. Using the clinical
device 42, a clinician can access current and historical
information about the operation of the system 10 from the memory
24, and can perform diagnostics for the system 10.
[0043] Using the clinical device 42, the clinician can also input
operating parameters for the system 10, including target levels of
support for each of the ventricles 12, 14 as determined from
examination of the patient. The clinician can input, for example, a
desired pump speed, blood flow, and/or pulsatility index for one or
both of the pumps 16, 18. The clinician can also select a control
mode with which the controller 20 operates the pumps 16, 18, or
enter new programming for the controller 20.
[0044] The controller 20 controls the operation of the pumps 16, 18
according to one of several different control modes. Using any of
the different control modes, the controller 20 automatically sets
the speed of one or both of the pumps 16, 18 to provide appropriate
ventricular support as physiological conditions of the patient
change. The control modes include, for example, controlling one or
more of the pumps 16, 18 based on (1) blood flow through the pumps
16, 18, (2) a pulsatility index for one of the ventricles 12, 14,
(3) a pulsatility index for each of the ventricles 12, 14 and a
heart rate, and (4) a pulsatility index for each of the ventricles
12, 14 and blood flow through the pumps 16, 18. For the operation
of the pumps 16, 18, both continuous operation and pulse-like
operation modes can be implemented.
[0045] A pulse-like operation mode is also described below. The
controller 20 can generate an artificial pulse by modulating a
rotor speed of one of the blood pumps 16, 18. In some
implementations, the control of a pump at a given time does not
simultaneously include pulse-like operation and a control based on
natural pulsatility of a ventricle 12, 14. For instance, the
controller 20 can alternate control modes between the pulse mode
and a continuous mode, or only one control mode can be selected for
implementation. Implementation of pump control that alternates
between control modes may be chosen based on a condition of the
patient. The control modes can be changed, for example, hourly,
daily, weekly, monthly, or according to a period having any
duration in length ranging from minutes to weeks. Furthermore, it
is possible to implement a pulse mode for one VAD that operates
simultaneously with a continuous mode for another VAD. Moreover, if
the ventricles are completely excised, either a fixed flow rate
mode or a pulse mode of operation can be selected for either
VAD.
[0046] In some implementations, both the pulsatility index control
and the artificial pulse control are performed simultaneously. As
described further below, the artificial pulse can be generated
without interfering with the pulsatility index calculations. For
example, the controller 20 can exclude data collected near the time
of an artificial pulse perturbation from the pulsatility index
calculation.
[0047] Examples of non-pulsatile control modes are described below,
followed by examples of pulsatile control modes.
[0048] For each of the control modes described below, the
controller 20 adjusts the speed of each pump 16, 18 in increments,
for example, by increasing or decreasing pump speeds by a set
amount, such as 100 rotations per minute (rpm). In the processes
described in FIGS. 3 to 6, when the controller 20 adjusts a pump
speed, the pump speed is adjusted by one increment. Later
repetitions of the processes, occurring periodically, can further
adjust the pump speeds. By gradually adjusting the speeds over
time, the controller 20 detects the response of the patient's
circulatory system and can prevent overcorrection of pump
speeds.
[0049] The size of the increments can vary based on the
characteristics of the pumps 16, 18, and the increments for the
pumps 16, 18 can be different. The increments can be selected for
each pump 16, 18 to effect a particular change in blood flow, such
as a change of 0.1 liters/minute (l/min). As an example, at a given
pressure, a change of 0.1 l/min may correspond to a change of 100
rpm for a first pump, and a change of 300 rpm for a second pump
with different operating characteristics. Over the range of
operation of the pumps 16, 18, the speed-flow response can be
generally linear, allowing for a consistent increment for each pump
16, 18. In some implementations, the increments for each pump 16,
18 can be varied over the operating range to compensate for a
non-linear flow response.
[0050] Because the output pressure for the right blood pump 18 is
lower than the output pressure for the left blood pump 16, the
speed change increment to effect a particular change in blood flow
for the right blood pump 18 is typically less than the increment
for the left blood pump 16. Thus when the pumps 16, 18 have the
same operating characteristics, each incremental speed change for
the left blood pump 16 is typically larger than the incremental
speed change for the right blood pump 18.
[0051] To limit the risk of overpumping and suction, the controller
20 decreases pump speeds more quickly than the controller 20
increases pump speeds. Accordingly, the speed adjustment increments
to decrease pump speed are greater than the speed adjustment
increments to increase pump speed. For example, the increment to
increase a pump speed may be 75 rpm, corresponding to a flow change
0.1 l/min, and the increment to decrease the pump speed may be 150
rpm, corresponding to a flow change of 0.2 l/min.
[0052] In some implementations, as an alternative to incremental
adjustment, the controller 20 adjusts the speeds of the pumps 16,
18 according to known head and flow (HQ) characteristics of the
pumps 16, 18 to reach a desired blood flow or blood pressure. The
controller 20 calculates pump speeds to correspond to the desired
blood flow or blood pressure and sets pumps 16, 18 to operate at
the calculated speeds.
[0053] The controller 20 operates the pumps 16, 18 within a speed
range, which is selected separately for each pump 16, 18. The upper
and lower limits of the speed ranges are selected based on the
prevailing condition of the patient's body and the patient's
circulatory support needs. Typically, due to the higher output
pressure facing the left blood pump 16, when the pumps 16, 18 have
the same operating characteristics, the left blood pump 16 operates
at speeds in a range higher than the range in which the right blood
pump 18 operates, although the ranges may overlap.
[0054] The controller 20 also performs calculations to detect and
avoid overpumping of the ventricles 12, 14, thus avoiding suction
and/or distending of the ventricles 12, 14. The controller 20
determines the pumping state of the ventricles 12, 14 based on, for
example, measured ventricular pressure, pump blood flow, and/or a
pulsatility index (described below) for the ventricle 12, 14. For
example, the controller 20 can detect and prevent suction of a
ventricle using the techniques described in U.S. Pat. No. 6,991,595
and/or the techniques described in U.S. patent application Ser. No.
12/394,264, each of which is incorporated herein by reference in
its entirety.
(1) Control Based on Blood Flow
[0055] In a flow-balancing control mode, the controller 20 sets the
speed of one of the pumps 16, 18 such that a predetermined
relationship between blood flow through the pumps 16, 18 is
maintained. The controller 20 designates one of the pumps 16, 18 as
a lead pump, and designates the other pump 16, 18 as a
flow-balancing pump. The lead pump is operated at, for example, a
fixed speed selected to provide a desired level of ventricular
support. The controller 20 sets the speed of the flow-balancing
pump based on blood flow through the lead pump.
[0056] Because the speed of the flow-balancing pump is
automatically adjusted by the controller 20, the system 10 responds
to changes in blood flow through the lead pump without manual
adjustment by a clinician. The controller 20 also adjusts the speed
of the flow-balancing pump when physiological conditions cause
blood flow through the lead pump to change. Regardless of the
control mode selected for the lead pump, the controller 20 varies
the speed of the flow-balancing pump to maintain a predetermined
relationship between blood flow through the flow-balancing pump and
blood flow through the lead pump.
[0057] By contrast, the controller 20 sets the speed of the lead
pump using a control mode independent of the speed of the
flow-balancing pump and the blood flow through the flow-balancing
pump. For example, the controller 20 may operate the lead pump at a
fixed speed selected by a clinician. Alternatively, the controller
20 varies the speed of the lead pump such that blood flow through
the lead pump is maintained at a target rate, or such that blood
flow through the lead pump is maintained in a target range.
[0058] In some instances, if blood flow through the right blood
pump 18 exceeds the blood flow through the left blood pump 16 for a
significant period of time, blood can accumulate in the pulmonary
system, causing the lungs to fill with fluid. To avoid this
condition, known as pulmonary edema, the controller 20 can adjust
the pump speeds such that, for example, blood flow through the left
blood pump 16 (which is typically the lead pump) is greater than or
equal to blood flow through the right blood pump 18 (which is
typically the flow-balancing pump). The controller 20 can also set
the pump speeds such that blood flow through the left blood pump 16
is greater than blood flow through the right blood pump 18 by a
particular percentage, such as 10%, or a particular flow rate, such
as 1.0 liters/minute. Typically, these relationships can be
maintained regardless of which of the pumps 16, 18 operates as the
lead pump or the flow-balancing pump.
[0059] In a normal heart, left ventricular output is typically
greater than right ventricular output by about 10%. While the
cardiac outputs fluctuate, the total right cardiac output should
generally be maintained at or below about 90% of the total left
cardiac output. In some implementations, the cardiac outputs from
the left ventricle 12 and the right ventricle 14 are assumed to be
equal. As a result, the controller 20 maintains blood flow through
the right blood pump 18 at less than 90% of the blood flow through
the left blood pump 16 to operate the system 10 safely.
[0060] Referring to FIG. 3, the controller 20 performs a process
300 to set the speed of the right blood pump 18, which, for
instance, is designated as the flow-balancing pump. Generally, the
left blood pump 16 is then operated as the lead pump. The speed of
the right blood pump 18 is increased after blood flow through the
left blood pump 16 has already increased, resulting in a low risk
of blood flow increases above the desired level due to an increase
in pump speed.
[0061] At the beginning of the process 300, in step 302, the
controller 20 calculates blood flow through the left blood pump 16.
In step 304, the controller 20 calculates blood flow through the
right blood pump 18. The blood flow through each of the pumps 16,
18 is determined as described above, for example, measured using
input from the blood flow sensors 36, 38 or calculated using
rotational speed and current draw of the pumps 16, 18. The
controller 20 determines average blood flow over an interval, such
as 1 second, 5 seconds, or 15 seconds, and can also determine an
instantaneous blood flow rate. In some implementations, rather than
calculating absolute blood flow through the pumps 16, 18, the
controller 20 calculates a relative measure of the blood flow
using, for example, the relative current draw of the pumps 16,
18.
[0062] In step 306, the controller 20 sets the speed of the right
blood pump 18 based on the blood flow through the left blood pump
16. For example, the controller 20 dynamically calculates a target
blood flow for the right blood pump 18 at, for example, 90% of the
blood flow through the left blood pump 16. The controller 20 then
compares the target blood flow to the calculated blood flow through
the right blood pump 18, and adjusts the speed of the right blood
pump 18 up or down so that the target blood flow is achieved. If
the blood flow through the right blood pump 18 is less than the
target blood flow, the controller 20 increases the speed of the
right blood pump 18. By contrast, if the blood flow through the
right blood pump 18 is greater than the target blood flow, the
controller 20 decreases the speed of the right blood pump 18.
[0063] Rather than determining a target blood flow, the controller
20 may compare the blood flow through the pump 16, 18 to determine
whether a predetermined relationship is satisfied, for example,
whether the blood flow through the right blood pump 18 is less than
or within a particular range relative to the blood flow through the
left blood pump 16. If the controller 20 determines that the
relationship is not satisfied, the controller 20 adjusts the speed
of the right blood pump 16 so that the relationship is
achieved.
[0064] The controller 20 repeats the steps of the process 300
approximately once each second to update the speed of the
flow-balancing pump and maintain the relative flow through the
pumps 16, 18. In some implementations, the controller 20 repeats
the process 300 a different periodic rate, substantially
continuously, in response to detected changes in blood flow, or
based on a measured number of heartbeats.
[0065] In some implementations, the speed of the flow-balancing
pump is adjusted in response to determining that blood flow through
one of the pumps has changed, rather than determining that the
desired relationship between the flows is no longer satisfied. Thus
the controller 20 can adjust the speed of the flow-balancing pump
to maintain the desired flow relationship, without requiring the
relationship to be lost before an adjustment is made.
(2) Control Based on a Pulsatility Index
[0066] Using a pulsatility index control mode, the controller 20
sets the speed of one of the pumps 16, 18 such that the
corresponding ventricle 12, 14 experiences a desired load. The
controller 20 designates one of the pumps 16, 18 as a lead pump,
and adjusts the speed of the lead pump to maintain a calculated
pulsatility index, discussed below, at a target level. As a result,
the load on the ventricle remains substantially consistent, even as
physiological conditions change. The controller 20 sets the speed
of the other pump 16, 18 based on blood flow through the lead pump,
using the flow balancing control mode described above.
[0067] The pulsatility of blood flow through a pump indicates the
load experienced by a ventricle supported by the pump. Pulsatility
refers to the amount of variation in blood flow through the pump.
The pump experiences varying input pressures during the cardiac
cycle, resulting in varying blood flow through the pump. Strong
contractions of the ventricle result in large variations in blood
flow during the cardiac cycle, or high pulsatility of blood flow
through the pump. Weak contractions result in lower variations in
blood flow, or lower pulsatility. High pulsatility indicates that a
large amount of blood flows out of the ventricle during systole due
to a strong contraction, whereas low pulsatility indicates that a
smaller amount of blood flows out of the ventricle due to weak
contraction.
[0068] The pulsatility of flow through the pump is correlated to
the peak filling of the ventricle during the cardiac cycle. The
greater the expansion and filling of a ventricle, the greater the
force with which the ventricle contracts to eject the blood in the
ventricle. Thus the pulsatility of flow through the pump, by
indicating the force of contraction of the ventricle, also
indicates the degree to which a ventricle fills with blood.
[0069] The controller 20 calculates a pulsatility index that
indicates the difference between the maximum flow and the minimum
flow through the pump during a particular time period. For example,
the pulsatility index, PI, is be a dimensionless number calculated
according to the equation, PI=(Q.sub.max-Q.sub.min)/Q.sub.ave,
where Q.sub.max is a maximum flow rate through the pump in the
period, Q.sub.min is a minimum flow rate through the pump in the
period, and Q.sub.ave is an average flow rate through the pump over
the period. The quantity Q.sub.ave is calculated, for example, as
the midpoint between Q.sub.max and Q.sub.min, or alternatively as
the total volume divided by the length of the time period of
interest.
[0070] The controller 20 uses the variation in current draw of the
pump over a control interval to calculate the pulsatility index.
Because the current draw of the pump is proportional to blood flow
through the pump, variation in the current draw indicates the
variation in blood flow. Alternatively, the controller 20 uses
input from the blood flow sensors to calculate the pulsatility
index.
[0071] The controller 20 calculates the pulsatility index over a
time period called a control interval. The control interval has a
duration of, for example, one second, in which approximately one to
two heartbeat cycles occur. The control interval can also be
varied, for example, with the speed of the heartbeat. The
pulsatility index can be averaged over multiple control intervals.
The controller 20 stores previous pulsatility indices and generates
an average of previously calculated pulsatility indices, for
example, an average of the pulsatility indices calculated for the
previous fifteen control intervals.
[0072] When a pump supporting a ventricle operates at a fixed
speed, the pump provides a generally fixed degree of ventricular
unloading. As circulatory needs of the patient increase and the
pump speed remains constant, the ventricle becomes increasingly
filled with blood, resulting in the ventricle experiencing an
increased load because the pump does not remove a sufficient amount
of blood from the ventricle. Without adjustment of the pump speed,
the ventricle may fill excessively because the ventricle is
incapable of adjusting to the varying physiological conditions, for
example, the ventricle may lack the ability to achieve a
contraction sufficient to eject the increased amount of blood
filling the ventricle.
[0073] To regulate the load on the ventricle, the controller 20
adjusts the speed of the pump using the calculated pulsatility
index and a target pulsatility index. The target pulsatility index
represents a desired level of load for the ventricle. When
circulatory demands increase, causing the calculated pulsatility
index to exceed the target pulsatility index, the controller 20
increases the speed of the pump to increase support, thus
decreasing the load experienced by the ventricle and reducing the
pulsatility index. Similarly, when the pulsatility index is below
the target level, the controller 20 decreases the speed of the pump
to increase the load experienced by the ventricle and thus increase
the pulsatility index. Thus when physiological conditions change,
rather than allowing the load on the ventricle to increase or
decrease, the controller 20 adjusts the pump allow the load on the
ventricle to remain substantially consistent under different
physiological conditions. Generally, increasing the speed of a pump
will increase ventricular unloading and thus reduce the pulsatility
index. By contrast, decreasing the speed of a pump will permit
increased loading in the ventricle and thus increase the
pulsatility index.
[0074] Referring to FIG. 4, the controller 20 performs a process
400 to control the left blood pump 16 as the lead pump based on a
pulsatility index for the left ventricle 12. Independent of the
process 400, the controller 20 also performs the process 300 (FIG.
3), setting the speed of the right blood pump 18 based on blood
flow through the left blood pump 16. Generally, as described above,
the left blood pump 16 is operated as the lead pump to limit the
risk of pulmonary edema.
[0075] Beginning the process 400, in step 402, the controller 20
calculates a left pulsatility index, PI.sub.L, for the left
ventricle 12, which is an average of the pulsatility indices
corresponding to the previous 15 control intervals. In step 404,
the controller 20 determines whether the left pulsatility index,
PI.sub.L, is above a target pulsatility index, which corresponds to
a particular load on the left ventricle 12. If the pulsatility
index, PI.sub.L, is greater than the target pulsatility index, the
left ventricle 12 is experiencing a greater load than desired. In
response, in step 406, the controller 20 increases the speed of the
left blood pump 16 to increase support to the left ventricle 12,
ending the process 400. The speed of the left blood pump 16 is
increased by a set increment, such as 100 rpm. Increasing the speed
of the left blood pump 16 causes the left ventricle 12 to become
less filled during subsequent cardiac cycles, decreasing the load
experienced by the ventricle 12 and reducing the pulsatility index,
PI.sub.L, toward the target pulsatility index.
[0076] If the controller 20 determines in step 404 that the left
pulsatility index, PI.sub.L, is not greater than the target
pulsatility index, the controller 20 determines in step 408 whether
the left pulsatility index, PI.sub.L, is less than the target
pulsatility index. If so, the left blood pump 16 is providing
excessive support, causing the left ventricle 12 to be
under-loaded. In response, the controller 20 decreases the speed of
the left blood pump 16, ending the process 400. Decreasing the
speed of the left blood pump 16 allows the left ventricle 12 to
fill more completely and provide a greater portion of the
circulatory output. To reduce the risk of suction of the left
ventricle 12, the controller 20 decreases the speed in step 410 by
a larger amount than the increase in speed in step 406, for
example, by 200 rpm.
[0077] In step 408, if the left pulsatility index, PI.sub.L, is not
less than the target pulsatility index, the load experienced by the
ventricle 12 and the level of support provided by the left blood
pump 16 are appropriate. The controller 20 maintains the current
speed of the left blood pump 16, ending the process 400.
[0078] The controller 20 repeats the process 400 to adjust the
support provided by the lead pump to meet to the changing needs of
the patient. In some implementations, the controller 20 performs
the steps of the process 400 at a particular interval, for example,
every 15 seconds. In some implementations, the pump speed is
adjusted each time a pulsatility index for a control interval is
calculated, using a running average of calculations for the
previous 15 control intervals.
[0079] In some implementations, the controller 20 determines in
step 404 and step 406, whether the pulsatility index, PI.sub.L, is
within a particular tolerance of the target pulsatility index. For
example, the controller 20 determines whether the pulsatility
index, PI.sub.L, is within an upper or lower bound of a target
pulsatility index range.
[0080] The techniques described can also be used to control the
right blood pump 18 as the lead pump, and to control the left blood
pump 16 as a flow-balancing pump. In this configuration, the speed
of the right blood pump 18 is based on comparisons between a
pulsatility index for the right ventricle 14 and a target
pulsatility index for the right ventricle 14.
(3) Control Based on Two Pulsatility Indices and Heart Rate
[0081] Using a dual pulsatility index control mode, the controller
20 sets the speeds of both of the pumps 16, 18 to regulate the
loads experienced by both ventricles 12, 14. The controller 20
adjusts the speeds of the pumps 16, 18 using a pulsatility index
calculated for each ventricle 12, 14 and a target pulsatility index
for each ventricle 12, 14. In addition, the controller 20 adjusts
the speeds of the pumps 16, 18 by comparing a heart rate of the
patient to a reference heart rate.
[0082] Referring to FIG. 5, the controller 20 sets the speeds of
the pumps 16, 18 by performing a process 500. In the process 500,
the left blood pump 16 is operated as the lead pump of the system
10, and the speed of both pumps 16, 18 is adjusted based on the
pulsatility indices for both ventricles 12, 14. The process 500
ends after the controller 20 adjusts the pump speeds or determines
that the current pump speeds should be maintained. The process 500
is repeated to adjust the pumps as physiological conditions
change.
[0083] The controller 20 sets the speed of the pumps 16, 18 using
(i) a right pulsatility index, PI.sub.R, for the right ventricle 14
and (ii) a left pulsatility index, PI.sub.L, for the left ventricle
12. The controller 20 calculates the pulsatility indices, PI.sub.R,
PI.sub.L, at the beginning of the process 500, or accesses the
pulsatility indices, PI.sub.R, PI.sub.L, from stored values in the
memory 24. The pulsatility indices, PI.sub.R, PI.sub.L, are
averages of pulsatility index calculations for the 15 most recent
control intervals.
[0084] The controller 20 stores (i) a target pulsatility index for
the right ventricle 14, or right target, T.sub.R, and (ii) a target
pulsatility index for the left ventricle 12, or left target,
T.sub.L. The targets, T.sub.R, T.sub.L, indicate desired loads on
the ventricles 12, 14, and in the process 500, the controller 20
varies the pump speeds to achieve the desired loads. Because the
left blood pump 18 is the lead pump for the process 500, the left
pulsatility index, PI.sub.L, and the left target, T.sub.L,
influence the control of the system 10 to a greater degree than the
right pulsatility index, PI.sub.R, and the right target, T.sub.R.
For example, the system 10 is controlled with a higher priority to
achieve the left target, T.sub.L, than to achieve the right target,
T.sub.R. In addition, the speed of the left pump 16 can be
increased or decreased without a corresponding change in the speed
of the right pump 18. The speed of the right pump 18, however,
changes only when the speed of the left pump 16 changes.
[0085] Beginning the process 500, in step 502, the controller 20
determines whether the left pulsatility index, PI.sub.L, exceeds
the left target, T.sub.L. If so, the controller 20 determines in
step 504 whether the right pulsatility index, PI.sub.R, is greater
than the right target, T.sub.R. If so, then the system 10 is
providing insufficient support to both ventricles 12, 14. As a
result, in step 506 the controller 20 increases the speed of the
left blood pump 16 and increases the speed of the right blood pump
18. Increasing the pump speeds off-loads the ventricles 12, 14
further and causes the pulsatility indices, PI.sub.R, PI.sub.L, to
decrease toward the targets, T.sub.R, T.sub.L.
[0086] Returning to step 504, if the right pulsatility index,
PI.sub.R, is not greater than the right target, T.sub.R, the
controller 20 increases the speed of the left blood pump 16 in step
510, increasing support to the left ventricle 12. Increased support
is needed because, as determined in step 502, the left pulsatility
index, PI.sub.L, exceeds the left target, T.sub.L, indicating
overloading of the left ventricle 12. By increasing the speed of
the left blood pump 16, the load on the left ventricle is reduced
and the pulsatility index, PI.sub.L, decreases toward the target
level, T.sub.L.
[0087] Returning to step 502, if the left pulsatility index,
PI.sub.L, is greater than the left target, T.sub.L, the controller
20 determines in step 512 whether the right pulsatility index,
PI.sub.R, is greater than the right target, T.sub.R. If so, then
the controller 20 decreases the speed of the left blood pump 16 in
step 514. When entering step 514, the left pulsatility index,
PI.sub.L, is known to be at or below the left target, T.sub.L, as
determined in step 502. By decreasing the speed of the left blood
pump 16, support for the left ventricle 12 is reduced, allowing the
left pulsatility index, PI.sub.L, to increase over subsequent
calculations. Because the right ventricle 14 remains overloaded
when the left ventricle 12 is under-loaded, it is assumed that
reducing the speed of the left blood pump 16 to avoid left
ventricular suction will not significantly affect the loading of
the right ventricle 14.
[0088] If the outcome of step 512 is negative, the controller 20
determines in step 516 whether the right pulsatility index,
PI.sub.R, is less than the right target, T.sub.R. If not, then the
right ventricle 14 is experiencing an appropriate load, the
controller 20 maintains the current speeds of the pumps 16, 18. If,
however, the right pulsatility index, PI.sub.R, is less than the
right target, T.sub.R, the controller 20 continues to step 518.
[0089] Entering step 518, comparisons between the pulsatility
indices, PI.sub.R, PI.sub.L, and the targets, T.sub.R, T.sub.L,
indicate that both ventricles 12, 14 are under-loaded, suggesting
that support for the ventricles 12, 14 should be decreased.
Nevertheless, the needs of the patient are not always fully
indicated by the pulsatility indices, PI.sub.R, PI.sub.L. For
example, when the patient begins to exercise, the patient's heart
rate increases but the ventricles 12, 14 do not immediately expand.
Net blood flow through the ventricles 12, 14 increases as the heart
rate increases, but the pulsatility indices, PI.sub.R, PI.sub.L,
for the pumps 16, 18 initially decrease. As the patient's needs for
support are increasing due to the increased exertion, it is
undesirable to decrease ventricular support.
[0090] To distinguish between actual under-loading of the
ventricles 12, 14 and false indications of under-loading, the
controller 20 compares a measured heart rate of the patient to a
reference heart rate. The reference rate target is set at a level
higher than a resting heart rate or an average heart rate for the
patient. For example, the reference heart rate is set at an offset
above a resting heart rate of the patient by a particular
percentage, such as 10%, or a particular amount, such 10 beats per
minute. In some implementations, the reference heart rate can be
set based on a running average of the patient's heart rate over a
time period. A baseline heart rate can be determined as an average
rate over, for example, the previous hour, and the reference heart
rate, for instance, can be set as an offset of 10 or 15 beats per
minute above the baseline rate.
[0091] When the heart rate is above the reference rate, the
exertion of the patient is likely above average, and the
ventricular support should not be decreased. Thus when the
controller 20 determines in step 518 that the patient's heart rate
is above the reference rate the controller 20 maintains the speeds
of the pumps 16, 18.
[0092] By contrast, when the heart rate is below the reference rate
in step 518, the patient is most likely not exercising, and the
ventricles 12, 14 are most likely under-loaded because the pumps
16, 18 are drawing too much blood from the ventricles 12, 14. As a
result, the controller 20 continues to step 520 and decreases the
speeds of both of the pumps 16, 18, allowing the loads experienced
by the ventricles 12, 14 to increase.
[0093] The controller 20 repeats the process 500, adjusting the
speeds of the pumps 16, 18 in response to changing physiological
conditions. The controller 20 recalculates the pulsatility indices,
PI.sub.R, PI.sub.L, and repeats the process 500 periodically to
allow the patient's circulatory system to respond to the changes in
ventricular support. Alternatively, the controller 20 repeats the
process substantially continuously or as new values for the
pulsatility indices, PI.sub.R, PI.sub.L, are calculated.
[0094] The process 500 is summarized as a set of control rules in
Table 1, below. Table 1 includes columns indicating conditions for
(i) the left pulsatility index, PI.sub.L, (ii) the right
pulsatility index, PI.sub.R, and (iii) the heart rate of the
patient. Table 1 also includes a column of actions performed by the
controller 20 in response to the conditions in each row. The
controller 20 performs the action in a given row of Table 1 when
the conditions in the row are determined to be present.
TABLE-US-00001 TABLE 1 Control Rules for the Left Blood Pump 16 as
Lead Pump (Process 500) Left Pulsatility Right Pulsatility Index
(PI.sub.L): Index (PI.sub.R): Heart Rate: Action Above Left Above
Right (any value) Increase the speeds of Target (T.sub.L) Target
(T.sub.R) both pumps 16, 18 Above Left At or Below (any value)
Increase the speed of Target (T.sub.L) Right Target (T.sub.R) the
left blood pump 16 Below Left Above Right (any value) Decrease the
speed of Target (T.sub.L) Target (T.sub.R) the left blood pump 16
Below Left At Right Target (any value) Maintain current speeds
Target (T.sub.L) (T.sub.R) of both pumps 16, 18 Below Left Below
Right Below Decrease the speeds Target (T.sub.L) Target (T.sub.R)
Reference of both pumps 16, 18 Heart Rate Below Left Below Right
Above Maintain current speeds Target (T.sub.L) Target (T.sub.R)
Reference of both pumps 16, 18 Heart Rate
[0095] The controller 20 can also set the speeds of the pumps 16,
18 with the right blood pump 18 designated as the lead pump, using
the control rules described in Table 2, below.
TABLE-US-00002 TABLE 2 Control Rules for the Right Blood Pump 18 as
Lead Pump Right Pulsatility Left Pulsatility Index (PI.sub.R) is:
Index (PI.sub.L) is: Heart Rate is: Action Above Right Above Left
(any value) Increase the speeds of Target (T.sub.R) Target
(T.sub.L) both pumps 16, 18 Above Right At or Below Left (any
value) Increase the speed of Target (T.sub.R) Target (T.sub.L) the
right blood pump 18 Below Right Above Left (any value) Decrease the
speed of Target (T.sub.R) Target (T.sub.L) the right blood pump 18
Below Right At Left Target (any value) Maintain current speeds
Target (T.sub.R) (T.sub.L) of both pumps 16, 18 Below Right Below
Left Below Decrease the speeds of Target (T.sub.R) Target (T.sub.L)
Reference both pumps 16, 18 Heart Rate Below Right Below Left Above
Maintain current speeds Target (T.sub.R) Target (T.sub.L) Reference
of both pumps 16, 18 Heart Rate
(4) Control Based on Two Pulsatility Indices and Blood Flow
[0096] Referring to FIG. 6, the controller 20 performs a process
600 that implements an alternative control mode using the
pulsatility indices, PI.sub.R, PI.sub.L, calculated for each
ventricle 12, 14. Rather than comparing a heart rate to a reference
heart rate, however, the controller 20 compares measured blood flow
to a target blood flow to maintain a generally constant blood flow
through the lead pump while regulating the load on the ventricles
12, 14.
[0097] In the process 600, the left blood pump 16 is operated as
the lead pump. The pulsatility indices, PI.sub.R, PI.sub.L, are
calculated and the targets, T.sub.R, T.sub.L, are set as described
above for the process 500. The controller 20 additionally stores a
target blood flow for the right blood pump 16.
[0098] The process 600 includes many of the same steps as the
process 500. In the process 600, however, the step 518 of the
process 500 for comparing a heart rate to a reference rate is
replaced with step 602, in which blood flow through the lead pump
18 is compared to the target blood flow. The process 600 also
includes an additional step 604, between steps 504 and 506, in
which blood flow through the lead pump 18 is compared to the target
blood flow.
[0099] At the point in the process 600 when step 602 is reached,
both of the pulsatility indices, PI.sub.R, PI.sub.L, have been
determined to be below their respective targets, T.sub.R, T.sub.L.
Under these conditions, if blood flow through the left blood pump
16 is greater than the target blood flow, support for the
ventricles 12, 14 should be reduced. The controller 20 reduces the
speed of both blood pumps 16, 18, allowing the pulsatility indices,
PI.sub.R, PI.sub.L, to rise toward the levels indicated by the
targets, T.sub.R, T.sub.L, and reducing the potential of
ventricular suction due to excessive unloading. In addition,
reducing the speed of the pumps 16, 18 allows the blood flow
through the left pump to decrease toward the target blood flow
level.
[0100] By contrast, if blood flow through the left blood pump 16 is
determined to be at or below the target blood flow, the speed of
the pumps 16, 18 should not be reduced, because a reduction in
speed would cause the blood flow through the left blood pump to
decrease. The controller 20 maintains the speed of the pumps 16, 18
so that the current blood flow through the left pump 16 is
maintained.
[0101] Referring now to step 604, when both ventricles 12, 14
experience a higher than desired load, the controller 20 compares
the blood flow through the left blood pump 16 to the target blood
flow. Step 604 is reached when the right pulsatility index,
PI.sub.R, exceeds the right target, T.sub.R, and the left
pulsatility index, PI.sub.L, exceeds the left target, T.sub.L. If
the blood flow through the left blood pump 16 is less than the
target blood flow, the controller 20 increases the speeds of both
pumps 16, 18 to increase support to both ventricles 12, 14 and
increase the blood flow through the left pump 16.
[0102] If blood flow through the left blood pump 16 is not less
than the target blood flow, the controller determines in step 602
whether blood flow through the left blood pump 16 is greater than
the target blood flow. If blood flow through the left blood pump 16
is greater than the target blood flow, the controller 20 reduces
the speed of the pumps 16, 18. If not, the flow through the left
blood pump 16 is at the target blood flow level, and the controller
20 maintains the current speed of the pumps 16, 18.
[0103] The controller 20 repeats the process 600, adjusting the
speeds of the pumps 16, 18 in response to changing physiological
conditions. For example, the controller 20 recalculates the
pulsatility indices, PI.sub.R, PI.sub.L, and repeats the process
600 approximately once each second as new values for the
pulsatility indices, PI.sub.R, PI.sub.L, are calculated. The
process 600 can also be repeated at other intervals or performed
substantially continuously.
[0104] In some implementations, the target blood flow is a moving
average of blood flow over a particular interval rather than a
fixed value. As a result, comparisons to the target blood flow
indicate whether blood flow through the pump is increasing or
decreasing. In step 602, for example, increasing blood flow through
the left blood pump 16 is increasing can be a strong indication
that the patient's level of activity is increasing, and thus that
the speeds of the pumps 16, 18 should be maintained.
[0105] In some implementations, the blood flow through the right
blood pump 18 is compared to a target blood flow, in addition to or
instead of comparing blood flow through the left blood pump 16. The
decisions in steps 602 and 604 can be based on blood flow through
both pumps 16, 18 to achieve a target blood flow for the right
blood pump 16 and a target blood flow for the left blood pump 16.
For example, in step 602 the pump speeds can be maintained when
either or both of the blood flows through the pumps 16, 18 are at
or below their respective target blood flow.
[0106] The process 600 can be modified to additionally adjust the
pump speeds based on measured a heart rate. The heart rate and
blood flow can together be compared to target values to determine
whether the patient's need for ventricular support is increasing.
For example, in step 602, the controller 20 can determine whether
the heart rate is above a reference rate and whether blood flow
through one or both of the pumps 16, 18 is above the target blood
flow. In some implementations, in step 602, the controller 20
maintains the current pump speeds unless the heart rate is below
the reference rate and the blood flow is above the target blood
flow, in which case the pump speeds are decreased.
[0107] The process 600 is summarized as a set of control rules in
Table 3, below. Table 3 includes columns indicating conditions for
(i) the left pulsatility index, PI.sub.L, (ii) the right
pulsatility index, PI.sub.R, and (iii) the blood flow through the
left blood pump 16. Table 3 also includes a column of actions
performed by the controller 20 in response to the conditions in
each row.
TABLE-US-00003 TABLE 3 Control Rules for the Left Blood Pump 16 as
Lead Pump (Process 600) Blood Flow Left Pulsatility Right
Pulsatility through left Index (PI.sub.L) is: Index (PI.sub.R) is:
pump 16 is: Action Above Left Above Right At Blood Flow Maintain
current Target (T.sub.L) Target (T.sub.R) Target Level speeds of
both pumps 16, 18 Above Left Above Right Above Blood Decrease the
Target (T.sub.L) Target (T.sub.R) Flow Target speeds of both Level
pumps 16, 18 Above Left Above Right Below Blood Increase the Target
(T.sub.L) Target (T.sub.R) Flow Target speeds of both Level pumps
16, 18 Above Left At or Below (any value) Increase the Target
(T.sub.L) Right Target (T.sub.R) speed of the left blood pump 16
Below Left Above Right (any value) Decrease the Target (T.sub.L)
Target (T.sub.R) speed of the left blood pump 16 Below Left At
Right (any value) Maintain current Target (T.sub.L) Target
(T.sub.R) speeds of both pumps 16, 18 Below Left Below Right Above
Blood Decrease the Target (T.sub.L) Target (T.sub.R) Flow Target
speeds of both Level pumps 16, 18 Below Left Below Right At or
Below Maintain current Target (T.sub.L) Target (T.sub.R) Blood Flow
speeds of both Target Level pumps 16, 18
[0108] The controller 20 can also set the speeds of the pumps 16,
18 with the right blood pump 18 designated as the lead pump, using
the control rules described in Table 4, below.
TABLE-US-00004 TABLE 4 Control Rules for the Right Blood Pump 18 as
Lead Pump Blood Flow Right Pulsatility Left Pulsatility through
right Index (PI.sub.R) is: Index (PI.sub.L) is: pump 18 is: Action
Above Right Above Left At Blood Flow Maintain current speeds Target
(T.sub.R) Target (T.sub.L) Target Level of both pumps 16, 18 Above
Right Above Left Above Blood Decrease the speeds Target (T.sub.R)
Target (T.sub.L) Flow Target of both pumps 16, 18 Level Above Right
Above Left Below Blood Increase the speed of Target (T.sub.R)
Target (T.sub.L) Flow Target both pumps 16, 18 Level Above Right At
or Below (any value) Increase the speed of Target (T.sub.R) Left
the right blood pump 18 Target (T.sub.L) Below Right Above Left
(any value) Decrease the speed of Target (T.sub.R) Target (T.sub.L)
the right blood pump 18 Below Right At Left (any value) Maintain
current speeds Target (T.sub.R) Target (T.sub.L) of both pumps 16,
18 Below Right Below Left Above Blood Decrease the speeds Target
(T.sub.R) Target (T.sub.L) Flow Target of both pumps 16, 18 Level
Below Right Below Left At or Below Maintain current speeds Target
(T.sub.R) Target (T.sub.L) Blood Flow of both pumps 16, 18 Target
Level
[0109] Any of the four control modes described above can be used to
control the pumps 16, 18 of the system 10 when the pumps 16, 18 are
configured to support the ventricles 12, 14. When the pumps 16, 18
are configured to replace the right and left ventricles of a heart,
however, only the flow balancing control mode is used. Without
pulsating ventricles to provide varying input pressures to the
pumps 16, 18, there is no variation of flow through the pumps 16,
18. As a result, pulsatility indices cannot be used as control
parameters for the system 10 when the pumps 16, 18 replace the
ventricles completely.
[0110] In some implementations, control can be implemented such
that the left blood pump 16 and the right blood pump 18 operate
independently unless the overpumping of a ventricle 12, 14 occurs.
For example, each pump 16, 18 is operated a fixed speed or based on
a pulsatility index without feedback between the pumps 16, 18. If
overpumping occurs, which may lead to suction and serious
disruptions of overall blood flow, control of the right blood pump
18 becomes limited based on the operation of the left blood pump
16.
[0111] In addition to, or as an alternative to, the techniques
described above, the maximum speed of the right blood pump 18 can
be limited so that the right blood pump cannot generate excessive
outlet pressures that could cause pulmonary edema.
[0112] In some implementations, one or both of the pumps 16, 18 may
be operated for periods of time to produce a pulsatile flow, as
described below. For example, the operating speeds of the pumps 16,
18 can be varied in a manner that generates or intensifies a
pulsatile flow through the pumps 16, 18.
[0113] The controller 20 can control one of the pumps 16, 18 to
produce an artificial pulse where operation at a fixed or constant
speed is described above. Control modes that induce an artificial
pulsatile flow can be used in an alternating sequence with control
modes that use pulsatility index calculations. As an example, the
controller 20 can control one or both of the pumps 16, 18 in a
manner that alternates between periods of pulsatile control and
periods of control that generate substantially continuous flow. For
example, the controller 20 can operate one or both of the pumps 16,
18 to generate a pulsatile flow during a first period. The
controller 20 can then operate one or both of the pumps 16, 18
based on pulsatility index calculations or blood flow during a
second period. The controller 20 alternates between the different
control modes at predetermined intervals. The period of time that
each control mode is active can have a predetermined duration.
[0114] In some implementations, the controller 20 operates one of
the pumps 16, 18 to generate a pulsatile flow while operating the
other pump 16, 18 to generate a substantially continuous flow.
Substantially continuous flow can be generated using a control mode
based on blood flow or a control mode based on pulsatility index
calculations for a ventricle supported by the pump 16, 18 operated
to generate the continuous flow. As an example, the controller 20
can operate one of the pumps 16, 18 using a pulsatile flow control
mode, as described further below. The controller 20 can
simultaneously operate the other pump 16, 18 such that a
predetermined relationship between blood flow through the pumps 16,
18 is maintained, using the techniques described above.
[0115] Various characteristics of the artificial pulse may differ
substantially from those of a physiologic pulse even while
producing a response in the body that is similar to that caused by
the physiologic pulse. It is generally understood that the dominant
source of dissipated energy that characterizes a meaningful pulse
is the pressure wave generated at the start of cardiac systole.
Accordingly, the artificial pulse described herein can include a
relatively brief perturbation of a nature designed to produce such
dissipated energy.
[0116] In some implementations, an artificial pulse cycle includes
a perturbation period that simulates the pulse pressure that occurs
at the leading edge of systole of a physiologic pulse. The
perturbation period can include, for example, a period during which
the blood pump 16 is operated at a low speed, followed immediately
by a period during which the blood pump 16 is operated at a higher
speed. The artificial pulse cycle can also include a period longer
than the perturbation period during which the pump 16 is operated
at an intermediate speed, for example, a speed maintained between
the speeds realized during the perturbation period.
[0117] Operating the pump at the intermediate speed can contribute
to a high operating efficiency. The efficiency achieved can be
greater than, for example, the efficiency of a pump that only
alternates between equal periods of operation at a high speed and
at a low speed. Typically, a continuous flow pump operates with
highest efficiency near the middle of its rotational speed range.
Therefore, it can be advantageous to operate such a pump at or near
a mid-range speed for at least a portion of an artificial pulse
cycle.
[0118] Some of the parameters that affect physiologic phenomena
include pulse pressure and the rate of blood pressure change
(dp/dt). For the blood pump 16, for example, pulse pressure and
time variation in blood pressure are affected by the angular
velocity of the rotor. Thus, the blood pump 16 can be selectively
controlled to produce a pulsatile blood flow pattern, including a
desired pulse pressure and/or a desired rate of pressure change, by
producing a pump speed pattern that includes a time period of
relatively high rotor rotation speeds and a time period of
relatively low rotor rotation speeds. In some implementations, the
pulse pressure produced by the blood pump 16 or produced by the
blood pump 16 and the patient's heart in combination can be
approximately 10 mmHg or more, such as from approximately 20 mmHg
to approximately 40 mmHg.
[0119] For example, the blood pump 16 can be operated to produce a
pump speed pattern 700, illustrated in FIG. 7. The pump speed
pattern 700 includes a first portion 710 with high pump speed
producing a relatively high blood pressure, and a second portion
720 with low pump speed producing a relatively low blood pressure.
Additionally, the pulsatile blood flow pattern can include a
transition between the first portion 710 and the second portion 720
that produces a desired rate of pressure change in the patient's
circulatory system, such as a rate of pressure change that
simulates a natural physiologic pulse and that produces desired
physiological effects associated with rate of pressure change. In
some implementations, the rate of pressure change produced by the
transition is, for example, between 500 mmHg to 1000 mmHg per
second.
[0120] The first portion 710 and/or the second portion 720 of the
pump speed pattern 700 can include multiple segments. In some
implementations, the segments each have predetermined durations. As
also shown in FIG. 7, the first high speed portion 710 of the pump
speed pattern 700 includes a first segment 710a and a second
segment 710b. In the first segment 710a, the rotor is rotated at a
first rotation speed .omega.1 for a first period of time from a
time T0 to a time T1. At the time T1, the rotation speed of the
rotor is rapidly decreased from the first rotation speed .omega.1
to a second rotation speed .omega.2, producing a stepped
transition. The rotor is rotated at the second rotation speed
.omega.2 for a second period of time from the time T1 to a time T2
during a second segment 710b of the first portion 710 of the pump
speed pattern 700. At the time T2, the rotation speed of the rotor
is decreased to a third rotation speed .omega.3 for a third period
of time from the time T2 to a time T4 during the second portion 720
of the pump speed pattern 700. This speed decrease may be as rapid
as the aforementioned speed increase, or more gradual to mimic
pressure changes during native diastole.
[0121] In the pump speed pattern 700, the second rotation speed
.omega.2 is a target high blood flow pump speed, and the first
rotation speed .omega.1 is a desired overshoot pump speed that is
selected to increase the rate of change of the blood pressure
during the first period. The first period of time from the time T0
to the time T1, during which the blood pump 16 is operated at the
first rotation speed .omega.1, is shorter than the second period of
time from the time T1 to the time T2, during which the blood pump
16 is operated at the second rotation speed .omega.2. The first
period of time can be from approximately 0.01 seconds to
approximately 1 second. In some implementations, the first period
of time is approximately 0.05 seconds in duration. In some
implementations, the first period of time can be approximately
equal to, or greater than the second period of time.
[0122] Additionally, the duration of the first period can be
selected to produce a desired pulse pressure, i.e., the difference
between blood pressure before the speed change time T1 and during
the time T1, and can be selected independently of the duration of
the second period of time. The first portion 710, including the
first and second time periods from the time T0 to the time T2, is
longer than the second portion 720. In some implementations, the
first and second time periods from the time T0 to the time T2 can
be shorter than, longer than, or substantially the same duration as
the second portion 720. For example, to increase the duration of
pumping at the higher flow rate relative to pumping at the lower
rate while still benefiting from the occasional pulse, it may be
advantageous for the first portion 710 to be longer than the second
portion 720. If desired, the speed of the blood pump 16 is
increased to the first rotation speed .omega.1 and the pump speed
pattern 700 can be repeated. The pump speed pattern 700 can be
repeated on a continuous or discontinuous basis, and the increase
of rotation speed of the rotor is also sufficiently rapid to
produce a desired rate of pressure change.
[0123] The concept of overshooting the rotation speed .omega.2 with
a greater speed, such as rotation speed .omega.1, is based upon
partly decoupling pulse pressure, i.e. the difference between the
blood pressures before and after the speed change, from the volume
flow rate at the higher speed. Thus, target pulse pressures and
volume flow rates can be attained at various flow conditions. Ideal
values will vary with particular pump design and requirements.
[0124] As shown in FIG. 7, the period 710b can be longer than the
period 710a. The period 710b can also be longer than the portion
720. In some implementations, the duration of the period 710b is
more than half of the duration of the pump speed pattern 700. For
example, the duration of the period 710b can be 60%, 70%, 80% or
more of the duration of the pump speed pattern 700. As an
alternative, depending on patient needs and pump characteristics,
the duration of the period 710b can be 50% or less of the duration
of the pump speed pattern 700, for example, 40%, 30%, 20% or
less.
[0125] Operating the pump at the rotation speed .omega.2 during the
period 710b can contribute to a high hydraulic efficiency during
the pump speed pattern 700. During the pump speed pattern 700, the
pulse pressure generated in a patient's body is generally
correlated to the change in pump rotation speed, for example the
magnitude of the speed change between the speeds .omega.3 and
.omega.1 at time T4. Therefore, to simulate a pressure change that
occurs at the beginning of systole of a physiologic pulse, a
significant speed differential between the rotation speeds .omega.3
and .omega.1 is generally desired. The speed differential can be,
for example, 1000 rpm, 2000 rpm, or more depending on the
characteristics of the blood pump 16. Due to the magnitude of the
speed differential, one or both of the speeds .omega.3 to .omega.1
may occur outside the range of highest operating efficiency of the
blood pump 16.
[0126] The rotation speed .omega.2 can be a speed that results in a
high hydraulic efficiency of the blood pump 16, for example, a
speed near the middle of the operating range of the blood pump 16.
During the pump speed pattern 700, the blood pump 16 can operate at
the speed .omega.2 that results in high efficiency for a
significant portion of the pump speed pattern 700, contributing to
a high efficiency. As described above, the blood pump 16 can
operate at the speed .omega.2 for more than half of the duration at
the pump speed pattern 700. Thus the blood pump 16 can operate in a
highly efficient manner for the majority of the pump speed pattern
700 and can also produce a pressure change that simulates the
beginning of systole of a physiologic heart. Accordingly, some
implementations of the pump speed pattern 700 can provide a higher
efficiency than control modes that attempt to mimic all aspects of
a native cardiac cycle.
[0127] The length of the period 710b relative to the length of the
pump speed pattern 700 can vary based on the frequency of the
artificial pulse. The duration of the period 710a and of the
portion 720, by contrast, can be independent of the pulse rate. To
produce the desired physiological response, a minimum duration for
the period 710a and the portion 720 can be selected, for example,
0.125 seconds. The period 710b can fill the remainder of the pump
speed pattern 700.
[0128] As an example, the pump speed pattern 700 can have a
duration of one second, for a frequency of 60 cycles per minute.
Given that the period 710a and the portion 720 have a combined
duration of 0.125 seconds, the period 710b can have a duration of
0.750 seconds, or 75% of the pump speed pattern 700. As another
example, when the pump speed pattern 700 has a duration of two
seconds (and thus a frequency of 30 cycles per minute), the
duration of the period 710b can be 1.75 seconds, which is 87.5% of
the duration of the pump speed pattern 700.
[0129] In some implementations, the rotation speed .omega.2 is
selected such that the operation of the blood pump 16 at the
rotation speed .omega.2 produces a flow rate that has a
predetermined relationship relative to the average flow rate during
the pump speed pattern 700. The flow rate during the portion 710b
can be within a predefined range of the average flow rate, for
example, within 30% or within 10% of the average flow rate. The
flow rate during the portion 710b can be substantially equal to the
average flow rate.
[0130] Selecting the rotation speed .omega.2 to produce a flow rate
that is substantially equal to the average flow rate can facilitate
a transition between a pulsatile control mode and another control
mode, such as a continuous flow control mode. In some
implementations, the blood pump 16 operates at a particular
constant speed for the greater part of the pump speed pattern 700.
Operation at the constant speed can occur during, for example, the
period 710b. By adjusting the speeds .omega.1 and .omega.3 and
duration of the period 710a and of the portion 720, the average
pump volume flow rate can be tuned to substantially match an
average pump volume flow rate that would be realized in a different
optional setting. Consequently, a clinician or patient can switch
from an artificial pulse mode to another control mode in a manner
that causes only a small difference or no difference in average
volume flow rate. This can provide a clinical advantage when the
artificial pulse is a selectable option among at least one
alternative, for example, a constant speed option.
[0131] As an example, a speed set by a clinician for a constant
speed mode can also be utilized for a constant speed portion of an
artificial pulse mode. The speed can be selected by the clinician
to produce a desired volume flow rate through the blood pump 16
during the constant speed mode (e.g., during continuous flow or
non-pulsatile operation of the blood pump 16). In the artificial
pulse mode, the same selected speed can be used as, for example,
the rotation speed .omega.2 during the period 710b of the pump
speed pattern 700. The speeds .omega.1, .omega.3 and the duration
of the period 710a and the portion 720 are calculated or chosen to
approximately balance the volume flow rate for the pump speed
pattern 700. For example, the reduced flow rate during the portion
720 can offset the increased flow rate during the portion 710a. As
a result, the net volume flow rate during the pump speed pattern
700 can substantially match the volume flow rate during the
constant speed mode. Thus in either the constant speed mode or the
artificial pulse mode, the volume flow rate can be approximately
the same, permitting the clinician to switch from one mode to
another without affecting the volume flow rate. This can help avoid
potentially dangerous conditions that could occur if switching from
one mode to another resulted in sudden changes in flow rate. For
example, a sudden decrease in volume flow rate could cause acutely
insufficient perfusion for the patient, and a sudden increase in
volume flow rate could cause ventricular suction and
arrhythmia.
[0132] As mentioned above, the second portion 710 of the pump speed
pattern 700 can also include multiple segments. For example, as
shown in FIG. 8, a pump speed pattern 800 includes a first portion
810 that has a first segment 810a and a second segment 810b and the
pump speed pattern 800 includes a second portion 820 that has a
first segment 820a and a second segment 820b. During the first
segment 810a, from the time T0 to the time T1, the blood pump 16 is
operated at the first rotation speed .omega.1. At the time T1, the
speed of the blood pump 16 is reduced to the second rotation speed
.omega.2, and the blood pump 16 is operated at the second rotation
speed .omega.2 for the second period of time from the time T1 to
the time T2. At the time T2, the speed of the blood pump 16 is
reduced from the second speed .omega.2 to the third rotation speed
.omega.3. The blood pump 16 is operated at the third rotation speed
.omega.3 for a third period of time from the time T2 to a time T3
during a first segment 820a of the second portion 820 of the pump
speed pattern 800. At the time T3, the speed of the blood pump 16
is increased from the third rotation speed .omega.3 to a fourth
rotation speed .omega.4, and the blood pump 16 is operated at the
fourth rotation speed .omega.4 during a fourth period of time from
the time T3 to the time T4 during a second segment 820b of the
second portion 820 of the pump speed pattern 800. If desired, the
speed of the blood pump 16 is increased to the first rotation speed
.omega.1 and the pump speed pattern 800 can be repeated. The pump
speed pattern 800 can be repeated on a continuous or discontinuous
basis, and the increase of rotation speed of the rotor is also
sufficiently rapid to produce a desired rate of pressure
change.
[0133] Similar to the concept of overshooting .omega.2 in pattern
700, the concept of overshooting the rotation speed .omega.4 with a
lower rotation speed, such as the rotation speed .omega.3, is also
based upon decoupling pulse pressure from the volume flow rate at
the lower rotation speed .omega.4. Thus, the pump speed pattern 800
more completely decouples target pulse pressures and volume flow
rates than the pump speed pattern 700, and ideal values can be
attained, or more closely approximated, at various flow
conditions.
[0134] While a single overshoot pump speed for a transition between
pump speeds are illustrated and described with reference to FIGS. 7
and 8, multiple overshoot pump speeds for one or more transitions
can be used. For example, FIG. 9 illustrates a pump speed pattern
900 that includes multiple overshoot pump speeds for each
transition. The pump speed pattern 900 includes a first portion 910
having a first segment 910a and a second segment 910b, and that
includes a second portion 920 having a first segment 920a and a
second segment 920b. The first segment 910a of the first portion
910 of the pump speed pattern 900 includes a first step 931 during
which the blood pump 16 is operated at the first rotation speed
.omega.1 to overshoot the target pump speed .omega.2 and a second
transition step 433 during which time the blood pump 16 is operated
at a fifth speed .omega.5 to transition from the first rotation
speed .omega.1 to the second rotation speed .omega.2. Similarly,
the first segment 920a of the second portion 920 includes a first
step 941 during which the blood pump 16 is operated at the third
rotation speed .omega.3 and a second segment 443 during which the
blood pump 16 is operated at a sixth speed .omega.6 to transition
between the third speed .omega.3 and the fourth rotation speed
.omega.4. If desired, the speed of the blood pump 16 is increased
to the first rotation speed .omega.1 and the pump speed pattern 900
can be repeated. The pump speed pattern 900 can be repeated on a
continuous or discontinuous basis, and the increase of rotation
speed of the rotor is also sufficiently rapid to produce a desired
rate of pressure change.
[0135] The concept of creating multiple stepwise rotation speed
changes is based upon producing the physiologic response that is
similar to that produced during human cardiac systole and diastole.
This is distinct from mimicking the nature of a native pulse
waveform in its entirety. As described above, greater hydraulic
efficiency can often be achieved by avoiding imitation of the
physiologic pressure waveform over the pulse cycle. It was
previously mentioned that an artificial pulse offers a multitude of
potential clinical advantages. For some or all of these potential
clinical advantages, the benefit of closely matching the energy
dissipated during a healthy native pulse varies. To the extent that
close matching facilitates achieving these potential clinical
advantages, the additional complexity of pattern 900 may be
warranted.
[0136] In contrast to the stepped or discontinuous transitions
discussed above with respect to FIGS. 7 to 9, smooth or continuous
transitions may be used in place of, or in combination with,
stepped transitions between different pump operation speeds. For
example, smooth transitions are illustrated in the pump speed
pattern 1000 of FIG. 10. The pump speed pattern 1000 includes a
first portion 1010 and a second portion 1020. The first portion
1010 includes a first segment 1010a during which the speed of the
pump 16 is decreased gradually, at a strategically-selected rate,
from the first rotation speed .omega.1 to the second rotation speed
.omega.2 from the time T0 to the time T1. The selected rate of pump
speed decrease can be, for example, a particular linear rate or a
particular non-linear rate. During the second segment 1010b of the
first portion 1010, from the time T1 to the time T2, the blood pump
16 is operated at the second rotation speed .omega.2. Similarly,
the second portion 1020 includes a first segment 1020a during which
the speed of the blood pump 16 is increased gradually, at a
strategically-selected rate, from the third rotation speed .omega.3
to the fourth rotation speed .omega.4 from the time T2 to the time
T3. During the second segment 1020b of the second portion 1020,
from the time T3 to the time T4, the blood pump 16 is operated at
the fourth rotation speed .omega.4. If desired, at time T4, there
is a step increase in the rotation speed of the rotor can be
rapidly increased to the first rotation speed .omega.1, and the
pump speed pattern 1000 is repeated.
[0137] The concept of creating multiple speed changes at a
strategically-selected rate is based upon producing the physiologic
response that is similar to that produced during human cardiac
systole and diastole. For example, if very accurate matching of
energy dissipation during a human pulse is necessary, the
additional complexity of pattern 1000 may be warranted.
[0138] The pump speed pattern 1000 illustrates the difference
between stepped transitions discussed above with respect to pump
speed patterns 700-900, produced by rapidly changing the rotation
speed of the rotor, and the gradual transitions of the first
segment 1010a of the first portion 1010 and the first segment 1020a
of the second portion 1020 of the pump speed pattern 1000. Such
gradual transitions can be included, for example, to mimic pressure
changes exhibited during native diastole, as may be achieved by the
gradual transition of the first segment 1010a of the first portion
1010 of the pump speed pattern 1000. In some implementations, one
or more of the rotation speed decreases of a pump speed pattern can
be gradual transitions. For example, a pump speed pattern can
include a gradual decrease in rotation speed from the first
rotation speed .omega.1 to the third rotation speed .omega.3 and a
stepped transition from the third pump speed .omega.3 back to the
first rotation speed .omega.1. Various combinations of stepped and
gradual transitions can be included in a pump speed pattern to
produce a desired arterial pressure wave form, or other desired
physiologic effect. Additionally, the type of transition between
rotation speeds can affect power consumption of the blood pump 16,
and the pump speed pattern can be selected based, at least in part,
on power consumption considerations.
[0139] For all the pump speed patterns discussed it should be
appreciated that although rotor speed is the technological
parameter utilized to impart an artificial pulse, any physiologic
effect is related to the consequential pressure and flow patterns,
including pulse pressure, the maximum time variation in rate of
blood pressure change (dp/dt), and the like. Rotor speed is not
intrinsically physiologically meaningful. The human vascular system
naturally dampens the native pulse produced by the heart, and it
will do the same for an artificial pulse produced as described. The
invention describes a utilitarian combination of factors that
result in a physiological meaningful pulse. Thus, the pump speed
patterns 700-1000 described above are exemplary combinations of
parameters that result in a physiologically meaningful pulse.
[0140] The controller 20 can generate an artificial pulse with one
of the pumps 16, 18 while the pump 16, 18 is controlled based on
pulsatility index calculations. The controller 20 can thus
implement pulsatility index control and artificial pulse control
concurrently for a single pump 16, 18. As described above,
pulsatility index calculations for one of the ventricles 12, 14 can
be used to dynamically set a speed for one or both of the pumps 16,
18. The pump speed can be adjusted in response to changing
physiologic conditions that are reflected in the pulsatility index.
An artificial pulse according to one of the waveforms 700-1000 can
be generated while pump speed is regulated based on pulsatility
index measurements.
[0141] Referring to FIG. 11, the pump speed pattern 700 is produced
by the left blood pump 16 while the blood pump 16 is controlled at
least in part based on the left pulsatility index, PIL. To combine
pulsatility index control and artificial pulse generation, the
controller 20 determines a speed for the pump 16 using the
pulsatility index, PIL, as described above. The controller 20 sets
the determined speed as the rotation speed .omega.2. As the
pulsatility index changes in response to changing physiologic
conditions, the controller 20 adjusts the rotation speed .omega.2
in the same manner that the controller 20 adjusts the pump speed
during a continuous flow control mode.
[0142] In some implementations, the controller 20 sets the speeds
.omega.1 and .omega.3 and the durations of the period 710a and the
portion 720 to maintain a volume flow rate for the pump speed
pattern 700 that is substantially equal to the volume flow rate
produced during operation of the pump 16 at the rotation speed
.omega.2. As the controller 20 adjusts the rotation speed .omega.2
based on changes in the pulsatility index, the controller 20 also
adjusts the speeds .omega.1 and .omega.3 to maintain an appropriate
average flow rate. As a result, the average volume flow rate
produced during the pump speed pattern 700 corresponds to the
average volume flow rate that would be produced using continuous
flow control at the rotation speed .omega.2.
[0143] Because changes in blood flow caused by the pump speed
pattern 700 can interfere with the accuracy of pulsatility index
calculations, the controller 20 calculates the pulsatility index
using blood flow rates measured during selected portions of the
pump speed pattern 700. For example, the controller 20 calculates
the pulsatility index using blood flow measured during a period in
which the pump 16 is operated at a constant speed. As a result, the
effects of the artificially generated pulse are excluded from the
pulsatility index calculations.
[0144] For the pulsatility index calculations, the controller 20
uses measurements of the blood flow that occurs during a period
1120. The period 1120 occurs during the period 710a, during which
the pump 16 is operated at the speed .omega.2. In particular, the
period 1120 begins at a time after the time T1, and the period 1120
can extend to the time T2, just before the pump speed is changed.
The beginning of the period 1120 occurs at a delay D after time T1,
permitting the flow rate through the pump to stabilize after the
speed change at time T1. The delay D can have a duration of, for
example, 0.05 seconds or 0.1 seconds.
[0145] To calculate the pulsatility index, the controller 20
excludes measurements of blood flow through the pump 16 outside the
period 1120. For example, the controller 20 can ignore or not
measure blood flow rates during an excluded period 1110. The
excluded period 1110 includes the period 720a and the portion 720,
during which the artificial pulse can obscure the changes in flow
rates due to the pulsatility of the ventricle 12. As shown, the
excluded period 1110 and the period 1120 alternate such that the
periods 1120 are interleaved between the excluded periods 1110.
[0146] The controller 20 synchronizes the timing of measurements
for pulsatility index calculation with the timing of the pump speed
pattern 700. For example, when blood flow is measured using the
current draw or power consumption of the pump 16, the controller 20
uses only current or power measurements corresponding to pump
activity during the 1120. Similarly, when blood flow is measured
using inflow pressure sensors, outflow pressure sensors, or flow
sensors, only sensor data received within the period 1120 is used.
Thus data that is influenced by artificial blood flow variations
caused by the artificial pulse are excluded from the pulsatility
index calculations.
[0147] In some implementations, the controller 10 increases the
likelihood that a natural physiologic pulse coincides with the
period 1120 by generating the artificial pulse at a non-physiologic
rate. In some implementations, the artificial pulse is generated at
a rate lower than 50 beats per minute, for example, at 30 beats per
minute. At pulse rates below typical physiologic heart rates, each
repetition of the pump speed pattern 700 can include an entire
physiologic pulse cycle, and can also include a fraction or more of
an additional cycle. Two or more complete natural pulse cycles can
occur during the pump speed pattern 700.
[0148] At pulse rates below typical physiological rates, the period
710a and thus the period 1120 comprise a larger portion of the pump
speed pattern 700, increasing the likelihood that each period 1120
will include at least one complete natural pulse cycle. The
duration of the period 1120 can be one second or more. As an
example, at an artificial pulse rate of 30 beats per minute, the
period 710a can have a duration of 1.75 seconds, permitting a
period 1120 of 1.70 seconds. Alternatively, periods 1120 that have
a duration of less than one second can also be used. Even though
each period 1120 may not include a complete natural pulse cycle,
averages of pulsatility index calculations for multiple periods
1120 or maximum or minimum rates across multiple periods 1120 can
be used to produce an accurate indication of pulsatility of the
ventricle.
[0149] In some implementations, the controller 20 synchronizes the
pump speed pattern 700 with the physiologic pulse rate. Thus the
controller 20 can time the period 1120 to include at least one
natural pulse cycle, to include multiple natural pulse cycles, or
to consistently include a particular portion of a natural pulse
cycle. For example, the artificial pulse may be generated at half
of the pulse rate of the natural heart, and the timing of the pump
speed pattern 700 can be set such that each period 1120 includes a
complete natural pulse cycle. The pump speed pattern 700 can be
synchronized such that successive periods 1120 include every other
natural pulse cycle of the patient. Other synchronizations can also
be used, for example, such that successive periods 1120 including
every third natural pulse cycle, or such that periods 1120 each
include two consecutive natural pulse cycles.
[0150] In use, the pump speed patterns 700-1000 can be generated by
the controller 20, which is configured to generate an electrical
drive signal to operate the blood pump 16. For example, the
controller 20 can include a computer system 1200, shown in FIG. 12,
that outputs an electrical current to operate the blood pump 16. In
order to produce the pump speed pattern 700 described above, the
controller 20 outputs a first electrical current from the time T0
to the time T1. At the time T1, the controller 20 reduces the
output electrical current to a second current that is lower than
the first electrical current, and outputs the second electrical
current from the time T1 to the time T2. At the time T2, the
controller 20 reduces the output electrical current from the second
current to a third current, and outputs the third electrical
current from the time T2 to the time T4.
[0151] The computer system 1200 includes one or more processors
1210, memory modules 1220, storage devices 1230, and input/output
devices 1240 connected by a system bus 1250. The input/output
devices 1240 are operable to communicate signals to, and/or receive
signals from, one or more peripheral devices 1260. For example, a
peripheral device 1260 can be used to store computer executable
instructions on the memory modules 1220 and/or the storage devices
1230 that are operable, when executed by the processors, to cause
the controller 20 to generate a waveform to control the operation
of the pump 16 and produce a pump speed pattern, such as the pump
speed patterns 700-1000.
[0152] Additionally, the controller 20 can include a sensor that
provides a signal that indicates activity of the heart. For
example, the controller 20 can include a sensor that provides a
signal indicative of power consumption of the blood pump 16. The
signal can be used to determine when the left ventricle 12
contracts. For example, the power consumption of the blood pump 16
may, for a given operating speed, increase as the left ventricle 12
contracts. Based on the determined heart activity, the controller
20 can adjust the generated control waveform. For example, the
controller 20 can automatically adjust the timing and duration of
the first portion 710 and the second portion 720 of the pump speed
pattern 700 such that the first portion 710 approximately coincides
with a contraction of the left ventricle 12. The pump 16 is
controlled such that the time T0 approximately coincides with a
beginning of a contraction of the left ventricle 12 and the time T2
approximately coincides with an end of the contraction of the left
ventricle 12. The time T4 approximately coincides with a beginning
of a subsequent contraction of the left ventricle 12. Thus, the
durations of the various portions and/or segments of the pump speed
patterns described above can be changed individually or
collectively for one or more repetitions of the pump speed
patterns. Using these techniques, the controller 20 can synchronize
the pulsatile operation of the pump with the natural physiologic
pulse of the heart.
[0153] Alternatively, the controller 20 can generate the control
waveform independently of the activity of the heart and/or to
operate in opposition to the activity of the heart, such as where
the first portion 710 occurs during left ventricular relaxation.
Similarly, the controller 20 can generate a control waveform that
includes a distinctly non-physiologic pulse rate, such as fewer
than 40 high-pressure periods per minute, and the waveform can be
generated independently of native heart function. In some examples,
the blood pump 16 can be operated to produce distinctly physiologic
pulse rates, such as between 50 and 110 high-pressure periods per
minute, and can be controlled dependently or independently of heart
function.
[0154] A number of implementations have been described.
Nevertheless, it will be understood that various modifications may
be made without departing from the spirit and scope of the claimed
invention. For example, the pump speed patterns described above can
be used with various types of blood pumps, including axial flow
blood pumps and centrifugal flow blood pumps. Similarly, the rotors
of blood pumps used to produce pulsatile blood flow patterns as
described above may be electromagnetically-suspended,
hydraulically-suspended, mechanically-suspended, or combinations
thereof. The rotors may also partially be passively
magnetically-suspended. However, the effect of an artificial pulse
may most accurately be simulated by a pump in which the rotor is
electromagnetically suspended, with or without partial passive
magnetic suspension, because in general, other things being equal,
electromagnetic suspension yields a high degree of responsiveness
of the rotor to speed change inputs. For example, mechanical
bearings associated with mechanical suspension and/or very narrow
rotor clearance gaps associated with hydraulic suspension hinder
rapid acceleration of the rotor compared to similar pumps that
employ electromagnetic suspension. Additionally, while the pump
speed patterns described above have been described with regard to a
measure of angular velocity, the pump speed patterns can be
produced with regard to one or more different measures of pump
speeds. Additionally, there may be a delay between a change in
drive signal generated by the controller 20 and a change in
operating speed of the blood pump. Thus, the controller 20 can be
operated such that changes in the output drive signal are effected
at a time to produce a corresponding change in pump operating speed
at a desired time, such as a time that approximately coincides with
selected activity of the heart.
[0155] In some implementations, the pump speed patterns 700-1000
can include additional portions or segments during which the blood
pump is operated at other speeds. For example, at desired times,
the blood pump can be operated to produce a pump speed pattern that
produces a desired physiologic effect, such as opening or closing
the aortic valve. Such operation of the blood pump can interrupt a
generally continuous repetition of a selected one or more of the
pump speed patterns described above, or others, including an
indefinite period of constant speed, and a selected pump speed
pattern can be resumed after the desired physiologic effect has
been produced. The pump speed patterns 700-1000 can also include
different portions or segments. For example, the second segment
710b of the first portion 710 of the pump speed pattern 700 can
include multiple pump speeds. Similarly, the transitions between
pump speeds, such as the reduction in pump speed from the first
rotation speed .omega.1 to the second rotation speed .omega.2, can
include constant, variable, exponential, combinations thereof, or
other rate of speed change over time such that the transition, such
as the first segment 1010a of the first portion 1010 of the pump
speed pattern 1000, is linear, curvilinear, parabolic, logarithmic,
sinusoidal, stepped, or combinations thereof.
[0156] In some implementations, one or more of the pump speed
changes in the pump speed patterns 700-1000 can be monotonic. A
transition from one speed to another may occur gradually over a
period of time, yet change directly from one speed to another. For
example, to decrease a pump speed from a first rotational speed to
a second rotational speed, the controller 20 can reduce the pump
speed without causing an intervening period of increasing pump
speed. Similarly, the transition from the first rotational speed to
the second rotational speed can occur without operating the pump
above the first rotational speed during the transition.
[0157] Additionally, a blood pump can be operated according to a
pump speed pattern that is selected according to a pump power
consumption rate associated with the pump speed pattern, a pump
efficiency associated with the pump speed pattern, a blood flow
rate associated with the pump speed pattern, and/or a rate of blood
pressure change associated with the pump speed pattern. For
example, in a first mode, the controller 20 can be operated to
produce a pump speed pattern that produces a desired rate of blood
pressure change. When a low power condition is detected, the
controller 20 can be switched to a power-saving mode to produce a
pump speed pattern that has a low power consumption rate, even if
the desired rate of pressure change is not produced in the
power-saving mode.
[0158] A number of implementations have been described.
Nevertheless, it will be understood that various modifications may
be made without departing from the spirit and scope of the claimed
invention. For example, the pump speed patterns described above can
be used with various types of blood pumps, including axial flow
blood pumps and centrifugal flow blood pumps. Similarly, the rotors
of blood pumps used to produce pulsatile blood flow patterns as
described above may be electromagnetically-suspended,
hydraulically-suspended, mechanically-suspended, or combinations
thereof. The rotors may also partially be passively
magnetically-suspended. However, the effect of an artificial pulse
may most accurately be simulated by a pump in which the rotor is
electromagnetically suspended, with or without partial passive
magnetic suspension, because in general, other things being equal,
electromagnetic suspension yields a high degree of responsiveness
of the rotor to speed change inputs. For example, mechanical
bearings associated with mechanical suspension and/or very narrow
rotor clearance gaps associated with hydraulic suspension hinder
rapid acceleration of the rotor compared to similar pumps that
employ electromagnetic suspension. Additionally, while the pump
speed patterns described above have been described with regard to a
measure of angular velocity, the pump speed patterns can be
produced with regard to one or more different measures of pump
speeds. Additionally, there may be a delay between a change in
drive signal generated by the controller 20 and a change in
operating speed of the blood pump. Thus, the controller 20 can be
operated such that changes in the output drive signal are effected
at a time to produce a corresponding change in pump operating speed
at a desired time, such as a time that approximately coincides with
selected activity of the heart.
[0159] In some implementations, the pump speed patterns 700 to 1000
can include additional portions or segments during which the blood
pump is operated at other speeds. For example, at desired times,
the blood pump can be operated to produce a pump speed pattern that
produces a desired physiologic effect, such as opening or closing
the aortic valve. Such operation of the blood pump can interrupt a
generally continuous repetition of a selected one or more of the
pump speed patterns described above, or others, including an
indefinite period of constant speed, and a selected pump speed
pattern can be resumed after the desired physiologic effect has
been produced. The pump speed patterns 700 to 1000 can also include
different portions or segments. For example, the second segment
710b of the first portion 710 of the pump speed pattern 700 can
include multiple pump speeds. Similarly, the transitions between
pump speeds, such as the reduction in pump speed from the first
rotation speed .omega.1 to the second rotation speed .omega.2, can
include constant, variable, exponential, combinations thereof, or
other rate of speed change over time such that the transition, such
as the first segment 1010a of the first portion 1010 of the pump
speed pattern 1000, is linear, curvilinear, parabolic, logarithmic,
sinusoidal, stepped, or combinations thereof.
[0160] In some implementations, the pump speed changes in the pump
speed pattern 700 can be monotonic. A transition from one speed to
another may occur gradually over a period of time, yet change
directly from one speed to another. For example, to decrease a pump
speed from a first rotational speed to a second rotational speed,
the controller 20 can reduce the pump speed without causing an
intervening period of increasing pump speed. Similarly, the
transition from the first rotational speed to the second rotational
speed can occur without operating the pump above the first
rotational speed during the transition.
[0161] Additionally, a blood pump can be operated according to a
pump speed pattern that is selected according to a pump power
consumption rate associated with the pump speed pattern, a pump
efficiency associated with the pump speed pattern, a blood flow
rate associated with the pump speed pattern, and/or a rate of blood
pressure change associated with the pump speed pattern. For
example, in a first mode, the controller 20 can be operated to
produce a pump speed pattern that produces a desired rate of blood
pressure change. When a low power condition is detected, the
controller 20 can be switched to a power-saving mode to produce a
pump speed pattern that has a low power consumption rate, even if
the desired rate of pressure change is not produced in the
power-saving mode.
[0162] As mentioned above, in some implementations, the blood pumps
16, 18 can be used to assist a patient's heart during a transition
period, such as during a recovery from illness and/or surgery or
other treatment. In other implementations, the blood pumps 16, 18
can be used to partially or completely replace the function of the
patient's heart on a generally permanent basis.
[0163] The subject matter and the functional operations described
in this specification can be implemented in digital electronic
circuitry, in tangibly-embodied computer software or firmware, in
computer hardware, including the structures disclosed in this
specification and their structural equivalents, or in combinations
of one or more of them. The subject matter described in this
specification can be implemented as one or more computer programs,
i.e., one or more modules of computer program instructions encoded
on a tangible non transitory program carrier for execution by, or
to control the operation of, data processing apparatus. The program
carrier can be a computer storage medium, for example, a
machine-readable storage device, a machine-readable storage
substrate, a random or serial access memory device, or a
combination of one or more of them, as described further below.
Alternatively or in addition, the program instructions can be
encoded on an artificially generated propagated signal, e.g., a
machine-generated electrical, optical, or electromagnetic signal,
that is generated to encode information for transmission to
suitable receiver apparatus for execution by a data processing
apparatus.
[0164] The term "data processing apparatus" encompasses all kinds
of apparatus, devices, and machines for processing data, including
by way of example a programmable processor, a computer, or multiple
processors or computers. The apparatus can include special purpose
logic circuitry, e.g., an FPGA (field programmable gate array) or
an ASIC (application specific integrated circuit). The apparatus
can also include, in addition to hardware, code that creates an
execution environment for the computer program in question, e.g.,
code that constitutes processor firmware, a protocol stack, a
database management system, an operating system, or a combination
of one or more of them.
[0165] A computer program (which may also be referred to or
described as a program, software, a software application, a module,
a software module, a script, or code) can be written in any form of
programming language, including compiled or interpreted languages,
or declarative or procedural languages, and it can be deployed in
any form, including as a stand-alone program or as a module,
component, subroutine, or other unit suitable for use in a
computing environment. A computer program may, but need not,
correspond to a file in a file system. A program can be stored in a
portion of a file that holds other programs or data, e.g., one or
more scripts stored in a markup language document, in a single file
dedicated to the program in question, or in multiple coordinated
files, e.g., files that store one or more modules, sub programs, or
portions of code. A computer program can be deployed to be executed
on one computer or on multiple computers that are located at one
site or distributed across multiple sites and interconnected by a
communication network.
[0166] The processes and logic flows described in this
specification can be performed by one or more programmable
computers executing one or more computer programs to perform
functions by operating on input data and generating output. The
processes and logic flows can also be performed by, and apparatus
can also be implemented as, special purpose logic circuitry, e.g.,
an FPGA (field programmable gate array) or an ASIC (application
specific integrated circuit).
[0167] Computers suitable for the execution of a computer program
can include, by way of example, general or special purpose
microprocessors or both, or any other kind of central processing
unit. Generally, a central processing unit will receive
instructions and data from a read only memory or a random access
memory or both. The essential elements of a computer are a
processing unit for performing or executing instructions and one or
more memory devices for storing instructions and data. A computer
can also include, or be operatively coupled to receive data from or
transfer data to, or both, one or more mass storage devices for
storing data, e.g., magnetic, magneto optical disks, or optical
disks. However, a computer need not have such devices. Moreover, a
computer can be embedded in another device, e.g., a pump, a pump
controller, or a portable storage device, e.g., a universal serial
bus (USB) flash drive or other removable storage module, to name a
few.
[0168] Computer readable media suitable for storing computer
program instructions and data include all forms of non-volatile
memory, media and memory devices, including by way of example
semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory
devices; magnetic disks, e.g., internal hard disks or removable
disks; magneto optical disks; and CD ROM and DVD-ROM disks. The
processor and the memory can be supplemented by, or incorporated
in, special purpose logic circuitry.
[0169] A number of implementations have been described.
Nevertheless, it will be understood that various modifications may
be made without departing from the spirit and scope of the
disclosure.
* * * * *