U.S. patent application number 16/860594 was filed with the patent office on 2020-08-13 for suction detection methods and devices.
The applicant listed for this patent is HeartWare, Inc.. Invention is credited to Michael C. Brown, Neil Voskoboynikov.
Application Number | 20200254165 16/860594 |
Document ID | 20200254165 / US20200254165 |
Family ID | 1000004784964 |
Filed Date | 2020-08-13 |
Patent Application | download [pdf] |
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United States Patent
Application |
20200254165 |
Kind Code |
A1 |
Brown; Michael C. ; et
al. |
August 13, 2020 |
SUCTION DETECTION METHODS AND DEVICES
Abstract
A method of detecting a suction condition during operation of a
rotary blood pump with an inlet connected to a ventricle of the
heart of a patient, an outlet connected to an artery of the
patient, a rotor, and a control circuit configured maintain the
rotor at a set rotational speed. The method includes measuring the
rotational speed of the rotor at a plurality of times during each
of a plurality of speed measurement intervals. A speed range is
determined between a minimum measured speed and a maximum measured
speed during each of the plurality of speed measurement intervals.
At least one additional parameter relating to the operation of the
blood pump is derived. A suction detection signal is generated if
both at least one determined speed range is above a speed range
limit and the at least one additional parameter is indicative of a
suction condition.
Inventors: |
Brown; Michael C.; (Dresher,
PA) ; Voskoboynikov; Neil; (Miami, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HeartWare, Inc. |
Miami Lakes |
FL |
US |
|
|
Family ID: |
1000004784964 |
Appl. No.: |
16/860594 |
Filed: |
April 28, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15665718 |
Aug 1, 2017 |
10675396 |
|
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16860594 |
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62369295 |
Aug 1, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61M 2205/3365 20130101;
A61M 1/1086 20130101; A61M 1/1029 20140204; A61M 1/122 20140204;
A61M 2205/3334 20130101 |
International
Class: |
A61M 1/12 20060101
A61M001/12; A61M 1/10 20060101 A61M001/10 |
Claims
1. A method of detecting a suction condition during operation of a
rotary blood pump with an inlet connected to a ventricle of a heart
of a patient, an outlet connected to an artery of the patient, and
a control circuit configured to maintain a rotor of the rotary
blood pump at a set rotational speed, comprising: (a) monitoring a
flow rate of blood through the pump; and (b) determining a duty
parameter representing a proportion of a time during one or more
cardiac cycles of the patient's heart when the flow rate is above a
crossover flow rate; and (c) generating a suction detect signal
based at least in part on the duty parameter.
2. The method of claim 1, wherein generating the suction detect
signal includes comparing the duty parameter in one or more cardiac
cycles with a duty limit corresponding to a set proportion of the
time during the one or more cardiac cycles and generating the
suction detect signal based at least in part on the comparison.
3. The method of claim 2, further comprising the step of generating
the suction detect signal if the duty parameter in one or more
cardiac cycles represents a proportion above the set
proportion.
4. The method of claim 3, wherein generating the suction detect
signal is performed if the duty parameter in a single cardiac cycle
represents a proportion above the set proportion.
5. The method of claim 4, further comprising deriving at least one
additional parameter relating to operation of the blood pump,
generating at least one additional signal if the at least one
additional parameter is indicative of a suction condition, and
altering operation of the blood pump if both the duty cycle signal
and the at least one additional signal are generated.
6. The method of claim 5, wherein deriving at least one additional
parameter includes measuring a speed of the pump at a plurality of
times during one or more speed measurement intervals at least
partially encompassing the one or more cardiac cycles and
determining a speed range between a minimum measured speed and a
maximum measured speed during each speed measurement interval.
7. The method of claim 5, wherein generating at least one
additional signal is performed by generating a speed range signal
if the speed range exceeds a range limit, and wherein the suction
detect signal is generated if both the speed range signal and the
duty signal are both generated.
8. The method of claim 1, further comprising determining an average
flow rate, and wherein the crossover flow rate is a function of the
average flow rate.
9. The method of claim 8, wherein the crossover flow rate is equal
to the average flow rate.
10. The method of claim 9, further comprising altering operation of
the blood pump responsive to the suction detect signal.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a Divisional of U.S. patent application
Ser. No. 15/665718, filed Aug. 1, 2017 and is related to and claims
priority to U.S. Provisional Patent Application Ser. No. 62/369295,
filed Aug. 1, 2016, entitled SUCTION DETECTION METHODS AND DEVICES,
the entirety of which is incorporated herein by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] n/a
TECHNICAL FIELD
[0003] The present invention relates to a method and system for
detecting suction conditions in a patient having an implantable
blood pump.
BACKGROUND
[0004] Implantable blood pumps may be used to provide assistance to
patients with late stage heart disease. Blood pumps operate by
receiving blood from a patient's vascular system and impelling the
blood back into the patient's vascular system. By adding momentum
and pressure to the patient's blood flow, blood pumps may augment
or replace the pumping action of the heart. For example, a blood
pump may be configured as a ventricular assist device or "VAD."
Where a VAD is used to assist the pumping action of the left
ventricle, the device draws blood from the left ventricle of the
heart and discharges the blood into the aorta.
[0005] To provide clinically useful assistance to the heart, blood
pumps impel blood at a substantial blood flow rate. For an adult
human patient, a ventricular assist device may be arranged to pump
blood at about 1-10 liters per minute at a differential pressure
across the pump of about 10-110 mm Hg, depending on the needs of
the patient. The needs of the patient may vary with age, height,
and other factors.
[0006] If a VAD is operated at an average flow rate in excess of
the average inflow rate of blood to the ventricle, the VAD will
create a suction condition within the ventricle, wherein the
ventricle is collapsed and essentially devoid of blood. This
condition is undesirable. For example, in such a condition the wall
of the ventricle may collapse in such a way that the wall occludes
the pump inlet, causing the flow rate through the pump to decline
rapidly, leading to inadequate blood perfusion. Moreover, if a
suction condition is maintained for a prolonged period, it can
cause damage to the heart. Accordingly, as disclosed, for example,
in U.S. Patent Application Publication Nos. 2015/0367048 ("the '048
Publication") and 2014/0100413 ("the '413 Publication"), the
disclosures of which are incorporated by reference herein, VAD
control systems have been arranged to detect suction conditions.
For example, the '048 Publication discloses methods in which the
control system associated with the pump acquires data representing
flow rate through the pump and examines this data to detect a
suction condition. In one embodiment, the control system examines
the minimum flow rate occurring during one or more cardiac cycles,
the amplitude of the flow rate waveform and the average flow rate
to yield a calculated value. The control system may examine a
plurality of these calculated values representing several cardiac
cycles and determine properties such as the mean, median, mode and
standard deviation of such values to determine whether or not a
suction condition exists. As disclosed in the '413 Publication, the
control system may compare a minimum flow rate occurring during a
current cardiac cycle against a threshold which is set based on
minima of previous cardiac cycles, and determine that a suction
condition exists if the minimum flowrate for the current cardiac
cycle is below the threshold. In either case, the control system
may respond to detection of a suction condition by altering
operation of the pump as, for example, by reducing the set
rotational speed of the rotor, in an effort to clear the suction
condition, by issuing an alarm signal, or both. Despite these and
other improvements in the art, still further improvements would be
desirable. For example, systems based on thresholds established
during previous cardiac cycles can be susceptible to false alarms
when the set rotational speed of the rotor is deliberately changed.
In some cases, the suction detection function is disabled during
speed changes, during pump startup or both. Moreover, if a suction
condition exists immediately after startup of the VAD or when the
detection system is re-enabled after being disabled, the thresholds
may be set based on conditions prevailing during the suction
condition, and the system may ignore the suction condition.
SUMMARY
[0007] The present invention advantageously provides a method of
detecting a suction condition during operation of a rotary blood
pump with an inlet connected to a ventricle of the heart of a
patient, an outlet connected to an artery of the patient, a rotor,
and a control circuit configured maintain the rotor at a set
rotational speed. The method includes measuring the rotational
speed of the rotor at a plurality of times during each of a
plurality of speed measurement intervals. A speed range is
determined between a minimum measured speed and a maximum measured
speed during each of the plurality of speed measurement intervals.
At least one additional parameter relating to the operation of the
blood pump is derived. A suction detection signal is generated if
both at least one determined speed range is above a speed range
limit and the at least one additional parameter is indicative of a
suction condition.
[0008] In another aspect of this embodiment, the speed range limit
is a function of the set speed during a respective one of the
plurality of speed measurement intervals.
[0009] In another aspect of this embodiment, the method further
includes determining an error signal based on at least one from the
group consisting of a difference between the measured speed and the
set speed, an integral of such difference over time, and a first
derivative of such difference.
[0010] In another aspect of this embodiment, generating the suction
detection signal includes generating the suction detection signal
if (i) occurs during one speed measurement interval at least
partially encompassing a cardiac cycle of the patient's heart and
(ii) is occurs during the same cardiac cycle of the patient's
heart.
[0011] In another aspect of this embodiment, the at least one
additional parameter includes a duty parameter representing a
proportion of time during one or more cardiac cycles of the
patient's heart when a flow rate is above a crossover flow
rate.
[0012] In another aspect of this embodiment, the method further
includes repeatedly determining an average flow rate, and wherein
the crossover flow rate is a function of the average flow rate.
[0013] In another aspect of this embodiment, the crossover flow
rate is equal to the average flow rate.
[0014] In another aspect of this embodiment, the method further
includes comparing the duty parameter to a duty limit constant, and
wherein the duty parameter varies directly with the proportion of
time the flow rate is above the crossover flow rate.
[0015] In another aspect of this embodiment, the method further
includes adjusting the set rotational speed of the rotor responsive
to the suction detect signal.
[0016] In another embodiment, a method of detecting a suction
condition during operation of a rotary blood pump with an inlet
connected to a ventricle of a heart of a patient, an outlet
connected to an artery of the patient, and a control circuit
configured to maintain a rotor of the rotary blood pump at a set
rotational speed comprises monitoring a flow rate of blood through
the pump. A duty parameter representing a proportion of a time
during one or more cardiac cycles of the patient's heart when the
flow rate is above a crossover flow rate is determined. A suction
detect signal is generated based at least in part on the duty
parameter.
[0017] In another aspect of this embodiment, generating the suction
detect signal includes comparing the duty parameter in one or more
cardiac cycles with a duty limit corresponding to a set proportion
of the time during the one or more cardiac cycles and generating
the suction detect signal based at least in part on the
comparison.
[0018] In another aspect of this embodiment, the method further
includes the step of generating the suction detect signal if the
duty parameter in one or more cardiac cycles represents a
proportion above the set proportion.
[0019] In another aspect of this embodiment, generating the suction
detect signal is performed if the duty parameter in a single
cardiac cycle represents a proportion above the set proportion.
[0020] In another aspect of this embodiment, the method further
includes deriving at least one additional parameter relating to
operation of the blood pump, generating at least one additional
signal if the at least one additional parameter is indicative of a
suction condition, and altering operation of the blood pump if both
the duty cycle signal and the at least one additional signal are
generated.
[0021] In another aspect of this embodiment, the method further
includes deriving at least one additional parameter includes
measuring a speed of the pump at a plurality of times during one or
more speed measurement intervals at least partially encompassing
the one or more cardiac cycles and determining a speed range
between a minimum measured speed and a maximum measured speed
during each speed measurement interval.
[0022] In another aspect of this embodiment, generating at least
one additional signal is performed by generating a speed range
signal if the speed range exceeds a range limit, and wherein the
suction detect signal is generated if both the speed range signal
and the duty signal are both generated.
[0023] In another aspect of this embodiment, the method further
includes determining an average flow rate, and wherein the
crossover flow rate is a function of the average flow rate.
[0024] In another aspect of this embodiment, the crossover flow
rate is equal to the average flow rate.
[0025] In another aspect of this embodiment, the method further
includes altering operation of the blood pump responsive to the
suction detect signal.
[0026] In yet another embodiment a ventricular assist device
comprises a rotary blood pump having a housing with an inlet and an
outlet, an impeller disposed in the housing between the inlet and
the outlet, the impeller being configured to impel blood within the
housing toward the outlet when rotating, and a motor configured to
rotate the impeller. A control circuit is in communication with the
blood pump and is configured to determine a rotational speed of the
impeller and a flow rate through the pump determine a duty
parameter representing a proportion of time during one or more
cardiac cycles of a patient's heart when the flow rate is above a
crossover flow rate, generate a suction detect signal based at
least in part on the duty parameter, and alter operation of the
pump responsive to detection of a suction condition.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] A more complete understanding of the present invention, and
the attendant advantages and features thereof, will be more readily
understood by reference to the following detailed description when
considered in conjunction with the accompanying drawings
wherein:
[0028] FIG. 1 is a diagrammatic view of a VAD in accordance with
one embodiment of the disclosure in conjunction with part of a
patient's vascular system;
[0029] FIG. 2 is an exploded perspective view depicting part of the
VAD of FIG. 1;
[0030] FIG. 3 is a block diagram of the control circuit of the VAD
of FIG. 1;
[0031] FIG. 4 is a graph depicting pump speed and flow rate;
[0032] FIG. 5 is a graph depicting flow rate and a moving average
flow rate during normal operation;
[0033] FIG. 6 is a graph similar to FIG. 5, but depicting a suction
condition; and
[0034] FIG. 7 is a flow chart depicting a portion of a method
according to one embodiment of the disclosure.
DETAILED DESCRIPTION
[0035] Now referring to the drawings in which like designators
refer to like elements, there is shown in FIGS. 1 and 2 a blood
pump VAD and control circuit constructed according to one
embodiment of the application. In this embodiment, the pump is a
centrifugal pump, such as the HVAD.RTM. Pump manufactured by
HeartWare Inc. in Miami Lakes, Fla., USA. The HVAD.RTM. Pump is
further described in U.S. Pat. Nos. 6,234,772 and 8,512,013, the
disclosures of which are incorporated by reference. The blood pump
101 includes a housing 105 including interlocking casings to form a
closed pumping chamber 103 between them. Blood is supplied to the
pump 101 through an axial inlet cannula 107 adapted for apical
insertion into a heart ventricle. The cannula 107 is affixed to or
may be integral with the housing 105 and is in fluid flow
communication with the pumping chamber 103. Blood exits the pumping
chamber 103 through an outlet 113 in a direction substantially
perpendicular to the longitudinal axis of the inlet cannula 107. As
best seen in FIG. 1, the outlet of the housing can be connected to
an artery of the patient, such as the aorta, by an outlet cannula
109.
[0036] A rotor or pump impeller 122 (FIG. 2) is located within the
pumping chamber 103. The rotor incorporates one or more permanent
magnets (not shown), and sets of electrical coils (not shown) are
disposed in fixed locations within housing 105. The coils and
magnets form a motor. In operation, blood entering the cannula 107
from a heart ventricle passes into the pumping chamber 103 where it
is engaged by the rotating impeller 122. Blood entering the pumping
chamber from the cannula 107 is redirected from axial flow exiting
the cannula to a radial flow within which the impeller 122 is
submerged.
[0037] A power and control cable 150 extends through a feedthrough
130 on the housing, and connects the coils within the housing to a
control circuit 140. Control circuit 140 is connected to a source
of electrical energy 142, which may include a storage battery, a
mains power connection, or both. As further discussed below,
control circuit 140 is arranged to energize the coils of the pump
in sequence so as to apply a rotating magnetic field within the
housing and drive rotor 122 in rotation so that, in operation, the
pump draws blood from the left ventricle V of the patient's heart
and propels the blood through outflow cannula 109 (FIG. 1) into the
patient's aorta.
[0038] The control circuit 140 monitors and further controls
operation of the pump 101. The control circuit functions may be
implemented at least in part by a general-purpose processor, as
shown in the example implementation of FIG. 3. As shown, the
control circuit 140 is implemented using a processor 210, a memory
220, data 230, instructions 240, and a pump interface 250.
Interface 250 may include components such as power semiconductors
connected to the coils of the pump, as well as one or more sensors
for detecting voltages on the pump coils. The control circuit 140
may optionally include an I/O interface 252 that connects the
control circuit 140 to one or more I/O devices 260 adapted to input
information into the control circuit, output information from the
control circuit, or both. The interface 250 may be an analog
interface (e.g., audio interface) or a digital interface, such as
Bluetooth, TCP/IP, Wi-Fi, and others. Where the control circuit is
implemented in an implantable structure adapted to be disposed
within the body of the patient, the I/O interface 252 may include
known elements for communicating signals through the skin of the
patient. Merely by way of example, the I/O device 260 may be a
speaker, a light, a display screen, a communications terminal
(e.g., computer, cell phone), a keyboard, or any other type of
input and/or output device.
[0039] Memory 220 stores information accessible by processor 210,
including instructions 240 that may be executed by the processor
210. The memory also includes data 230 that may be retrieved,
manipulated, or stored by the processor 210. The memory may be of
any type capable of storing information accessible by the
processor, such as a hard-drive, memory card, ROM, RAM, DVD,
CD-ROM, write-capable, and read-only memories. The processor 210
may be any well-known processor, such as commercially available
processors. Alternatively, the processor may be a dedicated
controller such as an ASIC.
[0040] Data 230 may be retrieved, stored, or modified by processor
210 in accordance with the instructions 240. The data may also be
formatted in any computer-readable format such as, but not limited
to, binary values, ASCII or Unicode. Moreover, the data may
comprise any information sufficient to identify the relevant
information, such as numbers, descriptive text, proprietary codes,
pointers, references to data stored in other memories (including
other network locations), or information that is used by a function
to calculate the relevant data. The instructions stored in the
memory may include one or more instruction sets or modules for
performing certain operations. One such module may be a flow
estimation module 242 for performing the steps required to estimate
a flow rate of blood through the pump. Another such module may be a
pump control module 244 for controlling the speed of the pump speed
control module. Another such module may be a suction detection
module for determining the presence of a suction condition and
taking actions in response to the same.
[0041] Although FIG. 2 functionally illustrates the processor and
memory as being within the same block, it will be understood that
the processor and memory may actually comprise multiple processors
and memories that may or may not be stored within the same physical
housing. The memory may include one or more media on which
information can be stored. In one configuration, the medium holding
the instructions retains the instructions in non-transitory form.
Some or all of the instructions and data may be stored in a
location physically remote from, yet still accessible by, the
processor. Similarly, the processor may actually comprise a
collection of processors that may or may not operate in
parallel.
[0042] The control circuit 140 is coupled to the pump and is
configured to collect pump data. The pump data includes speed of
rotation of the pump's rotor and amount of current used to drive
the pump. For example, the control circuit 140 may be configured to
apply pulse width modulated voltage to individual coils in sequence
to provide the rotating magnetic field that turns the rotor. At a
given time in the sequence, one or more of the coils is in an idle
state with no applied voltage. The moving magnets of the rotor
induce voltages in the idle coil, referred to as "back EMF." The
pump interface 250 may be arranged to sample the back EMF in one or
more of the coils during its idle state, and to supply the
resulting sequence of samples to the processor. The instructions in
pump speed control module 244 may cause processor 220 examine these
samples to detect a zero crossing or other recurring feature of the
back EMF waveform, and to determine the time between recurrences.
This time is inversely proportional to the speed of rotation of the
rotor. The processor repeatedly calculates the pump speed at a
repetition rate many times the heart rate of the patient, i.e., a
repetition time much shorter than a cardiac cycle. For example, the
processor may determine the speed of the pump at a repetition rate
of about 20 Hz or more and record the resulting sequence of
rotational speed data 234. In this embodiment, one or more of the
coils of the pump motor serve as sensors for determining the
rotational speed of the rotor. In other embodiments, one or more
separate sensors such as sensing coils separate from the rotor or
Hall effect devices may be used to detect magnetic field changes
associated with rotation, and the signals from these sensors may be
used to detect rotational speed. In still other embodiments, other
known techniques may be used for detecting the rotational
speed.
[0043] The pump speed control module 232 also includes instructions
that cause the processor to adjust the pump interface 250 so as to
maintain the rotational speed of the pump at a set speed. Stated
another way, these instructions cause the processor to act as an
element of a rotor speed control circuit. For example, if the
rotational speed is below the set speed, the processor may instruct
the pump interface to increase the rotational speed by increasing
the current in the coils, as by increasing the duty cycle of the
pulse width modulated voltages applied to the coils. The processor
may execute a proportional-integral-derivative ("PID") feedback
control scheme. In such a scheme, the processor compares the
rotational speed of the pump to the set speed and generates an
error signal that includes a proportional component representing
the difference between the actual speed and the set point; an
integral component representing the integral of the difference, and
a derivative component representing the rate of change in the
difference. The control circuit 140 has a small but finite response
time to a change in the actual speed. As used in this disclosure,
the term "response time" denotes the time from an instantaneous
step change in actual speed that causes the actual speed to deviate
from the set point to the time 90% of the deviation is corrected.
For example, the response time may be on the same order as the
repetition time between successive speed determinations or greater.
In this embodiment, the set speed is constant during normal
operation. The set speed may be changed by commands input to the
control by circuit through I/O device 260, or by actions taken by
supervisory control module 245. For example, the supervisory
control module may be responsive to physiological changes as, for
example, changes in the patient's level of activity.
[0044] In addition, the control circuit is operable to collect flow
rate data points 232 indicative of a flow rate of blood through the
pump. The instructions in flow rate estimation module 242 may cause
the processor to estimate the flow rate based on data including the
current in the coils, the speed of the rotor and the acceleration
of the rotor of the pump, and the viscosity of the patient's blood.
The speed and acceleration may be taken from the rotational speed
data 323 discussed above. The viscosity of the patient's blood
normally may be input to the control system as a constant (e.g.,
based on the patient's hematocrit level), or may be estimated as,
for example, by measuring the deceleration of the pump responsive
to momentary interruption of power supply to the pump as described
in U.S. Pat. No. 8,961,390, the disclosure of which is incorporated
by reference herein. Such measurements may be repeated periodically
as, for example, hourly or daily. The current in the coils is
directly related to the duty cycle of the pulse width modulated
voltage applied to the coils by the pump interface and can be
acquired from the pump interface. Alternatively, the current in the
coils can be acquired by a current-sensitive sensor (not shown)
electrically connected to the coils. As described in U.S. Pat. No.
8,897,873, the disclosure of which is incorporated by reference
herein, the instantaneous flow through the pump can be estimated
based on this information. Using such a model results in the
estimate having a dynamic range of about 15 Hz.
[0045] In other examples, other parameters indicative of flow may
be used, and/or different calculations may be employed, to estimate
a flow rate of blood. Alternatively, flow rate data points may be
gathered using direct measurements, such as with an ultrasonic flow
meter.
[0046] The suction detection module 241 includes instructions that
cause the processor to periodically examine the rotational speed
data 234 and determine a maximum and minimum speed during a series
of speed measurement intervals. Each interval may be slightly
longer than the maximum cardiac cycle time as, for example, about 2
seconds. For each interval, the processor determines a speed range
by subtracting the minimum rotational speed from the maximum
rotational speed. The processor compares the speed range for each
speed measurement interval with a range limit. The range limit may
be a fixed parameter stored in memory during setup of the system,
or may be a function of the set speed prevailing during the
measurement interval as, for example, a fixed percentage of the set
speed. If the speed range is above the range limit, the processor
issues a speed range signal as, for example, by setting a speed
range flag to a "true" condition. If not, the processor sets the
speed range flag to a "false" condition.
[0047] The speed range for each speed measurement interval
represents the interaction between the rotor speed control circuit
discussed above and physical events which tend to change the rotor
speed. As best seen in FIG. 4, during normal operation the flow
through the pump changes during each cardiac cycle due to the
pumping action of the heart. These changes tend to increase and
decrease the speed of the rotor, but the control circuit maintains
the rotor speed within a relatively narrow normal range RN.
However, during a suction condition, the flow changes to a greater
extent and at rapid rates. Although the present invention is not
limited by any theory of operation, these changes in flow may be
due to blockage of the pump inlet associated with collapse of the
ventricle. These changes in flow tend to cause greater and more
rapid changes in the rotor speed. Although the control circuit
counteracts this tendency, the net result is that the rotor speed
varies over a greater, abnormal range RX. The range limit desirably
is greater than the normal range RN but less that the expected
abnormal range RX during suction conditions, so that a range above
the range limit is indicative of a suction condition. The normal
range RN and the abnormal range RX depend in part on the
characteristics of the control circuit. For example, a control
circuit with a slower response time would yield greater normal and
abnormal ranges. The appropriate range limit for a VAD of a given
design, with a control circuit of a given design can be determined
by review of operational data.
[0048] Although a speed range above the range limit is indicative
of a suction condition, a speed range above the range limit may
also occur in other conditions as, for example, where the patient's
blood flow has very high pulsatility or during certain cardiac
arrhythmias.
[0049] The suction detection module also includes instructions that
cause the processor to examine the flow rate data during one or
more cardiac cycles. The processor maintains a moving average of
the flow rate over time. This moving average typically includes
flow rate data from a plurality of cardiac cycles. For example, the
processor may maintain a moving average consisting of the average
value of a set consisting of the last N flow rate estimates, i.e.,
the sum of the last N flow rate estimates divided by N. N is an
integer that desirably is greater than the number of flow rate
estimates obtained during a single cardiac cycle. Each time a new
flow rate estimate is derived by the flow estimation module as
discussed above, the processor deletes the oldest value in the set,
adds the new value to the set, and computes a new value of the
moving average.
[0050] The processor compares each flow rate estimate to the value
of the moving average at the time of such flow rate estimate, i.e.,
the first value of the moving average computed using that flow rate
estimate as part of the set of N values. Thus, the processor
determines whether each flow rate estimate is above or below the
moving average.
[0051] The processor determines the beginning of a cardiac cycle by
examining the results of this comparison. The time when the flow
rate rises above the moving average is taken as the beginning of
systole and as the end of one cardiac cycle and the beginning of
the next cardiac cycle. Thus, if the last previous flow rate
estimate was below the moving average, and a new flow rate estimate
is above the moving average, the processor recognizes the time of
the new estimate as the beginning of a new cardiac cycle.
[0052] The processor counts the number of flow rate estimates
during the entire cardiac cycle and also counts the number of flow
rate estimates which were above the moving average during the
cardiac cycle. Because the flow rate estimates are acquired
periodically, at a uniform rate, the number of flow rate estimates
during the cardiac cycle is directly proportional to the total
duration of the cardiac cycle. Likewise, the number of flow rate
estimates above the moving average is proportion to the time during
which the flow rate is above the moving average.
[0053] For each cardiac cycle, the system divides the number of
flow rate estimates within the cycle above the moving average by
the number of flow rate estimate by the number of flow rate
estimate in the cardiac cycle to arrive at a parameter referred to
herein as the "duty parameter." The duty parameter is directly
related to the portion of the cardiac cycle during which the flow
rate was above the moving average.
[0054] The processor compares the duty parameter for each cardiac
cycle with a set duty limit. As explained below, a duty parameter
above the duty limit is indicative of a suction condition.
[0055] Referring now to FIG. 5, the flow rate during normal
operation remains above the moving average for a relatively brief
time TA at the beginning of each cycle and is below the moving
average for a longer time TB near the end of each cycle. The total
duration of the cycle is indicated by TC. Thus, the duty parameter
is less than 0.5. Each flow rate estimate is indicated by a small
square on the flow rate curve. In the example shown in FIG. 5,
there are 31 flow rate estimates during the entire cardiac cycle.
Of these, 13 estimates are above the moving average (i.e., during
TA), whereas 18 were below the moving average (i.e., during TB).
The duty parameter is TA/TC, or 13/31, i.e., about 0.42 or 42%.
[0056] Referring now to FIG. 6, by contrast, in a suction condition
as depicted in FIG. 6, the time TA above the moving average is
prolonged and constitutes a greater proportion of the entire
cardiac cycle TC, whereas the time TB below the moving average is
shortened. In the particular example shown, there are 33 flow rate
estimates in the entire cardiac cycle. Of these, 24 flow rate
estimates are above the moving average (i.e., during TA), whereas 9
are below the moving average (i.e., during TB). The duty parameter
TA/TC is 24/33, i.e., about 0.73 or 73%.
[0057] The duty limit in this example desirably is set at a value
above that which occurs in normal operation, and below that which
occurs during suction conditions as, for example about 0.5 to about
0.6. If the duty parameter is above the duty limit, the processor
issues a duty signal representing the fact that that the duty
parameter is indicative of a suction condition as, for example, by
setting a duty flag to a "true" condition. If not, the processor
leaves the duty flag in a "false" condition.
[0058] Referring now to FIG. 7, for each cardiac cycle, the
processor checks whether the duty signal has been generated for
that cardiac cycle and also checks whether the range signal has
been generated for an interval which substantially or completely
overlaps that cardiac cycle. For example, the processor may check
if the duty flag was set during the last completed cardiac cycle,
and if the range signal was set for the speed measurement interval
ending immediately after the end of such cycle. If both conditions
are true, the processor issues a suction detection signal
indicating that a suction condition exists. The suction detection
signal may be sent through the I/O interface 252 (FIG. 3) and
converted to a human-perceptible signal such as a visual indication
on a display, an audible signal or the like, or may be transmitted
through the I/O interface to a remote monitoring device.
Alternatively or additionally, the processor may react to the
suction detection signal by altering operation of the pump
according to instructions in supervisory control module 245. For
example, the processor may reduce the set speed of the pump and
then check to determine if such reduction eliminates the suction
condition in subsequent cardiac cycles. If the suction condition
has been cleared, the processor may then increase the set speed
gradually while continuing to monitor for recurrence of the suction
condition. Routines for adjusting the set speed of a rotary pump
responsive to a suction condition are disclosed, for example, in
the aforementioned '048 Publication and '413 Publication.
[0059] The increase in the duty parameter discussed above is
strongly associated with the occurrence of a suction condition. For
example, the duty parameter typically does not increase above the
duty limit during arrhythmias, during conditions of high or low
pulsatility, or during changes in pump speed commanded by the
supervisory control system or manual inputs. Because the pump speed
range is used in combination with flow data in the system and
methods discussed above, the suction detect signal will not be
generated even if the speed range signal is erroneously set to
true. In operation, using speed and flow data previously recorded
during actual operation of the VAD with real patients and including
known suction events, the system was able to identify every known
suction event, and thus exhibited 100% sensitivity. The system did
not issue any suction detect signals in the absence of a known
suction event and thus exhibited 100% specificity.
[0060] Moreover, the system is extremely simple both in concept and
in implementation. There is no need to maintain and update values
based on computations or data from previous cycles. The suction
detect signal is generated based on data from a single speed
detection interval and a single cardiac cycle. This allows the
system to issue the suction detect signal promptly at the start of
a suction condition, so that corrective action can be taken
promptly.
[0061] The system and method discussed above can be varied in many
ways. In the system and method as discussed above, the moving
average flow rate serves as a crossover value for determination of
the duty parameter. The duty parameter represents the proportion of
the cardiac cycle duration during which the flow rate is above this
crossover value. In other embodiments, the crossover value may be
another flow rate. For example, the crossover value may be a
function of the moving average flow rate as, for a multiple of the
moving average flow rate such as 1.1 times the moving average or
0.9 times the moving average.
[0062] In the embodiments discussed above, the duty parameter is
equal to the time TA above the crossover value divided by the
duration of the cardiac cycle, and thus is directly proportional to
TA. However, the duty parameter can be calculated in many other
ways provided that the duty parameter is a function of any two or
more of TA, TB and TC. For example, the duty parameter can be
calculated as the time TB below the crossover value divided by the
duration of the cardiac cycle. In this instance, the duty parameter
would vary inversely with TA, so that a value of the duty parameter
less than a duty limit would be indicative of a suction condition.
In yet another variant, the duty parameter could be taken as the
difference between time above the crossover value and time below
the crossover value, i.e., (TA-TB) or vice-versa (TB-TA). In these
embodiments, the duty parameter is not proportional to TA or to
TA/TC, but nonetheless represents the proportion of time during the
cardiac cycle during which the flow rate is above the crossover
value. Stated another way, the duty parameter need not vary
linearly with TA/TC. In the embodiments discussed above, the duty
parameter is determined over a single cardiac cycle. However, the
duty parameter can be determined over a plurality of complete
cardiac cycles as, for example, by averaging duty parameters
determined from plural cycles or, equivalently, calculating the
duty parameter based on the sums of two or more of TA, TB and TC
for the plural cycles.
[0063] In the embodiments discussed above, the speed range is
calculated over intervals that are determined without regard for
cardiac cycles. Because each interval is longer than a single
cardiac cycle, each interval encompasses a complete cardiac cycle
and also includes some data collected during one or more other
cycles. In a variant, the speed range can be determined over
intervals corresponding exactly to cardiac cycles. For example,
each speed measurement interval may begin when the beginning of a
cardiac cycle is recognized based on the flow rate rising above the
moving average and end when the next cardiac cycle begins.
[0064] In the embodiments discussed above, the speed pump speed
data and flow data are each converted to a binary indication, i.e.,
true or false, and combined by an "and" operation, so that the
suction detect signal is generated only if both are true. However,
either or both of these indications can be provided in a format
that conveys additional information. For example, the duty
parameter and the measured speed range can be provided as multi-bit
values, and can be combined with one another in a more complex
manner. For example, one of these values may serve as a row address
of a two-dimensional lookup table, whereas the other value serves
as the column address. Each cell of the lookup table may store a
binary value, i.e., "no suction" or "suction," so that the suction
detect signal is generated only when the row and column addresses
point to a cell with the "suction" value. In a further variant of
such a scheme, each cell of the lookup table may store a multi-bit
number indicating a probability that a suction condition is
present. In a further variant, the measured speed range may be
compared with the range limit as discussed above, and the result
may be output as a multi-bit value that denotes the sign and
magnitude of the difference. Likewise, the duty parameter may be
compared to the duty limit, and the result may be output as a
multi-bit value the sign and magnitude of the difference. These
results may be used in a lookup table scheme. Alternatively, these
results may be combined as, for example, by addition or
multiplication, to yield a composite value, and the composite value
may be compared to a threshold value so that the suction signal is
generated only if the composite value exceeds the threshold.
[0065] In a further variant, the range limit, the duty limit, or
both may be based on data from previous measurement intervals or
cardiac cycles. For example, the range limit may be a selected
based in whole or in part on the speed ranges observed in previous
speed measurement intervals. The duty limit may be based in whole
or in part on the duty parameters measured in previous cardiac
cycles. However, such an arrangement is generally less
preferred.
[0066] As discussed above, the change in the duty parameter is
strongly associated with the presence of a suction condition. Thus,
the duty parameter can be used by itself as the sole indicator of
the presence or absence of a suction condition. In one such
embodiment, the suction detect signal is generated whenever the
duty parameter exceeds the duty limit. Alternatively, the decision
to issue the suction detect signal can be based on the duty
parameter can be used in conjunction with other information which
is indicative, to at least some degree, of the presence or absence
of a suction condition.
[0067] The speed range information discussed above can be combined
with information other than the duty parameter discussed above so
as to provide an indication as to whether or not a suction
condition exists which has greater specificity than that afforded
by speed range alone. For example, information derived from flow
rate data other than the duty parameter can be used in conjunction
with the speed range discussed above. In one such example, the
speed range determined during one or more measurement intervals may
be combined with flow rate data such as the waveform index taught
in the '048 Publication or with flow rate threshold information as
taught in the '413 Publication, and a suction detect signal may be
generated if both the speed range and the flow rate information are
indicative of a suction condition.
[0068] The pump depicted in FIGS. 1 and 2 and discussed above is a
centrifugal pump with an impeller arranged so that blood flows
substantially in a radial direction across the impeller. In other
examples, the blood pump may be an axial flow pump, such as that
used in the MVAD.RTM. ventricular assist device, also manufactured
by HeartWare Inc. As further described in U.S. Patent Application
Publication No. 2012/0245681, the disclosure of which is
incorporated by reference herein, in an axial flow blood pump, the
impeller drives the blood in a direction generally parallel to the
axis of the impeller. Still other pumps have impellers arranged to
drive the blood in mixed axial and radial flow. Any of these blood
pumps may be used.
[0069] The operations described above do not have to be performed
in the precise order described. Rather, various operations can be
handled in a different order or simultaneously. It should also be
understood that these operations do not have to be performed all at
once. For instance, some operations may be performed separately
from other operations. Moreover, operations may be added or
omitted. As these and other variations and combinations of the
features discussed above can be used, the foregoing description
should be taken as illustrating, rather than as limiting, the
present disclosure.
* * * * *