U.S. patent application number 13/172748 was filed with the patent office on 2012-01-05 for physiological demand responsive control system.
This patent application is currently assigned to Thoratec Corporation. Invention is credited to Peter Joseph Ayre.
Application Number | 20120004497 13/172748 |
Document ID | / |
Family ID | 28047438 |
Filed Date | 2012-01-05 |
United States Patent
Application |
20120004497 |
Kind Code |
A1 |
Ayre; Peter Joseph |
January 5, 2012 |
Physiological Demand Responsive Control System
Abstract
A demand responsive physiological control system for use with a
rotary blood pump; said system including a pump controller which is
capable of controlling pump speed of said pump; said system further
including a physiological controller, and wherein said
physiological controller is adapted to analyze input data relating
to physiological condition of a user of said pump; and wherein said
physiological controller determines appropriate pumping speed and
sends a speed control signal to said pump controller to adjust pump
speed; said system further including a physiological state detector
which provides said input data indicative of at least one
physiological state of said user, in use, to said physiological
controller.
Inventors: |
Ayre; Peter Joseph; (Crows
Nest, AU) |
Assignee: |
Thoratec Corporation
Pleasanton
CA
|
Family ID: |
28047438 |
Appl. No.: |
13/172748 |
Filed: |
June 29, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10529657 |
Nov 28, 2006 |
7988728 |
|
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PCT/AU03/01281 |
Sep 30, 2003 |
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13172748 |
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Current U.S.
Class: |
600/17 |
Current CPC
Class: |
A61M 2205/33 20130101;
A61M 2205/3334 20130101; A61M 60/148 20210101; A61M 60/824
20210101; A61M 2230/06 20130101; A61M 60/50 20210101; A61M
2205/3303 20130101; A61M 2230/63 20130101; A61M 60/562 20210101;
A61M 60/205 20210101 |
Class at
Publication: |
600/17 |
International
Class: |
A61M 1/12 20060101
A61M001/12 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2002 |
AU |
2002951685 |
Claims
1-30. (canceled)
31. A controller for use with a blood pump, comprising: a pump
controller configured for sending a pump speed signal to the pump
for controlling the pump speed; a physiological controller for
detecting a user's activity level based on a signal received from
an accelerometer; and the controller being further configured for
detecting a pumping state and the user's heart rate based on
signals received from the pump; wherein the pump speed signal is
derived from the detected pumping state and the user's heart rate
and activity level.
32. The controller of claim 31, wherein the accelerometer is
mounted in the controller.
33. The controller of claim 31, wherein the controller is
configured for deriving the user's heart rate based on an
instantaneous impeller speed of the pump.
34. The controller of claim 31, wherein the pump speed signal is
derived from the detected heart rate and activity level unless the
detected pumping state indicates that a Total Ventricular Collapse
(TVC) or a Partial Ventricular Collapse (PVC) pumping state
exists.
35. The controller of claim 31, wherein the controller is
configured for detecting the pumping state based on input power to
the pump and pump speed.
36. The controller of claim 35, wherein the controller is
configured for detecting at least one of a total ventricular
collapse, pump regurgitation and partial ventricular collapse
pumping states.
37. The controller of claim 35, wherein the controller is
configured for detecting a flow rate of the pump based on input
power to the pump.
38. A controller for use with a blood pump, comprising: a pump
controller capable of controlling pump speed; an accelerometer
mounted in the controller; and the controller further including a
physiological controller adapted to detect physical motion of the
user based on signals received from the accelerometer and generate
a signal for the pump controller to adjust pump speed according to
the detected physical motion; wherein the signal for the pump
controller is based on signals received from the accelerometer and
integrated over time to detect a continuous physical motion of the
user.
39. The apparatus of claim 38, wherein the detected physical motion
is based on both accelerometer signals and a detected heart rate of
the user.
40. The apparatus of claim 38, wherein the controller is configured
for selecting a pumping speed from a predetermined range of pumping
speeds according to the detected physical motion.
41. The apparatus of claim 40, wherein the controller further
includes a conditioning circuit for identifying a physiological
demand state representative of the physiological state of the user
based on the signals received from the accelerometer, wherein the
pumping speed is selected according to the identified physiological
demand state of the user.
42. The apparatus of claim 38, wherein the signal is generated from
the detected physical motion, pump power and instantaneous pump
speed.
43. The apparatus of claim 42, wherein the controller includes an
algorithm for predicting blood flow rate, heart rate, flow profile,
pulsatility and physiological demand based on the detected physical
motion, pump power and instantaneous pump speed, and wherein the
algorithm produces a preferred pump speed which is used to generate
the signal for the pump controller.
44. A controller for use with a blood pump, comprising: a pump
controller capable of controlling pump speed; and the controller
being configured for generating a speed control signal for the pump
controller based on a detected physiological state of the user and
pumping state of the pump; wherein the physiological state is
detected from data received from a non-invasive sensor; and wherein
the pumping state is detected from signals received from the
pump.
45. The controller of claim 44, wherein the controller includes an
alarm which is activated when the detected pumping state indicates
that a physiologically critical pumping state exists.
46. The controller of claim 45, wherein the physiologically
critical pumping state is a Total Ventricular Collapse (TVC) or a
Partial Ventricular Collapse (PVC).
47. The controller of claim 44, wherein the non-invasive sensor is
an accelerometer.
48. The controller of claim 44, wherein the pumping state is
detected from a pump impeller speed signal and an input power to
the pump.
49. The controller of claim 48, wherein the pump impeller speed
signal and input power to the pump is used by the controller in a
feedback loop to reduce pump speed when a physiologically critical
pumping state is detected.
50. The controller of claim 44, wherein the physiological state is
detected from data received from an accelerometer and a detected
heart rate.
51. The controller of claim 44, wherein the detected pumping state
is one of a Total Ventricular Collapse (TVC), Partial Ventricular
Collapse (PVC), Ventricle Ejecting (VE), Aortic Valve Closed (AC)
or Pump Regurgitation (PR).
Description
PRIORITY CLAIM
[0001] This application is divisional of U.S. application Ser. No.
10/529,657 filed Nov. 28, 2006, which is a national stage
application of PCT/AU03/01281 filed Sep. 30, 2003, which claims
priority to AU 2002951685 filed Sep. 30, 2002.
[0002] The present invention relates to a demand responsive
physiological control system and, more particularly, to such a
system particularly suited for use with blood pumps and, even more
particularly, those used to assist heart function such as, for
example, ventricular assist devices.
BACKGROUND
[0003] With particular reference to physiological control systems
in mammals and more particularly those of the human body it has
been noted that the control systems which the body itself uses to
control various organs are complex.
[0004] For example, the heart of a mammal may cause the amount of
blood that is to be circulated through the body to change not just
for what might be termed obvious reasons such as an increase in
physical exertion by a person, but may also occur for example, as a
result of anticipation of exertion. Furthermore the triggers which
can cause changes in heart rate and pumped blood volume may derive
from the nervous system directly or may derive from the action of
hormones or other chemical releases within the body.
[0005] It follows, where mechanical aids are introduced into the
body to assist the body's functions such as, for example,
implantable rotary blood pumps used as ventricular assist devices
that simplistic control mechanisms for these mechanical aids cannot
hope to anticipate or mimic the commands which the body may pass to
the heart.
[0006] For example, in early applications of ventricular assist
devices the control mechanisms simply set the ventricular assist
device to pump at a constant volume per unit time, adjusted at the
time of initial installation to best suit the patient in whom the
device has been installed.
[0007] Such systems use pump speed as the controlled variable.
Unfortunately, a set pump speed bears no relation to actual
physiological demand.
[0008] It is an object of the present invention to address or
ameliorate one or more of the above mentioned disadvantages.
BRIEF DESCRIPTION OF INVENTION
[0009] Accordingly, in one broad form the invention there is
provided a demand responsive physiological control system for use
with a rotary blood pump; said system including a pump controller
which is capable of controlling pump speed of said pump; said
system further including a physiological controller, and wherein
said physiological controller is adapted to analyze input data
relating to physiological condition of a user of said pump; and
wherein said physiological controller determines appropriate
pumping speed and sends a speed control signal to said pump
controller to adjust pump speed; said system further including a
physiological state detector which provides said input data
indicative of at least one physiological state of said user, in
use, to said physiological controller.
[0010] Preferably said the physiological state detector includes an
accelerometer to sense motion of the user, when in use.
[0011] Preferably said the accelerometer senses motion in at least
one axis.
[0012] Preferably said the accelerometer senses motion in three
orthogonal axes.
[0013] Preferably said system includes a pump monitor that detects
information relating to voltage and current of the pump and
delivers this information to said physiological controller.
[0014] Preferably said pump monitor detects an instantaneous pump
impeller speed of the rotary blood pump through measurements.
[0015] Preferably said pump monitor detects non-invasively.
[0016] Preferably said physiological controller uses said
information received from the pump monitor to derive mathematically
an appropriate pump speed.
[0017] Preferably said physiological controller assesses flow
dynamics and an average flow estimate, developed from speed and
input power supplied to the pump by the pump controller.
[0018] Preferably said physiological controller mathematically
determines a pumping state and if a deleterious state is determined
the speed control signal is changed accordingly.
[0019] In a further broad form of the invention there is provided a
physiological detector includes a means of detecting and
quantifying a heart rate of the user, when in use.
[0020] Preferably said physiological detector includes a means of
non-invasively detecting and quantifying a heart rate of the user,
in use.
[0021] Preferably said physiological controller can determine a
heart rate of the user by sensing speed of the pump.
[0022] Preferably said physiological controller can determine a
heart rate of the user using power inputted to the pump.
[0023] Preferably said pump is internally implantable within the
user.
[0024] Preferably said the pump is a ventricle assist device.
[0025] Preferably said the pump has a hydrodynamic bearing that
produces a relatively flat pump head versus pump flow curve.
[0026] Preferably said physiological controller is capable of
manual manipulation by the user.
[0027] Preferably said manual manipulation is within adjustable
predefined limits.
[0028] Preferably said physiological controller is adapted for
communication with a computer and wherein the physiological
controller is adapted for manipulation by a software user
interface.
[0029] Preferably said physiological controller includes an
alarm.
[0030] In a further broad form of the invention there is provided a
process for using physiological demand data to optimize pump speed
of a rotary blood pump wherein the process comprises of the
following steps: a heart rate of the user is non-invasively
determined; a level of physiological exertion of the user is
determined through non invasive means; an instantaneous pump speed
and input power is used to calculate instantaneous blood flow rate;
a pumping state is mathematically determined; the heart rate,
pumping state and level of physical exertion are compared to the
blood flow rate; and the pumping speed of the rotary blood pump is
changed to appropriately supply the user with the correct blood
flow rate.
[0031] In yet a further broad form of the invention there is
provided a pump control system for a pump for use in a heart assist
device; said system comprising data processing means which receives
body motion information and heart rate information thereby to
derive a speed control signal for impeller speed of an impeller of
said pump.
[0032] Preferably said body motion information is derived from an
accelerometer.
[0033] Preferably said accelerometer senses motion in a single
axis.
[0034] Preferably said accelerometer senses motion in three
orthogonal axes.
[0035] Preferably said heart rate information is derived from a
non-invasive sensor.
[0036] Preferably said heart rate information is derived from
voltage and current applied to an electric motor driving said
impeller.
[0037] In yet a further broad form of the invention there is
provided a method of control of pump speed of a blood pump; said
method comprising establishing a base set point speed; said method
further comprising establishing one or more criteria which, if
satisfied, cause establishment of at least a second set point
speed; said second set point speed higher than that of said base
set point speed.
BRIEF DESCRIPTION OF DRAWINGS
[0038] Embodiments of the present invention will now be described
with reference to the accompanying drawings wherein:
[0039] FIG. 1 is a diagram of a ventricular assist device
installation within a human body suitable for control by
embodiments of the present invention;
[0040] FIG. 2 is a block diagram of a physiological demand
responsive of control system applicable to the system of FIG. 1 in
accordance with a first preferred embodiment of the present
invention;
[0041] FIG. 3 illustrates graphically the behavior of the control
system of FIG. 2 under specified physiological conditions;
[0042] FIG. 4 is a graph of accelerometer behavior utilized as a
basis for an input to the control algorithm of the first preferred
embodiment;
[0043] FIG. 5 is a block diagram of a control system in accordance
with a second preferred embodiment of the present invention;
[0044] FIG. 6 is a block diagram of a pumping state detection
module for use with the second embodiment;
[0045] FIG. 7 is a flowchart for determining pump drive set point
for the arrangement of the second embodiment;
[0046] FIG. 8 illustrates graphically an HQ curve for a preferred
pump type particularly suited for use with the control system of
FIG. 2 or FIG. 5; and
[0047] FIG. 9 is a diagram of a preferred embodiment of the present
invention wherein said diagram shows preferred inputs and
outputs.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0048] With initial reference to FIG. 1 there is illustrated in
diagrammatic form a blood pump 10 installed within a human body 11
and arranged to function as a left ventricular assist device. The
pump 10 is arranged to operate in parallel with blood flow passing
through left ventricle 12. This is effected by inserting an inlet
cannula 13 into left ventricle 12 and directing blood flow through
the inlet cannula into an inlet of blood pump 10. Blood pump 10, in
operational mode, pumps the blood thus received into aorta 14 via
outlet cannula 15, as illustrated in FIG. 1.
[0049] The blood pump 10 can take a number of forms and rely on a
number of different pumping and drive technologies. Broadly, the
pump technology can be based on axial or centrifugal rotary pump
arrangements or on positive displacement technologies.
[0050] In particular, although not limiting forms, preferred
pumping technologies for the control system to be described below
include rotary pump technologies which rely on an impeller
supported for rotation within a casing and which causes blood to be
urged between an inlet and outlet of the casing as the impeller
rotates therein. In more particular preferred forms a centrifugal
form of pump can be utilized with the control system with the
characteristics of the pump tailored to compliment or otherwise
work particularly advantageously with the control system according
to various embodiments of the present invention.
[0051] Typically the pump 10 is driven by an electrical power
source, in this instance a battery pack 16 mounted externally of
the body. Electrical power from the battery pack 16 is controlled
by a controller unit 17, also mounted externally of the body. In
addition to communicating electrical power to the pump 10 the
controller 17 can also communicate with an external programming
source, in this case a personal computer 18 for the purposes of
initial setup and ongoing periodic monitoring and recalibration of
the pump and controller as customized for a specific patient.
[0052] Embodiments of a control system suited for use, although not
exclusively, with the controller 17 of the arrangement described
above and with reference to FIG. 1 will now be described.
DEFINITIONS
[0053] In the description which follows, the following definitions
of various terms as referenced therein are to be utilized:
[0054] "non-invasive" is applied to the derivation of various
physiological parameters of body 11 (including blood flow rates and
the like) by means which do not require sensors to be placed
(invasively) within the body.
[0055] "IRBP"--implantable rotary blood pump.
[0056] "LVAD"--left ventricular assist device.
[0057] "LVP"--left ventricular pressure.
[0058] "RMS" or "rms"--route mean square.
[0059] "V"--volts applied to pump motor.
[0060] "I"--current consumed by pump motor.
[0061] "SVR"--Systemic Vascular Resistance
[0062] "VR"--Venous Resistance
[0063] "H"--pump head pressure.
[0064] "N"--pump impeller rotation speed.
[0065] "Q"--flow rate of blood through pump.
[0066] "P" or "PWR"--pump power consumption.
[0067] ".omega."--angular velocity of impeller.
[0068] "t"--time.
Pumping States
[0069] "TVC"--total ventricular collapse.
[0070] "PR"--pump regurgitation.
[0071] "PVC"--partial ventricular collapse.
[0072] "AC"--aortic valve closed.
[0073] "VE"--ventricle ejecting
First Embodiment
[0074] With initial reference to FIGS. 1-4 a first preferred
embodiment of a control algorithm and control system is described
below and by way of example.
[0075] In this embodiment the aim is to provide a pump controller
which utilizes a control algorithm which takes as its two primary
inputs for decision making firstly an indication of the degree of
movement of body 11 per unit time as a coarse measure of exertion
and hence pumping load required of the heart and particularly left
ventricle 12 and secondly an indication of heart rate derived, in
this instance, non-invasively by monitoring of electrical
parameters driving pump 10.
[0076] The system described with reference to FIGS. 1-4 exhibits
the following characteristics:
[0077] 1. Allowing motor speed to vary and deriving control
information from those time varying signals; and
[0078] 2. Concept of using control of power input or speed to the
motor/pump.
[0079] The block diagram shown in FIG. 2 shows the signals that are
derived (non-invasively) from pump motor power and speed. To detect
these conditions the strategy is to measure speed instantaneously
every revolution of the impeller as a digital signal from the motor
commutation electronics. The haemo-dynamic controller electronics
measure the frequency of this signal which is proportional to
impeller speed. Using speed is an advantage since it is a digital
signal, which in practice has been found to be an inherently less
electrically noisy signal than that derived from measuring motor
current or power. Both instantaneous speed and root mean square
(rms) of speed are calculated. Also instantaneous pump input power
and rms of pump input power are calculated.
[0080] Many researchers only discuss constant speed or speed set
point. However the present control strategy allows impeller speed
and pump input power to be freely modulated by ventricular
contractions and uses the resulting dynamic information as feedback
to the control system. The characteristics of centrifugal IRBPs
mean that impeller speed is more sensitive to hydraulic load
variations than for axial IRBPs. Furthermore, allowing impeller
speed to vary in magnetically suspended IRBPs may affect suspension
control. A preferred pump uses a hydro dynamically suspended
impeller and therefore suspension controls are not needed.
Calculation of Instantaneous Impeller Speed N(t) and rms of
Impeller Speed Nrms(t)
[0081] Each pulse from the commutation controller represents
1/6.sup.th of a rotation of the impeller and is time stamped
relative to a reference time base. Therefore the angular velocity
.omega. of the impeller for each 60.degree. of rotation is
described by equation 1.
.omega. ( T ) = 2 .PI. 6 [ ( Tn + 1 ) - Tn ] equation 1
##EQU00001##
where Tn+1-Tn is the time difference between pulses (interrupts) in
seconds. .omega.(t) is converted to speed N(t) in rpm by
multiplying by 60/2II as-in equation 2
N ( t ) = 60 .omega. ( t ) 2 .pi. equation 2 ##EQU00002##
Rms speed is calculated in equation 3 from a moving window of
samples of N(t), the sample rate dependent on impeller speed. Each
instantaneous speed sample is time stamped at t.sub.1 to
t.sub.n.
Nrms ( t ) = o n [ N ( t ) ] 2 n equation 3 ##EQU00003##
Calculation of Instantaneous and Rms Electrical Input Power
Pin(t).
[0082] Calculation of pump electrical power is a direct way to
monitor the power consumption of the pump. Since the pump power and
speed is modulated by the heart which is an asymmetrical modulation
(due to the ejection fraction not being 50% rms) calculation of
both instantaneous power and speed is implemented. Power is
calculated using equation 4.
Pin(P)=Vm(i)Im(t) equation 4
Where Vm(t) and Im(t) are the "instantaneous" motor coil voltage
and summed phase current respectively, sampled. A moving window of
samples of Pin(t) is used to calculate P.sub.rms(t) using equation
5.
P rms ( t ) = o n [ Pin ( t ) ] 2 n equation 5 ##EQU00004##
Second Embodiment
[0083] With reference to FIGS. 5 to 7 inclusive there will now be
described a control system in accordance with a second preferred
embodiment:
[0084] In relation to this second embodiment the control strategy
is similar to that described with respect to the first embodiment
but, in addition, includes as a further control input derived from
non-invasively determined parameters the "pumping state" of pump
10. This feature provides a safety-override mechanism as
illustrated in the flowchart of FIG. 7 thereby to ensure that the
basic control strategy described with reference to the first
embodiment is less likely to put the patient at risk. Initially in
the description which follows invasively derived parameters are
discussed showing how the various pumping states have been defined
and come to be identified. A method of non-invasively deriving the
same parameters and pumping state determinations is then described
with both forms of derivation being summarized in table 1.
[0085] With reference to FIG. 4 experimental data suggests that
there is a correlation between heart rate and accelerometer output
where at least a single axis accelerometer is attached to a patient
and used as a measure of physical activity of the patient. This
observation is used for the control algorithm now to be
described.
[0086] FIG. 5 is a block diagram of the control arrangement
wherein, in addition to the input variables described with
reference to example 1 there is a "physical motion" input which can
be derived from an accelerometer associated with a patient. In the
simplest form the accelerometer can be a single axis accelerometer.
In alternative forms multiple axes of accelerometer sensing can be
utilized.
Detection of Physiologically Significant Pumping States
[0087] Physiologically critical pumping state detection methods are
used based on the non-invasive system observers pump speed and
electrical input power. Activity level is detected using heart rate
(detected from pump impeller instantaneous speed) and motion by
using an accelerometer although other measuring devices may be used
without departing from the scope of the present invention. These
non-invasive observers are utilized as inputs to a control
algorithm for a rotary blood pump to seek to ensure that pump
output is better adapted to patient rest and exercise states.
Identifying Pumping States
[0088] Methods were developed to detect pumping states based on
instantaneous measured pump power and speed. The methods developed
allow impeller speed and pump input power to be freely modulated by
ventricular contractions. This dynamic information is utilized as
feedback to the control system. Data from in-vitro and in-vivo
experiments shows that states TVC (total ventricular collapse) and
PR (pump regurgitation) produce low flow through the pump. State
TVC produces non-pulsatile low flow while state PR produces
pulsatile low flow less than 1 L/min. States PVC (Partial
Ventricular Collapse), AC (Aortic valve Closed) and VE (Ventricle
Ejecting) produce normal pump flows greater than IL/min. States PVC
and PR can be differentiated from state AC since flow pulsatility
is more evident. State PVC can be differentiated from state VE
since the dynamic flow symmetry is different from all other states.
The dynamic nature of the flow is reflected by pump speed and
power. Instantaneous measured pump speed is used to indicate flow
dynamics.
Detecting of State TVC, Ventricle Totally Collapsed Occluding the
Inlet Cannula.
[0089] Examining the in-vitro and in-vivo data it has been found
that state TVC can be consistently detected by fall in pump flow to
near 0 L/min accompanied by a reduction of flow pulsatility. It has
been observed Flow waveform symmetry may not be relevant for
detection of this state.
Detecting State PVA, Ventricle not Ejecting and Beginning to
Collapse onto the Cannula
[0090] The state PVC is indicated by a variation in symmetry of the
instantaneous speed waveform given a level of pulsatility. Given
that normal flow rates can still be observed during this state and
that flow pulsatility is large, the only parameter distinguishing
this state from the VE state is the flow symmetry.
State AC Ventricle not Ejecting and Positive Pump Flow.
[0091] By analyzing the cardiac cycle with the pump it has been was
found that there may be a portion of state AC where the aortic
valve remains closed, whilst however the pump flow is still
pulsatile. Assistance beyond this point causes pump flow
pulsatility to reduce. At high perfusion demands, as in exercise,
the failed ventricle may be supplemented to such an extent that the
flow through the pump is pulse-less. Theoretically if no left
ventricle contraction occurs then implantable rotary blood plump
flow will be non pulsatile. Contraction of the left ventricle with
the pump connected means that pump head is proportional to the
difference between the aortic pressure and the left ventricular
pressure (LVP). If the pump power is increased beyond the point
that the left ventricle is doing no work (the aortic valve no
longer opens) maximum LVP begins to decrease. This means that
minimum instantaneous pump differential pressure will begin to rise
relative to the RMS of the pump differential pressure over the
cardiac cycle. If the ventricle is weakened through heart failure
this will occur at relatively lower pump speeds and the mitral
valve will still continue to open and LVP maximum will decrease
towards zero with increasing speeds. During this interval the
mitral valve will open and close. Steady flow occurs when there is
no pulsatility in the speed signal and the mitral valve never
closes. The target speed at which this occurs will increase with
SVR or VR and cardiac contractility. Continuing to increase the
pump power will cause the transition from pulsatile to non
pulsatile flow. This means detection of the state VE and state AC
can only be achieved dynamically by considering the maximum
instantaneous speed Nmax(t) and the rms of instantaneous speed
Nrms(t) for the nth and (n-l)th cardiac cycle. A significant change
occurs only if there is a change in average pump speed set point,
after load or pre load. A method of detecting the AC state without
relying on transitions has been chosen which uses peak to peak flow
rate given that pump flow is greater than 1 L/min.
Detecting State VE, Ventricle Ejecting with Positive Pump Flow
[0092] State VE may be identified non invasively by pump flow rate
being larger than 1 L/min and peak to peak instantaneous voltage
(flow) being greater than a threshold value and the flow symmetry
being greater than that for the PVC state.
Detecting State PR: The Point at which Pump Flow Rate is Less than
Zero
[0093] The PR state may be indicated when the pump flow falls below
the lower flow limits Qmin which is set to be 1 L/min. This level
of Qmin is set at 1 L/min although not "0 L/min" may be was
considered a safe limit to be classed as retrograde flow.
Pumping State Detection Using Non-Invasive Pump Parameters
[0094] By analyzing pump parameters deriving from invasive-derived
parameters it is postulated that flow, flow amplitude and waveform
symmetry appear to be good indicators of pumping state using only
non-invasively-derived pump parameters. These variables can be
detected non-invasively using estimated pump flow (Qest), peak to
peak instantaneous speed Npp(n) and symmetry Nsym(n). Table 1 shows
the relationship of physiological parameters to non-invasive pump
parameters for each of the physiologically identified pumping
states which have been taken from in-vitro and in-vivo data sets
(n=3).
TABLE-US-00001 TABLE 1 A summary of physiological (invasive) and
pump (non-invasive) parameters used as the criteria to identify
pumping states. Identifying Parameter Non-invasive via pump
Invasion (Physiological) (from speed and power) AoP Pulse Press.
LVP max Qav Q.sub.estRMS(t) Npp(n) State mmHg (mmHg) (mmHg) (L/min)
(L/min) Q.sub.sym(n) (rpm) TVC <40 <10 <40 <1 <1 --
<30 PVC 60-180 >10 60-180 <1 >1 <0.4 >30 AC
60-180 <10 <AoP <1 >1 -- <30 VE 60-180 >10
>AoP >1 >1 >0.4 >30 PR 60-180 >10 >AoP >1
<1 >0.4 >30
[0095] Estimated pump flow Q.sub.est is derived from N.sub.rms(t)
and PWR.sub.rms(t). The RMS of instantaneous pump speed
N.sub.rms(t) and power PWR.sub.rmst) are derived from instantaneous
speed N(t) and power PWR(t). The haemo-dynamic controller
electronics measure the frequency of the speed signal, which is
proportional to impeller speed. Using speed rather than power as an
observer for dynamic changes is an advantage since it is a digital
signal, substantially free from electrical noise which may
contribute to error.
Detecting Low Flow
[0096] Equation 6 is used to model low and normal flow rate through
the pump based on RMS impeller speed and electrical input
power.
Q.sub.est.alpha.K+speed+Pwr+(Pwr).sup.2+(Pwr).sup.3 equation 6
[0097] A flow Index, Q.sub.pIndex, shown in equation 7 is developed
to distinguish between low flow rates and normal flow rates. by
incorporating Q.sub.est. If Q.sub.pIndex>50 this corresponds in
this example to a flow rate greater than 1 L/min. A
Q.sub.pIndex<50 means that flow is less than 1 L/min or "low
flow"
Q.sub.pIndex=50Q.sub.est equation 7
[0098] Both the TVC and the PR pumping states defined and discussed
produce low pump flow rates. States PVC, AC and VE produce "normal"
flow rates where the circulation is not compromised.
Detecting Pulsatile Flow
[0099] States TVC and AC produce near non pulsatile pump flow. The
difference between these states is that state AC occurs when the
circulation is supported and state TVC when it is not. These states
can be differentiated by comparing Q.sub.estIndex. States PVC, VE
and PR produces pulsatile flow. States PVC and VE produce flow
which supports the circulation whereas state PR compromises the
circulation due to back flow through the pump. It has been shown
that instantaneous speed amplitude is proportional to pump flow
amplitude. The flow pulsatility index Q.sub.p.p Index (equation 9)
is developed based on instantaneous speed amplitude N.sub.p-p (n)
(equation 8) which is equal to the difference between the maximum
and the minimum instantaneous impeller speed N.sub.max(n) and
N.sub.min(n) for the n.sup.th cardiac cycle. The index outputs a
value greater than 50 for pulsatile flow and less than 50 for non
pulsatile flow.
N.sub.p-p(n)=N.sub.max(n)-N.sub.min(n) equation 8
Q p - p Index = 50 Q p - p min N p - p ( n ) equation 9
##EQU00005##
Detecting Variations in Flow Symmetry
[0100] The in-vitro and in-vivo data show that instantaneous speed
reflects the inverse symmetry of pump flow whilst current reflects
the same symmetry, although speed exhibits less electrical noise.
Thus it is postulated that the symmetry of flow can be estimated by
using the inverted symmetry of instantaneous speed.
[0101] States PVC and VE both produce flow rates which support the
circulation and a degree of pulsatility. Differentiating between
states can be achieved by considering the symmetry of the flow wave
form which is reflected in instantaneous speed. The symmetry of
flow rate is an inversion of the instantaneous speed signal. The
flow symmetry index Q.sub.symIndex (equation 11) is developed by
using the inverted speed waveform symmetry defined by equation 10
with the symmetry threshold Q.sub.symMAX set at 0. The index is set
so that if the flow symmetry falls below 0.3 (speed symmetry rises
above 0.7) its output is less than 50.
N sym = ( N nrms ( n ) - N min ( n ) N p - p ( n ) equation 10 Q
sym Index = 50 Q sym max . N sym equation 11 ##EQU00006##
Determining the Current Pumping State From Non-Invasive
Indicators
[0102] The block diagram shown in FIG. 5 shows the module that
combines the detection methods discussed above derived from
instantaneous power and speed. The current state is determined by
the logic table shown in FIG. 6 where flow, flow pulsatility and
symmetry are used to decide the present pumping state.
Detection of Heart Rate Using Pump Speed
[0103] [While pulsatile flow is detected, the heart rate is
calculated by using the array of speed samples. For the entire
speed array N[t.sub.1-t.sub.n] of samples the frequency of speed is
calculated by using the derivative of speed and detecting the time
of the speed maxima and minima. The derivative of speed is defined
in equation 12.
N ( t n ) t = N ( t n ) - N ( t n - 1 ) .DELTA. t equation 12
##EQU00007##
[0104] HR is then calculated by time stamping the maxima and minima
of the speed signal given by HRa and HRb in is equations 13 and 14.
The average is then computed and used as HR using equation 15.
Speed maxima are detected by dN(t.sub.n)/dt changing from a
positive to a negative value. Speed minima are detected by
dN(t.sub.n)/dt changing from a negative to a positive value.
T.sub.max(n), t.sub.max(n-1), t.sub.min(n), t.sub.min(n-1) are the
time stamps for the maximum and minimum values of instantaneous
speed.
HRa = 60 [ t max ( n ) - t max ( n - 1 ) ] equation 13 HRb = 60 [ t
min ( n ) - t min ( n - 1 ) ] equation 14 ##EQU00008##
Detection of Physical Motion
[0105] An accelerometer is mounted in the controller electronics
and used to detect physical motion. The accelerometer output is
amplified by a differential amplifier and integrated to provide a
signal level indicating continuous physical motion.
[0106] A preferred embodiment of the present invention of the
physiological demand responsive controller is suited for used with
implantable third generation LVASs. Also, a further embodiment of
the present invention is designed to cooperate with a
Ventrassist.TM. left ventricle assist system (LVAS).
[0107] One of the preferred embodiments may automatically adjust
the pumping speed of an implanted third generation blood pump to an
optimal level for the varying physiological needs of the implanted
patient. The preferred embodiment may achieve this by periodically
iteratively changing the speed setpoint of the pump. When the
control system detects increased physiological demand by the
patient (e.g. by physical exertion) the controller will increase
the pumping speed accordingly. The pumping state of the patient's
heart and physiological demand of the patient will be computed by
the control system. in real time as functions of the pump's motor
power and speed setpoint of the pump. Additionally, the
physiological demand of the patient may also be detected by the use
of a three axis accelerometer.
[0108] This accelerometer may be able to detect the instantaneous
motion of the patient. Preferably, this instantaneous motion may
also be indicative of the relative motion of the patient. The
output of the accelerometer may preferably be directed into a
conditioning circuit to digitize and filter the output signal of
said accelerometer. This signal will then be passed from the
conditioning circuit to a computational module which derives a
physiological demand state (e.g. resting, sleeping, exercising, or
patient collapse) as a numerically represented version of the state
(e.g. 1=resting; 2=sleeping etc). The numerically represented
version of the physiological demand state may then be inputted into
the control system for a pumping device or medical device. The
pumping speed of the implantable blood pump may then be altered in
respect of the predetermined range for physiological demand for a
particular patient.
[0109] Preferably, the predetermined ranges of pumping speeds will
be set by a specialist doctor at the time of implantation of the
blood pump. Additionally, it may be preferable to allow doctors to
amend the predetermined range as they see fit.
[0110] Preferably in an embodiment of the present invention the
control system may include a specialized algorithm. This algorithm
may include a mathematical model of ideal pumping speed of an
implantable blood pump for suitable physiological conditions of the
patient. This algorithm may receive input or data included within
three broad areas of data. These areas of data may include pump
power, instantaneous pump speed and physical motion. The algorithm
within the controller system (see FIG. 9) may use these areas of
data to predict certain patient data. This patient data may include
blood flow rate, heart rate, flow profile, pulsatility and
physiological demand. The algorithm will then output the preferred
pump speed and the remainder of the controller system will use this
information to set a speed setpoint for the blood pump.
[0111] Preferably, an embodiment of the present invention will be
such that iterative changes will be able to be made in timely
manner.
[0112] In a further embodiment of the present invention, a
preferred physiological demand responsive controller system may be
adapted for use with a radial off flow type centrifugal blood
pump.
[0113] The above describes only some embodiments of the present
invention and modifications, obvious to those skilled in the art,
can be made thereto without departing from the scope and spirit of
the present invention.
[0114] The system is particularly suited for use with pumps which
exhibit a relatively flat HQ curve as described with reference to
FIG. 8. In particular, but not exclusively, pumps of the radial off
flow type as, for example, described in International Patent
Application PCT/AU98/00725 can exhibit this relatively flat
characteristic. The description of PCT/AU98/00725 is incorporated
herein by cross-reference.
* * * * *