Flow estimation in a blood pump
The flow rate of blood out of a blood pump is determined at least in part based on an acceleration of the pump's rotor and on the blood's viscosity. Considering the rotor acceleration when determining blood flow rate, increases the accuracy of the blood flow measurement thereby permitting the determination of a parameter related to the contractility of a patient's heart. The parameter may include a rate of pressure change of blood across the pump, a ratio of the rate of pressure change and a peak-to-peak value of the blood flow rate, or any other contractility index.
1. A blood pump system comprising:
a pump including a housing and a rotor disposed within the housing, the rotor being rotatable around an axis; and
a control circuit operatively coupled to the pump, the control circuit being configured to:
determine a speed of rotation of the rotor;
determine a derivative of the speed of rotation of the rotor;
determine a flow rate of blood exiting the pump, the determined blood flow rate having a dynamic frequency range of at least 15 hertz based at least in part on a calculated time-varying parameter which accounts for disturbances in the blood flow rate introduced by the pulsating action of the heart, the time-varying parameter based at least in part on the derivative of the speed of rotation.
2. The blood pump system of claim 1 , wherein the control circuit is further configured to:
determine an amount of current that is used to drive the pump;
wherein the blood flow rate is determined based on the amount of current and the speed of rotation of the rotor.
3. The blood pump system of claim 1 , wherein the control circuit is further configured to:
receive user input indicating a viscosity of the blood;
wherein the blood flow rate is further determined based on the indicated viscosity.
4. The blood pump system of claim 1 , wherein the blood flow rate is determined based on the equation:
Q
(
t
)
=
aI
(
t
)
+
bI
(
t
)
2
+
cI
(
t
)
3
+
ⅆ
ω
(
t
)
+
e
ω
(
t
)
2
+
g
ω
(
t
)
I
(
t
)
+
h
(
ω
(
t
)
2
)
I
(
t
)
+
k
-
m
ⅆ
ω
(
t
)
ⅆ
t
wherein:
Q(t) is a blood flow rate,
I(t) is amount of current used to drive the pump;
ω(t) is speed of the rotor;
a, b, c, d, e, g, h, and k are constants belonging to the set of real numbers, and m is a coefficient that depends on the speed of the pump rotor.
5. The blood pump system of claim 4 , wherein the value of the coefficient m is calculated using the formula:
m=n *ω( t )+ o
wherein n and o are constants belonging to the set of real numbers.
6. The blood pump system of claim 4 , wherein the value of the coefficient m is calculated using the formula:
m =( p*v−q )ω( t )+ r*v+s
wherein:
p, q, r, and s are constants belonging to the set of real numbers, and
v is viscosity of a patient's blood.
7. The blood pump system of claim 4 , wherein the control circuit is configured to receive user input indicating the values of at least some of the coefficients a, b, c, d, e, g, h, and k.
8. The blood pump system of claim 1 , wherein the control circuit is further configured to transmit an indication of the determined blood flow rate to a remote device.
9. The blood pump system of claim 1 , wherein the control circuit is further configured to determine a parameter related to contractility of a patient's heart based on the blood flow rate.
10. The blood pump system of claim 9 , wherein the control circuit is configured to determine the parameter related to the contractility of the patient's heart using the determined flow rate having the dynamic frequency range of at least 15 hertz.
11. The blood pump system of claim 8 , wherein the determined parameter is based on a rate of change of pressure head across the pump, the rate of change being determined based on the blood flow rate.
12. A method for determining a flow rate of blood that is used to provide a ventricular assistance to a patient's heart:
determining a speed of rotation of a rotor of the pump;
determining a derivative of the speed of rotation of the rotor; and
calculating, by a processor, the flow rate of the blood having a dynamic frequency range of at least 15 hertz based at least in part on a calculated time-varying parameter which accounts for disturbances in the blood flow rate introduced by the pulsating action of the heart, the time-varying parameter based at least in part on the derivative of the speed of rotation.
13. The method of claim 12 , further comprising:
determining an amount of current used to drive the pump;
wherein the blood flow rate is calculated also based on the amount of current.
14. The method of claim 12 , wherein the time-varying parameter is calculated also based on the viscosity of the blood and the speed of rotation of the rotor.
15. The method of claim 12 , wherein
the blood flow rate is calculated using the equation:
Q
(
t
)
=
aI
(
t
)
+
bI
(
t
)
2
+
cI
(
t
)
3
+
ⅆ
ω
(
t
)
+
e
ω
(
t
)
2
+
g
ω
(
t
)
I
(
t
)
+
h
(
ω
(
t
)
2
)
I
(
t
)
+
k
-
m
ⅆ
ω
(
t
)
ⅆ
t
wherein:
Q(t) is a blood flow rate,
I(t) is amount of current used to drive the pump;
ω(t) is speed of the rotor;
a, b, c, d, e, g, h, and k are constants belonging to the set of real numbers, and
m is a coefficient that depends on the speed of the pump rotor.
16. The method of claim 15 , wherein the value of the coefficient m is calculated using the formula:
m=n *ω( t )+ o , and
n and o are constants belonging to the set of real numbers.
17. The method of claim 15 , wherein the value of the coefficient m is calculated using the formula:
m =( p*v−q )ω( t )+ r*v+s
wherein:
p, q, r, and s are constants belonging to the set of real numbers, and
v is viscosity of the patient's blood.
18. The method of claim 15 , wherein the values of the coefficients a, b, c, d, e, g, h, and k are specified via user input.
19. The method of claim 12 , further comprising determining a rate of pressure change of blood across the pump based on the determined flow rate.
20. The method of claim 12 , further comprising determining a parameter related to the contractility of the patient's heart.
21. The method of claim 20 , wherein the determining the parameter related to the contractility of the patient's heart is performed using the calculated flow rate having the dynamic frequency range of at least 15 hertz.