Sensorless flow estimation for implanted ventricle assist device
A method of estimating the blood flow rate of a heart ventricle assist device which is positioned externally of, or implanted in, a patient. The assist device comprises a blood pump having a rapidly rotating, electrically powered impeller, and comprises briefly interrupting power to the impeller to cause its rotation to slow. From this, blood viscosity can be estimated, which viscosity is used to obtain real time, estimated blood flow rates and pressure heads. Apparatus for accomplishing this is disclosed.
1. A heart ventricle assist system comprising:
a blood pump having a magnetically rotatable impeller;
a plurality of stator drive coils magnetically coupled to said impeller to enable it to rotate;
a motor control circuit adapted to provide motor control signals to said drive coils thereby magnetically to rotate said impeller within said blood pump to establish a predetermined steady state blood flow rate, said motor control circuit continuously monitoring the rotational speed of said impeller; and
a controller adapted to interrupt said motor control signals at predetermined intervals, each interval consisting of a predetermined period of time, thereby to permit a decay in rotational speed of said impeller during each predetermined period of time, said controller adapted to measure values and time between minimum and maximum back EMF signals generated within said stator coils during each predetermined period and to determine therefrom a transient viscosity value of blood in said blood pump, said motor control circuit establishing a rotational speed of said impeller sufficient to create a steady state blood flow rate which is a function of said transient viscosity value between each of said predetermined intervals.
2. The system of claim 1 , wherein the predetermined period of time is less than 0.1 second.
3. The system of claim 2 in which the predetermined period of time is between 20 and 100 milliseconds.
4. The system of claim 1 , wherein voltage generated by said motor control circuit is greater than a positive peak of the back EMF signals.
5. The system of claim 1 , in which said controller comprises a back EMF signal biasing circuit configured to prevent clipping of the back EMF signals at levels below zero volts.
6. The system of claim 5 , wherein the back EMF signal biasing circuit is also configured to measure zero crossings of the back EMF signal when rotational speed of said impeller produces a steady state blood flow rate.
7. The system of claim 1 , wherein the controller is adapted to monitor the rotational position of said impeller relative to said drive coils during each predetermined period of time.
8. The system of claim 1 , wherein rotation of the impeller is not actively braked during any predetermined period of time.
9. The system of claim 1 , in which the length of the predetermined period of time is insufficient to permit physiologically detrimental interference to blood flow rate.