Method for physiologic control of a continuous flow total artificial heart

Abstract

A method is provided of controlling a pump including a electrical motor coupled to a rotor which carries first and second impellers at opposite ends thereof. The method includes: (a) driving the rotor using the motor, so as to circulate fluid from the first impeller through a first fluid circuit, the second impeller, a second fluid circuit, and back to the first impeller; (b) determining a resistance of the first fluid circuit, based on a first motor parameter which is a function of electrical power delivered to the motor; (c) determining a flow rate through the first fluid circuit based on a second motor parameter which is a function of electrical power delivered to the motor; and (d) varying at least one operational parameter of the pump so as to maintain a predetermined relationship between the flow rate and the resistance of the first fluid circuit.

Claims

1. An artificial heart system, comprising: (a) an artificial heart including an electrical motor coupled to a rotor which carries first and second impellers at opposite ends thereof, where: (i) the first impeller communicates with a patient's systemic vasculature; and (ii) the second impeller communicates with the patient's pulmonary vasculature; (b) a power source; and (c) a controller coupled to the power source and the artificial heart, the controller programmed to: (i) drive the rotor using the motor, so as to circulate blood from the first impeller through the systemic vasculature, the second impeller, the pulmonary vasculature, and back to the first impeller; (ii) sense electrical power delivered to the motor and determine a resistance of the systemic vasculature, based on a first motor parameter which is a function of the electrical power delivered to the motor; (iii) determine a flow rate through the systemic vasculature based on a second motor parameter which is a function of the electrical power delivered to the motor; and (iv) vary at least one operational parameter of the artificial heart so as to maintain a predetermined relationship between the systemic flow and the resistance of the systemic vasculature.

2. The artificial heart system of claim 1 wherein the controller is further programmed to modulate the speed of the rotor to generate a pulsatile flow.

3. The artificial heart system of claim 2 wherein the controller is further programmed to vary the degree of speed modulation based on the systemic vascular resistance.

4. The artificial heart system of claim 1 wherein the at least one operational parameter is selected from the group consisting of: are mean speed, electrical pulse rate, speed pulsatility, and pump duty cycle, and combinations thereof.

5. The artificial heart system of claim 1 wherein: (a) The rotor carries an a magnet assembly; (b) an electrical stator surrounds the rotor; and (c) the magnet assembly is axially shorter than the stator 18, such that the rotor is moveable axially within a housing to vary the relative performance of the first and second impellers in response to changes in fluid pressures at inlets to the first and second impellers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:

(2) FIG. 1 is a cross-sectional view of an artificial heart and control system constructed according to an aspect of the present invention;

(3) FIG. 2 is schematic block diagram of the pump of FIG. 1 coupled to a circulatory system;

(4) FIG. 3 is a graph showing a target operating characteristic;

(5) FIG. 4 is a block diagram showing a method of controlling a pump in accordance with an aspect of the present invention;

(6) FIG. 5 is a graph showing observed systemic vascular resistance values plotted against a first measured motor power characteristic;

(7) FIG. 6 is a graph showing observed systemic flow values plotted against a second measured motor power characteristic;

(8) FIG. 7 is a graph showing a control relationship for speed pulsatility;

(9) FIG. 8 is a set of graphs showing operation of an artificial heart with modulated speed under normal conditions;

(10) FIG. 9 is a set of graphs showing operation of an artificial heart with modulated speed which is experiencing intermittent suction;

(11) FIG. 10 is a graph showing normalized current flow versus time in an artificial heart operating at various speeds; and

(12) FIG. 11 is a graph of the statistical variation in the normalized current flow shown in FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

(13) Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 depicts a continuous flow total artificial heart 10 used to temporarily or permanently support an human patient. The artificial heart 10 includes a hollow housing 12 with opposed left and right inlets 14 and 16. An electrical stator 18 comprising a plurality of coil windings is disposed in the housing 12. While a total artificial heart 10 is used as an illustrative example, the principles of the present invention are equally applicable to other kinds of mechanical configurations and pumps, for example ventricular assist devices.

(14) A rotor 20 is disposed inside the stator 18. The rotor 20 includes a magnet assembly 22 comprising one or more permanent magnets arranged in an annular configuration. A left impeller 24 comprising an annular array of vanes is carried at the left end of the rotor 20 adjacent the left inlet 14. A right impeller 26 comprising an annular array of vanes is carried at the right end of the rotor 20 adjacent the right inlet 16. The left and right impellers 24 and 26 discharge into separate right and left peripheral outlets, which are not shown in FIG. 1. The left impeller 24 along with the portion of the housing 12 surrounding it may be referred to as a “left pump” while the right impeller 26 along with the portion of the housing 12 surrounding it may be referred to as a “right pump”.

(15) All of the portions of the artificial heart 10 which will come into contact with blood or tissue are constructed from known biologically compatible materials such as titanium, medical grade polymers, and the like.

(16) The rotor 20 and the stator 18 operate as a brushless DC motor through the application of varying electrical currents to the stator 18. The artificial heart 10 is coupled by a cable 28 to a controller 32, which is in turn powered by a power source 30, for example a battery, both of which are shown schematically in FIG. 1. The controller 32 is effective to provide pulsed DC current to the stator 18 in a known manner, and includes a microprocessor or other hardware suited to carry out a preprogrammed control method, as described in more detail below. The controller 32 may include subcomponents such as a CPU or main processor coupled to a known type of motor driver circuit. The degrees of freedom for the controller 32 are mean pump speed (RPM), DC pulse rate, speed pulsatility (i.e. RPM modulation about the mean), and/or duty cycle. The controller 32 is further configured to measure one or more control parameters, in particular electrical power (wattage) delivered to the artificial heart 10, and to receive a feedback signal from the artificial heart 10 indicative of the pump speed. Speed pulsatility (i.e. RPM modulation) may be used to create a pulse in a patient, and also provide an additional parameter for physiologic control.

(17) FIG. 2 shows a simplified schematic of the artificial heart 10 coupled to a patient's circulatory system. In operation, the left impeller 24 pushes blood through the body's systemic vasculature, which defines a fluid circuit “S” and is represented from a hydraulic standpoint by a systemic vascular resistance labeled “SVR”. Blood then flows back to the right atrium (right impeller inlet). The right impeller 26 pushes the blood through the body's pulmonary vasculature, which defines another fluid circuit “P” and is represented from a hydraulic standpoint by a pulmonary vascular resistance labeled “PVR”. Blood flows from the PVR back to the left atrium (left impeller inlet).

(18) If the systemic (i.e. left) flow is lower than the pulmonary (i.e. right) flow, then the left atrial pressure increases, and the right atrial pressure decreases. If the left output is greater than the right, then the atrial pressures reverse. Thus, an unbalance in flows is automatically accompanied by an unbalance in atrial (pump inlet) pressures.

(19) The magnet assembly 22 in the rotor 20 is axially shorter than the stator 18, allowing a degree of free axial movement of the rotor 20, in response to any unbalance of pump inlet (i.e. atrial) pressures. This axial movement changes the distances “D1” and “D2” (see FIG. 1) which represent axial operating clearances of the left impeller 24 and right impeller 26, respectively. This change in pump geometry affects the relative left/right performance in a direction to correct the atrial pressure imbalance. Thus, the artificial heart 10 is self-balancing, acting as an inlet pressure balancing regulator while at the same time pumping both systemic and pulmonary circulation.

(20) The artificial heart 10 is controlled as follows. First, a desired or targeted characteristic is determined by a physician. The characteristic describes the relationship between the volumetric flow rate in the systemic vasculature S and the SVR. In the example shown in FIG. 3, the characteristic is a linear plot between endpoints selected by the physician.

(21) Referring to FIG. 4, the controller 32 delivers power to the artificial heart 10 to spin the left and right impellers 24 and 26. The speed of the rotor pulse may be modulated in order to create a pulse in the patient. As used herein, “modulation” refers generally to any change in a cyclic property of the rotor speed, whether this change is made directly or indirectly, and may be accomplished by various means. For example, direct closed loop control of the rotor speed may be implemented. Alternatively, the electrical current supplied to the motor from the controller 32 may be modulated, and the resulting changes in rotor speed accepted. The rotor modulation signal (i.e. the speed or current wave form) may be sinusoidal, or a sine wave with a duty cycle transformation, or other wave forms such as ramp, triangular, or square. At block 100, the controller 32 senses the average electrical power (i.e. wattage) delivered to the motor, which can be measured by the controller 32 in a known manner, and the rotor speed, based on a feedback signal from the motor.

(22) Next, at block 110, the controller 32 computes two parameters: PSnorm, which is defined as average Watts divided by kRPM.sup.3, and PQnorm, defined as average Watts divided by kRPM.sup.2. Because these parameters are derived from or related to the power delivered to the motor using mathematical expressions, they may be described as being “functions of” power delivered to the motor.

(23) Next, at block 120, the SVR and the systemic flow rate are determined based on the computed parameters. FIG. 5 shows a sample of a suitable correlation for SVR (in dyn.Math.s/cm.sup.5) vs. PSnorm. The correlation is derived from empirical test data. It is depicted as a graph in FIG. 5, but it may be implemented or stored by the controller 32 in any equivalent fashion, for example as a graph, as a lookup table or matrix, or as a mathematical expression (e.g. a linear or polynomial curve fit).

(24) FIG. 6 shows a sample of a suitable correlation for systemic flow rate (in 1 pm) vs. PQnorm. The correlation is derived from empirical test data. It is depicted as a graph in FIG. 6, but it may be implemented or stored by the controller 32 in any equivalent fashion, for example as a graph, as a lookup chart or matrix, or as a mathematical expression (e.g. a linear or polynomial curve fit).

(25) Once the systemic flow rate and SVR have been determined, their relationship can be computed to determine if the current operating point lies on the prescribed characteristic shown in FIG. 3. If it does not, then one or more operational variables are increased or decreased until it does, at block 130. Examples of such operation variables include mean pump speed, pulse rate, speed pulsatility, and/or duty cycle. The process repeats at block 100 so long as pump operation continues.

(26) Independent of the control process, the self-balancing process described above is also taking place during operation of the artificial heart 10. The relative left/right performance of the two pumps can be further affected by the relative impedance seen by the pump outputs. In this pump configuration, speed modulation at high SVR can decrease the left pump output while increase the right pump output. This effect can be moderated by using the controller 32 to reduce or eliminate any speed pulsatility at high SVR values. For example, the controller 32 may be programmed to follow a characteristic of enforced speed pulsatility vs. SVR similar to the example shown in FIG. 7. This allows the artificial heart 10 to self-balance over a wider range of physiologic conditions, allowing an additional degree of freedom in balancing left/right performance.

(27) Operating the artificial heart 10 in a modulated mode can cause an intermittent suction of tissue around one of the left or right inlets 14 or 16 at the cyclic peak speed with physiologic decreases in blood volume returning to the artificial heart 10. This intermittent occlusion can cause erratic and amplified oscillation in axial movement of the rotor 20, and touching of the left or right impellers 24 or 26 against the pump housing 12, which is reflected in the speed and current signals processed in the controller 32. Intermittent suction and the associated effects are undesirable and can cause excessive wear or damage to the artificial heart 10. In addition to control of the artificial heart 10, the present invention provides a method for detecting this intermittent suction and responding to it through peak speed reduction.

(28) Normally, when the speed is a sinusoidal speed wave form, this will yield a current of a similar wave form, and vice-versa. An example of normal system response is shown in FIG. 8, showing the sinusoidal speed and a near sinusoidal current. In this example, the pump speed was modulated at frequency of 1.33 Hz (i.e. 80 beats per minute), and an amplitude of +/−25% of the mean speed of 2800 RPM. In FIG. 9, the same modulation at a mean speed of 3000 RPM resulted in intermittent suction, with a non-sinusoidal speed/current relationship (indicated at “A” and “B”), and spikes in the rate of current change (di/dt), clearly indicating the point of rubbing (high positive di/dt) at “C” and period of suction (high negative di/dt) at “D”. Based on these observations, a high absolute value of (di/dt) can serve as a predetermined limit or trigger for a response. The limit value for a particular application may be derived from empirical test data. Alternatively, the controller 32 may be programmed to evaluate the speed or current wave forms and trigger a response based on the existence of particular characteristics in the wave forms, such as the brief RPM drops indicated at A in FIG. 9, or the peak discontinuities shown at B in FIG. 9.

(29) Another suitable test for triggering peak speed reduction is analysis of normalized current. FIG. 9 shows normalized current (where the current signal is normalized by dividing by the cube of the speed signal) for an artificial heart 10 operating at 2600 RPM, 2800 RPM, and 3000 RPM. Normally, the resulting signal of current divided by speed cubed has small variation (see FIG. 11). However, when the speed is high enough to cause an abnormal suction condition, the variation in normalized current is dramatically higher. This is clearly seen in the example of operation at 3000 RPM. Rubbing is indicated at “C′” and suction at “D′” in FIG. 10, with a correspondingly high variation in normalized current in FIG. 11. Based on these observations, a high value of variation in normalized motor current can serve as a predetermined limit or trigger for a response. The limit value for a particular application may be derived from empirical test data.

(30) Regardless of which specific trigger or limit value is used to determine the presence of suction and/or rubbing, the controller 32 may be programmed to evaluate the parameter and look for the specified trigger and/or compare the parameter to the predetermined limit. If suction or rubbing is indicated, the controller 32 responds by reducing the peak speed. This can be done by lowering the mean speed, reducing the speed modulation amplitude, changing the duty cycle (portion of time at high speed), or any combination thereof.

(31) The foregoing has described a method of operating a total artificial heart. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.