Magnetically suspended fluid pump and control system

[0002] This application is based upon U.S. Provisional Patent Application Serial No. 60/119,356, filed Feb. 9, 1999, U.S. patent application Ser. No. 08/978,670, filed Nov. 26, 1997, now U.S. Pat. No. 5,928,131, and U.S. patent applications Ser. Nos. 09/273,384 and 09/288,413, now U.S. Pat. No. 6,179,773, filed Mar. 22, 1999, and Apr. 8, 1999, respectively, both of which are Divisional Patent Applications of above-referenced patent application Ser. No. 08/978,670. The disclosures of each of the above-referenced patent applications are hereby incorporated herein by reference.

[0003] 1. Field of the Invention

[0004] The present invention relates generally to pumps which employ magnetic suspension and rotation means to pump blood, and more particularly to a magnetically suspended and rotated blood pump that has no mechanical bearings or seals and has a pump means which is magnetically supported radially and axially.

[0005] 2. Description of the Prior Art

[0006] The use of rotary pumps (i.e. axial, centrifugal, mixed flow) to pump fluids and in particular blood is well known by those skilled in the art. A rotary pump, in general, consists of an outer housing, with inlet and outlet ports, and an impeller mounted on a shaft (with mechanical bearings and seals) within the outer housing for rotation about an axis. Mechanical bearings are susceptible to wear and premature failure and can generate sufficient heat and mechanical stresses to cause unacceptable blood damage. Shaft seals are also susceptible to wear and heat generation, which can lead to leakage, blood clot formation, bearing seizure, and bacterial growth. Examples of rotary pumps utilizing shaft mounted impellers with bearings and seals are disclosed in Reich et. al. U.S. Pat. No. 4,135,253; Possell U.S. Pat. No. 4,403,911; Moise U.S. Pat. No. 4,704,121; and Dorman U.S. Pat. No. 4,927,407.

[0007] Numerous pumps have been designed to circumvent the above problems by employing a lubricant flush of rotary pump mechanical bearings. Examples of such pumps are disclosed in Carriker et al. U. S. Pat. No. 4,944,722 and Wampler et al. U.S. Pat. No. 4,846,152. These types of pumps can have several problems including not having the ability to be fully implantable due to the need for a percutaneous supply line and external reservoir to achieve bearing flushing. Also the potential for infection and leakage exists due to the flushing fluid and percutaneous lines. In addition the mechanical bearings can still require replacement after time because they directly contact other pump structures during operation.

[0008] By employing a rotary fluid pump with a magnetically suspended impeller, all of the above mentioned problems can be avoided. Examples of such pumps are disclosed in Bramm et al. U.S. Pat. No. 5,326,344; Olsen et al. U.S. Pat. No. 4,688,998 and Moise U.S. Pat. No. 4,779,614. A problem which can be associated with all of the cited inventions is that a single gap is employed for both the blood flow pathway through the pump and for the magnetic suspension and rotation of the impeller. These two functions have directly opposing requirements on the size of the gap. As a blood flow pathway, the gap should be large to avoid blood damage. As a magnetic suspension and rotation gap, the gap should be small to minimize the size of the magnetic suspension and rotation components and also to allow for efficient use of energy to achieve impeller suspension and rotation. Consequently, for these types of pumps, any gap size selected can result in an undesirable compromise between blood damage, device size, and energy requirements. Another problem which can be associated with all of the cited inventions is that the magnetic fields created by the permanent magnets in the described pumps can change over time, with variations in temperature, and due to the influence of external ferromagnetic objects. Such changes in the magnetic fields can cause the equilibrium point of the pump rotors to change and can possibly cause the rotors to contact the stators. Contact between the pump rotors and stators can result in blood damage, pump damage, and inadequate pumping.

[0009] Examples of pumps having separate gaps for primary blood flow and impeller rotation are disclosed in Golding et al. U.S. Pat. No. 5,324,177 and Golding et al. U.S. Pat. No. 5,049,134. However, these pumps also use the rotation gap to implement hydrodynamic suspension bearings for the rotor. Such hydrodynamic bearings can subject the blood to excessive shear stresses which can unacceptably damage the fragile components of the blood. Additionally, the Golding et. al. pumps place the stationary magnetic components inside a center-bore of a rotating assembly. Such configurations generally cause the mass and rotational inertia of the rotating assembly to be larger than those in a system in which the stationary magnetic components are placed around the outer surface of the rotating assembly. Rotating assemblies having large masses and rotational inertias can be undesirable because the axial and radial bearing elements must be made relatively large in order to maintain proper alignment of the rotating assembly during shock, vibration, and acceleration.

[0010] The flow rate of blood pumps that are capable of creating negative inlet pressures must be dynamically adjusted to match the blood flow rate into the ventricle of the heart, typically the left ventricle. If too little flow is produced by the blood pump, the tissues and organs of the body may be inadequately perfused, and the blood pressure in the left ventricle will increase—potentially causing excessive pulmonary pressure and congestion. Conversely, if the flow rate of the blood pump is too high, excessive negative pressure may be created in the left ventricle and in the inlet to the pump. Excessive negative blood pressure is undesirable for the following reasons: 1) Unacceptable levels of blood damage may be caused by cavitation; 2) The pump may be damaged by cavitation; 3) The walls of the ventricle may collapse and be damaged; and 4) The walls of the ventricle may collapse and block the blood flow pathway to the pump.

[0011] By employing a control system to dynamically control the flow rate of the pump to avoid excessive negative blood pressure the above mentioned problems can be avoided. One example of such a control system is disclosed in Bramm et al., U.S. Pat. No. 5,326,344. Bramm describes a method of dynamically controlling the flow rate of a pump based on a signal derived from a single pressure sensor located within the pump inlet. One problem which can be associated with such a pressure sensing system is the difficulty in achieving long-term stability of such a sensor, particularly in light of the relatively low pressures (0 to 20 mm Hg) that must be resolved and the hostile environment in which the sensor is operated. Another problem which can be associated with such a pressure sensing system is that the effect of changing atmospheric pressure can cause inaccurate sensing of the pressure needed to properly control the pump.

[0012] The natural heart normally creates pulsatile blood flow. Artificial blood pumps that do not create pulsatile flow may cause thrombosis and may inadequately perfuse the organs and tissues of the body. The speed of rotary blood pumps can be varied to create pulsatile flow. Examples of pumps that can vary their speed to create pulsatile flow are disclosed in Isaacson et al. U.S. Pat. No. 5,211,546, Findlay U.S. Pat. No. 5,174,726, and Reich et al. U.S. Pat. No. 4,135,253. One problem that can be associated with varying the speed of the pumps as cited in all of the inventions is if the speed variation is excessive, the blood flow in the pump may become too low and/or reverse direction. If the blood flow in the pump becomes too low the blood may be damaged by constant shearing of the blood pool in the pump. If the blood flow in the pump reverses direction the ventricle may not be adequately unloaded.

[0013] Accordingly, there is a need for a blood pump which overcomes the aforementioned problems that can be associated with conventional blood pumps and also a system of dynamically controlling such a blood pump to avoid the previously described problems that can occur with control systems using pressure sensors.

[0014] A blood pump apparatus is provided which can include a stator member containing a magnetically suspended and rotated rotor member. The rotor can preferably be magnetically suspended within the stator both radially and axially. The blood pump can also have an associated magnetic suspension control system and a blood pump flow rate control system. The blood pump can preferably be a centrifugal pump wherein an impeller draws blood from the left ventricle of a the heart and delivers it to the aorta thereby reducing the pressure that must be generated by the left ventricle. The blood pump can also be of a relatively small size such that it can be completely implanted within the human body. If bi-ventricular cardiac assist is needed a second such blood pump can be implanted to assist the right ventricle. The impeller of the centrifugal pump can be an integral part of a rotor assembly. The rotor assembly can preferably be suspended by permanent magnet radial bearings and a Lorentz-force axial bearing. The Lorentz-force axial bearing can generate bidirectional axial forces in response to an applied current. The blood pump can also include an axial position sensor and an axial position controller. The axial position sensor can monitor the axial position of the rotor and provide feedback to the controller to maintain the axial position of the rotor. The axial position controller can also adjust the axial position of the rotor such that steady-state axial loads due to gravity, acceleration or the centrifugal pump impeller are offset by the inherent axial forces generated by the permanent magnet radial bearings. By offsetting the steady-state axial forces using the axial position controller, the power required by the Lorentz-force axial bearing is minimized. The rotor assembly can be rotated by an electric motor. The blood pump can also include an actuator to adjust the axial position of the magnetic suspension and rotation components in the stator, preferably, relative to the stator itself. Such mechanism can compensate for changes in the magnetic fields of the permanent magnets used in the blood pump.

[0015] A primary blood flow inlet path can preferably be through a relatively large center bore provided in the rotor. A first retrograde blood flow path can be through an annular gap which is formed between the rotor and the stator of the pump as a result of the radial magnetic suspension. In order to minimize the size of the device, all of the magnetic suspension and rotation forces can be applied across the relatively small annular gap. A second retrograde blood flow path can be through an axial gap which is provided between the impeller wall and the volute chamber. All blood contacting surfaces of the pump are continuously washed to avoid blood clots and protein deposition.

[0016] The speed of the centrifugal pump can be dynamically controlled to avoid excessive negative pressure in the left ventricle. The blood pump flow rate control system can include an electronic heart caliper. The heart caliper can be operatively attached to the outside surface of the heart and provide feedback to the blood pump flow rate control system. The heart caliper can be utilized to monitor the outside dimension of the left ventricle. The blood pump flow rate control system can preferably operate in two modes, continuous and pulsatile. In the continuous mode of operation, the pump speed can be controlled to hold the sensed left ventricle dimension at a first predefined setpoint. In the pulsatile mode of operation, the pump speed can be dynamically adjusted to cause the sensed left ventricle dimension to alternate between the first predefined setpoint and a second predefined setpoint. A flow sensor can be utilized to avoid excessively low or reverse flow through the blood pump when it is operated in the pulsatile mode.

[0017] Other details, objects, and advantages of the invention will become apparent from the following detailed description and the accompanying drawing figures of certain presently preferred embodiments thereof.

[0018] A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, wherein:

[0019] FIG. 1ais a cross section view of an embodiment of the blood pump having a magnetically suspended and rotated rotor assembly and a mechanism to adjust the axial position of the magnetic suspension and rotation components mounted in the stator;

[0020] FIG. 1bis a cross section view of another embodiment of the blood pump shown in FIG. 1a;

[0021] FIG. 2 is a view of the blood pump in FIG. 1ataken along line II—II;

[0022] FIG. 3ais a view of one preferred embodiment of the permanent magnet arrays used for radial suspension and axial bias force;

[0023] FIG. 3bis a view of a second preferred embodiment of the permanent magnet arrays used for radial suspension and axial bias force;

[0024] FIG. 4 is a perspective view of the blood pump of FIGS. 1aand 1bconnected to a circulatory system;

[0025] FIG. 5 is a schematic diagram of a circuit for sensing the axial position of the magnetically suspended rotor assembly;

[0026] FIG. 6 is a simplified schematic diagram of an axial position controller;

[0027] FIG. 7 is a graphical illustration of a minimum power axial position control method;

[0028] FIG. 8ais a sectional view of a heart caliper attached to a distended ventricle;

[0029] FIG. 8bis a sectional view of a heart caliper attached to a contracted ventricle;

[0030] FIG. 9 is an enlarged sectional view of an apparatus for electronically measuring the angle between two caliper arms shown in FIGS. 8aand 8b;

[0031] FIG. 10ais a sectional view of a sonomicrometry based heart caliper attached to a distended ventricle;

[0032] FIG. 10bis a sectional view of a sonomicrometry based heart caliper attached to a contracted ventricle;

[0033] FIG. 11 is a graphical illustration of a method for controlling a steady-state flow rate of the blood pump; and

[0034] FIG. 12 is a graphical illustration of a method for controlling the flow rate of the blood pump in a pulsatile manner.

[0035] Referring now to the drawing figures wherein like reference numbers refer to similar parts throughout the several views, presently preferred embodiments of a blood pump apparatus are shown in FIGS. 1aand 1bhaving a stator assembly 1 and a rotor assembly 2.

[0036] The stator assembly 1 can have an outer stator shell 3, a volute housing 4, an end cap 5, and a thin-walled stator liner 6. The stator shell 3, volute housing 4, end cap 5, and stator liner 6 can each be made from titanium. The stator liner 6 can have a thickness from about 0.005 to 0.015 inch, and preferably is about 0.010 inch. The outer stator shell 3, volute housing 4, and stator liner 6 can preferably be welded together to form a hermetically sealed annular stator chamber 54. The stationary magnetic suspension and motor components can be advantageously housed in the stator chamber 54.

[0037] The rotor assembly 2 can have a relatively large center bore 20 which can be the primary blood flow path 20′ through the pump. Preferably the center bore 20 is about 0.50 inch. The rotor assembly 2 can include an inner rotor support sleeve 7, a rotor end cap 8 and a thin-walled rotor liner 9. The inner rotor support sleeve 7, rotor end cap 8 and rotor liner 9 can each be made from titanium. The rotor liner 9 can have a thickness from about 0.005 to 0.015 inch and can preferably be about 0.010 inch. The rotor support sleeve 7, rotor end cap 8 and rotor liner 9 can preferably be welded together to form a hermetically sealed annular rotor chamber 55. The rotating magnetic suspension and motor components can be advantageously housed in the rotor chamber 55. The inner rotor support sleeve 7 can be fabricated with an integral impeller 10 or, alternately, the impeller 10 can be fabricated independently then welded or bonded to the rotor support sleeve 7.

[0038] The blood contacting surfaces of the blood pump can be chemically polished or abrasively polished by pumping an abrasive slurry through the pump. The blood contacting surfaces of the blood pump can also be coated with a diamond-like carbon film or a ceramic film. Such films enhance the long term bio-compatibility of the surfaces by improving their surface finish and abrasion resistance. Companies capable of providing such films include Diamonex Performance Products, Allentown, Pa., and Implant Sciences Corporation, Wakefield, Mass.

[0039] Impeller 10 can be a closed impeller with upper wall 56 and lower wall 57 enclosing impeller blades 36.

[0040] The primary blood flow path 20′, can be through the center bore 20 of the inner rotor support sleeve 7, through the closed impeller 10, to the outflow port 38.

[0041] To minimize the formation of thrombus, blood flow can be maintained across all blood contacting surfaces. A first retrograde blood flow path 21′, can be through the radial magnetic suspension gap 21 which is the radial magnetic suspension gap between the stator liner 6 and the rotor liner 9. The radial magnetic suspension gap 21 can preferably be about 0.010 inch. A second retrograde blood flow path 59, can be through an axial gap 60, provided between the lower impeller wall 57 and the end cap 5, and through an opening 67 in the lower wall 57 of the impeller 10. The axial gap 60 can preferably be about 0.005 inch.

[0042] When polarized as shown in FIG. 3a, radial magnetic repulsion forces are generated between permanent magnet array 11, mounted in the stator chamber 54, and permanent magnet array 12, mounted in the rotor chamber 55. The permanent magnet array 11 is comprised of ring magnets 64, 65, and 66. Ring magnet 64 is radially polarized from its outer to inner diameter, ring magnet 66 is radially polarized in the opposite direction from its inner to outer diameter and ring magnet 65 is axially polarized from right to left. With ring magnets 64, 65 and 66 polarized as described, magnetic flux is directed substantially in the radial direction toward permanent magnet array 12, mounted in the rotor chamber 55. Similarly, permanent magnet array 12 is comprised of ring magnets 61, 62, and 63. Ring magnet 63 is radially polarized from its outer to inner diameter, ring magnet 61 is radially polarized in the opposite direction from its inner to outer diameter and ring magnet 62 is axially polarized from right to left. With ring magnets 61, 62 and 63 polarized as described, magnetic flux is directed substantially in the radial direction toward permanent magnet array 11, mounted in the stator chamber 54.

[0043] In FIGS. 1aand 1b, as the rotor assembly 2, is moved radially downward relative to the stator assembly 1, the repulsion force between the lower portion, of permanent magnet arrays 11 and 12 increases while the repulsion force between the upper portion, of permanent magnet arrays 11 and 12 decreases. A net upward force is thus created which tends to restore the rotor assembly 2, to a radially aligned position. Likewise, as the rotor assembly 2, is moved radially upward relative to the stator assembly 1, the repulsion force between the upper portion, of permanent magnet arrays 11 and 12 increases while the repulsion force between the lower portion, of permanent magnet arrays 11 and 12 decreases. A net downward force is thus created that tends to restore the rotor assembly 2 to the radially aligned position. The described radial repulsion forces tend to cause the rotor assembly 2 to remain radially suspended with respect to the stator assembly 1. Ring magnets 61, 62, 63, 64, 65, and 66 can preferably be fabricated of magnetically hard material having a relatively high energy product such as Neodymium Iron Boron. Ring magnets 61, 62, and 63 can be bonded together to form permanent magnet array 12 which can be bonded to rotor support sleeve 7. Ring magnets 64, 65, and 66 can be bonded together to form permanent magnet array 12 which can be bonded to stator support sleeve 68.

[0044] Moreover, more than three adjacent ring magnets may also be used. For example, as shown in FIG. 3b, the two permanent magnet arrays 11′ and 12′ can each be made from five ring magnets bonded together. Magnet array 11′ includes rings 664-668 each having the polarities indicated by the arrows. Similarly, magnet array 12′ includes rings 659-663 each having the polarity indicated by the arrows. In the arrangement shown, two separate magnetic flux paths are established. Rings 664-666 and 659-661 direct the magnetic flux in the same manner previously described for rings 64-66 and 61-63 shown in FIG. 3. Additionally, rings 666-668 and rings 661-663 create a second magnetic flux path which directs the magnetic flux in the same manner as previously described, except in the opposite direction.

[0045] It is to be understood that other permanent magnet arrangements can be used to implement a radial bearing. One such alternate arrangement is described in Wasson U.S. Pat. No. 4,072,370, which is hereby incorporated herein by reference.

[0046] An assembly of permanent magnets 13, 14, 15, coils 16, 17, and back iron 18 cooperate to form a Lorentz-force actuator which can be employed as an axial bearing to axially support the rotor assembly 2. Permanent magnets 13, 14 and 15 cause magnetic flux 19, to flow radially from the outer surface of magnet 13, radially across the secondary blood flow path 21, radially through coil 16, axially through the stationary actuator back-iron 18, radially through coil 17, radially across the secondary blood flow path 21, radially through magnet 15, axially through the magnet 14, and radially through magnet 13. Permanent magnets 13, 14 and 15 can preferably be fabricated of a magnetically hard material having a relatively high maximum energy product such as Neodymium Iron Boron and can preferably be bonded to the rotor support sleeve 7. The stationary actuator back-iron 18 can preferably be fabricated of a soft magnetic material having a high saturation flux density. One such material is 48% Iron-48% Cobalt-2% Vanadium available as HIPERCO® 50A from Carpenter Technology Corporation, Reading Pa. Coils 16 and 17 can be fabricated from copper or silver wire and can preferably be bonded to the stationary actuator back-iron 18, which in turn can be bonded to the stator support sleeve 68. It is to be understood that coils 16 and 17 can be fabricated from round wire, square wire, rectangular wire, or flat tape. When coils 16 and 17 are energized such that current flows in a clockwise direction in coil 16 and in a counterclockwise direction in coil 17, as viewed from the pump inlet 20, a net axial Lorentz force is generated which tends to move the rotor assembly 2 to the right. If the direction of the currents in coils 16 and 17 is reversed such that current flows in a counterclockwise direction in coil 16 and in a clockwise direction in coil 17, as viewed from the pump inlet 20, a net axial Lorentz force is generated which tends to move the rotor assembly 2, to the left. A Lorentz-force actuator as described can be preferable to attractive ferromagnetic actuators because: a single Lorentz-force actuator is capable of producing bidirectional forces; the force output is a linear function of input current; the bandwidth is wider; the attractive radial force between the moving and stationary portions of the actuator is relatively low; and the generated force is parallel to the axial gap formed between the moving and stationary portions of the actuator.

[0047] A permanent magnet 31, armature windings 32 and back-iron 33 cooperate to form a slotless, brushless DC motor with a coreless armature. Such slotless, coreless motors are well understood by those skilled in the art and are described in U.S. Pat. No. 4,130,769, which is hereby incorporated herein by reference. A 2-pole permanent magnet ring 31 causes magnetic flux to flow radially from the its north pole 34, across the radial magnetic suspension gap 21, radially through the armature windings 32, circumferentially through the stator back-iron 33, radially through the armature windings 32, radially across the radial magnetic suspension gap 21 to the south pole 35 of the permanent magnet ring 31. Interaction between axial current flowing in the armature windings 32 and the radial magnetic flux produces torque between the rotor assembly 2 and the stator assembly 1. The permanent magnet ring 31 can preferably be fabricated of a magnetically hard material having a relatively high maximum energy product such as Neodymium Iron Boron. Alternatively, the permanent magnet ring 31 can be replaced with permanent magnet ring assemblies having more than 2 poles in order to reduce the size and/or increase the efficiency of the motor. The stator back-iron assembly 33, can be fabricated from a stack of magnetically soft lamination rings preferably having high resistivity and a high saturation flux density. One such material is 48% Iron-48% Cobalt-2% Vanadium and is available as HIPERCO® 50A from Carpenter Technology Corporation, Reading Pa. Electrically insulated laminations are used in the stator back-iron assembly 33 to minimize power losses caused by eddy currents which are induced by the rotating magnetic field produced by permanent magnet ring 31. It is to be understood that a conventional salient-pole brushless DC motor could be used in place of the described motor, however, a slotless, coreless, motor can be preferable because cogging torque can be eliminated in slotless motors allowing smoother, quieter operation as compared to salient-pole brushless DC motors, and slotless, coreless, motors generally have larger radial gaps between the permanent magnets in the rotor and the stator back-iron resulting in lower attractive radial forces. Attractive radial forces generated by the motor can be undesirable since they tend to oppose the repulsive radial suspension forces generated by the permanent magnet radial bearing magnets resulting in reduced radial suspension stiffness. Such slotless, brushless, coreless DC motors are available from companies such as Electric Indicator Company, Inc. Norwalk Conn.; Portescap U.S., Inc., Hauppauge N.Y.; Maxon Precision Motors, Inc., Fall River Mass.; and MicroMo Electronics, Inc., Clearwater Fla.

[0048] An assembly of coils 23, 24 and ferromagnetic rings 25, 26 cooperate to form an axial position sensor which is used to monitor the axial position of the rotor assembly 2 with respect to the stator assembly 1. The two coils 23, 24 can be fabricated from copper wire. A first ferromagnetic ring 25 causes the inductance of a first coil 23 to increase and the inductance of the second coil 24 to decrease as it is moved to the left. Likewise, the inductance of the first coil 23 decreases and the inductance of the second coil 24 increases as the first ferromagnetic ring 25 is moved to the right. A second ferromagnetic ring 26 can serve to both magnetically shield and increase the Q of the coils 23, 24. The two ferromagnetic rings 25, 26 can preferable be made of a ferrite material having a high permeability at the excitation frequency of the coils 23, 24. One such material is MATERIAL-W® available from Magnetics, Division of Spang & Co., Butler Pa. A pair of spacers 27, 28 can be used to radially locate the two ferromagnetic rings 25, 26.

[0049] As shown in FIG. 2, the impeller 10 rotates within volute housing 4. A series of impeller blades 36 propel blood centrifugally around the volute passage 37, and out the outflow port 38.

[0050] It is to be understood that other impeller-volute arrangements could be derived by those skilled in the art and the invention is therefore not to be limited to the particular configurations illustrated and described herein.

[0051] Referring now to FIG. 4, one method of connecting of the blood pump 51 to the circulatory system is schematically illustrated. Several cannulas 44, 46, 49, 90 can be provided to connect the pump 51 between the left ventricle of the heart and the aorta. A hole is cored in the apex of the left ventricle at location 43 and one cannula 44 directs blood from the left ventricular cavity to the flow sensor 91. The flow sensor 91 can preferably be provided to monitor the blood flow though the pump during the low-flow phase of the pump when the pump is used to provide pulsatile blood flow. A pump flow rate controller 92 can use the output signal of the flow sensor 91 to adjust the rotary velocity of the blood pump such that forward blood flow is maintained through the pump 51. Excessive blood damage can occur if the blood flow through the pump is too slow. The blood flow sensor 91 can be an electromagnetic sensor or an ultrasonic sensor. The construction and operation of electromagnetic flow sensors are well understood by those skilled in the art and are disclosed in Kolin U.S. Pat. No. 2,149,847, which is hereby incorporated herein by reference. The construction and operation of ultrasonic flow sensors are well understood by those skilled in the art and are disclosed in Drost U.S. Pat. No. 4,227,407, which is hereby incorporated herein by reference. Cannula 90 directs blood from the flow sensor 91 to the pump inlet 45. Another cannula 46 directs blood from the pump outlet 47 to an in-line valve assembly 48. Alternatively, a solenoid actuated valve could be used in place of valve assembly 48. The valve assembly 48 can preferably be provided to prevent retrograde blood flow from the aorta, through the pump, and into the left ventricle in the event of a failure of the blood pump or an associated control system. From the outlet of the valve assembly 48, another cannula 49 directs the blood to the ascending aorta 50. The pump flow rate controller 92 can also use the output signal of the heart caliper 400/500 to adjust the rotary velocity of the blood pump such that excessive distention or contraction of the ventricle is avoided. Excessive distention of the ventricle can occur if the pump velocity and the resulting flow rate is too low. Excessive contraction of the ventricle can occur if the pump velocity and the resulting flow rate is too high. For bi-ventricular cardiac assist, a second pump could be connected in like fashion between the right ventricle and pulmonary artery.

[0052] In some patient applications it will be highly desirable to provide pulsatile flow rate. While the pump mechanism shown can be operated as a continuous pump at any varying speed suitable for patient utilization, it may be desirable to create a cyclic or pulsatile flow rate. In this way, the pump can mimic the varying flow that may be desirable for some patients. The flow rate controller 92 as shown in FIG. 4 can be of various construction. It may be located separately from the pump as shown in FIG. 4 or may, in fact, be integrated inside the pump 51. Other placement and embodiments of the flow rate controller can be utilized in practicing the invention. The flow rate controller may preferably be microprocessor based using feed back control systems from the flow sensors. In other systems, feed back control of flow may not be required and other modulation techniques may be utilized. Any modulation or speed control of an electric motor can be utilized in the flow rate controller 92. These systems may use open loop or can use combinations of either flow sensors, pressure sensors, or both. As shown, the sensor 91 is located external from the pump housing. In other embodiments it may be more desirable to position sensors in or adjacent to the vessels carrying the blood or the heart itself. The flow rate controller 92 may contain software programming that provides for regulation for a various number of sensors such that regulation and modulation occurs with one set of sensors whereas another set of sensors such as the flow rate or other sensor indicative of the performance in the circulatory system is used as a limit control. These sensors could also be contained within the blood pump 51.

[0053] An axial position sense subsystem 141 can have the circuitry shown in FIG. 5. The subsystem 141 can utilize the ratio of the inductances of coils 23 and 24 to measure the axial position of the rotor assembly 2. The subsystem 141 can include an amplitude-stabilized sine-wave oscillator 100, which is used to excite coils 23 and 24 arranged as a half-bridge 101, and a synchronous demodulator 102. Synchronous demodulation is used to detect the relatively low amplitude signals output from the half-bridge circuit 101 because the synchronous demodulation technique effectively filters electrical noise at all frequencies except those centered about the excitation frequency. An oscillator 103 generates a square wave output 104, which is used to control analog switch 105. The output 106, of analog switch 105 is a square wave that alternates between the output voltage 107, of operational amplifier 108, and ground. A capacitor 109 and a resistor 110 form a highpass filter that removes the DC offset from signal 106. A lowpass filter 111 attenuates the upper harmonics of input signal 112 resulting in a sine-wave output signal 113. The lowpass filter 111 is of sufficient order and type to attenuate the third harmonic of the square wave input 112 by 40 dB or more. One possible configuration for low pass filter 111 is a 5^thorder Butterworth type. A capacitor 114 removes any DC offset from the output 113 of the lowpass filter 111. An AC sine-wave 115 is used to excite the half-bridge network 101. A pair of resistors 116, 117 and operational amplifier 118 form an inverting circuit with a gain of−1. A comparator 119 detects the sign of the sine-wave excitation signal 115. The output 120 of the comparator 119 is used to control an analog switch 121. When the sign of sine-wave 115 is negative, the output 122 of the analog switch 121 is connected to the non-inverted sine-wave signal 115. When the sign of sine-wave 115 is positive, the output 122, of the analog switch 121 is connected to the inverted sine-wave signal 123. The output 122 is thus the inverted, full-wave rectified representation of the excitation sine-wave signal 115. An operational amplifier 108, a pair of resistors 124, 144 and a capacitor 125 form an integrating difference amplifier. The output 107 of the operational amplifier 108 increases if the average full-wave rectified representation of the excitation sine-wave signal 115 is less than the applied precision reference voltage 126. Likewise the output 107 of the operational amplifier 108 decreases if the average full-wave rectified representation of the excitation sine-wave signal 115 is greater than the applied precision reference voltage 126. Through the described integrating action, the amplitude of the AC signal 106 is controlled as required to maintain the average full-wave rectified representation of the excitation sine-wave signal 115 equal to the applied precision reference voltage 126. As previously described, the ratio of the inductances of coils 23 and 24 is a function of the axial position of the rotor assembly 2 shown in FIGS. 1aand 1b. The amplitude of the output signal 127 of the half bridge circuit 101 formed by coils 23 and 24 thus varies with the axial position of the rotor assembly 2. A pair of resistors 128, 129 and an operational amplifier 130 form an inverting circuit with a gain of−1. The output 120, of the comparator 119 is used to control an analog switch 131. When the sign of sine-wave 115 is negative, the output 132 of the analog switch 131 is connected to the non-inverted output signal 127 of the half bridge circuit 101. When the sign of sine-wave 115 is positive, the output 132 of the analog switch 131 is connected to the inverted output signal 133 of the half bridge circuit 101. The output signal 132 is thus the inverted, full-wave rectified representation of the output signal 127 of the half bridge circuit 101. A lowpass filter 134 attenuates the AC components of the output signal 132. One possible configuration for the low pass filter 134 is an 8^thorder Butterworth type. Several resistors 135, 136, 137, along with an operational amplifier 138 and a precision reference voltage 126 shift and scale the output 139 of the lowpass filter 134 as required for downstream circuits. The output 140 of operational amplifier 138 is thus a representation of the axial position of the rotor assembly 2. Consequently, changes in the output 140 provide a measurement of the axial movement of the rotor assembly 2. The circuit illustrated in FIG. 5 is but one example of a circuit that can be used to detect changes in the ratio of the inductances of coils 23 and 24. It should be understood that other acceptable circuits may be derived by those skilled in the art.

[0054] Using the output 140 from the axial position sense subsystem 141, an axial position controller 200, shown in FIG. 6, can be used to both maintain the axial position of the rotor at a defined axial position setpoint and to adjust the axial position setpoint to minimize power dissipation in the Lorentz force actuator. The axial position controller 200 can have the basic circuitry shown in FIG. 6, including circuitry 201, which maintains the rotor at a defined axial position setpoint and circuitry 202, which adjusts the axial position setpoint for minimum power dissipation in the Lorentz-force actuator coils 16, 17. The axial position setpoint maintenance circuit 201, is comprised of the previously described axial position sense subsystem 141, a gain and servo compensation circuit 203, a switching power amplifier 204, the Lorentz-force actuator 205, and the rotor assembly 2. The axial position sense subsystem 141 outputs a signal 140, proportional to the axial position 215 of the rotor assembly 2. Several resistors 207, 208, 209 along with a capacitor 210 and an operational amplifier 211 form a gain and lead compensation network 203, which modifies the gain and phase of signal 140 as required to prevent unstable oscillation of the rotor assembly 2. The design of such gain and lead compensation networks is well understood by those skilled in the art of servo system design. The voltage output 212 of the gain and lead compensation network 203 is input to switching power amplifier 204. Switching power amplifier 204 outputs a current signal 213 that is proportional to the input voltage 212. The design of such transconductance switching amplifiers is well understood by those skilled in the art. The current signal 213 is applied to the coils 16, 17 of the Lorentz-force actuator 205. The Lorentz-force actuator 205 produces an axial force 214 proportional to the applied current signal 213. The axial force 214 is applied to the rotor assembly 2. The axial position 215 of the rotor assembly 2 changes in response to the applied axial force 214. The overall polarity of the described servo loop 201 is such that the force produced by the Lorentz-force actuator opposes displacement of the rotor assembly from the defined setpoint. Those skilled in the art will recognize that the function of the analog, gain and servo compensation circuit 203 can be implemented with software running on a microprocessor or digital signal processor.

[0055] In FIG. 7, the described minimum axial control power method is illustrated. The x-axis 300 of the graph represents the axial position of the rotor assembly 2 relative to the stator assembly 1. The y-axis 301 of the graph represents the axial force applied to the rotor assembly 2. Line 302 represents the inherent axial forces generated by the permanent magnet arrays 11, 12 for small axial displacements of the rotor assembly 2. At point 303 on the graph, the permanent magnets 11, 12 are magnetically aligned and generate no axial force. The slope of curve 302 is dependent on the design of the permanent magnet arrays 11, 12 and may be between 0.2 lb./0.001 inch to 1.0 lb./0.001 inch. Line 304 of FIG. 7 represents a steady-state axial load applied to the rotor assembly 2. The steady-state axial load 304 may be caused by gravity, acceleration, the centrifugal pump impeller, etc., Line 305 of FIG. 7 is the addition of lines 302 and 304 and represents the net force versus axial position of the rotor assembly 2 when the steady-state load 304 is applied. Point 306 defines the axial position of the rotor assembly 2 where the steady-state load force is canceled by the axial force produced by the permanent magnets 11,12. By adjusting the axial position setpoint of the rotor assembly 2 to the axial position defined by point 306, the steady-state actuator force output required to maintain the axial setpoint is zero. Since the power dissipated by the Lorentz-force actuator is proportional to the square of its output force, the net power dissipated by the actuator is minimized when the rotor assembly is operated at the axial position defined by point 306. Likewise, with no steady state load forces applied, the net power dissipated by the actuator is minimized when the rotor assembly is operated at the axial position defined by point 303.

[0056] The circuitry 202, shown in FIG. 6, can be employed to effectively adjust the axial setpoint position of the rotor assembly 2 for minimum power dissipation in the Lorentz-force actuator 205 using the previously described method. The steady-state axial position setpoint can be controlled by the voltage output 216 of the operational amplifier 217 and the resistor 218. The circuit formed by the resistor 219, capacitor 220 and the operational amplifier 217 inverts and integrates the voltage output 212 of the gain and lead compensation network 203. Signal 212 is directly proportional to the current flowing in the Lorentz-force actuator coils 16, 17. If the average voltage of signal 212 is positive, indicating that a net positive current is flowing in the actuator coils 16, 17, the output 216 of the operational amplifier 217 decreases and shifts the axial setpoint position of the rotor assembly 2 until the average current flowing in the actuator coils 16, 17 is zero. Likewise, if the average voltage of signal 212 is negative, indicating that a net negative current is flowing in the actuator coils 16,17, the output 216 of the operational amplifier 217 increases and shifts the axial setpoint position of the rotor assembly 2 until the average current flowing in the actuator coils 16,17 is zero. The steady-state axial setpoint position of the rotor assembly 2 is thus adjusted as required for minimum power dissipation in the Lorentz-force actuator 205. Those skilled in the art will recognize that the function of the analog, automatic setpoint adjustment circuitry 202 can be implemented with software running on a microprocessor or digital signal processor.

[0057] The magnetic fields created by the permanent magnets located in the stator assembly 1 and rotor assembly 2 may change over time, with variations in temperature, and due to the influence of external ferromagnetic objects. If the change in the magnetic fields causes the preferred axial operating point 306 (FIG. 7) to fall outside of the axial position limit of the rotor assembly 2, contact may occur between the rotor assembly 2 and the volute housing 4 or end cap 5 at points 71 and 72. To prevent such contact between the rotor and stator, an axial position adjustment actuator 70, shown generally in FIG. 1a, can be provided. The permanent magnet array 11, Lorentz force actuator components 16, 17, and 18, and motor components 32 and 33 can be bonded to stator support sleeve 68. The stator support sleeve 68 can move in the axial direction relative to the stator shell 3, volute housing 4, end cap 5, and stator liner 6. Therefore, adjustment of the axial position of the axially movable stator support sleeve 68 can be provided for using the axial position actuator 70 to adjust the position of the sleeve 68, and thus the magnet array 11 carried within the sleeve 68. A presently preferred embodiment of the axial position adjustment actuator 70 is shown in more detail in FIG. 1b, wherein the actuator 70 can preferably have an output shaft 74 of a motor-gearhead combination 73 attached to a lead screw 75. The lead screw 75 can mate with threads 76 in the stator support sleeve 68. The motor-gearhead 73 can preferably incorporate a stepper motor, although other types of motors such a DC brush motor or a DC brushless motor can be used. Miniature stepper motor-gearhead combinations are available from companies such as Donovan Micro-Tek, Inc., Simi Valley, Calif. A c-clip 77 can be used to axially capture lead screw 77. A tang 78 on stator support sleeve 68 can mate with a slot 79 in stator shell 3 to limit rotation of stator support sleeve 68 within stator shell 3. The axial position of stator support sleeve 68 and the attached magnetic suspension and rotation components can be adjusted relative to the stator shell 3, volute housing 4, end cap 5, and stator liner 6 by electrically controlling the motor-gearhead 73. By adjusting the axial position of the stator support sleeve 68 and the attached magnetic suspension and rotation components the preferred operating point 306 (FIG. 7) can be maintained within the axial position limits of the rotor assembly 2 to avoid contact between the rotor assembly 2 and the volute housing 4 or end cap 5 at points 71 and 72. It is to be understood that additional linear, rotary, and thrust bearings could be used to enhance the accuracy, efficiency, and reliability of the described axial adjustment mechanism. A magnet 80 and hall effect sensors 81 and 82 can be used by a controller to monitor and limit the travel of stator support sleeve 68. It is to be understood that other mechanisms could be used to adjust the axial position of the magnetic suspension components. Such mechanisms could utilize electromagnetic linear actuators, piezoelectric actuators, magnetostrictive actuators, or shape memory alloy based actuators.

[0058] The flow rate of any blood pump that is capable of creating negative inlet pressures must be dynamically adjusted to match the blood flow rate into the left ventricle. If too little flow is produced by the blood pump, the tissues and organs of the body may be inadequately perfused, and the blood pressure in the left ventricle will increase—potentially causing excessive pulmonary pressure and congestion. Conversely, if the flow rate of the blood pump is too high, excessive negative pressure may be created in the left ventricle and in the inlet to the pump. Excessive negative blood pressure is undesirable for the following reasons: 1) Unacceptable levels of blood damage may be caused by cavitation, 2) The pump may be damaged by cavitation, 3) The walls of the ventricle may collapse and be damaged, and 4) The walls of the ventricle may collapse and block the blood flow pathway to the pump. Preferably, the flow rate of the blood pump can be dynamically controlled to avoid these problems.

[0059] As shown in FIG. 4, the flow rate controller 92 for the blood pump can operate the pump such that the flow rate does not overly distend or contract the ventricle. Preferably, a heart measurement apparatus can provide the flow rate controller 92 with information about the dimension of the ventricle during normal distention and contraction. Such a heart measurement apparatus can be an electronic heart caliper, two types of which are illustrated in FIGS. 8a-10b.

[0060] In FIG. 8a, a cross section of a heart is illustrated, including a right ventricle 405 and a left ventricle 404 that is maximally distended by the pressure of the blood contained therein. In FIG. 8b, the left ventricle 404 has been partially depressurized. As blood is withdrawn from the left ventricle 404 the radial dimension of the outside surface 418 of the heart is reduced. By dynamically adjusting the flow rate of the blood pump to avoid excessive distention or contraction of the left ventricle, as indicated by the radial dimension of the exterior surface of the left ventricle, the average blood pump flow rate can be controlled to match the flow rate of blood into the left ventricle. One embodiment of an electronic heart caliper 400 is shown which can be employed to measure the radial dimension of the outside surface 418 of the heart. The heart caliper 400 can include two arms 401, 402 that can be suitably attached to the outside surface 418 of the heart and pivot about a point which can preferably be located inside a hermetically sealed enclosure 403. A measure of the radial dimension of the left ventricle 404 can be achieved by electronically measuring the angle between the caliper arms 401, 402. An angular measurement apparatus which can be used to measure the angle between the caliper arms 401, 402 is illustrated in FIG. 9. The angle measuring apparatus can preferably be contained within a hermetically sealed enclosure 403 in order to protect the internal components from the tissues and fluids of the body. A bellows 407, and end caps 408, 409 can preferably be welded together to form a hermetically sealed chamber. The bellows 407, and end caps 408, 409 are preferably made from titanium. A hermetic electrical feedthrough 410, which can use either a glass or brazed ceramic insulator 411, can be installed in titanium end cap 409. The caliper arms 401, 402 can be effectively connected to a pivot member 415 through respective end caps 408, 409 and respective control arms 414, 416. The caliper arm 401, end cap 408 and control arm 416 can be machined from a single piece of titanium or can be constructed individually and welded or bonded together. Likewise, the caliper arm 402, end cap 409 and control arm 414 can be machined from a single piece of titanium or can be constructed individually and welded or bonded together. A pivot 415 limits the motion of the caliper arms 401, 402 to an arc within a single plane. As the caliper arms 401, 402 move due to distention or contraction of the left ventricle, the extension 419 of control arm 416 moves closer or farther respectively from an eddy-current based position sensor 417 that can be bonded to the control arm 414. The eddy-current based position sensor 417 can be fabricated from a miniature ferrite pot core 412 and a copper coil 413. Such miniature ferrite pot cores are available from Siemens Components, Inc., Iselin, NJ. The eddy-current sensor coil can be connected to two electrical feedthroughs (only one of the two feedthroughs, 410, is shown in FIG. 9). As the metallic control arm extension 419 moves closer to the eddy-current based position sensor 417, the effective resistive loading of the coil increases causing a reduction of the coil's Q (Q is defined in the art as the ratio of reactance to the effective series resistance of a coil). An electronic circuit can be used to measure the change in the Q of the coil and provide a signal that corresponds to the relative position of the caliper arms 401, 402. Such electronic circuits, as described for measuring changes in Q, are well known in the art.

[0061] An alternative embodiment of an electronic heart caliper 500 is illustrated in FIGS. 10aand 10b. Similarly to FIGS. 8aand 8b, FIG. 10adepicts a left ventricle that is maximally distended by the pressure of the blood contained within it and FIG. 10bdepicts a left ventricle that has been partially depressurized. The heart caliper 500 can have a pair of arms 501, 502 that can be suitably attached to the outside surface of the heart and pivot about a point which can preferably be located inside a hermetically sealed enclosure 503. A measure of the radial dimension of the left ventricle can be achieved by measuring the time it takes for an ultrasonic pulse to travel from a sonomicrometer transducer 504 on one caliper arm 501 to an opposing sonomicrometer transducer 505 on the other caliper arm 502. Sonomicrometer transducers suitable for use in such a heart caliper are available from companies such as Triton Technology, Inc., San Diego Calif. and Etalon, Inc., Lebanon Ind. The details of sonomicrometry are well known by those skilled in the art. It should be understood that other suitable methods for measuring the relative distention and contraction of the ventricles, such as impedance and conductance measurement of the ventricle, could be derived by those skilled in the art and the invention is not to be limited to the particular methods described.

[0062] One way to implement the flow rate controller 92, to control the flow rate of the blood pump to avoid excessive distention or contraction of the left ventricle, is graphically illustrated in FIG. 11. The x-axis' 600 represent time. Line 601 represents a left ventricular radial dimension setpoint that is defined when the system is initially implanted and which may be periodically updated noninvasively using ultrasound imaging. Curve 602 represents the radial dimension of the left ventricle as sensed by either of the previously described electronic heart calipers 400, 500. Curve 603 represents the angular velocity of the disclosed centrifugal pump impeller. The flow rate of the disclosed centrifugal pump varies with the angular velocity of its impeller. When the sensed radial dimension exceeds the radial dimension setpoint as illustrated by point 604, the angular velocity of the centrifugal pump can be increased as illustrated by point 605. The increased angular velocity of the centrifugal pump causes its flow rate to increase and more rapidly remove blood from the left ventricle, which in turn causes the radial dimension of the left ventricle to be reduced towards the radial dimension setpoint line 601. Likewise, when the sensed radial dimension is less than the radial dimension setpoint as illustrated by point 606, the angular velocity of the centrifugal pump can be decreased as illustrated by point 607. The decreased angular velocity of the centrifugal pump causes its flow rate to decrease and reduce the rate at which blood is removed from the left ventricle, which in turn causes the radial dimension of the left ventricle to increase towards the radial dimension setpoint line 601.

[0063] Another way to implement the flow rate controller 92, to control the flow rate of the blood pump to avoid excessive distention or contraction of the left ventricle, and also to create pulsatile blood flow, is graphically illustrated in FIG. 12. A pulsatile flow rate more closely mimics the blood flow characteristics of a natural heart. In FIG. 12, the x-axis' 608 represent time. Lines 609 and 610 respectively represent upper and lower left ventricular radial dimension setpoints that are defined when the system is initially implanted and which may be periodically updated noninvasively using ultrasound imaging. Curve 611 represents the radial dimension of the left ventricle as sensed by either of the previously described electronic heart calipers 400, 500. Curve 612 represents the angular velocity of the disclosed centrifugal pump impeller. The angular velocity of the centrifugal pump can be periodically increased as indicated during time period 613. The increased angular velocity of the pump during time period 613 causes blood to be more rapidly removed from the heart, which in turn causes the radial dimension of the left ventricle to be reduced towards the lower radial dimension setpoint line 610. The angular velocity of the centrifugal pump can be reduced as indicated during time period 614 once the sensed radial dimension of the left ventricle nearly equals the lower radial dimension setpoint line 610. The reduced angular velocity of the pump during time period 615 causes the rate at which blood is removed from the left ventricle to be reduced, which in turn causes the radial dimension of the left ventricle to increase towards the upper radial dimension setpoint line 609. The angular velocity of the centrifugal pump can again be increased as indicated during time period 616 once the sensed radial dimension of the left ventricle nearly equals the upper radial dimension setpoint line 609. The described pulsatile pump flow rate serves to mimic the blood flow characteristics of the natural heart. It can be desirable to maintain forward blood flow through the pump to avoid blood damage caused by constant shearing of the blood pool in the vicinity of the impeller blades. A flow sensor 91 in FIG. 4 can be used to control the pump velocity during time period 615 in FIG. 12 such that adequate forward blood flow is maintained in the pump.

[0064] Although certain embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modification to those details could be developed in light of the overall teaching of the disclosure. Accordingly, the particular embodiments disclosed herein are intended to be illustrative only and not limiting to the scope of the invention which should be awarded the full breadth of the following claims and any and all embodiments thereof.