Patent Publication Number: US-2020282121-A1

Title: Impeller Displacement Based Flow Estimation

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is a Divisional of U.S. patent application Ser. No. 15/212,721 filed Jul. 18, 2016 (Allowed); which claims the benefit of U.S. Provisional Appln No. 62/194,700 filed Jul. 20, 2015, the disclosures of which are incorporated herein by reference in their entirety for all purposes. 
    
    
     BACKGROUND 
     Ventricular assist devices, known as VADs, often include an implantable blood pump and are used for both short-term (i.e., days, months) and long-term applications (i.e., years or a lifetime) where a patient&#39;s heart is incapable of providing adequate circulation, commonly referred to as heart failure or congestive heart failure. According to the American Heart Association, more than five million Americans are living with heart failure, with about 670,000 new cases diagnosed every year. People with heart failure often have shortness of breath and fatigue. Years of living with blocked arteries and/or high blood pressure can leave a heart too weak to pump enough blood to the body. As symptoms worsen, advanced heart failure develops. 
     A patient suffering from heart failure may use a VAD while awaiting a heart transplant or as a long term destination therapy. A patient may also use a VAD while recovering from heart surgery. Thus, a VAD can supplement a weak heart (i.e., partial support) or can effectively replace the natural heart&#39;s function. 
     The flow rate of blood pumped by a VAD is an important parameter for both control of the blood pump and for informing a health care professional regarding the level of circulatory support provided to the patient by the VAD. Direct measurement of blood flow rate has several drawbacks with existing technology. For example, the addition of components (e.g., a flow sensor) may increase complexity and reduce reliability. It is also generally undesirable to place structures in the flowpath because they can lead to thrombosis. Moreover, existing flow sensors suffer from drift and other factors which contribute to imprecise measurements over time. 
     Accordingly, existing VADs generally rely on an estimation of flow rate through the pump by indirect measurements. For example, the blood flow rate in a VAD can be estimated based on the amount of electrical power consumed by the VAD. 
     There is a need for improved devices and methods for measuring or estimating flow rate in a pump. 
     Additionally, there is the need for devices and methods for measuring clinical and/or pump parameters. 
     BRIEF SUMMARY 
     The following presents a simplified summary of some embodiments of the invention in order to provide a basic understanding of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some embodiments of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
     Improved blood circulation assist systems and related methods measure thrust load applied to a rotating impeller and estimate a flow rate of blood pumped by the blood pump based on impeller rotational speed and the thrust load. The thrust load can be measured via a sensor generating output indicative of a reaction force generated by the thrust load. The thrust load can also be measured via measurement of a displacement of the impeller induced by the thrust load. In at least some operational regimes, flow rate estimation based on the thrust load and impeller rotational speed is more accurate than flow rate estimation based on power consumption and impeller rotational speed. 
     Thus, in one aspect, a blood pump for a circulation assist system estimates a flow rate of blood pumped by the blood pump based on impeller rotational speed and thrust load applied to the impeller by blood impelled by the impeller. The blood pump includes a housing, an impeller, a motor stator, a support member, a sensor, and a controller. The housing forms a blood flow channel. The impeller is disposed within the blood flow channel. The motor stator is mounted to the housing and operable to magnetically rotate the impeller. The support member is coupled to the housing. The support member reacts a thrust load applied to the impeller by blood impelled through the blood flow channel by the impeller. The sensor generates a sensor output indicative of a magnitude of the thrust load reacted by the support member. The controller is operatively coupled with the motor stator and the sensor. The controller is configured to determine an impeller rotational speed for the impeller and estimate a flow rate of blood pumped by the blood pump based on the impeller rotational speed and the sensor output. 
     In many embodiments, the impeller is supported by the support member. For example, the impeller can be supported via a support bearing supported by the support member and the thrust load reacted by the support member is applied to the support member via the support bearing. An inlet stator can include the support member and be used to support the support bearing and/or the impeller. 
     In many embodiments, the sensor output is indicative of deformation of the support member induced by the thrust load reacted by the support member. For example, the sensor can include a strain gauge coupled to the support member and the sensor output can be indicative of strain in the support member induced by the thrust load reacted by the support member. 
     In many embodiments, the sensor output is indicative of a deflection of the support member induced by the thrust load reacted by the support member. For example, the sensor can include a deflection measuring sensor coupled to the support member and configured to measure a deflection of the support member induced by the thrust load reacted by the support member. 
     The systems and methods described herein allow for estimation of a pressure drop across the pump. In turn, the pressure estimate can be used clinically or to optimize operation of the pump. For example, an LVAD is typically connected to pull blood from the left ventricle and pump it to the aorta. The pressure drop across the pump is thus indicative of the difference between the left ventricular pressure and the aortic pressure. In other words, the pressure across the pump is representative of the pressure across the aortic valve. Knowing the pressure across the aortic valve has clinical value in its own right, and is useful in helping the clinician understand the functioning of the patient&#39;s heart. Accordingly, in many embodiments, the controller is configured to estimate a differential pressure across the pump based on the sensor output and output the pressure differential to an output device for output to a person. For example, the controller can determine the magnitude of the thrust load reacted by the support member and determine the differential pressure across the pump corresponding to the magnitude of the thrust load. The controller can then output the estimated pressure differential for display on a display device. In many embodiments, the blood pump is configured to pump blood from the patient&#39;s left ventricle to the patient&#39;s aorta. In such embodiments, the pressure differential corresponds to the pressure difference between the patient&#39;s left ventricle and the patient&#39;s aorta. 
     In many embodiments, the motor stator is coupled to the housing via the support member and transfers the thrust load applied to the impeller to the support member. For example, in many embodiments, the thrust load applied to the impeller is reacted via passive magnetic attraction between the impeller and the motor stator and the motor stator is coupled to the housing via the support member so as to react at least a portion of the thrust load reacted by the motor stator. 
     In another aspect, a method is provided for estimating blood flow rate in a blood circulation assist system. The method includes controlling a motor stator to magnetically rotate an impeller within a blood flow channel of a blood pump. A thrust load applied to the impeller by blood impelled through the blood flow channel by the impeller is reacted by a support member coupled to a housing of the blood pump. A sensor output indicative of a magnitude of the thrust load reacted by the support member is generated. An impeller rotational speed for the impeller is determined by a controller. A flow rate of blood impelled by the impeller is estimated by the controller based on the rotational speed and the sensor output. 
     In many embodiments, the method includes supporting the impeller via the support member. For example, the method can include supporting the impeller via a support bearing supported by the support member. The thrust load can be applied to the support member via the support bearing. 
     Any suitable sensor can be used in the method to generate the sensor output indicative of a magnitude of the thrust load reacted by the support member. For example, the sensor output can be indicative of strain in the support member induced by the thrust load reacted by the support member. The sensor output can be indicative of a deflection of the support member induced by the thrust load reacted by the support member. 
     The sensor output can be used to estimate other pump related parameters. For example, the method can include estimating, by the controller, a pressure differential across the blood pump based on the sensor output. The method can include outputting the pressure differential, by the controller, to an output device for output to a person. 
     In many embodiments of the method, the motor stator is coupled to the housing via the support member. In such embodiments, the method can further include reacting the thrust load applied to the impeller by the motor stator via passive magnetic attraction between the impeller and the motor stator and reacting the thrust load reacted by the motor stator by the support member. 
     In another aspect, a blood pump for a circulation assist system includes a housing, an impeller, a motor stator, at least one Hall-Effect sensor, and a controller. The housing forms a blood flow channel. The impeller is disposed within the blood flow channel. The motor stator is mounted to the housing and operable to magnetically rotate the impeller. A thrust load applied to the impeller by blood impelled through the blood flow channel by the impeller is reacted by the motor stator via passive magnetic attraction between the impeller and the motor stator. The at least one Hall-Effect sensor is configured to generate output indicative of a displacement of the impeller along the blood flow channel induced by the thrust load applied to the impeller. The controller is operatively coupled with the motor stator and the at least one Hall-Effect sensor. The controller is configured to determine an impeller rotational speed for the impeller, process the output generated by the at least one Hall-Effect sensor to determine the displacement of the impeller along the blood flow channel, and estimate a flow rate of blood pumped by the blood pump based on the impeller rotational speed and the displacement of the impeller. In many embodiments, the controller is further configured to estimate a differential pressure across the blood pump based on the displacement of the impeller. 
     In another aspect, a method is provided for estimating blood flow rate in a blood circulation assist system. The method includes controlling a motor stator to magnetically rotate an impeller within a blood flow channel of a blood pump. A thrust load applied to the impeller by blood impelled through the blood flow channel by the impeller is reacted via passive magnetic attraction between the impeller and the motor stator. Output generated by at least one Hall-Effect sensor is processed by a controller to determine a displacement of the impeller along the blood flow channel induced by the thrust load applied to the impeller. An impeller rotational speed for the impeller is determined by the controller. A flow rate of blood impelled by the impeller is estimated by the controller based on the impeller rotational speed and the displacement of the impeller. Many embodiments of the method further include estimating, by the controller, a pressure differential across the blood pump based on the displacement of the impeller. 
     For a fuller understanding of the nature and advantages of the present invention, reference should be made to the ensuing detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a blood pump of a circulation assist system implanted within a patient, in accordance with many embodiments. 
         FIG. 2  is a cross-sectional view of a blood pump in which an impeller is supported via end bearings and a thrust load applied to the impeller is reacted into an inlet stator via one of the end bearings, in accordance with many embodiments. 
         FIG. 3  is a simplified schematic diagram illustrating the generation of the thrust load on the impeller of the blood pump of  FIG. 2 . 
         FIG. 4  is a plot of differential pressure across the blood pump of  FIG. 2  as a function of flow rate and impeller rotation rate. 
         FIG. 5  is a simplified schematic diagram illustrating a blood pump in which a thrust load applied to an impeller is reacted by a support member supporting a motor stator used to rotate and magnetically position the impeller within a blood flow channel of the blood pump, in accordance with many embodiments. 
         FIG. 6  is an illustration of a mechanical circulatory support system implanted in a patient&#39;s body, in accordance with many embodiments. 
         FIG. 7  is an exploded view of certain components of the circulatory support system of  FIG. 6 . 
         FIG. 8  is an illustration of a blood pump in an operational position implanted in a patient&#39;s body. 
         FIG. 9  is a cross-sectional view of the blood pump of  FIG. 8 . 
         FIG. 10  is a partial cut-away perspective view of a stator of a blood pump. 
         FIG. 11  is an illustration of an embodiment of a Hall Sensor assembly for the blood pump of  FIG. 8 . 
         FIG. 12  is a schematic diagram of a control system architecture of the mechanical support system of  FIG. 6 . 
         FIG. 13  and  FIG. 14  are simplified schematic diagrams illustrating a blood pump in which a motor stator used to rotate and magnetically position the impeller within a blood flow channel of the blood pump and a deflection of the impeller along the blood flow channel is used to determine thrust applied to the impeller, in accordance with many embodiments. 
         FIG. 15  shows example aortic pressure and left ventricular pressure over a heart cycle. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, various embodiments of the present invention will be described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the embodiments. However, it will also be apparent to one skilled in the art that the present invention may be practiced without the specific details. Furthermore, well-known features may be omitted or simplified in order not to obscure the embodiment being described. 
     Referring now to the drawings, in which like reference numerals represent like parts throughout the several views,  FIG. 1  shows a blood pump assembly  100  implanted in a patient&#39;s body to supplement, or in some cases replace, the natural pumping function of a heart  10 . Aspects of the pump assembly  100  are similar to those disclosed in U.S. Pat. Nos. 5,951,263; 6,186,665; U.S. application Ser. No. 13/273,185 filed Oct. 13, 2011 and entitled PUMPING BLOOD; and U.S. application Ser. No. 12/394,244 filed Feb. 27, 2009 and entitled BLOOD FLOW METER, the entire contents of which patents and applications are incorporated herein by reference for all purposes. The blood pump assembly  100  includes a blood pump  100   a , which can be implanted to receive blood from the heart  10 , for example, from a left ventricle  12  of the heart  10 . As shown, the blood pump  100   a  pumps blood through an outflow conduit  130  to the patient&#39;s circulatory system, for example, to an aorta  17  or a peripheral artery  18 . The outflow conduit  130  for the blood pump  100   a  in  FIG. 1  is connected to the peripheral artery  18  thereby assisting the patient&#39;s circulation in a manner that might be used for a partial support patient. For a full support patient the outflow conduit  130  is connected to the aorta  17 . The blood pump assembly  100  can also be implanted such that the blood pump  100   a  receives blood from a right ventricle  14  of the heart  10  and supplies blood to, for example, a pulmonary artery. 
     Referring to  FIG. 2 , the blood pump  100   a  includes a housing  110  that defines a blood flow channel  112 . Blood enters the blood flow channel  112  through an inlet  101 , passes through a central tubular region  102  of the housing  110 , and exits through an outlet  103 . The housing  110  contains a motor stator  128 , which drives rotation of an impeller  120  located in the blood flow channel  112 . As the impeller  120  rotates, blades  122  on the impeller  120  impart energy to the blood flow, resulting in pressure and blood flow at the outlet  103 . The impeller  120  is suspended in the blood flow channel  112  by fore and aft mechanical, blood-immersed bearings  124 ,  126  that limit axial translation of the impeller  120 . The bearings  124 ,  126  also limit the impeller from shifting off its axis of rotation. 
     The blood pump  100   a  includes an inlet stator  130 , which is connected to the housing  110  and supports the inlet side bearing  124 . A strain gauge  132  is mounted to the inlet stator  130  and generates output indicative of strain in the inlet stator  130  induced by reacting the thrust load applied to the inlet side bearing  124  by the impeller  120 . The output from the strain gauge  132  is processed to determine the magnitude of the thrust load, which is then used in combination with the rotational rate of the impeller  120  to estimate flow rate of the blood pumped by the blood pump  100   a.    
       FIG. 3  is a simplified schematic diagram illustrating the generation of a thrust load  134  on the impeller  120  of the blood pump  100   a . Rotation of the impeller  120  impels the blood flow along the blood flow channel  112  and out of the outlet  103 . The rotating impeller  120  applies a net force to the blood, which in response exerts the thrust load  134  on the impeller  120 . The pressure applied on the impeller  120  by the blood (e.g., net pressure forces  136  applied to the blades  122 ) generates the thrust load  134 . The impeller  120  transfers the thrust load  134  to the inlet side bearing  124 . As seen from the perspective of the impeller  120 , the inlet side bearing  124  applies a reaction force  138  to the impeller  120 . As would be understood by one of skill, for every force there is an equal and opposite force, accordingly, the reaction force  138  is generally equal in magnitude and opposite in direction to the thrust load  134  applied to the impeller  120  by the blood impelled along the blood flow channel  112  by the impeller  120 . The thrust load  134  applied to the inlet side bearing  124  is reacted into the housing  110  via the inlet stator  130 , thereby inducing strain in the inlet stator  130 . The strain gauge  132  is attached to the inlet stator  130  so that the strain induced in the inlet stator  130  by the thrust load  134  induces a corresponding strain in the strain gauge  132 . The strain in the strain gauge  132  induces a change in the resistance of the strain gauge  132 , which is measured using known techniques. The change in the resistance of the strain gauge  132  is used to determine the corresponding thrust load  134  using any suitable known approach including calibration. 
     The instantaneous rate of flow of blood through the blood pump  100   a  can be estimated based on the determined thrust load  134  and the rotational rate of the impeller  120 . For example,  FIG. 4  is a plot of differential pressure across the blood pump  110   a  as a function of flow rate and impeller rotation rate. The differential pressure across the blood pump  110   a  can be calculated by dividing the determined thrust load  134  by a suitable cross-sectional reference area for the blood flow channel  112 . The rotational rate of the impeller  120  can be determined using the approaches as described herein. The rotational rate of the impeller  120  and the differential pressure can be used to determine a flow rate of blood through the pump  120  using the data of  FIG. 4 . For example, for a determined thrust load  134  corresponding to a differential pressure of 150 mm Hg and an impeller rotational rate of 12,000 rpm, the corresponding flow rate of blood is about 3.0 L/min. 
     In many embodiments, a suitable electronic controller is operatively coupled with the motor stator  128  and the strain gauge  132 . The controller can control the motor stator  128  via control of current applied to windings of the motor stator  128  so as to control rotation of the impeller  120 . As described herein, the controller can monitor a rotational rate of the impeller  120  using any suitable known approach, such a via control of the motor stator  128  and/or via processing output from one or more Hall-Effect sensors to determine the rotational rate of the impeller  120 . The controller can be operatively coupled with the strain gauge  132  so as to be able to measure changes in resistance of the strain gauge  132  and thereby determine a corresponding thrust load applied by the impeller  120  to the inlet stator  130  via the inlet side bearing  124 . The controller can then process the thrust load (or a differential pressure calculated from the thrust load) and the impeller rotation rate to generate a corresponding flow rate of blood via data relating the thrust load (or corresponding differential pressure) and the rotation rate to flow rate. 
     Flow Estimation in Magnetically Levitated Impeller Pumps 
       FIG. 5  is a simplified schematic diagram illustrating alternative embodiments of a blood pump  200   a  in which a thrust load  234  applied to an impeller  220  is reacted by one or more support members supporting a motor stator  228  configured to rotate and magnetically position the impeller  220  within a blood flow channel  212  of the blood pump  220   a . In accordance with embodiments described herein, the impeller  220  is magnetically levitated within the blood flow channel  212  via the motor stator  228 . The radial position of the impeller  220  within the blood flow channel (perpendicular to the inflow direction) is controlled via active control of current applied to the levitation coils of the motor stator. The axial position of the impeller  220  (parallel to the blood flow direction) along the blood flow channel  212  is passively controlled via passive magnetic attraction between the impeller  220  and stator poles of the motor stator  228 . The thrust load  234  applied to the impeller  220  positions the impeller  220  along the blood flow channel  212  to a position where the passive magnetic attraction between the impeller  220  and the motor stator  228  has an axial component (parallel to the blood flow direction) equal and opposite to the thrust load  234 . The axial component of the passive magnetic attraction between the impeller  220  and the motor stator  228  thereby applies an axial thrust load  240  to the motor stator  228 , thereby pushing the motor stator  228  in a direction parallel to the blood flow and towards the inlet  201 . The motor stator  228  can be mounted to the housing via one or more support members such that the axial thrust load  240  is transferred to the housing  210  via the one or more support members. From the perspective of the motor stator  228 , the one or more support members apply a reaction load  242  onto the motor stator  228 , thereby balancing the axial thrust load  240  applied to the motor stator  228  via the passive magnetic attraction between the motor stator  228  and the impeller  220 . One or more of the one or more supports can include a suitable number of strain gauges operatively coupled with a controller configured to monitor the strain gauges to determine the thrust load  240  applied to the motor stator  228  and thereby determine the corresponding thrust load  234  applied to the impeller  220 . As described above, the thrust load  234  and the rotation rate of the impeller  220  can be used to estimate the flow rate of blood through the blood pump  200   a.    
       FIG. 6  is an illustration of a mechanical circulatory support system  310  implanted in a patient&#39;s body  312 . The mechanical circulatory support system  310  includes an implantable blood pump assembly  314 , a ventricular cuff  316 , an outflow cannula  318 , an external system controller  320 , and power sources  322 . The implantable blood pump assembly  314  can include a VAD that is attached to an apex of the left ventricle, as illustrated, or the right ventricle, or both ventricles of the heart  324 . The VAD can include a centrifugal pump (as shown) that is capable of pumping the entire output delivered to the left ventricle from the pulmonary circulation (i.e., up to 10 liters per minute). Related blood pumps applicable to the present invention are described in greater detail below and in U.S. Pat. Nos. 5,695,471, 6,071,093, 6,116,862, 6,186,665, 6,234,772, 6,264,635, 6,688,861, 7,699,586, 7,976,271, 7,997,854, 8,007,254, 8,152,493, 8,652,024, and 8,668,473 and U.S. Patent Publication Nos. 2007/0078293, 2008/0021394, 2009/0203957, 2012/0046514, 2012/0095281, 2013/0096364, 2013/0170970, 2013/0121821, and 2013/0225909, all of which are incorporated herein by reference for all purposes in their entirety. With reference to  FIG. 6  and  FIG. 7 , the blood pump assembly  314  can be attached to the heart  324  via the ventricular cuff  316 , which can be sewn to the heart  324  and coupled to the blood pump  314 . The other end of the blood pump  314  connects to the ascending aorta via the outflow cannula  318  so that the VAD effectively diverts blood from the weakened ventricle and propels it to the aorta for circulation through the rest of the patient&#39;s vascular system. 
       FIG. 6  illustrates the mechanical circulatory support system  310  during battery  322  powered operation. A driveline  326  that exits through the patient&#39;s abdomen  328  connects the implanted blood pump assembly  314  to the external system controller  320 , which monitors system  310  operation. Related controller systems applicable to the present invention are described in greater detail below and in U.S. Pat. Nos. 5,888,242, 6,991,595, 8,323,174, 8,449,444, 8,506,471, 8,597,350, and 8,657,733, EP 1812094, and U.S. Patent Publication Nos. 2005/0071001 and 2013/0314047, all of which are incorporated herein by reference for all purposes in their entirety. The system  310  can be powered by either one, two, or more batteries  322 . It will be appreciated that although the system controller  320  and power source  322  are illustrated outside/external to the patient body, the driveline  326 , the system controller  320  and/or the power source  322  can be partially or fully implantable within the patient, as separate components or integrated with the blood pump assembly  314 . Examples of such modifications are further described in U.S. Pat. No. 8,562,508 and U.S. Patent Publication No. 2013/0127253, all of which are incorporated herein by reference for all purposes in their entirety. 
     With reference to  FIG. 8  to  FIG. 10 , a left ventricular assist blood pump assembly  400  having a circular shaped housing  410  is implanted in a patient&#39;s body with a first face  411  of the housing  410  positioned against the patient&#39;s heart H and a second face  413  of the housing  410  facing away from the heart H. The first face  411  of the housing  410  includes an inlet cannula  412  extending into the left ventricle LV of the heart H. The second face  413  of the housing  410  has a chamfered edge  414  to avoid irritating other tissue that may come into contact with the blood pump assembly  400 , such as the patient&#39;s diaphragm. To construct the illustrated shape of the puck-shaped housing  410  in a compact form, a stator  420  and electronics  430  of the pump assembly  400  are positioned on the inflow side of the housing toward first face  411 , and an impeller  440  of the pump assembly  400  is positioned along the second face  413 . This positioning of the stator  420 , electronics  430 , and impeller  440  permits the edge  414  to be chamfered along the contour of the impeller  440 , as illustrated in at least  FIGS. 7-9 , for example. 
     Referring to  FIG. 9 , the blood pump assembly  400  includes a dividing wall  415  within the housing  410  defining a blood flow conduit  403 . The blood flow conduit  403  extends from an inlet opening  401  of the inlet cannula  412  through the stator  420  to an outlet opening  405  defined by the housing  410 . The impeller  440  is positioned within the blood flow conduit  403 . The stator  420  is disposed circumferentially about a first portion  440   a  of the impeller  440 , for example about a permanent magnet  441 . The stator  420  is also positioned relative to the impeller  440  such that, in use, blood flows within the blood flow conduit  403  through the stator  420  before reaching the impeller  440 . The permanent magnet  441  has a permanent magnetic north pole N and a permanent magnetic south pole S for combined active and passive magnetic levitation of the impeller  440  and for rotation of the impeller  440 . The impeller  440  also has a second portion  440   b  that includes impeller blades  443 . The impeller blades  443  are located within a volute  407  of the blood flow conduit such that the impeller blades  443  are located proximate to the second face  413  of the housing  410 . 
     The puck-shaped housing  410  further includes a peripheral wall  416  that extends between the first face  411  and a removable cap  418 . As illustrated, the peripheral wall  416  is formed as a hollow circular cylinder having a width W between opposing portions of the peripheral wall  416 . The housing  410  also has a thickness T between the first face  411  and the second face  413  that is less than the width W. The thickness T is from about 0.5 inches to about 1.5 inches, and the width W is from about 1 inch to about 4 inches. For example, the width W can be approximately 2 inches, and the thickness T can be approximately 1 inch. 
     The peripheral wall  416  encloses an internal compartment  417  that surrounds the dividing wall  415  and the blood flow conduit  403 , with the stator  420  and the electronics  430  disposed in the internal compartment  417  about the dividing wall  415 . The removable cap  418  includes the second face  413 , the chamfered edge  414 , and defines the outlet opening  405 . The cap  418  can be threadedly engaged with the peripheral wall  416  to seal the cap  418  in engagement with the peripheral wall  416 . The cap  418  includes an inner surface  418   a  of the cap  418  that defines the volute  407  that is in fluid communication with the outlet opening  405 . 
     Within the internal compartment  417 , the electronics  430  are positioned adjacent to the first face  411  and the stator  420  is positioned adjacent to the electronics  430  on an opposite side of the electronics  430  from the first face  411 . The electronics  430  include circuit boards  431  and various components carried on the circuit boards  431  to control the operation of the pump  400  (e.g., magnetic levitation and/or drive of the impeller) by controlling the electrical supply to the stator  420 . The housing  410  is configured to receive the circuit boards  431  within the internal compartment  417  generally parallel to the first face  411  for efficient use of the space within the internal compartment  417 . The circuit boards also extend radially-inward towards the dividing wall  415  and radially-outward towards the peripheral wall  416 . For example, the internal compartment  417  is generally sized no larger than necessary to accommodate the circuit boards  431 , and space for heat dissipation, material expansion, potting materials, and/or other elements used in installing the circuit boards  431 . Thus, the external shape of the housing  410  proximate the first face  411  generally fits the shape of the circuits boards  431  closely to provide external dimensions that are not much greater than the dimensions of the circuit boards  431 . 
     With continued reference to  FIG. 9  and  FIG. 10 , the stator  420  includes a back iron  421  and pole pieces  423   a - 423   f  arranged at intervals around the dividing wall  415 . The back iron  421  extends around the dividing wall  415  and is formed as a generally flat disc of a ferromagnetic material, such as steel, in order to conduct magnetic flux. The back iron  421  is arranged beside the control electronics  430  and provides a base for the pole pieces  423   a - 423   f.    
     Each of the pole piece  423   a - 423   f  is L-shaped and has a drive coil  425  for generating an electromagnetic field to rotate the impeller  440 . For example, the pole piece  423   a  has a first leg  424   a  that contacts the back iron  421  and extends from the back iron  421  towards the second face  413 . The pole piece  423   a  can also have a second leg  424   b  that extends from the first leg  424   a  through an opening of a circuit board  431  towards the dividing wall  415  proximate the location of the permanent magnet  441  of the impeller  440 . In an aspect, each of the second legs  424   b  of the pole pieces  423   a - 423   f  is sticking through an opening of the circuit board  431 . In an aspect, each of the first legs  424   a  of the pole pieces  423   a - 423   f  is sticking through an opening of the circuit board  431 . In an aspect, the openings of the circuit board are enclosing the first legs  424   a  of the pole pieces  423   a - 423   f.    
     In a general aspect, the implantable blood pump  400  can include one or more Hall sensors that may provide an output voltage, which is directly proportional to a strength of a magnetic field that is located in between at least one of the pole pieces  423   a - 423   f  and the permanent magnet  441 , and the output voltage may provide feedback to the control electronics  430  of the pump  400  to determine if the impeller  440  and/or the permanent magnet  441  is not at its intended position for the operation of the pump  400 . For example, a position of the impeller  440  and/or the permanent magnet  441  can be adjusted, e.g., the impeller  440  or the permanent magnet  441  may be pushed or pulled towards a center of the blood flow conduit  403  or towards a center of the stator  420 . 
     Each of the pole pieces  423   a - 423   f  also has a levitation coil  427  for generating an electromagnetic field to control the radial position of the impeller  440 . Each of the drive coils  425  and the levitation coils  427  includes multiple windings of a conductor around the pole pieces  423   a - 423   f . Particularly, each of the drive coils  425  is wound around two adjacent ones of the pole pieces  423 , such as pole pieces  423   d  and  423   e , and each levitation coil  427  is wound around a single pole piece. The drive coils  425  and the levitation coils  427  are wound around the first legs of the pole pieces  423 , and magnetic flux generated by passing electrical current though the coils  425  and  427  during use is conducted through the first legs and the second legs of the pole pieces  423  and the back iron  421 . The drive coils  425  and the levitation coils  427  of the stator  420  are arranged in opposing pairs and are controlled to drive the impeller and to radially levitate the impeller  440  by generating electromagnetic fields that interact with the permanent magnetic poles S and N of the permanent magnet  441 . Because the stator  420  includes both the drive coils  425  and the levitation coils  427 , only a single stator is needed to levitate the impeller  440  using only passive and active magnetic forces. The permanent magnet  441  in this configuration has only one magnetic moment and is formed from a monolithic permanent magnetic body  441 . For example, the stator  420  can be controlled as discussed in U.S. Pat. No. 6,351,048, the entire contents of which are incorporated herein by reference for all purposes. The control electronics  430  and the stator  420  receive electrical power from a remote power supply via a cable  419  ( FIG. 8 ). Further related patents, namely U.S. Pat. Nos. 5,708,346, 6,053,705, 6,100,618, 6,222,290, 6,249,067, 6,278,251, 6,351,048, 6,355,998, 6,634,224, 6,879,074, and 7,112,903, all of which are incorporated herein by reference for all purposes in their entirety. 
     The impeller  440  is arranged within the housing  410  such that its permanent magnet  441  is located upstream of impeller blades in a location closer to the inlet opening  401 . The permanent magnet  441  is received within the blood flow conduit  403  proximate the second legs  424   b  of the pole pieces  423  to provide the passive axial centering force though interaction of the permanent magnet  441  and ferromagnetic material of the pole pieces  423 . The permanent magnet  441  of the impeller  440  and the dividing wall  415  form a gap  408  between the permanent magnet  441  and the dividing wall  415  when the impeller  440  is centered within the dividing wall  415 . The gap  408  may be from about 0.2 millimeters to about 2 millimeters. For example, the gap  408  can be approximately 1 millimeter. The north permanent magnetic pole N and the south permanent magnetic pole S of the permanent magnet  441  provide a permanent magnetic attractive force between the impeller  440  and the stator  420  that acts as a passive axial centering force that tends to maintain the impeller  440  generally centered within the stator  420  and tends to resist the impeller  440  from moving towards the first face  411  or towards the second face  413 . When the gap  408  is smaller, the magnetic attractive force between the permanent magnet  441  and the stator  420  is greater, and the gap  408  is sized to allow the permanent magnet  441  to provide the passive magnetic axial centering force having a magnitude that is adequate to limit the impeller  440  from contacting the dividing wall  415  or the inner surface  418   a  of the cap  418 . The impeller  440  also includes a shroud  445  that covers the ends of the impeller blades  443  facing the second face  413  that assists in directing blood flow into the volute  407 . The shroud  445  and the inner surface  418   a  of the cap  418  form a gap  409  between the shroud  445  and the inner surface  418   a  when the impeller  440  is levitated by the stator  420 . The gap  409  is from about 0.2 millimeters to about 2 millimeters. For example, the gap  409  is approximately 1 millimeter. 
     As blood flows through the blood flow conduit  403 , blood flows through a central aperture  441   a  formed through the permanent magnet  441 . Blood also flows through the gap  408  between the impeller  440  and the dividing wall  415  and through the gap  409  between the shroud  445  and the inner surface  408   a  of the cap  418 . The gaps  408  and  409  are large enough to allow adequate blood flow to limit clot formation that may occur if the blood is allowed to become stagnant. The gaps  408  and  409  are also large enough to limit pressure forces on the blood cells such that the blood is not damaged when flowing through the pump  400 . As a result of the size of the gaps  108  and  109  limiting pressure forces on the blood cells, the gaps  408  and  409  are too large to provide a meaningful hydrodynamic suspension effect. That is to say, the blood does not act as a bearing within the gaps  408  and  409 , and the impeller is only magnetically-levitated. In various embodiments, the gaps  408  and  409  are sized and dimensioned so the blood flowing through the gaps forms a film that provides a hydrodynamic suspension effect. In this manner, the impeller can be suspended by magnetic forces, hydrodynamic forces, or both. 
     Because the impeller  440  is radially suspended by active control of the levitation coils  427  as discussed above, and because the impeller  440  is axially suspended by passive interaction of the permanent magnet  441  and the stator  420 , no impeller levitation components are needed proximate the second face  413 . The incorporation of all the components for impeller levitation in the stator  420  (i.e., the levitation coils  427  and the pole pieces  423 ) allows the cap  418  to be contoured to the shape of the impeller blades  443  and the volute  407 . Additionally, incorporation of all the impeller levitation components in the stator  420  eliminates the need for electrical connectors extending from the compartment  417  to the cap  418 , which allows the cap to be easily installed and/or removed and eliminates potential sources of pump failure. 
     In use, the drive coils  425  of the stator  420  generates electromagnetic fields through the pole pieces  423  that selectively attract and repel the magnetic north pole N and the magnetic south pole S of the impeller  440  to cause the impeller  440  to rotate within stator  420 . For example, the one or more Hall sensors may sense a current position of the impeller  440  and/or the permanent magnet  441 , wherein the output voltage of the one or more Hall sensors may be used to selectively attract and repel the magnetic north pole N and the magnetic south pole S of the impeller  440  to cause the impeller  440  to rotate within stator  420 . As the impeller  440  rotates, the impeller blades  443  force blood into the volute  407  such that blood is forced out of the outlet opening  405 . Additionally, the impeller draws blood into pump  400  through the inlet opening  401 . As blood is drawn into the blood pump by rotation of the impeller blades  443  of the impeller  440 , the blood flows through the inlet opening  401  and flows through the control electronics  430  and the stator  420  toward the impeller  440 . Blood flows through the aperture  441   a  of the permanent magnet  441  and between the impeller blades  443 , the shroud  445 , and the permanent magnet  441 , and into the volute  407 . Blood also flows around the impeller  440 , through the gap  408  and through the gap  409  between the shroud  445  and the inner surface  418   a  of the cap  418 . The blood exits the volute  407  through the outlet opening  405 , which may be coupled to an outflow cannula. 
       FIG. 11  shows a Hall Sensor assembly  500  for the blood pump assembly  314 , in accordance with many embodiments. The Hall Sensor assembly  500  includes a printed circuit board (PCB)  502  and individual Hall Effect sensors  508  supported by the printed circuit board  502 . Eight axi-symmetric Hall Effect sensors  508  are placed in a rigid, plastic mechanical carrier  510  and the PCB  502  is placed onto the mechanical carrier  510 . The mechanical carrier  510  uses guide rails  512  to locate electrically neutral rigid PCB portions  514  attached to the top edges of the Hall Effect sensors  508  and to locate the PCB  502 . 
     The Hall Effect sensors  508  are configured to transduce a position of the impeller  440  of the pump  400 . In the illustrated embodiment, the Hall Effect sensors  508  are supported so as to be standing orthogonally relative to the PCB  502  and a longest edge of each of the Hall Effect sensors  508  is aligned to possess an orthogonal component with respect to the surface of the PCB  502 . Each of the Hall Effect sensors  508  generate an output voltage, which is directly proportional to a strength of a magnetic field that is located in between at least one of the pole pieces  423   a - 423   f  and the permanent magnet  441 . The voltage output by each of the Hall Effect sensors  508  is received by the control electronics  430 , which processes the sensor output voltages to determine the position and orientation of the impeller  440 . The determined position and orientation of the impeller  440  is used to determine if the impeller  440  is not at its intended position for the operation of the pump  400 . For example, a position of the impeller  440  and/or the permanent magnet  441  may be adjusted, for example, the impeller  440  or the permanent magnet  441  may be pushed or pulled towards a center of the blood flow conduit  403  or towards a center of the stator  420 . The determined position of the impeller  440  can also be used to determine impeller eccentricity or a target impeller eccentricity and/or a position along the blood flow conduit  403 . The position along the blood flow conduit  403  can be used as described herein in combination with rotation rate of the impeller  440  to estimate flow rate of blood pumped by the blood pump assembly  400 , as would be understood by one of skill from the description herein. 
       FIG. 12  is a schematic diagram of a control system architecture of the mechanical support system of  FIG. 6 . The driveline  426  couples the implanted blood pump assembly  400  to the external system controller  420 , which monitors system operation via various software applications. The blood pump assembly  400  itself also includes several software applications that are executable by the on board electronics  430  (e.g., processors, ASICs, etc.) for various functions, such as to control radial levitation and/or drive of the impeller of the pump assembly  400  during operation. The external system controller  320  can in turn be coupled to batteries  322  or a power module  330  that connect to an AC electrical outlet. The external system controller  320  can also include an emergency backup battery (EBB) to power the system (e.g., when the batteries  322  are depleted) and a membrane overlay, including Bluetooth capabilities for wireless data communication. An external computer having a system monitor  332  that is configurable by an operator, such as clinician or patient, may further be coupled to the circulatory support system for configuring the external system controller  320 , implanted blood pump assembly  400 , and/or patient specific parameters, updating software on the external system controller  320  and/or implanted blood pump assembly  400 , monitoring system operation, and/or as a conduit for system inputs or outputs. 
       FIG. 13  is a simplified schematic diagram illustrating a first axial displacement (D 1 ) of the impeller  440  (relative to a reference axial position of the impeller  440  with zero thrust load applied) that is induced by a first thrust load  434  applied to the impeller  440 .  FIG. 14  is a simplified schematic diagram illustrating a second axial displacement (D 2 ) of the impeller  440 , which is greater than the first axial displacement (D 1 ), that is induced by a second thrust load  435 , which is greater than the first thrust loads  434 . As described herein, the axial displacement of the impeller  440  (relative to the reference axial position of the impeller with zero thrust load applied) varies as a function of the magnitude of the thrust load applied to the impeller  440 . In many embodiments, the blood pump assembly  400  includes one or more sensors configured to generate output indicative of the axial position of the impeller  440 , and thereby indicative of the thrust load applied to the impeller  440 . For example, the output of the Hall Sensor assembly  500  can be processed to determine the axial displacement of the impeller  440  and thereby determine the thrust load applied to the impeller. The blood pump assembly  400  can also include one or more sensors configured to monitor the axial position of the impeller  440  and thereby monitor the thrust load applied to the impeller. For example, the blood pump assembly  400  can include one or more Hall-Effect sensors supported by the housing (e.g., adjacent a circumferential wall of the blood flow conduit  403  or in an inlet stator disposed in the blood flow conduit  403 ) that generates an output signal indicative of the axial position of the impeller  440  along the blood flow conduit  403 . The axial position of the impeller  440  can then be processed by the controller to determine a thrust load applied to the impeller  440 . As already described herein, the thrust load applied to the impeller  440  and the rotational rate of the impeller  440  can then be used by the controller to determine a flow rate of blood pumped by the blood pump  400 . 
     Pump Induced Pressure in Patient 
     As discussed herein, the thrust load from the impeller is indicative of the pressure differential across the pump. In many embodiments, the pressure differential across the pump is output to a suitable output device (e.g., a display, speaker) to inform the patient and/or a medical professional involved in the treatment of the patient with regard to a resulting pressure differential within the patient. For example, in the embodiment illustrated in  FIG. 6 , the blood pump  314  is attached to an apex of the left ventricle and pumps blood from the left ventricle to the patient&#39;s aorta. The pressure differential across the blood pump  314  is the difference between the left ventricular pressure and the aortic pressure. The pressure across the pump is therefore the same as the pressure across the aortic valve.  FIG. 15  shows example aortic pressure  516  and left ventricular pressure  518  over a heart cycle. During contraction of the left ventricle, the left ventricular pressure  518  increases from below the aortic pressure  516 . When the left ventricular pressure  518  equals and exceeds the aortic pressure  516 , the aortic valve opens to enable ejection of blood from the left ventricle into the aorta. At the end of the ejection of blood from the left ventricle, the left ventricular pressure  518  drops below the aortic pressure  516  thereby closing the aortic valve. By outputting the pressure differential across the pump to a suitable output device, the user and/or the medical professional is provided with information (e.g., pressure across the aortic valve when the blood pump is used as an LVAD) helpful in understanding the functioning of the patient&#39;s heart. 
     Other variations are within the spirit of the present invention. Thus, while the invention is susceptible to various modifications and alternative constructions, certain illustrated embodiments thereof are shown in the drawings and have been described above in detail. It should be understood, however, that there is no intention to limit the invention to the specific form or forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention, as defined in the appended claims. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The term “connected” is to be construed as partly or wholly contained within, attached to, or joined together, even if there is something intervening. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate embodiments of the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. 
     All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.