Patent Publication Number: US-10786611-B2

Title: VAD integrated flow sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/406,078 filed Jan. 13, 2017, now U.S. Pat. Ser. No. 9,861,730, issued Jan. 9, 2018, which application is a continuation of U.S. patent application Ser. No. 14/019219, filed Sep. 5, 2013, now U.S. Pat. No. 9,579,432 issued Feb. 28, 2017, which application is related to and claims priority from Provisional Patent Application Ser. No. 61/697087, filed Sep. 5, 2012, the entirety of all of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to blood pumps usable as implantable ventricular assist devices, and more particularly to an improved blood pump device with an integrated ultrasonic flow sensor. 
     In certain disease states, the heart lacks sufficient pumping capacity to maintain adequate blood flow to the body&#39;s organs and tissues. For example, conditions such as ischaemic heart disease and hypertension may leave the heart unable to fill and pump efficiently. This condition, also called congestive heart failure, may lead to serious health complications, including respiratory distress, cardiac asthma, and even death. In fact, congestive heart failure is one of the major causes of death in the Western world. 
     This inadequacy of the heart can be alleviated by providing a mechanical pump also referred to as a ventricular assist device (“VAD”) to supplement the pumping action of the heart. VADs may be used to assist the right ventricle, the left ventricle, or both. For example, a VAD may assist the left ventricle by mechanically pumping oxygenated blood from the left ventricle into the aorta. In this case, the pump is implanted within the body of the patient, an inflow conduit is attached to the left ventricle, and an outflow conduit is attached to the aorta. For example, where the pump is implanted below the heart or at the bottom of the heart, the outflow conduit may be a flexible conduit extending generally upwardly, from the outlet of the pump to the aorta. The pump receives blood from the left ventricle and then pushes it into the aorta for distribution throughout the body. This reduces the strain on the heart by reducing the volume of blood that the heart is responsible for moving. 
     U.S. Pat. Nos. 7,575,423, 7,976,271, 8,007,254, and 8,419,609, the disclosures of which are hereby incorporated by reference, disclose certain rotary blood pumps which can be used as ventricular assist devices. These pumps are electrically powered. Typically, these and other electrically powered implantable pumps are connected through a cable, commonly referred to as a “driveline”, to a control device which supplies electric power to the pump and controls its operation. The control device may be external to the patient&#39;s body, in which case the driveline extends through the skin. It has also been proposed to use implanted control devices which receive power from an external source by means of an implanted induction coil. 
     It is desirable to monitor certain parameters of the pump, including for instance the rate of blood flow through the VAD. Flow information can be used to detect abnormal operating conditions, such as blockage of the outflow conduit or a “suction” condition, where the left ventricle is not refilled fast enough to keep the pump supplied with blood, and also can be used to provide feedback control of the pump. However, blood flow through a VAD is difficult to monitor because it often cannot be measured directly. It would not be desirable to install a bulky sensor in the path of the flowing blood, as the sensor could obstruct the blood flow and reduce the effectiveness of the pump. 
     One solution that has been proposed is to measure blood flow indirectly. This can be achieved by measuring blood pressure at both the inflow and outflow sections of the pump, and then mathematically computing blood flow. Pressure sensors have been incorporated into VADs for the purpose of monitoring blood flow through the VAD. Blood flow also can be determined indirectly from operational parameters of the pump as, for example, the speed of the pump and the power used by the pump. 
     Other solutions have been proposed that involve measuring blood flow through the pump directly. This can be achieved, for instance, through the use of an ultrasonic flow probe. For example, it has been proposed to provide an ultrasonic flow probe around mounted on the outflow cannula. Similarly, European Patent EP1046403 discloses a blood circulation device with ultrasonic flow sensors attached to the inflow cannula or “blood feeding pipe.” In these proposed solutions, blood flow can be monitored directly for enhanced control over the therapeutic qualities of the pump. However, these solutions require an additional structure to hold the ultrasonic flow probe. Moreover, as further discussed below, certain types of ultrasonic flow measurement can be used only in a rigid conduit. Where the flow is measured along a flexible conduit, the additional structure typically must have appreciable bulk to hold a portion of the flexible conduit in a fixed configuration. Also, these arrangements require an additional cable extending to the additional structure housing the flow probe. These factors make it more difficult to implant the system in the body. 
     Thus, despite very considerable effort devoted in the art to development of ventricular assist devices, further improvement would be desirable. Particularly, there is a need for a VAD which can provide the benefits of direct flow measurement without substantially increasing the difficulty of implanting the device. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention provides a blood pump with an integrated flow sensor. The blood pump according to this aspect of the invention desirably includes an implantable pump for pumping blood having a rigid housing, a flow path extending within the housing and at least one movable element within the housing for impelling blood along the flow path, and a sensor for measuring the flow rate of blood through the pump. In this aspect of the invention, the sensor may be mounted to the housing of the pump. 
     A further aspect of the invention provides a blood pump including a first housing element having an interior surface at least partially defining the flow path and having an exterior surface defining a cavity. In this aspect, the sensor may be located within the cavity. The sensor may also include, for example, one, two or more ultrasonic transducers. 
     Yet another aspect of the invention includes a blood pump having a first and second platform. According to this aspect, the flow path may extend along a flow path axis extending in upstream and downstream directions. The first housing element may define a first platform facing downstream at an oblique angle to the flow path axis and a second platform surface upstream at an oblique angle to the flow path axis. Further, the ultrasonic transducers may include a first transducer mounted to the first platform and a second transducer mounted to the second platform. The platforms may, for example, have a slope of substantially 45 degrees to the flow path axis. The ultrasonic transducers may also be mounted to the platforms with an adhesive. 
     A further aspect of the invention also provides a second housing element. In this aspect, the first and second housing elements may cooperatively define at least a portion of the flow path. Further, the transducers may be arranged such that the ultrasound emitted from one of the transducers passes through the flow path to the second housing element and reflects from the second housing element and passes to the other one of the transducers. The housing may further include a cover overlying the cavity in the first housing element. Optionally, an electronic circuit may be disposed within the cavity and may also be connected to the sensor. 
     A still further aspect of the invention includes a blood pump having an inflow end and an outflow end. The sensor may be mounted adjacent the outflow end of the flow path. The sensor may also be mounted adjacent to the inflow end of the flow path. In some aspects of the invention, the pump may be a rotary pump. 
     A further aspect of the invention may provide a blood pump having one or more electrical elements for moving the movable element. In this aspect, the device may also include an external control unit that powers the electrical elements. Further, the device may also include a driveline for connecting the pump and the one or more ultrasonic transducers to the external control unit. The sensor may be connected to the control unit through the driveline. The driveline may also be the only connection between the pump and the control unit. 
     These and other aspects of the invention will be more readily understood with reference to the detailed description taken below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a blood pump usable as a ventricular assist device in accordance with one embodiment of the invention, in conjunction with the heart and certain blood vessels of a human patient. 
         FIG. 2  is an exploded view of pump used in the ventricular assist device of  FIG. 1 . 
         FIG. 3  is a fragmentary perspective view depicting a portion of the ventricular assist device of  FIGS. 1 and 2 . 
         FIG. 4  is a detailed view of the ventricular assist device shown in  FIGS. 1-3 , on a further enlarged scale. 
         FIG. 5  is a partial exploded view of the ventricular assist device of  FIGS. 1-4 . 
         FIG. 6  is a partial assembled view of the ventricular assist device shown in  FIGS. 1-5 . 
         FIG. 7  is a diagrammatic cross-sectional view of the ventricular assist device shown in  FIGS. 1-6  accordance with one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an implantable blood pump usable as a ventricular assist device (“VAD”)  100  in accordance with one embodiment of the invention. In this embodiment, the VAD  100  comprises a pump  110  having an outer housing  111  that includes and a first or lower housing element  113  and a second or upper housing element  112 . The housing elements are formed from biocompatible rigid materials such as titanium or a hybrid titanium-ceramic. The upper housing  112  may further comprise an inflow end  114  ( FIG. 2 ) defining an inlet opening  107 . In the implanted condition depicted in  FIG. 1 , the inflow end of the pump is inserted into heart of a mammalian subject such as a human patient, typically into the left ventricle so that the inflow opening is in communication with the interior of the ventricle. The VAD  100  may also include an apical ring  119  for securing the connection between the outer housing  111  of the pump and the heart. The sewing ring typically is sutured in place on the apex of the heart, and may include a clamp to secure the outer housing  111  to the sewing ring and thus secure the pump in place relative to the heart. 
     The VAD  100  may also include an outflow conduit  121  extending from the outer housing  111 . The outflow conduit  121  may comprise a flexible, biocompatible main tubing  122 . The main tubing  122  may also be encased along a portion of its length by an anti-kinking conduit  123 . The anti-kinking conduit  123  may be made by plastic interlocking links to prevent kinking. The main tubing  122  may further be surgically attached to a desired position  124  of the heart or the surrounding area, such as to the ascending aorta as depicted in  FIG. 1 . 
     The VAD  100  may further include a cable  130 , also referred to herein as a driveline. Driveline  130  typically includes a plurality of electrical conductors  131 . The driveline  130  electrically connects components of the pump  110  within the outer housing  111  to an external control unit  191 . Control unit  191  is arranged to supply electrical power to the pump, and to control the operation of the pump. All or part of control unit  191  may be implanted within the body of the subject, or may be external to the subject. 
     As shown in  FIG. 2  upper housing element  112  and lower housing element  113  define a flow path therein, which extends from the inlet opening  107  to an outflow end  115   a  and  115   b  cooperatively defined by the housing elements  112  and  113 . Permanent magnets (not shown) may be provided within one or both of the housing elements  112  and  113 , along with a set of electromagnet coils, one of which is schematically represented at  109 . In one embodiment, a permanent magnet stack may be contained within a center-post  117  on the lower housing element  113 . Pump  110  further includes a movable element  116  for impelling the blood along the flow path. In this embodiment, the movable element  116  is a wide-bladed impeller, which incorporates permanent magnets (not shown). The permanent magnets within impeller  116  cooperate with the permanent magnets in the housing elements to keep impeller  116  suspended and out of contact with the housing elements during operation. When the coil set  109  is energized with alternating current, the magnetic interaction between the coil set and the permanent magnets of the impeller spin the impeller around its axis, so that the impeller will drive blood along the flow path. 
     The upper  112  and lower  113  housing may further define a driveline interface  118   a  and  118   b  for receiving a power connector  127  on the driveline into the pump housing. In one embodiment, the upper housing  112  defines a top portion of the driveline interface  118 , and the lower housing  113  defines a bottom portion of the driveline interface  118   b.  The driveline interface  118  is provided with appropriate terminals (not shown) for making electrical contact with certain conductors of driveline  130  at power connector  127 . These terminals are electrically connected to the coil set. 
     As shown in  FIGS. 3-8 , and as further discussed below, the pump is provided with a flow sensor carried on or within the housing  111 . In each of  FIGS. 3-6 , the first or lower housing element  113  is front facing and visible. In this embodiment, the exterior surface of lower housing element  113  has a PCB cavity  361  and a further cavity  162  which accommodates a first platform  163  and a second platform  166 . In one embodiment, cavity  162  is about 0.5 mm in depth. Preferably, the first  163  and second  166  platforms have a height less than or equivalent to the depth of the cavity  166  so that the platforms may be fully accommodated within the cavity  162 . As best appreciated with reference to  FIG. 7 , platforms  163  and  166  overlie a portion  195  of the flow path cooperatively defined by housing elements  112  and  113 . This portion of the flow path is adjacent the outflow end  115   a,    115   b  depicted in  FIG. 2 . Within this portion  195  of the flow path, the blood flows in a downstream direction indicated by arrows D in  FIG. 7 , generally along a flow path axis  193 . As best seen in  FIGS. 4 and 7 , the first platform  183  defines a first surface  164  facing away from flow path  195  and facing generally upstream (the direction opposite to arrows D) at an angle oblique to the flow path axis  193 , as well as a second surface  165 . The second platform  166  is disposed downstream from the first platform. Second platform  166  defines a first surface  167  facing away from the flow path and facing downstream at an angle oblique to the flow path axis  193 , and also defines a second surface  168 . For example, the first surface of each platform may be disposed at an angle of 45 degrees to the axis of the flow path. 
     A window  172  ( FIG. 7 ) forms an interface between the first platform  163  and the interior of the flow path, and another window  172  forms an interface between the second platform and the flow path. Although the windows are depicted as separate elements from the platforms for clarity of illustration, the windows may be formed integrally with the platforms. Also, the surfaces of the windows bounding the flow path desirably are flush with the surrounding surfaces of housing element  113 . 
     The windows and platforms may be formed integrally with first housing element  113  or may be fixed directly to this housing element. The materials of the platforms and windows desirably provide a low-impedance path for ultrasound between first surfaces  164  and  187  and the interior of the flow path when the flow path is filled with blood. For example, the materials of the platforms and windows may have acoustic impedance reasonably close to that of blood so as to minimize reflection of ultrasound at the interface with the blood within the flow path. For example, the platforms and windows may be formed by casting a biocompatible polymer. 
     A first ultrasonic transducer  142  is bonded to the first surface  164  of the first platform  163 , whereas a second ultrasonic transducer  144  is bonded to the first surface  167  of the second platform  166 . The ultrasonic transducers may be conventional piezoelectric elements. The transducers are electrically connected by conductors  145  and  146  to electronic components on a printed circuit board  150  ( FIGS. 5 and 6 ) which is disposed in cavity  161  of housing element  113 . The components on the printed circuit board may include conventional components for driving one of the transducers (referred to herein as the “driven transducer”) at an ultrasonic frequency, typically in the megahertz range with an electrical signal, and for amplifying electrical signals from the other one of the transducers (referred to herein as the “receiving transducer”). The electronic components may also include components for comparing the phase of the electrical signals from the receiving transducer with the phase of the signals applied to drive the driven transducer. 
     Printed circuit board  150  is connected by conductors  151  to conductors  198  of driveline  130  at a connector  197  engaged in an opening  199  ( FIG. 5 ) communicating with cavity  561 . In this embodiment, connector  197  is separate from the power connector  127  ( FIG. 1 ) engaged with the driveline interface  118  ( FIG. 2 ). The conductors associated with connectors  197  and  127  extend within the outer sheath of driveline  130  over most of the length of the driveline, and diverge from one another only in the immediate vicinity of the pump. Conductors  198  of the driveline link the components on PCB  150  with an appropriate circuit in the control unit  191  ( FIG. 1 ). 
     A cover schematically indicated at  173  ( FIG. 7 ) overlies cavities  161  and  162 , and thus forms a part of the housing which cooperates with housing element  113  to enclose the platforms, transducers, printed circuit board and associated conductors. Cover  173  may be a biocompatible potting material such as an epoxy, or may be a plate fixed to housing element  113  by appropriate fasteners and sealed by an appropriate gasket to prevent entry of body fluids into the cavities  161  and  162 . 
     In operation, with the pump operating and forcing blood through the flow path, the control unit actuates the components on PCB  550  to drive one of the transducers. For example, the control unit and components on PCB  550  may cause the first or upstream transducer  143  to emit ultrasonic waves. These waves pass along a path  174  at an oblique angle to the direction of the blood flow (the downstream direction) and impinge on the wall of the flow path defined by the second or upper housing element  112  at a point  175 . The ultrasonic waves are reflected along a further portion of path  174 , also oblique to the downstream direction, back to the receiving transducer, in this case the second or downstream transducer  144 . The receiving transducer converts the ultrasonic waves to electrical signal. Because the path from the driven transducer to the receiving transducer has a component parallel to the direction of flow of the blood, the time of flight of the ultrasonic waves is influenced by the velocity of the blood according to the well-known Doppler effect. This causes the phase of the received ultrasonic waves to vary with the blood velocity, and thus with the flow rate. Because the housing elements  112  and  113  are rigid, the geometry of the system is fixed. As used in this disclosure, the term “rigid” should be understood as meaning that the housing elements do not distort in normal operation of the pump to a degree which would appreciably affect the phase difference between the received and emitted ultrasonic waves. The mathematical relationships used to convert phase difference to flow velocity, and to convert flow velocity to flow rate, are well known. The circuits used to measure phase difference are also well known and accordingly are not further described herein. 
     Because the flow measurement is performed by the ultrasonic sensors mounted in the pump housing, there is no need for a separate flow measurement device mounted along the outflow cannula. Moreover, because the connection between the flow sensor and the control unit is made through conductors of the same driveline used to convey power to the pump, there is no need to implant a separate cable leading to a flow sensor. 
     In a variant of the embodiment discussed above, the connectors  197  and  127  may be integrated into a single connector, mated to a single driveline interface on the pump housing. In yet another variant, conductors of the driveline which convey power pump coil system may also be used to convey ultrasonic frequency electrical signals to and from the PCB or the transducers in a multiplexing arrangement. In yet another embodiment, the PCB  150  may also be used to convey power to the electrically driven elements of the pump itself, such as the coil set  109  schematically shown in  FIG. 2 . 
     Also, although the pump depicted in  FIGS. 1-7  is a radial-flow impeller pump, the invention can be applied in other pumps, such as an axial-flow impeller pump as depicted in the aforementioned U.S. Pat. No. 8,419,609, and in conjunction with pumps such as diaphragm pumps and piston pumps. Further, although the invention has been described with reference to an ultrasonic flow sensor having two ultrasonic transducers, other types of flow sensors may be mounted to the pump housing. For example, flow sensors which measure the rate of heat transfer to the flowing blood from a heated element can be employed. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.