Patent Publication Number: US-2020288988-A1

Title: Blood Flow Rate Measurement System

Description:
BACKGROUND 
     Technical Field 
     The present invention relates to blood flow measurement systems and, more particularly, to catheter-mounted, turbine-driven blood flow measurement systems. 
     Related Art 
     Many patients with coronary artery disease (CAD) would benefit from stent percutaneous coronary intervention (CPI) or coronary artery bypass graft surgery. However, some of these patients are considered to be too high-risk for complications that may occur during these procedures. Risk factors include: advanced age; history of disease, such as kidney disease, stroke or diabetes; location of the CAD, including left main or bifurcated disease; challenging plaque types, including calcified or long lesions; chronic total occlusion of the coronary arteries; previous open-heart surgery; and advanced heart failure. 
     Interventional cardiologists treat some of these high-risk patients with an advanced catheter-based procedure, referred to as complete or complex higher risk indicated percutaneous coronary intervention (CHIP). However, high-risk CAD patients that have also been diagnosed with advanced heart failure or severe heart valve disease are particularly vulnerable during CHIP procedures, due to weakened heart muscle, which compromises blood pressure and cannot pump blood efficiently to the body. 
     For a high-risk CAD patient, a ventricular assist device may be used during CHIP to provide temporary support to the patient&#39;s heart by assisting in blood circulation through the patient&#39;s cardiovascular system. For example, a catheter with a heart pump at one end may be inserted via a standard catheterization procedure through a femoral artery, into the ascending aorta, across the aortic valve and into the left ventricle. Once in place, the heart pump supports blood movement from the left ventricle, through inlet ports near the tip and the cannula to outlet ports and into the ascending aorta. 
     Temporarily, such as for less than about six hours, supporting the heart with a ventricular assist device during CHIP is referred to as protected PCI. Exemplary ventricular assist devices include Impella 2.5® and Impella CP® heart pumps available from Abiomed, Inc., Danvers, Mass. Although the Impella 2.5 heart pump has a catheter diameter of only 9 Fr, and a pump motor diameter of only 12 Fr (14 Fr for the Impella CP heart pump), smaller diameter catheters and motor diameters are highly desirable. 
     SUMMARY OF EMBODIMENTS 
     An embodiment of the present invention provides a blood flow rate measurement system. The system includes a catheter, a turbine, a signal generator and a signal lead. The catheter has a length. The catheter is configured to be inserted into a blood vessel of a living being, such as a human being. The turbine is disposed proximate a distal end of the catheter. 
     The turbine includes at least one blade. The at least one blade is configured to rotate. The at least one blade is configured to rotate relative to the catheter. The at least one blade is configured to rotate in response to fluid flow through the blood vessel. The at least one blade is configured to rotate at a rotational speed that depends at least in part on speed of the fluid flow through the blood vessel. 
     The signal generator is mechanically coupled to the turbine. The signal generator is configured to generate a signal indicative of the rotational speed of the at least one blade. The signal lead is configured to carry the signal indicative of the rotational speed of the at least one blade. The signal lead is connected to the signal generator. The signal lead extends along the catheter. 
     In any embodiment, the signal generator may include an electrical generator. 
     In any embodiment, the signal lead may include first and second electrically conductive leads and/or an optical fiber. 
     Any embodiment may include a tachometer. The tachometer may be coupled to the signal lead. The tachometer may be configured to measure the speed of the fluid flow through the blood vessel, based on the signal indicative of the rotational speed of the at least one blade. 
     In any embodiment, the at least one blade may be radially collapsible. 
     Any embodiment may also include a duct configured to direct at least a portion of the fluid flow through the blood vessel toward the at least one blade. The at least one blade may be configured to rotate, relative to the catheter, at a rotational speed dependent at least in part on shape and size of the duct. 
     In any embodiment, the duct may be radially collapsible. 
     In any embodiment, the duct may be tapered. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood by referring to the following Detailed Description of Specific Embodiments in conjunction with the Drawings, of which: 
         FIG. 1  illustrates a percutaneous left-heart pump inserted into a blood vessel of a patient, according to the prior art. 
         FIG. 2  is an isometric illustration of a distal end of a heart pump catheter with a turbine, and an optional duct, located at the end of the catheter to measure blood flow rate, according to an embodiment of the present invention. 
         FIG. 3  is an isometric illustration of a distal end of a heart pump catheter with a turbine located near the end of the catheter to measure blood flow rate, according to another embodiment of the present invention. 
         FIG. 4  is a schematic diagram of a signal generator mechanically coupled to the turbine of  FIG. 2 or 3 , according to an embodiment of the present invention. 
         FIG. 5  is a detailed schematic diagram of the signal generator of  FIG. 4 , according to an embodiment of the present invention. 
         FIG. 6  is a detailed schematic diagram of the signal generator of  FIG. 4 , according to another embodiment of the present invention. 
         FIG. 7  is a detailed schematic diagram of the signal generator of  FIG. 4 , according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Embodiments of the present invention provide apparatus and methods for measuring blood flow, such as total blood flow due to natural heart action plus heart pump action, in a blood vessel of a patient when a catheter-based heart pump is inserted into the blood vessel, without relying on measurements of electric current drawn by a motor that drives the heart pump. Embodiments of the present invention include a turbine disposed at or near a distal end of the heart pump catheter. Rotation of the turbine blades is induced by blood or other fluid flowing through the blood vessel. The turbine is mechanically coupled to a signal generator, which generates a signal indicative of rotational speed of the turbine, which is dependent, at least in part, on speed of the fluid flowing through the blood vessel. The signal is carried by a lead to a proximal end of the catheter, external to the patient&#39;s body, where a tachometer calculates the blood flow rate from the rotational speed of the turbine. Advantageously, the lead is small in diameter. 
     Definitions 
     As used herein, the following terms shall have the following definitions, unless otherwise indicated: 
     A turbine is a rotary mechanical device that extracts energy from a fluid flow and converts the energy into useful work. The work produced by a turbine can be used for generating electrical power when combined with a generator. A turbine is a turbomachine with at least one moving part called a rotor assembly, which includes a shaft or drum with blades attached. Moving fluid acts on the blades so that they move and impart rotational energy to the rotor. (Wikipedia, turbine.) As used herein, turbines include, without limitation, Pelton, Francis and Kaplan turbines. 
     Reducing Diameter of Prior Art Heart Pumps 
       FIG. 1  shows an exemplary prior art heart pump  100  that includes an elongated catheter  102 , a distal portion  104  of which is inserted through a blood vessel  106  into a heart  108  of a patient. A proximal end  110  of the catheter  102  is connected to an external control unit  112 , such as an Automated Impella® Controller, available from Abiomed, Inc., Danvers, Mass. Conventional heart pumps, such as the Impella 2.5® and Impella CP® heart pumps, include electric motors that drive impeller blades. Blood is drawn in through inlet ports  114  and expelled through outlet ports  116 . Pumped blood flow is indicated by arrows  118 ,  120  and  122 . Speed of a motor  124 , and therefore speed of the pump  100  and amount of blood pumped, can be automatically ascertained by the control unit  112  by measuring electrical current drawn by the motor  124 . However, as noted, the motor  124  is relatively large in diameter. 
     Relocating the motor  124  from the distal portion  104  of the catheter  102  to a location outside the patient&#39;s body, and driving the impeller blades with a flexible drive shaft (not shown) extending through the catheter  102 , reduces the diameter of the heart pump  100 . However, such a long, flexible drive shaft is subject to significant friction and other losses along its length. Therefore, the current drawn by an external motor is not a reliable indicator of the rotational speed of the impeller blades or of the amount of blood pumped by the heart pump. 
     Turbine-Based Blood Flow Rate Measurement System 
     To overcome this problem, and to provide reliable measurements of flow rate of a fluid through a blood vessel, an embodiment of the present invention includes a turbine  200  at or near the distal end of a catheter  202  that also includes a heart pump (not shown), as illustrated in  FIGS. 2 and 3 . 
     One or more blades  204  of the turbine  200  are driven to rotate, about an axis  206 , by fluid, such as blood, flowing past the turbine  200  and impinging on the blades  204 . The axis  206  may, but need not necessarily, align with a longitudinal axis  208  of the catheter  202 . Optionally, a duct  210  (shown in phantom) may be included to protect the walls of the blood vessel during insertion and removal of the turbine  200 , and to prevent the blades  204  engaging the walls, thereby preventing rotation of the blades  204 , once the turbine  200  is in position. Optionally, the duct  210  may be tapered to increase rate of fluid flow  212  through the turbine  200 . The duct  210  may, for example, be attached to the catheter  200  by rigid or collapsible fins, represented by fin  214 . The optional duct  210  is omitted from  FIG. 3  for clarity. 
     The turbine  200  drives a signal generator  400 , as shown schematically in  FIG. 4 . The signal generator  400  generates a signal  404  that is indicative of rotational speed of the turbine blades  204 . The blades  204  are configured to rotate, relative to the catheter  202  ( FIG. 2 ), at a rotational speed that is dependent, at least in part, on speed of the fluid  402  flowing through the blood vessel, and configuration of the duct  210  (if present), such as the taper of the duct  210 . 
     The signal generator  400  is configured to generate the signal  404  indicative of the rotational speed of the blades  204 . A signal lead  406 , exemplified by two electric wires  408  and  410 , is configured to carry the signal  404  to a tachometer  412 . The tachometer  412  is configured to measure the speed of the fluid  402  flow (flow rate) through the blood vessel, based on the signal  404 . In one embodiment, the tachometer  412  calculates the flow rate by multiplying the rotational speed of the blades  204  by a factor. The factor may represent a linear or non-linear relationship between rotational speed of the blades  204  and flow rate of the fluid  402 . This relationship may be determined empirically or by modeling the blades  204 , the fluid  402 , blood vessel geometry, friction, etc. 
     In one embodiment, schematically illustrated in  FIG. 5 , the signal  404  indicative of the rotational speed of the turbine blades  204  is an AC signal, in which frequency of the signal  404  is proportional to the rotational speed of the blades  204  (not shown in  FIG. 5  for clarity). In one such embodiment, the blades  204  are mechanically coupled to a magnet  500 , so the magnet  500  rotates with the blades  204 , as indicated by an arrow  502 . A coil  504  is disposed proximate the magnet  500 . Each revolution of the magnet  500  induces a pulse (in this example, one cycle  506  of a sine wave) of the signal  404 . The tachometer  412  may count the pulses (cycles) received during a predetermined time interval to measure the frequency of the signal  404 . Although the coil  504  is shown with a core, any suitable core, such as an iron core or an air core, may be used. In addition, although the coil  504  is shown split into two portions, the coil need not be split. 
     Alternatively, the tachometer  404  may measure voltage of the signal  404 , which is proportional to the rotational speed of the turbine blades  204 . 
     In another embodiment, schematically illustrated in  FIG. 6 , the coil  504  ( FIG. 5 ) is replaced by a Hall effect sensor  600 . An output signal from the Hall effect sensor  600  may be processed by a threshold detector  602  (if needed) to generate the signal  404  indicative of the rotational speed of the turbine blades  204  (not shown in  FIG. 6  for clarity). In this embodiment, the signal  404  consists of rectangular pulses  604 . As with the first embodiment described with respect to  FIG. 5 , the frequency of the pulses  604  is proportional to the rotational speed of the blades  204 . 
     In yet another embodiment, schematically illustrated in  FIG. 7 , the leads  408  and  410  from the coil  504  described with respect to  FIG. 5  are connected to a light-emitting diode (LED)  700 . The LED  700  is optically coupled to a distal end of an optical fiber  702 . Each pulse (cycle) of the signal from the coil  504  causes the LED to flash, which sends an optical pulse along the optical fiber  702 . A series of these optical pulses collectively form the signal  404  indicative of the rotational speed of the blades  204 . Thus, in this embodiment, the optical fiber  702  is a lead  406  configured to carry the signal  404  to the tachometer  412 , and the tachometer  412  includes an optical sensor (not shown) to detect the optical pulses. 
     In any embodiment, the lead  406  configured to carry the signal  404  may be discrete and extend along a lumen of the catheter  202 . Alternatively, the lead  406  may be integral with the catheter  202 . For example, in some embodiments, the wires  408  and  410  are printed on an outside and/or inside surface of the catheter  202 , or imbedded within a wall of the catheter  202 . Similarly, in one embodiment, the optical fiber  702  is imbedded within the wall of the catheter  202 . 
     Although embodiments with one magnet per turbine have been described, each turbine can include more than one magnet, in which case the signal  404  may include more than one pulse per revolution of the blades  204 . 
     Although the heart pump pumps blood, the patient&#39;s heart action also pumps some blood. The total amount of blood flowing through the blood vessel is important to patient wellbeing. “Upstream” means in a direction opposite the direction of flow of blood or other fluid, and “downstream” means in the same direction as the flow of blood or other fluid. Advantageously, if the turbine is disposed a distance, in the upstream direction, from the heart pump inlet ports  114  ( FIG. 1 ) or a distance, in the downstream direction, from the heart pump outlet ports  116 , the blood flow rate measurement system embodiments described herein would measure total fluid flow in the blood vessel, not merely the amount of blood pumped by the heart pump. However, if the turbine is disposed between the heart pump inlet ports  114  and the heart pump outlet ports  116 , the blood flow rate measurement system would measure blood flow caused by heart action, plus additional blood flow around the heart pump caused by jet pumping driven by blood being ejected from the heart pump outlet ports  116 . 
     Collapsible Turbine Blades 
     In some embodiments, the blades  204  are radially collapsible, i.e., in a direction toward the axis  206 . In some such embodiments, the blades  204  are made of a flexible material that can be folded, shrunk or compacted to reduce outside diameter  216  ( FIG. 2 ) of the blades  204 , at least while the catheter is being inserted into a blood vessel. In some embodiments, the blades  204  are resilient. In some embodiments, the blades  204  are made of a shape-memory material that rebounds to its memorized shape upon being heated to a temperature equal to, or slightly less than, the temperature of circulating blood in a human body. In some embodiments, each blade  204  includes a plurality of struts that collapse or expand, depending on the mode (collapsed or expanded) of the blade  204 . Consequently, once the catheter is in position, the blades  204  unfold or otherwise rebound into an efficient shape for being driven by flowing fluid in the blood vessel. 
     Any suitable structure and/or method may be used to initially make the blades  204  compact, to expand the blades  204  once the turbine  200  is in place, and to collapse the blades  204  in preparation for removal of the turbine  200 . Exemplary structures and methods are described in U.S. Pat. Nos. 9,611,743, 9,416,783, 8,944,748, 9,416,791, 9,314,558, 9,339,596, 9,067,006, 9,642,984, 8,932,141, 8,814,933, 8,814,933 and 9,750,860, and U.S. Pat. Publ. Nos. 2018/0080326, 2014/0039465 and 2018/0296742, the entire contents of each of which are hereby incorporated by reference herein, for all purposes. Some of the structures and/or methods described in the aforementioned documents include wires or drive shafts to advance, retract and/or rotate components to expand or compress impeller blades and/or pumps. The same wires or drive shafts may be used in similar ways to actuate a structure configured to expand and/or compress the blades  204  of the turbine  200 . Optionally or alternatively, different or additional wires or drive shafts may be used to actuate the structure configured to expand and/or compress the blades  204  of the turbine  200 . Optionally or alternatively, the lead  406  or part of the lead  406  may be used to actuate the structure configured to expand and/or compress the blades  204  of the turbine  200 . 
     While the invention is described through the above-described exemplary embodiments, modifications to, and variations of, the illustrated embodiments may be made without departing from the inventive concepts disclosed herein. For example, although specific parameter values, such as dimensions and materials, may be recited in relation to disclosed embodiments, within the scope of the invention, the values of all parameters may vary over wide ranges to suit different applications. Unless otherwise indicated in context, or would be understood by one of ordinary skill in the art, terms such as “about” mean within ±20%. 
     As used herein, including in the claims, the term “and/or,” used in connection with a list of items, means one or more of the items in the list, i.e., at least one of the items in the list, but not necessarily all the items in the list. As used herein, including in the claims, the term “or,” used in connection with a list of items, means one or more of the items in the list, i.e., at least one of the items in the list, but not necessarily all the items in the list. “Or” does not mean “exclusive or.” 
     Although aspects of embodiments may be described with reference to flowcharts and/or block diagrams, functions, operations, decisions, etc. of all or a portion of each block, or a combination of blocks, may be combined, separated into separate operations or performed in other orders. References to a “module” are for convenience and not intended to limit its implementation. All or a portion of each block, module or combination thereof may be implemented as computer program instructions (such as software), hardware (such as combinatorial logic, Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), processor or other hardware), firmware or combinations thereof. 
     The tachometer, heart pump control unit, or portions thereof, may be implemented by one or more processors executing, or controlled by, instructions stored in a memory. Each processor may be a general purpose processor, such as a central processing unit (CPU), a graphic processing unit (GPU), digital signal processor (DSP), a special purpose processor, etc., as appropriate, or combination thereof. However, once programmed with these instructions, the combination of the processor and the memory collectively form a special purpose processor. 
     The memory may be random access memory (RAM), read-only memory (ROM), flash memory or any other memory, or combination thereof, suitable for storing control software or other instructions and data. Instructions defining the functions of the present invention may be delivered to a processor in many forms, including, but not limited to, information permanently stored on tangible non-transitory non-writable storage media (e.g., read-only memory devices within a computer, such as ROM, or devices readable by a computer I/O attachment, such as CD-ROM or DVD disks), information alterably stored on tangible non-transitory writable storage media (e.g., floppy disks, removable flash memory and hard drives) or information conveyed to a computer through a communication medium, including wired or wireless computer networks. Moreover, while embodiments may be described in connection with various illustrative data structures, systems may be embodied using a variety of data structures. 
     Disclosed aspects, or portions thereof, may be combined in ways not listed above and/or not explicitly claimed. In addition, embodiments disclosed herein may be suitably practiced, absent any element that is not specifically disclosed herein. Accordingly, the invention should not be viewed as being limited to the disclosed embodiments. 
     As used herein, numerical terms, such as “first,” “second” and “third,” for example as used to distinguish respective wires  408  and  410  from one another, are not intended to indicate any particular order or total number of items in any particular embodiment. Thus, for example, a given embodiment may include only a second wire and a third wire.