Patent Publication Number: US-2010121133-A1

Title: Apparatus and methods for measuring pressure and flow in cardiac assist devices and peripheral vasculature

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to apparatus and methods for measuring pressure and flow in cardiac assist device, such as left ventricular assist devices (LVAD), and in circulatory systems of patient&#39;s using cardiac assist devices. 
     For patients with conditions that compromise cardiac function, such as cardiomyopathy, for example, a cardiac assist device, such as a left ventricular assist device, is an option for treatment. In the case of end-stage NYHA Class 4 heart failure patients, the left ventricular assist device may be the only effective treatment option while the patient awaits transplant (“bridge to transplant”), or as a treatment instead of transplant (“destination therapy”). 
     The LVAD is a blood pump connected between the left ventricle and the aorta via blood-carrying conduits, such as is illustrated in the drawing FIGURE. The mechanical pump unloads the compromised heart by sharing the work required to provide circulation. In some cases the use of an LVAD has enabled recovery of sufficient cardiac function to permit removal of the LVAD. 
     Two general types of pumping mechanisms are employed in LVAD designs. First generation devices typically were based upon the use of a pulsatile pump in which a pumping chamber is passively filled by the patient&#39;s left ventricle. Upon detecting that the chamber is full, the pump actuates, compressing the LVAD chamber, providing the forceful ejection of the blood to the body, directed via unidirectional valves to control forward flow. Second generation LVADs employ smaller continuously running efficient rotary pump motor mechanisms. 
     With rotary pumps, motor speed controls blood flow. Unlike a passively filled pulsatile pump, there is no intrinsic way to ascertain the degree to which the heart is being emptied by the auxiliary mechanical pump. This is also the case with a class of pulsatile pumps that employ an active pumping mechanism for moving blood from a heart chamber, but for specificity in the examples described herein, a rotary pump is used for illustration. Without feedback as to the degree or rate of emptying the chamber, a too-slow pump may under-deliver pumping assistance, and an overly aggressive pump speed can collapse the heart chamber, with the potential of initiating undesirable arrhythmias. At present, algorithms based upon such indirect measures as motor speed and power consumption are used to derive information about pressure and flow, and inferentially about the degree of ventricular (or other chamber) emptying. 
     It would therefore be desirable to have a more direct measure of fluid pressure and flow in the LVAD/conduit system, and nearby circulatory system, to (1) provide information to monitor and control the device performance, (2) monitor the patient&#39;s physiologic parameters in real-time, and (3) monitor trends in the patient&#39;s cardiac function. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawing FIGURE, wherein like reference numerals designate like structural elements, and which is a schematic representation of an implanted pressure/flow monitoring apparatus comprising a left ventricular assist device, not drawn to size or proportion. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the sole drawing FIGURE, disclosed are measurement systems and apparatus comprising a cardiac assist device  10 , such as a ventricular assist device  10 , and in particular, a left ventricular assist device (LVAD)  10 , and one or more sensors  11 , and in particular, pressure sensors  11 . The LVAD  10  comprises a blood pump  12  connected between the left ventricle  13  and aorta  14  of a heart  15  via blood-carrying conduits  16 , referred to as an inflow conduit  16   a  and an outflow conduit  16   b . The LVAD  10  comprises an external console  12   a  that includes a controller and power supply coupled to the blood pump  12  via a percutaneous cable  17 . The apparatus also comprises a pressure monitor  11   a  that is coupled in a wired or wireless fashion to the one or more sensors  11  (pressure sensors  11 ). The external console  12   a  or the pressure monitor  11   a  may comprise an interrogator, for example, that is used to poll the sensors  11  to extract data, such as pressure data, therefrom. 
     Exemplary LVADs  10  include those that are internally implanted and those that are external to the body. In the case of implanted devices  10 , control of the pump  12 , and power, derived from a battery or compressed air, is provided by the external console  12   a , connected to the implanted pump  12  via the permanent percutaneous cable  17 . In the case of externally worn pumps  12 , percutaneous blood-carrying conduits  16   a ,  16   b  are necessary for connecting the pump  12  to the aorta  14 , and connecting the pump  12  to the ventricular cannula  16   a , which comprises the pump inflow conduit  16   a.    
     The pressure sensors  11  may be used to sense local blood pressure and determine fluid flow rates. Exemplary apparatus, comprising a pressure/flow monitoring LVADs  10  may include pressure sensors  11  at one or more of the following locations of the LVAD system and/or patient. With reference to the drawing FIGURE, these locations are: 
     (a) inside the inflow conduit  16   a  of the LVAD  10 , 
     (b) at a second location in the inflow conduit  16   a,    
     (c) inside the outflow conduit  16   b,    
     (d) at a second location in the outflow conduit  16   b,    
     (e) at a location within the aorta  14 , external to the outflow conduit  16   b , or 
     (F) at a second location within the aorta  14 . 
     Additionally or singularly, a pressure sensor  11  placed within the chamber of the heart  15 , affixed to a LVAD conduit  16  or cannula  16  at a location designated “LV” position in the drawing FIGURE, provides a direct measure of chamber pressure, and is valuable in providing information to assess the patient&#39;s cardiac condition. 
     While the drawing FIGURE illustrates the LVAD  10  in the context of an internally implanted pump  12 , one skilled in the art will readily understand how to implement the LVAD  10  with a pump  12  that is located exterior to the patient&#39;s body, connected via conduits  16  to a patient&#39;s heart  15  and circulatory system. Embodiments of the apparatus LVAD  10  may be enabled by wired and/or wireless pressure sensors  11  developed by the assignee of the present invention, discussed in patent applications mentioned below. 
     Furthermore, while this description refers to LVADs  10 , the concepts disclosed herein are equally applicable to similar cardiac assist devices  10 , and include those that assist right heart pumping, both atrial or ventricular, left atrial or ventricular assist devices, and multi-chamber devices or combination of devices, such as BiVAD systems, for example. For example, a right ventricular assist device receives bblood from the right ventricle and delivers it to the pulmonary artery. 
     Sensors  11  suited to enable the LVAD  10  include pressure sensors  11  that operate via direct electrical communication, wirelessly via radiofrequency (RF) communication and wirelessly via acoustic means. U.S. patent application Ser. No. 10/943,772 filed Sep. 16, 2004, U.S. patent application Ser. No. 11/314,046 filed Dec. 20, 2005, and U.S. patent application Ser. No. 11/157,375, filed Jun. 21, 2005 disclose pressure sensors  11  that are suited for use with the LVAD  10 , and the contents of these applications are incorporated herein by reference in their entirety. In particular, the disclosure contained in these applications relating to sensors and how to make and use those sensors is incorporated herein by reference. 
     Used singly at a location selected from (a) inside an inflow conduit  16   a  of the LVAD  10 , (b) at a second location in the inflow conduit  16   a , (c) inside an outflow conduit  16   b , or (d) at a second location in the outflow conduit,  16   b , as shown in the drawing FIGURE, the real-time pressure detected by a pressure sensor  11  may be used to monitor functioning of the mechanical pump  12  and control the speed of the mechanical pump  12 . Algorithms implemented in the external console  12   a , or pressure monitor  11   a , for example, that further refine the pressure/flow information may make use of pump motor speed and/or power as well. The sensors  11  may be wireless sensors communicating information to an internal or external electronic circuit in the external console  12   a  or pressure monitor  11   a , or sensors  11  that communicate to internal or external control circuitry in the external console  12   a  or pressure monitor  11   a  via wires. Such wires may be designed to run through, along, or integral to, the conduits  16  required for the LVAD  10 , although such routing of the wiring is not essential to the concept of the measurement systems described herein. 
     Sensors  11  in combination at two or more locations selected from (a) inside an inflow conduit of the LVAD, (b) at a second location in the inflow conduit, (c) inside an outflow conduit, or (d) at a second location in the outflow conduit, may provide not only pressure information, but also flow information, enabling better device monitoring and control. Flow information may be inferred by differential pressure by measurement of pressures at two points in the fluid flow path. 
     Flow through the aorta  15  has a contribution not only from that provided by the LVAD  10 , but also fluid from the intact heart  15  pumping in parallel. Thus, a sensor  11  (e) at a location within the aorta, external to the LVAD outflow conduit, and/or (f) at a second location within the aorta  15 , in conjunction with one or more sensors  11  in any of locations (a), (b), (c) or (d), may be used to infer how much load sharing is being provided by the LVAD  10 . Load sharing is a measure of how much work the patient&#39;s heart  15  is providing compared to that of the LVAD  10 . This flow information is useful not only on a real-time basis, but may be valuable for obtaining trend data for the purpose of guiding therapy directed at affecting cardiac recovery. 
     Regarding manufacture of the pressure-sensing LVAD  10 , biocompatible adhesive can be used to fix the sensors  11  at locations within the LVAD  10 . Alternatively, the sensors  11  may be anchored in desired locations via mechanical means such as hooks, barbs or wire basket or stent-like devices. Many other methods of incorporation are feasible and should be evident to one skilled in the art. 
     Thus, cardiac assist devices incorporating sensors, including pressure sensors, and related methods have been disclosed. It is to be understood that the above-described embodiments are merely illustrative of some of the many specific embodiments that represent applications of the principles discussed above. Clearly, numerous and other arrangements, including other types of sensors, can be readily devised by those skilled in the art without departing from the scope of the invention.