Patent Publication Number: US-11395910-B2

Title: Passive pump

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part application of: 
     U.S. patent application Ser. No. 16/879,164 to Gross, filed May 20, 2020, entitled, “Passive pump,” and 
     PCT Patent Application PCT/IL2021/050567 to Gross, filed May 18, 2021, entitled, “Passive pump,” which claims the priority from and is a continuation application of U.S. patent application Ser. No. 16/879,164 to Gross, filed May 20, 2020, entitled, “Passive pump.” 
     Each of these applications is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to heart repair. More specifically, the present invention relates to a method of repairing a failing heart utilizing a passive device which assists a ventricle of the heart. 
     BACKGROUND OF THE INVENTION 
     Heart failure is a condition in which the heart cannot pump enough blood to meet the body&#39;s needs. In some cases, the heart cannot fill with enough blood. In other cases, the heart cannot pump blood to the rest of the body with enough force. 
     Heart failure develops over time as the heart&#39;s pumping action grows weaker. The condition can affect one or both sides of the heart. 
     Systolic heart failure occurs when the contraction of the muscle wall of the left ventricle malfunctions, which compromises its pumping action. This causes a decrease in the ejection fraction below the normal range, and gradually, left ventricular remodeling occurs in the cardiac tissue which causes enlargement of the ventricle. The remodeling manifests as gradual increases in left ventricular end-diastolic and end-systolic volumes, wall thinning, and a change in chamber geometry to a more spherical, less elongated shape. This process is usually associated with a continuous decline in ejection fraction. In general, left-sided heart failure leaves the heart unable to pump enough blood into the circulation to meet the body&#39;s demands, and increasing pressure within the heart causes blood to back up in the pulmonary circulation, producing congestion. 
     SUMMARY OF THE INVENTION 
     In some applications of the present invention, apparatus is provided that is implantable at a heart of a patient and facilitates cyclical moving of fluid that is not blood of the patient into and out of a ventricle of the heart. That is, during ventricular diastole, a volume of the fluid is moved into the ventricle in a manner that produces a corresponding decrease in a total volume of blood that fills the ventricle during diastole. During ventricular systole, the volume of the fluid is moved out of the heart in a manner that produces a corresponding decrease in a total volume of the ventricle during isovolumetric contraction of the ventricle. In such a manner, the pressure rise in the ventricle occurs at a lower volume than the pressure rise would otherwise occur in a heart of a healthy subject. Typically, the fluid in the passive pump is moved between components of the passive pump responsively to pressure increases and decreases associated with stages of the cardiac cycle. Therefore, the pump is considered passive. The passive pump is typically not operably connected to machinery or circuitry which helps facilitate movement of the fluid between the components of the pump. 
     The apparatus typically comprises (a) a bag that is noncompliant, (b) a compliant balloon, (c) a conduit disposed between and in fluid communication with the bag and the compliant balloon, and (d) a volume of fluid disposed within an inner space defined by the apparatus. The volume of fluid is passable between the bag and the compliant balloon via the conduit passively and responsively to changes in pressure associated with respective stages of the cardiac cycle. 
     Except where indicated to the contrary, applications of the present invention described as utilizing a “fluid” may be implemented using either a liquid or a gas. 
     For some applications of the present invention, the apparatus comprises a second bag that is noncompliant, instead of the compliant balloon. In this case, a spring is typically, but not necessarily, coupled to the second bag to facilitate ejecting of the fluid from within the second bag. 
     Typically, the bag is positionable within the ventricle (e.g., the left ventricle or the right ventricle), and the compliant balloon, or second bag, is positionable outside the ventricle (e.g., in the right atrium, in the superior vena cava, in the inferior vena cava, or any suitable place outside of the heart). 
     For some applications, the bag is positionable within the ventricle, the compliant balloon is positionable outside of the heart, and the conduit is disposed transmyocardially. The compliant balloon is configured to expand upon transfer of the fluid into the balloon from the bag, and to contract upon passage of at least part of the fluid out of the balloon. During ventricular diastole, the balloon contracts and expels the fluid, through the conduit, into the bag within the ventricle. During ventricular systole, while the aortic valve of the heart is closed, the left ventricle contracts, causing a volume of the fluid to be expelled from the bag, through the conduit, and into the balloon, in a manner that produces a corresponding decrease in a total volume of the ventricle during isovolumetric contraction of the ventricle. It is advantageous to reduce the volume of the ventricle during isovolumetric contraction of the ventricle in order to facilitate reverse remodeling of the ventricle, such that the heart returns to a healthy geometry. Typically, this is accomplished since this application of the invention reduces the extent to which the ventricular wall is stretched at high pressure. This yields acute and chronic reduction in ventricular wall stress, which the inventor hypothesizes will result (typically over the course of months) in the desired reverse remodeling described hereinabove. 
     For some applications, the bag is positionable within the left ventricle of the patient while the compliant balloon or second bag is positionable outside of the left ventricle. For example, the compliant balloon or second bag may be positionable in the right atrium, the inferior vena cava, or the superior vena cava. 
     For some applications of the present invention, the passive pump comprises first and second noncompliant bags. A spring is coupled to one of the bags that is designated for positioning outside of the ventricle. The spring absorbs energy during filling of the bag, e.g., typically during ventricular systole, and releases the energy in order to expel the fluid from within the bag, e.g., during ventricular diastole, and into the other bag that is disposed within the ventricle. 
     There is therefore provided, in accordance with an application of the present invention apparatus, including: 
     a flexible intraventricular receptacle configured to be positioned within a ventricle of a heart of a patient, the flexible intraventricular receptacle being configured to assume a first volume upon passage of fluid that is not blood into the flexible intraventricular receptacle and a second volume upon passage of at least part of the fluid out of the flexible intraventricular receptacle, the second volume being smaller than the first volume; 
     an expandable extraventricular receptacle configured to be positioned outside of the ventricle, the expandable extraventricular receptacle being configured to expand upon transfer of the fluid into the expandable extraventricular receptacle from the intraventricular receptacle and to contract upon passage of at least part of the fluid out of the expandable extraventricular receptacle; and 
     a conduit disposed between and in fluid communication with the flexible intraventricular receptacle and the expandable extraventricular receptacle, the conduit being configured to allow passage of the fluid between the intraventricular and the extraventricular receptacles, 
     the apparatus is configured such that when the intraventricular receptacle is disposed within the ventricle, the extraventricular receptacle is disposed outside of the ventricle, the apparatus is configured to facilitate the passage of the fluid between the intraventricular and the extraventricular receptacles responsively to a cardiac cycle of the heart, in a manner in which:
         during ventricular diastole, the extraventricular receptacle contracts and expels the fluid, through the conduit, into the intraventricular receptacle, and   during ventricular systole, while an aortic valve of the heart is closed, a volume of the fluid is expelled from the intraventricular receptacle, through the conduit, into the extraventricular receptacle, in a manner that produces a corresponding decrease in a total volume of the ventricle during isovolumetric contraction of the ventricle, and       

     the apparatus further includes an energy-storage element coupled to the expandable extraventricular receptacle and configured to:
         absorb energy upon filling of the expandable extraventricular receptacle from a first state to a second, expanded state, and   release the energy to return the expandable extraventricular receptacle from the second, expanded state to the first state.       

     In an application, the extraventricular receptacle is configured to be positioned at an extracardiac location, and the conduit defines a transmyocardial conduit configured to be disposed passing through a wall of the heart. 
     In an application, the extraventricular receptacle includes a bellows, and the energy-storage element is coupled to the bellows in a manner in which the bellows is disposed between the intraventricular receptacle and the energy-storage element. 
     In an application, the energy-storage element is configured to: 
     assume a compressed, longitudinally-shortened state upon the filling of the bellows from the first state to the second, expanded state, and 
     return the bellows from the second, expanded state to the first state. 
     In an application, the energy-storage element includes a coil spring. 
     In an application, the energy-storage element includes a shape-memory material. 
     In an application, the apparatus includes a scaffolding surrounding the bellows and the energy-storing element, and the energy-storage element is coupled at a first end of the energy-storage element to an outer surface of the bellows, and at a second end of the energy-storage element to a portion of the scaffolding, during absorbing and releasing of the energy by the energy-storage element, the first end of the energy-storage element moves, respectively, toward and away from the second end of the energy-storage element. 
     In an application, the extraventricular receptacle includes a bellows, and the energy-storage element is coupled to the bellows by surrounding the bellows at least in part. 
     In an application, the energy-storage element includes a coil spring that at least partially surrounds the bellows and is configured to: 
     assume a longitudinally-elongated state upon the filling of the bellows from the first state to the second, expanded state, and 
     return the bellows from the second, expanded state to the first state. 
     In an application, the coil spring wraps around the bellows between folds of the bellows. 
     In an application, the energy-storage element includes a plurality of struts surrounding the extraventricular receptacle, a respective portion of each of the struts is configured to: 
     assume a longitudinally-elongated state upon the filling of the expandable extraventricular receptacle from the first state to the second, expanded state, 
     assume a longitudinally-shortened state, and 
     return the expandable extraventricular receptacle from the second, expanded state to the first state during a transition of the energy-storage element from the longitudinally-elongated state toward the longitudinally-shortened state. 
     In an application, in the longitudinally-shortened state, the respective portion of each of the struts is shaped so as to define a bend which projects laterally away from a central longitudinal axis of the apparatus. 
     In an application, each strut extends longitudinally along an external surface of the extraventricular receptable in a direction (1) from a portion of the extraventricular receptable that is furthest from the intraventricular receptable, and toward (2) the intraventricular receptable. 
     In an application, the apparatus includes a stent structure, and a first portion of the stent structure surrounds the conduit, and the energy-storage element defines a second portion of the stent structure that surround the extraventricular receptacle. 
     In an application: 
     a part of the first portion of the stent structure surrounds a portion of the intraventricular receptacle, 
     the stent structure is (i) compressed during delivery of the apparatus into a body of a patient, and (ii) expandable during implantation of the apparatus in the body of the patient, and 
     once expanded, the first portion of the stent structure is shaped so as to define (1) a narrowed portion at the conduit and (2) a wider portion than the narrowed portion, at the part of the first portion of the stent structure that surrounds the portion of the intraventricular receptacle. 
     In an application, the energy-storage element includes a mesh having a first portion surrounding the extraventricular receptacle, the first portion of the mesh is configured to: 
     absorb the energy upon the filling of the expandable extraventricular receptacle from the first state to the second, expanded state, and 
     release the energy to return the expandable extraventricular receptacle from the second, expanded state to the first state. 
     In an application, the apparatus is configured such that as the first portion of the mesh releases the energy, the first portion of the mesh transitions toward a longitudinally-shortened state in which the first portion of the mesh expands radially and shortens longitudinally, with respect to a central longitudinal axis of the apparatus. 
     In an application, a second portion of the mesh surrounds the conduit, and a third portion of the mesh surrounds a portion of the intraventricular receptacle. 
     In an application: 
     the mesh is (i) compressed during delivery of the apparatus into a body of a patient, and (ii) expandable during implantation of the apparatus in the body of the patient, and 
     once expanded, the second portion of the mesh is shaped so as to define a narrowed portion at the conduit and the third portion of the mesh is shaped so as to define a wider portion than the narrowed portion, at the portion of the intraventricular receptacle. 
     In an application, the apparatus further includes the fluid, and the fluid has a volume of 10-80 ml which is passable between the flexible intraventricular receptacle and the expandable extraventricular receptacle via the conduit. 
     In an application, the intraventricular receptacle is an intra-left-ventricular receptacle. 
     In an application, the apparatus further includes a stent structure, and the stent structure surrounds the conduit. 
     In an application, the apparatus further includes a scaffolding disposed within the intraventricular receptacle, the scaffolding being configured to prevent dislodging of the intraventricular receptacle from within the ventricle. 
     In an application, the apparatus further includes a rod disposed within the intraventricular receptacle, the rod being configured to prevent dislodging of the intraventricular receptacle from within the ventricle. 
     In an application, the expandable extraventricular receptacle is compliant. 
     In an application, wall compliance of the expandable extraventricular receptacle is at least three times wall compliance of the flexible intraventricular receptacle. 
     In an application, the expandable extraventricular receptacle and the flexible intraventricular receptacle are configured such that, in the absence of any external forces applied to the expandable extraventricular receptacle and the flexible intraventricular receptacle, (a) the expandable extraventricular receptacle undergoes an increase in volume when exposed to a change in internal pressure from 10 mmHg to 120 mmHg that is at least three times greater than (b) an increase in volume that the flexible intraventricular receptacle undergoes when exposed to a change in internal pressure from 10 mmHg to 120 mmHg. 
     In an application, the expandable extraventricular receptacle and the flexible intraventricular receptacle are configured such that, in the absence of any external forces applied to the expandable extraventricular receptacle and the flexible intraventricular receptacle, (a) the expandable extraventricular receptacle undergoes an increase in volume when exposed to a change in internal pressure from 10 mmHg to 120 mmHg that is at least 200%, and (b) the flexible intraventricular receptacle undergoes an increase in volume when exposed to a change in internal pressure from 10 mmHg to 120 mmHg that is less than 120%. 
     There is also provided, in accordance with an application of the present invention a method for repairing a heart, including: 
     identifying a heart of a patient as having a reduced ejection fraction; and 
     in response to the identifying, reducing wall stress of a ventricle of the heart by implanting apparatus that facilitates cyclical moving of fluid that is not blood of the patient into and out of the ventricle of the heart, the moving including:
         during ventricular diastole, moving a volume of the fluid into the ventricle in a manner that produces a corresponding decrease in a total volume of blood that fills the ventricle during diastole; and   during ventricular systole, moving the volume of the fluid out of the ventricle in a manner that produces a corresponding decrease in a total volume of the ventricle during isovolumetric contraction of the ventricle.       

     In an application, implanting the apparatus includes reducing a volume of blood expelled by the ventricle during systole. 
     In an application, moving the volume of the fluid out of the ventricle includes moving the volume of the fluid out of the heart. 
     In an application, implanting the apparatus includes implanting apparatus including a bag, a compliant balloon and a conduit disposed between and in fluid communication with the bag and the compliant balloon, in a manner in which (1) the bag is disposed within the ventricle, and (2) the compliant balloon is disposed outside the ventricle. 
     In an application, wall compliance of the balloon is at least three times wall compliance of the bag. 
     In an application, the bag is noncompliant. 
     In an application, implanting the apparatus includes positioning the bag in a left ventricle. 
     In an application, implanting the apparatus includes positioning the bag in a right ventricle. 
     In an application, implanting the apparatus includes: 
     positioning the balloon at an extracardiac space, and 
     positioning the conduit transmyocardially. 
     In an application, implanting the apparatus includes positioning the balloon in a right atrium, and the method further includes implanting the apparatus in a manner in which the conduit extends from the left ventricle to the right atrium. 
     In an application, implanting the apparatus includes positioning the balloon in a superior vena cava, and the method further includes implanting the apparatus in a manner in which the conduit extends from the left ventricle to the superior vena cava. 
     In an application, implanting the apparatus includes positioning the balloon in an inferior vena cava, and the method further includes implanting the apparatus in a manner in which the conduit extends from the left ventricle to the inferior vena cava. 
     In an application, implanting apparatus includes implanting apparatus including a first bag, a second bag and a conduit disposed between and in fluid communication with the first bag and the second bag, in a manner in which (1) the first bag is disposed within the ventricle, and (2) the second bag is disposed outside the ventricle. 
     In an application, the first and second bags are noncompliant. 
     In an application, implanting the apparatus includes positioning the first bag in a left ventricle. 
     In an application, implanting the apparatus includes positioning the first bag in a right ventricle. 
     In an application, implanting the apparatus includes: 
     positioning the second bag at an extracardiac space, and 
     positioning the conduit transmyocardially. 
     In an application, implanting the apparatus includes positioning the second bag in a right atrium, and the method further includes implanting the apparatus in a manner in which the conduit extends from the left ventricle to the right atrium. 
     In an application, implanting the apparatus includes positioning the second bag in a superior vena cava, and the method further includes implanting the apparatus in a manner in which the conduit extends from the left ventricle to the superior vena cava. 
     In an application, implanting the apparatus includes positioning the second bag in an inferior vena cava, and the method further includes implanting the apparatus in a manner in which the conduit extends from the left ventricle to the inferior vena cava. 
     In an application, the apparatus includes an energy-storage element coupled to the second bag and configured to:
         absorb energy upon filling of the second bag from a first state to a second, expanded state, and   release the energy to return the second bag from the second, expanded state to the first state.       

     In an application, the energy-storage element includes a plurality of struts surrounding the second bag, a respective portion of each of the struts is configured to: 
     assume a longitudinally-elongated state upon the filling of the second bag from the first state to the second, expanded state, 
     assume a longitudinally-shortened state, and 
     return the second bag from the second, expanded state to the first state during a transition of the energy-storage element from the longitudinally-elongated state toward the longitudinally-shortened state. 
     In an application, the energy-storage element includes a mesh having a first portion surrounding the second bag, the first portion of the mesh is configured to: 
     absorb the energy upon the filling of the second bag from the first state to the second, expanded state, and 
     release the energy to return the second bag from the second, expanded state to the first state. 
     In an application, the second bag includes a bellows, and the energy-storage element is coupled to the bellows. 
     In an application, implanting the apparatus includes acutely further reducing the ejection fraction and chronically increasing the ejection fraction. 
     There is further provided, in accordance with an application of the present invention apparatus, including: 
     an intraventricular bag configured to be positioned within a ventricle of a heart of a patient, the bag having, in the absence of any external forces applied thereto: (a) a first intraventricular bag volume when the bag has an internal pressure of 120 mmHg, and (b) a second intraventricular bag volume when the bag has an internal pressure of 10 mmHg, the first intraventricular bag volume being less than 110% of the second intraventricular bag volume; 
     an extraventricular bag configured to be positioned outside of the ventricle, the extraventricular bag having, in the absence of any external forces applied thereto: (a) a first extraventricular bag volume when the extraventricular bag has an internal pressure of 120 mmHg, and (b) a second extraventricular bag volume when the extraventricular bag has an internal pressure of 10 mmHg, the first extraventricular bag volume being at least 200% of the second extraventricular bag volume; 
     a conduit disposed between and in fluid communication with the intraventricular bag and the extraventricular bag, the apparatus thereby defining a total internal space disposed within the conduit, the intraventricular bag, and the extraventricular bag; and 
     disposed within the internal space, 10-80 ml of fluid that is (a) not blood and (2) passable between the intraventricular bag and the extraventricular bag via the conduit, 
     the apparatus is configured such that:
         during ventricular diastole, the extraventricular bag expels the fluid, through the conduit, and into the intraventricular bag, and   during ventricular systole, while an aortic valve of the heart is closed, a volume of the fluid is expelled from the intraventricular bag, through the conduit, into the extraventricular bag, and       

     the apparatus further includes an energy-storage element coupled to the extraventricular bag and configured to:
         absorb energy upon filling of the extraventricular bag from a first state to a second, expanded state, and   release the energy to return the extraventricular bag from the second, expanded state to the first state.       

     In an application, the extraventricular receptacle is configured to be positioned at an extracardiac location, and the conduit defines a transmyocardial conduit configured to be disposed passing through a wall of the heart. 
     In an application, the energy-storage element includes a plurality of struts surrounding the extraventricular bag, a respective portion of each of the struts is configured to: 
     assume a longitudinally-elongated state upon the filling of the extraventricular bag from the first state to the second, expanded state, 
     assume a longitudinally-shortened state, and 
     return the extraventricular bag from the second, expanded state to the first state during a transition of the energy-storage element from the longitudinally-elongated state toward the longitudinally-shortened state. 
     In an application, the energy-storage element includes a mesh having a first portion surrounding the extraventricular bag, the first portion of the mesh is configured to: 
     absorb the energy upon the filling of the extraventricular bag from the first state to the second, expanded state, and 
     release the energy to return the extraventricular bag from the second, expanded state to the first state. 
     In an application, the extraventricular bag includes a bellows, and the energy-storage element is coupled to the bellows in a manner in which the bellows is disposed between the first bag and the energy-storage element. 
     In an application, the energy-storage element is configured to: 
     assume a compressed, longitudinally-shortened state upon the filling of the bellows from the first state to the second, expanded state, and 
     return the bellows from the second, expanded state to the first state. 
     In an application, the energy-storage element includes a coil spring. 
     In an application, the extraventricular bag includes a bellows, and the energy-storage element is coupled to the bellows by surrounding the bellows at least in part. 
     In an application, the energy-storage element includes a coil spring that at least partially surrounds the bellows and is configured to: 
     assume a longitudinally-elongated state upon the filling of the bellows from the first state to the second, expanded state, and 
     return the bellows from the second, expanded state to the first state. 
     In an application, the coil spring wraps around the bellows between folds of the bellows. 
     There is also provided, in accordance with an application of the present invention apparatus, including: 
     a first bag having, in the absence of any external forces applied thereto: (a) a first first-bag volume when the first bag has an internal pressure of 120 mmHg, and (b) a second first-bag volume when the first bag has an internal pressure of 10 mmHg, the first first-bag volume being less than 110% of the second first-bag volume; 
     a second bag having, in the absence of any external forces applied thereto: (a) a first second-bag volume when the second bag has an internal pressure of 120 mmHg, and (b) a second second-bag volume when the second bag has an internal pressure of 10 mmHg, the first second-bag volume being less than 110% of the second second-bag volume; 
     an energy-storage element coupled to the second bag and configured to:
         absorb energy upon filling of the second bag from the first second-bag volume to the second second-bag volume, and   release the energy to return the second bag from the second second-bag volume to the first second-bag volume;       

     a conduit disposed between and in fluid communication with the first bag and the second bag, the apparatus thereby defining a total internal space disposed within the conduit, the first bag, and the second bag; and 
     disposed within the internal space, 10-80 ml of fluid that is (a) not blood and (b) passable between the first bag and the second bag via the conduit. 
     In an application, the second bag is configured to be positioned at an extracardiac location, and the conduit defines a transmyocardial conduit configured to be disposed passing through a wall of the heart. 
     In an application, the energy-storage element includes a plurality of struts surrounding the second bag, a respective portion of each of the struts is configured to: 
     assume a longitudinally-elongated state upon the filling of the second bag from the first state to the second, expanded state, 
     assume a longitudinally-shortened state, and 
     return the second bag from the second, expanded state to the first state during a transition of the energy-storage element from the longitudinally-elongated state toward the longitudinally-shortened state. 
     In an application, the energy-storage element includes a mesh having a first portion surrounding the second bag, the first portion of the mesh is configured to: 
     absorb the energy upon the filling of the second bag from the first state to the second, expanded state, and 
     release the energy to return the second bag from the second, expanded state to the first state. 
     In an application, the second bag includes a bellows, and the energy-storage element is coupled to the bellows in a manner in which the bellows is disposed between the first bag and the energy-storage element. 
     In an application, the energy-storage element is configured to: 
     assume a compressed, longitudinally-shortened state upon the filling of the bellows from the first state to the second, expanded state, and 
     return the bellows from the second, expanded state to the first state. 
     In an application, the energy-storage element includes a coil spring. 
     In an application, the second bag includes a bellows, and the energy-storage element is coupled to the bellows by surrounding the bellows at least in part. 
     In an application, the energy-storage element includes a coil spring that at least partially surrounds the bellows and is configured to: 
     assume a longitudinally-elongated state upon the filling of the bellows from the first state to the second, expanded state, and 
     return the bellows from the second, expanded state to the first state. 
     In an application, the coil spring wraps around the bellows between folds of the bellows. 
     The present invention will be more fully understood from the following detailed description of applications thereof, taken together with the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-B  are schematic illustrations of a passive pump comprising a noncompliant bag and a compliant balloon, in accordance with respective applications of the present invention; 
         FIG. 2  is a schematic illustration of implantation of the passive pump, in accordance with some applications of the present invention; 
         FIGS. 3A-G  are schematic illustrations of the operation of the passive pump, in accordance with some applications of the present invention; 
         FIG. 4A  is a graph and  FIG. 4B  is a table representing the association between the cardiac cycle and operation of the passive pump as shown in  FIGS. 3A-G , in accordance with some applications of the present invention; 
         FIG. 5  is a schematic illustration of a port system connected to the passive pump, in accordance with some applications of the present invention; 
         FIGS. 6A-B  are schematic illustrations of a passive pump comprising two noncompliant bags and a spring, in accordance with some applications of the present invention; 
         FIGS. 7A-B ,  8 , and  9  are schematic illustrations of the operation of a passive pump, in accordance with respective applications of the present invention; 
         FIG. 10  is a pressure/volume graph illustrating the respective pressure/volume loops of a failing heart, a healthy heart, and a heart with the passive pump, in accordance with some applications of the present invention; 
         FIG. 11  is a schematic illustration of the operation of a passive pump, in accordance with some applications of the present invention; 
         FIGS. 12A-B  are schematic illustrations of a passive pump comprising two noncompliant bags and a spring comprising a plurality of struts, in accordance with some applications of the present invention; 
         FIGS. 13A-B  are schematic illustrations of a passive pump comprising two noncompliant bags and braided mesh, in accordance with some applications of the present invention; 
         FIGS. 14A-B  are schematic illustrations of a passive pump comprising two noncompliant bags and bellows coupled to a spring, in accordance with some applications of the present invention; and 
         FIGS. 15A-B  are schematic illustrations of a passive pump comprising two noncompliant bags and bellows surrounded by a spring, in accordance with some applications of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference is now made to  FIG. 1A , which is a schematic illustration of a system  20  comprising a passive pump  21  which comprises a noncompliant bag  22  and a compliant balloon  24 , in accordance with some applications of the present invention. A conduit  26  is disposed between and in fluid communication with bag  22  and compliant balloon  24 . Passive pump  21  defines a total internal space disposed within conduit  26 , bag  22 , and compliant balloon  24 . Fluid is disposed within the internal space and is passable between bag  22  and compliant balloon  24  via conduit  26 . Typically, the internal space contains 10-80 ml of fluid, e.g., 20-40 ml of fluid. Typically, the fluid comprises fluid that is not blood of the patient. For some applications of the present invention, the fluid comprises a gas, such as carbon dioxide, or a liquid, such as saline. Alternatively or additionally, the fluid comprises a reduced-osmolarity fluid, e.g., a contrast-agent fluid which is used for imaging and is known to be acceptable for use in contact with blood of a patient. (It is expected that in general, there will be no contact between the fluid and the patient&#39;s blood.) 
     Bag  22  comprises a noncompliant, biocompatible material, e.g., polyethylene terephthalate (PET). Balloon  24  comprises a compliant, biocompatible material, e.g., polyolefin copolymer (POC), silicone, or polyurethane. For some applications of the present invention, wall compliance of the balloon  24  is at least three times wall compliance of bag  22 . 
     Passive pump  21  is configured for implantation at a heart of a patient. Typically, bag  22  is designated for positioning within a ventricle of the patient. For some applications, bag  22  is designated for positioning within a left ventricle of the heart of the patient. Alternatively, bag  22  is designated for positioning within a right ventricle of the heart of the patient. Thus, bag  22  defines a flexible, intraventricular receptacle. Balloon  24  is designated for positioning outside of the ventricle. For some applications, balloon  24  is designated for positioning outside of the heart of the patient. In such applications, balloon  24  defines an expandable extracardiac receptacle, and conduit  26  defines a transmyocardial conduit disposed between and in fluid communication with the flexible intraventricular receptacle (e.g., bag  22 ) and the expandable extracardiac receptacle (e.g., balloon  24 ). For applications in which conduit  26  is positioned transmyocardially, as shown in  FIGS. 2, 3A -G, and  5 , conduit  26  typically has an inner diameter of 6-10 mm, and a length of 2-5 cm, e.g., 3 cm. 
     For some applications, balloon  24  is designated for positioning outside of the ventricle of the heart of the patient (e.g., in the right atrium as shown in  FIGS. 7A-B , for example, in the superior vena cava, as shown in  FIG. 8 , for example, or in the inferior vena cava, as shown in  FIG. 9 , for example). In such applications, balloon  24  defines an expandable extraventricular receptacle, and conduit  26  defines a conduit disposed between and in fluid communication with the flexible intraventricular receptacle (e.g., bag  22 ) and the expandable extraventricular receptacle (e.g., balloon  24 ). 
     For some applications, balloon  24  is configured to be positioned at a site within the patient&#39;s vascular system, e.g., in a right atrium of the heart of the patient, or in the patient&#39;s superior vena cava or inferior vena cava, as shown in  FIGS. 7A-B ,  8  and  9 . 
     That is, for generally all applications of the present invention, balloon  24  of  FIGS. 1A-3G and 5  defines an extraventricular receptacle. 
     Reference is now made to  FIGS. 1A-B ,  2 ,  3 A-G,  4 A-B,  5 ,  6 A-B,  7 A-B,  8 - 11 ,  12 A-B,  13 A-B,  14 A-B, and  15 A-B. Passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  are configured for facilitating reverse remodeling in the heart of a patient experiencing heart failure. Passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  are configured, during ventricular systole, while an aortic valve of the heart is closed, for enabling passage of fluid that is not blood of the patient from within the ventricle to outside of the ventricle in a manner that produces a corresponding decrease in a total volume of the ventricle during isovolumetric contraction of the ventricle. That is, for example, since passive pump  21  is passive and bag  22  is noncompliant, some volume of the fluid exits the ventricle by exiting bag  22  during ventricular diastolic filling, such that pressure in the heart begins to rise at a lower total volume of the ventricle at the onset of isovolumetric contraction of the ventricle. Ultimately, passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  enable reverse remodeling of the heart because the left ventricle does not undergo as high wall stress while containing a high volume of blood, at the onset of and/or during isovolumetric contraction of the ventricle, as would occur in the absence of the applications of the present invention described herein. 
     Passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  are passive in that their respective systems  20 ,  40 ,  60 ,  90 ,  100 ,  120 ,  180 ,  200 ,  300 ,  400 , and  500  do not require any electrical or other source of power for the passive pumps to move the fluid within the pump. Additionally, passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  do not facilitate acute therapeutic motion of the ventricular wall, and do not acutely therapeutically push the ventricular wall, myocardium and/or the epicardium with a force originated due to energy harvested by the system from a motion of the ventricular chamber. Rather, passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  function responsively to a cardiac cycle of the heart in a manner in which, for example:
         at the onset of and during ventricular diastole, due to decreased pressure in the left ventricle, the extraventricular receptacle (e.g., balloon  24 ) contracts (as shown in the upper figure in  FIG. 1A , for example) and begins to expel the fluid, through the conduit (e.g., the transmyocardial conduit, for example conduit  26 ), into the intraventricular receptacle (e.g., bag  22 ), and   during ventricular systole, and even slightly before, while the aortic valve of the heart is closed, a volume of the fluid is expelled from the intraventricular receptacle (e.g., bag  22 ), through the conduit (e.g., the transmyocardial conduit, for example conduit  26 ), into the extraventricular receptacle (e.g., balloon  24 ), in a manner that produces a corresponding decrease in a total volume of the ventricle during isovolumetric contraction of the ventricle.       

     Thus, due to the cyclical moving of fluid that is not blood into and out of the ventricle, passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  cause acute as well as chronic reduction in wall stress of cardiac muscle surrounding the ventricle at the onset of ventricular systole. Additionally, for some embodiments of the present invention, passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  also cause the ejection fraction of the heart and the cardiac output to be (1) even further acutely reduced in patients that have been identified as already having a reduced ejection fraction and cardiac output, and (2) chronically increased. At the onset of and during ventricular diastole, systems  20 ,  40 ,  60 ,  90 ,  100 ,  120 ,  180 ,  200 ,  300 ,  400 , and  500  enable the moving of a volume of the fluid within passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  into the ventricle in a manner that produces a corresponding decrease in a total volume of blood that fills the ventricle during diastole. During ventricular systole and even slightly before, systems  20 ,  40 ,  60 ,  90 ,  100 ,  120 ,  180 ,  200 ,  300 ,  400 , and  500  enable moving the volume of the fluid within the pump out of the ventricle in a manner that produces a corresponding decrease in a total volume of the ventricle during isovolumetric contraction of the ventricle. 
       FIG. 1A  shows passive pump  21  in a state in which fluid is distributed in a manner in which fluid is passed into bag  22  such that bag  22  assumes a larger volume (upper image) than when the fluid is passed out of bag  22  and into balloon  24  (lower image) so that bag  22  assumes a lower volume. As fluid passes out of bag  22  and into balloon  24 , balloon  24  fills and/or expands to assume a larger volume (lower image) than when less fluid is disposed within balloon  24  (upper image). Since balloon  24  is compliant, it contracts to expel fluid disposed therein. Typically, one or more of the following numerical characteristics applies to bag  22  and balloon  24 :
         In the absence of external force applied to balloon  24  (e.g., the expandable extraventricular receptacle) and bag  22  (e.g., the flexible intraventricular receptacle), balloon  24  typically undergoes an increase in volume when exposed to a change in internal pressure from 10 mmHg to 120 mmHg that is at least three (e.g., at least five) times greater than any increase in volume that bag  22  undergoes when exposed to a change in internal pressure from 10 mmHg to 120 mmHg   In the absence of any external forces applied to balloon  24  (e.g., expandable extraventricular receptacle) and to bag  22  (e.g., the flexible intraventricular receptacle), (a) balloon  24  undergoes an increase in volume when exposed to a change in internal pressure from 10 mmHg to 120 mmHg that is at least 200%, and (b) (i) the volume of bag  22  having 120 mmHg internal pressure is less than (ii) a volume that is greater than 20% more than the volume of bag  22  having 10 mmHg internal pressure (and is for some applications substantially the same as the volume of bag  22  having 10 mmHg internal pressure).   In the absence of any external forces applied to bag  22 , (a) a first bag volume of bag  22  is 10-80 ml (e.g., 40 ml) when the intraventricular receptacle has an internal pressure of 120 mmHg, and (b) a second bag volume of bag  22  is 10-80 ml (e.g., 40 ml) when the bag has an internal pressure of 10 mmHg. Additionally, the first bag volume is less than 110% of the second bag volume. The second bag volume is typically within 10% of the first bag volume, e.g., the first and second bag volumes are substantially the same, because bag  22  is non-compliant.   In the absence of any external forces applied to balloon  24 , balloon  24  has (a) a first balloon volume of at least 30 ml, e.g., 80 ml, when balloon  24  has an internal pressure of 120 mmHg, and (b) a second balloon volume of at least 10 ml, e.g., 20 ml, when balloon  24  has an internal pressure of 10 mmHg.       

     It is to be noted that the passage of a given volume of fluid out of bag  22  corresponds to a similar or identical passage of fluid into balloon  24  and vice versa. That is, a volume increase in one receptacle substantially corresponds to a volume decrease in another receptacle. 
     Since bag  22  is configured for positioning within the ventricle, bag  22  is subjected to high pressure from the ventricle. As such, a fixation rod  28  is typically disposed within bag  22  which reinforces bag  22  and prevents everting and/or migration of bag  22  out of the ventricle. For some applications of the present invention, rod  28  is part of a scaffolding  29  disposed within bag  22 . For some applications of the present invention, rod  28  prevents everting of bag  22  through the transmyocardial access point of passive pump  21  to the ventricle. For some applications in which balloon  24  is positioned outside of the heart, balloon  24  may be surrounded by an optional cage (not shown). The cage may help protect balloon  24  by encasing balloon  24  or it may help facilitate expansion of balloon  24  by providing a defined space in which balloon  24  is allowed to expand. 
     Conduit  26  is reinforced, e.g., by being surrounded or internally lined, by a stent structure  30 . Structure  30  comprises a central tubular substructure  36 , a first flared section  32  which is configured to surround a portion of bag  22 , and a second flared section  34  which is configured to surround a portion of balloon  24 . 
     For applications in which conduit  26  is configured to be positioned within tissue of the patient, e.g., within myocardial tissue, conduit  26  comprises a tube surrounded by porous material, e.g., a fabric, which facilitates tissue growth around conduit  26  in order to enable sealing of conduit  26  and inhibit leakage of blood out of the ventricle. For some applications of the present invention, conduit  26  self-expands to position itself within the tissue of the patient. 
     Reference is now made to  FIG. 1B , which is a schematic illustration of a system  40  comprising a passive pump  42  which comprises a noncompliant bag  22  and a compliant balloon  24 , in accordance with some applications of the present invention. A conduit  26  is disposed between and in fluid communication with bag  22  and compliant balloon  24 . Passive pump  42  defines a total internal space disposed within conduit  26 , bag  22 , and compliant balloon  24 . Fluid is disposed within the internal space and is passable between bag  22  and compliant balloon  24  via conduit  26 . It is to be noted that system  40  is similar to and used in a similar fashion as system  20  described hereinabove with reference to  FIG. 1A , like reference numbers referring to like parts, with the exception that passive pump  42  does not comprise rod  28  and scaffolding  29 . 
       FIG. 2  is a schematic illustration of implantation passive pump  21  of  FIG. 1A , in accordance with some applications of the present invention. Typically, passive pump  21  is delivered transapically for applications in which passive pump  21  is configured to be positioned transmyocardially such that bag  22  is positioned within a ventricle  50  while balloon  24  is positioned at an extracardiac location. For some applications, passive pump  21  is delivered using a sub-xyphoid approach. Passive pump  21  is delivered to the heart in a compressed state within a delivery tool  58 . A distal portion of delivery tool  58  is passed through myocardial tissue  53  at apex  51  of the heart. Delivery tool  58  enables bag  22  to be positioned within a space of ventricle  50 . Tool  58  pushes passive pump  21  distally and/or tool  58  is retracted proximally in order to expose flared section  32  of stent structure  30  such that flared section  32  expands against and engages a wall of ventricle  50 . For some applications, flared section  32  comprises a biocompatible material is coated with a porous material, e.g., a fabric, which helps facilitate sealing of flared section  32  with respect to cardiac tissue by enhancing tissue growth into the outer surface of flared section  32 . Flared section  32  facilitates fixation of bag  22  in ventricle  50 . Flared section  32  is shaped so as to allow for bag  22  to fill and take shape as shown in the upper figure of  FIG. 1A . Tool  58  is then further retracted so that central tubular substructure  36  expands within and engages myocardial tissue  53 . Central tubular substructure  36  comprises a biocompatible material surrounded by porous material, e.g., a fabric, which helps facilitate sealing of central tubular substructure  36  with respect to cardiac tissue by enhancing tissue growth into substructure  36 . Tool  58  is then yet further retracted, so that flared section  34  expands against and engages the epicardium of the heart. For some applications, flared section  34  comprises a biocompatible material coated by a porous material, e.g., a fabric, which helps facilitate sealing of flared section  34  with respect to cardiac tissue by enhancing tissue growth into flared section  34 . Flared section  34  facilitates fixation of balloon  24  at the extracardiac location. Flared section  34  is shaped so as to allow for balloon  24  to fill, expand, and take shape as shown in the lower figure of  FIG. 1A . 
     Reference is now made to  FIGS. 3A-G , which are schematic illustrations of the operation of passive pump  21  of  FIG. 1A , in accordance with some applications of the present invention. Passive pump  42  operates in a like manner. 
     Reference is now made to  FIG. 4A , which is a graph and to  FIG. 4B , which is a table representing the association between the cardiac cycle and operation of passive pump  21  as shown in  FIGS. 3A-G , as well as passive pumps  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502 , mutatis mutandis, in accordance with some applications of the present invention. 
       FIG. 3A  shows a Stage A in which the heart undergoes isovolumetric relaxation while mitral valve  54  is closed and aortic valve  56  is closed. Stage A is when the heart undergoes diastole and blood has not yet entered left ventricle  50 . The volume in ventricle  50  is low and the pressure decreases. At Stage A, pressure in ventricle  50  is still sufficiently high (as shown in the near-bottom left side of the graph in  FIG. 4A ) that fluid from within passive pump  21  does not yet enter bag  22  disposed within ventricle  50  and remains within balloon  24  in its expanded state. Balloon  24  assumes a volume in its expanded state of for example 20-80 ml, e.g., 40 ml. As shown in  FIG. 3A , bag  22  assumes a very low volume and is near to empty or empty of the fluid. Rod  28  and scaffolding  29  maintain bag  22  from escaping from ventricle  50  by everting through myocardial tissue  53 . 
     Once the pressure in ventricle  50  drops to Stage B as represented in  FIG. 3B  and in the bottom left corner of the graph on  FIG. 4A , balloon  24  contracts due to its compliance and due to the reduced pressure in ventricle  50 , and expels the fluid through transmyocardial conduit  26  and into bag  22  within left ventricle  50 . Bag  22  fills to assume a greater volume than the volume it assumes in Stage A. Bag  22  fills to a volume of 20-80 ml, e.g., 40 ml. As shown in  FIG. 3B , balloon  24  has a very low volume and is near to empty or empty of the fluid. 
     Since (1) passive pump  21  is constructed in a manner in which bag  22  comprises a noncompliant material and balloon  24  is compliant, and (2) fluid passes between balloon  24  into bag  22  responsively to changes in pressure within ventricle  50 , passive pump  21  operates passively and in response to the cardiac cycle. 
     At Stage B, the heart is in diastole, and mitral valve  54  begins to open and blood begins to enter left ventricle  50  from left atrium  52 . Aortic valve  56  remains closed. Bag  22 , in its filled state, occupies space within ventricle  50  while blood of the patient fills ventricle  50 . Moving the volume of the fluid into ventricle  50 , i.e., into bag  22 , produces a corresponding decrease in a total volume of blood that fills ventricle  50  during diastole as is shown in  FIG. 3C . 
       FIG. 3C  shows Stage C, in which the diastolic phase is near completion and mitral valve  54  begins to close. Aortic valve  56  remains closed. Ventricle  50  is close to full with blood. Due to the volume of blood in ventricle  50 , some of the fluid within bag  22  is pushed into balloon  24 . At Stage C, the pressure in ventricle  50  is increased more than the pressure at Stage B. 
     Reference is now made to  FIG. 3D , which shows the heart in a stage of isovolumetric contraction at the onset of systole. Both aortic valve  56  and mitral valve  54  are closed. Once mitral valve  54  is closed, the pressure due to the volume of blood in ventricle  50  and ventricular contraction pushes the fluid from bag  22  into balloon  24 , such that bag  22  decreases in volume and balloon  24  increases in volume. Thus, pressure in ventricle  50  forces the fluid from within bag  22  through conduit  26  and into balloon  24 . Movement of the volume of fluid out of the heart and into balloon  24  produces a corresponding decrease in a total volume of ventricle  50  during isovolumetric contraction of ventricle  50 . It is advantageous to reduce the volume of ventricle  50  during isovolumetric contraction of ventricle  50  in order to facilitate reverse remodeling of the ventricle such that the heart returns to a healthy geometry. Reducing the volume of ventricle  50  increase the ventricular wall thickness during contraction, thereby reducing wall stress in the ventricle wall. The reduced stress on the ventricle wall enables the wall to gradually return to its normal non-stretched geometry, i.e., to undergo reverse remodeling. 
     That is, with reference to the graph of  FIG. 4A , the pressure of the ventricle isovolumetrically increases at a lower volume (as depicted by the dotted line representing the device-adjusted PV loop) rather than at a higher volume (as depicted by the solid line representing the normal PV loop). Lower volume during contraction yields less exertion per cubic millimeter of ventricular wall tissue, enabling the heart to heal and reverse-remodel. Thus, without the assistance of the passive pumps described herein, the pathologically remodeled heart would otherwise follow the graph represented by the solid line; that is, ventricle  50  would begin increasing in pressure during the isovolumetric phase of systole at a greater volume of ventricle  50 , thereby increasing stress in tissue of the ventricle wall. However, with the assistance of the passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502 , the remodeled, stretched ventricle  50  instead follows the graph represented by the dotted line; that is, ventricle  50  begins increasing in pressure during the isovolumetric phase of systole at a lower volume of ventricle  50 , thereby reducing the stress on tissue of the ventricle wall. Thus, the assistance of passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  generates acute as well as chronic reduction in wall stress of cardiac muscle surrounding the ventricle at the onset of ventricular systole due to cyclical moving of fluid that is not blood of the patient into and out of ventricle  50 , i.e., into and out of bag  22  and balloon  24  of passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502 . Additionally, for some embodiments of the present invention, passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  also generate (1) an acute further reducing of the ejection fraction of the heart due to cyclical moving of fluid that is not blood of the patient into and out of ventricle  50 , i.e., into and out of the intraventricular receptacle and the extraventricular receptacle of passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  as well as (2) a chronic increase in ejection fraction. This acute further reduction of the ejection fraction reduces the stress on the ventricle. 
     In  FIG. 3E , left ventricle  50  increases in pressure and continues isovolumetric contraction while mitral valve  54  and aortic valve  56  remain closed in Stage E. The increase in pressure in ventricle  50  together with the ventricle having filled with blood pushes the remaining fluid out of bag  22  and through conduit  26  and into balloon  24  in a manner in which bag  22  is near to empty or empty. Balloon  24  fills to hold the volume of fluid that was occupying space in ventricle  50  ( FIGS. 3B-D ) just before the onset of isovolumetric contraction. Moving the volume of fluid out of ventricle  50  produces a corresponding decrease in volume of ventricle  50  during isovolumetric contraction. 
     In Stage F as shown in  FIG. 3F , aortic valve  56  opens and the ejection stage of systole commences while the pressure in ventricle  50  increases due to the contraction of ventricle  50  in order to eject the blood. A lower volume of blood passes through aortic valve  56  during ejection than would otherwise pass through valve  56  in the absence of passive pump  21 . 
     At the end of systole, as shown in Stage G of  FIG. 3G , the blood has been ejected from ventricle  50 , however, the pressure in ventricle  50  is still high such that the fluid remains in balloon  24 . Aortic valve  56  is closed and the heart initiates isovolumetric relaxation. Once ventricle  50  sufficiently reduces in pressure during the isovolumetric relaxation stage of diastole, the heart returns to Stage A shown in  FIG. 3A . Once the pressure in ventricle is low enough (Stage B of  FIGS. 3B and 4A ), balloon  24  contracts due to its compliance and expels the fluid back into bag  22  ( FIG. 3B ), and the cycle repeats. 
     Reference is now made to  FIG. 5 , which is a schematic illustration of a system  60  comprising a port  64  connected to passive pump  21  of  FIG. 1A , in accordance with some applications of the present invention. Port  64  comprises a typically subcutaneous port which enables the physician to control a volume of fluid in passive pump  21 . For some applications of the present invention, passive pump  21  is implanted without any fluid, and the operating physician injects fluid into passive pump  21  via port  64 . For other applications, passive pump  21  is implanted with fluid and system  60  enables the physician to inject and/or extract fluid depending on the need of the patient. Port  64  comprises a membrane that is penetrable by a needle connected to a syringe  66  filled with fluid designated for injection through port  64 , through a tube  62  connecting port  64  to passive pump  21  and into the internal space defined by passive pump  21 . For some applications (as shown), tube  62  is coupled to balloon  24  by way of illustration and not limitation. For example, tube  62  may alternatively be connected to conduit  26  or to bag  22 . 
     Port  64  may be placed subcutaneously at the waist of the patient as shown, or at any suitable location in the body of the patient, e.g., the chest. 
     For some applications, a pressure sensor  68  (e.g., coupled to tube  62 ) senses the pressure in passive pump  21  and wirelessly transmits to an external device  70  information relating to the pressure in passive pump  21 . Sensor  68  may be coupled to tube  62  at any suitable location along tube  62  or to any portion of passive pump  21 . For some applications of the present invention, a coil is coupled to sensor  68  for supplying power to sensor  68 . For some applications of the present invention, sensor  68  is powered by radiofrequency or ultrasound energy. For some applications, the pressure measurement happens when the patient is in the doctor&#39;s office and the power antenna (e.g., radiofrequency transmitter or ultrasound transmitter) is placed next to the patient&#39;s chest. Based on the reading from sensor  68 , the physician decides whether to add fluid to pump  21  or to remove fluid from pump  21 . 
     For some applications of the present invention, two pressure sensors are coupled to conduit  26 , e.g., at either end of conduit  26  in order to measure flow through conduit  26 . The two pressure sensors allow the physician to derive the volume of each of bag  22  and balloon  24 . That is, in response to calculation of the difference in pressure between the two sensors, the physician can determine in which of either receptacle the pressure is higher. For example, if it is sensed and determined that the pressure in bag  22  is higher than the pressure in balloon  24 , then it can be determined that the flow is going from bag  22  to balloon  24 . 
     Reference is now made to  FIGS. 1A-5 . It is to be noted that for some applications, instead of comprising balloon  24  at the extraventricular location, passive pumps  21  and  42  may comprise a second noncompliant bag having the same or similar properties as bag  22 . 
       FIGS. 6A-B  are schematic illustrations of a system  80  comprising a passive pump  82  comprising two bags  22  and  84  and an energy-storage element  250  comprising a spring  86 , in accordance with some applications of the present invention. It is to be noted that system  80  is similar to system  20  described hereinabove with reference to  FIG. 1A , like reference numbers referring to like parts, with the exception that passive pump  82  does not comprise compliant balloon  24 . Bags  22  and  84  have similar wall compliance (for example, effectively no compliance at pressures less than 120 mmHg). For some applications, each of bags  22  and  84  has, in the absence of any external forces applied thereto: (a) a first volume when bags  22  and  84  each has an internal pressure of 120 mmHg, and (b) a second volume when the bags  22  and  84  each has an internal pressure of 10 mmHg, the first volume being less than 110% of the second volume. 
     Fluid is disposed within the internal space and is passable between bags  22  and  84  via conduit  26 . Typically, the internal space contains 10-80 ml of fluid, e.g., 20-40 ml of fluid. Typically, the fluid comprises fluid that is not blood of the patient. For some applications of the present invention, the fluid comprises a gas, such as carbon dioxide, or a liquid, such as saline. 
     It is to be noted that bag  84  has little to no wall compliance, however, the presence of energy-storage element  250  (e.g., spring  86 ) imparts compliance to the section of passive pump  82  that comprises bag  84  and energy-storage element  250  (e.g., spring  86 ). 
     Spring  86  comprises two broad structural elements  85  that are coupled together by a spring hinge  87 . Spring  86  is coupled to an external surface of bag  84 . Spring  86  has an energy-storage state ( FIG. 6A ) upon bag  84  assuming a greater volume, and an energy-released state ( FIG. 6B ). 
     Once the pressure in the ventricle decreases during diastole, spring  86  releases the energy stored in it, expelling the fluid from within bag  84  into bag  22 . That is, spring  86  is configured to (1) absorb energy upon filling of bag  84  (i.e., the extraventricular receptacle) from a first state to a second, expanded state ( FIG. 6A ), and (2) release the energy to return bag  84  (i.e., the extraventricular receptacle) from the second, expanded state to the first state ( FIG. 6B ). 
     It is to be noted that spring  86  comprises structural elements  85  and hinge  87  by way of illustration and not limitation and that spring  86  may comprise a spring having any suitable shape (e.g., helical). 
     Reference is now made to  FIGS. 3A-G ,  4 A-B, and  6 A-B. It is to be noted that passive pump  82  as described with reference to  FIGS. 6A-B  operates in Stages A-G as described in accordance with the operation of passive pump  21  with reference to  FIGS. 3A-G , the graph of  FIG. 4A , and the table of  FIG. 4B , mutatis mutandis. For such applications, bag  84  is an extraventricular receptacle. 
     Reference is now made to  FIGS. 1A-3G, 5, and 6A-9 . It is to be noted that systems  20 ,  40 ,  60 ,  90 ,  100 , and  120  may comprise energy-storage elements  250  described herein, and balloon  24  may comprise a bag such as a noncompliant bag. For example, e.g., the extraventricular receptacle may be coupled to (1) spring  86 , as described hereinabove with reference to  FIGS. 6A-B , (2) stent structure  250   a  comprising a plurality of struts  206  as described hereinbelow with reference to  FIGS. 12A-B , or (3) mesh  250   b  as described hereinbelow with reference to  FIGS. 13A-B . For other applications, the extraventricular receptacle may comprise a bellows  406  or  506  coupled to coil springs  250   c  and  250   d , respectively, as described hereinbelow with reference to  FIGS. 14A-15B . 
     Reference is now made to  FIGS. 7A-B , which are schematic illustrations of a system  90  comprising a passive pump  92  in which a receptacle comprising a bag  96  is positioned within left ventricle  50  and a second receptacle  94  is positioned in a right atrium  55 , in accordance with some applications of the present invention. Typically, a conduit  98  connects bag  96  and receptacle  94 . Conduit  98  passes through mitral valve  54  and crosses the interatrial septum, e.g., via the fossa ovalis and into right atrium  55 . Conduit  98  typically comprises a tube having an inner diameter of at least 5 mm in order to allow for passing of fluid between bag  96  and receptacle  94 . For some applications of the present invention, passive pump  92  comprises stent structure  30  surrounding the portion of conduit  98  designated for passing through the interatrial septum or through the fossa ovalis (not shown). For some applications, conduit  98  may comprise a tube covered in a porous material, e.g., a fabric, which enhances ingrowth of tissue into the outer surface of conduit  98  in order to seal conduit  98  within the interatrial septal tissue and prevent leaking. 
     For some applications, bag  96  is similar to or the same as bag  22  described hereinabove with reference to  FIGS. 1A-B ,  2 ,  3 A-G,  4 A-B,  5 , and  6 A-B. That is, bag  96  is noncompliant. 
     For some applications, receptacle  94  is similar to or the same as balloon  24  described hereinabove with reference to  FIGS. 1A-B ,  2 ,  3 A-G,  4 A-B, and  5 . That is, receptacle  94  is compliant and has wall compliance. Receptacle  94  is considered an extraventricular receptacle. 
     For some applications, receptacle  94  is similar to bag  22  described hereinabove with reference to  FIGS. 1A-B ,  2 ,  3 A-G,  4 A-B,  5 , and  6 A-B or to bag  84  described hereinabove with reference to  FIGS. 6A-B  (even without utilizing spring  86 ). That is, receptacle  94  is noncompliant. In such applications fluid passes from receptacle  94  to bag  96  disposed within left ventricle  50  when pressure in left ventricle  50  reduces from around 120 mmHg to around 5 mmHg. Since pressure in right atrium  55  remains around 15 mmHg, when pressure in left ventricle  50  drops to around 5 mmHg in Stage B (described hereinabove with reference to  FIG. 3B ), for example, as shown in  FIG. 7A , fluid passes from receptacle  94  within right atrium  55  and into bag  96  disposed within left ventricle  50 . 
     In  FIG. 7B , left ventricle  50  increases in pressure and continues isovolumetric contraction while mitral valve  54  and aortic valve  56  remain closed in Stage E (as described hereinabove with reference to  FIG. 3E ). The increase in pressure in ventricle  50  together with the ventricle having filled with blood pushes the fluid out of bag  96  and through conduit  98  and into receptacle  94  in a manner in which bag  96  is near to empty or empty. Receptacle  94  fills to hold the volume of fluid that was occupying space in ventricle  50  ( FIGS. 3B-D ) just before the onset of isovolumetric contraction. Moving the volume of fluid out of ventricle  50  produces a corresponding decrease in volume of ventricle  50  during isovolumetric contraction. 
     Reference is now made to  FIGS. 3A-G ,  4 A-B,  7 A-B, and  8 - 9 . It is to be noted that passive pump  92  as described with reference to  FIGS. 7A-B  and  8 - 9  operates in Stages A-G as described in accordance with the operation of passive pump  21  with reference to  FIGS. 3A-G , the graph of  FIG. 4A , and the table of  FIG. 4B , mutatis mutandis. 
     Reference is now made to  FIGS. 6A-B ,  7 A-B,  8 - 9 , and  12 A- 15 B. It is to be noted that systems  90 ,  100 , and  120  may comprise energy-storage elements  250  described herein (e.g., spring  86  coupled to receptacle  94  as described hereinabove with reference to  FIGS. 6A-B , stent structure  250   a  comprising a plurality of struts  206  as described hereinbelow with reference to  FIGS. 12A-B , mesh  250   b  as described hereinbelow with reference to  FIGS. 13A-B , and coil springs  250   c  and  250   d  as described hereinbelow with reference to  FIGS. 14A-15B ). Additionally, fluid is disposed within the internal space and is passable between receptacle  94  and bag  96  via conduit  98 . Typically, the internal space contains 10-80 ml of fluid, e.g., 20-40 ml of fluid. Typically, the fluid comprises fluid that is not blood of the patient. For some applications of the present invention, the fluid comprises a gas, such as carbon dioxide, or a liquid, such as saline. 
     Reference is again made to  FIGS. 7A-B . Passive pump  92  is implanted using a transcatheter/transvascular approach and advantageously does not require making an incision in myocardial tissue of the heart of the patient. 
     For some applications, conduit  98  travels from bag  96 , through a hole made in the interventricular septum, through the tricuspid valve, and to receptacle  94  positioned in right atrium  55 . 
     Reference is now made to  FIG. 8 , which is a schematic illustration of a system  100  comprising passive pump  92  in which bag  96  is positioned within left ventricle  50  and receptacle  94  is positioned in the superior vena cava  102 , in accordance with some applications of the present invention. Receptacle  94  is an extraventricular receptacle. It is to be noted that system  100  is similar to system  90  described hereinabove with reference to  FIGS. 7A-B , like reference numbers referring to like parts, with the exception that passive receptacle  94  of pump  92  is positioned in superior vena cava  102 . 
     Reference is now made to  FIG. 9 , which is a schematic illustration of a system  120  comprising passive pump  92  in which bag  96  is positioned within left ventricle  50  and receptacle  94  is positioned in the inferior vena cava  122 , in accordance with some applications of the present invention. It is to be noted that system  120  is similar to system  90  described hereinabove with reference to  FIGS. 7A-B , like reference numbers referring to like parts, with the exception that passive receptacle  94  of pump  92  is positioned in inferior vena cava  122 . Receptacle  94  is an extraventricular receptacle. 
     Reference is now made to  FIGS. 7A-B  and  8 - 9 . It is to be noted that bag  96  may be positioned within the right ventricle. It is to be additionally noted that, for some applications, use of gas within pump  92  may be preferable for embodiments in which pump  92  is positioned intravascularly. 
     Reference is now made to  FIG. 10 , which is a pressure/volume graph illustrating a pressure/volume loop  170  of a failing heart, a pressure/volume loop  150  of a healthy heart, and a pressure/volume loop  160  of a heart with the passive pumps described herein, in accordance with some applications of the present invention. Pressure/volume loop  170  of a failing heart shows reduced stroke volume increased left ventricular end-diastolic pressure and volume. 
     Reference is now made to  FIGS. 1A-B ,  2 ,  3 A-G,  4 A-B,  5 ,  6 A-B,  7 A-B,  8 - 11 ,  12 A-B,  13 A-B,  14 A-B, and  15 A-B. Upon implantation of passive pumps  21 ,  42 ,  82 ,  92 ,  182 ,  202 ,  302 ,  402 , and  502  described herein, the pressure/volume curve shifts left (i.e., loop  160 ), resulting in an increase in stroke volume and a decrease in end-diastolic pressure and end-diastolic volume as compared to loop  170 . This shift in the loop toward loop  160  enables the heart to reverse remodel. 
     Reference is now made to  FIG. 11 , which is a schematic illustration of a system  180  comprising a passive pump  182  in which a receptacle comprising a bag  186  is positioned within left ventricle  50  and a second receptacle  184  is positioned subcutaneously, in accordance with some applications of the present invention. Receptacle  184  is an extraventricular receptacle. For some applications, a pocket is created subcutaneously for receptacle  184  to be placed within. Typically, a conduit  188  connects bag  186  and receptacle  184 . Conduit  188  passes through the mitral valve into left atrium  52 , crosses the interatrial septum, e.g., via the fossa ovalis, into right atrium  55 , passes through superior vena cava  102  and into a subclavian vein  190 . Conduit  188  exits subclavian vein  190  and passes to a subcutaneous location, e.g., near a shoulder, as shown, or any suitable subcutaneous location. Conduit  188  typically comprises a tube having an inner diameter of at least 5 mm in order to allow for passing of fluid between bag  186  and receptacle  184 . For some applications of the present invention, passive pump  182  comprises stent structure  30  (described hereinabove with reference to  FIGS. 1A-B ) surrounding the portion of conduit  188  designated for passing through the interatrial septum or through the fossa ovalis (not shown). For some applications, conduit  188  comprises a tube surrounded by porous material, e.g., a fabric, which facilitates tissue growth around conduit  188  in order to enable sealing of conduit  188  and inhibit leakage of blood out of the vasculature through which conduit passes, e.g., through the interatrial septum or through the passage created in subclavian vein  190 . For some applications of the present invention, conduit  188  self-expands to position itself within openings created in the vasculature through which the conduit passes, e.g., through the interatrial septum or through the passage created in subclavian vein  190 . 
     For some applications, bag  186  is similar to or the same as bag  22  described hereinabove with reference to  FIGS. 1A-B ,  2 ,  3 A-G,  4 A-B,  5 , and  6 A-B. That is, bag  186  is noncompliant. 
     For some applications, receptacle  184  is similar to or the same as balloon  24  described hereinabove with reference to  FIGS. 1A-B ,  2 ,  3 A-G,  4 A-B, and  5 . That is, receptacle  184  is compliant and has wall compliance. 
     For some applications, receptacle  184  is similar to bag  22  described hereinabove with reference to  FIGS. 1A-B ,  2 ,  3 A-G,  4 A-B,  5 , and  6 A-B or to bag  84  described hereinabove with reference to  FIGS. 6A-B  (even without utilizing spring  86 ). That is, receptacle  184  is noncompliant. In such applications fluid passes from receptacle  184  to bag  186  disposed within left ventricle  50  when pressure in left ventricle  50  reduces from around 120 mmHg to around 5 mmHg Since pressure in the subcutaneous location is higher than 5 mmHg, when pressure in left ventricle  50  drops to around 5 mmHg in Stage B (described hereinabove with reference to  FIG. 3B ), fluid passes from receptacle  184  at the subcutaneous location, and into bag  186  disposed within left ventricle  50 . 
     Once left ventricle  50  increases in pressure and continues isovolumetric contraction while the mitral and aortic valves remain closed in Stage E (as described hereinabove with reference to  FIG. 3E ), the increase in pressure in ventricle  50  together with the ventricle having filled with blood pushes the fluid out of bag  186  and through conduit  188  and into receptacle  184  in a manner in which bag  186  is near to empty or empty. Receptacle  184  fills to hold the volume of fluid that was occupying space in ventricle  50  ( FIGS. 3B-D ) just before the onset of isovolumetric contraction. Moving the volume of fluid out of ventricle  50  produces a corresponding decrease in volume of ventricle  50  during isovolumetric contraction. 
     Reference is now made to  FIGS. 5 and 11 . It is to be noted that passive pump  182  may be coupled to a port  64 , as described hereinabove with reference to  FIG. 5 . Port  64  may be directly coupled to receptacle  184 . Alternatively, receptacle  184  has a penetrable film and functions as a port. 
     Reference is now made to  FIGS. 3A-G ,  4 A-B, and  10 - 11 . It is to be noted that passive pump  182  as described with reference to  FIG. 11  operates in Stages A-G as described in accordance with the operation of passive pump  21  with reference to  FIGS. 3A-G , the graph of  FIG. 4A , the table of  FIG. 4B , and the pressure/volume graph of  FIG. 10 , mutatis mutandis. 
     Reference is now made to  FIGS. 6A-B  and  11 . It is to be noted that system  180  may comprise springs  86  coupled to receptacle  184  as described hereinabove with reference to  FIGS. 6A-B . Additionally, fluid is disposed within the internal space and is passable between receptacle  184  and bag  186  via conduit  188 . Typically, the internal space contains 10-80 ml of fluid, e.g., 20-40 ml of fluid. Typically, the fluid comprises fluid that is not blood of the patient. For some applications of the present invention, the fluid comprises a gas, such as carbon dioxide, or a liquid, such as saline. 
     Reference is again made to  FIG. 11 . Passive pump  182  is implanted using a transcatheter/transvascular approach and advantageously does not require making an incision in myocardial tissue of the heart of the patient. 
     Reference is now made to  FIGS. 12A-B , which are schematic illustrations of a system  200  comprising a passive pump  202  comprising two bags  22  and  204  and an energy-storage element  250  comprising a plurality of struts  206 , in accordance with some applications of the present invention. Bag  204  typically functions as an extraventricular receptacle. It is to be noted that system  200  is similar to system  20  described hereinabove with reference to  FIG. 1A , like reference numbers referring to like parts, with the exception that passive pump  202  does not comprise compliant balloon  24 . Bags  22  and  204  have similar wall compliance (for example, effectively no compliance at pressures less than 120 mmHg). For some applications, each of bags  22  and  204  has, in the absence of any external forces applied thereto: (a) a first volume when bags  22  and  204  each has an internal pressure of 120 mmHg, and (b) a second volume when the bags  22  and  204  each has an internal pressure of 10 mmHg, the first volume being less than 110% of the second volume. 
     Fluid is disposed within the internal space and is passable between bags  22  and  204  via conduit  26 . Typically, the internal space contains 10-80 ml of fluid, e.g., 20-40 ml of fluid. Typically, the fluid comprises fluid that is not blood of the patient. For some applications of the present invention, the fluid comprises a gas, such as carbon dioxide, or a liquid, such as saline. 
     It is to be noted that bag  204  has little to no wall compliance, however, the presence of stent structure  250   a  imparts compliance to the section of passive pump  202  that comprises bag  204  and energy-storage element  250 . 
     For some applications, similarly as shown in  FIGS. 2, 3A -G,  5 ,  8 ,  9 , and  11 , bag  204  may be positioned outside of the heart and function as an extracardiac receptacle. For some applications in which bag  204  is an extracardiac receptacle, conduit  26  passes through myocardium and functions as a transmyocardial conduit. 
     Energy-storage element  250  comprises a stent structure  250   a  comprising a plurality of struts  206  surrounding bag  204 . Typically, struts  206  comprise a flexible material, e.g., nitinol. A respective portion  230  of each of struts  206  is configured to (1) assume a longitudinally-elongated state upon the filling of the expandable extraventricular receptacle (e.g., bag  204 ) from the first state to the second, expanded state ( FIG. 12A ), (2) assume an energy-released, longitudinally-shortened state ( FIG. 12B ), and (3) return the expandable extraventricular receptacle (e.g., bag  204 ) from the second, expanded state to the first state during a transition of energy-storage element  250  (e.g., struts  206 ) from the longitudinally-elongated state toward the longitudinally-shortened state. In the longitudinally-shortened state of portions  230  of struts  206  shown in  FIG. 12B , portions  230  of struts  206  are each typically shaped so as to define a bend which projects laterally away from a central longitudinal axis ax 1  of pump  202 . In such a manner, portions  230  expand radially and shorten longitudinally with respect to axis ax 1 . It is to be noted that the first state and the second expanded state of bag  204  (i.e., the expandable extraventricular receptacle) and the respective states of intraventricular bag  22  shown in  FIGS. 12A-B  are not drawn to scale. 
     Each strut  206  extends longitudinally along an external surface of the extraventricular receptable (e.g., bag  204 ) in a direction (1) from a portion  240  of the extraventricular receptable (e.g., bag  204 ) that is furthest from the intraventricular receptable (e.g., bag  22 ), and toward (2) the intraventricular receptable (e.g., bag  22 ). Pump  202  comprises stent structure  30  which has a first portion that surrounds conduit  26 . Energy-storage element  250  defines a second portion of stent structure  30  that surrounds the extraventricular receptacle (e.g., bag  204 ). A part of the first portion of stent structure  30  (i.e., a wider portion  212 ) surrounds a portion of the intraventricular receptacle (e.g., bag  22 ). Stent structure  30  is (i) compressed during delivery of pump  202  into the body of the patient, and (ii) expandable during implantation of pump  202  in the body of the patient. Once expanded, first portion  210  of stent structure  30  is shaped so as to define (1) a narrowed portion  210  at conduit  26  and (2) wider portion  212  than narrowed portion  210 , at the part of the first portion of the stent structure that surrounds the portion of the intraventricular receptacle (e.g., bag  22 ). 
     Stent structure  250   a  has a longitudinally-shortened state ( FIG. 12B ) and an energy-storage state ( FIG. 12A ) upon bag  204  assuming a greater volume. Once the pressure in the ventricle decreases during diastole, stent structure  250   a  releases the energy stored in it, expelling the fluid from within bag  204  into bag  22 . That is, stent structure  250   a  is configured to (1) absorb energy upon filling of bag  204  (i.e., the extraventricular receptacle) from a first state to a second, expanded state, and (2) release the energy to return bag  204  (i.e., the extraventricular receptacle) from the second, expanded state to the first state. 
     Reference is now made to  FIGS. 3A-G ,  4 A-B,  10 , and  12 A-B. It is to be noted that passive pump  202  as described with reference to  FIGS. 12A-B  operates in Stages A-G as described in accordance with the operation of passive pump  21  with reference to  FIGS. 3A-G , the graph of  FIG. 4A , the table of  FIG. 4B , and the pressure/volume graph of  FIG. 10 , mutatis mutandis. 
     Reference is now made to  FIGS. 13A-B , which are schematic illustrations of a system  300  comprising a passive pump  302  comprising two bags  22  and  304  and an energy-storage element  250  comprising a mesh  250   b , in accordance with some applications of the present invention. Bag  304  typically functions as an extraventricular receptacle. It is to be noted that system  300  is similar to system  20  described hereinabove with reference to  FIG. 1A , like reference numbers referring to like parts, with the exception that passive pump  302  does not comprise compliant balloon  24 . Bags  22  and  304  have similar wall compliance (for example, effectively no compliance at pressures less than 120 mmHg). For some applications, each of bags  22  and  304  has, in the absence of any external forces applied thereto: (a) a first volume when bags  22  and  304  each has an internal pressure of 120 mmHg, and (b) a second volume when the bags  22  and  304  each has an internal pressure of 10 mmHg, the first volume being less than 110% of the second volume. 
     Fluid is disposed within the internal space and is passable between bags  22  and  304  via conduit  26 . Typically, the internal space contains 10-80 ml of fluid, e.g., 20-40 ml of fluid. Typically, the fluid comprises fluid that is not blood of the patient. For some applications of the present invention, the fluid comprises a gas, such as carbon dioxide, or a liquid, such as saline. 
     It is to be noted that bag  304  has little to no wall compliance, however, the presence of mesh  250   b  imparts compliance to the section of passive pump  302  that comprises bag  304  and energy-storage element  250 . 
     For some applications, similarly as shown in  FIGS. 2, 3A -G,  5 ,  8 ,  9 , and  11 , bag  304  may be positioned outside of the heart and function as an extracardiac receptacle. For some applications in which bag  304  is an extracardiac receptacle, conduit  26  passes through myocardium and functions as a transmyocardial conduit. 
     Energy-storage element  250  comprises mesh  250   b  surrounding bag  204  and conduit  26 . Typically, mesh  250   b  comprises nitinol or stainless steel. A first portion  308  of mesh  250   b  is configured to (1) assume a longitudinally-elongated state upon the filling of the expandable extraventricular receptacle (e.g., bag  304 ) from the first state to the second, expanded state ( FIG. 13A ), (2) assume an energy-released, longitudinally-shortened state ( FIG. 13B ), and (3) return the expandable extraventricular receptacle (e.g., bag  304 ) from the second, expanded state to the first state during a transition of energy-storage element  250  (e.g., mesh  250   b ) from the longitudinally-elongated state toward the longitudinally-shortened state. As portion  308  of mesh  250   b  releases the energy, portion  308  of mesh  250   b  transitions toward an energy-released state in which first portion  308  of mesh  250   b  expands radially and shortens longitudinally, with respect to a central longitudinal axis ax 1  of the pump  302 . 
     Mesh  250   b  is (i) compressed during delivery of pump  302  into the body of the patient, and (ii) expandable during implantation of pump  302  in the body of the patient. Once expanded from the delivery state, mesh  250   b  is shaped so as to define a second, narrowed portion  310  of mesh  250   b  that surrounds conduit  26 , and a third, wider (typically flared) portion  312  of mesh  250   b  surrounds a portion of the intraventricular receptacle (i.e., bag  22 ). Wider portion  312  is typically wider than second, narrowed portion  310 . 
     Mesh  250   b  has a longitudinally-shortened state ( FIG. 13B ) and an energy-storage state ( FIG. 13A ) upon bag  304  assuming a greater volume. Once the pressure in the ventricle decreases during diastole, portion  308  of mesh  250   b  releases the energy stored in it, expelling the fluid from within bag  304  into bag  22 . That is, portion  308  of mesh  250   b  is configured to (1) absorb energy upon filling of bag  304  (i.e., the extraventricular receptacle) from a first state to a second, expanded state, and (2) release the energy to return bag  304  (i.e., the extraventricular receptacle) from the second, expanded state to the first state. It is to be noted that the first state and the second expanded state of bag  304  (i.e., the expandable extraventricular receptacle) and the respective states of intraventricular bag  22  shown in  FIGS. 13A-B  are not drawn to scale. 
     It is to be noted that pump  302  is shown as having mesh  250   b  surround a portion of bag  22 , conduit  26 , and bag  304 . It is to be noted that pump  302  may comprise mesh  250   b  surrounding bag  304 , while the portion of bag  22  and conduit  26  may be surrounded by stent structure  30 , as described hereinabove with reference to  FIGS. 1A-B , for example. 
     Reference is now made to  FIGS. 3A-G ,  4 A-B,  10 , and  13 A-B. It is to be noted that passive pump  302  as described with reference to  FIGS. 13A-B  operates in Stages A-G as described in accordance with the operation of passive pump  21  with reference to  FIGS. 3A-G , the graph of  FIG. 4A , the table of  FIG. 4B , and the pressure/volume graph of  FIG. 10 , mutatis mutandis. 
     Reference is now made to  FIGS. 14A-B , which are schematic illustrations of a system  400  comprising a passive pump  402  comprising bag  22 , an extraventricular receptacle  404  comprising a bellows  406 , and an energy-storage element  250  comprising a coil spring  250   c , in accordance with some applications of the present invention. It is to be noted that system  400  is similar to system  20  described hereinabove with reference to  FIG. 1A , like reference numbers referring to like parts, with the exception that passive pump  402  does not comprise compliant balloon  24 . Bag  22  and bellows  406  has similar wall compliance (for example, effectively no compliance at pressures less than 120 mmHg). For some applications, each of bag  22  and bellows  406  have, in the absence of any external forces applied thereto: (a) a first volume when bag  22  and bellows  406  each has an internal pressure of 120 mmHg, and (b) a second volume when the bag  22  and bellows  406  each has an internal pressure of 10 mmHg, the first volume being less than 110% of the second volume. 
     Bellows  406  is typically disposed between bag  22  (i.e., the intraventricular receptacle) and energy-storage element  250 . 
     Fluid is disposed within the internal space and is passable between bag  22  and bellows  406  via conduit  26 . Typically, the internal space contains 10-80 ml of fluid, e.g., 20-40 ml of fluid. Typically, the fluid comprises fluid that is not blood of the patient. For some applications of the present invention, the fluid comprises a gas, such as carbon dioxide, or a liquid, such as saline. 
     It is to be noted that bellows  406  has little to no wall compliance, however, the presence of spring  250   c  imparts compliance to the section of passive pump  402  that comprises bellows  406 . 
     For some applications, similarly as shown in  FIGS. 2, 3A -G,  5 ,  8 ,  9 , and  11 , bellows  406  may be positioned outside of the heart and function as an extracardiac receptacle. For some applications in which bellows  406  is an extracardiac receptacle, conduit  26  passes through myocardium and functions as a transmyocardial conduit. 
     Typically, energy-storage element  250  comprises coil spring  250   c  comprising a shape-memory material, e.g., nitinol. Energy-storage element  250  comprises coil spring  250   c  that is configured to (1) assume a compressed, longitudinally-shortened, state upon the filling of the expandable extraventricular receptacle (e.g., bellows  406 ) from a first state to a second, expanded state ( FIG. 14A ), and (2) return the expandable extraventricular receptacle (e.g., bellows  406 ) from the second, expanded state to the first state ( FIG. 14B ) during a transition of energy-storage element  250  (e.g., coil spring  250   c ) from the compressed, longitudinally-shortened state toward an energy-released state of spring  250   c . As coil spring  250   c  releases the energy, coil spring  250   c  lengthens longitudinally, with respect to a central longitudinal axis ax 1  of pump  402 . 
     Bellows  406  and coil spring  250   c  are surrounded by scaffolding  440 , e.g., a frame or a housing. Energy-storage element  250  is coupled at a first end  420  of energy-storage element  250  to an outer surface of bellows  406 , and at a second end  430  of energy-storage element  250  to a portion of scaffolding  440 . During the absorbing and releasing of energy by energy-storage element  250 , first end  420  of energy-storage element  250  moves, respectively, toward and away from second end  430  of energy-storage element  250 . For some applications, scaffolding  440  is coupled to stent structure  30  that surrounds conduit  26  and a portion of bag  22 , as described hereinabove with reference to  FIGS. 1A-B , for example. For some applications, scaffolding  440  is part of stent structure  30 . Scaffolding  440  and stent structure  30  are (i) compressed during delivery of pump  402  into the body of the patient, and (ii) expandable during implantation of pump  402  in the body of the patient. 
     Coil spring  250   c  has an energy-storage state ( FIG. 14A ) upon bellows  406  assuming a greater volume, and an energy-released state ( FIG. 14B ). Once the pressure in the ventricle decreases during diastole, coil spring  250   c  releases the energy stored in it, expelling the fluid from within bellows  406  into bag  22 . That is, coil spring  250   c  is configured to (1) absorb energy upon filling of bellows  406  (i.e., the extraventricular receptacle) from a first state to a second, expanded state, and (2) release the energy to return bellows  406  (i.e., the extraventricular receptacle) from the second, expanded state to the first state. It is to be noted that the first state and the second expanded state of bellows  406  (i.e., the expandable extraventricular receptacle) and the respective states of intraventricular bag  22  shown in  FIGS. 14A-B  are not drawn to scale. 
     Reference is now made to  FIGS. 3A-G ,  4 A-B,  10 , and  14 A-B. It is to be noted that passive pump  402  as described with reference to  FIGS. 14A-B  operates in Stages A-G as described in accordance with the operation of passive pump  21  with reference to  FIGS. 3A-G , the graph of  FIG. 4A , the table of  FIG. 4B , and the pressure/volume graph of  FIG. 10 , mutatis mutandis. 
     Reference is now made to  FIGS. 15A-B , which are schematic illustrations of a system  500  comprising a passive pump  502  comprising bag  22 , an extraventricular receptacle  504  comprising a bellows  506 , and an energy-storage element  250  comprising a coil spring  250   d , in accordance with some applications of the present invention. It is to be noted that system  500  is similar to system  20  described hereinabove with reference to  FIG. 1A , like reference numbers referring to like parts, with the exception that passive pump  502  does not comprise compliant balloon  24 . Bag  22  and bellows  506  have similar wall compliance (for example, effectively no compliance at pressures less than 120 mmHg). For some applications, each of bag  22  and bellows  506  has, in the absence of any external forces applied thereto: (a) a first volume when bag  22  and bellows  506  each has an internal pressure of 120 mmHg, and (b) a second volume when the bag  22  and bellows  506  each has an internal pressure of 10 mmHg, the first volume being less than 110% of the second volume. 
     Typically, energy-storage element  250  comprises coil spring  250   d  comprising a shape-memory material, e.g., nitinol. 
     Coil spring  250   d  surrounds bellows  506  at least in part. Coil spring  250   d  wraps around bellows  506  between folds  507  of bellows  506 . 
     Fluid is disposed within the internal space and is passable between bag  22  and bellows  506  via conduit  26 . Typically, the internal space contains 10-80 ml of fluid, e.g., 20-40 ml of fluid. Typically, the fluid comprises fluid that is not blood of the patient. For some applications of the present invention, the fluid comprises a gas, such as carbon dioxide, or a liquid, such as saline. 
     It is to be noted that bellows  506  has little to no wall compliance, however, the presence of spring  250   d  imparts compliance to the section of passive pump  502  that comprises bellows  506 . 
     For some applications, similarly as shown in  FIGS. 2, 3A -G,  5 ,  8 ,  9 , and  11 , bellows  506  may be positioned outside of the heart and function as an extracardiac receptacle. For some applications in which bellows  506  is an extracardiac receptacle, conduit  26  passes through myocardium and functions as a transmyocardial conduit. 
     Energy-storage element  250  comprises coil spring  250   d  that comprises a shape-memory material that is configured to (1) assume a longitudinally-elongated state upon the filling of the expandable extraventricular receptacle (e.g., bellows  506 ) from a first state to a second, expanded state ( FIG. 15A ), and (2) return the expandable extraventricular receptacle (e.g., bellows  506 ) from the second, expanded state to the first state ( FIG. 15B ) during a transition of energy-storage element  250  (e.g., coil spring  250   d ) from the longitudinally-elongated state toward an energy-released state of coil spring  250   d . As coil spring  250   d  releases the energy, coil spring  250   d  transitions toward the energy-released state in which coil spring  250   d  shortens longitudinally, with respect to a central longitudinal axis ax 1  of pump  502 . 
     Bellows  506  and coil spring  250   d  are surrounded by scaffolding  540 , e.g., a frame or a housing. For some applications, scaffolding  540  is coupled to stent structure  30  that surrounds conduit  26  and a portion of bag  22 , as described hereinabove with reference to  FIGS. 1A-B , for example. For some applications, scaffolding  540  is part of stent structure  30 . Scaffolding  540  and stent structure  30  are (i) compressed during delivery of pump  502  into the body of the patient, and (ii) expandable during implantation of pump  502  in the body of the patient. 
     Coil spring  250   d  has an energy-storage state ( FIG. 15A ) upon bellows  506  assuming a greater volume, and an energy-released state ( FIG. 15B ). Once the pressure in the ventricle decreases during diastole, coil spring  250   d  releases the energy stored in it, expelling the fluid from within bellows  506  into bag  22 . That is, coil spring  250   d  is configured to (1) absorb energy upon filling of bellows  506  (i.e., the extraventricular receptacle) from a first state to a second, expanded state, and (2) release the energy to return bellows  506  (i.e., the extraventricular receptacle) from the second, expanded state to the first state. It is to be noted that the first state and the second expanded state of bellows  506  (i.e., the expandable extraventricular receptacle) and the respective states of intraventricular bag  22  shown in  FIGS. 15A-B  are not drawn to scale. 
     Reference is now made to  FIGS. 3A-G ,  4 A-B,  10 , and  15 A-B. It is to be noted that passive pump  502  as described with reference to  FIGS. 15A-B  operates in Stages A-G as described in accordance with the operation of passive pump  21  with reference to  FIGS. 3A-G , the graph of  FIG. 4A , the table of  FIG. 4B , and the pressure/volume graph of  FIG. 10 , mutatis mutandis. 
     Reference is now made to  FIGS. 1A-15B . It is to be noted that although systems described herein are applied to assist and repair a failing left ventricle  50  of the heart of the patient, the systems described herein can also be applied to assist and repair a failing right ventricle, mutatis mutandis. It is to be noted that bags  22 ,  84 ,  96 , and  186 ; receptacles  94  and  184 ; balloon  24 ; and bellows  406  and  506  may comprise a polymer (e.g., polyurethane or any other suitable elastic material), which is configured to retain elasticity for an extended period of time. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.