Patent Publication Number: US-2021187258-A1

Title: Temporary interatrial shunts

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from U.S. Provisional Application 62/158,022, entitled “Percutaneous device for temporary stenting and shunting,” filed May 7, 2015, whose disclosure is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the present invention relate generally to the field of medical devices, and specifically to percutaneous devices for treatment of a subject. 
     BACKGROUND 
     Some medical conditions are treated by implanting a shunt between two body cavities, typically to release excess pressure from one of the cavities into the other. For example, a shunt may be implanted between the right and left atria of the heart for the treatment of pulmonary hypertension by decompression of the right atrium, or for the treatment of congestive heart failure by decompression of the left atrium. Implantable shunts of this sort are described, for example, in U.S. Pat. No. 8,091,556, whose disclosure is incorporated herein by reference. 
     U.S. Pat. No. 9,067,050, whose disclosure is incorporated herein by reference, describes an arteriovenous shunt assembly including a shunt and a pull wire operated flow control mechanism. The shunt has a tubular body that defines a fluid passageway between a first end and a second end thereof. The pull wire mechanism includes a portion disposed around the tubular shunt in at least one loop. The at least one loop may be selectively tightened or loosened remotely from the shunt to regulate the rate of blood flow through the tubular shunt. 
     U.S. Patent Application Publication 2014/0303710, whose disclosure is incorporated herein by reference, describes a recyclable and adjustable interventional stent for intravascular constriction. The stent main body is divided into three parts and shaped like a waist drum with expansion parts being arranged on the upper and lower parts of the stent main body respectively for supporting and positioning. A variable aperture part is arranged in the middle of the stent main body. The upper expansion part is or is not provided with a coating; the middle variable aperture part and the upper half part of the lower expansion part are covered with a pericardium subjected to anti-calcification treatment; and a metal wire ring is passed through the lowermost edge of the stem. A compound conveying guide pipe is composed of an outer sheath and a core. The core is a hollow pipe and a wire hanging groove is arranged on the outer side wall of the tip of the pipe to hang the metal wire ring of the lowermost edge of the stent. A fixing bolt on the outer sheath is used for fixing the relative position between the outer sheath and the core. The stent is used to replace conventional pulmonary artery banding as, adhesion not being formed around the heart and major vessels and pulmonary stenosis not being formed, difficulties during radical surgery are not increased. 
     U.S. Patent Publication 2013/0178784, whose disclosure is incorporated herein by reference, describes devices and methods for treating heart disease by normalizing elevated blood pressure in the left and right atria of a heart of a mammal. Devices may include an adjustable hydraulic diameter shunt portion which can be manually adjusted in vivo. Methods are provided for adjusting the flow rate of the devices in vivo. 
     U.S. Pat. No. 5,035,706, whose disclosure is incorporated herein by reference, describes a self-expanding stent formed of stainless steel wire arranged in a closed zig-zag configuration. The stent includes an endless series of straight sections joined at their ends by bends. The stem is compressible into a reduced diameter size for insertion into and removal from a body passageway. The bends of at least one end of the stent are formed into eyes for connection with the eyes at one end of a similarly constructed stent to permit single-step introduction of several lengths of stent into the passageway. A stent can include a monofilament thread passing through successive eyes at one end of the stem, the thread passing through each eye at least once and through some of the eyes a second time. The trailing ends of the thread extend from the stem and outside the body passageway. The stent can be retrieved from the body passageway by threading a tube of the free ends of the thread until the tube is adjacent the stent. The diameter at one end of the stem is reduced by pulling the free ends of the thread through the tube. A sheath concentrically disposed over the tube is introduced into the body passageway and over the remaining length of the stent to further compress the stent for removal from the passageway. 
     U.S. Pat. No. 6,221,096, whose disclosure is incorporated herein by reference, describes an intravascular stem having an elastic self-expandable cylindrical stent proper. The stent proper is connected to metal support wires that are long enough to reach outside of the body of a patient through a catheter. Manipulation of the support wires pushes the stem proper into a blood vessel from within the catheter, thereby allowing it to expand there, and then contracts and retracts the stent proper into the catheter, repeatedly. 
     U.S. Pat. No. 6,468,303, whose disclose is incorporated herein by reference, describes a collapsible medical device and associated method for shunting selected organs and vessels, wherein the medical device is shaped from a shape memory metal fabric. The device may be used, for example, to non-surgically create a transjugular intrahepatic portosystemic shunt. The device is preferably made from a continuous tubular metal fabric and includes two outer flanges that reduce device migration and includes a central passageway between the two outer flanges. The metal fabric may be heat treated within a mold in order to substantially set a desired relaxed shape of the device. The medical device includes a fastener for attaching to the end of a guide wire or delivery catheter. The medical device having the desired relaxed shape may be collapsed and delivered through a catheter or the like for deployment in a desired channel or opening in a patient&#39;s body and is retrievable after deployment. 
     SUMMARY OF THE INVENTION 
     An implanted interatrial shunt may cause various complications, such as distortion of the interatrial septum, cardiac arrhythmias, inability to use the transseptal approach for future interventions, paradoxical embolism, and/or blood desaturation. Hence, for cases in which an interatrial shunt is required only temporarily (i.e., for a short period of time, such as less than one week), an implanted shunt may not necessarily be the most appropriate solution for treatment. 
     Embodiments of the present invention therefore provide—as an alternative to an implanted shunt—shunting apparatus that may be placed within the subject for only a short period of time, e.g., for less than one week, or even less than one day (e.g., 2-3 hours, 3-6 hours, or 6-12 hours). The apparatus comprises a shunt, along with one or more wires that extend from the proximal portion of the shunt to the exterior of the subject. These wires, which are typically controlled via a control handle, may be used to collapse the shunt, whenever the shunt is no longer needed. Following the collapse of the shunt, the shunt may be easily withdrawn from the subject. 
     In some embodiments, another wire, which passes circumferentially along the intermediate portion of the shunt and also extends to the exterior of the subject, may be used to adjust the diameter of the shunt while the shunt is inside the subject, thus regulating the flow of blood across the interatrial septum. 
     Other temporary shunts described herein include a shunt that is coupled to a distal end of a sheath. The shunt is advanced, in a collapsed state, over a guidewire, until the shunt spans the interatrial septum. Subsequently, the guidewire is retracted while the sheath is held in place, such that a stopper coupled to the distal portion of the guidewire applies a longitudinally compressive force to the shunt, thus causing the shunt to open. The sheath is then locked with respect to the guidewire, such that the shunt remains open. Upon the conclusion of treatment, the sheath is unlocked, such that the shunt collapses, and subsequently, the shunt is removed from the subject. 
     (In general, within the context of medical applications, the term “shunt” may refer to (i) a passage that diverts a bodily fluid from one portion of the body to another, or (ii) a device that is used to establish, and/or maintain, such a passage. In the context of the present application, including the claims, the term “shunt” typically refers to the latter.) 
     There is therefore provided, in accordance with some embodiments of the present invention, apparatus that includes a shunt. The shunt includes a flared distal portion, a flared proximal portion, and an intermediate portion, disposed between the distal portion and the proximal portion. The apparatus further includes a constricting flexible longitudinal element passing circumferentially along the intermediate portion of the shunt, configured to constrict the intermediate portion of the shunt, and one or more proximal-portion-collapsing flexible longitudinal elements configured to collapse the proximal portion of the shunt. 
     In some embodiments, the constricting flexible longitudinal element includes a wire. 
     In some embodiments, the intermediate portion of the shunt is shaped to define a plurality of orifices, and the wire passes circumferentially along the intermediate portion by passing through the orifices. 
     In some embodiments, the proximal portion of the shunt is shaped to define a plurality of orifices, and the proximal-portion-collapsing flexible longitudinal elements include one or more wires, each of which passes through at least two of the orifices. 
     There is further provided, in accordance with some embodiments of the present invention, a method that includes placing a shunt between two chambers of a heart of a subject, such that one or more shunt-collapsing flexible longitudinal elements extend from a proximal portion of the shunt to an exterior of the subject. The method further includes, subsequently, using the shunt-collapsing flexible longitudinal elements, collapsing the shunt into a catheter. 
     In some embodiments, the method further includes, using the catheter, removing the shunt from the subject. 
     In some embodiments, removing the shunt from the subject includes removing the shunt from the subject after less than one week from the placement of the shunt. 
     In some embodiments, the two chambers of the heart are two atria of the heart. 
     In some embodiments, the two chambers of the heart are two ventricles of the heart. 
     In some embodiments, the method further includes, while the shunt is between the two chambers of the heart, constricting the shunt, using a constricting flexible longitudinal element that extends from the shunt to the exterior of the subject. 
     There is further provided, in accordance with some embodiments of the present invention, a method that includes placing a shunt between two atria of a subject, and, after less than one week from the placement of the shunt, withdrawing the shunt from the subject. 
     There is further provided, in accordance with some embodiments of the present invention, apparatus that includes a sheath, and a shunt coupled to a distal end of the sheath, the shunt being configured to open from a collapsed state to an open state upon a longitudinally compressive force being applied to the shunt. 
     In some embodiments, the shunt includes a plurality of wires, distal ends of which are joined together, and proximal ends of which are coupled to the sheath. 
     In some embodiments, the shunt is configured to open by the wires expanding radially outward from each other. 
     In some embodiments, in the open state, a proximal portion of the shunt and a distal portion of the shunt are wider than an intermediate portion of the shunt that is between the proximal portion of the shunt and the distal portion of the shunt. 
     In some embodiments, the apparatus further includes:
         a guidewire, and   a stopper coupled to a distal portion of the guidewire, the stopper being configured to apply the longitudinally compressive force to the shunt by pressing against a distal portion of the shunt when the sheath and the shunt are over the guidewire.       

     In some embodiments, the shunt is shaped to define a distal aperture configured to fittingly receive the stopper. 
     In some embodiments, the stopper includes a bead. 
     There is further provided, in accordance with some embodiments of the present invention, a method that includes passing a guidewire across a septum that separates between a first chamber of a heart of a subject and a second chamber of the heart, such that a stopper coupled to the guidewire is in the second chamber. The method further includes, subsequently, passing a shunt, in a collapsed state, over the guidewire, until a proximal portion of the shunt is in the first chamber, and a distal portion of the shunt is in the second chamber, and subsequently, using the stopper, opening the shunt from the collapsed state to an open state. 
     In some embodiments, opening the shunt includes opening the shunt by, using the stopper, pressing against the distal portion of the shunt. 
     In some embodiments, the shunt is shaped to define a distal aperture, and pressing against the distal portion of the shunt includes pressing against the distal portion of the shunt while the stopper is fittingly received by the distal aperture. 
     In some embodiments, the first chamber is a right atrium, and the second chamber is a left atrium. 
     In some embodiments, the first chamber is a right ventricle, and the second chamber is a left ventricle. 
     In some embodiments, the shunt is coupled to a distal end of a sheath, and the method further includes, subsequently to opening the shunt, maintaining the open state of the shunt by locking the sheath with respect to the guidewire. 
     In some embodiments, the method further includes:
         causing the shunt to collapse, by unlocking the sheath with respect the guidewire; and   subsequently, removing the shunt from the subject.       

     In some embodiments, removing the shunt from the subject includes removing the shunt from the subject after less than one week from the opening of the shunt. 
     The present invention will be more fully understood from the following detailed description of embodiments thereof, taken together with the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a temporary shunt apparatus inside a subject, in accordance with some embodiments of the present invention; 
         FIG. 2  is a schematic illustration of a temporary shunt apparatus, in accordance with some embodiments of the present invention; 
         FIG. 3A-C  collectively show a technique for removing a shunt from a subject, in accordance with some embodiments of the present invention; and 
         FIGS. 4A-B  are schematic illustrations showing the placement of a temporary shunt within an interatrial septum, in accordance with some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference is initially made to  FIG. 1 , which is a schematic illustration of a temporary shunt apparatus  28  inside a subject  20 , and to  FIG. 2 , which is a schematic illustration of temporary shunt apparatus  28 , in accordance with some embodiments of the present invention. 
     Apparatus  28  comprises a shunt  26 , which may be placed between two chambers of the heart  22  of subject  20 , such as within the interatrial septum  24  of heart  22 , between the right atrium  30  and the left atrium  32 . Alternatively, the shunt may be placed between the two ventricles of the heart, or between any other two body cavities. Shunt  26  typically comprises a flared distal portion  40 , a flared proximal portion  44 , and an intermediate portion  42 , which is disposed between distal portion  40  and proximal portion  44 . Distal portion  40  and proximal portion  44  anchor the shunt to septum  24  (i.e., prevent migration of the shunt from within the septum), while intermediate portion  42  provides a passageway across the septum, through which blood may flow. Typically, shunt  26  comprises a shape-memory material, such as Nitinol, such that the shunt expands to its natural shape (the shape shown in  FIGS. 1-2 ) upon being released from a delivery catheter, as further described below. (It is noted that, for clarity, shunt apparatus  28  is drawn disproportionately large, relative to heart  22 , in  FIG. 1 .) 
     In the context of the present application, including the claims, the proximal and distal portions of the shunt are “flared,” in that these portions extend radially outward from the intermediate portion of the stent. In some embodiments, as shown, each of the proximal and distal portions of the shunt comprises a plurality of leaves  25 , such as, for example, six leaves  25 , as shown. In other embodiments, the proximal portion and/or the distal portion does not comprise a plurality of leaves, but rather, is shaped to define a flared ring, or has some other suitable form. 
     To facilitate removal of the shunt, embodiments described herein provide one or more shunt-collapsing flexible longitudinal elements, which extend from proximal portion  44  to the exterior of the subject. For example, as shown in the figures, the shunt-collapsing flexible longitudinal elements may comprise wires  36 . Typically, while inside the subject, wires  36  are contained within a lumen  37  of a sheath  31  passing between proximal portion  44  and the exterior of the subject. (For example, sheath  31  may exit the subject via a femoral vein of the subject.) As further described below with reference to  FIG. 3A , as wires  36  are pulled, wires  36  exert an inward radial force on the proximal portion of the shunt, such that the proximal portion of the shunt is collapsed. Typically, as shown in  FIG. 1 , the proximal ends of wires  36  are coupled to control handle  34 , via which wires  36  may be pulled (or alternatively, released, such as to allow the proximal portion of the shunt to expand). 
     Typically, wires  36  remain coupled to the shunt throughout the time that the shunt is in place inside the subject. Due to wires  36  remaining coupled to the shunt, the shunt may be easily removed immediately upon receiving indication that further shunting is no longer required. In this respect, embodiments described herein differ from other, hypothetical shunting applications in which (i) the shunt is removed only in the event of a complication, and (ii) to remove the shunt, it is necessary to use a lasso or other specialized shunt-removal instrument. 
       FIG. 2  shows a particular embodiment in which proximal portion  44  is shaped to define a plurality of orifices  48 , and each of wires  36  passes through at least two of orifices  48 . For example, as shown, the end of each leaf  25  may be shaped to define an orifice  48 , and each wire may pass through the respective orifices of two adjacent leaves, such that the wire forms a loop that passes through the orifices. (Thus, as shown, a shunt having six proximal leaves is coupled to three wires  36 , each wire separately controlling the collapse of a respective pair of adjacent leaves.) To collapse the proximal portion of the shunt, the two proximal ends of each of the wires are pulled, as described below with reference to  FIG. 3A . 
     Alternatively to the embodiment shown, a single wire  36  may form a loop that passes through all of the orifices, this single wire controlling the collapse of the entire proximal portion. (In other words, by pulling on the two ends of this single wire, the entire proximal portion may be collapsed.) In yet other embodiments, wires  36  do not form loops; rather, a separate wire is coupled to each leaf. For example, each leaf may be coupled to the distal end of a respective wire. (Thus, for example, a shunt having six proximal leaves is coupled to six wires, one wire per leaf.) Similarly, wires  36  may be formed as extensions of the leaves, such that each leaf has a wire extension that extends to the exterior of the subject. In such embodiments, the proximal portion of the shunt may be collapsed by pulling on the single proximal end of each of the wires. 
     In some cases, it may be beneficial to adjust the diameter of intermediate portion  42  while the shunt is inside the subject. Hence, apparatus  28  typically further comprises a constricting flexible longitudinal element that passes circumferentially along intermediate portion  42 , and extends from the intermediate portion of the shunt to the exterior of the subject. For example, as shown, the constricting flexible longitudinal element may comprise a wire  38  that loops around the intermediate portion of the shunt, both ends of the wire passing through the vasculature of the subject (e.g., within lumen  37 , or within a separate lumen  39  of sheath  31 ), exiting from the subject (e.g., via a femoral vein), and being coupled to control handle  34 . The constricting flexible longitudinal element is configured to constrict the intermediate portion of the shunt, by exerting an inward radial force on the intermediate portion of the shunt. For example, to constrict the intermediate portion, and thus reduce the flow of blood across the septum, the control handle may be used to pull both ends of wire  38 . (Subsequently, the control handle may be used to hold wire  38  in place, to prevent the intermediate portion from re-expanding.) Conversely, to widen the intermediate portion, the control handle may be used to release wire  38 , thus allowing the intermediate portion to expand radially outward. 
     In some embodiments, as shown, the intermediate portion is shaped to define a plurality of orifices  50 , and the wire passes circumferentially along the intermediate portion by passing through the orifices. Alternatively or additionally to comprising a wire, the constricting flexible longitudinal element may comprise a band, strap, ribbon, or any other suitable type of longitudinal element. 
     In some embodiments, the adjustment of the diameter of the shunt is based on pressure monitoring. For example, as further described below, pressure sensors disposed on the shunt may be used to acquire intra-atrial pressure measurements, and the diameter of the shunt may be adjusted in response to such measurements. Alternatively or additionally, the diameter of the shunt may be adjusted in response to hemodynamic monitoring, such as by the application of flow imaging techniques such as pulsed wave (PW) or continuous wave (CW) Doppler echocardiography. 
     Typically, to place the shunt within the septum, the shunt is first collapsed and placed inside a delivery catheter  46 . Subsequently, catheter  46  is percutaneously inserted into the vasculature of the subject, such as via a femoral vein of the subject, and is then passed through the vasculature into right atrium  30 , e.g., via the inferior vena cava. (Alternatively, catheter  46  may be passed into the right atrium via the jugular vein and superior vena cava.) Subsequently, the distal end of the catheter is passed through the septum and into left atrium  32 , such that the distal and proximal portions of the shunt are on opposite sides of the septum. (As is known in the art, prior to passing the distal end of the catheter through the septum, a puncturing element may be used to create an opening in the septum, and, optionally, a dilator may be used to enlarge the opening, such that the distal end of the catheter may easily pass through the septum) The catheter is then withdrawn from over the shunt while the shunt is held in place. As the catheter is withdrawn from over the shunt, the shunt reassumes its natural, non-collapsed shape, such that distal portion  40  opens on the left-atrium side of the septum, and proximal portion  44  opens on the right-atrium side of the septum. 
     Following the deployment of the shunt, the catheter is withdrawn from the subject, leaving behind the shunt, along with the shunt-collapsing flexible longitudinal elements and the constricting flexible longitudinal element. Alternatively, the catheter may remain within the subject while the shunt is in place. For example, the catheter may remain within the subject such that the distal end of the catheter is near the proximal portion of the shunt. The catheter may thus be used to deliver medication to the shunt site, and/or pressure sensors in the catheter may be used to monitor the intra-atrial pressure. 
     By way of example,  FIG. 1  shows the catheter coupled to a control handle  34 , such that control handle  34  may be used to advance and withdraw the catheter. ( FIG. 1  shows a scenario in which the catheter has been withdrawn from within the subject, but remains coupled to the control handle, poised for reentry into the subject.) 
     Shunt  26  helps relieve excess intra-atrial pressure, by allowing blood to flow from the higher-pressure atrium to the lower-pressure atrium. Shunt  26  may thus be used to treat any relevant condition (e.g., pulmonary hypertension or congestive heart failure) for which the relief of excess pressure is beneficial, or, for example, to help prevent left ventricular dilation and remodeling following an acute myocardial insult. Typically, the subject remains hospitalized until the subject&#39;s physician decides that sufficient treatment has been provided, at which point the shunt is removed from the subject (as further described immediately below), and the subject is released from hospital. In some embodiments, shunt apparatus  28  comprises one or more pressure sensors, disposed, for example, on shunt  26 , on any of the longitudinal elements, and/or in sheath  31 . Such pressure sensors may be used to measure (e.g., continuously) the pressure in the subject&#39;s right atrium and/or left atrium, in order to monitor progression of the treatment, and ascertain the point in time at which the shunt may be removed from the subject. For example, one pressure sensor may be disposed on the proximal portion of the shunt, and another pressure sensor on the distal portion of the shunt, such that the pressure in both the left atrium and the right atrium is measured. 
     Reference is now made to  FIGS. 3A-C , which collectively show a technique for removing shunt  26  from subject  20 , in accordance with some embodiments of the present invention. It is noted that many of the details shown in  FIGS. 3A-C  are provided by way of example only, and that many variations of the illustrated technique are included within the scope of the present disclosure. 
     In  FIG. 3A , catheter  46  is reinserted into the subject, and is then advanced until the distal end of the catheter is close to proximal portion  44  of the shunt. Subsequently, wires  36  are pulled, as indicated by the arrow  54  shown in the figure, such that an inward radial force is exerted on proximal portion  44 . The inward radial force causes proximal portion  44  to collapse, as shown in  FIG. 3B . Following the collapse of the proximal portion of the shunt, as shown in  FIG. 3C , the catheter is advanced distally over the shunt. (In passing over the shunt, the catheter may at least partly pass through the interatrial septum.) As the catheter continues to pass over the shunt from the position shown in  FIG. 3C , the catheter collapses the distal end of the shunt, such that the shunt becomes entirely collapsed within the catheter. Subsequently, the catheter, containing the shunt, may be removed from the subject. 
     In some embodiments, the catheter is advanced while proximal portion  44  is collapsing, such that, as proximal portion  44  continues to collapse, the catheter passes over the shunt, until the distal end of the catheter crosses through the septum and reaches the distal portion of the shunt. (In such embodiments, the state shown in  FIG. 3B  does not actually come to transpire, since catheter  46  covers the proximal portion of the shunt before the proximal portion of the shunt is fully collapsed.) Then, as the pulling of wires  36  continues while the catheter is held in place or is pushed forward, the distal end of the catheter exerts a force on the distal portion of the shunt, such that the distal portion of the shunt collapses, and the shunt is drawn into the catheter. Such embodiments have the advantage that, due to the catheter being advanced over the shunt while wires  36  are pulled, the shunt is less likely to be pulled into the right atrium. 
       FIGS. 3A-C  show, by way of example, an embodiment in which sheath  31  extends to a stopper  52  contained inside of control handle  34 , wires  36  passing through stopper  52 . As the wires are pulled, stopper  52  prevents sheath  31  from moving proximally, such that most of the pulling force acts on proximal portion  44 , rather than on sheath  31 . (Although flexible, sheath  31  is resistant to buckling, such that the pulling force is effectively transferred to proximal portion  44 .) A similar mechanism may be used for wire  38 , which, as described above, controls the diameter of the intermediate portion of the shunt. (In the context of the present application, including the claims, the term “stopper” may refer to anything that stops something else. For example, stopper  52  is referred to as a “stopper,” in that it stops the proximal movement of sheath  31 .) 
     In some embodiments, two separate tubes run through a single lumen, or two separate lumens, of sheath  31 , one of these tubes holding wires  36 , and the other of these tubes holding wire  38 . Such tubes which may comprise, for example, HHS® (“Helical Hollow Strand”) Tubes—may provide additional resistance to buckling, such that the pulling force exerted on the wires is effectively transmitted to the shunt. In such embodiments, stopper  52  may be used to prevent the wire-holding tubes from moving proximally as the wires are pulled. 
     It is noted that the apparatus and methods described above may also be used for applications in which an implanted shunt is required. In such applications, during the implantation procedure, wires  36  may be used to facilitate the retrieval of shunt  26 , in the event that the shunt was riot placed at the proper location. Subsequently, upon confirmation that the shunt is properly situated, wires  36  may be detached from shunt  26 , and removed from the subject. Similarly, during the implantation procedure, the constricting flexible longitudinal element may be used to adjust the diameter of the shunt. Subsequently, upon completion of the adjustment, the constricting flexible longitudinal element may be locked in place, such as to maintain the diameter of the shunt, and then any proximally-extending portion of the constricting flexible longitudinal element may be detached and removed. 
     Reference is now made to  FIGS. 4A-B , which are schematic illustrations showing the placement of a temporary shunt  56  within interatrial septum  24 , in accordance with some embodiments of the present invention. Apparatus and methods described with reference to  FIGS. 4A-B  may be used for applications in which temporary shunting is desired, alternatively to apparatus and methods described with reference to earlier figures. Thus, for example, as described above for shunt  26 , shunt  56  may be removed less than one week from placement of the shunt. 
     Typically, shunt  56  is an expandable structure made from a suitable shape-memory material, such as Nitinol, such that shunt  56  may move from a collapsed state (shown in  FIG. 4A ) to a preconfigured open state (shown in  FIG. 4B ) upon being subjected to an appropriate force, and may then recollapse upon removal of the force. In general, shunt  56  may have any suitable form. For example, as shown in the figures, shunt  56  may comprise a plurality of wires  68 , which are joined together at their distal ends (e.g., by being connected to a common ring), and are coupled to a sheath  62  at their proximal ends. (Sheath  62  may be referred to as a “subselective sheath,” in that the diameter of sheath  62  is small, relative to the “vascular sheath” referred to below.) In the collapsed state of the shunt, wires  68  are approximately parallel to each other along the majority of the length of the wires. Upon the distal and proximal ends of the wires being pushed together (i.e., upon the shunt being longitudinally compressed), the wires expand radially outward from each other, such that the shunt assumes the preconfigured open state. Typically, in the shunt&#39;s preconfigured open state, the proximal portion of the shunt and the distal portion of the shunt are wider than the intermediate portion of the shunt that is between the proximal portion of the shunt and the distal portion of the shunt. The wider proximal and distal portions of the shunt anchor the shunt to the septum (i.e., prevent migration of the shunt front within the septum). 
     As described above with respect to shunt  26 , shunt  56  is percutaneously inserted into the subject. First, a guidewire  58  is inserted into the right atrium, e.g., via the femoral vein and the inferior vena cava, as shown. Subsequently, guidewire  58  is passed across septum  24  (from the right atrium to the left atrium), using conventional techniques known in the art, such that the distal portion of the guidewire is in the left atrium. (To introduce guidewire  58 , a vascular sheath (not shown), which may alternatively be referred to as a catheter, may be used, as is known in the art. If necessary, a dilating tool may be passed over the guidewire, and through the opening created by the guidewire, to enlarge the opening in the septum.) Subsequently to passing the guidewire across the septum, shunt  56  is passed, in a collapsed state, over the guidewire, until the distal portion of the shunt is in the left atrium, and the proximal portion of the shunt is in the right atrium. In other words, as shown in  FIG. 4A , shunt  56  is positioned over the guidewire such that the shunt spans the septum. 
     A stopper  60  is coupled to the distal portion of guidewire  58  (i.e., near the distal end of the guidewire, such as within one or two millimeters of the distal end). Subsequently to positioning the shunt such that the shunt spans the septum, the stopper is used to open the shunt from the shunt&#39;s initial collapsed state. In one embodiment of this technique, the guidewire is retracted (proximally pulled), as indicated by the arrow  70  in the figure, such that the stopper presses against the distal portion of the shunt. While the guidewire is retracted, sheath  62  (and hence, the proximal end of the shunt) is held in place or is advanced, such that, by pressing against the distal portion of the shunt, the stopper applies a longitudinally compressive force to the shunt. The compressive force causes the shunt to open, and hence, the opening in the septum to become enlarged, as shown in  FIG. 4B . (In alternative embodiments, the stopper may press against the distal portion of the shunt by virtue of the sheath being advanced while the guidewire is held in place.) 
     Stopper  60  is referred to herein as a “stopper,” in that stopper  60  stops the advancement of the shunt along the guidewire, by virtue of the stopper being large enough such that the shunt cannot pass over the stopper. Stopper  60  may have any form suitable for applying the longitudinally compressive force to the shunt. For example, the stopper may comprise a bead, as shown in the figure; alternatively, for example, stopper  60  may comprise a plate or rod which, when pressed against the shunt, applies the longitudinally compressive force. In some embodiments, the shunt is shaped to define a distal aperture  66 , shaped to fittingly receive the stopper. (In such embodiments, the stopper may be referred to as a “lock,” and distal aperture  66  may be referred to as a “fitting.”) As the guidewire is withdrawn in the proximal direction, the stopper engages with, i.e., is fittingly received by, distal aperture  66 . Subsequently, the stopper continues to press against the distal portion of the shunt while the stopper is fittingly received by the distal aperture. The engagement between the distal aperture and the stopper helps the pressing force to be more effectively applied to the shunt. 
     Typically, to maintain the open state of shunt  56 , it is necessary to maintain the longitudinally compressive force on the shunt. This may be accomplished by, using a locking mechanism, locking sheath  62  with respect to (or “against”) the guidewire, i.e., fixing the position of the sheath with respect to the guidewire. For example,  FIG. 4B  shows a lock  64  placed over the guidewire, proximally to the sheath. Lock  64  prevents the sheath from sliding backward along the guidewire, and the guidewire from sliding forward within the sheath, such that the longitudinally compressive force on the shunt is maintained. In some embodiments, the guidewire and sheath are coupled to control handle  34  ( FIG. 1 ), and the control handle is used to lock the sheath. 
     Subsequently, upon completion of the treatment, the sheath is unlocked, e.g., by removing lock  64 . The unlocking of the sheath allows the sheath to move proximally, and/or allows the guidewire to move distally, thus removing the longitudinally compressive force that had heretofore been applied to the shunt. The shunt therefore collapses. Subsequently, the sheath, and the shunt, are removed from the subject. 
     In some embodiments, prior to unlocking the sheath, a catheter is advanced over the sheath, until the distal end of the catheter is near the proximal end of the shunt. Subsequently, upon collapse of the shunt, the shunt is pulled into the catheter, and/or the catheter is advanced over the shunt. The catheter is then used to remove the shunt from the subject. In other embodiments, no catheter is used for removal of the shunt. Rather, upon collapse of the shunt, the sheath and shunt are proximally pulled, through the vasculature of the subject, out of the subject. 
     In some embodiments, one or more pressure sensors are disposed on guidewire  58 , shunt  56 , and/or sheath  62 . Such pressure sensors may be used to measure the pressure in the subject&#39;s right atrium and/or left atrium, as described above for apparatus  28 . 
     Although the description above relates mainly to interatrial shunting, it is noted that shunt  56 , and the above-described techniques for deployment thereof, may be used for shunting between any relevant two body cavities. For example, in an interventricular shunting application, guidewire  58  may be passed through the interventricular septum, from the right ventricle to the left ventricle, and the shunt may then be deployed over the guidewire and opened within the interventricular septum. 
     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.