Patent Publication Number: US-2022226556-A1

Title: Shunts with blood-flow indicators

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Application 63/138,451, entitled “Visualization of blood flow in a venous/arterial shunting system,” filed Jan. 17, 2021, whose disclosure is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to medical procedures, such as procedures in which blood of a subject is shunted from one blood vessel to another. 
     BACKGROUND 
     In some procedures, blood is shunted from one blood vessel to another. 
     SUMMARY OF THE INVENTION 
     There is provided, in accordance with some embodiments of the present invention, an apparatus for shunting blood. The apparatus includes a flow-indication chamber shaped to define an entry port and an exit port, and one or more moveable objects disposed within the flow-indication chamber and configured to move in response to a flowing of the blood from the entry port to the exit port. At least a portion of a wall of the flow-indication chamber is transparent so as to expose the moveable objects to sight. 
     In some embodiments, the moveable objects include a plurality of beads. 
     In some embodiments, at least one of the beads includes multiple faces. 
     In some embodiments, each of the beads is coated with an anticoagulant. 
     In some embodiments, the moveable objects include a rotational member configured to rotate in response to the blood exerting a force on the rotational member. 
     In some embodiments, the rotational member includes a wheel including a plurality of spokes. 
     In some embodiments, the wheel is positioned relative to the entry port and exit port such that each of the spokes, when perpendicular to an unimpeded path of the blood from the entry port to the exit port, intersects the unimpeded path. 
     In some embodiments, a most proximal point on each of the spokes that intersects the unimpeded path is located between 50% and 80% of a length of the spoke from a proximal end of the spoke. 
     In some embodiments, the apparatus further includes a filter chamber configured to couple to the flow-indication chamber and to hold a blood filter in the filter chamber. 
     In some embodiments, the apparatus further includes a threaded ring, 
     the filter chamber is shaped to define a filter-chamber port, 
     the flow-indication chamber is configured to screw into a first side of the threaded ring, and 
     the filter chamber is configured to screw into a second side of the threaded ring such that the entry port or exit port is fluidly connected with the filter-chamber port. 
     In some embodiments, the flow-indication chamber is further configured to hold a blood filter therein. 
     In some embodiments, the apparatus further includes a valve configured to regulate the flowing of the blood through a port selected from the group of ports consisting of: the entry port, and the exit port. 
     In some embodiments, the valve includes: 
     a pushable element passing through a wall of the flow-indication chamber and configured to cover the port upon being pushed into the flow-indication chamber; and 
     a spring coupled to the pushable element and to an inner wall of the flow-indication chamber, and configured to inhibit the pushable element from covering the port in an absence of any pushing force applied to the pushable element. 
     In some embodiments, the spring includes a tension spring. 
     There is further provided, in accordance with some embodiments of the present invention, a method including coupling an upstream end of a first conduit to a source blood vessel of a subject and a downstream end of the first conduit to an entry port of a shunt. The method further includes coupling an upstream end of a second conduit to an exit port of the shunt and a downstream end of the second conduit to a sink blood vessel of the subject, such that blood flows from the source blood vessel to the sink blood vessel via the shunt, thus causing movement of one or more moveable objects disposed within the shunt, the movement being visible through a wall of the shunt. 
     In some embodiments, the shunt includes a flow-indication chamber, which contains the moveable objects, and a filter chamber, which is coupled to the flow-indication chamber and holds a blood filter in the filter chamber. 
     There is further provided, in accordance with some embodiments of the present invention, an apparatus including a chamber shaped to define a fluid port, a first appendage protruding from the chamber and shaped to define a first aperture and a first row of one or more teeth, and a second appendage protruding from the chamber and shaped to define a second aperture and a second row of one or more teeth parallel to the first row of teeth. The second row is configured to interlock with the first row at multiple different relative positions of the first appendage and second appendage in which the second aperture is aligned with the first aperture with different respective degrees of alignment such that a tube, which carries blood to or from the fluid port through the first aperture and second aperture, is constricted with different respective degrees of constriction. 
     In some embodiments, the chamber is configured to hold a blood filter therein. 
     In some embodiments, the apparatus further includes one or more moveable objects disposed within the chamber and configured to move in response to a flowing of the blood through the chamber, at least a portion of a wall of the chamber being transparent so as to expose the moveable objects to sight. 
     In some embodiments, the first appendage and second appendage are continuous with a wall of the chamber. 
     In some embodiments, the first appendage and second appendage are configured to revert to a default relative position, in which the tube is not constricted, upon a release of the first row and second row from one another. 
     In some embodiments, 
     the first appendage includes a first back arm, which protrudes from the chamber, and a first front arm, which is angled with respect to the first back arm and is shaped to define the first aperture and the first row of teeth, and 
     the second appendage includes a second back arm, which protrudes from the chamber, and a second front arm, which is angled with respect to the second back arm and is shaped to define the second aperture and the second row of teeth. 
     In some embodiments, each of the teeth in the first row and in the second row is angled backward, such that an advancement of the first row and second row relative to one another constricts the tube. 
     In some embodiments, the tube is not completely constricted in any of the positions. 
     There is further provided, in accordance with some embodiments of the present invention, a method including sliding a first appendage, which protrudes from a chamber and is shaped to define a first aperture and a first row of one or more teeth, across a second appendage, which protrudes from the chamber and is shaped to define a second aperture and a second row of one or more teeth parallel to the first row of teeth, such that the second row interlocks with the first row at a relative position of the first appendage and second appendage in which the second aperture is misaligned with the first aperture, thereby constricting a tube that carries blood to or from a fluid port of the chamber through the first aperture and second aperture. The method further includes, subsequently to constricting the tube, releasing the first row and second row from one another, thereby causing the first appendage and second appendage to revert to a default relative position in which the tube is not constricted. 
     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 
         FIGS. 1-3  are schematic illustrations of an apparatus for shunting blood, in accordance with some embodiments of the present invention; and 
         FIGS. 4A-B  are schematic illustrations of a tube constrictor, in accordance with some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     In some cases, it may be necessary to shunt blood from one anatomical site to another. For example, during an operation to remove a clot from a carotid artery of a subject, it may be necessary to shunt blood from the carotid artery to a vein, such as a femoral vein, of the subject. In such cases, a shunting device (or “shunt”) is used to carry blood between the sites. However, there is a risk of the blood flow through the shunt slowing or stopping without the physician noticing. 
     To mitigate this risk, embodiments of the present invention provide a shunt comprising a flow-indication chamber containing one or more moveable objects, which are configured to move in response to the flow of blood through the flow-indication chamber. At least a portion of the wall of the flow-indication chamber is transparent, such that the moveable objects are visible. For example, the wall may comprise a transparent window, and the moveable objects may be disposed behind the window. Thus, a physician may readily check whether the blood is flowing properly through the shunt, by observing the degree of motion of the moveable objects. 
     In some embodiments, the moveable objects comprise multiple beads suspended in the blood, which rotate and/or change position as the blood flows. In other embodiments, the moveable objects comprise a wheel comprising a plurality of radiating spokes, which rotates as the blood flows across the spokes. 
     Another challenge is that sometimes it may be necessary to slow or stop the flow of blood through the shunt temporarily, e.g., to allow more blood to flow through the carotid artery to the brain of the subject. 
     To address this challenge, some embodiments of the present invention equip the shunt with a valve configured to control the rate of flow through the entry port or exit port of the shunt. For example, the valve may comprise a pushable element passing through the wall of the aforementioned flow-indication chamber, along with a spring within the flow-indication chamber, which couples the pushable element to an inner wall of the flow-indication chamber. In its resting state, the spring holds the pushable element away from the port, such that blood freely flows through the port. On the other hand, a pushing force sufficient to overcome the force of the spring may push the pushable element over the port, thereby slowing or stopping the flow of blood. 
     In other embodiments, the shunt is equipped with a tube constrictor configured to constrict the tube carrying the blood to or from the port. The tube constrictor comprises a pair of parallel arms, which are shaped to define respective apertures and respective rows of teeth. The rows of teeth are configured to interlock with one another at a default position, in which the apertures are aligned with one another, and at one or more other positions, in which the apertures are misaligned by varying degrees. Thus, while the tube passes through the apertures, the tube may be partially or fully constricted by sliding the arms over one another. Subsequently to constricting the tube, to resume regular blood flow, the rows of teeth may be released from one another such that the arms revert to their default positions. 
     Apparatus Description 
     Reference is initially made to  FIG. 1 , which is a schematic illustration of an apparatus  20  for shunting blood  22 , in accordance with some embodiments of the present invention. An inset portion  44  of  FIG. 1  shows part of the interior of apparatus  20 . 
     Apparatus  20 , which may be referred to as a “shunt,” comprises a flow-indication chamber  24  shaped to define an entry port  26  and an exit port  28 . Blood  22  enters flow-indication chamber  24  via entry port  26 , flows through the flow-indication chamber, and exits the flow-indication chamber via exit port  28 . 
     In some embodiments, entry port  26  is configured to couple to an entry tube  30  (or any other entry conduit, such as a catheter) through which blood  22  flows to apparatus  20 . For example, entry tube  30  may be fittingly inserted into entry port  26 , or the entry port may be fittingly inserted into the entry tube. 
     In some embodiments, apparatus  20  further comprises a filter chamber  32  configured to couple to flow-indication chamber  24  and to hold a blood filter  34  in filter chamber  32 . Blood filter  34  may be secured within the filter chamber using any suitable structural components, such as a plurality of ribs  86  as shown in  FIG. 3 , which is described below. Filter chamber  32  is shaped to define an entry port  38 , through which blood  22  enters the filter chamber, and an exit port  40 , through which the blood exits the filter chamber. 
     As shown in  FIG. 1 , the filter chamber may be coupled to the flow-indication chamber downstream from the flow-indication chamber, such that the blood flows through the filter chamber after flowing through the flow-indication chamber. (Optionally, a single common port may function as both exit port  28  and entry port  38 .) In such embodiments, exit port  40  is configured to couple to an exit tube  42  (or any other exit conduit, such as a catheter), which carries the blood from apparatus  20 . 
     Alternatively, the filter chamber may be coupled to the flow-indication chamber upstream from the flow-indication chamber. (Optionally, a single common port may function as both exit port  40  and entry port  26 .) In such embodiments, entry port  38  of the filter chamber is configured to couple to entry tube  30 , and exit port  28  of the flow-indication chamber is configured to couple to exit tube  42 . 
     In some embodiments, apparatus  20  further comprises a threaded ring  36 . Flow-indication chamber  24  is configured to screw into one side of threaded ring  36 , and filter chamber  32  is configured to screw into the other side of the threaded ring such that the two chambers are in fluid communication with one another. For example, as shown in  FIG. 1 , the flow-indication chamber may be screwed into the upstream side of ring  36  and the filter chamber may be screwed into the downstream side of ring  36 , such that the blood flows directly from exit port  28  into entry port  38 . Alternatively, the filter chamber may be screwed into the upstream side of ring  36  and the flow-indication chamber may be screwed into the downstream side of ring  36 , such that the blood flows directly from exit port  40  into entry port  26 . 
     In other embodiments, as shown in  FIG. 3 , the flow-indication chamber and filter chamber are coupled to one another via a coupling tube  84 . 
     In yet other embodiments, apparatus  20  does not comprise filter chamber  32 . In such embodiments, flow-indication chamber  24  may be configured to hold blood filter  34  therein. 
     Apparatus  20  may shunt blood  22  between any two suitable blood vessels of a human or animal subject. In other words, apparatus  20  may shunt blood  22  from any suitable “source” blood vessel of the subject to any suitable “sink” blood vessel of the subject. For example, apparatus  20  may shunt blood from an artery of the subject to a vein of the subject, with entry tube  30  delivering blood from the artery and exit tube  42  carrying the blood to the vein. As a specific example, apparatus  20  may shunt blood from a carotid artery to a femoral vein during an operation to remove a clot from the carotid artery. Alternatively, apparatus  20  may shunt blood from a higher-pressure artery to a lower-pressure artery. 
     To deploy apparatus  20 , the upstream end of entry tube  30  is coupled to the source blood vessel (e.g., via a stopcock and/or any other suitable equipment), and the downstream end of the entry tube is coupled to the entry port of apparatus  20  (e.g., entry port  26 , for embodiments in which the flow-indication chamber is upstream from the filter chamber). Similarly, the upstream end of exit tube  42  is coupled to the exit port of apparatus  20  (e.g., exit port  40 , for embodiments in which the flow-indication chamber is upstream from the filter chamber), and the downstream end of the exit tube is coupled to the sink blood vessel (e.g., via a stopcock and/or any other suitable equipment). Subsequently, blood flows from the source blood vessel to the sink blood vessel via apparatus  20 . 
     Blood-Flow Indicators 
     Apparatus  20  further comprises one or more moveable objects  46  disposed within flow-indication chamber  24  and configured to move in response to the flowing of blood  22  from entry port  26  to exit port  28 . At least a portion of the wall  52  of the flow-indication chamber is transparent so as to expose moveable objects  46  to sight. For example, wall  52  may be entirely transparent, as shown in  FIG. 3  (described below). Alternatively, as shown in  FIG. 1 , the wall may comprise at least one transparent window  50 . Thus, the physician may readily check the rate of blood flow through the flow-indication chamber, by observing the degree of movement of moveable objects  46 . In some embodiments, the transparent portion of wall  52  comprises a magnifying lens, configured to magnify moveable objects  46 . 
     In some embodiments, moveable objects  46  comprise a plurality of beads  48 , which rotate and/or change position as the blood flows. Typically, beads  48  have a density less than that of blood  22 , such that the beads remain suspended in the blood. Beads  48  may comprise any suitable hemocompatible material such as a metal, plastic, wood, latex, synthetic rubber, or any combination of the above. 
     In general, each of the beads may have any suitable shape. For example, beads  48  may comprise at least one spherical bead  48   a . Alternatively or additionally, beads  48  may comprise at least one bead comprising multiple faces; such a bead may move more in response to the blood flow, relative to spherical bead  48   a , due to the greater force applied to the bead by the blood. Example of beads comprising multiple faces include a cubical bead  48   b  and a pyramidical bead  48   c.    
     In general, larger beads may be more noticeable than smaller beads; hence, in some embodiments, for each bead  48 , the Cartesian distance between any two points on the outer surface of the bead is greater than 0.1 cm. Alternatively or additionally, for increased movement of the bead, the Cartesian distance between any two points on the outer surface of the bead may be less than 0.65 cm. 
     In some embodiments, each of the beads is coated with an anticoagulant, such as heparin. 
     In some embodiments, for greater visibility, the color of the beads contrasts with that of blood  22 . Suitable contrasting colors include black, blue, and white. Alternatively, the beads may have any other color. 
     In some embodiments, exit port  28  is covered with a filter configured to inhibit any of the beads from passing through. Alternatively or additionally, as described above, filter chamber  32  may be coupled to the flow-indication chamber downstream from the flow-indication chamber, such that any beads that pass through exit port  28  are filtered from the blood by filter  34 . 
     Reference is now made to  FIG. 2 , which is a schematic illustration of apparatus  20 , in accordance with some embodiments of the present invention. An inset portion  58  of  FIG. 2  shows part of the interior of apparatus  20 . 
     In some embodiments, moveable objects  46  comprise a rotational member configured to rotate in response to the blood exerting a force on the rotational member. 
     For example, moveable objects  46  may comprise a wheel  54  comprising a plurality of spokes  56  (which may also be referred to as “radial members”) and configured to rotate in response to blood  22  exerting a force on spokes  56 . For noticeability, the length of each spoke  56  may be greater than 0.6 cm, and/or the width of each spoke may be greater than 0.3 cm. Alternatively or additionally, to obviate the need for an overly large flow-indication chamber, the length of each spoke  56  may be less than 3.8 cm, and/or the width of each spoke may be less than 1.3 cm. 
     Using two dashed lines, inset portion  58  demarcates the unimpeded path  60  of the blood, i.e., the path from entry port  26  to exit port  28  that the blood would follow in the absence of wheel  54 . Typically, wheel  54  is positioned relative to the entry and exit ports such that each spoke, when perpendicular to path  60  at any point along the path, intersects the path. Thus, the flow of blood through the flow-indication chamber generally keeps the wheel rotating in a single direction. For example, in  FIG. 2 , the wheel rotates clockwise, as indicated by a rotation indicator  55 . 
     For example, denoting the end of the spoke closest to hub  62  as the proximal end of the spoke and the opposite end as the distal end of the spoke, the most proximal point on the spoke that intersects path  60  may be located between 50% and 80% of the length of the spoke from the proximal end of the spoke. (For example, if the spoke is 3 cm long, the most proximal point on the spoke that intersects path  60  may be located between 1.5 and 2.4 cm from the proximal end of the spoke.) Advantageously, this positioning of the wheel may increase the rotational force to which the wheel is subjected. 
     In some embodiments, the internal walls  61  of flow-indication chamber  24  constrict the space within the chamber in which the blood can flow, such that the blood follows path  60  at a greater speed and hence, applies greater force to the rotational member. 
     Typically, as shown in Section A-A, wheel  54  is mounted onto a shaft  64  (i.e., shaft  64  passes through hub  62 ), such that the wheel rotates about the shaft. Shaft  64  is coupled at each of its ends to wall  52 . 
     The rotational member (e.g., wheel  54 ) may be any suitable color, including a color that contrasts with that of blood, as described above for beads  48  ( FIG. 1 ). 
     In alternative embodiments, Doppler ultrasound is used to measure the rate of blood flow. For example, a fixture, shaped to define a socket, may be fitted over one of the tubes, and a standard Doppler ultrasound probe may be inserted into the socket. 
     Regulating Blood Flow 
     Reference is again made to  FIG. 1 . 
     In some embodiments, apparatus  20  further comprises a valve  66  configured to regulate the flow of blood through the flow-indication chamber. Thus, using valve  66 , a physician may control the rate at which blood is shunted. 
     In some embodiments, valve  66  comprises a pushable element  68  passing through wall  52  and configured to cover entry port  26  or exit port  28  upon being pushed into the flow-indication chamber. Typically, a gasket  76  (made of rubber, for example) seals the aperture in wall  52  through which the pushable element passes, such that blood does not leak through the wall. 
     In such embodiments, valve  66  further comprises a spring  78  coupled to the pushable element (e.g., by virtue of being coupled to a ledge  82  coupled to the pushable element) and to an inner wall of the flow-indication chamber (e.g., the inside of wall  52 ). Spring  78  is configured to inhibit the pushable element from covering the entry port or exit port in the absence of any pushing force applied to the pushable element. Thus, to slow or stop the flow of blood, the physician must continuously exert a pushing force to counteract the force applied by the spring, such that the physician is unlikely to forget that the flow has been slowed or stopped. 
     Typically, as shown in  FIG. 1 , spring  78  comprises a tension spring  80 . In the absence of any pushing force, spring  80  is maximally compressed, such that the tension spring holds the pushable element in its outermost position. 
     In some embodiments, pushable element  68  comprises a neck  70  and a foot  72 , which protrudes from the end of neck  70  that is inside the flow-indication chamber. As neck  70  is pushed further into the flow-indication chamber, foot  72  covers a greater portion of the entry port or exit port, thereby slowing the flow of blood. Upon the neck being maximally pushed, foot  72  completely covers the port, such that the flow is stopped. Optionally, the opposite end of neck  70 , which is outside the flow-indication chamber, may terminate at a head  74 , which is wider than the neck and thus facilitates the pushing of the neck into the flow-indication chamber. 
     In other embodiments, valve  66  comprises a pullable element passing through wall  52  and configured to cover entry port  26  or exit port  28  upon being pulled. 
     (It is noted that valve  66  may also be combined with any other suitable embodiment of moveable objects  46 , such as the embodiment of  FIG. 2 .) 
     Reference is now made to  FIG. 3 , which is a schematic illustration of apparatus  20 , in accordance with some embodiments of the present invention. 
     In some embodiments, apparatus  20  comprises tube constrictor  88 , which may also be referred to as a “locking clip.” Tube constrictor  88  comprises a first appendage  90   a , shaped to define a first aperture  92   a , and a second appendage  90   b , shaped to define a second aperture  92   b.    
     As further described below with reference to  FIGS. 4A-B , tube constrictor  88  is configured to control the rate of blood flow through a tube passing through apertures  92   a  and  92   b  by constricting the tube with varying degrees of constriction. Thus, apparatus  20  need not necessarily comprise valve  66  ( FIG. 1 ). 
     For example, tube constrictor  88  may provide two degrees of constriction: no (0%) constriction, and full (100%) or partial (e.g., 80%-90%) constriction. Alternatively, tube constrictor  88  may provide three or more degrees of constriction. An example of four degrees of constriction is 0%, 20%-40% (e.g., 33%), 60%-80% (e.g., 66%), and 90%-100%. 
     (In the context of the present application, including the claims, the tube may be considered to be constricted by x % if the rate of blood flow through the tube is x % of what the rate would be if the tube were not constricted at all.) 
     In some such embodiments, flow-indication chamber  24  is coupled to filter chamber  32  via coupling tube  84 , and tube constrictor  88  is configured to constrict the coupling tube. For example, as shown in  FIG. 3 , first appendage  90   a  and second appendage  90   b  may protrude from filter chamber  32  in the upstream direction, i.e., the first and second appendages may protrude beyond entry port  38 , and coupling tube  84  may carry blood to entry port  38  through the first and second apertures. Alternatively, the first and second appendages may protrude from flow-indication chamber  24  in the downstream direction, i.e., the first and second appendages may protrude beyond exit port  28 , and coupling tube  84  may carry blood from exit port  28  through the first and second apertures. 
     In other such embodiments, tube constrictor  88  is configured to constrict entry tube  30 . In other words, first appendage  90   a  and second appendage  90   b  protrude from flow-indication chamber  24  in the upstream direction, i.e., the first and second appendages protrude beyond entry port  26 , and entry tube  30  carries blood to entry port  26  through the first and second apertures. (In this case, flow-indication chamber  24  may be coupled to filter chamber  32  as in  FIGS. 1-2 , or filter chamber  32  may be omitted.) 
     In yet other such embodiments, tube constrictor  88  is configured to constrict exit tube  42 . In other words, first appendage  90   a  and second appendage  90   b  protrude from filter chamber  32  in the downstream direction, i.e., the first and second appendages protrude beyond exit port  40 , and exit tube  42  carries blood from exit port  40  through the first and second apertures. (In this case, flow-indication chamber  24  may be coupled to filter chamber  32  as in  FIGS. 1-2 , or flow-indication chamber  24  may be omitted.) 
     For those embodiments in which apparatus  20  comprises flow-indication chamber  24 , the flow-indication chamber may contain any suitable moveable objects  46 , such as wheel  54  or beads  48  ( FIG. 1 ). 
     In some embodiments, the first and second appendages are continuous with the wall of the chamber from which the appendages protrude, i.e., the wall extends beyond the chamber so as to define the appendages. In other embodiments, the appendages are coupled to the wall of the chamber, e.g., using any suitable adhesive. 
     Typically, each appendage comprises a back arm  100 , which protrudes from the chamber, and a front arm  102 , which is angled (e.g., at approximately 90 degrees) with respect to back arm  100  and is shaped to define the aperture through which the tube passes. 
     Reference is now made specifically to inset portion  110  of  FIG. 3 , which shows the back of front arm  102  of first appendage  90   a , i.e., the surface of the front arm that faces second appendage  90   b.    
     In addition to first aperture  92   a , first appendage  90   a  (e.g., front arm  102  of the first appendage) is shaped to define a first row  94   a  of one or more teeth  96 . Similarly, as shown in  FIGS. 4A-B , second appendage  90   b  (e.g., front arm  102  of the second appendage) is shaped to define a second row  94   b  of one or more teeth  96  parallel to first row  94   a . As further described below with reference to  FIGS. 4A-B , second row  94   b  is configured to interlock with first row  94   a  at multiple different relative positions of the first appendage and second appendage. In these different positions, second aperture  92   b  is aligned with first aperture  92   a  with different respective degrees of alignment such that coupling tube  84  (or any other tube passing through the apertures) is constricted with different respective degrees of constriction. 
     Reference is now made to  FIGS. 4A-B , which are schematic illustrations of tube constrictor  88 , in accordance with some embodiments of the present invention. 
     In  FIG. 4A , the first and second appendages are at a first relative position in which second aperture  92   b  is aligned with first aperture  92   a , such that coupling tube  84  is not constricted. Subsequently, one or both of the appendages may be shifted such that the appendages assume a second relative position in which the two apertures are less aligned with one another, and hence tube  84  is mostly constricted, as shown in  FIG. 4B . 
     In some embodiments, as shown in  FIGS. 4A-B , each tooth  96  is angled backward (e.g., toward back arm  100  of the appendage), such that the tooth comprises a longer front edge  104  and a shorter back edge  106 . In such embodiments, the tube may be constricted by advancing the two rows of teeth relative to one another, e.g., by pushing at least one front arm  102  toward the back arm  100  of the other appendage. For example, as indicated in  FIG. 4A  by pinch indicators  98 , the appendages may be pinched together, e.g., using a forefinger placed on one back arm  100  and a thumb placed on the other back arm. As one or both of the rows are advanced, front edges  104  slide across each other, until the rows of teeth interlock at the next position by virtue of the contact between back edges  106 , which inhibits the rows from sliding backward. 
     In other embodiments, each tooth  96  is angled forward, toward the tip of the appendage. In such embodiments, the tube may be constricted by moving at least one row of teeth backward relative to the other row of teeth, e.g., by pulling at least one front arm  102  away from the back arm  100  of the other appendage. 
     In some embodiments, the first and second appendages are configured to revert to a default relative position, in which the tube is not constricted, upon a release of first row  94   a  and second row  94   b  from one another, as indicated in  FIG. 4B  by release indicators  108 . In other words, at least one of the appendages is elastic, such that any movement from the default relative position causes the appendage to store elastic energy that, upon release of the rows from one another, causes the appendage to revert to the default position. Alternatively or additionally, upon release, the first and second appendages may revert to their default relative position due to elastic energy stored in the wall of the tube while the tube is constricted. 
     (It is emphasized that the appendages may be shaped to define fewer teeth than are shown in the figures. For example, one appendage may be shaped to define a single tooth, and the other appendage may be shaped to define N≥2 teeth, such that N degrees of constriction are provided.) 
     Typically, the tube is not completely constricted in any of the positions in which the rows of teeth interlock with one another. In other words, as shown in  FIG. 4B , even at the most constricted interlocked position, the tube may remain partly (e.g., 10%-20%) unconstricted. Thus, advantageously, full constriction of the tube requires that the physician continuously apply a force to one or both of the appendages, such that the physician is unlikely to forget that the flow has been stopped. For example, in the scenario shown in  FIG. 4B , full constriction of the tube may require a continuous pinching of the appendages. In the absence of a pinching force, the elastic energy stored in the appendages and/or the wall of the tube causes the appendages to revert to the most constricted interlocked position. 
     (For embodiments in which the teeth are angled backward, the most constricted interlocked position is that in which the frontmost tooth of one appendage locks against the backmost tooth of the other appendage. For embodiments in which the teeth are angled forward, the most constricted interlocked position is that in which the frontmost tooth of one appendage locks against the frontmost tooth of the other appendage.) 
     Alternatively to tube constrictor  88 , apparatus  20  may comprise any other clamp that protrudes from one of the chambers and is configured to constrict a tube in fluid communication with the chamber. The clamp may be continuous with the wall of the chamber from which the clamp protrudes, or coupled to the wall of the chamber. 
     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 embodiments 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. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.