Patent Publication Number: US-11648387-B2

Title: Blood pump

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. Ser. No. 15/574,948 to Schwammenthal, filed Nov. 17, 2017 (published as US 2018/0169313), which is a US national phase application of PCT Application No. PCT/IL/2016/050525 to Schwammenthal (published as WO 16/185473), filed May 18, 2016, which claims priority from U.S. Provisional Patent Application 62/162,881 to Schwammenthal, filed May 18, 2015, entitled “Blood pump,” which is incorporated herein by reference. 
    
    
     FIELD OF EMBODIMENTS OF THE INVENTION 
     Some applications of the present invention generally relate to medical apparatus. Specifically, some applications of the present invention relate to apparatus and methods associated with placing a pump in one or more of a subject&#39;s renal veins, and/or in the subject&#39;s vena cava. 
     BACKGROUND 
     It is common for cardiac dysfunction or congestive heart failure to develop into kidney dysfunction, which, in turn, causes congestive heart failure symptoms to develop or worsen. Typically, systolic and/or diastolic cardiac dysfunction causes systemic venous congestion, which gives rise to an increase in renal venous and interstitial pressure. The increase in the pressure causes fluid retention by the body to increase due both to kidney dysfunction and renal neurohormonal activation, both of which typically develop as a result of the increase in renal venous and interstitial pressure. The resulting fluid retention causes congestive heart failure to develop or worsen, by causing a blood volume overload at the heart and/or by increasing systemic resistance. Similarly, it is common for kidney dysfunction and/or renal neurohormonal activation to develop into cardiac dysfunction and/or congestive heart failure. This pathophysiological cycle, in which cardiac dysfunction and/or congestive heart failure leads to kidney dysfunction and/or renal neurohormonal activation, or in which kidney dysfunction and/or renal neurohormonal activation leads to cardiac dysfunction and/or congestive heart failure, each dysfunction leading to deterioration in the other dysfunction, is called the cardio-renal syndrome. 
     Increased renal venous pressure has been experimentally shown to cause azotemia, and a reduction in glomerular filtration rate, renal blood flow, urine output, and sodium excretion. It has also been shown to increase plasma renin and aldosterone, and protein excretion. Venous congestion may also contribute to anemia via three different pathways: A reduction in the kidney&#39;s erythropoietin production, hemodilution by fluid retention, and an inflammatory response leading to a reduced gastro-intestinal iron uptake. 
     Mechanistically, increased renal venous pressure may cause intracapsular pressure and, subsequently, interstitial peritubular pressure, to rise. A rise in peritubular pressure may impact tubular function (reduce sodium excretion), as well as diminish glomerular filtration, by raising the pressure in the Bowman capsule. 
     In heart failure patients, increased renal venous pressure may not only result from increased central venous (right atrial) pressure, but also from intraperitoneal fluid accumulations (ascites) exerting direct pressure on the renal veins. Reduction of intraabdominal pressure in heart failure patients by removal of fluid (e.g., via paracentesis, and/or ultrafiltration), has been shown to reduce plasma creatinine levels. 
     Increased venous return resulting from activation of the “leg muscle pump” during physical activity such as walking may raise systemic venous pressure, particularly in heart failure patients, and may result in reflux into the renal veins. 
     SUMMARY OF EMBODIMENTS 
     In accordance with some applications of the present invention, a subject is identified as suffering from cardiac dysfunction, congestive heart failure, reduced renal blood flow, increased renal vascular resistance, arterial hypertension, diabetes, and/or kidney dysfunction. In response thereto, blood pressure within the subject&#39;s renal veins is reduced by placing at least one pump in the subject&#39;s vena cava, and generating a low-pressure region within the subject&#39;s vena cava adjacent to junctions of the vena cava with the subject&#39;s renal veins, by activating the pump to pump blood away from the region. The pump is activated such that blood pressure within the low-pressure region is lower than central venous pressure of the subject. Typically, a downstream pump is placed within the vena cava downstream of the junctions of the vena cava with the subject&#39;s renal veins, and the pump pumps blood through the vena cava in the downstream direction, away from the junctions. For some applications, an upstream pump is placed within the vena cava upstream of the junctions of the vena cava with the subject&#39;s renal veins, and the pump pumps blood through the vena cava in the upstream direction, away from the junctions. Alternatively or additionally, an occlusion element, such as a balloon or a covered stent is placed in the vena cava upstream of the junctions, and is configured to partially occlude the vena cava upstream of the junctions. 
     For some applications, the upstream and downstream pumps are disposed on a single catheter. Typically, the catheter is inserted into the vena cava via a venous pathway, e.g., via the femoral vein, via the subclavian vein, or via the jugular vein. For some applications, the upstream pump, or the occlusion element is disposed on a first catheter, which is inserted via a vein that is below the subject&#39;s inferior vena cava (e.g., the femoral vein), and the downstream pump is disposed on a second catheter, which is inserted via a vein that is above the subject&#39;s inferior vena cava (e.g., the subclavian vein, or the jugular vein). 
     For some applications, the downstream pump and/or the upstream pump includes an impeller and a cage. For some applications, impellers of the downstream and the upstream pumps rotate in the same direction, but the downstream pump is configured to pump blood in the downstream direction and the upstream pump is configured to pump blood in the upstream direction. For some such applications, a single motor is used to impart rotational motion to both of the impellers, and there is a shaft disposed between the impellers that imparts rotational motion from a first one of the impellers to a second one of the impellers. Typically, for such applications, the impellers of the upstream and the downstream pumps are (a) of opposing handedness with respect to one another (i.e., one of the impellers is a left-handed impeller, and the other impeller is a right-handed impeller), and (b) are disposed upon the aforementioned shaft, such that the impellers are facing opposite directions to one another. 
     In general, in the specification and in the claims of the present application, the term “proximal” and related terms, when used with reference to a device or a portion thereof, should be interpreted to mean an end of the device or the portion thereof that, when inserted into a subject&#39;s body, is typically closer to a location through which the device is inserted into the subject&#39;s body. The term “distal” and related terms, when used with reference to a device or a portion thereof, should be interpreted to mean an end of the device or the portion thereof that, when inserted into a subject&#39;s body, is typically further from the location through which the device is inserted into the subject&#39;s body. 
     In general, in the specification and in the claims of the present application, the term “downstream” and related terms, when used with reference to a blood vessel, or with reference to a portion of a device that is configured to be placed inside a blood vessel, should be interpreted to mean a location within the blood vessel, or a portion of the device that is intended for placement at a location within the blood vessel, that is downstream, with respect to the direction of antegrade blood flow through the blood vessel, relative to a different location within the blood vessel. The term “upstream” and related terms, when used with reference to a blood vessel, or with reference to a portion of a device that is configured to be placed inside a blood vessel, should be interpreted to mean a location within the blood vessel, or a portion of the device that is intended for placement at a location within the blood vessel, that is upstream with respect to the direction of antegrade blood flow through the blood vessel, relative to a different location within the blood vessel. 
     There is therefore provided, in accordance with some applications of the present invention, apparatus including: 
     a catheter; 
     a first pump disposed on the catheter; 
     a second pump disposed on the catheter, proximally to the first pump; and 
     a control unit configured to control activation of the first and second pumps, 
     the first and second pumps being configured, when activated, to pump fluid in opposite directions from one another. 
     For some applications, the catheter is configured to be placed within a vena cava of a subject such that the first pump is disposed downstream of junctions of the vena cava with all renal veins of the subject, and such that the second pump is disposed upstream of junctions of the vena cava with all renal veins of the subject. 
     For some applications, the first and second pumps are configured to lower pressure within the subject&#39;s renal veins by: 
     the first pump pumping blood through the vena cava in a downstream direction, and 
     the second pump pumping blood through the vena cava in an upstream direction. 
     For some applications, the catheter is configured to be placed within the subject&#39;s vena cava by being inserted via a vein of the subject selected from the group consisting of: a subclavian vein, a jugular vein, and a femoral vein. 
     For some applications: 
     the first pump includes a first impeller configured to pump blood through the vena cava by rotating; and 
     the second pump includes a second impeller configured to pump blood through the vena cava by rotating. 
     For some applications: 
     the apparatus further includes a first cage, the first impeller being disposed inside the first cage, and the first cage configured to maintain a separation between the first impeller and an inner wall of the vena cava; and 
     the apparatus further includes a second cage, the second impeller being disposed inside the second cage, and the second cage being configured to maintain a separation between the second impeller and the inner wall of the vena cava. 
     For some applications, the first and second impellers are configured, when activated, to pump blood in opposite directions from one another by the first and second impellers being rotated in the same direction as one another, as viewed from an external reference point. 
     For some applications, the first and second impellers are of opposing-handedness with respect to one another, and are disposed upon the catheter such that the impellers face opposite directions from one another. 
     For some applications, the catheter is configured to be placed within a blood vessel of a subject, and the first and second pumps are configured to generate a region within the blood vessel that is of lower blood pressure than elsewhere within the blood vessel by pumping blood away from a region of the blood vessel between the first and second pumps. 
     For some applications, the catheter is configured to be placed within a main vein of a subject into which blood flows from a tributary venous system such that: 
     the first pump is placed in the main vein, downstream of the tributary venous system; and 
     the second pump is placed in the main vein, upstream of the tributary venous system. 
     For some applications, the catheter is configured to be placed within a blood vessel of a subject, and the first and second pumps are configured to generate a region within the blood vessel that is of higher blood pressure than elsewhere within the blood vessel by pumping blood toward a region of the blood vessel between the first and second pumps. 
     For some applications, the catheter is configured to be placed within a main artery of a subject that supplies a branching arterial system that branches from the main artery such that: 
     the first pump is placed in the main artery, downstream of the branching arterial system; and 
     the second pump is placed in the main artery, upstream of the branching arterial system. 
     For some applications: 
     the first pump includes a first impeller configured to pump fluid by rotating; and 
     the second pump includes a second impeller configured to pump fluid by rotating. 
     For some applications, the first and second impellers are configured, when activated, to pump fluid in opposite directions from one another by the first and second impellers being rotated in the same direction as one another, as viewed from an external reference point. 
     For some applications, the first and second impellers are of opposing-handedness with respect to one another, and are disposed upon the catheter such that the impellers face opposite directions from one another. 
     For some applications, the apparatus further includes a motor configured to cause the first and second impellers to pump fluid in opposite directions from one another by rotating the first and second impellers in the same direction as one another. 
     There is further provided, in accordance with some applications of the present invention, apparatus including: 
     a catheter; 
     a first impeller disposed on the catheter; and 
     a second impeller disposed on the catheter, proximally to the first impeller, 
     longitudinal centers of the first and second impellers being separated from one another by a distance of at least 3 cm, the distance being measured along a longitudinal axis of the catheter. 
     For some applications, the first and second impellers are of opposing-handedness with respect to one another, and are disposed upon the catheter such that the impellers face opposite directions from one another. 
     For some applications, the catheter is configured to be placed within a vena cava of a subject such that the first impeller is disposed downstream of junctions of the vena cava with all renal veins of the subject, and such that the second impeller is disposed upstream of junctions of the vena cava with all renal veins of the subject. 
     For some applications, the catheter is configured to be placed within the subject&#39;s vena cava by being inserted via a vein of the subject selected from the group consisting of: a subclavian vein, a jugular vein, and a femoral vein. 
     For some applications: 
     the apparatus further includes a first cage, the first impeller being disposed inside the first cage, and the first cage being configured to maintain a separation between the first impeller and an inner wall of the vena cava; and 
     the apparatus further includes a second cage, the second impeller being disposed inside the second cage, and the second cage being configured to maintain a separation between the second impeller and the inner wall of the vena cava. 
     For some applications: 
     the apparatus further includes a control unit configured to control rotation of the first and second impellers, and 
     the first and second impellers are configured, by rotating, to lower pressure within the subject&#39;s renal veins by: 
     the first impeller pumping blood through the vena cava in a downstream direction, and 
     the second impeller pumping blood through the vena cava in an upstream direction. 
     For some applications, the first and second impellers are configured to pump fluid in opposite directions from one another by the first and second impellers rotating in the same direction as one another, as viewed from an external reference point. 
     For some applications, the first and second impellers are of opposing-handedness with respect to one another, and are disposed upon the catheter such that the impellers face opposite directions from one another. 
     For some applications: 
     the apparatus further includes a control unit configured to control rotation of the first and second impellers, and 
     the first and second impellers are configured to pump fluid in opposite directions from one another, by the first and second impellers rotating in the same direction as one another, as viewed from an external reference point. 
     For some applications, the first and second impellers are of opposing-handedness with respect to one another, and are disposed upon the catheter such that the impellers face opposite directions from one another. 
     For some applications, the apparatus further includes a motor configured to cause the first and second impellers to pump fluid in opposite directions from one another by rotating the first and second impellers in the same direction as one another. 
     For some applications, the catheter is configured to be placed within a blood vessel of a subject, and the first and second impellers are configured to generate a region within the blood vessel that is of lower blood pressure than elsewhere within the blood vessel by pumping blood away from a region of the blood vessel between the first and second impellers. 
     For some applications, the catheter is configured to be placed within a main vein of a subject into which blood flows from a tributary venous system such that: 
     the first impeller is placed in the main vein, downstream of the tributary venous system; and 
     the second impeller is placed in the main vein, upstream of the tributary venous system. 
     For some applications, the catheter is configured to be placed within a blood vessel of a subject, and the first and second impellers are configured to generate a region within the blood vessel that is of higher blood pressure than elsewhere within the blood vessel by pumping blood toward a region of the blood vessel between the first and second impellers. 
     For some applications, the catheter is configured to be placed within a main artery of a subject that supplies a branching arterial system that branches from the main artery such that: 
     the first impeller is placed in the main artery, downstream of the branching arterial system; and 
     the second impeller is placed in the main artery, upstream of the branching arterial system. 
     There is additionally provided, in accordance with some applications of the present invention, apparatus including: 
     a catheter configured to be placed inside a blood vessel of a subject; 
     a blood pump disposed on the catheter; and 
     an occlusion element disposed on the catheter, and configured to partially occlude the subject&#39;s blood vessel, 
     longitudinal centers of the blood pump and the occlusion element being separated from one another by a distance of at least 3 cm, the distance being measured along a longitudinal axis of the catheter. 
     For some applications, the blood pump includes an impeller configured to pump blood through the subject&#39;s blood vessel by rotating. 
     For some applications, the apparatus further includes a cage, the impeller being disposed inside the cage, and the cage being configured to maintain a separation between the impeller and an inner wall of the blood vessel. 
     For some applications, the catheter is configured to be placed within a vena cava of a subject such that the blood pump is disposed downstream of junctions of the vena cava with all renal veins of the subject, and such that the occlusion element is disposed upstream of junctions of the vena cava with all renal veins of the subject. 
     For some applications, the blood pump is configured to lower pressure within the subject&#39;s renal veins by pumping blood through the vena cava in a downstream direction. 
     For some applications, the catheter is configured to be placed within the subject&#39;s vena cava by being inserted via a vein of the subject selected from the group consisting of: a subclavian vein, a jugular vein, and a femoral vein. 
     For some applications, the blood pump includes an impeller configured to pump blood through the vena cava by rotating. 
     For some applications, the apparatus further includes a cage, the impeller being disposed inside the cage, and the cage being configured to maintain a separation between the impeller and an inner wall of the vena cava. 
     For some applications, the blood pump and the occlusion element are configured to generate a region within the blood vessel that is of lower blood pressure than elsewhere within the blood vessel by the blood pump pumping away from a region of the blood vessel between the blood pump and the occlusion element. 
     For some applications, the catheter is configured to be placed within a main vein of a subject into which blood flows from a tributary venous system such that: 
     the blood pump is placed in the main vein, downstream of the tributary venous system; and 
     the occlusion element is placed in the main vein, upstream of the tributary venous system. 
     For some applications, the blood pump and occlusion element are configured to generate a region within the blood vessel that is of higher blood pressure than elsewhere within the blood vessel by the blood pump pumping blood toward a region of the blood vessel between the blood pump and the occlusion element. 
     For some applications, the catheter is configured to be placed within a main artery of a subject that supplies a branching arterial system that branches from the main artery such that: 
     the occlusion element is placed in the main artery, downstream of the branching arterial system; and 
     the blood pump is placed in the main artery, upstream of the branching arterial system. 
     There is further provided, in accordance with some applications of the present invention, a method for use with a tributary venous system of a subject that flows into a main vein of the subject, the method including: 
     reducing blood pressure within the tributary venous system by:
         placing a first pump in the main vein, downstream of the tributary venous system, and activating the first pump to pump blood through the main vein in a downstream direction; and   placing a second pump in the main vein, upstream of the tributary venous system, and activating the second pump to pump blood through the main vein in an upstream direction.       

     For some applications, the first and second pumps are disposed upon a single catheter, and placing the first and second pumps in the main vein includes inserting a distal end of the catheter into the main vein. 
     For some applications: 
     the main vein includes a vena cava of the subject, 
     the tributary venous system includes a renal venous system of the subject, 
     placing the first pump in the main vein, downstream of the tributary venous system, includes placing the first pump in the vena cava, downstream of junctions of the vena cava with all renal veins of the subject, 
     placing the second pump in the main vein, upstream of the tributary venous system, includes placing the second pump in the vena cava, upstream of the junctions of the vena cava with all of the subject&#39;s renal veins, 
     the method further includes identifying the subject as suffering from a condition selected from the group consisting of: cardiac dysfunction, congestive heart failure, reduced renal blood flow, increased renal vascular resistance, arterial hypertension, and kidney dysfunction, and 
     reducing pressure within the tributary venous system includes reducing pressure within renal veins of the subject, in response to the identifying. 
     For some applications, the first and second pumps are disposed upon a single catheter, and placing the first and second pumps in the vena cava includes inserting a distal end of the catheter into the subject&#39;s vena cava. 
     For some applications, inserting the distal end of the catheter into the subject&#39;s vena cava includes inserting the distal end of the catheter into the subject&#39;s vena cava via a vein of the subject selected from the group consisting of: a subclavian vein, a jugular vein, and a femoral vein. 
     For some applications: 
     placing the first pump in the main vein includes placing a first impeller in the main vein, downstream of the tributary venous system; and 
     placing the second pump in the main vein includes placing a second impeller in the main vein, upstream of the tributary venous system. 
     For some applications: 
     placing the first impeller inside the main vein includes inserting the first impeller into the main vein while the first impeller is disposed inside a cage that is configured to maintain a separation between the first impeller and an inner wall of the main vein; and 
     placing the second impeller inside the main vein includes inserting the second impeller into the main vein while the second impeller is disposed inside a cage that is configured to maintain a separation between the second impeller and the inner wall of the main vein. 
     For some applications, activating the first pump to pump blood through the main vein in the downstream direction includes rotating the first impeller in a given direction, and activating the second pump to pump blood through the main vein in the upstream direction includes rotating the second impeller in the same given direction, as viewed from an external reference point. 
     For some applications, the first and second impellers are of opposing-handedness to one another, and are disposed upon a single catheter such that the first and second impellers face in opposite directions from another, and placing the first and second pumps in the vena cava includes inserting a distal end of the catheter into the subject&#39;s vena cava. 
     For some applications, rotating the first and second impellers in the given direction includes using a single motor to rotate the first and second impellers. 
     There is additionally provided, in accordance with some applications of the present invention, a method for use with a tributary venous system of a subject that flows into a main vein of the subject, the method including: 
     reducing blood pressure within the tributary venous system by:
         placing a pump in the main vein, downstream of the tributary venous system, and activating the pump to pump blood through the main vein in a downstream direction; and   placing an occlusion element in the main vein at a location within the main vein that is upstream of the tributary venous system, such that the occlusion element partially occludes the main vein at the location.       

     For some applications, placing the occlusion element in the main vein includes placing a balloon in the main vein. 
     For some applications, placing the occlusion element in the main vein includes placing a frame that is covered with a blood-impermeable material in the main vein. 
     For some applications, the pump and the occlusion element are disposed upon a single catheter, and placing the pump and the occlusion element in the main vein includes inserting a distal end of the catheter into the main vein. 
     For some applications: 
     the main vein includes a vena cava of the subject, 
     the tributary venous system includes a renal venous system of the subject, 
     placing the pump in the main vein, downstream of the tributary venous system includes placing the pump in the vena cava, downstream of junctions of the vena cava with all renal veins of the subject, 
     placing the occlusion element in the main vein at the location within the main vein that is upstream of the tributary venous system includes placing the occlusion element in the vena cava upstream of the junctions of the vena cava with all of the subject&#39;s renal veins, 
     the method further includes identifying the subject as suffering from a condition selected from the group consisting of: cardiac dysfunction, congestive heart failure, reduced renal blood flow, increased renal vascular resistance, arterial hypertension, and kidney dysfunction, and 
     reducing pressure within the tributary venous system includes reducing pressure within renal veins of the subject, in response to the identifying. 
     For some applications, the pump and the occlusion element are disposed upon a single catheter, and placing the pump and the occlusion element in the vena cava includes inserting a distal end of the catheter into the vena cava. 
     For some applications, inserting the distal end of the catheter into the vena cava includes inserting the distal end of the catheter into the vena cava via a vein of the subject selected from the group consisting of: a subclavian vein, a jugular vein, and a femoral vein. 
     For some applications, placing the pump in the main vein includes placing an impeller in the main vein, downstream of the tributary venous system. 
     For some applications, placing the impeller inside the main vein includes inserting the first impeller into the main vein while the impeller is disposed inside a cage that is configured to maintain a separation between the first impeller and an inner wall of the main vein. 
     There is further provided, in accordance with some applications of the present invention, a method including: 
     identifying a subject as suffering from a condition selected from the group consisting of: cardiac dysfunction, congestive heart failure, reduced renal blood flow, increased renal vascular resistance, arterial hypertension, and kidney dysfunction; and 
     in response thereto, reducing blood pressure within renal veins of the subject, by:
         placing at least one pump in a vena cava of the subject; and   generating a low-pressure region within the subject&#39;s vena cava, adjacent to junctions of the vena cava with the subject&#39;s renal veins, blood pressure within the low-pressure region being lower than central venous pressure of the subject,   by activating the at least one pump to pump blood away from the region.       

     For some applications, generating the low-pressure region within the subject&#39;s vena cava includes: 
     placing a blood-impermeable sleeve in the subject&#39;s vena cava, such that a downstream end of the sleeve is coupled to a wall of the vena cava at a first location that is downstream of all of the renal veins of the subject, and such that an upstream end of the sleeve is coupled to the wall of the vena cava at a second location that is upstream of all the renal veins of the subject; and 
     activating the pump to pump blood from a location outside the sleeve that is in fluid communication with the subject&#39;s renal veins, to a location within the vena cava that is in fluid communication with an interior of the sleeve. 
     For some applications: 
     placing the at least one pump in the subject&#39;s vena cava includes:
         placing a first pump in the vena cava, downstream of junctions of the vena cava with all renal veins of the subject; and   placing a second pump in the vena cava, upstream of the junctions of the vena cava with all of the subject&#39;s renal veins; and       

     generating the low-pressure region within the subject&#39;s vena cava includes:
         activating the first pump to pump blood through the vena cava in a downstream direction; and   activating the second pump to pump blood through the vena cava in an upstream direction.       

     For some applications: 
     placing the at least one pump in the subject&#39;s vena cava includes:
         placing a pump in the vena cava, downstream of junctions of the vena cava with all renal veins of the subject; and   placing an occlusion element in the vena cava at a location within the vena cava that is upstream of the junctions of the vena cava with all of the subject&#39;s renal veins, such that the occlusion element partially occludes the vena cava at the location; and       

     generating the low-pressure region within the subject&#39;s vena cava includes activating the pump to pump blood through the vena cava in a downstream direction. 
     For some applications, placing the occlusion element in the vena cava includes placing a balloon in the vena cava. 
     For some applications, placing the occlusion element in the vena cava includes placing in the vena cava a frame that is covered with a blood-impermeable material. 
     There is additionally provided, in accordance with some applications of the present invention, apparatus including: 
     a catheter configured to be placed inside a blood vessel of a subject; 
     a first impeller disposed on the catheter; 
     a second impeller disposed on the catheter, proximally to the first impeller; and 
     a support structure disposed upon the catheter such that a longitudinal center of the support structure is disposed between the first and second impellers, the support structure being configured to support an inner wall of the blood vessel in an open configuration in a region between the first and second impellers. 
     For some applications, the longitudinal center of the support structure is disposed equidistantly from the first and second impellers. 
     For some applications, the support structure extends at least from a longitudinal center of the first impeller to a longitudinal center of the second impeller. 
     For some applications, the first and second impellers are of opposing-handedness with respect to one another, and are disposed upon the catheter such that the impellers face opposite directions from one another. 
     For some applications, the catheter is configured to be placed within a vena cava of a subject such that the first impeller is disposed downstream of junctions of the vena cava with all renal veins of the subject, and such that the second impeller is disposed upstream of junctions of the vena cava with all renal veins of the subject. 
     For some applications, the catheter is configured to be placed within the subject&#39;s vena cava by being inserted via a vein of the subject selected from the group consisting of: a subclavian vein, a jugular vein, and a femoral vein. 
     For some applications: 
     the apparatus further includes a first cage, the first impeller being disposed inside the first cage, and the first cage being configured to maintain a separation between the first impeller and an inner wall of the vena cava; and 
     the apparatus further includes a second cage, the second impeller being disposed inside the second cage, and the second cage being configured to maintain a separation between the second impeller and the inner wall of the vena cava. 
     For some applications, the first and second cages are integrally formed with the support structure. 
     For some applications, the first and second cages are separately formed from the support structure. 
     For some applications: 
     the apparatus further includes a control unit configured to control rotation of the first and second impellers, and 
     the first and second impellers are configured, by rotating, to lower pressure within the subject&#39;s renal veins by:
         the first impeller pumping blood through the vena cava in a downstream direction, and   the second impeller pumping blood through the vena cava in an upstream direction.       

     For some applications, the first and second impellers are configured to pump fluid in opposite directions from one another by the first and second impellers rotating in the same direction as one another, as viewed from an external reference point. 
     For some applications, the first and second impellers are of opposing-handedness with respect to one another, and are disposed upon the catheter such that the impellers face opposite directions from one another. 
     For some applications, the catheter is configured to be placed within a vena cava of a subject such that the first impeller is disposed upstream of junctions of the vena cava with all renal veins of the subject, and such that the second impeller is disposed downstream of junctions of the vena cava with all renal veins of the subject. 
     For some applications, the catheter is configured to be placed within the subject&#39;s vena cava by being inserted via a vein of the subject selected from the group consisting of: a subclavian vein, and a jugular vein. 
     For some applications: 
     the apparatus further includes a first cage, the first impeller being disposed inside the first cage, and the first cage being configured to maintain a separation between the first impeller and an inner wall of the vena cava; and 
     the apparatus further includes a second cage, the second impeller being disposed inside the second cage, and the second cage being configured to maintain a separation between the second impeller and the inner wall of the vena cava. 
     For some applications, the first and second cages are integrally formed with the support structure. 
     For some applications, the first and second cages are separately formed from the support structure. 
     For some applications: 
     the apparatus further includes a control unit configured to control rotation of the first and second impellers, and 
     the first and second impellers are configured, by rotating, to lower pressure within the subject&#39;s renal veins by:
         the first impeller pumping blood through the vena cava in an upstream direction, and   the second impeller pumping blood through the vena cava in a downstream direction.       

     For some applications, the first and second impellers are configured to pump fluid in opposite directions from one another by the first and second impellers rotating in the same direction as one another, as viewed from an external reference point. 
     For some applications, the first and second impellers are of opposing handedness with respect to one another, and are disposed upon the catheter such that the impellers face opposite directions from one another. 
     For some applications: 
     the apparatus further includes a control unit configured to control rotation of the first and second impellers, and 
     the first and second impellers are configured to pump fluid in opposite directions from one another, by the first and second impellers rotating in the same direction as one another, as viewed from an external reference point. 
     For some applications, the first and second impellers are of opposing-handedness with respect to one another, and are disposed upon the catheter such that the impellers face opposite directions from one another. 
     For some applications, the catheter is configured to be placed within a blood vessel of a subject, and the first and second impellers are configured to generate a region within the blood vessel that is of lower blood pressure than elsewhere within the blood vessel by pumping blood away from a region of the blood vessel between the first and second impellers. 
     For some applications, the catheter is configured to be placed within a main vein of a subject into which blood flows from a tributary venous system such that: 
     the first impeller is placed in the main vein, downstream of the tributary venous system; and 
     the second impeller is placed in the main vein, upstream of the tributary venous system. 
     For some applications, the catheter is configured to be placed within a main vein of a subject into which blood flows from a tributary venous system such that: 
     the first impeller is placed in the main vein, upstream of the tributary venous system; and 
     the second impeller is placed in the main vein, downstream of the tributary venous system. 
     There is additionally provided, in accordance with some applications of the present invention, apparatus including: 
     a catheter configured to be placed inside a blood vessel of a subject; 
     a first pump disposed on the catheter and configured to pump blood through the blood vessel in a first direction; 
     a second pump disposed on the catheter, proximally to the first pump, and configured to pump blood through the blood vessel in a second direction that is opposite to the first direction; and 
     a support structure disposed upon the catheter such that a longitudinal center of the support structure is disposed between the first and second pumps, the support structure being configured to support an inner wall of the blood vessel in an open configuration in a region between the first and second pumps. 
     For some applications, the longitudinal center of the support structure is disposed equidistantly from the first and second impellers. 
     For some applications, the support structure extends at least from a longitudinal center of the first pump to a longitudinal center of the second pump. 
     There is further provided, in accordance with some applications of the present invention, a method for use with a tributary venous system of a subject that flows into a main vein of the subject, the method including: 
     reducing blood pressure within the tributary venous system by:
         placing a first pump in the main vein, downstream of the tributary venous system, and activating the pump to pump blood through the main vein in a downstream direction;   placing a second pump in the main vein, upstream of the tributary venous system, and activating the pump to pump blood through the main vein in an upstream direction; and   placing a support structure within the main vein, such that a longitudinal center of the support structure is disposed between the first and second pump, and such that the support structure supports an inner wall of the main vein in an open configuration during the pumping of the blood by the first and second pumps.       

     For some applications, the first and second pumps are disposed upon a single catheter, and placing the first and second pumps in the main vein includes inserting a distal end of the catheter into the main vein. 
     For some applications: 
     the main vein includes a vena cava of the subject, 
     the tributary venous system includes a renal venous system of the subject, 
     placing the first pump in the main vein, downstream of the tributary venous system includes placing the first pump in the vena cava, downstream of junctions of the vena cava with all renal veins of the subject, 
     placing the second pump in the main vein, upstream of the tributary venous system includes placing the second pump in the vena cava, upstream of the junctions of the vena cava with all of the subject&#39;s renal veins, 
     the method further includes identifying the subject as suffering from a condition selected from the group consisting of: cardiac dysfunction, congestive heart failure, reduced renal blood flow, increased renal vascular resistance, arterial hypertension, and kidney dysfunction, and 
     reducing pressure within the tributary venous system includes reducing pressure within renal veins of the subject, in response to the identifying. 
     For some applications, the first and second pumps are disposed upon a single catheter, and placing the first and second pumps in the vena cava includes inserting a distal end of the catheter into the subject&#39;s vena cava. 
     For some applications, inserting the distal end of the catheter into the subject&#39;s vena cava includes inserting the distal end of the catheter into the subject&#39;s vena cava via a vein of the subject selected from the group consisting of: a subclavian vein, a jugular vein, and a femoral vein. 
     For some applications: 
     placing the first pump in the main vein includes placing a first impeller in the main vein, downstream of the tributary venous system; and 
     placing the second pump in the main vein includes placing a second impeller in the main vein, upstream of the tributary venous system. 
     For some applications: 
     placing the first impeller inside the main vein includes inserting the first impeller into the main vein while the first impeller is disposed inside a first cage that is configured to maintain a separation between the first impeller and an inner wall of the main vein; and 
     placing the second impeller inside the main vein includes inserting the second impeller into the main vein while the second impeller is disposed inside a second cage that is configured to maintain a separation between the second impeller and the inner wall of the main vein. 
     For some applications: 
     placing the first impeller inside the main vein includes inserting the first impeller into the main vein the first cage being integrally formed with the support structure; and 
     placing the second impeller inside the main vein includes inserting the second impeller into the main vein the second cage being integrally formed with the support structure. 
     For some applications: 
     placing the first impeller inside the main vein includes inserting the first impeller into the main vein the first cage being separately formed with respect to the support structure; and 
     placing the second impeller inside the main vein includes inserting the second impeller into the main vein the second cage being separately formed with respect to the support structure. 
     For some applications, activating the first pump to pump blood through the main vein in the downstream direction includes rotating the first impeller in a given direction, and activating the second pump to pump blood through the main vein in the upstream direction includes rotating the second impeller in the same given direction, as viewed from an external reference point. 
     For some applications, the first and second impellers are of opposing-handedness to one another, and are disposed upon a single catheter such that the first and second impellers face in opposite directions from another, and placing the first and second pumps in the vena cava includes inserting a distal end of the catheter into the subject&#39;s vena cava. 
     For some applications, rotating the first and second impellers in the given direction includes using a single motor to rotate the first and second impellers. 
     There is further provided, in accordance with some applications of the present invention, a method for use with a tributary venous system of a subject that flows into a main vein of the subject, the method including: 
     reducing blood pressure within the tributary venous system by:
         placing a first pump in the main vein, downstream of the tributary venous system, and activating the pump to pump blood through the main vein in a downstream direction;   placing an occlusion element in the main vein at a location within the main vein that is upstream of the tributary venous system, such that the occlusion element partially occludes the main vein at the location; and   placing a support structure within the main vein, such that a longitudinal center of the support structure is disposed between the first pump and the occlusion element, and such that the support structure supports an inner wall of the main vein in an open configuration during pumping of the blood by the first pump.       

     For some applications: 
     the main vein includes a vena cava of the subject, 
     the tributary venous system includes a renal venous system of the subject, 
     placing the first pump in the main vein, downstream of the tributary venous system includes placing the first pump in the vena cava, downstream of junctions of the vena cava with all renal veins of the subject, 
     placing the occlusion element in the main vein at the location within the main vein that is upstream of the tributary venous system includes placing the occlusion element in the vena cava upstream of the junctions of the vena cava with all of the subject&#39;s renal veins, 
     the method further includes identifying the subject as suffering from a condition selected from the group consisting of: cardiac dysfunction, congestive heart failure, reduced renal blood flow, increased renal vascular resistance, arterial hypertension, and kidney dysfunction, and 
     reducing pressure within the tributary venous system includes reducing pressure within renal veins of the subject, in response to the identifying. 
     There is additionally provided, in accordance with some applications of the present invention, apparatus including: 
     a catheter; 
     a first impeller disposed on the catheter; 
     a second impeller disposed on the catheter, proximally to the first impeller; and 
     a telescopic shaft extending between the first and second impellers, a length of the telescopic shaft being adjustable. 
     There is further provided, in accordance with some applications of the present invention, a method, including: 
     manufacturing an impeller by:
         cutting a tube such that the cut tube defines a structure having first and second end portions at proximal and distal ends of the structure, the end portions being connected to one another by a plurality of elongate elements;   causing the elongate elements to radially expand and form helical elongate elements, by axially compressing the structure;   coupling respective reinforcement elements to each of the helical elongate elements, such that each of the reinforcement elements extends from the helical elongate element toward a longitudinal axis of the structure; and   coupling a material to the helical elongate elements, such that each of the helical elongate elements with the material coupled thereto defines a respective blade of the impeller,   the reinforcement elements being configured to reinforce central portions of respective blades of the impeller.       

     For some applications: 
     the method further includes coupling an axial element to the structure such that the axial element extends along a longitudinal axis of the structure, and 
     coupling respective reinforcement elements to each of the helical elongate elements, includes coupling respective reinforcement elements to each of the helical elongate elements such that each of the reinforcement elements extends from a respective helical elongate element toward the axial element. 
     For some applications, the method further includes, subsequent to coupling the material to the helical elongate elements, removing the axial element from the structure. 
     For some applications: 
     the method further includes coupling axial elements to each of the helical elongate elements, such that each of the axial elements extends from a proximal end of a respective helical elongate element to a distal end of the helical elongate element, and 
     coupling respective reinforcement elements to each of the helical elongate elements, includes coupling respective reinforcement elements to each of the helical elongate elements such that each of the reinforcement elements extends from a respective helical elongate element toward a corresponding one of the axial elements. 
     For some applications, the method further includes, subsequent to coupling the material to the helical elongate elements, removing the axial elements from the structure. 
     There is additionally provided, in accordance with some applications of the present invention, apparatus including: 
     an impeller, including:
         an impeller frame that includes proximal and distal end portions and a plurality of helical elongate elements that wind from the proximal end portion to the distal end portion; and   a material that is coupled to the helical elongate elements, such that each of the helical elongate elements with the material coupled thereto defines a respective blade of the impeller,   each of the impeller blades including a reinforcement element extending from the helical elongate element of the blade toward a longitudinal axis of the impeller frame, and configured to reinforce a central portion of the impeller blade.       

     There is further provided, in accordance with some applications of the present invention, a method, including: 
     manufacturing an expandable impeller by:
         cutting a tube such that the cut tube defines a structure having first and second end portions at proximal and distal ends of the structure, the end portions being connected to one another by a plurality of elongate elements;   causing the elongate elements to radially expand and form helical elongate elements, by axially compressing the structure;   coupling axial elements to each of the helical elongate elements, such that each of the axial elements extends from a proximal end of a respective helical elongate element to a distal end of the helical elongate element; and   coupling a material to the helical elongate elements, such that each of the helical elongate elements, with a film of the material extending from the helical elongate element to the axial element, defines a respective blade of the impeller.       

     For some applications, the method further includes, subsequent to coupling the material to the helical elongate elements, removing the axial elements from the structure. 
     For some applications, the method further includes mounting the structure upon an axial shaft. 
     For some applications, mounting the structure upon the axial shaft includes mounting the structure upon a telescopic axial shaft. 
     There is additionally provided, in accordance with some applications of the present invention, apparatus including: 
     an impeller, including:
         an impeller frame that includes proximal and distal end portions and a plurality of helical elongate elements that wind from the proximal end portion to the distal end portion;   a material that is coupled to the helical elongate elements, such that each of the helical elongate elements with the material coupled thereto defines a respective blade of the impeller; and   an axially-elongatable telescopic shaft disposed along a longitudinal axis of the impeller frame.       

     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 A,  1 B,  1 C, and  1 D  are schematic illustrations of a blood-pump catheter placed within a subject&#39;s vena cava, an upstream pump being disposed upon the catheter, distally to a downstream pump, in accordance with some applications of the present invention; 
         FIG.  2    is a schematic illustration of the catheter of  FIGS.  1 A,  1 B,  1 C, and  1 D  inserted into the subject&#39;s vena cava via the subject&#39;s right jugular vein, in accordance with some applications of the present invention; 
         FIG.  3    is a schematic illustration of a blood-pump catheter inserted into a subject&#39;s vena cava via the subject&#39;s femoral vein, a downstream pump being disposed upon the catheter distally to an upstream pump, in accordance with some applications of the present invention; 
         FIG.  4    is a schematic illustration of upstream and downstream pumps disposed on respective blood-pump catheters, in accordance with some applications of the present invention; 
         FIGS.  5 A and  5 B  are schematic illustrations of a catheter that includes a downstream pump and an occlusion element, such as a balloon ( FIG.  5 A ), or a covered frame ( FIG.  5 B ), in accordance with some applications of the present invention; 
         FIG.  6    is a schematic illustration of a blood-impermeable sleeve configured to occlude blood flow from a subject&#39;s vena cava to the subject&#39;s renal veins, as described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference, and in accordance with some applications of the present invention; 
         FIGS.  7 A,  7 B, and  7 C  are schematic illustrations of respective stages of a method of manufacture of an impeller that includes reinforcement elements for reinforcing blades of the impeller, in accordance with some applications of the present invention; 
         FIGS.  8 A,  8 B, and  8 C  are schematic illustrations of a blood-pump catheter that includes an impeller that includes a telescopic axial shaft, in accordance with some applications of the present invention; 
         FIGS.  9 A,  9 B, and  9 C  are schematic illustrations of a blood-pump catheter that includes an impeller that includes a telescopic axial shaft, in accordance with some alternative applications of the present invention; 
         FIGS.  10 A,  10 B, and  10 C  are schematic illustrations showing respective steps of stages of a method of manufacture of an impeller that includes a telescopic axial shaft, in accordance with some applications of the present invention; 
         FIGS.  11 A,  11 B, and  11 C  are schematic illustrations of a blood-pump catheter placed within a subject&#39;s vena cava, an upstream pump being disposed upon the catheter, distally to a downstream pump, the upstream and downstream pumps being disposed at respective separations from one another in  FIGS.  11 A,  11 B, and  11 C , in accordance with some applications of the present invention; 
         FIG.  12    is a schematic illustration of a blood-pump catheter placed within a subject&#39;s vena cava, an upstream pump being disposed upon the catheter, distally to a downstream pump, and a support stent being disposed upon the catheter between the upstream and downstream pumps, in accordance with some applications of the present invention; 
         FIGS.  13 A,  13 B,  13 C,  13 D, and  13 E  are schematic illustrations of a blood-pump catheter for placing within a subject&#39;s vena cava, an upstream impeller being disposed upon the catheter, distally to a downstream impeller, the upstream and downstream impellers being disposed within a support cage that supports the walls of a portion of the vena cava between the upstream and downstream impellers, in accordance with some applications of the present invention; 
         FIGS.  14 A,  14 B, and  14 C  are graphs showing the pressure drop recorded in models of a subject&#39;s left and right renal veins, during experiments that were conducted using blood pumps, in accordance with some applications of the present invention; and 
         FIGS.  15 A and  15 B  are schematic illustrations of a bearing and axial shaft of a distal end of an impeller, in accordance with respective applications of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference is made to  FIGS.  1 A-D , which are schematic illustrations of a blood-pump catheter  20  placed within a subject&#39;s vena cava  22 , via a guide catheter  23 , an upstream pump  24 U being disposed upon the catheter, distally to a downstream pump  24 D, in accordance with some applications of the present invention. Typically, the distal portion of blood-pump catheter  20  is configured to be straight, when the catheter is in a non-constrained state, such that both the upstream and the downstream pumps are disposed along the axis of the catheter, within the vena cava. 
     Each of the upstream and downstream pumps  24 U and  24 D typically includes a radially-expandable impeller  28  disposed inside a radially-expandable impeller cage  30 . Typically, impeller  28  and cage  30  are shape-set such as to assume radially expanded configurations thereof in the absence of any radially constraining force acting upon the impeller and the cage. Further typically, an engagement mechanism engages the impeller and the cage with respect to one another, such that in response to the cage becoming radially constrained, the impeller becomes radially constrained, e.g., in accordance with apparatus and methods described in described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference. 
     It is noted that the term “impeller” is used herein to denote a bladed rotor, as shown in  1 A-D, for example. When the bladed rotor is placed inside a blood vessel (such as vena cava  22 ) and rotated, the bladed rotor functions as an impeller, by modifying the flow of blood through the blood vessel, and/or by generating a pressure difference between the upstream end and the downstream end of the impeller. 
     It is noted that reference numeral  24  is generally used to denote a blood pump in the present application. When a pump that is placed upstream is being referred to, reference numeral  24 U is used, and when a pump that is placed downstream is being referred to, reference numeral  24 D is used. Similarly, reference numeral  28  is generally used to denote an impeller in the present application. When an impeller that is placed upstream is being referred to, reference numeral  28 U is used, and when an impeller that is placed downstream is being referred to, reference numeral  28 D is used. 
     Blood-pump catheter  20  is typically placed inside the subject&#39;s vena cava  22 , and operated therein, in order to provide acute treatment of a subject suffering from cardiac dysfunction, congestive heart failure, low renal blood flow, high renal vascular resistance, arterial hypertension, diabetes, and/or kidney dysfunction. For example, the blood-pump catheter may be placed inside the subject&#39;s vena cava, and operated therein, for a period of more than one hour (e.g., more than one day), less than one week (e.g., less than four days), and/or between one hour and one week (e.g., between one day and four days). For some applications, the blood-pump catheter is chronically placed inside the subject&#39;s vena cava in order to provide chronic treatment of a subject suffering from cardiac dysfunction, congestive heart failure, low renal blood flow, high renal vascular resistance, arterial hypertension, diabetes, and/or kidney dysfunction. For some applications, a course of treatment is applied to a subject over several weeks, several months, or several years, during which the blood-pump catheter is intermittently placed inside the subject&#39;s vena cava, and the subject is intermittently treated in accordance with the techniques described herein. For example, the subject may be intermittently treated at intervals of several days, several weeks, or several months. 
     For some applications, blood-pump catheter  20  is inserted into vena cava  22 , via the subject&#39;s subclavian vein  40 , as shown in  FIG.  1 A . Typically, the blood-pump catheter is inserted under fluoroscopic imaging. Alternatively, the blood-pump catheter is inserted under ultrasound imaging, such as to reduce exposure of the subject to radiation and/or contrast agent. The catheter is placed into the vena cava such that upstream pump  24 U is disposed upstream of the junctions of the vena cava and all of the subject&#39;s renal veins  42 , and such that downstream pump  24 D is disposed downstream of the junctions of the vena cava and all of the subject&#39;s renal veins. Typically, the upstream pump is configured to pump blood through the vena cava in the upstream direction, away from the renal veins, and the downstream pump is configured to pump blood through the vena cava in the downstream direction, away from the renal veins. 
     The effect of both of pumps  24 U and  24 D pumping blood in the above-described manner is that, between the pumps, and adjacent to the junctions of the vena cava with the renal veins, there is a low-pressure region of the vena cava, within which blood pressure is lower than the subject&#39;s central venous pressure. Functionally, this region may be viewed as a compartment within the vena cava within which blood pressure is controlled (by controlling pumps  24 U and  24 D), regardless of the blood pressure elsewhere within the vena cava. This typically increases blood flow from the renal veins into the vena cava, lowers pressure within the subject&#39;s renal veins, and causes renal perfusion to increase. The effect of pumps  24 U and  24 D on blood flow through the renal veins and the vena cava is indicated by arrows  44  in  FIG.  1 B . 
     As described hereinabove, the effect of operating blood pumps  24 U and  24 D is that between the pumps there is a low-pressure region of the vena cava. However, typically, the pumps are operated simultaneously such that the pressure within other portions of the vena cava is substantially unchanged relative to when blood-pump catheter  20  is not in operation. For example, the pumps are typically operated simultaneously such that the pressure within the vena cava downstream of downstream pump  24 D is not substantially increased relative to when blood-pump catheter  20  is not in operation. Similarly, the pumps are typically operated simultaneously such that the pressure within the vena cava upstream of upstream pump  24 U is not substantially increased relative to when blood-pump catheter  20  is not in operation. This is because the pumps are typically operated simultaneously such that outside of the region between the two pumps, the effects of the pumping by the upstream and downstream pumps cancel each other with respect to pressure. It is noted that there is likely to be some increase in the pressure within the vena cava downstream of downstream pump and upstream of upstream pump due to the increased blood flow from the renal veins into the vena cava. 
     Similarly, the pumps are typically operated simultaneously such that venous return to the vena cava from regions upstream of the upstream pump and downstream from the downstream pump is substantially unchanged relative to when blood-pump catheter  20  is not in operation. In this manner, the pumps are typically operated simultaneously such as to have a generally synergistic effect on pressure and flow in the region between the pumps, but to have an antagonistic effect on pressure and flow outside of the region, such that, outside of the region, the effects of the two pumps typically substantially cancel each other out. 
     Typically, blood-pump catheter  20  pumps blood in a manner that enhances the rate of blood flow through the renal veins and into the vena cava, but does not cause a substantial change in the direction of the blood flow relative to the natural direction of flow through the renal veins, or from the renal veins to the vena cava (i.e., relative to blood flow in the absence of pumping by the blood-pump catheter). That is to say, the blood-pump catheter pumps blood in the downstream direction through the renal veins and then directly into the portion of the vena cava that is adjacent to the renal veins, rather than, for example, pumping the blood from the renal veins into a different portion of the subject&#39;s veins (such as, an upstream location within the vena cava). It is noted that, due to the pumping of the downstream pump in the downstream direction, there is likely to be some blood flow from the renal veins to the portion of the vena cava that is below the renal veins. Further typically, blood-pump catheter  20  enhances blood flow through the renal veins without removing blood from the subject&#39;s venous system into a non-venous receptacle, such as an artificial lumen of a blood pump. 
     As described hereinabove, typically blood-pump catheter  20  is placed inside the vena cava of a subject suffering from cardiac dysfunction, congestive heart failure, low renal blood flow, high renal vascular resistance, arterial hypertension, diabetes, and/or kidney dysfunction. Typically, operating the blood-pump catheter in the vena cava of such a subject causes a lowering and flattening of the subject&#39;s renal vein pressure profile, even though the subject&#39;s central venous pressure is elevated, e.g., as described with reference to  FIG.  4 B  of WO 14/141284 to Schwammenthal, which is incorporated herein by reference. 
     Typically, due to the reduction in pressure in the renal vein that is caused by the pumping of blood by blood-pump catheter  20 , perfusion of the kidney increases. In turn, this may cause pressure in the renal veins to rise relative to the pressure in the renal veins immediately subsequent to initiation of the pumping, due to increased blood flow into the renal vein. Typically, even after perfusion of the kidney increases, the pump is configured to maintain the pressure in the renal vein at a lower value than the pressure in the renal vein before the initiation of the pumping. For some applications, in addition to lowering the subject&#39;s renal vein pressure, and/or increasing perfusion of the subject&#39;s kidney, blood-pump catheter  20  performs ultrafiltration on the subject&#39;s blood. 
     It is noted that, for some applications, due to the reduction in pressure in the renal vein that is caused by the pumping of blood by blood-pump catheter  20 , the subject&#39;s renal vascular resistance decreases, in accordance with physiological mechanisms that are described, for example, in an article by Haddy et al., entitled “Effect of elevation of intraluminal pressure on renal vascular resistance” (Circulation Research, 1956), which is incorporated herein by reference. It is further noted that a treatment of the subject that increases renal perfusion by increasing blood pressure in the subject&#39;s renal arteries would typically not effect the aforementioned physiological mechanisms. 
     Typically, when blood-pump catheter  20  is used to reduce pressure in the subject&#39;s renal veins, it is expected that there will be an improved responsiveness by the subject to administration of diuretics to the subject, due to the reduction in renal venous pressure. Therefore, for some applications, a reduced dosage of diuretics may be administered to the subject relative to a dosage of diuretics that would be administered to the subject in the absence of performing the techniques described herein. Alternatively, a regular dosage of diuretics may be administered to the subject, but the diuretics may have a greater effect on the subject, due to the reduction in renal venous pressure. 
     Typically, high central venous pressure leads to a high level of blood pressure within the heart, which in turn leads to the release of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) by the subject, both of which act as natural diuretics. For some applications, when blood-pump catheter  20  is used to reduce pressure in the subject&#39;s renal veins, there is expected to be an improved responsiveness by the subject to the release of the natural diuretics by the subject, due to the reduction in renal venous pressure. For some applications, since the subject&#39;s central venous pressure is not lowered by using blood-pump catheter  20 , it is expected that the subject will continue to release atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP), even while the subject&#39;s renal venous pressure is reduced by the use of the blood pumps. Thus, for some applications, using blood-pump catheter  20  may result in the subject continuing to release atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP), as well as resulting in the effectiveness of the aforementioned natural diuretics being greater than the effectiveness of the diuretics in the absence of the use of blood-pump catheter  20 . 
     Typically, each of upstream and downstream pumps  24 U and  24 D includes an impeller  28 , for example, an impeller as shown in any one of  FIGS.  7 A-C ,  8 A-C,  9 A-C, and/or  10 A-C, and/or any one of the impellers described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference. In accordance with respective applications, impeller  28  may have a single blade, two blades (e.g., as described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference, and as shown in  FIGS.  8 A-C  and  FIGS.  9 A-C ), three blades (e.g., as described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference, and as shown in  FIGS.  7 A-C  and  FIGS.  10 A-C ), or more than three blades. For some applications, one or both of blood pumps  24 U and  24 D includes more than one impeller. Typically,  ceteris paribus , by using more than one impeller in at least one of the pumps, in order to generate a given flow of blood with the pump, the force that impacts each of the impellers within the pump is smaller than if a single impeller were to be used in the pump. 
     For some applications, one or both of the pumps includes radially expandable cage  30 . Typically, cage  30  is configured to hold open the inner wall of the vena cava and to separate the inner wall of the vena cava from the impeller, such that the vena cava does not become injured by the impeller. As described hereinabove, typically, impeller  28  and  30  are shape-set such as to assume radially expanded configurations thereof in the absence of any radially constraining force acting upon the impeller and/or the cage. Further typically, an engagement mechanism engages the impeller and the cage with respect to one another, such that in response to the cage becoming radially constrained the impeller becomes radially constrained, e.g., in accordance with apparatus and methods described in described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference. 
     Referring now to  FIG.  1 C , typically, when blood-pump catheter  20  is placed inside vena cava  22 , impeller  28  and cage  30  are substantially not radially constrained, due to the relatively low radial force exerted by the vena cava wall on the cage. Typically, a span SP of impeller  28 , when the impeller is in a non-constrained configuration thereof inside the vena cava is more than 14 mm (e.g., more than 16 mm), and/or less than 28 mm (e.g., less than 22 mm), e.g., 14-28 mm, or 16-22 mm Typically, a diameter D of cage  30 , when the cage is in a non-constrained configuration thereof inside the vena cava is more than 14 mm (e.g., more than 16 mm), and/or less than 40 mm (e.g., less than 35 mm), e.g., 14-40 mm, or 16-35 mm. Further typically, when blood-pump catheter  20  is used to enhance blood flow from the renal veins into the subject&#39;s vena cava, as described herein, a longitudinal distance D 1  between centers of the impellers of the upstream and downstream pumps, measured along the longitudinal axis of the catheter, is typically more than 3 cm (e.g., more than 6 cm), and/or less than 18 cm (e.g., less than 14 cm), e.g., 3-18 cm, or 6-14 cm. For some applications, distance D 1  is adjustable and is set based upon measurements that are performed upon a subject, as described in further detail hereinbelow, with reference to  FIGS.  11 A-C . 
     Typically, impellers of pumps  24 U and  24 D are coupled to one or more motors  46  ( FIG.  1 A ), which impart rotational motion to the impellers, via one or more shafts, the shaft(s) being housed inside blood-pump catheter  20 . In accordance with respective applications, the motors are disposed outside of the subject&#39;s body (as shown), or are placed inside the subject&#39;s body (not shown). 
     For some applications, in order for the impellers to pump blood in opposite directions (i.e., in order for the upstream impeller to pump blood upstream, and the downstream pump to pump blood downstream), the impellers are rotated in opposite directions from one another, as viewed from an external reference point. 
     Referring now to  FIG.  1 D , typically, impellers  28  of upstream and downstream pumps  24 U and  24 D are rotated in the same rotational direction as one another, as viewed from an external reference point (e.g., in the direction of arrow  48  (i.e., clockwise), or counterclockwise), but the impellers are disposed on the catheter such that the rotation of the impellers in this direction of rotation causes the impellers to pump blood in respective, opposite directions. It is noted that the rotational direction of the impellers “as viewed from an external reference point” should be interpreted to mean the direction of rotational motion of the impellers as observed from any point that is not undergoing the same rotational motion as either of the impellers. (For illustrative purposes,  FIG.  1 D  shows the impellers in the absence of the cages, although typically, the impellers are used together with cages, as described hereinabove.) 
     Typically, for such applications, a single motor is used to rotate both of the impellers. A shaft  50  is used to impart the rotational motion from the motor to the proximal impeller. An additional shaft  51 , which is in series with shaft  50 , couples the proximal impeller to the distal impeller and imparts the rotational motion from the proximal impeller to the distal impeller. For some applications, by using a single series of shafts to impart rotation to impellers  28  of both upstream and downstream pumps  24 U and  24 D, the diameter of blood-pump catheter  20  is reduced relative to if parallel shafts were used, in order to impart rotation to the upstream and downstream impellers. 
     For some applications, the angles and/or orientations of the impeller blades of impellers  28  of upstream and downstream pumps  24 U and  24 D may be such as to cause the impellers to pump blood in respective, opposite directions. For some applications, as shown in  FIG.  1 D , each propeller is shaped and/or oriented in the mirror image of the other, the axis of reflection being orthogonal to the longitudinal axes of the impellers. Typically, the upstream and downstream impellers are of opposing handedness to one another, a first one of the impellers being a left-handed impeller, and the other one of the impellers being a right-handed impeller. It is generally the case that impellers of opposing handedness that are positioned parallel to one another, facing the same direction as one another, and rotating in opposite rotational directions from one another, generate flow in the same direction as one another. In accordance with the present invention, the upstream and downstream impellers are typically disposed upon shaft  51  such that the impellers are facing in opposite directions to one another. As described hereinabove, the impellers are typically rotated in the same rotational direction as one another, as viewed from an external reference point. The result of the impellers (a) being of opposing handedness to one another, and (b) facing in opposite directions, is that, when the impellers are rotated in the same direction as one another about an axis defined by shaft  51 , the impellers pump blood in opposite directions from one another. 
     Typically, the blades of the downstream impeller are oriented such that, as the downstream impeller rotates in the direction of arrow  48 , the downstream impeller pumps in the downstream direction. The blades of the upstream impeller are oriented such that, as the upstream impeller rotates in the direction of arrow  48 , the upstream impeller pumps in the upstream direction. 
     As described in further detail hereinbelow, for some applications, upstream and downstream pumps  24 U and  24 D and blood-pump catheter  20  are placed within a main artery upstream and downstream of bifurcations of the artery with one or more branching arterial systems that branch from the main artery and supply a given organ, mutatis mutandis. For such applications, the blades of the downstream impeller are oriented such that, as the downstream impeller is rotated, the downstream impeller pumps in the upstream direction (toward the bifurcations). The blades of the upstream impeller are oriented such that, as the upstream impeller rotates is rotated, the upstream impeller pumps in the downstream direction (toward the bifurcations), such that blood flow into the branching arterial system is increased, thereby increasing perfusion of the organ. 
     For some applications, the blades of the impellers of the upstream and downstream pumps are configured to pump blood in the same direction as one another (e.g., in the antegrade direction). For example, the impellers may be of the same handedness as one another, placed upon catheter  20  such that the impellers are facing in the same direction as one another, and rotated in the same direction as one another, as viewed from an external reference point. Alternatively, the two impellers may be of opposing handedness to one another, placed within the vena cava such that the two impellers are facing in the same direction as one another, and rotated in opposite directions to one another, as viewed from an external reference point. 
     For some applications, blades of the upstream and downstream impellers are disposed at an angle alpha with respect to the longitudinal axes of the impellers, the blades of the respective impellers being oriented in opposite directions. For some applications, angle alpha is greater than 15 degrees (e.g., greater than 25 degrees), and/or less than 45 degrees (e.g., less than 35 degrees), e.g. 15-45 degrees, or 25-35 degrees. 
     For some applications, impellers  28  of upstream and downstream pumps  24 U and  24 D are rotated at respective rotation rates, in order to cause the pumping of blood in the upstream and downstream directions to be performed at respective rates. Alternatively, the impellers are rotated at the same rotation rate (and, typically, in the same direction), but the impellers are sized, shaped, and/or oriented such that the rate at which blood is pumped, respectively, in the upstream and downstream directions, by the respective impellers, is not equal. 
     Typically, a control unit  52  and a user interface  54  are disposed outside the subject&#39;s body. Further typically, the control unit receives inputs from one or more pressure sensors  56 ,  58 , and/or  60 , e.g., as shown in  FIGS.  1 A-D . 
     In accordance with some applications: 
     (a) a pressure sensor  56  is disposed on the upstream side of upstream blood pump  24 U and is configured to measure pressure within the vena cava upstream of the low-pressure region of the vena cava, which is typically indicative of venous pressure within the subject&#39;s lower body; 
     (b) a pressure sensor  58  disposed between the two blood pumps, and is configured to measure pressure within the low-pressure region of the vena cava between the two blood pumps, which is typically indicative of blood pressure within the subject&#39;s renal veins; and/or 
     (c) a pressure sensor  60  is disposed on the downstream side of downstream blood pump  24 D and is configured to measure pressure within the vena cava downstream of the low-pressure region of the vena cava, which is typically indicative of the subject&#39;s central venous pressure close to the subject&#39;s right heart. 
     For some applications, blood-pump catheter  20  includes pressure sensor  58  disposed between the two blood pumps, and is configured to measure pressure within the low-pressure region of the vena cava between the two blood pumps, which is typically indicative of blood pressure within the subject&#39;s renal veins, and the blood-pump catheter does not include pressure sensor  56 , or pressure sensor  60 . 
     For some applications, control unit  52  controls pumps  24 U and  24 D, e.g., by controlling rotation of impellers  28 , responsively to one or more of the above-described inputs. Typically, user interface  54  displays the subject&#39;s current lower-body venous pressure, renal venous pressure, and/or central venous pressure, based upon the signals generated by sensors  56 ,  58 , and/or  60 . Typically, based upon the current values of the subject&#39;s lower-body venous pressure, renal venous pressure, and/or central venous pressure, a user (such as a healthcare professional) inputs a target value for the subject&#39;s renal venous pressure, via the user interface. In response thereto, control unit  52  controls the speed of the rotation of the impellers, such that the impellers pump blood away from the renal veins at a flow rate that is such as to reduce the renal venous pressure toward the target level, as indicated by the user. For some applications, in response a signal received from sensor  60  indicating that the central venous pressure is at the target renal venous pressure, the control unit stops the impellers rotating. For some applications, the control unit receives an input from an additional sensor (such as a flow sensor and/or an oxygen-saturation sensor, and/or a thermal flow sensor, e.g., as described with reference to  FIGS.  22 A   i - 22 C ii  of WO 14/141284 to Schwammenthal, which is incorporated herein by reference), and the control unit controls the speed of the rotation of the impellers responsively to an input from the additional sensor. 
     It is noted that control unit  52  typically includes a computer processor that comprises circuitry and that is configured to execute the actions described herein. Typically, the operations described herein that are performed by the computer processor transform the physical state of a memory, which is a real physical article that is in communication with the computer processor, to have a different magnetic polarity, electrical charge, or the like, depending on the technology of the memory that is used. Control unit  52  is typically a hardware device programmed with computer program instructions to produce a special-purpose computer. For example, when programmed to perform the techniques described herein, control unit  52  typically acts as a special-purpose, renal-venous-pressure-modulating computer processor. 
     It is further noted that user interface  54  typically includes any type of user interface configured to receive inputs from a user and/or to provide outputs to the user. For example, the user interface may include one or more input devices (such as a keyboard, a mouse, a trackball, a joystick, a touchscreen monitor, a touchpad, a voice-command interface, a smartphone, a tablet computer, and/or other types of input devices that are known in the art), and/or one or more output devices (such as a monitor, an audio output device, a smartphone, a tablet computer, and/or other types of output devices that are known in the art). 
     Reference is now made to  FIG.  2   , which is a schematic illustration of blood-pump catheter  20  being inserted into the subject&#39;s vena cava  22  via the subject&#39;s right jugular vein  62  (through guide catheter  23 ), in accordance with some applications of the present invention. For some applications, instead of being inserted via the subclavian vein (as shown in  FIG.  1 A , for example), blood-pump catheter  20  is inserted into the vena cava via the subject&#39;s right jugular vein, or via another vein that is above the subject&#39;s inferior vena cava. In all other aspects, blood-pump catheter  20  and the functioning thereof are generally as described with reference to  FIGS.  1 A-D . 
     Reference is now made to  FIG.  3   , which is a schematic illustration of blood-pump catheter  20  being inserted into the subject&#39;s vena cava  22  via the subject&#39;s femoral vein  64  (through guide catheter  23 ), downstream pump  24 D being disposed upon the catheter distally to upstream pump  24 U, in accordance with some applications of the present invention. For some applications, instead of being inserted via the subclavian vein (as shown in  FIG.  1 A , for example), blood-pump catheter  20  is inserted into the vena cava, via the subject&#39;s femoral vein  64 , or via another vein that is below the subject&#39;s inferior vena cava. Typically, downstream blood pump  24 D is disposed on blood-pump catheter  20  distally to upstream blood pump  24 U. Blood-pump catheter  20  is configured to be placed within the vena cava, such that the upstream pump is disposed upstream of the junctions of the vena cava with all of the subject&#39;s renal veins  42 , and such that the downstream pump is disposed downstream of the junctions of the vena cava with all of the subject&#39;s renal veins. Other than the dispositions of the upstream and downstream blood pumps with respect to blood-pump catheter  20 , blood-pump catheter  20 , as shown in  FIG.  3   , and the functioning thereof, is generally similar to that described with reference to blood-pump catheter  20  as shown in  FIGS.  1 A-D . 
     Reference is now made to  FIG.  4   , which is a schematic illustration of upstream and downstream pumps  24 U and  24 D being disposed on respective catheters  66  and  68 , in accordance with some applications of the present invention. For some applications, a first catheter  66  is inserted into vena cava  22  through a guide catheter  67  that is inserted via the subject&#39;s femoral vein  64 , or via another vein that is below the subject&#39;s inferior vena cava. Upstream blood pump  24 U is disposed on the first catheter, and is configured to be placed within the vena cava upstream of the junctions of the vena cava with all of the subject&#39;s renal veins, and to pump blood through the vena cava in the manner described hereinabove. A second catheter  68  is inserted into the vena cava through a guide catheter  69  that is inserted via the subject&#39;s jugular vein  62  (as shown), via the subclavian vein (not shown), or via a different vein that is above the subject&#39;s inferior vena cava. Downstream blood pump  24 D is disposed on the second catheter, and is configured to be placed within the vena cava downstream of the junctions of the vena cava with all of the subject&#39;s renal veins, and to pump blood through the vena cava in the manner described hereinabove. 
     For applications in which the upstream and downstream blood pumps include impellers, typically, respective motors  70  and  72  are used to control rotation of the impellers. Further typically, control unit  52  controls both pumps (e.g., by controlling the rates of rotation of the impellers). For some applications, pressure sensors  56 ,  58  and  60  are disposed upon the first and/or second catheters, and are configured to detect indications of, respectively, lower body venous pressure, renal venous pressure, and central venous pressure. The control unit is configured to control the operation of the upstream and downstream pumps responsively to the detected indications, in accordance with the techniques described hereinabove. 
     Reference is now made to  FIGS.  5 A-B , which are schematic illustrations of blood-pump catheter  20 , the catheter including downstream pump  24 D and an occlusion element, such as a balloon  80  ( FIG.  5 A ), or a covered frame  82  ( FIG.  5 B ), in accordance with some applications of the present invention. For some applications, downstream pump is placed inside vena cava  22 , downstream of the junctions of the vena cava with all of the subject&#39;s renal veins. The downstream pump pumps blood through the vena cava, in the downstream direction, away from the junctions of the vena cava with the renal veins, in the manner described hereinabove. As an alternative to, or in addition to using an upstream pump as described hereinabove, the occlusion element is placed inside the vena cava upstream of the junctions of the vena cava with the subject&#39;s renal veins. Typically, the occlusion element is configured to partially occlude the subject&#39;s vena cava upstream of the junctions of the vena cava with the subject&#39;s renal veins. The occlusion element is configured to partially occlude the subject&#39;s vena cava such that, in response to the pumping of the downstream blood pump, there is not a substantial increase of blood flow from the subject&#39;s lower body toward the subject heart, but such that a region of low pressure within the vena cava is generated, between the occlusion element and the downstream blood pump, within which the blood pressure is lower than the subject&#39;s central venous pressure. Typically, by generating a region of low pressure, blood flow from the renal veins into the vena cava increases, thereby lowering renal blood pressure and enhancing renal perfusion. It is noted that the occlusion element is configured to partially occlude, but not to totally occlude, the vena cava, in such a manner as to generate a region of low pressure within the vena cava, but to allow a substantial flow of blood through the vena cava. 
     When blood-pump catheter  20  is used to enhance blood flow from the renal veins into the subject&#39;s vena cava, as described herein, a longitudinal distance D 2  between the longitudinal center of the impeller of the downstream pump and the longitudinal center of the occlusion element, measured along the longitudinal axis of the catheter, is typically more than 3 cm (e.g., more than 6 cm), and/or less than 18 cm (e.g., less than 14 cm), e.g., 3-18 cm, or 6-14 cm. 
     As used in the present application, including in the claims, a “longitudinal axis” of a structure is the set of all centroids of cross-sectional sections of the structure along the structure. Thus the cross-sectional sections are locally perpendicular to the longitudinal axis, which runs along the structure. (If the structure is circular in cross-section, the centroids correspond with the centers of the circular cross-sectional sections.) As used in the present application, including in the claims, the term “longitudinal center” denotes the center of a structure along the direction of the structure&#39;s longitudinal axis. 
     For some applications, the occlusion element is balloon  80 , as shown in  FIG.  5 A . Alternatively or additionally, the occlusion element is covered frame  82 , as shown in  FIG.  5 B . For example, the frame may be a rigid frame made of a shape-memory element (such as nitinol) that is covered with a blood-impermeable material  83  (e.g., polyester, polyurethane, and/or a different polymer). 
     As described hereinabove, typically, the occlusion element is configured to partially occlude the vena cava upstream of the junctions of the vena cava with the subject&#39;s renal veins. For some applications, the diameter to which the occlusion element is expanded is controllable. For example, inflation of the balloon may be controllable, or the stent may be expandable (e.g., by heating the stent, or by applying an electrical current to the stent). For some applications, the extent to which the occlusion element occludes the vena cava is controlled by a control unit (e.g., control unit  52 ) responsively to the blood pressure detected by blood pressure sensor  56 ,  58 , and/or  60 , in response to an input from a different sensor (such as a flow sensor and/or an oxygen-saturation sensor, and/or a thermal flow sensor, e.g., as described with reference to FIGS. 22Ai-Cii of WO 14/141284 to Schwammenthal, which is incorporated herein by reference), and/or in response to an input from a user. For some applications, the rate at which pump  24 D pumps blood away from the renal veins (e.g., the rate at which impeller  28  of the pump is rotated), as well as the extent to which the occlusion element occludes the vena cava is controlled by a control unit responsively to the blood pressure detected by blood pressure sensor  56 ,  58 , and/or  60 , in response to an input from a different sensor (such as a flow sensor and/or an oxygen-saturation sensor, and/or a thermal flow sensor, e.g., as described with reference to  FIGS.  22 A   i -C ii  of WO 14/141284 to Schwammenthal, which is incorporated herein by reference), and/or in response to an input from a user. 
     Although  FIGS.  5 A and  5 B  show the downstream blood pump and the occlusion element disposed on a catheter that is inserted into the vena cava from above the junctions of the vena cava with the subject&#39;s renal veins (e.g., via the subject&#39;s subclavian vein or jugular vein), for some applications, the downstream blood pump and the occlusion element are disposed on a catheter that is inserted into the vena cava from below the junctions of the vena cava with the subject&#39;s renal veins (e.g., via the subject&#39;s femoral vein),  mutatis mutandis . Alternatively or additionally, the occlusion element is disposed on a first catheter which is inserted into the vena cava from below the junctions of the vena cava with the subject&#39;s renal veins (e.g., via the subject&#39;s femoral vein), and the downstream blood pump is disposed on a second catheter, which inserted into the vena cava from above the junctions of the vena cava with the subject&#39;s renal veins (e.g., via the subject&#39;s subclavian vein, or jugular vein). 
     Reference is now made to  FIG.  6   , which is a schematic illustration of a blood-impermeable sleeve  84  configured to occlude blood flow from a subject&#39;s vena cava to the subject&#39;s renal veins, as described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference. Typically, the sleeve is placed within the vena cava such that a downstream end  86  of the sleeve is coupled to the wall of the vena cava at a first location  88  that is downstream of all renal veins  42  of the subject (e.g., left and right renal vein in a typical subject that has two renal veins), and such that an upstream end  90  of the sleeve is coupled to a wall of the vena cava at a second location  92  that is upstream of all renal veins of the subject. Thus, the sleeve isolates the blood in the renal veins into a compartment that is separated from blood flow through the center of the vena cava. Typically, a rigid structure, e.g., a stent  94  as shown, is configured to couple the upstream and downstream ends of the sleeve to the vena cava. 
     A pump  96  is configured to pump blood through inlet holes  97 , from a location that is exterior to sleeve  98  (i.e., from the isolated compartment) to a location that is in fluid communication with the interior of the sleeve (e.g., a location within the vena cava upstream or downstream of the sleeve). Thus, the pump pumps blood out of the subject&#39;s renal veins and into the subject&#39;s vena cava. The sleeve prevents backflow of blood from the vena cava into the renal veins. 
     For some applications, sleeve  84  and stent  94  are inserted into the subject&#39;s vena cava, while a guidewire  99  is disposed inside a pump-accommodating sleeve  95 . Subsequent to anchoring sleeve  84  and stent  94  to the vena cava, pump  96  is inserted through the pump-accommodating sleeve, by advancing the pump over the guidewire. 
     Sleeve  84  and pump  96  are generally as described with reference to  FIGS.  10 A-D  of WO 14/141284 to Schwammenthal, which is incorporated herein by reference. 
     It is noted that the effect of inserting sleeve  84  into the vena cava and activating pump  96  in the described manner is that a low-pressure region is generated within the subject&#39;s vena cava, adjacent to junctions of the vena cava with the subject&#39;s renal veins, blood pressure within the low-pressure region being lower than central venous pressure of the subject. Similarly, by using blood-pump catheter  20  as described with reference to  FIGS.  1 A- 5 B , a low-pressure region is generated within the subject&#39;s vena cava, adjacent to junctions of the vena cava with the subject&#39;s renal veins, blood pressure within the low-pressure region being lower than central venous pressure of the subject. The effect of generating the low-pressure region within the vena cava is typically that blood flow from the renal veins to the vena cava is increased, thereby reducing renal venous pressure, and increasing renal perfusion. 
     Reference is now made to  FIGS.  7 A-C , which are schematic illustrations of respective stages of a method of manufacture of impeller  28  (which is typically a bladed rotor), the impeller including reinforcement elements  100  for reinforcing blades  102  of the impeller, in accordance with some applications of the present invention. It is noted that  FIGS.  7 A-C  show a three-bladed impeller. However, the techniques described with reference to  FIGS.  7 A-C  may be practiced with an impeller having a different number of blades (e.g., one blade, two blades, or more than three blades), mutatis mutandis, in accordance with some applications of the present invention. 
     In accordance with the techniques described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference, for some applications, a tube (e.g., a nitinol, a stainless steel, or a plastic tube) is cut (e.g., laser cut), such that the cut tube defines a structure  104  having first and second end portions, e.g., rings  106 , at ends of the structure, the rings being connected to each other by a plurality of elongate elements. The tube is then axially compressed, such that the elongate elements form respective helical elongate elements  108 , and typically, the structure is shape-set in the axially compressed state of the structure. Structure  104  forms the frame of impeller  28 . 
     For some applications, as shown in  FIGS.  7 A-C , the impeller defines three blades  102 , in accordance with some applications of the present invention. For such applications, the tube is cut define three elongate elements. Alternatively or additionally, the impeller defines two blades (e.g., as described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference), or a different number of blades. 
     As described hereinabove, typically, in the axially compressed state of structure  104 , each of the elongate elements of structure  104  forms a helical elongate element  108 . Each of the helical elongate elements originates from a first one of the end portions (e.g., rings  106 ) and terminates at the second one of the end portions (e.g., rings  106 ). The pitches of each of the helical elongate elements are typically within 20 percent of one another, the helical elongate elements typically having the same pitch as one another. For some applications, the pitch of the helical elongate elements varies along the length of the helical elongate elements. The radii of each of the helical elongate elements are typically within 20 percent of one another, and, typically, the helical elongate elements have the same radius as one another. For some applications, the helices defined by the three elongate elements are not symmetrical with respect to one another. The longitudinal axis of each one of the helical elongate elements is typically parallel to the longitudinal axis of the other one of the helical elongate elements, and is typically parallel to the longitudinal axis of the impeller. For some applications, each of the elongate elements defines more than one eighth of a winding of a helix, and/or less than half a winding of a helix, e.g., between one eighth of a winding and half a winding of a helix. 
     It is noted that, although each of the elongate elements is described as being helical, for some applications, the elongate elements do not define precise mathematical helices, but each of the elongate elements defines a generally helical shape in that the elongate element spirals radially outwardly from a first one of end portions (e.g., rings), while extending axially away from the first one of the end portions, and then spirals radially inwardly toward the second one of the end portions, while extending axially toward the second one of the end portions. 
     As described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference, subsequent to axially compressing structure  104 , a material  110  (e.g., a flexible polymeric material, such as silicone, polyurethane, and/or polyester) is coupled to at least a portion of structure  104 , e.g., to helical elongate elements  108  of structure  104 . Typically, material  110  is coupled to the portion of structure  104  by structure  104  being dipped into material  110 , while material  110  is in a liquid state thereof. For example, structure  104  may be dipped into liquid silicone, a silicone-based elastomer, and/or a different elastomer. Subsequently, the material is dried (e.g., by a curing and/or a polymerization process), such that a film of the material forms that is supported by the helical elongate elements  108  of structure  104 . 
     For some applications, during the drying of material  110 , structure  104  is rotated about its longitudinal axis, such as to facilitate the formation of a film of material  110  having a uniform thickness. For some applications, material  110  is coupled to structure  104  in a different manner to the above-described manner, e.g., via suturing and/or electrospinning a flexible polymeric material (such as silicone, polyurethane, and/or polyester) to the helical elongate elements of structure  104 . The helical elongate elements  108  with the material coupled thereto define the impeller blades. For some applications, the material is dried (e.g., by curing, and/or polymerization) onto the helical elongate elements such that the helical elongate elements with the material coupled thereto forms a three-bladed impeller, as shown in  FIG.  7 B-C . 
     For some applications, sutures  112  are tied around a portion of the frame of the impeller, in accordance with techniques described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference. For some applications, in order to facilitate the formation of a film of material  110  on structure  104 , and/or in order to facilitate coupling of material  110  to helical elongate elements  108 , sutures  112  are tied around a portion of structure  104 . For example, the sutures may be tied around helical elongate elements  108  of structure  104 , as shown in  FIG.  7 A , which shows sutures  112  tied around helical elongate elements  108  before material  110  has been coupled to structure  104 . 
     As is generally described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference, for some applications, sutures  112  increase the surface area with which material  110  comes into contact, while material  110  is in its liquid state. Alternatively or additionally, the surface of the sutures is more rough and/or porous than that of elongate elements  108  (which are typically made of nitinol). Therefore, material  110  becomes coupled to the sutures with a greater coupling strength than that of the coupling between material  110  and elongate elements  108 . For some applications, the sutures act as mediators between a material from which the elongate elements are made, which typically has a relatively high stiffness (and is typically nitinol), and material  110 , which is typically an elastomer having a relatively low stiffness. The sutures thereby enhance the strength of the coupling between material  110  and helical elongate elements  108 , when the material dries. For some applications, by enhancing the strength of the coupling between material  110  and helical elongate elements  108 , the sutures prevent gaps from forming between the material and helical elongate elements  108 , during and/or after the drying of material  110 . In this manner, the sutures facilitate the formation of a film of material  110  between the helical elongate elements. 
     For some applications, before material  110  is coupled to structure  104  (i.e., the frame of the impeller), reinforcement elements  100  are coupled to the structure. Typically, a respective reinforcement element is coupled to each of helical elongate elements  108 , such that there is a respective reinforcement element corresponding to each one of the impeller blades. Further typically, each of the reinforcement elements extends from a helical elongate element toward the longitudinal axis of the structure  104 . For some applications, each of the reinforcement elements extends from a longitudinal center of a helical elongate element toward the longitudinal axis of the structure  104 . Typically, the reinforcement elements are strings (e.g., strings made of polyester, polyamide, silicone, nylon, synthetic or biological polymers) and/or wires (e.g., wires made of nitinol, stainless steel, cobalt chrome or other metal alloys). 
     For some applications, an axial element  114  is placed along the longitudinal axis of structure  104 , and the reinforcement elements are coupled to the axial element, such that each of the reinforcement elements extends from a helical elongate element to the axial element. For some applications, the axial element is a string (e.g., a string made of polytetrafluoroethylene, extruded polytetrafluoroethylene, and/or nylon), and/or a wire (e.g., a wire made of nitinol, stainless steel, cobalt chrome or other metal alloys). 
     With reference to  FIG.  7 B , subsequent to coupling reinforcement elements  100  to structure  104 , material  110  is coupled to the structure, typically using techniques as described hereinabove, e.g., by structure  104  being dipped into material  110 , while material  110  is in a liquid state thereof. For some applications, including a reinforcement element in the space between a helical elongate element and the longitudinal axis of structure  104 , facilitates the formation of a film of material  110  that encompasses the entire aforementioned space, and that does not include any holes. Typically, the reinforcement elements reinforce central portions of the impeller blades. For example, the reinforcement element may reinforce the film along the longitudinal center of the film, where, in the absence of the reinforcement element, the film may be thin and may be given to allow the formation of a hole through the film. The reinforcement element provides an additional surface to which the film of the material can become attached. For some applications, the reinforcement elements are used to facilitate the formation of a film of material  110  that encompasses the space between a helical elongate element and the longitudinal axis of structure  104  for an impeller that is suitable for placing in the vena cava, e.g., an impeller having a span of more than 14 mm (e.g., more than 16 mm). As shown in  FIG.  7 B , typically, each of the blades of the impeller includes a respective reinforcement element, in accordance with the techniques described hereinabove. 
     With reference to  FIG.  7 C , for some applications, subsequent to the coupling of material  110  to structure  104 , axial element  114  is removed from the longitudinal axis of structure  104 . As described hereinabove, for some applications, the axial element is a string (e.g., a string made of polytetrafluoroethylene, extruded polytetrafluoroethylene, and/or nylon), and/or a wire (e.g., a wire made of nitinol, stainless steel, cobalt chrome or other metal alloys). For such applications, the string and/or the wire is typically removed from the longitudinal axis of structure  104 , such that reinforcement elements  100  remain in place. Typically, material  110  defines a hollow central lumen  116  along the longitudinal axis of structure  104 , subsequent to the removal of the axial element from the longitudinal axis. Further typically, the film of material  110  defines a thickened portion  118  that surrounds the hollow central lumen and that is thickened relative to the thickness of the material at other portions of the film. For some applications, the impeller is strengthened by virtue of the thickened portion of the film that surrounds the hollow central lumen, relative to an otherwise similarly shaped impeller that does not define the thickened portion of film. 
     For some applications of the present invention, axial element  114  is placed inside structure  104  before material  110  is coupled to the structure, even in the absence of reinforcing elements inside the structure. In this manner, the film of material forms a thickened portion  118  that surrounds the hollow central lumen and that is thickened relative to the thickness of the material at other portions of the film, in accordance with the techniques described hereinabove. 
     As is generally described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference, for some applications, the three-bladed impeller shown in  FIGS.  7 A-C  does not include an axial support member (such as a shaft) between the proximal and distal ends of the helical elongate elements and along the axis of the impeller, for providing support to material  110 . More generally, for some applications, the impeller does not include a support member (such as a shaft) for providing support to material  110  in addition to the helical elongate elements, between the proximal and distal ends of the helical elongate elements. Furthermore, rotational motion is imparted from the proximal end portion (e.g., proximal ring  106 ) of the impeller to the distal end portion (e.g., distal ring  106 ) of the impeller via the helical elongate elements  108  of the impeller (e.g., substantially solely via the helical elongate elements), and not via an axial support member (such as a shaft). 
     For some applications, material  110  of the impeller itself is molded such as to facilitate the insertion of an axial support member therethrough. For example, an elastomer (such as silicone or a silicone-based elastomer) may be used as material  110 , and the elastomer may be molded to form a hollow central lumen therethrough. In accordance with the techniques described hereinabove, for some applications, axial element  114  is placed inside structure  104  before material  110  is coupled to the structure. In this manner, the film of material is shaped to define hollow central lumen  116 . For some applications, an axial support member is coupled to the impeller by being passed through the hollow central lumen defined by the elastomer. Alternatively or additionally, an impeller that includes an axial support member is provided using the techniques described hereinbelow with reference to  FIGS.  8 A- 10 C . 
     Reference is now made to  FIGS.  8 A-C , which are schematic illustrations of blood-pump catheter  20 , the blood-pump catheter including impeller  28 , the impeller including an axially elongatable telescopic axial shaft  120 , in accordance with some applications of the present invention. It is noted that  FIGS.  8 A-C  show a two-bladed impeller. However, the techniques described with reference to  FIGS.  8 A-C  may be practiced with an impeller having a different number of blades (e.g., one blade, three blades, or more than three blades), mutatis mutandis, in accordance with some applications of the present invention. 
     As is generally described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference, typically, during insertion of cage  30  and impeller  28  into the subject&#39;s vasculature, the cage and the impeller are crimped by axially elongating the cage and the impeller, such that the cage and the impeller become radially compressed. The cage and the impeller are inserted into the vasculature, while the cage and the impeller are maintained in radially-compressed configurations by an insertion device, e.g., catheter  23 . The cage and the impeller are released from the distal end of the insertion device into the renal vein, typically by retracting the insertion device. In response to being released from the distal end of the insertion device, the cage and the impeller automatically radially expand, and axially contract. 
     As shown in the transition from  FIG.  8 A  to  FIG.  8 B  and then to  FIG.  8 C , for some applications, as the cage expands radially, the distal end of the cage moves closer to the proximal end of the cage. (It is noted that distal end of the cage may move closer to the proximal end of the cage by either or both ends of the cage moving. That is, the distal end of the cage may move proximally, and/or the proximal end of the cage may move distally.) The distal end of the cage engages an inner tubular member  122  of telescopic shaft  120 , such that the movement of the distal end of the cage closer to the proximal end of the cage causes the inner tubular member of the telescopic shaft to slide proximally with respect to an outer tubular member  124  of the telescopic shaft. In this manner the telescopic shaft contracts in the axial direction causing impeller blades  102  to radially expand. It is noted that, typically, the impeller is shape-set such as to radially expand in the absence of any radial constraining force acting upon the impeller. Typically, a rotating shaft member  128  is disposed inside both the inner and outer tubular members of the telescopic shaft and is configured to impart rotational motion to the impeller blades. 
     Typically, cage  30  is configured to hold open the inner wall of the vena cava and to separate the inner wall of the vena cava from the impeller, such that the vena cava does not become injured by the impeller, in accordance with the general techniques described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference. For some applications, in response to the cage becoming radially contracted and axially elongated (e.g., in response to the renal vein exerting radial pressure on the cage), the distal end of the cage moves away from the proximal end of the cage. The distal end of the cage engages inner tubular member  122  of telescopic shaft  120 , so as to cause the inner tubular member of the telescopic shaft to slide distally with respect to an outer tubular member  124  of the telescopic shaft. In this manner the telescopic shaft elongates in the axial direction causing impeller blades  102  to radially contract. 
     The cage is typically engaged with respect to the telescopic shaft such that, even at a circumferential location at which a separation between the impeller and the inner surface of the cage is smallest, a separation between the impeller and the inner surface of the cage is maintained (i.e., the impeller and the inner surface of the cage are still separated from each other), even if the cage radially contracts. A fortiori, even at the circumferential location at which a separation between the impeller and the outer surface of the cage is smallest, the cage is engaged with respect to the telescopic shaft such that the separation between the impeller and the outer surface of the cage is maintained (i.e., the impeller and the outer surface of the cage are still separated from each other), even if the cage radially contracts. Since the inner wall of the renal vein is supported by the outer surface of the cage, the separation between the impeller and the outer surface of the cage is typically also the separation between the impeller and the inner wall of the renal vein at the location at which the inner wall of the renal vein is closest to the impeller. Thus, the cage is engaged with respect to the telescopic shaft such that a separation between the impeller and the inner wall of the renal vein is maintained, even at the location at which the inner wall of the renal vein is closest to the impeller, and even when the renal vein exerts pressure on the cage such that the cage radially contracts. 
     Reference is now made to  FIGS.  9 A-C , which are schematic illustrations of blood-pump catheter  20 , the blood-pump catheter including impeller  28 , the impeller including axially elongatable telescopic axial shaft  130 , in accordance with some applications of the present invention. It is noted that  FIGS.  9 A-C  show a two-bladed impeller. However, the techniques described with reference to  FIGS.  8 A-C  may be practiced with an impeller having a different number of blades (e.g., one blade, three blades, or more than three blades), mutatis mutandis, in accordance with some applications of the present invention. 
     As is generally described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference, typically, during insertion of cage  30  and impeller  28  into the subject&#39;s vasculature, the cage and the impeller are crimped by axially elongating the cage and the impeller, such that the cage and the impeller become radially compressed. The cage and the impeller are inserted into the vasculature, while the cage and the impeller are maintained in radially compressed configurations by an insertion device, e.g., catheter  23 . The cage and the impeller are released from the distal end of the insertion device into the renal vein, typically by retracting the insertion device. In response to being released from the distal end of the insertion device, the cage and the impeller automatically radially expand, and axially contract. 
     As shown in the transition from  FIG.  9 A  to  FIG.  9 B  and then to  FIG.  9 C , for some applications, as the cage expands radially, the distal end of the cage moves closer to the proximal end of the cage. (It is noted that distal end of the cage may move closer to the proximal end of the cage by either or both ends of the cage moving. That is, the distal end of the cage may move proximally, and/or the proximal end of the cage may move distally.) The distal end of the cage engages an outer tubular member  132  of telescopic shaft  130 , such that the movement of the distal end of the cage closer to the proximal end of the cage causes the outer tubular member of the telescopic shaft to slide proximally with respect to an inner tubular member  134  of the telescopic shaft. In this manner the telescopic shaft contracts in the axial direction causing impeller blades  102  to radially expand. It is noted that, typically, the impeller is shape-set such as to radially expand in the absence of any radial constraining force acting upon the impeller. Typically, a rotating shaft member  138  is disposed inside both the inner and outer tubular members of the telescopic shaft and is configured to impart rotational motion to the impeller blades. 
     Typically, cage  30  is configured to hold open the inner wall of the vena cava and to separate the inner wall of the vena cava from the impeller, such that the vena cava does not become injured by the impeller, in accordance with the general techniques described in WO 14/141284 to Schwammenthal, which is incorporated herein by reference. For some applications, in response to the cage becoming radially contracted and axially elongated (e.g., in response to the renal vein exerting radial pressure on the cage), the distal end of the cage moves away from the proximal end of the cage. The distal end of the cage engages outer tubular member  132  of telescopic shaft  130 , so as to cause the outer tubular member of the telescopic shaft to slide distally with respect to inner tubular member  134  of the telescopic shaft. In this manner the telescopic shaft elongates in the axial direction causing impeller blades  102  to radially contract. 
     The cage is engaged with respect to the telescopic shaft such that, even at a circumferential location at which a separation between the impeller and the inner surface of the cage is smallest, a separation between the impeller and the inner surface of the cage is maintained (i.e., the impeller and the inner surface of the cage are still separated from each other), even if the cage radially contracts. A fortiori, even at the circumferential location at which a separation between the impeller and the outer surface of the cage is smallest, the cage is engaged with respect to the telescopic shaft such that the separation between the impeller and the outer surface of the cage is maintained (i.e., the impeller and the outer surface of the cage are still separated from each other), even if the cage radially contracts. Since the inner wall of the renal vein is supported by the outer surface of the cage, the separation between the impeller and the outer surface of the cage is typically also the separation between the impeller and the inner wall of the renal vein at the location at which the inner wall of the renal vein is closest to the impeller. Thus, the cage is engaged with respect to the telescopic shaft such that a separation between the impeller and the inner wall of the renal vein is maintained, even at the location at which the inner wall of the renal vein is closest to the impeller, and even when the renal vein exerts pressure on the cage such that the cage radially contracts. 
     Reference is now made to  FIGS.  10 A-C , which are schematic illustrations showing respective steps of a method of manufacture of impeller  28 , the impeller including a telescopic axial shaft (e.g., telescopic shaft  120 , or telescopic shaft  130 ), in accordance with some applications of the present invention. It is noted that  FIGS.  10 A-C  show a three-bladed impeller. However, the techniques described with reference to  FIGS.  10 A-C  may be practiced with an impeller having a different number of blades (e.g., one blade, two blades, or more than three blades), mutatis mutandis, in accordance with some applications of the present invention. 
     For some applications, structure  104 , which includes helical elongate elements (and which is generally as described hereinabove) forms the frame of the impeller  28 . Material  110  (e.g., a flexible polymeric material, such as silicone, polyurethane, and/or polyester) is coupled to at least a portion of structure  104 , e.g., to helical elongate elements  108  of structure  104 . For some applications, sutures  112  are tied around a portion of the frame of the impeller, in accordance with techniques described in WO 14/141284, which is incorporated herein by reference. As described hereinabove, with reference to  FIGS.  7 A-C , for some applications, impeller  28  includes reinforcement elements  100  for reinforcing blades  102  of the impeller. 
     For some applications, before material  110  is coupled to structure  104  (i.e., the frame of the impeller), reinforcement elements  100  are coupled to the structure. Typically, a respective reinforcement element is coupled to each of helical elongate elements  108 , such that there is a respective reinforcement element corresponding to each one of the impeller blades. For some applications, axial elements  140  are placed on the helical elongate elements in the vicinity of the longitudinal axis of structure  104 , typically, such that the axial elements extend from the proximal end to the distal end of respective helical elongate elements. Respective reinforcement elements are coupled to (e.g., sutured to) respective axial elements, such that each of the reinforcement elements extends from a helical elongate element to a respective axial element. For some applications, the axial elements are strings (e.g., strings made of polytetrafluoroethylene, extruded polytetrafluoroethylene, and/or nylon), and/or a wire (e.g., a wire made of nitinol, stainless steel, cobalt chrome, or other metal alloys). 
     With reference to  FIG.  10 B , subsequent to coupling reinforcement elements  100  to structure  104 , material  110  is coupled to the structure, typically using techniques as described hereinabove, e.g., by structure  104  being dipped into material  110 , while material  110  is in a liquid state thereof. For some applications, including a reinforcement element in the space between a helical elongate element and an axial element  140  facilitates the formation of a film of material  110  that encompasses the entire aforementioned space, and that does not include any holes. For example, the reinforcement element may reinforce the film along the longitudinal center of the film, where, in the absence of the reinforcement element, the film may be thin and may be given to allow the formation of a hole through the film. The reinforcement element provides an additional surface to which the film of the material can become attached. For some applications, the reinforcement elements are used to facilitate the formation of a film of material  110  that encompasses the space between a helical elongate element and the axial elements for an impeller that is suitable for placing in the vena cava, e.g., an impeller having a span of more than 14 mm (e.g., more than 16 mm). As shown in  FIG.  10 B , typically, each of the blades of the impeller includes a respective reinforcement element, in accordance with the techniques described hereinabove. Further typically, for each of the blades of the impeller, an axial element is placed in the vicinity of the longitudinal axis of structure  104 . 
     With reference to  FIG.  10 C , for some applications, subsequent to the coupling of material  110  to structure  104 , axial elements  140  are removed from structure  104 . As described hereinabove, for some applications, the axial elements are strings (e.g., strings made of polytetrafluoroethylene, extruded polytetrafluoroethylene, and/or nylon), and/or a wire (e.g., a wire made of nitinol, stainless steel, cobalt chrome or other metal alloys). For such applications, the strings and/or the wires are typically removed from structure  104 , such that reinforcement elements  100  remain in place. 
     Typically, subsequent to the steps described hereinabove with reference to  FIGS.  10 A-C , structure  104  is mounted upon a telescopic shaft, e.g., telescopic shaft  120  (described with reference to  FIGS.  8 A-C ), or telescopic shaft  130  (described with reference to  FIGS.  9 A-C ). 
     Reference is now made to  FIGS.  11 A-C , which are schematic illustrations of blood-pump catheter  20  placed within a subject&#39;s vena cava  22 , upstream pump  24 U being disposed upon the catheter, distally to downstream pump  24 D, the upstream and downstream pumps being disposed at respective separations from one another in  FIGS.  11 A,  11 B, and  11 C , in accordance with some applications of the present invention. For some applications, the upstream and downstream pumps are coupled to one another via a telescopic shaft  150 , the length of which is adjustable, such that the distance between the upstream and downstream pumps is adjustable. For some applications, upon being placed inside the subject&#39;s body, the response of the subject&#39;s renal venous pressure to the pumping of pumps  24 D and  24 U is measured, when the pumps are disposed at respective distance from one another. Based upon the measured responses of the subject&#39;s renal venous pressure, the pumps are deployed at a given separation from one another and are operated. 
     For some applications, regardless of the separation between the upstream and downstream pumps  24 U and  24 D, the pumps are deployed in the vena cava such that the distance from the right renal vein  42 R to the longitudinal center of the impeller of the upstream pump  24 U is approximately equal to the distance between the left renal vein  42 L and the longitudinal center of the impeller of the downstream pump  24 D. Alternatively, the distance from the right renal vein  42 R to the longitudinal center of the impeller of the upstream pump  24 U is different from the distance between the left renal vein  42 L and the longitudinal center of the impeller of the downstream pump  24 D. 
     It is noted that blood-pump catheter  20 , as shown in  FIGS.  11 A-C , includes only blood pressure sensor  58 , in accordance with some applications of the present invention. Alternatively, blood-pump catheter  20  includes one or more additional sensors, e.g., blood pressure sensors  56  and  60 , as described hereinabove. 
     Reference is now made to  FIG.  12   , which is a schematic illustration of blood-pump catheter  20  placed within a subject&#39;s vena cava  22 , upstream pump  24 U being disposed upon the catheter, distally to downstream pump  24 D, and a support stent  160  being disposed upon the catheter between the upstream and downstream pumps, in accordance with some applications of the present invention. As described hereinabove, typically during operation of pumps  24 U and  24 D, a region of low pressure is generated within the vena cava between the two pumps. Typically, stent  160  is configured to support the walls of the vena cava at the low pressure region, such that the vena cava does not become obstructed at the low pressure region, due to the walls of the vena cava collapsing. For some applications, stent  160  has a generally similar shape to cage  30 . Although  FIG.  12    shows stent  160  disposed upon a blood-pump catheter, upon which the upstream pump is disposed distally to the downstream pump, for some applications, stent  160  is disposed upon a blood-pump catheter, upon which the downstream pump is disposed distally to the upstream pump, as described hereinabove. 
     Reference is now made to  FIGS.  13 A-E , which are schematic illustrations of blood-pump catheter  20  placed within a subject&#39;s vena cava  22 , upstream impeller  28 U being disposed upon the catheter, distally to downstream impeller  28 D, the upstream and downstream impellers being disposed within a support cage  170  that supports the walls of a portion of the vena cava between the upstream and downstream impellers, in accordance with some applications of the present invention. Although some of  FIGS.  13 A-E  shows support cage  170  disposed upon a blood-pump catheter, upon which the upstream pump is disposed distally to the downstream pump, for some applications, support cage  170  is disposed upon a blood-pump catheter, upon which the downstream pump is disposed distally to the upstream pump, as described hereinabove. 
     As described hereinabove, typically during operation of pumps  24 U and  24 D, a region of low pressure is generated within the vena cava between the two pumps. Typically, support cage  170  is configured to support the walls of the vena cava at the low pressure region, such that the vena cava does not become obstructed at the low pressure region, due to the walls of the vena cava collapsing. 
     Support cage typically extends at least from the longitudinal center of the downstream impeller to the longitudinal center of the upstream impeller. For some applications, a length L 1  ( FIG.  13 B ) of the support cage, when the cage is in a non-constrained configuration thereof inside the vena cava, is more than 3 cm (e.g., more than 6 cm), and/or less than 18 cm (e.g., less than 14 cm), e.g., 3-18 cm, or 6-14 cm. Further typically, a diameter D 3  ( FIG.  13 B ) of support cage  170 , when the cage is in a non-constrained configuration thereof inside the vena cava, is more than 14 mm (e.g., more than 16 mm), and/or less than 35 mm (e.g., less than 25 mm), e.g., 14-35 mm, or 16-25 mm. 
     For some applications, as shown in  FIGS.  13 A-C , the impellers are placed inside support cage  170 , in the absence of individual cages that are disposed around the respective impellers. For such applications, the support cage is typically configured to (a) support the walls of the vena cava at the low pressure region, as described hereinabove, and (b) to maintain a separation between the impellers and the inner wall of the vena cava, in a generally similar manner to that described hereinabove with respect to cage  30 . 
       FIG.  13 A  shows a blood-pump catheter as described, placed inside the subject&#39;s vena cava  22 .  FIG.  13 B  shows a three-dimensional view of impellers and support cage  170  as described, in the absence of the subject&#39;s anatomy.  FIG.  13 C  shows a three-dimensional view of impellers and support cage  170 , in the absence of the subject&#39;s anatomy, and with the support cage including support elements  172 , in accordance with some applications of the present invention. As described hereinabove, for some applications, an axial shaft  51  is disposed between the proximal and distal impellers and is configured to impart rotational motion from the proximal impeller to the distal impeller. For some applications, support elements  172  extend from the support cage, and are coupled to axial shaft  51 , such as to maintain the disposition of shaft  51  along the longitudinal axis of the support cage. In this manner, the disposition of the axial shaft is typically maintained along the longitudinal axis of the vena cava. Further typically, the support elements maintain the longitudinal axes of the proximal and distal impellers in alignment with one another, and in alignment with the longitudinal axis of the vena cava. 
     For some applications, as shown in  FIGS.  13 D-E , impellers  28 D and  28 U are placed inside support cage  170 , in the presence of individual cages that are disposed around the respective impellers. Typically, the individual cages in which the impellers are disposed are generally similar to cage  30 , as described hereinabove. 
     For some applications, support cage  170  is shaped to define individual cages  174  that are formed as a single integrated structure together with the support cage, as shown in  FIG.  13 D . For example, the individual cages and the support cage may be cut from a single piece of metal or alloy (e.g., nitinol). Alternatively, individual cages  30  may be formed separately from support cage  170 , as shown in  FIG.  13 E . For such applications, the impellers may be placed inside individual cages  30 , as described hereinabove, and individual cages  30  may then be placed inside support cage  170 . For some such applications, individual cages  30  are placed inside support cage  170  inside the subject&#39;s body (e.g., inside the subject&#39;s vena cava). Alternatively, individual cages  30  are placed inside support cage  170  outside the subject&#39;s body (e.g., within guide catheter  23 , shown in  FIG.  1 A ), and the individual cages are deployed inside the subject&#39;s body (e.g., inside the subject&#39;s vena cava) together with the support cage. 
     In general,  FIGS.  12    and  FIGS.  13 A-E  show examples of blood-pump catheter  20  in which the blood-pump catheter includes a support structure (e.g., stent  160 , or support cage  170 ), a longitudinal center of which is disposed between the upstream and downstream blood pumps (e.g., between the upstream and downstream impellers). For some applications, the longitudinal center of the support structure is disposed equidistantly from the upstream and downstream blood pumps (e.g., the upstream and downstream impellers). The support structure is configured to support an inner wall of the vena cava in an open configuration during the pumping of the blood by the first and second pumps. For some applications (not shown), a support structure, such as the structure shown in  FIGS.  12  and  13 A -E, is used in conjunction with a blood-pump catheter that includes a downstream pump and an upstream occlusion element (e.g., as shown in  FIGS.  5 A-B ), mutatis mutandis. For such applications, the support structure is configured to support an inner wall of the vena cava in an open configuration during the pumping of the blood by the downstream pump. 
       FIGS.  14 A-C  are graphs showing the pressure drop recorded in models of a subject&#39;s left and right renal veins, during experiments that were conducted using pumps, in accordance with some applications of the present invention. 
     In the experiments, a model of the vena cava and renal veins was used. The model was made of flexible silicone filled with saline. Upstream and downstream pumps as described herein were placed inside the vena cava, respectively below and above the renal veins. The pumps were activated to pump the saline through the vena cava in the manner described herein, and the drop in pressure in left and right renal veins was measured relative the pressure in the left and right renal veins before the pumps were activated. 
       FIG.  14 A  shows a plot of the measured pressure reduction (dP) in the left and right renal veins for respective rates of revolutions per minute (RPM) of the pumps (which was always the same for both pumps), for when the pumps were placed in the vena cava in the absence of either a support stent (as shown in  FIG.  12   ) or a support cage (as shown in  FIGS.  13 A-E ) between the upstream and downstream pumps. 
       FIG.  14 B  shows a plot of the measured pressure reduction (dP) in the left and right renal veins for respective rates of revolutions per minute (RPM) of the pumps (which was always the same for both pumps), for when the pumps were placed in the vena cava in the presence of a support stent (as shown in  FIG.  12   ) between the upstream and downstream pumps. 
       FIG.  14 C  shows a plot of the measured pressure reduction (dP) in the left and right renal veins for respective rates of revolutions per minute (RPM) of the pumps (which was always the same for both pumps), for when the pumps were placed in the vena cava in the presence of a support cage between the upstream and downstream pumps, the support cage being configured as shown in  FIG.  13 E . 
     As may be observed in  FIGS.  14 A-C , the greatest pressure reduction was achieved when the pumps were used in conjunction with a support cage that extends at least from the longitudinal center of the downstream impeller to the longitudinal center of the upstream impeller (the results of which are shown in  FIG.  14 C ). In addition, the most even pressure reduction of both the left and right renal veins was achieved when the pumps were used in conjunction with a support cage that extends at least from the longitudinal center of the downstream impeller to the longitudinal center of the upstream impeller. When a support stent as shown in  FIG.  12    was disposed between the upstream and downstream pumps (the result of which are shown in  FIG.  14 B ), there was still a greater and more even pressure reduction than when the upstream and downstream pumps were used in the absence of any supporting structure between the upstream and downstream pumps (the results of which are shown in  FIG.  14 A ). 
     Therefore, the results shown in  FIGS.  14 A-C  indicate that the efficacy of the reducing renal venous pressure by pumping blood through the vena cava using upstream and downstream pumps as described herein may be improved by placing a support structure inside the vena cava between the upstream and downstream pumps, in accordance with techniques described herein. Furthermore, the results indicate that the efficacy of the aforementioned technique may be particularly improved by placing a support cage inside the vena cava that extends at least from the longitudinal center of the downstream pump (e.g., the longitudinal center of the impeller of the downstream pump) to the longitudinal center of the upstream pump (e.g., longitudinal center of the impeller of the upstream pump). Therefore, for some applications of the present invention, apparatus and methods as described in  FIGS.  12 - 13 E  are used. 
     Reference is now made to  FIGS.  15 A-B , which are schematic illustrations of an axial shaft  180  of a distal end of impeller  28  and a bearing  182  within which the axial shaft rotates, in accordance with respective applications of the present invention. Typically, axial shaft  180  and bearing  182  are disposed inside a housing  183 . For some applications, the axial shaft and the bearing are configured to reduce the formation of clots of blood that may be generated at the interface between the axial shaft and the bearing due to the heat generated by the axial shaft rotating within the bearing. 
     For some applications, as shown in  FIG.  15 A , the axial shaft and the bearing are sized such that there is a gap  184  between the axial shaft and the bearing. Typically, there is a difference between the outer diameter of the axial shaft and the inner diameter of the bearing that is greater than 0.02 mm (e.g., greater than 0.03 mm) and/or less than 0.1 mm (e.g., less than 0.07 mm), e.g., 0.02 mm-0.1 mm, or 0.03 mm-0.07 mm, such that gap  184  is defined between the axial shaft and the bearing. Typically, the gap facilitates the flow of blood therethrough which reduces friction at the interface between the axial shaft and the bearing. 
     For some applications, bearing  182  is supported inside housing  183  by a supporting element  185 . Typically, the supporting element is sized such that, at locations at which the bearing does not separate between the supporting element and axial shaft  180 , there is a gap  187  between the supporting element and the axial shaft. Typically, gap  187  between the supporting element and axial shaft is larger than gap  184  between the axial shaft and the bearing. 
     For some applications, housing  183  includes a protrusion  186  from its inner wall. The protrusion is configured to prevent the inner wall of the housing from wearing away due to contact between the distal end of the axial shaft and the inner wall of the housing. 
     For some applications, as shown in  FIG.  15 B , a distal end  188  of the axial shaft is frustoconically shaped. For some applications, due to the frustoconical shape of the distal end of the axial shaft, even if blood clots develop inside housing  183 , the vector of the resisting force that is exerted upon the distal end of the axial shaft by the blood clots is such that it does not impede rotation of the axial shaft. 
     In experiments that were conducted by the inventors of the present application, impellers as described herein were rotated inside a Tygon® tube that contained blood. The blood was maintained at 37 degrees Celsius by being placed in a water bath. When a pump was used in which the difference between the outer diameter of the shaft and the inner diameter of the bearing was less than 0.02 mm, and in which the distal end of the axial shaft had not been shaped frustoconically, after one hour the motor current started to increase and showed instability. After another 10 minutes the impeller stopped rotating due to high resistance to rotation. The front axis assembly of the device was opened and clot formation was observed. Subsequently, a pump was operated under the same conditions, but the pump was configured such that the difference between the outer diameter of the shaft and the inner diameter of the bearing was between 0.04 mm and 0.05 mm, and the distal end of the axial shaft had been shaped frustoconically. In this case, the device worked for over 48 hours. These results indicate that using a pump that defines a gap, as described, between axial shaft  180  and bearing  182 , and/or in which the distal end of axial shaft  180  is frustoconically shaped may reduce the likelihood of clot formation, relative to a pump that is not so configured. 
     In general, in the specification and in the claims of the present application, the term “proximal” and related terms, when used with reference to a device or a portion thereof, should be interpreted to mean an end of the device or the portion thereof that, when inserted into a subject&#39;s body, is typically closer to a location through which the device is inserted into the subject&#39;s body. The term “distal” and related terms, when used with reference to a device or a portion thereof, should be interpreted to mean an end of the device or the portion thereof that, when inserted into a subject&#39;s body, is typically further from the location through which the device is inserted into the subject&#39;s body. 
     In general, in the specification and in the claims of the present application, the term “downstream” and related terms, when used with reference to a blood vessel, or with reference to a portion of a device that is configured to be placed inside a blood vessel, should be interpreted to mean a location within the blood vessel, or a portion of the device that is intended for placement at a location within the blood vessel, that is downstream, with respect to the direction of antegrade blood flow through the blood vessel, relative to a different location within the blood vessel. The term “upstream” and related terms, when used with reference to a blood vessel, or with reference to a portion of a device that is configured to be placed inside a blood vessel, should be interpreted to mean a location within the blood vessel, or a portion of the device that is intended for placement at a location within the blood vessel, that is upstream with respect to the direction of antegrade blood flow through the blood vessel, relative to a different location within the blood vessel. 
     It is noted that blood pumps  24 U and  24 D, the catheters upon which the blood pumps are disposed (e.g., blood-pump catheter  20 , catheter  66 , and catheter  68 ), and the occlusion elements described with reference to  FIGS.  5 A-B , and other devices described herein, are generally described as being placed within the subject&#39;s vena cava, such that the upstream pump or the occlusion element is disposed upstream of junctions of the vena cava with the subject&#39;s renal veins, and the downstream pump is disposed downstream of the junctions of the vena cava with the subject&#39;s renal veins. However, it is noted that the scope of the present invention includes placing upstream pump  24 U or the occlusion element in any main vein upstream of a tributary venous system, and placing downstream pump  24 D downstream of said tributary venous system, and configuring the pump(s) (e.g., via the direction of rotation of impellers of the pumps, or the orientation of the pumps) to generate preferential flow from the tributaries into the main vein, mutatis mutandis. For example, the pump(s) could be used to generate flow from the subject&#39;s hepatic veins into the subject&#39;s vena cava, in order to increase perfusion of the subject&#39;s liver,  mutatis mutandis . For some applications, the upstream pump or the occlusion element is placed within a main vein upstream of two or more tributary venous systems into the main vein (e.g., within the vena cava upstream of the renal venous system and the hepatic venous system). The downstream pump is placed downstream of the two or more tributary venous systems. The pump(s) are configured to generate preferential flow from both of the tributary venous systems into the main vein by pumping blood through the main vein, in the manner described herein. 
     For some applications, upstream pump  24 U or the occlusion element is placed in a main vein upstream of a tributary venous system, and downstream pump  24 D is placed downstream of said tributary venous system, and the pump(s) are configured (e.g., via the direction of rotation of impellers of the pumps, or the orientation of the pumps) to reduce flow from the tributaries into the main vein. For some such applications, the blades of the downstream impeller are oriented such that, as the downstream impeller is rotated, the downstream impeller pumps in the upstream direction (toward the junction between the tributary system and the main vein). The blades of the upstream impeller are oriented such that, as the upstream impeller is rotated, the upstream impeller pumps in the downstream direction (toward the junction between the tributary system and the main vein). 
     For some applications, the upstream and downstream pumps  24 U and  24 D, the catheter(s) upon which the blood pumps are disposed (e.g., blood-pump catheter  20 , catheter  66 , and catheter  68 ), and/or the occlusion elements described with reference to  FIGS.  5 A-B , and other devices described herein, are placed within a main artery upstream and downstream of bifurcations of the artery with one or more branching arterial systems that branch from the main artery and supply a given organ,  mutatis mutandis . For such applications, the upstream pump is typically configured to pump in the downstream direction (toward the bifurcations) and the downstream pump is configured to pump in the upstream direction (toward the bifurcations), such that blood flow into the branching arterial system is increased, thereby increasing perfusion of the organ. Alternatively or additionally, the occlusion element is placed downstream of the bifurcations of the artery with the one or more arterial systems and is configured to partially occlude the artery downstream of the bifurcations. For example, the upstream pump may be placed in the subject&#39;s aorta upstream of the subject&#39;s renal arteries and the downstream pump may be placed in the subject&#39;s aorta downstream of the subject&#39;s renal arteries, the pumps acting to pump blood into the renal arteries and toward the subject&#39;s kidneys. For some applications, upstream and downstream pumps, and/or occlusion elements are placed on both the arterial and venous sides of the subject&#39;s body in order to increase perfusion of a given organ or set of organs, in the manner described herein. 
     Although some applications of the present invention are described with reference to blood pumps  24 D and  24 U, according to which the blood pumps include impellers, the scope of the present invention includes using any other type of pump for pumping blood in the manner described herein, mutatis mutandis. For example, a roller pump, an Archimedes screw pump, a centrifugal pump, a pneumatic pump, and/or a compression pump may be used. 
     The scope of the present invention includes combining any of the apparatus and methods described herein with any of the apparatus and methods described in one or more of the following applications, all of which are incorporated herein by reference: 
     International Patent Application PCT/IL2015/050532 to Schwammenthal (published as WO 15/177793), filed May 19, 2015, entitled “Blood pump,” which claims priority from U.S. Provisional Patent Application 62/000,192 to Schwammenthal, filed May 19, 2014, entitled “Blood pump;” 
     International Patent Application PCT/IL2014/050289 to Schwammenthal (published as WO 14/141284), filed Mar. 13, 2014, entitled “Renal pump,” which claims priority from (a) U.S. Provisional Patent Application 61/779,803 to Schwammenthal, filed Mar. 13, 2013, entitled “Renal pump,” and (b) U.S. Provisional Patent Application 61/914,475 to Schwammenthal, filed Dec. 11, 2013, entitled “Renal pump;” 
     U.S. patent application Ser. No. 14/567,439 to Tuval (published as US 2015/0157777), filed Dec. 11, 2014, entitled “Curved catheter,” which claims priority from U.S. Provisional Patent Application 61/914,470 to Tuval, filed Dec. 11, 2013, entitled “Curved catheter;” and 
     International Patent Application PCT/IL2013/050495 to Tuval (published as WO 13/183060), filed Jun. 6, 2013, entitled “Prosthetic renal valve,” which claims priority from U.S. Provisional Patent Application 61/656,244 to Tuval, filed Jun. 6, 2012, entitled “Prosthetic renal valve.” 
     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.