Patent Publication Number: US-2022218976-A1

Title: Blood pumps

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority from: 
     U.S. Provisional Patent Application 62/851,769 to Friedland, entitled “Blood pumps,” filed May 23, 2019; 
     U.S. Provisional Patent Application 62/870,822 to Friedland, entitled “Blood pumps,” filed Jul. 5, 2019; 
     U.S. Provisional Patent Application 62/890,177 to Friedland, entitled “Blood pumps,” filed Aug. 22, 2019; and 
     U.S. Provisional Patent Application 62/983,786 to Friedland, entitled “Blood pumps,” filed Mar. 2, 2020. 
     Each of the above-referenced applications 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 a vein or an artery. 
     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. 
     Typically, in patients suffering from acute heart failure, elevated systemic venous pressures cause increased renal parenchymal pressure and increased intraabdominal pressure, factors that can contribute to deterioration of renal perfusion and function. In addition, high systemic venous pressures may impede lymphatic drainage of pulmonary interstitial fluid resulting in aggravation and prolongation of pulmonary congestion in patients with acute pulmonary edema. 
     SUMMARY OF EMBODIMENTS 
     In accordance with some applications of the present invention, the blood stream of a region of the vena cava that is adjacent to junctions of the vena cava with the renal veins is separated from the subject&#39;s vena-caval bloodstream upstream and downstream of the junctions. Blood is pumped from the region of the vena cava that is adjacent to the junctions to a suprarenal location within the vena cava, using a pump. The subject&#39;s blood is allowed to flow from an infrarenal location within the subject&#39;s vena cava to the suprarenal location, without being pumped by the pump, by directing the blood to bypass the pump. 
     For some applications, such methods include (a) at least partially occluding blood flow through a vein by placing a downstream occluding element (such as a material) at a downstream location that is downstream of junctions of the vein with one or more tributary vessels (e.g., one or more tributary veins), (b) pumping blood from a region of the vein that is adjacent to junctions of the vein with the tributary vessels through the downstream occluding element, using a pump, and (c) directing upstream venous blood flow from an upstream location that is upstream of junctions of the vein with the tributary vessels to downstream of the occluding element, without the upstream venous blood flow being pumped by the pump, by directing the upstream venous blood flow to bypass the pump. 
     Typically, such methods are performed using a blood-pump catheter, which includes a material that is configured to be placed at a downstream location that is downstream of junctions of the vein with the tributary vessel and to at least partially occlude blood flow through the vein at the downstream location. The material typically defines a blood-outlet opening. A blood pump is typically configured to pump blood from the region of the vein that is adjacent to junctions of the vein with the tributary vessels through the blood-outlet opening defined by the material. A blood-flow pathway (i.e., a blood-flow channel) typically passes between an inlet opening disposed at an upstream location that is upstream of junctions of the vein with the tributary vessel and an outlet opening that opens to a downstream side of the material. The blood-flow pathway is configured to direct upstream venous blood flow from the upstream location to downstream of the material, without the upstream venous blood flow being pumped by the pump. For some applications, the blood-flow pathway includes a funnel and one or more tubes. The funnel is configured to be disposed at an upstream location that is upstream of junctions of the vein with the tributary vessels, and the one or more tubes are configured to extend from the funnel to a downstream side of the material. The funnel and the one or more tubes are configured to direct upstream venous blood flow from the upstream location to downstream of the material, without the upstream venous blood flow being pumped by the pump. 
     In general, the scope of some applications of the present invention includes apparatus and methods for use with a venous system of a subject that includes tributary veins that flow into a main vein (e.g., renal veins that flow into the vena cava). Typically, such methods include separating bloodstream within the region of the main vein that is adjacent to junctions of the main vein with the tributary veins from bloodstream within the main vein upstream and downstream thereto, pumping blood from the region of the main vein that is adjacent to junctions of the vein with the tributary veins to a location within the main vein that is downstream of the tributary veins, using a pump, and allowing the subject&#39;s blood to flow from a location within the main vein that is upstream of the tributary veins to the location within the main vein that is downstream of the tributary veins without being pumped by the pump, by directing the blood to bypass the pump. The scope of some applications of the present invention further includes apparatus and methods for use with a venous system of a subject that includes tributary vessels (e.g., lymphatic vessels) that flow into a vein. Typically, such methods include separating flow within a region of the vein that is adjacent to junctions of the vein with the tributary vessels from bloodstream within the vein upstream and downstream thereto, pumping blood from the region of the vein that is adjacent to junctions of the vein with the tributary vessels to a location within the vein that is downstream of the region, using a pump, and allowing the subject&#39;s blood to flow from a location within the vein that is upstream of the tributary vessels to the location within the vein that is downstream of the tributary vessels without being pumped by the pump, by directing the blood to bypass the pump. 
     For some applications of the present invention, a blood-pump catheter includes an impeller configured to pump blood by rotating, and an impeller housing that is configured to be disposed around the impeller. A blood-pump-support frame is typically configured to be disposed upstream of impeller housing, and to at least partially align a longitudinal axis of the impeller housing with a local longitudinal axis of the blood vessel, by contacting the inner wall of the blood vessel. A material is configured to extend from the impeller housing to the blood-pump-support frame, such as to contact the vessel wall and to occlude the blood vessel in the region of the blood vessel that surrounds the impeller. For some applications, the material defines a blood-outlet opening, and the impeller is configured to pump blood through the blood-outlet opening. 
     For some applications, a blood-pump catheter is configured for use with a subject&#39;s descending aorta. The blood-pump catheter includes a material configured to be placed within the descending aorta and to separate the aorta into upstream and downstream compartments by at least partially occluding blood flow through the aorta. The material typically defines a blood-outlet opening. A blood pump (e.g., an impeller-based blood pump) is configured to pump blood in a downstream direction through the descending aorta, through the blood-outlet opening. 
     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. 
     The terms “downstream” and “upstream” and related terms should be interpreted as being defined with respect to the direction of antegrade blood flow through a subject&#39;s vasculature. Thus, by way of example, the suprarenal vena cava is downstream of the infrarenal vena cava, and the descending aorta is downstream of the aortic arch. When used with reference to a device, the term “upstream” should be interpreted as referring to a portion of the device that it configured to be disposed at a relatively upstream location, and the term “downstream” should be interpreted as referring to a portion of the device that it configured to be disposed at a relatively downstream location. 
     There is therefore provided, in accordance with some applications of the present invention, apparatus for use with a venous system of a subject that includes one or more tributary vessels that flow into a vein at junctions between the vein and the one or more tributary vessels, the apparatus including: 
     a blood-pump-catheter including:
         a material configured to be placed at a downstream location that is downstream of the junctions and to at least partially occlude blood flow through the vein at the downstream location, the material defining a blood-outlet opening;   a blood pump configured to pump blood from a region of the vein that is adjacent to the junctions, and through the blood-outlet opening defined by the material;   a blood-flow pathway that passes between (a) a pathway-inlet opening disposed at an upstream location that is upstream of the junctions and (b) a pathway-outlet opening that opens to a downstream side of the material,   the blood-flow pathway being configured to direct upstream venous blood flow from the upstream location to downstream of the material, without the upstream venous blood flow being pumped by the pump.       

     In some applications, the blood pump includes an elongate blood-pump tube, the elongate blood-pump tube defining a blood-inlet area which is disposed within an upstream portion of the elongate blood-pump tube, a blood-outlet area that is that is disposed within a downstream portion of the elongate blood-pump tube, and an impeller configured to pump blood into the blood-inlet area, through the elongate blood-pump tube, and out of the blood-outlet area. 
     In some applications, the blood pump includes an impeller that is disposed within a frame, the impeller being configured to pump blood by rotating within the frame. 
     In some applications, the blood-pump catheter is configured for use with one or more lymphatic vessels that flow into a vein, and the blood-pump catheter is configured to be placed within the vein, such that: 
     the material is placed at a downstream location that is downstream of junctions of the vein with the one or more lymphatic vessels and such as to at least partially occlude blood flow through the vein at the downstream location, 
     the blood pump is configured to pump blood from a region of the vein that is adjacent to the junctions of the vein with the one or more lymphatic vessels, and through the blood-outlet opening defined by the material, and 
     the blood-flow pathway passes between (a) the pathway-inlet opening, the pathway-inlet opening being disposed at an upstream location that is upstream of the region of the vein that is adjacent to the junctions of the vein and the one or more lymphatic vessels and (b) the pathway-outlet opening, the pathway-outlet opening opening to the downstream side of the material, 
     the blood-flow pathway being configured to direct upstream venous blood flow from the upstream location to the downstream side of the material, without the upstream venous blood flow being pumped by the pump. 
     In some applications, the blood-pump catheter is configured to be placed within a vena cava of the subject, such that: 
     the material is placed at a downstream location that is downstream of junctions of the vena cava with one or more renal veins of the subject and such as to at least partially occlude blood flow through the vena cava at the downstream location, 
     the blood pump is configured to pump blood from a region of the vena cava that is adjacent to the junctions of the vena cava with the one or more renal veins through the blood-outlet opening defined by the material, and 
     the blood-flow pathway passes between (a) the pathway-inlet opening, the pathway-inlet opening being disposed at an upstream location that is upstream of junctions of the vena cava and the one or more renal veins and (b) the pathway-outlet opening, the pathway-outlet opening opening to the downstream side of the material, 
     the blood-flow pathway being configured to direct blood flow from an infrarenal vena-caval bloodstream to downstream of the material, without the infrarenal vena-caval bloodstream being pumped by the pump. 
     In some applications, the apparatus further includes a percutaneous left-ventricular assist device configured to assist with left-ventricular functioning of the subject. 
     In some applications, the blood pump includes an elongate blood-pump tube, the elongate blood-pump tube defining a blood-inlet area which is disposed within an upstream portion of the elongate blood-pump tube, a blood-outlet area that is that is disposed within a downstream portion of the elongate blood-pump tube, and an impeller configured to pump blood into the blood-inlet area, through the elongate blood-pump tube, and out of a blood-outlet area. 
     In some applications, the elongate blood-pump tube is configured to be disposed such that the blood-outlet area is disposed within a pulmonary artery of the subject and is configured to pump blood into the subject&#39;s pulmonary artery via the elongate tube. 
     In some applications, the apparatus further includes an elongate tube that extends downstream from the material and that defines suprarenal blood-inlet openings that are configured to be disposed within a suprarenal vena cava of the subject, and the blood-inlet area of the elongate blood-pump tube is configured to be disposed downstream of the suprarenal blood-inlet openings and the elongate blood-pump tube is configured to pump blood from a suprarenal vena-caval bloodstream into the subject&#39;s pulmonary artery. 
     In some applications, the apparatus further includes an elongate tube that is configured to extend into a pulmonary artery of the subject, and the blood pump is configured to pump blood into the subject&#39;s pulmonary artery via the elongate tube. 
     In some applications, the elongate tube defines suprarenal blood-inlet openings that are configured to be disposed within a suprarenal vena cava of the subject, and the blood pump is configured to be disposed downstream of the suprarenal blood-inlet openings and is configured to pump blood from a suprarenal vena-caval bloodstream into the subject&#39;s pulmonary artery. 
     In some applications, diameters of one or more portions of the blood-flow pathway are modulable, such that flow from the infrarenal vena-caval bloodstream to a right atrium of the subject is modulable. 
     In some applications, the apparatus further includes a blood pressure sensor configured to detect blood pressure of the subject, and a computer processor configured to receive the subject&#39;s blood pressure and to automatically modulate the diameters of the one or more portions of the blood-flow pathway, in response to the detected blood pressure. 
     In some applications, the diameters of the one or more portions of the blood-flow pathway are configured to be modulated in response to an input. 
     In some applications, the blood-flow pathway includes a funnel having a wide end and a narrow end, and one or more tubes, and the wide end of the funnel defines the pathway-inlet opening, and the narrow end of the funnel passes into the one or more tubes. 
     In some applications, the funnel and the one or more tubes include a single continuous blood-flow pathway. 
     In some applications, diameters of one or more portions of the blood-flow pathway are modulable, such that flow through the blood-flow pathway is modulable. 
     In some applications, the apparatus further includes a blood pressure sensor configured to detect blood pressure of the subject, and a computer processor configured to receive the subject&#39;s blood pressure and to automatically modulate the diameters of the one or more portions of the blood-flow pathway, in response to the detected blood pressure. 
     In some applications, the diameters of the one or more portions of the blood-flow pathway are configured to be modulated in response to an input. 
     There is further provided, in accordance with some applications of the present invention, apparatus including: 
     a blood-pump-catheter including:
         a material disposed upon the blood-pump catheter and configured to at least partially occlude blood flow through a vein, the material defining a blood-outlet opening;   a blood pump configured to pump blood through the blood-outlet opening defined by the material;   a blood-flow pathway that passes between (a) a pathway-inlet opening disposed at a location that is proximal with respect to the material and (b) a pathway-outlet opening that opens to a location that is distal with respect to the material,   the blood-flow pathway being configured to direct blood flow from the pathway-inlet opening to the pathway-outlet opening, without the blood flow being pumped by the pump.       

     There is further provided, in accordance with some applications of the present invention, a method for use with a venous system of a subject that includes tributary vessels that flow into a vein at junctions between the vein and the tributary vessels, the method including: 
     separating a bloodstream of a region of the vein that is adjacent to the junctions into a compartment that is separate from bloodstream within the vein that is upstream of the junctions and from bloodstream within the vein that is downstream of the junctions; 
     pumping blood from the compartment to downstream of the compartment, using a pump; and 
     allowing the subject&#39;s blood to flow from a location within the vein that is upstream of the compartment to a location that is downstream of the compartment without being pumped by the pump, by directing the blood to bypass the pump. 
     In some applications, pumping blood from the compartment to downstream of the compartment includes pumping blood from the compartment to downstream of the compartment via an elongate blood-pump tube, the elongate blood-pump tube defining a blood-inlet area disposed within an upstream portion of the elongate blood-pump tube, a blood-outlet area that is disposed within a downstream portion of the elongate tube, and an impeller configured to pump blood into the blood-inlet area, through the elongate blood-pump tube, and out of the blood-outlet area. 
     In some applications, pumping blood from the compartment to downstream of the compartment includes pumping blood from the compartment to downstream of the compartment by rotating an impeller. 
     In some applications, the method is for use with one or more lymphatic vessels that flow into a vein, and the method includes: 
     separating bloodstream within a region of the vein that is adjacent to junctions of the vein with the one or more lymphatic vessels into a compartment that is separate from bloodstream within the vein downstream of the region of the vein that is adjacent to the junctions of the vein with the one or more lymphatic vessels, and from bloodstream within the vein upstream of the region of the vein that is adjacent to the junctions of the vein with the one or more lymphatic vessels; 
     pumping blood from the compartment to downstream of the compartment, using the pump; and 
     allowing the subject&#39;s blood to flow from a location within the vein that is upstream of the compartment to a location that is downstream of the compartment without being pumped by the pump, by directing the blood to bypass the pump. 
     In some applications, the method is for use with renal veins and a vena cava of the subject, and the method includes: 
     separating bloodstream within a region of the vena cava that is adjacent to the junctions of the vena cava with the one or more the renal veins into a vena-caval compartment that is separate from bloodstream within the vena cava downstream of the vena-caval compartment, and from bloodstream within the vena cava upstream of the vena-caval compartment; 
     pumping blood from the vena-caval compartment to downstream of the vena-caval compartment, using the pump; and 
     allowing the subject&#39;s blood to flow from a location that is upstream of the vena-caval compartment to a location that is downstream of the vena-caval compartment without being pumped by the pump, by directing the blood to bypass the pump. 
     In some applications, the method further includes assisting left-ventricular functioning of the subject using a percutaneous left-ventricular assist device. 
     In some applications, pumping blood from the vena-caval compartment to downstream of the vena-caval compartment includes pumping blood from the vena-caval compartment to downstream of the vena-caval compartment via an elongate blood-pump tube, the elongate blood-pump tube defining a blood-inlet area which is disposed within an upstream portion of the elongate blood-pump tube, a blood-outlet area that is that is disposed within a downstream portion of the elongate blood-pump tube, and an impeller configured to pump blood into the blood-inlet area, through the elongate blood-pump tube, and out of the blood-outlet area. 
     In some applications, pumping blood from the vena-caval compartment to downstream of the vena-caval compartment via the elongate blood-pump tube includes pumping blood into the subject&#39;s pulmonary artery via the elongate tube. 
     In some applications, the method further includes pumping blood from a suprarenal vena-caval bloodstream of the subject into the subject&#39;s pulmonary artery, via the elongate blood-pump tube. 
     In some applications, pumping blood from the vena-caval compartment to downstream of the vena-caval compartment includes pumping blood into a pulmonary artery of the subject via an elongate tube. 
     In some applications, the method further includes pumping blood from a suprarenal vena-caval bloodstream of the subject into the subject&#39;s pulmonary artery via the elongate tube. 
     In some applications, directing the blood to bypass the pump includes directing the blood to bypass the pump via a blood-flow pathway, the method further including modulating blood flow to a right atrium of the subject by modulating diameters of one or more portions of the blood-flow pathway. 
     In some applications, the method further includes detecting blood pressure of the subject, and modulating diameters of one or more portions of the blood-flow pathway includes automatically modulating the diameters of the one or more portions of the blood-flow pathway, in response to the detected blood pressure. 
     In some applications, modulating diameters of one or more portions of the blood-flow pathway includes modulating diameters of one or more portions of the blood-flow pathway in response to an input. 
     In some applications, directing the blood to bypass the pump includes directing the blood to bypass the pump via a blood-flow pathway, the method further including modulating blood flow through the blood-flow pathway by modulating diameters of one or more portions of the blood-flow pathway. 
     In some applications, the method further includes detecting blood pressure of the subject, and modulating diameters of one or more portions of the blood-flow pathway includes automatically modulating the diameters of the one or more portions of the blood-flow pathway, in response to the detected blood pressure. 
     In some applications, modulating diameters of one or more portions of the blood-flow pathway includes modulating diameters of one or more portions of the blood-flow pathway in response to an input. 
     There is further provided, in accordance with some applications of the present invention, a method for use with a venous system of a subject that includes tributary vessels that flow into a vein at junctions between the tributary vessels and the vein, the method including: 
     at least partially occluding blood flow through the vein by placing a downstream occluding element at a downstream location that is downstream of junctions of the vein with the tributary vessels; 
     pumping blood from a region of the vein that is adjacent to the junctions of the vein with the tributary vessels through the downstream occluding element, using a pump; and 
     directing upstream venous blood flow from an upstream location that is upstream of junctions of the vein with the tributary vessels to downstream of the occluding element, without the upstream venous blood flow being pumped by the pump, by directing the upstream venous blood flow to bypass the pump. 
     There is further provided, in accordance with some applications of the present invention, a method for use with a venous system of a subject that includes tributary vessels that flow into a vein at junctions between the tributary vessels and the vein, the method including: 
     separating a bloodstream of a region of the vein that is adjacent to the junctions from bloodstream within the vein that is upstream of the region of the vein that is adjacent to the junctions and from bloodstream within the vein that is downstream of the region of the vein that is adjacent to the junctions; 
     allowing blood to flow from the tributary vessels to a location that is downstream of the tributary vessels, via a first flow path; 
     allowing the subject&#39;s blood to flow from a location within the vein that is upstream of the tributary vessels to a location within the vein that is downstream of the tributary vessels, via a second flow path; and 
     controlling blood flow via at least one of the first and second flow paths. 
     There is further provided, in accordance with some applications of the present invention, apparatus for use with an arterial system of a subject that includes a main artery and one or more branching arteries that branch from the main artery at junctions between the main artery and the one or more branching arteries, the apparatus including: 
     a blood-pump-catheter including:
         a material configured to be placed at a downstream location that is downstream of the junctions between the main artery with the branching arteries and to at least partially occlude blood flow through the main artery at the downstream location, the material defining a blood-inlet opening;   a blood pump configured to pump blood from the main artery toward the branching arteries, through the blood-inlet opening;   a funnel configured to be disposed at an upstream location that is upstream of junctions of the main artery with the branching arteries; and   a blood-flow pathway that passes between (a) a pathway-inlet opening disposed at an upstream location that is upstream of junctions of the main artery with the branching arteries and (b) a pathway-outlet opening that opens to a downstream side of the material,   the blood-flow pathway being configured to direct upstream main arterial blood flow from the upstream location to downstream of the material, without the upstream main arterial blood flow being pumped by the pump.       

     There is further provided, in accordance with some applications of the present invention, a method for use with an arterial system of a subject that includes a main artery and one or more branching arteries that branch from the main artery at junctions between the main artery and the one or more branching arteries, the method including: 
     separating bloodstream within a region of the main artery that is adjacent to the junctions into a compartment that is separate from bloodstream within the main artery that is downstream from the compartment and from bloodstream within the compartment; 
     pumping blood from the main artery to the branching arteries, using a pump; and 
     allowing the subject&#39;s blood to flow from a location within the main artery that is upstream of the compartment to a location that is downstream of the compartment without being pumped by the pump, by directing the blood to bypass the pump. 
     There is further provided, in accordance with some applications of the present invention, a method for use with an arterial system of a subject that includes a main artery and one or more branching arteries that branch from the main artery at junctions between the main artery and the one or more branching arteries, the method including: 
     at least partially occluding blood flow through the main artery by placing a downstream occluding element at a downstream location that is downstream of junctions of the main artery with the branching arteries; 
     pumping blood from the main artery to the branching arteries, through the downstream occluding element, using a pump; and 
     directing upstream main artery blood flow from an upstream location that is upstream of junctions of the main artery with the branching arteries to downstream of the occluding element, without the upstream main artery blood flow being pumped by the pump, by directing the upstream main artery blood flow to bypass the pump. 
     There is further provided, in accordance with some applications of the present invention, apparatus for use with a descending aorta of a subject, the apparatus including: 
     a blood-pump-catheter including:
         a material configured to be placed within the descending aorta and to separate the aorta into upstream and downstream compartments by at least partially occluding blood flow through the aorta, the material defining a blood-outlet opening;   a blood pump configured to pump blood in a downstream direction through the descending aorta, through the blood-outlet opening.       

     There is further provided, in accordance with some applications of the present invention, a method for use with a descending aorta of a subject, the method including: 
     separating the descending aorta into upstream and downstream compartments by placing an occluding element into the descending aorta that at least partially occludes the descending aorta; and 
     pumping blood from the upstream compartment to the downstream compartment using a blood pump. 
     There is further provided, in accordance with some applications of the present invention, apparatus including: 
     a blood-pump catheter configured to be placed inside a blood vessel of a subject, the blood-pump catheter including:
         an impeller configured to pump blood by rotating;   an impeller housing that is configured to be disposed around the impeller;   a blood-pump-support frame configured to be disposed upstream of the impeller housing, and to at least partially align a longitudinal axis of the impeller housing with a local longitudinal axis of the blood vessel, by contacting the inner wall of the blood vessel; and   a material configured to extend from the impeller housing to the blood-pump-support frame, such as to contact the vessel wall and to occlude the blood vessel in the region of the blood vessel that surrounds the impeller, the material defining a blood-outlet opening, the impeller being configured to pump blood through the blood-outlet opening.       

     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. 1A, 1B, and 1C  are schematic illustrations of a blood-pump catheter placed within a subject&#39;s vena cava, in accordance with some applications of the present invention; 
         FIGS. 2A, 2B, and 2C  are schematic illustrations of a blood-pump catheter placed within a subject&#39;s vena cava, in accordance with some applications of the present invention; 
         FIG. 2D  is a schematic illustration of a blood-pump catheter placed within a subject&#39;s vena cava, the blood pump having a non-axisymmetric outflow tube, in accordance with some applications of the present invention; 
         FIG. 2E  is a schematic illustration of a blood-pump catheter placed within a subject&#39;s vena cava, the blood pump having a non-axisymmetric outflow tube and a non-axisymmetric impeller, in accordance with some applications of the present invention; 
         FIG. 2F  is a schematic illustration of the blood-pump catheter of  FIG. 2E  in the absence of the subject&#39;s anatomy, in accordance with some applications of the present invention; 
         FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G, and 3H  are schematic illustrations of a blood-pump catheter, in accordance with some further applications of the present invention. 
         FIGS. 4A and 4B  are schematic illustrations of a blood-pump catheter placed within a subject&#39;s descending aorta in the vicinity of a subject&#39;s renal arteries, in accordance with some applications of the present invention; 
         FIGS. 5A and 5B  are schematic illustrations of a blood-pump catheter placed within a subject&#39;s descending aorta (e.g., the thoracic aorta or the abdominal aorta), in accordance with some applications of the present invention; 
         FIGS. 6A, 6B, 6C, 6D, 6E, 6F, 6G, and 6H  are schematic illustrations of an impeller of a blood pump or a portion thereof, in accordance with some applications of the present invention; 
         FIG. 7  is a schematic illustration of an impeller disposed inside a frame of a blood pump, in accordance with some applications of the present invention; 
         FIGS. 8A and 8B  are schematic illustrations of the impeller and the frame of the blood pump, respectively, in non-radially-constrained and radially-constrained states thereof, in accordance with some applications of the present invention; and 
         FIG. 8C  is a schematic illustration of a typical bearing assembly that is used in prior art axial impeller-based blood pumps. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference is made to  FIGS. 1A-C , which are schematic illustrations of a blood-pump catheter  20  placed within a subject&#39;s vena cava  22 , in accordance with some applications of the present invention. Typically, the blood-pump catheter includes a blood pump  24  that includes an impeller housing  26 , and an impeller  50  disposed inside the impeller housing. Impeller housing  26  (e.g., a frame  34  of the housing) typically functions to separate the inner wall of the vena cava from the impeller (e.g., in case the vena cava collapses inwardly due to intra-abdominal pressure), such that the vena cava does not become injured by the impeller and the impeller does not become deformed by pressure from the inner wall of the vena cava. Further typically, an axial shaft  92  (shown in  FIG. 7 ) that passes through the impeller is supported by radial bearings  116 ,  118  disposed, respectively, at proximal and distal ends of the impeller housing. 
     It is noted that, for illustrative purposes, in  FIGS. 1A-C  (as well as in  FIGS. 2A-E ,  3 A-H,  4 A-B, and  5 A-B), a three-dimensional view of blood pump  24  is shown, but cross-sectional views are shown of other elements of the blood-pump catheters. 
     For some such applications, a blood-pump-support frame  32  is disposed upstream of impeller housing  26 , and is configured to come into contact with the inner wall of the vena cava. Blood-pump-support frame  32  is configured to align the longitudinal axis of impeller housing  26 , and, in turn, impeller  50 , with the local longitudinal axis of the vena cava, by contacting the inner wall of the vena cava. (It is noted that, for some applications, the blood-pump-support frame may not fully align the longitudinal axis of impeller with the local longitudinal axis of the vena cava. However, typically, the blood-pump-support frame maintains the longitudinal axis of the impeller in greater alignment with the local longitudinal axis of the vena cava, relative to alignment of the longitudinal axis of the impeller with the local longitudinal axis of the vena cava in the absence of the blood-pump-support frame.) Typically, ceteris paribus, the efficacy of the pumping of blood by impeller  50  is greater, the greater the alignment of the longitudinal axis of the impeller with the local longitudinal axis of the vena cava. It is noted that a slight misalignment of the of the longitudinal axis of the impeller with the local longitudinal axis of the vena cava (e.g., as shown in  FIG. 2E ), will nevertheless typically still result in efficacious pumping of blood by the impeller. 
     For some applications, impeller housing  26  includes a frame  34  (e.g., a rigid or semi-rigid frame) made of a shape-memory element (such as nitinol) that is at least partially covered with a material  36  (e.g., a blood-impermeable material, e.g., polyester, polyurethane, and/or a different polymer). Typically, for such applications, the stiffness of frame  34  is sufficiently great that pressure exerted upon the frame  34  by the inner wall of the vena cava does not deform the frame. Typically, material  36  extends from the impeller housing to blood-pump-support frame  32  such as to contact the vessel wall and to occlude the blood vessel in the region of the blood vessel that surrounds the impeller and/or that is upstream of the impeller. The material typically defines a hole therethrough at a distal portion of the impeller housing. The material is configured to occlude backflow of blood around the outside of the impeller, but such to allow antegrade blood flow in the central region of the vessel in the vicinity of the impeller, by the hole acting as a blood-outlet opening  31 . For some applications, a portion of the impeller housing is covered (i.e., lined) on the inside of the impeller housing with an inner lining  39 , e.g., as described hereinbelow with reference to  FIG. 7 . For some such applications, the inner lining of the impeller housing at least partially overlaps with material  36 . For some applications, the inner lining extends the blood-outlet opening defined by material  36 . 
     Typically, a drive cable  78  is disposed within an outer tube  77  of blood-pump catheter  20 . (The drive cable is shown in  FIG. 8A , for example.) Further typically, the drive cable extends from a motor  79  which is disposed outside the subject&#39;s body, to axial shaft  92  (shown in  FIG. 7 ), upon which the impeller is disposed. The motor imparts rotational motion to the axial shaft (and thereby imparts rotational motion to the impeller), via the drive cable. Typically, the motor is controlled by a computer processor  10 . For some applications, the motor is controlled based upon blood pressure measurements that are performed by a pressure sensor  75 . 
     For some applications, the use of material  36  in the above-described manner reduces a likelihood of there being retrograde blood flow in the region of the blood vessel that surrounds the impeller, caused by turbulence that is introduced by the impeller. For some applications, the material acts to separate the suprarenal vena cava into upstream and downstream compartments, and the impeller is configured to pump blood from the upstream compartment to the downstream compartment, thereby reducing pressure in the upstream compartment, in the vicinity of the renal veins. 
     For some applications (not shown), blood-pump-support frame  32  and frame  34  of impeller housing  26  are formed as a single frame having an upstream portion that is configured to come into contact with the inner wall of the vena cava, and a downstream portion that is configured to house the impeller. 
     Typically, the maximum diameter of blood-pump-support frame  32  (i.e., the diameter of the blood-pump-support frame  32  at the longitudinal location(s) at which the diameter of the blood-pump-support frame  32  is at its maximum), when the blood-pump-support frame is in a radially non-constrained configuration thereof, is at least 1.1 times (and, for some applications, at least 1.3 times) greater than the maximum diameter of frame  34  of impeller housing  26  (i.e., the diameter of frame  34  at the longitudinal location(s) at which the diameter of frame  34  is at its maximum) when frame  34  is in a radially non-constrained configuration thereof. 
     Typically, blood pump  24  is placed downstream of junctions of the vena cava with the subject&#39;s renal veins  25 , and is configured to pump blood in the downstream direction, such as to reduce pressure in the subject&#39;s renal veins. For some applications, blood-pump catheter  20  includes an occlusion element  40  that is configured to be placed upstream of junctions of the vena cava with the subject&#39;s renal veins, and to partially occlude the vena cava at this location. For example, the occlusion element may include a balloon  42 , as shown in  FIG. 1A . Alternatively or additionally, the occlusion element includes a blood-impermeable material  44  that defines holes  45  therethrough, as shown in  FIG. 1B . For some applications, the material is supported by a frame  46 . Typically, material  44  is configured such that there is some blood flow via the holes in the material, but the blood flow is reduced relative to in the absence of the material. 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. For some applications (not shown), a nozzle is used as the upstream occlusion element, e.g., as described in US 2019/0239998 to Tuval, which is incorporated herein by reference. 
     Occlusion element  40  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&#39;s 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. 
     For some applications, blood-pump catheter  20  does not include an upstream occlusion element, e.g., as shown in  FIG. 1C . As described hereinabove, for some applications, material  36  acts to separate the suprarenal vena cava into upstream and downstream compartments, and impeller  50  is configured to pump blood from the upstream compartment to the downstream compartment, thereby reducing pressure in the upstream compartment, in the vicinity of the renal veins. For some applications, blood-pump catheter  20  is thereby configured to generate a region of low pressure within the vena cava, in the vicinity of the renal veins, even in the absence of an upstream occlusion element, as shown in  FIG. 1C . 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. 
     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. 
     With respect to the catheter shown in  FIGS. 1A-B , it is noted that such a catheter, which includes blood pump  24  that is disposed distally with respect to an upstream occlusion element, is suitable for placement into the vena cava from a vein that is below the junctions of the vena cava with the subject&#39;s renal veins, e.g., the femoral vein. However, the scope of the present invention includes a catheter that has a pump and an occlusion element disposed thereon, but with the upstream occlusion element disposed distally with respect to the downstream pump. Such a catheter is typically inserted via a vein that is disposed above the inferior vena cava, e.g., the subclavian vein or the jugular vein, mutatis mutandis. Similarly, a catheter as generally shown in  FIG. 1C  may be configured for insertion into the suprarenal vena cava via a vein that is disposed above the inferior vena cava, e.g., the subclavian vein or the jugular vein, mutatis mutandis. Typically, in such cases, the dispositions of components of the blood-pump catheter with respect to the shaft of the catheter are in the opposite direction from that shown in  FIGS. 1A-C . 
     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. 
     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 and has additional effects, e.g., as described with reference to  FIG. 4B  of US 2016/0022890 to Schwammenthal, which is incorporated herein by reference. 
     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 frame may be expandable (e.g., by heating the frame, or by applying an electrical current to the frame). For some applications, the extent to which the occlusion element occludes the vena cava is controlled by a computer processor  10  responsively to the blood pressure detected by blood pressure sensor  75 , 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 US 2016/0022890 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  pumps blood away from the renal veins (e.g., the rate at which impeller  50  of the pump is rotated), and/or the extent to which the occlusion element occludes the vena cava, is controlled by computer processor  10 , responsively to the blood pressure detected by blood pressure sensor  75 , 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 US 2016/0022890 to Schwammenthal, which is incorporated herein by reference), and/or in response to an input from a user. For some applications, a blood pressure sensor as described hereinbelow with reference to  FIGS. 1C and 2C  is used in conjunction with the above-described apparatus and methods. 
     Although some applications of the present invention are described with reference to blood pumps, 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. 
     With reference to  FIGS. 1A-C , it is noted that the scope of the present invention includes applying similar apparatus and methods to any venous system of a subject that includes tributary veins that flow into a main vein. For example, similar techniques could be applied at junctions of the hepatic veins and the vena cava. For some applications, generally similar apparatus and methods are used within the subclavian vein or jugular vein, at junctions of the vein with one or more lymphatic vessels. For some such applications, blood-pump catheter  20  is configured to increase flow of lymphatic fluid from the lymphatic vessels into the vein, mutatis mutandis. 
     Reference is now made to  FIGS. 2A and 2B , which are schematic illustrations of a blood-pump catheter  70  placed within a subject&#39;s vena cava  22 , in accordance with some applications of the present invention. Typically, blood-pump catheter  70  includes blood pump  24 , which typically includes impeller housing  26  and impeller  50 , all of which are generally as described hereinabove. Impeller housing  26  (e.g., frame  34  of the housing) typically functions to separate the inner wall of the vena cava from the impeller (e.g., in case the vena cava collapses inwardly due to intra-abdominal pressure), such that the vena cava does not become injured by the impeller and the impeller does not become deformed by pressure from the inner wall of the vena cava. Further typically, an axial shaft  92  (shown in  FIG. 7 ) that passes through the impeller is supported by radial bearings  116 ,  118  that are disposed, respectively, at proximal and distal end of the impeller housing. 
     For some such applications, a blood-pump-support frame  32  is disposed upstream of impeller housing  26 , and is configured to come into contact with the inner wall of the vena cava. Blood-pump-support frame  32  is configured to align the longitudinal axis of impeller housing  26 , and, in turn, impeller  50 , with the local longitudinal axis of the vena cava, by contacting the inner wall of the vena cava. (It is noted that, for some applications, the blood-pump-support frame may not fully align the longitudinal axis of impeller with the local longitudinal axis of the vena cava. However, typically, the blood-pump-support frame maintains the longitudinal axis of the impeller in greater alignment with the local longitudinal axis of the vena cava, relative to alignment of the longitudinal axis of the impeller with the local longitudinal axis of the vena cava in the absence of the blood-pump-support frame.) Typically, ceteris paribus, the efficacy of the pumping of blood by impeller  50  is greater, the greater the alignment of the longitudinal axis of the impeller with the local longitudinal axis of the vena cava. As noted above, a slight misalignment of the of the longitudinal axis of the impeller with the local longitudinal axis of the vena cava (e.g., as shown in  FIG. 2E ), will nevertheless typically still result in efficacious pumping of blood by the impeller. 
     For some applications, impeller housing  26  includes a frame  34  (e.g., a rigid or semi-rigid frame) made of a shape-memory element (such as nitinol) that is at least partially covered with a material  36  (e.g., a blood-impermeable material, e.g., polyester, polyurethane, and/or a different polymer). Typically, for such applications, the stiffness of frame  34  is sufficiently great that pressure exerted upon the frame  34  by the inner wall of the vena cava does not deform the frame. Typically, material  36  extends from the impeller housing to blood-pump-support frame  32  such as to contact the vessel wall and to occlude the blood vessel in the region of the blood vessel that surrounds the impeller and/or that is upstream of the impeller. The material typically defines a hole therethrough at a distal portion of the impeller housing. The material is configured to occlude backflow of blood around the outside of the impeller, but such to allow antegrade blood flow in the central region of the vessel in the vicinity of the impeller, by the hole acting as a blood-outlet opening  31 . For some applications, a portion of the impeller housing is covered (i.e., lined) on the inside of the impeller housing with an inner lining  39 , e.g., as described hereinbelow with reference to  FIG. 7 . For some such applications, the inner lining of the impeller housing at least partially overlaps with material  36 . For some applications, the inner lining extends the blood-outlet opening defined by material  36 . 
     Typically, a drive cable  78  is disposed within an outer tube  77  of blood-pump catheter  70 . (The drive cable is shown in  FIG. 8A , for example.) Typically, the drive cable extends from motor  79  which is disposed outside the subject&#39;s body, to axial shaft  92  (shown in  FIG. 7 ), upon which the impeller is disposed. The motor imparts rotational motion to the axial shaft (and thereby imparts rotational motion to the impeller), via the drive cable. Typically, the motor is controlled by computer processor  10 . For some applications, the motor is controlled, based upon blood pressure measurements that are performed by pressure sensor  75 . 
     For some applications, the blood-pump catheter further includes a funnel  72  that extends into a tube  74 . The funnel is configured to be placed upstream of junctions of the vena cava with the subject&#39;s renal veins, and is configured to direct all blood flow from the infrarenal vena cava into tube  74 . In turn, tube  74  directs the blood flow from the infrarenal location to a suprarenal location, in such a manner that this blood flows to the suprarenal location by bypassing blood pump  24  (i.e., without being pumped by blood pump  24 ). For example, tubes  76  may extend from tube  74  to a suprarenal location while bypassing the blood pump. As indicated by the blood flow arrows in  FIG. 2A , blood thereby flows from the infrarenal vena cava to the suprarenal vena cava without being pumped by blood pump  24 . By contrast, blood is pumped from a region of the vena cava that is adjacent to the junctions of the vena cava with the renal veins to the suprarenal vena cava, by blood pump  24 , as indicated by the blood flow arrows in  FIG. 2B . 
     In this manner, by deploying the blood-pump catheter  70  within the vena cava, the bloodstream of the region of the vena cava that is adjacent to the junctions of the vena cava with the renal veins is separated into a compartment  82  that is separate from the infrarenal vena-caval bloodstream  84  and the suprarenal vena-caval bloodstream  86 . Blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  occurs passively (e.g., by flowing through funnel  72 , tube  74 , and tubes  76 ). Blood is pumped actively from compartment  82  to the suprarenal bloodstream, using blood pump. In this manner, a region of low pressure within the vena cava is generated in compartment  82 , 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 specific example of the construction of funnel  72 , tube  74 , and tube  76 , is shown by way of example, and the scope of the present applications includes using any other construction of a blood-flow pathway  60  (i.e., blood-flow channel) to direct blood from the infrarenal vena cava to the suprarenal vena cava without being pumped by blood pump  24 , mutatis mutandis. Typically, funnel  72 , tube  74  and tubes  76  comprise respective portions of a single continuous blood-flow pathway. The blood-flow pathway typically passes between a pathway-inlet opening  61  disposed at an upstream location that is upstream of junctions of the vena cava with the renal veins and a pathway-outlet opening  63  that opens to a downstream side of material  36 . Typically, funnel  72  defines a wide end  85  and a narrow end  87 , as shown in  FIG. 2A . The wide end of the funnel typically defines the inlet opening into the blood-flow pathway, and the narrow end of the funnel passes into one or more tubes (e.g., tubes  74  and  76 ). 
     Typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  is substantially unchanged relative to the blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  in the absence of the blood-pump catheter. For some applications, the diameters of one or more portions of blood-flow pathway  60  are controlled such as to control blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86 . For example, the diameters of one or more portions of the blood-flow pathway (e.g., funnel  72 , tube  74 , and/or tube(s)  76 ) may be modulated (e.g., using an inflatable/deflatable component that is disposed inside or outside one of these elements). In this manner, preload to the right atrium may be controlled, for example. 
     For some applications, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  is controlled responsively (e.g., automatically in response) to one or more pressure measurements, for example, pressure measurements that are performed as described herein. Alternatively or additionally, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  may be modulated periodically (e.g., in the above-described manner). For example, a healthcare professional (or another person) may periodically modulate blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  (e.g., in the above-described manner), in response to changes in the subject&#39;s condition. 
     For some applications, the diameters of one more portions of blood-flow pathway  60  (e.g., funnel  72 , tube  74 , and tubes  76 ) are not modulable, but the diameter of at least a portion of the blood-flow pathway is such that blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  is reduced (relative to the level of blood flow that would occur in the absence of these components), such as to reduce preload to the right atrium. 
     Reference is now made to  FIG. 2C , which is a schematic illustration of blood-pump catheter  70 , the blood-pump catheter including blood-pressure measurement tubes  71 ,  73 , in accordance with some applications of the present invention. Reference is also again made to  FIG. 1C , which shows blood-pump catheter  20  including blood-pressure measurement tubes  73 , in accordance with some applications of the present invention. 
     For some applications, at least one blood-pressure-measurement tube  71  is configured to extend to at least an outer surface of funnel  72  and/or tube  74 , such that an opening at the distal end of the blood-pressure-measurement tube is in direct fluid communication with the patient&#39;s bloodstream outside funnel  72  and/or tube  74  (i.e., the renal venous bloodstream). Pressure sensor  75  measures pressure of blood within the blood-pressure-measurement tube. Typically, by measuring pressure of blood within the blood-pressure-measurement tube, the pressure sensor thereby measures the subject&#39;s blood pressure outside funnel  72  and/or tube  74 , which is indicative of the renal venous blood pressure. Typically, blood-pressure-measurement tube  71  extends from outside the subject&#39;s body to an opening at the distal end of the tube, and the pressure sensor is disposed toward a proximal end of the tube, e.g., outside the subject&#39;s body. For some applications, computer processor  10 , receives an indication of the measured blood pressure and controls the pumping of blood by the impeller, in response to the measured blood pressure. 
     For some applications of blood-pump catheter  20 , blood-pump catheter  70 , or blood-pump catheter  90  (described hereinbelow), at least one blood-pressure-measurement tube  73  is configured to extend to at least an outer surface of material  36 , such that an opening at the distal end of the blood-pressure-measurement tube is in direct fluid communication with the patient&#39;s bloodstream outside material  36  (e.g., the suprarenal vena-caval blood pressure). Pressure sensor  75  measures pressure of blood within the blood-pressure-measurement tube. Typically, by measuring pressure of blood within the blood-pressure-measurement tube, the pressure sensor thereby measures the subject&#39;s blood pressure outside material  36 , which is indicative of the suprarenal vena-caval blood pressure, in the example shown in  FIGS. 1C and 2C . Typically, blood-pressure-measurement tube  73  extends from outside the subject&#39;s body to an opening at the distal end of the tube, and the pressure sensor is disposed toward a proximal end of the tube, e.g., outside the subject&#39;s body. For some applications, computer processor  10 , receives an indication of the measured blood pressure and controls the pumping of blood by the impeller, in response to the measured blood pressure. For some applications (e.g., when used in conjunction with blood-pump catheter  70 ), the computer processor modulates blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  (e.g., as described hereinabove), in response to the measured blood pressure. For some applications (e.g., when used in conjunction with blood-pump catheter  20 , as shown in  FIGS. 1A and 1B ), the computer processor modulates the diameter to which occlusion element  40  is expanded, in response to the measured blood pressure. 
     As described hereinabove, typically, a drive cable  78  is disposed within an outer tube  77  of blood-pump catheters  20 ,  70 , and  90 . (The drive cable is shown in  FIG. 7A , for example.) Typically, the drive cable extends from motor  79  which is disposed outside the subject&#39;s body, to axial shaft  92 , upon which the impeller is disposed. The motor imparts rotational motion to the axial shaft (and thereby imparts rotational motion to the impeller), via the drive cable. For some applications, the one or more blood-pressure measurement tubes  71 ,  73  are disposed within outer tube  77 , surrounding the drive cable, along at least a portion of the lengths of the blood-pressure measurement tubes. 
     As shown in  FIG. 1C  and  FIG. 2C , for some applications of blood-pump catheter  20  or blood-pump catheter  70 , infrarenal vena-caval blood pressure is measured using at least one blood-pressure-measurement tube that defines an opening  83  in outer tube  77  at its distal end. The blood-pressure-measurement tube is configured to extend from outside the subject&#39;s body to an outer surface of outer tube  77  within the subject&#39;s infrarenal vena cava, such that the opening at the distal end of the blood-pressure-measurement tube is in direct fluid communication with the subject&#39;s infrarenal-vena-caval bloodstream. Pressure sensor  75  is configured to measure the subject&#39;s infrarenal vena-caval blood pressure by measuring blood pressure within the blood-pressure-measurement tube. For some applications (e.g., when used with blood-pump catheter  20  as shown in  FIGS. 1A-1C ), renal venous pressure is measured in a generally similar manner (e.g., via a blood-pressure-measurement tube that defines an opening that is disposed in a vicinity of junctions of the renal veins with the vena cava). For some applications, computer processor  10 , receives an indication of the measured blood pressure and controls the pumping of blood by the impeller, in response to the measured blood pressure. For some applications (e.g., when used in conjunction with blood-pump catheter  70 ), the computer processor modulates blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  (e.g., as described hereinabove), in response to the measured blood pressure. For some applications (e.g., when used in conjunction with blood-pump catheter  20 , as shown in  FIGS. 1A and 1B ), the computer processor modulates the diameter to which occlusion element  40  is expanded, in response to the measured blood pressure. 
     It is typically the case that there is a relationship between the amount of power that is required to power the rotation of the impeller at a given rotation rate and the pressure difference that the impeller generates. (Typically, the impeller lowers the pressure in the region of the vena cava that is upstream of material  36 , by pumping blood to the region of the vena cava that is downstream of material  36 , thereby lowering the blood pressure in the upstream region and generating a pressure difference between the upstream region and the downstream region.) For some applications, calibration measurements are performed, such that the relationship between (a) power consumption by the motor that is required to rotate the impeller at a given rotation rate and (b) the pressure difference that is generated by the impeller, is known. 
     For some applications, blood pressure upstream of impeller  50  and covering material  36  is measured, for example, using the blood-pressure-measurement tube that defines opening  83  in outer tube  77  at its distal end, and/or using a pressure sensor that is disposed within the vena cava at an infrarenal location (e.g., a piezo-electric pressure sensor, and/or an optical sensor). For some such applications, blood pressure downstream of impeller  50  and covering material  36  is then calculated by the computer processor, based upon (a) the measured upstream blood pressure, (b) the power consumption by the motor that is required to rotate the impeller at a given rotation rate at that time, and (c) the predetermined relationship between power consumption by the motor that is required to rotate the impeller at a given rotation rate and the pressure difference that is generated by the impeller. For some applications, the above-described technique is performed while maintaining the rotation rate of the impeller at a constant rate. Alternatively or additionally, the rotation rate of the impeller is varied, and the variation of the rotation rate of the impeller is accounted for in the above-described calculations. 
     Typically, blood-outlet opening  31  in material  36  has a known cross-sectional area. For some applications, the flow through the blood-outlet opening that is generated by the impeller is determined based on the determined pressure difference that is generated by the impeller, and the known cross-sectional area of the opening. For some applications, such flow calculations incorporate calibration parameters in order to account for factors such as flow resistance that are specific to the blood-pump catheter (or type of blood-pump catheter) upon which the calculations are performed. 
     For some applications, the above-described apparatus and methods for measuring blood pressure are used in conjunction with blood-pump catheter  20  described hereinabove with reference to  FIGS. 1A-C , in conjunction with blood-pump catheter  70  described hereinabove with reference to  FIGS. 2A-F , in conjunction with blood-pump catheter  90  described hereinbelow with reference to  FIGS. 3A-B , and/or in conjunction with blood-pump catheter  20  described hereinbelow with reference to  FIGS. 4A-B , mutatis mutandis. 
     Reference is now made to  FIG. 2D , which is a schematic illustration of blood-pump catheter  70  placed within the subject&#39;s vena cava  22 , the blood pump having a non-axisymmetric outflow tube  76 , in accordance with some applications of the present invention. Blood-pump catheter  70  as shown in  FIG. 2D  is generally similar to that shown and described with reference to  FIGS. 2A-C . However, in the example shown in  FIGS. 2A-C , the blood-pump catheter included two tubes  76  disposed on respective sides of the longitudinal axis of the blood-pump catheter, and extending from tube  74  to a suprarenal location, while bypassing blood pump  24 . By contrast, in the example shown in  FIG. 2D , the blood-pump catheter includes only a single tube  76  disposed on one side of the longitudinal axis of the blood-pump catheter, and extending from tube  74  to a suprarenal location, while bypassing blood pump  24 , in accordance with some applications of the present invention. In other respects, blood-pump catheter  70  as shown in  FIG. 2D  is generally similar to that shown in and described with reference to  FIGS. 2A-C . For some applications (not shown), blood-pump catheter includes more than two tubes  76  disposed at respective locations around the longitudinal axis of the blood-pump catheter, and extending from tube  74  to a suprarenal location, while bypassing blood pump  24 . 
     Reference is now made to  FIG. 2E , which is a schematic illustration of blood-pump catheter  70  placed within the subject&#39;s vena cava  22 , the blood pump having a non-axisymmetric outflow tube  76  and a non-axisymmetric blood pump  24 , in accordance with some applications of the present invention. Reference is also made to  FIG. 2F , which is a schematic illustration of the blood-pump catheter of  FIG. 2E  in the absence of the subject&#39;s anatomy, in accordance with some applications of the present invention. As described hereinabove with reference to  FIG. 2D , for some applications, the blood-pump catheter includes only a single tube  76  disposed on one side of the longitudinal axis of the blood-pump catheter, and extending from tube  74  to a suprarenal location, while bypassing blood pump  24 . For some such applications, blood pump  24  is disposed in a non-axisymmetric orientation on the opposite side of the longitudinal axis of the blood-pump catheter, as shown. In other respects, blood-pump catheter  70  as shown in  FIGS. 2E and 2F  is generally similar to that shown in and described with reference to  FIGS. 2A-D . 
     For some applications, techniques as described with reference to  FIGS. 2A-E  are performed in the absence of blood pump  24 . That is, the bloodstream within a region of the vena cava that is adjacent to junctions of the vena cava with the renal veins is separated into a compartment  82  that is separate from the infrarenal vena-caval bloodstream  84  and the suprarenal vena-caval bloodstream  86 , in the absence of blood pump  24 . For some such applications, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  is then controlled passively (e.g., as described hereinabove), without affecting blood flow from the renal veins to the suprarenal vena-caval bloodstream  86 . Alternatively or additionally, blood flow from the renal veins to the suprarenal vena-caval bloodstream  86  is controlled passively, without affecting blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86 . 
     With reference to  FIGS. 2A-E , it is noted that the scope of the present invention includes applying similar apparatus and methods to any venous system of a subject that includes tributary veins that flow into a main vein. For example, similar techniques could be applied at junctions of the hepatic veins and the vena cava. Typically, such methods include separating bloodstream within the region of the main vein that is adjacent to junctions of the main vein with the tributary veins from bloodstream within the main vein upstream and downstream thereto, pumping blood from the region of the main vein that is adjacent to junctions of the vein with the tributary veins to a location within the main vein that is downstream of the tributary veins, using a pump, and allowing the subject&#39;s blood to flow from a location within the main vein that is upstream of the tributary veins to the location within the main vein that is downstream of the tributary veins without being pumped by the pump, by directing the blood to bypass the pump. For some applications, generally similar apparatus and methods are used within the subclavian vein or jugular vein, at junctions of the vein with a lymph duct. For some such applications, blood-pump catheter  70  is configured to increase flow of lymphatic fluid from the lymph duct into the vein, mutatis mutandis. In general, the scope of some applications of the present invention further includes apparatus and methods for use with a venous system of a subject that includes tributary vessels (e.g., lymphatic vessels) that flow into a vein. Typically, such methods include separating flow within a region of the vein that is adjacent to junctions of the vein with the tributary vessels from bloodstream within the vein upstream and downstream thereto, pumping blood from the region of the vein that is adjacent to junctions of the vein with the tributary vessels to a location within the vein that is downstream of the region, using a pump, and allowing the subject&#39;s blood to flow from a location within the vein that is upstream of the tributary vessels to the location within the vein that is downstream of the tributary vessels without being pumped by the pump, by directing the blood to bypass the pump. 
     For some applications, such methods include at least partially occluding blood flow through a vein by placing a downstream occluding element (such as material  36 ) at a downstream location that is downstream of junctions of the vein with tributary vessels, pumping blood from the region of the vein that is adjacent to junctions of the vein with the tributary vessels and through the downstream occluding element, using a pump, directing upstream venous blood flow from an upstream location that is upstream of junctions of the vein with the tributary vessels to downstream of the occluding element, without the upstream venous blood flow being pumped by the pump, by directing the upstream venous blood flow to bypass the pump. Typically, such methods are performed using a blood-pump catheter (such as, blood-pump catheter  70 ), which includes material  36 , which is configured to be placed at a downstream location that is downstream of junctions of the vein with the tributary vessel and to at least partially occlude blood flow through the vein at the downstream location, the material defining blood-outlet opening  31 . Blood pump  24  is typically configured to pump blood from the region of the vein that is adjacent to junctions of the vein with the tributary vessels through the blood-outlet opening defined by the material. Blood-flow pathway  60  typically passes between a pathway-inlet opening  61  disposed at an upstream location that is upstream of junctions of the vein with the tributary vessels and a pathway-outlet opening  63  that opens to a downstream side of material  36 . For some applications, the blood-flow pathway comprises funnel  72  and one or more tubes (e.g., tubes  74  and  76 ). Funnel  72  is configured to be disposed at an upstream location that is upstream of junctions of the vein with the tributary vessel, and one or more tubes  74 ,  76  are configured to extend from the funnel to a downstream side of the material. The funnel and the one or more tubes (or alternative blood-flow pathway) are configured to direct upstream venous blood flow from the upstream location to downstream of the material, without the upstream venous blood flow being pumped by the pump. Typically, funnel  72  defines a wide end  85  and a narrow end  87 , as shown in  FIG. 2A . The wide end of the funnel typically defines the inlet opening into the blood-flow pathway, and the narrow end of the funnel passes into one or more tubes (e.g., tubes  74  and  76 ). 
     Reference is now made to  FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G, and 3H , which are schematic illustrations of blood-pump catheter  70 , in accordance with some further applications of the present invention. Blood-pump catheter  70  as shown and described with reference to  FIGS. 3A-H  is generally similar to that described with reference to  FIGS. 2A-F , except for the differences described hereinbelow. It is noted that, as shown in  FIGS. 3A-H , the blood-pump catheter has a generally similar design to that shown in  FIGS. 2E and 2F  (in that (a) only a single tube  76  is disposed on one side of the longitudinal axis of the blood-pump catheter, and extends from tube  74  to a suprarenal location, while bypassing blood pump  24 , and (b) blood pump  24  is disposed in a non-axisymmetric orientation on the opposite side of the longitudinal axis of the blood-pump catheter). However, the scope of the present application includes combining features of blood-pump catheter  70  described with reference to  FIGS. 3A-H  with any one of the designs of blood-pump catheter  70  described hereinabove with reference to  FIGS. 2A-D , mutatis mutandis. 
     Referring now to  FIG. 3A , for some applications, an elongate tube  109  extends from frame  34  to the subject&#39;s pulmonary artery  110 , such that blood-outlet opening  31  is disposed in the pulmonary artery. (It is noted that, for some applications, material  36  defines a blood-outlet opening, and elongate tube extends from the blood-outlet opening defined by the material to the blood-outlet opening disposed within the pulmonary artery.) Tube  109  is typically made of a blood-impermeable material. For example, tube  109  may include polyurethane, polyester, and/or silicone. Alternatively or additionally, the tube is made of polyethylene terephthalate (PET) and/or polyether block amide (e.g., PEBAX®). Blood-pump catheter is configured to pump blood directly from compartment  82  into the subject&#39;s pulmonary artery. In this manner, the blood-pump catheter reduces renal venous pressure without increasing preload on the subject&#39;s right atrium. In accordance with the description of  FIGS. 2A-F , typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  bypasses the elongate tube. Typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  occurs passively, by flowing through blood-flow pathway  60  (e.g., funnel  72 , tube  74 , and tube(s)  76 ). 
     Referring to  FIG. 3B , for some applications, elongate tube  109  includes a portion  111  that is disposed within suprarenal vena-caval bloodstream  86  and that defines suprarenal blood-inlet openings  112 . Impeller  50  and frame  34  are typically disposed downstream of the suprarenal blood-inlet openings, and in addition to pumping blood into elongate tube  109  from compartment  82 , the impeller is configured to pump blood into elongate tube  109  from the suprarenal vena-caval bloodstream, via blood-inlet openings  112 . As described with reference to  FIG. 3A , typically, elongate tube  109  extends from frame  34  to the subject&#39;s pulmonary artery  110 , such that blood-outlet opening  31  is disposed in the pulmonary artery. Blood-pump catheter  70  as configured in  FIG. 3B  is typically configured to reduce renal venous pressure (by pumping blood from the compartment  82 ) and to assist with the subject&#39;s right heart function, by pumping blood from the suprarenal vena-caval bloodstream to the pulmonary artery. In accordance with the description of  FIGS. 2A-F , typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  bypasses the elongate tube. Typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  occurs passively, by flowing through blood-flow pathway  60  (e.g., funnel  72 , tube  74 , and tube(s)  76 ). 
     Referring to  FIG. 3C , for some applications, in addition to using blood-pump catheter  70  to reduce renal venous pressure (and, optionally, to assist with right heart function, as described with reference to  FIG. 3B ), a left-ventricular assist device  113  is deployed within the subject&#39;s left ventricle  114 , in order to assist with the subject&#39;s left heart function. For some applications, the left-ventricular assist device is a left-ventricular assist device as described in U.S. Ser. No. 16/750,354 to Tuval (filed Jan. 23, 2020, entitled “Distal tip element for a ventricular assist device”), US 2019/0209758 to Tuval, and/or US 2019/0175806 to Tuval, all of which applications are incorporated herein by reference. For some applications, the left-ventricular assist device includes a left-ventricular assist device manufactured by Abiomed® (Massachusetts, USA), such as Impella 2.5®, Impella CP®, Impella 5.5®, and/or Impella 5.0®. It is noted that although left-ventricular assist device  113  is shown as being used in combination with the example of blood-pump catheter  70  shown in  FIG. 3B , the scope of the present application includes using left-ventricular assist device  113  in combination with any of the examples of blood-pump catheter  70  described herein. 
     Referring now to  FIG. 3D , for some applications, in place of impeller  50  and frame  34 , an elongate blood-pump tube  104  (which typically includes an impeller), is disposed within the blood-outlet opening in covering material  36 . Typically, covering material  36  and/or inner lining  39  ( FIG. 7 ) is sealingly coupled to the outer surface of the elongate blood-pump tube. For example, the elongate blood-pump tube may include an Impella RP®, manufactured by Abiomed® (Massachusetts, USA). Typically, the elongate blood-pump tube includes a blood-inlet area  105  (disposed within an upstream portion of the elongate blood-pump tube), which is disposed at an inferior (i.e., upstream) position with respect to covering material  36  and/or inner lining  39  ( FIG. 7 ). An impeller  106  (which is typically disposed distally and in close proximity to the blood-inlet area) pumps blood from compartment  82  into the blood-inlet area, through the elongate blood-pump tube, and out of a blood-outlet area  107  (disposed within a downstream portion of the elongate blood-pump tube). In the example shown in  FIG. 3A , the blood-outlet area is disposed in the suprarenal vena cava, such that blood is pumped into suprarenal vena-caval bloodstream  86 . In accordance with the description of  FIGS. 2A-F , typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  bypasses the elongate blood-pump tube. Typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  occurs passively, by flowing through blood-flow pathway  60  (e.g., funnel  72 , tube  74 , and tube(s)  76 ). 
     Referring to  FIG. 3E , for some applications, elongate blood-pump tube  104  is disposed such that the blood-inlet area  105  and impeller  106  are disposed within the suprarenal vena cava. For some such applications, a reinforced tube  115  (e.g., a tube made of a polymer supported by a shape-memory-material (e.g., nitinol) frame and/or a shape-memory-material (e.g., nitinol) braid) extends from the outlet opening of covering material  36  and/or inner lining  39  ( FIG. 7 ) to the blood-inlet area of the elongate blood-pump tube, and forms a seal between covering material  36  and/or inner lining  39  and the blood-inlet area of the elongate blood-pump tube. In this manner, impeller  106  draws blood from compartment  82  into blood-inlet area  105 . Typically, the elongate blood-pump tube extends from the blood-inlet area to blood-outlet area  107 , which is disposed in the subject&#39;s pulmonary artery  110 . In some alternative applications (not shown), the elongate blood-pump tube itself extends from upstream of covering material  36  until pulmonary artery  110  (i.e., in the absence of reinforced tube  115 ). Elongate blood-pump tube is configured to pump blood from compartment  82  into the subject&#39;s pulmonary artery. In this manner, blood-pump catheter  70  reduces renal venous pressure without increasing preload on the subject&#39;s right atrium. In accordance with the description of  FIGS. 2A-F , typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  bypasses the elongate blood-pump tube. Typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  occurs passively, by flowing through blood-flow pathway  60  (e.g., funnel  72 , tube  74 , and tube(s)  76 ). 
     Referring now to  FIG. 3F , for some applications, reinforced tube  115  defines blood-inlet openings  117 , which are disposed in the suprarenal vena cava. Impeller  106  and blood-inlet area  105  are typically disposed downstream of blood-inlet openings  117 , and in addition to pumping blood into elongate blood-pump tube  104 , the impeller is configured to pump blood into elongate blood-pump tube  104  from the suprarenal vena-caval bloodstream, via blood-inlet openings  117 . As described with reference to  FIG. 3E , typically, elongate blood-pump tube  104  extends from the suprarenal vena cava to the subject&#39;s pulmonary artery  110 , such that blood-outlet area  107  is disposed in the pulmonary artery. Blood-pump catheter  70  as configured in  FIG. 3F  is typically configured to reduce renal venous pressure (by pumping blood from the compartment  82 ) and to assist with the subject&#39;s right heart function, by pumping blood from the suprarenal vena-caval bloodstream to the pulmonary artery. In accordance with the description of  FIGS. 2A-F , typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  bypasses the elongate blood-pump tube. Typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  occurs passively, by flowing through blood-flow pathway  60  (e.g., funnel  72 , tube  74 , and tube(s)  76 ). 
     Referring now to  FIG. 3G , for some applications, blood-inlet openings  117  are disposed in the subject&#39;s right ventricle  119 , and impeller  106  and blood-inlet area  105  are disposed downstream of the blood-inlet openings. In addition to pumping blood into elongate blood-pump tube  104 , the impeller is configured to pump blood into elongate blood-pump tube  104  from the right ventricle, via blood-inlet openings  117 . Typically, elongate blood-pump tube  104  extends from the right ventricle to the subject&#39;s pulmonary artery  110 , such that blood-outlet area  107  is disposed in the pulmonary artery. Blood-pump catheter  70  as configured in  FIG. 3G  is typically configured to reduce renal venous pressure (by pumping blood from the compartment  82 ) and to assist with the subject&#39;s right heart function, by pumping blood from the right ventricle to the pulmonary artery. In accordance with the description of  FIGS. 2A-F , typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  bypasses the elongate blood-pump tube. Typically, blood flow from the infrarenal vena-caval bloodstream  84  to the suprarenal vena-caval bloodstream  86  occurs passively, by flowing through blood-flow pathway  60  (e.g., funnel  72 , tube  74 , and tube(s)  76 ). 
     Referring now to  FIG. 3H , for some applications, blood-pump catheter  70  includes elongate blood-pump tube  104  (e.g., in accordance with any one of the examples described with reference to  FIGS. 3D-G ), and the blood-pump catheter is used in conjunction with left-ventricular assist device  113 , which is deployed within the subject&#39;s left ventricle  114 , in order to assist with the subject&#39;s left heart function (e.g., as described hereinabove with reference to  FIG. 3C ). For some applications, the left-ventricular assist device is a left-ventricular assist device as described in U.S. Ser. No. 16/750,354 to Tuval (filed Jan. 23, 2020, entitled “Distal tip element for a ventricular assist device”), US 2019/0209758 to Tuval, and/or US 2019/0175806 to Tuval, all of which applications are incorporated herein by reference. For some applications, the left-ventricular assist device includes a left-ventricular assist device manufactured by Abiomed® (Massachusetts, USA), such as Impella 2.5®, Impella CP®, Impella 5.5®, and/or Impella 5.0®. It is noted that although left-ventricular assist device  113  is shown as being used in combination with the example of blood-pump catheter  70  shown in  FIG. 3F , the scope of the present application includes using left-ventricular assist device  113  in combination with any of the examples of blood-pump catheter  70  described herein. 
     Reference is now made to  FIGS. 4A and 4B , which are schematic illustrations of a blood-pump catheter  90  placed within a subject&#39;s descending aorta  80  in the vicinity of a subject&#39;s renal arteries  94 , in accordance with some applications of the present invention. The structure of blood-pump catheter  90  is generally similar to that of blood-pump catheter  70  described hereinabove with reference to  FIGS. 2A-F . However, blood pump  24  of blood-pump catheter  90  is typically configured to pump blood in the upstream direction, such as to enhance perfusion of an organ (such as a kidney) as described in further detail hereinbelow. 
     Typically, blood-pump catheter  90  includes blood pump  24 , which typically includes impeller housing  26  and impeller  50 , all of which are generally as described hereinabove. Impeller housing  26  (e.g., frame  34  of the housing) typically functions to separate the inner wall of the aorta from the impeller, such that the aorta does not become injured by the impeller and the impeller does not become deformed by pressure from the inner wall of the aorta. Further typically, an axial shaft  92  (show in  FIG. 7 ) that passes through the impeller is supported by radial bearings  116 ,  118  disposed, respectively, at the proximal and distal ends of the impeller housing. 
     For some such applications, a blood-pump-support frame  32  is disposed upstream of impeller housing  26 , and is configured to come into contact with the inner wall of the aorta. Blood-pump-support frame  32  is configured to align the longitudinal axis of impeller housing  26 , and, in turn, impeller  50 , with the local longitudinal axis of the aorta, by contacting the inner wall of the aorta. (It is noted that, for some applications, the blood-pump-support frame may not fully align the longitudinal axis of the impeller with the local longitudinal axis of the aorta. However, typically, the blood-pump-support frame maintains the longitudinal axis of the impeller in greater alignment with the local longitudinal axis of the aorta, relative to alignment of the longitudinal axis of the impeller with the local longitudinal axis of the aorta in the absence of the blood-pump-support frame.) Typically, ceteris paribus, the efficacy of the pumping of blood by impeller  50  is greater, the greater the alignment of the longitudinal axis of the impeller with the local longitudinal axis of the aorta. As noted above, a slight misalignment of the of the longitudinal axis of the impeller with the local longitudinal axis of the blood vessel in which it is placed (e.g., as shown in  FIG. 2E ), will nevertheless typically still result in efficacious pumping of blood by the impeller. 
     For some applications, impeller housing  26  includes a frame  34  (e.g., a rigid or semi-rigid frame) made of a shape-memory element (such as nitinol) that is at least partially covered with a material  36  (e.g., a blood-impermeable material, e.g., polyester, polyurethane, and/or a different polymer). Typically, for such applications, the stiffness of frame  34  is sufficiently great that pressure exerted upon the frame  34  by the inner wall of the aorta does not deform the frame. Typically, material  36  extends from the impeller housing to blood-pump-support frame  32  such as to contact the vessel wall and to occlude the blood vessel in the region of the blood vessel that surrounds the impeller and/or that is upstream of the impeller. The material typically defines a hole therethrough at a distal portion of the impeller housing. The hole is configured to act as a blood-inlet opening  98 , during pumping of blood by the impeller. For some applications, a portion of the impeller housing is covered (i.e., lined) on the inside of the impeller housing with an inner lining  39 , e.g., as described hereinbelow with reference to  FIG. 7 . For some such applications, the inner lining of the impeller housing at least partially overlaps with material  36 . For some applications, the inner lining extends the blood-inlet opening defined by material  36 . 
     Typically, a drive cable  78  is disposed within an outer tube  77  of blood-pump catheter  90 . (The drive cable is shown in  FIG. 8A , for example.) Typically, the drive cable extends from motor  79  which is disposed outside the subject&#39;s body, to axial shaft  92  (shown in  FIG. 7 ), upon which the impeller is disposed. The motor imparts rotational motion to the axial shaft (and thereby imparts rotational motion to the impeller), via the drive cable. Typically, the motor is controlled by computer processor  10 . For some applications, the motor is controlled, based upon blood pressure measurements that are performed by pressure sensor  75 . 
     For some applications, the blood-pump catheter further includes a blood-flow pathway  60 . For some applications, blood-flow pathway  60  includes funnel  72  that extends into tube  74 . The funnel is configured to be placed upstream of junctions of the aorta with the subject&#39;s renal arteries, and is configured to direct all blood flow from the suprarenal aorta into tube  74 . In turn, tube  74  directs the blood flow from the suprarenal aortic location to an infrarenal aortic location, in such a manner that this blood flows to the infrarenal location by bypassing blood pump  24  (i.e., without being pumped by blood pump  24 ). For example, one or more tubes  76  may extend from tube  74  to an infrarenal location while bypassing the blood pump. As indicated by the blood flow arrows in  FIG. 4A , blood thereby flows from the suprarenal aorta to the infrarenal aorta without being pumped by blood pump  24 . By contrast, blood is pumped from the infrarenal aorta to the renal arteries  94 , by blood pump  24 , as indicated by the blood flow arrows in  FIG. 4B . Typically, the pumping of blood into the renal arteries enhances renal perfusion. 
     In this manner, by deploying the blood-pump catheter  90  the bloodstream within a region of the aorta that is adjacent to junctions of the aorta with the renal arteries is separated into a compartment  97  that is separate from the suprarenal aortic bloodstream  95  and the infrarenal aortic bloodstream  99 . Blood flow from the suprarenal aortic bloodstream  95  to the infrarenal aortic bloodstream  99  occurs passively (e.g., by flowing through blood-flow pathway  60 ). Blood is pumped actively from infrarenal aortic bloodstream  99  to compartment  97 , using blood pump. 
     It is noted that the specific example of the construction of funnel  72 , tube  74 , and tube  76 , is shown by way of example, and the scope of the present applications includes using any other construction of blood-flow pathway to direct blood from the suprarenal aortic bloodstream to the infrarenal aortic bloodstream without being pumped by blood pump  24 , mutatis mutandis. For some applications, funnel  72 , tube  74  and tube  76  comprise respective portions of a single continuous blood-flow pathway. The blood-flow pathway typically passes between pathway-inlet opening  61  disposed at an upstream location that is upstream of junctions of the aorta with the renal arteries and pathway-outlet opening  63  that opens to a downstream side of material  36 . Typically, funnel  72  defines wide end  85  and narrow end  87 . The wide end of the funnel typically defines the inlet opening into the blood-flow pathway, and the narrow end of the funnel passes into one or more tubes (e.g., tubes  74  and  76 ). 
     Typically, blood flow from the suprarenal aortic bloodstream  95  to the infrarenal aortic bloodstream  99  is substantially unchanged relative to the blood flow from the suprarenal aortic bloodstream  95  to the infrarenal aortic bloodstream  99  in the absence of the blood-pump catheter. For some applications, the diameters of one or more portions of the blood-flow pathway are controlled, such as to control blood flow from the suprarenal aortic bloodstream  95  to the infrarenal aortic bloodstream  99 . For example, the diameters of one or more portions of the blood-flow pathway (e.g., funnel  72 , tube  74 , and/or tube(s)  76 ) may be modulated (e.g., using an inflatable/deflatable component that is disposed inside or outside one of these portions). In this manner, blood supply to the lower body may be controlled, for example. 
     With reference to  FIGS. 4A-B , it is noted that the scope of the present invention includes applying similar apparatus and methods to any arterial system of a subject that includes a main artery that flows into branching arteries (which typically supply an organ). For example, similar techniques could be applied at junctions of the aorta with the hepatic arteries (which supply the liver). Typically, such methods include separating bloodstream within a region of the main artery that is adjacent to junctions of the main artery with the branching arteries from bloodstream within the main artery upstream and downstream thereto, enhancing perfusion of the organ, by pumping blood from the main artery to the region of the main artery that is adjacent to junctions of the main artery with the branching arteries, using a pump, and allowing the subject&#39;s blood to flow from a location within the main artery that is upstream of the branching arteries to a location within the main artery that is downstream of the branching arteries without being pumped by the pump, by directing the blood to bypass the pump. For some applications, such methods include at least partially occluding blood flow through the main artery by placing a downstream occluding element (e.g., material  36 ) at a downstream location that is downstream of junctions of the main artery with the branching arteries, enhancing perfusion of the organ, by pumping blood from the region of the main artery that is adjacent to junctions of the main artery with the branching arteries and through the downstream occluding element, using a pump (e.g., blood pump  24 ), and directing upstream main artery blood flow from an upstream location that is upstream of junctions of the main artery with the branching arteries to downstream of the occluding element, without the upstream main artery blood flow being pumped by the pump, by directing the upstream main artery blood flow to bypass the pump. Typically, a blood-pump catheter such as blood-pump catheter  90  is used, the blood-pump catheter including material  36  configured to be placed at a downstream location that is downstream of junctions of the main artery with the branching arteries and to at least partially occlude blood flow through the main artery at the downstream location, the material defining blood-inlet opening  98 . Blood pump  24  is typically configured to pump blood from the main artery toward the region of the main artery that is adjacent to junctions of the main artery with the branching arteries, through the blood-inlet opening. Blood-flow pathway  60  typically passes between pathway-inlet opening  61  disposed at an upstream location that is upstream of junctions of the main artery with the branching arteries and pathway-outlet opening  63  that opens to a downstream side of material  36 . For some applications, the blood-flow pathway comprises funnel  72  and one or more tubes (e.g., tubes  74  and  76 ). Funnel  72  is configured to be disposed at an upstream location that is upstream of junctions of the main artery with the branching arteries, and one or more tubes  74 ,  76  (or alternative blood-flow pathways) are configured to extend from the funnel to a downstream side of the material. The funnel and the one or more tubes (or alternative blood-flow pathway) are typically configured to direct upstream main arterial blood flow from the upstream location to downstream of the material, without the upstream main arterial blood flow being pumped by the pump. Typically, funnel  72  defines wide end  85  and narrow end  87 . The wide end of the funnel typically defines the inlet opening into the blood-flow pathway, and the narrow end of the funnel passes into one or more tubes (e.g., tubes  74  and  76 ). 
     Reference is now made to  FIGS. 5A-B , which are schematic illustrations of blood-pump catheter  20  placed in a subject&#39;s descending aorta  80 , in accordance with some applications of the present invention. Typically, blood-pump catheter  20  is generally as described hereinabove, the blood-pump catheter including blood pump  24 , which typically includes impeller housing  26  and impeller  50 , all of which are generally as described hereinabove. Impeller housing  26  (e.g., frame  34  of the housing) typically functions to separate the inner wall of the aorta from the impeller, such that the aorta does not become injured by the impeller and the impeller does not become deformed by pressure from the inner wall of the aorta. Further typically, axial shaft  92  (shown in  FIG. 7 ) that passes through the impeller is supported by radial bearings  116 ,  118  that are disposed, respectively, at proximal and distal ends of the impeller housing. 
     For some such applications, a blood-pump-support frame  32  is disposed upstream of impeller housing  26 , and is configured to come into contact with the inner wall of the aorta. Blood-pump-support frame  32  is configured to align the longitudinal axis of impeller housing  26 , and, in turn, impeller  50 , with the local longitudinal axis of the aorta, by contacting the inner wall of the aorta. (It is noted that, for some applications, the blood-pump-support frame may not fully align the longitudinal axis of the impeller with the local longitudinal axis of the aorta. However, typically, the blood-pump-support frame maintains the longitudinal axis of the impeller in greater alignment with the local longitudinal axis of the aorta, relative to what the alignment of the longitudinal axis of the impeller with the local longitudinal axis of the aorta would be in the absence of the blood-pump-support frame.) Typically, ceteris paribus, the efficacy of the pumping of blood by impeller  50  is greater, the greater the alignment of the longitudinal axis of the impeller with the local longitudinal axis of the aorta. 
     For some applications, impeller housing  26  includes a frame  34  (e.g., a rigid or semi-rigid frame) made of a shape-memory element (such as nitinol) that is at least partially covered with a material  36  (e.g., a blood-impermeable material, e.g., polyester, polyurethane, and/or a different polymer). Typically, for such applications, the stiffness of frame  34  is sufficiently great that pressure exerted upon the frame  34  by the inner wall of the aorta does not deform the frame  34 . Typically, material  36  extends from the impeller housing to blood-pump-support frame  32  to the impeller housing such as to contact the vessel wall and to occlude the blood vessel in the region of the blood vessel that surrounds the impeller and/or that is upstream of the impeller. The material typically defines a hole therethrough at a distal portion of the impeller housing. The material is configured to occlude backflow of blood around the outside of the impeller, but such to allow antegrade blood flow in the central region of the vessel in the vicinity of the impeller, by the hole acting as a blood-outlet opening  31 . For some applications, a portion of the impeller housing is covered (i.e., lined) on the inside of the impeller housing with an inner lining  39 , e.g., as described hereinbelow with reference to  FIG. 7 . For some such applications, the inner lining of the impeller housing at least partially overlaps with material  36 . For some applications, the inner lining extends the blood-outlet opening defined by material  36 . 
     Typically, a drive cable  78  is disposed within an outer tube  77  of blood-pump catheter  90 . (The drive cable is shown in  FIG. 8A , for example.) Typically, the drive cable extends from motor  79  which is disposed outside the subject&#39;s body, to axial shaft  92  (shown in  FIG. 7 ), upon which the impeller is disposed. The motor imparts rotational motion to the axial shaft (and thereby imparts rotational motion to the impeller), via the drive cable. Typically, the motor is controlled by computer processor  10 . For some applications, the motor is controlled, based upon blood pressure measurements that are performed by pressure sensor  75 . Covering material  36  effectively separates the descending aorta into two compartments, and the blood pump pumps blood from the upstream compartment to the downstream compartment. In this manner, the blood pump typically augments aortic blood flow and/or reduces left ventricular afterload. 
     Referring to  FIG. 5A , for some applications, the blood pump is inserted into the descending aorta via an artery that is below the descending aorta, such as the femoral artery. Alternatively or additionally, the blood pump is inserted into the descending aorta via an artery that is above the descending aorta, e.g., via the radial artery, or the subclavian artery, as shown in  FIG. 5B . For some applications, the blood-pump catheter is inserted into the subject&#39;s aortic arch, rather than the descending aorta, mutatis mutandis. 
     For some applications, impeller  50 , impeller housing  26 , frame  34 , and/or other components of the blood pump catheters described herein have similar characteristics to components described in U.S. Ser. No. 16/750,354 to Tuval (filed Jan. 23, 2020, entitled “Distal tip element for a ventricular assist device”), US 2019/0209758 to Tuval, US 2019/0239998 to Tuval, and/or US 2019/0175806 to Tuval, all of which applications are incorporated herein by reference. For example, some of these characteristics are described hereinbelow with reference to  FIG. 6A-8C . 
     Reference is now made to  FIGS. 6A-C , which are schematic illustrations of impeller  50  or portions thereof, in accordance with some applications of the present invention. Typically, the impeller includes at least one outer helical elongate element  52 , which winds around a central axial spring  54 , such that the helix defined by the helical elongate element is coaxial with the central axial spring. Typically, the impeller includes two or more helical elongate elements (e.g., three helical elongate elements, as shown in  FIGS. 6A-C ). For some applications, the helical elongate elements and the central axial spring are made of a shape-memory material, e.g., a shape-memory alloy such as nitinol. Typically, each of the helical elongate elements and the central axial spring support a film  56  of a material (e.g., an elastomer, such as polyurethane, and/or silicone) therebetween. For some applications, the film of material includes pieces of nitinol embedded therein, for example in order to strengthen the film of material. For illustrative purposes, the impeller is shown in the absence of the material in  FIG. 6A .  FIGS. 6B and 6C  show respective views of the impeller with the material supported between the helical elongate elements and the spring. 
     Each of the helical elongate elements, together with the film extending from the helical elongate element to the spring, defines a respective impeller blade, with the helical elongate elements defining the outer edges of the blades, and the axial spring defining the axis of the impeller. Typically, the film of material extends along and coats the spring. For some applications, sutures  53  (e.g., polyester sutures, shown in  FIGS. 6B and 6C ) are wound around the helical elongate elements, e.g., as described in US 2016/0022890 to Schwammenthal, which is incorporated herein by reference. Typically, the sutures are configured to facilitate bonding between the film of material (which is typically an elastomer, such as polyurethane, or silicone) and the helical elongate element (which is typically a shape-memory alloy, such as nitinol). For some applications, sutures (e.g., polyester sutures, not shown) are wound around spring  54 . Typically, the sutures are configured to facilitate bonding between the film of material (which is typically an elastomer, such as polyurethane, or silicone) and the spring (which is typically a shape-memory alloy, such as nitinol). 
     Enlargements A and B of  FIG. 6C  show two alternative ways in which the sutures are tied around helical elongate elements  52 . For some applications, the sutures are tied around the outer surface of the helical elongate elements, as shown in enlargement A. Alternatively, the helical elongate elements define grooves  48  on their outer surfaces, and the sutures are embedded within the grooves, as shown in enlargement B. By embedding the sutures within the grooves, the sutures typically do not add to the outer profile of the impeller, and the outer profile of the impeller is defined by the outer surfaces of the helical elongate elements. 
     Typically, proximal ends of spring  54  and helical elongate elements  52  extend from a proximal bushing (i.e., sleeve bearing)  64  of the impeller, such that the proximal ends of spring  54  and helical elongate elements  52  are disposed at a similar radial distance from the longitudinal axis of the impeller, as each other. Similarly, typically, distal ends of spring  54  and helical elongate elements  52  extend from a distal bushing  58  of the impeller, such that the distal ends of spring  54  and helical elongate elements  52  are disposed at a similar radial distance from the longitudinal axis of the impeller, as each other. Typically, spring  54 , as well as proximal bushing  64  and distal bushing  58  of the impeller, define a lumen  62  therethrough (shown in  FIG. 6C ). 
     Reference is now made to  FIG. 7 , which is a schematic illustration of impeller  50  disposed inside frame  34  of blood-pump catheter  20 ,  70  or  90 , in accordance with some applications of the present invention. For some applications, within at least a portion of frame  34 , inner lining  39  lines the frame. In accordance with respective applications, the inner lining partially overlaps or fully overlaps with covering material  36  over the portion of the frame that the inner lining lines. In the application shown in  FIG. 7 , the inner lining lines (i.e., covers) the inside of a cylindrical portion  38  of the frame and covering material  36  does not cover the cylindrical portion of the frame. For such applications, the inner lining extends the blood-inlet opening or the blood-outlet opening of the blood pump. 
     As shown in  FIG. 7 , typically there is a gap G, between the outer edge of impeller  50  and inner lining  39 , even at a location at which the span of the impeller is at its maximum. For some applications, it is desirable that the gap between the outer edge of the blade of the impeller and the inner lining  39  be relatively small, in order for the impeller to efficiently pump blood. However, it is also desirable that a gap between the outer edge of the blade of the impeller and inner lining  39  be maintained substantially constant throughout the rotation of the impeller within frame  34 , for example, in order to reduce the risk of hemolysis. 
     For some applications, when the impeller and frame  34  are both disposed in non-radially-constrained configurations, gap G between the outer edge of the impeller and the inner lining  39 , at the location at which the span of the impeller is at its maximum, is greater than 0.05 mm (e.g., greater than 0.1 mm), and/or less than 1 mm (e.g., less than 0.4 mm), e.g., 0.05-1 mm, or 0.1-0.4 mm. For some applications, when the impeller is disposed in its non-radially-constrained configuration, the outer diameter of the impeller at the location at which the outer diameter of the impeller is at its maximum is more than 7 mm (e.g., more than 8 mm), and/or less than 10 mm (e.g., less than 9 mm), e.g., 7-10 mm, or 8-9 mm. For some applications, when frame  34  is disposed in its non-radially-constrained configuration, the inner diameter of cylindrical portion  38  of frame  34  (as measured from the inside of inner lining  39  on one side of the frame to the inside of inner lining on the opposite side of the frame) is greater than 7.5 mm (e.g., greater than 8.5 mm), and/or less than 10.5 mm (e.g., less than 9.5 mm), e.g., 7.5-10.5 mm, or 8.5-9.5 mm. For some applications, when the frame is disposed in its non-radially-constrained configuration, the outer diameter of cylindrical portion  38  of frame  34  is greater than 8 mm (e.g., greater than 9 mm), and/or less than 13 mm (e.g., less than 12 mm), e.g., 8-13 mm, or 9-12 mm. 
     Typically, axial shaft  92  passes through the axis of impeller  50 , via lumen  62  of the impeller. Further typically, the axial shaft is rigid, e.g., a rigid tube. For some applications, proximal bushing  64  of the impeller is coupled to the shaft such that the axial position of the proximal bushing with respect to the shaft is fixed, and distal bushing  58  of the impeller is slidable with respect to the shaft. The axial shaft itself is radially stabilized via proximal radial bearing  116  and distal radial bearing  118 . (It is noted that in  FIG. 7 , distal radial bearing  118  is not visible since it is disposed under distal-tip element  108 . However, the distal radial bearing is visible in embodiments shown in other figures, e.g.,  FIGS. 1A-C ,  2 A-E, and  4 A-B.) In turn, the axial shaft, by passing through lumen  62  defined by the impeller, radially stabilizes the impeller with respect to the inner surface of frame  34 , such that even a relatively small gap between the outer edge of the blade of the impeller and the inner surface of frame  34  (e.g., a gap that is as described above) is maintained, during rotation of the impeller. 
     Referring again to  FIGS. 6A-C , for some applications, the impeller includes a plurality of elongate elements  67  extending radially from central axial spring  54  to outer helical elongate elements  52 . The elongate elements are typically flexible but are substantially non-stretchable along the axis defined by the elongate elements. Further typically, each of the elongate elements is configured not to exert force upon the helical elongate element, unless force is acting upon the impeller that is causing the helical elongate element to move radially outward, such that (in the absence of the elongate element) a separation between the helical elongate element and the central axial spring would be greater than a length of the elongate element. For example, the elongate elements may include strings (such as polyester, and/or another polymer or a natural material that contains fibers) and/or wires (such as nitinol wires, and/or wires made of a different alloy, or a metal). 
     For some applications, the elongate elements  67  maintain the helical elongate element (which defines the outer edge of the impeller blade) within a given distance with respect to the central axial spring. In this manner, the elongate elements are configured to prevent the outer edge of the impeller from being forced radially outward due to forces exerted upon the impeller during the rotation of the impeller. The elongate elements are thereby configured to maintain the gap between the outer edge of the blade of the impeller inner lining  39  of frame  34 , during rotation of the impeller. Typically, more than one (e.g., more than two) and/or fewer than eight (e.g., fewer than four) elongate elements  67  are used in the impeller, with each of the elongate elements typically being doubled (i.e., extending radially from central axial spring  54  to an outer helical elongate element  52 , and then returning from the helical elongate element back to the central axial spring). For some applications, a plurality of elongate elements, each of which extends from the spring to a respective helical elongate element and back to the spring, are formed from a single piece of string or a single wire, as described in further detail hereinbelow. 
     For some applications, the impeller is manufactured in the following manner. Proximal bushing  64 , distal bushing  58 , and helical elongate elements  52  are cut from a tube of shape-memory material, such as nitinol. The cutting of the tube, as well as the shape setting of the shape-memory material, is typically performed such that the helical elongate elements are defined by the shape-memory material, e.g., using generally similar techniques to those described in US 2016/0022890 to Schwammenthal. Typically, spring  54  is inserted into the cut and shape-set tube, such that the spring extends along the length of the tube from at least the proximal bushing to the distal bushing. For some applications, the spring is inserted into the cut and shape-set tube while the spring is in an axially compressed state, and the spring is configured to be held in position with respect to the tube, by exerting a radial force upon the proximal and distal bushings. Alternatively or additionally, portions of the spring are welded to the proximal and distal bushings. For some applications, the spring is cut from a tube of a shape-memory material, such as nitinol. For some such applications, the spring is configured such that, when the spring is disposed in a non-radially-constrained configuration (in which the spring is typically disposed during operation of the impeller), there are substantially no gaps between windings of the spring and adjacent windings thereto. 
     For some applications, subsequent to spring  54  being inserted into the cut and shape-set tube, elongate elements  67 , as described hereinabove, are placed such as to extend between the spring and one or more of the helical elongate elements, for example, in the following manner. A mandrel (e.g., a polyether ether ketone (PEEK) and/or a polytetrafluoroethylene (PTFE) mandrel) is inserted through the lumen defined by the spring and the bushings. A string or a wire is then threaded such that it passes (a) from the mandrel to a first one of the helical elongate elements, (b) back from the first of the helical elongate elements to the mandrel, (c) around the mandrel, and to a second one of the helical elongate elements, (d) back from the second one of the helical elongate elements to the mandrel, etc. Once the string or the wire has been threaded from the mandrel to each of the helical elongate elements and back again, the ends of the string or the wire are coupled to each other, e.g., by tying them to each other. For some applications, sutures  53  (e.g., polyester sutures) are wound around the helical elongate elements, in order to facilitate bonding between the film of material (which is typically an elastomer, such as polyurethane, or silicone) and the helical elongate elements (which is typically a shape-memory alloy, such as nitinol), in a subsequent stage of the manufacture of the impeller. For some applications, sutures (e.g., polyester sutures, not shown) are wound around spring  54 . Typically, the sutures are configured to facilitate bonding between the film of material (which is typically an elastomer, such as polyurethane, or silicone) and the spring (which is typically a shape-memory alloy, such as nitinol), in the subsequent stage of the manufacture of the impeller. 
     Typically, at this stage, a structure  59  has been assembled that is as shown in  FIG. 6A . The structure includes the cut and shape-set tube that defines the proximal and distal bushings, the helical elongate elements, and the spring (and, optionally, the elongate elements, and the sutures). This structure is dipped into the material that defines film  56 . For some applications, the assembled structure is dipped into the material with the mandrel disposed through the lumen defined by the spring and the bushings, although it is noted that the mandrel is not shown in  FIG. 6A . Typically, the material from which the film is made is silicone and/or polyurethane (and/or a similar elastomer), and the assembled structure is dipped into the material, while the material is in an uncured, liquid state. Subsequently, the material is cured such that it solidifies, e.g., by being left to dry. Once the material has dried, the mandrel is typically removed from the lumen defined by the bushings and the spring. 
     The result of the process described above is typically that there is a continuous film of material extending between each of the helical elongate elements to the spring, and also extending along the length of the spring, such as to define a tube, with the spring embedded within the tube. The portions of the film that extend from each of the helical elongate elements to the spring define the impeller blades. For applications in which the impeller includes elongate elements  67 , the elongate elements are typically embedded within these portions of the film. 
     Typically, impeller  50  is inserted into the subject&#39;s body, while impeller  50  is in a radially-constrained configuration. In the radially-constrained configuration, both helical elongate elements  52  and central axial spring  54  become axially elongated, and radially constrained. Typically film  56  of the material (e.g., silicone and/or polyurethane) changes shape to conform to the shape changes of the helical elongate elements and the axial support spring, both of which support the film of material. Typically, using a spring to support the inner edge of the film allows the film to change shape without the film becoming broken or collapsing, due to the spring providing a large surface area to which the inner edge of the film bonds. For some applications, using a spring to support the inner edge of the film reduces a diameter to which the impeller can be radially constrained, relative to if, for example, a rigid shaft were to be used to support the inner edge of the film, since the diameter of the spring itself can be reduced by axially elongating the spring. 
     For some applications, proximal bushing  64  of impeller  50  is coupled to axial shaft  92  such that the axial position of the proximal bushing with respect to the shaft is fixed, and distal bushing  58  of the impeller is slidable with respect to the shaft. For some applications, when the impeller is radially constrained for the purpose of inserting the impeller into the subject&#39;s body or for the purpose of withdrawing the impeller from the subject&#39;s body, the impeller axially elongates by the distal bushing sliding along the axial shaft distally. Subsequent to being released inside the subject&#39;s body, the impeller assumes its non-radially-constrained configuration (in which the impeller is typically disposed during operation of the impeller), as shown in  FIGS. 6A-C . 
     It is noted that, for illustrative purposes, in some of the figures, impeller  50  is shown without including all of the features of the impeller as shown and described with respect to  FIGS. 6A-C . The scope of the present application includes using an impeller with any of the features shown and described with respect to  FIGS. 6A-C  in combination with any of the apparatus and methods described herein. 
     Reference is now made to  FIGS. 6D, 6E, and 6F , which are schematic illustration of impeller  50  or portions thereof, in accordance with some applications of the present invention. As described hereinabove, for some applications, impeller  50  includes sutures  53 . Sutures  53  are wound around the helical elongate elements  52  and are configured to facilitate bonding between the film of material (which is typically an elastomer, such as polyurethane, or silicone) and the helical elongate element (which is typically a shape-memory alloy, such as nitinol). 
     As an alternative or in addition to sutures  53 , for some applications, coils  68  are wound around (or placed over) the helical elongate elements, as shown in  FIG. 6D . For example, a tightly-wound coil (e.g., a tightly-wound nitinol coil) may be wound around (or placed around) each of the helical elongate elements. The coil typically facilitates bonding between the film of material and the helical elongate element by increasing the surface area to which the material bonds at the interface between the material and the helical elongate element. For some applications, structure  59  is formed modularly (e.g., as described hereinbelow with reference to  FIG. 6F .) For some such applications, the coils are placed around each of the elongate elements  52  (e.g., by sliding the entire coil over the elongate element in a single action), prior to the elongate elements being coupled to the proximal and distal bushings of the impeller. 
     As a further alternative to or in addition to sutures  53 , for some applications, sleeves  69  are placed around the helical elongate elements, as shown in  FIG. 6E . For example, such sleeves may be made of a polymer, such as polyester. The sleeves typically facilitate bonding between the film of material and the helical elongate elements by increasing the surface area to which the material bonds at the interfaces between the material and the helical elongate elements. For some applications, the sleeve acts as a mediator between a material from which the elongate elements are made, which typically has a relatively high stiffness (and is typically nitinol), and the material from which film  56  is made, which is typically an elastomer having a relatively low stiffness. The sleeve thereby enhances the strength of the coupling between the material and the helical elongate elements, when the material dries. For some applications, sleeves  69  are applied to structure  59 . For some such applications, longitudinal slits are formed in the sleeves in order to allow the sleeves to be placed around the helical elongate elements  52 . Subsequent to being placed around helical elongate elements  52  the slits are closed (e.g., by suturing or adhering the slits closed). For some applications, structure  59  is formed modularly (e.g., as described hereinbelow with reference to  FIG. 6F .) For some such applications, the sleeves are placed around elongate elements  52 , prior to the elongate elements being coupled to the proximal and distal bushings of the impeller. 
     As yet a further alternative to or in addition to sutures  53 , for some applications, elongate elements  52  are shaped to have a rounded (e.g., a circular) cross section, as shown in the right portion of  FIG. 6F  (which shows a cross-sectional view of an elongate element having a rounded cross-section). The left portion of  FIG. 6F  shows a cross-sectional view of elongate element  52  with material of film  56  coupled to the elongate element, in a case in which the elongate element has a non-rounded cross section (e.g., a square or a rectangular cross section). As shown, it is sometimes the case that the material (e.g., the silicone and/or the polyurethane) from which the film is made forms a thinner layer at the corners of an elongate element having a non-rounded cross-section. By contrast as shown in the left portion of  FIG. 6F , when the elongate element has a rounded cross section, the material typically forms a layer having a substantially uniform thickness at the interface with the elongate element. Therefore, for some applications, the elongate elements have rounded cross sections. 
     For some applications, proximal and distal bushings  64 ,  58  and elongate elements  52  are cut from an alloy tube, e.g., as described hereinabove. For such applications, after the tube is cut, the elongate elements typically have non-rounded edges. Therefore, for some applications, subsequent to the tube being cut, the edges of the elongate elements are rounded, for example, using grinding, sandblasting, tumble finishing, etching, plasma, surface-charging, and/or by adding rounded edges to the elongate elements. Alternatively, the proximal and distal bushings and the elongate elements may be formed in a modular manner, and may subsequently be coupled to each other (e.g., via welding, and/or swaging). For some such applications, the elongate elements that are coupled to the proximal and distal bushings have rounded cross sections. As described hereinabove with reference to  FIG. 6E , for some applications, sleeves  69  are placed on the elongate elements prior to the elongate elements being coupled to the proximal bushing and/or prior to the elongate elements being coupled to the distal bushing. 
     For some applications, alternative or additional techniques are used to facilitate bonding between the film of material and the helical elongate elements. For example, the helical elongate elements may be treated using a surface treatment (such as, grinding, sandblasting, tumble finishing, etching, plasma, surface-charging, etc.), in order to roughen the outer surface of the helical elongate elements. 
     In accordance with the above description of  FIGS. 6A-F , for some applications of the present invention, impeller  50  is manufactured by forming a structure having first and second bushings  64 ,  58  at proximal and distal ends of the structure, the first and second bushings being connected to one another by at least one elongate element  52 . The at least one elongate element is made to radially expand and form at least one helical elongate element, at least partially by axially compressing the structure. An elastomeric material is coupled to the at least one helical elongate element, such that the at least one helical elongate element with the elastomeric material coupled thereto defines a blade of the impeller. Typically, the coupling is performed such that a layer of the material is disposed around a radially outer edge of the at least one helical elongate element, the layer of material forming the effective edge of the impeller blade (i.e., the edge at which the impeller&#39;s blood-pumping functionality substantially ceases to be effective). Further typically, the method includes performing a step to enhance bonding of the elastomeric material to the at least one helical elongate element in a manner that does not cause a protrusion from the effective edge of the impeller blade. For example, sutures  53  may be placed within grooves defined by the at least one helical elongate element, such that the sutures do not protrude from the radially outer edge of the helical elongate element, the sutures being configured to enhance bonding of the elastomeric material to the at least one helical elongate element. Alternatively or additionally, tightly-wound coil  68  may be placed around the at least one helical elongate element, such that the elastomeric material forms a substantially smooth layer along a radially outer edge of the coil, the coil being configured to enhance bonding of the elastomeric material to the at least one helical elongate element. Further alternatively or additionally, sleeve  69  may be placed around the at least one helical elongate element, such that the elastomeric material forms a substantially smooth layer along a radially outer edge of the sleeve, the sleeve being configured to enhance bonding of the elastomeric material to the at least one helical elongate element. For some applications, a rounded cross section is provided to the at least one helical elongate element, such that the elastomeric material forms a layer having a substantially uniform thickness at an interface of the elastomeric material with the helical elongate element. As noted hereinabove, it is typically desirable that gap G between the outer edge of the blade of the impeller and the inner lining  39  (shown in  FIG. 7 ) be relatively small. Therefore, it is desirable that there be no protrusion from the effective edge of the impeller blade, since this would occupy some of the gap between the outer edge of the impeller blade (thereby requiring a larger gap), without increasing the effectiveness of the blood-pumping functionality of the impeller. 
     Reference is now made to  FIGS. 6G and 6H , which are schematic illustrations of elongate elements  67  extending between each of the helical elongate elements  52  and spring  54 , in accordance with some applications of the present invention. For some applications, a respective looped elongate element  67  extends between each of the helical elongate elements and the spring. Typically, the looped elongate elements are closed loops that have predefined lengths and are (substantially) non-stretchable. The lengths of the looped elongate elements are typically predefined, such as to maintain the helical elongate element (which defines the outer edge of the impeller blade) within a given distance with respect to the central axial spring, and to thereby maintain the gap between the outer edge of the blade of the impeller and the inner surface of frame  34 , during rotation of the impeller, as described hereinabove. For some applications, the impeller is formed by looping first ends of the looped elongate elements around each of the helical elongate elements as indicated in the enlarged portions of  FIGS. 6G and 6H . Subsequently, spring  54  is inserted through proximal and distal bushings  64 ,  58 , and through second ends of the looped helical elongate elements. 
     For some applications, at a longitudinally-central location of spring  54 , the spring is shaped to define a tube  88  (i.e., without windings), as shown in  FIGS. 6G and 6H . Typically, the second ends of the looped elongate elements loop around the tube at the longitudinally-central location of the spring. Typically, this reduces a risk of the looped elongate elements tearing, relative to if the second ends of the looped elongate elements were to loop around windings of the spring. For some applications (not shown), the tube defines a groove therein and the second ends of the looped elongate elements are configured to be held within the groove. 
     For some applications, the looped elongate element is looped around the body of the helical elongate element, as shown in the enlarged portions of  FIG. 6G . Enlargements A and B of  FIG. 6G  show two alternative ways in which the looped elongate element is looped around the body of the helical elongate element. For some applications, the looped elongate element is looped around the outer surface of the helical elongate element, as shown in enlargement A. Alternatively, the helical elongate elements define grooves  48  on their outer surfaces, and the looped elongate element is looped around a groove  48  (such as to become embedded within the groove), as shown in enlargement B. By embedding the looped elongate element within the grooves, the looped elongate element typically does not add to the outer profile of the impeller, and the outer profile of the impeller is defined by the outer surfaces of the helical elongate elements. 
     For some applications, the helical elongate element is shaped to define two holes  91  disposed in close proximity to each other, and the looped elongate element may be looped through the holes, as shown in the enlarged portions of  FIG. 6H . Enlargements A and B of  FIG. 6H  show two alternative ways in which the looped elongate element is looped through holes  91 . For some applications, the looped elongate element is looped around the outer surface of the helical elongate element and through holes  91 , as shown in enlargement A. Alternatively, the helical elongate elements define grooves  48  on their outer surfaces, and the looped elongate element is looped around groove  48  and through holes  91  (such as to become embedded within the groove), as shown in enlargement B. By embedding the looped elongate element within the grooves, the looped elongate element typically does not add to the outer profile of the impeller, and the outer profile of the impeller is defined by the outer surfaces of the helical elongate elements. 
     Reference is now made to  FIGS. 8A and 8B , which are schematic illustrations of impeller  50  and frame  34  of blood-pump catheter  20 ,  70 , or  90 , respectively in non-radially-constrained and radially-constrained states thereof, in accordance with some applications of the present invention. The impeller and the frame are typically disposed in radially-constrained states during the transcatheteral insertion of the impeller and the frame into the subject&#39;s body, and are disposed in non-radially-constrained states during operation of the blood-pump catheter. As described hereinabove, typically covering material  36  extends from frame  34 . However, for illustrative purposes, the frame and the impeller are shown in the absence of covering material  36  in  FIGS. 8A-B . As indicated in  FIG. 8B , the frame and the impeller are typically maintained in radially-constrained configurations by a delivery catheter  143 . 
     Reference is also made to  FIG. 8C , which shows a typical bearing assembly that is used in prior art axial impeller-based blood pumps.  FIG. 8C  is shown for the purpose of acting as a point of reference for some of the applications of the invention described herein. As shown in  FIG. 8C , a bearing assembly typically includes a radial bearing (indicated by ellipse  200 ) and a thrust bearing (indicated by circle  202 ). The radial bearing is configured to reduce radial motion of the impeller, by maintaining the axis of the impeller at a given radial position. In response to an impeller pumping blood in a first direction, forces acting upon the impeller typically push the impeller to move in the opposite direction to the first direction. The purpose of a thrust bearing is to oppose such motion of the impeller and to maintain the axial position of the impeller. In the example shown in  FIG. 8C , in response to the impeller pumping blood in the direction of arrow  204 , the impeller gets pushed in the direction of arrow  206 , and the thrust bearing opposes this motion. Typically, due to the frictional forces that are exerted upon them, bearings undergo a substantial amount of heating and wear. Thrust bearings are typically exposed to substantial heating and wear, due to the fact that the frictional forces that are exerted upon them are typically spread over opposing surfaces having a smaller contact area between them, than is the case for radial bearings. 
     As described hereinabove, typically, axial shaft  92  passes through the axis of impeller  50 , via lumen  62  of the impeller. Typically, proximal bushing  64  of the impeller is coupled to the shaft via a coupling element  65  such that the axial position of the proximal bushing with respect to the shaft is fixed, and distal bushing  58  of the impeller is slidable with respect to the shaft. The axial shaft itself is radially stabilized via proximal radial bearing  116  and distal radial bearing  118 . 
     Typically, a coupling portion  30  of frame  34  is coupled to proximal radial bearing  116 , for example, via snap-fit coupling, and/or via welding. Typically, at the distal end of frame  34  distal strut junctions  33  are placed into grooves defined by the outer surface of distal radial bearing  118 , the grooves being shaped to conform with the shapes of the distal strut portions. The proximal end of a distal-tip element  108  typically holds the distal strut portions in their closed configurations around the outside of distal radial bearing  118 , as shown. For some applications, the device includes a distal extension  121  that extends distally from the distal radial bearing. Typically, the extension is configured to stiffen a region of the distal-tip element into which the distal end of shaft  92  moves. 
     As described above, axial shaft  92  is radially stabilized via proximal radial bearing  116  and distal radial bearing  118 . In turn, the axial shaft, by passing through lumen  62  defined by the impeller, radially stabilizes the impeller with respect to the inner surface of frame  34 , such that even a relatively small gap between the outer edge of the blade of the impeller and the inner surface of frame  34  or inner lining  39  (e.g., a gap that is as described above) is maintained, during rotation of the impeller, as described hereinabove. For some applications, axial shaft  92  is made of stainless steel, and proximal bearing  116  and/or distal bearing  118  are made of hardened steel. Typically, when crimping (i.e., radially constraining) the impeller and the frame for the purpose of inserting the impeller and the frame into the subject&#39;s body, distal bushing  58  of the impeller is configured to slide along the axial shaft in the distal direction, such that the impeller becomes axially elongated, while the proximal bushing remains in an axially fixed position with respect to the axial shaft. More generally, the impeller changes from its radially-constrained configuration to its non-radially-constrained configuration, and vice versa, by the distal bushing sliding over the axial shaft, while the proximal bushing remains in an axially fixed position with respect to the axial shaft. 
     Typically, the impeller itself is not directly disposed within any radial bearings or thrust bearings. Rather, bearings  116  and  118  act as radial bearings with respect to the axial shaft. Typically, the blood-pump catheter does not include any thrust bearing that is configured to be disposed within the subject&#39;s body and that is configured to oppose thrust generated by the rotation of the impeller. For some applications, one or more thrust bearings are disposed outside the subject&#39;s body (e.g., within a motor unit), and opposition to thrust generated by the rotation of the impeller is provided solely by the one or more thrust bearings disposed outside the subject&#39;s body. For some applications, a mechanical element and/or a magnetic element is configured to maintain the impeller within a given range of axial positions. For example, a magnet that is disposed at the proximal end of the drive cable may be configured to maintain the impeller within a given range of axial positions. 
     For some alternative applications of the present invention, a thrust bearing is used to maintain the axial position of the impeller, and the thrust bearing is disposed within a portion of the blood-pump catheter that is proximal to the impeller, such that the thrust bearing does not come into contact with the subject&#39;s blood. For example, the thrust bearing may be disposed within an outer tube in which the drive shaft of the impeller is disposed. Alternatively or additionally, the thrust bearing may be disposed outside the subject&#39;s body. For some such applications, since the thrust bearing is disposed outside the subject&#39;s body, the thrust bearing&#39;s dimensions are not constrained by virtue of needing to be deployed within a small anatomical location. Therefore, in such cases, the contact area between the two opposing surfaces of the thrust bearing is typically greater than 20 square mm. For some applications (not shown), the thrust bearing is disposed distally to the impeller and in contact with the subject&#39;s blood, such that the thrust bearing is cooled by the subject&#39;s blood. 
     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. Computer processor  10  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, computer processor  10  typically acts as a special-purpose, blood-pump computer processor. 
     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: 
     U.S. Ser. No. 16/750,354 to Tuval, filed Jan. 23, 2020, entitled “Distal tip element for a ventricular assist device;” 
     US 2019/0209758 to Tuval, which is a continuation of International Application No. PCT/IB2019/050186 to Tuval (published as WO 19/138350), entitled “Ventricular assist device, filed Jan. 10, 2019, which claims priority from:
         U.S. Provisional Patent Application 62/615,538 to Sohn, entitled “Ventricular assist device,” filed Jan. 10, 2018;   U.S. Provisional Patent Application 62/665,718 to Sohn, entitled “Ventricular assist device,” filed May 2, 2018;   U.S. Provisional Patent Application 62/681,868 to Tuval, entitled “Ventricular assist device,” filed Jun. 7, 2018; and   U.S. Provisional Patent Application 62/727,605 to Tuval, entitled “Ventricular assist device,” filed Sep. 6, 2018;       

     US 2019/0269840 to Tuval, which is the US national phase of International Patent Application PCT/IL2017/051273 to Tuval (published as WO 18/096531), filed Nov. 21, 2017, entitled “Blood pumps,” which claims priority from U.S. Provisional Patent Application 62/425,814 to Tuval, filed Nov. 23, 2016; 
     US 2019/0175806 to Tuval, which is a continuation of International Application No. PCT/IL2017/051158 to Tuval (published as WO 18/078615), entitled “Ventricular assist device,” filed Oct. 23, 2017, which claims priority from U.S. 62/412,631 to Tuval filed Oct. 25, 2016, and U.S. 62/543,540 to Tuval, filed Aug. 10, 2017; 
     US 2019/0239998 to Tuval, which is the US national phase of International Patent Application PCT/IL2017/051092 to Tuval (published as WO 18/061002), filed Sep. 28, 2017, entitled “Blood vessel tube,” which claims priority from U.S. Provisional Patent Application 62/401,403 to Tuval, filed Sep. 29, 2016; 
     US 2018/0169313 to Schwammenthal, which is the US national phase of International Patent Application PCT/IL2016/050525 to Schwammenthal (published as WO 16/185473), filed May 18, 2016, entitled “Blood pump,” which claims priority from U.S. Provisional Patent Application 62/162,881 to Schwammenthal, filed May 18, 2015, entitled “Blood pump;” 
     US 2017/0100527 to Schwammenthal, which is the US national phase of 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;” 
     U.S. Pat. No. 10,039,874 to Schwammenthal, which is the US national phase of 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. Pat. No. 9,764,113 to Tuval, issued Sep. 19, 2017, 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 
     U.S. Pat. No. 9,597,205 to Tuval, which is the US national phase of 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.