How to purge a blood pump

Abstract

Devices, systems, and methods are provided based on techniques for purging a blood pump. The techniques may include providing a blood pump having an impeller and providing a fluid including bicarbonate. The techniques may include flowing the fluid through a first gap between a bearing and an outer surface of a rotatable shaft coupled to the impeller, the bearing and gap being disposed within a lumen of a tubular member, and depending on the fluid flow rate and the impeller speed, the fluid may flow through the first gap and into a second gap between the bearing and a surface of the impeller facing the bearing. The bicarbonate reduces denaturation and adsorption of blood proteins in any one or more gaps.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 312,277, filed February 21, 2022, and U.S. Provisional Patent Application No. 63 / 398,991, filed August 18, 2022, each of which is incorporated by reference in its entirety into this document.

[0002] Technical Field

[0002] The present disclosure relates to blood pumps, particularly intravascular blood pumps, for assisting blood flow in a patient's blood vessels, as well as methods for purging such pumps during operation while inserted within a patient's body. [Background technology]

[0003] background

[0003] Different types of blood pumps are known, such as axial, centrifugal, or mixed blood pumps, where blood flow is induced by both axial and radial forces. One example of a blood pump is the Impella line of blood pumps (e.g., Impella 2.5®, Impella CP®, Impella 5.5®, etc.) manufactured by Abiomed (Danvers, Mass.). Intravascular blood pumps can be inserted into a patient's blood vessel, such as the aorta, by a catheter.

[0004]

[0004] In some pump designs, a purge fluid is deployed to prevent blood from entering the mechanism and reduce the impact of blood on the pump mechanism. Typically, the purge fluid contains an anticoagulant, such as heparin (typically the sodium salt of heparin). Heparin is believed to prevent blood from clotting in the gaps between pump components, such as the impeller shaft and the housing. Heparin is a commonly used anticoagulant that is typically administered in controlled doses.

[0005] In one example, the purge fluid is delivered by a purge cassette, and the purge fluid enters an internal purge lumen that carries the purge fluid through a filter assembly, into the blood pump catheter, through the catheter, and into a purge channel in the motor assembly. The flow of the purge fluid is regulated by an automatic controller.

[0006]

[0006] However, such conventional techniques generally lead to the denaturation of circulating proteins and the deposition of biomaterials in the gaps between pump components. Summary of the Invention [Means for solving the problem]

[0007] overview

[0007] In various aspects, a blood pump may be provided. The blood pump may include a tubular body having a distal end and a proximal end and a lumen extending therethrough, the lumen having a central axis. An impeller may be coupled to a flexible shaft at least partially disposed within the lumen. A bearing may be disposed within the lumen and about a portion of the flexible shaft. The bearing may be configured to form a first gap in a radial direction between an outer surface of the flexible shaft and an inner surface of the bearing, and a second gap in an axial direction between the bearing and a surface of the impeller facing the bearing. The blood pump may be configured to flow a fluid including bicarbonate through the first gap and into the second gap. In some embodiments, the first gap may be between 4 and 9 μm. In some embodiments, the second gap may be between 90 and 110 μm.

[0008] In some embodiments, the bicarbonate may be sodium bicarbonate. In some embodiments, the fluid may include between 12.5 mEq / L and 100 mEq / L of sodium bicarbonate, such as between 25 mEq / L and 50 mEq / L of sodium bicarbonate. In some embodiments, the fluid may include a pharmaceutical therapeutic or prophylactic agent, such as an anticoagulant or a patient specific pharmaceutical therapeutic (e.g., a small molecule drug). In some embodiments, the fluid may further include aqueous dextrose.

[0009] In some embodiments, the concentration of the bicarbonate-containing fluid in the volume of the space defined by the first gap may be at least 50% of the total fluid in the first gap. In some embodiments, the first gap may be filled with a bicarbonate-containing fluid. In some embodiments, the concentration of the bicarbonate-containing fluid in the volume of the space defined by the second gap may be no more than 40% of the total fluid in the second gap.

[0010] In various aspects, a system may be provided. The system may include a pump as disclosed herein, and may include a purge bag containing a fluid including bicarbonate, where the purge bag may be operably coupled to the pump. In some embodiments, the first gap may be between 4 and 9 μm. In some embodiments, the second gap may be between 90 and 110 μm.

[0011] In some embodiments, the bicarbonate may be sodium bicarbonate. In some embodiments, the fluid may include between 12.5 mEq / L and 100 mEq / L of sodium bicarbonate, such as between 25 mEq / L and 50 mEq / L of sodium bicarbonate. In some embodiments, the fluid may include a pharmaceutical therapeutic or prophylactic agent, such as an anticoagulant or a patient-specific pharmaceutical therapeutic (e.g., a small molecule drug). In some embodiments, the fluid may further include hydrous dextrose.

[0012] In some embodiments, the concentration of the bicarbonate-containing fluid in the volume of the space defined by the first gap may be at least 50% of the total fluid in the first gap. In some embodiments, the first gap may be filled with a bicarbonate-containing fluid. In some embodiments, the concentration of the bicarbonate-containing fluid in the volume of the space defined by the second gap may be no more than 40% of the total fluid in the second gap.

[0013]

[0013] In various aspects, a kit may be provided. The kit may include a pump as disclosed herein. The kit may include a purge bag containing a fluid including bicarbonate. The kit may include a purge cassette configured to couple the purge bag to the pump.

[0014] In various aspects, a method for purging a pump may be provided. The method may include providing a pump having an impeller as disclosed herein and a fluid including bicarbonate. The method may include flowing the fluid through a first gap between a bearing and an outer surface of a rotatable shaft coupled to the impeller, the bearing and the gap being disposed within a lumen of a tubular member.

[0015] In some embodiments, the bicarbonate may be sodium bicarbonate. In some embodiments, the fluid may include between 12.5 mEq / L and 100 mEq / L of sodium bicarbonate, such as between 25 mEq / L and 50 mEq / L of sodium bicarbonate. In some embodiments, the fluid may include a pharmaceutical therapeutic or prophylactic agent, such as an anticoagulant or a patient-specific pharmaceutical therapeutic (e.g., a small molecule drug). In some embodiments, the fluid may further include hydrous dextrose.

[0016]

[0016] In some embodiments, the method may include controlling the flow rate of the fluid so that the concentration of the bicarbonate-containing fluid within the volume of space defined by the first gap is at least 50% of the total fluid within the first gap.

[0017] In some embodiments, flowing the fluid through the first gap may include flowing the fluid through a second gap after passing through the first gap, the second gap being between the bearing and a surface of the impeller facing the bearing. In some embodiments, the method may include controlling a flow rate of the fluid such that a concentration of the fluid including bicarbonate in a volume of space defined by the second gap is less than or equal to 40% of the total fluid in the second gap.

[0018]

[0018] In some embodiments, the method may include automatically adjusting the flow rate of the fluid based on operating characteristics of the pump.

[0019]

[0019] In various aspects, a medical device may be provided. The medical device may include a tubular body having a distal end and a proximal end, and a lumen may extend through the tubular body. The lumen may have a central axis. The medical device may include an outer sheath disposed about the tubular body. The medical device may include a first gap formed radially between an outer surface of the tubular body and an inner surface of the outer sheath. The medical device may include a second gap formed axially between the tubular body and the outer sheath. A fluid including bicarbonate may be configured to flow through the first and second gaps to purge the first and second gaps. In some embodiments, the first gap may be between 4 and 9 μm. In some embodiments, the second gap may be between 90 and 110 μm.

[0020] In some embodiments, the bicarbonate may be sodium bicarbonate. In some embodiments, the fluid may include between 12.5 mEq / L and 100 mEq / L of sodium bicarbonate, such as between 25 mEq / L and 50 mEq / L of sodium bicarbonate. In some embodiments, the fluid may include a pharmaceutical therapeutic or prophylactic agent, such as an anticoagulant or a patient-specific pharmaceutical therapeutic (e.g., a small molecule drug). In some embodiments, the fluid may further include hydrous dextrose.

[0021] In some embodiments, the concentration of the bicarbonate-containing fluid in the volume of the space defined by the first gap may be at least 50% of the total fluid in the first gap. In some embodiments, the first gap may be filled with a bicarbonate-containing fluid. In some embodiments, the concentration of the bicarbonate-containing fluid in the volume of the space defined by the second gap may be no more than 40% of the total fluid in the second gap.

[0022] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will now be described, by way of example only, with reference to the accompanying drawings, which are not intended to be drawn to scale. In the drawings, each identical or nearly identical component shown in various drawings is represented by a similar reference number. For clarity, not every component may be referenced in every drawing. [Brief description of the drawings]

[0023] [Figure 1]

[0023] The blood flow and purge flow through the gap between the shaft and the housing in the pump are shown. [Diagram 2]

[0024] FIG. 1 is a schematic diagram of an intravascular blood pump inserted in front of the left ventricle with its inflow cannula positioned within the left ventricle. [Diagram 3]

[0025] 1 is a schematic longitudinal cross-sectional view of an exemplary prior art blood pump. [Figure 4]

[0026] FIG. 4 is an enlarged view of a portion of the blood pump of FIG. 3 according to a second embodiment. [Diagram 5]

[0027] 13 is an enlarged view of a portion of a blood pump according to another embodiment. [Figure 6A]

[0028] A simulated condition of a portion of a blood pump shows bicarbonate-reduced coagulability of blood in the presence of heparin, where coagulation parameters include clotting initiation time ("R Time") and clot formation time ("K Time") in FIG. 6A. [Figure 6B]

[0028] Simulation conditions of a portion of a blood pump show bicarbonate-decreased coagulability of blood in the presence of heparin, where the coagulation parameters include the clotting rate (alpha angle) in FIG. 6B. [Figure 6C]

[0029] 1 shows TEG-ACT data demonstrating the synergistic effect on clotting in the presence of heparin and bicarbonate. [Figure 7]

[0030] 1 illustrates a kit according to some embodiments. [Figure 8]

[0031] 1 is a flow chart of one embodiment of a method. [Figure 9]

[0032] 1 illustrates an embodiment of a system. [Figure 10]

[0033] 1 is a schematic diagram of a medical device according to some embodiments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Detailed Description

[0034] Blood pumps are deployed in patients who require critical care and life support. It is therefore important to improve any aspect of the device that may adversely affect pump operation. For example, in some pump designs, a purge fluid may be deployed to prevent blood from entering the pump mechanism and reduce the impact of blood on the pump mechanism. Generally, the purge fluid includes an anticoagulant, such as heparin (e.g., a sodium salt of heparin). Without wishing to be bound by theory, it is believed that the heparin prevents blood from clotting in the gaps between pump components, such as between the impeller shaft and the housing. Heparin is a commonly used anticoagulant that is typically administered in controlled doses.

[0025]

[0035] In some cases, however, the physician may not want to administer heparin to the patient's blood via the purge fluid. For example, during any kind of surgical procedure, administering heparin may be counterproductive since it may prevent blood clotting and therefore healing and hemostasis. Also, the amount of heparin administered to the patient's blood, together with the heparin in the purge fluid, is difficult to control for various reasons. In some cases, the amount of heparin may be more than what is desired by the physician, and the amount of heparin administered to the patient may be difficult to precisely control. Therefore, the physician may prefer to provide heparin to the patient separately from the operation of the blood pump, when necessary (and therefore only in the amount required). Furthermore, some patients are heparin intolerant, as they are prone to heparin-induced thrombocytopenia (HIT). Therefore, heparin-containing purges are not suitable for these patients at all. Also, heparin salts may cause unwanted wear on the metallic pump bearings. Therefore, there is a need for an intravascular blood pump that can run on a purge fluid that does not contain heparin, or at least has reduced content, if desired.

[0026]

[0036] It has been found that certain fluids, either by themselves or in combination with heparin (eg, systemic heparinization), can provide significant benefits when present within certain gaps within an intravascular blood pump.

[0027]

[0037] As disclosed herein, the inventors have recognized the benefits of a purge solution containing sodium bicarbonate. The inventors have also recognized the benefits of using such a purge solution with systemically anticoagulated blood. For example, as described herein, the inventors have recognized the synergistic or supportive role of sodium bicarbonate during purging when used with systemic anticoagulation therapy as part of an overall anticoagulation strategy. In some embodiments, the sodium bicarbonate purge fluid may provide environmental conditions that delay the onset of clotting and inhibit fibrin formation. This, in turn, may reduce the overall rate of clot formation where there is a mixture of purge fluid and systemically anticoagulated blood in the axial gap.

[0028]

[0038] In some embodiments, the purge fluid may include aqueous glucose. For example, in some embodiments, the purge fluid may include 5% glucose in an aqueous solution. In some embodiments, the purge fluid may include 12.5 mEq / L to 100 mEq / L sodium bicarbonate. In some embodiments, the purge fluid may include 25 mEq / L to 50 mEq / L sodium bicarbonate.

[0029]

[0039] In some embodiments, the purge fluid may also include a pharmaceutical therapeutic or prophylactic agent. In some embodiments, the pharmaceutical therapeutic or prophylactic agent may be an anticoagulant (e.g., heparin). In some embodiments, the pharmaceutical therapeutic or prophylactic agent may include one or more small molecule drugs that may be used to treat a patient, such as blood thinning agents, anti-inflammatory agents, etc.

[0030]

[0040] Turning now to the drawings, Figure 1 shows a pump 100 having a tubular member having a drive section 110 and a pump section 130, and a catheter 115 attached to a proximal end 120 of the drive section 110 (e.g., the end of the drive section closer to the physician, i.e., the "rear end" of the drive section) and having a line extending to the drive section 110 for powering, with the pump section 130 fastened to a distal end 125 of the drive section. The drive section 110 may include a motor housing 150 having an electric motor 151 disposed therein, the motor shaft 160 of the electric motor projecting distally from the drive section 110 into the pump section 130. The pump section 130 may in turn include a tubular pump housing 165 within which an impeller 170 rotates, the impeller mounted on the end of the motor shaft 160 projecting from the motor housing 150. The motor shaft 160 may be mounted in two bearings 171, 172 in the motor housing, which are maximally displaced from each other to ensure a true and precisely centered guidance of the impeller 170 in the motor housing 150. Different bearing types may be used in different pump designs. As shown in FIG. 1, the bearing 171 may include a radial ball bearing and the bearing 172 may include an axial-radial plain bearing. As shown in FIG. 1, the blood 140 may exit the outflow cage of the pump housing 165. Blood that would otherwise enter the motor housing 150 is further neutralized by the purge fluid 135 that is passed through the motor housing and the impeller-side seal bearing. Thus, the purge fluid may pass through the gap of the impeller-side radial plain bearing to prevent blood from entering the housing. This is done by having a purge fluid pressure higher than the pressure present in the blood.

[0031]

[0041] As shown in FIG. 1, the purge fluid 135 may fill the pump's motor housing 150 and form a lubricating film within the pump's bearings 171, 172. For example, as described in U.S. Patent Application Publication No. 2015 / 0051436, the purge fluid 135 may form a lubricating film within the bearing gap 180 of the pump's axial plain bearing. The purge fluid is described as being supplied through a purge fluid supply line and flowing through a radial bearing gap 173 located at the distal end of the motor housing 150 and then through the bearing gap 180 of the axial plain bearing. Purge fluid supplied in this manner may contribute to hemodilution and shorten blood retention time under the impeller 170.

[0032]

[0042] In some embodiments, to ensure that the purge fluid 135 reaches the distal radial bearing 172 at a pressure higher than the existing blood pressure, at least one of the surfaces forming the bearing gap of the axial plain bearing is provided with a channel penetrating the bearing gap 180 from the radially outer side to the radially inner side, so that the purge fluid can flow through this channel to the distal radial bearing. This channel does not necessarily have to be in the bearing gap surface, but can also be realized as a separate channel or bore. However, in some embodiments, providing a channel in one of the bearing gap surfaces can be advantageous in that the lubricant film in the bearing gap does not heat up too much, since part of the lubricant film is continuously replaced by the later flowing purge fluid. In some embodiments, the channel is in the stationary bearing gap surface in order to minimize the radial transport capacity.

[0033]

[0043] FIG. 2 illustrates the use of a blood pump to support the left ventricle in this particular example. As shown in this drawing, the blood pump may include a catheter 14 and a pump device 10 attached to the catheter 14. The pump device 10 may include a motor section 11 and a pump section 12, which are arranged coaxially one behind the other to create a rod-like configuration. The pump section 12 may include an extension in the form of a flexible suction hose 13, a tubular member often referred to as a "cannula." The pump section 12 may be provided with an impeller to cause blood flow from a blood flow inlet to a blood flow outlet, and the rotation of the impeller is caused by an electric motor located in the motor section 11. The blood pump may be arranged to be primarily within the ascending aorta 15b, which leads to the aortic arch 15a. In the closed state, the aortic valve 18 abuts the outside of the pump section 12 or its suction hose 13, which is substantially within the left ventricle 17. The blood pump with suction hose 13 in front can be advanced to the position shown by advancing catheter 14, optionally using a guidewire, so that suction hose 13 can be passed back through aortic valve 18 so that blood is drawn through suction hose 13 and pumped into aorta 16.

[0034]

[0044] As will be appreciated, the use of the blood pump need not be limited to the application depicted in Fig. 2, which is merely a representative example of an application. Thus, the pump may also be inserted through other peripheral blood vessels, such as the subclavian artery. Alternatively, a reverse application to the right ventricle may be envisioned. As will be further appreciated, in other embodiments, other suitable pump arrangements may be used.

[0035]

[0045] FIG. 3 shows an exemplary embodiment of a blood pump such as that described in US Patent Application Publication No. 2015 / 0051436 A1, which may also be suitable for use in the context of the present disclosure, except that the enclosed front end, marked with "I", may be modified (such modification is shown in FIG. 4). Thus, the motor section 11 may have an elongated housing 20 in which an electric motor 21 may be housed. A stator 24 of the electric motor 21 may have, in the usual way, a number of circumferentially distributed windings as well as a longitudinal return path 28. The return path 28 may form an outer cylindrical sleeve of the elongated housing 20. The stator 24 may surround a rotor 26, which consists of a permanent magnet connected to a motor shaft 25 and magnetized in an active direction. The motor shaft 25 may extend over the entire length of the motor housing 20 and project distally from the latter through an opening 35. There may be an impeller 34 having pump vanes 36 protruding therefrom, which may rotate within a tubular pump housing 32 which may be rigidly connected to the motor housing 20 .

[0036]

[0046] The proximal end of the motor housing 20 may have a flexible catheter 14 sealingly attached thereto. An electrical cable 23 may extend through the catheter 14 for powering and controlling the electric motor 21. Additionally, a purge fluid line 29 may extend through the catheter 14 and penetrate the proximal end wall 22 of the motor housing 20. Purge fluid may be supplied to the interior of the motor housing 20 through the purge fluid line 29 and exit through an end wall 30 at the distal end of the motor housing 20. The purge pressure may be selected to be higher than the existing blood pressure, thereby preventing blood from entering the motor housing. For example, in some embodiments, the purge pressure may be between 300 and 1400 mmHg, depending on the application.

[0037]

[0047] In some embodiments, the same purged seal may be combined with a pump driven by a flexible drive shaft and a remote motor.

[0038]

[0048] As the impeller 34 rotates, blood may be drawn into a distal opening 37 of the pump housing 32 and conveyed axially rearward within the pump housing 32. The blood exits the pump section 12 through a radial exit opening 38 of the pump housing 32 and flows further along the motor housing 20. In some embodiments, this ensures that heat generated within the motor can be carried away. The pump section may also be operated in the reverse conveying direction, allowing blood to be drawn along the motor housing 20 and out the distal opening 37 of the pump housing 32.

[0039]

[0049] The motor shaft 25 can be mounted in radial bearings 27, 31 at the proximal end of the motor housing 20 on the one hand and at the distal end of the motor housing 20 on the other hand. The radial bearings, in particular the radial bearing 31 at the opening 35 at the distal end of the motor housing, are configured as plain bearings. Furthermore, the motor shaft 25 can also be mounted axially in the motor housing 20, and an axial bearing 40 is likewise configured as a plain bearing. The axial plain bearing 40 serves to absorb the axial force of the motor shaft 25 acting in the distal direction when the impeller 34 conveys blood from distal to proximal. If the blood pump is also used to convey blood in the reverse direction or only in the reverse direction, a corresponding axial plain bearing 40 (also or only) can be provided in a corresponding manner at the proximal end of the motor housing 20.

[0040]

[0050] FIG. 4 shows in detail the part marked "I" in FIG. 3, but structurally modified according to an embodiment of the present disclosure. In particular the radial plain bearing 31 and the axial plain bearing 40 can be seen. The bearing gap of the radial plain bearing 31 is formed on the one hand by the peripheral surface 25A of the motor shaft 25 and on the other hand by the surface 33A of the through bore in the bush or sleeve 33 of the end wall 30 of the motor housing 20, and defines an outer clearance diameter of about 1 mm, although the outer clearance diameter may be larger. In one example, the gap width of the bearing gap of the radial plain bearing 31 is 2 μm or less, not only at the front end or impeller side of the gap, but also over its entire length. In some embodiments, the gap width is 1 μm to 2 μm. The length of the bearing gap may range from 1 mm to 2 mm, preferably from 1.3 mm to 1.7 mm, for example 1.5 mm. The surfaces forming the gap of the radial plain bearing 31 may have a surface roughness of 0.1 μm or less. These dimensions vary depending on the type of pump and are given as examples and not as limitations.

[0041]

[0051] The bearing gap of the axial plain bearing 40 may be formed on the one hand by the axial inner surface 41 of the end wall 30 and by the surface 42 facing it. This facing surface 42 may be part of a ceramic disk 44 that is mounted on the motor shaft 25 distal to the rotor 26 and rotates together with the rotor 26. A channel 43 in the bearing gap surface 41 of the end wall 30 ensures that the purge fluid can flow between the bearing gap surfaces 41 and 42 of the axial plain bearing 40 to the radial plain bearing 31 and exit distally from the motor housing 20. The axial plain bearing 40 represented in FIG. 3 is a normal plain bearing. Contrary to the depiction, the axial gap of the axial plain bearing 40 is very small, a few microns (for example, less than 10 μm).

[0042]

[0052] Instead of the axial plain bearing 40 and the radial plain bearing 31, a combined radial-axial plain bearing 40 can also be realized, which has a concave bearing shell, from which a convex bearing surface extends. Such a variant is represented in FIG. 4 by a spherical plain bearing 40. The bearing clearance surface 41 can be of spherical concave design, and the opposing bearing clearance surface 42 can be of correspondingly spherical convex design. The channel 43 can again be in the stationary bearing clearance surface 41 of the end wall 30. Alternatively, the stationary bearing clearance surface 41 of the end wall 30 can be of convex form, and the opposing bearing clearance surface 42 can be of concave form. The surfaces 42, 43 can also be conical instead of spherical. In some embodiments, corresponding radial-axial plain bearings are provided on both sides of the motor housing 20, which do not allow any radial offset when the shaft 25 moves axially. The advantage of a combined axial-radial plain bearing can be a higher load capacity.

[0043]

[0053] During operation, the blood pump may be attached to a purge fluid source, and the fluid passes through a purge fluid line to the motor housing. The purge fluid then flows through the axial plain bearing and further through the distal radial bearing. In the axial plain bearing, the purge fluid may form a lubricating film in the bearing gap. However, the pressure at which the purge fluid flows through the motor housing may adversely affect the width of the bearing gap. Specifically, a higher purge fluid pressure may require a smaller bearing gap width, thereby resulting in a thinner lubricating film between the sliding surfaces. The thinner the lubricating film, the higher the motor current required to drive the electric motor to overcome the frictional forces. This complicates the control of the blood pump, since the current carrying volume is usually established by a stored characteristic curve based only on the motor current and the rotational speed (both known quantities). As the purge fluid pressure also affects the motor current, this factor also needs to be taken into account. Considering the fact that the same blood pump type may be operated in a wide variety of applications with different purge fluid pressures from 300 to 1400 mmHg, it is important to avoid a dependency of the motor current on the purge fluid pressure.

[0044]

[0054] Such a dependency is avoided when a purge fluid is selected that has a viscosity significantly higher than that of water (η=0.75 mPas at 37° C.). The viscosity of the purge fluid can be controlled by the concentration of glucose in the purge fluid. Aqueous glucose solutions are widely administered to patients for a variety of reasons. The amount of glucose in the aqueous solution can be about 5% to about 50%. In one embodiment, the purge fluid can contain 5% glucose in water (D5W, i.e., 278 mmol / liter). The viscosity can be increased by including a solution with a higher concentration of glucose in water (e.g., D20W, D40W, etc.). When a highly viscous purge fluid is used, a fluid film can be maintained even at high pressures, and thus the friction of the axial plain bearing becomes independent of the purge fluid pressure. In some embodiments, the axial plain bearing can be configured as a simple plain bearing and does not need to be configured as a hydrodynamic plain bearing when the purge fluid has a viscosity of about 1.2 mPas or more at 37° C.

[0045]

[0055] In some embodiments, the pump impeller can induce shear stresses in the blood passing through the pump. Shear stresses can be induced in the gaps between the impeller and the outer surface of the ceramic bearing and between the impeller shaft and the inner ring of the bearing (e.g., ceramic bearing, ball bearing, etc.). Due to the shear stresses experienced by the blood, proteins in the blood denature and polymerize as the blood passes through the pump. The deposition of denatured and aggregated proteins can cause activation of the coagulation cascade, which in turn can cause the accumulation of biodeposits on the pump mechanism (e.g., impeller, outflow cage, etc.). Small gaps between parts (i.e., purge gaps) are particularly vulnerable to blockage by biodeposits. The accumulation of biodeposits can increase the motor current required to operate the pump. The increased motor current or biodeposits can reduce pump performance or even cause the pump to stop.

[0046]

[0056] FIG. 5 illustrates another embodiment of the purge fluid flowing through the purge gap. As shown in this figure, the pump may include a first gap 290 and a second gap 292. In some embodiments, the first gap 290 may include a small radial gap 291 between the outer surface of the shaft 25 and the inner surface of the sleeve bearing 289, which may control the purge flow rate as the purge fluid 288 flows through the purge gap. In some embodiments, the small radial gap 291 may extend radially from 4 μm to 9 μm, such as 5-6 μm in some pumps and 7-8 μm in other pumps. In some embodiments, the first gap may include primarily purge fluid. In some embodiments, the concentration of purge fluid in the fluid filling the volume of the space defined by the first gap may be at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or at least 99% of the total fluid in the first gap. In some embodiments, the first gap is filled entirely with purge fluid. In some embodiments, due to co-mixing of flows, some blood components may be able to reach the distal end of this gap. Due to small dimensions, heat, and high shear, accumulation of biological material at this point may include denatured proteins, which may result in elevated purge pressures and increased friction, leading to high motor currents. In some embodiments, the most likely mechanism of action of bicarbonate by an illustrative pump (e.g., Impella® pump) in the first gap is to neutralize the acidic pH of the D5 carrier solution, reducing denaturation of proteins in the blood and their adsorption within the radial gap.

[0047]

[0057] In some embodiments, the second gap 292 may include an axial gap 293 between the impeller 34 and the sleeve bearing 289. In some embodiments, the axial gap 293 may extend axially (e.g., a distance parallel to the axis 299 of the impeller shaft) about 90-110 μm, such as about 100 μm. In some embodiments, the second gap may be where the purge fluid and blood first mix. The purge fluid may flow along the backside of the impeller, with systemic blood being drawn into the gap and setting up a clockwise flow pattern in this microenvironment. In some embodiments, the volume of blood drawn into the second gap may be about 0.75-1.1 cubic millimeters, depending on the pump. In some embodiments, the purge fluid and blood do not mix completely in the axial gap. As will be appreciated, the mixture ratio of blood to purge fluid in the second gap may vary as the purge flow varies. For example, at high purge flows (e.g., 30 ml / hr), the ratio may be about 60% blood based, while at low purge flows (e.g., about 0 ml / hr), the ratio may approach 100% blood. That is, in some embodiments, depending on the flow rate of the purge fluid, the concentration of purge fluid in the fluid filling the volume of the space defined by the second gap may be at least 40% of the total fluid in the first gap, or no more than 40%, no more than 30%, no more than 20%, or no more than 10%.

[0048]

[0058] As mentioned above, the purge fluid used in purged blood pumps typically contains the anticoagulant heparin (e.g., 50 units / ml) in 5%-dextrose (D5W) to mitigate the adverse effects of shear on the blood flowing through the pump. In some embodiments, heparin is provided in the purge fluid to prevent shear-induced biomaterial or biodeposit formation and the resulting undesirable deposition / accumulation of biological material within the pump, for example between the impeller shaft and the inner ring of the bearing in high shear regions. However, as mentioned above, there may be challenges associated with the addition of heparin to the purge fluid. Specifically, heparin: a) may complicate the systematic administration of anticoagulants (i.e., the heparin dose the patient receives via the purge fluid must be considered); b) heparin, as an anticoagulant, may increase the bleeding tendency of the patient; c) may make it more difficult to control bleeding in such patients, especially when surgical instruments are used in post-operative patients; and d) cannot be used in heparin-induced thrombocytopenia (HIT) patients. Also, heparin may be administered systemically to some patients, making it difficult to coordinate the administration of two heparin sources.

[0049]

[0059] As described herein, the glucose concentration can determine the viscosity of the purge fluid and therefore affect the purge flow rate. A purge fluid with a low glucose concentration is less viscous and flows more quickly with less pressure through the purge system. A purge fluid with a higher glucose concentration (more viscous) will result in a lower purge flow rate and require a higher purge pressure. For example, a decrease in glucose concentration from 20% to 5% can result in an increase in purge flow rate of approximately 30% to 40%.

[0050]

[0060] The purge flow rate is generally in the range of about 2 ml / hr to about 30 ml / hr. This produces a purge pressure of about 1100 mmHg to about 300 mmHg. The typical purge flow rate of the blood pumps described herein, e.g., Impella CP, Impella 2.5, Impella 5.0 / LD, and RP, is about 5 ml / hr to about 20 ml / hr. All of these pumps have ball bearing rotor / stator systems with similar tolerances leading to similar purge operating ranges. The typical purge flow rate of the Impella 5.5 is about 2 to about 10 ml / hr. This lower flow rate results from the deployment of a ceramic bearing rotor / stator system designed with a reduced purge gap (radial) to reduce or eliminate the amount of heparin delivered to the patient. For surgical patients, surgeons prefer not to administer heparin for several days after surgery. Therefore, for these patients, a purge fluid that does not contain heparin is preferred.

[0051]

[0061] Therefore, there remains a need for purge fluids / purge fluid additives that can alleviate pump performance issues due to pumping. It has been observed that denatured proteins are more prone to aggregation because protein unfolding exposes the hydrophobic regions of the protein, which causes undesirable bio-deposition. In the absence of denaturation and aggregation, the hydrophobic segments are hidden and the protein molecules are repelled due to the charged groups of the protein.

[0052]

[0062] Soluble calcium ions are known to mediate clotting. Human serum albumin in blood controls calcium ions. At higher pH values, albumin holds calcium ions more strongly. This mechanism reduces the effective concentration of calcium available for clotting. Therefore, providing a purge fluid with an additive that increases the pH of the purge fluid reduces the amount of calcium that supports clotting in high stress areas. Described herein are relatively high pH buffers for use in purge fluids that increase the pH of blood and therefore mitigate clotting.

[0053]

[0063] Contemplated herein are pH adjusting and buffering agents added to the purge fluid that avoid the problems of heparin but achieve other objectives of the purge fluid (such as biodeposit mitigation; bearing wear reduction; higher than blood pressure, etc.). One example of a suitable pH adjusting and buffering agent is sodium bicarbonate. However, pH adjusting and buffering agents other than sodium bicarbonate are also contemplated. These pH adjusting and buffering agents include, for example, salts of small organic acids such as citrate, lactate, gluconate, acetate, pyruvate, etc. In one example, sodium bicarbonate has a pH of about 7.4 to about 9.1. In one example, the pH of the purge fluid with bicarbonate is about 8.4. Other ranges are contemplated, such as, but not limited to, about 7.5 to about 9.1, 7.6 to about 9.1, 7.7 to about 9.1, 7.8 to about 9.1, 7.9 to about 9.1, 8.0 to about 9.1, 8.1 to about 9.1, 8.2 to about 9.1, 8.3 to about 9.1, 8.4 to about 9.1, 8.5 to about 9.1, 8.6 to about 9.1, 8.7 to about 9.1, 8.8 to about 9.1, 8.9 to about 9.1, and 9.0 to about 9.1. The pH of blood is about 7.3 to about 7.4. By adding sodium bicarbonate to the purge fluid, the pH of blood in contact with the purge fluid is increased. The elevated pH within these purge gaps (e.g., first and second gaps 290, 292 as shown in FIG. 5) reduces biodeposits resulting from blood clotting due to the high shear pump environment. Due to this effect, the presence of sodium bicarbonate may reduce the formation of insoluble biodeposits even as clotting proceeds to the formation of individual fibrin molecules.

[0054]

[0064] In one embodiment, a bicarbonate-containing solution mixed with aqueous dextrose, e.g., 5% dextrose in water (D5W), 20% dextrose in water (D20W), 40% dextrose in water (D40W), etc., can be added to the blood to increase the local pH of the blood in the gap (higher shear region) and prevent protein aggregation by increasing the charge of serum proteins, thus reducing the formation of biodeposits. The amount of bicarbonate in the bicarbonate mixed with aqueous dextrose solution can be from about 1.5 milliequivalents per liter (mEq / L) to about 50 mEq / L. In some embodiments, the amount of sodium bicarbonate can be from 12.5 mEq / L to 100 mEq / L, e.g., from 25 mEq / L to 50 mEq / L. In such embodiments, the purge solution may be heparin-free. In such embodiments, the pH in the gap can be configured to maintain optimal presentation of positively charged key amino acids of the heparin target.

[0055]

[0065] Described herein are systems and methods that use sodium bicarbonate as a replacement for heparin in a purge fluid used to maintain patency of a purge system for a blood pump, such as in the first and second gaps. The sodium bicarbonate may be deployed in the purge system and not used for systemic anticoagulation of the patient in which the pump is deployed. Instead, systemic anticoagulation using heparin, bivalirudin, and / or argatroban may be used to prevent thromboembolic events, even when sodium bicarbonate is used in purging. As will be appreciated, other suitable anticoagulants may be used in other embodiments.

[0056]

[0066] According to some embodiments, systemic anticoagulation with sodium bicarbonate purge liquid (e.g., using heparin) can limit the risk of biomaterial deposition and embolization. For example, the sodium bicarbonate purge fluid can maintain a slightly higher neutral pH in the blood and purge fluid mixture in the purge gaps (e.g., first and second gaps 290 and 292). This can create environmental conditions that 1) support protein stability in the radial gap to maintain patency of the purge flow, and 2) delay the onset of clotting and inhibit fibrin formation with an overall reduced clot formation rate where the purge fluid and systemically anticoagulated blood mix in the second gap (e.g., axial gap).

[0057]

[0067] As shown in Figures 6A, 6B, and 6C, the effect of sodium bicarbonate in the purge gap environment was simulated by performing in vitro coagulation experiments using thromboelastography (TEG). As shown in the figures, in the presence of sodium bicarbonate, systemically anticoagulated blood may exhibit delayed onset of clot formation and reduced clot formation rate in systemically anticoagulated conditions compared to reference blood (R and K (6A), angle parameters (6B), and TEG-ACT (6C)). In such an embodiment, the effect of sodium bicarbonate in the purge gap environment was simulated using 60% blood and 40% purge fluid (to represent the purge fluid concentration in a particular purge gap environment at a high purge flow rate of, for example, 30 mL / hr). It will be appreciated that even though the purge fluid concentration may be 40% in the local microenvironment, the overall concentration of purge fluid in the patient's bloodstream is substantially less than that. For example, the total dose of bicarbonate delivered to a patient may be 12 mEq (or less) over an 8 hour period. In some embodiments, with a purge fluid containing sodium bicarbonate mixed with blood, the pH within the Impella interstitial space may maintain an optimal presentation pattern of the positively charged key amino acids of the heparin target, and conditions for systemic action of heparin are optimal.

[0058]

[0068] As shown in Figure 6C, TEG-ACT was used to evaluate the interaction of purge fluid mixed with systemically heparinized blood in gap 2 (e.g., axial gap). Again, simulations were performed with 60% blood and 40% purge fluid (to represent purge fluid at a high purge fluid flow rate of, e.g., 30 mL / hr). As shown in Figure 6C, in the presence of bicarbonate and heparin, a synergistic effect on blood coagulability was observed.

[0059]

[0069] FIG. 7 illustrates a kit according to some embodiments of the present disclosure. As shown in this figure, in some embodiments, the kit 700 includes a pump 100, such as a pump including one or more features described herein. In some embodiments, the pump may include an inlet region, a cannula, one or more sensors, an outlet region, a motor housing, and a catheter. In some embodiments, as shown in this figure, the kit may include a purge cassette 702 configured to deliver a purge fluid to and through the catheter to the pump, such as to prevent blood from entering the motor. As will be appreciated, the purge cassette is configured to attach to the purge fluid, such as to a purge bag 704. In some embodiments, the purge bag includes a premixed solution with sodium bicarbonate. For example, in some embodiments, the purge fluid may include 5% glucose in an aqueous solution with 12.5 to 100 mEq / L, such as 25 mEq / L to 50 mEq / L of sodium bicarbonate. In some embodiments, the amount of sodium bicarbonate is 25 mEq / L.

[0060]

[0070] Referring to FIG 8, a method may be provided. The method may include providing 810 the necessary components. This may include providing 812 a pump as disclosed herein, and may include providing 814 a purge fluid as described herein. Other components may be shown in FIG 9.

[0061]

[0071] 9, blood pump assembly 900 may include a blood pump 910 fluidly connected to a container 951 (e.g., a purge bag) containing a purge fluid as disclosed herein through a purge device 953. Blood pump assembly 900 may also include a controller 930 (e.g., an Automated Impella Controller® from Abiomed, Inc., Danvers, Mass.), a display 940, a connector cable 960, a plug 970, and a repositioner 980. As shown, controller 930 may include display 940. Controller 930 may monitor and control blood pump 910. In operation, the purging device 953 may deliver a purging fluid as disclosed herein through the first lines 950, 955 (e.g., tubing), through one or more components 956, 957, 958, 959, and through the catheter tube 917 to the blood pump 910, for example to prevent blood from entering a motor (not shown) in the motor housing of the pump. A connector cable 960 may provide an electrical connection between the blood pump 910 and the controller 930. A plug 970 connects to the catheter tube 917, the purging device 953, and the connector cable 960. In some embodiments, the plug 970 may include a memory for storing operating parameters in case the patient needs to be transferred to another controller. A repositioning unit 980 may be used to reposition the blood pump 910. As shown in this figure, the fluid lines 950, 955 may be separate from the connector cable 960 having one or more electrical wires.

[0062]

[0072] The method may include operating 820 the pump, which may include rotating 822 the impeller of the pump. The method may also include flowing 824 a purge fluid through a gap of the present disclosure of the pump. This may be done by controlling a purge device 953, which may include, for example, a positive displacement pump. This may include flowing the fluid through a first gap between a bearing and an outer surface of a rotatable shaft coupled to the impeller, the bearing and gap being disposed within the lumen of the tubular member. In some embodiments, this may include flowing the fluid through a second gap after passing through the first gap, the second gap being between the bearing and a surface of the impeller facing the bearing.

[0063]

[0073] In some embodiments, the method may include controlling 830 a flow rate of fluid into the pump during operation. In some embodiments, this may include controlling the flow rate such that a concentration of bicarbonate-containing fluid in a volume of space defined by a first gap is at least 50% of the total fluid in the first gap. In some embodiments, this may include controlling the flow rate of fluid such that a concentration of bicarbonate-containing fluid in a volume of space defined by a second gap is no more than 40% of the total fluid in the second gap.

[0064]

[0074] In some embodiments, the controller may be configured to control the flow rate of fluid into the pump based on the speed of the impeller. For example, at low impeller rotational speeds, the flow rate of fluid may be relatively low, and as the speed of the impeller increases, the flow rate of fluid automatically increases to offset the increase in pressure of blood entering one or more gaps of the pump. In some embodiments, the blood flow is controlled to maintain a substantially constant ratio of blood to bicarbonate-containing fluid in each gap. For example, if a flow rate is initially selected to provide a blood / bicarbonate-containing fluid ratio of 80% / 20%, and then the impeller speed is increased (or decreased) (thereby changing the 80 / 20 ratio), the method may include controlling the flow rate of fluid into the pump to increase (or decrease) accordingly to keep the ratio substantially equal. In some embodiments, this is done by correlation based on empirical data. In some embodiments, the controller is configured to solve a predetermined function for flow rate versus, for example, impeller speed.

[0065]

[0075] While the embodiments disclosed herein involve affecting the microenvironment of purge gaps (e.g., axial and radial gaps) in a blood pump (e.g., a percutaneous blood pump) with a purge fluid containing sodium bicarbonate, it will be understood that such sodium bicarbonate purges may be used with other components of a blood pump system, or even other medical devices (or other medical device parts) having similar purge gaps. For example, in some embodiments, as shown in the schematic diagram of FIG. 10, a sodium bicarbonate purge fluid 1030 may be used to purge an axial gap 1026 and a radial purge gap 1024 between a tubular body 1010 (e.g., a catheter) having a distal end 1012 and a proximal end 1011 (which may include a lumen 1013 extending therethrough, the lumen having a central axis 1005) and an outer sheath 1020 disposed about the tubular body (e.g., an accessory sheath, such as a sheath used to insert and / or hold the pump in place within a patient's body). Purge fluid may be configured to flow in direction 1031 (e.g., move generally axially when in the radial purge gap and radially in the axial gap) through the radial and axial purge gaps to purge them. In an illustrative embodiment, this may be used to flush the sheath's side arm 1040 (which may be operably connected to the radial purge gap, e.g., via a port 1022 in the side of the outer sheath 1020), e.g., to minimize clot formation. In some embodiments, the axial gap may be located near the first end of the outer sheath, as shown in FIG. 10. However, it will be understood that the axial gap may be located at any suitable portion along the length of the outer sheath, e.g., near the side arm of the outer sheath.

[0066]

[0076] In some embodiments, the maximum distance D1 between the first edge of the axial gap and the end of the outer sheath may be ≦10 cm. In some embodiments, D1 ≦5 cm. In some embodiments, D1 ≦4 cm. In some embodiments, D1 ≦3 cm. In some embodiments, D1 ≦2 cm. In some embodiments, D1 ≦1 cm. In some embodiments, the maximum distance D2 between the first edge of the axial gap and the center of a port (e.g., port 1022) in the outer sheath may be ≦5 cm. In some embodiments, D2 ≦4 cm. In some embodiments, D2 ≦3 cm. In some embodiments, D2 ≦2 cm. In some embodiments, D2 ≦1 cm.

[0067]

[0077] As will be appreciated, as shown in this figure, axial and radial gaps may be formed between the outer diameter of the catheter and the inner diameter of the sheath. In some embodiments, it will be appreciated that the blood-to-bicarbonate-containing fluid ratio is higher than that experienced in the bearing gap shown and described (e.g., greater than 80% / 20%, up to 100% blood). Although not shown, it will be appreciated that a fluid line may be connected to the sheath to allow purge fluid to be transferred to the purge gap. As noted above, a controller may be used to control the rate at which purge fluid is delivered to the sheath.

[0068]

[0078] Although specific embodiments of the present technology have been described, it will be apparent to those skilled in the art that the present technology may be embodied in other specific forms without departing from its essential characteristics. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive. It will be further understood that any reference herein to subject matter known in the art is not an admission that such subject matter is generally known by those skilled in the art to which the technology pertains, unless expressly stated to the contrary.

Claims

1. a tubular body having a distal end and a proximal end and a lumen extending therethrough, the lumen having a central axis; an impeller coupled to a flexible shaft disposed at least partially within the lumen; a bearing within the lumen, the bearing disposed about a portion of the flexible shaft and configured to define a first radial gap between an outer surface of the flexible shaft and an inner surface of the bearing and a second axial gap between the bearing and a surface of the impeller facing the bearing; A blood pump comprising: The blood pump is configured to cause a fluid including bicarbonate to flow through the first gap and into the second gap.

2. 2. The blood pump of claim 1, wherein the bicarbonate is sodium bicarbonate.

3. 3. The blood pump of claim 2, wherein the fluid contains 12.5 mEq / L to 100 mEq / L of sodium bicarbonate.

4. The blood pump of claim 1 , wherein the fluid further comprises a medicinal therapeutic or prophylactic agent.

5. 5. The blood pump of claim 4, wherein the medicinal therapeutic or prophylactic agent is an anticoagulant.

6. The blood pump of claim 1 , wherein the fluid further comprises aqueous glucose.

7. 2. The blood pump of claim 1, wherein a concentration of the fluid containing the bicarbonate within a volume of space defined by the first gap is at least 50% of the total fluid within the first gap.

8. The blood pump of claim 7 , wherein the first gap is filled with the fluid including the bicarbonate.

9. 9. The blood pump of claim 8, wherein a concentration of the fluid containing the bicarbonate within a volume of space defined by the second gap is less than 40% of the total fluid within the second gap.

10. The blood pump according to claim 1 , wherein the first gap is between 4 and 9 μm.

11. The blood pump according to claim 1 , wherein the second gap is 90 to 110 μm.

12. 1. A pump comprising: a tubular body having a distal end and a proximal end and a lumen extending therethrough, the lumen having a central axis; an impeller coupled to a flexible shaft disposed at least partially within the lumen; a bearing within the lumen, the bearing disposed about a portion of the flexible shaft and configured to define a first radial gap between an outer surface of the flexible shaft and an inner surface of the bearing and a second axial gap between the bearing and a surface of the impeller facing the bearing; Including, a pump configured to flow a fluid including bicarbonate through the first gap and into the second gap; a purge bag containing the fluid including the bicarbonate, the purge bag being operably coupled to the pump; Including, the system.

13. The system of claim 12 , wherein the bicarbonate is sodium bicarbonate.

14. 14. The system of claim 13, wherein the fluid contains 12.5 mEq / L to 100 mEq / L of sodium bicarbonate.

15. The system of claim 12 , wherein the fluid further comprises a pharmaceutical therapeutic or prophylactic agent.

16. The system of claim 15 , wherein the pharmaceutical therapeutic or prophylactic agent is an anticoagulant.

17. The system of claim 12 , wherein the fluid further comprises aqueous dextrose.

18. 13. The system of claim 12, wherein a concentration of the fluid containing the bicarbonate within a volume of space defined by the first gap is at least 50% of the total fluid within the first gap.

19. 20. The system of claim 18, wherein the first gap is filled with the fluid including the bicarbonate.

20. 20. The system of claim 19, wherein a concentration of the fluid containing the bicarbonate within a volume of space defined by the second gap is less than or equal to 40% of the total fluid within the second gap.

21. The system of claim 12, wherein the first gap is between 4 and 9 μm.

22. The system of claim 12, wherein the second gap is between 90 and 110 μm.

23. 1. A pump comprising: a tubular body having a distal end and a proximal end and a lumen extending therethrough, the lumen having a central axis; an impeller coupled to a flexible shaft disposed at least partially within the lumen; a bearing within the lumen, the bearing disposed about a portion of the flexible shaft and configured to define a first radial gap between an outer surface of the flexible shaft and an inner surface of the bearing and a second axial gap between the bearing and a surface of the impeller facing the bearing; Including, a pump configured to flow a fluid including bicarbonate through the first gap and into the second gap; a purge bag containing the fluid including the bicarbonate; a purge cassette configured to couple the purge bag to the pump; Including the kit.

24. 1. A method for purging a pump, comprising: providing said pump having an impeller and a fluid including bicarbonate; and passing the fluid through a first gap between a bearing and an outer surface of a rotatable shaft coupled to the impeller, the bearing and gap being disposed within a lumen of a tubular member; A method comprising:

25. 25. The method of claim 24, wherein the bicarbonate is sodium bicarbonate.

26. 26. The method of claim 25, wherein the fluid contains 12.5 mEq / L to 100 mEq / L of sodium bicarbonate.

27. 25. The method of claim 24, wherein the fluid further comprises a pharmaceutical therapeutic or prophylactic agent.

28. 28. The method of claim 27, wherein the pharmaceutical therapeutic or prophylactic agent is an anticoagulant.

29. 25. The method of claim 24, wherein the fluid further comprises aqueous dextrose.

30. 25. The method of claim 24, further comprising controlling a flow rate of the fluid such that a concentration of the fluid containing the bicarbonate in a volume of space defined by the first gap is at least 50% of the total fluid in the first gap.

31. The method of claim 24, wherein the first gap is between 4 and 9 μm.

32. 25. The method of claim 24, wherein flowing the fluid through a first gap further comprises flowing the fluid through a second gap after passing through the first gap, the second gap being between the bearing and a surface of the impeller facing the bearing.

33. 33. The method of claim 32, further comprising controlling a flow rate of the fluid such that a concentration of the fluid containing the bicarbonate in a volume of space defined by the second gap is less than or equal to 40% of the total fluid in the second gap.

34. The method of claim 33, wherein the second gap is between 90 and 110 μm.

35. 25. The method of claim 24, further comprising automatically adjusting a flow rate of the fluid based on an operating characteristic of the pump.

36. a tubular body having a distal end and a proximal end and a lumen extending therethrough, the lumen having a central axis; an outer sheath disposed about the tubular body; a first gap formed radially between an outer surface of the tubular body and an inner surface of the outer sheath; a second gap formed axially between the tubular body and the outer sheath; A medical device comprising: A medical device, wherein a fluid comprising bicarbonate is configured to flow through the first and second gaps to purge the first and second gaps.

37. 37. The medical device of claim 36, wherein the bicarbonate is sodium bicarbonate.

38. 38. The medical device of claim 37, wherein the fluid contains 12.5 mEq / L to 100 mEq / L of sodium bicarbonate.

39. 37. The medical device of claim 36, wherein the fluid further comprises a pharmaceutical therapeutic or prophylactic agent.

40. 40. The medical device of claim 39, wherein the pharmaceutical therapeutic or prophylactic agent is an anticoagulant.

41. 37. The medical device of claim 36, wherein the fluid further comprises aqueous dextrose.

42. 37. The medical device of claim 36, wherein a concentration of the fluid containing the bicarbonate within a volume of space defined by the first gap is at least 50% of the total fluid within the first gap.

43. 43. The medical device of claim 42, wherein the first gap is filled with the fluid comprising the bicarbonate.

44. 44. The medical device of claim 43, wherein the concentration of the fluid containing the bicarbonate within the volume of space defined by the second gap is less than or equal to 40% of the total fluid within the second gap.

45. The medical device of claim 36, wherein the first gap is between 4 and 9 μm.

46. The medical device of claim 36, wherein the second gap is 90 to 110 μm.

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