Abstract:
Systems and methods convey the blood through a gap defined between an inner surface that is located about an axis and an outer surface that is concentric with the inner surface. At least one of the inner and outer surfaces carries a membrane that consists essentially of either a hemofiltration membrane or a hemodialysis membrane. The systems and methods cause relative movement between the inner and outer surfaces about the axis at a selected surface velocity, taking into account the size of the gap. The relative movement of the two surfaces creates movement of the blood within the gap, which creates vortical flow conditions that induce transport of cellular blood components from the membrane while plasma water and waste material are transported to the membrane for transport across the membrane. Shear-enhanced transport of waste materials and blood plasma water results.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to systems and methods that remove waste materials and liquid from the blood of an individual whose renal function is impaired or lacking.  
         BACKGROUND OF THE INVENTION  
         [0002]    For various reasons, including illness, injury or surgery, patients may require replacement or supplementation of their natural renal function in order to remove excess fluid or fluids containing dissolved waste products from their blood. Several procedures known for this purpose are hemodialysis, hemofiltration, hemodiafiltration and ultrafiltration.  
         SUMMARY OF THE INVENTION  
         [0003]    The invention provides shear-enhanced systems and methods for removing waste materials and liquid from the blood.  
           [0004]    The systems and methods convey the blood through a gap defined between an inner surface that is located about an axis and an outer surface that is concentric with the inner surface. At least one of the inner and outer surfaces carries a membrane that consists essentially of either a hemofiltration membrane or a hemodialysis membrane. The systems and methods cause relative movement between the inner and outer surfaces about the axis at a selected surface velocity, taking into account the size of the gap. The relative movement between the inner and outer surfaces creates movement of the blood within the gap, which creates a vortical flow condition that induces transport of cellular blood components from the membrane while plasma water and waste material are transported to the membrane for transport across the membrane.  
           [0005]    The circulatory forces of the vortical flow condition clear the membrane surface of occluding cellular components to maintain efficient operation. The circulatory forces also supplement the shear forces exerted on the blood by viscous drag. Due to the circulatory forces, the concentration of waste materials in the blood plasma water becomes more homogenous. As a result, the transport of waste materials and associated blood plasma water across the membrane is significantly enhanced. Shear-enhanced waste removal makes possible the use of smaller processing devices and/or processing at reduced blood flow rates. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a schematic view of a system that includes a blood processing unit for removing waste material and plasma water from the blood;  
         [0007]    [0007]FIG. 2 is a side section view of one embodiment of a blood processing unit that the system shown in FIG. 1 can incorporate for the purpose of performing shear-enhanced hemofiltration;  
         [0008]    [0008]FIG. 3 is a side section view of another embodiment of a blood processing unit that the system shown in FIG. 1 can incorporate for the purpose of performing shear-enhanced hemodialysis;  
         [0009]    [0009]FIG. 4 is a side section view of another embodiment of a blood processing unit that the system shown in FIG. 1 can incorporate for the purpose of performing shear-enhanced hemodialysis;  
         [0010]    [0010]FIG. 5 is a side section view of another embodiment of a blood processing unit that the system shown in FIG. 1 can incorporate for the purpose of performing shear-enhanced hemofiltration and hemodialysis;  
         [0011]    [0011]FIG. 6 is an enlarged and simplified perspective view of a gap formed between a stationary and rotating concentric surfaces, of a type that the blood processing units shown in FIGS.  2  to  5  incorporate, in which vortical flow conditions provide shear-enhanced waste material and plasma water removal; and  
         [0012]    [0012]FIG. 7 is an enlarged side sectional view of the vortical flow conditions shown in FIG. 5 that provide shear-enhanced waste material and plasma water removal. 
     
    
       [0013]    The invention may be embodied in several forms without departing from its spirit or essential characteristics. The scope of the invention is defined in the appended claims, rather than in the specific description preceding them. All embodiments that fall within the meaning and range of equivalency of the claims are therefore intended to be embraced by the claims.  
       DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    [0014]FIG. 1 shows a system  10  for removing waste material (e.g., urea, creatinine, and uric acid) and plasma water from the blood of an individual whose renal function is impaired or lacking. The system  10  includes a blood processing unit  12  that receives whole blood from the individual. The individual typically has one or more surgically installed vascular access devices, such as an arterial-venous fistula, to facilitate coupling the blood processing unit  12  to the circulatory system of the individual. In the illustrated embodiment, arterial whole blood is drawn from the individual through an inlet path  14 . An inlet pump  16  governs the blood inlet flow rate.  
         [0015]    The blood processing unit  12  includes a membrane  18 , along which the whole blood drawn from the individual is conveyed. The membrane  18  can have different functional and structural characteristics, which affect the manner in which waste material is transported by the membrane  18 . Generally speaking, waste material carried in blood plasma water can be separated by the membrane  18  from the whole blood either by convective transport, which is driven by pressure differentials across the membrane (in a process known as hemofiltration), or by diffusion, which is driven by concentration gradients across the membrane (in a process known as hemodialysis). The waste materials and associated blood plasma water are removal from the blood processing unit  12  through a waste path  20  for discard.  
         [0016]    The pores of the membrane  18  desirably have a molecular weight cut-off that block the passage of cellular blood components and larger peptides and proteins (including albumin) across the membrane. These components are retained in the blood, which is conveyed from the blood processing unit  12  through an outlet path  22  for return to the individual. In the illustrated embodiment, the treated blood is returned to the venous blood circulatory system of the individual.  
         [0017]    Fresh physiologic fluid, called replacement fluid, is typically supplied from a source  24  to the plasma water and toxin-depleted blood. The replacement fluid restores, at least partially, a normal physiologic fluid and electrolytic balance to the blood returned to the individual.  
         [0018]    The relative volumes of waste plasma water removed and replacement fluid supplied can be monitored, e.g., by gravimetric means, so that a desired volumetric balance can be achieved. An ultrafiltration function can also be performed by the blood processing unit  12 , by which plasma water is replaced in an amount slightly less than that removed. Ultrafiltration decreases the overall fluid level of the individual undergoing treatment, which typically increases due to normal fluid intake between treatment sessions.  
         [0019]    The blood processing unit  12  includes a processing cartridge  26 , in which the membrane  18  is housed. The cartridge  26  is desirable disposable and, in one representative embodiment (see FIG. 2), includes a generally cylindrical housing  28 , which is sized to be conveniently manipulated by an operator. The housing  28  can be oriented for use either horizontally or vertically, or any intermediate position.  
         [0020]    An elongated cylindrical rotor  30  (which can also be called a “spinner”) is rotatably supported within the housing  28  between oppositely spaced pivot bearings  32  and  34 . The rotor  30  rotates within the housing  28 , which is held stationary during use. However, other manners of operation are possible, and the housing need not be held stationary.  
         [0021]    An annular gap  36  is formed between the outer surface of the rotor  30  and the interior wall  38  of the housing  28 . Whole blood in the inlet path  14  is conveyed through a blood inlet port  40  into the gap  36  for processing by the inlet pump  16 . After processing, the blood is discharged from the gap  36  through an oppositely spaced outlet port  42 , which communicates with the blood return path  22 .  
         [0022]    In the illustrated embodiment, a magnetic drive assembly  44  provides rotation to the rotor  30 . A ring of magnetic material  46  in the rotor  30  is acted upon by a rotating magnetic field generated by an external, rotating magnetic drive member  48 , which releasably engages the adjacent end of the housing  28  for use. The rotor  30  rotates relative to the stationary interior wall  38  of the housing  28 . The magnetic drive member  48  rotates the rotor  30  at a predetermined angular velocity.  
         [0023]    Further details regarding devices employing a spinning rotor and a stationary housing for blood filtration can be found in U.S. Pat. Nos. 5,194,145 and 4,965,846, which are incorporated herein by reference.  
         [0024]    Further details of construction and operation of the processing cartridge  26  can differ, depending upon the type of blood processing sought to be performed. If hemofiltration is to be performed, the membrane  18  comprises an appropriate hemofiltration membrane (as FIG. 2 shows). If hemodialysis is to be performed, the membrane  18  comprises an appropriate hemodialysis membrane (as FIGS. 3 and 4 show). If hemodialysis with hemofiltration is to be performed, the processing cartridge  26  can include both a hemofiltration membrane and a hemodialysis membrane (as FIG. 5 shows).  
         [0025]    A. Hemofiltration  
         [0026]    In the embodiment shown in FIG. 2, the rotor  30  has an internal cavity  50  bounded by a grooved cylindrical wall  52  forming a network of channels  56 . A hemofiltration membrane  54  covers the outer surface of grooved wall  52 . The hemofiltration membrane  54  can comprise, e.g., a biocompatible synthetic material such as polysulfone, polyacrylonitrile, polymethylmethacrylate, polyvinyl-alcohol, polyamide, polycarbonate, etc., and cellulose derivatives. The pores of hemofiltration membrane  54  desirably allow passage of molecules up to about 30,000 Daltons, and desirably not greater than about 50,000 Daltons, to avoid the passage of albumin (molecular weight of 68,000 Daltons).  
         [0027]    The network of channels  56  convey blood plasma water passing through the membrane  54  into the cavity  50 . An outlet port  58  communicates with the cavity  50  to convey blood plasma water from the processing cartridge  26 .  
         [0028]    In operation, as the rotor  30  is rotated, the pump  16  conveys whole blood into the gap  36 . The whole blood flows within the gap  36  in contact with the hemofiltration membrane  54 .  
         [0029]    In response to the transmembrane pressure created by the pump  16 , waste material and associated blood plasma water flow from the gap  36  through membrane  54  into the channels  56 . Waste material and associated blood plasma water are discharged from the processing cartridge through the outlet port  58 . Cellular blood components continue to flow within the gap  36  for discharge through the outlet port  42 .  
         [0030]    It should be appreciated that, alternatively, the hemofiltration membrane  54  can be mounted on the stationary wall  38  of the housing  28 , instead of being mounted on the spinning rotor  30 , as FIG. 2 shows. In this arrangement, the network of channels  56  communicating with the waste outlet port  58  would be formed in the stationary wall  38 , and the membrane  54  would overlay the channels in the same fashion shown in FIG. 2. It should also be appreciated that, alternatively, a hemofiltration membrane  54  can be mounted on both the spinning rotor  30  and the stationary wall  28  and used in tandem for waste material and plasma water removal.  
         [0031]    B. Hemodialysis  
         [0032]    In the embodiment shown in FIG. 3, the interior wall  38  of the housing  28  has a network of channels  60  communicating with an inlet port  62  and an outlet port  64 . A semipermeable hemodialysis membrane  66  overlays the network of channels  60 . The membrane  66  can, e.g., comprise a medium to high flux membrane, for example, a polysulfone, cellulose triacetate or acrylonitrile membrane. Such membranes are typically well suited to fluid and small solute (less the 10,000 Daltons) removal. One side of the membrane  66  faces the annular gap  36  and the rotor  30 , which, in the illustrated embodiment, carries no membrane. The other side of the membrane  66  faces the channels  60 .  
         [0033]    In operation, as the rotor  30  is rotated, the pump  16  conveys whole blood into the gap  36 . The whole blood flows within the gap  36  in contact with membrane  66 . Fresh dialysate is circulated by a pump  70  from a source  68  through the channels  60  via the ports  62  and  64 . Desirably (as FIG. 3 shows), the dialysate is circulated through the channels  60  in a flow direction opposite to the direction of whole blood flow in the gap  36 .  
         [0034]    As blood flows through the gap  36 , plasma water is conveyed across the membrane  66  due to transmembrane pressure created by the pump  16 . Targeted waste materials are also transferred across the membrane  66  by diffusion, due to a difference in concentration of these materials in the blood (high concentrations) and in the fresh dialysate (low concentrations). In response to the high-to-low concentration gradient, waste materials flow from the gap  36  through the membrane  66  into the dialysate. The waste materials are discharged with the spent dialysate out of the processing cartridge  26  to, e.g., a drain. Cellular blood components continue to flow within the gap  36  for discharge through the outlet port  42  for return to the individual.  
         [0035]    As shown in FIG. 4, in an alternative embodiment, the rotor  30  can include a network of channels  72  through which dialysate can be circulated in the manner just described. In this arrangement, a hemodialysis membrane  74  overlays the network of channels  72  on the rotor  30 . One side of the membrane  74  faces the annular gap  36 . The other side of the membrane faces the channels  72 .  
         [0036]    It should be appreciated that the hemodialysis membrane  74  on the rotating rotor  36  can also be used in combination with the hemodialysis membrane  66  on the stationary interior wall  38  of the housing  28 , or by itself (in which case the stationary interior wall  38  of the housing  28  would be free of a membrane.  
         [0037]    As shown in FIG. 5, the processing cartridge  26  can include a hemodialysis membrane  66  mounted on either the rotor  30  or the interior housing wall  38  and a hemofiltration membrane  54  mounted on the other location. In this arrangement, the processing cartridge  26  accommodates hemdialysis with hemofiltration, a process also called hemodiafiltration.  
         [0038]    C. Shear-Enhanced Waste Removal  
         [0039]    In an annular gap  36  as just described, which is defined between two concentric surfaces (e.g., the rotor  30  and the interior housing wall  38 ), rotation of the inner surface relative to the outer surface can induce vortical flow conditions in the blood residing in the gap  36 . The vortical flow conditions take the form of successive, alternately circulating, annuli TV (see FIG. 6) in the gap  36  between the two concentric surfaces. This vortex action can be a type that can be generally classified as “Taylor vortices” (as designated as TV in FIG. 6). The nature of the Taylor vortices can vary among laminar stable Taylor vortices, wavy non-stable Taylor vortices, turbulent Taylor vortices, or other intermediate vortical flow conditions.  
         [0040]    Taylor vortices will develop in the blood occupying the gap  36 , regardless of whether the membrane is mounted on the inner surface or on the outer surface, or both surfaces. Taylor vortices develop in the blood occupying the gap  36  as a result of relative movement between the inner and outer surfaces, regardless of whether one of the surfaces is held stationary while the other rotates, or whether both surfaces are allowed to rotate. To achieve desired vortical flow conditions, it is believed that the inner surface should be rotated relative to the outer surface, and, if the outer surface is allowed to rotate, the rate of rotation of the inner surface should exceed the rate of rotation of the outer surface.  
         [0041]    The amplitude of the vortex action, which is characterized by the Taylor number, is a function of the rate of rotation of the rotating surface and the radial dimension of the gap  36 . At a given radial dimension, increasing the rate of rotation will increase the amplitude of the vortex action, leading to a higher Taylor number. Given a rate of rotation, narrowing the radial dimension of the gap  36  will also increase the amplitude vortex action, leading to a higher Taylor number. It is believed that radial dimension of the gap  36  and the rate of rotation should be selected to yield a Taylor number that is greater than the critical Taylor number, at which vortical flow conditions develop.  
         [0042]    Transmembrane pressure is also desirably monitored and maintained (by controlling operation of the pump  16 ) at a magnitude that maximizes fluid transport across the membrane without driving cellular blood components into the membrane pores, which can cause membrane plugging, hemolysis, and trauma to fragile cellular blood components residing within the gap  36 .  
         [0043]    When maintained within desired limits, the vortical flow conditions provide a sweeping action in the gap  36  (see FIG. 7) that transports cellular blood components away from the membrane while blood plasma water carrying the targeted uremic toxins is transported to the membrane for passage through the pores of the operative membrane. The circulation caused by the vortical flow conditions removes adherent cellular blood components from the surface of the operative membrane and replenishes available blood plasma water for transport through the membrane pores. The vortical flow conditions thereby clear the membrane surface of occluding cellular components to maintain efficient operation at desirable transmembrane pressure levels. The circulatory forces also supplement the shear forces exerted on the blood by viscous drag, which is tangential to the spinning membrane surface. Furthermore, due to the circulatory forces, the concentration of waste materials in the blood plasma water becomes more homogenous. In all, the transport of waste materials and associated blood plasma water across the membrane is significantly enhanced. Shear-enhanced waste removal makes possible the use of smaller processing devices and/or processing at reduced blood flow rates.  
         [0044]    If desired, ultrafiltration volume can be augmented by placing, either upstream or downstream of the processing cartridge  26 , an auxiliary processing cartridge  76  (shown in phantom lines in FIG. 1). The auxiliary processing cartridge  76  subjects the blood to plasma water removal (either by hemodialysis or hemofiltration) in addition to the plasma water removal by the processing cartridge  26 . An auxiliary processing cartridge  76  can also be used in series with the processing cartridge  26 , to provide waste removal by hemofiltration to augment waste removal by hemodialysis conducted by the processing cartridge  26 , or vice versa.  
         [0045]    Various features of the invention are set forth in the following claims.