Abstract:
A system is disclosed for removing gas bubbles from blood during circulatory assist procedures. Such bubbles are generated, along with particulate matter, in an extracorporeal circulatory bypass system by the pump, oxygenator and other components. Filters are used in the line to remove particulates and bubbles from the blood before they are pumped back to the patient but current filters are inefficient at removing small bubbles and debris that can cause neurological defects and renal and other organ failures in the patient. An active filter apparatus and method is disclosed that forces the bubbles to the center of the system where they are removed from the blood before the blood exits the filter. The filter comprises an axially elongate cylindrical shell with a blood inlet, a blood outlet and a gas outlet. A motor driven impeller spins the blood within the shell and forces the gas toward the center by centripetal force, utilizing the buoyancy effects of the bubbles in blood, whereby the bubbles can be bled off at the center of the filter element.

Description:
PRIORITY CLAIM 
       [0001]    This application is a continuation of, and claims priority benefit from U.S. patent application Ser. No. 10/791,075, filed Mar. 1, 2004, entitled METHOD AND APPARATUS FOR REMOVAL OF BUBBLES FROM BLOOD, which claims priority benefit under 35 USC § 119(e) from U.S. Provisional Application No. 60/447,266, filed Feb. 13, 2003, entitled METHOD AND APPARATUS FOR REMOVAL OF BUBBLES FROM BLOOD, the entire contents of both of which are hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The field of this invention is cardiac circulatory assist and, more specifically, cardiopulmonary bypass. 
       BACKGROUND OF THE INVENTION 
       [0003]    During cardiovascular surgical procedures, the heart is often arrested and the patient is placed on cardiopulmonary bypass. In addition, a subset of patients with cardiopulmonary complications and or disease will be placed on partial longer-term cardiopulmonary bypass. These patients include, but are not limited to: neonates with severe pulmonary lung disease, bridge to transplant patients, liver transplant patients and patients with severe myocardial trauma accompanied by pump failure. Such cardiopulmonary bypass is used to support the patient&#39;s circulation and/or pulmonary function while the heart is being surgically repaired or the failing organ is allowed to recover. Typical surgical repair procedures include valve replacement, annuloplasty, coronary artery bypass grafting, total heart replacement, cardiac assist placement, repair of tetralogy of Fallot, repair of atrial and ventricular septal defects, heart and/or lung transplantation, liver transplantation and the like. Cardiopulmonary bypass devices use a cannula to remove blood from the patient where it is oxygenated, purged of carbon dioxide, heated or cooled, filtered and pumped back into the systemic circulation of the patient. Blood filters are used in the cardiopulmonary bypass system to trap particulates and gas bubbles that are generated in the extracorporeal loop. Blood filters prevent particulates and gas bubbles from being pumped back into the patient. The most common gas entrained within the blood of an extracorporeal circuit is air. Such particulates and gas bubbles, also known as emboli, can cause blockage in the arterioles and capillary beds and lead to ischemic cell death. Consequences of such ischemic cell death may affect organ function (viz. intestine, pancreas, kidney, brain, etc.) and result in sepsis, renal failure and neurological defects such as loss of memory, cognitive function, and changes in personality. 
         [0004]    Modern blood filters do trap emboli and remove debris before they are pumped back into patients but it has been scientifically validated that small gas bubbles, primarily air, and certain particulate substances missed by these filters are returned to the patients and compromise patient recovery. Patients who undergo cardiopulmonary bypass are often subject to some degree of neurological deficit as a result of the gas bubbles and other embolic materials. This phenomenon is sometimes characterized as “Pump Head”. 
         [0005]    Current blood filters are considered to be adequate for removing larger debris and large gas bubbles from the blood, but patient outcomes would be improved if small gas bubble and particulate removal efficiencies were higher. One of the primary problems with current blood filters is that when the mesh size is increased to screen out smaller particles and bubbles, the pressure drop across the filter becomes unacceptably high at normal blood flow rates. Such unacceptably high pressure gradients can potentially cause tubing or connection failures resulting in blood leaks or air leaks into the system, either of which could be catastrophic. Typical examples of the prior art in blood filters include U.S. Pat. No. 4,919,802 to Katsura, U.S. Pat. No. 4,411,783 to Dickens et al., U.S. Pat. No. 5,279,550 to Habib et al., U.S. Pat. No. 5,5,632,894 to White et al., and U.S. Pat. No. 5,683,355 to Fini et al. These patents disclose filters and bubble traps that are static devices employing filter screens to collect the debris and bubbles. 
         [0006]    Additionally, U.S. Pat. Nos. 4,411,783, 4,919,802, and 5,632,894 disclose use of tangential blood inflow and a gas vent at the top center of the filter to improve bubble removal. The tangential inflow generates centrifugal effects to move the bubbles to the center of the device. However, since these are not active systems, they are unable to generate the rotational velocities necessary to adequately rid the blood of small bubbles that can cause neurological defects. A recent publication by Schoenburg (Ref J Thorac Cardiovasc Surg 2003:126:1455-60) describes an air bubble trap, which incorporates a three channel helix to cause the blood to passively rotate around the axis of the tube causing the centrifugal forces to direct air bubbles to the center of the flow stream where they are evacuated via a special collection tube. None of these devices impart rotational motion using an active drive system, which can rotate the blood at much higher rates and thus generate higher separation forces on the bubbles to remove them from the blood. 
         [0007]    New devices and methods are needed to more efficiently remove gas bubbles from the blood of a patient undergoing circulatory support without traumatizing blood elements and without unacceptably increasing the pressure drop across the filter to dangerous levels. 
       SUMMARY OF THE INVENTION  
       [0008]    This invention relates to a blood filter, blood-air filter, or trap for removing air or other gas bubbles and particulate matter (both large and small) from the blood of a patient during assisted circulation. The present invention is an active device that accepts blood at its inlet, actively rotates the blood to drive the bubbles toward the center of the device under centripetal force, and allows separation of the blood from the aforementioned bubbles. More dense materials, such as blood cells, move toward the periphery of the filter or are otherwise trapped by filter meshes. The device comprises a chamber or housing with a blood inlet and a blood outlet. In addition, the chamber has a third outlet for removing gas from the blood. The device additionally comprises a stirring rod or impeller to spin the blood circumferentially within the chamber. This stirring rod or impeller is coupled to a rotary motor that generates the rotational energy necessary to separate bubbles from the blood. The present invention actively removes gas bubbles and debris from the blood, including the tiny gas bubbles and particulates which current blood filters are unable to remove. The gas bubbles have less mass than the same volume of blood, i.e. the bubbles are buoyant in blood, so that rotation causes them to move toward the center of the blood filter by centripetal force. The centripetal force accelerates the bubbles until the bubbles reach a radial velocity where the drag force balances the centrifugal force. The blood filter of the present invention is designed to remove the majority of bubbles of size greater than 7 to 10 microns in diameter in the time the blood takes to traverse the volume of the filter. Thus, this is a single-pass bubble filter for a large majority of the bubbles. The design is optimized for bubbles 7 to 10 microns in diameter or larger. Bubbles smaller than 7 to 10 microns are considered less harmful to patients than larger bubbles because they will pass through the capillary beds of the patient. 
         [0009]    In accordance with another aspect of the invention, a method is described to remove bubbles from blood. This method includes the steps of passing the blood into a circular, axially elongate or cylindrical chamber and actively spinning the blood within the chamber at high rotational rates to move the bubbles to the center of the chamber. In a further aspect of the invention, the air is removed from the blood at the center of the chamber and the blood is drawn off along the outer periphery of the chamber where it is ultimately returned to the patient. 
         [0010]    The present invention distinguishes over the cited prior art because it uses an active component to spin the blood to forcibly remove gas bubbles from the blood. The invention is most useful during surgery when cardiopulmonary bypass is instituted to maintain the patient on temporary cardiopulmonary support. It is also useful for removal of gas and bubbles during intravenous infusion of liquids to a patient. Patients with increased risk of pulmonary emboli are especially vulnerable during intravenous infusion and would benefit from such protection. 
         [0011]    For purposes of summarizing the invention, certain aspects, advantages and novel features of the invention are described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner. that achieves one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    A general architecture that implements the various features of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. Throughout the drawings, reference numbers are re-used to indicate correspondence between referenced elements. 
           [0013]      FIG. 1  illustrates a breakaway view of the blood filter of the current invention showing a cross-sectional view of the internal rotating component and the blood inflow port as well as the motor drive and pole clamp. 
           [0014]      FIG. 2A  illustrates a breakaway view of the disposable blood filter of the current invention. An impeller that utilizes vanes is shown in cross-section. 
           [0015]      FIG. 2B  illustrates a top view of the vane-type impeller through a cross-sectional view of the disposable blood filter of the present invention. 
           [0016]      FIG. 3A  illustrates a front exterior view of the blood filter and motor drive. 
           [0017]      FIG. 3B  illustrates a side exterior view of the blood filter and motor drive, showing the blood inlet port and blood outlet port. 
           [0018]      FIG. 4  illustrates a schematic drawing of the cardiopulmonary bypass loop with the blood filter of the current invention in place. 
           [0019]      FIG. 5  illustrates a sectional view of another embodiment of the blood filter using a conical impeller with no vanes. 
           [0020]      FIG. 6  illustrates a sectional view of another embodiment of the blood filter using an axial inlet port and a screen-type cylindrical impeller. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIG. 1  illustrates a breakaway view of a blood filter assembly  8  of the present invention. The blood filter assembly  8  comprises a disposable blood filter  10  and a motor drive  26 . The disposable blood filter  10  comprises a shell  12 , an impeller  14 , a blood outlet port  16 , a gas outlet or central port  18 , a blood inlet port  20 , an optional baffle  22 , and a bearing shaft  24 . The optional baffle  22  optionally comprises a plurality of vent holes  28 . The impeller  14  comprises a magnetic coupler  30 . The shell  12  optionally comprises a plurality of lock down tabs  46 , a gas trap  57  and a bleed valve  58 . 
         [0022]    The motor drive  26  comprises a motor  32 , a power cable  34 , a power switch  36 , a central shaft  38 , a magnetic driver  40 , a housing  42 , and a plurality of optional lock-down or clamping mechanisms  44  to hold the disposable blood filter shell  12  to the motor drive housing  42 . The motor drive  26  optionally comprises a power-on lamp  48 , an extension arm  54 , and a pole clamp  50 . The optional pole clamp  50  further comprises a setscrew  52 . 
         [0023]    The shell  12  of the disposable blood filter  10  is an axially elongate cylinder or vessel, most preferably disposed with its axis parallel to the direction of gravity. The top of the shell  12  is, preferably, conical. The gas outlet port  18  is preferably disposed along the central axis at either the top or the bottom of the shell  12 . The blood inlet port  20  and the blood outlet port  16  are, preferably located on the periphery of the shell  12 . The blood inlet port  20  may be located lower or higher on the periphery of the shell  12  than the blood outlet port  16  but the gas outlet port  18 , otherwise known as a gas vent, is most preferably located higher than the blood inlet port  20  and the blood outlet port  16 . The gas outlet port  18  is located at the entrance of the gas trap  57  and the bleed valve  58  is located at or near the highest point of the gas trap  57 . The gas outlet port  18 , in another embodiment, is located at the center of the bottom of the blood filter. The gas and blood, which is removed from either gas outlet port  18  is routed back to the venous reservoir of the cardiopulmonary bypass system thus minimizing blood loss during the surgical procedure. The bottom-mounted gas outlet port  18  may be able to take advantage of fluid patterns generated within the shell  12  to enhance separation of gas from the blood. 
         [0024]    The diameter of the blood inlet port  20  and the blood outlet port  16  is generally 1.2 cm and ranges from 0.2 cm to 3.0 cm. The diameter of the gas outlet port  18  is from 0.1 cm to 2.0 cm. The diameter of the shell  12  is generally from 1 cm to 30 cm, more preferably from 3 cm to 15 cm and most preferably 5 to 10 cm. The length of the shell  12  ranges from 2 cm to 30 cm. Smaller lengths and diameters of the shell  12  are preferable because the priming volume of the disposable blood filter  10  is minimized with minimized dimensions and a small priming volume reduces patient blood lost during a bypass procedure. 
         [0025]    The baffle  22  is a cylindrical structure located inside the conical top of the disposable filter  10  under the gas outlet port  18 . The series of vent holes  28  perforate the circumferential periphery of the baffle  22 . The diameter of the baffle  22  is optimized to shunt the blood with gas bubbles away from the blood outlet port  16 . The length of the baffle  22  is generally such that the lowermost portion of the baffle  22  is at or below the height of the blood outlet port  16 . The maximum radius of the baffle  22  is equal to or less than the distance from the innermost extent of the blood outlet port  16  from the center of the shell  12 . 
         [0026]    The gas outlet port  18  directs gas out of the disposable blood filter  10  and into the gas trap  57  where the small gas bubbles coalesce into macroscopic amounts of gas that is then bled off through the bleed valve  58 . The gas trap  57  is, preferably, transparent so that the clinician may monitor the buildup of macroscopic amounts of gas within the gas trap. The bleed valve  58  is either a manual valve, such as a stopcock, or it is an automatic valve that opens when a pre-determined amount of gas builds up within the gas trap  57 . The blood and foam collected in the gas trap  57  are preferably returned to a reservoir for recombination with the rest of the blood in the extracorporeal circulation. 
         [0027]    The bearing shaft  24  holds the impeller  14  at the center of the bottom inside surface of the shell  12 , which is along the central axis of the disposable filter  10 . The impeller  14  rotates freely around the bearing shaft  24 . The impeller  14  may be designed as a simple axially elongate stirring bar with its axis perpendicular to the axis of the shell  12 , like that used by laboratory stirrers. Preferably, the impeller  14  is an axially elongate structure with its axis parallel to that of the shell  12  and a plurality of vanes that engage the blood and force the blood to spin. More preferably, the impeller  14  is a smooth axially elongate cylinder, cone, or other axially elongate shape that rotates and causes the blood to rotate by viscous effects. Such a smooth cylinder is known in the art to move the blood gently, through shear effects, causing minimal damage to blood components such as red cells and leucocytes. In this embodiment, the impeller  14  contains the magnetic coupler  30 . The magnetic coupler  30  is preferably a permanent magnet with a north and a south pole which are disposed at diametrically opposed positions on the impeller  14  and distributed so that the center of mass and the center of force is aligned with the rotational central axis of the impeller  14 . Typical permanent magnet materials include, but are not limited to, samarium cobalt, neodymium iron boron, ceramics, and the like. A coupling magnet on a drive unit will be similarly configured and will attract opposing polarities on the magnetic coupler  30  in the impeller  14 . The magnetic coupler  30  is in one embodiment, embedded and enclosed within the impeller  14 . Typical methods of embedding the magnetic coupler  30  include injection molding, insert molding, machining the cavity and inserting the magnetic coupler  30  followed by gluing or bonding a cap over the magnetic coupler  30 . The impeller  14  with the magnetic coupler  30  is preferably balanced carefully so that the impeller  14  does not vibrate or wobble when it spins. 
         [0028]    The lockdown tabs  46  are located around the bottom outside edge of the cylindrical shell  12  of the disposable filter. Correspondingly, the motor drive  26  has lockdown or clamping mechanisms  44  located around the top outside edge of the cylindrical housing  42 . The lockdown tabs  46  mate with the lockdown mechanisms  44  and when the lockdown mechanisms  44  are in the locked position, the disposable filter  10  is attached to the motor drive  26 . In order to allow for disposability of the blood handling components, the lock-down or clamping mechanisms  44  permit reversible fastening of the blood filter shell  12  to the motor drive  26 . This is important since cross-contamination of patients&#39; blood must be prevented in order to control the spread of infectious diseases. The motor drive  26  may be reusable. In this embodiment, the clamping mechanism  44  is a set of latches that grasp protrusions  46  on shell  12  and hold it to the housing  42  of the motor drive  26 . In other embodiments, the clamping mechanism  44  may also be a bayonet mount, spring-loaded catch, magnetic latch or other fastening mechanism. 
         [0029]    The motor  32  of the motor drive  26  is affixed to the housing  42 . The central shaft  38  is affixed to, and protrudes from, the rotating armature of the motor  32 . The motor drive  26  most preferably uses an electric motor  32  powered by a 6 to 24 volt direct current (DC) power supply. Such DC power supplies comprise batteries or electronics to convert alternating current electricity to direct current. The motor  32  could also be designed to use standard 110 VAC to 220 VAC. A direct current power source is preferable to an alternating current power source because patient and hospital staff protection is maximized with the DC system. The motor  32  is powered through the power cable  34 . The power switch  36  and the power on light  48  are physically affixed to the housing  42  and electrically connected to the power line  34 . The power on light  48  turns on only when the motor  32  is electrically energized by activating the power switch  36 . The electric motor  32  spins at a pre-determined constant speed. The central shaft  38  rotates from 100 to 10,000 RPM and most preferably from 500 to 5,000 RPM. Alternative embodiments of the motor  32  include, but are not limited to, compressed air or hydraulically driven motors. 
         [0030]    In this embodiment, the magnetic driver  40  is affixed to the shaft  38  and rotates with the shaft  38 . The magnetic driver  40  is located near the perimeter of the housing  42  so that when the disposable blood filter  10  is positioned against the motor drive  26 , the magnetic driver  40  is magnetically engaged to the magnetic coupler  30  that is affixed to the impeller  14  of the disposable blood filter  10 . The motor  32  spins the shaft  38  and the magnetic driver  40 . The magnetic driver  40  has a magnetic field that acts through the housing  42  of the motor drive  26  and through the shell  12  of the disposable blood filter  10 . The magnetic field interacts with the magnetic coupler  30  in the impeller  14  and causes the impeller  14  to rotate at the same rate as that of the motor  32 . The magnetic driver  40  is preferably a bar magnet that spins about its central region with north and south poles diametrically opposed and equidistant from the center of rotation. 
         [0031]    The magnetic driver  40  and magnetic coupler  30  may both be permanent magnets. Alternatively, at least one of either the magnetic driver  40  or the magnetic coupler  30  may be permanent magnets with the other being a material that is magnetically attracted to a magnet. In another embodiment, the magnetic coupler  30  or the magnetic driver  40  may be electromagnets energized by batteries or by another type of electrical power supply. Typical permanent magnets are fabricated from materials such as, but not limited to, neodymium iron boron, iron, ceramics, samarium cobalt and the like. Materials that are magnetically attracted to a magnet include, but are not limited to, iron or metallic alloys of iron. The magnetic coupler  30  is desirable because it allows for a sealed disposable blood filter  10  to be attached to the reusable motor drive  26 . In an alternate embodiment, a direct coupling between the central shaft  38  and the impeller  14  may be made using interlocking fingers on the impeller  14  that mate with the shaft  38  through a rotary seal. 
         [0032]    Attachment of the blood filter assembly  8  to a cardiopulmonary bypass system is accomplished using the optional pole clamp  50 . The pole clamp  50  is connected to the housing  42  of the motor drive  26  by the arm  54  and is secured to a pole by the setscrew  52 . By attaching the reusable motor drive  26  of the blood filter assembly  8  to a pole or other part of a pump console in the cardiopulmonary bypass system, interchange of the disposable blood filter  10  is more easily accomplished. 
         [0033]    Typical materials from which the disposable blood filter shell  12  and baffle  22  are fabricated include polycarbonate, polypropylene, polyethylene, polystyrene, polyvinyl chloride, fluorinated ethylene polymer (FEP), poly tetrafluoroethylene (PTFE), polysulfone, and the like. These same materials are used to fabricate the housing  42  of the motor drive  26 , although metals such as aluminum, stainless steel and the like would also work. Optionally, the interior of the shell  12  of the disposable blood filter  10  may be treated with an antithrombogenic material such as heparin and a bonding agent. The impeller  14  is made from materials that include polycarbonate, polypropylene, polyethylene, polystyrene, polyvinyl chloride, fluorinated ethylene polymer (FEP), polysulfone, poly tetrafluoroethylene (PTFE), and the like. 
         [0034]      FIG. 2A  shows a breakaway view of the shell  12  of the disposable blood filter  10 , which comprises the blood inlet port  20  and the impeller  14 . The impeller  14  further comprises the bearing shaft  24 , the magnetic coupler  30  and a plurality of vanes  15 . 
         [0035]    Referring to  FIG. 2A , the vanes  15  are affixed to, or are integral to, the impeller  14  and appear as fins, rotors or propeller blades. The magnetic coupler  30  is embedded within or affixed to the impeller  14 . 
         [0036]    The vanes  15  are rotated by the impeller  14 , which in turn, is rotated by the magnetic coupler  30  around the bearing shaft  24 . The blood enters the shell  12  through the blood inlet port  20  and is rotated by the vanes  15  on the impeller  14 . 
         [0037]      FIG. 2B  shows a top cross-sectional view of the shell  12  of the disposable blood filter  10 . In this embodiment, the impeller  14  has four vanes  15 . Any number of vanes  15  from one to  50  may be employed in the impeller  14 . The length and diameter of the vanes  15  are roughly equal to the overall length and diameter of the impeller  14 . 
         [0038]      FIG. 3A  shows an exterior view of the blood filter assembly  8 , comprising the disposable blood filter  10  and the motor drive  26 , viewing along the axis of the blood inlet port  20  and blood outlet port  16 . Also shown in  FIG. 3A  are the gas outlet port  18 , the gas trap  57 , the bleed valve  58 , the lock-down mechanisms  44 , and the lock-down tabs  46  on the shell  12 .  FIG. 3B  shows an exterior view of the blood filter assembly  8 , comprising the disposable blood filter  10  and the motor drive  26 , viewing perpendicular to the axis of the blood inlet port  20  and the blood outlet port  16 . Also shown in  FIG. 3B  are the gas outlet port  18 , the gas trap  57 , the bleed valve  58 , the lock-down mechanisms  44 , and the lock-down tabs  46  on the shell  12 .  FIGS. 3A and 3B  clearly show the tangential disposition of the blood inlet port  20  and the optional tangential disposition of the blood outlet port  16 . The blood inlet port  20  is disposed so that blood enters the disposable filter  10  in a direction tangential to the shell  12  to assist with generation of a rotational fluid field within the shell  12 . 
         [0039]      FIG. 4  shows a schematic diagram of a typical cardiopulmonary bypass circuit  60  comprising the blood filter assembly  8  of the present invention. The cardiopulmonary bypass circuit  60  further comprises a patient  62 , a venous drainage cannula  64 , a venous reservoir  66 , a circulatory assist pump  68 , a heat exchanger  70 , an oxygenator  72 , an optional gas pump  74 , a gas bleed line  76 , a particulate filter  78 , and an arterial inlet cannula  80 . 
         [0040]    The venous circuit of the patient  62  is connected to a blood inlet of the venous reservoir  66  through the venous drainage cannula  64 . An outlet of the venous reservoir  66  connects to an inlet of the circulatory assist pump  68  and an outlet of the circulatory assist pump  68  connects to an inlet of the heat exchanger  70 . An outlet of the heat exchanger  70  connects to an inlet of the oxygenator  72  and an outlet of the oxygenator  72  connects to the blood inlet port  20  of the blood filter assembly  8 . The gas outlet port  18  of the blood filter assembly  8  connects, by way of the gas trap  57  and bleed valve  58 , to an inlet of the gas pump  74 . An outlet of the gas pump  74  connects to an inlet of the venous reservoir  66  through the gas bleed line  76 . The blood outlet port  16  of the blood filter assembly  8  connects to an inlet of the particulate filter  78 . An outlet of the particulate filter  78  connects to the patient  62  through the arterial inlet cannula  80 . 
         [0041]    In yet another embodiment, the disposable blood filter assembly  10  is integrated into the venous reservoir  66  to minimize the need for additional priming volume. Since the venous reservoir  66  holds between 10 cc and 1000 cc of blood, the disposable blood filter  10  may be affixed thereto or integrated therein so that the internal volume of the disposable blood filter  10  does not add significantly to the priming volume of the cardiopulmonary bypass circuitry. In this embodiment, the drive unit or motor drive  26  for the filter  10  attaches to a component of the venous reservoir  66  to rotate the impeller  14  of the blood filter  10 . Typically, during cardiopulmonary bypass, venous blood is removed from the patient  62  by the venous drainage cannula  64  and is collected, generally by gravity feed, in venous reservoir  66  where it is de-foamed using standard technology such as de-foaming sponges and bonded surfactants. The venous reservoir  66  generally comprises a blood-air interface and blood entering the reservoir entrains air and other gasses into the blood. In addition, a suction line, used to remove blood from the operative field, returns air and blood to the venous reservoir  66 . The de-foaming devices in the venous reservoir  66  are incapable of removing micro-bubbles or small gas bubbles that have become entrained in the blood, thus the need for a blood filter. The blood is pumped from the venous reservoir  66  and through the rest of the cardiopulmonary bypass circuit  60  by the circulatory assist pump  68 . The blood passes through the heat exchanger  70  where it is cooled for the majority of the procedure to reduce the metabolic requirements of the patient  62 . Typical hypothermia temperatures range from 28 to 35 degrees centigrade. Toward the end of the procedure, the heat exchanger  70  is used to warm the blood to normothermia, approximately 37 degrees centigrade. The blood is next pumped through the oxygenator  72  where it is oxygenated and cleared of carbon dioxide. From the oxygenator  72 , the blood is pumped to the blood filter assembly  8 . 
         [0042]    Referring to  FIGS. 1 ,  3 A,  3 B, and  4  the blood filter assembly  8  of the present invention is designed to move gas bubbles present in the blood toward the center of the shell  12  so that blood may flow from the outside of the shell  12  through the blood outlet port  16 , free of these bubbles. The blood enters the blood filter assembly  8  through the blood inlet port  20 . Preferably, the blood inlet port  20  is positioned tangential to the shell  12  of the disposable filter  10 . The rotating impeller  14  pushes the blood and causes the blood to rotate. Tangential entry of the blood into the disposable filter  10  imparts a rotational velocity to the blood, thus requiring less shear stress on the blood for the motor  32  to turn the impeller  14  and rotationally accelerate the blood to the required velocity. 
         [0043]    The gas bubbles, many as small or smaller than 10 to 25 microns in diameter, need to be moved to the center of the disposable blood filter  10  in the time it takes for the blood to make a single pass through the filter  10 . By way of example, a typical blood flow rate through the cardiopulmonary bypass circuit  60  is approximately 5 liters per minute. A typical diameter for the blood filter  10  is 7.5 centimeters. With a 10-centimeter height, the blood filter will have a priming volume of about 440 cubic centimeters. That means blood will dwell within the blood filter  10  for about 5 seconds. The gas bubbles, therefore, have about 5 seconds to move radially inward to within the diameter of the baffle  22  and, thus, be separated from the blood that flows through the blood outlet port  16 . Rotational rates specified for this blood filter assembly  8  are sufficient to move bubbles as small as 7 to 10 microns to the center of the blood filter  10  within 5 seconds by means of centrifugal force. 
         [0044]    Buoyancy causes the gas bubbles to rise, relative to gravitational attraction, and pass out of the gas outlet port  18  and into the gas trap  57 , although the gas removal may be augmented by an optional external pump  74 , powered by electricity, for example. 
         [0045]    Gas and some blood, removed from the gas outlet port  18  of the disposable blood filter  8  are collected in the gas trap  57  and pumped back into the venous reservoir  66  by optional gas pump  74  through the gas bleed line  76  where the blood component can be reclaimed. The optional gas pump  74  is a continuously operating pump. Optionally, gas pump  74  is a demand pump and pumps only when the volume of gas collects in sufficient quantity to warrant return to the venous reservoir  66 . This may be accomplished using a fluid level sensor mounted in the blood filter assembly  8  or gas bleed line  76  that controllably turns power to the gas pump  74  on and off. The bleed valve  58  is optional and not necessary if the gas pump  74  is used. 
         [0046]    Referring again to  FIG. 4 , the blood is pumped from the blood filter assembly  8  through the blood outlet port  16  to the particulate filter  78 . The particulate filter  78  may be integral to the blood outlet port  16 . The particulate filter  78  filters solid debris and particulates, generally larger than 25 microns, using screens or filter meshes. The oxygenated blood is cleared of most particulates greater than 25 microns and most gas bubbles greater than 7 to 10 microns when it is returned to the patient  62  via the arterial inlet cannula  80 . 
         [0047]      FIG. 5  shows another embodiment of the disposable blood filter  10  wherein the impeller  14  is an axially elongate, smooth shape without any vanes or protrusions. This type of impeller  14  uses viscosity to create shear forces that cause the blood to spin. Referring to  FIGS. 1 and 5 , the impeller  14  is driven through the magnetic coupler  30  that is adapted to interact with the magnetic driver  40 . The preferred shape of the impeller  14  is conical and helps reduce the priming volume of the system. The blood inlet port  20 , the blood outlet port  16 , and the gas outlet port  18  are disposed in the same configuration as that shown in  FIG. 1 . 
         [0048]      FIG. 6  shows yet another embodiment of the disposable blood filter  10  wherein the impeller  14  is an axially elongate perforated structure such as a cylinder or cone. The impeller  14 , in this embodiment, comprises a filter mesh wall  56 . The filter mesh wall  56  is made from a mesh material or screen to provide particulate filtering for the blood that eliminates the need for a secondary particulate filter. The mesh material or screen has a maximum pore size of 25 to 35 microns to limit the size of particulates that can pass through the mesh wall  56 . 
         [0049]    The blood outlet port  16  is disposed tangential to the shell  12  of the disposable blood filter  10 . However, the blood inlet port  20  is disposed along the central axis of the disposable blood filter  10 . The blood inlet port  20 , optionally, rotates with the impeller  14  to pre-rotate the blood as it enters the filter system and to reduce shear forces acting on the blood at the center of the disposable blood filter  10 . The blood enters the filter  10  inside the impeller  14 . The gas outlet port  18  is disposed coaxially around the blood inlet port  20  to allow for gas entrapment and removal. The blood outlet port  16  is disposed outside the filter mesh wall  56  of impeller  14  and blood must pass through the filter mesh walls  56  to reach the blood outlet port  16 . 
         [0050]    In another embodiment, the blood is spun by magnets that directly interact with the ionic potential of the blood. This embodiment requires multiple high output electromagnets that are disposed circumferentially around the perimeter of the disposable blood filter  10 . These electromagnets are fired sequentially to form a rotational magnetic field on the blood. A central magnet or a plurality of central magnets is disposed on the core of the disposable blood filter  10  and serves as the alternative pole for the magnets disposed circumferentially around the filter. The blood inlet port  20  and blood outlet port  16  are disposed tangential to the shell  12  of the disposable blood filter  10 . The gas outlet port  18  is disposed as close to the axis of the disposable blood filter  10  as possible, given the central magnet structure, at its highest point. 
         [0051]    In another embodiment of this device, the blood filter assembly  8  also serves as a primary pump in a cardiopulmonary bypass circuit since centrifugal type pumps are regularly used in a large number of clinical cases. Centrifugal pumps are considered less damaging to the blood than their less-expensive roller-pump alternatives. 
         [0052]    In yet another embodiment, the blood filter assembly  8  can be used as a hemoconcentrator. A one-pass hemoconcentrator is useful in separating non-cellular fluids from the cells in the blood at the end of the bypass procedure. The rotational rates of the hemoconcentrator of the current invention will enable such separation of cells. The blood cells are forced to the perimeter of the shell  12  of the disposable blood filter  8  where they are drawn off through the blood outlet port  16 . Non-cellular materials, such as plasma, migrate to the center of the filter where the non-cellular materials are drawn out through the central port  18 . Rotational spin rates of 1,000 to 20,000 RPM, and more preferably 5,000 to 10,000 RPM, are required to cause adequate centrifugation effects to separate the cellular components from the non-cellular components in a device of 5 to 15 cm diameter. 
         [0053]    In a further embodiment, a pressure less than the ambient pressure within the cardiopulmonary bypass circuit  60  is applied to the interior of the disposable blood filter shell  12 . The pressure within the cardiopulmonary bypass circuit  60  is, generally, within the range of 0 to 200 mm Hg. By locally reducing the pressure within the blood filter shell  12 , the bubble size will be increased and the efficiency of the bubble separation will be likewise increased. The internal pressure within the disposable blood filter  12  is reduced by adding a pump to forcefully remove blood from the interior of the shell  12  through either the blood outlet port  16  or the gas outlet port  18 . Additionally, an optional restriction, or narrowing of the channel, is added to the blood inlet port  20 . 
         [0054]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.