Abstract:
A medical fluid delivery system includes a fluid disposable configured to hold and transport a medical fluid and an air separation chamber in fluid communication with the fluid disposable. The air separation chamber includes at least one fluid baffle floating within the air separation chamber and configured to separate air from medical fluid traveling through the chamber.

Description:
PRIORITY 
     This application claims priority to and the benefit as a divisional application of U.S. patent application entitled, “Dialysis Systems Having Air Traps With Internal Structures To Enhance Air Removal”, Ser. No. 11/865,577, filed Oct. 1, 2007, the entire contents of which are incorporated herein by reference and relied upon. 
    
    
     BACKGROUND 
     The examples discussed below relate generally to medical fluid delivery. More particularly, the examples disclose systems, methods and apparatuses for dialysis such as hemodialysis (“HD”) automated peritoneal dialysis (“APD”). 
     Due to various causes, a person&#39;s renal system can fail. Renal failure produces several physiological derangements. It is no longer possible to balance water and minerals or to excrete daily metabolic load. Toxic end products of nitrogen metabolism (urea, creatinine, uric acid, and others) can accumulate in blood and tissue. 
     Kidney failure and reduced kidney function have been treated with dialysis. Dialysis removes waste, toxins and excess water from the body that normal functioning kidneys would otherwise remove. Dialysis treatment for replacement of kidney functions is critical to many people because the treatment is life saving. 
     One type of kidney failure therapy is Hemodialysis (“HD”), which in general uses diffusion to remove waste products from a patient&#39;s blood. A diffusive gradient occurs across the semi-permeable dialyzer between the blood and an electrolyte solution called dialysate to cause diffusion. Hemofiltration (“HF”) is an alternative renal replacement therapy that relies on a convective transport of toxins from patient&#39;s blood. This therapy is accomplished by adding substitution or replacement fluid to the extracorporeal circuit during treatment (typically ten to ninety liters of such fluid). That substitution fluid and the fluid accumulated by the patient in between treatments is ultrafiltered over the course of the HF treatment, providing a convective transport mechanism that is particularly beneficial in removing middle and large molecules (in hemodialysis there is a small amount of waste removed along with the fluid gained between dialysis sessions, however, the solute drag from the removal of that ultrafiltrate is not enough to provide convective clearance). 
     Hemodiafiltration (“HDF”) is a treatment modality that combines convective and diffusive clearances. HDF uses dialysate flowing through a dialyzer, similar to standard hemodialysis, to provide diffusive clearance. In addition, substitution solution is provided directly to the extracorporeal circuit, providing convective clearance. 
     Most HD (HF, HDF) treatments occur in centers. A trend towards home hemodialysis (“HHD”) exists today in part because HHD can be performed daily, offering therapeutic benefits over in-center hemodialysis treatments, which occur typically bi- or tri-weekly. Studies have shown that a patient receiving more frequent treatments removes more toxins and waste products than a patient receiving less frequent but perhaps longer treatments. A patient receiving more frequent treatments does not experience as much of a down cycle as does an in-center patient who has built-up two or three days worth of toxins prior to a treatment. In certain areas, the closest dialysis center can be many miles from the patient&#39;s home causing door-to-door treatment time to consume a large portion of the day. HHD can take place overnight or during the day while the patient relaxes, works or is otherwise productive. 
     Another type of kidney failure therapy is peritoneal dialysis, which infuses a dialysis solution, also called dialysate, into a patient&#39;s peritoneal cavity via a catheter. The dialysate contacts the peritoneal membrane of the peritoneal cavity. Waste, toxins and excess water pass from the patient&#39;s bloodstream, through the peritoneal membrane and into the dialysate due to diffusion and osmosis, i.e., an osmotic gradient occurs across the membrane. Osmotic agent in dialysis provides the osmotic gradient. The spent dialysate is drained from the patient, removing waste, toxins and excess water from the patient. This cycle is repeated. 
     There are various types of peritoneal dialysis therapies, including continuous ambulatory peritoneal dialysis (“CAPD”), automated peritoneal dialysis (“APD”), tidal flow dialysate and continuous flow peritoneal dialysis (“CFPD”). CAPD is a manual dialysis treatment. Here, the patient manually connects an implanted catheter to a drain to allow spent dialysate fluid to drain from the peritoneal cavity. The patient then connects the catheter to a bag of fresh dialysate to infuse fresh dialysate through the catheter and into the patient. The patient disconnects the catheter from the fresh dialysate bag and allows the dialysate to dwell within the peritoneal cavity, wherein the transfer of waste, toxins and excess water takes place. After a dwell period, the patient repeats the manual dialysis procedure, for example, four times per day, each treatment lasting about an hour. Manual peritoneal dialysis requires a significant amount of time and effort from the patient, leaving ample room for improvement. 
     Automated peritoneal dialysis (“APD”) is similar to CAPD in that the dialysis treatment includes drain, fill and dwell cycles. APD machines, however, perform the cycles automatically, typically while the patient sleeps. APD machines free patients from having to manually perform the treatment cycles and from having to transport supplies during the day. APD machines connect fluidly to an implanted catheter, to a source or bag of fresh dialysate and to a fluid drain. APD machines pump fresh dialysate from a dialysate source, through the catheter and into the patient&#39;s peritoneal cavity. APD machines also allow for the dialysate to dwell within the cavity and for the transfer of waste, toxins and excess water to take place. The source can include multiple sterile dialysate solution bags. 
     APD machines pump spent dialysate from the peritoneal cavity, though the catheter, and to the drain. As with the manual process, several drain, fill and dwell cycles occur during dialysis. A “last fill” occurs at the end of APD and remains in the peritoneal cavity of the patient until the next treatment. 
     In any of the above modalities, entrained air and other gases are a concern. Entrained air can cause inaccuracies when pumping dialysate for either PD or HD. Entrained air can cause a reduction in effective surface area in a hemodialysis filter when it accumulates on the filter fibers, leading to a reduction in the effectiveness of the therapy. Entrained air entering a patient&#39;s peritoneum during PD can cause discomfort. Entrained air entering a patient&#39;s bloodstream during HD can have severe consequences. Accordingly, a need exists to provide an apparatus that ensures that entrained air is removed from dialysate or blood prior to delivering such fluids to the patient. 
     SUMMARY 
     The present disclosure relates to air and gas removal for dialysis systems and extracorporeal devices, e.g., blood separation, blood warming, etc. The structures disclosed herein can be performed in any type of peritoneal dialysis treatment or blood dialysis treatment such as hemodialysis, hemofiltration, hemodiafiltration and continuous renal replacement therapy. The embodiments below are disclosed in connection with a dialysis cassette that is loaded into a dialysis instrument. The dialysis cassette is part of an overall dialysis set which can include one or more supply bag, or connection to the dialysate generation system, one or more drain bag, a heater bag and associated tubing connecting the bags to the dialysis cassette. The user places the dialysis cassette within the dialysis instrument for therapy. The dialysis cassette can include one or more pump chamber, flow path and/or valve chamber. The dialysis instrument includes one or more pump actuator that actuates the pump chamber of the disposable cassette. The dialysis instrument also includes one or more valve actuator that actuates the valve chamber of the disposable cassette. The disposable cassette can also include a fluid heating pathway that operates with a fluid heater of the dialysis instrument. The disposable cassette can also include various regions for sensing pressure, fluid composition, fluid temperature, and fluid levels. 
     While air traps  50  are shown herein in connection with a disposable set described below, the separation chambers are alternatively stand-alone apparatuses that operate independent of the disposable cassette. Further, the present disclosure mainly discusses air but other gases can also be present and therefore the present air separation chambers can also trap these gases. In PD for example, gases from the patient can become entrained in fluid being pumped form the system. Also, gases from dialysate concentrate, such as bicarbonate can become entrained in fresh dialysate. It is expressly contemplated for the air separation chambers of the present disclosure to remove these additional types of gases. 
     As mentioned above, air in dialysate or dialysis fluid as well as air in blood needs to be removed before any of these fluids are either delivered to a dialyzer or patient. Air can be present in the system via air trapped in supply bags, air trapped in the tubes leading from the supply bags to the disposable cassette, air not completely primed from the disposable cassette itself and air that is released from solution when the dialysis fluid is mixed and/or heated. Air can also signal a leak in the disposable unit. 
     The air traps discussed below are shown generally in connection with a dialysis fluid, such as dialysate, having entrained air. It should be appreciated however that the embodiments are applicable equally to the removal of air from blood pumped from a patient to a hemodialyzer or hemofilter. As used herein, the term dialysis fluid includes, without limitation, mixed dialysate, mixed infusate, mixed replacement fluid, concentrated components of any of these, and blood. 
     In one embodiment, the disposable cassette defines an air separation chamber that has a fluid inlet and a fluid outlet. An inlet valve and an outlet valve are paired with the fluid inlet and fluid outlet of the air separation chamber, respectively. The air separation chamber also includes an air vent outlet, which is in fluid communication with one or more air vent valve. The air removed from fluid in the air trap is sent to atmosphere, to a holding vessel such as an empty bag or a fluid filled bag (e.g, saline bag or dialysate bag), or to a drain, for example, whichever is desired. 
     In one embodiment, the air separation chamber is configured with respect to the other components of the disposable cassette such that when the cassette is loaded into the dialysis instrument, the fluid inlet and fluid outlet are located towards a bottom or bottom wall of the air separation chamber, while the air outlet is located at or near the top of the dialysis instrument. Such configuration allows buoyancy forces to lift air bubbles from the dialysis fluid to the top of the air separation chamber for venting. 
     The dialysis cassette in one embodiment includes a rigid portion, which can be a hard plastic. The rigid portion is formed to have pump chambers (e.g., for diaphragm pumps) or pump tubing (for peristaltic pumping), fluid pathways and valve chambers. The rigid portion also defines some or all of the air separation chamber. It is contemplated that the disposable cassette will have flexible sheeting welded to one or both sides of the rigid portion of the cassette. The flexible sheeting allows a pneumatic or mechanical force to be applied to the pump chambers (e.g., diaphragm) and valve chambers to operate those chambers. It is also contemplated that at least one outer surface of the air separation chamber consumes a portion of one or both flexible sheets. In addition, one or both sides of the dialysis cassette can be rigid. 
     The disposable cassette can have a base wall or mid-plane that divides the disposable cassette into first and second sides. For example, in one embodiment the flow paths are provided on one side of the disposable cassette (one side of the base wall), while the pump and valve chambers are provided on the other side of the disposable cassette. The air separation chamber in one embodiment is provided on either the first or second side, whichever is more convenient. Here, the air separation chamber has one side surface that is a rigid mid-plane and a second side surface that is cassette sheeting. The cassette sheeting is welded to an air separation chamber inlet wall, an air separation chamber outlet wall, an air separation chamber top wall and an air separation chamber bottom wall, which each extends from and is formed with the mid-plane of the rigid portion. 
     It is expressly contemplated however to make the outer wall of the disposable cassette from a rigid material rather than from cassette sheeting. For example, a rigid piece could be welded, adhered or otherwise sealingly bonded to the air separation walls extending from the mid-plane of the rigid portion, or could be formed as one piece along with the mid-plane of the rigid portion during manufacture. 
     In still a further alternative embodiment, the mid-plane is not present within the air separation chamber, but the air separation chamber is bonded on two-sides by flexible sheeting. Still further alternatively, the mid-plane is not provided, however, the outer walls of the air separation chamber are rigid and adhere to the top, bottom, inlet and outlet walls via a suitable sealing process. 
     The air separation chamber includes one or more baffle or separation wall that is configured to disrupt the flow of fluid through the air separation chamber, promoting the separation of air from the dialysis fluid. In one embodiment, the baffle or separation wall is a single wall formed of the rigid material and extending upward from a base wall of the air separation chamber. The single baffle wall can be parallel to the inlet and outlet walls. Alternatively, the baffle is angled relative to one or both the inlet and outlet air chamber walls. The inlet and outlet air chamber walls can themselves be squared with the side walls of the disposable cassette. Alternatively, the inlet and outlet air separation chamber walls are angled with respect to the cassette side walls. For example, the air separation chamber can form a parallelogram shape with the inlet and outlet walls at a nonorthogonal angle with respect to the top and bottom walls of the air separation chamber. The baffle can be angled the same as or different than the inlet and outlet walls. 
     In one embodiment, inlet and outlet pathways leading to and from the air separation chamber inlet and outlet are inline with each other and are disposed at least substantially perpendicular to an air vent pathway leading out of the top of the disposable cassette. Here, the inlet and outlet pathways can be at least substantially horizontally disposed when the disposable cassette is loaded into the dialysis instrument. Alternatively, the inlet and outlet pathways are not aligned with each other and are provided at a non-horizontal angle. Indeed, in one embodiment, the inlet and outlet pathways are at least substantially vertically disposed and parallel to the air vent pathway when the cassette is loaded for operation. 
     The baffle is not limited to being a single plate and can instead be a polygonal shape formed from one or more wall extending within the air separation chamber. For example, the polygonal shape can have a wall that includes a straight or nonlinear surface. The nonlinear surface can extend from the bottom wall of the air chamber upwardly towards the air vent, change direction towards the outlet of the air chamber and then extend downwardly to the bottom wall of the air separation chamber. The polygonal baffle removes a volume within the air separation chamber such that the volume is separated from the dialysis fluid. The removed volume can be a solid rigid material or can be a hollow volume beneath the baffle wall. 
     The surface of the polygonal baffle forces the fluid to change direction at least once between the dialysis fluid inlet and the dialysis fluid outlet of the air separation chamber. The polygonal shape of the baffle can have one or more straight side so as to form a triangle or rectangle within the air separation chamber. The polygonal shape can be curved so as to form a semicircle within the air separation chamber. Further alternatively, the polygonal shape can have a combination of straight and curved surfaces. 
     While many of the embodiments below show the single wall or polygonal baffle extending from the bottom wall of the air separation chamber, it is expressly contemplated to extend the baffle from a wall different from the bottom wall. The baffle can instead be located above the bottom wall and extend for example from the mid-plane of the rigid portion. This baffle can also be a single wall baffle or a polygonal baffle having multiple walls or a single continuous wall. The baffle an be utilized as a barrier which forces the fluid to overflow or creates a constriction between another structural feature such that the fluid is forced to flow through. 
     It is further contemplated to provide multiple baffles within the air separation chamber. The multiple baffles can be any combination of single wall, polygonal shape, attached to or not attached to the bottom wall, and perpendicular or angled with respect to the dialysate inlet and outlet. A second baffle can for example extend from a surface of the first baffle. For example, a second single wall baffle can extend from a first polygonal baffle. Further alternatively, multiple single wall baffles can extend from the same polygonal baffle or from different polygonal baffles. 
     In still a further alternative embodiment, the baffle is configured to float within the air separation chamber. That is, the baffle is not connected to any of the walls of the rigid portion of the disposable cassette and instead is moveable independently within the air separation chamber. For example, the air separation chamber can include a plurality of spheres or other shaped members, which are too large to fit through any of the dialysate inlet, dialysis fluid outlet or air chamber openings. The plural spheres provide increased surface contact area with the fluid to better remove air bubbles from the liquid. Additionally, the free motion accorded to the baffle (or plurality of baffles) allows their portion to disrupt air bubbles that may have accumulated through surface tension onto various surfaces within the chamber or on themselves. This disruption allows the bubbles to be coalesced into larger bubbles and carried to the air/fluid interface of the chamber for venting. Alternatively, a loose fitting geometry, such as a coiled or twisted strip of plastic, is moveable within the air separation chamber to provide a large contact surface area with the dialysis fluid and to route fluid towards the air/fluid interface of the chamber where bubbles can be separated from the fluid. To this end, the spheres or other shaped members can be textured to provide even additional surface contact area. In an alternate stationary geometry, the geometry can be rigid on the air trap. 
     It is further contemplated to provide a nozzle at the air separation chamber inlet, which causes the inlet dialysis fluid to form a spray or mist, which further aids in removing air from the dialysis fluid. Still further, it is contemplated to provide a vibrator, such as an ultrasonic or mechanical vibrator within the disposable cassette, which contacts a portion of the disposable set, such as a portion of the disposable cassette. The vibrator vibrates the disposable cassette to further aid in dislodging air bubbles from the dialysis stream and to aid in their coalescence that increases the bubbles&#39; buoyancy force, thereby inducing them to float to the surface of the chamber for venting. In an embodiment, the ultrasonic or other type of vibrator is positioned within the dialysis instrument to contact the disposable cassette at the air separation chamber. The vibrator provides a force in addition to buoyancy to separate air from the fluid, forming an air separation area. The air separation area attacks entrained air on multiple fronts, one using the above described baffles and another using the ultrasonic or other type of vibration. It is further contemplated to add the nozzle at the inlet to this two-pronged air separation attack to form a three-pronged attack. 
     It is accordingly an advantage of the present disclosure to provide improved air separation chambers for the removal of air from the dialysis fluid or blood flowing through a disposable dialysis fluid apparatus. 
     Additional features and advantages are described herein, and will be apparent from, the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is one elevation view of one embodiment of a dialysis fluid air trap of the present disclosure. 
         FIG. 2  is another elevation view of one embodiment of a dialysis fluid air trap of the present disclosure. 
         FIG. 3  is further elevation view of one embodiment of a dialysis fluid air trap of the present disclosure. 
         FIG. 4  is still another elevation view of one embodiment of a dialysis fluid air trap of the present disclosure. 
         FIG. 5  is still a further elevation view of one embodiment of a dialysis fluid air trap of the present disclosure. 
         FIG. 6  is yet another elevation view of one embodiment of a dialysis fluid air trap of the present disclosure. 
         FIG. 7  is yet a further elevation view of one embodiment of a dialysis fluid air trap of the present disclosure. 
         FIG. 8  is an eighth elevation view of one embodiment of a dialysis fluid air trap of the present disclosure. 
         FIG. 9  is a ninth elevation view of one embodiment of a dialysis fluid air trap of the present disclosure. 
         FIG. 10  is simulations of the dialysis fluid air trap of  FIG. 9  in operation. 
         FIG. 11  is a tenth elevation view of one embodiment of a dialysis fluid air trap of the present disclosure. 
         FIG. 12  is an eleventh elevation view of one embodiment of a dialysis fluid air trap of the present disclosure. 
         FIG. 13  is a twelfth elevation view of one embodiment of a dialysis fluid air trap of the present disclosure. 
         FIG. 14  is an elevation view of a dialysis instrument and cassette operable with the instrument, wherein the instrument includes a vibrator for vibrating the disposable cassette to remove air from fluid flowing through the cassette. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings and in particular to  FIG. 1 , dialysis cassette  10  having air trap  50  illustrates one embodiment of the present disclosure. Dialysis cassette  10  is operable with any type of dialysis instrument, such as a peritoneal dialysis instrument, hemodialysis, hemofiltration, hemodiafiltration or continuous renal replacement therapy instrument. Dialysis cassette  10  can hold a dialysis fluid, such as dialysate or blood. The dialysis fluid can be premixed or cassette  10  can carry a component of dialysate such as a dialysate concentrate. 
     Dialysis cassette  10  in one embodiment is part of a disposable set, which includes one or more supply bag, a drain bag, a heater bag, and tubing running from those bags (not illustrated) to dialysis cassette  10 . Dialysis cassette  10  in one embodiment is disposable, however, dialysis cassette  10  could be cleaned for multiple uses in which case the air traps described herein are used multiple times. Dialysis cassette  10  includes a rigid portion have a cassette top wall  12 , a cassette side wall  14  and a cassette bottom wall  16 . Suitable materials for the rigid portion include polyvinyl chloride (“PVC”), acrylic, ABS, polycarbonate, and polyolefin blends. The rigid portion of cassette  10  also includes a base wall or mid-plane  18 , which separates cassette  10  into first and second sides. 
     The side of mid-plane  18  illustrated in  FIG. 1  includes pump chambers  20   a  and  20   b , which here are part of a pneumatically and/or electromechanically operated diaphragm pump. Alternatively, cassette  10  includes peristaltic pumping tubes that operate with a peristaltic pump actuator of the dialysis instrument. Cassette  10  also includes valve chambers, such as air separation chamber inlet valve chamber  22 , air separation chamber outlet valve chamber  24  and air separation chamber air vent valve chambers  26   a  and  26   b . The valve chambers can also be pneumatically and/or electromechanically operated. 
     The other side of cassette  10 , which is divided by mid-plane  18  (not illustrated), can include flow paths and/or other valve chambers and/or pump chambers. It should be appreciated that cassette  10  can have different structural layouts without affecting the performance air separation chamber  50 . Air separation chamber  50  can be located on either side of mid-plane  18  for space purposes or for other reasons related to component layout. 
     In the illustrated embodiment, the rigid portion of cassette  10  defines the wall or walls of pump chambers  20   a  and  20   b , which in the illustrated embodiment operate with a flexible cassette sheeting  28 , which is welded, heat sealed or solvent bonded to rigid walls  12 ,  14 ,  16 , etc., of the rigid portion of cassette  10 . Suitable cassette sheeting  28  includes polyvinyl chloride (“PVC”), polypropylene/polyethylene blends, polypropylene or Kraton blends, polyester, polyolefin, and ULDPE. The suitable PVC sheeting can include, for example, monolayer PVC films, non-DEHP PVC monolayer films, monolayer non-PVC and multilayer non-PVC films (wherein different layers are chosen to provide strength, weldability, abrasion resistance and minimal “sticktion” to other materials such as rigid cassette materials). Multiple layers can be coextruded or laminated with or without a gas barrier. 
     Cassette sheeting  28  is also used to open and close valve chambers, such as chambers  22 ,  24 ,  26   a  and  26   b . The dialysis instrument includes a controller unit that operates a program that controls when valves  22 ,  24 ,  26   a  and  26   b  are opened or closed. The controller unit can include, but is not limited to, a processor, memory, hardware (e.g. sensors, actuators, I/O boards, etc.), software, and algorithms. For example, inlet and outlet valves  22  and  24  can be sequenced during priming to fill the air separation chamber. Inlet and outlet valves  22  and  24  are open during dialysis fluid delivery and/or blood pumping to remove air from those fluids. While inlet and outlet valves  22  and  24  are shown directly in front of and behind the air separation chambers, it is also contemplated to move one or both the inlet and outlet valves  22  and  24  further away from the air separation chamber. One or both of inlet and outlet valves  22  and  24  can be configured to control flow to multiple places within cassette  10 , including the air separation chamber. 
     The controller unit is also programmed to operate vent valves  26   a  and  26   b  so as to remove air from the air separation chamber in a manner so as not to effect the sterility of the dialysis fluid flowing through cassette  10 . To this end, the controller unit can operate with a signal from an optical or ultrasonic sensor monitoring the level of fluid within the air separation chamber. Alternatively, the controller unit can operate with an air pressure signal from a pressure sensor monitoring the pressure of air in the chamber. In either case, the signal is monitored to determine when to perform the air purge valve sequence of valves  26   a  and  26   b . Alternatively, the controller unit is programmed to perform the valve sequence for valves  26   a  and  26   b  at set intervals. 
     Cassette  10  in  FIG. 1  also includes a plurality of rigid ports extending from one of the walls, such as cassette top wall  12 . In the illustrated embodiment, cassette  10  includes a vent port  30 , which operates with vent valves  26   a  and  26   b  and air separation chamber  50 . Cassette  10  also includes other ports, such as one or more fluid supply port  32 , a drain port  34 , a to- or from-heater port  36  and other ports, such as a patient port and a heater bag port. 
     Vent port  30  can vent air from air separation chamber  50  to atmosphere or to drain in different embodiments. Cassette  10  can include other apparatuses (not illustrated), such as pressure sensing areas, heater flow path areas, and additional pumping areas, such as heparin and/or saline pumping areas. 
     Air trap  50  refers generally to each of the air traps  50   a  to  50   l  discussed herein.  FIG. 1  shows one embodiment of the air separation chamber or air trap of the present disclosure, namely, air separation chamber  50   a . Air separation chamber  50   a  includes an inlet wall  52 , a bottom wall  54 , an outlet wall  56  and a top wall  58 . Walls  52  to  58  can extend from mid-plane  18 , such that mid-plane  18  forms one of the broad sides of air separation chamber  50 . Alternatively, mid-plane  18  extends along the outside of walls  52  to  58  but not inside air separation  50 , such that walls  52  to  58  extend the entire thickness of cassette  10 . Here, both broad surfaces of air separation chamber  50  can be made of flexible sheeting  28 . 
     Alternatively, one or both of the broad surfaces of air separation chamber  50  are made of the rigid material, wherein sheeting  28  is welded to the broad surfaces of air separation chamber  50 . For example, the profile shape of air separation chamber  50  can be welded or solvent bonded to walls  52  to  58 . Thereafter, the sheeting is welded or solvent bonded to the edges of the rigid broad sides of air separation chamber  50 . 
     In the case where mid-plane  18  forms one of the broad sides of air separation chamber  50 , the outer broad surface of air separation  50  can be flexible sheet  28  or a rigid piece, welded or solvent bonded to walls  52  to  58 . 
     Inlet valve  22  opens or closes an inlet pathway  62 , while outlet valve  24  opens or closes an outlet pathway  64 . Inlet pathway  62  communicates with air separation chamber  50  via inlet  66 , which is formed in wall  52  of air chamber  50 . Outlet pathway  64  communicates with air separation chamber  50  via an outlet  68  formed in wall  56  of air separation chamber  50 . It should be appreciated that while valves  22  and  24  are shown as inlet and outlet valves, respectively, each valve can be either an inlet or an outlet valve, e.g., for priming purposes both valves  22  and  24  may be inlet valves that prime fill chamber  50   a  up to a predetermined fluid level within the chamber. 
     Vent valves  26   a  and  26   b  open and close a vent line  70 . Vent  70  communicates with vent port  30  and with air separation chamber  50  via a vent outlet  72  formed in top wall  58  of air separation chamber  50 . Dual vent valves  26   a  and  26   b  allow the controller unit of the dialysis instrument to isolate a slug of air in vent line  70  before vent valve  26   b  is opened, allowing the air to escape via vent port  30  to atmosphere or drain. In the programmed sequence, with vent valve  26   b  closed, vent valve  26   a  is opened allowing vent line  70  to become pressurized with air. Once line  70  becomes pressurized, valve  26   a  is closed and valve  26   b  is opened, relieving the pressure in vent line  70 . 
     With air separation chamber  50   a , inlet pathway  62  and outlet pathway  64  are aligned with each other and are at least substantially perpendicular to vent line  70 . Walls  52  and  56  are at least substantially orthogonal to walls  54  and  58 , forming a square or rectangular air separation chamber  50 . 
     Air separation chamber  50   a  includes a single baffle  80   a , which as illustrated is a single wall extending vertically upwardly from bottom wall  54  past inlet  62  and outlet  64 . Single baffle  80   a  is also integral with mid-plane  18  in one embodiment. Baffle  80   a  forces the flow of dialysis fluid  82  vertically upward from inlet  62  against the direction of gravity g, along a first surface of baffle wall  80   a . Baffle  80   a  and outlet wall  56  then force dialysis fluid  82  down a return surface of baffle wall  80   a , to outlet flow path  64  and to outlet valve  24 . As the flow of dialysis fluid  82  rises and flows over separation wall or baffle  80   a , the fluid it is slowed down due to increased cross-sectional area of air chamber  50   a . Air is collected in the upper section  84  of chamber  50   a . The primary purging action of air chamber  50   a  is the force of buoyancy. 
     Referring now to  FIG. 2 , air separation chamber  50   b  operable with cassette  10  includes many of the same features as air separation chamber  50   a . Here however, inlet wall  52  and outlet wall  56  are tapered outwardly from bottom wall  54  to top wall  58 , producing an air separation chamber having a substantially trapezoidal shape. The shape of air separation chamber  50   b  causes the dialysis fluid  82  flow cross-section to increase gradually in a vertical direction, enabling further slowing of the fluid, and allowing more time for buoyancy forces to lift air bubbles from the dialysis fluid  82 . 
     Single wall baffle  80   b  in air separation chamber  50   b  is tilted away from the 90° vertical position of baffle  80   a , towards outlet line  64  and outlet valve  24 . Tilted baffle  80   b  causes the cross-section of dialysis fluid flow  82  on the inlet side of chamber  50   b  to increase even more as dialysis fluid  82  flows vertically upward until reaching the free end of baffle  80   b , further slowing the fluid and allowing more time for buoyancy forces to lift air bubbles from the dialysis fluid  82 . 
     Single wall baffles  80   a  and  80   b  and indeed any of the single wall baffles describe herein (baffles  80 ) extend in one embodiment the total thickness of the air separation chamber, for example, all the way from mid-plane  18  to the cassette sheeting  28 . Alternatively, wall or baffle  80  does not extend all the way across the width of the air separation chamber. In such case, additional gusseting or support can be provided. Also, additional support or gusseting can be provided to baffles  80  when the air separation chamber is bounded on both broad sides by flexible sheeting  28 . 
       FIG. 3  illustrates a further alternative air separation chamber  50   c  operable with cassette  10 , in which inlet wall  52  and outlet wall  56  are positioned at a non-orthogonal angle with respect to bottom wall  54  and top wall  58 . The shape of air trap  50   c  is substantially that of a parallelogram. Baffle  80   c  is at least substantially the same as baffle  80   b  and is at least substantially aligned with the angled walls  52  and  56  in air separation chamber  50   c . Besides increasing cross-sectional flow area on the inlet side of baffles  80   b  and  80   c , angling baffles  80   b  and  80   c  in the direction shown also extends or lengthens the dialysis fluid  82  flow path along the left or inlet portion of the air separation chamber  50   a.    
     Air separation chamber  50   d  of  FIG. 4  operable with cassette  10  illustrates that inlet path  62  and outlet path  64  are not aligned and are not orthogonal to vent line  70 . The shape of air trap  50   d  is once again polygonal. Angled baffle  80   d  is the same as or similar to baffles  80   b  and  80   c . However, as illustrated in  FIG. 4 , the direction of the inlet and outlet pathways,  62  and  64  respectively, can be in directions other than horizontal or vertical. Air separation chamber  50   d  allows a cross-sectional area on the inlet portion of the valve chamber to increase, such that fluid velocity slows as it fills over baffle  80   d . Outlet flowpath  64  angled as shown tends to lengthen the exit flow path, between baffle  80   d  and outlet wall  56 , of chamber  50   d  since in this configuration buoyancy and drag forces acting on the air bubbles are not in opposite directions. The buoyancy force is always opposite to the direction of gravity whereas the drag force is opposite the velocity of a particle. The bubbles would rise up and they will only feel a smaller component of the drag force opposing their rise. 
     Air separation chamber  50   e  of  FIG. 5  operable with cassette  10  illustrates a number of additional concepts. Here, inlet and outlet pathways  62  and  64  are vertical when cassette  10  is placed in an operable position. Dialysis fluid flow  82  in pathways  62  and  64  is accordingly aided or impeded by the force of gravity g. 
     Air separation chamber  50   e  also has multiple baffles  80   e   2  and  80   e   3 . Bottom wall  54  includes or has multiple surfaces or walls which force dialysis fluid  82  upwardly through inlet pathway  62 , over a curved or nonlinear portion of bottom wall  54 , down a vertical wall of bottom wall  54 , and out outlet pathway  64 . Air separation chamber  50   e  includes first and second flow restrictions or baffles  80   e   2  and  80   e   3 . Baffle  80   e   2  is not connected to bottom wall  54  and instead extends from mid-plane  18 . Baffle  80   e   2  forms a narrow channel  86  between the baffle and the top surface of bottom wall  54 . Dialysis fluid flow  82  is forced through channel  86 . Second baffle  80   e   3  extends from bottom wall  54  and thus forces fluid to flow up over bottom wall  54 . 
     The net effect of the two baffles  80   e   2  and  80   e   3  of air separation chamber  50   e  is the creation, essentially, of three fluid regions  82   a ,  82   b  and  82   c  of dialysis fluid  82  within the air separation chamber. Each region resides above the curved surface of bottom wall  54 . In the regions, dialysis fluid  82  flows into chamber  50   e  through inlet pathway  62  and into left chamber  82   a . Baffle  80   e   2  forces fluid to flow through a constriction  86  into middle region  82   b . Fluid velocity in region  82   a  decreases due to the restriction through opening to  84 , aiding de-gassing due to buoyancy force. Fluid pressure builds in region  82   a  and a difference in fluid level as illustrated results between regions  82   a  and  82   b.    
     Dialysis fluid  82  rises for a second time in large region  82   b , resulting in a slowed flow and a second opportunity to de-gas via buoyancy forces. When the dialysis fluid level rises in region  82   b  to the free end of second baffle  80   e   3 , the dialysis fluid  82  flows over baffle  80   e   3  and begins to fill third region  82   c . The surface of baffle  80   e   1  is channeled slightly to allow the dialysis fluid  82  to pool in both regions  82   b  and  82   c  while filling. Dialysis fluid  82  rises to the edge of outlet pathway  64  and then flows out pathway  64 , leaving air separation chamber  50   e . Depending on the dialysis fluid  82  flowrate into region  82   c , the fluid level in the region may be the same as (as shown) or lower than that of  82   b . Region  82   c  aids in de-gassing any air bubbles remaining in dialysis fluid  82 . 
     Air separation chamber  50   f  of  FIG. 6  operable with cassette  10  is very similar to air separation chamber  50   e  of  FIG. 5 . Here however, second baffle plate  80   e   3  is not provided and instead curved wall  54  includes a hump at its exit side to decrease dead zones in the fluid flow in region  82   b  (i.e., areas of low or no fluid flow or stagnation). Also, baffle  80   f   2  is modified to have a triangular shape, further decreasing the dead zones and increasing circulation zones. Angled exit  82   c  increases the amount of air bubbles that ride upwardly along the outlet-side surface of baffle  80   f   2  because the drag force along the baffle  80   f   2  and the buoyancy force are not co-linear. Outlet flowpath  64  angled as shown tends to lengthen the exit flow path, between baffle  80   f   2  and outlet wall  56 , of chamber  50   e  since in this configuration buoyancy and drag forces acting on the air bubbles are not in opposite directions. The buoyancy force is always opposite to the direction of gravity whereas the drag force is opposite the velocity of a particle. The bubbles would rise up and they will only feel a smaller component of the drag force opposing their rise. 
     Air separation chamber  50   g  of  FIG. 7  operable with cassette  10  illustrates a further modification of the air separation chamber  50   e  of  FIG. 5 . Here, three polygonal baffles  80   g   1  to  80   g   3  are each polygonal shape and positioned to create free flow regions. Again, the angled surfaces of polygonal baffles  80   g   1 ,  80   g   2  and  80   g   3  increase the ability of those surfaces to carry bubbles upward due to a drag force and buoyancy force differential. In this configuration, buoyancy and drag forces acting on the air bubbles are not in opposite directions. The buoyancy force is always opposite to the direction of gravity whereas the drag force is opposite the velocity of a particle. The bubbles would rise up and they will only feel a smaller component of the drag force opposing their rise.  FIG. 7  further illustrates that once diameter D 1  and length L are determined, the dimensions of each of the baffles  80   g   1 ,  80   g   2  and  80   g   3  as well as inlet wall  52 , outlet wall  56 , bottom wall  54  and top wall  58  are also set. Fluid in regions  82   a  and  82   b  can fill to the dimensions shown, with the fluid in region  82   a  filling to a slightly higher level. 
     The flow pattern of air separation chamber  50   g  is similar to that of chambers  50   e  and  50   f . In a similar manner, dialysis fluid  82  is forced from region  82   a  to region  82   b  through opening  86 , allowing the dialysis fluid  82  to fill and de-gas for a second time in region  82   b . Dialysis fluid  82  eventually rises to the free end of polygonal baffle  80   g   3  and flows over baffle  80   g   3  to outlet pathway  64 . Depending on the dialysis fluid flowrate into region  82   c , the fluid level in the region may be the same as (as shown) or lower than that of  82   b . Region  82   c  aids in de-gassing any air bubbles remaining in the dialysis fluid  82 . 
     Air separation chamber  50   h  of  FIG. 8  operable with cassette  10  shows a further modification over these air separation chambers of  FIGS. 5 to 7 . Here, outlet wall  56  is also angled to help air bubbles travel upwards towards air collection portion  84  via third flow region  82   c . Similar to air separation chamber  50   g , air separation chamber  50   h  includes three polygonal baffles  80   h   1  to  80   h   3 , which are each positioned to create free flow regions. 
     Referring now to  FIG. 9 , air separation chamber  50   i  operable with cassette  10  illustrates one preferred air separation chamber of the present disclosure. While chambers  50   e  to  50   h  of  FIGS. 5 to 8  are very effective at removing air from dialysis fluid  82 , chambers  50   e  to  50   h  consume a fair amount of space within cassette  10 . It is desirable from a manufacturability and cost standpoint to make cassette  10  smaller rather than larger. It has been found that the first portion of air separation chambers  50   g  and  50   h  alone provides a very effective air removal chamber. Thus it is believed that air separation chamber  50   i  provides a smaller but effective chamber. Similar structures to air separation chamber  50   i  are included in first regions  82   a  of chambers  50   g  and  50   h  and are also very effective in removing gas bubbles from the fluid as discussed above. 
     Air separation chamber  50   i  as seen includes polygonal baffle  80   i , which has a triangular shape, including angled inlet surface  90  and angled outlet surface  92 . Surfaces  90  and  92  can be straight (as shown) or curved. Angled inlet surface  90  forms a first dialysis fluid region  82   a  with inlet wall  52 . The angled wall provides an increase in the cross-sectional flow area that slows the dialysis fluid  82  as it rises within region  82   a.    
     Angled outlet surface  92  forms a second dialysis fluid region  82   b  with outlet wall  56 . As fluid fills past the apex of baffle  80   i , the cross-sectional area approximately doubles, further slowing the flow of dialysis fluid  82  and allowing buoyancy forces to push air bubble from the fluid. Fluid exit  64  extends the outlet flow path similar to air separation chamber  52   d  such that the flow path is extended as much as possible in the air trap. Fluid pathway  64  acts as a constricted exit having a smaller cross-sectional area as compared with fluid inlet  62 . Air collects in region  84  and is purged through air purge line  70 . 
     As seen additionally in  FIGS. 7 and 8 , in one implementation if inlet wall  52  and top wall  58  are each 2L in length, sides  90  and  92  have a vertical component of length L. The apex of baffle  80   i  or the intersection of sides  90  and  92  occurs approximately at a distance L from inlet wall  52  and outlet wall  56 . This implementation as seen below strikes an effective balance by separating chamber  50   i  into different regions while allowing an ample common area for dialysis fluid  82  to release air bubbles at the interface with air collection portion  84 . 
       FIG. 10  illustrates an output of a simulation of air separation chamber  50   i , showing pathways taken by larger air bubbles, approximately five-hundred microns in diameter, trapped within dialysis fluid  82  when flowing through air separation chamber  50   i  at a certain flowrate and a certain fluid level. 
     Referring now to  FIG. 11 , air separation chamber  50   j  operable with cassette  10  illustrates an additional concept of providing a nozzle  74  at inlet  62 . Nozzle  74  creates a mist or spray of fluid leaving the nozzle due to the low pressure at the exit of the nozzle. The formation of the spray causes de-gassing of the dialysis fluid  82  due to the increased dialysis surface area that the mist creates, and in particular in combination with a negative pressure that may be present in chamber  50   j , which would help to pull air from the fluid. One embodiment for providing a nozzled flow into an air separation chamber is described in co-pending application entitled “Dialysis System Having Non-Invasive Fluid Velocity Sensors”, Ser. No. 11/876,619, filed Oct. 22, 2007, the pertinent portions of which are incorporated here expressly by reference. Nozzle  74  sprays inlet dialysis fluid  82  against a splash wall  76 . Splash wall  76  causes air to de-gas from the dialysis fluid  82  due to impact and also protects against fluid spray exiting through air line  70 . 
     Dialysis fluid  82  falling down along splash plate  76  pools in a first liquid region  82   a . A baffle  80   j  forces the pooled fluid from region  82   a  through opening  86  caused by baffle  80   j  into a second liquid region  82   b . Fluid region  82   b  provides another opportunity for liquid to de-gas due to buoyancy forces before the dialysis fluid  82  leaves exit fluid pathway  64 . 
     Nozzle  74  may cause the exiting fluid to foam, which would not be desirable for de-gassing blood in an HD blood circuit for example. However, air separation chamber  50   j  is suitable for any dialysate circuit. 
     Referring now to  FIGS. 12 and 13 , air separation chambers  50   k  and  50   l , each operable with dialysis fluid cassette  10  illustrate further alternative embodiments of the present disclosure. Air separation chamber  50   k  of  FIG. 12  includes textured spheres or members  94 , which are placed loosely within air separation chamber  50   k . That is, spheres or members  94  are free to move within chamber  50   k . The particles are sized so as not to be able to fit into, or block flow through, any of inlet line  62 , outlet line  64  or air line  70 . Suitable spheres can be obtained from McMaster-Carr, model number 9587K13, 1383K44, or similar. Spheres or members  94  introduce additional surface area for bubbles to attach to and be pulled from dialysis fluid  82 . Spheres  94  also serve to agitate fluid flow through chamber  50   k , which has the dual benefit of precipitating air that may be dissolved in the fluid and dislodging and/or coalescing bubbles that may have accumulated on the interior chamber surface or sphere surface. The bubbles move upwardly and eventually de-gas into air portion  84 . Spheres or members  94  also agitate the flow of liquid within air separation chamber  50   k , which also helps to free air bubbles from the dialysis fluid  82 . 
     Air separation chamber  50   l  of  FIG. 13  illustrates a helical or coiled ramp  96 , which can be textured to produce additional surface area. Ramp  96  in one embodiment is free to move within chamber  50   l . Ramp  96  can be made of a suitable medical grade plastic, such as any of the material listed above for the rigid portion of cassette  10 . Ramp  96  pulls bubbles out of the dialysis fluid  82  and also serves to turbulate or agitate fluid flow through chamber  50   l.    
     Referring now to  FIG. 14 , dialysis machine  100  illustrates a further alternative air separation apparatus and technique of the present disclosure. Dialysis machine  100  includes a main or instrument portion  102  and a door  104 , which opens and closes with respect to main portion  102  to accept cassette  10 . Cassette  10  can have any of the air separation chambers  50  discussed above. In an embodiment, the air separation chamber in operation is pressed against a contact transducer or vibrator  106 , which is configured to vibrate the liquid  82  at the air separation chamber. One suitable contract transducer  106  is provided by Xactec Corporation, model number CM-HP-1/2-1.00. While one preferred embodiment is to vibrate the liquid at the air separation chamber  50 , it should be appreciated that contract transducer  106  can be configured to shake the entire cassette  10  or other portions of the cassette, such as a pump chamber to relieve air bubbles from the dialysis fluid  82 . 
     It is accordingly contemplated to provide a multi-prong attack for removing and trapping air from dialysis liquid. Each of: (i) inducing vibrations into the air separation chamber; (ii) providing the baffles for buoyancy removal; and/or (iii) providing the nozzle (with any of the air separation chambers  50  described herein) to mist the dialysis fluid  82  into a spray and to increase fluid surface area, ultimately enables the gas to more readily pull from the fluid via negative pressure in order to remove gas from the liquid. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.