Patent Publication Number: US-7591812-B1

Title: Passive venous air purging chamber with vent/sucker blood handling capabilities

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a Continuation-in-Part of International Application number PCT/US2005/002198 filed Jan. 24, 2005 titled “A Soft-Shell Venous reservoir-Cardiotomy Reservoir with Improved Air Removal Capability” and published under the PCT as WO 2006/057650 A2 Jun. 1, 2006 that claims the priority date of the provisional Patent Application Ser. No. 60/538,277 filed Jan. 22, 2004, the disclosures of those applications being incorporated herein by reference thereto. 

   GOVERNMENT INTERESTS 
   This invention was in part made with government support under SBIR Grants # R44HL-55034 and R44-HL66810 awarded by the National Institute Health, National Heart, Lung, and Blood Institute. As such the government may have certain rights in the invention. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The full invention consists of an air purging system in combination with a compliant storage chamber having at least one pliable wall that forms a venous reservoir and a two-chamber cardiotomy reservoir incorporated atop the storage chamber. The combined units provide a collapsible “closed” venous reservoir unitized with cardiotomy reservoir, with vented blood separated from the sucker blood, having air removal features that improve that of a hardshell venous-cardiotomy reservoir unit. The innovative characteristics of each of the three devices support their use on their own. 
   2. Description of the Prior Art 
   During cardiopulmonary bypass, blood flow from the venous side of the patient to a venous reservoir depends on the resistance of the fluid conduit between patient and reservoir and the pressure difference between patient and reservoir. When the reservoir is maintained at atmospheric pressure, that pressure difference is the height difference between patient and reservoir; the resulting flow is referred to as gravity drainage. Venous drainage by gravity alone provides inadequate return during procedures such as minimally invasive cardiac surgery and bypass via femoral cannulation. Usually it is the resistance of the venous cannula that limits the flow rate. Vacuum assisted venous drainage (VAVD) is a technique that overcomes flow limitations by applying suction to the venous reservoir thereby increasing the pressure difference between the venous cannulation site and venous reservoir. VAVD allows for a decrease in the inner diameter (ID) of the venous line, thereby reducing prime volume and enabling the use of a smaller internal diameter cannula, which translates to an easier insertion, better surgical view and a smaller surgical incision. 
   Clinically, venous bags are used because they provide significant safety features: if the bag accidentally empties, it collapses, thereby preventing gross air from being pumped to the patient, they have no, or very little, air-blood interface, and they require no antifoam agents that can embolize into the blood. Designs that allow VAVD with venous bags have been previously described by Tamari&#39;s U.S. Pat. Nos. 6,337,049 and 6,773,426, and Cambron&#39;s U.S. Pat. No. 6,537,495. The present invention also allows the user the option of VAVD. 
   The only venous reservoir that combines a flexible wall and a rigid wall to form a “closed” variable blood chamber is that described in U.S. Pat. Nos. 4,424,190 and 4,959,062. Each of these devices has its outlet at the lowest point of the venous reservoir (see 24 in FIG. 24 of U.S. Pat. No. &#39;062, and 36 in FIG. 3 of U.S. Pat. No. &#39;190). This choice makes it more difficult to assure that the pliable wall seals against its mating rigid wall when the reservoir empties. The &#39;190 patent has a drawback: once that reservoir empties, its pliable wall is sucked in at its outlet and shuts off as it should. However, when the volume returns, the flexible wall remains stuck to the outlet and does not open until the reservoir is almost full. The &#39;062 patent&#39;s design alleviates that flaw however it introduced another problem: when the reservoir empties the rib prevents the flexible wall from completely sealing against the outlet port allowing air from the cardiotomy reservoir to be sucked in, an undesirable outcome. The present invention provides a design that allows the outlet to close completely when the reservoir empties and to easily open when less than ⅓ of the volume of a full reservoir returns. 
   When air enters blood, the blood foams. Hard shell reservoirs have defoamers within the venous inlet chamber (e.g. see defoamer 6 in distribution chamber 22 of FIG. 1 in Fini, U.S. Pat. No. 6,287,270). In the open, hard shell reservoir, air escapes by floating to the top of the reservoir where it is purged to atmosphere. In prior art venous bag reservoirs, air also floats to the top but it must be actively eliminated. This can be done manually with a syringe, or more frequently with a roller pump intermittently operating to remove air accumulating at the top of the bag. This is shown in  FIG. 1  where foam that accumulates at the top of collapsible venous reservoir  1103  is removed by suction pump  1114  that pumps that foam/air to cardiotomy reservoir  1115 . With either method, a sudden large volume of air can overwhelm the air removal system and cause disastrous consequences, especially without a vigilant perfusionist. Further, sucking blood and foam through the small diameter of the tubes and stopcocks used at the top of current bags results in blood damage. Various solutions have been proposed. Tamari&#39;s U.S. Pat. Nos. &#39;049 and &#39;426 eliminate air from a venous bag automatically by utilizing a floating ball valve, a hydrophobic membrane, or a level sensor. All three systems require suction to remove the air. Also, with current available membrane materials, the membrane has limited effective life. 
   Tamari&#39;s &#39;049 and &#39;426 patents also describe a blood chamber with a level sensor that activates a vacuum source to remove the air when the blood level in the chamber drops below that sensor. The design provides automated means to remove air from that chamber and that chamber in combination with a blood chamber having at least one flexible wall allows automated air removal and VAVD with a venous bag but does not, as the present invention, allow for passive air removal. 
   Active air removal required for venous bags also results in more difficult priming. Thus, once a bag is primed and the prime solution is recirculated, the air in the rest of the circuit is returned to the bag, again filling it with air. A better system would not allow air to return to the primed bag. This would reduce priming time and the possibility that air can remain in the bag. 
   State-of-the-Art soft-shell venous reservoirs with a screen are poorly designed: a large portion of their screen contacts the internal walls of the bag rendering that portion of the screen ineffective and thereby increasing the resistance to blood flow across the screen. This is illustrated in  FIG. 1   a , a line drawing of a prior art bag made by Cobe,  FIG. 1   aa , a line drawing of a cross section along line  1   aa - 1   aa ′ of the bag shown in  FIG. 1   a , and  FIG. 1   ab , an enlargement of the circled section of the bag shown in  FIG. 1   aa . As illustrated, at low volume most of the screen contacts the walls of the bag and is unavailable for blood flow. The contact area of the screen contacting the walls of the bag increases as the volume in the reservoir decreases. Two typical prior art venous bags exhibiting these problems are the ones described in Hess&#39;s U.S. Pat. No. 5,573,526 and in Stewart&#39;s U.S. Pat. No. 5,720,741. The latter is depicted in the present application in  FIG. 1   a  as PRIOR ART. One aspect of the present invention eliminates contact between the screen and the walls of the chamber housing it, thereby allowing the use of a screen with smaller area and/or smaller pores. Though Tamari&#39;s &#39;049 and &#39;426 patents illustrate designs that prevent the screen from contacting the wall of the reservoir, (e.g. FIGS. 7a and 7b of &#39;049), those designs in combination with a venous reservoir require that controlled vacuum be applied to the top of the venous bag to remove air. 
   As the pore size of filter screen used to trap bubbles decreases so does the effective open area and resistance to flow. For example, the preferred screen used for State-of-the-Art venous bag has a pore size of 105μ and an open area of 52% as compared to an open area of only 30% for a screen with a pore size of 37μ. However, as described in reference to screen  2   c  in  FIG. 3   a , the use of smaller pore screen reduces the size and number of bubbles. The larger effective screen area of the present invention allows using a smaller pore size screen and still maintaining a total open area that is equal to, or is even larger than, State-of-the-Art bags thereby maintaining a blood velocity across the screen that equals to or is lower than that associated with current bags. A lower velocity translates to lower number of bubbles crossing the screen. 
   At low blood levels, or if there is air in current soft shell reservoirs, the indication of blood level is inaccurate and the alarms, mostly designed for hard shell reservoirs, are not reliable. The present invention provides more reliable means to alarm at low blood levels. 
   A softshell reservoir with integrated cardiotomy reservoir is described in Elgas&#39;s U.S. Pat. No. 5,935,093. That softshell blood reservoir incorporates an integral flexible cardiotomy section in which a filter/defoamer unit is supported in a semirigid cage. The reservoir also incorporates a storage section and a mixing section. The three sections can selectively communicate with each other. A major shortcoming of this unit is that if the cardiotomy, in fluid communication with the venous reservoir, empties, then air can enter the venous reservoir and exit its outlet. The unit also does not allow VAVD and requires active removal of air from the venous reservoir. The present invention prevents air from the cardiotomy to exit the venous reservoir, allows VAVD and air is exhausted to atmosphere passively. 
   Almost all hard shell venous reservoirs incorporate a cardiotomy reservoir; the user gets a single unit (e.g. Thor et al&#39;s U.S. Pat. No. 5,411,705 and Fini&#39;s &#39;270 patent). The lower cost, shorter setup time and ease of air removal are the major reasons that more hard shell reservoirs are used than collapsible venous reservoirs. In one form, the present invention provides the user with a collapsible venous reservoir and a cardiotomy reservoir as a single unit that is easy to setup, and is clinically safer than the State-of-the-Art unified hard shell venous-cardiotomy reservoir unit. 
   During CPB, “clean” blood includes blood aspirated from a venting site (e.g. the aortic root cardioplegia cannula, LV vent), blood with entrapped air withdrawn from the top of a venous bag (i.e. as used by NovoSci), and blood purged from the top of the arterial filter. Clean blood is distinctly different from the “dirty” blood sucked from the surgical field, mostly of which comes from the pericardial sack. Before clean blood is returned to the patient, its entrapped air must be removed. Vent blood may entrain a large volume of air. If that blood is added directly to venous blood, it generates a large volume of foam (and a concomitant large blood-gas area) and can easily overwhelm the active air removal systems of current minicircuits (see below). Before sucker blood can be returned it must be filtered to remove both debris (e.g. particles of fat and tissue, clots) and entrapped air. Retransfusion of cardiotomy suction blood and mediastinal shed blood increases postoperative systemic inflammatory response, hemolysis, acellular lipid deposits in the microvasculature, thrombin, neutrophil, and platelet activation, and the release of neuron-specific enolase. It is considered preferable to send this blood to a cell saver. When using venous bags both the clean and dirty blood are processed by passing through a cardiotomy reservoir, a disadvantage because the clean blood is exposed to the inlet of the filter for the dirty blood with its aforementioned deleterious consequences. 
   There are no cardiotomies that filter the clean blood separate from the dirty blood and keep the two apart. Some State-of-the-Art hard shell reservoirs allow clean blood to bypass the dirty blood filter by directing the clean blood into the venous blood inlet chamber. However, none provide means to purge the air out of the clean blood prior to it combining with the venous blood, thereby limiting the clinically undesirable generation of large volume of blood foam. Fini&#39;s &#39;270 patent partially addresses this by having a valve that lets the user to either use the dirty blood (flow to the venous chamber) or to accumulate it in a chamber and then direct it to a cell saver. Yokoyama, et al.&#39;s U.S. Pat. No. 6,908,446 illustrates a cardiotomy reservoir with a filter for the vented blood and a separate filter for dirty blood. This prevents clean blood from directly contacting foreign substances filtered off from the dirty blood but does not prevent clean blood accumulating in the cardiotomy from contacting the debris in the filter once the blood level reached the debris. The present invention addresses these drawbacks by separating the clean blood from the dirty blood as well as giving the user a choice to return the dirty blood to the bag or to a cell saver. 
   Blood enters the cardiotomy reservoir of prior art devices either at the top of the cardiotomy or via a tube that extends vertically downward to the bottom half of the reservoir. The former separates blood from air as it enters the reservoir but results in foam from the entering blood splashing into the blood in the reservoir. The latter, when the blood level is above the outlet of the entrance tube, results in foam from the air in the entering blood bubbling in the blood in the reservoir. The present invention allows at least the clean blood to enter along an angle that purges most of the air prior to the clean blood joining blood already in the cardiotomy while limiting splashing. 
   Many of the State-of-the-Art cardiotomies include a perfusion connector that accommodates a ⅜″ ID tubing, but none have a ⅜″ ID tubing extending from the top of the cardiotomy to its bottom as to allow a clotted cardiotomy filter to be bypassed using a new cardiotomy connected to the inlet of that ⅜″ tubing. 
   Cardiotomy reservoirs are also used to accommodate the overflow of blood volume that exceeds the capacity of the venous bag (the largest one can accommodate less than 2.0 L). Larger blood volume capacity is frequently required during aortic or mitral valve replacement during which the excess volume rises into the cardiotomy reservoir resulting in clean venous blood contacting the aggressive filter and the filtrate (e.g. fat, bone chips etc.) trapped at the bottom of the filter of the cardiotomy reservoir that can result in the aforementioned undesirable outcomes. 
   The blood level at which the clean blood contacts the antifoam loaded defoamer in the venous reservoir and the volume at which the blood contacts the aggressive cardiotomy reservoir filter is summarized in Table 1 for typical State-of-the-Art hard shell venous-cardiotomy reservoir units. In one form of the present invention, the volume capacity of the closed venous blood chamber and/or the “clean” chamber of the cardiotomy reservoir is higher than current systems, thereby limiting clean blood contact with the “dirty” chamber and/or defoamer while limiting blood-to-air interface (see below). 
   Minimizing the blood-to-air interface is a major design objective of devices used in the cardiopulmonary bypass circuit. The last two columns in Table 1 provide the surface area of air that the blood is exposed to when the reservoir contains 500 or 1000 ml of blood. As will be shown, when the venous reservoir of the present invention is filled, it has 7 to 30 times smaller blood-to-air area than State-of-the-Art hard shell venous reservoirs. The values assume that there is no air in the venous blood or foam above the blood. It should be noted that the maximum venous blood volume that can be accommodated by the State-of-the-Art units without contacting the defoamer, of either the HSVR or CR, or the dirty blood filter is 1,200 ml. 
   
     
       
         
             
           
             
               TABLE 1 
             
           
          
             
                 
             
             
               Nominal values for adult Prior Art hard shell reservoirs v. the 
             
             
               present invention. 
             
          
         
         
             
             
             
             
          
             
                 
               Volume to 
                 
                 
             
             
                 
               contact 
               Volume to 
             
          
         
         
             
             
             
             
          
             
                 
               HSVR* 
               contact 
               Blood-Air area at 
             
          
         
         
             
             
             
             
             
          
             
               Model 
               Defoamer 
               CR** 
               500 ml 
               1000 ml 
             
             
                 
             
          
         
         
             
             
             
             
             
             
             
             
             
          
             
               One form of the 
               3,000 
               ml 
               3500 
               ml 
               &lt;20 
               cm 2   
               &lt;20 
               cm 2   
             
             
               present 
             
             
               Invention*** 
             
             
               Terumo - Capiox 
               3,000 
               ml 
               1,200 
               ml 
               78 
               cm 2   
               240 
               cm 2   
             
             
               18SX 
             
             
               Dideco - Synthesis 
               600 
               ml 
               1,400 
               ml 
               155 
               cm 2   
               213 
               cm 2   
             
             
               Dideco - Avant 903 
               600 
               ml 
               3,500 
               ml 
               303 
               cm 2   
               303 
               cm 2   
             
             
               Jostra - VHK 4200 
               300 
               ml 
               500 
               ml 
               181 
               cm 2   
               181 
               cm 2   
             
             
               Medtronic - Affinity 
               1,000 
               ml 
               400 
               ml 
               232 
               cm 2   
               232 
               cm 2   
             
             
               CVR NT 
             
             
               Cobe - VVR4000i 
               200 
               ml 
               800 
               ml 
             
             
               Gish - CAPVRF 44 
               1200 
               ml 
               1200 
               ml 
               131 
               cm 2   
               135 
               cm 2   
             
             
                 
             
             
               *HSVR—Hard shell venous reservoir, 
             
             
               **CR—Cardiotomy reservoir, 
             
             
               ***Typical values 
             
          
         
       
     
   
   The defoamer of State-of-the-Art venous reservoirs is located at least along the top section of the reservoir (e.g. Terumo&#39;s SX-25) or lineup the entire screen area of the inlet section of the reservoir (Cobe&#39;s VVR 4000i). Currently it is not known whether early collapsing of blood foam that lowers blood-air interface but increases blood contact with the antifoam, is clinically better than reducing blood contact with the antifoam but increasing blood-to-air interface. One aspect of the present invention incorporates a top and bottom defoamer, wherein the bottom defoamer is smaller by volume and extends downward from the top defoamer into the inlet chamber of the air purging chamber, a geometry that provides flexibility in adjusting early defoaming capabilities while reducing the defoamer contact with the blood. 
   Blood flow from the cardiotomy reservoir to a venous bag can be intermittent or continuous depending on the frequency the suckers are used and the volume aspirated from the field. The outlet port of the cardiotomy reservoir used for adults is universally ⅜″ internal diameter as is the tube connecting the outlet of the cardiotomy reservoir to the inlet of the venous bag (see tube  153  in  FIG. 1 ). Further, the outlet port faces straight vertically or horizontally. That combination results in the air in tube  153  to be trapped by the incoming blood above it, and dragged with that blood into the venous bag thereby adding air to the blood in the venous bag. A form of the present invention provides a fluid path between the cardiotomy reservoir and venous bag with decreased tendency to trap air. 
   Unpublished studies by the Inventor have shown the efficiency to remove venous air while minimizing blood volume and blood damage by blood contacting air can be expressed by a dimensionless parameter that equals the ratio of screen area available for blood flow between the venous inlet and outlet and the blood-to-air interface area. A higher ratio should result in better clinical outcome. The values for that parameter for some State-of-the-Art hard shell venous reservoirs are given in  FIGS. 8   c  and  8   d  at a range of volumes. The data suggests that Terumo&#39;s hard shell venous reservoir (e.g. Capiox 18SX) should handle venous air best. Unpublished studies by the Inventor confirmed that Terumo&#39;s 18SX has the lowest bubble count at its outlet when air is added to the venous blood flowing into its inlet.  FIGS. 8   c  and  8   d  also show that the values of the dimensionless parameter for the present invention (i.e. APC and VR+APC) are significantly higher than Terumo&#39;s values. For that reason, as well as others described below, the present invention handles air better than any other State-of-the-Art venous reservoir. 
   Hard shell venous reservoirs utilize low level detectors to either shut the arterial pump and/or actuate a tubing clamp (e.g. see Sorin&#39;s ECC.O system below) to stop outlet flow from the reservoir and prevent the reservoir from emptying and air from being pumped to the patient. A higher change in blood level per change in volume (i.e. a reservoir with smaller cross sectional area) allows for a control more sensitive to smaller changes in blood volume in the reservoir. As shown in  FIGS. 8   a  and  8   b , Terumo&#39;s hard shell venous reservoirs have the smallest cross sectional area with the highest blood level of any State-of-the-Art reservoirs. At low volumes, Terumo&#39;s reservoirs also have the largest screen area and smallest blood-to-air interface, see  FIG. 8   d . The present invention improves on these parameters, providing an elongated, small diameter air purging chamber with a larger screen area and smaller blood-to-gas interface per blood volume. 
   Numerous minicircuits that minimize operating volume and eliminate the blood-to-air interface have been introduced (e.g. Cardiovention&#39;s CORx covered by U.S. Pat. No. 6,773,670, Medtronic&#39;s Resting Heart system covered by U.S. Pat. Nos. 6,302,860, 6,524,267, Terumo&#39;s Reduced Prime Optimized Circuit also called ROC SAFE  and Sorin&#39;s ExtraCorporeal Circulation Optimized, ECC.O). Each of these circuits has replaced a venous reservoir with a venous air filter to trap and remove air from the venous line and the cardiotomy reservoir. The premise of these circuits is to reduce prime volume to reduce hemodilution. Lower priming volume results in a reduction in post-bypass transfusions, reduces crystalloid fluid administration and retains plasma colloid osmotic pressure during CPB, and reduces post-CPB extravascular lung water and weight gain (edema). 
   Terumo&#39;s minicircuit includes an air sensor that controls both the centrifugal pump and an electronic venous occluder that allow the user to remove the air manually. Stopping blood flow to remove air may be clinically harmful, especially in cases where air enters the venous line repeatedly. 
   Medtronic&#39;s, CardioVention&#39;s and Cobe&#39;s minicircuits utilize a sensor that detects air in the venous line and actuates suction to remove that air and the associated electronics assure that the suction is applied only when so indicated by the sensor. Doing otherwise removes blood from the reservoir or risking air entering the outlet of the air purging chamber. A more recent invention is described by Olsen et al in US Patent Application number 20040220509 entitled “Active air removal from an extracorporeal blood circuit”. That design requires complicated electronics as displayed by FIGS. 14 through 56 of that application. 
   The greatest weakness of the State-of-the-Art minicircuits is their poor air removal characteristics and lack of compliance between the patient and the centrifugal (arterial) pump. Each of the active air removal systems requires a special controlling system that senses air and applies suction to remove all the air from the top of the venous filter. This requires the user to assist the upward rise and removal of air by reducing blood flows by as much as 50%. Thus, in cases where air is intermittently entrained in the venous line, the user has to reduce the blood flow to the patient repeatedly. Cobe recommends that the roller pump removing air from the venous line be set to pump 450 ml/min for 6 sec each time air is sensed in the venous line. In a worst-case scenario, 0.5 cc of air in the venous blood passing the air sensor every 6 sec (5 cc/min of air) would result in pumping over 400 ml of blood every minute out of the CPB circuit and into the cell saver or cardiotomy. Even if the air removal pump is actuated once per minute due to a small air bubble in the venous line, a patient on bypass for 90 minutes would lose 45 ml/min or, in aggregate, over 4 L. Medtronic&#39;s Active Air Removal Device and Venous Air Removal Device (VARD) can be fooled by foam that forms at the top of the venous filter, a shortfall that can result in removal of a significant volume of blood. This is especially true at high blood flow rates and may be the reason that the Medtronic&#39;s&#39; instructions warn that blood flow should be reduced to 1.0 L/min for 30 sec at least once every hour. Further, per the “Instructions for Use”, wall suction is used to remove the air and therefore, the blood sucked with the air is also lost. Later instructions suggested that the cell saver&#39;s cardiotomy be used instead of wall suction. However, large blood volumes directed to the cardiotomy exposes that blood to the aforementioned harmful affects of the filtrate of the dirty blood, the antifoam agents, and to the large blood-air interface associated with the defoamer of the cardiotomy, all of which degrade the benefits of the minicircuits. A form of the present invention provides efficient venous air removal unmatched by any minicircuit, or State-of-the-Art hard shell or soft shell reservoirs. 
   The air purging chambers of State-of-the-Art minicircuits essentially placed arterial filters in the venous line. These filters&#39; ratio of height to diameter is less than 2 and they do not communicate with ambient atmosphere. This design is not conducive to detecting small changes in volume. The present invention provides a much larger height to diameter ratio and a much larger change in height for the same change in volume, a characteristic that allows level sensors to react to much smaller volume changes. 
   A major objective for “venous filters” with air removal capabilities is to reduce the priming volume of the CPB circuit by removing the venous reservoir while still removing air and foam from the venous line prior to it entering the arterial pump. At least one aspect of the present invention provides further reduced volume and effectively, passively or actively, deaerates venous blood and collapses foamed blood, much like hard-shell venous reservoirs do. This action is achieved without the need to reduce pump flow or suck blood out of the circuit, and requires less contact with the defoamer while lowering foam volume (by defoaming the blood earlier), without exposing venous blood to the “dirty” blood of a cardiotomy, and with minimal or no blood loss due to air removal. 
   None of the minicircuits enables the user to handle clean blood except through the cell saver, which mandates a loss of plasma and platelets. The present invention deaerates clean blood before it is combined with venous blood without requiring it to pass through either a cardiotomy reservoir or a cell saver. In one form this is achieved with a two chamber cardiotomy, one chamber for the dirty blood and the other for the clean blood, with means that allow the user to start or stop blood flow from the dirty to the clean blood. Blood from the clean blood chamber is in fluid communication with the inlet of the venous reservoir. 
   All minicircuits utilize a centrifugal pump (CP) to draw venous blood (Kinetic Assisted Venous Drainage or KAVD) and generate arterial line pressure. KAVD has 2 major draw backs: 
   1. Centrifugal pumps handle air poorly; any large bubbles that pass the air-removal system and enter the centrifugal pumps are divided into much smaller bubbles that are less buoyant and are thus more difficult to remove. Large bubbles that could be trapped at the top of the arterial filter appear as smaller bubbles, able to cross the arterial filter. This problem is further exacerbated at the higher pump speeds needed when a single centrifugal pump is used both to draw venous blood and to generate the arterial line pressure.
 
2. Centrifugal pumps maintain a fixed pressure difference between inlet and outlet. Thus, when the heart is manipulated causing the venous cannula to temporarily obstruct, venous flow stops. Flow cessation due to occlusion upstream of the pump inlet results in an inlet suction at least equivalent to the positive pressure at the centrifugal pump&#39;s outlet before the flow stopped. For example, at a flow of 5 L/min and a pump outlet pressure of +250 mmHg, an obstruction at the pump inlet would translate to the pump generating an inlet pressure of −250 mmHg or higher. A high transient suction applied at the tip of the venous cannula could suck the vessel against the cannula&#39;s open end, causing it to occlude, preventing further flow, and perhaps causing both intima and blood damage until the pump is stopped. This condition may also result in cavitation (bubble formation within the pump), a situation that could result in significant blood damage. The present invention operates in the safer VAVD mode that allows the user to set a maximum negative pressure with a vacuum regulator yet allow reverting to operating in the KAVD mode. In either the KAVD or VAVD mode of operation, the system provides better air handling than current venous reservoirs or removes air from the venous blood before it reaches the bubble dispersing centrifugal pump. Further, the presence of air at the top of the air purger of the present invention provides compliance that reduces the spikes of negative pressure inherent with current KAVD minicircuits by allowing the pump controller more time to decrease pump flow and absorbing the large pressure spikes associated with sudden stoppage of blood flow at the pump inlet. Unlike all other minicircuits, the present invention, besides providing a choice between KAVD and VAVD, also allows venous drainage by gravity. Further in gravity mode it may incorporate passive means that prevent blood from spilling over the top of the blood chamber.
 
   When vacuum is applied to State-of-the-Art blood reservoirs then the water vapor at 37° C. of the air being purged condenses on the cooler walls of the vacuum tubing and can drip back into the blood thereby compromising sterility. Current systems eliminate that problem by inserting a sterile vapor trap between the vacuum source and the blood chamber. This increases setup time and cost. The present invention incorporates a vapor trap thereby eliminating the need for a stand alone trap. 
   Current vacuum assisted venous drainage capable bags (e.g. See FIG. 4 of the Instruction for Use for The V-Bag) require one connection to apply vacuum to the rigid chamber housing the bag and another to apply vacuum to the cardiotomy. A single connection to both venous bag and cardiotomy chambers would reduce setup and cost. 
   The aforementioned shortfalls of the State-of-Art minicircuits, venous reservoirs (whether bags or hard shell) and cardiotomy reservoirs are improved upon by the innovative designs of the present invention as more fully described below. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention in its simplest form consists of a very efficient, low volume, chamber that removes air from the venous line, hereinafter referred to as the air purging chamber (APC). The air purging chamber consists of a vertical blood chamber having an inlet, an outlet and an air purge port. The inlet and outlet preferably separated by a screen that allows blood to cross from the inlet to the outlet but retains air bubbles in the inlet chamber. The screen also defines an inlet chamber and an outlet chamber. Gas (e.g. air) bubbles in the blood entering the bottom of the inlet chamber rise to the top of the blood column contained by the inlet chamber and are purged to atmosphere. The air purging chamber separates air bubbles from the venous blood and purges that air to atmosphere as is the case with the hard shell venous reservoir without the need for the suction or vacuum required with prior State-of-Art soft-shell venous reservoir or minicircuits. A defoamer, located at the top of the inlet chamber, collapses foam that forms by the air bubbles in the blood. The blood in the inlet chamber flows across the screen into the outlet chamber and downward towards the outlet port located along the bottom of the outlet chamber. Combining this air purger with a cardiotomy reservoir and a reservoir having at least one flexible wall as a single unit provides venous reservoir with superior air removal characteristics, with the safety benefits of prior art collapsible venous reservoirs while avoiding such disadvantages as air entrapment, required vigilance for air removal, poor level control, slower priming, and lack of a cardiotomy reservoir. The functionality of this innovative venous reservoir is further enhanced by also providing VAVD capabilities with a built-in vapor trap, a small blood-to-gas interface even when the venous reservoir is full. The innovative cardiotomy with a separate chamber for clean blood and separate chamber for dirty blood and a vapor trap completes this superior design. 
   From the brief description of the invention and the aforementioned drawbacks of current reservoirs and venous air purging of minicircuits, it is the objective of the present invention to provide an improved venous air purging chamber that provides bubbles entering its inlet a path to move unhindered upward and passively purged to atmosphere and that can operate either with VAVD where suction is utilized to remove air or by gravity drainage, where the venous air is removed passively to atmosphere. 
   In one preferred embodiment, the top of the air purging chamber is designed to also accept deaerated vented blood without it contacting the dirty blood in the cardiotomy reservoir. 
   In another preferred embodiment, the top of the air purging chamber also incorporates means to deaerate and defoam vented blood without it mixing with the dirty blood in the cardiotomy reservoir. 
   It is another form the present invention provides an improved venous air purging chamber with defoaming capabilities. 
   Yet another objective of the present invention is to provide an improved low volume venous air purging chamber with a large effective screen area available for blood flow that provides defoaming capabilities and that, in one form, accepts clean blood without that blood being exposed to dirty blood. 
   Yet another objective of the present invention is to provide an improved venous air purging chamber with a large effective screen area available for blood flow having a lower operating volume than State-of-the-Art air purgers used for minicircuits. 
   Yet another objective of the present invention is to provide an improved passive venous air purging chamber having a large effective screen area and a low operating volume that for the adult patient its blood-to-air interface area is lower than 30 cm 2  for at least 8 cm of its lower height and or 60 cm 2  for at least 13 cm of its lower height. 
   Another objective is to provide a venous air purger acting as a venous reservoir that provides a “tall” and relatively narrow liquid column having a ratio of height to diameter greater than 2 such that even at low blood volumes a large screen area is still available for blood flow and, relative to State-of-the-Art reservoirs, large changes in height correspond to small changes in volume. 
   It is another objective of the present invention to provide an improved venous air purging chamber that provides defoaming capabilities even at low filling volume while minimizing blood contact with the defoamer. 
   It is another objective of the present invention to provide a blood chamber with defoaming capabilities that incorporates a top and bottom defoamer, wherein the bottom defoamer has a lower volume, is longer and extends downward into the blood chamber. Such chamber can for example be the venous air purging chamber. 
   Another objective is to provide a cardiotomy reservoir having at least a clean blood and a dirty blood chambers as a single unit with the clean and dirty chambers in a fluid communication that can be closed or opened by the end user. 
   Another objective is to provide fluid communication between the outlet of an air purging chamber and the inlet to a compliant storage chamber that is, at least in part, formed of the rigid structure. 
   Another objective is to provide a compliant blood chamber that once primed remains air free. 
   Another objective is to provide a compliant storage chamber with at least one pliable wall that once primed, it shuts off prior to air entering it. 
   Another objective is to provide a fluid path between an air purging chamber and a compliant storage chamber with an adjustable resistance to liquid flow thereby allowing the user to adjust the height of the blood column in the air purging chamber and thereby increase the screen area available for blood flow. 
   Yet another objective of the present invention is an improved venous blood reservoir, having at least one pliable wall combined with a cardiotomy reservoir in fluid communication such that a single air port can be used to apply the same vacuum to the blood in each of the chambers. 
   One more objective of the present invention is to incorporate a vapor trap in the vacuum side of the air purging chamber, the cardiotomy reservoir or compliant blood chamber, said trap replacing a stand alone vapor trap thereby reducing cost and setup time. 
   Another objective of the present invention is to provide an improved venous blood reservoir with at least one pliable wall having passive means to eliminate air. 
   Yet another objective of the present invention is to provide a venous reservoir with at least one flexible wall combined with a cardiotomy as a single unit that can be used with either gravity drainage or with VAVD thus, reducing cost of inventory and simplifying the user&#39;s set up. 
   A further objective of the present invention is to provide a single unit composed of a venous blood reservoir having at least one pliable wall in fluid communication with at least a second blood chamber said second chamber having at least one rigid wall wherein the blood level in the second chamber can be higher than the blood level of the chamber with the pliable wall. 
   A further objective of the present invention is to provide a venous blood reservoir having at least one pliable wall and at least second blood chamber having at least one rigid wall wherein the second chamber is located above the venous reservoir and the chambers combined as a single unit with an outlet that, once primed, shuts off before air can exit its outlet. 
   A further objective of the present invention is to provide a venous blood reservoir and a cardiotomy reservoir combined as a single unit with the venous blood reservoir able to handle a blood flow of 6 l/min and accommodates at least 1,300 ml without the venous blood contacting a defoamer and/or the filter of the dirty blood. 
   A further objective of the present invention is to provide a venous blood reservoir having at least one pliable wall and a cardiotomy reservoir combined as a single unit and with the cardiotomy consisting of a clean blood chamber and a dirty blood chamber, said cardiotomy chambers in fluid communication with each other via a fluid path that can be opened/closed by the user. 
   A further objective of the present invention is to provide a cardiotomy reservoir consisting of a clean blood chamber and a dirty blood chamber; said chambers in fluid communication with each other, and means that allow the user to open/close said fluid communication. 
   A further objective of the present invention is to provide a blood path between the outlet of a cardiotomy reservoir and a second chamber that incorporates means that limit the volume of air entrapped and pumped from the cardiotomy reservoir into the second chamber. 
   A further objective of the present invention is to provide a venous blood reservoir with improved passive air removal capabilities, having a blood volume capacity of at least 300 ml and a flow capacity of at least 5 l/min, wherein the venous blood-to-air interface is no more than 50 cm 2  for at least the lower 10 cm of the blood reservoir. 
   A further objective of the present invention is to provide a stand-alone venous air purging chamber with passive and improved passive air removal capabilities even when operating at a blood volume less than 150 ml that can handle a blood flow of at least 4 l/min. 
   Yet another objective is to offer the combination of a rigid air purging chamber and a venous reservoir having at least one flexible wall with passive air removal capabilities. 
   One more objective of the present invention is to provide a venous air purging chamber with passive and improved air removal capabilities having a ratio of screen area to blood-to-air-interface area of at least 4.0. 
   Another objective of the present invention is to provide the manufacturing flexibility to accommodate future clinical finding that could determine which is more beneficial to the patient: collapsing the foam to reduce blood-gas interface or reducing blood contact with the defoamer. 
   Other objectives, features and advantages of the present invention will become apparent by reference to the following detailed description, an illustrative description not to be taken as limiting the present invention, of the presently preferred embodiments thereof with reference to the accompanying drawings therein. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a line drawing of the pertinent components of a typical cardiopulmonary bypass circuit showing the prior art soft shell venous reservoir and a single chamber cardiotomy reservoir. 
       FIG. 1   a  is a line drawing illustrating that at a low blood level in a prior art venous bag the effective screen area is very small relative to total area of the screen in the bag and that suction is required to remove air entering the bag. 
       FIG. 1   aa  is a line drawing of a cross section along line  1   aa  and  1   aa ′ of prior art bag shown in  FIG. 1   a  illustrating that at low blood volume very little of the screen of this bag is available for blood flow. 
       FIG. 1   ab  is a line drawing of an enlargement of the circled section of the bag shown in  FIG. 1   aa  showing that at low volume with the prior art bag very little of the screen is available for blood flow. 
       FIG. 2  is a line drawing of the pertinent components of a typical cardiopulmonary bypass circuit showing the soft shell venous reservoir integrated with the air purging chamber and a two chambered cardiotomy reservoir of the present invention. 
       FIG. 3   a  is a line drawing of the front view of one preferred embodiment of the present invention illustrating a venous air purging chamber that purges air without the user input. 
       FIG. 3   aa  is a line drawing illustrating a means to prevent blood from overfilling the air purging chamber using a vertically extended air exhaust tube. 
       FIG. 3   b  is a line drawing illustrating another view of the air purging chamber illustrated in  FIG. 3   a  taken along line  3   b - 3   b ′ of  FIG. 3   a.    
       FIG. 3   c  is a line drawing of the front view of another preferred embodiment of the present invention illustrating a venous air purging chamber similar to that illustrated in  FIG. 3   a  that also incorporates a deaerating and defoaming chamber for vented (clean) blood. 
       FIG. 3   d  is a line drawing illustrating another view of the air purging chamber illustrated in  FIG. 3   c  taken along line  3   d - 3   d ′ of  FIG. 3   c.    
       FIG. 4   a  is a line drawing illustrating one preferred embodiment of the present invention where the air purging chamber shown in  FIG. 3   a  is combined with a venous reservoir having at least one flexible wall and with the innovative two-chambers cardiotomy reservoir. 
       FIG. 4   b  is a line drawing illustrating another cross sectional view of the air purging chamber combined with the venous and the two chambers cardiotomy reservoir illustrated in  FIG. 4   a  taken along line  4   b - 4   b ′ of  FIG. 4   a.    
       FIG. 4   c  is a line drawing illustrating another cross sectional view of the air purging chamber combined with the venous and cardiotomy reservoir illustrated in  FIG. 4   a  taken along line  4   c - 4   c ′ of  FIG. 4   a.    
       FIG. 4   d  is a line drawing illustrating another cross sectional view of the air purging chamber combined with the venous and cardiotomy reservoir illustrated in  FIG. 4   a  taken along line  4   d - 4   d ′ of  FIG. 4   a.    
       FIG. 5   a  is a line drawing illustrating the blood level in the air purging chamber relative to the blood level in the venous reservoir at high flow and low volume conditions. 
       FIG. 5   b  is a line drawing illustrating the blood level in the air purging chamber relative to the blood level in the venous reservoir at low flow and low volume conditions. 
       FIG. 6  is a line drawing illustrating the air purging chamber used with State-of-the-Art venous bag and an innovative cardiotomy reservoir that limits air from being dragged along its exit tubing into the air purging chamber. 
       FIG. 6   a  is a line drawing illustrating the exit tubing into the air purging chamber shown in  FIG. 6  and the direction of flow of blood and air bubbles. 
       FIG. 7  is a line drawing illustrating a cardiotomy and a compliant chamber combined as a single unit by sharing a common rigid wall. 
       FIG. 8   a  presents data comparing the blood-to-air interface area of the present invention (VR-APC) to that of the State-of-the-Art venous reservoirs as a function of blood volume. 
       FIG. 8   b  presents data comparing the blood-to-air interface area of the present invention (VR-APC) to that of the State-of-the-Art venous reservoirs as a function of the height of the blood in the reservoir. 
       FIG. 8   c  presents the data of  FIG. 8   b  expanding the lower volume range. 
       FIG. 8   d  presents the ratio of the screen area available for venous blood flow to the area of the blood-to-air interface of the present invention (APC and VR+APC) to the State-of-the-Art hard shell venous reservoir. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference should now be made to the drawings wherein the same reference numerals are used throughout to designate the same or similar parts. It should be noted that the use of cardiopulmonary bypass, as shown in  FIG. 1 , is for descriptive purposes, and should not be taken as a limitation to the use of the devices described hereinafter. Further, the description of the inventions that follow are for illustrative purposes, other embodiments are possible, and therefore what follows is suggestive of the art but should not be construed as a limitation of the scope or the spirit of the invention. Hereinafter, unless otherwise indicated, the terms soft-shell venous reservoir, venous bag and bag are used interchangeably and refer to a blood chamber with at least one flexible wall. 
     FIG. 1  is a schematic representation of a typical cardiopulmonary bypass circuit according to the prior art showing the location of the venous reservoir relative to the patient and the other components in the circuit. Tubing  123 , receiving venous blood from patient  1102 , is coupled to venous reservoir  1103 . The blood is drawn from venous reservoir  1103  via tube  135  by arterial pump  1104  and pumped through membrane oxygenator  1105  wherein oxygen is supplied to the blood and carbon dioxide is removed. The blood from the oxygenator flows via tubing  157  to arterial filter  1107  and then via tubing  172  and an arterial cannula (not shown) back to the patient. One main function of the venous reservoir is to eliminate air contained in the venous blood before it is pumped back to the patient. Air that enters venous bag  1103  rises to the top of the reservoir. This air can not be passively removed to atmosphere, but requires active removal as shown in  FIG. 1  where the air is pumped out by suction pump  1114  into cardiotomy reservoir  1115 . Suction pump  1114  is usually one of 3 to 5 pumps composing a heart-lung machine, which is part of a hardware required for cardiopulmonary bypass. The defoamed blood is returned from cardiotomy reservoir  1115  back to venous bag  1103  via connecting tubing  153 . As described in the Description of the Prior Art, blood flowing in tubing  153  can entrain air that is then pumped into the venous bag, a non-desirable outcome. 
   The ineffective air handling of prior art soft shell venous reservoirs is illustrated by line drawings given in  FIGS. 1   a ,  1   aa  and  1   ab .  FIG. 1   a  is a line drawing of State-of-the-Art venous reservoir as typically represented by Cobe&#39;s venous reservoir (Model # VRB, Cobe Lakewood, Colo. and described in U.S. Pat. No. 5,352,218, issued to Buckley, et al. Oct. 4, 1994 titled “Venous reservoir bag assembly”). Though this bag has a large screen (defined by the dashed lines in  FIG. 1   a ), at low volumes only a very small area of the screen (defined by the double headed arrow in  FIG. 1   ab ) is available for blood entering the screen area to flow to the outlet port. This is further exasperated at low blood volume, the bag is not fully expanded at the screen area causing most of the screen, defined by the double arrow, to contact the collapsed walls and not available for blood flow. At low volumes and high flow, large bubbles  27  entering the Cobe bag are dragged across the screen and break up into smaller bubbles ( 27   a ) that are more difficult to remove due to their lower buoyancy. As described below in reference to  FIGS. 5   a  and  5   b , the present invention provides a larger screen area for blood flow at a lower blood volume. 
     FIG. 1   a  also illustrates that air removal from State-of-the-Art venous reservoirs having at least one pliable wall requires active suction. Thus air is removed with a syringe or usually with one of the roller pumps (e.g.  1114 ) sucking the air out. 
     FIG. 2  is a schematic representation of a system according to present invention showing the relative location of the collapsible venous reservoir in a typical cardiopulmonary bypass circuit and the incorporation of the air purging system. The circuit shown in  FIG. 2  is identical to  FIG. 1 , except that in  FIG. 2 , venous bag  1103  of the present invention incorporates cardiotomy reservoir with chamber  1115   a  for “clean” blood and separate chamber  1115   b  for “dirty” blood as well as incorporating air purging chamber  1116 . Incorporating the cardiotomy reservoir eliminates the need for tubing  136  between the top of the collapsible venous reservoir and cardiotomy reservoir  1115  and suction pump  1114  required for the prior art systems shown in  FIG. 1 . If desired, the fluid communication between cardiotomy  1115   a  and air purging chamber  1116  can be made, as well known in the art, via perfusion connecting ports and perfusion tubing. 
     FIGS. 3   a  and  3   b  are line drawings of a front view and top view, respectively, illustrating one preferred embodiment of air purging chamber  1116 . In general, the air purging chamber is a disposable consisting of an inlet chamber and an outlet chamber having a common screened wall. Screen wall  2   c  forms at least a portion of the outside wall of inlet chamber  2  and at least a portion of the inside wall of outlet chamber  3 . While there are various configurations that can be used to form the air purging chamber, one of the more efficient and simpler designs is illustrated by  FIGS. 3   a  and  3   b , and  3   c  and  3   d . Blood enters inlet chamber  2  via inlet port  1 . Inlet chamber  2  is a vertical cylinder defined by screened wall  2   c  that extends from its bottom level  2   a  to its top level  2   b . The internal diameter of chamber  2  preferably is larger than the internal diameter of inlet port  1 . As shown, inlet port  1  extends, and expands in diameter into outlet chamber  3  formed by rigid walls  3   c  and is sealed to bottom  2   a  of inlet chamber  2  to form a continuous fluid path with inlet chamber  2 . The screened walled cylinder forming inlet chamber  2  is housed in, and preferably is centrally located along its axis within a second cylindrical structure formed by rigid vertical wall  3   c  forming outlet chamber  3 . Venous blood that contains air bubbles  27  enters air purging chamber  1116  via inlet port  1  into inlet chamber  2 . Bubble movement upward in chamber  2  is enhanced by the larger diameter of inlet chamber  2  relative to that of the inlet port. The large internal diameter reduces the blood velocity thereby reducing drag and allowing longer time for the bubbles to move upward. Lower velocity also lowers the tendency of larger bubbles to break into smaller ones; larger bubbles have higher buoyancy and less of a chance of crossing screen  2   c  into outlet chamber  3 . With sufficient pressure across screen  2   c , bubbles can cross into chamber  3  and travel to outlet  4  of chamber  3 , a very undesirable outcome. The upward direction of blood flow at inlet  1  further enhances desirable upward movement of the bubbles. It should be noted that by extending the top most level  2   b  of screen  2   c  to level  3   b  of outlet chamber  3  limits the chance of air crossing into outlet chamber  3 . Though not shown, it should be obvious that inlet port  1  can be positioned along top  3   b  with a tubing connected to its bottom and extending downward into inlet chamber  2 , much like many of the State-of-the-Art hard shell venous reservoirs. 
   The effective area of screened wall  2   c  is defined as the area available for blood in inlet chamber  2  to flow across screen  2   c  into outlet chamber  3 . For air purging chamber  1116  that area equals the product of the periphery of screen wall  2   c  (i.e. 3.14×diameter of chamber  2 ) and the height difference between blood level  3   d  in chamber  3  and bottom level  2   a  of chamber  2 . Because none of the screen area in contact with the blood (wetted area) is blocked by the walls  3   c  of outlet chamber  3 , as is the case with State-of-the-Art venous bags discussed in reference to  FIGS. 1   a ,  1   aa  and  1   ab , blood flows across that entire wetted area. A larger effective area translates to lower velocity across each pore in the screen which lowers the drag force that can push bubbles across that screen into outlet chamber  3 . Once the blood enters outlet chamber  3 , it flows downward in the annular space defined by cylindrical screen wall  2   c  and rigid wall  3   c  until it exits through outlet port  4 . Outlet port  4  in fluid communication with outlet chamber  3  is located at a vertical level that is preferably lower than bottom level  2   a  of screen  2   c , a location that reduces the chance of bubbles crossing from inlet chamber  2  to the outlet port  4 . However, if minimizing operating volume is more important then the outlet can be located at level  2   a . Air entering inlet chamber  2  may cross to outlet chamber  3  along a non wetted screen area and then flow upward to air exhaust tube  3   e , or escape along the top of chamber  2 . 
   Air purging chamber  1116  preferably incorporates defoamer  20  (preferably made of reticulated polyurethane foam having a pore size in the range of 5 to 50 ppi treated with antifoam agents such as silicone) that breaks up foam formed by air traveling up the blood column in inlet chamber  2 . Defoamer  20  preferably is located at the top most section of inlet chamber  2 , a location that limits its contact with the blood in chamber  2  but avails it to defoam blood foam that rises to the top of chamber  2 . The defoamer may also incorporate open channel  20   a  that provides unrestricted fluid communication between inlet chamber  2 , the top of chamber  3 , and air exhaust port  3   e  utilizing structure  20   c . Air is exhausted via channel  3   e  in wall  3   c  to atmosphere. Thus, foam (very large bubbles) entering channel  20   a  is exposed to defoamer  20  where it is broken up and collapsed. For cases where excess foam is generated, or if defoamer  20  loses some of its defoaming capacity, then having outlet of channel  3   e  directed from exhausting to atmosphere to the inlet of a cardiotomy reservoir, as shown in reference to tube  137  in  FIG. 6 , and defoamer  15   cc  illustrated in  FIGS. 4   a  and  4   b  where further defoaming can be achieved. 
   Since foam has a very a large blood-to-air interface, it is advantageous to limit its life and collapse it sooner rather than later. To that end, an additional length of a defoamer,  20   b , is used. Defoamer  20   b  preferably has a nominal width equaling to 10% to 50% of the inside periphery of inlet chamber  2 , and extends from the bottom of defoamer  20 , downward up to 75% of the length of inlet chamber  2 . It may also have a smaller surface area than defoamer  20 . These dimensions reduce the undesirable blood contact with the defoamer yet still collapse blood foam that has not reached defoamer  20  and thereby significantly reducing the blood to gas interface associated with foam. The variable defoaming capabilities achieved by varying the width to less than the perimeter of chamber  2  and extending the defoamer into the blood chamber accommodates future clinical finding that would optimize which is more beneficial to the patient: collapsing the foam to reduce blood-gas interface or reducing blood contact with the defoamer. 
   The dimensions of air purging chamber  1116  are balanced between decreasing the velocity of the blood and increasing the screen area to enhance bubble removal (i.e. large internal diameter) and limiting the prime volume (i.e. smaller internal diameter). It should also have low resistance to blood flow. With that in mind, it has been determined that for an air purging chamber designed for adult patients, the optimum internal effective diameter of outlet chamber  3  is between ⅞″ and 2.0″ (having a horizontal cross sectional area of 4 to 32 cm 2 ) and inlet chamber  2  preferably has a nominal internal diameter that is ⅛″ to 3/16″ smaller than that of the outlet chamber. Another definition of the effective diameter of the inlet chamber is such that it results in annular cross sectional area (area of the outlet chamber less that of the inlet chamber) of at least 1 cm 2 , or the effective cross sectional area between that of a ⅜″ ID tubing (0.71 cm 2 ) and that of ½″ ID tubing (1.27 cm 2 ). The preferred cross sectional area of the annular space between the inlet and outlet chamber can also be defined as approximating the cross sectional area of venous line  123  shown in  FIG. 2 . The effective diameter is the average diameter of the chamber, (e.g. for an ellipse it would be average of the major and minor diameters). The height of inlet chamber  2 , when used in combination with a venous bag, is preferably greater than the maximum blood level in the venous bags, or typically between 7″ and 22″. 
   When air purger  1116  is used for minicircuits without a venous reservoir, then its height can be as low as 4″, a height that can be extended with a smaller diameter tube as described hereinafter in reference to tube  132  shown in  FIG. 3   aa . The screen filter is preferably made of medical grade polyester, or from other appropriate blood compatible material, having a pore size between 20μ and 150μ but most preferably having a pore size between 20μ and 40μ. Most preferably the outlet chamber has an internal diameter between 1.25″ and 1.75″ and a height that prevents blood level  2   d  in inlet chamber  2  from contacting defoamer  20  even when the bag is filled. Blood still may contact extended defoamer  20   b . The top of the screen preferably extends to the top of the outlet chamber and may be closed at its top, a design that limits the blood foam crossing the defoamer radially and spilling into the outlet chamber. 
   When air purger  1116  is used as a stand alone device or in combination with a cardiotomy as for example shown by  FIG. 5   a  less venous bag  1103 , then it is advantageous to provide it with a larger volume capacity. This can be achieved using the design dictated by the aforementioned dimensionless of the ratio of screen area available for blood flow between the venous inlet and outlet and the blood-to-air interface area. As shown in  FIG. 8   d , the largest ratio for State-of-the-Art hard shell venous reservoirs is below 3. Since a larger dimensionless ratio (larger screen area for blood flow and/or lower blood gas interface area) is clinically advantageous, the air purger incorporates dimensions, for the adult patient, that provide a ratio that is at least 4 when operating at 100 ml without a complaint venous reservoir and at 300 ml when operating with a complaint venous reservoir. This design allows for a combination of diameters composed of long small diameter at the bottom and a shorter larger diameter at the top. In some respect this design simulates current hard shell venous reservoirs but its much larger ratio renders it superior. 
   A larger volume for the stand alone unit can also be achieved by either having a larger diameter top, as for example shown in  FIG. 3   c , or increasing the effective diameter of outlet chamber  3  as its height increases. Typical values for such a unit are illustrated in  FIGS. 8   b  and  8   c  by the data labeled “APC-Compound”. Note that the cross sectional area of these higher volume capacity units still have a blood-to-air interface that is significantly lower than State-of-the-Art venous reservoirs. Unlike present reservoirs, the venous air purger always provides a blood-to-air interface area that is less than 40 cm 2  for at least the first 10 cm from bottom  3   a  of outlet chamber  3  and less than 60 cm 2  for at least the first 14 cm from bottom  3 . Should a higher blood level in chamber  3  lower gravity drainage, then vacuum applied to air exhaust port  3   e  compensates for that shortfall. 
   Defoamer  20  preferably has an OD equal to the internal diameter of chamber  2  and a length of ¾″ to 6″. If the defoamer is to be supported by the screen, then the OD of defoamer  20  can be slightly larger than the internal diameter of inlet chamber  2 , thus the slightly compressed defoamer inserted into chamber  2  would spring back and its outer surface grab the inner surface of chamber  2 . The inside diameter of open channel  20   a  is preferably between ⅛″ and ½″, a diameter that allows free air flow yet maximizes the volume of defoamer per unit length. 
   It should be pointed out that the most preferred dimensions, at a prime volume of 100 ml, provide a nominal screen area of 110 cm 2 , a total screen area that is smaller than the most other venous bags (e.g. the screen used in Cobe&#39;s bag is 200 cm 2 ). Further, as the pore size of filter screen used to retain bubbles decreases, the effective open area decreases and the resistance to flow increases. The preferred screen used for State-of-the-Art venous bags has a pore size of 105μ with an open area of 52%. However, the use of smaller pore screen reduces the size and number of bubbles that cross screen  2   c . A test comparing bubble counts at the outlet of the air purging chamber showed that when 130 cc/min of air are pumped into a 6 L/min blood flow entering the venous air purging chamber, the bubble counts of 15μ size bubbles were 6,410, and 763 bubbles/min for screens having a pore size of 37μ, 65μ and 85μ respectively. The smaller pore size screen had significantly lower bubbles crossing despite its lower open area (31% for the 37μ, 38% for the 65μ and 46% for the 85μ pore size screens). For a preferred internal diameter of 1.2″ for chamber  2 , a screen with a pore size of 37μ having an open area of 31%, the open area per one inch height of screen is over 1.1 in 2 . Since the screen makes no contact with wall  3   c  of outlet chamber  3 , its entire wetted area is available for blood flow. It is the larger effective screen area of the present invention that allows using a smaller pore size screen and still maintaining a total open area that is equal to, or is even larger than, State-of-the-Art venous bags thereby maintaining a blood velocity across the screen that equals to or is lower than that associated with current venous bags. Lower velocity translates to lower number of bubbles crossing the screen. 
   As with many devices handling blood outlet chamber  3  preferably is made of clear biocompatible thermoform plastic such as PETG, PVC or polycarbonate or other similar materials. The entire blood contacting surfaces of air purging chamber  1116 , and if possible the defoamer too, are preferably passiviated by one of the many coats available such as the heparin coating by Carmeda (Carmeda AB, Upplands Vasby Sweden.) 
   The preferred dimensions and the design of venous air purging chamber provide a smaller diameter air remover than State-of-the-Art venous or arterial filters and allow the user to operate at a lower blood volume in line with minicircuit technology while providing superior air handling that includes passive air elimination. This is illustrated by  FIGS. 8   a ,  8   b ,  8   c  and  8   d.    
   It should be understood that the aforementioned specifications are for illustrative purposes and that other combination of dimensions can also achieve desirable results. For example, the circular cross sections can be replaced with elliptical, star, or rectangular cross sections. The design of the air purging chamber is not limited to the designs shown in  FIGS. 3   a  and  3   b . For example the air purging chamber can also be effective by incorporating some of the designs suggested for arterial filters (e.g. U.S. Pat. No. 5,484,474 or 4,743,371) or bubble traps (e.g. U.S. Pat. No. 6,019,824). 
   It should be further understood that the aforementioned specifications are given for adult patients with expected blood flow between 4 and 7 L/min. The specifications can be easily scaled down to accommodate lower flows and lower prime volumes associated with pediatric and infant patients. Scaling can utilize well know equation relating resistance to flow as a function of cross sectional areas, such as Poiseuille law dictating flow, Q, to be a function of diameter, d, to the fourth power, Q=f(d 4 ) while still having the diameter of the inlet chamber sufficiently large to allow bubbles to float freely up the blood column. For example, for infants on bypass using ¼″ internal diameter tubing as venous line  123  in  FIG. 1 , the nominal internal diameter of inlet chamber of the air purging chamber can be ⅜″ and the internal diameter of the outlet chamber can be 1/16″ to 3/32″ larger. 
   The air purging chamber can incorporate means assuring that, when inflow exceeds outflow, blood does not overfill reaching and spilling out of exhaust port  3   e . One such means is level sensor  32   b  shown in  FIG. 3   a . Thus, when sensor  32   b  detects that the blood level in outlet chamber  3  is too high, it sends a signal to alarm the user and/or to controller  33  that increases pump speed to accommodate the increased inflow from the patient and prevent blood overflowing chamber  3 . The controller can also actuate tubing clamp  132   a  placed on air exhaust tube  132  shown in  FIG. 3   aa  to close tube  132  thereby preventing blood from rising to further and spilling. The placement of sensor  32   b  can be adjustable to preferably prevent venous blood from contacting main defoamer  20 , but if clinically necessary, moved higher to allow blood level  2   d  to reach defoamer  20  but still prevent blood from spilling out. 
   Another and a simpler means that assures blood does not overfill and spills from the air purging chamber is shown in  FIG. 3   aa . Here, air exhaust tube  132  extends vertically up so its open end is leveled approximately at the level of the patient&#39;s heart. Thus, when venous inflow exceeds flow out of the air purging chamber, then the blood level in the tube  132  rises, reducing gravity drainage (the height difference between the patient and blood level in the air purger) and decreasing venous flow from the patient. Venous inflow stops once the blood level in tube  132  reaches the level of the patient. The internal diameter of tube  132  is preferably at least ⅜″ but more preferably is at least ½″. 
   The air purging chamber can also incorporate blood level maintaining means for maintaining the blood level in chamber  3  above outlet port  4  thereby preventing gross air (as opposed to microbubbles) from exiting the outlet blood chamber. As an example, sensor  32  on wall  3   c  operably connected to controller  33  is attached to wall  3   c  of outlet chamber  3  in  FIGS. 3   a , and  3   aa  and is used as a low level sensor that maintains blood level  3   d  at a safe level above outlet  4  of chamber  3 . When detector  32  senses that the blood level dropped below a safe level, it sends a signal to control unit  33 . Control unit  33  is capable of at least sounding an alarm, slowing down or shutting off the arterial pump, clamping off outflow (e.g. via tube clamp  32   a ), or increasing inlet flow (e.g. during VAVD, increasing the vacuum—not shown) to assure that blood level  3   d  does not reach outlet port  4  causing air to exit outlet port  4 . Level detectors already incorporated in State-of-the-Art heart-lung machines can be used for this purpose. Thus the combination of State-of-the-Art control units and the innovative disposable air purging chamber, provides the user a system that effectively purges air out of the venous line passively and automatically while preventing air from exiting outlet  4  of air purging chamber  1116 . It should be pointed out that the rigid wall of air purging chamber  1116  can be easily adapted to the State-of-the-Art level detectors. This is in contrast to the difficulties encountered when trying to use these level detectors by placing the sensor on the pliable wall of current venous bags. As described above, the air purging chamber acts much like a hard shell reservoir except that its blood-to-air interface area and hold up volume are significantly lower. 
   The combination of adjustable means to limit the high and low levels of blood in air purger  1116  also allows adjustment of the maximum operating blood volume in outlet chamber  3  and inlet chamber  2 . For example, a small patient requiring low flow and having low blood volume is better served by operating at a low minimum level (low flow allows more time to react before the outlet chamber empties) and a low “high” level setting, the combination minimizing the volume in the air purger. On the other hand, for a large patient that requires high flow and more operating volume, both the low volume and high level sensor would be raised to provide a larger screen area for the higher flow and more volume for the controller to react a low volume condition. 
   It should be clear that inlet pressure sensed by pressure transducer  32   ca  shown in  FIG. 3   aa  can send a signal to control unit  33  that controls pump speed in a manner similar to that described level sensor  32 . Thus, a low hydraulic pressure sensed at  32   ca  indicates a low blood level in outlet chamber  3  and signals controller  33  to react to prevent further drop in blood level. During VAVD, a second pressure transducer,  32   cb  measuring the vacuum applied to the blood chamber, is also used. Here, the signal used for controller  33  is the difference between transducer  32   ca  and transducer  32   cb , the difference being the hydraulic pressure due to the blood level in the chamber. 
   The air purging chamber allows many new designs not possible with Sate-of-Art devices. For example line drawings of  FIGS. 3   c  and  3   d  illustrate an air purging reservoir incorporating means to accept vented (“clean”) blood. Combining the vented blood directly with the venous blood in chamber  2  is prohibitive because the high volume of air in the vented blood would cause excess foaming. Incorporating an air purging chamber for vented blood at the top of the air purging chamber assures that the large volume of air, so common to vented blood, is eliminated prior to the vented blood combining with the blood in inlet chamber  2 . As shown, the top section of the air purging chamber is radially enlarged preferably to 1.5 to 2.5 times the diameter of chamber  3  so channel  20   a  can accommodate vent chamber  22 . Vent chamber  22  is formed of screen wall  22   c , much like chamber  2  is formed by screen wall  2   c , and incorporates defoamer  15  at the top section of chamber  22 . Vent chamber  22  is housed in the enlarged top section of outlet chamber  3  defined by outside cylindrical wall  3   cc  inside cylindrical screen wall  2   cc . Defoamer  20  is contained and may be supported by screen  2   cc . Vent chamber is centrally suspended at the top and preferably does not contact venous blood defoamer  20 . Annular channel  20   a  between vent chamber  22  and venous defoamer  20  serves to exhaust venous air. Vented blood enters vent chamber  22  via inlet vent port  23  located atop the air purging chamber and flows downward to bottom  22   a  of screen  22   c . Further, the bottom of screen  22   c  is sealed on a diagonal with its lowest point contacting fluid path  22   b  described below. It is important that bottom  23   a  of vent tube  23  be at a higher level than the maximum blood level  2   d  of inlet chamber  2 . This would assure that the large air volume in the vented blood does not bubble into the venous blood entering chamber  2 . Foam associated with the vented blood entering vent chamber  22  is collapsed when contacting defoamer  15  disposed within the inside periphery of vent chamber  22 . Preferably, the only fluid communication between port  23  and enlarged channel  20   a  is via screen  22   c . This limits the foam and bubbles from the vented blood from entering channel  20   a . Defoamer  20  is supported by circular screen  2   cc  and defoamer  15  is supported by screen  22   c . With the exception noted in the next paragraph, the lower section of air purging chamber  1116   a  is identical to that described for air purging chamber  1116  in reference to  FIG. 3   a . Inlet chamber  2  forms unobstructed fluid communication with enlarged channel  20   a  via thin walled fitting  24 . Thus, screen  2   c  is sealed to the smaller diameter bottom section of fitting  24  and screen  2   cc  is sealed to the larger diameter top section of fitting  24 . 
   Also shown in  FIG. 3   c  is fluid path  22   b  between bottom  22   a  of vent chamber  22  to inlet chamber  2 . Path  22   b  prevents, or at least reduces, splashing where the vented blood enters blood in chamber  2  and combines with venous blood at level  2   d . Less splashing results in less air bubbles entrapment and foam formation. Path  22   b  can be for example polyurethane foam as used for defoamer  20  but without the antifoam agents. Other materials that wick blood and prevent its free fall, such as screens, can also serve as the fluid path. Air entering inlet chamber  2  crosses to outlet chamber  3  along a non wetted screen area and then flows upward to air exhaust tube  3   e . Air entrapped within the vented blood entering vent chamber  22 , is forced across the non wetted section of screen  22   c  and then exhausts via port  3   e . It should be clear that defoamer  15  may also facilitate defoaming venous blood foam formed in chamber  2 . 
     FIG. 3   e  illustrates air purger  1116   a , shown in  FIG. 3   c , in its preferred tilted angle of between 10° and 45° and most preferably between 15° and 30°. Angling the air purger allows air bubbles  27  to separate from the blood and move vertically up to higher wall  2   c -H and the blood to lower wall  2   c -L. Here, fluid path  22   b  is in contact with screen wall  2   c -L. This allows vented blood flowing along path  22   b , to continue flowing along screen wall  2   c -L thereby further reducing splashing of vented blood as it combines with venous blood in chamber  2  at level  2   d.    
     FIG. 3   c  also illustrates that a design where bottom  3   a  of outlet chamber  3  and bottom  2   a  of inlet chamber  2  can essentially be at the same vertical height. This design reduces the dead space associated with that shown in  FIG. 3   a  where some blood can always be left in inlet tube  1  because its opening is higher than outlet port  4 . In this configuration, outlet port  4  has expanded section  4   c  about the area of screen  2   c  at bottom  3   a  of outlet chamber  3  to provide slower and more uniform velocity. Such a design would require that chamber  3  be formed by blow molding or combined by two halves made by vacuum forming or that outlet port be injected molded as a separate piece and fixed to the bottom of chamber  3 . Outlet Port  4  can also be aligned parallel to inlet port  1 . 
   Another preferred means to prevent air from entering blood pump  1104  of  FIG. 2  is to combine air purging chamber  1116  with closed compliant chamber  7  having outlet port  6  that shuts off before blood level  3   d  reaches the level of its outlet port  4  of outlet chamber  3 .  FIG. 4   a  is a line drawing illustrating a front view of a unit combining air purging chamber  1116  with compliant venous reservoir  1103  having at least one pliable wall. As shown, the unit also incorporates dual chamber cardiotomy reservoir ( 1115   a  and  1115   b  in  FIG. 2 ). A line drawing of a cross sectional view of the combined unit shown in  FIG. 4   a  taken along line  4   b - 4   b ′ is illustrated in  FIG. 4   b . A line drawing of a cross sectional view of the combined unit shown in  FIG. 4   a  taken along line  4   c - 4   c ′ is illustrated in  FIG. 4   c . A line drawing of a cross sectional view of the combined unit shown in  FIG. 4   a  taken along line  4   d - 4   d ′ is illustrated in  FIG. 4   d . Here inlet channel  1   a  extends from top inlet  1  of the combined unit downward to form a fluid communication with inlet chamber  2  of air purging chamber  1116 . Air purging chamber  1116  is essentially the same as described in reference to  FIGS. 3   a  and  3   b . Outlet  4  of outlet chamber  3  extends as channel  4   b  to form fluid communication with inlet  5  of compliant storage chamber  7 . To assure low resistance to flow in channel  4   b , preferably channel  4   b  is as short as possible and has an effective diameter of ½ inch. Compliant storage chamber  7  is formed by rigid concaved backwall  8   c  and matching in shape concaved pliable diaphragm  25 , seen in  FIGS. 4   c  and  4   d . Diaphragm  25 , which can be vacuum-formed from films such as PVC, polyurethane or EVA, nestles in concaved backwall  8   c  and is sealed along its outside periphery to the outside periphery of rigid wall  8   c  to form variable volume closed blood chamber  7 . Chamber  7  has an inlet port  5  and outlet port  6 , both located along bottom  7   a  of blood chamber  7  as illustrated in  FIGS. 4   a  and  4   d . Blood is pumped from blood chamber  7  via outlet  6  that is in fluid communication with arterial pump  1104 , shown in  FIG. 2 , via typical perfusion tubing (e.g. tube  135  in  FIG. 2 ). As blood fills blood chamber  7 , pliable diaphragm  25  moves outward, expanding the volume capacity of chamber  7  to accommodate the additional blood volume. The only air to be removed from chamber  7  is that associated with initial priming of chamber  7 . Air in blood chamber  7  is removed through tube  37 , shown in  FIG. 4   d , having one open end along top  7   b  of chamber  7  and the other open end accessible for the user to pull the air out. A one-way valve or a stopcock can be adapted to tube  37  to facilitate air removal and preventing its return. Air movement upward in chamber  7  may be enhanced by at least one protrusion  8   ca  incorporated into rigid backwall  8   c , shown in  FIGS. 4   a  and  4   c . The protrusion serves the same purpose as described in reference to air removal tubes shown in  FIG. 1   b  of aforementioned Tamari&#39;s U.S. Pat. No. &#39;426. In short, protrusion  8   ca  forms channels between its wall and flexible diaphragm  25  to allow air to move up and be purged via air purging port  37 . In addition, protrusions  8   ca  also provide greater strength and stiffness to backwall  8   c , thereby allowing wall  8   c  to be thinner, reducing its weight and cost. As described below, once primed and air free, no air should enter blood chamber  7 , a great advantage over State-of-the-Art venous bags which require constant vigilance, suction to remove that air or some automated means. 
   The preferred dimensions of compliant chamber  7  depend on its use. For adult patients, the blood volume capacity should be between 1 and 3 liters, but can be reduced to 20 to 500 ml when designed to be used in association with minicircuits. The lowest volume is that required for chamber  7  to provide the shot-off feature of outlet  6  of the compliant chamber  7  described below. 
   A critical design feature of the combining air purging chamber  1116  with venous reservoir  1103  is that the vertical location of outlet port  4  of the air purging chamber  1116  is below the lowest vertical location of chamber  7  of venous reservoir  1103 . Height difference “y 1 ” shown in  FIG. 4   a  between top of outlet port  4  and bottom of outlet port  6  should be sufficient to assure that as blood level  3   d  of chamber  3  drops below the bottom of outlet  6  of blood chamber  7 , the negative pressure due to the hydraulic height difference between blood level  3   d  and the bottom of outlet  6  is sufficient to cause pliable diaphragm  25 , shown in  FIGS. 4   c  and  4   d , to collapse against backwall  8  and close outlet  6 . Closed port  6  isolates outlet chamber  3  from the negative pressure generated by pump  1104 , shown in  FIG. 2  and stops flow out of compliant chamber  7 , thereby stops flow out of chamber  3  via outlet port  4  preventing blood level  3   d  from dropping to outlet port  4  and air flowing from emptied blood chamber  3  into blood chamber  7  of venous reservoir  1103 . As blood level  3   d  in air purger chamber  3  rises above bottom  7   a  of complaint chamber  7 , then blood starts to refill compliant chamber  7  until its outlet  6  reopens. 
   The aforementioned combination illustrated in  FIGS. 4   a - d  and  5   a - b  provides automatic and passive air purging capabilities, as does a hard shell venous reservoir and the clinically preferred close blood chamber as does a soft shell venous reservoir. It also prevents air from entering the compliant venous blood chamber. Any other venous bag that contains air, allows that air to be pumped to the patient when it is drained of blood. With the aforementioned combined unit, air purging is handed in a separate chamber than compliant chamber  7  forming the closed venous reservoir and therefore no air can be pumped to the patient. 
   The lower section of compliant storage chamber  7 , that between bottom  7   a  and transition level  7   c  in  FIGS. 4   a  and  4   d  having a height of “h”, is shallower, more vertical in shape, and accommodates much less volume per height than the upper section, that between transition  7   c  and upper level  7   b , defined as “H-h” in  FIG. 4   d . The shallower section provides a larger change in height per unit change in blood volume and therefore a better visualization of small volume changes in that section. The larger increase in blood height also results in a greater force to move compliant diaphragm  25  from a closed position to an open position thereby overcoming the shortfall of prior art venous reservoir as previously described for aforementioned U.S. Pat. No. 4,424,190. The desirable early opening of a closed outlet is further assisted by virtue of outlet port  6  located on lower rigid wall  8   cb  that is flat and close to vertical (e.g. between 70° and 90° of the horizontal plane). The vertical shape allows diaphragm  25  more easily to “peel” away from rigid wall  8   cb  shown in  FIG. 4   d . The depth of the lower section is preferably between ¼″ and 1″ and most preferably between ⅜″ and ⅝″. Height “h” of the lower section is preferably between 1″ and 4″ but can be higher if a low volume compliant storage chamber is desired to better meet minicircuit requirements. To further assure that pliable diaphragm  25  does open without requiring a large volume increase in chamber  7 , outlet  6  of chamber  7  is not located at the lowest fluid level of chamber  7 , as is the case for prior art compliant venous reservoirs, but preferably at least 1 to 4 lengths of the diameter for outlet port  6  away from bottom  7   a . This design also enhances intimate contact between diaphragm  25  and rigid backplate  8  and improves the closure of outlet port  6 . 
   It should be clear that when using the air purging chamber for minicircuit applications, compliant chamber  7  can be limited in volume by reducing the chamber to that defined by a height sufficient to allow diaphragm  25  to remain open at the designed blood flows (e.g. 6 liters/min for adult patients) and a blood level in outlet chamber  3  that provides sufficient screen area for efficient air removal. The minimum volume that is needed for minicircuits would have compliant chamber  7  serve as a shut off valve to assure that blood flow from air purging chamber to complaint chamber  7  stops prior to air exiting outlet port  4  of outlet chamber  3 . The minimum operating volume of the compliant chamber is preferably between 20 ml and 200 ml. Since blood level  3   d  can rise above the vertical height of “h” (the maximum height for compliant chamber  7  when used only as a shut off valve) it will provide a higher hydraulic pressure than that afforded by “h”. This higher pressure will result in a higher force available to reopen port  6  without the large volume associated with filling section “H-h” shown in  FIG. 4   d.    
   It should be noted that if a low level sensor is used with the air purger, then even if outlet  6  of compliant chamber does not fully close, it still would collapse to increase resistance to flow out of complaint chamber  7  and would allow more time for controller  33  to react as previously described. More time allows for greater safety. 
   The Inventor has shown that bubble counts at the outlet of a venous bag decrease as its blood level increases (Tamari Y, et. al.: A New Top-Loading Venous Bag Provides Vacuum Assisted Venous Drainage. Perfusion 17:383-390, 2002). Thus, the higher blood level in the air purging chamber relative to blood level in the compliant storage chamber is an inherent safety characteristic of the system that assures more favorable conditions for air removal. Further, with State-of-the-Art venous bags, as the blood volume in the bag decreases, the screen area available for blood flow from the inlet to the outlet of the bag decreases. A decrease in the screen area results in an increase in blood velocity across each pore of the screen which can cause more bubbles at the inlet side of the screen to cross to the outlet side of the screen. With the present design, the screened section in the air purging chamber can extend below the bottom of compliant storage chamber  7 , assuring that even at very low blood levels in compliant storage chamber  7 , the blood column in the air purging chamber is high enough to provide a large screen area to inhibit bubbles in the venous line from reaching the outlet of the compliant storage chamber. This is clearly shown in  FIGS. 4   a ,  5   a ,  5   b  and  6  where bottom  2   a  of screen inlet chamber  2  is vertically lower than bottom  7   a  of blood chamber  7 . Another advantage is that whenever blood flows from the air purging chamber to the compliant storage chamber, the height of the blood column in the air purging chamber is always higher than the blood level in the compliant storage chamber. 
     FIGS. 5   a  and  5   b  are line drawings illustrating the design superiority of the present invention in handling air. In each, air purging chamber  1116  is combined with venous reservoir  1103  in a similar manner to the combined air purging chamber  1116  and venous reservoir  1103  as described in reference to  FIGS. 4   a ,  4   c  and  4   d . Cardiotomy chambers  1115   a  and  1115   b  shown in  FIGS. 5   a  and  5   b  do not affect the discussion below.  FIG. 5   a  illustrates a condition that is most challenging to any venous reservoir: eliminate incoming venous air at low blood volume and high blood flow. With the combined air purging chamber  1116  and venous reservoir  1103 , air is handled in an efficient manner by assuring that a large screen area is available. This is achieved by three means. First, bottom  2   a  of screen  2   c  forming cylindrical inlet chamber  2  can be located below bottom  7   a  of blood chamber  7 . The height difference between bottom  2   a  and bottom  7   a  provides additional screen area with a smaller increase in operating volume than that possible with State-of-the-Art venous bags. Second, the blood volume where air is eliminated (i.e. chamber  2 ) is present as a tall vertical blood column surrounded by screened wall  2   c  provides a larger uninhibited screen area than the shallow but wide blood level of the aforementioned state-of-art venous bags that exposes a much smaller effective screen area as described in reference to  FIGS. 1   a ,  1   aa , and  1   ab . Third, because there is some resistance to flow between outlet chamber  3 , outlet port  4 , interconnecting channel  4   b  and inlet port  5  of blood chamber  7 , then under any flow conditions, blood level  3   d  is higher than blood level  7   d  in blood chamber  7  of venous reservoir  1103  (i.e. the pressure required to overcome the aforementioned resistance to flow). 
   Further, for any given design, the difference in height between blood level  3   d  in air purging chamber  1116  and blood level  7   d  in venous reservoir  1103  increases as blood flow increases as is illustrated by comparing low flow conditions illustrated in  FIG. 5   b  to high flow conditions illustrated in  FIG. 5   a . This inherent property provides additional safety: at higher flows when a fixed screen area is more vulnerable to bubbles crossing the screen (higher velocity and drag), the current invention presents a larger screen area for blood flow at least partially compensating for the higher flow and keeping the velocity across each pore low. 
   The inherent difference between blood level  3   d  in outlet chamber  3  of the air purger and blood level  7   d  in compliant chamber  7  of the present invention can be manipulated to improve air removal from the incoming venous blood. For example, channel  4   b  could incorporate an adjustable resistance that controls the height of level  3   d  relative to level  7   d . Thus, at very low volumes, the resistance to flow between chamber  3  and chamber  7  can be increased to maintain higher blood level  3   d . Conversely, with a high blood volume in chamber  7  the resistance to flow between chamber  3  and chamber  7  can be decreased because blood level  3   d  is already high. This can be achieved by incorporating an adjustable resistance, to the flow fluid path between the air purging chamber and compliant storage chamber  7 , such as tubing clamp  4   bb  shown in  FIG. 5   a , thereby allowing the end user to adjust the height of the blood column in the air purging chamber to accommodate different flow and air volume in the incoming venous blood. It should be noted that simple fluid physics dictates that when blood flow ceases, blood level  3   d  in chamber  3  equals blood level  7   d  in venous blood chamber  7 . The effective air handling described above, is in direct contrast to prior art soft shell venous reservoirs as previously described in reference illustrated in  FIGS. 1   a ,  1   aa  and  1   ab.    
     FIGS. 5   a  and  5   b  also illustrate that interconnecting channel  4   b  could form U-shape bottom  4   a  having a vertical level that is lower than the level of outlet  4 . Lower level  4   a  provides a higher hydraulic pressure to close outlet  6  by pliable film  25  when blood level  3   d  drops below the level of outlet port  6 , as described in reference to  FIGS. 4   a  and  4   d . Further, if lowest level  4   a  is lower than the bottom of outlet  6  of blood chamber  7 , then the vertical height of outlet  4  of outlet chamber  3  can be raised above outlet  6 . 
     FIG. 5   b  illustrates another innovation of the present system; when air purger  1116  is combined with compliant venous reservoir  1103 , then interconnecting tube  4   b  can incorporate one more port in fluid communication via tube  4   c  with outlet port  6  of reservoir  1103 . Tube  4   c  is also in fluid communication with outlet port  6   a . This combination allows the user to clamp outlet port  6  and interconnecting tube  4   b  (between tube  4   c  and inlet  5  of reservoir  1103 ) thereby bypassing reservoir  1103 . Bypassing reservoir  1103  allows flowing at a lower operating volume. Thus, one device allows the use of only the venous air purger for cases that do not require much external blood volume capacity (e.g. routine coronary bypass cases) thereby minimizing prime and operating volume. For cases where a large volume capacity is required, tube  4   c  is clamped between tube  4   b  and outlet  6   a  thereby having venous blood flowing through and using the capacity of complaint reservoir  1103 . 
   Yet another advantage of the combined unit described in reference to  FIGS. 4   a - 4   d  is that it limits the venous blood gas interface to the cross sectional area of outlet chamber  3  and inlet chamber  2  at blood level  3   d , an area that is significantly lower than State-of-the-Art hard shell reservoirs.  FIG. 8   a  illustrates that the blood-to-air interface area of the combination of the complaint venous reservoir and venous air purger (APC-VR) is unaffected by the blood volume in the venous bag and is much lower than State-of-the-Art hard shell venous reservoirs. Thus, at a blood level of 1,000 ml, the area of the blood-to-air interface of the air purging chamber having the most preferred dimensions is only between 8 and 15 cm 2  compared to 180 cm 2  to 305 cm 2  for State-of-the-Art hard shell reservoir. At a blood level of 500 ml, the area of the blood to gas interface of the air purging chamber remains between 8 cm and 15 cm 2  as compared to an area from 78 cm 2  to 305 cm 2  for the State-of-the-Art hard shell venous reservoir shown in Table 1. This significant reduction in blood-gas interface results in significant reduction in blood damage and should translate to better clinical outcome.  FIG. 8   a  also shows that the blood-to-air interface area for the stand alone venous air purger (APC-Compound) is also a lower than that of the State-of-the-Art venous reservoirs. The blood-to-air interface area for the former being no more than 50 cm 2  at a volume of 500 ml and no more than 80 cm 2  at a volume of 800 ml. 
     FIGS. 4   a - 4   d  also illustrate another permutation possible with the present invention: a unitized air purging chamber  1116 , compliant blood chamber  7  forming venous reservoir  1103  and a two-chamber (“clean” blood chamber  1115   a  and “dirty” blood chamber  1115   b ) cardiotomy reservoir. The two-chamber cardiotomy reservoir shown is for illustrative purposes only. Essentially it is similar to many of the cardiotomy reservoirs in the market (e.g. see cardiotomy reservoir 4 shown in FIG. 1 of aforementioned U.S. Pat. No. 6,287,270) except that it provides clean blood chamber  10   a  and dirty blood chamber  10   b , separated by wall  16 . Cardiotomy section for dirty blood  1115   b  has at least one inlet port  11   a  that accepts sucker blood and directs that blood to cardiotomy filter  11  composed of defoamer and a screen and/or felt  11   c  to filter debris and air associated with suction blood. The filtered blood then accumulates in chamber  10   b . Cardiotomy section for clean blood  1115   a  separates air from the vented blood; it does not need aggressive filter  11  used for dirty blood chamber  10   b . Further, the blood filtered in this chamber is “clean”, there is no need to store it, and it can be combined with venous blood without concerns for the inflammatory response associated with the dirty blood in chamber  10   b . Thus, it needs only screen  14   c  to reduce the bubble size and volume in the clean blood prior to it combining with the venous blood. 
   Defoamer  15  used to collapse foam formed by the air in the clean blood in chamber  10   a  can be much smaller, for example 20% in volume of the defoamer in filter  11  of chamber  10   b . Inlet port  15   a  of clean blood cardiotomy  1115   a  accepts clean blood and directs it to closed chamber  14  formed, at least partially, by screen wall  14   c . Preferably, chamber  14  is angled, as shown in  FIG. 4   d , a design that has the advantages described by the following example: if the walls of inlet chamber  14  are formed by screen  14   c , then air is separated from the blood by having the blood flow across the lower portion of screen  14   c  while air is removed along the top section of the screen. The pore size of screen  14   c  is sufficiently small to assure that blood wicks and flows along wall  14   c  till it reaches bottom  14   ca , or blood accumulating in chamber  10 , rather than dripping across the screen. This limits splashing and the previously described negative results associated with splashing. Defoamer  15  located along the top section of screened chamber  14  preferably does not contact blood flowing along the lower wall of chamber  14  but is available to collapse blood foam rising to it. The filter for the clean blood is similar to that described in reference to inlet chamber  2  shown in  FIGS. 3   a  and  3   b  with the exception that the inlet for chamber  14  is at the top. Preferably only blood foam contacts defoamer  15  while vented blood contacts the defoamer only when its level in chamber  10   a  rises to the level of defoamer  15 . 
   Channel  19  forming the fluid communication between the bottom of the cardiotomy and the inlet to the air purger, is preferably angled to reduce blood velocity as well as allow air bubbles to rise to the top of the moving column as explained in detail in reference to yet to be described channel  153  shown in  FIGS. 6 and 6   a . The entrance to channel  19 , at the bottom of chamber  10   a , is widened as indicated by  19   b  in  FIGS. 4   a  and  4   b , a design that allows air bubbles in channel  19  to separate from the flowing blood and escape and thereby limit the air volume trapped as clean blood intermittently flows down channel  19 . Channel  19  should have at least the top portion enlarged to an equivalent diameter greater than ⅜″ and preferably at least ½″. Blood velocity in channel  19  can also be reduced by structure  19   c , that can be for example polyurethane foam as used for defoamer  20  but without an antifoam agent. Reducing the blood velocity in channel  19  reduces air bubbles entrapment, especially at the level where the flowing vented blood combines with the blood at the lower level, and foam formation in channel  19 . To reduce air from venous blood entering channel  19 , lowest level  19   a  of channel  19  and lowest level  1   aa  of inlet channel  1   a  are lower than level  1   ab  where channel  19  and channel  1   a  combine and the deaerated vented blood combines with the venous blood. 
   Chambers  10   a  and  10   b  are in fluid communication along their common top wall via always open channel  16   a  and along their common floor via ports  17  and  18 . Ports  17  and  18  are in fluid communication, for example via tubing  13 , that can be closed or opened with valve  13   a  to allow, or prevent, flow of dirty blood into clean chamber  10   a  and then to combine with the venous blood in chamber  2 . As shown in  FIG. 4   b , valve  13   a  is a sliding tubing clamp. Dirty blood accumulated in the chamber  10   b  blood can be removed via port  21   a  atop chamber  10   b , through tubing  21   aa  with its open end at the bottom of chamber  10   b.    
   Unique clinical advantages are provided by the combination of the two chambers that allow separating clean blood from dirty blood, the ability to store dirty blood or combine it with the venous blood, having an aggressive filter for the dirty blood and an air purger for the clean blood, and reducing the volume of air that is dragged as the vented blood flows down channel  19 . It should be clear that the filter for the clean blood consisting of screen  14   c  and defoamer  15  can also be fitted in channel  20   a  formed by defoamer  20  located at the top section of inlet chamber  2  in a manner similar to that described in reference to vent chamber  22  shown in  FIGS. 3   c  and  3   d.    
   The volume of chambers  1115   a  and  1115   b  can be between 1.0 and 1.5 liters, but preferably, the volume of clean blood chamber  1115   a  is greater than dirty chamber  1115   b . This distribution provides a larger capacity to store venous blood that may overflow from venous reservoir  1103  without that blood contacting filter  11  of dirty chamber  1115   b.    
   It should be understood that at least two of the three units: air purging chamber, compliant chamber and cardiotomy chamber can be combined to form a single unit. For example, it is possible to combine just the cardiotomy and complaint chamber by having those two units share a common rigid wall as described in reference to  FIGS. 4   a - d  but without the air purging chamber as shown by line drawing illustrated in  FIG. 7 . Since the unit illustrated in  FIG. 7  shares the same design for the clean  1115   a  and dirty  1115   b  blood chambers combination forming the cardiotomy reservoir, as shown in  FIGS. 4   a - 4   d , only the deviations from the unit illustrated in  FIGS. 4   a - 4   d  are described. The combination unit consists of venous reservoir  7103  having at least one flexible wall and one rigid wall, said walls sealed along their outside periphery to form closed complaint chamber  707  much like compliant chamber  7  previously described in reference to  FIGS. 4   a - 4   d . Venous blood enters chamber  707  via inlet port  701  and exits via outlet port  706 , both preferably located along bottom  707   a  of chamber  707 . Air entrained in the venous blood is purged via air purging port  703  located along the top most point in chamber  707  and preferably in fluid communication with chambers  710   a  and  710   b  of clean and dirty cardiotomies respectfully. Fluid communication of the air side of the cardiotomies and the venous reservoir, allows for air to escape to atmosphere, or for equal vacuum to be applied to all three chambers through single exhaust port  716   b . To facilitate use, channel  703  preferably extends up to the top of the cardiotomy reservoir. Channel  703  preferably is large in diameter (e.g. ½″ ID) to form a low resistance channel for air bubbles to move up the blood column in channel  703 . Defoamer  720  may be located along the top  707   b  of compliant chamber  707  and may also extend into air purging port  703  (shown as  720   a ). Air sensor  732  may also be added for the same purpose and function as previously described in reference to sensor  32  in  FIGS. 3   a  and  3   aa . The addition of a defoamer to the top of compliant chamber  707  and or air purging port  703  is to aggressively collapse foam thereby reduce the large blood-air interface associated with foam. This method also allows port  703  to attach to a regulated vacuum source (not shown) to purge the air without the need to pump the foam out of chamber  707  and without the need to have the foam/blood exiting port  703  to contact the aggressive filter of a cardiotomy reservoir. 
     FIG. 7  shows that outlet port  706  is located in the lower portion of chamber  707  and is designed with considerations previously described for outlet  6  shown in reference to  FIGS. 4   a - 4   d . Thus, level  719   a  is vertically lower than outlet port  706 . This assures that outlet port  706  closes prior to blood level  719   d  in channel  719  drops to lowest point  719   a  of channel  719 . As described before in reference to port  6  closing, when the blood in chamber  707  empties, the negative pressure due to the hydraulic height difference between  719   a  of channel  719  and outlet  706  of blood chamber  707  is sufficient for the flexible wall of chamber  707  to collapse against the opening of outlet  706 , closing that port, preventing further emptying of blood chamber  707  and preventing air from passing point  719   a  and into chamber  707 . 
   The unit shown in  FIG. 7  does not have an air purger such as  1116  shown in  FIG. 6 . It therefore does incorporate a screen (not shown) between inlet port  705  and outlet port  706 . 
   The dual chamber cardiotomy reservoir atop the venous reservoir is almost identical to that described in reference to clean  1115   a  and dirty  1115   b  blood chambers shown in  FIGS. 4   a - 4   d . It has one large chamber  710  divided by separating wall  716  into chamber  710   a  for clean blood and chamber  710   b  for dirty blood. Wall  716  extends from the bottom to the top of chamber  710  leaving a fluid communication  716   a  between the top of chambers  710   a  and  710   b . Since clean blood need not be kept out of closed compliant chamber  707 , then the volume of chamber  710   a  need only be sufficient to accommodate the screened chamber  714  used to deaerate and defoam the clean blood. Thus, most of the volume in the cardiotomy can be assigned to chamber  710   b , thereby maximizing the volume available for dirty blood at the expense of volume for the clean blood. If there is no need for additional blood holding capacity than that available in compliant chamber  707  then the volume of chamber  710   a  should be no more than 20%, of the volume of chamber  710   b . Clean blood enters via port  715   a  into clean chamber  714  formed of screen wall  714   c  that is sealed at its bottom. Defoamer  715  positioned at the top section of chamber  714  collapses foam in chamber  714 . Clean blood flows through screen  714   c , with the screen reducing the bubble size and air volume in the exiting clean blood. Similarly, dirty blood enters via port  711   a  into aggressive filter  711  that also includes a defoaming agent. The filtered dirty blood flows into chamber  710   b  where it can be stored or flow to chamber  710   a  via port  718  and  717 . Ports  717  and  718  are in fluid communication via tubing  713  that can be closed or opened with valve  713   a  to allow or prevent the dirty blood to flow to clean chamber  710   a  and then to combine with the venous blood in chamber  707 . Blood in chamber  710   a  flows down channel  719  and combines with venous blood at inlet port  701 . It is preferred that lowest vertical level  719   a  of channel  719  is lower than inlet port  701  and that inlet port  701  is bottom  707   a  of chamber  707 . This minimizes air from the venous blood rising into channel  719  or air from channel  719  rising up venous line via inlet port  701 . 
   It should be obvious that it may be advantageous to have clean blood chamber  1115   a  larger than dirty blood  1115   b , for example for patients with a large blood volume that needs to be stored without it contacting the filter of the dirty blood. 
     FIGS. 4   c  and  4   d  also illustrate that compliant blood chamber  7  can be fitted with front plate  28  sealed along the outside periphery of wall  8   c  to form sealed air chamber  26  with flexible diaphragm  25  as one wall and front plate  28  as the other wall. Channel  12  forms a fluid communication between air chamber  26  at opening  12   a  and cardiotomy reservoir chamber  10  at opening  12   b  assuring that the air pressure in these two chambers is equal. Exhaust port  21  in fluid communication with chamber  10  is open to ambient atmosphere, or as shown in  FIG. 4   d , in fluid communication with vacuum regulator  29 . The latter can be used for vacuum assisted venous return (VAVD). Note that exhaust port  21  is in fluid communication with the air side of chamber  10   a  of the clean blood, chamber  10   b  of the dirty blood, and inlet chamber  2  and outlet chamber  3  of the air purging chamber. It is also in fluid communication with the air side of diaphragm  25 . This allows a single port to apply vacuum to all four chambers. Freely moving compliant diaphragm  25  assures that the blood pressure in compliant storage chamber  7  equals the air pressure in air chamber  26 . Thus, when vacuum is applied to port  21 , it is transmitted to air chamber  10 , and then via channel  12 , to air chamber  26  and then, to the air side of moving diaphragm  25 , to the blood side of diaphragm  25  forming the flexible wall of chamber  7 . This design is much like that described in reference to  FIGS. 10   a ,  10   b  and  10   c  of aforementioned Tamari&#39;s &#39;049 patent, vacuum can be applied to flexible diaphragm  25  that allows vacuum assisted venous drainage (VAVD). The shape of front plate  28  preferably is similar in shape to the shape of diaphragm  25  thereby allowing diaphragm  25  to fully expand and maximize the volume chamber  7  can fill to with blood. The same vacuum is also applied to inlet chamber  2  and outlet chamber  3  of air purging chamber  1116  via channel  3   e  shown in  FIGS. 4   a  and  5   a  and  5   b . Thus, the unrestricted fluid communication between air chamber  26  and cardiotomy chamber  10  and between cardiotomy chamber and inlet chamber  2  and outlet chamber  3  of air purging chamber  1116  (or  1116   a ) allows a single air port in any of these chambers to be used to apply vacuum to the other chambers and maintain these other chambers at the same vacuum as the chamber with that single air port. In the example shown that port is port  21  atop chamber  10  of cardiotomy  111   a  shown in  FIGS. 4   a ,  4   b  and  4   d.    
   As previously described, during VAVD it is necessary to trap water vapor in the air exhausted from the venous/cardiotomy reservoir that condenses on the cooler walls of the vacuum tubing and prevent the condensate from dripping back into the blood.  FIGS. 4   a  and  4   d  show another innovation: a vapor trap incorporated into the reservoir designed for VAVD. For example, bottom section  21   b  of exhaust port  21 , extends below the top of chamber  12 . The top of chamber  12  is in fluid communication with the air space above clean chamber  1115   a  and dirty chamber  1115   b  and the bottom of chamber  12  is in fluid communication with air chamber  26  of venous reservoir  1103 . Thus, any water condensate that drips down out of exhaust tube  21   b , falls into, and is trapped by, chamber  12  and eventually flows into air chamber  26  of venous reservoir  1103 . 
   It should be understood that the description for gravity drainage (i.e. no vacuum applied to any port), where air is purged to atmosphere, also applies to VAVD applications except “atmosphere” is changed to the vacuum applied. 
   As shown in  FIGS. 4   a  to  4   d , air purging chamber  1116 , venous reservoir  1103 , cardiotomy reservoir  1115 , as well as channels  1 ,  3   e ,  4  and  12  interconnecting these chambers, are at least partially formed with common rigid plate  8 , a design that reduces manufacturing costs and simplifies setup. Thus for example, rigid wall  8   a  forms at least a part of the wall forming inlet channel  1   a  of air purging chamber  1116 ; rigid wall  8   b  forms at least one part of the wall forming outlet chamber  3  of air purging chamber  1116 ; rigid wall  8   c  forms at least a part of the wall forming compliant storage chamber  7 ; rigid wall  8   d  forms at least a part of channel  4   b  connecting outlet chamber  3  to compliant storage chamber  7 ; rigid wall  8   e  forms channel  19  connecting chamber  10  of cardiotomy  1115  to the bottom of inlet channel  1 ; rigid wall  8   f  forms at least one part of the wall forming chamber  10  of cardiotomy  1115 . Common wall  8  may also form channel  3   e  connecting the top of outlet chamber  3  to the top chamber  10  of cardiotomy  1115 , outlet port  6  of compliant storage chamber  7  as well as the numerous connectors atop cardiotomy reservoir  1115 . 
   It should be clear that rigid wall  8  can have a matching clamshell like mirror image structure that fits to complete the walls required to form the aforementioned chambers and channels. Other designs include having outlet chamber  3  and cardiotomy chamber  10  with its divider  16 , and concave section  8   c  injected molded and then fitted with top covers that incorporate ports such as air port  21  or inlet port  1   a  shown in  FIG. 4   a.    
     FIG. 6  is a line drawing illustrating a design where tubing is used as channel  4   b  connecting outlet  4  of the air purging chamber, previously described in reference to  FIGS. 3   a - 3   d , to compliant reservoir  7 , and tubing  153  is used to connect the outlet of cardiotomy reservoir to inlet  1  of air purging chamber  1116 . Using tubing has two advantages: it simplifies the mold and it enables the user to clamp off the tubing when needed. It also illustrates that air purging chamber  1116  can be used with State-of-the-Art venous bags eliminating air from entering the bag to provide greater safety. 
     FIG. 6  also illustrates another means to reduce air being dragged from cardiotomy  1115  to inlet chamber  2  via channel  153 . Here tube  153  is slanted to allow air bubbles  27  that have entered tube  153  to rise to the radial top of channel  153  and then, as illustrated in  FIG. 6   a  by the upward pointing arrow, move up channel  153  along its top most wall. Raised section  159   a  of outlet port  159  located at the bottom of cardiotomy  1115 , facilitates air bubble escape from channel  153  by preventing blood from entering channel  153  at highest outlet point  159   a  where air escapes from tube  153  during intermittent blood flow. To further reduce air bubbles entrapment and facilitate air bubbles movement up channel  153 , channel  153  should have at least its top portion enlarged to an equivalent diameter greater than ⅜″ and preferably at least ½″ and incorporate structure  153   c  serving the same function as structure  19   c  previously described in reference to  FIG. 4   a.    
   Incorporating all the elements of the invention provides a closed venous reservoir that:
     1) Has superb air handling.   2) Passively removes air.   3) Primes easier and faster than any other closed venous reservoir.   4) Incorporates a cardiotomy reservoir and allows VAVD.   5) Deaerates vented (clean) blood without that blood contacting the filter of dirty blood cardiotomy by incorporating an innovative two chamber cardiotomy reservoir.   6) Shuts off when empty and requires little volume to reopen.   7) It can operate at low volume without the danger of pumping air.   8) Once primed, air cannot enter it. The new reservoir is therefore safer.   9) A low level alarm can be placed on it as easily as on any hard shell venous reservoir.   10) Has the lowest blood-to-air interface of any hardshell reservoirs.   11) Has the highest ratio of screen area to blood-to-air interface.   12) Its air purging chamber can be used with or without its compliant chamber allowing lower operating volume.   

   It should be understood that a comprehensive description of each of the applications of the invention is beyond the scope of a patent application and therefore the aforementioned descriptions are given as illustrations and should not be used to limit the intent, spirit, or scope of the invention.