Abstract:
The need for a venous reservoir in a heart-lung machine is obviated by using a vacuum-purged negative-pressure air filter in the venous return line ahead of the main blood pump. The purging vacuum for the venous air filter can also be used to purge air from the cardiotomy reservoir if a backflow-preventing valve is used on the venous air filter.

Description:
This application is a continuation under 35 U.S.C. 120 of application Ser. No. 09/251,619, Feb. 17, 1999 now U.S. Pat. No. 6,302,860, which patent is herein expressly incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to blood filters used in cardiopulmonary bypass circuits, and more particularly to a negative pressure blood filter for use in the venous line whether using assisted venous return techniques or not. 
     BACKGROUND OF THE INVENTION 
     Conventional cardiopulmonary bypass uses an extracorporeal blood circuit which includes a venous drainage line, venous reservoir, blood pump, oxygenator, and arterial filter. Blood circulation is accomplished by draining blood from the patient by gravity through the venous drainage line to the venous reservoir. From there, blood drains down to the blood pump, placing this portion of the circuit at a negative pressure with respect to atmosphere. The pump supplies positive pressure to return the blood to the patient through the oxygenator and filter. The venous reservoir holds blood volume as required, while both the venous reservoir and arterial filter remove air bubbles from the blood. These may cause health problems if returned to the patient in the arterial blood flow. Air can enter the circuit from a number of sources, including around the venous cannula and through various unanticipated intra-operative events. A further complication arises if a centrifugal pump is used, in which case a large volume of air will de-prime the pump, depriving it of its pumping capability. 
     In order to remove air from an extracorporeal circuit prior to its use, the circuit is primed with an appropriate solution. During surgery, this solution dilutes the patient&#39;s blood, and it is therefore desirable to minimize the volume required. The venous reservoir contains a relatively large volume of fluid, and recently it has been proposed to eliminate this component of the circuit. Several problems arise, however. Without the venous reservoir between the patient and the oxygenator, any air in the venous line will either accumulate in the centrifugal pump (if used) or be pumped into the oxygenator. Furthermore, if a large bolus of air is introduced, it may de-prime the pump and oxygenator. Although arterial filters are designed to capture air bubbles, they are not designed to handle larger volumes of air such as may occur from the causes described above. Also, arterial filters are located downstream of both the pump and the oxygenator, and therefore cannot prevent air problems that would occur in those devices. Furthermore, conventional arterial filters are designed to operate at positive blood pressures. 
     SUMMARY OF THE INVENTION 
     The present invention improves upon the design of an arterial filter to allow it to be used as a venous filter at a negative pressure and to capture larger volumes of air. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a conventional heart-lung machine; 
     FIG. 2 is a schematic diagram of an AVR type heart-lung machine; 
     FIG. 3 is a schematic diagram of a heart-lung machine in accordance with the invention; and 
     FIG. 4 is a partly schematic vertical section of the inventive filter. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Conventional heart-lung equipment, as schematically depicted in FIG. 1, draws the blood of a patient  10  during cardiovascular surgery through a venous line  12 , oxygenates it, and returns the oxygenated blood to the patient  10  through an arterial line  14 . Cardiotomy blood and surgical field debris are aspirated by a suction device  16  and are pumped by pump  18  into a cardiotomy filter  20 . 
     In a conventional extracorporeal blood circuit, venous blood from line  12 , as well as defoamed and filtered cardiotomy blood from filter  20 , are discharged into a venous reservoir  22 . In the reservoir  22 , air entrapped in the venous blood (as, for example, air drawn into the blood circuit through the sutures, not shown, that attach the venous line  12  to a vein of the patient  10 ) rises to the surface of the blood in the reservoir  22  and is vented to atmosphere through a purge line  24 . The purge line  24  is typically about a 6 mm ID line, and the air space above the blood in reservoir  22  is substantial. 
     In the conventional circuit of FIG. 1, a pump  26  draws blood from the reservoir  22  and pumps it through an oxygenator  28  and an arterial filter  30  into the arterial line  14 . The arterial filter is basically a bubble trap that traps any microair bubbles larger than about 20-40 μm and discharges them to atmosphere through a typically about 1.5 mm ID purge line  32 . 
     As shown in FIG. 2, it has recently been proposed to produce an assisted venous return (AVR) and to eliminate the reservoir  22 , which accounts for a major portion of the priming volume of the extracorporeal blood circuit, by moving the arterial filter  30  into the venous line  12 , upstream of the pump  26 . 
     The filter  30  does not have an air space between its inlet and outlet, as the venous reservoir  22  does. Consequently, the negative pressure caused on the outlet side of filter  30  in FIG. 2 by the pump  26  is transmitted as suction to the venous line  12 , thereby assisting the venous return from the patient  10 . 
     In accordance with the invention (FIGS.  3  and  4 ), a filter  30  of the type and size conventionally used as an arterial filter is adapted for efficient use as an AVR filter by several modifications. First, evacuation of air is facilitated by increasing the size of the purge port  34  to accept, e.g., a 6 mm ID purge line. Secondly, a vacuum greater than that normally used for venous drainage is applied to the purge port  34  to actively purge air from the filter  30 . Thirdly, a check valve  36  is incorporated into the purge port to prevent air or blood from the cardiotomy reservoir  20  (which is at ambient pressure but is conveniently purged by the same vacuum that purges filter  30 ) from being drawn into the filter  30  by the negative pressure in filter  30 , when the purging vacuum is not active. Fourthly, an air sensor  38  is provided in the filter  30  and is connected to activate the purge vacuum when, and only when, air is present in the filter  30 . This prevents blood from being aspirated by the purging vacuum. 
     The purging vacuum may be produced by a pump  40 , or it may be produced by connecting the purge line  42  to the vacuum outlet conventionally provided in operating rooms. 
     FIG. 4 shows in somewhat schematic form the filter  30  of this invention. Blood enters the filter  30  through an inlet  44  and is drawn through the filter element  46  and into the outlet  48  by the action of pump  26 . A screen (not shown) or other conventional bubble-trapping device traps any air bubbles in the blood stream and causes them to rise to the top  50  of filter  30 . Normally, the filter  30  is filled with blood. When air begins to accumulate at the top of filter  30 , this fact is sensed by the air sensor  38 . The sensor  38  activates the vacuum in the purge line  22 . The vacuum at the purge port  34  overcomes the negative pressure in the filter  30  and draws out any accumulated air, shutting off under the control of sensor  38  when all the air has been removed. 
     It is understood that the exemplary venous filter for assisted venous return described herein and shown in the drawings represents only a presently preferred embodiment of the invention. Indeed, various modifications and additions may be made to such embodiment without departing from the spirit and scope of the invention. Thus, other modifications and additions may be obvious to those skilled in the art and may be implemented to adapt the present invention for use in a variety of different applications.