Abstract:
The bubble trap is used in extracorporeal blood flow circuits of the type used for open heart surgery. The trap is placed in the external blood flow and it removes small micro bubbles from the blood prior to delivery to the body. The device accelerates the blood flow radially and the small bubbles migrate toward the center of the accelerated flow. These bubbles are concentrated at this location and the blood that contains the micro bubbles is extracted and recirculated before the degassed blood flow is returned to the body.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application Ser. No. 09/545,637 is a continuation-in-part of U.S. patent application No. 08/998,500, filed on Dec. 26, 1997, now abandoned, which is a continuation of U.S. patent application No. 08/571,490, filed on Dec. 13, 1995 which has Issued as U.S. Pat. No. 5,824,212. 
     This application claims the benefit of Provisional Application No. 60/128,346, filed on Apr. 8, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to “bubble trap” devices that are used for removing gas bubbles from the extracorporeal circulation of blood. 
     BACKGROUND OF THE INVENTION 
     Open heart surgery as well as other modern surgical procedures require that the patient&#39;s blood be routed to an extracorporeal blood pump and oxygenator system. Extracorporeal support of blood perfusion provides many opportunities form air to be mixed with the circulating blood. Consequently it has become conventional practice to place a fine mesh filter called a “bubble trap” close to the blood return cannula. This device serves to trap gas bubbles before they are introduced into the body. This is an essential safety precaution as it is well known that gas bubbles can cause embolisms to form in the vasculature. Since the typical aortic return cannula commonly used in open heart surgery is located near the vessels that communicate with the brain, the possibility of a stroke from small bubbles is a distinct clinical concern. Recent evidence suggests that the presence of even very small micro bubbles is undesirable in perfusion procedures. 
     Bubbles having a diameter of just a few micrometers are impossible to remove using conventional filter technology. A porous mesh filter sufficiently small to “trap” a small bubble has a very high flow resistance and this results in a very high-pressure differential across the mesh which is undesirable. For this reason among others there is a continuing need to improve bubble trap technology. 
     SUMMARY OF THE INVENTION 
     The bubble trap of the present invention is inserted into the external “blood loop” and blood is forced through the dynamic bubble trap by the blood pump. Typically the device is placed just ahead of the outlet cannula to act as a final-filter for the removal of bubbles just prior to the delivery of blood to the patient. The bubble trap device splits the blood flow into two streams. The first stream is fully bubble free and it is delivered to the patient. The secondary stream is smaller and it contains the micro bubbles removed from the in coming blood flow. This secondary flow is returned to the extracorporeal circuit upstream of the trap for additional degassing. 
     The blood flows through the bubble trap device from end to end and thus this flow is primarily axial in direction. Within the bubble trap device the blood flow is subjected to a strong radial acceleration so that there is a strong radial velocity imparted to the blood flow as well. A specialized helical separation chamber is used to impart this radial acceleration. The helix within the separation section comprises a center body and one or more blades. The design and the cross sectional areas of the separation zone are optimized to treat the blood cells gently while applying enough force to the small bubbles to concentrate them for removal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Throughout the various figures like reference numerals represent identical or equivalent structures, wherein: 
     FIG. 1 is a schematic diagram showing a cross section of a dynamic bubble trap device; 
     FIG. 2 is a schematic diagram showing a cross section of a dynamic bubble trap device; 
     FIG. 3 is a schematic diagram showing a cross section of a dynamic bubble trap device; and, 
     FIG. 4 is a schematic diagram showing a cross section of a dynamic bubble trap device. 
    
    
     DETAILED DESCRIPTION 
     Throughout the description the dynamic bubble trap is described as a stand alone device placed in an extracorporeal blood circulation path for ease of explanation. However, it should be understood that the bubble trap device technology can be incorporated into other blood handling devices without departing from the scope of the invention. The preferred structures shown in the figures are illustrative and variations in the design can be carried out within the scope of the invention. 
     Fig. 1 is a schematic drawing which shows the dynamic bubble trap  10  in partial cross section. The device includes a body  12  that has an exterior wall  14  and an interior wall  16 . The overall shape of the device is elongate and approximately cylindrical, The blood flow through the device is primarily axial along the axis  28  of the body  12 . A helix is placed inside the device  10  and this helical section is formed by a center body  18  coupled to a blade  20 . In the figure, portions of the blade are shown in phantom dotted lines to clarify the figure. The blade  20  extends between the interior wall  16  and the surface of the center body  18 . Various construction techniques can be used to realize the device  10 . It has proved convenient to form the center body  18  and blade  20  as a unitary structure and to segment the body  12  to receive the helical section as an insert. Various other assembly techniques can be used as well. 
     As seen in FIG. 1 the device  10  includes a primary blood flow inlet  22  to receive blood flow  30  which contains micro bubbles. The device  10  also has a primary blood flow outlet  24  formed in the body for delivering bubble free blood flow  32 . A secondary blood flow outlet  26  is also provided for the recirculation of secondary blood flow  34  which contains micro bubbles. 
     In operation, the primary inlet  22  and the primary outlet  24  are connected in an extracorporeal blood flow loop at or near the discharge or blood return cannula. The secondary recirculation from secondary blood flow outlet  26  is connected by the user to a location that returns this blood stream to the extracorporal flow system “upstream ” of the bubble trap device  10 . This allows the micro bubbles to be dissipated and this portion of blood to be further degassed in the system. 
     During operation the bubble trap device  10  divides the inlet blood flow  30 , into a bubble free primary outlet blood flow  32  and a secondary blood flow  34  for recirculation. The device  10  is powered by the pressure gradient imparted to the primary blood flow  30  by the extracorporeal blood pump. 
     For the purposes of describing flow regimes within the device  10  the device maybe considered to have a supply section  40 ; an inlet section  42 ; a radial acceleration section  44 ; an outlet section  46  ands a separation section  48 . 
     The inlet blood flow  30  is introduced into the device  10  at through the supply section  40 . In this section  40  the blood flows smoothly in the axial direction defined by the axis  28 . Next the blood flows from the supply section  40  to the inlet section  42  where the cross section of the device may change. As seen in the figure this area may be reduced to gently accelerate the flow along the axial direction and introduce the blood flow into the helical blades in the radial acceleration section  44 . 
     In FIG. 1 blade  20  cooperates with the exterior wall  14  and the center body  18  to form two helical channels which imparts a rotary motion to the blood flow with respect to the axis  28 . The blade  20  divides the flow path into two channels shown as blood flow channel  50  and blood flow channel  52 . the two channels are parallel and both channels are defined by the blade  20 . Although only one blade is shown in the figure for simplicity of illustration, more than one blade can be used. If multiple blades are used then there will be additional flow channels in the device  10 . 
     In FIG. 1 the drawing shows the contour of the center body  18  as a “teardrop” shape which smoothly changes in diameter along the axis  28 . The shape of this center body  50  defines the cross section of the flow path within the radial acceleration section  44 . Alternate center body contours arc operable and shown in other figures. 
     The center body  18  seen in FIG. 1 is axially and radially symmetric and it is shown forming a gently converging channel to minimize disruption to the blood cells as they are radially accelerated in channels  50  and  52  defined by the stationary blade  20 . The blade  20  seen in all the figure exhibits constant pitch however the pitch of the blade may be constant or the pitch may vary along the direction of flow. In the figures a single blade and a constant pitch configuration is shown for clarity and simplicity of description. The detail design of this portion of the device results in acceleration of the blood while minimize damage to the blood and it may be useful to vary both center body contour and blade number and pitch to achieve this result. 
     Immediately after the annular acceleration section  44  the blood flows into the outlet section  46  which functions in the exemplary embodiment as a diverging nozzle to slow the axial velocity of the blood passing through the device. In general the design of this section is compromised in favor of minimizing the pressure change on the blood cells. For this reason the included angle  36  defining the taper of this section may vary from about 5 degrees to about 45 degrees. The outlet section  46  cooperates with the separation section  48  to separate the micro bubbles from the blood flow. The blood removed through the secondary outlet  26  contains the concentrated flow of micro bubbles that have migrated toward the centerline of the flow along axis  28  under the force imparted by the radial acceleration section. 
     The shape of the inlet or bubble pick up  25  of the secondary blood flow outlet  26  may take any suitable form but round or circular opening s have proven effective. 
     FIG. 2 shows a schematic partial cross section of the bubble trap device  10  with an alternate form of center body  60 . In this example the center body  60  is essentially cylindrical in form and has blunt entry surfaces. The interior wall  16  is concave to cooperate with the cylindrical body to accelerate the flow. In general the blood flow channel is defined by the space between the interior wall  16  and the center body  18  and either surface or both may vary in shape and contour. The dimension D may be taken as an average or characteristic dimension for the size of the helical flow section. 
     In this figure the blood flow is shown by flow stream line  62  which is shown entering the radial acceleration section  44 . The blood spirals around the radial acceleration section  44  as depicted by flow streamline  64  which is intended to depict rotary motion about the axis  28 . The blood exits the radial acceleration section  44  and continues to spiral around the axis  28  during transit through the outlet section  46  and the separation section  48 . 
     It is believed that a substantial amount of time is require to allow the small micro bubbles (8 micron diameter) to migrate under the accelerations imparted by the radial acceleration section  44 . It has been determined that the length of the separation section has an important impact on the efficiency of the device. It has been found empirically and supported by a mathematical model, that the relationships between the flow rate, diameters and lengths can result in optimum separation with minimum recirculation flow rate from the secondary outlet  26 . These relationships are counterintuitive and can be expressed as ratios of certain geometric relationships. In general the length “L” of the combined separation section  46  and outlet section  48  should be at least  3  times as long as the diameter “D” of the annular acceleration section  44 . In general the length of the combined separation section  48  and outlet section  463  be not more than  10  times as long as the diameter “D” of the annular acceleration section  44 . It must be understood that these are estimates and that operation outside of these ranges is possible but less practical and efficient. 
     The nominal flow rate through the bubble trap is approximately 2-7 liters a minute for an adult and the nominal diameter “D” of the device is between 1 and 4 centimeters. The width of the channel in the screw section  46  depends on the detail design and some experimentation should be expected to minimize damage to the blood while imparting high radial accelerations. It must be remembered that the pitch of this section also controls the path length for the blood. 
     FIG. 3 shows an alternate embodiment of the invention. In this embodiment the outlet section  56  and separation section  58  have rounded contours that may be expressed as radii. Although the actual shape may be quite complex the curve may be approximated by a circle of radius “R 1 ” between points “A” and “B” and a second radius “R 2  between points “B” and “C”. In this embodiment the outlet section  56  blends smoothly with the separation section  58 . The center body  38  is blunt on its leading edge and truncated on its trailing edge. Although this shape is not preferred it is associated with effective extraction and concentration of micro bubbles. 
     FIG. 4 shows a preferred design with the overall length of the combined outlet section and separation section  68  is more than 3 diameters away from the radial acceleration section  66 . No distinct outlet section is apparent in this design. The included angle  36  defining the “straight” taper of the separation section is larger than 5 degrees. In this embodiment the center body  70  is shown in partial cut away to reveal the contour more clearly. In this instance the center body  70  is blunt at both ends and has a slightly steeper section after the maximum diameter station along the center body. The reference numeral  72  shows the maximum diameter station and this location is more than half way along the length of the radial acceleration section  66 . In this fashion the center body is “sharper” at the leading edge of the flow in the radial acceleration section  66  than at the trailing edge of the radial acceleration section. 
     It is difficult to measure the operating pressures inside the radial acceleration section  66 , however computation suggest that the maximum pressure gradient is achieved at a location approximately 25% of the total length of the radial acceleration section  66 . This position is measured along the axis  28  and is depicted in the figure by reference numeral  74 . The value of the pressures at this position are approximately −30 mm Hg at the surface of the center body shown at location  74  and a value of +5 mm Hg at the periphery of the flow next to the interior wall of the radial acceleration section indicated in the figure by reference numeral  76 . As the flow moves along the radial acceleration section, the pressures change smoothly. The computed pressure at the exit of the radial acceleration section at the periphery at location  78  is near 0.0 mm Hg and the pressure at the end of the center body at location  80  is approximately −20 mmHg. These computed figures correspond to a geometry of an efficient and successful bubble trap. It is intended that variations from these computed and expected values are within the scope of the invention.