BACKGROUND OF THE INVENTION The present invention relates to pumps and more particularly to centrifugal blood pumps in which impeller blades are positioned so that hemolysis is minimized.

Delicate surgical procedures require that the site of surgery remain motionless. This requirement made early heart surgery difficult to impossible as interruption of the heart's pumping action for the required length of surgical time would invariably be fatal.

During the 1960s, prolonged and non-fatal stoppage of the heart became possible by use of newly developed "heart-lung" machines. These machines consisted of a mechanical blood pump combined with a blood oxygenator. The heart-lung machines were capable of taking over the function of the natural heart and lungs for periods of up to several hours, enabling the development of techniques leading to today's extensive practice of open-heart surgery.

The first practical mechanical blood pumps used were peristaltic or "roller" pumps. The pumping action of a roller pump derives from the compression of a section of the flexible plastic tubing which carries the blood through the heart-lung machine. A moving roller presses the tubing against a semicircular platen, moving the blood forward in the tubing. The speed of the moving roller and the diameter of the tubing control the rate of blood flow.

Although the roller pump was and is simple and reliable, it has two characteristics which can endanger the patient undergoing surgery. First, if flow is inadvertently obstructed, the resulting increase in pressure produced by a roller pump may exceed the bursting strength of the tubing circuit.

Second, if air is accidentally introduced into the tubing circuit, it will be

pumped to the patient along with the blood. Either of these conditions may result in serious or fatal consequences to the patient.

In 1976, centrifugal blood pumps began to replace the roller pump as the "heart" of the heart-lung machine. The pumping action of a centrifugal blood pump derives from the rotation of an impdler within a pumping chamber. One impeller design associated with centrifugal blood pumps is a disk-shaped device with multiple blades positioned on a surface. The impeller is rotated about a central axis of rotation by way of a rotational drive source. After the blood enters the pumping chamber via an inlet, it makes contact with the impeller blades and is rotated along with the impeller. The impeller rotates at a predetermined speed so that a required pressure and flow rate is maintained.

Pump pressure is controlled by the rotational speed of the impeller. At operational speeds, excessive pressure cannot be produced. Additionally, the centrifugal forces in the pump form a natural air trap and, with massive introduction of air, deprime and discontinue pumping altogether. The above-mentioned safety features, and the decreased blood damage, or hemolysis, caused by centrifugal blood pumps is now widely recognized and has led to their extensive use for open heart surgery. In the early 1980s it was demonstrated that a mechanical blood pump could be used as a heart-assist pump for patients who could not be separated from the heart-lung machine following surgery. The readily available centrifugal blood pumps were quickly adapted to this situation as well as to the more routine use during heart surgery. The fragility of blood , however, presents several problems for the design of mechanical blood pumps. Excessive shear forces cause rupture of the red blood cells. Hemolysis is a measure of the rate at which red blood cells are damaged. Despite the risk associated with excessive forces which may cause hemolysis, constant motion and high flow velocity rates are needed (especially

over local areas of friction, such as seals) to maintain required pump pressure and to prevent points of high temperature which may cause blood damage and the accumulation of clot deposits. Thus, a balance must be established between adequate rotational speed of the pump impeller and an acceptable level of hemolysis.

Previous centrifugal blood pumps have reduced hemolysis by decreasing the rotational speed of the impeller. The required pump pressure and flow rate of blood in these pumps is maintained by increasing the diameter of the pump. In other words, increasing the diameter counteracts the decrease in rotational speed of the impeller. Some pumps, however, have dimensional constraints which do not allow an increase in pump diameter. A means to reduce hemolysis without increasing the diameter of the centrifugal blood pump is not found in the prior art.

SUMMARY OF THE INVENTION With the present invention, a reduction in hemolysis is achieved by positioning a plurality of blades of the impeller adjacent to the spindle which intersects the center and is the axis of rotation of the impeller. This configuration reduces hemolysis because the velocity of the blade tangential to the axis of rotation is at its minimum at the center of the impeller. Thus, hemolysis is reduced because the first contact of the blood with the impeller blade occurs at a point where the impeller blades have a relatively low tangential velocity with respect to the axis of rotation. Since the blades of the present invention are positioned adjacent to the center of the impeller, the blood is slowly accelerated from a point of relatively low tangential velocity to a maximum tangential velocity which occurs when the blood reaches the outer blade end.

The present invention is a centrifugal pump for pumping biological fluids such as blood. The pump has a housing which defines a

pumping chamber. An impeller, supported by a spindle, rotates about an axis of rotation located at the center of the impeller. The impeller itself is disposed within the pumping chamber. The pumping chamber has an inlet directed at the center of the impeller and an outlet provided along the periphery of the impeller. The impeller includes a plurality of blades positioned on a first surface. An inner end of each blade is adjacent to the spindle which supports the impeller. A plurality of inner blade ends form a cone-shaped area, or spin inducer, around the circumference of the spindle. The location of the spin inducer near the center of the impeller causes the blood to rotate at a relatively low tangential velocity when it enters the high flow inlet area.

The inner ends of the impeller blades, which actually form the cone-shaped spin inducer, are positioned at the center of the impeller adjacent the spindle. The velocity of each inner blade end tangential to the axis of rotation is at a minimum. The spin inducer is able to reduce the amount of hemolysis because the first contact of the blood with the impeller is at a point of relatively low tangential velocity. As the blood continues to travel into the pumping chamber, it is accelerated to a maximum velocity, tangential to the axis of rotation, which occurs at the outer blade end. The gradual acceleration decreases the turbulence of the flow within the pumping chamber and results in less hemolysis than in prior art centrifugal pumps.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an exploded perspective view of the present invention. Figure 1 A is a close-up of a full impeller blade. Figure 2 is a cross-sectional side view of the present invention. Figure 3 is a close-up view of the spin inducer shown in the cross-sectional side view of the Figure 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention is shown as centrifugal blood pump 10 in Figures 1-3. Figure 1 shows that blood pump 10 is composed of three distinct parts: upper enclosure 12, base 14, and impeller 16. Upper enclosure 12 includes inlet 17, journal bearing 18 and plug

20. Ridge 22 on inlet 17 facilitates attachment of inlet tubing (not shown) from a reservoir/oxygenator or from the patient. Journal bearing 18 and plug 20 are mounted in aperture 24 at the bend in inlet 17.

Base 14 includes cylindrical side wall 32, bottom wall 34 and outlet 36. Bottom wall 34 contains aperture 34A in order to receive journal bearing 45. Outlet 36 is a tubular member which extends from cylindrical side wall 32 to free end 38. Ridge 40 facilitates attachment of outlet tubing (not shown) to free end 38.

Impeller 16 includes spindle 42, impeller platform 44, full impeller blades 46 and short impeller blades 48. Central circulation hole 50 and spin inducer 52 are also shown.

Figure 1A shows a close-up view of full impeller blade 46. Inner blade end 46A and outer blade end 46B are shown. A plurality of inner blade ends 46 A forms spin inducer 52 (shown in Figures 1, 2 and 3). Full impeller blade 46 is approximately four inches in total length. The width of inner blade end 46A, which is measured laterally from spindle 42 (not shown) to the end of the blade, is approximately 0.25 inches. The width of outer blade end 46B, which is measured vertically from impeller platform 44 (not shown) to the end of the blade, is approximately 0.38 inches. The clearance between inlet 17 and inner blade end 46A is less than the clearance between upper enclosure 12 and outer blade end 46B.

Figure 2 shows a cross sectional view of centrifugal blood pump 10 as it appears assembled. In this view, housing 60 (formed by upper enclosure 12 and base 14) encloses impeller 16. Pumping chamber P is a

volume defined by upper enclosure 12, inlet 17, cylindrical side wall 32 and bottom wall 34.

Impeller 16 is supported by spindle 42 and rotates about an axis of rotation defined by spindle 42 in a direction indicated by arrow R. An inlet end of spindle 42 is inserted into journal bearing 18 while an opposite end of spindle 42 is received by journal bearing 45. Bottom wall aperture 34A (shown in Figure 1) receives journal bearing 45. Attached to platform 44 are ull impeller blades 46 and short impeller blades 48. Full blades 46 extend across platform 44 and central circulation hole 50 (shown in Figure 1). Short blades 48 have a length approximately half the radius of platform 44. Both full blades 46 and short blades 48 are tapered. Inner blade ends 46A are supported by and positioned adjacent spindle 42 to form cone-shaped spin inducer 52.

Platform 44 has positioned on its bottom surface, magnets (not shown), which are coupled with magnets carried by a drive rotor (not shown) positioned below bottom wall 34 of base 14. In this configuration of a centrifugal blood pump, an electric motor is connected to the rotor in order to rotate the magnets on the bottom of platform 44 which are coupled with the magnets on the rotor. Both magnets on platform 44 and the rotor rotate about the axis of rotation. The magnets on the bottom of platform 44 and the magnets carried by the rotor are coupled together so that the impeller rotates at the same speed as the rotor.

In another configuration, the impeller is mounted on a drive shaft which extends outside the pumping chamber to a rotational drive source. As the drive source is activated, the impeller rotates along with the shaft. In either configuration, however, rotational speed adequate to create the required pump pressure and rate of flow of blood must be maintained.

Blood from the patient enters inlet 17 and is directed toward spin inducer 52. At this point, blood flow is generally in the axial direction. Journal bearing 18 receives an end of spindle 42. Journal bearing 18 is press-fit

into aperture 24 and is capped with epoxy plug 20. Impeller 16 is supported by spindle 42. While impeller 16 is rotated by methods discussed above, blood contacts spin inducer 52 and experiences gradual acceleration as it travels through pumping chamber P. Full impeller blades 46 and short impeller blades 48 cause the blood to circulate within pumping chamber P toward outlet 36. Figure 3 shows a close-up view of a portion of inlet 17 and inner blade ends 46A, which form spin inducer 52. Spin inducer 52 is supported by spindle 42. Journal bearing 18 is shown press-fit into aperture 24 and capped with epoxy plug 20. Inlet tubing (not shown) is secured by ridge 22 and extends away from inlet 17.

High shear rate and turbulence can cause rupture (hemolysis) of the red blood cells Therefore, it is desirable to accelerate the blood cells slowly so that there is no drastic change in speed or in flow direction of the blood. Unlike the bladed impellers found in prior art centrifugal blood pumps, spin inducer 52 serves to reduce the impact of the turbulent flow which is normally associated with the inlet area. Specifically, spin inducer 52 acts as a means to slowly accelerate the blood. The tangential velocity of inner blade ends 46A, which form spin inducer 52, is less than the tangential velocity of outer blade ends 46B. Although the rotational speed remains constant at all points on impeller 16, the tangential velocity of the blade decreases as one moves toward the center of impeller 16.

Hemolysis is reduced because the relatively slow tangential velocity and acceleration of the blood as it enters inlet 17 and impacts spin inducer 52 causes less blood damage than prior art pumps which do not have a spin inducer.

Positioning impeller blades such that spin inducer 52 is formed allows for the blood to travel in a manner such that dramatic increases in tangential velocity (i.e. acceleration) do not occur. Thereby, the blood experiences less turbulence in the high flow inlet area and hemolysis is reduced.

In an alternative embodiment of the present invention, the inlet may be vertical instead of the curved shape shown in Figures 1-3. In this configuration, the spindle is inserted into a journal bearing which is supported by struts which extend from the inlet wall. Similar to the embodiment shown in Figures 1-3, however, the inlet is flared so that the spin inducer may fit inside.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.