Field of the Disclosure
The invention relates to blood separation systems and methods. More particularly, the invention relates to systems and methods for improving the flow rate of plasma separated from blood during a blood separation procedure.
Description of Related Art
Various blood processing systems now make it possible to collect particular blood constituents, rather than whole blood, from donors. Typically, in such systems, whole blood is drawn from a donor, the particular blood component or constituent is removed and collected, and the remaining blood constituents are returned to the donor. By thus removing only particular constituents, potentially less time is needed for the donor's body to return to normal, and donations can be made at more frequent intervals than when whole blood is collected. This increases the overall supply of blood constituents, such as plasma and platelets, made available for health care.
Whole blood is typically separated into its constituents through centrifugation. This requires that the whole blood be passed through a centrifuge after it is withdrawn from, and before it is returned to, the donor. To avoid contamination and possible infection of the donor, the blood is preferably contained within a sealed, sterile fluid flow system during the entire centrifugation process. Typical blood processing systems thus include a permanent, reusable centrifuge assembly containing the hardware (drive system, pumps, valve actuators, programmable controller, and the like) that spins and pumps the blood, and a disposable, sealed and sterile fluid processing assembly that is mounted in cooperation on the hardware. The centrifuge assembly engages and spins a disposable centrifuge chamber in the fluid processing assembly during a collection procedure. The blood, however, makes actual contact only with the fluid processing assembly, which assembly is used only once and then discarded.
As the whole blood is spun by the centrifuge, the heavier (greater specific gravity) components, such as red blood cells, move radially outwardly away from the center of rotation toward the outer or “high-G” wall of a separation chamber included as part of the fluid processing assembly. The lighter (lower specific gravity) components, such as plasma, migrate toward the inner or “low-G” wall of the separation chamber. Various ones of these components can be selectively removed from the whole blood by forming appropriately located channeling seals and outlet ports in the separation chamber of the fluid processing assembly. For example, therapeutic plasma exchange involves separating plasma from cellular blood components, collecting the plasma, and returning the cellular blood components and a replacement fluid to the patient.
Proper separation requires, however, that the interface between the separated components be located within a particular zone between the high-G and low-G walls of the separation chamber. For example, when performing a therapeutic plasma exchange procedure, the interface between the plasma and the cellular blood components affects the performance of the system. If the interface is located too close to the low-G wall, then the collected plasma may become unduly populated or contaminated by cellular blood components. On the other hand, if the interface is located too far from the low-G wall, there may be no contamination of the plasma, but the separation efficiency of the system may be decreased with less plasma collected over time.
Various known centrifuges, such as those shown and described in U.S. Pat. No. 6,254,784 to Nayak et al. and U.S. Pat. No. 6,312,607 to Brown et al. (which are incorporated herein by reference), are operable to automatically keep the interface within a desired zone as the centrifuge operates. Typically, the separation chamber of the fluid processing assembly is loaded between the bowl and spool of a centrifuge. A radially inwardly ramped surface is located on the radially outer wall of the separation channel in the bowl wall of the separation chamber. The interface between the generally dark, opaque red blood cell layer and the generally light, clear plasma layer appears as a line on the ramped surface. Where, exactly, the line appears on the ramped surface is a function of the position of the interface between the high-G and low-G walls of the separation chamber. Accordingly, the position of the line on the ramped surface can be used to gauge the position of the interface between the high-G and low-G walls.
Automatic control over the location of the interface has been achieved by sensing the position of the line on the ramped surface and thereafter adjusting the centrifuge operating parameters to place and keep the line within desired limits. In particular, by controlling the rate at which plasma is withdrawn from the separation chamber, the line can be “moved” up (radially inwardly) or down (radially outwardly) on the ramped surface, such as by decreasing or increasing the plasma flow rate. Typically, an optical sensor assembly is used to sense the position of the line on the ramped surface. As the centrifuge spins past the sensor, the sensor develops an electrical pulse having a width related to the position of the line on the ramped surface. As the line moves closer to the high g wall of the separation chamber, the pulse width increases. As the line moves closer to the low-G wall, the pulse width narrows. By sensing the width of the pulses developed by the optical sensor and thereafter using the pulse width to increase or decrease the rate at which plasma is withdrawn from the separation chamber, the line can be kept within desired positional limits on the ramped surface and the interface maintained in the desired radial position or range of positions.
U.S. Pat. No. 8,556,793 (which is incorporated herein by reference) describes a system employing two optical sensor assemblies. A first optical sensor assembly monitors the location of the interface on the ramped surface, while a second optical sensor assembly monitors outlet tubing carrying separated plasma to assess various characteristics of the plasma flow (including turbidity and the concentration of free plasma hemoglobin). Experience has shown that it is possible for the first optical sensor assembly to detect a spillover condition (which is indicative of cellular components spilling into the separated plasma) while the second optical sensor assembly indicates that the plasma is sufficiently clear (i.e., that the cellular components have not spilled into the separated plasma). This may happen for any of a number of reasons, for example, due to the presence of cold agglutinins. Detection of a spillover condition may cause a temporary decrease in the plasma flow rate through the outlet tubing in order to relocate the interface on the ramped surface, which eliminates the spillover condition. However, if the second optical sensor assembly shows that the plasma is sufficiently clear, then the slowed plasma flow rate may only lengthen the procedure, rather than providing the intended benefit. Accordingly, the need remains for a centrifugal blood processing system that can reconcile conflicting observations of two optical sensor assemblies for safe, efficient blood processing.