Patent Publication Number: US-2019184076-A1

Title: Improvements in or relating to blood salvage and autotransfusion

Description:
FIELD OF THE INVENTION 
     This invention relates to improved apparatus for salvage and autotransfusion of blood evacuated from wound sites. The apparatus is of particular utility in austere situations in which, for instance, electrical power is not available. As such, the apparatus may be useful in military applications, including battlefield medical facilities. 
     BACKGROUND TO THE INVENTION 
     Uncontrolled haemorrhage is the leading cause of death on the battlefield, where two thirds of these deaths are a result of non-compressible (truncal) haemorrhage. In the recent conflicts in Iraq and Afghanistan, uncontrolled blood loss was the cause of death in 85% of the potentially survivable battlefield cases and in 80% of those who died in a military treatment facility. Between 2001 and 2011 in these war zones, 25% of the 4596 combat deaths were potentially survivable, and 90% of the deaths occurred before the injured reached a medical facility. Of the 4090 mortally wounded troops 1391 died instantly and 2699 died before reaching a treatment facility—only 506 reached a hospital before succumbing to injury. This data highlights an obvious need for improving field treatment capability. 
     Over 25% of military casualties require aggressive and urgent haemostatic resuscitation to increase survivability. Additionally, 30-40% of these military personnel that need blood transfusions when they reach emergency departments also suffer from acute traumatic coagulopathy (ATC), which is associated with an 80% mortality rate. Variations of blood transfusion techniques are possible, although very few blood restoration modalities are available to a field medic in austere conditions, due to blood provision and storage constraints. The urgent deployment of blood products (fresh frozen plasma and packed red blood cells) has been shown to attenuate ATC, although there are considerable logistical concerns and clinical risk with this type of intervention. For these reasons, there is a need to develop innovative and less logistically intense mechanisms for treating casualties that require blood at point of injury. 
     More recent damage control resuscitation (DCR) protocols have modified the ratios of blood elements administered to war casualties and signalled the necessary implementation of whole fresh warm blood (WFWB) administration in combat hospitals, where ‘buddy transfusions’ have exceeded 6000 units in recent combat operations. Although WFWB transfusions in the battlefield are feasible—having been shown to aid effectiveness of clotting and oxygen transport—there is still potential for catastrophic outcomes such as haemolytic reactions. Also, donors are unable to provide more than a single unit, require fluid therapy and are limited to light duty for at least 72 hours following donation, and are restricted from flying duties for this time period. 
     Military blood transfusion relies on donations from the general public and military personnel for treating massive blood loss in casualties. In 2013, 8000 blood components from UK donors were shipped overseas to combat zones, which required safe and sterile transportation over thousands of miles to military hospitals. Donated blood has been shown to increase death rates, accentuate bleeding, and carries risk of biological reactions. Worryingly, transfusion reactions are difficult to recognise in severely or multiply injured casualties. Haemolytic reactions present acutely with fever, dyspnea and renal failure and delayed reactions may occur. These deleterious consequences are avoided where the patient&#39;s own blood is collected and recycled—a process known as auto-transfusion. 
     An auto-transfusion device known as HemoSep® is already known for blood cell salvage in non-military clinical applications. HemoSep® is a portable intraoperative blood salvage device that achieves effective blood recycling—concentrating the residual blood during and after an operative procedure, returning all cell species including platelets and red blood cells back to the patient—and it has been successfully and broadly used in a wide range of applications. The concentrate comprises all cell types, including platelets, which are crucial to haemostasis but which are not salvaged by alternative, less-accessible devices. 
     The principle of the HemoSep® system is described in WO-A-2009/141589, and the processing bag used in the HemoSep® system is described in WO-A-2011/061533. The disclosures of both of those documents are hereby incorporated by reference, in their entirety. 
     The HemoSep® device salvages blood lost during high blood loss surgery, by concentrating the residual blood and recycling it so that it can be transfused back to the patient. The device consists of a blood bag which employs a superabsorbent material to absorb blood plasma and a mechanical agitator to concentrate blood sucked from the surgical site or drained from the heart-lung machine after the surgery. The separated cells are then returned to the patient by intravenous transfusion. 
     HemoSep® was designed as a haemoconcentration technology that produces a blood concentrate of all cell species, preserving the platelets, white blood cells and clotting residuals. 
     The process does not require centrifugation and associated blood transfer steps, as well as removing the need for highly trained technical personnel. The concept was based upon the use of a membrane-controlled superabsorber-driven plasma removal process, which is carried out in one vessel and requires no additional blood transfer steps or flushing procedures. Residual blood is introduced to the device, concentrated using the membrane-controlled superabsorber process and transfused back to the patient using a transfer bag. Another benefit of HemoSep® technology is that it produces a gelatinous waste product—essentially dilute plasma in a gel matrix—which is safer and easier to dispose of than the large volumes of fluid associated with more common centrifugation processes. The technique for removing the plasma from the blood product, leading to concentration of the cellular components, is fairly simple but involves a number of critical steps and controls. 
     The HemoSep® system comprises a blood reservoir to which blood is transported under suction from the wound site, and from which the blood is pumped to the HemoSep® bag. The HemoSep® bag is responsible for the removal of plasma from the blood, thereby producing a blood cell concentrate. To achieve this the HemoSep® bag contains an inner bag formed from a porous membrane that contains a superabsorbent material. The membrane has a pore size which prevents the migration of cellular species from the blood into the superabsorber section of the device. Free passage of the critically dilute plasma, however, is not restricted and as this fluid passes from the blood through the membrane into the superabsorber. This results in a concentration of the cellular components of the blood held within the bag. Once the appropriate level of haemoconcentration is achieved, the blood held in the bag is transferred into a transfer bag for subsequent transfusion to the patient. 
     In order to reduce the time required for absorption of plasma, and also to prevent the pores in the membrane becoming blocked by blood cells, the HemoSep® system further comprises a powered orbital shaker. 
     HemoSep® offers a wide range of benefits. It provides fast, simple cell concentration with improved patient outcomes:
         Patient&#39;s own blood is transfused, reducing the risk of contamination and reaction   Decreased need for donor blood and associated transfusion products, leading to a reduction in donor dependency   Decreased blood loss   Assists in the reduction of post-operative bleeding, resulting in improved patient recovery   Maintenance of platelet population and associated preservation of normal clotting function   Removal of pro-inflammatory molecules, resulting in a reduction in post-operative complications   Reduction in donor demand   Improved patient recovery   Preservation of more normal clotting function   Reduction in post-operative complications       

     There are also profound advantages of using HemoSep® over current auto-transfusion cell-salvage systems, which are significantly more expensive in terms of initial equipment costs and cost of disposables, require specialist technicians and also remove viable platelets, clotting factors and plasma proteins essential to whole blood. In addition, the HemoSep® technology has a significantly smaller footprint and weight when compared to alternatives. 
     The HemoSep® system is simple, portable and easy to use. These characteristics are necessary for use in battlefield situations. However, the HemoSep® system also requires the use of a powered shaker to reduce processing time and to prevent clogging of the membrane pores, and a supply of suitable electrical power may not be available in a battlefield situation. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a field-deployable technology that allows recovery of cells that can be efficiently and quickly recovered to the patient. It is a further object of the invention to develop a convenient and elegant technology for military deployment where access to power and specialist personnel is often lacking, for routine application by non-specialists in an operational setting, where cells harvested by this device are directly transfused back into the casualty (autotransfusion), to offer critical primary intervention on the battlefield for haemorrhaging soldiers. It is a further object of the invention a device that requires no electrical power for the process for cell concentration or blood harvesting, and that can be operated by one member of the recovery team. 
     According to a first aspect of the invention, there is provided apparatus for blood processing, said apparatus comprising
         a blood collection reservoir;   a blood collection conduit adapted and arranged to convey, in use, blood from a patient&#39;s wound site to the blood collection reservoir;   a blood transfer conduit adapted to convey, in use, blood from the blood collection reservoir to the patient or an intermediate blood processing unit; and   a manually operable pump unit comprising first and second peristaltic pumps, the first peristaltic pump being mounted about and acting upon the blood collection conduit, and the second peristaltic pump being mounted about and acting upon the blood transfer conduit,   wherein the pump unit is provided with an actuator adapted to switch the second peristaltic pump between an operative condition and an inoperative condition, in the operative condition the second peristaltic pump being engaged with the blood transfer conduit to convey blood from the blood collection reservoir while the first peristaltic pump is engaged with the blood collection conduit to convey blood from a wound site to the blood collection reservoir, and in the inoperative condition the second peristaltic pump being disengaged from the blood transfer conduit.       

     The first and second peristaltic pumps are preferably housed within a single common housing. The housing is preferably provided with a grip or handle by which it can be grasped by an operator in use. 
     Manual operation of the pump unit is preferably carried out by rotation of a handle that is connected to a suitable drive mechanism within the pump unit. The pump unit preferably includes gearing to increase the rate of rotation of the peristaltic pumps in proportion to the rate of rotation of the handle. Typically, the gearing ratio (ie the rate of rotation of the pumps relative to the rate of rotation of the handle) is from 1.5:1 to 10:1, more typically from 2:1 to 4:1. 
     Preferably, the flow rate of the first peristaltic pump is greater than 1 L/min, more preferably greater than 2 L/min or greater than 5 L/min or greater than 10 L/min. The flow rate of the second peristaltic pump, on the other hand, will generally be lower than that of the first peristaltic pump and is preferably less than 2 L/min, or less than 1 L/min or less than 0.5 L/min. The flow rates of the first and second peristaltic pumps may be in a ratio of between 2:1 and 10:1, more typically between 4:1 and 8:1. The flow rates of the pumps may be adjusted by appropriate selection of dimensions for the pumps and the respective conduits. 
     The first and second peristaltic pumps are preferably of generally conventional configuration, comprising sets of rollers (typically three in number) that orbit around the main axis of the pump and are periodically brought into engagement with, and at least partially occlude, the conduit with which the pump is associated. 
     The blood collection conduit and the blood transfer conduit are typically flexible tubes, most commonly of plastics material. The internal diameter of the blood collection conduit may be between about 5 mm and about 20 mm, more typically between about 10 mm and 15 mm. The internal diameter of the blood transfer conduit may be between about 2 mm and about 10 mm, more typically between about 3 mm and about 8 mm. 
     The blood collection conduit preferably terminates in a suction wand or the like of generally conventional form, the tip of the wand being inserted, in use, into blood at the wound site. Preferably, a supply of anti-coagulant is provided, the anti-coagulant being entrained in the collected blood in a manner that is known per se. 
     The actuator may take any suitable form effective to engage or disengage the second peristaltic pump. 
     In a first group of embodiments, in the inoperative condition the rollers of the second peristaltic pump are disengaged from the drive mechanism by which they are rotated in the operative condition. In such embodiments, the actuator has the nature of a clutch mechanism. 
     In another group of embodiments, the rollers of the second peristaltic pump are caused to rotate even in the inoperative condition, ie they remain coupled to the drive mechanism in that condition, but in the inoperative condition the rollers are displaced from the blood transfer conduit. In such embodiments, the actuator may be a push button or the like, depression of which brings the rollers into engagement with the conduit, and release of which causes the rollers to be released from engagement with the conduit. 
     Whilst the pump unit of the system of the invention is manually operable, such that it can be operated in the absence of electrical power, the pump unit is preferably configured such that where electrical power is available, it can be engaged with a supply of such power. To that end, the manually operable handle of the pump unit may be detachable to allow the pump unit to be mounted on a docking station or the like with an electrically driven spindle that engages the drive mechanism of the peristaltic pumps. The source of electrical power is most preferably a source of DC power. 
     Blood harvested from a wound site and conveyed to the blood collection reservoir by the first peristaltic pump will generally be in the form of a blood/air mixture that is very turbulent and prone to foaming. The flow needs to be made more laminar, without causing haemolysis. To achieve this, the invention further provides a defoaming device of novel construction. Such a defoaming device comprises an upstream, generally hemispheroidal funnel having an opening at the base thereof, and a downstream cup of complementary form to the funnel, juxtaposed surfaces of the funnel and the cup (ie the downstream or underside surface of the funnel and the upstream or upper surface of the cup) being spaced apart to form a gap, such that blood entering the funnel and passing through the opening in the funnel into the cup flows in random directions over the surface of the cup and flows out of the defoaming device through the gap. 
     The blood collection reservoir preferably further comprises a filter, downstream of the defoaming device, that is effective for removal of particulate matter, eg particles having a size greater than about 50 μm. Such a filter may take the form of a fabric tube depending from the defoaming device and that is closed at its distal end. Such a tube may contain a defoaming sponge to further reduce the occurrence of foaming. 
     An intermediate blood processing unit is preferably adapted to produce a red blood cell concentrate by removal of plasma from the blood conveyed to it. The blood processing unit may therefore be generally of the form of the known HemoSep® bag described above. In such a bag, blood plasma passes across a porous membrane and is absorbed by a superabsorbent material. Blood cells, however, are unable to pass through the membrane. 
     As also described above, however, the HemoSep® system utilises an electrically powered shaker to reduce the time required for haemoconcentration of the blood. Such a system is incompatible with the situations in which the present invention is envisaged to be of greatest utility, ie situations in which electrical power is unavailable. Surprisingly, however, it has been found that clinically satisfactory levels of haemoconcentration (ie sufficiently high levels of packed cell volume) may be achieved in acceptably short periods of time, without shaking, if the number and density of the pores in the membrane is increased substantially, such that the effective open area of the membrane is considerably increased. It has been found that despite vast increases in the number of pores, the membrane retains sufficient integrity to function as an effective barrier to blood cells. 
     Thus, according to a further aspect of the invention, there is provided a blood processing unit for the haemoconcentration of blood, which unit comprises a porous filter membrane having pores with an average size of less than 5 μm and wherein the membrane has an effective open area of at least 25%. 
     Preferably, the effective open area of the membrane is at least 30% or at least 40%. 
     By “effective open area” is meant the aggregate area of the pores present in the membrane relative to the overall area of the membrane. 
     Preferably, the pores in the membrane have an average size of less than 2 μm. 
     The system of the invention may further comprise a blood transfusion reservoir, typically a transfusion bag of conventional form, to which processed blood may be transferred from the blood processing unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in greater detail, by way of illustration only, with reference to the accompany drawings, in which 
         FIG. 1  is a schematic view of a blood salvage and autotransfusion system in accordance with the invention; 
         FIG. 2  is a schematic view, partially cut away, of a manually operated pump forming part of the system of  FIG. 1 ; 
         FIG. 3  shows (a) a side view, (b) a view from above, and (c) an exploded view, of a working embodiment of a pump of the form depicted schematically in  FIG. 2 ; 
         FIG. 4  is a diagrammatic view of an actuation mechanism forming part of the pump of  FIG. 3 ; 
         FIG. 5  shows (a) a perspective view of a docking station for the electrical operation of the pump of  FIG. 3 , and (b) another view of the docking station together with the pump, prior to engagement of the pump with the docking station; 
         FIG. 6  shows (a) a blood collection bag forming part of the system of  FIG. 1  and—separately—a defoaming device and a polyester sock that are fitted to the blood collection bag, (b) a side view of the blood collection bag with the defoaming device and sock fitted, and (c) a plan view of the blood collection bag with the defoaming device and sock fitted; 
         FIG. 7  shows views of the defoaming device of  FIG. 6 , in particular (a) a perspective view from below of an outer component of the defoaming device, (b) a further perspective view of the outer component, from a different angle, (c) a perspective view from above of an inner component of the defoaming device, and (d) a perspective view of the defoaming device with the inner and outer components engaged; 
         FIG. 8  is a schematic side view of the defoaming device of  FIG. 7 , generally along the arrow X in  FIG. 7( d ) , together with a disc by which the defoaming device is attached to a polyester sock and a tube connector disc; 
         FIG. 9  is a plan view of a blood processing bag forming part of the system of  FIG. 1 ; 
         FIG. 10  is a cross-sectional view of the blood processing bag of  FIG. 9 , on the line A-A in  FIG. 9 ; 
         FIG. 11  shows scanning electron micrograph (SEM) images of a membrane used in the construction of the blood processing bag of  FIGS. 9 and 10 , at (a) 1000× and (b) 3000× magnification, showing the porosity of the membrane; and 
         FIG. 12  is a plot comparing the performance of a blood processing bag constructed using the membrane of  FIG. 11 , in comparison to a prior art blood processing bag. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to  FIG. 1 , a blood salvage and autotransfusion system according to the invention comprises the following principal components:
         manually operable peristaltic pump unit  10     suction wand  20     blood collection bag  30     blood processing bag  40     blood transfusion bag  50     anti-coagulant reservoir  60         

     The above components are connected by suitable conduits that are of generally conventional form and construction, typically being flexible tubes of plastics material fitted with suitable connectors for connection to the respective components. 
     In use, the tip of suction wand  20  is inserted into blood at a wound site. As described in greater detail below, operation of the peristaltic pump unit  10  draws blood from the wound site to the blood collection bag  30 . The fluid drawn from the wound site is a turbulent blood/air mixture. All air and major contaminants (eg clots, bone fragments etc) must be removed before the blood can be retransfused to the patient; blood collection bag  30  must perform that task efficiently without causing significant haemolysis. A defoaming device and other measures to achieve that objective are described below with reference to  FIGS. 6 to 8 . 
     Anti-coagulant is simultaneously drawn from anti-coagulant reservoir  60  and entrained in the blood, in a conventional manner. It should be noted, however, that anti-coagulant will not be needed where the wound is a chest wound, as shed blood from the chest is missing key coagulation factors and will not clot. The primary function of the anti-coagulant is then to boost the oxygen-carrying capacity of the patient&#39;s blood. Treatment with anti-coagulant will generally be valuable for blood shed from other wound sites and where the processed blood is intended to be stored, rather than immediately retransfused back to the patient. 
     As also described in greater detail below, peristaltic pump unit  10  can be operated to draw blood from blood collection bag  30  and transfer it to blood processing bag  40 . Blood processing bag  40  is, apart from certain inventive modifications that are described with reference to  FIGS. 11 and 12 , similar in form to the processing bag disclosed in WO-A-2011/061533, the teaching of which is hereby incorporated by reference in its entirety. 
     Blood processing bag  40  serves to remove plasma from the blood introduced into it, so producing a red blood cell concentrate suitable for transfusion back to the patient from whom the blood has been taken. Thus, when the desired degree of concentration has been achieved, the red blood cell concentrate is drained from blood processing bag  40  to transfusion bag  50 , which is of generally conventional form and will not be described in any greater detail. 
     In some cases, where autotransfusion of blood back to the patient is urgent, blood processing bag  40  may be omitted from the system. 
     Turning now to  FIGS. 2 and 3 , peristaltic pump unit  10  is lightweight, compact and manually operable, such that it is suitable for deployment in battlefield situations, or other scenarios in which electrical power is not available. 
     As illustrated schematically in  FIG. 2 , peristaltic pump unit  10  comprises a generally cylindrical housing  101  having an integrally formed grip  102  by which pump unit  10  may be held be an operator during use. A cranked, detachable handle  103  is connected to a drive axle  104  mounted centrally within housing  101 , and is used by the operator to manually drive the pumps contained within pump unit  10 . 
     Housing  101  accommodates two separate peristaltic pumps  10 A, 10 B, each of which is based upon a conventional peristaltic roller mechanism. Each such mechanism comprises a set of three rollers that are held in fixed relation to each other and which orbit about an axis (corresponding to the axis of drive axle  104 ). The rollers periodically bear against, and occlude, a flexible tube that extends through pump unit  10 , between the rollers and the internal wall of housing  101 . 
     Pump  10 A is capable of relatively high flow (typically &gt;10 L/min), which the other pump  10 B has a relatively low flow configuration (typically &lt;500 mL/min). The two pumps  10 A, 10 B are connected to common drive axle  104 , but operate independently using an actuation mechanism (not visible in  FIG. 2 ) to activate the low flow pump  10 B when required. The rollers of high flow pump  10 A act upon a relatively large bore tube that extends from suction wand  20  to blood collection bag  30 , while the rollers of low flow pump  10 B act upon a relatively small bore tube that connects blood collection bag  30  with blood processing bag  40 . 
     The high flow pump  10 A can be operated constantly to evacuate blood from the patient to the blood collection bag  30 , and operation of the low flow pump  10 B does not interfere with that function. The drive axle  104  is manually rotated by means of cranked handle  103 , with gearing (not visible in  FIG. 2 ) enabling easy attainment of a suitable rotation speed. High flow pump  10 A is capable of constantly extracting blood from the wound site while moving the blood into blood collection bag  30 , but blood is drawn from blood collection bag  30  to blood processing bag  40  only when desired. That is achieved by means of the actuation mechanism, by means of which the rollers of low flow pump  10 B are brought into engagement with the tube that connects blood collection bag  30  with blood processing bag  40 , so pumping blood along that tube. When it is desired to cease operation of low flow pump  10 B, the actuation mechanism is released, causing the rollers of low flow pump  10 B to be disengaged from the tube, so stopping the pumping action of low flow pump  10 B. 
     An embodiment of the pump illustrated schematically in  FIG. 2  is shown in greater detail in  FIG. 3 , in which components corresponding to those described in relation to  FIG. 2  are given the same reference numerals. 
     As described above, pump unit  10  comprises a generally cylindrical housing  101 . A manually operable, cranked handle  103  is mounted on a central axis of housing  101 . 
     As illustrated in  FIG. 3( c ) , handle  103  has a multi-part construction, as is conventional for such a rotatable handle. As can also be seen in  FIG. 3( c ) , housing  101  comprises three housing elements  101 A, 101 B, 101 C, together with a pair of end plates  105 , 106 . End plate  105  and housing element  101 A are located adjacent to handle  103 . Housing element  101 A accommodates gearing  107  that controls the rate of rotation of the pump in relation to the the rate of rotation of handle  103 , typically in a ratio of 2:1. 
     Housing element  101 B, together with a first roller assembly  108  accommodated within it, constitutes high flow pump  10 A. Housing element  101 B is formed with a pair of tubular protrusions  109  through which passes the relatively large bore tube on which high flow pump  10 A acts, such that when first roller assembly  108  is rotated within housing element  101 B, the rollers of that assembly periodically bear against and occlude that tube, thereby generating a pumping action. 
     Similarly, housing element  101 C and a second roller assembly  110  accommodated within it constitute the low flow pump  10 B. Housing element  101 C is formed with a pair of tubular protrusions  111  through which passes the relatively small bore tube on which low flow pump  10 B acts. In the case of the low flow pump  10 B, however, the rollers of the second roller assembly  110  bear against that tube—and so exert a pumping action—only when the low flow pump  10 B is brought into operation by an actuator button  112 . 
     The manner in which the actuator button  112  operates is illustrated schematically in  FIG. 4 . 
       FIG. 4( a )  shows diagrammatically the low flow pump  10 B in an inoperative condition. The second roller assembly  110  is represented as a pair of rollers R mounted on spars having a parallelogram arrangement between a fixed point F and the actuator button A. The tube that passes through the low flow pump  10 B is denoted T. 
     In the  FIG. 4( a )  arrangement, the rollers R are offset from tube T. Although the roller assembly rotates (about the axis extending between F and A), the rollers R exert no action upon tube T. 
     When it is desired to bring low flow pump  10 B into operation, actuator A is depressed along the direction of the arrow in  FIG. 4( a ) . Since the point F is fixed, the rollers R are displaced to the left (as viewed in  FIG. 4( a ) ) and are also moved apart into the configuration shown in  FIG. 4( b ) . In that operative configuration, the rollers R bear against—and, as they rotate, periodically occlude—tube T. Hence, in this configuration, the low flow pump  10 B is actuated. When it is desired to cease operation of low flow pump  10 B, actuator A is returned to its  FIG. 4( a )  position, thereby disengaging rollers R from tube T. 
     As described above, pump unit  10  is manually operable, and so is suitable for use in situations in which electrical power is not available. Where such power is available, however, it is clearly desirable for it to be used to operate pump unit  10 . To achieve that objective, manually operable, cranked handle  103  of pump unit  10  is detached and instead pump unit  10  is engaged with docking station  70  of  FIG. 4 . Docking station  70  comprises a base  701  having suitable supports for housing  101  of pump unit  10  and an upstand  702  upon which is mounted a DC Geared Motor  703  (24 VDC 24 Nm 125 rpm, Mellor Electric, RS Components, UK). Motor  703  is coupled (Ruland Aluminium Flexible Beam Coupling, RS Components, UK) with the pump and powered with a DC power supply (Digimess Concept Series, Digimess Instruments Ltd, Derby, UK). 
     The pumping characteristics of pump unit  10  when engaged with docking station  70  were then tested using fresh bovine blood collected from a local abattoir on the morning of assessment. Flow rate was controlled by altering the voltage output of the power supply. Testing was performed with the motor running at a maximum rate of 84 rpm at 24V DC. 
     The following flow rates were achieved: 
     High flow pump flow rate: 3 L/min 
     Low flow pump flow rate: 600 mL/min 
     To assess whether the flow rates and turbulence would cause blood haemolysis, blood samples were taken before and after each flow test. High flow pump tests were performed using blood and a blood/air mix (to mimic blood evacuated from a wound site), and the low flow pump was tested using blood. Blood samples were placed into microhaematocrit capillary tubes (Brand GmbH+Co KG, Wertheim, Germany), and spun in a centrifuge at 2000 revolutions per minute for three minutes. Visual inspection showed that there was no evident haemolysis—red discolouration of plasma confirms presence of haemolysis—and this was corroborated with spectrophotometric analysis that showed only trace levels of haemolysis before and after pumping. 
     As noted above, the fluid drawn from the wound site is a turbulent blood/air mixture. All air and major contaminants (eg clots, bone fragments etc) must be removed before the blood can be retransfused to the patient; blood collection bag  30  must perform that task efficiently without causing significant haemolysis. To address that objective, blood collection bag  30  is fitted with a defoaming device and a polyester sock that will now be described with reference to  FIGS. 6 to 8 . 
     Turning first to  FIG. 6 , blood collection bag  30  is of broadly conventional form, but is fitted with defoaming device  31  that constitutes the inlet through which blood drawn from the wound site enters blood collection bag  30 . The construction of defoaming device  31  and the manner in which it is fitted to blood collection bag  30  is described in greater detail below, with reference to  FIGS. 7 and 8 . A polyester sock  32  is fitted to defoaming device  31 , within blood collection bag  30 . Sock  32  comprises a tube of thinly woven polyester fibres that is closed at one end. The open end of the tube is fitted to the defoaming device  31 , as described below, such that blood flowing into blood collection bag  30  through defoaming device  31  passes into the interior of sock  32  and then through the walls of sock  32  into blood collection bag  30 . The spacing of the fibres from which sock  32  is woven means that sock  32  filters out of the blood particulates having a size greater than approximately 50-70 μm. 
     The blood/air mixture entering blood collection bag  30  is very turbulent and prone to foaming. The flow needs to be made more laminar, without causing haemolysis. This is achieved primarily by means of defoaming device  31 , but to further reduce foaming sock  32  contains a polyester defoaming sponge that is coated with an anti-foaming agent, such as a hydrophobic silica dispersed in silicone oil. 
     Defoaming device  31  is shown in detail in  FIGS. 7 and 8 , and comprises two principal components: a funnel element  311  and a cup  312 . Funnel  311  and cup  312  are both generally hemispherical in form with planar peripheral flanges and part-spherical hoods ( 313 , 314  respectively) extending from parts of those flanges. The flange of cup  312  is provided with three upstanding pins  315  that engage in corresponding openings  316  in the flange of funnel  311 . Funnel  311  has a central opening  317 . 
     In the assembled condition shown in  FIG. 7( d ) , the cup  312  is fixed to the funnel  311 , with the major surfaces of the two components being slightly spaced apart. Thus, a gap exists between the upper surfaces of cup  312  and the lower surfaces of funnel  311  and a similar gap exists between the juxtaposed surfaces of the respective hoods  313 , 314 . 
     Defoaming device  31  is inserted into the open end of sock  32  and the edges of the mouth of sock  32  are folded over the flange of funnel  311  and held in place by a disc  318 —see  FIG. 8 —that has a central opening. Disc  318  is fixed to the flange of funnel  311  by means of adhesive, thereby captivating the edges of sock  32  between disc  318  and defoaming device  31 . The assembly of defoaming device  31  and sock  32  is inserted into blood collection bag  30  through a suitably shaped and dimensioned opening in the wall of blood collection bag  30  and fixed in place by attachment (by adhesive or ultrasonic welding or other suitable technique) of a tube connector  319  having a disc-shaped base. Tube connector  319  adheres to the flange of funnel  311  and captivates the edge of the opening in the wall of blood collection bag  30 , thereby completing the assembly. Defoaming device  31  is oriented such that hoods  313 , 314  are positioned uppermost within blood collection bag  30 . This assists with ensuring that sock  32  hangs vertically within blood collection bag  30  during use, and also helps to maintain an open configuration of sock  32 . 
     The end of the conduit leading from pump unit  10  to blood collection bag  30  is fitted onto tube connector  319 . In use, the turbulent blood/air mixture transported from the wound site to blood collection bag  30  by pump unit  10  passes through tube connector  391 , through the central opening of disc  318  and into funnel  311 . The mixture is channelled through the central opening  317  of funnel  311  into cup  312 . The mixture then flows in random directions along the surface of cup  312 , exiting defoaming device  31  and entering sock  32  through the gap between cup  312  and funnel  311 . 
     Finally, referring to  FIGS. 9 to 12 , there is shown the blood processing bag  40  that forms part of the system of  FIG. 1 . 
     Blood processing bag  40  comprises an outer bag  401  formed of a tough impermeable material, an inner bag  402  formed of a porous membrane material and contained within outer bag  401 , and a superabsorbent material  403  encapsulated within inner bag  402 . The outer bag  401  has an inlet port  404 , through which blood may—by operation of the low flow peristaltic pump  10 B of pump unit  10  causing it to be drawn from blood collection bag  30 —pass into blood processing bag  40  and enter the cavity  405  formed between outer bag  401  and inner bag  402 , and an outlet port  406 , though which processed blood may exit blood processing bag  40  and be drained into transfusion bag  50 . 
     Outer bag  401  is formed of polyvinylchloride (PVC) sheets and inner bag  402  is formed of porous polycarbonate membrane. Both outer bag  401  and inner bag  402  are formed by fastening two sheets of material together around their edges by heat welding. The material of outer bag  401  is impermeable to blood plasma, whereas the material of inner bag  402  permits blood plasma, but not blood cells suspended in the plasma, to pass through it and into the interior of inner bag  402 . For instance, inner bag  402  is formed of material having pores with a maximum size of no greater than 5 μm, and more commonly 1-2 μm, to permit blood plasma, but not red blood cells, to pass through. 
     The area around the edge of outer bag  401  where the two polyvinylchloride (PVC) sheets are welded together defines a welded portion  407 . This welded portion  407  projects from each upper corner of outer bag  401  to form extensions  408 . Each extension  408  has an aperture  409  to allow blood processing bag  40  to be hung from a suitable support. 
     The area around the edge of inner bag  402  where the two porous polycarbonate membranes are welded together also defines a welded portion  410 . The superabsorbent material  403  is entirely encapsulated by inner bag  402 . The welded portion  410  around the edge of the inner bag  402  extends outwardly to form a number of tabs  411 , which are trapped within the welded region  407  at the periphery of the outer bag  401  at a number of points to suspend the inner bag  402  within the outer bag  401 . 
     To the extent described above, blood processing bag  40  is as described in WO-A-2011/061533. The manner in which blood processing bag  40  differs from that earlier disclosure, however, is illustrated in  FIGS. 11 and 12 . 
       FIG. 10  shows SEM images of the porous membrane material used to form inner bag  402 . The number and density of pores present in the membrane material is considerably increased, in comparison to that previously used, such that the effective open area (EOA) of the membrane is around 45%. For prior art membranes used for the same application, the EOA was only about 7%. Surprisingly, it has been found that membranes with such a high EOA retain sufficient structural integrity, whilst increasing the rate of passage of blood plasma across the membrane material and so reducing the time required for a given degree of blood concentration. 
     To assess the performance of inner bags  402  constructed using the high EOA membrane material of the invention, haemoconcentration efficacy was assessed using fresh bovine blood (n=10). To reduce the initial packed cell volume (PCV) before haemoconcentration assessment, the blood was diluted with saline (0.9 g sodium chloride per 100 ml). Baseline measures was recorded and the bags were primed with 100 ml of saline immediately before blood was introduced. Bags were filled with 350 ml of the blood-saline mix and placed on an orbital shaker. Blood samples were taken after 20 minutes and after 40 minutes. Blood samples were collected with microcapillary tubes and spun in a microhaematocrit centrifuge (Hawksley, Sussex, UK) for 3 minutes at 2000 rpm. Manual estimation of the haematocrit was performed using a Hawksley tube reader (Hawksley, England, UK). A comparison was made with historical files (n=19) relating to the bags made with conventional membrane material having an EOA of about 7%. 
     The results are shown in  FIG. 11 , where data are presented as the mean±SD. Results for high EOA bags are shown by the broken line; comparative historical data for low EOA bags is shown by the solid line. 
     Baseline PCV for the high EOA bags was 20.4±1.17%, which increased to 38.4±1.5% at 20 minutes processing and 49.4±2.1% at 40 minutes. Baseline PCV historical data (low EOA bags) was 21.8±2.15%, which increased to 31.9±4.3% at 20 minutes and 37.3±5.8% at 40 minutes. Thus, with the high EOA bags, PCV reached clinically acceptable levels in less than 20 minutes.