Abstract:
A fluid warming cassette system in which the cassette has a stiffening frame structure and an integral handle is provided to support a parenteral fluid container. The fluid container is desirably thin to minimize heat exchange inefficiencies. The frame structure permits the thin fluid container to be inserted into the narrow space between fixed position warming plates of a warming unit. The frame structure has a quadrilateral shape with sides and ends. The fluid container is attached, at its periphery to the sides and ends of the frame structure, within the quadrilateral shape. Part of the frame structure is formed into a handle to assist in both the insertion and removal of the cassette from a warming unit.

Description:
RELATED APPLICATIONS/PRIORITY 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/257,831 filed Oct. 25, 2005, which is a continuation of U.S. patent application Ser. No. 10/210,643, filed Jul. 31, 2002, now U.S. Pat. No. 7,010,221, which is a continuation of U.S. patent application Ser. No. 09/415,405, filed on Oct. 8, 1999, now U.S. Pat. No. 6,464,666. 
     
    
     BACKGROUND 
       [0002]    This invention is generally related to parenteral fluid warming systems and, more particularly, to the structure of a warming cassette that includes a stiffening frame with an attached fluid container and that is used in a parenteral fluid warming apparatus. 
         [0003]    Fluid warming apparatuses, designed to warm and administer parentarel fluids and blood products (hereinafter “fluids”), are in common use. Generally, these fluids are administered using a disposable fluid container which includes a fluid pathway and one or more heat exchange surfaces. The fluid container may be made of plastic film material or thin metal. A warming cassette incorporates such a fluid container, imparting structural support to the container for handling and for being received and supported in the warming unit. 
         [0004]    Specifics of a multi-layered fluid container compatible with a supporting cassette frame structure are discussed in applicants U.S. patent application Ser. No. 09/415,558, entitled “PRESSURE TOLERANT PARENTERAL FLUID AND BLOOD CONTAINER FOR A WARMING CASSETTE”, invented by Augustine et al., filed on Oct. 8, 1999, now abandoned. 
         [0005]    A warming cassette is placed into a warming unit to heat fluids as they flow through the fluid pathway. Heat is transferred to the fluid through the fluid container by contact with a heat source such as heated metal plates, heated liquid, or heated gas. Metal plate, “dry heat” exchanger warming units are widely known. However, in the last 10 to 15 years, water bath heat exchangers have become the norm in the United States. 
         [0006]    While convenient to use, water bath heat exchangers can pose health risks. The warm water in these systems is often circulated for long periods of time without being changed or sterilized. The warm water provides an excellent growth medium for microbes. After several weeks of use, bacteria and fungi can be cultured from these water baths. For these reasons, a “dry heat” system is probably safest for warming medical fluids. However, there are significant fluid thermodynamic problems, as well as convenience, reliability, and cost issues that must be solved for a “dry heat” system to replace the water bath systems. 
         [0007]    The American Association of Blood Banks (AABB) mandates that blood products and IV fluids must not be heated above a temperature of 42° C., so as to prevent blood cell damage and thermal injury to a patient. A temperature of 42° C. is easy to maintain under steady-state flow conditions, a low flow rates. However, as the flow rate of the fluid increases, the rate of heat transfer to the fluid must keep pace in order to achieve a target fluid temperature. The boost in the rate of heat transfer is most obviously achieved by using larger heater and by increasing the temperature difference (−T) between the heater and the fluid. Both solutions effectively drive more heat into the fluid. Unfortunately, these solutions are not necessarily effective when the fluid flow rates are highly dynamic. Large heaters and high temperature differentials are not responsive enough to sudden changes in fluid flow rates. For example, in the case of a sudden change from a high fluid flow rate to a low one, the high temperature limit can be exceeded, potentially causing thermal damage to the fluid or patient. 
         [0008]    The problems of thermal efficiency and temperature responsiveness over a wide range of flow rates can be met by improving the thermal conductivity of the fluid cassette materials, and minimizing the thickness of the fluid at the point of heat transfer. This implies a thin, flat fluid container, constructed from properly selected materials. 
         [0009]    Plastic film materials are commonly used in the manufacture of disposable fluid warming cassettes. However, plastic is a poor heat transfer material. Metal foils, or metal conduits have been used with plastic materials in warming cassettes to enhance thermal conductivity; however, it is difficult to bond metal to plastic materials, and leakage can occur along bonding seams between these materials. Further, metal foils generally increase the cost of cassette manufacturing. 
         [0010]    Fluid temperature response may also be improved by reducing the thickness of the fluid channel in the fluid container. In this regard, the space between the heater plates is then reduced to be compatible with thin cassettes. Assume, for example, that an optimal balance between fluid flow resistance and heat transfer for a particular warming unit design yields a distance of 0.048 inches between the heater plates of the unit. It is very difficult to insert an appropriately dimensioned cassette into such a warming unit simply by sliding it between the warming plates. The plastic materials of which such cassettes are made impart little rigidity. Consequently, such a cassette may kink or tear when being slid into or out of such a small space. As a result, “clamshell” solutions have been proposed that spread the warming plates apart when a cassette is inserted or removed from a warming unit. 
         [0011]    The limitations of the clamshell design are manifest. Moving parts add to the warming unit&#39;s cost, and reduce reliability. It is very difficult to maintain an accurate 0.048 in. spacing across the entire plate surface, when hinges, clasps, and other moving parts are required. Finally, insertions of the cassette into such a warming system becomes a multi-step process, which is both time consuming and inconvenient. 
         [0012]    Other problems occur with the use of plastic fluid containers in fixed plate warming units. For example, the fluid channel formed between the plastic films of a fluid container must be contained entirely within the space between the heater plates. However, some portion of the cassette must extend outside of the heater plates in order to provide structure that can be grasped to extract the cassette. If the portion of the cassette that extends outside of that space includes an unsupported portion of the fluid container, the container can rupture when the fluid pressure is increased to increase the flow rate. 
         [0013]    It would be advantageous if an efficient and low cost fluid cassette could be developed for a “dry heat” parenteral fluid warming system. Advantage would be gained if the fluid cassette permitted the rapid heating of parenteral fluid under high pressures. Further, it would also be advantageous if the cassette could be made rigid, yet thermally conductive, without the use of metal. 
         [0014]    It would be advantageous if a cassette fluid container could be made with plastic walls stiff enough for insertion in between close-set parallel warming plates of a warming unit, yet thin enough to efficiently transfer heat from the plates to the fluid. 
         [0015]    It would be advantageous if the above-mentioned cassette could be easily inserted into and removed from the warming unit without being kinked or torn. It would further be advantageous if the cassette had a handle for insertion of the cassette between the warming plates of a fluid warming unit. It would be advantageous if the above-mentioned cassette handle extended outside the unit for convenient handling. 
       SUMMARY 
       [0016]    Accordingly, a warming cassette for parenteral fluids, used in a parenteral fluid warming system, is provided. The cassette comprises a flexible fluid container made from thermally conductive material and attached to a planar frame structure, which imparts structural rigidity to stiffen and support the fluid container. The fluid container and the frame structure are all of a piece, permanently bonded, joined or connected together in a unitary, integrated structure. The frame structure is in the shape of a planar figure bounded by sides. Preferably the figure is a quadrilateral, with sides, a distal end, and a proximal end. A handle is provided on the proximal end. A fluid container is disposed inside the shape of the frame structure, attached along its periphery to the sides and ends. Optionally, the container could be attached to just the sides or just the ends. The handle provides an element that may be grasped to manipulate the cassette for insertion into and extraction from a warming unit. 
         [0017]    Optionally, the warming cassette is provided with a keying mechanism that prevents it from being inserted either upside down, or backwards in a warming unit. The keying mechanism also prevents the cassette from being inserted too far into the warming unit. The keying mechanism comprises lands on the sides of the frame structure. To key the cassette, the lands mate with corresponding grooves in the warming unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1A  is a perspective view of a warming cassette according to this invention for use in an intravenous fluid warming system. 
           [0019]      FIG. 1B  is side sectional view of the cassette taken along A 1 -A 1  of  FIG. 1B . 
           [0020]      FIG. 2A  is a more detailed depiction of the warming cassette of  FIG. 1A . 
           [0021]      FIG. 2B  is an exploded view of the warming cassette of  FIG. 1A . 
           [0022]      FIGS. 3A and 3B  illustrate the warming cassette  10  of  FIG. 1A , detailing an optional bubble trap feature. 
           [0023]      FIGS. 4A through 4C  illustrate details of the present invention keying system used to selectively orient the cassette in the warming apparatus 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    This invention is a system for warming fluids during intravenous infusion. The system includes a cassette designed for use with a “dry heat” warming unit in which heater plates are disposed in a parallel, spread-apart orientation, separated by a thin laminar space. The cassette is received in the space between the heater plates, in close contact with the heater plates. 
         [0025]    The warming cassette has a unitary integral structure that includes two portions: a frame structure, and a fluid container attached, or joined, to the frame structure. The fluid container portion of the cassette is made of two sheets of thermally conductive plastic film material. Preferably, the two sheets of plastic film material are bonded together in a pattern which creates a fluid channel between the sheets. A fluid channel with a serpentine pattern is illustrated, although other patterns are contemplated. Preferably the plastic film is 0.004 in. (4 mil) thick, or less. 
         [0026]      FIG. 1A  is a perspective illustration of a warming cassette  10  according to the invention for use with an intravenous fluid warming unit  12 . The warming unit  12  is a “dry heat” unit with warming plates  14  and  16 . The plates  14  and  16  are maintained spread apart at a fixed distance, and the cassette  10  is inserted between the plates  14  and  16  so that the fluid in the cassette  10  is heated before infusion into a body. 
         [0027]      FIG. 2A  is a more detailed depiction of the warming cassette  10  of  FIG. 1A ;  FIG. 2B  is an exploded view of the cassette  10  showing two of its elements. The cassette  10  comprises a flexible fluid container  20  and a frame structure  22  to which the fluid container  20  is attached, joined, or bonded. The frame structure  22  stiffens the cassette within a first plane represented by orthogonal X and Y axes. The X and Y axes are referred to herein as first and second directions, respectively. Likewise, the fluid container  20  is substantially planar and disposed in the first plane. The frame structure  22  is considered to be rigid with respect to the flexible fluid container  20 . However, the frame structure  22  also has some degree of flexibility. Preferably, the frame structure  22  is semi-rigid. 
         [0028]    The frame structure  22  is in the shape of a planar figure bounded by sides; preferably the figure is a closed figure. For example,  FIGS. 2A and 2B , the figure is a quadrilateral having sides  24  and  26 , a distal end  30 , and a proximal end  28 . The figure of the quadrilateral defines an opening  31  within which the fluid container  20  is received. The fluid container  20  has a periphery including opposing, parallel sides  21   a  and  21   b , and opposing, parallel ends  21   c  and  21   d . The cassette  10  is assembled by receiving the fluid container  20  within the opening  31  and joining the fluid container  20  to the frame structure  22  by bonding, or otherwise permanently joining or connecting, the periphery  21   a ,  21   b ,  21   c , and  21   d  of the fluid container  20  to the sides and ends  24 ,  26 ,  28 , and  30  of the frame structure  22 . Alternatively, the peripheral bonds could include just the sides  24  and  26 , or just the ends  28  and  30 . As a consequence of such bonding, joining, or connecting, the fluid container cannot be separated from the frame structure. The result, best seen in  FIG. 2A  is a unitary integrated warming cassette structure that may be handled, manipulated, or otherwise used or processed as a single piece. The inventors contemplate that the quadrilateral shape of the frame structure  22  may be practiced in an alternate embodiment in which distal end  30  is omitted, or is not continuous with the sides  24  and  26 . As best seen in  FIG. 2A , the sides  24  and  26  are oriented, and provide stiffness in the first direction (X-axis) while the ends  28  and  30  are oriented, and provide stiffening in the second direction (Y-axis). Taken together, the sides  24  and  26  and the ends  28  and (optionally end  30 ) provide stiffness generally in the X Y plane. 
         [0029]    In some aspects of the invention the proximal end  28  includes a handle portion  29 , formed to be manipulable by hand. That is, the proximal end  28  is given sufficient surface area, extending away from the fluid container  20  to accommodate finger purchase. When the cassette  10  is engaged with warming device  12  (see  FIG. 1 ), the handle portion  29  is not received (at least, not entirely received) between the plates  14  and  16 . The handle portion  29  remains accessible while the rest of the cassette is being heated between plates  14  and  16 . 
         [0030]    The handle portion  29  serves several purposes: 
         [0031]    first, the handle portion  29  maintains the sides  24  and  26  in proper alignment and position for easy, one handed indexing with the warming unit; 
         [0032]    second, the handle portion  29  may include a hole  29   a  for accommodating and holding a bubble trap which is part of the tubing connected to the fluid outlet (see  FIG. 3A ); 
         [0033]    third, the handle portion  29  includes a stopping mechanism  65  which mechanically prevents handle portion  29  from entering fluid warming unit  12  and assures proper insertion depth (see  FIGS. 1B ,  2 A,  2 B,  3 A and  3 B); 
         [0034]    fourth, the handle portion  29  preferably includes a mechanism  38  for supporting fluid inlet and fluid outlet tubing (see  FIG. 2A ), and providing strain relief preventing undue tension being applied to the tubing. Without this kind of strain relief, there is the risk of tension on the tubing, resulting in tearing the plastic film material. Attaching the tubes helps to prevent kinking of the tubing as it leaves the warming unit; and 
         [0035]    fifth, the handle portion  29  includes a substantially flat area  56  which may be used for labeling (see  FIG. 3A ). Since the majority of the cassette  10  is inside the warming unit during use, it is convenient to have labeling visible to the user even during use. Handle portion  29  is always external to the warming unit and, therefore, is an ideal platform for such labeling. 
         [0036]    The frame structure  22  can be formed from a material selected from the group consisting of polyester, polyamide (Nylon®, DuPont), polyethylene glycol terephthalate (Mylar®, DuPont), and ionomer resins (Surlyn®, DuPont). The frame structure  22  can be manufactured by die cutting, injection molding, and thermal processes. 
         [0037]    The fluid container  20  can be made from one or more materials selected from the group consisting of polyvinyl chloride (PVC), polyurethane, polypropylene, polyethylene, polyester, and other polymeric materials. 
         [0038]    The fluid container  20  includes a fluid channel  32  and at least a first port  34  for fluid communication with the fluid channel  32 , which is highlighted with cross-hatched lines in  FIGS. 2A and 2B . A first tube  36  is joined to the first port  34 . Optionally, the first tube is attached to the frame structure  22  at its proximal end  28 . Alternately, the first tube  36  is, at least partially, formed to be an integral part of the handle portion  29 . In  FIG. 2A , the area of attachment is represented with double cross-hatched lines and labeled with reference numeral  38 . The fluid container  20  also includes a second port  40  in fluid communication with the fluid channel  32 . A second tube  42  is joined to the second port  40 . 
         [0039]      FIGS. 3A and 3B  illustrate the warming cassette  10  of  FIG. 1 , detailing an optional bubble trap feature.  FIG. 3B  is an enlargement of Section A of  FIG. 3A . The warming cassette  10  optionally includes a bubble trap  50  attached to the handle portion  29  for support. The bubble trap  50  traps any air bubbles that may have inadvertently been introduced into the inlet tubing from the IV bag or may have been created by “out-gassing” during the warming of the fluids. The bubble trap  50  has an input  52  connected to the second port  40 . The bubble trap  50  has an output  54  to supply fluid, and a gas exhaust port (not shown) to vent gases escaping from the communicated fluid. The output  54  is operatively connected to the patient&#39;s IV catheter (not shown). 
         [0040]    The bubble trap  50  can be mechanically attached or bonded through thermal, adhesive, or chemical means to the handle portion  29 . Attaching the bubble trap  50  to the handle portion  29  makes it less likely that the trap  50 , or its associated tubing will be inadvertently disconnected from the cassette  10 . 
         [0041]    The handle portion  29  optionally includes a label surface  56 , highlighted with cross-hatched lines in  FIGS. 3A and 3B . The cassette  10  then may receive a label (not shown) overlying the second stiffening member label surface  56 . The label can be visible to the eye, or configured for electronic identification, such as a bar code. 
         [0042]      FIGS. 4A through 4C  illustrate details of keying elements used to orient the cassette  10  in the warming unit  12 . The intravenous warming unit  12  includes the first and second opposing warming plates  14  and  16 , adapted to accept the warming cassette  10  in a first orientation.  FIG. 4A  is a simplified end view of the warming unit  12 . The warming plates  14  and  16  have been separated for the purpose of clarifying the invention. Two grooves  60  are formed in the upper plate  14  to cooperate with a key mechanism on the warming cassette  10 .  FIG. 4B  illustrates the warming unit  12  with the warming plates  14  and  16  assembled for normal operation. 
         [0043]    Refer now to  FIGS. 1A-3B  and  4 C.  FIG. 4C  is a sectional view taken along A 4 -A 4  of  FIG. 1A . In these figures, there are illustrated two lands  64  that act as a key mechanism with the grooves  60  to mate the cassette  10  with the warming plates in a predetermined orientation. Preferably, the lands  64  are formed integrally with the sides  24  and  26 , and extend longitudinally thereon. When the warming cassette  10  is received between the plates  14  and  16 , the lands  64  key the warming cassette  10  by permitting the cassette to be inserted or slid into the space between the plates only if the lands  64  are received in the grooves  60 . Otherwise, the lands  64  will prevent the cassette from being inserted into the warming unit  12  between the plates  14  and  16 . 
         [0044]    Referring to  FIGS. 1B-3B , a stop mechanism is illustrated in the form of a ridge  65  that extends parallel to the distal end  28  on an upper surface of the handle portion  29 . The ridge  65  is high enough to contact the upper plate  14  when the fluid pathway  32  is fully received between the plates  14  and  16 ; this contact stops the cassette from being inserted any further between the plates  14  and  16 . Manifestly another ridge, or an alternate ridge, can be provided on the lower surface of the handle portion  29 . 
         [0045]    The unitary, integrated warming cassette  10  of  FIGS. 1A and 2A  can be inserted into the warming unit  12  by a user, employing one hand to grasp the integral handle portion  29 , orienting the warming cassette  10  so that the lands  64  are aligned with the grooves  60 , inserting the distal end  30  between the plates  14  and  16  and sliding the warming cassette  10  inwardly between the plates  14  and  16  until the stopping mechanism  65  halts further insertion. 
         [0046]    Other variations and embodiments of the prevent invention will occur to those skilled in the art with reflection upon the disclosed examples of the fluid warming system.