Abstract:
A fluid warming device may include a housing comprising a main body, a heat exchange body receiving compartment, and a cover movably coupled to the main body between an open position and a closed position; a heater assembly disposed within the main body and having a heat conducting surface disposed proximate the heat exchange body receiving compartment; and a heat exchange body removably disposable in the heat exchange body receiving compartment of the main body and having an input port and an output port to couple the heat exchange body to tubing to flow a fluid to be warmed through the heat exchange body. In another aspect, a fluid warming system increases or decreases power to a heater assembly to adjust the fluid temperature to ensure that the fluid is at an appropriate temperature when it reaches the patient.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of co-pending U.S. patent application Ser. No. 12/983,420, filed Jan. 3, 2011 and issued on Apr. 3, 2012 as U.S. Pat. No. 8,150,244, which is a divisional of U.S. patent application Ser. No. 11/385,085, filed Mar. 21, 2006 and issued as U.S. Pat. No. 7,865,072, which claims benefit of United States provisional patent application Ser. No. 60/663,857, filed Mar. 21, 2005. Each of the aforementioned related patent applications is herein incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Intravenous (IV) fluid warming devices are known that heat an IV fluid prior to introducing the fluid into a patient. Warmed IV fluids that are administered at very low flow rates can cool as they flow down the IV tubing to the patient. Often this heat loss is ignored. One prior art approach has been to simply heat the fluid to 41° C. to try to overcome the loss for all flow rates. 
         [0003]    In another aspect of fluid warming, most IV fluid warmers heat fluid through flexible plastic walls. Because these walls are inefficient in transferring heat, heaters are exposed to both sides of the disposable set. This requires inserting the disposable set into a slot or hinged clam shell configuration. Both of these designs do not allow the user to see the fluid passing through the heat exchanger. As these plastic walls are flexible, the pressure necessary for good heat transfer cannot be guaranteed as it is supplied only by the gravitational force of the IV fluid bag height. Cleaning of configurations with slots is difficult and typically requires special tools or even disassembly, such as in the case of blood spills. 
         [0004]    One type of exemplary medical fluid warming system is described in U.S. 2005-0008354. In this device, fluid passes along a generally serpentine fluid flow path through a removable/disposable heat exchange body. The heat exchange body is in thermal contact with a resistive film heater via thermally conductive layers interposed between the heat exchange body and the heater. Temperature sensors are provided that sense the temperature of the heat exchange body and of the heater. 
       SUMMARY OF THE INVENTION 
       [0005]    In some embodiments, a fluid warming device may include a housing comprising a main body, a heat exchange body receiving compartment, and a cover movably coupled to the main body between an open position and a closed position; a heater assembly disposed within the main body and having a heat conducting surface disposed proximate the heat exchange body receiving compartment; and a heat exchange body removably disposable in the heat exchange body receiving compartment of the main body and having an input port and an output port to couple the heat exchange body to tubing to flow a fluid to be warmed through the heat exchange body. 
         [0006]    In some embodiments, a method of minimizing heat loss through IV tubing to a patient may include a) determining a temperature drop across a heat exchange body of an IV fluid warming device; b) determining a temperature drop of the IV tubing to the environment, the IV tubing extending from the IV fluid warming device to the patient; c) determining if the temperature drop along the IV tubing is greater than a determined temperature limit; d) determining if the total temperature drop along the IV tubing and across the heat exchange body is greater than a determined drop limit; e) if the determined values from c) or d) are greater than their respective limits, determining the actual fluid temperature as a fluid output temperature minus the drop limit; f) if either of the determined values from c) or d) are not greater than their respective limits, determining the actual fluid temperature as a fluid output temperature minus the heat exchanger temperature drop; and g) adjusting power to the heat exchanger to achieve a desired output temperature. 
         [0007]    In some embodiments a method of minimizing heat loss through IV tubing to a patient may include a) providing a heat exchange body, coupled between an IV fluid source and the patient via IV tubing, to a heat exchange body receiving compartment of an IV fluid warming device that comprises a housing including a main body, the heat exchange body receiving compartment, a cover movably coupled to the main body between an open position and a closed position, and a heater assembly disposed within the main body and having a heat conducting surface disposed proximate the heat exchange body receiving compartment such that the heat exchange body is in heat exchange communication with the heater assembly; b) determining a temperature drop across the heat exchange body by dividing heater power of the heater assembly by the thermal resistance of the heater assembly; c) determining a temperature drop of the IV tubing to the environment, the IV tubing extending from the IV fluid warming device to the patient, by determining the difference between a fluid target temperature and ambient temperature, multiplying this difference by a radiation loss constant to get a result, and dividing the result by heater power of the heater assembly; d) determining if the temperature drop along the IV tubing is greater than a determined temperature limit; e) determining if the total temperature drop along the IV tubing and across the heat exchange body is greater than a determined drop limit; f) if the determined values from d) or e) are greater than their respective limits, determining the actual fluid temperature as a fluid output temperature minus the drop limit; g) if either of the determined values from d) or e) are not greater than their respective limits, determining the actual fluid temperature as a fluid output temperature minus the heat exchanger temperature drop; and h) adjusting power to the heat exchanger to achieve a desired output temperature. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0009]      FIG. 1  is an isometric view of a fluid warming device illustrating slidable covers in a closed position according to the present invention; 
           [0010]      FIG. 2  is an isometric view of the fluid warming device of  FIG. 1  illustrating the slidable covers in a half closed position; 
           [0011]      FIG. 3  is an isometric exploded view of the fluid warming device of  FIG. 1  with the slidable covers in an open position and a disposable set removed; 
           [0012]      FIG. 4  is an isometric view of a main body of the housing of the device of  FIG. 1  with the slidable covers fully removed; 
           [0013]      FIG. 5  is a plan view of the device of  FIG. 2 ; 
           [0014]      FIG. 6  is a cross sectional view taken along line A-A of  FIG. 5 ; 
           [0015]      FIG. 7  is a schematic view of a disposable set and heater assembly of the fluid warming device of  FIG. 1 ; 
           [0016]      FIG. 8  is a side view of a further embodiment of a fluid warming device illustrating gripping faces on the slidable covers; 
           [0017]      FIG. 9  is a flow chart illustrating a system for adjusting power to a heater of a fluid warming device to accommodate heat loss in an IV tubing at low flow rates; 
           [0018]      FIG. 10  is a graph illustrating heat loss across a disposable set heat exchange body and along a length of IV tubing and their combined heat losses; and 
           [0019]      FIG. 11  is a graph illustrating temperature loss for various lengths of IV tubing and various flow rates. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    A fluid warming device  10  according to the present invention is illustrated in  FIGS. 1-7 . The fluid warming device includes a housing  12  having a main body  14  and two sliding covers  16 . Within the housing, supported by the main body, are a removable heat exchange body  18  and a heater assembly  20 . The sliding covers are independently slidable to a closed position in which they retain the removable heat exchange body in place, as described more fully below. The slidable covers are preferably identical. 
         [0021]    The removable heat exchange body  18  and the heating assembly  20  are illustrated schematically in  FIG. 7 . The heat exchange body, also called a disposable or removable set, includes an input port  22  connectable to an IV tubing line from a source of IV fluid, which may include an infusion pump. The disposable set also includes an output port  24  connectable to a further IV tubing line to deliver the IV fluid to the patient. Within the disposable set, the IV fluid flows along a flow path (not shown) having a serpentine or other suitable configuration between the input and output ports to optimize heat transfer to the fluid. See, for example, US Patent Pub. No. 2005-0008354, the disclosure of which is incorporated by reference herein. The disposable set is formed from any suitable material, such as aluminum, to facilitate heat transfer to the fluid flowing therein. When inserted in the housing with the sliding covers in a closed position, the disposable set is held in thermal contact with the heater assembly, so that heat transfer from the heater assembly to the disposable set causes heating of an IV fluid flowing therethrough. 
         [0022]    The heater assembly  20  is affixed within the main body  14  of the housing  12 . The heater assembly includes a heater  26  and one or more thermally conductive layers  28 ,  30  interposed between the disposable set and the heater. Preferably, the heater is an electrically powered resistive thin film heater. A power line  32  to the heater from a suitable power source is provided. Alternatively, the device may include a battery compartment or a connection to a battery pack, for example, for portable operation. Temperature sensors  34 ,  36  are provided that sense the temperature of the disposable set and of the heater. See, for example, US Patent Pub. No. 2005-0008354. The thermally conductive layers also electrically insulate the disposable set from the resistive heater. One thermally conductive layer  28  may suitably comprise a phase transition material, and the other thermally conductive layer  30  may suitably comprise a material such as a graphite to optimize heat transfer between the heater and the disposable set. See, for example, US Patent Pub. No. 2005-0008354. It will be appreciated that other or further thermally conductive layers may be provided. The main body  14  includes a compartment  38  on one side to receive the disposable set  18  in contact with an exposed surface  40  of the uppermost thermally conductive layer  30 . 
         [0023]    As noted above, the heat exchange body or disposable set  18  is removable from the housing. The disposable set can be removed from the main body of the housing by sliding the two opposed sliding covers outwardly in opposite directions. In this manner, the removable set can be lifted out of the housing with the IV tubing still attached to the input and output connectors, without breaking the fluid path. Finger cutouts  42  may be provided for ease of grasping the disposable set in the main body. 
         [0024]    Any suitable sliding mechanism to allow the covers to move axially into the closed position can be provided. In the embodiment shown, the main body  14  of the housing includes protruding longitudinal tracks  46  along two opposed longitudinal outer wall surfaces of the main body. See  FIG. 5 . The sliding covers include complementary longitudinal recesses  48  along inner wall surfaces that mate with the tracks and allow the covers to slide axially along the main body. When in the closed position, the sliding covers extend over the edges of the disposable set within the recess of the main body, thereby retaining the disposable set therein. See  FIG. 1 . The covers also compress the disposable set to the outermost thermally conducting surface  40  of the heater assembly. This compression provides the necessary pressure for proper heat transfer between the heater assembly and the disposable set. Preferably, the covers are retained in the closed position by frictional engagement with the disposable set. Alternatively, any suitable latching or retaining mechanism may be provided. 
         [0025]    Also, the covers  16  do not block the view of the bulk of the mid portion of the disposable set  18 , allowing the operator to view the fluid passing through the disposable set. The disposable set is also keyed to the main body  14  in any suitable manner so that it fits within the compartment in the correct orientation. For example, one end  47  of the disposable set may be rounded to fit within an correspondingly rounded portion  49  of the compartment  38 . The disposable set may include an arrow  50  thereon to provide an indication of the direction of flow, so that the disposable set is inserted in the housing in the correct orientation. The covers do not block this arrow. Also, the main body preferably includes indicator lights, such as LEDs, thereon. For example, one LED  52  may provide an indication of temperature at the output port, and another LED  54  may provide an indication that the heater is connected to the power source. The covers do not block these indicator lights either. 
         [0026]    In one embodiment, the covers  16  can be maintained in two positions on the main body or can be removed fully from the main body. While on the main body, the covers can be in a fully closed position or an open position. The covers can include magnets or Hall Effect devices or other proximity sensors that interface with a corresponding component within the main body to determine the positions of the covers and cause operation of any appropriate switches. In a further embodiment, the covers can be maintained in a third or intermediate, half closed, position on the main body, described further below. 
         [0027]    More particularly, in the fully closed position, (see  FIG. 1 ), the covers  16  apply full pressure to the disposable set  18  to ensure good thermal contact with the heater assembly. In this position, the sliding covers can also be used to turn the power on to commence warming and/or to activate any audible or visible alarm(s). In the half closed position (see  FIG. 2 ), the disposable set is still held in place by the covers, but warming is stopped, the audible alarm is silenced, and the visual indicators are turned off. The status LED could be flashed in battery operation to inform the user that the warmer is connected to the battery and draining. When the covers are in the open position (see  FIG. 3 ), the disposable set  18  can be inserted and removed. No heating takes place, the audible alarm is silenced, and visual indicators are turned off. The status LED could be flashed in battery operation to inform the user that the heater is connected to the battery and draining. 
         [0028]    Any suitable latching or retaining mechanism can be provided to retain the covers in the desired positions relative to the main body. For example, as shown in  FIGS. 5 and 6 , recessed surfaces  62  are provided on the main body  14  that latch with corresponding tabs  64  on the covers in the open position, preventing the covers from readily coming off the main body. Also, the tabs  64  abut surfaces  63  to hold the covers in the closed position. Finger grips  68  are provided to aid in grasping the covers to push or pull them to the desired position. The closed (and power on) position can be indicated by arrows  70  and an adjacent “ON” marking on the covers. Similarly, the open (and power off) position can be indicated by arrows  72  and an adjacent “OFF” marking on the covers. The covers can be fully removed from the main body in any suitable manner, for example, by the insertion of a suitable tool, such as a screw driver or dime, to lift the tab  64  over the surfaces  62 . Alternatively, a latching or retaining mechanism can be configured to release simply by the use of sufficient force. Removal of the covers allows the device to be readily cleaned. Alternatively, passageways in the interior surfaces of the covers and a water tight main body housing allow cold sterilization by dipping in a sterilization fluid without complete removal of the covers. 
         [0029]    Referring to  FIG. 8 , the sliding covers  14  may include opposed faces  74  that include gripping teeth thereon to form gripping faces. The gripping faces can be used to grip hospital clothing or bedding and hold the warmer in place to reduce stress on the IV line when the covers are fully closed. 
         [0030]    In another aspect of the present invention, power to the heater can be increased or decreased to adjust the fluid temperature to ensure that the fluid is at an appropriate temperature when it reaches the patient. More particularly, some IV fluids that have been warmed are administered at very low flow rates. These fluids cool as they travel down the IV tubing to the patient. The greater the difference between ambient temperature and the fluid temperature, the greater the radiated heat losses from the IV tubing. 
         [0031]    A suitable controller is provided to perform the calculations and communicate with the heater to make the desired adjustments. Heater power is determined by the difference between a target temperature (typically in the range of 39 to 41° C.), and the actual fluid temperature. 
         [0032]    Referring to  FIG. 9 , the temperature drop across the heat exchanger is calculated (step  102 ). This temperature drop is equal to the heater power divided by the thermal resistance of the heater assembly. The thermal resistance can be readily determined by one of skill in the art from the thickness, thermal conductivity and area of the materials between the heater and the fluid and stored as a constant. 
         [0033]    Then, the controller calculates the temperature loss of the IV tubing to the environment (step  104 ). First, the difference between the fluid target temperature and the ambient temperature is determined. The temperature loss is equal to this temperature difference multiplied by the radiation loss and divided by the heater power. The ambient temperature is measured by a suitable sensor located within the warming device in close contact with the housing, which is very close to ambient temperature. The radiation loss is a constant that is derived from experimentation with various lengths of the IV tubing and various flow rates. See  FIG. 11 . 
         [0034]    Next, at step  106 , the controller determines if the IV tubing loss is greater than 1° C. Also at step  106 , the controller also determines if the total drop along the IV tubing and across the heat exchanger is greater than a drop limit. The drop limit is the maximum temperature that the fluid can be artificially raised so that the allowable surface temperature on the heat exchanger is not exceeded, for example, no greater than 3° C. from the desired target temperature. If the answer at step  106  is Yes, the actual fluid temperature is calculated at step  108  as the measured fluid output temperature minus the drop limit. If the answer at step  106  is No, the actual fluid temperature is calculated at step  110  as the fluid output temperature in the IV tubing drop minus the IV tubing drop (from step  102 ) minus the heat exchanger drop (from step  104 ). Using the calculated value of the actual temperature, heater power is adjusted appropriately. 
         [0035]    In this manner, heat loss along the IV tubing can be more efficiently controlled The system allows the fluid warming device to be located a bit farther from the infusion site and still deliver normothermic fluid. 
         [0036]    The invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims.