Abstract:
The present invention relates to a portable apparatus for warming biocompatible fluids for use in the treatment of injured patients. The present invention may be used to warm intravenous fluids for trauma resuscitation or to warm air from a ventilator circuit. The portable nature of the present invention makes it highly suitable for field applications, such as a forward surgical hospital near a combat zone.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a portable apparatus for warming biocompatible fluids for use in the treatment of injured patients. The present invention may be used to warm intravenous fluids for trauma resuscitation or to warm air from a ventilator circuit. The portable nature of the present invention makes it highly suitable for field applications, such as a forward surgical hospital near a combat zone. 
   2. Description of the Prior Art 
   Hypothermia is quite common in injured patients, including patients experiencing trauma. Hypothermia produces a number of physiologic derangements which worsen the effects of major injury. Several relevant enzyme systems begin to lose efficiency as their ambient temperature falls. For example, the myocardium, which is dependent on the function of membrane-channel type enzymes for normal electrical function, shows a predictable series of atrial followed by ventricular arrhythmias as core temperature falls below 34° C. Cardiac output is further compromised by poor function of intrinsic myocardial components, with bovine myocardium showing a linear decrease in developed tension with decreasing temperature. 
   Hypothermia also exacerbates hemorrhagic shock in multiple ways. The onset of coagulopathy which accompanies hypothermia, has been shown to result from malfunction of both clotting factors and platelets. 
   While profound hypothermia may be tolerated by immersion or cardiac surgery patients, the presence of hypothermia in trauma patients predicts significantly higher mortality. Mortality doubles for heterogeneous groups of trauma patients at 34° C., and survival after trauma is very rare when the core temperature falls below 32° C. This effect is greater for more severely injured patients. 
   The development of hypothermia comes from several factors. Body heat is convectively lost to the environment, and this effect is enhanced by bleeding or the presence of large surface area burns. The body loses both central thermoregulation and peripheral shivering after traumatic injury. Less heat is produced peripherally as perfusion decreases in shock. 
   The administration of intravenous fluids is used in trauma resuscitation. The administration of fluid at ambient temperature, however, induces hypothermia. This condition is worse in more severely injured patients, who require the most fluid and have the least ability to tolerate the additional insult of decreased core temperature. Hypothermia and mortality clearly increase after the administration of five liters of crystalloid or five units of packed red blood cells, and the onset of hypothermia increases the incidence of coagulopathy in injured patients, particularly in the presence of acidosis. 
   As used herein, the term “biocompatible fluid” refers to any fluid that is appropriate for infusion into the human body, including normal saline and its less concentrated derivatives, Ringer&#39;s lactate, and hypertonic crystalloid solutions; blood and fractions of blood including plasma, platelets, albumin and cryoprecipitate; intravascular volume expanding blood substitutes including hetastarch, polymerized hemoglobin, perfluorocarbons; medications reconstituted with saline or sterile water; and medical gasses including air, oxygen, helium, nitric oxide, and combinations thereof. 
   Prior art methods of treating hypothermia include direct intravenous fluid warming. The fluid that is warmed may be the blood other biocompatible liquid. 
   Prior art devices used to warm one or more biocompatible fluids for use in the treatment of trauma have used electricity as their heating source. These systems are referred to herein as “biocompatible liquid infusion systems.” Electrically heated biocompatible fluid infusion systems have several drawbacks. If the source of electrical energy is alternating current from a central generating station, the unit can then only be used in locations where such alternating current is available. This significantly limits the locations where the units may be used. Locations such as non-industrialized nations or battlefield locations are likely not have readily available sources of alternating current to power such systems. 
   Batteries may also be used to generate electrical energy. It is believed that sufficient power to heat a single liter of fluid to 20° C. within a ten minute time period would require a rechargeable battery the size and weight of a large laptop computer. In such a case, the weight of the battery would exceed the weight of a liter of saline fluid. The size and weight of such a unit would limit its portability. Additionally, the battery would require recharging after each liter of biocompatible fluid is delivered. 
   The present invention overcomes the limitations of prior art biocompatible fluid infusion systems by providing a biocompatible liquid infusion system that is not dependent upon electrical energy as a heat source. The present invention is light enough and compact enough to be used in field hospital environments which are remotely located from large central hospitals and from sources of alternating current. The present invention may also be used to warm air delivered to a patent via a ventilation circuit. 
   SUMMARY OF THE INVENTION 
   The present invention is directed toward a portable biocompatible fluid warming system that may be used for infusing biocompatible liquids into a patient for the treatment of trauma. The present invention uses heat from hydrocarbon combustion. Hydrocarbon combustion can take place in the absence of an open flame. As an example, in one embodiment, the present invention may be used with a gaseous hydrocarbon such as butane which is allowed to flow onto a platinum mesh and then ignited. The butane combines with oxygen and liberates heat which then heats the platinum mesh. In this embodiment, the temperature of the mesh stabilizes at the ignition temperature of the butane, thereby allowing combustion to occur on the surface of the platinum mesh. 
   The present invention functions as a heat exchanger which takes the heat resulting from the hydrocarbon combustion process described above and transfers this heat to a biocompatible liquid. 

   
     DESCRIPTION OF THE FIGURES 
       FIG. 1  is a side cutaway view of one embodiment of the outer housing of the present invention. 
       FIG. 2  is an isometric view of one embodiment of the present invention. 
       FIG. 3  is an exploded isometric view of one embodiment of the present invention. 
       FIG. 4  is a block diagram of the process control instrumentation of a preferred embodiment of the present invention. 
       FIG. 5  is a side view of the gas delivery and ignition components of the present invention. 
       FIG. 6  is a side view of one embodiment of the actuator of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In a preferred embodiment, the present invention is directed toward a portable warmer of a biocompatible fluid comprising an outer steel housing  10  comprising a first outer diameter  12 , a first inner diameter  14 , and at least one flow channel  16  located between the first inner diameter and the first outer diameter as shown in  FIGS. 2 and 3 . 
   The term “diameter” as used herein refers to the length of an axis which bisects a cross sectional area of the housing. For cylindrical geometries the diameter is constant at a given point along the longitudinal axis of the cylindrical housing at various azimuths. For noncylindrical geometries the diameter at a given point along the longitudinal axis of the housing may vary as a function of the azimuth. 
   In a preferred embodiment, the outer diameter of the steel housing is no more than 20 centimeters. In another preferred embodiment, the outer housing is cylindrical. In another preferred embodiment, the outer housing is made of stainless steel. 
   The flow channel comprises an inlet section  18  and an outlet section  20 , as shown in  FIG. 1 . In a preferred embodiment, the flow channel is helical, as shown in  FIG. 1 . In another preferred embodiment, the mass of the portable warmer described herein is less than or equal to two kilograms. 
   This embodiment of the invention further comprises an inner aluminum housing  22  having a second outer diameter  24  sized to fit snugly within said outer housing and an inner wall defining a second inner diameter  26  and an internal volume as shown in  FIGS. 2 and 3 . In the preferred embodiment depicted in  FIG. 2 , the internal volume defined by inner diameter  26  extends longitudinally the length of outer housing  10 . In a preferred embodiment both the outer and inner housings are cylindrical. In a preferred embodiment, the inner housing comprises at least two ports  29  to permit fluid flow between regions on opposite sides of the inner housing as shown in  FIG. 3 . In another preferred embodiment, the inner housing comprises at least two grooves in which fluid can flow. 
   This preferred embodiment further comprises a multiplicity of heat transfer protrusions  32  affixed to the inner wall as shown in  FIG. 3 . In one preferred embodiment, the heat transfer protrusions are fins. In another preferred embodiment, the heat transfer protrusions are ring like disks as shown in  FIG. 3 . In a preferred embodiment, the invention further comprises a metallic mesh  34  located within the internal cylindrical volume as shown in  FIG. 5 . In a preferred embodiment, the metallic mesh is made from a metal selected from the group consisting of palladium and platinum. 
   This invention further comprises a gas delivery line  36  comprising a distal end region  38  located within the internal volume and a proximal end region  40  located outside the internal volume as shown in  FIG. 5 . A valve  42  is located in the gas delivery line. In a preferred embodiment, the valve is a needle valve. 
   The invention further comprises a spark igniter  44  located in the internal volume and situated close enough to the valve such that when the valve is open and gas flows through the gas delivery line and the valve into the internal volume, the igniter can ignite the gas and cause the wire mesh to be heated to a temperature in excess of 420° C., as shown in  FIG. 5 . In a preferred embodiment, the invention further comprises a source of combustible gaseous hydrocarbon  46  in fluid communication with the proximal end of the gas delivery line as shown in  FIG. 5 . In a preferred embodiment, the gaseous hydrocarbon is selected from the group consisting of methane, ethane, propane, and butane. 
   Another embodiment of the present invention comprises process controls for controlling the temperature of the fluid output from the portable fluid warmer. In this embodiment, the invention further comprises a temperature sensor  50  positioned to sense the temperature of a fluid flowing through the outlet section of the flow channel and to transmit a temperature signal  52  indicative of the temperature of a fluid flowing through the outlet section of the flow channel as shown in  FIG. 4 . In a preferred embodiment, the temperature sensor is selected from the group consisting of a thermistor, a thermocouple, and a solid state thermal sensor. 
   In another preferred embodiment, the invention further comprises a controller  54  operatively connected to receive the temperature signal from the sensor and transmit a control signal  56  responsive to the temperature signal as shown in  FIG. 4 . In one preferred embodiment, the controller is a microcontroller. In another preferred embodiment, the controller is an analog controller. In a preferred embodiment, when the temperature signal indicates that the temperature of the fluid flowing through the outlet section of flow channel exceeds a preselected temperature threshold, a control signal to increase the degree of closure of the valve is generated. In another preferred embodiment, the actuator comprises worm gear  60  mechanically coupled to a spur gear  62  as shown in  FIG. 6 . In this embodiment, the spur gear is mechanically coupled to the valve. 
   In this embodiment, the invention further comprises a valve actuator  58  operatively connected to the valve and to the controller to control the degree of closure of the valve in response to the control signal as shown in  FIG. 4 . In a preferred embodiment, the valve actuator is coupled to receive the control signal from the controller. In a preferred embodiment, the valve actuator is a servo-controller. 
   In other embodiments, temperature may be regulated by controlling fuel flow into the inner cylinder. Additionally, temperature may be controlled by mixing small amounts of unheated fluid with the heated fluid exiting the portable warming device. In another embodiment, fluid temperature may be controlled by changing the thermal conductance of the layer between the inner cylinder and the flow channels. 
   The foregoing disclosure and description of the invention are illustrative and explanatory. Various changes in the size, shape, and materials, as well as in the details of the illustrative construction may be made without departing from the spirit of the invention.