Electric heating apparatus for warming a parenteral fluid

Feb. 20, 1968 a. f. du fresne etal 3,370,153

ELECTRIC HEATING APPARATUS FOR WARMING A PARENTERAL FLUID

Filed March 6, 1964

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Feb. 20, 1968 a. f. du fresne et al 3,370,153

ELECTRIC HEATING APPARATUS FOR WARMING A PARENTERAL FLUID

Filed March 6, 1964 ₂ Sheets-Sheet 2

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INVENTORY

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United States Patent Office

3,370,153

Patented Feb. 20, 1968

3,370,153 ELECTRIC HEATING APPARATUS FOR WARMING A PARENTERAL FLUID

Armand F. Du Fresne, Duarte, and Walter H. Goodwin, Sierra Madre, Calif., assignors to Dupaco Incorporated, Arcadia, Calif., a corporation of California

Filed Mar. 6, 1964, Ser. No. 349,869

Claims. (Cl. 219—302)

This invention relates to a fluid warmer and, more particularly, to an apparatus for warming cold parenteral fluids and the like during intravenous injection or transfusion procedures.

It has recently been established that there is a direct relation between hypothermia and the occurrence of ventricular fibrillation and cardiac asystole. The probability that ventricular fibrillation and cardiac asystole will occur and will be fatal rises as the temperature of the heart is lowered. It has also been established that hypothermia is induced when large quantities of cold blood are transfused to a person. Extensive transfusions of blood often are required where a patient hemorrhages during a surgical operation and blood is usually applied substantially directly from a blood bank where blood normally is now stored at a temperature of about 4° C. It is apparent that the blood to be transfused must be warmed to about body temperature if cardiac malfunction is to be prevented.

This invention provides a simple, effective, and efficient apparatus for warming cold blood and other parenteral fluids to approximately body temperature during the process of infusing such fluids into the patient. It is a feature of this invention that the temperature of the fluid leaving the warmer does not fall drastically as the rate of flow of the fluid through the warmer increases and conversely does not increase dangerously if the flow is slowed even to the point of stoppage. This feature is obtained by providing a maximum amount of contact between the exterior surfaces of a coil of tubing through which the warmed fluid is caused to flow and a metallic heat source, “source” being used in the thermodynamic sense and being the opposite of a heat sink. Another feature of the invention is that the coil through which the fluid to be warmed is caused to flow is disposable.

Generally speaking, this invention provides apparatus for warming a circulating fluid and comprises a metallic body which defines therein an upwardly open well adapted to receive a quantity of liquid. The body defines thick walls around the well. Means are disposed in the walls for heating the walls and liquid retained in the well. Control means are operatively coupled to the heating means to maintain the block and liquid in the well at a substantially constant predetermined temperature. A coil of flexible tubing, through which the fluid to be warmed is circulated, is removaly disposed in the well and is immersed in the liquid contained in the well. Means cooperate with the coil for urging the coil into intimate contact with the well walls. In a preferred embodiment of the invention, the distance transversely of the well is less than the normal transverse dimension of the coil. The coil is forcibly engaged in the well so that the tendency of the coil to return to its normal shape assures intimate contact between the coil and the walls of the well.

The above-mentioned and other features of the invention are more fully set forth in the following detailed description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a perspective view of a fluid warmer according to this invention;

FIG, 2 is a partially cross-sectioned elevation view of a fluid warmer showing the fluid circulating coil engaged therein;

FIG. 3 is an end view of the normal configuration of the fluid circulating coil shown in FIG. 2;

FIG. 4 is an elevation view of the coil shown in FIG. 3;

FIG. 5 is an enlarged cross-sectional elevation view of 5 a second preferred fluid fluid circulating coil according to this invention;

FIG. 6 is a schematic diagram of the electrical circuitry of the fluid warmer; and

FIG. 7 is a partially cross-sectioned elevation view of Ιθ a second preferred embodiment of a fluid warmer according to this invention.

Referring to the drawings, FIG. 1 shows a fluid warmer 10 according to this invention. The warmer includes a base 11 which carries an upstanding massive rectangular 15 metallic heat source block 12. The block has an end face 13 in which an upwardly open cavity or well 14 is formed. A second cavity 15 (see FIG. 2) is formed in the opposite end of the heat source block separate from well 14. A three conductor electrical cable 16 is sealed to the interior 20 of cavity 15 and at its end remote from the warmer carries a male-type electrical connector 17. A pair of indicator lamps 18, 19 are mounted in the block adjacent cavity 15. A carrying handle 20 is pivotally mounted to the upper end of the block. The heat source block preferably is 25 fabricated from a lightweight metal such as aluminum or aluminum alloy, although any metal may be used.

Well 14 has opposing planar and substantially vertical side wall surfaces 21 and 22 adjacent which the block has substantial thickness. The block, as shown in FIG. 1, also 30 has substantial thickness adjacent the ends of the well. A pair of resistance heaters 23 (only one of which is shown in FIG. 2) are embedded in the block, one adjacent each of surfaces 21, 22. Each heater is arranged vertically of the block and has a pair of conductors which extend 35 into cavity 15. An adjustable temperature responsive switch mechanism or thermostat 24 is also embedded in the block adjacent one of surfaces 21, 22 and has conductors which extend into cavity 15. The thick walls of the well assure that heat released from heaters 23 is dis40 tributed substantially uniformly over well surfaces 21, 22.

A small diameter hole 25 is drilled into block 12 from end surface 13 parallel to the well. Hole 25 is adapted to receive a thermometer 26 (see FIG. 1) which provides a visual check on the temperature of the block at any 45 given moment. A duct 27 (see FIG. 7) extends upwardly from hole 25 into the well adjacent the open end of the well and assures that hole 25 is filled with fluid when the well is filled. This insures efficient heat transfer to the thermometer to insure accurate temperature indication. ⁵⁰ FIGS. 3 and 4 show a parenteral fluid circulating coil 30 for use in well 14. The coil preferably is fabricated from about 25 feet of flexible and somewhat resilient plastic tubing having an outer diameter of inch and an inner diameter of Vs inch, although any size tubing may be used. A small diameter tubing is preferred, however, so that the internal volume of the coil is minimized. The tubing is formed into a plurality of identical elongated loops 31 having a transverse dimension d which is greater than the distance between opposing well surfaces ^u 21, 22. Adjacent loops of the tubing are abutted against each other, as shown in FIG. 4, and are fused to one another by compressing the loops against each other at an elevated temperature with or without the presence of a solvent. Adjacent ends of the length of tubing are left uncoiled to define inlet and outlet ducts 32 and 33, respectively, for the circulating coil. Male and female connectors 34 and 35 are connected to the inlet and outlet ducts so that the coil may be connected into a blood ₇6 transfusion circuit, for example. Preferably coil 30 is disposable.

Fluid warmer 10 is operated by filling well 14 and hole

3,370,153 ³ with water or some other heat transfer liquid having a high specific heat. Circulating coil 30' is then inserted into the well as shown in FIG. 2. Since the distance across the well between wall surfaces 21, 22 is less than the distance d, this dimensional difference provides a mechanism which cooperates with the coil, as the coil is inserted into the well, so that the outer surfaces of the coil are intimately engaged with the walls of the cavity. The inherent resiliency of the coil further ensures intimate physical contact between the coil and the walls of the well. Connector 17 is then coupled to a source of electrical power and the warming device is allowed to stand for approximately 5 minutes.

Thermostat 24 operates to supply electrical power to resistance heaters 23 only when the temperature sensed by the thermostat is less than a predetermined temperature which is programmed into the thermostat. Preferably, the temperature programmed into the thermostat is 40° C. Since the thermostat is disposed closely adjacent the well, the temperature sensed by the thermostat is essentially the temperature of the water in the well. After the temperature of the water has stabilized, this condition being indicated by indicator light 19 being OFF while indicator light 18 is lit, the circulating coil is connected into the blood transfusion circuit. Normally the blood supply to which the circulating coil is connected is a bottle of blood just removed from a blood bank. The blood which enters the coil is at approximately 4° C.

Prior art devices for warming blood use a water bath in which is disposed a coil through which blood circulates. The water of the water bath may be heated prior to the time it is poured into the device with no heat being added to the water thereafter, or the water bath may be heated as blood flows through the coil. The heat transfer rate across the coil from the bath to the blood is low in prior art devices because of the two films the heat must traverse to the blood, i.e., the films at the exterior and interior surfaces of the coil tubing. Accordingly, to provide a blood exit temperature of about body temperature, 37° C., it was necessary for prior art warming devices to use a relatively long length of tubing or to use very hot water. The former expedient is not preferred since this means that a large amount of blood remains in the tubing after the transfusion is complete. The latter expedient is not favored since the blood will become too hot in the periods, which are common, when the blood flow is stopped, as when a new bottle of blood is placed in the transfusion circuit.

Where plastic tubing is used, the specific heat conductivity (k) of the plastic is low and this further limits the heat transfer rate across the tube. Plastic tubing, however, is preferred so that the coil can be disposable. In medical applications devices which cannot be easily cleaned are not favored and therefore the disposability of the coil is a decided advantage.

It is a feature of the present invention that the exit temperature of the blood from coil 30 is approximately body temperature although the circulating coil through which the blood flows is fabricated from a short length of plastic tubing and a water bath is used. This feature of the invention centers around the use of a massive container for the water, which container is heated to a temperature which also approximates body temperature, and around intimate thermal contact of the coil with the container. The water container is heat source block 12 with which, as noted above, the exterior surfaces of coil 30 are intimately engaged. Heat flows directly from the block to the walls of the coil tubing and to the blood across the film which exists along the interior of the tubing. Because there is no film between the exterior of the coil and the block, heat transfer from the block to the blood is much more efficient than a water bath heat transfer mechanism. This is true even though a coil of plastic tubing is used to carry the blood through the device. Approximately 40 percent of the surface of the coil is engaged with the heated block and, accordingly, the blood is heated primarily by heat flowing from the block. The remainder of the coil surface is exposed to the water in well 14 for heat transfer to the blood from the water. Even though 5 the block is heated to a temperature only one or two degrees above normal body temperature, this temperature is sufficient to heat blood flowing through the coil to about body temperature. The block, however, is not so warm as to dangerously overheat blood if the flowrate through the ₁₀ coil slows during a transfusion or is interrupted as when a blood supply body is being replaced.

As shown in FIGS. 1 and 2, the block preferably defines walls which are as thick around the sides and the ends of well 14 as the well is wide between wall surfaces 15 21, 22. This construction assures that the block functions as a substantially infinite source of controlled heat to the well, to coil 30 and to the water in the well. The massive construction and configuration of block 12 assures that heat lost to the space around the heating 20 device has little effect upon the operation of the device. Accordingly, the block can be heated to a temperature only a few degrees above body temperature and still have the temperature of the liquid leaving the coil at substantially body temperature. This means that the 25 device may safely accommodate flow rates through the coil from nothing to the maximum possible flowrate.

Fluid warming device 10 was operated for a period of ten minutes with the water in the well at a temperature of 40° C. Water was introduced to the coil at a tem30 perature of 12.5° C. and was flowed through the coil at a rate of 100 cc. per minute. After three minutes the temperature of the water leaving the coil stabilized at 34.7° C. and remained at this temperature for succeeding minutes. The same coil was then placed in a con35 ventional water bath which initially had a temperature of 41.5° C. Water at a temperature of 12° C. was flowed through the coil at a rate of 100 cc. per minute. The temperature of the water leaving the coil was 33.4° C. after one minute and at the end of ten minutes had 40 fallen to 31.8° C. The temperature of the water in the bath also fell during the same ten minutes interval; it was observed that the temperature differential between the water bath and the water leaving the coil stabilized at approximately 7° C. This comparison clearly shows 45 that apparatus according to this invention has a much more efficient heat transfer mechanism than prior art devices.

FIG. 5 shows that the area of contact between the fluid circulating coil and the adjacent surface of well 50 14 may be maximized by fabricating coil 30 from a tubing 38 which has a rectangular cross-section. Individual loops of the coil are fused to the adjacent loops as described above.

In some instances it may be desirable to provide posi55 tively acting means for urging the adjacent legs of coil 30 into mechanical contact with well surfaces 21, 22. FIG. 2 illustrates a coil biasing device 40 which includes a pair of substantially identical rectangular plates 41 each of which defines a plurality of transverse slots 42. The plates 60 are connected to each other by a plurality of compression springs 43. The plates have a width corresponding substantially to the width of coil 30 as shown in FIG. 4, and a length corresponding to the uncurved length of the legs of the coil when the coil is disposed in cavity 14. The 65 biasing device is positioned inside the coil and the coil is then inserted into cavity 14. As the coil is deformed, plates 41 are moved toward each other against the bias of springs 43. Springs 43, however, have a low spring constant so that the force with which each plate 41 forces 70 the adjacent leg of the coil into engagement with well surfaces 21 or 22 is not sufficient to collapse the tubing. Slots 42 are provided so that warm water in the well may circulate over the opposing surfaces of the coil legs.

FIG. 6 is a schematic diagram of the electrical com75 ponents of warmer 10. Connector 17 is connected to three

3.370.153 conductors 45, 46, 47, the latter being grounded to block 12. In the warmer, indicator lamp 18 is connected across conductors 45 and 46 and is lit when power is being supplied to the warmer. Thermostatic switch 24 and heater elements 23 are series connected in parallel with pilot lamp 18. Indicator lamp 19 is connected in parallel with the heater elements and is lit when the heaters are energized.

FIG. 7 is a partially cross-sectioned elevation view ol a second fluid warmer 50 according to this invention. The warmer includes a cylindrical block 51 fabricated of aluminum or some other lightweight metal. The block defines an upwardly open cylindrical cavity or well 52 in which a helical parenteral fluid circulating coil 53 is disposed. The walls of the well, defined by the block, are thick and have embedded therein a plurality of electrical heaters 23 (one of which is shown) and a thermostatic switch (not shown) for controlling the heaters.

Coil 53 has a vertical axis about which the helix of the coil is formed. The walls of the well are vertical and define a helical groove 55 circumferentially of the well. The groove is concave toward the well and has a radius corresponding to the outer radius of the tubing from which coil 53 is fabricated. The pitch of the helical groove and the pitch of the coil helix are identical, and the diameter of the coil corresponds to the diameter of the well as measured in the groove. Accordingly, the coil is snugly engageable in the block so that individual loops of the coil mate with and are disposed in corresponding loops of helical groove 55. The coil is disposed in the well so that the upper end of the coil lies below duct 27 which connects the well with thermometer receptacle 25.

Over 50% of the surface area of coil 53 is engaged with block 51 and accordingly warmer 50 is less dependent on heat transfer from the water in well 52 to fluid flowing through the coil than is the case with warmer 10. Warmer 10, however, is smaller than warmer 50 and is therefore presently preferred since its light weight makes it easier for nurses and the like to handle.

While the invention has been described above in conjunction with specific apparatus, this has been by way of example only and is not to be considered as limiting the scope of this invention.