TEMPERATURE SENSOR ASSEMBLIES FOR ELECTRIC WARMING BLANKETS
An electric warming blanket for warming patients during surgery and other medical procedures includes a flexible heater and a temperature sensor assembly coupled thereto; a first layer of water resistant material coupled to a second layer of water resistant material, about a perimeter of the heater, forms a substantially hermetically sealed space for the heater and the temperature sensor assembly. The blanket may further include a thermal insulation layer disposed between the temperature sensor assembly and the first layer of water resistant material. The temperature sensor assembly may provide input of an average temperature over a portion of a surface area of the heater to a temperature controller, when the heater and sensor assembly are coupled to the controller.
The present application claims priority to co-pending provisional applications Ser. No. 60/825,573, entitled HEATING BLANKET SYSTEM filed on Sep. 13, 2006; Ser. No. 60/722,106, entitled ELECTRIC WARMING BLANKET INCLUDING TEMPERATURE ZONES AUTOMATICALLY OPTIMIZED, filed Sep. 29, 2005; and Ser. No. 60/722,246, entitled HEATING BLANKET, filed Sep. 29, 2005; all of which are incorporated by reference in their entireties herein.
RELATED APPLICATIONSThe present application is related to the following commonly assigned utility patent applications, all of which are filed concurrently herewith and all of which are hereby incorporated by reference in their entireties: A) ELECTRIC WARMING BLANKET HAVING OPTIMIZED TEMPERATURE ZONES, Practitioner docket number 49278.2.5.2; B) NOVEL DESIGNS FOR HEATING BLANKETS AND PADS, Practitioner docket number 49278.2.7.2; C) FLEXIBLE HEATING ELEMENT CONSTRUCTION, Practitioner docket number 49278.2.15; D) BUS BAR ATTACHMENTS FOR FLEXIBLE HEATING ELEMENTS, Practitioner docket number 49278.2.16; and E) BUS BAR INTERFACES FOR FLEXIBLE HEATING ELEMENTS, Practitioner docket number 49278.2.17.
TECHNICAL FIELDThe present invention is related to heating or warming blankets or pads and more particularly to those including electrical heating elements.
BACKGROUNDIt is well established that surgical patients under anesthesia become poikilothermic. This means that the patients lose their ability to control their body temperature and will take on or lose heat depending on the temperature of the environment. Since modern operating rooms are all air conditioned to a relatively low temperature for surgeon comfort, the majority of patients undergoing general anesthesia will lose heat and become clinically hypothermic if not warmed.
Over the past 15 years, forced-air warming (FAW) has become the “standard of care” for preventing and treating the hypothermia caused by anesthesia and surgery. FAW consists of a large heater/blower attached by a hose to an inflatable air blanket. The warm air is distributed over the patient within the chambers of the blanket and then is exhausted onto the patient through holes in the bottom surface of the blanket.
Although FAW is clinically effective, it suffers from several problems including: a relatively high price; air blowing in the operating room, which can be noisy and can potentially contaminate the surgical field; and bulkiness, which, at times, may obscure the view of the surgeon. Moreover, the low specific heat of air and the rapid loss of heat from air require that the temperature of the air, as it leaves the hose, be dangerously high —in some products as high as 45° C. This poses significant dangers for the patient. Second and third degree burns have occurred both because of contact between the hose and the patient's skin, and by blowing hot air directly from the hose onto the skin without connecting a blanket to the hose. This condition is common enough to have its own name—“hosing.” The manufacturers of forced air warming equipment actively warn their users against hosing and the risks it poses to the patient.
To overcome the aforementioned problems with FAW, several companies have developed electric warming blankets. However, there is still a need for electrically heated blankets or pads that can be used safely and effectively warm patients undergoing surgery or other medical treatments. These blankets need to be flexible in order to effectively drape over the patient (making excellent contact for conductive heat transfer and maximizing the area of the patient's skin receiving conductive as well as radiant heat transfer), and should incorporate means for precise temperature control.
Precise temperature control is important because non-uniform heat distribution can occur within an electric warming blanket. Unfortunately, many temperature sensors used to provide feedback to a temperature controller do not dependably report an accurate average temperature of the blanket because they sense temperature from too small of an area. For example, if the temperature of a measured location is cooler than the average blanket temperature, the temperature sensor will cause the controller to deliver more power to the heater and the resulting average temperature of the heater will be higher than desired.
Further, an electric blanket can overheat if the temperature sensor is thermally grounded to a cool object. This condition can occur if a cool object such as a metal pan is placed on top of the heater in the area of the temperature sensor. The sensor “feels” cool and tells the temperature controller to deliver more power to the heater.
Accordingly, there is a need for a blanket that utilizes a temperature sensor that takes temperature measurements that are representative of the average temperature of the blanket. Further, there is a need for a blanket with a temperature sensor that will not cause the blanket to overheat if a cool object is placed in proximity to it. Various embodiments of the invention described herein solve one or more of the problems discussed above.
BRIEF DESCRIPTION OF THE DRAWINGSThe following drawings are illustrative of particular embodiments of the present invention and therefore do not limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description. Embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, wherein like numerals denote like elements.
FIGS. 1B-C are end views of two embodiments of the subassembly shown in
The following detailed description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides practical illustrations for implementing exemplary embodiments of the present invention. Examples of constructions, materials, dimensions, and manufacturing processes are provided for selected elements, and all other elements employ that which is known to those of skill in the field of the invention. Those skilled in the art will recognize that many of the examples provided have suitable alternatives that can be utilized. The term ‘blanket’, used to describe embodiments of the present invention, may be considered to encompass heating blankets and pads.
According to an exemplary embodiment, a conductive fabric comprising heating element 10 comprises a non-woven polyester having a basis weight of approximately 130 g/m2 and being 100% coated with polypyrrole (available from Eeonyx Inc., Pinole, Calif.); the coated fabric has an average resistance, for example, determined with a four point probe measurement, of approximately 15-20 ohms per square inch at about 48 volts, which is suitable to produce the preferred watt density of 0.2 to 0.4 watts/sq. in. for surface areas of heating element 10 having a width, between bus bars 15, in the neighborhood of about 20 inches. Such a width is suitable for a lower body heating blanket, some embodiments of which will be described below. A resistance of such a conductive fabric may be tailored for different widths between bus bars (wider requiring a lower resistance and narrower requiring a higher resistance) by increasing or decreasing a surface area of the fabric that can receive the conductive coating, for example by increasing or decreasing the basis weight of the fabric. Resistance over the surface area of the conductive fabrics is generally uniform in many embodiments of the present invention. However, the resistance over different portions of the surface area of conductive fabrics such as these may vary, for example, due to variation in a thickness of a conductive coating, variation within the conductive coating itself, variation in effective surface area of the substrate which is available to receive the conductive coating, or variation in the density of the substrate itself. Local surface resistance across a heating element, for example heater 10, is directly related to heat generation according to the following relationship:
Q (Joules)=I2(Amps)×R(Ohms)
Variability in resistance thus translates into variability in heat generation, which is measured as a temperature. According to preferred embodiments of the present invention, which are employed to warm patients undergoing surgery, precise temperature control is desirable. Means for determining heating element temperatures, which average out temperature variability caused by resistance variability across a surface of the heating element, are described below in conjunction with FIGS. 2A-B.
A flexibility of blanket subassembly 100, provided primarily by flexible heating element 10, and optionally enhanced by the incorporation of flexible bus bars, allows blanket subassembly 100 to conform to the contours of a body, for example, all or a portion of a patient undergoing surgery, rather than simply bridging across high spots of the body; such conformance may optimize a conductive heat transfer from element 10 to a surface of the body. However, as illustrated in
The uniform watt-density output across the surface areas of preferred embodiments of heating element 10 translates into generally uniform heating of the surface areas, but not necessarily a uniform temperature. At locations of heating element 10 which are in conductive contact with a body acting as a heat sink, for example, body 16, the heat is efficiently drawn away from heating element 10 and into the body, for example by blood flow, while at those locations where element 10 does not come into conductive contact with the body, for example lateral portions 11, 12 as illustrated in
According to embodiments of the present invention, zones of heating element 10 may be differentiated according to whether or not portions of element 10 are in conductive contact with a body, for example, a patient undergoing surgery. In the case of conductive heating, gentle external pressure may be applied to a heating blanket including heating element 10, which pressure forces heating element 10 into better conductive contact with the patient to improve heat transfer. However, if excessive pressure is applied the blood flow to that skin may be reduced at the same time that the heat transfer is improved and this combination of heat and pressure to the skin can be dangerous. It is well known that patients with poor perfusion should not have prolonged contact with conductive heat in excess of approximately 42° C. 42° C. has been shown in several studies to be the highest skin temperature, which cannot cause thermal damage to normally perfused skin, even with prolonged exposure. (Stoll & Greene, Relationship between pain and tissue damage due to thermal radiation. J. Applied Physiology 14(3):373-382. 1959 and Moritz and Henriques, Studies of thermal injury: The relative importance of time and surface temperature in the causation of cutaneous burns. Am. J. Pathology 23:695-720, 1947) Thus, according to certain embodiments of the present invention, the portion of heating element 10 that is in conductive contact with the patient is controlled to approximately 43° C. in order to achieve a temperature of about 41-42° C. on a surface a heating blanket cover that surrounds element 10, for example, a cover or shell 20, 40 which will be described below in conjunction with
Referring now to the end view of
Returning now to
According to the illustrated embodiment, heat spreader 212 is sized to contact an enlarged surface area so that a temperature sensed by sensor 21 is more representative of an average temperature over a region of heater 10 surrounding sensor 21, which is positioned such that, when a heating blanket including heater 10 is placed over a body, the regions surrounding sensor 21 will be in conductive contact with the body. As previously described, it is desirable that a temperature of approximately 43° C. be maintained over a surface of heater 10 which is in conductive contact with a body of a patient undergoing surgery. Other types of heat spreaders, in addition to metallic foils, include metallic meshes or screens, or an adhesive/epoxy filled with a thermally conductive material.
Heat spreader 212 is a desirable component of a temperature sensor assembly, according to some embodiments of the present invention, since conductive fabrics employed by heating element 10, such as those previously described, may not exhibit uniform resistance across surface areas thereof. Heat spreader 212, having a surface area that does not exceed approximately four square inches, according to a preferred embodiment, may be effective in averaging out relatively small scale spatial resistance variation, for example, about 3% to 10% variability over less than about one or two inches. Such a limitation on heat spreader 212 surface area may be necessary so that heat spreader 212 does not become too bulky, since the larger the surface area, the greater the thickness of spreader 212 needed in order to maintain effective heat transfer across spreader 212 and to sensor 21. In addition, if spreader 212 is too thick, a thermal mass of spreader 212 will cause spreader 212 to respond too slowly to changes in heat loss or gain by heating element. According to an exemplary embodiment of the present invention, spreader 212 has a surface area of no greater than approximately four square inches and a thickness of no greater than approximately 0.001 inch. Some alternate embodiments of the present invention address a non-uniform resistance across a surface area of element 10 by employing a distributed temperature sensor, for example, a resistance temperature detector (RTD) laid out in flat plane across a surface of heater 10, or by employing an infrared temperature measurement device positioned to receive thermal radiation from a given area of heater 10. An additional alternate embodiment is contemplated in which an array of temperature sensors are positioned over the surface of heater 10, being spaced apart so as to collect temperature readings which may be averaged to account for resistance variance.
According to a preferred embodiment, assembly 421 includes a second, redundant, temperature sensor mounted to substrate 211, close enough to sensor 21 to detect approximately the same temperature; while sensor 21 may be coupled to a microprocessor temperature control, the second sensor, for example, a chip thermistor similar to sensor 21, may be coupled to an analog over-temperature cutout that cuts power to element 10, and/or sends a signal triggering an audible or visible alarm. The design of the second sensor may be the same as the first sensor and need not be described again. Another safety check may be provided by mounting an identification resistor to substrate 211 in order to detect an increase in resistance of element 10, due, for example, to degradation of the material of element 10, or a fractured bus bar; the optional identification resistor monitors a resistance of heating element 10 and compares the measured resistance to an original resistance of element 10.
According to some embodiments of the present invention, for example as illustrated in
According to some embodiments of the present invention, shell 20 includes top and bottom sheets extending over either side of assembly 250; the two sheets of shell 20 are coupled together along a seal zone 22 (shown with cross-hatching in the cut-away portion of
Returning now to
FIGS. 3C-D further illustrate a pair of securing strips 217, each extending laterally from and alongside respective lateral portions 11, 12 of heating element 10 and each coupled to side 13 of heating element 10 by the respective row of stitching 345. Another pair of securing strips 271 is shown in
With reference to
With further reference to
In the foregoing detailed description, the invention has been described with reference to specific embodiments. However, it may be appreciated that various modifications and changes can be made without departing from the scope of the invention as set forth in the appended claims. Although embodiments of the invention are described in the context of a hospital operating room, it is contemplated that some embodiments of the invention may be used in other environments. Those embodiments of the present invention, which are not intended for use in an operating environment and need not meet stringent FDA requirements for repeated used in an operating environment, need not including particular features described herein, for example, related to precise temperature control. Thus, some of the features of preferred embodiments described herein are not necessarily included in preferred embodiments of the invention which are intended for alternative uses.
Claims
1. An electric warming blanket for warming patients during surgery and other medical procedures, comprising:
- a flexible heater having a surface area and a substantially uniform watt density output across the surface area when the heater is electrically powered;
- a temperature sensor assembly coupled to the heater and providing input of an average temperature over a portion of the surface area of the heater to a temperature controller when the heater and the sensor assembly are coupled to the controller; and
- a first layer of water resistant material coupled to a second layer of water resistant material about a perimeter of the heater to form a substantially hermetically sealed space for the heater and the temperature sensor assembly.
2. The blanket of claim 1, wherein the heater comprises a conductive fabric.
3. The blanket of claim 1, wherein the heater comprises carbon.
4. The blanket of claim 1, wherein the heater comprises a nonconductive layer coated with a conductive material.
5. The blanket of claim 4, wherein the nonconductive layer comprises a woven polyester and the conductive material comprises polypyrrole.
6. The blanket of claim 1, wherein the heater comprises a fabric incorporating closely spaced conductive elements.
7. The blanket of claim 1, wherein the portion is disposed along the surface area so at to be in conductive contact with the patient when the blanket is placed over the patient to warm the patient.
8. The blanket of claim 1, wherein the temperature sensor assembly includes a temperature sensor coupled to a heat spreader, the heat spreader extending over the portion of the surface area.
9. The blanket of claim 8, wherein the portion of the surface area is no greater than approximately four square inches.
10. The blanket of claim of claim 8, wherein the heat spreader comprises a metal foil.
11. The blanket of claim 1, wherein the temperature sensor assembly includes a distributed temperature sensor comprising a resistance temperature detector (RTD) laid out in a flat plane across the portion of the surface area.
12. The blanket of claim 1, wherein the temperature sensor assembly includes an array of temperature sensors spaced apart over the portion of the surface area.
13. An electric warming blanket for warming patients during surgery and other medical procedures, comprising:
- a flexible heater including a first side and a second side, at least one of the first and second sides having a surface area and a substantially uniform watt density output across the surface area when the heater is electrically powered;
- a temperature sensor assembly coupled to the first side of the heater;
- a first layer of water resistant material disposed over the first side of the heater, being un-adhered thereto, and forming a top surface of the blanket when the blanket is placed over the patient;
- a second layer of water resistant material disposed over the second side of the heater, being un-adhered thereto, and forming a bottom surface of the blanket, adjacent to the patient, when the blanket is placed over the patient,
- the first layer of water resistant material coupled to the second layer of water resistant material about a perimeter of the heater to form a substantially hermetically sealed space for the heater and the temperature sensor; and
- a layer of thermal insulation disposed between the temperature sensor assembly and the first layer of water resistant material.
14. The blanket of claim 13, wherein the flexible heater comprises a conductive fabric.
15. The blanket of claim 13, wherein the heater comprises carbon.
16. The blanket of claim 13, wherein the heater comprises a nonconductive layer coated with a conductive material.
17. The blanket of claim 16, wherein the nonconductive layer comprises a woven polyester and the conductive material comprises polypyrrole.
18. The blanket of claim 13, wherein the flexible heater comprises a fabric incorporating closely spaced conductive elements.
19. The blanket of claim 13, wherein the layer of thermal insulation comprises flexible polymeric foam.
20. The blanket of claim 13, wherein the layer of thermal insulation comprises high loft fibrous polymeric non-woven material.
21. The blanket of claim 13, wherein the layer of thermal insulation comprises non-woven cellulose material.
22. The blanket of claim 13, wherein the layer of thermal insulation comprises air.
23. An electric warming blanket for warming patients during surgery and other medical procedures, comprising:
- a flexible heater having a surface area and a substantially uniform watt density output across the surface area when the heater is electrically powered; and
- a temperature sensor assembly coupled to the heater,
- the sensor assembly including a temperature sensor and a heat spreader, and
- the heat spreader comprising a metal foil disposed between the temperature sensor and the heater.
24. The blanket of claim 23, wherein the temperature sensor comprises a surface mount chip thermistor.
25. The blanket of claim 23, wherein the heat spreader extends over a portion of the surface area of the heater, the portion being no greater than approximately four square inches.
26. The blanket of claim 23 wherein the heater spreader has a thickness that is no greater than approximately 0.001 inch.
Type: Application
Filed: Sep 29, 2006
Publication Date: Mar 29, 2007
Inventors: Scott Augustine (Bloomington, MN), Scott Entenman (St. Paul, MN), Keith Leland (Medina, MN), Gordon Lawrence (Minneapolis, MN)
Application Number: 11/537,189
International Classification: H05B 3/34 (20060101); H05B 3/54 (20060101);