Abstract:
A continuous body core temperature monitor comprises a pliable ear plug that conforms to the shape of an ear canal and incorporates a temperature sensor that is clamped between the plug and the ear canal wall. The external surface of the plug is connected to an external temperature sensor and a heating element that compensate for a heat lost from the ear canal to the environment by maintaining the temperature gradient between the temperature sensor and the heating element close to zero.

Description:
This application is a continuation of application Ser. No. 09/927,179, filed on Aug. 8, 2001 (now U.S. Pat. No. 6,773,405), which claims the benefit of U.S. provisional application Ser. No. 60/233,104, filed on Sep. 15, 2000 (abandoned), the disclosures of which are fully incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method of monitoring temperature of a human body and devices for achieving same and, more particularly, to such a method and device which monitors the internal core temperature of a person undergoing continuous medical observation. 
     DESCRIPTION OF PRIOR ART 
     Frequently, during surgical and other medical procedures related to humans and animals, there is a need for continuous monitoring of the body core temperature. Core temperature here means temperature of blood flowing around the brain and other vital internal organs. It has been recognized long time ago that the core temperature is an accurate parameter for assessing the physiological functions and metabolic activity of a body. 
     Traditionally, there are several known devices for continuous assessing body temperature of a patient. All these devices primarily differ by the measurement site. Specifically, they are 1) an esophageal probe, 2) a rectal probe, 3) skin temperature probes, and 4) an intermittent instant ear thermometers, often called tympanic. The last device presently can not provide a continuous monitoring. The first two devices yield accuracy well acceptable for the diagnostic and monitoring purposes and account for the majority of present temperature recordings. These traditional devices are invasive, may require sterile probes (esophageal), often inconvenient and, as a rule, not acceptable for patients outside the operating rooms. A skin temperature monitoring is used sporadically as it is more influenced by the ambient temperature. The need for an easy, inexpensive, accurate, and comfortable way of continuous temperature monitoring is substantial. 
     It has been recognized long time ago that the tympanic region of the ear canal follows the body core temperature with high fidelity. The region includes the tympanic membrane and the adjacent walls of the ear canal. This premise has been the basis for the tympanic thermometers, including both the contact and non-contact (infrared) types. An example of a contact transducer is a miniature thermistor (produced, for example, by Vital Signs, Inc.) that is positioned directly on the surface of a tympanic membrane with the connecting wires secured inside the ear canal. Generally, this can be performed only on an anesthetized patient with a risk of damaging the tympanic membrane and thus is rarely used in medical practice. Another example is a contact temperature transducer that is incorporated into an ear plug (U.S. Pat. No. 3,274,994). Examples of continuous noncontact optical infrared probes are given in U.S. Pat. Nos. 3,282,106 and 3,581,570. 
     Contact detectors are much simpler than noncontact, but they both suffer from the same effect—difficulty of a reliable placement inside the ear canal. Placement of a contact temperature transducer inside the ear canal without a reliable securing of it at any specific position may cause a high inaccuracy in measurement, due to unpredictable effects of the ambient temperature and placement technique of the probe. An attempt to incorporate a temperature transducer into an ear plug similar to a hearing aid device is exemplified by U.S. Pat. No. 5,333,622 issued to Casali, et al. Yet, the teaching does not resolve the accuracy problem due to heat loss. Besides, such a probe requires an individual tailoring of its shape. It should be noted that besides a temperature measurement, there are some other types of measurements that may require a secure adaptive positioning of a transducer inside a body cavity. 
     Therefore, it is a goal of this invention to produce a sensing device that can be positioned securely and reliably in a body cavity; 
     Another goal of the invention is to make an ear temperature transducer with a contact probe that is automatically secured at an ear canal wall; 
     It is another goal of this invention to produce an ear temperature transducer that tracks the core temperature with high fidelity; 
     It is another goal of this invention to make an ear temperature transducer that is less influenced by the ambient temperature; 
     It is another goal to provide an ear temperature transducer that doesn&#39;t cause a discomfort for a patient and can remain in the ear canal for a prolonged time; 
     SUMMARY OF THE INVENTION 
     The goals of this invention is achieved by the novel ear temperature detector. The detector is comprised of an ear plug carrying the temperature sensing device wherein the sensing device is characterized by its increased thermal coupling to a wall of an ear canal and decreased coupling to the environment. This is attained by pre-shaping the plug into a smaller size and allowing to change its shape upon the insertion, until the sensing device is clamped between the plug and the skin. To correct for a thermal gradient across the ear plug, the plug has low thermal conductivity and its external temperature is monitored. Alternatively, temperature of the external portion of the plug is actively controlled by a heater attached to the plug. The heater forms a thermal shield around the temperature sensing device, thus negating a thermal gradient across the plug. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a temperature detector inserted into an ear and secured on a helix. 
         FIG. 2  is a cross-sectional view of an ear temperature detector in a storage state 
         FIG. 3  is a cross-sectional view of a temperature detector in expanded state 
         FIG. 4  is a temperature detector with the electronic module inside the plug 
         FIG. 5  is a fork version of an ear plug 
         FIG. 6  shows a block diagram of a temperature monitor 
         FIG. 7  depicts a block diagram of a temperature monitor with an additional heater 
         FIG. 8  is an electrical circuit diagram of a controlled heater with thermistor sensors. 
         FIG. 9  is a radio telemetry version of a temperature monitor 
         FIG. 10  depicts a practical assembly of an ear temperature detector 
         FIG. 11  shows a tympanic sensor with a compensating heater 
         FIG. 12  illustrates a cross-sectional view of a surface temperature sensor 
         FIG. 13  is an electrical circuit diagram of a controlled heater with a thermocouple sensor 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention describes a device and method for obtaining information from a body cavity. At least three essential elements are required for this invention to work: a temperature transducer, a thermal insulator, and an external temperature sensor. 
     A preferred embodiment is illustrated herewith by showing how this can be accomplished with improved accuracy when the information is temperature and the body cavity is an ear canal of a human or other animal. The major task for accomplishing the stated goals is to increase a thermal coupling between the ear canal walls and a temperature transducer, while minimizing such coupling to the external environment.  FIG. 1  illustrates an ear plug  4  that is inserted into ear  1 , but not reaching the tympanic membrane  3 . Temperature transducer  5  is clamped between plug  4  and ear canal walls  2 . The transducer is connected to the electronic module  8  via wires  9 . There may be more than one transducer attached to the plug, but for the preferred embodiment just one is a sufficient example. The module is positioned in the external supporting disk  7  that contains external temperature sensor  21 . The entire assembly may be secured on ear  1  by carrier clamp  6  that has shape suitable for encircling the helix of an ear. Naturally, other conventional methods of securing may work as well. Since the ear canal wall temperature is close to that of tympanic membrane  3 , it is assumed that transducer  5  can monitor the tympanic temperature, unless plug  4  and wires  9  sink a significant portion of thermal energy from the transducer, resulting in erroneous temperature measurement. The position of transducer  5  inside the ear canal has to be consistent and always between plug  4  and walls  2 . The output signal is measured via conductors  70 . 
     To achieve the desired results, transducer  5  is attached to a specific portion of plug  4 . That portion preferably should be at the distal end of the plug that would be inserted into a body cavity, such is an ear canal.  FIG. 2  shows plug  4  in a storage stage, that is, before it is inserted into an ear. The plug has two ends—base II and tip  12 . The base is attached to an external enclosure. The enclosure is in form of disk  7  that may have a protruding pin  45  inside the plug for better mechanical and thermal coupling between disk  7  and plug  4 . In a storage state or just prior the insertion into the ear canal, tip  12  is compressed to a size that is smaller than the inner dimension of the ear canal. To retain such reduced shape for a long time, the tip may he inserted into storage sleeve  10  that provides a constraining compression. The sleeve may be a plastic tube. Plug  4  is fabricated of pliable material that may be collapsed when squashed (compressed) and recover its original shape (expand) when external pressure is released. The plug serves as a thermal insulator. Its thermal conductivity should be minimal, thus foams are the best choice of the material. An example of such a material is water-born hydrophilic foam. The foam should not have a significant dimensional memory so that it returns to the original expanded shape after prolonged storage in the collapsed (compressed) shape. 
     For a better thermal speed response, temperature transducer  5  is secured on the surface of tip  12 . The transducer should have a small size and may be of any suitable design—thermistor, thermocouple, semiconductor, etc. Wires  9  should sink out as little heat as possible, thus they need lo be fabricated as thin as practical and should have an extended length inside or on the surface of plug  4 . To increase the length, wires  9  may be formed into loop  13  that is positioned between transducer  5  and electronic module  8 , regardless of position of the module (explained below). 
     Before insertion of the plug into an ear canal, sleeve  10  is removed and discarded. Shape of tip  12  slowly returns to that which was prefabricated before the installation of sleeve  10 . Alternatively, tip  12  may be squashed by an operator just before the insertion. The rate of the shape recovery should be sufficiently slow to allow enough time for the insertion of plug  4  into an ear canal. Practically, the shape recovery time should be greater than 3 seconds. After the collapsed tip  12  is inserted into an ear canal, its continuous shape recovery forces plug  4  to conform with the shape of an ear canal. The expansion of plug  4  stops when it completely fills up the adjacent ear canal volume. This allows transducer  5  to be forcibly compressed against ear canal wall  2 , while still being electrically connected to electronic module  8  via wires  9 , as shown in  FIG. 1 . 
     Electronic module  8  may contain the amplifier, power supply, signal conditioner, transmitter and other components, or it may be a simple connecting device. In some embodiments, module  8  may be positioned directly inside plug  4  as shown in  FIG. 4 . In this case, the size of module  48  should be small enough to allow compression of tip  12  before the insertion. Module  8  may be used for many other purposes, in addition to or instead of measuring temperature. An example is generating sound. In that case, opening  44  in plug  4  may be required for better sound coupling to the lympanic membrane. 
     It should be stressed that an ear canal is just an example of an application and the identical concept of an expandable plug with an attached transducer can be used for producing an insert for other body cavities, for example, nasal. Further, there maybe other than temperature transducers attached to the plug, for example acoustic. 
     Another possible embodiment of plug  4  is shown in  FIG. 5  where the plug is made in shape of flexible fork  36  having a spring action. The end of the fork is squeezed by fingers  35  before the insertion and let go after. The fork has arm  37  that carries transducer  5 . After the fork is released, it expands so that its arm  37  compresses transducer  5  against ear canal wall  2 . To improve thermal separation of transducer  5  from the outside, the fork may be supplied with insulator  38 . Other components, like the wires, the loop, the electronic module, etc, are not shown in  FIG. 5 . 
     The expanded plug  4  performs an important function—positioning and clamping transducer  5  on an ear canal wall surface. The other critical function—minimizing effects of the ambient temperature may be accomplished by at least two methods. One method is a mathematical correction and the other is an active compensation. The method of a mathematical correction of an error is performed by the use of an additional ambient temperature sensor that is positioned either directly on disk  7  as external sensor  21  ( FIGS. 2 and 3 ), or in/on the external monitor  16  as ambient sensor  20  ( FIG. 6 ). Note that for this method, heater  14  is not required and only one sensing device—either ambient sensor  20  or external sensor  21  is needed. Disk  7  of an ear device is connected to monitor  16  via cable  15  ( FIGS. 6 and 7 ). Monitor  16  may contain signal processor  17 , power supply  18 , display  19  and other components. Ambient sensor&#39;s  20  or external sensor&#39;s  21  signal is processed and used to correct for errors in the ear temperature measurement. The degree of correction needs to be established experimentally for a particular plug design. The corrected body temperature t b  may be determined through a temperature gradient, for example, as:
 
 t   b   =t   s +μ( t   s   −t   a )  (1)
 
where μ is the experimental constant, t a  is the temperature measured by ambient sensor  20  or external sensor  21  and t s  is the reading of ear temperature transducer  5 .
 
     The above method of error correction, however, has it&#39;s limitations. One is the uncertainty in the value of constant μ. Another limitation is the use of ambient sensor  20 . Having ambient sensor  20  placed at monitor  16 , makes the mathematical correction less effective when, for example, the patient is laying on the ear which is being monitored and thus having the external ear temperature significantly different from that of ambient monitored by sensor  20 . 
     A more effective method of the error reduction is an active heat loss compensation that is shown in  FIG. 7 . It is based on forming a thermal shield around temperature transducer  5 . Disk  7  carries heater  14  and external temperature sensor  21 , positioned on or near heater  14  with a good thermal coupling between them. Note that disk  7  is located outside of the ear canal, directly at it&#39;s opening. Heater  14  also may be seeing in  FIGS. 2 ,  3  and  4 . The heater controller, that is positioned either inside disk  7  or in monitor  16 , as shown in  FIG. 7 , receives signal from external sensor  21  and controls temperature of heater  14  to a required level, that should be close to the actual body temperature as monitored by transducer  5 . Thus, heater  14  minimizes temperature gradient between temperature transducer  5  and heater  14 . It acts as a thermal shield between temperature transducer  5  and the environment. Circuit diagram of  FIG. 8  further illustrates this method. A reference point for the heater control is provided by temperature transducer  5  positioned inside the ear canal and compressed by plug  4  to the ear canal wall. Both temperature transducer  5  and external sensor  21  are connected to the Wheatstone bridge circuit with two pull-up resistors  30  and  31 . Thermal coupling between transducer  5  and the ear canal walls needs to be much better than between temperature transducer  5  and the external components, that is, external sensor  21  and heater  14 . This is primarily accomplished by the use of thermally insulating plug  4 . An excessive thermal coupling between temperature transducer  5  and external sensor  21  may result in undesirable instability of the control circuit. Error amplifier  32  compares the output signals from temperature transducer  5  and external sensor  21  and controls heater controller  22 , that in turn, via conductors  33 , adjusts electric power to heater  14 . This circuit maintains temperature of heater  14  close to that of temperature transducer  5 . This results in a negligible heat transfer through plug  4  and elimination of the error in temperature measured by temperature transducer  5 . Turning again to Eq. 1, we can see that with the active heating of the above thermal shield method, temperatures at both sides of plug  4  equalize: t a ≈t s  and thus value of μ become irrelevant, so that t b =t s . In other words, transducer  5  now directly measures temperature of the body with no influence of the ambient temperature. 
     As a variant of  FIG. 8 ,  FIG. 13  illustrates use of a thermocouple temperature transducer having two dissimilar conductors  100  and  101 . A thermocouple has two junctions, hot junction  102  and cold junction  103 . Hot junction  102  is positioned inside the body cavity at one end of plug  4 , while cold junction  103  is thermally attached to heater  14  and external sensor  21  near the other end of plug  4 . 
     Heater controller  22  receives signal from thermocouple amplifier  32  and operates such as to bring thermocouple output voltage  105  close to zero. This will establish a minimal thermal gradient across plug  4  so that external sensor  21  indicates the internal body temperature. 
     The use of cable  15  as shown in  FIGS. 6 and 7  may not be desirable, as it may restrict movement of a patient. The cable can be eliminated if disk  7  carries transmitter  24  and power source  25 , as illustrated in  FIG. 9 . Accordingly, monitor  18  needs to contain antenna  27  and receiver  23 . The link between the patient and the monitor may be via radio waves  26 , or optical (both involve electromagnetic radiation). Alternatively, transmitter  24  and/or power source  25  can be located outside of disk  7 , but in that case, an intermediate packaging for these components (not shown) would be required. It should be noted, that in the wireless communication with the monitor, method of a passive error correction is preferable, so that transmitter  24  will send information concerning blot transducer  5  and external sensor  21 . 
     A practical way to produce an ear temperature monitoring device with a thermal shield is shown in  FIG. 10 . Reusable cup  39  may contain electronic module  8 , cable  15 , second contacts  29 , heater  14 , and external sensor  21 . A detachable part is disposable insert  41  that contains plate  40 , plug  4 , and transducer  5  attached via wires  9  to first contacts  28 . Before operation, insert  41  is moved in direction  42  to mate with cup  39 . Both cup  39  and insert  41  are engaged and retained together during the temperature monitoring with the help of lock  43 . Contacts  28  and  29  provide connection between wires  9  and electronic module  8 . After the monitoring in completed, disposable insert  41  may be detached from cup  39  and discarded. 
     OTHER EMBODIMENTS 
     A thermal shield method similar to one shown in the preferred embodiment can be employed to reduce effects of the environment with other types of the medical temperature sensors. The general operating principle is basically the same as described above.  FIG. 11  shows an example of a temperature transducer  52  that is directly attached to tympanic membrane  3  of ear  1 . This embodiment does not necessarily require an expanding plug  4  that has been shown in the prior illustrations. Wires  9  pass through or near heating insert  50  that is inserted into ear opening  51 . The heating insert contains external sensor  21  and thermally attached to it heater  14 , whose temperature is controlled to approach that measured by transducer  52 . As above, wires should be as thin as practical and heater  14  should be thermally de-coupled from transducer  52 . Since the thermal gradient across wires  9  between transducer  52  and external sensor  21  becomes small, effects of the ambient temperature also become small, while the accuracy of measurement improves. 
     Another embodiment of the same thermal shield method is depicted in  FIG. 12 . This is a surface temperature measuring device that can measure a “deep” (subcutaneous) body temperature. The device is comprised of housing  55  secured to skin  60  or another surface of a subject, skin temperature sensor  56 , heater  14 , thermal insulator  58 , second temperature sensor  57 , wires  9 , and cable  59 . Housing  55  is formed from metal, as represented by the cross-section lines. Note that wires  9  pass through insulator  58  and through or near heater  14 . Insulator  58  can be a body of polymer foam or even an air gap. In operation, the temperature of second temperature sensor  57  is controlled to approach that of skin temperature sensor  56 , by providing thermal energy to heater  14 . This forms a thermal shield above skin temperature sensor  56  and minimizes heat loss from skin  60  and, subsequently, to an improved accuracy in temperature measurement. In most practical cases, for an acceptable accuracy, a typical temperature difference between sensors  56  and  57  should be no greater than 2° C. and preferably equal to zero. 
     While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.