LOW-POWER HEAT FLUX MODULATOR FOR CORE BODY TEMPERATURE

SENSING

DESCRIPTION The following relates to the medical arts. It finds application in core body temperature monitoring, and will be described with illustrative reference thereto. The following finds more general application in temperature monitoring generally, and in other applications that may benefit from a controllable heat flux modulator or controllable heat flux modulation. Core body temperature is a probative vital sign for detecting and diagnosing many diseases and other medical conditions. However, precise and accurate core body temperature measurement is surprisingly difficult to perform. Known techniques, such as oral or rectal thermometry, can be inaccurate due to misplacement of the thermometer or deviations of the oral or rectal temperature from the true core body temperature. Invasive techniques such as an arterial line catheter can be more accurate, but invasive procedures have disadvantages such as causing discomfort or pain to the patient, possibly introducing infection, and so forth.

More elaborate core body temperature thermometry techniques have been developed, in which skin temperature is measured in conjunction with thermal perturbation to provide data sufficient to solve a relevant heat transfer relationship relating core body temperature with the measured skin temperature. Examples of such techniques are disclosed, for example, in Tokita et al., U.S. Publ. Appl. 2002/0191675 Al and in European Patent Application No. EP06126697 filed December 20, 2006, both of which are incorporated herein by reference in their entireties. The approach of EP06126697 employs a temperature sensor, such as a thermistor or thermocouple, contacting the skin to provide a skin temperature, and a second temperature sensor separated from the skin by a layer of thermal insulation. The difference between the temperatures on the opposite sides of the insulation layer measured by the skin temperature sensor and the second temperature sensor can be related to the heat flux out of the skin and into the ambient, and enable correction of the skin temperature to determine the core body temperature. In order to achieve high sensing accuracy that

accounts for patient-to-patient variability and to provide robustness with respect to sensor-patient contact quality, thermal resistance of the tissue under the probe is run-time calibrated by modulating the heat flux from the body to the ambient using a heater, evaporator, or by varying the thermal conductance, for example by varying the effective thickness of the insulation layer by electromechanical means.

The use of heating, evaporation, or electromechanical manipulation to generate a thermal perturbation have disadvantages. Heaters consume substantial amounts of electrical power which is problematic in battery-powered core body temperature thermometers. Evaporators tend to produce slow thermal changes, which delays or complicates the core body temperature measurement. Electromechanical actuators add substantial complexity to the system, are difficult to control in a precise manner, and can compromise device robustness and reliability.

The following provides a new and improved apparatuses and methods which overcome the above-referenced problems and others.

In accordance with one aspect, a thermal device is disclosed, comprising a heat flux modulator including a thermally insulating body, first and second heat exchangers thermally isolated from each other by the thermally insulating body, the first and second heat exchangers each comprising microtubing, and a fluid pump operatively connected with the first and second heat exchangers and operable to drive a working fluid through the heat exchangers so as to selectively provide thermal communication between the first and second heat exchangers.

In accordance with another aspect, a core body temperature measuring device is disclosed, including the thermal device as set forth in the immediately preceding paragraph.

In accordance with another aspect, a core body temperature measuring device comprising: a heat flux modulator comprising first and second microtubing thermally isolated from each other and a fluid pump operatively connected with the first and second heat exchangers and operable to drive a working fluid through the heat exchangers so as to selectively provide thermal communication between the first and second heat exchangers; a probe having a contacting surface configured to contact skin, the heat flux modulator arranged to modulate heat flux from the skin contacted by the

contacting surface of the probe; temperature sensors arranged to measure (i) a temperature of the skin contacted by the contacting surface of the probe and (ii) a parameter correlated with heat flux from the skin contacted by the contacting surface of the probe; and a core body temperature processor configured to determine a core body temperature reading based on measurements received from the temperature sensors.

In accordance with another aspect, a temperature measurement method is disclosed, comprising: providing a heat flux modulator including first and second heat exchangers separated by a thermally insulating body, the heat flux modulator a passive thermal conductance; selectively flowing a working fluid through the first and second heat exchangers to selectively adjust a thermal conductance of the heat flux modulator to a value higher than the passive thermal conductance; and measuring a temperature of a region relatively closer to the heat flux modulator at a plurality of different adjusted values of the thermal conductance of the heat flux modulator; and determining a temperature of a region relatively further from the heat flux modulator based on the measured temperatures of the region relatively closer to the heat flux modulator at the plurality of different adjusted values of the thermal conductance of the heat flux modulator

One advantage resides in providing a core body temperature measurement device with reduced power consumption.

Another advantage resides in providing a compact and energy-efficient heat flux modulator.

Another advantage resides in improved core body temperature measurement.

Still further advantages of the present invention will be appreciated to those of ordinary skill in the art upon reading and understand the following detailed description.

FIGURE 1 diagrammatically shows a core body temperature measuring device including a heat flux modulator.

FIGURE 2 diagrammatically shows a heat flux modulator with a two-dimensional array of heat exchangers to provide lateral heat flux control.

FIGURE 3 diagrammatically shows one of the heat exchangers of the heat flux modulator of FIGURE 2.

FIGURES 4-7 diagrammatically show top views of heat exchangers with various layouts configured to provide enhanced lateral uniformity.

With reference to FIGURE 1, a core body temperature measuring device includes a heat flux modulator 8 including a thermally insulating body 10, which in the illustrated embodiment is generally planar, and first and second heat exchangers 12, 14 thermally isolated from each other by the thermally insulating body 10. The first and second heat exchangers 12, 14 include respective microtubing 16, 18 and respective thermally conductive bodies 22, 24 in thermal communication with the respective microtubing 16, 18. In the illustrative embodiments the thermally conductive bodies 22, 24 are generally planar and arranged parallel with and spaced apart by the generally planar thermally insulating body 10. In some embodiments, for example, the thermally insulating body 10 may be a plate of a thermally insulating material such as plastic, polystyrene silica aerogel, or so forth serving as a substrate for the thermally conductive bodies 22, 24 embodied as metallic or other thermally conductive layers deposited on opposing principal sides of the thermally insulating body 10. In some embodiments, the thermally insulating body 10 is contemplated to be embodied as a volume of air defined by an air gap between the thermally conductive bodies 22, 24 which in these embodiments are spaced apart by discrete spacer elements (not shown).

The heat flux modulator 8 further includes a fluid pump 26 connected across the first and second heat exchangers 12, 14, and more particularly connected across the first and second illustrated microtubings 16, 18. The fluid pump 26 is operable to drive a working fluid (not shown) through the heat exchangers 12, 14, and more particularly through the illustrated microtubing 16, 18, so as to selectively provide thermal communication between the first and second heat exchangers 12, 14. In some embodiments, the fluid pump 26 and microtubing 16, 18 define a closed-path fluid flow system. Alternatively, the fluid flow system can be open-path, for example having a fluid inlet feeding into the fluid pump and a fluid outlet exiting the fluid pump. The working fluid can be a liquid or a gas, and is selected to have suitable relevant fluid properties such as density and viscosity, and suitable relevant thermal properties such as heat capacity. The working fluid may also be selected to have other desired properties such as nontoxicity. Although the illustrated fluid pump 26 is a discrete unit connected with the microtubing 16, 18, it is also contemplated to have the fluid pump integrated with the microtubing. For

example, such an integrated fluid pump is suitably implemented using an electrostatic operational principle employing electrodes embedded in the microtubing.

The microtubing 16, 18 can be variously embodied, for example as fine plastic or vinyl tubing arranged in a spiral, serpentine (as shown), or other lateral arrangement, or as a planar structure defining a generally linear fluid flow cavity with a serpentine or otherwise-arranged layout, or so forth. In some embodiments, the heat exchangers are metallic plates, e.g. copper plates, with a serpentine or otherwise- arranged fluid flow cavity. Although each illustrated microtubing unit 16, 18 has a single inlet and a single outlet both connected with the fluid pump 26, in other contemplated embodiments the microtubing may define a plurality of independent, e.g. parallel, fluid paths. If the microtubing is sufficiently densely arranged over the area of each heat exchanger, then the thermally conductive bodies 22, 24 are optionally omitted.

The first and second heat exchangers 12, 14 are at respective first and second temperatures Ti, T ₂ each of which are generally uniform across the area of the respective heat exchanger 12, 14 due to the thermal conductivity of the thermally conductive bodies 22, 24 and the distribution of fluid flow provided by the serpentine microtubing 16, 18. However, the first and second temperatures T ₁ , T ₂ may in general be different from each other due to the thermal isolation provided by the thermally insulating body 10. That is, the temperatures T ₁ , T ₂ of the two heat exchangers 12, 14 may in general be different. The thermally insulating body 10 is to be understood to be substantially thermally insulating, that is, to have a relatively high thermal resistance or correspondingly a relatively low thermal conductance. Thus, the temperatures T ₁ , T ₂ are related based on the thermal conductance of the thermally insulating body 10. For the thermally insulating body 10 being a planar with a thickness L, and area A, and made of a material of isotropic thermal conductivity k (in suitable dimensions such as Watts/(meter- Kelvin)), the thermal conductance of the thermally insulating body 10 is kAIL in suitable dimensions such as Watts/Kelvin. This is a passive, substantially diffusion-based heat exchange mode.

On the other hand, when the fluid pump 26 is operating to drive the working fluid through the microtubing 16, 18, this provides thermal communication between the first and second heat exchangers 12, 14 bypassing the substantial thermal isolation of the thermally insulating body 10, so that when the fluid pump 26 is operating the temperatures T ₁ , T ₂ of the two heat exchangers 12, 14 will tend to be driven toward a common

temperature. This is an active, substantially convection-based heat exchange mode. In some embodiments, the heat exchangers 12, 14 and fluid pump 26 provide thermal communication that is high enough to be modeled as a thermal "short circuit" such that the temperatures T ₁ , T ₂ of the two heat exchangers 12, 14 can be approximated as being equal when the fluid pump 26 is operating at a steady state.

In some operational approaches the heat flux modulation is binary, that is, switchable between a minimum heat flux state in which the pump 26 is off and a maximum heat flux state inwhich the pump 26 is operating at full capacity or at a level sufficient to provide a virtual thermal short circuiting of the thermally insulating body 10. It is also contemplated to operate the fluid pump 26 at a variable speed or to switch the pump 26 on and off at a rapid switching speed so as to provide heat flux in a combination of the passive, diffusion-based and active, convection-based modes. These operational approaches provide analog or pseudo-analog heat flux modulation that is not merely switchable between minimum and maximum heat flux states, but rather can assume heat flux values in a continuous range bounded by minimum and maximum heat flux states.

In the core body temperature measuring device of FIGURE 1, the heat flux modulator 8 is disposed on or in thermal communication with a probe thermally insulating body 30 having a contacting surface 32 distal from the heat flux modulator 8 configured to contact a body of interest, which in FIGURE 1 is a portion of skin 34 of a human or animal subject. The contacting surface 32 is thermally isolated from the second heat exchanger 14 of the heat flux modulator 8 by the probe thermally insulating body 30. For skin 34 at a skin temperature T _skin and assuming the probe thermally insulating body 30 is planar with area A _p , thickness L _p , and and made of a material of isotropic thermal conductivity k _p , the probe thermally insulating body 30 has a thermal conductance of k _p A _p IL _p in, for example, Watts/Kelvin. The temperature T ₂ of the second heat exchanger 14 is, in the device of FIGURE 1, measureable using one or more temperature sensors 40 disposed in thermal contact with the second heat exchanger 14. Similarly, the skin temperature T _8Hn is, in the device of FIGURE 1, measureable using one or more temperature sensors 42 disposed in thermal contact with the skin 34. The contacting surface 32 of the probe thermally insulating body 30 is assumed herein to be at the skin temperature T _skin , which is reasonable if the contacting surface 32 is in intimate contact with the skin 34. Optionally, the contacting surface 32 may be coated with or include a metal film or other thermally

conductive structure or material to enhance thermal uniformity across the area of the contacting surface 32, and to improve thermal contact with the skin 34.

Although the skin temperature T _skin is readily measured, the temperature that is actually desired to be measured is a core body temperature T _core that is representative of a core region 44 diagrammatically indicated in FIGURE 1 as a plane. The core region 44 is separated from the skin 34 by various biological tissue such as skin tissue, muscle tissue, fat tissue, bone tissue, or the like. The core body temperature T _core can be estimated from the accessible temperatures T ₂ , T _8R j _n using the following relationship: dϊ d ² T dt dx ² where T generically denotes a temperature, α = λ/pc _p , λ denotes thermal conductivity, p denotes density, and c _p denotes specific heat. In a suitable coordinate system, x denotes a depth of the core region 44 with x=0 corresponding to a point inside the body at temperature T _core and x=h _s corresponding to the surface of the skin 34. The boundary conditions for Equation (1) include the core body temperature T _core (to be determined) at x=0, and the measured temperature T _skin at x=h _s , that is, at the surface of the skin 34. If the contacting surface 32 is contacting or otherwise in good thermal communication with the skin 34, then this surface 32 is at temperature T _Skm to a good approximation. The heat flux out of the skin portion 34 is denoted q _s herein.

Assuming the skin portion 34 can be represented as a plane of thickness h _s and thermal conductivity λ _s , the heat flux out of the skin portion q _s (that is, heat transfer rate on a per-unit area basis) can be written as: q _s = -λ^- at x = h _s (2), dx and a solution of Equation (1) can be approximated as:

A _s 2a _s dt At equilibrium, Equation (3) reduces to:

T _core = T _skm + ^ q _s (4),

A s

which demonstrates that the core body temperature T _cor e is higher than the skin temperature T _skin by a temperature drop across the gap between the skin 34 and the core region 44 corresponding to Qιjλ^)-q _s .

By using the heat flux modulator 8, the values of the quantities T _Sk i _n , q _s , and dT s.k _h in can be measured for different moments in time t,={ti,...,t _n } _to produce a matrix of dt coupled equations:

i - £ - ^f h

(5), h h in which the unknown quantities are T _cor e, — and and where:

2a. ξ ^≡ iAO (6), and η ≡ (7). dt '

It is assumed here that T _core ,— and ~^— are time-independent during the time interval λ s 2a,

{ti,...,t _n }over which the set of measurements are acquired. The system of Equations (5) can be solved using a least squares minimisation procedure or other suitable coupled equations solver to provide the core body temperature T _cor e, and also the heat flux q _s through the surface of the skin 34. The sampling moments I ₁ are suitably chosen such that to ensure that the system of Equations (5) is well-conditioned.

To calibrate this system on a per-patient or per-measurement basis, the heat flux out of the skin 34 can be measured since it is equal to the heat flux through the probe thermally insulating body 30 which can be computed based on the thermal conductance k _p A _p lL _p (which is a fixed value for a given geometry and material) and the temperature difference between the temperatures T ₂ , T _skin measured by respective temperature sensors 40, 42. The core body temperature computation of Equations (5)-(7) is merely an illustrative example, and other approaches and algorithms can be used to measure core

body temperature T _core based on one or both measureable temperatures T ₂ , T _8Hn and heat flux modulation provided by the heat flux modulator 8.

With continuing reference to FIGURE 1, the outputs of the temperature sensors 40, 42 are suitably received by a core body temperature processor 50, which also serves as a controller for controlling the fluid pump 26 to modulate the heat flux. The core body temperature processor computes a core body temperature reading based on the algorithm of Equations (5)-(7) or another suitable algorithm. The computed core body temperature reading is suitably displayed on a core body temperature display 52 which may, for example, be a computer display, a dedicated core body temperature measuring device display, or so forth. Additionally or alternatively, the computed core body temperature reading may be stored in an electronic patient record.

Heat flux modulation can be controlled in various ways. In one approach, the processor 50 is programmed or otherwise configured to modulate an electrical control signal 54 between the first and second values to provide pulse modulation control of heat flux modulation provided by the heat flux modulator 8. For example, a pulse- width modulation (PWM) can be used, in which the PWM cycling is sufficiently fast to produce a pseudo-analog heat flux value determined by widths of the PWM pulses. In another approach, the processor 50 is programmed or otherwise configured to modulate an electrical control signal 54 in an analog fashion, in which an analog value of the electrical control signal 54 controls an operational speed of the fluid pump 26 so as to provide analog control of the heat flux modulation provided by the heat flux modulator 8.

With reference to FIGURE 2, in some embodiments it is useful to modulate or control the heat flux laterally. FIGURE 2 shows an arrangement in which a two-dimensional array of heat flux modulators 8 are distributed across a surface of the probe thermally insulating body 30 which in the embodiment of FIGURE 2 has a lateral area coextensive with the two-dimensional array of heat flux modulators 8, rather than with a single heat flux modulator. Controlled lateral heat flux modulation is achievable by individually addressed operation of the individual heat flux modulators 8 of the array.

With reference to FIGURE 3, another variation is diagrammatic ally illustrated, in which a single first heat exchanger 16 on one side of the heat flux modulating device is connected to a multiplicity of second heat exchangers 18 on the other side (e.g., two second heat exchangers 18 in the embodiment shown in FIGURE 3). Valves V are

optionally used to block flow of the working fluid through the pumps 26 in an undesired direction.

With reference to FIGURES 4-7, various arrangements of a central heat flux modulator 8 _C surrounded by one or more peripheral heat flux modulators 8 _P can be employed to provide controllable or calibrated lateral heat flux uniformity. FIGURE 4 shows an arrangement in which a rectangular central heat flux modulator 8 _C is surrounded on all four sides by peripheral heat flux modulators 8 _P . FIGURE 5 shows an arrangement in which a circular central heat flux modulator 8 _C is surrounded by an annular peripheral heat flux modulator 8 _P . FIGURE 6 shows an arrangement similar to that of FIGURE 5, but with the annular peripheral heat flux modulator broken up into four semi- annular peripheral heat flux modulators 8 _P . FIGURE 7 shows an arrangement similar to that of FIGURE 6, but with two rings of semi-annular peripheral heat flux modulators 8 _P .

The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.