SINGLE HEAT FLUX SENSOR ARRANGEMENT

Abstract

The invention describes a single heat flux sensor arrangement (1) comprising a sensor (10) comprising a layer (100) of thermally insulating material, an inner temperature measurement means (S1) arranged at an inner region of the insulating layer (100) and an outer temperature measurement means (S2) arranged at an outer region of the insulating layer (100); an evaluation unit (11) adapted to receive a temperature input from the inner 5 temperature measurement means (S1) and to receive a temperature input from the outer temperature measurement means (S2); and a heating means (12, S2) realized to deliberately raise the temperature of a region of the insulating layer (100). The invention further describes a non-invasive method of measuring a core body temperature (T0) of a subject (3).

Claims

1. A single heat flux sensor arrangement for measuring a core body temperature of a subject, comprising: a sensor comprising only two temperature sensors separated by a layer of thermally insulating material, wherein the sensor comprises an inner temperature sensor arranged at an inner region of the insulating layer and an outer temperature sensor arranged at an outer region of the insulating layer; a heating means realized to deliberately raise the temperature in a region of the insulating layer; and an evaluation unit adapted to receive a temperature input from the inner temperature, to receive a temperature input from the outer temperature, to calculate a value of thermal resistivity for that subject from the temperature values and the thermal resistivity of the insulating material according to $R.Math..Math.0=\frac{T.Math..Math.12-T.Math..Math.11}{T.Math..Math.11-T.Math..Math.21+T.Math..Math.22-T.Math..Math.12}.Math.R.Math..Math.1$ and to calculate the core body temperature of the subject on the basis of the calculated thermal resistivity.

2. A single heat flux sensor arrangement according to claim 1, wherein the evaluation unit is realized to determine an equilibrium temperature condition of the sensor, wherein an equilibrium temperature condition is attained when each temperature sensor reports a stable value over time.

3. A single heat flux sensor arrangement according to claim 2, wherein the evaluation unit is realized to activate the heating means in response to a first equilibrium temperature condition of the sensor.

4. A single heat flux sensor arrangement according to claim 2, wherein the evaluation unit is realized to deactivate the heating means in response to a second equilibrium temperature condition of the sensor.

5. A single heat flux sensor arrangement according to claim 1, wherein the heating means is realized to raise the temperature of the outer region of the insulating layer.

6. A single heat flux sensor arrangement according to claim 1, wherein a temperature measurement means comprises a thermistor.

7. A single heat flux sensor arrangement according to claim 6, wherein the heating means is realized to apply an electric current through the outer thermistor.

8. A single heat flux sensor arrangement according to claim 1, wherein the heating means comprises a current source.

9. A single heat flux sensor arrangement according to claim 1, comprising a memory module realized to record temperature-related data collected by the single heat flux sensor arrangement.

10. A single heat flux sensor arrangement according to claim 1, realized as a wearable device.

11. A single heat flux sensor arrangement according claim 1, comprising a display unit realized to display a temperature profile for the core body temperature of the subject.

12. A single heat flux sensor arrangement according to claim 1, comprising a wireless interface for communication with a remote monitoring station.

13. A method of measuring a core body temperature of a subject, comprising the steps of: attaching a single heat flux sensor arrangement according to claim 1 to the subject; determining a first equilibrium temperature and subsequently activating the heating means to heat the outer region of the insulating layer of the sensor arrangement; determining a second equilibrium temperature; calculating a value of thermal resistivity for that subject; calculating the core body temperature of the subject on the basis of the calculated thermal resistivity value and the equilibrium temperatures.

14. A method according to claim 13, comprising a step of de-activating the heating means after determining the second equilibrium temperature.

15. A method according to claim 13, wherein the single heat flux sensor arrangement is attached to the skin of a human subject, and a value of skin thermal resistivity is determined.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 shows a block diagram of the single heat flow sensor according to the invention;

[0026] FIG. 2 shows a simplified diagram of an ideal single heat flux sensor;

[0027] FIG. 3A illustrates a first stage in the method according to the invention;

[0028] FIG. 3B illustrates a second stage in the method according to the invention;

[0029] FIG. 4 shows an embodiment of the single heat flow sensor according to the invention.

[0030] In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] FIG. 1 shows an exemplary embodiment of the inventive single heat flow sensor arrangement 1. The sensor arrangement 1 comprises a sensor 10, which mainly consists of a thermal insulation layer 100 or pad 100. The pad 100 is applied to the skin 2 of a patient, and the sensor arrangement 1 will be used to determine the patient's core body temperature.

[0032] The sensor 10 has an inner thermistor S1 arranged at or close to an inner surface of the pad 100, and an outer thermistor S2 arranged at or close to an outer surface of the pad 100. An evaluation unit 11 is electrically connected to the thermistors and can evaluate the electrical signals that it receives in order to determine a temperature T1 at the inner region of the pad 100 and a temperature T2 at the outer region of the pad 100. To this end, the evaluation unit 11 can comprise a suitable microprocessor, FPGA, etc.

[0033] In this exemplary embodiment, the single heat flow sensor 1 has a memory module 111 for recording a temperature profile, and is also equipped with a transmitter 112 for sending a temperature profile and/or recorded temperature measurements to a remote monitoring station. The diagram also indicates that the single heat flow sensor 1 can be connected to or equipped with a display unit 113 (for example in the manner of a smart watch) for showing a temperature profile to a user, for example the patient or a caregiver.

[0034] The inventive sensor arrangement 1 has a heating means 12 realized to raise the temperature of the outer thermistor S2. In this exemplary embodiment, the heating means 12 is a current source 12, and is controlled by the evaluation unit 11 to apply a current through the outer thermistor S2 for a predefined duration. This allows additional information to be collected, so that the thermal resistivity of the skin 2 can be determined, as will be explained in the following.

[0035] FIG. 2 illustrates the principle of operation of a single heat flux sensor applied to the skin 2 of a person. This relatively simple sensor construction uses only two temperature sensors S1, S2 separated by the layer of insulating material 100. The heat flow I.sub.heat through the skin 2 and through the sensor pad 100 will be the same, and can be expressed as

[00001]$\begin{array}{cc}{I}_{\mathrm{heat}}=\frac{T.Math..Math.1-T.Math..Math.2}{R.Math..Math.1}=\frac{T.Math..Math.0-T.Math..Math.1}{R.Math..Math.0}& \left(1\right)\end{array}$

where T1 is the temperature at the inner sensor S1, T2 is the temperature at the outer sensor S2, R1 is the thermal resistivity of the insulating material 100, and R0 is the skin thermal resistivity. The thermal resistivity R1 of the insulating material may be presumed to be known. An expression for the core body temperature can be obtained by re-arranging equation (1):

[00002]$\begin{array}{cc}T.Math..Math.0=T.Math..Math.1+\frac{\left(T.Math..Math.1-T.Math..Math.2\right).Math.R.Math..Math.0}{R.Math..Math.1}& \left(2\right)\end{array}$

[0036] Equations (1) and (2) are based on the presumption that the material layers 2, 100 are infinitely wide, so that the heat can be presumed to flow only in the vertical direction, as indicated by the arrows. In a practical application, there will also be a lateral heat flow component which will detract from the accuracy of the measurement results. Furthermore, only the thermal resistivity R1 of the sensor pad is known, and the skin thermal resistivity R0 must be estimated. This can significantly detract from the accuracy of the measurement results, since the skin thermal resistivity can vary depending on which part of the body the sensor is attached to, and values of skin thermal resistivity can vary significantly between people depending on various factors such as age, physiology, etc.

[0037] The inventive sensor can overcome this limitation of the conventional sensors by actually measuring the skin thermal resistivity. FIGS. 3A and 3B will be used to explain the stages of the method. Initially a thermistor of the sensor 10 may be assumed to have ambient or room temperature T.sub.room. After attaching the sensor 10 to the skin 2, heat flow from the patient's body will raise the temperature of the sensor pad 100, so that the thermistors also report rising temperatures. A temperature profile P.sub.x for one of the thermistors in the first stage of the method is shown in FIG. 3A. Ultimately, the temperature of the thermistor will reach an equilibrium temperature T.sub.EQ1. At this point, using equation (2) above, the core body temperature can be expressed as

[00003]$\begin{array}{cc}T.Math..Math.0=T.Math..Math.1.Math..Math.1+\frac{\left(T.Math..Math.11-T.Math..Math.21\right).Math.R.Math..Math.0}{R.Math..Math.1}& \left(3\right)\end{array}$

where T11 is the initial inner thermistor temperature value and T21 is the initial outer thermistor temperature value. As explained above, R1 is a known quantity, and T11, T21 are measured values. The remaining unknowns are the skin thermal resistivity R0 and the core body temperature T0.

[0038] In the second stage, the outer thermistor S2 is heated for a brief duration by turning on the heating means at time t.sub.on to apply a current through this thermistor S2. The heating arrangement can be controlled such that the temperature at the outer thermistor S2 is only slightly raised, i.e. so that it will not be raised above core body temperature level. A new equilibrium temperature TEQ2 is reached after a while, as illustrated in FIG. 3B. The thermistor temperatures are recorded and current is switched off at time t.sub.off. Again, using equation (2) above, the core body temperature can be expressed as

[00004]$\begin{array}{cc}T.Math..Math.0=T.Math..Math.12+\frac{\left(T.Math..Math.12-T.Math..Math.22\right).Math.R.Math..Math.0}{R.Math..Math.1}& \left(4\right)\end{array}$

where T11 is the second inner thermistor temperature value; T21 is the second outer thermistor temperature value T21. In this case also, R1 is a known quantity; T11, T21 are measured values, and the remaining unknowns are the skin resistivity R0 and the core body temperature T0. By reversing the thermal flux, T22 will be higher than T12, so that equation (4) describes a core temperature value that is lower than T12.

[0039] The inventive method allows the skin thermal resistivity R0 to be expressed in terms of the measured quantities, by equating equations (3) and (4) above and solving for skin thermal resistivity R0 to give:

[00005]$\begin{array}{cc}R.Math..Math.0=\frac{T.Math..Math.12-T.Math..Math.11}{T.Math..Math.11-T.Math..Math.21+T.Math..Math.22-T.Math..Math.12}.Math.R.Math..Math.1& \left(5\right)\end{array}$

[0040] Instead of assuming that the patient's skin resistivity is the same as an average value, the patient's skin resistivity R0 has been measured to a satisfactory degree of accuracy and is now patient-specific as well as location-specific. This allows equation (2) to be used throughout the temperature monitoring procedure to accurately report the patient's core body temperature. In this way, the core body temperature of the patient can be accurately estimated in an entirely non-invasive manner.

[0041] FIG. 4 shows the inventive single heat flow sensor 1 realized as a wearable device 1, and indicates two of several possible locations, namely on the chest or around the arm. In each case, the sensor pad has been attached to the skin 2 of a patient 3. A visual readout of the patient's core body temperature T0, as computed using the method explained above, can be shown as a temperature profile P.sub.3. The wearable device might include a small display so that the patient or caregiver can observe the core body temperature development. Alternatively, the wearable device can send the information to a remote monitoring station 4 where the temperature profile P.sub.3 may be shown on a display.

[0042] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

[0043] For the sake of clarity, it is to be understood that the use of a or an throughout this application does not exclude a plurality, and comprising does not exclude other steps or elements. The mention of a unit or a module does not preclude the use of more than one unit or module.