WO2010020919A1

WO2010020919A1 - Monitoring of a fluid accumulation in a body of a person

Info

Publication number: WO2010020919A1
Application number: PCT/IB2009/053582
Authority: WO
Inventors: Robert Pinter; Anke Schmeink; Cristina Bescos Del Castillo; Joerg Habetha; Matthew Harris
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2008-08-22
Filing date: 2009-08-13
Publication date: 2010-02-25

Abstract

Fluid accumulation in the body is an indicator for heart failure. The fluid accumulation causes a change in the magnetic permeability. The change in magnetic permeability is determined with an inductance, wherein the body is encircled by a conductive loop (110) that has the inductance. A change in the determined inductance is indicative for a change in fluid accumulation. The conductive loop is coupled to an oscillator (600) which oscillator frequency is dependent on the inductance. A change in the oscillator frequency is indicative for a change in fluid accumulation. When the conductive loop is placed around the chest a change in the average oscillator frequency is indicative for a change in fluid accumulation.

Description

Monitoring of a fluid accumulation in a body of a person

FIELD OF THE INVENTION

The present invention relates generally to the field of cardiovascular disease management and in particular to a method and device for non-invasively monitoring of a fluid accumulation in a body of a person. The invention further relates to a use of a determined change in an inductance of a conductive loop to monitor the fluid accumulation in the body of the person.

BACKGROUND OF THE INVENTION

Heart Failure is a complication of a plurality of heart diseases such as coronary artery disease and heart attack. With heart failure, blood moves through the heart and body at a slower rate and pressure in the heart increases. As a result the heart cannot pump enough oxygen and nutrients to meet the body's need. The kidneys respond by causing the body to retain fluid (water) and sodium. If fluid builds up in the arms, legs, ankles, feet, lungs or other organs, the body becomes congested. Heart Failure is a progressive condition characterized by frequent hospital admissions and ultimately high mortality rates. The prevalence of Heart Failure increases dramatically with age, occurring in 1 to 2 percent of persons aged 50 to 59 and in up to 10 percent of individuals older than the age of 75. With the overall aging population, it is expected that the number of chronic cardiovascular disease patients will increase in the coming years and thus the cost of the disease management will rise.

There are several causes of fluid accumulation that may result in swelling of the body. Generalized swelling of the body resulting from fluid accumulation for example may be caused by heart failure, kidney failure, a liver condition or an infection. Parts of the body may also swell from fluid accumulation in response to injury or disease. Also an allergic reaction may cause swelling. Therefore any type of swelling symptom needs professional diagnosis to determine its cause.

Heart Failure patients tend to accumulate fluid in their limbs. The severity of this fluid unbalance varies with the severity of the heart failure and the signs of low perfusion in some parts of the body. US5788643 discloses an early warning monitoring system for determining changes in the status of patients with chronic congestive heart failure, with the goal of intervening before the onset of acute congestive heart failure. In the process for monitoring patients with chronic congestive heart failure a high frequency current is passed between electrodes applied to two limbs of a patient. The current, voltage and phase angle between the measured current and voltage are measured to enable the calculation of congestive heart failure indicia values.

A disadvantage is that the contact impedance between an electrode and the limbs may change over time, thereby creating a source of inaccuracy in the determination of fluid accumulation.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method of non-invasively monitoring of a fluid accumulation in a body that does not rely on the contact impedance between an electrode and the body. The object is achieved with the method as defined in claim 1. The invention is based on the insight that fluid accumulation does not only change an electrical parameter of the body such as the conductivity but also a magnetic parameter like the magnetic permeability. In the method according to the invention a conductive loop is placed around the body, for example around a leg or an ankle. In the context of the invention the term 'body' means the body as a whole as well as any part of the body, e.g. leg, arm, wrist, ankle, torso, etc. The conductive loop has an inductance that is not dependent on contact impedance with the body, but is dependent on the relative magnetic permeability of the body that is surrounded by the conductive loop. The inductance is further dependent on parameters such as a length of the conductive loop and a predetermined number of windings in the conductive loop. The inductance is also dependent on a cross sectional area A of the conductive loop that surrounds the body. The inductance will change as a result of the fluid accumulation in the body. Therefore a change of the inductance of the conductive loop is determined, the determined change being indicative of the fluid accumulation in the body. As the method is not dependent on a current flow through the body there is no need for an electrode having a contact impedance in order to achieve the object of the invention. An advantage is that the conductive loop will not hinder the movements of the person being monitored or cause discomforts such as may be the case with electrodes that need to be attached to the skin. As the largest amount of fluid is accumulated in the lungs the earliest signs of fluid accumulation may be obtained by measuring a change of magnetic permeability with the conductive loop positioned around the thorax. However due to the breathing of the person the cross sectional area will vary causing a periodical change in the inductance. The fluid accumulation in the thorax is a slow process in comparison with a breathing period ( = 1 / breathing frequency).Therefore in a further embodiment of the invention a time average of the inductance is determined, for example by averaging determined inductance values over 10 breathing cycles. Fluid accumulation in the thorax causes a change in said time averaged inductance. Thus in the further embodiment a change in the average of the determined inductance is indicative of fluid accumulation in the thorax, and by monitoring the average of the determined inductance the fluid accumulation in the thorax of the person is monitored.

In a further embodiment of the method the inductance is determined by determining a parameter of an electrical signal in the conductive loop, wherein the parameter is chosen from a group comprising AC voltage, AC current, frequency. The electrical signal has a value that is a function of time, for example V_A = sin(2πfo.t) or I_A = (2πfo.t), wherein V_A and I_A are the amplitudes of AC voltage and AC current, respectively, and fo is the frequency of the signal. The value of the impedance of the conductive loop is given by X_L = 2πfo-L(θ), wherein L(θ) is the inductance of the conductive loop which is dependent on the fluid accumulation θ in the body of the person. For example the conductive loop may be coupled to an AC current source lA(fo, t) having a current with a predetermined amplitude I_A and predetermined frequency fo resulting in an electrical signal in the conductive loop across the terminals of the conductive loop of VL (θ, fo, t) = I_A(fo, t) -2π-fo-L(θ). The AC voltage VL (θ, fo, t) therefore relates to the inductance of the conductive loop, and a change in the AC voltage is indicative for fluid accumulation in the person. When the conductive loop is placed around the thorax of the person the value of the inductance L(θ, fβ,t) becomes dependent on the breathing of the person. The value of the impedance of the conductive loop is given by X_L = 2π-fo -L(θ, fβ, t). For example the conductive loop may be coupled to an AC current source lA(fo, t) resulting in an electrical signal in the loop V_L(Θ, fo, fβ, t) = I_A(fo, t) -2π-fo -L(θ, fβ, t). The changes in the inductance L(θ, fβ, t) of the conductive loop due to fluid accumulation θ are slow when compared to a period of breathing ( 1/ frequency fβ , which is for example 10 breaths per minute), whereas the breathing frequency fβ may be low in comparison with frequency fo of the current which for example is chosen in the range 20OkHz - 90OkHz. Due to the breathing of the person the amplitude of V_L(Θ, fo, fβ, t) is modulated with the breathing frequency fβ. The average value of the inductance will slowly change as a result of the fluid accumulation in the body. As a result also the average of the AC voltage V_L(Θ, fo, fβ, t) will slowly change. The average of the AC voltage V_L (θ, fo, t) is therefore indicative for fluid accumulation in the body of the person.

It should be noted that the change of the average of the AC voltage V_L (θ, fo, t) occurs very slowly, according to the slow process of fluid accumulation, i.e. the change may become visible only if the results of consecutive measurements are compared to each other, each one of the consecutive measurements being conducted once per day, for example. The conductive loop also may be coupled to an oscillator having an oscillation frequency fosc being dependent on the inductance of the conductive loop, the oscillator causing a signal in the conductive loop having a frequency fosc Due to the fluid accumulation θ in the person the frequency fosc will change. The inductance is obtained by determining the parameter 'frequency' of fosc When the conductive loop is placed around the thorax the breathing of the person causes the value of fosc to vary periodically with the breathing frequency fβ, and due to the fluid accumulation θ there is also a superimposed slow change in fosc The average of the parameter 'frequency' of fosc therefore relates to the average of the inductance and is consequently indicative of fluid accumulated in the body. It should be noted that the change of the average of the frequency fo occurs very slowly, according to the slow process of fluid accumulation, i.e. the change may become visible only if the results of consecutive measurements are compared to each other, each one of the consecutive measurements being conducted once per day, for example.

In a further embodiment a change of the inductance (or a parameter of an electrical signal in the conductive loop relating to the inductance) is determined by comparing the determined inductance (or said parameter) with a baseline and providing an alarm when the change is larger than a predetermined threshold. This provides the advantage that the person is warned that a medicine should be taken or a visit to a doctor is required. The alarm is especially helpful for persons who forget to take their medicine. The alarm may also be provided to a care giver in case the person is for example suffering from dementia.

The value of the predetermined threshold and baseline may need to be determined and set for every person individually. In a further embodiment a rate of change of the change of inductance is determined and a further alarm is provided when the determined rate of change is larger than a further predetermined threshold. This provides the advantage that the person or doctor is warned earlier when there are indications of rapidly changing fluid accumulation levels which may be an omen for a deterioration of a heart disease or insufficiently working medicine.

In a further embodiment the inductance (or parameter of the electrical signal in the conductive loop relating to the inductance) is determined after a medication administration to the person and the baseline is adjusted dependent on the determined inductance (or parameter). The baseline provides a reference for the determination of the change of the inductance. To set the reference to a right value the inductance (or parameter) is determined after the medicine intake. The reference may be different for every person and is dependent on the effect the medicine has on the fluid accumulation in the person.

In a further embodiment the inductance (or parameter) is determined on a regular basis, for example twice daily, thereby providing sample values of the determined inductance (or parameter) which are stored a data memory for analysis by a doctor. With the stored data the changes in the inductance are determined. This provides the advantage that the doctor acquires insight in the trend of the accumulation of fluid and the effect of medicine on the accumulation of fluid. With the acquired insight a doctor may adjust the medication or the quantity and/or frequency a medicine should be taken.

In a further embodiment a new value for the predetermined threshold and/or the further predetermined threshold and/or the baseline may be received and the threshold and/or the further threshold and/or the baseline may be adjusted in response thereto. For example based on the acquired insight the doctor may want to change the medication as well as the value of the predetermined threshold for the alarm.

The invention further relates to a device for non-invasively monitoring of a fluid accumulation in a body of a person, the device being characterized in comprising a conductive loop that is arranged to be positioned around the body, the conductive loop having an inductance, the device further comprising determining means coupled to the conductive loop arranged to determine a change of the inductance, the change of the inductance being indicative of the fluid accumulated in the body. Due to fluid accumulation in the body the magnetic permeability of the body that is encircled by the conductive loop changes causing a change in the inductance of the conductive loop. In an embodiment of the device the conductive loop is incorporated in a flexible belt that the person once a day positions around his chest, enabling the device to determine a momentary value of the fluid accumulation in the body of the person by averaging the measurements over a number of breathing cycles, as described above. After the measurement has been completed the person can remove the belt. The device in this embodiment allows a daily measurement. In a further embodiment, the belt described above is applied to an ankle of the person having the advantage that no averaging of the measurements over a number of breathing cycles may be needed. In a further embodiment the conductive loop is integrated in a garment, for example by weaving of a conductive fiber in the fabric of a vest or a sock providing the advantage that it allows a continuous monitoring of the person. The invention further relates to a garment comprising the device. Examples of the garment are a belt, vest or sock as described above.

In a further embodiment the determining means of the device are arranged to determine whether an increase or decrease in the fluid accumulation has taken place. This may for example be realized by comparing a determined inductance with a baseline to obtain the change in the inductance. When the change is larger than a predetermined threshold the determining means will raise an alarm. The baseline may have a value that is set by the doctor and that is regarded as a safe or preferred value for the person. In a further embodiment the baseline is set to a previously determined value of the inductance. For example this previously determined value may be the value measured 24 hours ago. The alarm may be provided to the person by sound producing means such as a speaker, visual means such as a LED or a display on the device, or by tactile means such as vibration generator. The alarm may act as a reminder for the person to take a prescribed medicine. The alarm may also be provided wireless to a doctor or care giver of a person that is for example mentally impaired. In a further embodiment of the device the determining means are arranged to receive a signal indicating a medication administration to the person. The device may have receiving means such as a number of buttons enabling the doctor or the person to change settings of the device. For example with the buttons the value for the baseline, predetermined threshold and further predetermined threshold may be set or a button may provide the signal indicating a medication administration to the person. In a further embodiment of the device the receiving means operate wireless.

In a further embodiment of the device the determining means are further arranged to determine a rate of change of the determined change of the inductance. For example by determining periodical samples of the inductance (for example once per hour or once per day) and calculating a second derivative of the determined inductance as a function of time the rate of change in the change of the inductance may be obtained.

In a further embodiment of the device the determining means are further arranged to adapt the sample frequency (of determining a sample of the inductance) in dependence on the difference between successive determined samples of the inductance. In a situation with e.g. daily measurements, if there is a change of the inductance detected from one day to the next which for this person is unusually large, then the frequency of taking measurements may also be increased, e.g. to an hourly measurement, in order to realize a closer monitoring. The determining means are further arranged to provide a further alarm when the determined rate of change is larger or smaller than a further predetermined threshold. This is advantageous when for example the medicine is not effective in fast reducing the fluid accumulation. In a person who is in a critical condition the rate of change may be smaller than a threshold value, indicating to the doctor that the improvement comes slower than usually. In this case, changing the medication of the person may be necessary. A further advantage of this embodiment is that an early detection of rapid fluid accumulation is provided which enables a change in the treatment preventing a dangerous condition of the person and thereby preventing hospitalization.

There are several methods to determine the momentary value of the inductance. A method is to couple a known AC voltage to the inductance and to determine the resulting AC current which is inversely proportional to the inductance. A further method is to force a known AC current through the inductance and to determine the resulting AC voltage which is proportional to the inductance. In yet a further method the inductance is used as a timing element, preferably for a known oscillator, and the frequency which is proportional or inversely proportional to the inductance is determined. For example, in a further embodiment the determining means of the device comprise an AC voltage source V_L (fo, t) coupled to the conductive loop and causing an AC current I_A(Θ, fo, t) to flow in the conductive loop. Said AC voltage source may provide a voltage with a predetermined frequency fo and amplitude. The determining means are further arranged to determine the AC current. Said determined AC current relates to the inductance according to the equation I_A(Θ, fo, t) = V_L (fo, t) / (2π-fo-L(θ)), and is indicative for the fluid accumulation in the body of the person.

A further embodiment of the determining means comprises an AC current source lA(fo, t) having a predetermined frequency fo and amplitude, wherein the AC current source is coupled to the conductive loop and causes an AC voltage V_L (θ, fo, t) across the terminals of the conductive loop, according to the equation V_L (θ, fo, t) = I_A(Θ, fo, t) •2π-fo-L(θ). The determining means are further arranged to determine the AC voltage wherein the AC voltage relates to the inductance and is indicative for the fluid accumulation in the body of the person.

In yet another example, a yet further embodiment of the determining means comprises an oscillator coupled to the conductive loop wherein a frequency f_osc of the oscillator is dependent on the inductance, for example the frequency may be inversely proportional to the inductance, e.g. f_osc(θ) ~ 1/ L(θ) or f_osc(θ) ~ 1/ VL(Θ). The determining means are further arranged to determine the frequency f_osc, the determined frequency being indicative for the fluid accumulation in the body of the person.

In a further embodiment of the device the determining means comprise filter means. When the fluid accumulation is monitored with the conductive loop positioned around the thorax of the person the cross-sectional area of the conductive loop will change with the breathing frequency of the person. With the filter means a contribution of the breathing to a change in the inductance is separated from a further contribution of fluid accumulation to a further change in the inductance. For example 'high frequency' changes in the inductance due to the respiratory action may be separated from 'low frequency' changes in the inductance due to the accumulation of fluid with the help of a low-pass filter having its cut-off frequency between the 'high frequency' and 'low frequency'. A low-pass filter may for example be realized with time averaging.

For example, the further embodiment of the device wherein the determining means comprise the AC current source I_A(I_O, t) having a predetermined frequency fo and amplitude, the AC current source being coupled to the conductive loop and causing an AC voltage V_L (θ, fo, fβ, t) across the terminals of the conductive loop. The filter means may filter the caused AC voltage V_L (θ, fo, fβ, t) to obtain an output voltage in which the frequency components of fo and fβ have been attenuated and which relates to the inductance and is indicative for the fluid accumulation in the body of the person.

In a further embodiment of the device the determining means comprise an oscillator coupled to the conductive loop wherein a frequency f_osc of the oscillator is dependent on the inductance, for example the frequency may be inverse proportional to the inductance: f_osc (θ) ~ 1/ VL(Θ), such as has been discussed above. Due to the breathing of the person the oscillator frequency will vary with the breathing frequency fβ. By measuring the frequency, the inductance is obtained according to the relation L(θ, fβ, t) ~ 1/Vf_osc (θ, fβ , t). The contribution of the breathing to the change in the inductance is suppressed by time averaging of the measured inductance L(θ, fβ, t), or by determining the average oscillator frequency fosc,Av(θ), the inductance L(θ) being inversely proportional to the determined average oscillator frequency. The time averaging may for example be performed over 10 breathing cycles of the person, or in another example by averaging over 1 minute.

In a further embodiment the device further comprises transmitting means to send data relating to the determined inductance preferably wireless to a doctor. For example, the doctor may obtain insight in the condition of the person by receiving the determined inductance which is indicative of the fluid accumulation in the body of the person. From the received data the doctor can extract the time between the intakes of medicine as well as the effect of the medicine on the accumulated fluid in the body. This provides the advantage that the doctor can monitor the fluid accumulation in the body of the person without the need to actually see the person. This is also more convenient for the person as less frequent visits to the doctor are required.

The invention further relates to the use of a change of an inductance of a conductive loop to determine fluid accumulation in a body of a person having the conductive loop placed around its body.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Fig. 1 illustrates a conductive loop;

Fig. 2 illustrates the conductive loop positioned around a thorax of a person; Fig. 3 shows a graph of the inductance of the conductive loop, as it varies over time due to the breathing of the person wearing the conductive loop around the thorax;

Fig. 4 shows a graph of a parameter relating to the average inductance, as it changes from measurement to measurement;

Fig. 5 illustrates a method of monitoring and treatment of a heart failure person; Fig. 6 shows a device for non-invasively monitoring of fluid accumulation in a body of a person.

DETAILED DESCRIPTION OF THE EMBODIMENTS Fig. 1 illustrates a conductive loop that is incorporated in a flexible band or belt 100. In the shown example the conductive loop has one winding, but the conductive loop may comprise a plurality of windings. The loop is made of conductive material such as for example metal fiber that is incorporated in the fabric of the flexible belt, for example by weaving. The conductive loop has two terminals 120 for coupling to an AC source, such as for example an AC current source or an AC voltage source. The inductance L of the conductive loop is given by

N L = μ₀ • μ_r • / • I — I - A Equation 1

wherein 1 is the length of the metal fiber 110 forming the outline of A, N is the number of windings (in the example of Fig. 1 N=I here), μo is the permeability of the vacuum, μ_r is the relative permeability of the material inside the conductive loop and A is the cross-sectional area of the conductive loop.

Fig. 2 shows a person 200 with the band positioned around its thorax. If the person inhales the cross sectional area A, which is the area encircled by the conductive loop, and the length of the metal fiber 110 increase, resulting in an increase of the inductance L. If the person exhales, the cross sectional area A and the length of the metal fiber 110 decrease resulting in a decrease of the inductance L. The thorax of the person has a relative magnetic permeability that is dependent on the fluid accumulation in the thorax.

(N λ² L = μ₀ ^■ μ_r(θ) • / ^■ j ^■ A(f_B,t) Equation 2

wherein θ represents the fluid accumulation, fβ the breathing frequency, and t the time. As an example to show the influence of fluid accumulation on the relative magnetic permeability, table 1 shows a change of the relative permeability of a volume of 1 liter air when said volume is gradually filled with a saline solution (100 gram NaCl dissolved in 0.75 liter H₂O).

Table 1

As equation 2 shows, the inductance L will periodically increase and decrease dependent on the breathing frequency fβ of the person whereas a scaling factor μ_r(θ) of the inductance changes as fluid accumulation θ in the person takes place. It is noted that the contribution of changes in the relative magnetic permeability to changes in the inductance L are small in comparison to changes in the inductance caused by breathing. Yet the change in the inductance L caused by fluid accumulation can be distinguished from the changes caused by breathing using a filter that separates the relatively fast changes caused by breathing from the relatively slow change caused by fluid accumulation. Data on the inductance as a function of time t can for example be obtained by coupling of an AC current source with predetermined amplitude and frequency to the current loop and measuring the voltage across said current loop. For example by using a mathematical operation of low-pass filtering (averaging) the 'slow' change in inductance L caused by fluid accumulation can be extracted from data on the inductance value L as a function of time t as given by equation 2. Or, for example by using a low-pass filter (averaging) coupled to the terminals of the conductive loop the 'slow' change in the AC voltage is obtained at an output of the low-pass filter, the 'slow' change being dependent on a change of the inductance L of the conductive loop caused by the fluid accumulation. In a further embodiment data on the inductance as a function of time t can for example be obtained by coupling an oscillator to the current loop, the operating frequency of the oscillator being for example inversely proportional to the value of the inductance of the loop, and measuring the frequency of the voltage across the terminals of the loop. Table 2 shows the influence of fluid accumulation on the operating frequency f_osc of an oscillator that is coupled to a conductive loop when a volume of 1 liter air enclosed by the conductive loop is gradually filled with a saline solution (100 gram NaCl dissolved in 0.75 liter H₂O).

Table 2

The physical explanation of the frequency variation caused by the fluid accumulation is the following. The oscillator forces an AC current to flow in the conductive loop which results in a time dependent primary magnetic field in the saline solution. This primary magnetic field induces in the saline solution eddy currents that produce a secondary magnetic field which is oriented such that it weakens the current loops primary field. This effect is expressed with the help of the relative magnetic permeability μ_r. The decrease of μ_r causes a decrease in the inductance L and an increase in the frequency f_osc of the oscillator. Fig. 3 shows two graphs of a frequency f_osc 310 (Y-axis) of an oscillator as a function of time 300 (X-axis) wherein said oscillator is coupled to the conductive loop 100 that is positioned around the thorax of the person 200 of Fig. 2. The frequency 340 of the oscillator is inversely proportional to the value of the inductance L of the conductive loop. Examples of such an oscillator are a Colpitts oscillator or a Hartley oscillator. Due to the breathing of the person the inductance changes periodically with a period T_B 360 which is the inverse of the breathing frequency fβ. The frequency of the oscillator varies with a respiratory action of the person around an average frequency fo,Av 320, 330 whereas an amplitude 370 of the variation of the frequency is proportional to the breathing depth of the person. If the person inhales 350, the cross sectional area A of the conductive loop increases, causing an increase in the inductance L which causes a decrease of the frequency of the oscillator. If the person exhales 340 said cross sectional area A decreases causing a decrease in the inductance L and an increase in the frequency of the oscillator. The average oscillator frequency f₀ 320, 330 of the two graphs shown in Fig. 3 is different. The average frequency fo, _AV 330 of the oscillator shown in the upper graph is higher than the average frequency 320 shown in the lower graph due to the presence of more electrically conductive fluid in the thorax of the person 200, caused by fluid accumulation. Thus the increase of the average oscillator frequency fo, _AV 330 is indicative of fluid accumulation in the thorax. Fig. 4 shows a trend of measurements 410 of the average oscillator frequency fo, AV 430 (Y-axis) as a function of time 420 (X-axis). Each point in the trend may for example correspond to a daily measurement, the trend 410 being indicative of changes in the fluid accumulation in the thorax of the person. If the average oscillator frequency observed during a daily measurement exceeds a predetermined threshold 400 the medication of the person should be adjusted. The value of the predetermined threshold 400 can be determined individually for every person based on measurement data that was taken during a number of days after a medication was adjusted by a doctor, for example the first seven days.

Fig. 5 illustrates a method of monitoring of a fluid accumulation in a person. In such a method a device for non-invasively monitoring of a fluid accumulation in a thorax of a person as defined in claim 7 is used. In an embodiment the conductive loop may be incorporated in a garment that is being worn by the person whereas additional circuitry that is detachably coupled to the conductive loop is for example included in a pocket of the garment. This embodiment allows for a continuous monitoring of the fluid accumulation in the person. In a further embodiment the device is incorporated in a flexible belt that, for example, each morning is being used by the person for a daily measurement after which the belt is taken off. The device may for example comprise an oscillator coupled to the conductive loop, the frequency of the oscillator being inversely proportional to the inductance of the conductive loop. Based on his knowledge of the heart disease and the medical history of the person, a doctor adjusts the medication 500. During a number of days the average oscillator frequency fo, AV is determined 510. The determined values of the average oscillator frequency fo, _AV are used to calculate 520 a value for a baseline and for the predetermined threshold. After the value of the baseline and the predetermined threshold have been calculated the average oscillator frequency on a next day following the number of days is determined 530 and compared with the predetermined threshold 540. When the predetermined threshold is exceeded 560 the person may need to see the doctor again for an adjustment of the medication 500. When the determined average oscillator frequency is below the predetermined threshold the monitoring of the person continues 550 and next determined values will be compared with the predetermined threshold. Fig. 6 shows a device for non-invasively monitoring of a fluid accumulation in a body of a person. The device comprises a flexible belt 100 having an incorporated conductive loop 110. The belt may be positioned around the chest of a person in a way as shown in Fig. 2. The belt follows the contours of the chest such that the area enclosed by the conductive loop expands when the person inhales and shrinks when the person exhales. As a consequence the inductance of the conductive loop will change in dependence of the breathing frequency fβ of the person. The device further comprises determining means 610 that are coupled to the conductive loop. The determining means may be located in a pocket on the belt and detachably coupled to the terminals 120 of the conductive loop. The determining means comprise an oscillator 600 coupled to the terminals 120 of the conductive loop. The frequency f_osc of the oscillator is dependent on the inductance. The frequency of the oscillator is measured by a frequency to voltage (f-to-V) converter 620. An output of the f-to- V converter will provide a voltage that varies with the breathing frequency fβ and is further dependent on the fluid accumulation in the thorax enclosed by the conductive loop 110. The output of the f-to-V converter is coupled to a low-pass filter 630. A filter output will provide a filter output voltage 635 that is dependent on the fluid accumulation in the thorax enclosed by the conductive loop 110. The baseline 670 is subtracted 640 from the filter output voltage 635 to obtain a change of the inductance which by means of a comparison 650 with a predetermined threshold 680 may result in an alarm. The subtraction 640 may for example be realized with a further comparator providing an output voltage 645 that is compared with the predetermined threshold 680 when the filter output voltage 635 surpasses the baseline 670. The alarm may be wireless transmitted using a transmitter 660 using for example AM or FM modulation. In a further embodiment standardized wireless links such as for example Bluetooth or Zigbee are used for transmission of the alarm. As will be understood by the skilled person the determining means may be built using digital electronics such as a processor, a memory, an AD converter, etc. For example, the output of the f-to-V converter may be coupled to an Analogue to Digital (AD) converter enabling the filter function to be implemented using digital filters. The values for the determined inductance may be stored in a memory, the change in the inductance being calculated by the processor. Also values for the base line and predetermined threshold may be stored in memory.

The invention is summarized as follows. Fluid accumulation in the body of a mammal is an indicator for heart failure. The fluid accumulation causes a change in the magnetic permeability. The invention relates to the use of an inductance and a change of the inductance to monitor changes in the magnetic permeability of a body to monitor a fluid accumulation in said body. In the method and device according to the invention the change in magnetic permeability is determined with an inductance, wherein the body is encircled by a conductive loop 110 that has said inductance. A change in a determined inductance is indicative for a change in fluid accumulation. In a preferred embodiment of the method and device the conductive loop is coupled to an oscillator 600, the oscillator frequency of which is dependent on the inductance. A change in the oscillator frequency fosc is indicative for a change in the fluid accumulation. When the conductive loop is placed around the chest, the short-term changes of the oscillator frequency due to respiratory chest movement have to be averaged away. So in this case, a change in the average oscillator frequency fosc, _AVE is indicative for a change in the fluid accumulation.

Claims

CLAIMS:

1. A method of non-invasively monitoring of a fluid accumulation in a body of a person (200), the method being characterized in comprising placing a conductive loop (110) around the body, the conductive loop having an inductance; - determining a change of the inductance, the determined change of the inductance being indicative of the fluid accumulation in the body.

2. A method according to claim 1 wherein the change of the inductance is determined by determining the inductance and subtracting a baseline from the determined inductance, the method further comprising providing an alarm when the determined change is larger than a predetermined threshold.

3. A method according to claim 2 wherein the inductance is determined after a medication administration to the person; - the baseline is adjusted dependent on the determined inductance.

4. A method according to claim 1, 2 or 3 further comprising determining a rate of change of the determined change of the inductance; providing a further alarm when the determined rate of change is larger than a further predetermined threshold.

5. A method according to any one of claims 2-4 wherein the inductance is determined by determining a parameter of an electrical signal in the conductive loop, the parameter being chosen from a group comprising voltage value, current value, frequency value.

6. A method according to any one of claims 1-5 wherein the determined inductance is a time averaged inductance, the time averaging being preferably over n t breathing cycles of the person, n being an integer larger than 0.

7. A device for non-invasively monitoring of a fluid accumulation in a body of a person (200), the device being characterized in comprising a conductive loop (110) arranged to be positioned around the body, the conductive loop having an inductance, the device further comprising determining means (610) coupled to the conductive loop arranged to determine a change of the inductance, the determined change of the inductance being indicative of the fluid accumulation in the body.

8. A device according to claim 7 wherein the determining means are arranged to determine the inductance and determine the change (645) of the inductance in dependence of a subtraction (640) of a baseline (670) from the determined inductance, the determining means being further arranged to provide an alarm in dependence of a comparison (650) of the determined change of the inductance with a predetermined threshold (680).

9. A device according to claim 8 wherein the determining means are arranged to receive a signal indicating a medication administration to the person, the device being further arranged to determine the inductance in response to the received signal, and to adjust the baseline (670) in dependence of said determined inductance.

10. A device according to claim 7, 8 or 9 wherein the determining means are further arranged to determine a rate of change of the determined change of the inductance, the determining means being further arranged to provide a further alarm in dependence of a comparison of the determined rate of change with a further predetermined threshold.

11. A device according to any one of claims 8-10 wherein the determining means comprise an AC voltage source coupled to the conductive loop, the AC voltage source being arranged to cause an AC current in the conductive loop, the determining means being further arranged to determine the AC current, the determined inductance being related to the determined AC current.

12. A device according to any one of claims 8-10 wherein the determining means comprise an AC current source coupled to the conductive loop, the AC current source being arranged to cause an AC voltage across the terminals of the conductive loop, the determining means being further arranged to determine the AC voltage, the determined inductance being related to the determined AC voltage.

13. A device according to any one of claims 8-10 wherein the determining means comprise an oscillator (600) coupled to the conductive loop, a frequency of the oscillator being dependent on the inductance, the determining means being further arranged to determine the frequency of the oscillator, the determined inductance being related to the determined frequency.

14. A device according to claim 11 wherein the determining means further comprise a filter arranged to provide a time average of the determined AC current, the determined inductance being dependent on the provided time average of the determined AC current.

15. A device according to claim 12 wherein the determining means further comprise a filter arranged to provide a time average of the determined AC voltage, the determined inductance being dependent on the provided time average of the determined AC voltage.

16. A device according to claim 13 wherein the determining means are further arranged to provide an average of the determined frequency, the determined inductance being dependent on the provided average of the determined frequency.

17. A garment comprising the device according to anyone of claims 7 to 16.

18. Use of a determined change in an inductance of a conductive loop to monitor a fluid accumulation in a body of a person having the conductive loop placed around its body.

19. Use according to claim 18 wherein the determined change in the inductance is obtained by determining a time average of the inductance, and subtracting a baseline from the time average.