WO2009031119A1

WO2009031119A1 - Monitoring the degree of hydration of the human body

Info

Publication number: WO2009031119A1
Application number: PCT/IB2008/053589
Authority: WO
Inventors: Yan Shi; Ladislav Jankovic
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-09-04
Filing date: 2008-09-04
Publication date: 2009-03-12

Abstract

The invention relates to the field of monitoring the degree of hydration of the human body based on measuring the time for refilling an occluded blood vessel with blood, especially by measuring vein refilling time (VRT) and/or capillary refilling time (CRT). According to the invention, a method is proposed in which the blood vessel is occluded in order to remove the blood from the blood vessel; occlusion of the blood vessel is released in order to allow the blood to refill the blood vessel; tissue comprising the blood vessel with pulse light during refill of the blood vessel is irradiated; an acoustical signal which is photoacoustically generated by absorption of each light pulse is detected; the acoustical signal is analyzed with respect to its amplitude; and the time necessary for regaining a predefined amplitude of the acoustical signal after release of occlusion is measured. With this method, specific, accurate and easy monitoring of the degree of hydration of the human body is possible.

Description

Monitoring the degree of hydration of the human body

FIELD OF THE INVENTION

The invention relates to the field of monitoring the degree of hydration of the human body based on measuring the time for refilling an occluded blood vessel with blood, especially by measuring vein refilling time (VRT) and/or capillary refilling time (CRT).

BACKGROUND OF THE INVENTION

Water accounts for up to 70 % of the mass of the human body and is essential for maintaining normal physiological functions. It is well known that dehydration, i. e. water loss, impares normal physiology. Physiological responses caused by dehydration depend on the amount of water loss. For example, water loss corresponding to 2 % decrease in body weight leads to reduced exercise performance and alertness. Further dehydration can lead to serious consequences, such as tissue damage, heart attack, stroke and even death.

Dehydration is encountered in a wide range of population from newborns to the elderly. Further, dehydration is particularly common under certain environmental and clinical conditions such as hot weather, vomiting, and diarrhea. There has been a steady increase in the number of hospitalizations due to dehydration in the past decade with substantial mortality occurred especially for elderly persons and children, respectively. Early detection of dehydration followed by proper care can reduce these numbers significantly because of the preventable and reversable nature of dehydration. Currently, dehydration detection relies on laboratory tests or appearance of certain symptoms, e. g. thirst or a dry mouth. However, symptomatic dehydration detection is inaccurate, subjective and unspecific and, thus, does not allow for dehydration quantification, i. e. determination of the degree of hydration/dehydration of the human body. Further, symptomatic dehydration detection usually detects a relatively late stage of dehydration. Furthermore, laboratory tests performed by medical professionals are invasive and cost- and time-consuming.

One simple dehydration test is the visual observation of vein refilling after occluding the vein with a finger. Observation of the speed and degree of vein refilling reflects the volume of fluid since veins contain the greatest amount of extracellular water in the body. This means that veins serve as a reservoir for body fluid, and therefore, their condition reflects the total body water content. Visual observation of vein refilling after occluding with a finger is typically performed for the elderly as well as for infants. In the case of the elderly, generally, small veins of the foot are used, while in the case of children, often, small capillaries below the fingernails are used. Visual observation of vein refilling, however, does not allow for accurate and in-time dehydration detection.

Using the visual observation method described above, dehydration can be classified in four categories: (i) blood returns instantly, i. e. vein is filled, (ii) direction of blood flow is easily observed, i. e. vein is still filled, (iii) blood returns slowly, i. e. vein requires about 3 s to refill, and (iv) vein remains collapsed after release of occlusion.

In US 2004/0249290 Al, a diagnostic medical instrument is described that is used in a capillary refilling time test procedure in which a skin area which overlies blood- filled capillaries, which normally display a pink color, is depressed to expel blood from the capillaries. When the pressure is released, blood is permitted to flow back into the capillaries, and, thus, the skin looses its white colour and regains its pink colour again. The instrument includes a colour sensor trained on the skin area and responsive to light reflected therefrom. By measuring the time necessary for the skin to regain its pink colour, capillary refilling time is measured in order to determine the degree of hydration of the patient. However, measurements with this instrument are not reliable and secure, especially when used for people with a darker skin type.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method and a device for monitoring the degree of hydration of the human body based on measuring the time for refilling an occluded blood vessel with blood, the method and device being specific, accurate and easy to use.

According to the invention, this object is addressed by a method for monitoring the degree of hydration of the human body based on measuring the time for refilling an occluded blood vessel with blood, the method comprising the following steps: occluding the blood vessel, in order to remove the blood from the blood vessel; releasing occlusion of the blood vessel in order to allow the blood to refill the blood vessel; irradiating tissue comprising the blood vessel with pulsed light during refill of the blood vessel; detecting an acoustical signal which is photoacoustically generated by absorption of each light pulse; analyzing the acoustical signal with respect to its amplitude; and measuring the time necessary for regaining a predefined amplitude of the acoustical signal after release of occlusion.

Accordingly, it is an important idea of the invention, to use photoacoustic techniques in order to measure the necessary refilling time of a blood vessel after release of occlusion, in order to estimate the degree of hydration. Photoacoustic measurements of vein refilling time or capillary refilling time take advantage of the fact that blood strongly absorbs visible and near-infrared (NIR) light. When a short light pulse is shone on human tissue, e. g. a fingertip, the light is absorbed by blood in capillaries in the subsurface portion of the fingertip, thus, creating photoacoustically generated acoustic waves. These waves are detectable, e. g. using an acoustical detector. Depending on the amount of blood present in the capillaries, the detected acoustical signals vary significantly in amplitude, with a larger signal corresponding to a greater amount of blood in the capillaries. This process can be monitored with great temporal accuracy and, as a result, vein refilling time or capillary refilling time can be determined quantitatively by analyzing the intensity of the acoustical signal and, thus, the degree of hydration of the human body can be determined and indicated. It should be noted that the term "amplitude" may be used for any measure or quantity of the acoustical signal. Especially, the amplitude can be the peak height of the detected acoustical signal, the signal intensity, or an integral of amplitude or intensity over a predefined range. Generally, the light pulse may comprise different wavelengths. However, according to a preferred embodiment of the invention, the light pulse comprises light of a predefined wavelength. Further, it is preferred that this wavelength relates to an absorption wavelength of blood. As stated above, this way, a great variation of the amplitude of the generated acoustical signal in dependence of the amount of blood present is generated. Accordingly, the use of a predefined wavelength for the light pulse which relates to an absorption wavelength of blood allows for a better S/N-ratio. With respect to this, it is further preferred to irradiate light in the visible range at or around 550 nm and/or NIR (near- infrared) light with a wavelength equal to or below 800 nm. These wavelengths are preferred since strong absorption of blood is in the visible part of the spectrum around 550 nm and in the NIR spectrum beginning at 800 nm. In general, the length of the light pulses can vary in a wide range. However, according to a preferred embodiment of the invention, the length of the light pulses is in the range from 1 - 500 ns. It has been found that such pulse durations provide for an accurate estimation of a temporal change of refill of the blood vessel. Further, energy per pulse may vary in a wide range, too. However, according to a preferred embodiment of the invention, energy per pulse is equal to or below 10 mJ/cm². It has been observed that such pulse energies provide for sufficient acoustical signals without doing any harm to the human skin.

For the invention, different types of ultrasound transducers can be used. In general, the ultrasound transducer frequency can be from 0.2 to 20 MHz. Further, the transducer can be either a single element transducer (focused or unfocused) or a transducer array.

It might be sufficient for monitoring the degree of hydration to irradiate light and to measure the generated acoustical signal while the blood vessel is refilling with blood after release of occlusion without measuring any further parameters. However, according to a preferred embodiment of the invention, at least the ambient temperature is measured in order to provide compensation for results. It has been observed that the measurement of the ambient temperature is advantageous for increasing the measuring accuracy, since vein refilling time and capillary refilling time are strongly affected by ambient temperature. Though it is preferred to provide a warm environment, this might not always be possible and, thus, the additional measurement of the ambient temperature can be used for standardization of the measured refilling times.

In general, there are different ways to determine the predefined amplitude of the acoustical signal after release of occlusion which defines the end of the measured refilling time. However, according to a preferred embodiment of the invention the predefined amplitude of the acoustical signal after release of occlusion is determined before occlusion as an absolute value or is determined as a constant value when no further temporal changes of the amplitude exceeding a predefined threshold occur. This means that the predefined value can be defined as the absolute amplitude before occlusion. Accordingly, in this case, the end of the refilling time can be determined when the original absolute signal is regained.

However, since in general, the refilling curve is asymptotic the predefined value is defined to be less than the absolute amplitude before occlusion. With respect to this, according to a preferred embodiment of the invention, the predefined value is defined as an absolute value which is calculated to be a predefined percentage of less than 100 % of the absolute value before occlusion. Further, it is especially preferred to define the predefined as a value in the range of 80 - 95 % of the absolute value before occlusion.

However, defining the predefined amplitude as a constant value when practically no further significant changes of the signal occur any more is advantageous, too, especially in cases where the original absolute value might not be regained, e. g. due to a change of posture of the patient. With respect to this, it is especially preferred to define an absolute or relative threshold value for the temporal change. When the temporal change falls under this threshold value the predefined value is considered to be reached and refilling time is estimated. According to the invention, above mentioned object is further addressed by a device for monitoring the degree of hydration of the human body based on measuring the time for refilling an occluded blood vessel with blood, comprising: an occlusion means for occluding the blood vessel, in order to remove the blood from the blood vessel; a light source for radiating tissue comprising the blood vessel with pulse light; an acoustical detector for detecting an acoustical signal which is photoacoustically generated by absorption of each light pulse; an analyzer for analyzing the acoustical signal with respect to its amplitudes; a measuring unit for measuring the time necessary for regaining a predefined amplitude of the acoustical signal after release of occlusion.

Such device is especially meant to be used with a method as described above.

There are multiple different possibilities to provide for an occlusion means which is capable of removing blood from the blood vessel under consideration. However, according to a preferred embodiment of the invention, the occlusion means is formed by a transparent film which can act on the tissue. Especially, a film with high mechanical strength is preferred in order to guarantee sufficient occlusion. Further, it is preferred that such a material of the film is used which is transparent for the used optical waves or at least does not attenuate the irradiated light substantially. It is also preferred that a material is used which has a matched acoustic impedance, especially matched to the acoustic impedance of water with a low acoustic attenuation coefficient. Alternatively, if the film is thin compared to the ultrasound wavelength, matching problems can be omitted, to. An especially suited material for the transparent film comprises polyester. As a result, such a transparent film can be provided as an occlusion means which transmits visible and NIR light nearly undisturbed and which transmits acoustical signals like ultrasonic waves well, too. In general, the light incident on the human tissue and the acoustical signal received from the human tissue can propagate through air between the light source and the acoustical detector, respectively. However, according to a preferred embodiment of the invention, a coupling medium is provided between the irradiated tissue and the light source and/or the acoustical detector, respectively. It has been observed that especially water or oil are well suited as a coupling medium, respectively, especially when held at temperatures near normal body temperature.

As already stated above with respect to a preferred embodiment of the method for monitoring the degree of hydration, the determination of the ambient temperature is useful for increasing measurement accuracy. Accordingly, according to a preferred embodiment of the invention, the device for monitoring the degree of hydration based on a photoacoustical effect further comprises a temperature sensor for measuring the ambient temperature.

Finally, different light sources and acoustical detectors can be used for the device. However, according to a preferred embodiment of the invention, a laser, especially a laser diode, a cluster of laser diodes (diode laser module), or an extended light source (such as light emitting diodes), is used as a light source, and/or an ultrasonic transducer is used as an acoustical detector.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings:

Fig. 1 is a diagrammatic view of a device for monitoring the degree of hydration of the human body based on measuring the time for refilling occluded capillaries of a fingertip with blood according to a preferred embodiment of the invention, and

Fig. 2 shows the received photoacoustical signals for cases with and without blood in the capillaries of the fingertip, respectively.

DETAILED DESCRIPTION OF AN EMBODIMENT

From Fig. 1 the measuring set up for a method and device for monitoring the degree of hydration of the human body based on measuring the capillary refilling time in a fingertip 2 is shown. As a light source 1 a laser is provided which sends out short laser pulses to illuminate the fingertip 2. Due to the absorption of the laser light in the fingertip 2 a photoacoustical signal within surface layers of the tissue is generated. This photoacoustical signal is recorded by an acoustical detector 3 which is formed by an ultrasonic transducer operated in listening mode. The light source 1 and the acoustical detector 3 are synchronized with the help of a master clock 4 to ensure prompt data acquisition. In order to improve coupling between the light source 1 and the fingertip 2 as well as between the fingertip 2 and the acoustical detector 3, coupling medium 5, i. e. warm water, is provided between the fingertip 2 on the one side and the light source 1 and the acoustical detector 3, respectively, on the other side. Preferably, this coupling medium 5 is held at a temperature near normal body temperature. In order to occlude the capillaries in the fingertip 2 under consideration, an occlusion means 6 is provided which is formed by a thin transparent film. This occlusion means 6 can act on the fingertip 2 in such a way that capillaries are occluded and blood is expelled from the fingertip 2. After release of the occlusion means 6 measurement of capillary refilling time in order to determine the degree of hydration can be performed as follows:

With a constant repetition rate in the range of 1 kHz to 10 kHz the light source 1 emits ns-laser pulses at a wavelength of 532 nm at a light intensity of 2 mJ/cm², through the coupling medium 5 and the transparent occlusion means 6, onto the fingertip 2. Photoacoustical signals due to absorption of the laser pulses in the fingertip 2 are detected by the acoustical detector 3 which is coupled to the fingertip via the coupling medium 5 at a center frequency of 1 MHz.

The acoustical signal received by acoustical detector 3 is then transferred to a desktop housing 7 which incorporates an analyzer 8 for analyzing the acoustical signal with respect to its amplitude, and a measuring unit 9 for measuring the time necessary for regaining a predefined amplitude of the acoustical signal after release of occlusion by the occlusion means 6. Further, the desktop housing comprises a display 10 for displaying the degree of hydration estimated by the determined refilling time.

Further, a temperature sensor 11 for measuring the ambient temperature is provided which is coupled to the measuring unit 9. This temperature sensor 11 allows for standardization of the measured capillary refilling time depending on the ambient temperature which strongly influences refilling of capillaries and, thus, with the aid of such an ambient temperature measurement the measuring accuracy for the estimated degree of hydration is strongly increased. From Fig. 2, a photoacoustical signal received for one laser pulse in the case of capillaries filled with blood (full line) and capillaries without blood (doted line) can be seen. Actually, the acoustical signal received after a laser pulse extends over a certain time range since the run time for an acoustical signal which is generated in deeper layers is longer than for such an acoustical signal which is generated in layers which are nearer to the surface. In Fig. 2, instead of the time of detection of the acoustical signal, the estimated depth is depicted where the acoustical signal is generated by absorption of the laser light.

The signal for capillaries filled with blood shows two main peaks located in Fig. 2. In the case of capillaries without blood the main peaks amplitude significantly decreases, in the present case by a factor of approximately 5. This means that by determining the peak height or by determining an integral signal relating to predefined depth range, the amount of blood in the capillaries can be estimated with great accuracy. By consecutively determining the amount of blood in the capillaries after each laser pulse, the capillary refilling time and, thus, the degree of hydration of the human body can be calculated and finally indicated on the display 10 of the desktop housing 7.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A method for monitoring the degree of hydration of the human body based on measuring the time for refilling an occluded blood vessel with blood, the method comprising the following steps: occluding the blood vessel, in order to remove the blood from the blood vessel; releasing occlusion of the blood vessel in order to allow the blood to refill the blood vessel; irradiating tissue comprising the blood vessel with pulsed light during refill of the blood vessel; detecting an acoustical signal which is photoacoustically generated by absorption of each light pulse; analyzing the acoustical signal with respect to its amplitude; and measuring the time necessary for regaining a predefined amplitude of the acoustical signal after release of occlusion.

2. The method according to claim 1, wherein the light pulse comprises light of a predefined wavelength which relates to an absorption wavelength of blood.

3. The method according to claim 2, wherein the irradiated light is visible light at or around 550 nm and/or NIR light with a wavelength equal to or below 800 nm.

4. The method according to any of claims 1 to 3, wherein the length of a pulse is in the range from 1 to 500 ns, the repetition rate of the pulses is equal or higher than 10 Hz, preferably in the range between 1 kHz and 10 kHz, and/or the energy per pulse is equal to or below 10 mJ/cm².

5. The method according to any of claims 1 to 4, wherein the ambient temperature is measured in order to provide compensation for results. PH008690

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6. The method according to any of claims 1 to 5, wherein the predefined amplitude of the acoustical signal after release of occlusion is determined before occlusion as an absolute value, especially as a predefined percentage of less than 100 %, preferably in the range from 80 to 95 %, of the absolute value before occlusion, or is determined as a constant value when no further temporal changes of the amplitude exceeding a predefined threshold occur.

7. A device for monitoring the degree of hydration of the human body based on measuring the time for refilling an occluded blood vessel with blood, comprising: an occlusion means (6) for occluding the blood vessel, in order to remove the blood from the blood vessel; a light source (1) for irradiating tissue comprising the blood vessel with pulsed light; an acoustical detector (3) for detecting an acoustical signal which is photoacoustically generated by absorption of each light pulse; an analyzer (8) for analyzing the acoustical signal with respect to its amplitude; and a measuring unit (9) for measuring the time necessary for regaining a predefined amplitude of the acoustical signal after release of occlusion.

8. The device according to claim 7, wherein the occlusion means (6) is formed by a transparent film which can act on the tissue.

9. The device according to claim 7 to 8, wherein a coupling medium (5), especially water or oil, is provided between the irradiated tissue and the light source (1) and/or the acoustical detector (3), respectively.

10. The device according to any of claims 7 or 9, wherein a temperature sensor (11) is provided for measuring ambient temperature in order to provide compensation for results.