WO2009053920A1

WO2009053920A1 - Monitoring the degree of hydration of the human body

Info

Publication number: WO2009053920A1
Application number: PCT/IB2008/054371
Authority: WO
Inventors: Natallia E. Uzunbajakava; Aleksey Kharin; Markus Laubscher; Runze Wu; Golo Von Basum; Thomas Vollmer; Cristian N. Presura
Original assignee: Koninklijke Philips Electronics N.V.; Philips International Property And Standards Gmbh
Priority date: 2007-10-25
Filing date: 2008-10-23
Publication date: 2009-04-30

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 I is proposed wherein tissue comprising the blood vessel is irradiated with linearly polarized light of at least one predefined wavelength; the part of the reflected light which is orthogonally polarized with respect to the irradiated light is collected as an image; 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; the image is analyzed with respect to the flow of blood cells; and the time necessary for regaining a predefined flow of blood cells 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 a first aspect of the invention, this is 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: irradiating tissue comprising the blood vessel with linearly polarized light of at least one predefined wavelength; collecting the part of the reflected light which is orthogonally polarized with respect to the irradiated light as an image; occluding the blood vessel, in order to remove the blood from the blood vessel; release of occlusion of the blood vessel in order to allow the blood to refill the blood vessel; analyzing the image with respect to the flow of blood cells; measuring the time necessary for regaining a predefined flow of blood cells after release of occlusion.

This means that according to the first aspect of the invention, orthogonal polarization spectral imaging (OPSI) is used in order to monitor blood flow back in the blood vessel after release of occlusion. The OPSI principle is based on the idea of irradiating linearly polarized light on tissue, while only that component of the light propagating back from the tissue is detected which is orthogonally polarized with respect to the incident light. Thus, it is made use of the fact that the light reflected from the surface of the tissue shows the same polarization direction as the incident light, while the light coming back from the deeper tissue layers which contain blood vessels has a different polarization.

Accordingly, OPSI allows to detect blood flow in blood vessels below the surface of the tissue. Compared to the conventional camera detection described above, the method according to the first aspect of the invention allows for an image, i.e. a two- dimensional representation, of blood vessels located below the surface, i.e. inside the tissue, and, thus, for blood flow detection in deeper lying blood vessels. The blood vessels examined by this method can generally be of any type, especially the blood vessels can be veins or capillaries. Accordingly, the time measured can especially be vein refilling time or capillary refilling time, respectively. Further, it should be noted that the order of the steps as explained above does not mean that these steps are carried out consecutively. In contrast, it is preferred that the tissue is irradiated with light and reflected light is collected for analyzing the blood cell flow and, thus, for determining the refilling time, at least during refilling of the blood vessel, i.e. starting with release of occlusion and ending with total refill. Further, though it is preferred that irradiation and collection of light is performed continuously, it is also possible to irradiate and/or collect the light only at discrete times. Concerning the analysis of the image with respect to the flowing blood cells, in general, this can be done continuously, too. However, it is preferred to perform this analysis at a predefined rate, like at an image rate of a video camera used for capturing images of the blood vessels and the blood cells flowing therein.

Generally, it can be sufficient to irradiate linearly polarized light of only one predefined wavelength. However, according to a preferred embodiment of the first aspect of the invention, the irradiated linearly polarized light comprises at least two different predefined wavelengths, from the collected part of the reflected light which is orthogonally polarized with respect to the irradiated light two different images for the two different wavelengths are generated, respectively, the images are subtracted from each other in order to generate a differential image; and the differential image is analyzed with respect to the flow of blood cells. Using two different wavelengths provides for the possibility to use such wavelengths, respectively, which are differently absorbed by blood vessels. Accordingly, an increase of contrast can be achieved and background signals coming from surrounding tissue can eliminated to a large extent.

According to the first aspect of 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: a light source for irradiating tissue comprising the blood vessel with linearly polarized light of at least one predefined wavelength; a light detector for collecting the part of the reflected light which is orthogonally polarized with respect to the irradiated light as an image; an occlusion means for occluding the blood vessel, in order to remove the blood from the blood vessel; an analyzer for analyzing the image with respect to the flow of blood cells; and a measuring unit for measuring the time necessary for regaining a predefined flow of blood cells after release of occlusion.

This device is preferably used with a method according to the first aspect of the invention described above. Since the light detector collects reflected light as an image, a two-dimensional light detector like a camera is used. According to a second aspect of the invention, above mentioned 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: irradiating tissue comprising the blood vessel with light of at least one predefined wavelength; collecting a reflected part of the light in order to generate a magnified image of the blood cells flowing through the blood vessel; occluding the blood vessel, in order to remove the blood from the blood vessel; release of occlusion of the blood vessel in order to allow the blood to refill the blood vessel; analyzing the image with respect to the flow of blood cells; measuring the time necessary for regaining a predefined flow of blood cells after release of occlusion.

Accordingly, the method according to the second aspect of the invention uses a microscopy based method with a magnified image of a blood vessel in order to monitor the blood flow through the blood vessel. This also means that the method according to the second aspect of the invention uses a magnifying imaging system in order to generate a magnified image of the blood cells flowing through the blood vessel. Using this magnified image, the flow rate of the blood cells and, thus, refilling time can be determined.

Both the imaging techniques according to the first and second aspect of the invention, respectively, are used to monitor regaining blood flow in the blood vessel after release of occlusion with the help of a two-dimensional image. Generally, this can be achieved in multiple different ways. However, according to a preferred embodiment of the invention, analyzing the image with respect to the flow of blood cells comprises the detection of the number of blood cells flowing through the blood vessel per time unit and/or the detection of the temporal change of the number of blood cells flowing through the blood vessel per time unit. According to the second aspect of 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: a light source for irradiating the tissue comprising the blood vessel with light of at least one predefined wavelength; a microscope imaging system for collecting a reflected part of the light in order to generate a magnified image of the blood cells flowing through the blood vessel; a light detector for collecting the magnified image of the blood cells flowing through the blood vessel; an occlusion means for occluding the blood vessel, in order to remove the blood from the blood vessel; an analyzer for analyzing the image with respect to the flow of blood cells; a measuring unit for measuring the time necessary for regaining a predefined flow of blood cells after release of occlusion.

This device is preferably used with a method according to the second aspect of the invention described above.

According to a third aspect of the invention, above mentioned 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: irradiating tissue comprising the blood vessel with light of at least one predefined wavelength, the wavelength relating to an absorption band of a main component ofblood; collecting the reflected light or the light which is transmitted through the tissue, respectively; occluding the blood vessel, in order to remove the blood from the blood vessel; release of occlusion of the blood vessel in order to allow the blood to refill the blood vessel; determining the intensity of the reflected light or the transmitted light, respectively; measuring the time necessary for regaining a predefined intensity of the reflected light or the transmitted light, respectively, after release of occlusion. Accordingly, according to the third aspect of the invention, diffuse reflectance or diffuse transmittance is used in order to monitor the regaining flow ofblood cells in the blood vessel under observation by the intensity of the reflected part or the transmitted part of the light, respectively. This means that a one-dimensional signal is acquired.

In general, different wavelengths can be used for such a diffuse reflectance or diffuse transmittance measurement. However, according to a preferred embodiment of the invention, the wavelength of the incident light relates to an absorption band of a main component ofblood. Further, it is preferred that the concentration of the main component of blood used is sufficient to provide for a good S/N-ratio. For that, generally, different main components ofblood can be used. However, according to a preferred embodiment of the third aspect of the invention, haemoglobin is used as main component of blood and, thus, the irradiated wavelength relates to an absorption band of haemoglobin.

According to the third aspect of 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: a light source for irradiating tissue comprising the blood vessel with light of at least one predefined wavelength, the wavelength relating to an absorption band of a main component of blood; a light detector for collecting the reflected light or the light transmitted through the tissue, respectively; an occlusion means for occluding the blood vessel, in order to remove the blood from the blood vessel; an analyzer for determining the intensity of the reflected light or the transmitted light, respectively; and a measuring unit for measuring the time necessary for regaining a predefined intensity of the reflected light or the transmitted light, respectively, after release of occlusion.

This device is preferably used with a method according to the third aspect of the invention described above.

In general, for the methods and devices according to the first, second and third aspect of the invention, respectively, different wavelengths can be used for the incident light. However, for the methods and devices according to the first and second aspect of the invention, respectively, i.e. for OPSI and for the microscopic application, according to a preferred embodiment of the invention, green light, preferably at or around 450 nm, is used. When using a second wavelength in order to enhance contrast of the generated image, according to a preferred embodiment of the invention, red light, preferably at or around 660 nm, is used. When combining green light and red light, the contrast of a differential image is increased since these different wavelengths are differently absorbed by blood and tissue material, respectively. Accordingly, disturbing background signals can be eliminated efficiently. The use of green light, preferably at or around 450 nm, is especially efficient for detecting the blood flow in capillaries which are lying underneath the surface of the skin. If it is desired to monitor the blood flow in deeper lying veins, according to a preferred embodiment of the invention, NIR light (near- infrared light), especially with a wavelength in the range from 760 - 900 nm, is used. It has been observed that NIR light can provide for a greater penetration depth, thus, leading to a better S/N-ratio for deeper lying blood vessels.

In general, for the method and device according to the third aspect of the invention, light with different wavelengths can be used, too. However, for reflectance measurements according to a third aspect of the invention, green light, especially at or around 450 nm, and/or NIR light, especially in the range from 760 - 900 nm, is preferred. For transmission measurements NIR light is especially preferred because of its great penetration depth in human tissue.

In general, the methods and devices according to the invention allow for different methods for determining the degree of hydration of the human body. However, according to a preferred embodiment of the invention, the time necessary for regaining a predefined flow of blood cells after release of occlusion is compared with predefined times in order to determine the degree of hydration of the human body. This means that according to this embodiment of the invention, the measured times for regaining a predefined flow of blood cells in the blood vessel under observation is compared to expected values which relate to different degrees of hydration, e.g. such degrees of hydration as described with respect to the background of the invention above.

With respect to determining a predefined flow of blood cells after release of occlusion, in general, a predefined absolute value of blood flow can be used. However, according to a preferred embodiment of the invention, the predefined flow of blood cells is a constant flow of blood cells. This embodiment makes it easy to reliably detect a total refill of the blood vessel under consideration since absolute values of blood cell flow can vary from person to person and even for one person, e.g. depending on posture and mental stress.

However, alternatively, according to a preferred embodiment of the invention, it is also possible that the predefined flow of blood cells has been defined by analyzing the flow of blood cells before occlusion of the blood vessel as an absolute value. According to this preferred embodiment of the invention it is possible to determine the time for refilling the blood vessel after release of occlusion by simply determining that the original flow of blood cells has been regained. For the devices according to the invention, different types of occlusion means can be used. In general, an occlusion means for use with a device according to the first, second or third aspect of the invention, respectively, can be opaque. According to a preferred embodiment of the invention, however, the occlusion means, at least in part, is transparent for the irradiated and/or reflected light, respectively. This provides for the possibility to radiate incident or reflected light through the occlusion means, e.g. in order to provide for a simple beam structure and, thus, for an easy design of the device.

Further, according to a preferred embodiment of the invention, the occlusion means can also comprise a beam splitter. This eliminates the need for a separate beam splitter in the case that irradiated light and reflected light is guided through the occlusion means in different directions, respectively.

Further, the occlusion means can be provided as an active or a passive unit, respectively.

This means that, according to a preferred embodiment of the invention for an active actuator, the occlusion means comprises an actuator that actively applies a force on the tissue that comprises the blood vessel. For example, a simple actuator according to such a preferred embodiment of the invention can be made of a bloc of solid material which, when pressed into the tissue, removes the blood from the blood vessel.

Further, according to a preferred embodiment of the invention, the actuator is provided in a cuff, which can be fixed around a limb of the human body, e.g. a foot. This makes the application of the measurement of the refilling time of the blood vessel under consideration very easy since such a cuff can be used in a similar way as a cuff for blood pressure measurements. Especially, according to a preferred embodiment of the invention, designs of according devices are possible which are suited for self-monitoring consumer devices intended for home use, thus eliminating the need for specially educated medical staff.

According to an alternative preferred embodiment of the invention for an active actuator, the actuator comprises at least one roller wheel that can be rolled over the tissue comprising the blood vessel. Preferably, this type of actuator comprises two roller wheels which are placed on the skin in the proximity of the blood vessel under consideration. While one wheel stays fixed at the initial position, the other wheel moves along the vein towards the heart, pushes the blood away from the blood vessel, and then stays fixed at another position. At the moment the two wheels are released, the signal of refilling of the blood vessel is taken and the refilling time required is estimated.

Further, another type of occlusion means is the combination of two actuators shifted for some distance from each other. One of the actuators is further away from the heart than the other. The actuator further away occludes first, then, the second actor occludes at a near heart side with some time delay. During this time delay blood can leave the blood vessel. Then, after release of both actuators the measurement is taken in the middle of the actuators. According to an alternative embodiment of the invention, the occlusion means is provided as a passive unit which provides a counterpart for pressing with a part of the human body. For example, according to this embodiment of the invention, the occlusion means comprises a pressing plate which can be pressed with the tip of a finger of the patient. This way, occlusion of a blood vessel under consideration can be achieved without actively providing a pressing force.

In the case of passive occlusion means, but also in the case of active occlusion means, according to a preferred embodiment of the invention, the occlusion means comprises a force sensor for sensing the force applied. Especially in the case of the occlusion means as a passive unit, this way it can be estimated if a sufficient force by pressing with the part of the human body is applied in order to achieve the required occlusion of the blood vessel.

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 vein refilling time monitoring using the OPSI method on a foot vein according to a first embodiment of the invention,

Fig. 2 is a diagrammatic view of capillary refilling time monitoring using the OPSI method on a finger nail according to a second embodiment of the invention,

Fig. 3 is a diagrammatic view of vein/capillary refilling time monitoring using diffuse transmittance on a fingernail according to a third embodiment of the invention,

Fig. 4 is a diagrammatic view of vein/capillary refilling time monitoring using diffuse reflectance on a fingernail according to a forth embodiment of the invention, Fig. 5 is a diagrammatic view of a microscopy imaging based capillary refilling time meter according to a fifth embodiment of the invention,

Fig. 6 is a diagrammatic view of a microscopy imaging based capillary refilling time meter according to a sixth embodiment of the invention, and

Fig. 7 is a diagrammatic view of an imaging device for use with the microscopy imaging devices according to the fifth and sixth embodiment of the invention, respectively.

DETAILED DESCRIPTION OF EMBODIMENTS From Fig. 1 a diagrammatic view of a device according to a first embodiment of the invention can be seen. There, a device for monitoring the degree of hydration of the human body based on measuring the time for refilling an occluded vein 1 of a foot 2 with blood is shown. This device for vein refilling time monitoring uses the OPSI method which means that tissue of the foot 2 comprising the vein 1 is irradiated with linearly polarized light of one predefined wavelength, and the part of the reflected light which is orthogonally polarized with respect to the irradiated light is collected as an image.

For that, the device according to the first embodiment of the invention comprises a cuff 3 which can be fixed around the foot 2 and which comprises a light source 4 for irradiating the tissue with linearly polarized green light of 450 nm. The light emitted from the light source 4 is sent through a beam splitter 5 to an occlusion means 6 which is formed by a transparent actuator. In the occlusion means 6 the light emitted from the light source 4 is directed onto the tissue comprising the vein 1 of the foot 2.

A light detector 7 is provided that collects that part of the reflected light which is orthogonally polarized with respect to the irradiated light as an image. For that, the reflected light is lead through the occlusion means 6 to the beam splitter 5 where the beam of the reflected light is directed onto the light detector 7. For selectively detecting only that part of the reflected light which is orthogonally polarized with respect to the incident light, the beam splitter 5 is a polarizing beam splitter so that only linearly polarized light is directed onto the light detector 7. The light detector 7 is provided as a video camera for receiving an image of the vein 1 under consideration.

According to the first embodiment shown in Fig. 1, the device for monitoring vein refilling time is operated using a rechargeable accumulator 8 as a power supply which is provided within the cuff 3. Image data is sent wirelessly from light detector 7 to an analyzer 9. In the analyzer 9 the image signal from light detector 7 is processed with respect to the flow of blood cells. It is determined how the flow of blood cells per time unit changes in order to determine the necessary time for refilling the vein 1 after release of occlusion by the occlusion means 6. The analyzer 9 is combined with a measuring unit 10 in a desktop housing 11 which also comprises a display 17 for indicating the determined degree of hydration. The analyzer 9 serves for analyzing the image with respect to the flow of blood cells, wherein the measuring unit 10 serves for measuring the time necessary for regaining a predefined flow of blood cells after release of occlusion. The regain in blood flow can be estimated e.g. as difference in contrast in the images obtained before and after occlusion, respectively. Vein refilling time monitoring using the OPSI method with the device according to the first embodiment of the invention shown in Fig. 1 is performed as follows: By pressing the actuator 6 onto the vein 1 , vein 1 is occluded. The tissue containing vein 1 is irradiated with linearly polarized light, and the reflected part of the light which is orthogonally polarized with respect to the incident light is detected in order to generate an image of the vein 1 and the the blood cell flow therethrough. By determining the time which is necessary to regain the original blood cell flow through the vein 1 after release of occlusion, the vein refilling time and, thus, the degree of hydration, is measured and displayed. From Fig. 2, a diagrammatic view of a device according to a second embodiment of the invention can be seen. This device according to the second embodiment of the invention serves for capillary refilling time monitoring using the OPSI method on a fingernail. The device comprises a two-part actuator 12 that serves as a "clamp" for a fingertip 13. The two-part actuator 12 comprises a lower actuator part 14 and an upper actuator part 15. The lower actuator part 14 is movable relative to the upper actuator part 15. Accordingly, the fingertip 13 can be "clamped" in the two-part actuator in order to receive occlusion of the capillaries. The upper actuator part 15 is transparent which allows light coming from a light source 4 to irradiate the tissue with its capillaries under the fingernail 16. According to the second preferred embodiment of the invention, green light with a wavelength of 450 nm together with red light at a wavelength of 660 nm is irradiated. With the help of a polarizing beam splitter 5 only that part of the green light and the red light, respectively, is detected by a light detector 7 which has a polarization direction which is orthogonal to the polarization direction of the incident light. In case more than one wavelength is used for illumination, multiple light detectors are used, i.e. one for each wavelength, in order to generate a respective signal for each wavelength.

As already described with respect to the first embodiment of the invention, the image signals are sent from light detector 7 wirelessly to a desktop housing 11 comprising an analyzer 9 and a measuring unit 10. There, the "red" and "green" pictures are subtracted from each other leading to a differential image which shows an enhanced contrast of the image of the capillaries in which flow of the blood cells is to be determined. Operation of the device according to the second embodiment of the invention is similar to operation of the device according to the first embodiment of the invention: After release of occlusion the time for regaining original blood cell flow is measured in order to determine capillary refilling time and thus the degree of hydration. The embodiments described with reference to Fig. 1 and 2 can include at least one focusing element, such as a lens to form an image of the selected tissue area containing blood vessels on the detector. An additional lens or a combination of lenses can be used to illuminate a sufficiently large skin area, where blood vessels are located. From Fig. 3, a diagrammatic view of vein/capillary refilling time monitoring using diffuse transmittance on a fingernail according to a third embodiment of the invention can be seen. This device comprises a light source for emitting NIR light with a predefined wavelength in the range between 760 and 900 nm. This incident light is transmitted through a transparent actuator which can "clamp" a fingertip together with a light detector 7 in order to achieve occlusion of veins and/or capillaries. When the "clamp" formed by transparent actuator 6 and light detector 7 is released the diffuse transmittance signal received by light detector 7 is determined and, thus, vein/capillary refilling time is measured via the temporal change of the intensity of the transmitted light.

From Fig. 4, a diagrammatic view of vein/capillary refilling time monitoring using diffuse reflectance on a fingernail according to a forth embodiment of the invention can be seen. Here, also a two-part actuator 12 is provided which is formed by an upper actuator part 15 which is transparent and a lower actuator part 14 which can be moved relative to the upper actuator part 15 in order to "clamp" the fingertip 13 in the two-part actuator 12 for achieving occlusion. In contrast to the third preferred embodiment described above, in the present case, instead of diffuse transmittance, diffuse reflectance is measured. This means that that light reflected back from the irradiated tissue material is collected by a light detector 7. Similar to the third preferred embodiment described above, when the "clamp" is released the diffuse reflectance signal received by light detector 7 is determined and, thus, vein/capillary refilling time is measured via the temporal change of the intensity of the reflected light.

According to the following preferred embodiments of the invention which are described with reference to Figs. 5 - 7, images of the capillaries are acquired by imaging an area under the skin with a microscope. In Fig. 5 a fifth embodiment of the invention is shown which comprises an imaging device 18 which is described in greater detail with reference to Fig. 7, and a passive occlusion means 19.

The occlusion means 19 comprises a transparent pressing plate 20 and a force sensor 21 for sensing the force applied on the pressing plate 20. Occlusion of capillaries in the fingertip 13 is achieved according to the fifth preferred embodiment of the invention by pressing the fingertip 13 onto the pressing plate 20 until the force sensor 21 senses a sufficient force for occluding the capillaries in the fingertip 13. After release of occlusion refilling of the capillaries is imaged by imaging device 18.

Imaging device 18 is shown in greater detail in Fig. 7. There, it can be seen that imaging device 18 comprises a microscope imaging system 22 for generating a magnified image of the capillaries in the fingertip 13. The fingertip 13 which is pressed on the transparent pressing plate 20 is irradiated with incident light of 450 nm by a light source 4 formed by a LED. In order to reduce refraction disturbances between the fingertip 13 and the pressing plate 20, the pressing plate 20 is provided within an oil container 23 filled with such an amount of oil 24 that provides for an essentially undisturbed optical path between the pressing plate 20 and the fingertip 13.

A magnified image of the capillaries of the fingertip 13 is achieved with the aid of the microscope imaging system 22 and a light detector 7 which is formed by a video camera. Similarly to the first embodiment of the invention, the image signal received by the light detector 7 is transmitted to an analyzer 9 and a measuring unit 10 for further processing, especially for analyzing the image with respect to blood cell flow within the capillaries in order to determine refilling time after release of occlusion by pressing against the transparent pressing plate 20.

Finally, from Fig. 6 a diagrammatic view of a microscopy imaging based capillary refilling time meter according to a sixth embodiment of the invention can be seen. General functions of this device are similar to the ones described with respect to the device according to the fifth embodiment of the invention. However, in contrast to the device according to the fifth embodiment, in the present case, an active occlusion means 19 is provided which acts as a "clamp" actuator similar to the one described with respect to the third and forth embodiment of the invention. The preferred wavelengths for use with the methods and devices described herein are:: 576 nm, 556 nm, 540 nm, 415 nm which relate to oxyhemoglobin; 928 nm, 756 nm, 556 nm, 432 nm which relate to deoxyhemoglobin; and 796 nm, 500 nm, 674 nm as reference wavelengths. These wavelengths correspond to the maxima of the absorption bands of hemoglobin, and in particular, oxy- and deoxyhemoglobin as well as to iso-points as well as minima of absorption. The latter can be used as reference to acquire tissue signal nonspecific with respect to blood vessels, which is later subtracted from the specific signal. All the embodiments can include additional optical components, such as focusing elements, e.g., lenses, filters for selecting a particular wavelength, polarizing elements for selecting a preferential polarization, matching fluid to match the refractive index of the illumination optics with the one of the skin. Additionally, in case multiple wavelengths are used at least one dichroic beamsplitter can be used to selectively reflect/transmit the wavelength of interest. Light sources may include broadband halogen lamps, LEDs, or lasers. 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: irradiating tissue comprising the blood vessel with linearly polarized light of at least one predefined wavelength; collecting the part of the reflected light which is orthogonally polarized with respect to the irradiated light as an image; occluding the blood vessel, in order to remove the blood from the blood vessel; release of occlusion of the blood vessel in order to allow the blood to refill the blood vessel; analyzing the image with respect to the flow of blood cells; measuring the time necessary for regaining a predefined flow of blood cells after release of occlusion.

2. The method according to claim 1, wherein the irradiated linearly polarized light comprises at least two different predefined wavelengths, from the collected part of the reflected light which is orthogonally polarized with respect to the irradiated light two different images for the two different wavelengths are generated, respectively, the images are subtracted from each other in order to generate a differential image; and the differential image is analyzed with respect to the flow of blood cells.

3. 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: irradiating tissue comprising the blood vessel with light of at least one predefined wavelength; collecting a reflected part of the light in order to generate a magnified image of the blood cells flowing through the blood vessel; occluding the blood vessel, in order to remove the blood from the blood vessel; release of occlusion of the blood vessel in order to allow the blood to refill the blood vessel; analyzing the image with respect to the flow of blood cells; measuring the time necessary for regaining a predefined flow of blood cells after release of occlusion.

4. The method according to any of claims 1 to 3, wherein analyzing the image with respect to the flow of blood cells comprises the detection of the number of blood cells flowing through the blood vessel per time unit and/or the detection of the temporal change of the number of blood cells flowing through the blood vessel per time unit.

5. 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: irradiating tissue comprising the blood vessel with light of at least one predefined wavelength; collecting the reflected light or the light which is transmitted through the tissue, respectively; occluding the blood vessel, in order to remove the blood from the blood vessel; release of occlusion of the blood vessel in order to allow the blood to refill the blood vessel; determining the intensity of the reflected light or the transmitted light, respectively; measuring the time necessary for regaining a predefined intensity of the reflected light or the transmitted light, respectively, after release of occlusion.

6. The method according to claim 5, wherein the predefined wavelength relates to an absorption band of a main component of blood, especially haemoglobin.

7. The method according to any of claims 1 to 6, wherein the irradiated light is red light and/or green light or NIR light.

8. 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: a light source (4) for irradiating tissue comprising the blood vessel with linearly polarized light of at least one predefined wavelength; a light detector (7) for collecting the part of the reflected light which is orthogonally polarized with respect to the irradiated light as an image; an occlusion means (6) for occluding the blood vessel, in order to remove the blood from the blood vessel; an analyzer (9) for analyzing the image with respect to the flow of blood cells; and a measuring unit (10) for measuring the time necessary for regaining a predefined flow of blood cells after release of occlusion.

9. 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: a light source (4) for irradiating tissue comprising the blood vessel with light of at least one predefined wavelength; a light detector (7) for collecting the reflected light or the light transmitted through the tissue, respectively; an occlusion means (6) for occluding the blood vessel, in order to remove the blood from the blood vessel; an analyzer (9) for determining the intensity of the reflected light or the transmitted light, respectively; and a measuring unit (10) for measuring the time necessary for regaining a predefined intensity of the reflected light or the transmitted light, respectively, after release of occlusion.

10. The device according to claim 9, wherein the predefined wavelength relates to an absorption band of a main component of blood, especially haemoglobin.

11. 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: a light source (4) for irradiating the tissue comprising the blood vessel with light of at least one predefined wavelength; a microscope imaging system (22) for collecting a reflected part of the light in order to generate a magnified image of the blood cells flowing through the blood vessel; a light detector (7) for collecting the magnified image of the blood cells flowing through the blood vessel; an occlusion means (19) for occluding the blood vessel, in order to remove the blood from the blood vessel; an analyzer (9) for analyzing the image with respect to the flow of blood cells; a measuring unit (10) for measuring the time necessary for regaining a predefined flow of blood cells after release of occlusion.

12. The device according to any of claims 8 to 11, wherein the light source (4) emits red light and/or green light or NIR light.

13. The device according to any of claims 8 to 12, wherein the occlusion means (6, 19), at least in part, is transparent for the irradiated and/or reflected light, respectively.

14. The device according to any of claims 8 to 13, wherein the occlusion means (6, 19) comprises an actuator that actively applies a force on the tissue that comprises the blood vessel.

15. The device according to claim 14, wherein the actuator is provided in a cuff

(3) which can be fixed around a limb of the human body.

16. The device according to any of claims 8 to 13, wherein the occlusion means (19) is a passive unit which provides a counterpart for pressing with a part of the human body.

17. The device according to any of claims 8 to 16, wherein the occlusion means (19) comprises a force sensor (21) for sensing the force applied.