Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration or pH-value ; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid or cerebral tissue
  • A61B5/1455—Measuring characteristics of blood in vivo, e.g. gas concentration or pH-value ; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid or cerebral tissue using optical sensors, e.g. spectral photometrical oximeters
  • A61B5/14551—Measuring characteristics of blood in vivo, e.g. gas concentration or pH-value ; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid or cerebral tissue using optical sensors, e.g. spectral photometrical oximeters for measuring blood gases
  • A61B5/14552—Details of sensors specially adapted therefor
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration or pH-value ; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid or cerebral tissue
  • A61B5/14535—Measuring characteristics of blood in vivo, e.g. gas concentration or pH-value ; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid or cerebral tissue for measuring haematocrit
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
  • A61B5/6801—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be attached to or worn on the body surface
  • A61B5/6813—Specially adapted to be attached to a specific body part
  • A61B5/6825—Hand
  • A61B5/6826—Finger
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
  • A61B5/6801—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be attached to or worn on the body surface
  • A61B5/683—Means for maintaining contact with the body
  • A61B5/6838—Clamps or clips
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/72—Signal processing specially adapted for physiological signals or for diagnostic purposes
  • A61B5/7235—Details of waveform analysis
  • A61B5/7239—Details of waveform analysis using differentiation including higher order derivatives

Definitions

• Pulse oximetry involves emitting light at two or more separate wavelengths (e.g., red and infrared light) across a body part, such as a fingertip. The total attenuation of each of the two wavelengths is measured using sensors (i.e., based on reflectance or transmittance measurements). Pulse oximetry utilizes the pulsing nature of the blood vessels, in order to isolate the blood in the artery from other tissues which the light passes through (e.g., skin, bone, muscle, fat, fingernail, etc). The various blood vessels in the body change volume in a cyclic manner, due to the pumping action of the heart as the blood circulates throughout the body.
• two or more separate wavelengths e.g., red and infrared light
• the total attenuation of each of the two wavelengths is measured using sensors (i.e., based on reflectance or transmittance measurements).
• Pulse oximetry utilizes the pulsing nature of the blood vessels, in order to isolate the
• a second set of measurements are acquired while the body part is set to another temperature.
• Optical parameters are determined at each temperature, along with the temperature dependence of the optical parameters.
• the hemoglobin concentration or the hematocrit value is determined based on a calibration relationship that relates the optical parameters at a given temperature, and the dependence of the optical parameters on temperature with the hemoglobin concentration or hematocrit value.
• Figure 5 is a schematic illustration of a method for monitoring the hematocrit of a subject, operative in accordance with an embodiment of the disclosed technique.
• Figure 6 is a schematic illustration of a method for measuring at least one parameter related to the blood of a subject, operative in accordance with another embodiment of the disclosed technique.
• the disclosed technique overcomes the disadvantages of the prior art by providing a device and method for non-invasive continuous monitoring of blood related parameters of a subject, such as the hematocrit value (Hct).
• the method is essentially analytic (i.e., does not rely on calibration), and is based , on simple optical measurements.
• the method utilizes the temporal changes in the optical properties of different tissue components in the inspected body tissue.
• the method provides hematocrit measurement using a single optical transmission at a single wavelength, by isolating the optical parameters of the blood component from those of other tissue components.
• an emitter emits light at an isosbestic wavelength for oxyhemoglobin and deoxyhemoglobin toward a body part, such as a finger.
• C w volumetric percentage of water in the total tissue sample (not including the water in the blood).
• Ms Msfilood ' Cb + Ms, rest ' ⁇ rest ( ⁇ )
• ⁇ ' s . b i ood reduced scattering coefficient for the whole blood component in the tissue sample
• ⁇ ' s ,r est reduced scattering coefficient for all components in the tissue sample other than water and blood.
• SAT fraction of blood oxygen saturation (amount of oxygenated RBCs in relation to total RBCs); ⁇ ox - the absorption cross-section of oxygenated RBCs;
• V a .pi asma absorption coefficient for the plasma in the blood.
• this parameter includes the absorption due to all components in the blood other than the RBCs.
• the absorption cross-section of the oxygenated and deoxygenated RBCs ( ⁇ ox and ⁇ deox ) is related to the ability of a particle to scatter light, and is dependent on various characteristics of a single RBC (e.g., size, shape, complex refractive index, etc). These parameters can be calculated using Mie theory, and can be easily obtained from available publications.
• the reduced scattering coefficient for the whole blood component ( ⁇ ' s ,bioo d ) is related to the hematocrit value according to the following exemplary equation (experimentally determined):
• the temporal change of the volumetric percentage of each tissue component is due to the pulsing nature of the blood flow within the tissue.
• the various blood vessels in the body change volume in a cyclic manner, due to the pumping action of the heart as the blood circulates throughout the body.
• DC percentage of blood in the tissue sample ( — - ) is the temporal change in dt
• the temporal variation of the blood component is much greater than the temporal variation of the other tissue components (i.e.,
• equation (5) can be simplified to:
• equation (6) can be simplified to:
• the wavelength of approximately 803nm is one such isosbestic wavelength.
• Equation (3) can be simplified to: Equations (7) and (8) can be combined to yield:
• Equation (11 ) relates the temporal rate of change of the reduced
• tissue sample The disclosed technique involves measuring these two
• Figure 1 is a
• FIG. 1 schematic illustration of a device, generally referenced 100, for monitoring
• Figure 2 is a schematic representation of the disclosed technique.
• Device 100 includes an emitter 102, a plurality of reflectance detectors 104 ! ...104 M , a plurality of transmittance detectors 124i...124 N , a controller 106, a processor 108, an interface 110, a display 112, an operator interface 114, a body contact means 116 and a holding means 118.
• Controller 106 is coupled with emitter 102, and with processor 108.
• Interface 110 is coupled with detectors 1O4 1 ...1O4 N and detectors 124 ! ...124 N and with processor 108.
• Processor 108 is further coupled with display 112 and with operator interface 114.
• Device 100 includes a sensor module 126 coupled with a controller module 128.
• Sensor module 126 includes emitter 102, reflectance detectors 104-(...104 N , transmittance detectors 124 1 ...124 N , body contact means 116, and holding means 118.
• Controller module 128 includes controller 106, processor 108, interface 110, display 112, and operator interface 114. Although sensor module 126 and controller module 128 are typically separate and distinct units (i.e., each enclosed in separate housings), it is appreciated that this represents an exemplary implementation of device 100, and that the various components of device 100 may be organized differently.
• Emitter 102 emits radiation toward a body part 120 of the subject.
• Body part 120 is depicted in Figure 2 as a finger, but it is appreciated that the body part may be another region in the body which is easily accessible and convenient for the medical staff to apply the emitted light.
• body part 120 may also be the: toe, outer ear, eardrum, earlobe, mouth, eye, other regions of the hand (e.g., the palm, the webbing between the fingers), and the like.
• Emitter 102 may be a light emitting diode (LED), or another type of light emitting element.
• Emitter 102 may direct the light to body part 120 through an optical element, such as a waveguide or an optical fiber.
• Reflectance detectors 104-] ...104 N are arranged at varying distances with respect to the optical axis of emitter 102. Detectors 104-
• Reflectance detectors 104i...104 N and transmittance detectors 124-) ... 124N may be operative to detect light only at the desired wavelength (i.e., a narrow band sensor).
• a bandpass filter may be used to block light outside the desired wavelength from reaching reflectance detectors 104 ! ...104 N or transmittance detectors 124 1 ...124 N .
• ...124N may be photodiodes, or other types of light detection elements.
• Reflectance detectors 104- 1 ...104 N may be arranged as a series of discrete light detection elements, as depicted in Figure 2, or as a continuous array of light detection elements.
• FIG 3 is a schematic illustration of a continuous reflectance detector arrangement, constructed and operative in accordance with another embodiment of the disclosed technique.
• ...204 N are analogous to reflectance detectors 104.
• Processor 108 obtains data from reflectance detectors 104-
• Interface 110 includes any relevant components for passing data from the detectors to processor 108, such as a driver, an amplifier, and an analog to digital converter.
• Processor 108 may send data to be displayed on display 112.
• Display may be, for example, a microdisplay unit embedded within device 100, or an output device (e.g., a monitor) associated with an external computer.
• Controller 106 is an optional element, which may be used to adjust various parameters of the emitted light, such as the intensity, wavelength, duration, and the like.
• Operator interface 114 is an optional element, which enables the person implementing the disclosed technique (e.g. a physician, a medical aide, and the like) to select the relevant parameters of the emitted light, in accordance with relevant criteria (e.g., the type of the body part, the medical condition of the subject, and the like).
• Body contact means 116 is optionally disposed on the region of body part 120 at which the light is emitted.
• Body contact means 116 is for example a fiber optic face plate, which serves as an optical interface between emitter 102 and detectors 104 ⁇ ...104 N on one side and body part 120 on the other side.
• Body contact means 116 directs light from emitter 102 toward body part 102 in a substantially perpendicular direction, and further directs the reflected light from body part 120 toward reflectance detectors 104 ! ...104 N .
• Body contact means 116 prevents any adverse crosstalk between emitter 102 and detectors 104-i...104 N .
• Body contact means 116 further functions as a protective layer, protecting the body part tissue from emitter 102 and detectors 104-
• Body contact means 116 may be integrated in the housing of sensor module
• Holding means 118 is an optional element, which enables the person implementing the disclosed technique to hold device 100 in a stable manner, and to accurately and effectively direct the emitted light onto body part 120.
• holding means 118 may include a strap or adhesive material for attaching to the hand.
• Device 100 may optionally include a power supply (not shown), for powering the various components, for example via a battery or voltage mains.
• Device 100 may also optionally include a separate memory (not shown) for storing data.
• emitter 102 emits light ( referenced 132 in Figure 2) at the isosbestic wavelength toward body part 120. Since biological tissue is a turbid medium, which both scatters and absorbs light, some of the photons will be absorbed in the tissue and some will be scattered (either transmitted through or reflected back).
• the isosbestic wavelength is preferably approximately 803nm (e.g., 803 ⁇ 5nm). Other possible isosbestic wavelengths are: approximately 390nm, approximately 422nm, approximately 452nm, approximately 500nm, approximately 529nm, approximately 545nm, approximately 570nm and approximately 584nm.
• Reflectance detectors 104 ! ...104 M detect the diffuse reflectance (referenced 134 in Figure 2), in accordance with a spatially resolved reflectance measurement technique.
• the absorption coefficient and the reduced scattering coefficient of the blood component of body part 120 i.e., ⁇ a and ⁇ s ) are then extracted from the spatially resolved reflectance measurements.
• One technique for extracting the desired parameters employs a semi-analytic model, which relates the optical parameters of the tissue to the diffuse reflectance measurements.
• a semi-analytic model which relates the optical parameters of the tissue to the diffuse reflectance measurements.
• the calibration model is created using a large series of experimental observations of reflectance distributions for various optically well defined mediums, such as using polynomial regression.
• An example of such a calibration model is disclosed in Dam et al., Applied Optics, Vol.40, No.7, pp.1155-1164 (March, 2001).
• Processor 108 calculates the temporal derivatives from the extracted parameters based on a series of continuous measurements obtained by detectors 104- ! ...104 N . It is noted that device 100 may include any number of detectors 104i...104 N which are sufficient for obtaining spatially resolved reflectance measurements. Alternatively, device 100 may include a plurality of emitters and a single reflectance detector for implementing the spatially resolved reflectance measurements.
• the diffuse reflectance measurements are combined with diffuse transmittance measurements, to provide additional validation and increased accuracy.
• Figure 4 is a schematic illustration of the device of Figure 1 adapted for diffuse transmittance measurements, constructed an operative in accordance with a further embodiment of the disclosed technique.
• the Beer-Lambert law expresses the attenuation of light propagating through a uniformly absorbing medium.
• a modified form of the Beer-Lambert law can provide a more precise model for describing the total attenuation through biological tissue, by taking into account the significant contribution of the scattering phenomenon to the total attenuation, as follows:
• T the transmittance through the medium
• l out the intensity of the light which passes through the medium
• I 1n the intensity of the incident light
• d the distance that the light travels through the medium
• Equation (13) yields the total attenuation through the medium.
• transmittance detectors 124 ⁇ 122 2 , 122 3 and 124 4 are disposed opposite from the location of emitter 102 across body part 120 (e.g., if emitter 120 is positioned directly above body part 120, then detectors 124i, 124 2 , 124 3 , 124 4 are positioned directly below body part 120 across from emitter 102).
• Detectors 124-], 124 2 , 124 3 , and 124 4 are arranged at various distances (referenced d-i, d 2 , d 3 and d 4 , respectively), respective of emitter 102.
• 124 3 , 124 4 detects the light passing through body part 120 along its respective distance.
• the distance derivative of equation (13) is then calculated (by processor 108) based on the measurements obtained by the detectors, to provide the total attenuation value ( ⁇ ).
• a simple iterative regression algorithm may be applied to extract the variables of ⁇ a and ⁇ s , by determining the values that provide an optimal fit (minimal deviation) for the reflectance measurements and the transmittance measurements.
• Other mathematical techniques known in the art may also be used to extract the desired coefficients based on the combination of the reflectance measurements and the transmittance measurements.
• Processor 108 then computes the temporal derivatives
• detectors are depicted in Figure 3 for exemplary purposes only, and device 100 may use any number of detectors 124 N which are sufficient to detect the transmittance. Generally, a single transmittance detector is sufficient for obtaining diffuse transmittance measurements, and additional transmittance detectors may be utilized for validation of the measurement results. It is further appreciated that the reflectance measurements and transmittance measurements may be performed simultaneously (i.e., where reflectance detectors 10 ⁇ ...104 N carry out the spatially resolved reflectance measurements, and transmittance detectors 124.) ...124N carry out the diffuse transmittance measurements).
• the disclosed technique enables the measurement of hematocrit using only a single wavelength. This is advantageous, since emitting light at different wavelengths onto a living biological tissue may lead to various distortions in the subsequent analysis. Each wavelength may have a different penetration depth, resulting in inconsistent tissue volumes being inspected. Moreover, each wavelength may have a different dependency with respect to the various tissue components.
• the hematocrit may be measured by emitting light at two separate wavelengths, rather than a single isosbestic wavelength.
• the absorbance of oxyhemoglobin and deoxyhemoglobin are not necessarily equal, and equation (3) cannot be reduced to equation (9).
• equations (3) and (4) into equation (10) produces:
• This equation, or another relevant equation, can then be solved at two separate wavelengths (at least one of which is non-isosbestic) to determine both variables. For example, light may be emitted at approximately 690nm and approximately 860nm (preferably at two wavelengths where the absorbance spectra of oxyhemoglobin and deoxyhemoglobin are substantially far apart). Similarly, a third wavelength may be used, in order to provide additional validation to the measurement results.
• the hematocrit may be measured by emitting light at three separate wavelengths in the operational spectral range, by solving for three unknown variables in three separate equations.
• the temporal variation of the absorption coefficient in the blood component is a function of three variables: the hematocrit value (HcO, the oxygen saturation level (SAT), and the temporal variation of the
• the wavelength dependence of the absorption coefficient of the blood component ( ⁇ a , ⁇ Ood ) is included in the absorption cross-section of the oxygenated and deoxygenated RBCs ( ⁇ ox and ⁇ deO ⁇ ), which are also wavelength dependent (and can be calculated using Mie theory, for example, as mentioned earlier).
• This approach also eliminates the use of the reduced scattering coefficient of the blood component ( ⁇ ' s ), which may be a less reliable measurement, in determining the hematocrit (or other parameters).
• the disclosed technique allows continuous monitoring of the hematocrit value of the subject over a period of time, based on simple non-invasive optical measurements. This is particularly valuable for diagnosing or treating various medical conditions.
• One useful application of continuous hematocrit monitoring is for optimizing fluid status for a patient undergoing hemodialysis.
• Hemodialysis is a medical procedure, generally for treating patients suffering from kidney failure, in which waste products (e.g., ureic toxins and excess water in the blood plasma) are filtered out of the blood and the cleansed blood is restored to the body in a continuous circuit.
• the plasma refill rate of the patient is the rate at which the patient body transfers fluid back into the blood from the extravascular space, and the rate of dialysis filtration must be adapted with this rate.
• the change in blood volume in the intravascular space can be detected, since the RBC count remains constant during dialysis and thus an increase in Hct indicates a reduction in plasma volume.
• the rate of dialysis filtration can then be set accordingly.
• a hemorrhaging patient who is receiving fluid or undergoing blood replacement can also be monitored in a similar manner. Hematocrit monitoring may also be used as part of other types of medical diagnostics and treatments, such as for anemia, dehydration, myeloproliferative disorders, chronic obstructive pulmonary disease, and other conditions.
• the hematocrit measurement may also be used as a test for determining suitability to donate blood or plasma.
• FIG. 5 is a schematic illustration of a method for monitoring the hematocrit of a subject, operative in accordance with an embodiment of the disclosed technique.
• procedure 152 light is emitted from an emitter, at an isosbestic wavelength for oxyhemoglobin and deoxyhemoglobin, toward a body part containing at least one blood vessel.
• emitter 102 emits light at an isosbestic wavelength (e.g., approximately 803nm) toward body part 120.
• spatially resolved reflectance measurements are obtained, by capturing the light reflected by the body part, with a plurality of reflectance detectors arranged at varying distances with respect to the optical axis of the emitter.
• reflectance detectors 104 1 ...104 N are arranged at varying distances (e.g., in a radial pattern) with respect to the optical axis of emitter 102.
• ...104 N detect the light reflected from body part 120, obtaining a series of spatially resolved reflectance measurements.
• diffuse transmittance measurements are obtained, by capturing the light passing through the body part, with at least one transmittance detector disposed opposite the emitter across the body part.
• transmittance detectors 124 1 f 122 2 , 122 3 and 124 4 are disposed underneath body part 120 (i.e., opposite from the location of emitter 102 across body part 120), where each detector is positioned at a different distance (referenced d ⁇ d 2 , d 3l d 4 , respectively) with respect to the optical axis of emitter 102.
• Each of detectors 124-,, 124 2 , 124 3 , 124 4 detects the light passing through body part 120 along its respective distance, thereby obtaining a series of diffuse transmittance measurements.
• the total attenuation ( ⁇ ) through body part 120 is calculated based on the measurements.
• the absorption coefficient ( ⁇ a ) and the reduced scattering coefficient ( ⁇ ' s ) of the blood component of the body part are extracted from the measurements.
• processor 108 calculates the absorption coefficient and the reduced scattering coefficient of the blood component of body part 120 ( ⁇ a and ⁇ ' s ) from the spatially resolved reflectance measurements, and optionally, from the diffuse transmittance measurements as well.
• the parameters may be extracted using a semi-analytic model or using an experimentally based calibration model. An iterative regression algorithm may be used to combine the reflectance measurements and the transmittance measurements.
• processor 108 calculates the temporal derivatives of the absorption
• the hematocrit value of the body part is calculated using the temporal derivatives.
• processor 108 calculates the hematocrit based on the extracted temporal derivatives, by solving for the Hd variable in equation (11) or another similar relevant equation.
• procedure 178 at least one of: the absorption coefficient ( ⁇ a ) and the reduced scattering coefficient ( ⁇ ' s ) of the blood component of the body part at the at least one wavelength, is extracted from the measurements.
• procedure 180 the temporal derivatives of the at least
• procedure 182 at least one parameter related to the blood of the body part is calculated using the temporal derivatives.

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