Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/48—Biological material, e.g. blood, urine; Haemocytometers
  • G01N33/483—Physical analysis of biological material
  • G01N33/487—Physical analysis of biological material of liquid biological material
  • G01N33/49—Blood
• • 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, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue
  • A61B5/14535—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, 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
  • A61B5/48—Other medical applications
  • A61B5/4869—Determining body composition
• • 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
  • A61B5/7264—Classification of physiological signals or data, e.g. using neural networks, statistical classifiers, expert systems or fuzzy systems

Definitions

• Hematocrit is traditionally obtained by acquiring a patient blood sample from a vein via a syringe, or by use of a capillary tube from a finger stick, or puncture.
• the blood contained in an elongated vessel, is then centrifuged and the height percentage of the column of blood in the vessel which is solid represents the hematocrit.
• FIG. 3A shows a representation of the total impedance in a limb at a low blood volume
• Zy is equivalent to Z R in parallel with Z B , where Z B is the blood present at higher volume that is not present at lower volume.
• impedance Z R is calculated through equation (17), below:
• blood impedance Z B includes both a magnitude and phase, the phase appears to be the stronger indicator of hematocrit.
• both phase and magnitude of Z B may be used in pattern analysis in a neural network.
• a large number of steps may be used to average out arterial pulsation noise, but takes more time and, therefore, there is a greater risk that the blood volume will undesirably and unpredictably change over time with a longer measurement. It has been found by the inventors t at the phase change increases (as a negative number) from about 10 kHz to in the region of 1.6 MHz and then begins to decrease (although there may be an inflection point at well below 1.6 MHz), (de Vries, P.M.J.M., et al., "Implications of the dielectrical behavior of human blood for continuous on-line measurement of hematocrit", Med. Biol. Eng.
• a "scan” refers to the process of applying signals of various frequencies in steps between a lower and upper frequency limit to electrode 48A. As described above, this creates a current between electrodes 48A and 48B, and a voltage between electrodes 50A and 50B. It takes about one 55th of a second to gather V,, V Q , , and C Q signals at each frequency. Digital filter 128 requires about 9 milliseconds to achieve the desired 60 Hz bandwidth.
• digital filter 128 processes P ⁇ for 9 milliseconds and then processes Pyj for 9 milliseconds at one frequency. The processes is then repeated for 9 milliseconds for P CT and then 9 milliseconds for Py, at another frequency.
• the corresponding digital filter in mixer and filter 106 similarly processes P CQ and P VQ .
• Neural network 52 may be in PC 42 or an adjacent PC or other computer. Accordingly, in FIG. 4, neural network 52 is shown in dashed lines.
• the neural network could consider parameters including frequency, magnitude, phase, and derivations thereof.
• the neural network could consider parametes including the patient's age, weight, sex, temperature, illiness, heat applied to the limb, blood pressure, and arm elevation and position. Of course, it is not necessary that the neural network consider each of these parameters.
• the neural network would also consider the hematocrit measurements from centrifuging capillary tubes corresponding to the patient from which the other factors were obtained.
• the neural network is able to process out the small vessel effect and produce the hematocrit value due to blood contained in large vessels.
• Air pump, solenoid(s'). and pressure cuff 28 There are various methods of changing the blood volume. For example, if limb 44 is a finger, blood volume may be change through venous restriction about the upper arm of the patient, or arterial occlusion of the wrist of the patient.
• microprocessor 94 When it is time to decrease the pressure in cuff 156, microprocessor 94 activates solenoid 168 through which tube 162 is connected to an exhaust. Air pump 152 may be turned on under separate switch or under the control of micro-processor 94. The volume change should be maximized by adjusting the tilt and height of the patient's arm.
• frequency generator 100 low pass filters 116 and 128, and mixer and filters 104 and 106 are performed in hardware (including programmed dedicated hardware with, for example, adders, multipliers, and gate arrays) as opposed to a microprocessor.
• some or all of the functions may be performed in PC 42, in another microprocessor system, or otherwise in software.
• resistance 10 in circuit path 12 representing the response of the extracellular or plasma component
• the parallel circuit path 14 representative of the erythrocyte or red blood coipuscle component
• whole blood impedance is attributable primarily to the extracellular blood component circuit path 12, while at higher frequencies (for example, 1 MHz) the capacitive nature of the cell membrane of the red blood corpuscles results in a more significant impedance contribution from circuit path 14, reducing the magnitude of the whole blood impedance.
• FIG. 15 of the drawings comprises a representative sector of a demodulated voltage signal envelope over a period of time as measured by sensors attached to an electrically-stimulated extremity of a patient according to the present invention, the measured voltage being directly proportional to and therefore representative of the total impedance of the whole blood plus the surrounding tissue.
• the signal envelope includes a dominant DC or baseline component and a small AC or pulsatile component.
• the DC component is generated by the patient's tissue, non- pulsatile arterial blood, and venous and capillary blood of the stimulated body portion.
• the AC component is attributable only to the pulsatile blood, and is therefore truly representative of whole blood impedance for a given frequency.
• AC components at different frequencies will have substantially identical voltage envelope shapes, differing only in magnitude due to the aforementioned frequency-dependent nature of the whole blood impedance response.
• the impedance effects of the patient's extravascular tissue are eliminated and a hematocrit determination may be made using the ratio of a low-frequency pulsatile impedance to a high-frequency pulsatile impedance.
• each frequency excites the tissue of body portion 220 with a constant current, and the resulting voltage signal at each frequency is measured from inner sensor electrodes 224. Since the current excitation is constant, the envelope of the measured voltage at each frequency is directly proportional to the tissue impedance at that frequency.
• Segments of the converted analog values from Detectors 230 and 232 are then repeatedly extracted over identical time periods by Microcomputer 236, correlated to further reduce noise effects, and then normalized by dividing by the voltage baseline of their respective carrier waveforms before a series of ratios of the time-matched digitized pulsatile component signal segments at frequencies A and B are calculated.
• the ratios are averaged in a preferred embodiment using weighted averaging techniques well known in the art, relative weighting being based upon the change in voltage magnitude versus time for the time period over which the digitized signals are extracted. Stated another way, the greater the ⁇ V per _U for a pair of time-matched component segments, the more significant the resulting ratio and the more heavily the ratio is weighted in the averaging process.
• the frequency range of greatest interest previously believed to lie between 50 kHz and 1 MHz, has been proven to be somewhat different and expanded at the high frequency end.
• the preferred frequency range has subsequently been established to he substantially between 100 kHz and 10 MHz to 20 MHz.
• C TM Cell Membrane Capacitance of Tissue
• This modified small signal approach is effected by applying a mechanical "assist" to the limb under measurement. To understand the basis for this "assist,” consider what happens when a blood pressure cuff is applied to a limb and taken through an inflation- deflation cycle.
• the tiny fraction of blood that is able to completely traverse the occlusion zone is nearly pure plasma, because plasma is less viscous than whole blood and the resistance of the nearly totally occluded artery is very high. As the cuff pressure continues to decrease, the resistance presented to the blood also decreases, and more cellular components are able to flow.
• the desirable effect being sought is one where the artery remains occluded for at least a small portion of the cardiac cycle and where the blood traversing the occlusion zone is representative of whole blood, at least over time.
• the cuff pressure is in the region of mean arterial pressure.
• This pressure zone is non-critical and corresponds to the pressure region where the amplitude of the plethysmographic component of the signal becomes a maximum.
• the cuff is applied to the body portion (limb) in question proximate the stimulation and sensor electrodes. It is feasible to place the cuff either proximally, distally or over the electrodes, there at present being no identified preferred location for the cuff relative to the electrodes.
• Pressure in the cuff and inflation and deflation thereof may be controlled via a pump, bleed valve and sensor (pressure transducer) as known in the art, which devices are preferably under control of the microcomputer of the hematocrit determination apparatus.
• a large shift in blood is effected by the system and method described in connection with FIGS. 3-10.
• the nature of the method is such that blood flow artifact is eliminated.
• the same concept of subtracting out the background tissue impedance is employed, using the equations that result from solving the parallel model.
• the measurement points of interest using a blood pressure cuff and impedance determination electrodes and circuitry are found as follows: the cuff is inflated initially to suppress the plethysmographic signal; as the cuff is deflated, systolic pressure is the point at which the plethysmographic waveform reappears; as cuff deflation continues, mean arterial pressure is the point of maximum intensity of the plethysmographic signal; as cuff deflation continues still further, diastolic pressure is that at which the morphology of the plethysmographic waveform ceases to undergo further change with continued cuff deflation. D.

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• Animal Behavior & Ethology (AREA)
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• General Physics & Mathematics (AREA)
• Immunology (AREA)
• Measurement And Recording Of Electrical Phenomena And Electrical Characteristics Of The Living Body (AREA)
• Measuring Pulse, Heart Rate, Blood Pressure Or Blood Flow (AREA)
• Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)

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Cited By (26)

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• 1996
  • 1996-04-03 JP JP53176596A patent/JP3844779B2/ja not_active Expired - Fee Related
  • 1996-04-03 CN CN 96194348 patent/CN1244779A/zh active Pending
  • 1996-04-03 EP EP96910716A patent/EP0955871A4/en not_active Withdrawn
  • 1996-04-03 WO PCT/US1996/004547 patent/WO1996032883A1/en active Application Filing
  • 1996-04-03 CA CA002218281A patent/CA2218281C/en not_active Expired - Fee Related
  • 1996-04-03 AU AU53837/96A patent/AU5383796A/en not_active Abandoned

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