Method and apparatus for improving the accuracy of noninvasive hematocrit measurements

A device for measuring hematocrit values using optical spectrophotometry comprising:

• a probe housing configured to be placed proximal to a tissue location which is being monitored;
• an occlusion device connected to said housing and configured to magnify the fractional change in the vascular blood volume to a value greater than said fractional change produced by normal arterial pulsations;
• light emission optics connected to said housing and configured to direct radiation at said tissue location, wherein said light emission optics comprise at least one of a (a) incandescent light source, (b) white light source and (c) light emitting diodes (“LEDs”) which are tuned to emit radiation at at least a first and a second wavelength, where said at least first wavelength is within a band of wavelengths where hemoglobin is the dominant absorber and where said at least second wavelength is within a band where water is the dominant absorber;
• a photodiode connected to said housing and configured to receive radiation from said tissue location;
• a processing device connected to said housing and configured to process radiation from said light emission optics and said light detection optics to compute said hematocrit values, wherein said processing device:
• receives at least two sets of optical measurements at an at least a first and a second wavelength, where for each wavelength two optical measurements are obtained corresponding to measurements before and after a venous occlusion conducted by said occlusion device, to obtain before and after occlusion measurements at each wavelength;
• combines said before and after measurements at each wavelength to determine a blood pulse spectrum at each wavelength;
• combines said blood pulse spectra at each wavelength to obtain a ratio of said blood pulse spectra;
• combines said ratio with measurements of tissue water fractions to determines the blood hematocrit value, such that 1 H ≅ 0.34 1 - f w - f p ⁢ ⁢ p ⁢ ( 1 + R ⁢ μ a H ⁢ ⁢ b ⁡ ( λ 1 ) + Δ ⁢ ⁢ μ s ⁡ ( λ 1 ) μ a w ⁡ ( λ 2 ) + Δ ⁢ ⁢ μ s ⁡ ( λ 2 ) ) , where :
• H is the hematocrit value;
• ƒw is the tissue water fraction;
• ƒpp is the plasma protein fraction;
• R is the ratio of magnitudes of the blood pulse spectrum;
• μaHb(λ1) is the sum of the absorption coefficient of the two forms of hemoglobin at a first wavelength;
• μaw(λ2) is the absorption coefficient of water at a second wavelength;
• Δμs(λ1) is the difference between the scattering coefficients of the blood and surrounding tissue at a first wavelength;
• Δμs(λ2) is the difference between the scattering coefficients of the blood and surrounding tissue at a second wavelength;
• and
• 0.34 is the fraction of the red cell volume occupied by hemoglobin, which is assumed to be constant.

A method of measuring a percent hematocrit near a tissue location using optical spectrophotometry comprising:

• placing a probe housing proximal to said tissue location;
• occluding the venous blood flow adjacent to said tissue location;
• emitting radiation at at least two wavelengths using light emission optics configured to direct radiation at said tissue location;
• detecting radiation using light detection optics configured to receive radiation from said tissue location;
• processing said radiation from said light emission and said light detection optics using a processing device;
• computing said percent hematocrit, where said percent hematocrit is determined by:
• receiving at least two sets of optical measurements at an at least a first and a second wavelength, where for each wavelength two optical measurements are obtained corresponding to measurements before and after a venous occlusion, to obtain before and after occlusion measurements at each wavelength;
• combining said before and after measurements at each wavelength;
• determining a blood pulse spectrum at each wavelength;
• obtaining a ratio of said blood pulse spectra;
• combining said ratio with measurements of tissue water fractions;
• and
• determining said percent hematocrit value such that 1 H ≅ 0.34 1 - f w - f p ⁢ ⁢ p ⁢ ( 1 + R ⁢ μ a H ⁢ ⁢ b ⁡ ( λ 1 ) + Δ ⁢ ⁢ μ s ⁡ ( λ 1 ) μ a w ⁡ ( λ 2 ) + Δ ⁢ ⁢ μ s ⁡ ( λ 2 ) ) , where :
• H is the hematocrit value;
• ƒw is the tissue water fraction;
• ƒpp is the plasma protein fraction;
• R is the ratio of magnitudes of the blood pulse spectrum;
• μaHb(λ1) is the sum of the absorption coefficient of the two forms of hemoglobin at a first wavelength;
• μaw(λ2) is the absorption coefficient of water at a second wavelength;
• Δμs(λ1) is the difference between the scattering coefficients of the blood and surrounding tissue at a first wavelength;
• Δμs(λ2) is the difference between the scattering coefficients of the blood and surrounding tissue at a second wavelength;
• and
• 0.34 is the fraction of the red cell volume occupied by hemoglobin, which is assumed to be constant.

A method of computing percent hematocrit based on optical measurements, wherein said percent hematocrit is determined such that 1 H ≅ 0.34 1 - f w - f p ⁢ ⁢ p ⁢ ( 1 + R ⁢ μ a H ⁢ ⁢ b ⁡ ( λ 1 ) + Δ ⁢ ⁢ μ s ⁡ ( λ 1 ) μ a w ⁡ ( λ 2 ) + Δ ⁢ ⁢ μ s ⁡ ( λ 2 ) ) , where :

• H is the hematocrit value;
• ƒw is the tissue water fraction;
• ƒpp is the plasma protein fraction;
• R is the ratio of magnitudes of the blood pulse spectrum;
• μaHb(λ1) is the sum of the absorption coefficient of the two forms of hemoglobin at a first wavelength;
• μaw(λ2) is the absorption coefficient of water at a second wavelength;
• Δμs(λ1) is the difference between the scattering coefficients of the blood and surrounding tissue at a first wavelength;
• Δμs(λ2) is the difference between the scattering coefficients of the blood and surrounding tissue at a second wavelength;
• and
• 0.34 is the fraction of the red cell volume occupied by hemoglobin, which is assumed to be constant.

A method of measuring a percent hematocrit near a tissue location using optical spectrophotometry comprising:

• irradiating said tissue location and processing received signals from said tissue location to measure tissue water;
• occluding the venous blood flow adjacent to said tissue location;
• repeat irradiating said tissue location;
• detecting radiation from said tissue following said repeat irradiating;
• and
• calculating hematocrit values using tissue water measurements.