Measuring hematocrit and estimating hemoglobin values with a non-invasive, optical blood monitoring system

• US 8,130,369 B2
• Filed: 11/05/2008
• Issued: 03/06/2012
• Est. Priority Date: 11/05/2008
• Status: Active Grant

• Current Assignee: Fresenius Medical Care Holdings Incorporated (Fresenius Medical Care AG)
• Original Assignee: Fresenius Medical Care Holdings Incorporated (Fresenius Medical Care AG)
• Inventors: Barrett, Louis LeeGrande, Peterson, David Wayne, Sammann, Kristian Allen
• Primary Examiner(s): Chowdhury, Tarifur
• Assistant Examiner(s): Lapage, Michael P
• Application Number:
  US12/265,392
• Publication Number:
  US 20100110416A1
• Time in Patent Office: 1,217 Days
• Field of Search: 356 39- 42, 436/70
• US Class Current: 356/39
• CPC Class Codes: A61B 5/14535 for measuring haematocrit
• CPC Class Codes: A61B 5/14557 specially adapted to extracorporeal circuits
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes
• CPC Class Codes: A61M 1/367 Circuit parts not covered by the preceding subgroups of group A61M1/3621
• CPC Class Codes: A61M 2205/3313 used specific wavelengths
• CPC Class Codes: A61M 2230/207 hematocrit
• CPC Class Codes: Y10T 436/11 Automated chemical analysis

Abstract

An optical blood monitoring system with a ratiometric model determines hematocrit values for a hemodialysis patient, from which hemoglobin values for the patient are estimated. The ratiometric model is calibrated, normally against a cell counter, using blood from a blood bank. The blood from a blood bank is preserved in a long term preservative which is typically different than that found in clinical settings. The hematocrit value determined by the ratiometric model is scaled by scaling factor so that the estimated hemoglobin level output from the monitor consistently matches that measured in a clinical setting. The hematocrit scaling factor is substantially about 1.033 when the patient'"'"'s blood sample is stored in a short term preservative ethylene diamine tetra acetic, and is substantially about 1.06 when the hematocrit is measured in the blood sample without preservative being added to the blood sample. The hemoglobin value can also be adjusted for altitude.

Claims

1. In a hemodialysis system, a method for monitoring at least one blood constituent, the method comprising the steps of:

providing a blood chamber in extracorporeal tubing through which a patient'"'"'s blood flows for hemodialysis;

providing a ratiometric model within a controller for an optical monitoring device that determines a hematocrit value as a function of at least a detected value of light intensity having a first wavelength and a detected value of light intensity having a second wavelength, wherein the ratiometric model is calibrated at least in part by testing the ratiometric model by using the optical monitoring device with whole blood from a supply of blood preserved in a first anticoagulant, measuring the hematocrit value of whole blood from the same supply of blood preserved in the first anticoagulant using an in vitro method, and calibrating the ratiometric model to the measured hematocrit value taken using the in vitro method;

emitting light at a first wavelength through the extracorporeal blood chamber and the patient'"'"'s blood flowing through the blood chamber;

emitting light at the second wavelength through the extracorporeal blood chamber and the patient'"'"'s blood flowing through the blood chamber;

detecting the intensity of the light at the first wavelength after it has passed through the extracorporeal blood chamber and the patient'"'"'s blood flowing therethrough;

detecting the intensity of the light at the second wavelength after it passes through the extracorporeal blood chamber and the blood flowing therethrough;

calculating a hematocrit value using the ratiometric model from at least the detected light intensity at the first wavelength and the detected light intensity at the second wavelength; and

scaling the hematocrit value calculated by the ratiometric model by a scaling factor to result in a scaled hematocrit value that accounts for a second anticoagulant in the patient'"'"'s blood.

2. The method as recited in claim 1 wherein the flow of blood through the blood chamber is assumed to be at a steady state during the monitoring process and wherein the ratiometric model is of the following form:

3. A method as recited in claim 2 wherein during initial calibration of the ratiometric model the value I_0Λ

1 is set to a predetermined value and the ratiometric model is calibrated by adjusting I<sub>0Λ

2</sub> until the hematocrit value output from the model matches the hematocrit value determined by the in vitro method.

4. A method as recited in claim 2 wherein the light intensity of the first wavelength is substantially isobestic for red blood cells and the light intensity of the second wavelength is substantially isobestic for water, and the function f is a second order polynomial.

5. A method as recited in claim 4 wherein the function f is defined by the following equation:

6. A method as recited in claim 1 wherein the first wavelength is approximately 800 nm, the second wavelength is approximately 1300 nm, and the method further comprises emitting light at a third wavelength through the extracorporeal blood chamber and blood flowing therethrough, and detecting the intensity of the light at the third wavelength, and determining oxygen saturation levels based on the detected light intensity at the first wavelength and the detected light intensity at the third wavelength.

7. A method as recited in claim 1 wherein the first anticoagulant is citrate phosphate dextrose, the scaling factor is substantially about 1.033 in order that the scaled hematocrit value output from the monitor matches the measured hematocrit value of blood taken from the patient and preserved with potassium ethylene diamine tetra acetic when measured using the in vitro method.

8. A method as recited in claim 1 wherein the first anticoagulant is citrate phosphate dextrose, the scaling factor is substantially about 1.06 in order that the scaled hematocrit value output from the monitor matches the hematocrit value of blood taken from the patient without the addition of anticoagulant or preservative when measured using the in vitro method.

9. A method as recited in claim 1 wherein the method further comprises the step of dividing the scaled hematocrit value by a constant substantially equal to 2.94 in order to estimate the patient'"'"'s hemoglobin value.

10. A method as recited in claim 1 wherein the value of the scaling factor is determined based on statistical analysis of test data on a variety of blood samples.

11. A method as recited in claim 1 wherein the blood used to calibrate the ratiometric model is preserved in citrate phosphate dextrose, without additional preserving salts.

12. For use with a hemodialysis system that draws blood from a patient, passes the drawn blood through extracorporeal tubing and through a dialyzer, and returns the dialyzed blood through extracorporeal tubing to the patient, a optical blood monitor associated with the hemodialysis system, the monitor comprising:

a sensor assembly that monitors the patient'"'"'s blood flowing through the hemodialysis system, the sensor comprising;

a blood chamber having an inlet and an outlet which are connected inline with extracorporeal tubing for the hemodialysis system, and a fluid passageway through which the drawn blood from the patient flows,

a first photo emitter for emitting light at a first wavelength through the blood chamber and the drawn blood flowing through the blood chamber,

a second photo emitter for emitting light at a second wavelength through the blood chamber and the drawn blood flowing through the blood chamber, and

at least one photo detector for detecting the intensity of the light at each wavelength after it passes through the blood chamber and the drawn blood;

a controller that comprises a ratiometric model that calculates an hematocrit value as a function of at least a detected value of light intensity at the first wavelength and a detected value of light intensity at the second wavelength, wherein the ratiometric model is calculated at least in part by testing the ratiometric model with whole blood from a supply of blood preserved in a first anticoagulant and measuring the hematocrit value of whole blood from the same supply of blood preserved in the first anticoagulant using an in vitro method, and calibrating the ratiometric model to the hematocrit value measured using the in vitro method, and further wherein the controller comprises means for scaling the hematocrit value calculated by the ratiometric model by a scaling factor to account for a second anticoagulant in the patient'"'"'s blood.

13. An optical blood monitor as recited in claim 12 wherein the ratiometric model is of the following form:

14. An optical blood monitor as recited in claim 13 wherein the first wavelength is substantially isobestic with red blood cells and the second wavelength is substantially isobestic with water, and the function f is a second order polynomial.

15. An optical blood monitor as recited in claim 14 wherein the function f is defined by the following equation:

16. An optical blood monitor as recited in claim 14 wherein the sensor assembly further comprises a third photo emitter for emitting light at a third wavelength through the blood chamber and the drawn blood flowing through the blood chamber, and further wherein the sensor assembly includes a photo detector that can detect the intensity of the third wavelength after it passes through the blood chamber and the drawn blood for measurement of oxygen saturation.

17. An optical blood monitor as recited in claim 16 wherein the first wavelength is approximately 800 nm, the second wavelength is approximately 1300 nm and the third wavelength is approximately 660 nm.

18. An optical blood monitor as recited in claim 14 wherein the first wavelength is approximately 810 nm and the second wavelength is approximately 1300 nm.

19. An optical blood monitor as recited in claim 14 wherein the predetermined proportionality factor is substantially about 1.033 in order that the adjusted hematocrit value output from the monitor matches measured hematocrit value of blood taken from the patient and stored in a test tube containing potassium ethylene diamine tetra acetic when measured using an in vitro method.

20. An optical blood monitor as recited in claim 14 wherein the predetermined proportionality factor is substantially about 1.06 in order that the adjusted hematocrit value output from the monitor matches measured hematocrit value of blood taken from the patient without being treated with anticoagulant or preservatives when measured using an in vitro method.