Method and apparatus for determining oxygen saturation in vivo

• US 4,651,741 A
• Filed: 05/30/1985
• Issued: 03/24/1987
• Est. Priority Date: 05/30/1985
• Status: Expired due to Term

• Current Assignee: Edwards Lifesciences Corporation
• Original Assignee: Baxter Travenol Laboratories Incorporated
• Inventors: Passafaro, James D.
• Primary Examiner(s): Husar, Stephen F.
• Application Number:
  US06/739,546
• Time in Patent Office: 663 Days
• Field of Search: 128/633, 128/634, 128/632, 128/637, 356/39, 356/40, 356/41, 356/309
• US Class Current: 600/328
• CPC Class Codes: A61B 5/14535 for measuring haematocrit
• CPC Class Codes: A61B 5/14542 for measuring blood gases A61B5/14551 takes precedence
• CPC Class Codes: A61B 5/1459 invasive, e.g. introduced into the body by a catheter A61B5/1464 takes precedence
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes

Abstract

Accurate in vivo measurement of blood oxygen saturation by fiberoptical means is made possible throughout a wide range of hematocrit or total hemoglobin values by using a single R/IR ratio I and calculating the SO₂ value therefrom through the use of a second-order polynomial of the form

SO.sub.2 =Ak.sup.2 I.sup.2 +BkI+C

in which A, B and C are hematocrit or total hemoglobin-dependent coefficients. These values may be contained in a lookup table accessed by a hematocrit or total hemoglobin value selection, and k is a purely multiplicative calibration constant which can be readily determined for any individual fiberoptic system.

Claims

1. The method of measuring blood oxygen saturation in vivo comprising:

obtaining a pair of signals indicative of the reflectivity of the blood being measured in the red and infrared portions of the light spectrum, respectively;

forming the ratio I of said signals;

establishing the hematocrit level of said blood; and

determining the oxygen saturation SO₂ of said blood by the equation

space="preserve" listing-type="equation">SO.sub.2 =Ak.sup.2 I.sup.2 +BkI+C

in which A, B, and C are Hematocrit-dependent coefficients and k is a calibration constant.

2. The method of claim 1 in which k is calculated by storing the value of said ratio at the time of taking a blood sample, ascertaining the actual saturation values of said sample in vitro, determining a standard value of said ratio corresponding to said actual saturation and hematocrit values, and making k equal to the ratio of said determined value to said stored value.

3. Apparatus for measuring blood oxygen saturation in vivo, comprising:

optical means for illuminating blood within a blood conduit and providing a reflection therefrom;

sensing means for providing signals representative of the intensity of said reflection at a pair of wavelengths in the red and infrared portion of the spectrum, respectively;

divider means for forming the ratio I of said red to said infrared signals;

hematocrit selection means for selecting a hematocrit value corresponding to the hematocrit of said blood;

saturation determining means for determining an oxygen saturation value SO₂ from said ratio in accordance with the transfer function

space="preserve" listing-type="equation">SO.sup.2 =Ak.sup.2 I.sup.2 +BkI+C

in which A, B, and C are hematocrit-dependent coefficients and in which k is a calibration constant; and

display means for displaying said determined SO₂ value.

4. The apparatus of claim 3 further comprising:

memory means for storing the value of said ratio at the time of taking a blood sample;

entry means for entering the actual SO₂ and hematocrit values of said sample;

computing means for computing an intensity ratio corresponding to said SO₂ value; and

quotient-forming means for forming the quotient of said computed ratio value to said stored value, said quotient being k.

5. The apparatus of claim 3 further comprising calibration means for calculating k so as to provide an SO₂ display consistent with the known SO₂ value of a standard reflector during in vitro calibration.

6. The apparatus of claim 3 further comprising look-up table means for storing the values of A, B and C for a plurality of hematocrit levels, said look-up table means being controlled by the selection of a hematocrit level at which blood oxygen saturation is to be measured.

7. The apparatus of claim 3, wherein A, B and C have uniform proportionality to each other for different systems.

8. The apparatus of claim 3 wherein the absolute values of A, B and C decrease with increasing hematocrit.

9. The apparatus of claim 8 wherein the absolute value of B decreases more than the absolute values of A and C between 10% and 60% hematocrit.