System for correction of trends associated with pulse wave forms in oximeters

• US 5,216,598 A
• Filed: 10/15/1991
• Issued: 06/01/1993
• Est. Priority Date: 10/04/1989
• Status: Expired due to Fees

• Current Assignee: Colin Medical Technology Corporation (Omron Corporation)
• Original Assignee: Colin Electronics Co., Ltd.
• Inventors: Edgar, Reuben W., Branstetter, Ronald L.
• Primary Examiner(s): Hayes, Gail O.
• Application Number:
  US07/774,147
• Time in Patent Office: 595 Days
• Field of Search: 364/413.09, 128/637, 128/653.1, 128/653.2, 128/653.3, 128/659, 128/660.01, 128/660.02
• US Class Current: 600/330
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7207 of noise induced by motion artifacts

Abstract

A noninvasive optical oximeter measures oxygen saturation of arterial blood. A patient'"'"'s arterial blood is illuminated with light at two different wavelengths and the intensity of the reflected light is sensed by a photodetector and an output signal is created in response thereto. The output signal is processed to form a ratio representing the AC component of the reflected light at each wavelength over the DC component. The oxygen saturation of the blood is calculated by correlating the quotient of these ratios with an oxygen reference curve uniquely representative of the blood oxygen characteristics of a particular patient. The pulse amplitude signals for the AC component at each wavelength are compensated for the effects of upward or downward trends and, therefore, the accuracy of the blood oxygen saturation calculation is significantly increased.

Claims

1. An oximeter for measuring the oxygen saturation of blood comprising:

first and second sources of electromagnetic radiation having first and second wavelengths of radiation, respectively, for illuminating a sample of said blood;

a sensor for detecting electromagnetic radiation generated by said first and second sources which has illuminated said blood sample and for producing electrical signals corresponding to the intensity of the detected electromagnetic radiation at each of said first and second wavelengths, each of radiation at each of said first and second wavelengths, each of said electrical signals having an AC pulsatile component and a DC component;

a signal correction circuit for modifying said AC pulsatile component of each of said electrical signals to remove upward and downward trend effects therefrom;

a signal processor for combining said modified AC pulsatile components and said DC components of said electrical signals for producing a measurement signal related to the oxygen saturation of said blood sample.

2. The oximeter of claim 1 further comprising a display, and said signal processor comprises means for comparing said measurement signal with a reference curve representative of blood oxygen saturation characteristics bored in an electronic storage memory, and for controlling said display in dependence on said comparison to provide an indication of the measured oxygen saturation.

3. The oximeter of claim 1 wherein each of said AC pulsatile components comprises pulses having maximum and minimum signal levels defined by a diastolic-to-systolic time interval between a diastolic time at which the detected light intensities are at their respective local maximum level, and a systolic time at which the detected light intensities are at their respective local minimum level;

and said signal correction circuit comprises;

means for determining, for at least one of said electrical signals, a current pulse time interval and a current pulse median amplitude representing, respectively, the average diastolic-to-systolic time interval and the median of the difference between the systolic and diastolic signal levels for at least one of the most recent pulses; and

a prior pulse time interval and a prior pulse median amplitude representing, respectively, the average diastolic-to systolic time interval and the median of the difference between the systolic and diastolic signal levels for at least one previous pulse;

means for determining a trend slope factor corresponding to the ratio of a first quantity, comprising the difference between said current and said previous pulse median amplitudes, and a second quantity, comprising the difference between said current and said previous pulse time intervals;

means for adjusting the measured systolic and diastolic signal levels of the current pulse for each of said electrical signals according to a trend factor corresponding to the product of said trend slope factor and the diastolic-to-systolic time interval of the current pulse, thereby producing a detrended current pulse.

4. The oximeter of claim 3 wherein said current and said prior pulse diastolic-to-systolic time intervals and median amplitudes are each averaged parameters derived from a plurality of pulses.

5. The oximeter of claim 3 further comprising means for comparing the amplitude, defined as the difference between the diastolic and systolic signal levels of a pulse, of a detrended current pulse with predetermined maximum and minimum amplitude limits, and for selecting one of the detrended current pulse and the current pulse from which the detrended pulse was derived for further processing in dependence on said amplitude comparison.

6. The oximeter of claim 3 wherein said signal correction circuit comprises means for rejecting a detected pulse in said AC components for further processing which has a pulse amplitude, defined as the difference between the diastolic and systolic signal levels of the pulse, which is outside a range limit defined by maximum and minimum pulse amplitude limits determined based on the pulse amplitude of at least one previously detected pulse.

7. The oximeter of claim 6 wherein said maximum pulse amplitude limit corresponds to an average pulse amplitude of a plurality of previous pulses divided by a scale factor;

and said minimum pulse amplitude limit corresponds to said average pulse amplitude multiplied by said scale factor.

8. The oximeter of claim 7 wherein said scale factor is approximately equal to three.

9. The oximeter of claim 3 further comprising:

pulse detection window means for rejecting detected pulses in said AC components for further processing which do not have a diastolic-to-systolic time interval which exceeds the duration of a pulse detection window determined based on the diastolic-to-systolic time interval of at least one previous unrejected pulses.

10. The oximeter of claim 9 wherein the duration of said pulse detection window is based on an average of the diastolic-to-systolic time intervals for a plurality of previous unrejected pulses.

11. The oximeter of claim 1 further comparison noninvasive monitoring probe housing said first and second sources of electromagnetic radiation and said sensor, said monitoring prove being positionable over a portion of a patient'"'"'s tissue such that said electromagnetic radiation illuminates a sample of the patient'"'"'s arterial blood in tissue, and said sensor detects electromagnetic radiation reflected from said blood sample.

12. Signal generating and conditioning apparatus for use in measurement of the oxygen saturation of arterial blood in tissue, said apparatus comprising:

first and second sources of electromagnetic radiation having first and second wavelengths of radiation, respectively, for illuminating a sample of said blood;

a sensor for detecting electromagnetic radiation generated by said first and second sources which has illuminated said blood sample and for producing electrical signals corresponding to the intensity of the detected electromagnetic radiation at each of said first and second wavelengths, each of said electrical signals having an AC pulsatile component and a DC component; and

a signal correction circuit for modifying said AC pulsatile component of each of said electrical signals to remove upward and downward trend effects therefrom.

13. The signal generating and conditioning apparatus of claim 12 wherein said each of said AC pulsatile components comprises pulses having maximum and minimum signal levels defined by a diastolic-to-systolic time interval between a diastolic time at which the detected light intensities are at their respective local maximum level, and a systolic time at which the detected light intensities are at their respective local minimum level;

and said signal correction circuit comprises;

means for determining, for at least one of said electrical signals, a current pulse time interval and a current pulse median amplitude representing, respectively, the average diastolic-to-systolic time interval and the median of the difference between the systolic and diastolic signal levels for at least one of the most recent pulses; and

a prior pulse time interval and a prior pulse median amplitude representing, respectively, the average diastolic-to systolic time interval and the median of the difference between the systolic and diastolic signal levels for at least one previous pulse;

means for determining a trend slope factor corresponding to the ratio of a first quantity, comprising the difference between said current and said previous pulse median amplitudes, and a second quantity, comprising the difference between said current and said previous pulse time intervals;

means for adjusting the measured systolic and diastolic signal levels of the current pulse for each of said electrical signals according to a trend factor corresponding to the product of said trend slope factor and the diastolic-to-systolic time interval of the current pulse, thereby producing a detrended current pulse.

14. The signal generating and conditioning apparatus of claim 13 further comprising means for comparing the amplitude, defined as the difference between the diastolic and systolic signal levels of the pulse, of a detrended current pulse with predetermined maximum and minimum amplitude limits, and for selecting one of the detrended current pulse and the current pulse from which the detrended pulse was derived for further processing in dependence on said amplitude comparison.

15. The signal generating and conditioning apparatus of claim 14 wherein said signal correction circuit comprises means for rejecting a detected pulse in said AC components for further processing which has a pulse amplitude which is outside a range limit defined by maximum and minimum pulse amplitude limits determined based on the pulse amplitude of at least one previously detected pulse.

16. The signal generating and conditioning apparatus of claim 15 further comprising pulse detection window means for rejecting detected pulses in said AC components for further processing which do not have a diastolic-to-systolic time interval which exceeds the duration of a pulse detection window determined based on the diastolic-to-systolic time interval of at least one previous unrejected pulses.