Method for ambient light subtraction in a photoplethysmographic measurement instrument

• US 5,954,644 A
• Filed: 03/24/1997
• Issued: 09/21/1999
• Est. Priority Date: 03/24/1997
• Status: Expired due to Fees

• Current Assignee: Datex-Ohmeda Incorporated (General Electric Company)
• Original Assignee: Ohmeda, Inc. (Linde Plc)
• Inventors: Martin, Alan, Aronow, Kurt, Dettling, Allen
• Primary Examiner(s): Peffley, Michael
• Assistant Examiner(s): YARNELL, BRYAN KEITH
• Application Number:
  US08/823,526
• Time in Patent Office: 911 Days
• Field of Search: 600/310, 600/322-328, 600/336, 600/340, 600/473, 600/476, 356/39-41
• US Class Current: 600/322
• CPC Class Codes: A61B 5/14552 Details of sensors specially adapted therefor

Abstract

An improved photoplethysmographic measurement system is disclosed in which a portion of a time division multiplexed (TDM) signal represents an ambient light level, and other TDM signal portions represent detected levels of two or more centered wavelengths of transmitted light. The ambient and detected light portions of the signal are simultaneously applied to the inputs of an instrumentation amplifier(s) so as to produce a continuous output voltage that is proportional to a difference in voltage between the ambient and detected light portions of a TDM signal. Such an approach provides for ambient light level subtraction with reduced noise and componentry.

Claims

1. A photoplethysmographic measurement system for use in determining a value for at least one characteristic derived from a patient'"'"'s blood using a time division multiplexed signal in an environment including an ambient light level, said system comprising:

means for emitting light through a portion of said patient'"'"'s anatomy, said light being emitted at two or more different primary wavelengths;

means for detecting said ambient light level and transmitting levels for each said primary wavelength that has passed through said portion of the patient'"'"'s anatomy such that the detected transmitted levels for each wavelength include the ambient light level and so that the levels of detected ambient light and detected transmitted light form respective ambient and detected light signals within said time division multiplexed signal;

first and second amplification means each of which includes a first input, a second input and an output, each said amplification means being configured for producing a corresponding amplified output that is proportional to a difference between signals present on its first and second inputs multiplied by a corresponding predetermined gain;

means for applying said ambient light signal to said first input of each amplification means while, at the same time, applying said detected light signal to the second input of the first and second amplification means, such that the first amplification means produces a first output that is proportional to the difference between the detected signal corresponding to a first wavelength and the ambient light level and the second amplification means produces a second output that is proportional to the difference between the detected signal corresponding to a second wavelength and the ambient light level, wherein the ambient light level, as indicated by the ambient light signal, is substantially removed from the first and second outputs contemporaneous with the corresponding amplification by said corresponding predetermined gain; and

signal processing means for receiving said first and second outputs and for determining said value of said characteristic based upon the first and second outputs.

2. A photoplethysmographic measurement system in accordance with claim 1, further comprising:

means for displaying the value of said characteristic.

3. A photoplethysmographic measurement system in accordance with claim 1, further comprising:

means for providing an alarm if the determined value of said characteristic falls above or below predetermined threshold levels.

4. A photoplethysmographic measurement system in accordance with claim 1, wherein:

said system determines a value for at least one of pulse rate, blood oxygen saturation level, oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin and methemoglobin.

5. A photoplethysmographic measurement system in accordance with claim 1, each said amplification means further comprising:

an amplifier having a non-inverting input and an inverting input which correspond to said first and second inputs, respectively.

6. A photoplethysmographic measurement system in accordance with claim 5, each said amplification means further comprising:

a series of predetermined resistance values which are selectively connectable with said amplifier for setting the gain of each respective amplification means.

7. A photoplethysmographic measurement system in accordance with claim 6 further comprising:

switching means cooperating with said processing means for connecting the appropriate resistance value to said amplifier so as to set the gain.

8. A photoplethysmographic measurement system in accordance with claim 1 wherein:

said detected light signal includes a first portion generated in response to said first wavelength and a second portion generated in response to said second wavelength, said means for applying the ambient and detected light signals to the amplification means further including a plurality of sample and hold means for continuously providing said ambient light signal to the first input of each of the amplification means and for continuously providing said first portion of the detected light signal to the second input of the first amplification means while continuously providing the second portion of the detected light signal to the second input of the second amplification means such that the output of each amplification means is continuous.

9. A photoplethysmographic measurement system in accordance with claim 8, each said sample and hold means further comprising:

filter means for shaping the respective ambient or detected light signal applied thereto.

10. A photoplethysmographic measurement system in accordance with claim 9, said filter means further comprising:

at least on e low-pass filter.

11. A photoplethysmographic measurement system in accordance with claim 10, said filter means further comprising:

two low-pass filters wherein one low-pass filter includes a roll-off frequency of approximately 7 Hz and another low-pass filter includes a roll-off frequency of approximately 26 Hz.

12. In a photoplethysmographic measurement system, a method of determining a value for at least one characteristic derived from a patient'"'"'s blood using a time division multiplexed signal in an environment including an ambient light level, said method comprising the steps of:

emitting light through a portion of said patient'"'"'s anatomy, said light being emitted with two or more different primary wavelengths;

detecting said ambient light level and transmitted levels for each said primary wavelength that has passed through said portion of the patient'"'"'s anatomy such that the detected transmitted levels for each primary wavelength include the ambient light level and so that the levels of detected ambient light and detected transmitted light form respective ambient and detected light signal portions of said time division multiplexed signal;

providing first and second amplification means each of which includes a first input, a second input and an output, each said amplification means being configured for producing a corresponding amplified output on its output that is proportional to a difference between signals present on its first and second inputs multiplied by a corresponding predetermined gain;

applying said ambient light signal to said first input of each amplification means while, at the same time, applying said detected light signal to the second input of the first and second amplification means, such that the first amplification means produces a first output that is proportional to the difference between the detected signal corresponding to a first primary wavelength and the ambient light level and the second amplification means produces a second output that is proportional to the difference between the detected signal corresponding to a second primary wavelength and the ambient light level, wherein the ambient light level, as indicated by the ambient light signal, is substantially removed from the first and second outputs contemporaneous with the corresponding amplification by said corresponding predetermined gain; and

processing said first and second outputs so as to determine said characteristic.

13. The method of claim 12, further comprising the step of:

displaying said characteristic.

14. The method of claim 12, further comprising the step of:

providing an alarm if the predetermined value of said characteristic falls above or below predetermined threshold values.

15. The method of claim 12, and processing step further comprising the step of:

determining a value for at least one of pulse rate, blood oxygen saturation level, oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin and methemoglobin.

16. The method of claim 12, further comprising the step of:

initially adjusting the gain of each amplification means and, thereafter, readjusting each gain only in response to predetermined physiological changes within said portion of the patient'"'"'s anatomy or when said light is transmitted through a different portion of the patient'"'"'s anatomy such that switching noise resulting from gain adjustment of the amplification means is minimized.

17. The method of claim 12 wherein said detected light signal includes a first portion generated in response to said first wavelength and a second portion generated in response to said second wavelength, said step for applying the ambient and detected light signals further comprising the steps of:

continuously providing the ambient light signal to the first input of each amplification means; and

continuously providing said first portion of the detected light signal to the second input of the first amplification means while continuously providing the second portion of the detected light signal to the second input of the second amplification means such that the output of each amplification means is continuous.

18. The method of claim 12 further comprising the step of:

de-multiplexing said time division multiplexed signal once within the overall system prior to applying the signal to the amplification means such that multiplex/de-multiplex switching noise is substantially reduced.

19. In a photoplethysmographic measurement system, a method of determining a value of at least one characteristic derived from a patient'"'"'s blood in an environment including an ambient light level, said method comprising the steps of:

emitting light through a portion of said patient'"'"'s anatomy, said light being emitted with two or more different primary wavelengths;

detecting said ambient light level and transmitted levels for each said primary wavelength that has passed through said portion of the patient'"'"'s anatomy such that the detected transmitted levels for each primary wavelength include the ambient light level and so that the levels of detected ambient light and detected transmitted light form separate portions of a time division multiplexed signal;

demultiplexing said time division multiplexed signal into a first signal corresponding to detected transmitted light at said first primary wavelength, a second signal corresponding to said detected transmitted light at said second primary wavelength and an ambient light signal corresponding to said detected ambient light level;

separately conditioning said first signal, said second signal and said ambient light signal to generate a first conditioned output, a second conditioned output and an ambient output, respectively;

directly and substantially contemporaneously using said first conditioned output and said ambient output at a first amplifier to generate a first ambient compensated and amplified output corresponding to said first primary wavelength, said first ambient compensated and amplified output being proportional to a first difference between said first conditioned output and said ambient output multiplied by a first predetermined gain;

directly and substantially contemporaneously using said second conditioned output and said ambient output at a second amplifier to generate a second ambient compensated output corresponding to said second primary wavelength, said ambient compensated and amplified output being proportional to a second difference between said second conditioned output and said ambient output multiplied by a second predetermined gain; and

processing said first and second ambient compensated outputs to determine said value of said characteristic.

20. The method of claim 19, wherein:

said step for producing said first ambient compensated output includes the step of substantially contemporaneously amplifying said first conditioned output and said ambient output while removing said ambient light level; and

said step for producing said second ambient compensated output includes the step of substantially contemporaneously amplifying said second conditioned output and said ambient output while removing said ambient light level.

21. The method of claim 19, further comprising the step of:

after said processing step, displaying said characteristic.

22. The method of claim 19, further comprising the step of:

after said processing step, producing an alarm if said characteristic falls above or below predetermined threshold levels.

23. A photoplethysmographic measurement system for use in determining a value of at least one characteristic derived from a patient'"'"'s blood in an environment including an ambient light level, said system comprising:

means for emitting light through a portion of said patient'"'"'s anatomy, said light being emitted with two or more different primary wavelengths;

means for detecting said ambient light level and transmitted levels for each said primary wavelength that has passed through said portion of the patient'"'"'s anatomy such that the detected transmitted levels for each wavelength include the ambient light level and so that the levels of detected ambient light and detected transmitted light form separate portion of a time division multiplexed signal;

means for demultiplexing said time division multiplexed signal to provide a first signal corresponding to detected transmitted light at a first primary wavelength, a second signal corresponding to said detected transmitted light at said second primary wavelength and an ambient light signal corresponding to said detected ambient light;

a first amplifier for producing a first ambient compensated output corresponding to said first primary wavelength by removing the ambient light level from said first signal using said ambient light signal, said first amplifier being configured for producing said first ambient compensated output by substantially contemporaneously amplifying said first signal and said ambient light signal by a first predetermined gain while removing said ambient light level;

a second amplifier, separate from said first amplifier, for producing a second ambient compensated output corresponding to said second primary wavelength by removing said ambient light level from said second signal using said ambient light signal, said second amplifier being configured for producing said second ambient compensated output by substantially contemporaneously amplifying said second signal and said ambient light signal by a second predetermined gain while removing said ambient light level; and

processing means for determining said value of said characteristic based on said first and second ambient compensated outputs.

24. The photoplethysmographic measurement system of claim 23, wherein:

said first means is configured for producing said first ambient compensated output by substantially contemporaneously amplifying said first signal and said ambient light signal while removing said ambient light level; and

said second means is configured for producing said second ambient compensated output by substantially contemporaneously amplifying said second signal and said ambient light signal while removing said ambient light level.

25. The photoplethysmographic measurement system of claim 23, further comprising:

means for displaying said value of said characteristic.

26. The photoplethysmographic measurement system of claim 23, further comprising:

means for providing an alarm if the determined value of said characteristic falls above or below predetermined threshold levels.