Method and apparatus for reducing ambient noise effects in electronic monitoring instruments

• US 5,713,355 A
• Filed: 04/18/1996
• Issued: 02/03/1998
• Est. Priority Date: 10/23/1992
• Status: Expired due to Term

• Current Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Original Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Inventors: Bernstein, Michael, Richardson, Charles A., Okikawa, Jerry K., Bennett, Terrence R.
• Primary Examiner(s): Bahr, Jennifer
• Assistant Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US08/637,002
• Time in Patent Office: 656 Days
• Field of Search: 128/633, 128/664, 128/665, 356/41
• US Class Current: 600/336
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal

Abstract

The present invention provides a method and apparatus for adapting to noise sources affecting a pulse oximeter. Various available frequencies are evaluated to determine their respective noise levels and one is selected to act as the operating demultiplexer frequency. During normal operation of the pulse oximeter, the various available demultiplexer frequencies are periodically scanned to determine which has the lowest associated noise. The noise level associated with the operating frequency is used to determine the signal-to-noise ratio of the pulse oximeter signals and thereby qualify certain signals from the pulse oximeter. Those pulses associated with a signal-to-noise ratio below a predetermined threshold are rejected and excluded from use in calculating blood oxygen saturation.

Claims

1. A method for qualifying signal pulses in a pulse oximeter having at least one light source, the qualified signal pulses being used to determine blood saturation, the method comprising the following steps:

(a) monitoring the signal pulses in a pulse oximeter while the light source is not emitting light, the signal so monitored representing a dark noise level;

(b) monitoring frequency components above a threshold frequency of the signal pulses in the pulse oximeter while the light source is emitting light, the frequency components so measured representing a high frequency noise level;

(c) monitoring a signal amplitude corresponding to the signal pulses while the light source is emitting light;

(d) dividing the signal amplitude by the dark noise level to obtain a first signal-to-noise ratio;

(e) dividing the signal amplitude by the high frequency noise level to obtain a second signal-to-noise ratio;

(f) comparing the first and second signal-to-noise ratios with corresponding predetermined threshold values;

(g) rejecting signal pulses having associated first or second signal-to-noise ratios below the corresponding threshold values, such that the rejected pulses are not used to determine blood oxygen saturation; and

(h) accepting signal pulses having associated signal-to-noise ratios equal to or above the corresponding threshold values, such that the accepted pulses are available to determine blood oxygen saturation.

2. The method recited in claim 1 further comprising the following steps:

(i) displaying blood saturation values calculated from the accepted pulses;

(j) maintaining a display of a most recent blood saturation value when a pulse is rejected; and

(k) updating the blood saturation values displayed in step (i) whenever one of the accepted pulses is used to calculate blood saturation.

3. The method of claim 1 wherein the pulse oximeter includes a red and an infrared light source and wherein the signal amplitude is divided by the high frequency noise level for both the red and infrared light sources.

4. The method recited in claim 3 wherein the dark noise level is obtained when the red light source periodically is not emitting light.

5. The method recited in claim 1 wherein the step of monitoring frequency components is performed by

digitizing each of the signal pulses to create a digitized signal;

and

passing the digitized signal through a high pass filter to provide the high frequency noise level.

6. Apparatus for qualifying signal pulses in a pulse oximeter, the qualified signal pulses being used to determine blood saturation, the apparatus comprising:

(a) a detector for collecting the signal pulses from the pulse oximeter sensor;

(b) signal processing means for monitoring a signal-to-noise ratio associated with the signal pulses, and for comparing the signal-to-noise ratio with a predetermined threshold value;

(c) means for rejecting pulses having associated signal-to-noise ratios below the threshold value and accepting pulses having associated signal-to-noise ratios above the threshold value, such that the rejected pulses are not used in determining blood oxygen saturation; and

(d) a display for displaying blood saturation values determined from accepted pulses.

7. The apparatus recited in claim 6 wherein the display is operable to display a most recent blood saturation value determined from an accepted pulse when a pulse is rejected and wherein the apparatus is operable to update the blood saturation value displayed when an accepted pulse is used to calculate blood saturation.

8. The apparatus recited in claim 6 further comprising a high pass filter to filter the signal pulses from the pulse oximeter sensor to produce a filtered signal, the filtered signal providing a noise value used by the signal processing means for monitoring a signal-to-noise ratio associated with the signal pulses.