Signal processing apparatus and method

• US 7,471,971 B2
• Filed: 03/02/2004
• Issued: 12/30/2008
• Est. Priority Date: 04/14/1997
• Status: Expired due to Fees

• Current Assignee: Masimo Corporation
• Original Assignee: Masimo Corporation
• Inventors: Diab, Mohamed K., McCarthy, Rex
• Primary Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US10/791,683
• Publication Number:
  US 20050096517A1
• Time in Patent Office: 1,764 Days
• Field of Search: 600/300, 600/310, 600/322, 600/323, 600/324, 600/336, 600/481, 600/508, 600/509, 600/515, 600/502, 702191-197
• US Class Current: 600/336
• CPC Class Codes: A61B 5/0205 Simultaneously evaluating both cardiovascular conditions and different types of body conditions, e.g. heart and respiratory condition
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• CPC Class Codes: A61B 5/02433 for infrared radiation
• CPC Class Codes: A61B 5/1455 using optical sensors, e.g. spectral photometrical oximeters
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/14552 Details of sensors specially adapted therefor
• CPC Class Codes: A61B 5/6826 Finger
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• CPC Class Codes: A61B 5/7214 using signal cancellation, e.g. based on input of two identical physiological sensors spaced apart, or based on two signals derived from the same sensor, for different optical wavelengths
• CPC Class Codes: A61B 5/7225 Details of analog processing, e.g. isolation amplifier, gain or sensitivity adjustment, filtering, baseline or drift compensation A61B5/30, A61B5/316 take precedence; transformers for specific applications H01F38/00; amplifiers H03F
• CPC Class Codes: A61B 5/7257 using Fourier transforms
• CPC Class Codes: A61B 5/726 using Wavelet transforms
• CPC Class Codes: A61B 5/7278 Artificial waveform generation or derivation, e.g. synthesising signals from measured signals A61B5/327 takes precedence
• CPC Class Codes: A61B 5/742 using visual displays A61B5/339 takes precedence
• CPC Class Codes: G06F 2218/04 Denoising

Abstract

A method and an apparatus to analyze two measured signals that are modeled as containing desired and undesired portions such as noise, FM and AM modulation. Coefficients relate the two signals according to a model defined in accordance with the present invention. In one embodiment, a transformation is used to evaluate a ratio of the two measured signals in order to find appropriate coefficients. The measured signals are then fed into a signal scrubber which uses the coefficients to remove the unwanted portions. The signal scrubbing is performed in either the time domain or in the frequency domain. The method and apparatus are particularly advantageous to blood oximetry and pulserate measurements. In another embodiment, an estimate of the pulserate is obtained by applying a set of rules to a spectral transform of the scrubbed signal. In another embodiment, an estimate of the pulserate is obtained by transforming the scrubbed signal from a first spectral domain into a second spectral domain. The pulserate is found by identifying the largest spectral peak in the second spectral domain.

Claims

1. In a signal processor for processing at least two measured signals M₁ and M₂, where said signal M₁ comprises a physiological signal portion S₁ and may comprise a noise portion N₁, and where said signal M₂ comprises a physiological signal portion S₂ and may comprise a noise portion N₂, a method comprising:

determining a value for a coefficient c used in a combination of M₁ and M₂ such that the combination of M₁ and M₂ approximates S₁, wherein determining the coefficient c comprises comparing frequency domain representations of the two measured signals M₁ and M₂;

using said coefficient c to remove at least some of the noise portion N₁from the measured signal M₁, thereby producing an approximation A₁ to said physiological signal S₁; and

determining a measured output value for one or more physiological parameters based at least in part on said approximation A₁.

2. The method of claim 1, where A₁, M₁ and M₂ comprise frequency domain signals.

3. The method of claim 2, wherein S₁ is indicative of a heart plethysmograph, further comprising calculating a pulse rate of the heart.

4. The method of claim 1, further comprising displaying A₁ on a display.

5. The method of claim 1, wherein said physiological parameter comprises arterial oxygen saturation.

6. The method of claim 1, further comprising determining, based upon the comparison of M₁ and M₂, whether M₁ comprises N₁.

7. The method of claim 6, wherein said value for the coefficient c is selected to reduce its effect if M₁ does not comprise N₁.

8. The method of claim 1, wherein the combination of M₁ and M₂ comprises a linear combination of M₁ and M₂.

9. The method of claim 8, wherein the linear combination of M₁ and M₂ comprises cM₁−

M₂ such that A₁=cM₁−

M₂.

10. The method of claim 9, wherein an error value, e, given by the relation e=S₁−

(cM₁−

M₂) is reduced.

11. The method of claim 1, wherein comparing the two measured signals M₁ and M₂ comprises calculating ratios of values of a frequency domain representation of M₁ to corresponding values of a frequency domain representation of M₂.

12. The method of claim 11, wherein said ratios are calculated using values that correspond to peaks in said frequency domain representations of M₁ and M₂.

13. The method of claim 11, further comprising determining whether the ratios have a predetermined degree of similarity.

14. The method of claim 1, comprising displaying the measured output value on a display.