Signal processing apparatus and method

• US 6,067,462 A
• Filed: 05/19/1998
• Issued: 05/23/2000
• Est. Priority Date: 04/14/1997
• Status: Expired due to Term

• Current Assignee: Masimo Corporation
• Original Assignee: Masimo Corporation
• Inventors: Diab, Mohamed K., McCarthy, Rex
• Primary Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US09/081,539
• Time in Patent Office: 735 Days
• Field of Search: 600/300, 600/310, 600/322, 600/323, 600/324, 600/336, 600/481, 600/508, 600/509, 600/515, 702/191-197
• US Class Current: 600/310
• 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 signals M₁ and M₂, the signal M₁ containing a primary signal portion S₁ and a secondary signal portion N₁, the signal M₂ containing a primary signal portion S₂ and a secondary signal portion N₂, said signals M₁ and M₂ having the flowing relationship:

space="preserve" listing-type="equation">M.sub.1 =S.sub.1 +N.sub.1

space="preserve" listing-type="equation">M.sub.2 =S.sub.2 +N.sub.2,

a method comprising the steps of;

determining a value for a coefficient c, such that an error value e, given by the relation e=S₁ -(cM₁ -M₂) is partially minimized, where said coefficient c is related to a ratio of a frequency-domain representation of M₁ and a frequency-domain representation of M₂ ; and

using said coefficient c in a waveform scrubber to remove some of the secondary signal portion N₁ from the signal M₁ to produce an approximation A₁ to said primary signal portion S₁, where A₁ =cM₁ -M₂.

2. The method of claim 1, further comprising the step of displaying said approximation A₁ on a display.

3. The method of claim 1, wherein said signals M₁ and M₂ are obtained from physiological signals, the method further comprising the step of processing said approximation A₁ to determine a physiological parameter.

4. The method of claim 1, wherein at least a portion of said primary signal portion S₁ is indicative of a heart plethysmograph, said method further comprising the step of calculating a pulse rate.

5. The method of claim 4, wherein said physiological parameter is arterial oxygen saturation.

6. The method of claim 4, wherein said physiological parameter is an ECG signal.

7. A physiological monitor comprising:

a first input configured to receive a first signal M₁ having a primary portion, S₁, and a secondary portion N₁, said signal M₁ related to a physiologic parameter;

a second input configured to receive a second signal M₂ having a primary portion S₂ and a secondary portion N₂, said first and said second signals M₁ and M₂ being in accordance with the following relationship;

space="preserve" listing-type="equation">M.sub.1 =S.sub.1 +N.sub.1

space="preserve" listing-type="equation">M.sub.2 =S.sub.2 +N.sub.2

where S₁ and S₂, and N₁ and N₂, are related by;

space="preserve" listing-type="equation">S.sub.1 ≈

r.sub.a S.sub.2

space="preserve" listing-type="equation">N.sub.1 ≈

r.sub.v N.sub.2

and where r_a and r_v are coefficients;

a first signal processor, said first signal processor configured to compute said r_a coefficient and said r_v coefficient using a transformed representation of said first signal M₁ and a transformed representation of said second signal M₂ ; and

a waveform scrubber having a first input configured to receive said first signal M₁, and having a second input configured to receive said second signal M₂, said waveform scrubber providing an output signal corresponding to an approximation of S₁.

8. The physiological monitor of claim 7, further comprising a second signal processor, said second signal processor configured to compute a selected blood constituent value from at least a portion of said output signal.

9. The physiological monitor of claim 8, wherein said selected blood constituent is arterial blood oxygen saturation.

10. The physiological monitor of claim 8, wherein said selected blood constituent is venous blood oxygen saturation.

11. The physiological monitor of claim 8, wherein said selected blood constituent comprises carbon monoxide saturation.

12. The physiological monitor of claim 8, wherein said selected blood constituent value corresponds to a physiological concentration of a selected blood constituent.

13. In a signal processor for processing at least two measured signals M₁ and M₂, the signal M₁ having a primary signal portion S₁ and a secondary signal portion N₁, the signal M₂ having a primary signal portion S₂ and a secondary signal portion N₂, such that:

space="preserve" listing-type="equation">M.sub.1 =S.sub.1 +N.sub.1

space="preserve" listing-type="equation">M.sub.2 =S.sub.2 +N.sub.2

where S₁ and S₂, and N₁ and N₂, are related by;

space="preserve" listing-type="equation">S.sub.1 ≈

r.sub.a S.sub.2

space="preserve" listing-type="equation">N.sub.1 ≈

r.sub.v N.sub.2

and where r_a and r_v are coefficients, a method comprising the steps of;

determining a value for a coefficient c, such that an error value e, given by the relation e=S₁ -(cM₁ -M₂) is partially minimized; and

using said coefficient c in a waveform scrubber to remove some of the signal N₁ from the measured signal M₁ to produce an approximation A₁ to said signal S₁, where A₁ =cM₁ -M₂, and where A₁, M₁, and M₂ are expressed as frequency domain signals.

14. A physiological monitor comprising:

a first input configured to receive a first measured signal M₁ having a primary portion, S₁, and a secondary portion N₁ ;

a second input configured to receive a second measured signal M₂ having a primary portion S₂ and a secondary portion N₂, said first and said second measured signals M₁ and M₂ being in accordance with the following relationship;

space="preserve" listing-type="equation">M.sub.1 =S.sub.1 +N.sub.1

space="preserve" listing-type="equation">M.sub.2 =S.sub.2 +N.sub.2

where S₁ and S₂, and N₁ and N₂, are related by;

space="preserve" listing-type="equation">S.sub.1 ≈

r.sub.a S.sub.2

space="preserve" listing-type="equation">N.sub.1 ≈

r.sub.v N.sub.2

and where r_a and r_v are coefficients;

a first signal processor comprising a neural network configured to compute said r_a coefficient and said r_v coefficient using a transformed representation of said signal M₁ and a transformed representation of said signal M₂ ; and

a waveform scrubber having a first input configured to receive said first measured signal and having a second input configured to receive said second measured signal, said waveform scrubber providing an output corresponding to an approximation of S₁.