Signal processing apparatus - Patent Review 5490505

Claims

We claim:

1. In combination:

a detector responsive to a first signal which travels along a first propagation path and a second signal which travels along a second propagation path, to provide a representation of said first and said second signals on an output, a portion of said first and second propagation paths being located in the same propagation medium, wherein said representation of said first signal on said output has a primary signal portion and a secondary signal portion, said primary signal portion of said first signal being subject to attenuation along substantially the entire first propagation path and wherein said representation of said second signal on said output has a primary signal portion and a secondary signal portion, said primary signal portion of said second signal being subject to attenuation along substantially the entire second propagation path; and

a first signal processor having inputs coupled to said detector, said first signal processor responsive to said representation of said first and second signals from said detector to combine said first and second signals to generate either a primary or secondary reference signal which is a function significantly of either of, respectively, said primary or said secondary signal portions of said first and second signals.

2. The combination recited in claim 1, further comprising a second signal processor responsive to the secondary reference signal and to said representation of said first signal to derive therefrom an output signal which is a function of significantly said primary signal portion of said first signal.

3. The combination recited in claim 2, wherein said second signal processor comprises a correlation canceler.

4. The combination recited in claim 2, wherein said second signal processor comprises an adaptive noise canceler.

5. The combination recited in claim 4, wherein said adaptive noise canceler comprises a joint process estimator.

6. The combination recited in claim 5, wherein said joint process estimator comprises a least-squares lattice predictor and a regression filter.

7. The combination recited in claim 1, further comprising a second signal processor responsive to said primary reference signal and to said representation of said first signal to derive therefrom an output signal which is a function significantly of said secondary signal portion of said first signal.

8. The combination recited in claim 1, further comprising a second signal processor responsive to said secondary reference signal and to said representation of said first signal to derive therefrom an output signal which is a function significantly of said primary signal portion of said second signal.

9. The combination recited in claim 1, further comprising a second signal processor responsive to said secondary reference signal and to said representation of said first signal to derive therefrom an output signal having a significant component which is a function of said secondary signal portion of said second signal.

10. The combination recited in claim 1, wherein said detector is configured to detect a physiological function represented by said first and second signals.

11. The combination recited in claim 10, wherein said detector is adapted to measure a blood constituent.

12. The combination recited in claim 11, wherein the blood constituent measured by said detector is blood gas.

13. The combination recited in claim 10, wherein said detector comprises a sensor that is responsive to electromagnetic energy.

14. The combination recited in claim 1, further comprising electromagnetic means connected to said detector for measuring a plethysmographic waveform depending upon said first and second signals received by said detector through said propagation medium, said propagation medium including living tissue.

15. The combination recited in claim 1, further comprising a pulse oximeter connected to said detector, said pulse oximeter monitoring a physiological condition depending upon said first and second signals received by said detector through said propagation medium, said propagation medium including living tissue.

16. The combination recited in claim 1, further comprising a blood pressure monitor connected to said detector and configured to derive a physiological condition depending upon said first and second signals received by said detector through said propagation medium, said propagation medium including living tissue.

17. The combination recited in claim 1, further comprising an electrocardiograph connected to said detector said electrocardiograph adapted to determine an electrocardiogram condition depending upon said first and second signals received by said detector means through said propagation medium, said propagation medium including living tissue.

18. The combination recited in claim 17, wherein said electrocardiograph includes a tripolar electrode sensor having three concentrically arranged electrodes.

19. An apparatus for computing arterial and venous signals in living tissue, said apparatus comprising:

a detector configured to receive a first signal which travels along a first propagation path and a second signal which travels along a second propagation path, at least a portion of said first and second propagation paths being located in a propagation medium, wherein said first signal has an arterial signal portion that is indicative of arterial blood and another signal portion that is indicative of venous blood, and said second signal has an arterial signal portion that is indicative of arterial blood and another signal portion that is indicative of venous blood; and

signal processor means having an input coupled to said detector and responsive to said first and second signals and to combine said first and second signals to generate a signal having a significant component which is a function of either of said arterial or said other signal portions of said first and second signal.

20. The apparatus recited in claim 19, wherein the other signal portion of each of said first and second signals includes an indication of human respiration.

21. A signal processor comprising:

a detector responsive to a first signal which travels along a first propagation path and a second signal which travels along a second propagation path, to provide a representation of said first and said second signals on an output, at least a portion of said first and second propagation paths being located in the same propagation medium, wherein said representation of said first signal on said output has a primary signal portion and a secondary signal portion, and wherein said representation of said second signal on said output has a primary signal portion and a secondary signal portion;

a first signal processor having inputs coupled to said detector, said first signal processor responsive to said representations of said first and second signals from said detector to combine said first and second signals to generate either a primary or secondary reference signal which is a function significantly of either of, respectively, said primary or said secondary signal portions of said first and second signals; and

a second signal processor responsive to the secondary reference signal and to said representation of said first signal to derive therefrom an output signal which is a function of significantly said primary signal portion of said first signal.

22. A signal processor comprising:

a detector responsive to a first signal which travels along a first propagation path and a second signal which travels along a second propagation path, to provide a representation of said first and said second signals on an output, at least a portion of said first and second propagation paths being located in the same propagation medium, wherein said representation of said first signal on said output has a primary signal portion and a secondary signal portion, and wherein said representation of said second signal on said output has a primary signal portion and a secondary signal portion;

a first signal processor having inputs coupled to said detector, said first signal processor responsive to said representations of said first and second signals from said detector to combine said first and second signals to generate either a primary or secondary reference signal which is a function significantly of either of, respectively, said primary or said secondary signal portions of said first and second signals; and

a second signal processor responsive to said primary reference signal and to said representation of said first signal to derive therefrom an output signal which is a function significantly of said secondary signal portion of said first signal.

23. A signal processor comprising:

a detector responsive to a first signal which travels along a first propagation path and a second signal which travels along a second propagation path, to provide a representation of said first and said second signals on an output, at least a portion of said first and second propagation paths being located in the same propagation medium, wherein said representation of said first signal on said output has a primary signal portion and a secondary signal portion, and wherein said representation of said second signal on said output has a primary signal portion and a secondary signal portion;

a first signal processor having inputs coupled to said detector, said first signal processor responsive to said representations of said first and second signals from said detector to combine said first and second signals to generate either a primary or secondary reference signal which is a function significantly of either of, respectively, said primary or said secondary signal portions of said first and second signals; and

a second signal processor responsive to said secondary reference signal and to said representation of said first signal to derive therefrom an output signal which is a function significantly of said primary signal portion of said second signal.

24. A signal processor comprising:

a detector responsive to a first signal which travels along a first propagation path and a second signal which travels along a second propagation path, to provide a representation of said first and said second signals on an output, at least a portion of said first and second propagation paths being located in the same propagation medium, wherein said representation of said first signal on said output has a primary signal portion and a secondary signal portion, and wherein said representation of said second signal on said output has a primary signal portion and a secondary signal portion;

a first signal processor having inputs coupled to said detector, said first signal processor responsive to said representations of said first and second signals from said detector to combine said first and second signals to generate either a primary or secondary reference signal which is a function significantly of either of, respectively, said primary or said secondary signal portions of said first and second signals; and

a second signal processor responsive to said secondary reference signal and to said representation of said first signal to derive therefrom an output signal having a significant component which is a function of said secondary signal portion of said second signal.

FIELD OF THE INVENTION

The present invention relates to the field of signal processing. More specifically, the present invention relates to the processing of measured signals, containing a primary and a secondary signal, for the removal or derivation of either the primary or secondary signal when little is known about either of these components. The present invention also relates to the use of a novel processor which in conjunction with a correlation canceler, such as an adaptive noise canceler, produces primary and/or secondary signals. The present invention is especially useful for physiological monitoring systems including blood oxygen saturation.

BACKGROUND OF THE INVENTION

Signal processors are typically employed to remove or derive either the primary or secondary signal portion from a composite measured signal including a primary signal portion and a secondary signal portion. If the secondary signal portion occupies a different frequency spectrum than the primary signal portion, then conventional filtering techniques such as low pass, band pass, and high pass filtering could be used to remove or derive either the primary or the secondary signal portion from the total signal. Fixed single or multiple notch filters could also be employed if the primary and/or secondary signal portion(s) exit at a fixed frequency(s).

It is often the case that an overlap in frequency spectrum between the primary and secondary signal portions exists. Complicating matters further, the statistical properties of one or both of the primary and secondary signal portions change with time. In such cases, conventional filtering techniques are totally ineffective in extracting either the primary or secondary signal. If, however, a description of either the primary or secondary signal portion can be made available correlation canceling, such as adaptive noise canceling, can be employed to remove either the primary or secondary signal portion of the signal leaving the other portion available for measurement.

Correlation cancelers, such as adaptive noise cancelers, dynamically change their transfer function to adapt to and remove either the primary or secondary signal portions of a composite signal. Correlation cancelers require either a secondary reference or a primary reference which is correlated to either the secondary signal or the primary signal portions only. The reference signals are not necessarily a representation of the primary or secondary signal portions, but have a frequency spectrum which is similar to that of the primary or secondary signal portions. In many cases, it requires considerable ingenuity to determine a reference signal since nothing is usually known a priori about the secondary and/or primary signal portions.

One area where composite measured signals comprising a primary signal portion and a secondary signal portion about which no information can easily be determined is physiological monitoring. Physiological monitoring apparatuses generally measure signals derived from a physiological system, such as the human body. Measurements which are typically taken with physiological monitoring systems include electrocardiographs, blood pressure, blood gas saturation (such as oxygen saturation), capnographs, heart rate, respiration rate, and depth of anesthesia, for example. Other types of measurements include those which measure the pressure and quantity of a substance within the body such as breathalyzer testing, drug testing, cholesterol testing, glucose testing, arterial carbon dioxide testing, protein testing, and carbon monoxide testing, for example. Complications arising in these measurements are often due to motion of the patient, both external and internal (muscle movement, for example), during the measurement process.

Knowledge of physiological systems, such as the amount of oxygen in a patient's blood, can be critical, for example during surgery. These data can be determined by a lengthy invasive procedure of extracting and testing matter, such as blood, from a patient, or by more expedient, non-invasive measures. Many types of non-invasive measurements can be made by using the known properties of energy attenuation as a selected form of energy passes through a medium.

Energy is caused to be incident on a medium either derived from or contained within a patient and the amplitude of transmitted or reflected energy is then measured. The amount of attenuation of the incident energy caused by the medium is strongly dependent on the thickness and composition of the medium through which the energy must pass as well as the specific form of energy selected. Information about a physiological system can be derived from data taken from the attenuated signal of the incident energy transmitted through the medium if either the primary or secondary signal of the composite measurement signal can be removed. However, non-invasive measurements often do not afford the opportunity to selectively observe the interference causing either the primary or secondary signal portions, making it difficult to extract either one of them from the composite signal.

The primary and/or secondary signal portions often originate from both AC and/or DC sources. The DC portions are caused by transmission of the energy through differing media which are of relatively constant thickness within the body, such as bone, tissue, skin, blood, etc. These portions are easy to remove from a composite signal. The AC components are caused by physiological pulsations or when differing media being measured are perturbed and thus, change in thickness while the measurement is being made. Since most materials in and derived from the body are easily compressed, the thickness of such matter changes if the patient moves during a non-invasive physiological measurement. Patient movement, muscular movement and vessel movement, can cause the properties of energy attenuation to vary erratically. Traditional signal filtering techniques are frequently totally ineffective and grossly deficient in removing these motion induced effects from a signal. The erratic or unpredictable nature of motion induced signal components is the major obstacle in removing or deriving them. Thus, presently available physiological monitors generally become totally inoperative during time periods when the measurement site is perturbed.

A blood gas monitor is one example of a physiological monitoring system which is based upon the measurement of energy attenuated by biological tissues or substances. Blood gas monitors transmit light into the tissue and measure the attenuation of the light as a function of time. The output signal of a blood gas monitor which is sensitive to the arterial blood flow contains a component which is a waveform representative of the patient's arterial pulse. This type of signal, which contains a component related to the patient's pulse, is called a plethysmographic wave, and is shown in FIG. 1 as curve s. Plethysmographic waveforms are used in blood pressure or blood gas saturation measurements, for example. As the heart beats, the amount of blood in the arteries increases and decreases, causing increases and decreases in energy attenuation, illustrated by the cyclic wave s in FIG. 1.

Typically, a digit such as a finger, an ear lobe, or other portion of the body where blood flows close to the skin, is employed as the medium through which light energy is transmitted for blood gas attenuation measurements. The finger comprises skin, fat, bone, muscle, etc., shown schematically in FIG. 2, each of which attenuates energy incident on the finger in a generally predictable and constant manner. However, when fleshy portions of the finger are compressed erratically, for example by motion of the finger, energy attenuation becomes erratic.

An example of a more realistic measured waveform S is shown in FIG. 3, illustrating the effect of motion. The primary plethysmographic waveform portion of the signal s is the waveform representative of the pulse, corresponding to the sawtooth-like pattern wave in FIG. 1. The large, secondary motion-induced excursions in signal ampl