Inventors: M o h a m e d K. Diab, Mission Viejo; Massi E. Kiani, Laguna Niguel; Walter M Weber, Laguna Hills, all of C A (US)
(73)
Assignee: Masimo Corporation, Irvine, C A (US)
( *)
Notice:
Subject to any disclaimer, the term of this patent is extended or adjusted under 35 U.S.C. 154(b) by 0 days.
(21)
Appl. No.: 08/943,511
(22)
Filed:
(io) Patent No.: US 6,263,222 B l (45) Date of Patent: Jul. 17,2001
92/15955
9/1992 (WO) . OTHER PUBLICATIONS
Rabiner, Lawrence et al. Theory and Application of Digital Signal Processing, p. 260, 1975. Tremper, Kevin et al.. Advances in Oxygen Monitoring, pp. 1 3 7 - 1 5 3 , 1987. (List continued on next page.) Primary Examiner—Eric F. Winakur (74) Attorney, Agent, or Firm—Knobbe, Bear, LLP (57)
(63)
Continuation of application No. 08/572,488, filed on Dec. 14, 1995, now Pat. No. 5,685,299, which is a continuation of application No. 08/132,812, filed on Oct. 6, 1993, now Pat. No. 5,490,505, which is a continuation-in-part of application No. 07/666,060, filed on Mar. 7, 1991, now abandoned.
(51) (52) (58)
Int. CI. 7 A 6 1 B 5/00 U.S. CI 600/310; 600/336 Field of Search 600/300, 310, 600/322, 323, 330, 336, 473, 476, 479, 485, 500, 5 0 1 ; 356/39-41 References Cited U.S. PATENT D O C U M E N T S 3,647,299 3,704,706 4,063,551 4,086,915
3/1972 12/1972 12/1977 5/1978
Lavallee . Herczfeld et al. . Sweeney . Kofsky et al. .
(List continued on next page.) FOREIGN PATENT D O C U M E N T S 3328862 341327 2166326 2235288 1674798
ABSTRACT
Oct. 6, 1997 Related U.S. Application Data
(56)
Martens, Olson &
2/1985 11/1989 4/1986 2/1991 9/1991
(DE) (EP) (GB) (GB) (SU)
. . . . .
A signal processor which acquires a first signal, including a first primary signal portion and a first secondary signal portion, and a second signal, including a second primary signal portion and a second secondary signal portion, wherein the first and second primary signal portions are correlated. The signals may be acquired by propagating energy through a medium and measuring an attenuated signal after transmission or reflection. Alternatively, the signals may be acquired by measuring energy generated by the medium. A processor of the present invention generates a primary or secondary reference signal which is a combination, respectively, of only the primary or secondary signal portions. The secondary reference signal is then used to remove the secondary portion of each of the first and second measured signals via a correlation canceler, such as an adaptive noise canceler, preferably of the joint process estimator type. The primary reference signal is used to remove the primary portion of each of the first and second measured signals via a correlation canceler. The processor of the present invention may be employed in conjunction with a correlation canceler in physiological monitors wherein the known properties of energy attenuation through a medium are used to determine physiological characteristics of the medium. Many physiological conditions, such as the pulse, or blood pressure of a patient or the concentration of a constituent in a medium, can be determined from the primary or secondary portions of the signal after other signal portion is removed. 23 Claims, 2 0 Drawing Sheets
2JA
,-24t>
US 6,263,222 B l Page 2
U.S. PATENT DOCUMENTS 4,095,117 6/1978 Nagy . 4,407,290 10/1983 Wilber . 4,537,200 8/1985 Widrow . 4,649,505 3/1987 Zinser, Jr. et al. . 4,723,294 2/1988 Taguchi. 4,773,422 9/1988 Isaacson et al. . 4,799,493 1/1989 DuFault . 4,800,495 1/1989 Smith . 4,819,752 4/1989 Zelin . 4,824,242 4/1989 Frick et al. . 4,848,901 7/1989 Hood, Jr. . 4,860,759 8/1989 Kahn et al. . 4,863,265 9/1989 Flower et al. . 4,867,571 9/1989 Frick et al. . 4,869,253 9/1989 Craig, Jr. et al. . 4,869,254 9/1989 Stone et al. . 4,883,353 11/1989 Hausman . 4,892,101 1/1990 Cheung et al. . 4,907,594 3/1990 Muz . 4,911,167 3/1990 Corenman et al. . 4,927,264 5/1990 Shiga et al. . 4,928,692 5/1990 Goodman et al. . 4,948,248 8/1990 Lehman . 4,955,379 9/1990 Hall . 4,956,867 9/1990 Zurek et al. . 4,960,126 10/1990 Conlon et al. . 5,003,977 * 4/1991 Suzuki et al 5,057,695 10/1991 Hirao et al. . 5,246,002 9/1993 Prosser . 5,273,036 12/1993 Kronberg et al. . 5,431,170 7/1995 Mathews . 5,458,128 10/1995 Polanyi et al. . 5,494,032 * 2/1996 Robinson et al 5,685,299 * 11/1997 Diab et al
600/476
600/323 600/300
OTHER PUBLICATIONS Harris, Fred et al., "Digital Signal Processing with Efficient Polyphase Recursive All-Pass Filters", Presented at International Conference on Signal Processing, Florence, Italy, Spet. 4-6, 1991, 6 pages. Haykin, Simon, Adaptive Filter Theory, Prentice Hall, Englewood Cliffs, NJ, 1985.
Widrow, Bernard, Adaptive Signal Processing, Prentice Hall, Englewood Cliffs, NJ 1985. Brown, David P., "Evaluation of Pulse Oximeters using Theoretical Models and Experimental Studies", Master's thesis, University of Washington, Nov. 25, 1987, pp. 1-142. Cohen, Arnon, "vol. I" Time and Frequency Domains Analysis, Biomedical Signal Processing, CRC Press, Inc., jBoca Raton, Florida, pp. 152-159. Severinghaus, J.W., "Pulse Oximetry Uses and Limitations", pp. 1-4, ASA Convention, New Orleans, 1989. Mook, G.A., et al., "Spectrophotometirc determination of Oxygen saturation of blood independent of the presence of indocyanine green". Cardiovascular Research, vol. 13, pp. 233-237, 1979. Neuman, Michael R., "Pulse Oximetry: Physical Principles, Technical Realization and Present Limitations", Continuous Transcutaneous Monitoring, Plenum Press, New York, 1987, pp. 135-144. Mook, G.A., et al., "Wavelength dependency of the spectrophotometric determination of blood oxygen saturation", Clinical Chemistry Acta, vol. 26, pp. 170-173, 1969. Klimasauskas, Casey, "Neural Nets and Noise Filtering", Dr. Dobb's Journal, Jan. 1989, p. 32. Melnikof, S. "Neural Networks for Signal Processing: A Case Study", Dr. Dobbs Journal, Jan. 1989. p. 36-37. Jingzheng, Ouyang et al., "Digital Processing of HighResolution Electrocardiograms—Detection of HisPurkinje Activity from the Body Surface", Biomedizinische Technik, 33, Oct. 1, 1988, No.10, Berlin, W. Germany, pp. 224-230. Chen, Jiande, et al., "Adaptive System for Processing of Electrogastric Signals", Images of the Twenty-First Century, Seattle, WA, vol. 11, Nov. 9-12, 1989. pp. 698-699. Varanini, M. et al., "A Two Channel Adaptive Filtering Approach for Recognition of the QRS Morphology", Proceedings of the Computers in Cardiology Meeting, Venice, Sep. 23-26, 1991, Institute of Electrical and Electronics Engineers, pp. 141-144. * cited by examiner
U.S. Patent
Jul. 17,2001
Sheet 1 of 20
US 6,263,222 B l
FIG. I
^-t
BONE MUSCLE TISSUE ARTERIAL BLOOD VENOUS BLOOD
FIG. 2
FIG. 3 S = s+n
-t
U.S. Patent
Jul. 17, 2001
US 6,263,222 B l
Sheet 2 of 20
22b Xa
20-
Aa"
SIGNAL CONDITIONER
^a
'22a S
246
SIGNAL CONDITIONER
X]5-s\bhnXb
A/D
24a _£_ A/D
FIG. 4a n(t)=
n X a ( l ) - CO a n x b (t) -28
CORRELATION CANCELER
DISPUY s
'Xa(t)
U.S. Patent
Jul. 17, 2001
Sheet 3 of 20
US 6,263,222 B l
U.S. Patent
Jul. 17, 2001
US 6,263,222 B l
Sheet 4 of 20
Q:
O
O
m
^ ^
^
0
ID
U.S. Patent
Jul. 17,2001
Sheet 5 of 20
US 6,263,222 Bl
>-
O
o
TRANSFER FUNCTION
U.S. I•atent
jui. 17,2001
siu:et 6 of 20
US 6,263,222 B l
V
lo V
r4Z
Ai Ai A1 A1 A1 A1A1 A2A2 A2 A2 A2 A2A2\ ••
An An An An An An An
FK 7.
V
1
6CL
lo
V
V
|Ai Ai A2A2 A3 A3 A4A4
Ai A2 A3 A4
Ai A2 As A4
r<2
Ai A2 A3 A4
A1A1 A2A2 A3 A^ A4A4
As ^ 5 As Ae A e A e /Ae Ae As Ae As \
:
( An A n An A n An An An
Fl G. 6b
fli V
lo \
Ai Ai A2A2 A3 A A4A3
[
Ai Ai Ai Ai Ai A2 A2 A2 A2A2 A3 A A3 A3A4 A4 A 4 A 4 A3
As A . As Ae A e A e Ae " A e As Ae As An An An An t ^\n An An )
FK3.6c
I V
)
U.S. Patent
Jul. 17, 2001
Sheet 7 of 20
US 6,263,222 B l
65 OH
=>oi III O
Ld 1— 1— X Ld O
y^
^N on I 01— < C£ O
UJ 1—
z
U.S. Patent
Jul. 17, 2001
Sheet 8 of 20
US 6,263,222 B l
o
3"
3
3
3 m 10
3
o
3 0 3m
^ ^
3
3
CM
3
RELATIVE CORRELATION CANCELER ENERGY OUTPUT
o o
U.S. Patent
Jul. 17, 2001
Sheet 9 of 20
US 6,263,222 B l
i o
3 3
00
3
3 (/) ID
o
3
>
o
3 3 3
o o
3
^
m
3 3
ro
3 3
RELATIVE CORRELATION CANCELER ENERGY OUTPUT
U.S. Patent
Jul. 17, 2001
US 6,263,222 B l
Sheet 10 of 20
LEAST SQURES LATTICE PREDICTOR
REGRESSION FILTER A-
%
00
U.S. Patent
Jul. 17, 2001
Sheet 11 of 20
US 6,263,222 B l
/20 INITIALIZE NOISE CANCELLER INPUT NEW SAMPLES
.fjO
[S
Aa(t) AND S X b (t)]
TIME UPDATE OF [Z"1] ELEMENTS
I
f40 CALCULATE REFERENCE [n'(t) OR S'(t)] FOR TWO MEASURED SIGNAL SAMPLES
/SO ZERO-STAGE UPDATE
•teo m = 0 •f70 ORDER UPDATE mih-STAGE OF LSL-PREDICTOR
/80 ORDER UPDATE mth-STAGE OF REGRESSION FILTER(S) m = m + 1
200 CALCULATE OUTPUT
-2fO TO DISPLAY
FIG.9
U.S. Patent
Jul. 17, 2001
LEAST SQURES LATTICE PREDICTOR
Sheet 12 of 20 REGRESSION FILTER
US 6,263,222 B l REGRESSION FILTER
U.S. Patent
Jul. 17, 2001
U S 6,263,222 B l
Sheet 13 of 20
1
n r'
A
\^-J
/ o
y v
I
"1
3/g
i
S /
A
*
v .T %
L _
A
oto
§S
v /
^
V C3 V*-
'
1
§
I— <
S ^J \
s
o ^ .
o
Q_
to o
Q_
/"\_
I
>-
o o az
O
/L^-_A V i
CO UJ
'<
o o
I— < UJ > Q
Li_ •«-
^
S!
Q:
o %
^
C^ O
>
a: Q
I CO
U.S. Patent
Jul. 17, 2001
US 6,263,222 B l
Sheet 14 of 20
FIG. 12
1000~ 600
500
X(nm)
I
I
i H- — 1 1 !
- ———
!
4/0\ 4_
1 ^ - - - + -
I 1
1 1
1 1 1 1
1 1 1 1
!
, 1
_l
•_•
!
!
Hb/ /
/
/
/
/
/
/
I /
^ U i ^
i .4. 1 1 1
,^l !
!
i i i i i
!
i/
1 --. ! / ^
i i i i
Hb02
i
1 1
1
1000
900
800
X(nm)
FIG. 14
1
1- 1 —
700
600
U.S. Patent
Jul. 17, 2001
Sheet 15 of 20
US 6,263,222 B l
E c o > <
CN
o
J3
CO CO
U.S.
Patent
Jul. 17, 2001
Sheet 16 of 20
U S 6,263,222 B l
S
Xa(t) = S X r e d l (t)
S
\b(t) = sXred2(t)
FIG. 15
FIG. 16
s
FIG. 17
FIG. 18
FIG. 19
xcM=SxiR(t)
U.S.
Patent
Jul. 17, 2001
Sheet 17 of 20
U S 6,263,222 B l
s'xaCt) = s " \ r e d l (*)
FIG. 20
I
2/o
2/c
s
Xc(t) = s \ I R ( t )
FIG. 21 • • t
"AaM = "Aredl W
FIG. 22
23c
FIG. 23
n
Xc(t)=nXIR(t)
J / -—t
U.S. Patent
Jul. 17,2001
Sheet 18 of 20
US 6,263,222 Bl
s
\a(t)= s Xred(t)
FIG. 24
S
Xb(t) = S XIR(t)
FIG. 25
FIG. 26
FIG. 27 •-t
U.S.
Patent
Jul. 17, 2001
Sheet 19 of 20
US 6,263,222 B l
SXaOO^AredlOO
FIG. 28
I
£Sa
s
xb(t) = 4aR(t)
FIG. 29
ri
Xa(t) = n Xredl(t)
FIG. 30 •-t
n
Xb(t) = n xiR(t)
FIG.3I • • t
U.S. Patent
Jul. 17, 2001
Sheet 20 of 20
US 6,263,222 B l
v>
E i_
o o UJ 5!
a
(/)
Q
<
C7)
TO on
o
UJ
a; O
o c o o UJ
O
US 6,263,222 Bl 1 SIGNAL PROCESSING APPARATUS
2
generally measure signals derived from a physiological system, such as the human body. Measurements which are PRIOR RELATED APPLICATIONS typically taken with physiological monitoring systems include electrocardiographs, blood pressure, blood gas satuThis is a continuation of application Ser. No. 08/572,488 s r a t i o n ( s u c h a s oxygen saturation), capnographs, heart rate, filed Dec. 14, 1995, now U.S. Pat. No. 5,685,299, which is respiration rate, and depth of anesthesia, for example. Other a continuation of Ser. No. 08/132,812 filed Oct. 6,1993, now types of measurements include those which measure the U.S. Pat. No. 5,490,505, which is a continuation-in-part of pressure and quantity of a substance within the body such as U.S. patent application Ser. No. 07/666,060 filed Mar. 7, breathalyzer testing, drug testing, cholesterol testing, glu1991, now abandoned. cose testing, arterial carbon dioxide testing, protein testing, and carbon monoxide testing, for example. Complications FIELD OF THE INVENTION arising in these measurements are often due to motion of the rj* . • .• , . . ., n , j r • , patient, both external and internal (muscle movement, for Ine present invention relates to the field of signal pro, x , • , • », -r. n .. . • .• , . . example), during the measurement process, cessmg. More specifically, the present invention relates to r /> & ±the processing of measured signals, containing a primary 15 Knowledge of physiological systems, such as the amount of ox and a secondary signal, for the removal or derivation of y g e n l n a Patient's blood, can be critical, for example durm either the primary or secondary signal when little is known g s u r g e r y- T h e s e d a t a c a n b e determined by a lengthy lnvaslve about either of these components. The present invention also Procedure of extracting and testing matter, such as blood from a relates to the use of a novel processor which in conjunction ' P a t l e n t ' o r b y m o r e expedient, non-invasive with a correlation canceler, such as an adaptive noise 20 measures. Many types of non-invasive measurements can be made b usln the known canceler, produces primary and/or secondary signals. The y g properties of energy attenuation as a s e l e c t e d f o r m o f ener present invention is especially useful for physiological g y P a s s e s t h r o u g h a medium. Ene monitoring systems including blood oxygen saturation. r g y is c a u s e d to b e incident on a medium either derived from or contained within a patient and the amplitude BACKGROUND OF THE INVENTION 25 of transmitted or reflected energy is then measured. The amount of attenuation of the incident energy caused by the Signal processors are typically employed to remove or m e d i u m i s str ongly dependent on the thickness and compoderive either the primary or secondary signal portion from a sition of the m e d i u m through w h i c h the energy m u s t p a s s as composite measured signal including a primary signal porwell as the specific form of energy selected. information tion and a secondary signal portion. If the secondary signal about a physiological system can be derived from data taken portion occupies a different frequency spectrum than the from the attelluated signal of the illcident energy transmitted primary signal portion, then conventional filtering techthrough the medium if either the primary or secondary signal mques such as low pass, band pass, and high pass filtering o f t h e c o m p o s i t e m e a s u r e m e n t s i g n a i c a n be removed, could be used to remove or derive either the primary or the However, non-invasive measurements often do not afford secondary signal portion from the total signal. Fixed single t h e o p p o r t u n i t y t o s e i e c t i v e i y observe the interference causor multiple notch filters could also be employed if the i n g e i t h e r t h e p r i m a r y o r s e C o n d a r y s i g n a i portions, making primary and/or secondary signal portion(s) exit at a fixed i t d i f f i c u l t t o e x t r a c t e i t h e r o n e o f t h e m f r o m t h e C o m posite frequency(s). signal It is often the case that an overlap in frequency spectrum T h e p r i m a r y a n d / o r secondary signal portions often origibetween the primary and secondary signal portions exists. 40 n a t e f r o m both AC and/or DC sources. The DC portions are Complicating matters further, the statistical properties of one c a u s e d by transmission of the energy through differing or both of the primary and secondary signal portions change m e d i a w h i c h a r e of relatively constant thickness within the with time. In such cases, conventional filtering techniques b o d y ; s u c h a s bone, tissue, skin, blood, etc. These portions are totally ineffective in extracting either the primary or a r e e a s y t o remove from a composite signal. The AC secondary signal. If, however, a description of either the 4S components are caused by physiological pulsations or when primary or secondary signal portion can be made available differing m e d i a being measured are perturbed and thus, correlation canceling, such as adaptive noise canceling, can c h a n g e i n thickness while the measurement is being made, be employed to remove either the primary or secondary S i n c e m o s t m a t e r i a l s i n a n d derived from the body are easily signal portion of the signal leaving the other portion availcompressed, the thickness of such matter changes if the able tor measurement. 50 p a ti e n t moves during a non-invasive physiological measureCorrelation cancelers, such as adaptive noise cancelers, ment. Patient movement, muscular movement and vessel dynamically change their transfer function to adapt to and movement, can cause the properties of energy attenuation to remove either the primary or secondary signal portions of a vary erratically. Traditional signal filtering techniques are composite signal. Correlation cancelers require either a frequently totally ineffective and grossly deficient in removsecondary reference or a primary reference which is corre- 55 ing these motion induced effects from a signal. The erratic lated to either the secondary signal or the primary signal or unpredictable nature of motion induced signal compoportions only. The reference signals are not necessarily a nents is the major obstacle in removing or deriving them, representation of the primary or secondary signal portions. Thus, presently available physiological monitors generally but have a frequency spectrum which is similar to that of the become totally inoperative during time periods when the primary or secondary signal portions. In many cases, it m measurement site is perturbed. requires considerable ingenuity to determine a reference A blood gas monitor is one example of a physiological signal since nothing is usually known a priori about the monitoring system which is based upon the measurement of secondary and/or primary signal portions. energy attenuated by biological tissues or substances. Blood One area where composite measured signals comprising a gas monitors transmit light into the tissue and measure the primary signal portion and a secondary signal portion about 65 attenuation of the light as a function of time. The output which no information can easily be determined is physisignal of a blood gas monitor which is sensitive to the ological monitoring. Physiological monitoring apparatuses arterial blood flow contains a component which is a wave-
US 6,263,222 Bl 3 form 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 cvclic wave s in FIG 1 rr • ,, ,. .. , r, , ^ lypically, a digit such as a iineer, an ear lobe, or other /• ^1ii i i .> i a i ^ LI i • • portion of the body where blood flows close to the skin, is , , .. ,. ., , , . , ,. , . . employed as the medium through which light energy is „ ,, , ". ^ in Vi , „ transmitted tor blood gas attenuation measurements. Ihe • i• ?^ i , ^ i i finger comprises skin, tat, bone, muscle, etc., shown sche°. „ . r „ T „ » i r i•i ^ \ • -i ^ matically in FIG. 2, each of which attenuates energy incident , ^ . ,, ,• , , , , , \_ on ithe finger in a generally predictable and constant manner. i a i »• r^i n J TT However, when fleshy portions of the finger are compressed ,, r i i .r .i ^ i. erratically, tor example by motion of the finger, energy , 4.i. attenuation becomes erratic.
4 primary signal portions from either of the first or second measured signals. The remaining secondary signal portions from the first and second measured signals are combined to form the secondary reference. This secondary reference is 5 correlated to the secondary signal portion of each of the first a ™ second measured signals. The secondary reference is then used to remove the secondary portion of each of the first and second measured signals via a correlation canceler, such as an adaptive noise l-m u canceler. The correlation canceler is a device which takes a „ , , , . „ , „ ,, ^ , • , •,, first and second input and removes from the first input all . , ^ ,. , , ^ , ^ i1 ,. signal components which are correlated to the second input, .0 . ,. , „ , . . . i1 . „ Any unit which performs or nearly performs this function is , . ., ,^ , -, :• , A , iherein considered to be a correlation canceler. An adaptive , .. , , , ., , , , ^ l:K correlation canceler can be described by analogy to a > , . , , ^^ ,•, , • „ , 1i . , dynamic multiple notch filter which dynamically changes its ^ „ „ ^ i . , , , transfer function in response to a reference signal and ithe i • i * r r JT , measured signals to remove frequencies from the measured , ^ ^ , ^. ? c -, T,, signals that are also present in the reference signal. 1 hus, a ^ !=. , , i . , i. , . ^i . , „ 2n typical adaptive correlation canceler receives the signal trom w h i c h i t i s d e s i r e d t o r e m o v e a c o m p o n e n t a n d a re f erence s i g n a l T h e o u t p u t o f t h e correlation canceler is a good approximation to the desired signal with the undesired comoonent removed Alternatively, the first and second measured signals may
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 signals 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 amplitude be e s s e d t0 ate a ima reference which does not hide the primary plethysmographic signal s. It is easy to see contain ^ seconda si al t i o n s f r o m e i t h e r o f t h e flrst how even small variations in amplitude make it difficult to or s e c o n d m e a s u r e d si als T h e remaillin ima si al distinguish the primary signal s in the presence of a secportions from ^ flrst and second m e a s u r e d signals are ondary signal component n. ^ c o m b i l l e d t o f o r m t h e p r i m a r y reference. The primary refAspecific example of a blood gas monitoring apparatus is may then be used to remove the primary portion of eKnce a pulse oximeter which measures the arterial saturation of e a c h of the first and second measured signals via a correoxygen in the blood. The pumping of the heart forces freshly i a t j o n canceler. The output of the correlation canceler is a oxygenated blood into the arteries causing greater energy g 0 o d approximation to the secondary signal with the priattenuation. The arterial saturation of oxygenated blood may 3 5 m a r y signal removed and may be used for subsequent be determined from the depth of the valleys relative to the processing in the same instrument or an auxiliary instrupeaks of two plethysmographic waveforms measured at m e n t . i n this capacity, the approximation to the secondary separate wavelengths. Patient movement introduces signal s i g n a i m a y be used as a reference signal for input to a second portions mostly due to venous blood, or motion artifacts, to correlation canceler together with either the first or second the plethysmographic waveform illustrated in FIG. 3. It is 40 measured signals for computation of, respectively, either the these motion artifacts which must be removed from the first or second primary signal portions, measured signal for the oximeter to continue the measurePhysiological monitors can often advantageously employ ment of arterial blood oxygen saturation, even during pens i g n a l processors of the present invention. Often in physiods when the patient moves. It is also these motion artifacts ological measurements a first signal comprising a first which must be derived from the measured signal for the 4S p r i m a r y p o r t i o n a n d a flrst s e c o n d a r y p o r t i o l l a n d a s e c 0 I l d oximeter to obtain an estimate of venous blood oxygen s i g n a l comprising a second primary portion and a second saturation. Once the signal components due to either arterial s e c o n d a r y p o r tion are acquired. The signals may be acquired blood or venous blood is known, its corresponding oxygen b y propagating energy through a patient's body (or a matesaturation may be determined. r i a l w h i c h ^ derived from the body, such as breath, blood,
' for e x a m p l e ) o r inside a vessel and measuring an attenuated signal after transmission or reflection. This invention is an improvement of U.S. Patent appliAlternatively, the signal may be acquired by measuring cation Ser. No. 07/666,060 filed Mar. 7, 199f and entitled energy generated by a patient's body, such as in electrocarSignal Processing Apparatus and Method, which earlier diography. The signals are processed via the signal processor application has been assigned to the assignee of the instant 55 of the present invention to acquire either a secondary application. The invention is a signal processor which reference or a primary reference which is input to a correacquires a first signal and a second signal that is correlated lation canceler, such as an adaptive noise canceler. to the first signal. The first signal comprises a first primary One physiological monitoring apparatus which can signal portion and a first secondary signal portion. The advantageously incorporate the features of the present second signal comprises a second primary signal portion and go invention is a monitoring system which determines a signal a second secondary signal portion. The signals may be which is representative of the arterial pulse, called a plethysacquired by propagating energy through a medium and mographic wave. This signal can be used in blood pressure measuring an attenuated signal after transmission or refleccalculations, blood gas saturation measurements, etc. A tion. Alternatively, the signals may be acquired by measurspecific example of such a use is in pulse oximetry which ing energy generated by the medium. 65 determines the saturation of oxygen in the blood. In this The first and second measured signals are processed to configuration, we define the primary portion of the signal to generate a secondary reference which does not contain the be the arterial blood contribution to attenuation of energy as SUMMARY OF THE INVENTION
50 o r t i s s u e
US 6,263,222 Bl 5
6
it passes through a portion of the body where blood flows receiving the first and second signals. The processor is close to the skin. The pumping of the heart causes blood flow adapted to combine the first and second signals to generate to increase and decrease in the arteries in a periodic fashion, a secondary reference having a significant component which causing periodic attenuation wherein the periodic waveform is a function of the first and said second secondary signal is the plethysmographic waveform representative of the 5 portions. The processor may also be adapted to combine the arterial pulse. We define the secondary portion of the signal first and second signals to generate a primary reference to be that which is usually considered to be noise. This having a significant component which is a function of the portion of the signal is related to the venous blood contri- first and second primary signal portions bution to attenuation of energy as it passes through the body. The above described aspect of the present invention may Patient movement causes this component to flow in an 10 further comprise a signal processor for receiving the secunpredictable manner, causing unpredictable attenuation ondary reference signal and the first signal and for deriving and corrupting the otherwise periodic plethysmographic therefrom an output signal having a significant component waveform. Respiration also causes secondary or noise comwhich is a function of the first primary signal portion of the ponent to vary, although typically at a much lower frequency first signal. Alternatively, the above described aspect of the than the patients pulse rate. present invention may further comprise a signal processor A physiological monitor particularly adapted to pulse for receiving the secondary reference signal and the second oximetry oxygen saturation measurement comprises two signal and for deriving therefrom an output signal having a light emitting diodes (LED's) which emit light at different significant component which is a function of the second wavelengths to produce first and second signals. A detector primary signal portion of the second signal. Alternatively, registers the attenuation of the two different energy signals 20 the above described aspect of the present invention may after each passes through an absorptive media, for example further comprise a signal processor for receiving the primary a digit such as a finger, or an earlobe. The attenuated signals reference and the first signal and for deriving therefrom an generally comprise both primary and secondary signal poroutput signal having a significant component which is a tions. Astatic filtering system, such as a bandpass filter, function of the first secondary signal portion of the signal of removes a portion of the secondary signal which is outside 2 5 the first signal. Alternatively, the above described aspect of of a known bandwidth of interest, leaving an erratic or the present invention may further comprise a signal procesrandom secondary signal portion, often caused by motion sor for receiving the primary reference and the second signal and often difficult to remove, along with the primary signal and for deriving therefrom an output signal having a sigportion. nificant component which is a function of the second secNext, a processor of the present invention removes the 30 ondary signal portion of the second signal. The signal primary signal portions from the measured signals yielding processor may comprise a correlation canceler, such as an a secondary reference which is a combination of the remainadaptive noise canceler. The adaptive noise canceler may ing secondary signal portions. The secondary reference is comprise a joint process estimator having a least-squarescorrelated to both of the secondary signal portions. The lattice predictor and a regression filter, secondary reference and at least one of the measured signals 35 The detector in the aspect of the signal processor of the are input to a correlation canceler, such as an adaptive noise present invention described above may further comprise a canceler, which removes the random or erratic portion of the sensor for sensing a physiological function. The sensor may secondary signal. This yields a good approximation to the comprise a light or other electromagnetic sensitive device, primary plethysmographic signal as measured at one of the Additionally, the present invention may further comprise a measured signal wavelengths. As is known in the art, 4Q pulse oximeter for measuring oxygen saturation in a living quantitative measurements of the amount of oxygenated organism. The present invention may further comprise an arterial blood in the body can be determined from the electrocardiograph. plethysmographic signal in a variety of ways. Another aspect of the present invention is a physiological The processor of the present invention may also remove monitoring apparatus comprising a detector for receiving a the secondary signal portions from the measured signals 45 first physiological measurement signal which travels along a yielding a primary reference which is a combination of the first propagation path and a second physiological measureremaining primary signal portions. The primary reference is ment signal which travels along a second propagation path, correlated to both of the primary signal portions. The A portion of the first and second propagation paths being primary reference and at least one of the measured signals located in the same propagation medium. The first signal has are input to a correlation canceler which removes the pri- 50 a first primary signal portion and a first secondary signal mary portions of the measured signals. This yields a good portion and the second signal has a second primary signal approximation to the secondary signal at one of the meaportion and a second secondary signal portion. The physisured signal wavelengths. This signal may be useful for ological monitoring apparatus further comprises a reference removing secondary signals from an auxiliary instrument as processor having an input for receiving the first and second well as determining venous blood oxygen saturation. 55 signals. The processor is adapted to combine the first and One aspect of the present invention is a signal processor second signals to generate a secondary reference signal comprising a detector for receiving a first signal which having a significant component which is a function of the travels along a first propagation path and a second signal first and the second secondary signal portions. Alternatively, which travels along a second propagation path wherein a the processor may be adapted to combine the first and portion of the first and second propagation paths are located 60 second signals to generate a primary reference having a in a propagation medium. The first signal has a first primary component which is a function of the first and second signal portion and a first secondary signal portion and the primary signal portions. second signal has a second primary signal portion and a The physiological monitoring apparatus may further cornsecond secondary signal portion. The first and second secprise a signal processor for receiving the secondary referendary signal portions are a result of a change of the 65 ence and the first signal and for deriving therefrom an output propagation medium. This aspect of the invention additionsignal having a significant component which is a function of ally comprises a reference processor having an input for the first primary signal portion of the first signal.
US 6,263,222 Bl 7 Alternatively, the physiological monitoring apparatus may further comprise a signal processor for receiving the secondary reference and the second signal and for deriving therefrom an output signal having a significant component which is a function of the second primary signal portion of the second signal. Alternatively, the physiological monitoring apparatus may further comprise a signal processor for receiving the primary reference and the first signal and deriving therefrom an output signal having a significant component which is a function of the first secondary signal portion of the first signal. Alternatively, the physiological monitoring apparatus may further comprise a signal processor for receiving the primary reference and the second signal and deriving therefrom an output signal having a significant component which is a function of the second secondary signal portion of the second signal. A further aspect of the present invention is an apparatus for measuring a blood constituent comprising an energy source for directing a plurality of predetermined wavelengths of electromagnetic energy upon a specimen and a detector for receiving the plurality of predetermined wavelengths of electromagnetic energy from the specimen. The detector produces electrical signals corresponding to the predetermined wavelengths in response to the electromagnetic energy. At least two of the electrical signals are used each having a primary signal portion and an secondary signal portion. Additionally, the apparatus comprises a reference processor having an input for receiving the electrical signals. The processor is configured to combine said electrical signals to generate a secondary reference having a significant component which is derived from the secondary signal portions. Alternatively, the processor may be configured to combine said signals to generate a primary reference having a significant component which is derived from the primary signal portions. This aspect of the present invention may further comprise a signal processor for receiving the secondary reference and one of the two electrical signals and for deriving therefrom an output signal having a significant component which is a function of the primary signal portion of one of the two electrical signals. Another aspect of the present invention may further comprise a signal processor for receiving the primary reference and one of the two electrical signals and for deriving therefrom an output signal having a significant component which is a function of the secondary signal portion of one of the two electrical signals. This may be accomplished by use of a correlation canceler, such as an adaptive noise canceler, in the signal processor which may employ a joint process estimator having a least-squareslattice predictor and a regression filter. Yet another aspect of the present invention is a blood gas monitor for non-invasively measuring a blood constituent in a body comprising a light source for directing at least two predetermined wavelengths of light upon a body and a detector for receiving the light from the body. The detector, in response to the light from the body, produces at least two electrical signals corresponding to the at least two predetermined wavelengths of light. The at least two electrical signals each have a primary signal portion and a secondary signal portion. The blood oximeter further comprises a reference processor having an input for receiving the at least two electrical signals. The processor is adapted to combine the at least two electrical signals to generate a secondary reference with a significant component which is derived from the secondary signal portions. The blood oximeter may further comprise a signal processor for receiving the secondary reference and the two electrical signals and for
8 deriving therefrom at least two output signals which are substantially equal, respectively, to the primary signal portions of the electrical signals. Alternatively, the reference processor may be adapted to combine the at least two 5 electrical signals to generate a primary reference with a significant component which is derived from the primary signal portions. The blood oximeter may further comprise a signal processor for receiving the primary reference and the two electrical signals and for deriving therefrom at least two 10 output signals which are substantially equivalent to the secondary signal portions of the electrical signal. The signal processor may comprise a joint process estimator, The present invention also includes a method of determining a secondary reference from a first signal comprising 15 a first primary signal portion and a first secondary portion and a second signal comprising a second primary signal portion and a second secondary portion. The method comprises the steps of selecting a signal coefficient which is proportional to a ratio of predetermined attributes of the first 20 primary signal portion and predetermined attributes of the second primary signal portion. The first signal and the signal coefficient are input into a signal multiplier wherein the first signal is multiplied by the signal coefficient thereby generating a first intermediate signal. The second signal and the 25 first intermediate signal are input into a signal subtractor wherein the first intermediate signal is subtracted from the second signal. This generates a secondary reference having a significant component which is derived from the first and second secondary signal portions. 30 The present invention also includes a method of determining a primary reference from a first signal comprising a first primary signal portion and a first secondary signal portion and a second signal comprising a second primary signal portion and a second secondary signal portion. The 35 method comprises the steps of selecting a signal coefficient which is proportional to a ratio of the predetermined attributes of the first secondary signal portion and predetermined attributes of the second secondary signal portion. The first signal and the signal coefficient are input into a signal 40 multiplier wherein the first signal is multiplied by the signal coefficient thereby generating a first intermediate signal. The second signal and the first intermediate signal are input into a signal subtractor wherein the first intermediate signal is subtracted from the second signal. This generates a primary 45 reference having a significant component which is derived from the first and second primary signal portions. The first and second signals in this method may be derived from electromagnetic energy transmitted through an absorbing medium, 50 The present invention further embodies a physiological monitoring apparatus comprising means for acquiring a first signal comprising a first primary signal portion and a first secondary signal portion and a second signal comprising a second primary signal portion and a second secondary signal 55 portion. The physiological monitoring apparatus of the present invention also comprises means for determining from the first and second signals a secondary reference, Additionally, the monitoring apparatus comprises a correlation canceler, such as an adaptive noise canceler, having a 60 secondary reference input for receiving the secondary reference and a signal input for receiving the first signal wherein the correlation canceler, in real or near real time, generates an output signal which approximates the first primary signal portion. Alternatively, the physiological 65 monitoring device may also comprise means for determining from the first and second signals a primary reference, Additionally, the monitoring apparatus comprises a correla-
US 6,263,222 Bl 9 tion canceler having a primary reference input for receiving the primary reference and a signal input for receiving the first signal wherein the correlation canceler, in real or near real time, generates an output signal which approximates the first secondary signal portion. The correlation canceler may 5 further comprise a joint process estimator. A further aspect of the present invention is an apparatus for processing an amplitude modulated signal having a signal amplitude complicating feature, the apparatus comprising an energy source for directing electromagnetic 10 energy upon a specimen. Additionally, the apparatus comprises a detector for acquiring a first amplitude modulated signal and a second amplitude modulated signal. Each of the first and second signals has a component containing information about the attenuation of electromagnetic energy by 15 the specimen and a signal amplitude complicating feature. The apparatus includes a reference processor for receiving the first and second amplitude modulated signals and deriving therefrom a secondary reference which is correlated with the signal amplitude complicating feature. Further, the appa- 20 ratus incorporates a correlation canceler having a signal input for receiving the first amplitude modulated signal, a secondary reference input for receiving the secondary reference, wherein the correlation canceler produces an output signal having a significant component which is 25 derived from the component containing information about the attenuation of electromagnetic energy by the specimen. Alternatively, the apparatus may also include a reference processor for receiving the first and second amplitude modulated signals and deriving therefrom a primary reference 30 which is correlated with the component containing information about the attenuation of electromagnetic energy by the specimen. Further, the apparatus incorporates a correlation canceler having a signal input for receiving the first amplitude modulated signal, a primary reference input for receiv- 35 ing the primary reference, wherein the correlation canceler produces an output signal having a primary component which is derived from the signal amplitude complicating teature. Still another aspect of the present invention is an appa- 40 ratus for extracting a plethysmographic waveform from an amplitude modulated signal having a signal amplitude complicating feature, the apparatus comprising a light source for transmitting light into an organism and a detector for momtoring light from the organism. The detector produces a first 45 light attenuation signal and a second light attenuation signal, wherein each of the first and second light attenuation signals has a component which is representative of a plethysmographic waveform and a component which is representative of the signal amplitude complicating feature. The apparatus 50 , . , , j, „ . . ^ ^T . also includes a reference rprocessor tor receiving the first and ,,.,,_ . , 1 1 • • .11 _c i. second light attenuation signals and deriving therefrom a , „ m i c 1 .i • ! secondary reference. 1 he secondary reference and the signal , ,. ,. j. „ , , j. 1V amphtude complicating teature each have a frequency spectrum. The frequency spectrum of the secondary reference is 55 correlated with the frequency spectrum of the signal amplitude complicating feature. Additionally incorporated into this embodiment of the present invention is a correlation canceler having a signal input for receiving the first attenuation signal and a secondary reference input for receiving 60 the secondary reference. The correlation canceler produces an output signal having a significant component which is derived from the component which is representative of a plethysmographic waveform. The apparatus may also include a reference processor for receiving the first and 65 second light attenuation signals and deriving therefrom a primary reference. Additionally incorporated in this embodi-
10 ment of the present invention is a correlation canceler having a signal input for receiving the first attenuation signal and a primary reference input for receiving the primary reference. The correlation canceler produces an output sign a i having a significant component which is derived from t h e component which is representative of the signal complicating feature, The present invention also comprises a method of removi n g o r determining a motion artifact signal from a signal derived from a physiological measurement wherein a first s i g n a l h a v i l l g a physiological measurement component and a motion artifact component and a second signal having a physiological measurement component and a motion artifact component are acquired. From the first and second signals a secondary reference which is a primary function of the first a n d se cond signals motion artifact components is derived. T h i s method of removing a motion artifact signal from a s i g n a l derived from a physiological measurement may also comprise the step of inputting the secondary reference into a correlation canceler, such as an adaptive noise canceler, to produce an output signal which is a significant function of t h e physiological measurement component of the first or s e c o n d s i g n a L Alternatively, from the first and second sign a l s a primary reference which is a significant function of the physiological measurement components of the first and s e c o n d s i g I l a i s may be derived. This approach may also comprise the step of inputting the primary reference into a correlation canceler to produce an output signal which is a significant function of the first or second signal's motion a rtif a ct component BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an ideal plethysmographic waveform, FIG. 2 schematically illustrates the cross-sectional structure of a typical finger. F I G . 3 illustrates a plethysmographic waveform which includes a motion-induced erratic signal portion, F I G 4fl i l l u s t r a t e s a schematic diagram of a physiological monitor, to compute primary physiological signals, incorporating a processor of the present invention, and a correlation canceler. F I G 4b i llu strates a schematic diagram of a physiological monitor, to compute secondary erratic signals, incorporating a processor of the present invention, and a correlation canceler FIG 5fl i l l u s t r a t e s a n e x le o{ a n a d a p t i v e n oise ! d in a physiological canceler which could be physiological signals, which monitor) to c o m im also inco a t e s the VIOces;ioI oi the present invention. „ „ -, •,, ^ , , r, ,FIG. 5o illustrates an example of an adaptive noise , ... ,, , , , . , . , . , canceler which could be employed in a physiological ^ , Ac \ . , v i. monitor, to compute secondary motion artifact signals, r ,. , , , ,, „., .. which also incorporates the processor ol the present inven'
FIG FIG
-
5c
. . . . , . , illustrates the transfer function of a multiple notch
- 6a illustrates a schematic absorbing material comP g N constituents within an absorbing material, F I G 6b illustrates another schematic absorbing material comprising N constituents, including one mixed layer, within an absorbing material. FIG. 6c illustrates another schematic absorbing material comprising N constituents, including two mixed layers, within an absorbing material. FIG. 7a illustrates a schematic diagram of a monitor, to compute primary and secondary signals, incorporating a rlsm
US 6,263,222 Bl 11
12
processor of the present invention, a plurality of signal coefficients (%, (t>2, . . . a>„, and a correlation canceler. FIG. 7b illustrates the ideal correlation canceler energy or power output as a function of the signal coefficients w^ w 2 , . . . u)n. In this particular example, FIG. 7c illustrates the non-ideal correlation canceler energy or power output as a function of the signal coefficients ooj, a> 2 ,. . . a>„. In this particular example, (o3=wa and a>7=ot> FIG. 8 is a schematic model of a joint process estimator comprising a least-squares lattice predictor and a regression filter. FIG. 9 is a flowchart representing a subroutine capable of implementing a joint process estimator as modeled in FIG. g FIG. 10 is a schematic model of a joint process estimator with a least-squares lattice predictor and two regression filters. FIG. 11 is an example of a physiological monitor incorporating a processor of the present invention and a correlation canceler within a microprocessor. This physiological monitor is specifically designed to measure a plethysmographic waveform or a motion artifact waveform and perform oximetry measurements. FIG. 12 is a graph of oxygenated and deoxygenated hemoglobin absorption coefficients vs. wavelength. FIG. 13 is a graph of the ratio of the absorption coefficients of deoxygenated hemoglobin divided by oxygenated hemoglobin vs. wavelength. FIG. 14 is an expanded view of a portion of FIG. 12 marked by a circle labeled 13. FIG. 15 illustrates a signal measured at a first red wavelength Xa=Xredl=650 nm for use in a processor of the present invention employing the ratiometric method for determining either the primary reference n'(t) or the secondary reference s'(t) and for use in a correlation canceler, such as an adaptive noise canceler. The measured signal comprises a primary portion sXa(t) and a secondary portion n Xa (t). FIG. 16 illustrates a signal measured at a second red wavelength Xb=Xred2=685 nm for use in a processor of the present invention employing the ratiometric method for determining the secondary reference n'(t) or the primary reference s'(t). The measured signal comprises a primary portion sxb(t) and a secondary portion nxfc(t). FIG. 17 illustrates a signal measured at an infrared wavelength Xc=Xm=940 nm for use in a correlation canceler. The measured signal comprises a primary portion s xc(t) a n d a secondary portion nXc(t). FIG. 18 illustrates the secondary reference n'(t) determined by a processor of the present invention using the ratiometric method. FIG. 19 illustrates the primary reference s'(t) determined by a processor of the present invention using the ratiometric method. FIG. 20 illustrates a good approximation s"Xa(t) to the primary portion sXa(t) of the signal sXa(t) measured at Xa=^redl=650 nm estimated by correlation cancellation with a secondary reference n'(t) determined by the ratiometric method. FIG. 21 illustrates a good approximation s"Xe(t) to the primary portion sAc(t) of the signal s^c(t) measured at Xc=}JR=940 nm estimated by correlation cancellation with a secondary reference n'(t) determined by the ratiometric method.
F I G . 2 2 illustrates a good approximation n"Xa(i) to the secondary portion n X a (t) of the signal S X a (t) measured at Xa=Xredl=650 n m estimated by correlation cancellation with a primary reference s'(t) determined b y the ratiometric method. F I G . 2 3 illustrates a good approximation n" X c (t) to the secondary portion n X c (t) of the signal S X c (t) measured at Xc=MR=940 n m estimated b y correlation cancelation with a primary reference s'(t) determined by the ratiometric method, FIG. 24 illustrates a signal measured at a red wavelength Xa=Xred=660 nm for use in a processor of the present invention employing the constant saturation method for determining the secondary reference n'(t) or the primary reference s'(t) and for use in a correlation canceler. The measured signal comprises a primary portion sXa(t) and a secondary portion n^a(t). FIG. 25 illustrates a signal measured at an infrared wavelen th g ^b=XIR=940 nm for use in a processor of the P r e s e n t invention employing the constant saturation method for determining the secondary reference n'(t) or the primary reference s'(t) and for use in a correlation canceler. The measured signal comprises a primary portion sxfc(t) and a secondar y portion nX6(t). FIG - 2 6 illustrates the secondary reference n'(t) determined by a processor of the present invention using the constant satura ion t method, FIG. 27 illustrates the primary reference s'(t) determined ^ Y a processor of the present invention using the constant saturation method, FIG. 28 illustrates a good approximation s"}>.a(t) to the primary portion sXa(t) of the signal SXa(t) measured at Xa=Xred=660 nm estimated by correlation cancelation with a secondary reference n'(t) determined by the constant saturation method. FIG. 29 illustrates a good approximation s"Xj,(t) to the primary portion sxfc(t) of the signal S xi (t) measured at )\.b=XIR=940 nm estimated by correlation cancelation with a secondary reference n'(t) determined by the constant saturation method, FIG. 30 illustrates a good approximation n"Xa(t) to the secondary portion nXa(t) of the signal SXa(t) measured at Xa=Xred=660 nm estimated by correlation cancelation with a primary reference s^t) determined by the constant saturation method. FIG. 31 illustrates a good approximation n" xi (t) to the secondary portion nxfc(t) of the signal Sxfc(t) measured at Xb=XIR=940nm estimated by correlation cancelation with a primary reference s'(t) determined by the constant saturation method, F I G . 32 depicts a set of 3 concentric electrodes, i.e. a tripolar electrode sensor, to derive electrocardiography (EGG) signals, denoted as Sj, S 2 and S3, for use with the present invention. Each of the ECG signals contains a primary portion and a secondary portion.
5
20
25
30
35
50
55
DETAILED DESCRIPTION OF THE IJNVEJNIION The present invention is a processor which determines either a secondary reference n'(t) or a primary reference s'(t) for use in a correlation canceler, such as an adaptive noise canceler. A correlation canceler may estimate a good 65 approximation s"(t) to a primary signal s(t) from a composite signal S(t)=s(t)+n(t) which, in addition to the primary portion s(t) comprises a secondary portion n(t). It may also be go
US 6,263,222 Bl 13
14
used to provide a good approximation n"(t) to the secondary signal Sxb(t) from the first measured signal SXa(t). The signal signal n(t). The secondary portion n(t) may contain one or coefficient factors (x>a and a)v are determined to cause either more of a constant portion, a predictable portion, an erratic the primary signal portions sXa(t) and sXj,(t) or the secondary portion, a random portion, etc. The approximation to the signal portions nXa(t) and nXj,(t) to cancel when the two primary signal s"(t) or secondary signal n"(t) is derived by 5 signals SXa(t) and Sxfo(t) are subtracted. Thus, the output of removing as many of the secondary portions n(t) or primary the reference processor 26 is either a secondary reference portions s(t) from the composite signal S(t) as possible. The signal n'(t)=nXa(t)-Manxfo(t), in FIG. 4a, which is correlated constant portion and predictable portion are easily removed to both of the secondary signal portions n^(t) and nxb(t) or with traditional filtering techniques, such as simple a primary reference signal s'(t)=sXa(t)-mvsxfe(t), in FIG. 4b, w h i c h is subtraction, low pass, band pass, and high pass filtering. The 10 correlated to both of the primary signal portions s erratic portion is more difficult to remove due to its unpre^ ( t ) a n d s^iCO- A reference signal n'(t) or s'(t) is input, alon w l t h o n e of the dictable nature. If something is known about the erratic g measured signals SXa(t) or Sxfc(t), to a signal, even statistically, it could be removed, at least correlation canceler 27 which uses the reference signal n'(t) partially, from the measured signal via traditional filtering o r s '(t) to remove either the secondary signal portions nXa(t) techniques. However, it is often the case that no information 15 o r M O or the primary signal portions sXa(t) or s xi (t) from the is known about the erratic portion of the noise. In this case, measured signal SXa(t) or Sxfo(t). The output of the traditional filtering techniques are usually insufficient. Often correlation canceler 27 is a good approximation s"(t) or n"(t) to elther the no information about the erratic portion of the measured pnmary s(t) or the secondary n(t) signal signal is known. Thus, a correlation canceler, such as an components. The approximation s"(t) or n"(t) is displayed on adaptive noise canceler may be utilized in the present 20 display zs. invention to remove or derive the erratic portion. An adaptive noise canceler 30, an example of which is s h o w n in block Generally, a correlation canceler has two signal inputs and diagram form in FIG. 5a, is employed to remove e l t h e r o n e o f t h e erratlc one output. One of the inputs is either the secondary > secondary signal portions reference n'(t) or the primary reference s'(t) which are % a (t) and nxfc(t) from the first and second signals SXa(t) and correlated, respectively, to the secondary signal portions n(t) 25 S ^ t ) - T h e a d a P t i v e n o i s e canceler 30, which performs the and the primary signal portions s(t) present in the composite functions of a correlation canceler, in FIG. 5a has as one ln ut a s a m l e of the signal S(t). The other input is for the composite signal S(t). P P secondary reference n'(t) which is Ideally, the output of the correlation canceler s"(t) or n"(t) correlated to the secondary signal portions nXa(t) and nxfc(t). The corresponds, respectively, to the primary signal s(t) or the secondary reference n'(t) is determined from the two secondary signal n(t) portions only. Often, the most difficult 30 measured signals S^(t) and S xb (t) by the processor 26 of the task in the application of correlation cancelers is determinP r e s e n t invention as described herein. A second input to the ing the reference signals ri(t) and s'(t) which are correlated adaptive noise canceler, is a sample of either the first or to the secondary n(t) and primary s(t) portions, respectively, second composite measured signals SXa(t)=sXa(t)+nXa(t) or J, of the measured signal S(t) since, as discussed above, these A6(.t)=sxi.(,t)+nxfc(t)portions are quite difficult to isolate from the measured 35 T h e adaptive noise canceler 30, in FIG. 5b, may also be signal S(t). In the signal processor of the present invention, employed to remove either one of primary signal portions either a secondary reference n'(t) or a primary reference s'(t) s^(t) and sxfc(t) from the first and second signals SXa(t) and is determined from two composite signals measured S xi (t). The adaptive noise canceler 30 has as one input a simultaneously, or nearly simultaneously, at two different sample of the primary reference s'(t) which is correlated to wavelengths, Xa and Xb. 40 t h e primary signal portions sXa(t) and sX6(t). The primary A block diagram of a generic monitor incorporating a reference s'(t) is determined from the two measured signals signal processor, or reference processor, according to the SXa(t) and Sxfo(t) by the processor 26 of the present invention as present invention, and a correlation canceler is shown in described herein. A second input to the adaptive noise FIGS. 4a and 4b. Two measured signals, SXa(t) and Sxfe(t), canceler 30 is a sample of either the first or second measured are acquired by a detector 20. One skilled in the art will 45 signals s xa( t ) =s x a (t)+nx a (t) or S xi (t)=s xfc (t)+n xi (t). realize that for some physiological measurements, more than The adaptive noise canceler 30 functions to remove one detector may be advantageous. Each signal is condifrequencies common to both the reference n'(t) or s'(t) and tioned by a signal conditioner 22a and 22b. Conditioning the measured signal SXa(t) or Sxfc(t). Since the reference includes, but is not limited to, such procedures as filtering signals are correlated to either the secondary signal portions the signals to remove constant portions and amplifying the 50 nXa(t) and nxfc(t) or the primary signal portions sXa(t) and signals for ease of manipulation. The signals are then s ^ t ) , the reference signals will be correspondingly erratic converted to digital data by an analog-to-digital converter or well behaved. The adaptive noise canceler 30 acts in a 24a and 24b. The first measured signal SXa(t) comprises a manner which may be analogized to a dynamic multiple first primary signal portion, labeled herein sXa(t), and a first notch filter based on the spectral distribution of the reference secondary signal portion, labeled herein n Xa (t). The second 55 signal n'(t) or s'(t). measured signal SXj,(t) is at least partially correlated to the Referring to FIG. 5c, the transfer function of a multiple first measured signal SXa(t) and comprises a second primary notch filter is shown. The notches, or dips in the amplitude signal portion, labeled herein sX6(t), and a second secondary of the transfer function, indicate frequencies which are signal portion, labeled herein nxi,(t). Typically the first and attenuated or removed when a composite measured signal second secondary signal portions, nXa(t) and nxfc(t), are 60 passes through the notch filter. The output of the notch filter uncorrelated and/or erratic with respect to the primary signal is the composite signal having frequencies at which a notch portions sXfl(t) and 8^,(1). The secondary signal portions was present removed. In the analogy to an adaptive noise nXa(t) and nX6(t) are often caused by motion of a patient. The canceler 30, the frequencies at which notches are present signals SXa(t) and 8^(1) are input to a reference processor change continuously based upon the inputs to the adaptive 26. The reference processor multiplies the second measured 65 noise canceler 30. signal SXj,(t) by either a factor ma=Sxa(t)/Sxfc(t) or a factor The adaptive noise canceler 30 shown in FIGS. 5a and 5b (Bv=nXa(t)/nXj,(t) and then subtracts the second measured produces an output signal, labeled herein as s \ a ( t ) , sXj,(t),
US 6,263,222 Bl 16
15 n" Xa (t) or n"Xj,(t) which is fed back to an internal processor 32 within the adaptive noise canceler 30. The internal processor 32 automatically adjusts its own transfer function according to a predetermined algorithm such that the output of the internal processor 32, labeled b(t) in FIG. 5a or c(t) in FIG. 5b, closely resembles either the secondary signal portion n X a (t) or n X6 (t) or the primary signal portion s Xa (t) or s x i ( t ) . The output b(t) of the internal processor 32 in FIG. 5a is subtracted from the measured signal, S Xa (t) or S xfc (t), yielding a signal output s"ka(t)=sXa(i)+n^a(t)-bXa(t) or a signal output s" X6(r) _ jXfc (t)+n X6 (t)-b xfc (t). The internal processor optimizes s \ a ( t ) or s \ 6 ( t ) such that s"^ a (t) or s " ^ ! ) is approximately equal to the primary signal s Xa (t) or s xi ,(t), respectively. The output c(t) of the internal processor 32 in FIG. 5b is subtracted from the measured signal, S Xa (t) or SxiO), yielding a signal output given by n \ a ( t ) = s X a ( t ) + n X a (t)- c x«(t) or a signal output given by n\ 6 (t)=s x f c (t)+n x f c (t)cXj,(t). The internal processor optimizes n" Xa (t) or n " x i ( t ) such that n" X a (t) or n"xb(l) is approximately equal to the secondary signal n X a (t) or nXj,(t), respectively. One algorithm which may be used for the adjustment of the transfer function of the internal processor 32 is a least-squares algorithm, as described in Chapter 6 and Chapter 12 of the book Adaptive Signal Processing by Bernard Widrow and Samuel Stearns, published by Prentice Hall, copyright 1985. This entire book, including Chapters 6 and 12, is hereby incorporated herein by reference. Adaptive processors 3 0 in FIGS. 5a and 5b have been successfully applied to a number of problems including antenna sidelobe canceling, pattern recognition, the elimination of periodic interference in general, and the elimination of echoes on long distance telephone transmission lines. However, considerable ingenuity is often required to find a suitable reference signal n'(t) or s'(t) since the portions nxa(t)> nXi(t)> s x a ( 0 and s xfc (t) cannot easily be separated from the measured signals S Xa (t) and SXj,(t). If either the actual secondary portion n X a (t) or n X6 (t) or the primary signal portion s Xa (t) or sXj,(t) were a priori available, techniques such as correlation cancellation would not be necessary. The determination of a suitable reference signal n'(t) or s'(t) from measurements taken by a monitor incorporating a reference processor of the present invention is one aspect of the present invention. Generalized Determination of Primary and Secondary
To obtain the reference signals n'(t) and s'(t), the measured signals S X a (t) and SXj,(t) are transformed to eliminate, respectively, the primary or secondary signal components. One way of doing this is to find proportionality constants, iaa 5 and (Bv, between the primary signals s Xa (t) and s xfc (t) and secondary signals n X a (t) and nXj,(t) such that:
(3)
10
These proportionality relationships can be satisfied in many measurements, including but not limited to absorption measurements and physiological measurements. Additionally, in most measurements, the proportionality constants coa and (Bv 15 can be determined such that: n}At)*^an}.b({) (4)
20
Multiplying equation (2) by (joa and then subtracting equation (2) from equation (1) results in a single equation wherein the primary signal terms 8^,(1) and sXj,(t) cancel, leaving:
25
«W=SX<,(0-<»,AA(')=«JJO-»,.«U.(0;
30
(5a)
a non-zero signal which is correlated to each secondary signal portion n Xa (t) and nXj,(t) and can be used as the secondary reference n'(t) in a correlation canceler such as an adaptive noise canceler. Multiplying equation (2) by a>v and then subtracting equation (2) from equation (1) results in a single equation wherein the secondary signal terms n X a (t) and n x i ( t ) cancel, leaving:
35
!!'(t)=S}.a{{)-^A.b(()=!i}.a({)-^vS}.l,(t);
40
(5b)
a non-zero signal which is correlated to each of the primary signal portions s Xa (t) and s xfc (t) and can be used as the signal reference s'(t) in a correlation canceler such as an adaptive noise canceler. Example of Determination of Primary and Secondary
45
Reference Signals in an Absorptive System
Correlation canceling is particularly useful in a large number of measurements generally described as absorption measurements. An example of an absorption type monitor An explanation which describes how the reference signals n'(t) and s'(t) may be determined follows. A first signal is 50 which can advantageously employ correlation canceling, such as adaptive noise canceling, based upon a reference measured at, for example, a wavelength Xa, by a detector n'(t) or s'(t) determined by a processor of the present yielding a signal S X a (t): invention is one which determines the concentration of an (1) VW^W+^W energy absorbing constituent within an absorbing material where s Xa (t) is the primary signal and n Xa (t) is the secondary 55 when the material is subject to change. Such changes can be caused by forces about which information is desired or signal. primary, or alternatively, by random or erratic secondary A similar measurement is taken simultaneously, or nearly forces such as a mechanical force on the material. Random simultaneously, at a different wavelength, Xb, yielding: or erratic interference, such as motion, generates secondary S}.i,(t)=s}.b({)+n}.b(t)(2) 60 components in the measured signal. These secondary components can be removed or derived by the correlation Note that as long as the measurements, S Xa (t) and S xfo (t), are canceler if a suitable secondary reference n'(t) or primary taken substantially simultaneously, the secondary signal reference s'(t) is known. components, n Xa (t) and n x i ( t ) , will be correlated because any random or erratic functions will affect each measurement in A schematic N constituent absorbing material comprising nearly the same fashion. The well behaved primary signal 65 a container 42 having N different absorbing constituents, components, s Xa (t) and sXj,(t), will also be correlated to one labeled A j , A 2 , A3, . . . A^, is shown schematically in FIG. another. 6a. The constituents A 1 through A ^ in FIG. 6a are arranged Reference Signals
US 6,263,222 Bl 18
17
in a generally orderly, layered fashion within the container layer of constituent A5 is affected by perturbations different 42. An example of a particular type of absorptive system is than each of the layers of other constituents A1 through A 4 one in which light energy passes through the container 42 and A 6 through A^. An example of such a situation is when and is absorbed according to the generalized Beer-Lambert layer A5 is subject to forces about which information is Law of light absorption. For light of wavelength Xa, this 5 deemed to be primary and, additionally, the entire material attenuation may be approximated by: is subject to forces which affect each of the layers. In this case, since the total force affecting the layer of constituent I=I0 exp(-Z",._ .acA) (6) Aj is different than the total forces affecting each of the other layers and information is deemed to be primary about the Initially transforming the signal by taking the natural loga10 forces and resultant perturbation of the layer of constituent rithm of both sides and manipulating terms, the signal is A5, attenuation terms due to constituents Aj through A4 and transformed such that the signal components are combined A 6 through AN make up the secondary signal portion n Aa (t). by addition rather than multiplication, i.e.: Even if the additional forces which affect the entire material cause the same perturbation in each layer, including the layer (7) •S^lnM^-ifa^,. 15 of A5, the total forces on the layer of constituent A5 cause it to have different total perturbation than each of the other where I 0 is the incident light energy intensity; I is the layers of constituents Aj through A4 and A 6 through A^. transmitted light energy intensity; e jAiI is the absorption It is often the case that the total perturbation affecting the coefficient of the Vh constituent at the wavelength Xa; x/t) is layers associated with the secondary signal components is the optical path length of \'h layer, i.e., the thickness of material of the i'h layer through which optical energy passes; 20 caused by random or erratic forces. This causes the thickness of layers to change erratically and the optical path length of and c/t) is the concentration of the i'h constituent in the each layer, x/t), to change erratically, thereby producing a volume associated with the thickness x/t). The absorption random or erratic secondary signal component nXfl(t). coefficients Cj through e^ are known values which are However, regardless of whether or not the secondary signal constant at each wavelength. Most concentrations c^t) through c^t) are typically unknown, as are most of the 25 portion n^a(t) is erratic, the secondary signal component nXa(t) can be either removed or derived via a correlation optical path lengths x/t) of each layer. The total optical path canceler, such as an adaptive noise canceler, having as one length is the sum of each of the individual optical path input, respectively, a secondary reference n'(t) or a primary lengths x^t) of each layer. reference s'(t) determined by a processor of the present When the material is not subject to any forces which cause change in the thicknesses of the layers, the optical path 30 invention as long as the perturbation on layers other than the layer of constituent A5 is different than the perturbation on length of each layer, x/t), is generally constant. This results the layer of constituent A5. The correlation canceler yields a in generally constant attenuation of the optical energy and good approximation to either the primary signal sXa(t) or the thus, a generally constant offset in the measured signal. secondary signal nXa(t). In the event that an approximation Typically, this portion of the signal is of little interest since knowledge about a force which perturbs the material is 35 to the primary signal is obtained, the concentration of the constituent of interest, c5(t), can often be determined since usually desired. Any signal portion outside of a known in some physiological measurements, the thickness of the bandwidth of interest, including the constant undesired primary signal component, x5(t) in this example, is known or signal portion resulting from the generally constant absorpcan be determined. tion of the constituents when not subject to change, should The correlation canceler utilized a sample of either the be removed. This is easily accomplished by traditional band 40 secondary reference n'(t) or the primary reference s'(t) pass filtering techniques. However, when the material is determined from two substantially simultaneously measured subject to forces, each layer of constituents may be affected signals SXa(t) and S xi (t). SXa(t) is determined as above in by the perturbation differently than each other layer. Some equation (7). S xi (t) is determined similarly at a different perturbations of the optical path lengths of each layer x^t) may result in excursions in the measured signal which 45 wavelength Xb. To find either the secondary reference n'(t) or the primary reference s'(t), attenuated transmitted energy represent desired or primary information. Other perturbais measured at the two different wavelengths Xa and Xb and tions of the optical path length of each layer x/t) cause transformed via logarithmic conversion. The signals SXa(t) undesired or secondary excursions which mask primary and SX6(t) can then be written (logarithm converted) as: information in the measured signal. Secondary signal components associated with secondary excursions must also be 50 •S)v a (0= e 5.)v a C5X 5 (t)+2 4 ^ 1 e, Ao c,x,.+2 A '^ ji Kb
US 6,263,222 Bl 20
19 (13b)
a given volume may be made with any number of constituents in the volume subject to the same total forces and It is often the case that both equations (12) and (13) can be therefore under the same perturbation or change. To detersimultaneously satisfied. Multiplying equation (11) by wa mine the saturation of one constituent in a volume comprisand subtracting the result from equation (9) yields a noning many constituents, as many measured signals as there zero secondary reference which is a linear sum of secondary are constituents which absorb incident light energy are signal components: necessary. It will be understood that constituents which do not absorb light energy are not consequential in the deter(14a) nXt)=S*Jt)-JiJt)-'hJfrjiKb(f) mination of saturation. To determine the concentration, as =2\.ie1-iac^1<0+2A,,-^e1-^cA-(0-24,-.iCOfleatcjj:,.(0+2Jv1-.6(ofle1-, 10 many signals as there are constituents which absorb incident light energy are necessary as well as information about the sum of concentrations. It is often the case that a thickness under unique motion contains only two constituents. For example, it may be Multiplying equation (11) by oov and subtracting the result from equation (9) yields a primary reference which is a 1 5 desirable to know the concentration or saturation of A 5 within a given volume which contains A 5 and A 6 . In this linear sum of primary signal components: case, the primary signals sXa(t) and sXj,(t) comprise terms (14b) sXt)=SiA()-A.b{t)=Si.a(t)-^i.b(t) related to both A 5 and A 6 so that a determination of the concentration or saturation of A 5 or A 6 in the volume may (15b) =C X (0£5,) .-<'V 5*5(O 5,Xi 20 be made. A determination of saturation is discussed herein. It will be understood that the concentration of A 5 in a (16b) =c x (i)[i^ -bi i^ \. volume containing both A 5 and A 6 could also be determined A sample of either the secondary reference n'(t) or the if it is known that A 5 +A 6 =l, i.e., that there are no constituprimary reference s'(t), and a sample of either measured ents in the volume which do not absorb incident light energy signal SXa(t) or 8^,(1), are input to a correlation canceler 27, 25 at the particular measurement wavelengths chosen. Then such as an adaptive noise canceler 30, an example of which measured signals SXa(t) and Sxfc(t) can be written (logarithm is shown in FIGS. 5a and Sb and a preferred example of converted) as: which is discussed herein under the heading PREFERRED CORRELATION CANCELER USING A JOINT PROCESS (18a) •S)va(0=e5.)vaC5X5_6(0+e6,xocsj:5_6(r)+nXa(0 ESTIMATOR IMPLEMENTATION. The correlation can- 30 (18b) celer 27 removes either the secondary portion nXa(t) or nXj,(t), or the primary portions, sXa(t) or sX6(t), of the (19a) •S)vi(0=e5.)viC5X5_6(0+e6,x6Csj:5_6(r)+nxfc(0 measured signal yielding a good approximation to either the primary signals s"Xa(t)»eSjXacsx5(t) or s"X6(t)=e5iXfcc5x5(t) or (19b) =sKb{t)+n^b(t) the secondary signals n"Xa(t)=nXa(t) or n"xfc(t)=nX6(t). In the 35 It is also often the case that there may be two or more event that the primary signals are obtained, the concentrathicknesses within a medium each containing the same two tion c5(t) may then be determined from the approximation to constituents but each experiencing a separate motion as in the primary signal s \ a ( t ) or s " ^ ! ) according to: FIG. 6c. For example, it may be desirable to know the (17) 40 concentration or saturation of A 5 within a given volume C5(()""S"Ka{{)lt5;f^5{{)~!i"}.b(t)l^}.l^5{i). which contains A 5 and A 6 as well as the concentration or As discussed previously, the absorption coefScients are saturation of A3 within a given volume which contains A3 constant at each wavelength Xa and Xb and the thickness of and A 4 , A3 and A 4 having the same constituency as A s and the primary signal component, x5(t) in this example, is often Ag, respectively. In this case, the primary signals sXa(t) and s known or can be determined as a function of time, thereby xi(t) again comprise terms related to both A 5 and A 6 and 45 allowing calculation of the concentration c5(t) of constituent portions of the secondary signals nXa(t) and nxfc(t) comprise A5. terms related to both A3 and A 4 . The layers, A3 and A 4 , do not enter into the primary equation because they are Determination of Concentration or Saturation assumed to be perturbed by random or erratic secondary forces which are uncorrelated with the primary force. Since In a Volume Containing More Than One constituents 3 and 5 as well as constituents 4 and 6 are taken Constituent to be the same, they have the same absorption coefScients. Referring to FIG. 6i>, another material having N different i• 3,Xa 5,Xa' 3,Ai> 5Afc' 4 , A a 6, Aa a n d e Xi. 6,Ai>' Generally speaking, however, A3 and A 4 will have different constituents arranged in layers is shown. In this material, concentrations than A 5 and A 6 and will therefore have a two constituents A 5 and A 6 are found within one layer different saturation. Consequently a single constituent having thickness x 5 6(t)=x5(t)+x6(t), located generally ranwithin a medium may have one or more saturations associdomly within the layer. This is analogous to combining the ated with it. The primary and secondary signals according to layers of constituents A 5 and A 6 in FIG. 6a. A combination this model may be written as: of layers, such as the combination of layers of constituents A 5 and A 6 , is feasible when the two layers are under the 60 ^(0=[e5,x o c 5 +e 6 ^c 6 >: 5 _ 6 (0 (20a) same total forces which result in the same change of the, optical path lengths x5(t) and x6(t) of the layers. nxa(0=[%.i.oC3+e6,^c4>:3_4(t)+22,..1e,.Xoc,.x,.(t)+2A';.7e;,xacA.(t) (20b) Often it is desirable to find the concentration or the n)va(0=[%A0c3+e6A0C4te.4(')+n)va(0 (20c) saturation, i.e., a percent concentration, of one constituent within a given thickness which contains more than one 65 e •s')vi(0=[ 5.)v6C5+e6,XAc:5-6(r) (21a) constituent and is subject to unique forces. A determination c z2 e A of the concentration or the saturation of a constituent within n)vi(0=[%A6 3+e6A6C4te.4(')+ ^i ,-.x6C,J:,.(r)+2 ',-.7e,-X(,cfc,.(')-(21b) £5^'!v£5M>-
:
5
5 5
5
e
U
:K[l
v
:Kb
e
e
=e
e
=e
e
=e
=e
4>
US 6,263,222 Bl 22
21 'W')=l>5,)vfaC3+e6,«.C4>:3,4(0+'W')
(21c)
where signals nXa(t) and nxb(t) are similar to the secondary signals nXa(l) and n xi (t) except for the omission of the 3, 4 layer. Any signal portions whether primary or secondary, outside of a known bandwidth of interest, including the constant undesired secondary signal portion resulting from the generally constant absorption of the constituents when not under perturbation, should be removed to determine an approximation to either the primary signal or the secondary signal within the bandwidth of interest. This is easily accomplished by traditional band pass filtering techniques. As in the previous example, it is often the case that the total perturbation or change affecting the layers associated with the secondary signal components is caused by random or erratic forces, causing the thickness of each layer, or the optical path length of each layer, x/t), to change erratically, producing a random or erratic secondary signal component n Xa (t). Regardless of whether or not the secondary signal portion nXa(t) is erratic, the secondary signal component nXa(t) can be removed or derived via a correlation canceler, such as an adaptive noise canceler, having as one input a secondary reference n'(t) or a primary reference s'(t) determined by a processor of the present invention as long as the perturbation in layers other than the layer of constituents A 5 and A 6 is different than the perturbation in the layer of constituents A 5 and A 6 . Either the erratic secondary signal components nj^(t) and nX6(t) or the primary components sXa(t) and sXj,(t) may advantageously be removed from equations (18) and (19), or alternatively equations (20) and (21), by a correlation canceler. The correlation canceler, again, requires a sample of either the primary reference s'(t) or the secondary reference n'(t) and a sample of either of the composite signals SXa(t) or SXj,(t) of equations (18) and (19)
thereby causing the terms other than nXa(t) and n^b(t) to be linearly dependent. Then, proportionality constantsff>avand uie may be found for the determination of a non-zero primary and secondary reference ^6,}.a-V->a^6Thb n