Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS HAVING REDUNDANT SENSORS FOR CONTINUOUS
MONITORING OF VITAL SIGNS AND RELATED METHODS
Technical Field
[0001] This invention relates to devices for
monitoring vital signs such as blood pressure,
pulse rate, and oxygen saturation. The invention
has particular application in devices for
determining blood pressure by measuring a pulse
wave velocity or pulse transit time.
Background
(0002] Equipment for monitoring the vital signs
of subjects is widely used in clinical settings.
Such devices may monitor various physiological
signs including blood pressure, oxygen
saturation, pulse rate and the like. Such devices
typically include one or more sensors placed at
suitable locations on the subject's body. Various
different types of sensors may be used. The
sensors may be of invasive types or of non-
invasive types. Signals from the sensors are
carried to the vital signs monitoring equipment
where they are amplified, conditioned, and
processed to determine values for the
physiological parameters being measured.
[0003] In general, it is desirable to provide
non-invasive monitoring of vital signs. While
invasive systems are sometimes used, surgery is
required to introduce sensors of invasive types
into the subject's body. The sensors typically
have leads which emerge from the subject's body
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through a fistula. The fistula can provide a
pathway for infection.
[0004] As an example of non-invasive monitoring
of a vital sign, blood oxygen saturation may be
measured by providing a small clip-on sensor
which includes one or more light sources and one
or more light detectors. Variations in the oxygen
saturation of the subject's blood cause resulting
variations in the intensity of light reaching the
detector. These variations are superimposed upon
a variation in the intensity of light reaching
the detector which results from the subject's
heartbeat pulses. A device equipped with this
type of sensor can also be used to measure pulse
rate. various such devices are known.
[0005] One type of system for measuring a
subject's blood pressure relies upon the fact
that the speed at which pulse waves propagate
through a blood vessel is dependent upon blood
pressure. Consequently, if one detects the
arrival of a pulse at two different points of a
subject's circulatory system there will be, in
general, a difference in the time at which the
pulse wave arrives at the two points. This time
difference varies according to the blood
pressure. One system for measuring blood pressure
as a function of such a time difference is
described in PCT application No. PCT/CA00/010552,
and in the commonly owned and co-pending
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application entitled CONTINUOUS NON-INVASIVE
BLOOD PRESSURE MONITORING METHOD AND APPARATUS
which is being filed simultaneously herewith,
both of which are fully incorporated herein by
reference. Such systems require at least two
sensors, one for detecting the arrival of the
pulse wave at each of the two points. This type
of device may use sensors of the same type as are
used to detect oxygen saturation although other
types of sensor could also be used.
10006] One problem with such vital signs
monitoring equipment is that the accuracy of
measurements obtained can depend upon the
stability of the signals received from the
sensors. Artifacts may be caused by movement of
the subject. In the worst case, a sensor may
become disconnected from the subject and
monitoring may be interrupted until the sensor is
replaced.
(0007] There is a need for cost effective
methods and apparatus for monitoring one or more
vital signs of a subject which provide improved
accuracy and are affected less by artifacts than
current vital signs monitoring equipment.
Summar~r of Invention
(0008] This invention provides an apparatus for
monitoring one or more vital signs of a subject
by using a number of sensors. Each sensor
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originates a signal, typically a pulse signal. A
selection system determines performance criteria
for a number of groups of signals. Each group
includes one or more signals. The apparatus
computes a estimated value for a vital sign by
either computing a value from each group of
signals and taking a weighted average with
weights based upon the performance criterion or
by selecting one of the groups of signals for
which the performance criterion is best and
computing the output value from that group of
signals.
[0009] The output values are relatively
insensitive to artifacts and to errors caused by
the disconnection or malfunctioning of one
sensor. Further advantages and features of the
invention are described below.
Brief Description of Drawings
[0010] In drawings which illustrate non-
limiting embodiments of the invention:
Figure 1 is a block diagram of a vital
signs monitoring system according to the
invention;
Figure 2 is a view illustrating possible
sensor locations for a vital signs monitoring
system according to the invention; and,
Figure 3 is a block diagram of a vital
signs monitoring system according to a specific
embodiment of the invention.
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Description
[0011] Throughout the following description,
specific details are set forth in order to
provide a more thorough understanding of the
invention. However, the invention may be
practiced without these particulars. In other
instances, well known elements have not been
shown or described in detail to avoid
unnecessarily obscuring the invention.
Accordingly, the specification and drawings are
to be regarded in an illustrative, rather than a
restrictive, sense.
[0012] Figure 1 shows a system 10 according to
the invention. Figure 1 comprises a plurality of
sensors 12. The illustrated embodiment comprises
three sensors, 12A, 12B and 12C. As shown in
Figure 2, sensors 12A, 12B and 12C may be located
on a subject's earlobe, finger and toe. Sensors
12 include at least one redundant sensor. That
is, there is at least one more sensor 12 than is
required for the type of measurement being made
by system 10. The signal from each sensor 12 is
conditioned and digitized in a signal conditioner
13. In the illustrated embodiment there are three
separate signal conditioners 13A, 13B and 13C
which correspond respectively with sensors 12A,
12B and 12C.
[0013] The resulting digitized signals are
passed to a controller 14. In the illustrated
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embodiment of the invention, controller 14
comprises a processor 14A. Processor 14A runs
software l5 stored in a program memory 15A.
Software 15 receives the signals from all of
sensors 12A through 12C by way of a suitable
interface 18.
(0014] In the illustrated example there are
three sensors. This means that there are three
pairs of sensors. Software 15 computes a blood
pressure for the subject from the signals of each
of the three pairs of sensors. This yields three
computed values for the subject's blood pressure.
Computation of the subject's estimated blood
pressure may be done, for example, according to
the methods described in PCT patent application
No. PCT/CA00/00691 which is incorporated herein
by reference. The methods described in that
application involve measuring a time difference
between a pair of signals to obtain a
differential pulse transit time (DPTT). Separate
DPTT values are derived for systolic and
diastolic portions of the signals. A known
relationship between DPTT and blood pressure is
used to compute an estimated blood pressure from
a DPTT value. The known relationship is obtained
during a calibration process which involves
measuring the subject's blood pressure by a
separate accurate mechanism, and substantially
simultaneously measuring the DPTT. These methods
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may be separately applied to each pair of signals
from sensors 12.
[0015 Controller 14 includes an automated
selection system 16. Selection system 16 takes
the signals from sensors 12 in distinct groups.
In this case, "distinct" means that each group
has a combination of signals from a different set
of one or more sensors 12. Each group includes
sufficient signals for the determination of a
vital sign of interest. Where the groups include
more than one signal, a signal from one sensor 12
may be included in more than one group. Selection
system 16 identifies the best group of sensors 12
to be used for monitoring the vital sign in
question. For example, where the vital sign is a
blood pressure determined from a DPTT, each group
of sensors comprises a pair of two sensors. For
vital signs, such as pulse or blood oxygen
saturation, which can be measured on the basis of
the signal from one sensor the groups of sensors
include one sensor each.
[,0016] The determination of the best group of
sensors to use is preferably made based upon the
stability of the signals originating at the
sensors. For example, the following equation may
be used to provide a performance criterion for
the pair of signals (pi,pj) to be used in a DPTT
blood pressure estimation:
Cij - ( Corr ( j'Ji, x7j ) ~6i6j ~ max
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Where, Cjj is the maximum value of the correlation
coefficient of the paired signals (pt,pj) and g
and ~j are deviation measures for the two signals
which are given as follows:
N-1
(Z7
6i - N- 1 k~o [ pi ( k) - }.~i ) 2
N- 1
_ 1 ~ [ p~ ( k) - .~] ~ 2 ~3~
N-lk.o
The correlation function (Corr) between the two
signals can thus be calculated as:
1 N+ 1
Corr (pi..p~) - N+ 1 k~o [pi (k) - ui~ [p~ (k) - l~~l t4)
where N is the number of samples used to
calculate DPTT and determine the performance of
the considered pair in a certain period of time.
,ui and ,uj are respectively average signal values
for the paired signals from the sensors under
consideration.
[0017] ui may be given by the following
equation:
1 N-1
u. - - ~ p. (k) (5)
Nk- o
[0018] ,u~ may be given by the following
equation:
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1 N-1
p. . _ - ~ p ( k ) t6)
Nk= o ~
[0019] The performance criterion Cjj given by
Equation (1) has a number of features which makes
it suitable for use as a performance criterion
for the pair of sensors under consideration. In
particular, Cij is independent of the amplitudes
of pulse signals pi and Pj and 1<_C3jsl.
[0020] To reduce the complexity of these
computations, a histogram-based calculation may
be used in practice. Histogram-based calculation
techniques which may be applied in this invention
are described in Smith, "The Scientist and
Engineer's Guide to Digital Signal Processing
(Second Edition)" California Technical
Publishing, 1999. Histogram-based techniques have
the advantage that computational complexity is
not dependent upon the number of samples
collected.
[0021] Selection system 16 preferably comprises
a function which tests for unacceptable signal
values (as might result, for example, from the
disconnection of a sensor) and, when such
conditions are detected, forces the affected
performance criterion Cj j to be zero (or some
other value that will cause the selection system
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16 to not select the pairs of sensors affected by
the unacceptable signal).
[0022] In the preferred embodiment of the
invention, selection system 16 computes a blood
pressure as a weighted average of the blood
pressures computed from the signals of each of
three pairs of sensors. This weighted average may
be expressed as follows:
P = ~ a . P.
i7 iJ iJ
where there are three sensors, this reduces to:
P a12P12 + a23P23 + a31P31 ~8~
where a12, a23 and a31 are weighting factors with
ai2+a23+a31=1 and P12, p23 and P31 are blood pressures
calculated from the signals produced by each of
the three pairs of sensors respectively.
[0023] The weighting factors a~j (where i and j
are indices which represent the sensors in the
pair of sensors under consideration) may be given
by the following equation, where CjJ is the
performance criterion for the pair of sensors
under consideration:
C..
m
a i~
~9~
~. Ci7
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[0024] In the alternative, selection system 16
may select one of the pairs of sensors which
provides the best value for the subject's blood
pressure (i.e. for which the performance
criterion is the highest). In this case,
apparatus 10 presents the blood pressure derived
from the signals of that pair of sensors 12 as
the subject's blood pressure. This alternative
embodiment of the invention is a special case of
the weighted average according to equation (8) in
which the values of atj which do not correspond to
the group of signals having the best performance
criterion are all zero and the a~j corresponding
to the group of signals having the best
performance criterion is 1.
[0025] A display 20 displays the computed blood
pressure. A user input device, 24 such as a
button panel, a graphical user interface, a touch
screen, or the like is provided to permit users
to control the operation of apparatus 10.
Preferably, user input device 24 permits a user
to control whether selection system 16 selects
signals from a specified pair of sensors 12,
selects signals from the pair of sensors 12 which
has the best performance criterion, or uses
signals from all of sensors 12 in a blended
average such as that of equation (7).
[0026] A digital input/output (I/O) connection
26 permits results to be delivered to other
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devices, for example, a printer, or a data
collector/concentrator, or a computer being used
for data analysis. A non-invasive blood pressure
(NIBP) measurement module 28 provides reference
blood pressures for calibration and re-
calibration purposes. Display 20, user input 24,
I/O connection 26 and measurement module 28
communicate with processor 14A by way of one or
more suitable interfaces 18A.
[0027] Apparatus 10 according to the invention
preferably uses a similar strategy to that
described above for obtaining and displaying a
value representing the oxygen saturation of a
subject's blood and the subject's pulse rate.
Each of the three sensors produces a signal which
can be used to derive an oxygen saturation value
and a pulse rate value. As the performance
criteria are calculated in real time for CNIBP
estimation, one signal can be selected from the
best pair of sensors for the purpose of obtaining
an oxygen saturation value and a pulse rate
value.
[0028] Once again, the system may be set to
display a best one of the oxygen saturation or
pulse rate values or, in the alternative, may
present an oxygen saturation value which is a
blended average of the oxygen saturation or pulse
rate values derived from the signals originating
from each sensor.
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[0029] Those skilled in the art will notice
that system 10 has at least one redundant sensor.
If any one sensor becomes disconnected or
malfunctions then the signal performance of all
parameter values which are calculated based upon
a signal from that sensor will be poor or
useless. Such values will be given either a very
small weighting or no weighting at all in the
computation of the parameter to be displayed. In
the preferred embodiment of the invention,
processor 14 causes apparatus 10 to generate a
visual or audible warning if the performance
criterion for one group of sensors is lower than
a threshold value. Most preferably the visual or
audible warning is generated if the performance
criterion remains below the threshold value for
longer than a predetermined time period.
[0030] It is preferable to re-calibrate system
10 relatively frequently. For example, it is
preferable to compare the blood pressure values
produced by system 10 to a calibration value
obtained by another measurement technique
approximately every 30 minutes. Re-calibration is
especially important if a sensor is relocated.
Re-calibration is also desirable if the
extremities of the subject where sensors are
located are moved in relation to the subject's
heart. For example, if a sensor is on a subject's
finger and the arm to which the finger is
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attached is elevated then it would be preferable
to re-calibrate the system after the subject has
assumed a comfortable position with the arm
elevated. Similarly, if a vasoactive medication
is administered to the subject, or the dose of a
continuously delivered vasoactive medication is
altered, re-calibration is desirable.
[0031 Figure 3 illustrates apparatus 10A
according to an alternative embodiment of the
invention. Apparatus 10A comprises a separate
processor 40 for computing the performance
criterion and the blood pressure determined by
each pair of sensors. For example, processor 40A
computes the performance criterion and an
estimated blood pressure from the signals
measured by sensors 12A and 12B. Processor 40B
computes the performance criterion and an
estimated blood pressure from the signals
produced by sensors 12B and 12C. Sensor 40C
computes the performance criterion and an
estimated blood pressure from the signals
produced by sensors 12A and 12C. Each of
processors 40 also computes values for oxygen
saturation for one of the sensors. The results of
the computations by the processors 40 are
delivered to a host processor 42. Host processor
42 coordinates the operation of apparatus 10A and
also computes an appropriate value for the
systolic and diastolic blood pressures, pulse
rate, blood volume and oxygen saturation from the
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received signals. In doing so, processor 42 may
implement the functions of selection system 16
which is described above.
(0032] As will be apparent to those skilled in
the art in the light of the foregoing disclosure,
many alterations and modifications are possible
in the practice of this invention without
departing from the spirit or scope thereof. For
example:
~ the number of sensors may be varied. If four
sensors are used then there are potentially
six pairs of sensors from which pulse transit
time information for blood pressure
estimation may be derived. There are four
sensors from which pulse and oxygen
saturation information may be derived.
~ various functions which are described above
as being performed by software running on a
computer processor may be performed using
appropriate hardware.
[0033] Accordingly, the scope of the invention
is to be construed in accordance with the
substance defined by the following claims.
