U.S. patent application number 13/317851 was filed with the patent office on 2012-06-14 for apparatus and method for detecting a fetal heart rate.
This patent application is currently assigned to MONICA HEALTHCARE LIMITED. Invention is credited to John CROWE, Barrie HAYES-GILL, David JAMES, Jean-Francois PIERI.
Application Number | 20120150053 13/317851 |
Document ID | / |
Family ID | 10862460 |
Filed Date | 2012-06-14 |
United States Patent
Application |
20120150053 |
Kind Code |
A1 |
HAYES-GILL; Barrie ; et
al. |
June 14, 2012 |
Apparatus and method for detecting a fetal heart rate
Abstract
An apparatus and method for detecting the heart rate of a fetus.
The apparatus includes three detectors detecting heart beats of the
fetus, each detector including at least two electrodes detecting
ECG signals, the detectors being positioned on an abdomen of a
mother in use. The apparatus also includes a processor coupled to
the detectors, the processor being adapted to process the ECG
signals received from the detectors and determine a heart rate of
the fetus. The method of determining the heart rate of the fetus
includes determining a position of the fetus within a womb, placing
the detectors on an abdomen of the mother, monitoring the ECG
signals obtained from the detectors for a predetermined length of
time, and processing the ECG signals obtained from the detectors
positioned on the abdomen of the mother to determine heart beats of
the mother.
Inventors: |
HAYES-GILL; Barrie;
(Nottingham, GB) ; JAMES; David; (Nottinghamshire,
GB) ; CROWE; John; (Nottingham, GB) ; PIERI;
Jean-Francois; (Cannes, FR) |
Assignee: |
MONICA HEALTHCARE LIMITED
Nottingham
GB
|
Family ID: |
10862460 |
Appl. No.: |
13/317851 |
Filed: |
October 31, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12385068 |
Mar 30, 2009 |
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13317851 |
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10089997 |
Sep 23, 2002 |
7532923 |
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12385068 |
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Current U.S.
Class: |
600/511 |
Current CPC
Class: |
A61B 5/02411 20130101;
A61B 5/4362 20130101; A61B 5/344 20210101 |
Class at
Publication: |
600/511 |
International
Class: |
A61B 5/0444 20060101
A61B005/0444 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 8, 1999 |
GB |
9923955.0 |
Oct 6, 2000 |
GB |
PCT/GB00/03843 |
Claims
1. A method of determining the heart rate of the fetus of using an
apparatus having three detectors for detecting ECG signals
representative of the heart beat of the fetus, the method
comprising: determining a position of the fetus within a womb;
placing the detectors on an abdomen of the mother, the detectors
being positioned in accordance with the position of the fetus;
monitoring the ECG signals obtained from the detectors for a
predetermined length of time greater than one hour; and processing
the ECG signals obtained from the detectors positioned on the
abdomen of the mother to determine heart beats of the mother by
determining when the ECG signals reach a maximum, the heart rate of
the mother from the time interval between maternal heart beats, and
thereby the heart rate of the fetus, wherein each detector has two
electrodes, the electrodes of one detector being positioned such
that one electrode lies beneath the umbilicus but above the
symphysis pubis and the other is positioned at the uterus
fundus.
2. The method according to claim 1, wherein the detectors include a
common electrode, the electrodes of the detectors being arranged in
a kite shape.
3. The method according to claim 1, wherein the predetermined
length of time is greater than 12 hours.
4. The method according to claim 1, wherein the determining the
position of the fetus within the womb comprises palpating the
mothers abdomen.
5. The method according to claim 1, wherein the processing the ECG
signals comprises: suppressing portions of the ECG signals
representative of the heart beat of the mother; detecting heart
beats of the fetus by determining when the remaining ECG signals
reach a maximum; and determining the heart rate by determining the
time interval between adjacent heart beats.
6. The method according to claim 5, wherein the processing the ECG
signals further comprises repeating the suppressing, detecting, and
determining on the signals detected by each detector and then
aggregating the obtained heart rates over a predetermined time
period of not less than one hour.
7. The method according to claim 5, wherein the suppressing
portions of the ECG signal representative of the heart beat of the
mother comprises: locating maternal ECG signals representing the
heart beat of the mother; and subtracting the maternal ECG signals
from the ECG signals obtained from each detector.
8. The method according to claim 5, wherein the determining the
heart rate by determining the time interval between adjacent heart
beats comprises: determining the standard deviation of each time
interval for the heart beats detected during the predetermined
time; and selecting the time intervals having a standard deviation
lower than a predetermined value.
9. The method according to claim 8, wherein the predetermined value
is approximately 7 ms for four consecutive time intervals.
10. The method according to claim 8, wherein the method further
comprises: designating time intervals not selected, to be erroneous
time intervals; and modifying the erroneous time intervals in
accordance with the selected time intervals.
11. The method according to claim 10, wherein the modifying the
erroneous time intervals comprises: comparing the erroneous time
interval to the selected time intervals; determining the number of
errors within the erroneous time interval; and dividing the
erroneous time interval into a number of corrected time intervals
by adding a number of heart beats corresponding to the number of
errors to thereby subdivide the erroneous time interval.
12. The method according to claim 11, the method further comprising
averaging the time intervals and the corrected time intervals to
determine a heart rate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of application Ser. No.
12/385,068, filed Mar. 30, 2009, pending, which is a divisional of
U.S. Ser. No. 10/089,997, filed Sep. 23, 2002, now U.S. Pat. No.
7,532,923. This application is based upon and claims the priority
of Great Britain application no. 9923955.0, filed Oct. 8, 1999, PCT
application no. PCT/GB00/03843, filed Oct. 6, 2000 and U.S. patent
application Ser. No. 10/089,997, filed Sep. 23, 2002, the contents
being incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an apparatus and a method
for detecting the heart rate of a fetus.
[0004] 2. Description of the Related Art
[0005] It is useful to be able to detect the heart beat of fetuses
as this can provide information regarding the health of the fetus
during the progress of pregnancy. Currently there are four main
methods for detecting fetal heart rates during pregnancy and these
involve the use of Doppler ultrasound, a SQUID magnetometer,
phonocardiography, and abdominal fetal electrocardiography.
[0006] The Doppler ultrasound technique consists of directing a 2
MHz (or other similar frequency) crystal transducer at the fetus on
the mother's abdomen. The signal reflected from the fetus is
shifted by a small frequency (known as the Doppler shift) which is
due to the pulsation of the fetal heart, hence (after suitable
processing) producing a fetal heart rate (fHR) trace. Portable
Doppler systems exist but as with all single channel Doppler
systems the transducer has to be periodically re-positioned to
point at the fetus and this requires the intervention of clinically
trained staff. Such systems are therefore limited to use in a
hospital environment.
[0007] A multi-channel Doppler ultrasound unit has also been
described in the document entitled "Fetal heart rate recorder for
long-duration use in active full-term pregnant women", by Shono et
al from Obstetrics and Gynecology, 1994 83, 2, page 301, which aims
to be ambulatory. This consists of six Doppler transducers
positioned on the abdomen with each being optimally positioned for
the various stances of the mother during her day. However, due to
the nature of the ultrasound signal being directed at the fetus
then long term recordings of fHR using ultrasound may be considered
as invasive. Although this invasive nature has not been clinically
substantiated, the use of Doppler ultrasound is still limited to
short time periods. Finally, the Doppler ultrasound technique only
provides an averaged fHR and gives no information about: the
beat-to-beat variability; nor about morphological information such
as the one contained in the shape of the Fetal electrocardiogram
(fECG) complex.
[0008] A SQUID magnetometer has been described in "Application of
SQUID magnetometer in fetal electrocardiography" Applied
Superconductivity, H Rogalla ed Inst, Physics Conf. Series, IOP,
1997, pp 21-26 by Rijpma et al. This describes a SQUID magnetometer
that can record the magnetic field associated with the electric
field generated by the fetal electrocardiogram (fECG). A transducer
is placed on the mother's abdomen above the fetus and the
corresponding fetal magnetic field can be detected using a
sensitive SQUID magnetometer. However, the system requires complex
circuitry and the SQUID transducer must be cooled to liquid helium
temperatures, thus resulting in these systems being both expensive
and large. Although long term recordings can be made they are
nevertheless carried out in an environment, which due to the nature
of the surrounding equipment, is neither comfortable nor easily
accessible to a wider community.
[0009] Phonocardiography has been in existence for over 80 years
and has mainly been applied to adults. The technique consists of
using a microphone which can detect the audible sound of the blood
flowing through the heart. Such a technique can be applied to the
fetal heart during pregnancy but is highly susceptible to
extraneous audible noises.
[0010] It is also possible to record an fECG by the insertion of a
fine needle through the mother's abdomen and into the womb. This
technique will give not only a reliable fHR but will also produce a
reliable fECG complex. However, the technique is highly invasive
and in some cases extremely risky, and accordingly can only be
performed by a highly skilled medical team.
[0011] Another abdominal technique has been implemented by Visser
et al and is described in "Diurnal and other cyclic variations in
human fetal heart rate near term", Am. J. Obst. & Gynec., 142,
5, page 535. This system uses a single channel sub-cutaneous needle
electrode to record long term fHR diurnal variations. Although not
as invasive the technique does require some penetration of the skin
and its avoidance is preferred.
[0012] The recording of fECG from the mother's abdomen has also
been carried out using passive surface skin electrodes via a single
analogue channel ECG machine. This is described in "Method and
apparatus for indicating repetition intervals of a specified
component of a composite electrical signal, particularly useful for
displaying fetal R waves" U.S. Pat. No. 4,945,917 by Akselrod et
al. The technique uses two electrodes which are placed
approximately 10 cm to 20 cm either side of the umbilicus. The
apparatus consists of an analogue front end having amplification
and filtering. The output of this is fed into a bed-side computer
for subsequent digitization and processing. As a result of the
system requiring separate processing apparatus to process the
obtained data as it is generated, the unit is not portable. The
system is therefore only used for short time intervals, typically
in the region of 20 minutes. Also, the system only uses a single
channel and this means that for a large proportion of the time
during which measurements are made, the fetal heart rate cannot be
detected.
[0013] Multichannel abdominal fECG units have been presented in the
literature in "The potential distribution generated by the fetal
heart at the maternal abdomen", J. Perinat. Med. 14, page 435 by
Oostendorp et al 1986. These system are used for vector
cardiography (VCG) and for obtaining the shape of a single fECG
complex. Measurements of this form are made in order to determine
the shape of the electrocardiogram so as to determine fetal cardiac
health. As a result, although the shape of individual heart beats
are measured, the duration between these beats, and hence the fetal
heart rate, is never determined.
[0014] In any event, in order to make the required measurements the
system (which will require a large recording bandwidth) must
therefore use a large number of recording channels, typically 32.
As a result of this, the system utilizes large bedside units which
are permanently positioned in a hospital. Not only does this
therefore require that the mother spend a period of time in
hospital for the monitoring to take place, but this also means that
the equipment is not used for long-term fetal heart beat detection.
In fact, measurement is typically made over a 45 second time
interval. Finally, the electrodes are positioned indiscriminately
on the abdomen without reference to fetal position.
[0015] Typical results obtained from a single channel abdominal
fetal ECG machine are shown in FIGS. 1A and 1B. FIG. 1A shows the
data obtained from Agood@ signal which shows definite fetal ECG's
(F) along with the large maternal ECG's (M). However, Agood@ data
is infrequently obtained and typically in 60% of cases the data is
Abad@ meaning that the fetal ECG (fECG) is undetectable. An example
of such Abad@ data, in which only the maternal ECG signal can be
seen, is shown in FIG. 1B.
[0016] In a trial using this technique 300 recordings were made and
the results of these were grouped into `definite` fECG observation
and `not definite`. 38% of recordings were classed as `definite`.
Hence out of 10 mothers visiting the hospital the technique, in
this form, will only be successful on typically 4 pregnancies--a
figure which makes the technique unacceptable for routine spot
checks of abdominal fECG.
[0017] During these recordings a note of fetal position for each
recording was made by palpating the mothers abdomen. This was
simply because the mother's abdomen during pregnancy can be
considered as a sphere of diameter typically 50 cm and the fetus
(sometimes with a fetal heart as small as 2 cm at early periods of
gestation) could reside anywhere inside this sphere. Hence the use
of a single pair of electrodes in the center of the abdomen only
provides a compromise. The largest fECG would occur where
conduction through the abdomen and other tissues is at its
highest.
[0018] Analysis of the results shows that the largest fECG occurs
at the shortest path from the fetal heart. For example, it was
found, from the above 300 recordings, that when the fetus was
engaged at the front-right of the abdomen the percentage success
rate of `definite` increased to 46% whilst those on the front-left
had a percentage of only 32%. A possible explanation for this is
that when the fetus is on the front-right and the two electrodes in
the center of the abdomen, the fetal heart is close to these
centrally positioned electrodes hence a larger fECG complex is
detected. However, with the fetus on the front-left, the fetal
spine masks the fetal heart from these electrodes hence attenuating
the signal. Further, when the fetus was orientated on the far
left-hand or right-hand side of the abdomen, referred to as
posterior, the percentage success rate, using electrodes positioned
at the center of the abdomen, drops dramatically to 18%.
[0019] The definition of achieving acceptable results is based
around two important factors:
[0020] 1. The percentage success--This is the percentage of time a
fetal heart rate trace can be extracted from the data.
[0021] 2. The reliability of obtaining at least one 20 minute
continuous fHR recording that can be used by the clinician for
analysis.
[0022] As can be determined from the results described above, these
two parameters have in the past been so disappointing that results
were never considered acceptable and therefore the technique has
not been routinely adopted in clinical practice.
[0023] Accordingly, it is not currently possible to record reliably
a long-term fetal heart rate using a portable and non-invasive
technique in the home.
[0024] Another example of an acoustic fetal monitor is described in
U.S. Pat. No. 4,781,200.
SUMMARY OF THE INVENTION
[0025] In accordance with a first aspect of the present invention,
we provide apparatus for detecting the heart rate of a fetus, the
apparatus including a detector for detecting heart beats of the
fetus, the detector comprising at least two electrodes for
detecting ECG signals, the detector being positioned on the abdomen
of the mother in use and a processor coupled to the detectors, the
processor being adapted to process the ECG signals received from
each detector and determine the heart rate of the fetus,
characterized in that the processor is further adapted to determine
the heart rate of the mother by detecting heart beats of the mother
by determining when the ECG signals reach a maximum and determining
the heart rate by determining the time interval between adjacent
heart beats.
[0026] Accordingly, the present invention provides apparatus which
is capable of detecting ECG signals produced by the fetal heart,
and hence determine the heart rate of the fetus. In the preferred
approach, at least two detectors detect the heart beats allowing
two separate channels to be defined for carrying the ECG signals
from a respective detector. Accordingly, should the fetus move such
that one of the detectors is no longer able to detect the heart
beat, then an ECG signal can still be obtained from the other
detector via the respective channel.
[0027] Although the heart rate will not be detectable for the
entirety of this hour, this still ensures that sufficient data is
collected to allow the fetal heart rate to be accurately
determined.
[0028] Furthermore, this allows a portable device to be produced
which can be carried by the mother, thereby allowing the
measurements to be made over a longer time interval than has
previously been achieved.
[0029] Optionally, where two or more detectors are provided, a
common electrode forms one of the electrodes of each detector. The
use of a common electrode is particularly advantageous as it
reduces the number of electrodes that must be positioned on the
mother's abdomen. It also leads to several advantages regarding the
signal processing. However, alternatively each detector may
comprise two respective electrodes that are not common to any other
detectors.
[0030] The electrodes are typically passive conductive cutaneous
electrodes which, in use, electrically detect signals representing
the electrical activity in the region of the mother's abdomen.
[0031] Typically the apparatus further comprises a signal processor
for amplifying and filtering the signals detected by the detectors.
This helps remove noise which is detected by the detectors, thereby
improving the quality of the fetal heart beat signal. However, if
the signal is strong enough, amplification or filtering may not be
required.
[0032] Typically, the processor generates virtual ECG signals as a
weighted sum of the ECG signals detected by the detectors, the
virtual ECG signals representing the ECG signals that would have
been obtained from a virtual detector positioned at a virtual
location on the abdomen of the mother. By using multiple detectors
and processing the signals obtained from the detectors, it is
possible to derive ECG signals that would have been generated by a
detector positioned at an alternative location on the mother's
abdomen. This advantageously allows an ECG signal to be generated
for an optimum detector location even if this detector location was
not actually used. However, the processor may alternatively simply
aggregate the ECG signals obtained from the detectors or may simply
obtain the fetal heart rate results from any respective
detector.
[0033] Typically each virtual ECG signal is generated dynamically
so as to represent the ECG signals that would be received from a
detector dynamically located on the mother's abdomen. This allows
the optimum detector position to move as the fetus moves within the
womb, thereby ensuring that an optimum signal is obtained at all
times.
[0034] Typically the apparatus further comprises an output for
displaying an ECG trace of the heart beat of the fetus (and the
mother) in accordance with the detected ECG signals. However, any
suitable form of output of the heart beat may be produced.
[0035] Typically the processor is adapted to determine the heart
rate of the fetus (and the mother) from the ECG signals by carrying
out the steps of: [0036] a. suppressing portions of signal
representative of the heart beat of the mother; [0037] b. detecting
heart beats of the fetus by determining when the remaining signal
reaches a maximum; and, [0038] c. determining the heart rate by
determining the time interval between adjacent heart beats.
[0039] Accordingly, by removing the portions of the signals which
are representative of the mother's heart beat, this should only
leave the fetal heart beat and any detected noise. The fetal heart
beat can be detected by determining locations where the signal
reaches a maximum (or minimum) amplitude. However, any suitable
method of detecting the fetal heart beat within a signal, such as
estimating the point at which the heart beat should appear, or
comparing the signal to a predetermined threshold could be
used.
[0040] Typically the processor is further adapted to perform the
steps (a), (b), and (c) on the ECG signals detected by each
detector and then aggregate the obtained heart rate over a
predetermined time period of not less than one hour. However, any
suitable method of processing the ECG signals may be used.
[0041] Typically the apparatus is portable such that it can be
carried around by the mother without placing any undue burden on
the mother. This allows the apparatus to be used over a long time
periods without disturbing the mothers normal routine.
[0042] It will be realized that although at least one detector is
specified, the present invention may be utilized with any suitable
number of detectors which does not cause discomfort to the
mother.
[0043] In accordance with a second aspect of the invention, we
provide a method of determining the heart rate of the fetus by
using apparatus having a detector for detecting ECG signals
representative of the heart beat of the fetus, the method
comprising: [0044] 1. determining the position of the fetus within
the womb; [0045] 2. placing the detector on the abdomen of the
mother, the detector being positioned in accordance with the
position of the fetus; [0046] 3. monitoring the ECG signals
obtained from the detector for a predetermined length of time, the
predetermined length of time being greater than one hour; and,
[0047] 4. processing the ECG signals obtained from the detector to
determine the heart rate of the fetus.
[0048] Accordingly, the present invention also relates to a method
of operating apparatus having one or more detectors to thereby
obtain the best fetal heart rate detection. This is achieved by
positioning the or each detector on the abdomen of the mother in
accordance with the position of the fetus within the womb, thereby
maximizing the chance of obtaining a signal from any one detector.
Additionally by measuring the signals over a longer duration, this
allows a larger amount of data to be detected which can be analyzed
to obtain information regarding the heart beat of the fetus. This
is particularly advantageous as the heart beat of the fetus is
generally only detectable for about 40% of the time for any given
detector. This is due to noise and movement of the fetus within the
womb. Furthermore, by having multiple detectors, should the baby
move so that the fetal heart beat can no longer be detected by one
of the detectors, then there is a high probability that it will
then be detected by another detector.
[0049] Typically, the signals obtained from each channel are
monitored for a predetermined length of time. This is preferably
greater than 12 hours. It will be realized however that longer
durations may also be used.
[0050] Typically the method of determining the position of the
fetus within the womb comprises palpating the mother's abdomen.
Thus, this uses a simple non-invasive procedure for determining the
position of the fetus. Alternatively however ultrasound, or other
suitable techniques, could be used to locate the fetus.
[0051] The method of processing the ECG signals typically comprises
the steps of: [0052] a. suppressing portions of the ECG signals
representative of the heart beat of the mother; [0053] b. detecting
heart beats of the fetus by determining when the remaining ECG
signals reach a maximum; and, [0054] c. determining the heart rate
by determining the amount of time between adjacent heart beats.
[0055] Thus, this advantageously provides a method of processing
the signals to determine the heart rate. However, any suitable
method, such as comparing the ECG signals to a threshold, can be
used.
[0056] Typically, where there is more than one detector, the method
of processing the signals further comprises repeating steps (a),
(b), and (c) on the ECG signals detected by each detector and then
aggregating the obtained heart rate over a predetermined time
period of not less than one hour. By aggregating the ECG signals
obtained from different detectors, this means that the heart rate
can be determined at any time during which at least one of the
detectors is detecting fetal ECG signals. Alternatively however the
signals can be processed so as to generate virtual ECG signals and
this is achieved by determining a weighted sum of the ECG signals
obtained from each of the respective detectors.
[0057] Typically the step of removing portions of signals
representative of the heart beat of the mother comprises locating
maternal ECG signals representing the heart beat of the mother, and
subtracting the maternal ECG signals from the ECG signals obtained
from each detector. This therefore advantageously removes the
portion of the ECG signal which is due to the ECG signal generated
by the mother's heart beat. However alternatively, the portion of
the ECG signals which are due to the mother's heart beat can be
ignored. In this case, any ECG signal obtained during the mother's
heart beat is simply removed from the ECG signals which are then
analyzed to determine the fetal heart rate.
[0058] It will be realized from this that the invention can
advantageously be used to detect the heart beat of the mother,
although these could be detected by separate apparatus. This is
achieved by: [0059] 1. detecting heart beats of the mother by
determining when the ECG signals reach a maximum; and, [0060] 2.
determining the heart rate by determining the time interval between
adjacent heart beats.
[0061] However, it is not essential that the heart rate of the
mother is detected.
[0062] Preferably the method of determining the heart rate by
determining the time interval between adjacent heart beats
comprises: [0063] a. determining the standard deviation of each
time interval for the heart beats detected during the predetermined
time; and, [0064] b. selecting the time intervals having a standard
deviation lower than a predetermined value.
[0065] However, the selection of erroneous time intervals could
simply be determined by comparing heart rate indicated by the time
interval to a threshold to identify time intervals indicating heart
rates that are physically impossible. Alternatively, the fetal
heart rate could be determined directly from the raw ECG signals,
without analysis of the time intervals and the like.
[0066] The predetermined value is preferably approximately 7 ms for
four consecutive time intervals, although any suitable value could
be chosen by the user.
[0067] Typically the method further comprises: [0068] a.
designating time intervals not selected to be erroneous time
intervals; and, [0069] b. modifying the erroneous time intervals in
accordance with the selected time intervals.
[0070] Alternatively however, the erroneous time intervals could
simply be ignored, although this results in a reduced amount of
data from which the end heart rate is calculated.
[0071] Typically the method of modifying the time intervals
comprises: [0072] a. comparing the erroneous time interval to the
selected time intervals; [0073] b. determining the number of errors
within the erroneous time interval; and, [0074] c. dividing the
erroneous time interval into a number of corrected time intervals
by adding a number of heart beats corresponding to the number of
errors to thereby subdivide the erroneous time interval.
[0075] However, any suitable method may be used.
[0076] The method generally further comprises averaging the time
intervals and the corrected time intervals to determine a heart
rate.
[0077] Typically the apparatus further comprises a signal processor
for amplifying and filtering the ECG signals detected by the
detector. Although this may not be required if the signals are of
sufficient strength that the amplification is not required.
[0078] By accepting that the signal can only be detected in any
patient for approximately 40% of the time using a single detector,
we have developed a system that is portable, can be used for 24
hours or longer, has more than one recording channel and is
extremely low noise. By initially deducing the baby position a
small array of electrodes can be placed around the fetus on the
mother=s abdomen--thus increasing the detection rate. By recording
over 24 hours using our improved technique will typically result in
at least 10 hours of data. This long term, non-invasive collection
of fHR data allowing mothers freedom to function in their normal
environment has never been achieved before.
BRIEF DESCRIPTION OF THE DRAWINGS
[0079] Examples of the present invention will now be described with
reference to the accompanying drawings, in which:
[0080] FIG. 1A shows a "good" output from a single channel
detector;
[0081] FIG. 1B shows a "poor" output from a single channel
detector;
[0082] FIG. 2 is a block diagram of apparatus for detecting the
heart beat in accordance with the present invention;
[0083] FIG. 3 is a first example of the detector arrangement of the
apparatus shown in FIG. 2;
[0084] FIGS. 4A and 4B are examples of the output obtained from the
detector arrangement of FIG. 3;
[0085] FIG. 5 is a second example of an output obtained from the
electrode arrangement of FIG. 3;
[0086] FIG. 6 is an example of fetal heart rate together with an
aggregate fetal heart rate obtained from the detector arrangement
of FIG. 3;
[0087] FIG. 7A shows a second example of an electrode arrangement
used in the apparatus of FIG. 2;
[0088] FIG. 7B shows a third example of an electrode arrangement
used in the apparatus of FIG. 2;
[0089] FIG. 8 shows an example of the output obtained from the
electrode arrangement of FIG. 7A;
[0090] FIG. 9 shows an example of the heart rate obtained from the
electrode arrangement of FIG. 7A;
[0091] FIG. 10 shows an example of the method of processing the
output obtained from the apparatus of FIG. 2;
[0092] FIGS. 11A to 11E are graphs of the amplitude of an ECG
signal against time for various processing stages; and,
[0093] FIGS. 12A to 12D show the variations in fetal heart rate
over time for various processing stages.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0094] FIG. 2 is a block diagram showing apparatus for detecting
the heart beat of a fetus. The apparatus comprises three detectors
1,2,3 which are coupled to an amplification and filter stage 4. The
output of the amplification and filter stage 4 is coupled to an
analogue-to-digital converter 5, which is in turn connected to a
processor 6. The processor 6 is coupled to a memory 7 and a display
8.
[0095] The system operates as follows. Each detector 1,2,3 consists
of two passive cutaneous conductive electrodes positioned on the
abdomen of the mother so as to detect ECG signals generated in the
region of the mother's abdomen. An example of a suitable electrode
arrangement is shown in FIG. 3. In this case, the electrodes e1,e2
correspond to detector 1, electrodes e3,e4 correspond to detector 2
and electrodes e5,e6 correspond to detector 3. Reference numeral 10
represents the fetus, with 10A representing the head and 10B the
fetal back.
[0096] Electrical signals detected by the detectors 1,2,3 are fed
into the amplification and filter stage 4 via a respective analogue
channel. Each analogue channel has a very low noise (better than 75
nV (RMS) equivalent to an average of 17 nv/Hz.sup.1/2).
[0097] Although this example is described with respect to three
channels, one or more channels, coupled to respective detectors,
could be used. Increasing the number of channels and detectors will
increase the success of heart beat detection, however this will be
at the expense of inconveniencing the mother by having several
electrodes placed on her abdomen. In any event, further increases
in the success of heart rate detection can be achieved by
monitoring generated ECG signals for long time periods as will be
explained in more detail below. This can be achieved by the use of
the apparatus of the invention which is portable, allowing readings
to be taken over extended time periods without inconveniencing the
mother.
[0098] The signals output from each detector 1,2,3 are transferred
to the amplification and filter stage 4 for amplification and
subsequent filtering. This is typically achieved using two stages
of amplification and filtering which are software programmable. The
overall gain can be set typically from 1000 to 5000 and will
typically have a bandwidth of approximately 4-80 Hz. However, in
cases with low patient noise a bandwidth of 0.5-250 Hz can be
set.
[0099] The amplified and filtered signals are passed onto the
analogue-to-digital convertor 5 which operates to convert each of
the signals into a digital signal which is then passed onto the
processor 6. Accordingly, the processor unit receives three digital
signals corresponding to a respective one of the detectors
1,2,3.
[0100] The processor handles memory storage, real time processing
and display of the digitized signals. The data is stored in the
memory 7, which is typically some form of large capacity flash
semiconductor storage. This form of device is particularly
advantageous as it may be located inside the instrument, is
non-volatile and can be removed for subsequent downloading of
data.
[0101] The processing of the data to extract the fetal heart rate
can be implemented either "on-line" within the processor 6, or
"off-line". In the on-line case, the result of the processing can
simply be displayed on the display 8. Otherwise, the data is
typically downloaded onto a PC for additional processing.
[0102] As mentioned in the introduction, whilst testing previous
techniques, 300 recordings were made using a single channel
detector. Using the results of these previous 300 recordings,
knowledge determined from these was then applied in carrying out a
test of the apparatus shown in FIG. 2. In this case, the electrode
arrangement shown in FIG. 3 was utilized.
[0103] The results of one such test are shown in FIGS. 4A and 4B
which show the amplitude of signals detected by the detectors 1,2,3
against time, with the output of each channel being shown on a
similar scale graph. In this case, the test was carried out during
a 10 minute period and this shows that during the early part of the
10 minute period, shown in FIG. 4A, the fetal heart beat is
detectable using detector 1. However in a later part of the 10
minute period, which is shown in FIG. 4B, the fetus has moved and
the fetal heart beat is now detected by detector 2.
[0104] Accordingly, knowledge of fetal position and the presence of
an array of electrodes (i.e. more than one detector) leads to an
increase in the percentage success of fetal heart beat detection.
Thus using more than one detector 1,2,3 and positioning the
detector electrodes around the periphery of the located fetus
dramatically increases the success rate. It is important to note
that the central point of this array of electrodes on two totally
different patients could be separated by as much as 50 cm. Hence
the knowledge of fetal position is important with regard to correct
electrode positioning.
[0105] The electrode arrangement shown in FIG. 3 is a 3-channel
system with differential inputs. In this case, 6 electrodes
e1,e2,e3,e4,e5,e6 are positioned as shown, with an additional
ground connection electrode (not shown) located on the back of the
patient. The exact positioning of the electrodes will vary from
case-to-case, although the electrodes of a given detector (e.g. the
electrodes e3 and e4) are typically positioned such that e4 lies
beneath the umbilicus but above the symphysis pubis and e3 at the
fundus (this distance is typically 20 cm apart). This six electrode
technique offers the attraction of 3 separate channels thus
reducing any common muscle noise (i.e. electromyogram or EMG).
[0106] As the apparatus shown in FIG. 2 uses a minimum number of
components, it can be incorporated into a small portable device
which may measure 14 cm by 10 cm by 3 cm or smaller. In addition to
this, the apparatus is implemented using semiconductor electronics
and is therefore extremely light such that it can be easily carried
by the mother. As a result, the apparatus is portable in the sense
that it can be strapped to the mother and the electrodes attached
without it being intrusive into the mother's everyday routine. This
allows the mother to attach the apparatus for extended periods of
time, such as 24 hours, allowing measurements to be made over
longer time periods.
[0107] A short extract from a 24 hour recording using three
detectors, simultaneously, is shown in FIG. 5, which again shows
the amplitude of the electrical signal detected against time. In
this case, at the time of the extract, the fetal heart rate can be
seen in the ECG signals obtained from both the detector 1 and the
detector 2.
[0108] Once the raw data has been obtained, the processor 6
operates to extract the fetal heart rate (fHR) for each channel
using techniques described in more detail below. This results in
the output traces shown in FIG. 6. This shows the time (in hours)
during which heart beat detection occurred. Time periods when a
heart beat could be detected are shown as a solid bar-graph with
the output obtained from detectors 1,2,3 labeled 01,02,03
respectively. Times when no heart beat could be detected are
indicated by blank portions B. The individual percentage success
rates for times when a fetal heart beat could be detected in this
case were:
TABLE-US-00001 Channel fHR extraction success rate 1 (01) 36% 2
(02) 24% 3 (03) 19%
[0109] However, the processor 6 is configured to determine an
aggregate of these 3 channels which is shown in FIG. 6 as P1, here
we obtain a percentage success of 67%. This increase occurs because
when the fECG signal is not detected by one of the detectors 1,2,3,
at least for some of the time it is detectable by one of the other
detectors 1,2,3.
[0110] However, it is more acceptable for the mother if fewer
electrodes are used. This can be achieved by using a single
detector electrode common to all three detectors 1,2,3 with the
common electrode being coupled to either an inverting or
non-inverting input of the amplification and filter stage 4.
[0111] In this case only four electrodes e7,e8,e9,e10 are required
to form the three detectors 1,2,3 as shown for example in FIGS. 7A
and 7B. Of the two arrangements the one shown in FIG. 7A is more
suitable for fetuses at a later stage of gestation. Here the fetus
is more stable and does not move around as much. The `kite` shape
shown in FIG. 7B is suitable for fetuses in the early stages of
gestation where fetal position varies considerably.
[0112] In the case of FIGS. 7A and 7B each detector will measure
ECG vectors with respect to the common electrode e10 thus allowing
other mathematical combinations to be produced. These are known as
virtual detector outputs, as the processed output represents the
output that would have been obtained from a detector having
respective electrodes positioned elsewhere on the mother's
abdomen.
[0113] The virtual detector outputs are calculated using Kirchoff's
voltage law which allows the processor 6 to mathematically combine
the ECG signals obtained from each of the detectors 1,2,3. This can
be achieved because the four electrode arrangements use a common
electrode e10.
[0114] In this case, the virtual detector outputs are generated
using a weighted sum of the amplitude signals obtained from each
detector 1,2,3. This allows the virtual detector output to indicate
an increased presence of fECG signals than is obtained with any one
of the detectors 1,2,3 on its own. In other words, a more optimum
virtual electrode position is simulated with the relative position
of the electrodes depending on the weighting coefficients which are
used when determining the weighted sum.
[0115] An example of this is shown in FIG. 8 which shows the
amplitude of the ECG signals detected by the detectors against
time. The signals detected by the detectors 1,2,3 are labeled
detector 1, detector 2 and detector 3 respectively. Virtual
detector 4 and virtual detector 5 represent the results of the
calculation of the virtual detector output signals that would have
been generated by two different virtual electrode positions. In
this case, virtual detector 4 is determined by subtracting the
output from detector 1 from the output from detector 2, whereas
virtual detector 5 is determined by subtracting the output from
detector 1 from the output detector 3. Again, the location of fetal
heart beats is indicated by the label F.
[0116] The success for the detection of the heart rate for each
channel is shown in FIG. 9, which is a bar-graph showing the time
in hours during which heart rates were detected. Again, times
during which heart rates were detected are represented by a solid
bar. In this example the output obtained from the detectors 1,2,3
are labeled 01,02,03 respectively, and the output calculated for
the virtual detectors 4,5 are labeled V04,V05. The percentage
success for each channel and each virtual channel are tabulated
below:
TABLE-US-00002 Detector fHR extraction success rate 1 (01) 15% 2
(02) 31% 3 (03) 4% Virtual detector (V04) 34% Virtual detector
(V05) 44%
[0117] Again if we take an aggregate of channels 1, 2 and 3 we get
an improvement to 35%. However, an aggregate of all five channels
(including the virtual channels) gives an increased percentage of
48% as shown by the bar labelled P1 in FIG. 9. Hence, for this
patient alone this results in an increase in the percentage from 4%
(worst case) on a single channel system to 48% by the use of 3
channels and mathematical combinations to form these virtual
channels.
[0118] It is also possible to calculate a dynamically changing
optimum virtual detector output in real time and hence produce a
single virtual detector output having the largest fECG complex
possible. This is achieved by using a weighted sum of the signals
output from all three detectors, with individual weighting
coefficients, adjusted iteratively such that the fetal
signal-to-noise ratio is maximized.
[0119] A secondary advantage of the presence of 3 detectors is the
addition of redundancy to the system in the event of a failure on
one detector caused by saturation. Saturation can occur when one
electrode is depressed or leant upon. This is a common problem in
abdominal fECG monitoring. By having 3 detectors, this reduces the
probability of this occurring.
[0120] The worry with traditional single channel abdominal fECG
techniques has been that if employed for spot checks (i.e. a 20
minute ante-natal visit) only 4 out of every 10 mothers would
present successful abdominal fECG traces. Using the 3 channel
system described above and then making use of the virtual channels
still does not provide a certainty of detecting a fetal ECG.
However, if this apparatus is used to record data over 24 hours
then the probability of detecting a continuous 20 minute fHR trace
which can be analyzed by the clinician is a near certainty.
[0121] Hence, assuming that the instrument and patient are noise
free then in order to achieve this high percentage the procedure
summarized below should be followed:
[0122] i. Find fetal location.
[0123] ii. Use multi-channel recorder (three, but any number
greater than two would preferably suffice).
[0124] iii. Position electrodes in an array but use a common
electrode so that virtual channels can be generated.
[0125] iv. Record for 24 hours.
[0126] v. Compute virtual channels either real time or
off-line.
[0127] vi. Extract fHR on all channels including virtual
channels.
[0128] vii. Calculate the total aggregate of all fHR channels
including virtual channels.
[0129] In order for the processor 6 to extract the heart rate from
the digitized output signals, one of the following techniques can
be used.
Technique 1
[0130] The first technique is outlined in FIG. 10, which shows a
block diagram of the steps involved in the first technique for
extracting the fetal heart rate from the obtained ECG signals. This
will be described with respect to FIGS. 11A to 11E which show the
amplitude of an ECG signal against time, during the processing
steps.
[0131] The raw ECG signals received from one of the detectors 1,2,3
are shown in FIG. 11A. The signals resulting from muscle noise are
labeled Mn, the fetal heart beat is labeled F, and the mothers
heart beat M. Coincident fetal and maternal heart beats are labeled
M&F, whilst coincident fetal heart beats and muscle noise are
labeled F&Mn.
[0132] The first stage, shown as step 10, is to correlate the
obtained ECG signals with the ECG complex of the mother. This is
achieved by filtering the raw ECG signals first using a 10-80 Hz
filter to reduce the amount of noise. The results of this filtering
are shown in FIG. 11B.
[0133] Next, a maternal template is established using the average
of 5 maternal complexes at initialization and then 32 complexes
during the remaining time. This template is correlated (often
called matched filtered) with the raw ECG to produce the trace
shown in FIG. 11C. The maternal ECG's can then be located by
detecting the maximum of this correlation.
[0134] Two options are then available for removing the maternal
ECG. Firstly, as shown at step 30, the maternal average ECG complex
template can be subtracted from the ECG signal. The remaining ECG
trace, which is shown in FIG. 11D, consists of fetal ECG and
remnant noise.
[0135] Alternatively, as set out in step 20 in FIG. 10, the
maternal ECG can be blanked out. This is usually used when
subtraction of the maternal ECG leaves a remnant signal that is too
often mistaken for a fetal ECG complex. The blanking technique
(labeled in FIG. 10) can be used here. This involves locating the
maternal as before (i.e. with correlation) and then simply drawing
a straight line (or a simple interpolated function such as a
spline) between the edges of the maternal ECG complex on the raw
abdominal ECG trace. Although this can also remove fetal ECG
complexes which are coincident with the maternal it can be
regenerated during `post-processing` which will be described in
more detail below.
[0136] Once completed, the fetal heart beat must be detected and
this can again be achieved using two different methods. The first
method, shown as step 40 in FIG. 10, is to correlate the signal to
detect this fetal ECG complex. The other method, step 50, involves
band-pass filtering the signal using a 25-40 Hz filter or similar.
Both techniques enhance the fetal ECG complex and hence improve the
signal-to-noise ratio. However, the band-pass filtering technique
usually results in a more stable output and an example of this is
shown as FIG. 11E.
[0137] Finally the fetal heart beats are determined by identifying
the maxima of the resulting ECG trace in step 60.
[0138] It should be noted that if a fetal heart beat is located
during the maternal subtraction window then this fetal is tagged in
the fHR data file as `coincident`--referred to as "coincident
flag". This is because it may in fact be an artefact caused by
maternal subtraction and can be allowed for during
post-processing.
Technique 2
[0139] The second technique involves a Non-Linear Filtering
technique described by Thomas Schreiber. Details of this can be
found for example in either of the following references: [0140]
Kantz D., Schreiber T., `Nonlinear time series analysis`, Cambridge
Univ. Press, 1997. [0141] Richter M., Schreiber T., Kaplan D. T.,
`Fetal ECG extraction with nonlinear state-space projections`, IEEE
Trans. Biomed. Eng., Vol. 45, No. 1, pp 133-137, January 1998.
[0142] The outputs from this non-linear technique produce
`semi-clean` mECG and fECG traces. This data is then passed through
a band-pass FIR filter. The bandwidth of the filter is 4 Hz to 40
Hz for the mECG and 25 Hz to 40 Hz for the fECG to remove any
remnant noise not cleaned by the non-linear filtering. The maxima
of the output of the filter are then located to obtain again the
raw beat to beat heart rate files for maternal and fetal.
[0143] In both the above mentioned techniques, the fetal heart rate
is measured by determining the time interval between adjacent heart
beats and then using this to derive a heart rate. Once the data
representing the heart rates has been obtained, it is then possible
to carry out further post-processing of the raw maternal and fetal
heart rate data to further improve the results of the heart rate
determination.
[0144] The processing described in the above two techniques
generates `raw beat-to-beat heart rate values` for both maternal
and fetal. Incorrect heart rate values can exist in both fetal and
maternal data and these are caused by electrical noise (either EMG
or man made) detected on the mothers abdomen and an inadequate
heart rate extraction algorithm.
[0145] It is possible to remove these errors by passing the raw
heart rate data through a post-processing procedure. This is
usually only necessary for the raw fetal heart rate data since the
maternal consists of very few errors. All fHR data files are passed
through this post-processing algorithm. On some occasions, when no
errors exist, this post processing will not change the raw fetal
heart rate data and hence a true beat to beat heart rate file is
available.
[0146] Wrong detection or missed ECG complexes will generate
`spikes` up or down on the heart rate trace, creating a variability
in the heart rate trace that does not actually exist. In the case
of a genuine arrhythmia (i.e. large single beat variations in heart
rate values caused by cardiac conduction disorders) these beats can
be incorrectly suppressed by the proposed post-processing
technique. In cases with patients having arrhythmia then the raw
heart rate data file should be used--however, this occurs very
infrequently (less than 0.2% of patients).
[0147] Also, it is found that on average about 10% of occasions the
fetal ECG is coincident with the maternal ECG. If the mECG removal
is carried out by blanking (instead of subtracting the mECG
template) then this will create an artificial fetal bradycardia.
The running of the post-processing is therefore essential to remove
artificial fetal bardycardia.
[0148] Another problem is that the above fetal heart rate
extraction algorithm produces an output whether the fECG signal is
present or not. It is very important that in the case of no fECG
signal then no fHR trace must be displayed to the clinicians.
[0149] Hence, it is necessary for all these reasons, that the beat
to beat trace must be post-processed before being plotted for
clinical analysis.
[0150] Finally it is usual to present fHR data using the standard
Cardiotocograph (CTG) output format (obtained from Doppler
ultrasound machines) so that easy comparison can be made between
the two methods. The standard Doppler ultra-sound machines do not
give beat to beat value but present an average value. The
post-processing to be presented also has the ability to average the
heart rate so as to be compatible with the CTG traces.
[0151] The post-processing scheme is the same for both the maternal
and fetal heart beat interval values (both referred to as "RR
intervals") and is made up of two passes.
[0152] The first stage selects from the RR intervals those which
can be considered as `good` (i.e "sure") values. It is not
important to select all the `good` values, but it is very important
at this stage not to select a wrong RR interval. The condition of
selection must then be very strict since the correction of
subsequent data is based on this `good` data.
[0153] The second stage will look either side of these selected
`good` values and decide if the beat to beat variability (as
entered by the user) is correct. If it is not, the software will
attempt to correct the data. This second pass can correct from one
to four consecutive errors.
First Stage
[0154] The first stage involves analyzing the RR values and
selecting those having a small standard deviation. A running window
of 4 consecutive RR's are taken and the standard deviation of the
data must not exceed typically 7 ms (but set by the user), else the
data will be rejected.
[0155] On average, it is found that typically 5% of the raw fetal
heart rate data are selected at this first stage. The selected
`good` RR intervals are the basis of the eventual subsequent
corrections and cannot therefore be corrected themselves. The
correction scheme applies to the correction of both the fetal and
maternal RR interval data.
[0156] For the fetal heart rate data there is an extra condition.
Here, in order to be selected by the first pass, the heart beats
detected must not have their "coincidence flag" set. This is to
avoid a bad mECG subtraction creating an artificial fHR.
Second Stage
[0157] The RR intervals not selected above are the values that can
be corrected in this second pass. A RR interval is not corrected if
it differs by less than 10 percent from a running average (on the
three last `good` RR) or on a neighbor RR with a small standard
deviation (set by the user). If the RR interval is not in that
range the algorithm looks at the following RR values and will
perform a correction if a maximum of four consecutive errors have
occurred.
[0158] There are two possible error sources:
[0159] 1. A heart beat has been missed.
[0160] 2. There was a detection error, i.e. the detected signal
does not correspond to a heart beat.
[0161] By calling T a detected true heart beat, E a detection error
and M a missed heart beat, the possible sequences that the second
part of the post-processing can correct are: [0162] One error: TET,
TMT. [0163] Two consecutive errors: TEET, TEMT, TMET, TMMT. [0164]
Three consecutive errors: TEEET, TEEMT, TEMET, TMEET, TEMMT, TMEMT,
TMMET, TMMMT. [0165] Four consecutive errors: TEEEET, TEEEMT,
TEEMET, TEMEET, TMEEET, TEEMMT, TEMEMT, TMEEMT, TEMMET, TMEMET,
TMMEET, TEMMMT, TMEMMT, TMMEMT, TMMMET, TMMMMT.
[0166] Consider the example where only one error occurs. If this is
a detection error, then the wrong RR is simply removed. If there
was a missed RR, the corrected RR interval will correspond to a
heart beat placed in the middle of the interval between the
previous and next `good` heart beat.
[0167] If there is ambiguity with two possible faults, priority is
given to the fault pattern which corresponds to the smallest number
of errors. In general if a fECG signal is present, most of the
faults will be made of just one error.
[0168] This correction scheme allows recovery of the discarded
fetal heart beats when using the extraction with mECG blanking.
[0169] Finally, if no `good` data (from the first pass) is found
within a one minute interval, the fECG signal will be considered
not present, no correction will be done and no output will be
plotted within that interval.
[0170] The post processing can also operate to average the
beat-to-beat heart rate data. The interest of doing so is that it
allows a better visual comparison with the standard Doppler CTG
traces, which usually contain some kind of averaging. Typically a
two-second averaging is used.
[0171] FIGS. 12A to 12D show the fHR versus time of an example of
the post-processing of raw abdominal fHR data. FIG. 12A shows the
raw fHR data, FIG. 12B shows the post-processed fHR data and FIG.
12C shows the post-processed data after implementing a 2 second
average. As a comparison FIG. 12D shows a simultaneous Doppler
ultrasound CTG trace which shows excellent correlation illustrating
that the post-processing has correctly processed the raw abdominal
fHR data.
* * * * *