U.S. patent number RE31,158 [Application Number 06/094,160] was granted by the patent office on 1983-02-22 for apparatus for indicating uterine activity in labor.
This patent grant is currently assigned to National Research Development Corporation. Invention is credited to Michael C. Carter, Philip J. Steer.
United States Patent |
RE31,158 |
Carter , et al. |
February 22, 1983 |
Apparatus for indicating uterine activity in labor
Abstract
Apparatus for providing an indication of uterine activity in
labor is described. Intrauterine pressure above the basal pressure
existing between contractions is integrated over intervals of
between ten and thirty minutes to give the required indication. An
algorithm is disclosed for calculating dosage of a labor inducing
substance, the algorithm containing safeguards to prevent
hyperstimulation.
Inventors: |
Carter; Michael C. (London,
GB2), Steer; Philip J. (London, GB2) |
Assignee: |
National Research Development
Corporation (London, GB2)
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Family
ID: |
10237452 |
Appl.
No.: |
06/094,160 |
Filed: |
November 14, 1979 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
757024 |
Jan 5, 1977 |
04114188 |
Sep 12, 1978 |
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Foreign Application Priority Data
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Jun 23, 1976 [GB] |
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26042/76 |
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Current U.S.
Class: |
600/591; 600/561;
600/587; 604/890.1 |
Current CPC
Class: |
G16H
40/67 (20180101); A61B 5/4356 (20130101); G16H
20/17 (20180101); A61M 5/1723 (20130101); A61B
5/7239 (20130101) |
Current International
Class: |
A61M
5/172 (20060101); A61M 5/168 (20060101); A61B
5/03 (20060101); G06F 19/00 (20060101); G06F
015/42 (); A61B 005/04 (); A61B 005/10 () |
Field of
Search: |
;364/415
;128/748,778 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1145360 |
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Mar 1969 |
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GB |
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1214153 |
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Dec 1970 |
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GB |
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1298685 |
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Dec 1972 |
|
GB |
|
1345720 |
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Feb 1974 |
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GB |
|
1375869 |
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Nov 1974 |
|
GB |
|
1450081 |
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Sep 1976 |
|
GB |
|
1478766 |
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Jul 1977 |
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GB |
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Other References
Electonic Circuits Manual, p. 494. .
Instrument for Measuring the Contractile Activity of the Uterus, V.
Pronin, Biomedical Engineering, vol. 7, No. 4, pp. 212-215,
Jul.-Aug. 1973. .
An Automated Method for the Frequency Analysis of Myoelectric
Signals Evaluated by an Investigation of the Spectral Changes
Following Strong Sustained Contractions, S. Johansson, L. E.
Larsson & R. Ortegren, Med. & Bio. Engineering, vol. 8, No.
3, pp. 257-264, 1970. .
A State Variable Averaging Filter for Electromyogram Processing,
Med. & Bio. Eng., vol. 10, pp. 559-560, 1972. .
Uterine Contractility Data Processing System, Kepka et al.,
American Journal Obstetrics Gynecology, Oct. 1, 1975, pp. 246-250.
.
Quantitation of Uterine Activity, Hon et al., Obstetrics and
Gynecology, vol. 2, No. 3, Sep. 1973, pp. 368-370. .
Uterine Activity in Induced Labour, Steer, et al., British Journal
of Obstetrics and Gynecology, vol. 82, No. 6, Jun. 1975, pp.
433-441. .
Quantitation of Uterine Activity in 100 Primiparous Patients,
Miller et al., American Journal Obstetrics and Gynecology, Feb. 15,
1976, pp. 398-405..
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Primary Examiner: Wise; Edward J.
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
We claim:
1. Apparatus for indicating uterine activity in labor
including:
means adapted to be coupled to a uterine pressure transducer for
providing a first signal representative of intrauterine
pressure,
means for providing a second signal .[.which is equal to.].
.Iadd.representative of .Iaddend.the first signal except .[.when
a.]. .Iadd.during each .Iaddend.contraction .[.occurs.]. when .[.it
takes up a value equal to that of.]. the .Iadd.value the
.Iaddend.first signal .Iadd.would have had .Iaddend.in the absence
of .[.the.]. .Iadd.that .Iaddend.contraction .Iadd.is
represented.Iaddend.,
.[.means for subtracting the second signal from the first 65
signal, to provide a third signal,.]. .Iadd.means for deriving a
third signal from the first and second signals, the third signal
being representative of the additional uterine pressure, which
occurs during contractions, above the uterine pressure existing in
the absence of contractions.Iaddend., and
means for integrating the third signal over intervals of at least
ten but not more than thirty minutes to provide an activity signal
indicative of uterine activity in labor.
2. Apparatus according to claim 1 wherein the first and second
signals are analog signals.
3. Apparatus according to claim 1 wherein the means for providing
the second signal includes means for holding, during each
contraction, a signal equal to the first signal at the beginning of
the contraction, the signal held forming the second signal during
contractions.
4. Apparatus according to claim 1 wherein the means for providing
the second signal include
detection means for detecting first and second changes in the first
signal likely to be indicative of the onset and completion,
respectively, of a contraction, and
a sample-and-hold circuit for sampling the first signal except
between the detection of said first and subsequent second changes
by the detection means when the last value sampled is held, the
ouput of the sample and hold circuit providing the said second
signal.
5. Apparatus according to claim 4 wherein the detection means
includes a differentiation circuit with output coupled to a first
input of a first comparator by way of a full-wave rectifier
circuit, and
gating means for applying a signal to a second input of the first
comparator which is of opposite polarity to output signals of the
full-wave rectifier circuit when the rate of change of the first
signal falls to zero during a contraction,
the first comparator providing signals maintaining the
sample-and-hold circuit in its hold state whenever the signal
applied to the first comparator is of the same polarity as and
greater than that applied to its second input and when the applied
signals are of opposite polarities.
6. Apparatus according to claim 5 wherein the gating means includes
a second comparator connected to receive the said first signal as
one input and a reference signal representative of an intrauterine
pressure usually only attained near the beginning of a contraction
as another input, the output of the second comparator forming the
second input for the first comparator.
7. Apparatus according to claim 5 wherein the gating means
includes
a further differentiation circuit,
a half-wave rectifier circuit with input coupled to the output of
the further differentiation circuit,
an addition circuit for adding a reference signal to the output of
the further full-wave rectifier circuit, the reference signal being
representative of an intrauterine pressure usually only attained
near the beginning of a contraction, and
an inverter for inverting the output signal of the addition circuit
and applying the inverted signal to the second input of the first
comparator.
8. Apparatus according to claim 1 wherein the third signal is
integrated over approximately 15 minutes.
9. Apparatus for automatically calculating the dosage of a labour
stimulating substance including
activity-indicating means, adapted to be coupled to a uterine
pressure transducer, for providing an activity signal
representative of the integral of the additional intrauterine
pressure which occurs during contractions above the pressure
existing between contractions, the said integral being taken over
at least 10 but not more than 30 minutes,
.[.first comparison means for comparing the most recently obtained
value of an activity signal with the value of a recently obtained
previous activity signal,.]. and
dose calculating means for determining the dosage of a labour
stimulating substance in dependence upon values of the activity
signal .[.and for stabilising or reducing the dosage, at least when
the value of the activity signal is above a predetermined minimum
and below a predetermined maximum, if the first comparison means
indicates that the said most recently obtained value of the
activity signal is equal to or less than the said recently-obtained
previous value of the activity signal,.].
the dosage being so calculated, in operation, that when employed in
stimulating labour, labour progresses without substantial
interruption and without the activity signal exceeding a dangerous
value.
10. Apparatus according to claim .[.9.]. .Iadd.23
.Iaddend.including second comparison means for comparing a recent
value of the activity signal with a first reference signal
representing an upper limit for the values of the activity signal
which it is dangerous to exceed, the dose calculating means being
coupled to the second comparison means and constructed to stabilize
or reduce the calculated dosage when the upper limit is
exceeded.
11. Apparatus according to claim 10 wherein the first reference
signal is equivalent to an activity signal value of about 1500
kPas.
12. Apparatus according to claim 10 including third comparison
means for comparing a recent value of the activity signal with a
second reference signal representing a lower limit for the values
of activity signals corresponding to a possibility of labour
ceasing, the dose calculating means being coupled to the third
comparison menas and constructed to increase the dosage if the
lower limit is not exceeded.
13. Apparatus according to claim 12 wherein the second reference
signal is equivalent to an activity signal value of about 500
KPas.
14. Apparatus according to claim 10 wherein the activity indicating
means includes
means adapted to be coupled to a uterine pressure transducer for
providing a first signal representative of intrauterine
pressure,
means for providing a second signal which is equal to the first
signal except when a contraction occurs when it takes up a value
equal to that of the first signal in the absence of the
contraction,
means for substracting the second signal from the first signal, to
provide a third signal, and
means for integrating the third signal over intervals of at least
10 but not more than thirty minutes to provide the activity
signals.
15. Apparatus according to claim 10 wherein the dose calculating
means includes a read-only memory or a programmable read-only
memory programmed according to an algorithm for determining the
dosage which takes account of the values of recent activity
signals.
16. Apparatus according to claim 15 wherein at, at least one point
in the said algorithm, the dosage determined is stabilized or
reduced if a later activity signal is equal to, or less than, an
earlier activity signal.
17. Apparatus according to claim 16 wherein the said algorithm is
such that at, at least one point therein, the dosage determined is
stabilized or reduced if the value of a recent activity signal
exceeds an upper limit for integral signals which it is dangerous
to exceed.
18. Apparatus according to claim 17 wherein the said algorithm is
such that at, at least one point therein, the dosage determined is
increased if the value of a recent activity signal is below a lower
limit for integral signals indicating that labour may cease.
19. Apparatus according to claim 10 wherein the dose-calculating
means is coupled to dispensing means for controlling the rate of
infusion of a labour stimulating substance into a patient. .[.20.
Apparatus for deriving a corrected signal from a signal with a
varying datum, including means for receiving a first signal having
a varying datum,
means for providing a second signal which is equal to the first
signal except when the first signal deviates from the datum when it
takes up a value equal to that of the first signal in the absence
of deviation, including detection means for detecting first and
second changes in the first signal likely to be indicative of the
onset and completion, respectively, of deviation from the datum,
and a sample-and-hold circuit for sampling the first signal except
between the detection of said first and subsequent second changes
by the detection means when the last value sampled is held, the
output of the sample and hold circuit providing the said second
signal, said detection means further including a differentiation
circuit with output coupled to a first input of a first comparator
by way of a full-wave rectifier circuit, and gating means for
applying a signal to a second input of the first comparator which
is of opposite polarity to output signals of the full-wave
rectifier circuit when the rate of change of the first signal falls
to zero during deviation from the datum, the first comparator
providing signals maintaining the sample-and-hold circuit in its
hold state whenever the signal applied to the first comparator is
of the same polarity as the greater than that applied to its second
input and when the applied signals are of opposite polarities,
and
means for providing a third corrected signal by subtracting the
second signal from the first signal..]. .[.21. Apparatus according
to claim 20 wherein the gating means includes a second comparator
connected to receive the said first signal as one input and a
reference signal representative of a value of the first signal
usually only attained near the beginning of a deviation from the
datum as another input, the output of the second comparator forming
the second input for the first comparator..]. .[.22. Apparatus
according to claim 20 wherein the gating means includes
a further differentiation circuit,
a half-wave rectifier circuit with input coupled to the output of
the further differentiation circuit,
an addition circuit for adding a reference signal to the output of
the further full-wave rectifier circuit, the reference signal being
representative of a value of the first signal usually only attained
near the beginning of a deviation from the datum, and
an inverter for inverting the output signal of the addition circuit
and applying the inverted signal to the second input of the first
comparator..]. .Iadd. 23. An apparatus as in claim 9 further
including first comparison means for comparing the most recently
obtained value of an activity signal with the value of a recently
obtained previous activity signal and wherein said dose calculating
means stabilises or reduces the dosage, at least
when the value of the activity signal is above a predetermined
minimum and below a predetermined maximum, if the first comparison
means indicates that the most recently obtained value of the
activity signal is equal to or less than the said recently-obtained
previous value of the activity signal. .Iaddend.
Description
The present invention relates to apparatus for providing an
indication of uterine activity in labour, and the apparatus may be
extended to determine suitable dosages of labour inducing
substances, that is substances which cause contractions to occur,
such as for example oxytocin or prostaglandins.
The correct rate of infusion of oxytocin has mainly in the past
been determined by observation of the frequency of contractions but
this method suffers from the disadvantage that the rate of infusion
may occasionally be too low to cause labour to continue or so high
that the foetus is in danger.
In attempting to find a more accurate measurement of uterine
activity other factors such as pressure, duration and profile of
contractions have been measured.
Automatic apparatus for controlling oxytocin infusion is available
but this apparatus operates by increasing the infusion rate until
preset frequency and pressure values are reached. Such limits
cannot take into account the wide variation in normal uterine
activity and as a result inadequate stimulation appears to occur in
approximately 4% of cases, but seriously gross stimulation appears
to occur in 8% of cases. This hyperstimulation carries grave risks
to the foetus.
According to the present invention there is provided apparatus for
indicating uterine activity in labour including means adapted to be
coupled to an intrauterine pressure transducer for providing a
first signal representative of intrauterine pressure, means for
providing a second signal which is equal to the first signal except
when a contraction occurs when it takes up a value equal to that of
the first signal in the absence of the contraction, means for
subtracting the second signal from the first signal, to provide a
third signal and means for integrating the third signal over
intervals of at least ten but not more than 30 minutes to provide
an activity signal indicative of uterine activity in labour.
Apparatus according to the invention may be used to calculate
dosage of a labour stimulating substance if the apparatus includes
dose calculating means for determining dosage in dependence at
least upon the activity signal, the apparatus being such that when
the dosage determined is employed, labour progresses without
substantial interruption and without the said integral exceeding a
dangerous value.
The main advantage of the present invention is that the said
activity signal is a more satisfactory measure of uterine activity
than other parameters which have been readily available before
since the basal pressure, that is the pressure which occurs between
contractions is subtracted before integration. However, one of the
problems encountered in determining the said integral is that the
basal pressure varies between contractions and also the pressure is
often different before a contraction is compared with after. This
problem is largely overcome by deriving the second signal and by
subtracting the second signal from the first before
integration.
Preferably each said integral is taken over about fifteen minutes,
this being a compromise between sensitivity and the random changes
due to the last contraction being just in or just out of the sample
period.
The progress of labour can be determined by comparison of
successive activity signals each obtained at the end of a fifteen
minute interval. Hence the dose-calculating means may include a
plurality of stores for storing digital values representing
successive activity signals, and dosage, for example rate of
infusion of oxytocin, can be determined, at least partially, from
the relationship between successive activity signals.
In a typical induced birth the integral signal rises steadily and
then reaches a "plateau" where further activity signals are
slightly smaller in value than previous such signals. The present
inventors have found that when the plateau region is reached dosage
can be reduced by half, usually without materially affecting the
progress of labour. The dose calculating means may therefore
include a first comparator for comparing the current activity
signal with a previous such signal and means for stabilising or
preferably reducing, by for example half, the dosage when the value
of the current activity signal is equal to, or smaller than, the
former signal.
If the dosage is continually increased after the plateau has been
reached the activity signal will eventually again start to rise in
value and this is potentially dangerous. Thus, as a safety measure
to prevent hyperstimulation, the means for determining dosage may
include a second comparator for comparing the current activity
signal with a first reference signal representative of
hyperstimulation. The reference signal should be equivalent to a
value of the said integral of between 1300 to 1800 kilo-Pascal
seconds (kPas) and preferably 1500 kPas for a 15 minute integration
integral and means are provided for decreasing the dosage,
preferably by half when the second comparator indicates the
reference signal has been exceeded.
On the other hand the activity signals may indicate that labour is
not yet fully in progress and for this reason a third comparator
may be provided for comparing the current activity signal with a
second reference signal equivalent to between 300 and 800 kPas,
preferably 500 kPas for a fifteen minute integration period. Means
are then included for increasing the dosage in response to the
output of the third comparator.
The dose calculating means may include a programmable read-only
memory (PROM) coupled to the three comparators, where provided. A
program entered into the PROM provides outputs on different
terminals of the memory in accordance with the outputs of the
comparators and outputs of the PROM fed back to indicate progress.
In general the program may carry out tests for integrals exceeding
1500 kPas at various times in the program halving the dosage if
this figure is exceeded; carry out tests for integrals below 500
kPas, increasing the dose when this figure is not exceeded; and
increasing the dose steadily once every 15 minutes until the value
of the current activity signal is below that of a former such
signal when the dosage is halved.
The above mentioned means for increasing, stabilising and reducing
the dose may thus be set up in the PROM by means of the
program.
Means may be provided for entering a value into the dose calculator
to set the dose to an initial value or another value decided on by
different methods.
Indicator means may be provided for indicating the dosage
determined and/or a signal from the means for determining dosage
may be used as a control signal for controlling the rate of
infusion of oxytocin.
Another aspect of the invention relates to the provision of a
stable baseline when a waveform contains very low frequency
components and normal AC coupling techniques cannot therefore be
used. The contraction waveform has a variable basal pressure or
datum and, as explained above, it is the provision of the third
signal which allows the activity signal to be determined from a
stable datum. However, there are many other applications where a
corrected output signal from a signal having a variable datum is
required.
Hence according to a further aspect of the invention there is
provided apparatus for deriving a corrected signal from a signal
with a varying datum, including means for receiving a first signal
having a varying datum, means for providing a second signal which
is equal to the first signal except when the first signal deviates
from the datum when it takes up a value equal to that of the first
signal in the absence of deviation, and means for providing a third
corrected signal by subtracting the second signal from the first
signal.
It will be recognised that the baseline connection circuits
hereinafter described are examples of apparatus according to the
second apsect of the invention. One example of the application of
the second aspect of the invention is as follows: where it is
required to provide an indication of the relationship between
changes in intrauterine pressure and foetal heart rate, the
absolute pressure is unimportant so that a guard-ring
tocodynamometer (described by C. N. Smyth in the Journal of
Obstetrics & Gynaecology of the British Commonwealth, 77,908)
may be used. Large fluctuations in the signal baseline often occur
with resultant difficulties in chart recording and the baseline
correction circuit may be used to reduce these fluctuations.
Certain embodiments of the invention will now be described, by way
of example, with reference to the accompanying drawings, in
which:
FIG. 1 is a block diagram of apparatus according to the
invention.
FIGS. 2a and 2b show graphs of pressure against time before and
after application to the baseline correction circuit which forms
part of the circuit of FIG. 1.
FIGS. 3a and 3b are block diagrams of the baseline correction
circuits which may be used in the circuit of FIG. 1,
FIGS. 4a to 4g are waveforms of signals which occur in the
apparatus of FIG. 1,
FIG. 5 is a block diagram of the alarm systems of FIG. 1,
FIG. 6 is an algorithm for programming the PROM of FIG. 1,
FIG. 7 is a graph showing the integral of pressure versus rate of
infusion of oxytocin,
FIG. 8 is a block diagram of the PROM of FIG. 1,
FIG. 9 is a block diagram of the dosage calculator of FIG. 1,
and
FIG. 10 is a part-block part-circuit diagram of a baseline
correction circuit.
The embodiment of FIG. 1 will first be described in outline and
then details will be given.
An electrical signal representative of intrauterine pressure is
derived from the external socket of a commercially available
cardiotocograph or an intrauterine pressure transducer and passed
by way of a buffer amplifier 11 which drives a pressure indicating
meter 12 to a baseline correction circuit 13. Since it is the
integral of the areas of those parts of the pressure versus time
curve (shown as the curve in FIG. 2a) which are above the basal
pressure which is of interest, the baseline correction circuit 13
substitutes a steady baseline 14 in the curve of FIG. 2b for the
varying basal pressure 15 in the curve of FIG. 2a. The corrected
baseline 14 is at zero pressure and the corrected pressure signal
is passed to an integrator circuit 16.
An alarm circuit 17 sounds an alarm if the catheter which forms
part of the cardiotocograph becomes blocked and the pressure signal
drops to around zero. The alarm also sounds if the basal pressure
reaches an abnormal level for long periods.
Both curves 14 and 15 of FIGS. 2a and 2b show artefacts of short
duration and to prevent these artefacts from being integrated by
the circuit 16 a monostable circuit 18 operates switch means 19
which momentarily disconnects the input to the integrator.
When the patient is placed on a bedpan a switch 21a is operated
arresting a timer circuit 22 and operating switch means 21c to
disconnect the input signal to the integrator.
At the end of a timing period of fifteen minutes as determined by
the timer circuit 22 the integrated value at the output of the
integrator 16 is sampled by a first sample and hold circuit 23 and
at the same time the previous contents of the circuit 23 are passed
to a sample and hold circuit 24, the previous contents of which are
passed on to a sample and hold circuit 25. The values held by the
circuits 23 to 25 which indicate the progress of labour as
represented by the integrals are applied to three comparator
circuits 26, 27 and 28. The comparator 26 gives a high output
signal level when the integral value held by the sample and hold
circuit 23 exceeds 500 kPas. This high output is applied to a store
30 and also to an inverter circuit 29 to give a high output at a
separate terminal of the store 30 when the contents of the circuit
23 indicate an integral below 500 kPas. If the integral value held
by the circuit 23 exceeds 1,500 kPas the comparator 27 provides a
high output level which is also applied to a separate terminal of
the store 30. The comparator 28 compares the contents of circuits
23 and 25 and when the output of the former is greater than the
output of the latter a comparator 28 gives a high output signal
applied again to a separate terminal of the store 30.
The various outputs of the comparators 27 and 28 as stored by the
store 30 are applied to a PROM 31 and the output from the PROM is
coded and fed back by way of a store 32 to the PROM input. The code
fed back to the PROM input and the comparator states as indicated
by the store 30 determine which outputs of the PROM will be high
according to the program entered into the PROM. The output signals
from the PROM are fed to a dose calculator 33 which at the end of
each fifteen minute period either indicates that the dose should be
increased or halved. The current dose levels are indicated by a
digital indicator 34 and a pump 35 is coupled to the calculator so
that its speed is proportional to the current dose. The motor
drives a pump infusing oxytocin or oxytocin agents into the
patient. A thumbwheel switch 36 is connected to the calculator 33
to allow any dose to be set on the switch and entered into the
calculator.
One form of the baseline correction circuit 13 is shown in more
detail in FIG. 3a where the output of the buffer amplifier 11 is
shown applied directly to one input of a differential amplifier 37
and by way of a sample-and-hold circuit 38 to another input of the
amplifier. When the circuit 38 is in the "sample" mode both inputs
to the amplifier are identical so the output from the amplifier
(see the lower curve FIG. 2) which is passed to the integrating
circuit 16 is zero, ignoring the effect of artefacts. At the onset
of a contraction the circuit 38 is switched to the "hold" mode for
the duration of the contraction. The output from the amplifier 37
is therefore the varying contraction signal minus the basal
pressure signal which existed at the onset of the contraction. At
the end of the contraction the circuit 38 is switched back to
"sample" and its output returns to zero. The circuit 38 may be
based on the National Semiconductor LH0022 series operational
amplifier and constructed as set out in the manufactuer's data.
To obtain the switching signals for the sample-and-hold circuit 38
the pressure versus time signal, shown again in FIG. 4(a) is
differentiated by a circuit 40 and then applied to a full-wave
rectifier 41 the output signals of these circuits being shown in
FIGS. 4(b) and 4(c), respectively. A comparator 42 controlling the
switching of the circuit 38 receives the rectified and
differentiated pressure signal which it compares with a reference
signal derived by a comparator 43. The comparator 43 changes state
when the pressure exceeds 2.7 kPa and its output signal is
therefore as shown in FIG. 4(d). When the rectified differentiated
pressure signal is greater than the output from the comparator 43
at the beginning of a contraction (see FIG. 4(e)), the comparator
42 switches the circuit 38 to "hold", and the circuit is switched
back to "sample" when the contraction dies away and a signal from
the comparator 43 again exceeds that from the rectifier 41. The
object of using the comparator 43 is to avoid the circuit 38
switching back to "sample" when the differentiated pressure signal
is zero at the peak of a contraction.
The output of the comparator 42 appearing at a terminal 44 is used,
as will be explained later, to trigger the alarm circuit 17 and the
artefact suppression circuit 18.
Another baseline correction circuit which may be used as the
circuit 13 is shown in FIG. 3b where the comparator 43 of FIG. 3a
is replaced by a further differentiator 40' and half-wave rectifier
41, the further differentiator 40' having a longer time constant
than the differentiator 40. The two differentiators have different
time constants so that the zero differentiator outputs resulting
from maximum pressure do not occur at the same time. Different time
constants are achieved by using components having different
resistance and/or capacitance values. A reference voltage
equivalent to 1.3 kPa is added to the signal from the further
halfwave rectifier 41' in a circuit 96 and the resultant signal is
passed to an inverter 97 before application to the comparator 42.
Thus when a contraction commences the signal from the rectifier 41
becomes larger than that from the inverter 97 but as the
contraction fades the signal from the inverter 97 becomes larger
than the signal from the rectifier 41. Hence the comparator 42
again provides a suitable control signal for the sample and hold
circuit 38. This alternative baseline correction circuit provides
faster response in recognizing onset of contractions and can be
used where the pressure transducer is of a type which does not
provide a calibrated output signal with a known relationship in
terms of kiloPascals and therefore may have a basal pressure
reading exceeding the reference voltage equivalent to 2.7 kPa
applied to the comparator 43.
The half-wave rectifier may be replaced by a full-wave rectifier
and the time constant of the differentiator 40' may be shorter than
that of the differentiator 40.
The circuit of FIG. 3(b) is shown in more detail with minor
modifications in FIG. 10. An inverter 101 is connected to a
differentiator 102 followed by a voltage addition circuit 103 and a
full-wave rectifier circuit 104, these circuits being equivalent to
the circuits 97, 40, 96 and 41, respectively. A differentiator and
full-wave rectifier circuit 105 is connected to a comparator 106,
whose output is connected to a sample and hold circuit 107. Finally
a differential amplifier 108 is connected at the output of the
sample and hold circuit and to receive the pressure signal from the
inverter 101. The equivalence to FIG. 3(b) will be apparent, the
voltage addition and inverter circuits being connected to the
differentiator 40 and the full-wave rectifier 41 instead of the
circuits 40' and 41', and both input signals to the differential
amplifier 108 being inverted.
In FIG. 10 all resistors are 10 K ohm except resistors 110, 111 and
112 which are 1 M ohm, 330 K ohm and 2.2 M ohm, respectively, and
all amplifiers shown as included in the various circuits are type
741 except that of the sample and hold circuit 107 which is an FET
amplifier (for example National Semiconductor LH 0022 series,
operational amplifier). Capacitors 113 to 117 have values 122
.mu.F., 0.68 .mu.F., 200 .mu.F., 4.7.mu.F. and 2.2 .mu.F.,
respectively. FET 118 is type P1086E and the potentiometer 119 has
a maximum value of 10 K ohms.
When a short duration artefact occurs in the pressure signal, the
output signal appearing at terminal 44 of FIG. 3 switches the
monostable circuit 18 to its unstable condition and isolates the
input of the integrator 16 by switching off a field effect
transistor (FET) (not shown) which forms the switch means 19. The
circuit 18 remains in its unstable state for two seconds and then
switches on the FET reconnecting the integrating circuit. Since as
can be seen from FIG. 2 a large number of short duration artefacts
occur their total area would give rise to considerable error if the
signal reaching the integrator contained the artefacts. On the
other hand although a two second interruption in the connection to
the integrator 16 occurs each time a contraction takes place, the
error in integrated output signal is small since the two seconds
duration of this interruption is small compared with the length of
a contraction which is usually about a minute.
When the patient is placed on a bedpan the switch 21a is manually
operated and the state of a bistable circuit 21(b) changes applying
a signal to a gate (not shown) which receives as another input, a
signal from a comparator, such as the comparator 43 in FIG. 3a.
Thus, provided the pressure exceeds 2.7 kPa, an FET (not shown)
which forms the switch means 21c then isolates the input of the
integrator circuit 16. At the same time a clock pulse oscillator
(not shown) in the timer 22 is switched off so suspending the
interval for which integration is carried out while intrauterine
pressure is abnormally high. When the pressure returns to below 2.7
kPa the trailing edge of the pulse (see FIG. 4f) which appears at
the terminal 44 resets the bistable circuit 21b reconnecting the
input signal to the integrator and restarting the timer 22.
The alarm circuit 17 will now be described in more detail with
reference to FIG. 5. If the catheter which forms part of the
pressure transducer becomes blocked the pressure signal drops to
about zero. Since the pressure signal is supplied to a comparator
46 which also receives a reference voltage equivalent to 0.6 kPa,
the comparator sets a monostable circuit 47 to its unstable state
when the pressure drops to below 0.6 kPa. The output signal from
the comparator 46 also passes to a gate 48 but it is not gated to
an alarm 49 until the gate 48 receives a signal from the monostable
circuit 47. The signal is provided when the monostable returns to
its stable state after ninety seconds so that if the abnormal low
pressure lasts for more than ninety seconds the alarm 49 is
sounded. At the same time the comparator output signal is gated to
a display 51 indicating that a blockage has occurred.
The output from terminal 44 of the comparator 42 (shown in FIG. 3a)
is also connected by way of a connection 39 to change the
monostable circuit 47 to its unstable state. Thus when hypertonus
occurs and the basal pressure is raised above 2.7 kPa, a signal is
applied to a gate 52 and if the hypertonus continues for more than
90 seconds the signal from the terminal 44 is gated to the alarm
circuit 49, causing the alarm to sound. At the same time this
signal is gated to a display 53 which indicated hypertonus. When
the circuit of FIG. 3b is used, the output of a further comparator
(not shown) is applied by way of the connection 39 to the
monostable circuit 47. The pressure signal and a reference signal
equivalent to 2.7 kPa are applied as inputs to the further
comparator.
Both alarm circuits are reset by operating an alarm reset switch 45
so that the monostable circuit 47 is triggered to its unstable
state closing the gates 48 and 52. The alarms cannot therefore be
disabled, only reset every 90 seconds.
If during the ninety second interval following the operation of the
switch 45 in resetting the alarm, a high pressure condition returns
to below 2.7 kPa and then a contraction occurs again raising
pressure, the alarm would sound again if it were not for the
provision of a gate 50. If either alarm condition temporarily
ceases as sensed by the gate 50, while the ninety second interval
occurs, the monostable circuit 47 is immediately set by the gate 50
to its stable state. Hence the alarm will then not sound again
unless an alarm condition still exists 90 seconds after the
monostable circuit 47 is re-triggered for example by a new
contraction.
The programming and operation of the PROM 31 according to the
algorithm shown in FIG. 6, will now be explained. Progress through
this algorithm is controlled by the output signals of the
comparator circuits 26, 27 and 28 as each new integrator output
signal is read into the sample-and-hold circuit 23, the previous
contents of this circuit is shifted through the sample-and-hold
circuits 24 and 25.
The initial oxytocin dose should be between 0 milli units/minute (m
U/min) and 4 m U/min, preferably 2 m U/min where the Unit is an
international unit equal to 0.5 gms of dried posterior pituitary
gland of an ox.
The algorithm if FIG. 6 is based on the integral of pressure, taken
over fifteen minute intervals, versus rate of infusion of oxytocin
curve shown in FIG. 7. The curve shown is idealised and assumes a
steadily increasing rate of infusion. As dosage is increased the
integral of pressure rises steadily until a "knee" is reached at
point 55. The incremental dose should be between 1 m U/min/15 min
and 4 m U/min/15 min, preferably 2 m U/min/15 min.
There is then a slight drop in the integral until a sharp rise
occurs at point 56. If the point 56 is passed the combination of
pressure in contractions and frequency of contractions implied by
the integral of pressure curve may mean that the foetus suffers
from lack of oxygen.
The rate of infusion can be stabilised at the point 55 and in fact
the rate of infusion of oxytocin can be halved when the point 55 is
reached without reducing the progress of labour significantly. In
practice it has been found that the dosage should be reduced by an
amount not less than one quarter and not more than three quarters
of the current dose, preferably by one half. Thus the algorithm of
FIG. 6 depends on detecting the point 55 and reducing the rate of
infusion by half when detection occurs.
However certain safeguards need to be built into the algorithm. The
inventors have found by statistical study that the integral must
reach 500 kPas before labour can be said to have commenced and that
the integral must be kept above this figure to ensure steady
progress. Again it has been discovered that in most cases it is
dangerous for the foetus to allow the integral of pressure to rise
above 1500 kPas. Both these figures therefore mark significant
tests carried out at different points in the algorithm.
A typical progression through the algorithm will now be described.
When the first output signal from the integrator 16 is shifted to
the sample-and-hold circuit 23 the algorithm is entered at point 57
and a test 58 is carried out to determine whether the integral is
greater than 1500 kPas. Normally the integral will be below this
figure so that when the next integrator output signal occurs and is
passed to the sample-and-hold circuit 23, while the contents of
that circuit are passed to the circuit 24 as indicated by "shift"
at 59, a test 61 will be carried out to determine whether the new
integral is less than 500 kPas. At this stage the left-hand branch
at 61 will usually be taken and the dose will be increased by 2 m
U/min as indicated at 62. When the next shift occurs at 63, a test
64 is carried out to determine again whether the new integral is
less than 1500 kPas. If so the dose is again increased by 2 m U/min
at 65. A further test to determine whether the next integral is
below 1500 kPas is carried out at 66 followed by a test at 67 for
an integral less than 500 kPas. Again at this stage it is assumed
that the integral is below this figure and the dose is again
increased by returning to the point 62. The portion 62 to 67 of the
algorithm is then cycled as long as is necessary for the test 67 to
indicate that the integral has risen above 500 kPas. When this
occurs a test is carried out at 68 to determine whether the current
integral S.sub.1 is greater than that which occurred half an hour
previously and is now the contents S.sub.3 of the sample-and-hold
circuit 25. Usually the point 55 in FIG. 7 will not have been
reached the first time the test 68 is carried out so that the
portion of the algorithm 65, 66, 67 and 68 is repeated until
S.sub.1 is less than S.sub.3 indicating that the point 55 has been
reached. At this time the oxytocin dose is halved at 70 and then
following the next shift 71 two tests 72 and 73 are carried out to
ensure that the current integral is not greater than 1500 kPas or
less than 500 kPas and if both these conditions are fulfilled the
dose remains stable while the algorithm cycles through the portion
71, 72 and 73. At points 58, 64, 66 and 72 the safety measure of
reducing to half dose is provided if the current integral is
greater than 1500 kPas. Test 73 ensures that labour progresses by
increasing the dose and returning to point 62 if the current
integral falls below 500 kPas. Should the current integral exceed
500 kPas at 61, which is normally early on in induced labour, the
stable portion 71, 72, 73 of the algorithm is entered directly at
71.
In order to simplify FIG. 1, an output coder 80 and a selection
circuit 83 used with the PROM 31 are omitted and shown only in FIG.
8.
When the first integrated output signal from the circuit 16 reaches
the sample-and-hold circuit 23 the outputs from the comparators 26,
27 and 28, and the inverter 29 are entered into the store 30 along
the four input lines 75 to 78, respectively. These four input
signals are immediately passed to output lines 1, 3, 4 and 2,
respectively, which also appear as column headings under
"comparator output" in Table I (see below) which is the truth table
for the PROM 31. If the current integral is not greater than 1500
kPas comparator output 3 will be zero and an output will appear on
terminal A of the PROM. Referring now to Table II (see below) which
is the truth table for the output coder 80, it will be seen that
under these conditions a 1 appears at a terminal Y.sub.3 of the
coder. The signals appearing on terminals Y.sub.1, Y.sub.2 and
Y.sub.3 are stored by the store 32 and passed back to input
terminals of the PROM, Table I having columns headed Y.sub.1,
Y.sub.2 and Y.sub.3.
TABLE I ______________________________________ Com- Position Coded
Output parator In From Prom Output Output From Prom Algorithm
Y.sub.1 Y.sub.2 Y.sub.3 1/4 2 3 A B C D E
______________________________________ 0 0 0 */* * 0 1 57 and 58 0
0 0 */* * 1 1 59 0 0 1 0/* * * 1 61 0 0 1 1/* 0 0 1 and 72 0 0 1
1/* 0 1 1 0 1 0 */* * 0 1 63 and 64 0 1 0 */* * 1 1 0 1 1 */* * 1 1
66 to 0 1 1 */* 1 0 1 0 1 1 */0 0 0 1 68 0 1 1 */1 0 0 1 72 1 0 0
*/* * 1 1 and 1 0 0 */* 0 0 1 73 1 0 0 */* 1 0 1 72 1 0 1 */* * 1 1
and 1 0 1 */* 0 0 1 73 1 0 1 */* 1 0 1
______________________________________
TABLE II ______________________________________ CODED FEEDBACK PROM
OUTPUT SIGNAL A B C D E Y.sub.1 Y.sub.2 Y.sub.3
______________________________________ 0 0 0 0 0 0 0 0 1 0 0 0 0 0
0 1 0 1 0 0 0 0 1 0 0 0 1 0 0 0 1 1 0 0 0 1 0 1 0 0 0 0 0 0 1 1 0 1
______________________________________
In Table I "1" indicates a high level in positive logic, "0"
indicates a low level and * indicates that the state is
irrelevant.
It will now be clear how the algorithm of FIG. 6 is put into effect
since each time a shift occurs a new set of inputs appear for the
PROM and the outputs obtained from the PROM which depend on the
program entered therein also appear at the terminals A to E. These
signals when applied to the ROM 80 provide new feedback signals and
where appropriate provide command signals on connections 81 and 82
to increase or halve the dose, respectively. Whenever the B output
or C output occurs a signal appears on the connection 81 to
increase the dose by 2 m U/min and whenever the D output occurs a
signal appears on the connection 82 to halve the dose.
Thus as the various inputs for the PROM change in accordance with
the outputs of the comparators 26, 27 and 28 and the signals fed
back by way of the store 32 (representing the current position in
the algorithm), progress is made through the algorithm. Table I can
be related to the algorithm by using the first column of the
table.
For example, after the dose has been increased at 65 and a shift
has occurred, test 66 in the algorithm is reached. If the current
activity signal is greater than 1500 kPas as indicated by input 3
being a one, then regardless of other inputs to the PROM from other
comparators, the dose must be halved so output D is given. If on
the other hand the current activity signal is less than 1500 kPas,
as indicated by input 3 being zero, test 67 can be carried out. If
input 2 is one, a return to test 62 is made but if input 2 is zero
test 68 is made in dependence on input 4 and either the dose is
increased (an output at B) or halved (an ouput at D). Since this
example will have made clear how the algorithm of FIG. 6
progresses, no further examples are given.
Suitable circuits for the PROM are Texas Instruments Programmable
Read-Only Memories Types SN 54186 or SN 74186 and these can be
programmed according to the makers' instructions to provide the
required outputs according to Table I. The stores 30 and 32 may be
Texas Instruments four-bit latches type Ser. No. 54116 or Ser. No.
74116, and the output coder 80 may be constructed from four NOR
gates of Texas Instruments type Ser. No. 5432 or Ser. No. 7432.
Either the output from comparator 28 or from the comparator 26, by
way of the store 30, are passed to one input of the PROM 31. This
selection is achieved by means of a circuit 83 which selects the
inverter output when the PROM output C is high. It will be apparent
from further study of FIG. 8 and Tables I and II how the remainder
of the algorithm of FIG. 6 is put into effect.
Other algorithms may of course by used many of which can be put
into operation simply by changing the programming of the PROM
rather than substituting different circuits. A modification which
can be made to the algorithm of FIG. 6 is to take the S.sub.1 not
greater than S.sub.3 output of test 68 to a further "halve dose"
operation (not shown) and then back to point 59.
The invention can also be put into practice when the signal
representative of intrauterine pressure is a digital signal. The
baseline correction circuit, the integrator, the sample and hold
circuits and the comparator circuits of FIG. 1 are then digital
circuits. Integration is carried out digitally and the sample and
hold circuits may be replaced by digital stores, for example types
Ser. No. 54116 or Ser. No. 74116. The comparators may then be
digital types Ser. No. 5485 or Ser. No. 7485. Clearly digital
signals and circuits can be introduced at any point in the circuit
of FIG. 1 and as an example it is convenient to use an integrator
of the type which receives an analog input signal but has a digital
output signal.
The output from the PROM 31 controls the dose calculator 33 which
is shown in more detail in FIG. 9. At switch-on, two stores 85 and
86 are cleared and at the end of each timing period as signalled
along a channel 87 from the time 22, the contents of the store 86
are applied by way of the store 85 to an adder circuit 88. If now
the signal on connection 81 from the ROM output 80 of FIG. 8
indicates that "2" should be added, the adder circuit carries out
the addition and passes its output by way of a divide-by-two
circuit 89 and a data selector 90 back to the store 86. When 2 m
U/min are to be added to the dose as in this sequence of operations
the divide-by-two circuit 89 is inactive since it is never required
to divide at the same time as the dose is increased. However at
other points in the algorithm there will be no command on the
connection 81 to add, but rather a command on the connection 82 to
divide-by-two and this will be carried out by circuit 89.
Thus it will be seen that the current dose is stored by the circuit
86 and it is the output of this circuit which controls the digital
read-out 34 by way of a binary to seven-segment decoder 92.
The contents of the store 86 also control the pump 35 by way of a
digital-to-analogue converter (not shown).
Should it be required to set the dose to a particular value, this
can be carried out by using the thumbwheel switch 36. The switch is
set to the value required and then a dose select switch 94 is
operated manually so that the value represented by the setting of
the switch 36 is entered by way of the data selector 90 into the
store 86.
The timer 22 is controlled by a clock pulse oscillator (not shown)
with a repetition period of 0.9 seconds but this is divided by a
thousand to give a timing period of 900 seconds. At the end of each
900 second period a number of sequential timing pulses each of 0.9
seconds duration are provided and supplied as required to the
sample-and-hold circuit 23 to sample the integrator output, to the
sample-and-hold circuits 24 and 25, the stores 30 and 32 and the
dose calculator 33 to ensure that data is transferred as required
in the foregoing description. Since the transfer of data items by
sequential pulsing of circuits is well known it will not be
described further except to mention that conventional arrangements
are also made for clearing or resetting the various circuits
mentioned and for providing the required selection of sequential
timing pulses to allow the setting of the thumbwheel switch 36 to
be entered into the dose calculator 33. Where the integrator 16
provides a digital output the duration of the timing pulses is
preferably reduced to, for example, 27.5 milliseconds.
Although an embodiment of the invention has been specifically
described it will be clear that the invention may be put into
practice in many other ways. For example the algorithms mentioned
can be changed, the programming of the PROM may also be changed and
various portions of the circuit of FIG. 1 can be replaced by others
having substantially the same function. The PROM may be replaced by
a read-only memory constructed to operate a suitable algorithm. The
apparatus and/or the algorithm may be modified, for example by
modifying the comparator 28 to provide an output signal for equal
input signals, to stabilise or reduce the dosage when successive
integrals are equal. While the invention is particularly useful in
controlling the progress of human births it may also find
application at the births of animals.
* * * * *