The present invention relates to improved tocodynamometers in general, and to improvements in the sensor and in the circuitry of such devices. In particular, the present invention relates to an improved transducer for and the circuitry associated with converting linear deflections of a sensing element of the tocodynamoraeter into electrical signals and amplifying the signals sufficiently to activate a suitable display.

Tocodynamometers are used for measuring intra-amniotic pressures in the uterus during pregnancy. Intrauterine pressure changes can be used as an index of uterine activity to provide information concerning the onset of labour. The information obtained can be used in the diagnosis of and the prognosis for events, either normal or abnormal, occurring during the pregnancy, particularly during labour.

Research has shown that uterine activity during pregnancy may be a valuable indicator of the likelihood of premature labour. Throughout a normal pregnancy a woman experiences "Braxton Hicks" contractions which occur randomly and are a normal condition in pregnancy. The frequency and strength of these contractions gradually increase towards term, commonly ending in labour. If they increase in strength and

frequency before the end of the normal 40 week gestation period, premature labour is likely to. ensure.

Uterine contractions associated with the onset of labour are not normally detected by the patient until labour is advanced and usually beyond the point of arrest. Hence, a device which could be used by a high risk patient (i.e. a patient with a history of premature labour) at home, to identify early contractions, is desirable.

One way of determining the uterine activity is by the so called "internal" methods for measuring intrauterine pressure changes in antenatal patients, which have some attendant significant risk to both mother and foetus. One such internal method is by the internal catheter method which is unacceptable in many cases because it is an invasive method requiring ruptured membrances before it can be implanted. Additionally, such internal methods are not suitable for some types of assessments throughout the pregnancy such as serial assessments.

External devices have also been used to measure intra- uterine pressure as an alternative for and/or as a complement to the internal methods. However, current external uterine activity monitors are large, bulky machines primarily designed for use in hospitals under controlled conditions. Also, many external devices are regarded as inaccurateand have not always been entirely successful, since the information obtained by their use could only be used in semi-quantative assessments.

External devices measure the tension in the uterine wall during contractions or monitor the internal pressure through recording the pressure required to render the uterus flat. One such device is described by C. N. Smyth in an article entitled "The Guard Ring Tocodynamometer" in the Journal of Obstetrics and Gynaecology, 1957, 64, at page 59, which has been reported as obtaining measurements of an accuracy comparable to those obtained by internal methods.

In the device of Smyth there is a sensing head comprising

a movable pressure sensitive plate surrounded by a guard plate or guard ring. The guard ring is held exactly level with the measuring area and flattens an additional surround of body tissue. The pressure sensitive plate which is free to move in response to the sensed pressure, is directly connected to a spring which is supported on or by the guard ring. The amount of deflection of the spring corresponds to the magnitude of the pressure sensed and is measured by resistance strain gauges directly connected to the spring. The resistance strain gauge may, in one embodiment, be connected to a heatstone bridge circuit.

The present invention, on the other hand, sets out to provide a device which is more sensitive, more accurate and more reliable than that described by Smyth by having an improved transducer in the sensing head for accurately converting its linear movement into an electrical signal and by having improved circuitry to amplify the electrical signal to operate a suitable display means.

In accordance with one aspect of the present invention it has been found to be particularly advantageous to utilise a simple and compact form of transducer which converts linear displacements to electrical signals by altering the inductance between a core member and a tube which surrounds the core. Another aspect of the present invention realtes to the use of novel circuitry coupled to the output of the transducer in order that more reliable and sensitive information can be obtained.

According to one aspect of the present invention there is provided a sensing device suitable for use in detecting pressure or pressure changes comprising a movable pressure responsive element located adjacent or contiguous with a fixed pressure insensitive guard body, said element being connected to a transducer for converting linear movement of the element into electrical signals , said transducer comprising a first portion fixed with respect to the guard

body and a second portion connected to the moveable element capable of corresponding movement therewith and movable with respect to the second portion so that relative movement of said first and second portions produces an electrical signal whereby the magnitude of the electrical signal produced is proportional to the extent of movement of the pressure responsive element.

Typically, the device of the present invention is arranged so that changes in intrauterine pressure result in small linear deflections of the pressure responsive element which is typically a centrally located sensing element or button located in the guard body or transducer base which is generally annular. The amount of linear deflection being directly proportional to the change in pressure. Typically, the linear deflection of the central button is inboard within the annular transducer base.

Typically, the button is directly coupled to a spring means which provides support for and a resistance against any movement of the button. .Preferably the spring means is a flat or leaf spring or similar biased to maintain one end of the button in a common plane with the outer surface of the guard ring. Typically, the spring is calibrated to ensure that under maximum load, deflection outside of the linear range of the transducer does not occur. Typically, in one form of the present invention deflection of the button is detected by means of a displacement trans¬ ducer on a stainless steel shaft, connected to the flat or leaf spring and the button. Movement of the button results in movement of the core and hence a proportional change in transducer output.

Typically, the transducer is a Linear Variable Differen¬ tial transducer (LVDT) , such as that supplied by Schaevitz. Typically, the change in transducer output is detected and applied to a differential amplifier. Further stages of amplification raise the signal level for application to a visual information device typically a strip chart recorder.

Typically, the device further includes a facility for electrically zeroing the system once the transducer is fitted to the patient.

The present invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows cross-sectional view of one form of a tocodynamometer constructed in accordance with the present invention;

Figure 2 shows a plan view of the tocodynamometer of Figure 1 but with the cup-shaped cover removed;

Figure 3 is an underside view of the tocodynamometer of Figure 1;

Figures 4a and b show cross-sectional and plan views respectively of parts of the device shown in Figure 1; Figure 5 shows a detail of the mounting of the core of the transducer;

Figure 6 is a plan view of the preferred form of spring of the tocodynamometer;

Figure 7 shows a schematic block diagram of a basic form of the invention;

Figure 8 shows circuit refinements which can be utilized with the invention;

Figure 9 is a circuit diagram for an oscillator;

Figure 10 is a circuit diagram for an oscillator buffer; Figure 11 shows the circuit diagram for the transducer demodulator;

Figure 12 shows the complete circuitry for the oscillator buffer and transducer demodulator;

Figure 13 is a circuit diagram for a differential amplifier;

Figure 14 shows an amplifier for a chart recorder;

Figure 15 shows a circuit diagram for the level detector circuits;

Figure 16 shows a circuit diagram for the display/counter circuits;

Figure 17 is a circuit diagram for a display strobe circuit;

Figure 18 shows a circuit diagram for a timer; and Figure 19 shows a circuit for the power supply. The tocodynamometer illustrated in Figures 1 to 6 com¬ prises a cup-shaped housing 12 at the lower face of which is mounted an annular base member 1. Base member 1 forms a guard ring and serves to flatten an area of the uterine wall to provide a pressure base. Located in the opening of the base member 1 is a movable sensing button 2 which is coupled to a transducer and operates to produce an electrical output from the transducer when the button 2 is moved inboard relative to the base 1. In use of the apparatus the base 1 is placed on the abdomen of the patient and is preferably held in position by means of a belt (not shown) . The device is preferably located in the vicinity of the umbilicus thus ensuring there will be a fluid filled portion of the uterus beneath the movable button 2. The movable button 2 moves in accordance with pressure variations within the fluid in the uterus and this information can be used to predict the onset of labour more accurately than is possible with known techniques.

On the top side of the base 2 when in its normal in use position, there is provided a support bridge 6, preferably non ferous, which supports a transducer 20. The transducer 20 comprises a Schaevitz linear variable differen¬ tial transducer - 010 MHR. This form of transducer requires approximately 3 to 4 volts peak to peak applied to it at a frequency of between 400 Hz and 20 kHz for optimum performance. The transducer 20 comprises an outer tube 7 which is carried by the bridge 6 and an inner core 8 which is carried out by a stainless steel needle 9. Core 8 is attached to needle 9 by means of heatshrink tubing 8a as shown in detail in Figure 5. The heatshrink tubing 8a is first shrunk onto needle 9 at two closely spaced apart locations and the core 8

which is provided with an internal screw thread is screwed down onto the two heatshrink tubes, locking the core in the desired position on needle 9. The mounting of the needle 9 on the movable button 2 is also illustrated in more detail in Figure 5. The upper end of the wire 9 passes through an opening in a cap 10 formed on the tube 7 and serves to act as a bearing guide for the needle so as to retain it in position when the needle moves upwards and downwards in use thereby accurately maintaining core 8 at a spaced apart location relative to outer tube 7. The core 8 passes through the centre of the tube 7 and its relative position determines the output of the transducer.

As best seen in Figure 2, a leaf spring 4 is mounted on the upper face of the base 2 and extends generally transversely of the bridge 6. -The spring 4, which is shown in plan view in Figure 6, is connected to the movable button 2 by means of screws and mounting bosses 5. Spring 4 serves to limit the movement of button 2 and provides a resistance to movement of button 2 inboard of the device. Displacement £>f removable button 2 caused by uterine pressure variations causes the core 8 to move in the tube 7 and by means of electrical connections (not shown) between these elements an electrical output can be obtained which is proportional to the displace¬ ment of the button 2 relative to the base 1. In practice pressure variations due to contractions would occur from 0 to 30 per hour and would be in the range from 2 to 5 mm of mercury for weak contractions, 5 to 15 mm of mercury for mild contractions and 15 to 100 mm of mercury for strong contractions. Typically, the device of the present invention may detect up to 30 contractions of different or the same intensities per hour.

Figure 7 shows the basic form of circuitry associated with the transducer 20. The circuit includes an oscillator detector 22 which supplies the transducer 20 with its exciting voltage and also detects output from the transducer. Output from the detector 22 is applied to a differential amplifier 24

whose output is coupled to a receiver demodulator 26. The output from the receiver demodulator is coupled to a chart recorder 28 or to any other suitable display means. The circuit also includes a power supply circuit 30 and a zero setting circuit 32 for the differential amplifier 24.

Figure 8 shows the circuitry for monitoring contraction levels within three predetermined pressure bands-and to count the number of contractions so detected. The circuit includes a filter 32 which receives output from the receiver demodulator 26. The filter removes spurious spikes which might be caused when the patient coughs or laughs or changes position. Events of this sort can cause a sudden change in abdominal pressure of up to 40 mm of mercury. Experiments have shown however that spikes due to this cause are not a serious problem and are easily detected from the required information. The filter can be omitted where only a chart recorder is used but it is preferred to use the filter if the counters are employed. Output from the filter 32 is applied to discriminating circuits 34, 36 and 38 which effectively monitor the magnitude of the signals derived from the receiver demodulator 26 which are proportional to the displacement of the movable button 2 and hence of the pressure variations within the abdomen. The outputs of the discrimina¬ tors 34, 36 and 38 are applied to counters 40, 42 and 44 respectively and then to digital displays 46, 48 and 50 respectively. A circuit also includes a base line detector circuit 52 which locks the displays whilst a contraction is being monitored so that only when the contraction has passed and the pressure returns to a base line will the appropriate display change. This prevents the displays cycling as a contraction progresses which might cause patient anxiety. Figure 9 shows in more detail the oscillator circuit which is used in the oscillator detector 22. It produces a stable sine wave of 25 kHz at approximately 3.0 volts peak to peak and hence good isolation is required between it and

the oscillator.

Figure 10 shows a typical circuit for the oscillator buffer which uses an operational amplifier with a unity gain amplifier and 47 micro Farad coupling capacitors to provide low impedance coupling to and from the amplifier.

Figure 11 shows the preferred circuit diagram for the transducer demodulator, the circuit including a centre- tapped winding 54 and.diodes 56 and 58. The outputs appear at the outputs of the diodes 56 and 58. Figure 12 shows a complete circuit diagram for the oscillator detector 22. The outputs appearing at the diodes 56 and 58 are applied to the positive and negative inputs of a differential amplifier 24, in the- manner illustrated in Figure 13. The centre tap of the winding 54 is connected to the wiper of a potentio- meter 60. The gain of the differential amplifier 24 is approximately 20. The output of the differential amplifier is used to drive the chart recorder amplifier which is shown in Figure 14. The output from the differential amplifier 24 is also used as an inp _^ ut to zero set amplifiers and indicators which are not shown in the drawings. These will show that the transducer is correctly placed on the patient and that the differential amplifier output is zero volts so that any changes in uterine pressure will be recognised as a positive going voltage. The zeroing circuitry is therefore switched across the output of the differential amplifier initially for calibration and then switched out during recording. The circuitry may include an arrangement for calibration which includes a balanced potentiometer on the front panel of the control unit which is adjusted to obtain final nulling of the differential amplifier output. The attainment of a null is indicated by a succession of LED's around the balance potentiometer which progressively extinguish as the null point is approached from either direction.

The discriminating circuits 34, 36 and 38 of Figure 8 are illustrated in more detail in Figure 15. Each circuit includes a Schmitt trigger operational amplifier 62 associated

with potentiometers 64. The potentiometers 64 are adjusted so as to vary the reference levels of the amplifiers so that the amplifiers are triggered at voltage levels corresponding to pressure variations of about 2.0 mm, 5.0 mm and 15 mm of mercury contraction pressures. The circuit also shows an enable/disable circuit 66 which operates to disable the displays 46, 48 and 50 when a contraction is in progress and when the counters are incrementing. When the contraction pressure has returned to the base line, the circuit 66 then updates the display to the new counter value.

Figure 16.shows a convenient circuit realisation for the counters 40 to 44 and displays 46 to 50. The counters preferably comprise two sets of cascaded UP/DOWN decade counters so as to give a maximum count capability of 99 on each level. The U /DOWN facility of the counters has been utilized by having each counter increment at its respective trigger circuit trips, but if the contraction peaks at a higher pressure than the cotanters that have been incremented, the last counter will remain set to the new value and as the contraction subsides, the lower counters will decrement back to their original value. The front panel of the control unit includes a reset switch to reset all counters to zero. The front panel also includes a display ON/OFF switch to blank the display if desired. This does not affect the operation of the counters so that the counter outputs can be read by reactivating the display circuitry.

Figure 17 shows a preferred form of clock pulse generator 70 for the displays. A suitable circuit is available in integrated form such as an LM555. The circuitry may include a timer which indicates the end of a predetermined monitoring period such as one half an hour. An integrator timer LM322 is suitable for this device and it is coupled as illustrated in Figure 18.

A convenient form of power supply for producing the power supplies at -15, earth, +5 and +15 volts is shown in Figure 19.

It is to be understood that the inventive concept in any of its aspects can be incorporated in many different constructions so that the generality of the preceding des¬ cription is not to be superceded by the particularity of the attached drawings. Various alterations, modifications and/or additions may be incorporated into the invention particularly described without departing from the spirit or ambit of the invention.