Apparatus for the infusion of a labour-inducing drug into the bloodstream of a patient during childbirth

Abstract

 In apparatus for the automatic infusion of labour-. inducing drugs during childbirth, in which the dose rate is steadily increased until a particuiar level of contraction activity is detected and then held constant, this constant rate can result in over-stimulation of labour. The improvement comprises the inclusion of a dose rate control system in which the duration and periodicity of the labour contractions are detected in the form of a pulse train (curve 35) from which a signal (curve 36) is produced by integration, which signal is representative of the average value of the mark-space ratio of the pulse train. This average value is compared with a predetermined value (48, curve 36) and the dose rate is increased or decreased according to whether the average value is less or greater than the predetermined value.

Description

[0001] This invention relates to apparatus for the infusion of a labour inducing drug into the bloodstream of a patient during childbirth



maintain satisfactory progress of labour will lie in the range 1 to 128 milliunits of oxytocin per minute, with some 80% of cases laying in the range 4 to 32 milliunits per minute.

[0002] If the dose rate is too low, the uterus does nor. contract strongly enough to cause labour to progress, but if the dose rate is to high, the uterus may contract too strongly and/or too frequently. stay result in maternal or foetal distress. It is found desirable to commence the infusion at a low dose rate and to increase this are gradually during a period of time until labour is established.

sein of the patient. The pump, of the positive displacement peristaltic cype, is driven by an electric motor provided with a control circuit such that the rotational speed of the motor, and hence the delivery rate of the pump, is determined by the value of a d.c. reference potential applied the control circuit. The control circuit may comprise a servo system in which a feedback signal representative of the actual speed of the motor is compared with the reference signal, and the motor drive current controlled to minimise any difference. The feedback signal may be derived from the back-EMF of a d.c. motor or from a tacho-generator coupled to the motor output shaft. In an alternative arrangement the pump-is driven by stepping motor which is itself driven by a train of pulses derived from tne output of a voltage-controlled oscillator whose frequency is determinedby the value of the reference potential.

[0003] Since the output per revolution of the pump is substantially constant, the dose rate is directly proportional to the pump speed, and hence to the value of the reference potential.

[0004] The reference potential is obtained from the slider of a reference potentiometer connected across a stabilised d.c. supply. A constant- speed motor, referred to hereinafter as the dose rate motor, is arranged to drive the potentiometer shaft through reduction gearing. Before commencing an infusion the reference potentiometer is set to produce a minimum dose rate, e.g. 1 mU oxytocin per minute. At the start of the infusion the dose rate motor is switched on and drives the reference potentiometer so as to increase the dose rate as the infusion proceeds. The arrangement is such that, in the absence of command signals from a control system to be described hereinafter, the dose rate is doubled approximately every fifteen minutes until the rate of 32mU oxytocin pe- minute is reached, when the dose rate motor is automatically switched off and the dose rate thereafter remains constant.

[0005] The control system used heretofore is based on the assumption that when labour has been induced and is progressing normally, uterine contraction occur every 2 to 2

minutes, that the usual duration of a contraction lies in the range 30 to 60 seconds, that the intra-uterine ressure developed during a normal contraction lies in the range 30 to 60 mm Hg and that the resting pressure between contractions lies in the range 5 to 15 mm Hg.

[0006] Intra-uterine pressure is measured by a transducer coupled to the patient by means of a fluid-filled catheter whose open end is inserted into the amniotic fluid. The electrical signal from the transducer is fed via a stabilised buffer amplifier to a first input of a comparator circuit to a second input of which is applied a potential corresponding to a pressure of 35 mm Hg. The comparator thus produces an output signal during a contraction in which the intra-uterine pressure equals or exceeds 35 mm Hg. This signal is used to trigger a delay circuit (timer) having a delay period of 2

minutes. The timer ? when triggered, switches off the dose rate motor for the duration of its delay period.

[0007] After the start of an infusion, therefore, the dose rate motor runs to increase the dose rate from its initial, low, value until a first contraction occurs. The motor is then switched off and the dose rate held constant for 2

minutes. If no further contraction has occurred in that period, the timer then resets, the dose rate motor is switched on again and the dose rate further increased, until a second contraction is detected, when. the timer is again triggered and the motor switched off for a further 2

minutes.

[0008] It will be seen that as the interval between successive contractions decreases, the more frequently is the dose rate motor switched off and the more slowly the dose rate increased, until contractions are occurring at intervals of 2

minutes or less. The delay timer cannot then reset, the motor is held continuously in the off condition and the dose rate is held constant.

[0009] The control circuit also includes means for initiating an alarm and stopping the infusion if abnormal conditions, i.e. spasm or excessive infra-uterine pressure, should cecur. Spasm is the condition in which a contraction occurs but is not followed by a relaxation after the normal interval. This condition interferes with the supply of blood to the foetus and may cause foatal anoxia.

[0010] To detect spasm, the leading edge of a output signal for the above-mentioned comparator is used to trigger a second timer having a period of, for example; two minutes. If the duration of the output signal exceeds the period of the second timer, a signal is generated which traggers an alarm circuit. This latter circuit, when triggered, produces an audible and/cr visual signal to call medical assistance to the patient and also switches off the pump motor of the infusion apparatus; so stopping the supply of oxytocin to the patient.

[0011] It is considered that a normal contraction should not result in a sustained pressure greater than 80 mm Hg. The output signal from a pressure transducer buffer amplifier is applied to a second comparator circuit which also has applied to it a potential corresponding to a pressure of 80 mm Hg. Any output signal from the second camparator is used to trigger a third timer circuit having a delay period of 10 seconds. Only if the duration of the output signal exceeds the delay of the third timer is a signal generated which is effective to trigger the alarm circuit described above. The 10 second delay prevents alarm condition warnings being given as a result of transient high pressure signals caused by the patients exertions in, for example, changing position, coughing and sneezing.

[0012] Although automatic infusion apparatus of the general type described hereinbefore is widely used for the induction of labour, it is found to have certain shortcomings. There is necessarily some delay before the effect on the patient of a particular dose rate of oxytocin becomes apparent. Since in this apparatus the dose rate is progressively increased.until a particular level of contraction activity is detected, and thereafter held constant, there is a tendency for the constant rate to be somewhat higher than is necessary to maintain that level of activity. Moreover , it is found with most patients that induction of labour by the administration of oxytocin stimulates the production of natural hormones having a similar effect. In such cases, once labour is established it may be necessary to reduce the oxytocin dose rate rather than to maintain it constant in order to avoid excessive contraction activity with risk of damage to the baby.

[0013] it is an object of the present invention to provide, in apparatus for the infusion of labour inducing drugs, improved means for controlling the dose rate so as to minimise the risk of over-stimulation of labour.

[0014] According to tha present invention, apparatus for the diameter infusion of labour inducing drugs includes a done rate control system comprising means for producing a pulse train wherein with successive pulse corresponds to a contraction of the patientisa arus and wherein the duration of each pulse is representative of the duration of the corresponding contration means for generating a further signal representative of the average value of the mark-space ratio or the pulse. train and means responsive to the further signal for increasing the dose rate when the said average value is less than a predetermined . for reducing the dose rate when the average value excueds are se predetermined value.

[0015] In order that the invention and the manner in which is to be performed may be clearly understood, an embodiment - thereof will be described, by way of example, with reference to the assompanying drawing of which

. Figure 1 is a schematic circuit diagram of a control system according to the invention, and

Figure 2 illustrates waveforms at particular points in the circuit of Figure 1.



restricted for more than 20% of the total time.

[0016] Referring first to Figure 1, a pressure transducer adapted to produce an electric signal reprssentative of pressures the ranqe 0 to produce an electric signal representative of pressures in the range 0 to 100 mm Hg is indicated symbolically at 1. The transducer is connected to a patient by means of a fluid-filled catheter, not shown in the drawings, whose open end is immersed in the patient's amniotic fluid. Other reliable pressure-measuring means may alternatively be used. The output signal from the transcucer 1 is replied to a feedback-stabilise suffer amplifier 2 whose output is connected via a capacitor a and a d.c. restoration circuit 4 to an input of a further buffer amplifier 5. The purpose of the d.c. restoration circuit 4, which has a time constant of the order of 15 minutes, is to remove any apparent fluctuation in the base Line of the pressure siqnals due to changes in hydrostatic height of the uterus with respect to the transducer.

[0017] The signal appearing at the output terminal of the amplifier 5, representative cf the patient' s intra-uterine pressure, is applied to an inverting input terminal 6 of a comparator amplifier 7. A non-inverting input terminal 8 of the comparator 7 is connected to the slider of a potentiometer 9, itself connected between the positive supply rail (+V) and ground (0V). The potentiometer 9 is set to produce at the terminal 8 a potential representative of a selected pressure, typically 15 mm kg. The arrangement is sucn that when the potential at the terminal 6 is les: than the potential at the terminal 8, the output terminal 10 of the comparator 7 is at a high potential substantially equal to that of the positive supply reil (+V). When the potential at 6exceeds that at 8, terminal 10 is at a low potential, substantially equal to ground pulerti (OV ) Consequently during rest intervals between contractions, when the intra-uterine pressure is lower than the Selectedpressure, the potentia at terminal 10 is high. During contractions however,terminal 10 is at low potential.

[0018] The Comparator output terminal 10 is connected to an integrator circuit. indicated generally bythe reference 12operation will bedescribed herein after. An output terminal 13 of the integrator 12 is connected to an inventinginput terminal14 of a second comparator

[0019] The motor 20 therefore commences to drive the dose rate potentiometer in a forward direction, increasing the potential applied to the input of the control unit 27 and increasing the dose apte. The law of the potentiometer 24 and the ratio of the qear unit 25 are chosen so that the dose rate is doubled in each successive period typically, 23minutes.

[0020] This situation persists until the dose rate/hasbeen increased. sufficiently to cause contraction to commence. then a first contractive occurs the patient's intra-uterine pressure exturis 15mm kg. utput 10 of the comparator 7 therefore goes to ground potential(OV)for The duration of the Contraction, and returns the high potential(+V) at the end of the contraction. The integrator 12 herefore charges during the contraction but commerces to discharge age a at the end of the Contraction

[0021] The integrator 12 comprises an operational amplifier 29 provided with a capacitor 30 and resistor 76 connected in parallel between an output terminal 13 and an inverting terminal 31of the ampifier 29. The utput 10 of the comparator 7 is connect to the terminal 31 via an input resistor 32. A non-inverting input- terminal 33 of the amplifier 29 is connected to the slider of a potent ometer 34, itself connected between the positive supply rail and ground, a an input resitsor 77.

[0022] The nett current into the integrator 12 is given by the voltage at the terminal 10 divided by the valu of the input resistor 32 minus the voltage at the slider of the poten ometer 34 divided by the value of the Input resistor 77. It will be see that when the terminal 10 at zoro volts i.e, during a contraction, the output terminal 13of the Integrator 12 linearly the positive potential,andwhenthe terminal10returns to +V volts uring a subsequert rest period. the output terminal13 discharges 1. earlytowards zero The relative rates charge anddischarge are dependenton the setting of the potentiometer 24. Thisis typically set so at the integrated charges from zerotoa otential V in one unit of ti and discharges from the potential V to zero in four units of time.

[0023] Referringnow to Figurethe curve 35 represents the variation with time of the potential the output terminal 10 of the Comparator 7 and the curve 36 represents the corresponding variationof the potential at the output terminal13 of the integrator 12.

[0024] The necative-going pulse 37 of 1 minute duration corresponds to the first contraction after the start of an infusion. As shown at 39 the integrator charges to a potential V1 end during the pulse 37 and discharges to zero during the first four minutes of the subsequent rest period 38. It will be appreciated that if the pulse 37 were of shorter or longer duration, the integrator would charge to a lower or higher potential as the case might be, but would discharge to nero in a period four times the duration of the pulse.

[0025] A second pulse 40, corresponding to a second contraction, is shown-as occurring after a rest period 38 of 5 minutes duration. This followed by a further rest period 41 of four minutes duration. The integrator charges during the pulse 40, as shown at 42, and discharges to zero immediately before the onset of the next pulse 43. This is followed by a rest period 44, shown as being of three minutes duration, and further pulse 45. The integrator 12 again charges during pulse 43, but in this case cannot fully discharge during the rest period, Consequently it charges to a potential greater than V1 during the pulse 45

[0026] As described hereinbefore, output of the integrator12is compared in the comparator 15 with a threshold potential setby the potentiometer 16. The threshold level may typically be set equaltoV1as shown in Figure 2 by the broken line 48.

[0027] It will be seen that for contractions of one minute duration followed by rest periods of not less that 4 minutes, the integrator output does not exceed the threshold level V1. If however,theduration of the contractions exceeds 20% of the total time. The integrated voltage accumulates over successive periods so that it exceedsthe threshold level V1 for a greater proportion of each successive cycle.

[0028] The relay 18 is de-energised reversing the drive to the dose rate potentiometer 24 when and only when the integrator output exceeds the threshold level V1. It follows that, from the start of an infusion the dose rate is increased until conctractionsoccurwith a duration and frequency such that they occupysay20% of thetotaltime. Thereafter the dose rate is decreasedorincreased as any be necessary to assist the uterus to continue contracting for periods averaging about 20% of the total time.

[0029] The Figures of 1 minute duration for individual contractions and 20% ratio of contraction time to total time are purely exemplary. Other values may be chosen by adjustment of the potentiometers 34 and 16. Also, although the integrator circuit is here described as an analogue device, the same functions can be realised digitally by using an up-down counter driven by clock pulses which are gated by the contraction signal.

[0030] The control system further comprises means for operating an alarm and for switching off the pump drive motor 28 if any of the three following conditions obtain:-

(i) The uterus continues to contract, on average, for periods substantially greater than 20% of the total time, despite reduction of dose rate.

(ii) An individual contraction persists for more than two minutes (spasm).

(iii) The intra-uterine pressure exceeds a danger level of (say) 80 mm Hg for 10 seconds or more.

(i) and (ii) above are achieved by providing a further integrator circuit 50 fed from the output 10 of the comparator 7. The integrator 50 is similar to the integrator 12 described hereinbefore save that its input resistor 51 has a value double that of the resistor 32. The integrator 50 therefore has an integration time constant double that of the integrator 1.2. For any given width of input pulse, its output is therefore half that of the integrator 12, as shown by the curve 52 in Figure 2.

[0031] The output 52 of the integrator 50 is fed to a comparator 53 which also receives a threshold potential V2 set by a potentiometer 54 and shown in Figure 2 by the broken line 55. Typically V2 is set equal to V1. The comparator output will remain high until the potential at the output 52 exceeds the threshold level V2, whereupon it will fall to zero.

[0032] At 56 in Figure 2 is shown in broken outline a pulse at the output 10 of the comparator 7 having a duration of 3 minutes. The corresponding output from the integrator 50 is indicated by the broken line 57. It will be seen that the integrator output exceeds

[0033] A circuit of known form is employed to provide an alarm in the excess pressure condition (iii) above. Briefly, the output of the buffer amplifier 2 is fed to a non-inverting input of a comparator 65 whose inverting input is supplied with a potential corresponding to an intra-uterine pressure of 80 mm Hg derived from a slider of a preset potentiometer 66 connected between the stabilised positive line and ground. The output 67 of the comparator 64 is normally at zero potential and goes positive if and only if the intra-uterine pressure exceeds 80 mm Hg.

[0034] The output 67 of the comparator 65 is connected to a D input of a D type flip-flop unit 68 and also to an input of a timer circuit 69 which produces a positive-going output signal having a duration of 10 seconds from the instant at which its input goes positive. The output of the timer 69 is connected to a clock'input (C) of the flip-flop 68 via a NAND gate 70. The arrangement is such that the flip-flop 78 is clocked by the back edge of the timer output pulse, i.e 10 seconds after the output of the comparator 65 went positive. Hence, if the excess pressure has persisted and the D input is still positive when the flip-flop 68 is clocked, the Q output of the flip-flop is set positive.

[0035] A light emitting diode 71 driven by a transistor 72 from the Q output is illuminated when the flip-flop 68 is in the set (alarm) condition. A push button switch 73 permits of resecting the flip-flop 68 after an alarm.

[0036] An inverse output of the latch 61 and a Q output of the flip-flop 68 are connected to respective inputs of a NAND gate 74. The output of the gate 74 therefore goes positive if either the latch 61 or the flip-flop 67 (or both) is in the set (alarm) condition. The output of the gate 74 is connected to the pump motor control circuit 27 and is effective, when positive, to stop the pump motor 28 and with it the infusion pump, so terminating the infusion if any one or more of the alarm conditions (i) to (iii) above occurs. The output of the gate 74 may be further connected to an audible alarm unit 75.

Claims

terminating the infusion if the said pressure exceeds a given lever for a predetermined period.

6. Apparatus as claimed in any previous Claim characterised in that the means for generating a further signal comprises an integrator circuit adapted to charge at a first rate during each pulse of the pulse train and to discharge at a second rate during each interval between pulses of the pulse train.

7. Apparatus as claimed in Claim 6, characterised by means for varying the rates of charge and of discharge of the integrator circuit.

8. Apparatus as claimed in any of Claims 1 to 5, characterised in that the means for producing a pulse train comprises a pressure transducer for producing an electric signal representative of the patient's intra-uterine pressure, d.c. restoration means effective to reduce variations in the electric signal caused solely by changes in the relative hydrostatic height of the patient and the transducer and comparator means for producing an output signal when themagnitude of the electric signal exceeds a predetermined value.

Drawing