Fetal heart rate monitoring system

• US 4,086,917 A
• Filed: 02/02/1976
• Issued: 05/02/1978
• Est. Priority Date: 03/08/1974
• Status: Expired due to Term

• Current Assignee: Medical Instruments & Technology Corporation
• Original Assignee: Medical Instruments & Technology Corporation
• Inventors: Burks, Nathaniel W., Ball, Newton E., Britt, David L.
• Primary Examiner(s): Kamm, William E.
• Application Number:
  US05/654,275
• Time in Patent Office: 820 Days
• Field of Search: 128/2.05 P, 128/25.0 R, 128/25.0 T, 128/2.05 Z, 128/2.06 A, 128/2.06 B, 128/2.06 E, 128/2.06 F, 128/2.06 R, 128/2.1 A, 128/2.1 R, 174/48, 174/49, 317/101 CB
• US Class Current: 600/453
• CPC Class Codes: A61B 5/0002 Remote monitoring of patients using telemetry, e.g. transmission of vital signals via a communication network A61B5/07 takes precedence; transmission systems for measured values G08C; transmission H04B; transmission of digital information H04L; wireless communication networks H04W
• CPC Class Codes: A61B 5/02411 of foetuses
• CPC Class Codes: A61B 5/4362 Assessing foetal parameters
• CPC Class Codes: A61B 8/02 Measuring pulse or heart rate
• CPC Class Codes: A61B 8/0866 involving foetal diagnosis; pre-natal or peri-natal diagnosis of the baby

Abstract

Each ultrasonic transducer in a plurality provides a particular signal having characteristics related to the heart rate of an associated fetus. Each particular signal is processed in an associated labor room monitor and modulated on a different carrier frequency for transmission on power lines in the hospital. At a remote location, a nurses station power unit is modular with nurses station monitors. The line signals are individually demodulated in an associated one of the nurses station monitors to provide an indication of heart rate. Alternatively, the transducer assemblies can be coupled to delivery room monitors to provide an indication of heart rate.

The transducer assemblies are uniquely shielded to inhibit interference of spurious low frequency signals with the particular signals. The particular signals are partially detected in the transducer assemblies so that the assemblies can be used with both the labor room and delivery room monitors.

In the monitors, the particular signals are separated into signals each characterized by a particular fundamental frequency. A voting network elects to process that signal most indicative of the fetal heart rate.

Audio and video indications are provided to signal a heart rate beyond a preferred range and to signal a heart rate of substantially zero. If the transducer assembly becomes dislocated, the indicated zero heart rate activates means for storing the previous heart rate. When the assembly is relocated, the previous heart rate is used to avoid delay associated with regenerating heart rate information.

Claims

1. A system for use with a prospective mother to monitor the heart rate of a fetus within the prospective mother, comprising:

a sensor assembly disposed in contact with the prospective mother and in proximity to the heart of the fetus, the sensor assembly being responsive to the heart beat of the fetus to provide a first electrical signal having particular characteristics dependent upon the heart rate of the fetus;

a monitor respective to the first electrical signal to provide signals representing the heart rate of the fetus;

first means included in the monitor and variable to define a preferred range of heart rates for the fetus;

second means included in the monitor and responsive to the signals from the monitor to provide a first indication when the particular characteristics of such signals indicate a heart rate outside of the preferred range of heart rates;

third means included in the monitor and responsive to the signals from the monitor to provide a second indication when the particular characteristics of such signals indicate a heart rate of substantially zero;

the first indication signaling a distressed fetus and the second indication signaling loss of contact between the sensor assembly and the mother and the fetus;

fourth means operatively coupled in the monitor means to the second means to provide an audible indication when the heart rate of the fetus is outside of the preferred range;

fifth means operatively coupled in the monitor means to the second means to provide a visual indication when the heart rate of the fetus is outside of the preferred range;

sixth means operatively coupled in the monitor means to the fourth and fifth means to activate a particular one of the fourth and fifth means;

the sensor means being located such that the particular characteristics of the first signal indicate a particular heart rate and, when the sensor means is dislocated, the particular characteristics of the first signal indicate a heart rate of substantially zero;

seventh means responsive to the production by the monitor of signals indicating a heart rate of substantially zero to store for at least a particular period of time a quantity indicative of the last heart rate of the fetus before the production of the signals indicating a heart rate of substantially zero; and

eighth means responsive to the signals stored by the seventh means to jump the first indication of the monitor to the heart rate stored by the seventh means after the resumption in the monitor of signals indicating a heart rate of the fetus other than zero, whereby

a rapid indication of the heart rate is provided as soon as the sensor means is relocated.

2. A system for monitoring the heart rate of each of a plurality of fetuses in each of a plurality of expectant mothers, including:

a first plurality of transducers each constructed to be disposed in external proximity to the abdomen of a different one of the expectant mothers to send first signals to a particular frequency to the fetus in that monitor;

a second plurality of non-directional transducers each constructed to be disposed in external proximity to the abdomen of a different one of the expectant mothers and each responsive to the heart beat of the fetus in that mother to produce a second signal having frequency characteristics dependent upon the frequency of the signals from the heart rate of the associated one of the transducers and upon the associated fetus;

a plurality of means each operatively coupled to an individual one of the transducers in the second plurality of demodulating the signals from the operatively coupled transducer to produce signals representing the heart beat of the fetus and representing noise;

a plurality of means each coupled to an individual one of the last mentioned means for processing the signals from the individual one of the last mentioned means to recover the signals representing the heart beat of the fetus and to deemphasize the signals representing noise;

a plurality of transmitting means for generating a plurality of carrier signals each having an individual carrier frequency, each of the transmitting means being responsive to the processed signals representing the heart beat of the fetus from the individual one of the transducers in the second plurality to modulate the carrier signals from such transmitting means with the processed signals from the individual one of the transducers in the second plurality to provide one of a plurality of modulated carrier signals each having the individual frequency;

at least a pair of electrical conductors extending from the plurality of transmitting means to a remote location and responsive to the modulated carrier signals from the plurality of transmitting means to introduce simultaneously the modulated carrier signals to the remote location;

a plurality of frequency responsive means disposed at the remote location, each of the frequency responsive means being responsive to an individual one of the different carrier frequencies in the pair of electrical conductors to receive the modulated carrier signals from only an individual one of the transmitting means;

a plurality of demodulating means each associated with an individual one of the frequency responsive means for demodulating the modulated carrier signals to provide an indication of the heart rates of an individual one of the fetuses,

a master unit for providing electrical power for the plurality of frequency responsive means and the plurality of demodulating means;

a plurality of modular units constructed to be connected to the master unit in series and each of the modular units including a first bus for receiving the modulated carrier signals from the modular units preceding in the series and for introducing the modulated carrier signals to the modular units following in the series,

each of the processing means including a plurality of band pass amplifiers each operative in an individual band of frequencies within the range of frequencies in which the heart rate of the fetus occurs and the processing means further including a plurality of detector means each associated with an individual one of the band pass amplifiers and the processing means further including voting means responsive to the signals from the plurality of detector means for selecting the signals from an individual one of the detector means in accordance with the relative amplitudes of such signals and the processing means including means responsive to the signals selected by the voting means for passing only such signals to the transmitting means;

means variable to define a preferred range of heart rates for the associated fetus;

threshold detection means responsive to the associated modulated carrier signals to provide an alarm signal when the heart rate of the associated fetus in not within the preferred range of heart rates;

a bus disposed in each of the demodulating means and connected to introduce the alarm signal from the demodulating means following in the series to the demodulating means preceding in the series; and

alarm means responsive to the alarm signal on the bus to provide an audible indication when the heart rate of any one of the fetuses is beyond the preferred range of heart rates for the associated fetus.

3. A system for monitoring the heart rate of each of a plurality of fetuses in each of a plurality of expectant mothers, including:

a first plurality of transducers each constructed to be disposed in external proximity to the abdomen of a different one of the expectant mothers to send first signals to a particular frequency to the fetus in that monitor;

a second plurality of non-directional transducers each constructed to be disposed in external proximity to the abdomen of a different one of the expectant mothers and each responsive to the heart beat of the fetus in that mother to produce a second signal having frequency characteristics dependent upon the frequency of the signals from the heart rate of the associated one of the transducers and upon the associated fetus;

a plurality of means each operatively coupled to an individual one of the transducers in the second plurality of demodulating the signals from the operatively coupled transducer to produce signals representing the heart beat of the fetus and representing noise;

a plurality of means each coupled to an individual one of the last mentioned means for processing the signals from the individual one of the last mentioned means to recover the signals representing the heart beat of the fetus and to deemphasize the signals representing noise;

a plurality of transmitting means for generating a plurality of carrier signals each having an individual carrier frequency, each of the transmitting means being responsive to the processed signals representing the heart beat of the fetus from the individual one of the transducers in the second plurality to modulate the carrier signals from such transmitting means with the processed signals from the individual one of the transducers in the second plurality to provide one of a plurality of modulated carrier signals each having the individual frequency;

at least a pair of electrical conductors extending from the plurality of transmitting means to a remote location and responsive to the modulated carrier signals from the plurality of transmitting means to introduce simultaneously the modulated carrier signals to the remote location;

a plurality of frequency responsive means disposed at the remote location, each of the frequency responsive means being responsive to an individual one of the different carrier frequencies in the pair of electrical conductors to receive the modulated carrier signals from only an individual one of the transmitting means;

a plurality of demodulating means each associated with an individual one of the frequency responsive means for demodulating the modulated carrier signals to provide an indication of the heart rates of an individual one of the fetuses;

a master unit for providing electrical power for the plurality of frequency responsive means and the plurality of demodulating means;

a plurality of modular units constructed to be connected to the master unit in series and each of the modular units including a first bus for receiving the modulated carrier signals from the modular units preceding in the series and for introducing the modulated carrier signals to the modular units following in the series,

each of the processing means including a plurality of band pass amplifiers each operative in an individual band of frequencies within the range of frequencies in which the heart rate of the fetus occurs and the processing means further including a plurality of detector means each associated with an individual one of the band pass amplifiers and the processing means further including voting means responsive to the signals from the plurality of detector means for selecting the signals from an individual one of the detector means in accordance with the relative amplitudes of such signals and the processing means including means responsive to the signals selected by the voting means for passing only such signals to the transmitting means,

the threshold detector means being constructed to provide the alarm signal with first characteristics when the heart rate of the associated fetus is higher than the preferred range of heart rates and with second characteristics when the heart rate of the associated fetus is lower than the preferred range of heart rates and wherein the system further comprises;

a first indicator included in each of the demodulating means and responsive to the first characteristics of the alarm signal to provide a first visual indication;

a second indicator included in each of the demodulating means and responsive to the second characteristics of the alarm signal to provide a second visual indication;

means included in each of the demodulating means for sustaining the indication of the first and second indicators even after the heart rate of the associated fetus returns to the preferred range of heart rates; and

resettable means operable after the heart rate of the associated fetus has returned to the preferred range of heart rates for inhibiting the alarm signal and the first and second visual indications.

4. Apparatus for monitoring the heart rate of a fetus in an expectant mother, comprising:

a housing;

a first ultrasonic transducer included in the housing and constructed to be disposed externally of the abdomen of the expectant mother to transmit signals at a particular frequency to the fetus;

a second ultrasonic transducer included in the housing and constructed to be disposed externally on the abdomen of the expectant mother to receive from the fetus the signals at the particular frequency but modulated by the heart rate of the fetus;

an ultrasonic transducer assembly, including the first and second transducers, constructed to provide for the transmission of the signals at the particular frequency to the fetus and to receive the modulated signals from the fetus;

the transducer assembly including means included in the housing for matching the impedance of the first transducer to the impedance of the abdomen of the expectant mother and means for matching the impedance of the second transducer to the impedance of the abdomen of the expectant mother;

monitor means responsive to the modulated characteristics of the received signal to detect such modulations for the production of indications representing the heart rate of the fetus, the monitor means including means for passing the modulations and rejecting artifact noise;

coupling means releasably connecting the transducer assembly with the monitor means to introduce the modulated signals to the monitor means;

means for providing a carrier signal having a particular frequency;

means for modulating the carrier signals with the signals passing through the monitor means and for transmitting the signals to a distant position;

means at the distant position for demodulating the signals transmitted to the distant position;

the transducer assembly including the first and second transducers being constructed to be disposed in proximity to the heart of the fetus externally on the abdomen of the expectant mother, the first transducer being constructed to produce signals of the particular frequency and the second transducer being responsive to the heart beat of the fetus to provide the particular signal with frequency modulations dependent upon the heart rate of the fetus;

signal characterization network means fixedly connected to the transducer assembly and operatively coupled to the first transducer to obtain the production of the signals of the particular frequency and operatively coupled to the second transducer and responsive to the signal received by the second transducer of the transducer assembly to partially detect the signal modulating the particular frequency, the signal characterization network means having characteristics responsive to the resonant frequency of the transducer assembly to maximize the power of the modulated signal and to introduce the modulated signal to the monitor means,

the rejecting means in the monitor means including means responsive to abrupt signals of large magnitude resulting from changes in the disposition of the transducer assembly unrelated to the heart rate of the fetus for inhibiting such signals from affecting the indications of fetal heart rate and means responsive in the monitor means to the disposition of the transducer assembly out of proximity with the heart of the fetus for retaining for at least a particular period of time the last indication provided by the monitor means of the fetal heart rate,

the transducer assembly further comprising;

a first cable including a first conductor for introducing the signal of the particular frequency to the first transducer;

a second cable including a second conductor for introducing the modulated signal to the signal characterization network means from the second transducer;

a first metallic shield surrounding the first conductor in the first cable;

a second metallic shield surrounding the second conductor in the second cable;

a third metallic shield at least partially surrounding the first and second transducers in the transducer assembly;

a fourth metallic shield at least partially surrounding the signal characterization network and coupled at one end to the monitor means and coupled at the other end to the first and second cables; and

means electrically connecting the first, second, third and fourth metallic shields to provide a reference potential for the first, second, third and fourth metallic shields.

5. The combination set forth in claim 4 wherein the monitor means includes a plurality of band pass amplifiers each constructed to pass signals having frequencies in an individual range and wherein the modulations in the received signal are introduced to the band pass amplifiers to obtain the passage of signals through the amplifiers in accordance with the frequency characteristics of the modulations and the frequency characteristics of the amplifiers and wherein the rejecting means in the monitor means includes means for weighing the amplitudes of the signals passing through each of the band pass amplifiers in accordance with the frequency characteristics of each of the amplifiers.

6. A system for monitoring the heart rate of a fetus in an expectant mother, comprising:

a housing;

a first transducer included in the housing and constructed to be disposed on the abdomen of the expectant mother to transmit signals at a particular frequency to the fetus;

a second transducer included in the housing and constructed to be disposed on the abdomen of the expectant mother to receive from the fetus the signals at the particular frequency modulated by the heart rate of the fetus;

first means included in the housing for matching the impedance of the first transducer to the impedance of the abdomen of the expectant mother;

second means included in the housing for matching the impedance of the second transducer to the impedance of the abdomen of the expectant mother;

transducer assembly means including the first and second transducers from the first and second means for providing for the transmission of the signals at the particular frequency and the reception of the modulated signals and a high degree of artifact noise;

a plurality of filter means collectively responsive to the particular spectrum of frequencies in the modulated signals and individually tuned sharply to pass a filtered signal including frequencies within a relatively narrow and individual portion of the particular frequency spectrum, each of the filter means being constructed to pass filtered signals including fundamental frequencies and harmonic frequencies within its relatively narrow and individual portion of the frequency spectrum;

third means responsive to the filtered signals passing through the plurality of filter means for enabling the particular one of the filter means passing the filtered signal having the greatest magnitude;

fourth means responsive to the fundamental frequency of the signal passed by the enabled one of the filter means for demodulating such signal to provide an indication of the heart rate of the fetus;

a voting network including a plurality of voting means each associated with an individual one of the filter means for providing a voting signal having a magnitude dependent upon the magnitude of the associated filtered signal; and

a channel selection network responsive to the voting signals from the plurality of voting means for enabling the filtered signal associated with the voting signal of greatest magnitude,

the individual voting means including;

fifth means for increasing the magnitude of the voting signal in the voting means relative to the magnitude of the harmonics in the voting means in accordance with the magnitude of the fundamental frequency of the signal introduced to the voting means whereby

each of the voting signals has a magnitude primarily dependent upon the magnitude of the fundamental frequency.

7. The system set forth in claim 6 including sixth means responsive to the production by the signal means of a signal outside of the particular spectrum of frequencies for producing an audible signal representing the production of such a signal and seventh means responsive to the production by the signal means of a signal outside of the particular spectrum of frequencies for providing a visual indication representing the production of such a signal and eighth means operatively coupled to the sixth and seventh means for activating the sixth means at first particular times and the seventh means at second particular times and ninth means responsive to a change in the disposition of the transducer means to a position removed from the heart of the fetus for providing a visual indication even upon the activation of the sixth means by the eighth means.

8. Apparatus for monitoring the heart rate of a fetus in an expectant mother, comprising:

a housing;

a first ultrasonic transducer included in the housing and constructed to be disposed externally of the abdomen of the expectant mother to transmit signals at a particular frequency to the fetus;

a second ultrasonic transducer included in the housing and constructed to be disposed externally on the abdomen of the expectant mother to receive from the fetus the signals at the particular frequency but modulated by the heart rate of the fetus;

an ultrasonic transducer assembly, including the first and second transducers, constructed to provide for the transmission of the signals at the particular frequency to the fetus and to receive the modulated signals from the fetus;

the transducer assembly including means included in the housing for matching the impedance of the first transducer to the impedance of the abdomen of the expectant mother and means for matching the impedance of the second transducer to the impedance of the abdomen of the expectant mother;

monitor means responsive to the modulated characteristics of the received signal to detect such modulations for the production of indications representing the heart rate of the fetus, the monitor means including means for passing the modulations and rejecting artifact noise;

coupling means releasably connecting the transducer assembly with the monitor means to introduce the modulated signals to the monitor means;

means for providing a carrier signal having a particular frequency;

means for modulating the carrier signals with the signals passing through the monitor means and for transmitting the signals to a distant position;

means at the distant position for demodulating the signals transmitted to the distant position;

means responsive to a change in the disposition of the transducer assembly means to a position removed from the heart of the fetus for providing for at least a particular period of time a signal having a frequency representing the last rate of the heart beat of the fetus before such change in disposition; and

means responsive to a restoration of the transducer assembly means on the abdomen of the expectant mother for restoring the signal having the last rate of the heart beat;

the monitor including a plurality of filter means collectively responsive to all of the heart rates within the preferred range and individually responsive to a narrow spectrum of frequencies within the preferred range and the monitor further including voting means responsive to the signals from the filter means in the plurality for enabling only the filter means passing the signal of the greatest magnitude.

9. The system set forth in claim 8 wherein each of the filter means is operative to attenuate harmonics within its narrow spectrum in relation to fundamental frequencies within its narrow spectrum to cause the voting means to be responsive essentially only to the fundamental frequencies in the preferred range.

10. The system set forth in claim 9 wherein means are included in the monitor for inhibiting any effect on the signals from the monitor of changes in the disposition of the sensor assembly relative to the prospective mother.

11. Apparatus for monitoring the heart rate of a fetus in an expectant mother, comprising:

a housing;

a first ultrasonic transducer included in the housing and constructed to be disposed externally of the abdomen of the expectant mother to transmit signals at a particular frequency to the fetus;

a second ultrasonic transducer included in the housing and constructed to be disposed externally on the abdomen of the expectant mother to receive from the fetus the signals at the particular frequency but modulated by the heart rate of the fetus;

an ultrasonic transducer assembly, including the first and second transducers, constructed to provide for the transmission of the signals at the particular frequency to the fetus and to receive the modulated signals from the fetus;

the transducer assembly including means included in the housing for matching the impedance of the first transducer to the impedance of the abdomen of the expectant mother and means for matching the impedance of the second transducer to the impedance of the abdomen of the expectant mother;

monitor means responsive to the modulated characteristics of the received signal to detect such modulations for the production of indications representing the heart rate of the fetus, the monitor means including means for passing the modulations and rejecting artifact noise;

coupling means releasably connecting the transducer assembly with the monitor means to introduce the modulated signals to the monitor means;

means for providing a carrier signal having a particular frequency;

means for modulating the carrier signals with the signals passing through the monitor means and for transmitting the signals to a distant position;

means at the distant position for demodulating the signals transmitted to the distant position;

the transducer assembly including the first and second transducers being constructed to be disposed in proximity to the heart of the fetus externally on the abdomen of the expectant mother, the first transducer being constructed to produce signals of the particular frequency and the second transducer being responsive to the heart beat of the fetus to provide the particular signal with frequency modulations dependent upon the heat rate of the fetus,

signal characterization network means fixedly connected to the transducer assembly and operatively coupled to the first transducer to obtain the production of the signals of the particular frequency and operatively coupled to the second transducer and responsive to the signal received by the second transducer of the transducer assembly to partially detect the signal modulating the particular frequency, the signal characterization network means having characteristics responsive to the resonant frequency of the transducer assembly to maximize the power of the modulated signal and to introduce the modulated signal to the monitor means,

means responsive in the monitor means to the disposition of the transducer assembly out of proximity to the heart rate of the fetus for retaining for at least a particular period of time the last indications provided by the monitor means of the fetal heart rate; and

means for restoring the last indications provided by the monitor means of the fetal heart rate when the transducer assembly is returned in proximity to the heart rate of the fetus.

12. Apparatus for monitoring the heart rate of a fetus in an expectant mother, comprising:

a housing;

a first ultrasonic transducer included in the housing and constructed to be disposed externally of the abdomen of the expectant mother to transmit signals at a particular frequency to the fetus;

a second ultrasonic transducer included in the housing and constructed to be disposed externally on the abdomen of the expectant mother to receive from the fetus the signals at the particular frequency but modulated by the heart rate of the fetus;

an ultrasonic transducer assembly, including the first and second transducers, constructed to provide for the transmission of the signals at the particular frequency to the fetus and to receive the modulated signals from the fetus;

the transducer assembly including means included in the housing for matching the impedance of the first transducer to the impedance of the abdomen of the expectant mother and means for matching the impedance of the second transducer to the impedance of the abdomen of the expectant mother;

monitor means responsive to the modulated characteristics of the received signal to detect such modulations for the production of indications representing the heart rate of the fetus, the monitor means including means for passing the modulations and rejecting artifact noise;

coupling means releasably connecting the transducer assembly with the monitor means to introduce the modulated signals to the monitor means;

means for providing a carrier signal having a particular frequency;

means for modulating the carrier signals with the signals passing through the monitor means and for transmitting the signals to a distant position;

means at the distant position for demodulating the signals transmitted to the distant position;

the transducer assembly including the first and second transducers being constructed to be disposed in proximity to the heart of the fetus externally on the abdomen of the expectant mother, the first transducer being constructed to produce signals of the particular frequency and the second transducer being responsive to the heart beat of the fetus to provide the particular signal with frequency modulations dependent upon the heart rate of the fetus;

signal characterization network means fixedly connected to the transducer assembly and operatively coupled to the first transducer to obtain the production of the signals of the particular frequency and operatively coupled to the second transducer and responsive to the signal received by the second transducer of the transducer assembly to partially detect the signal modulating the particular frequency, the signal characterization network means having characteristics responsive to the resonant frequency of the transducer assembly to maximize the power of the modulated signal and to introduce the modulated signal to the monitor means;

means responsive in the monitor means to the disposition of the transducer assembly out of proximity to the heart rate of the fetus for retaining for at least a particular period of time the last indications provided by the monitor means of the fetal heart rate;

the rejecting means in the monitor means including means responsive to abrupt signals of large magnitude resulting from changes in the disposition of the transducer assembly unrelated to the heart rate of the fetus for inhibiting such signals from affecting the indications of fetal heart rate and means responsive in the monitor means to the disposition of the transducer assembly out of proximity with the heart of the fetus for retaining for at least a particular period of time the last indication provided by the monitor means of the fetal heart rate; and

means for restoring the last indications provided by the monitor means of the fetal heart rate when the transducer assembly is returned in proximity to the heart rate of the fetus.

13. Apparatus for monitoring the heart rate of a fetus in an expectant mother, comprising:

a housing;

a first ultrasonic transducer included in the housing and constructed to be disposed externally of the abdomen of the expectant mother to transmit signals at a particular frequency to the fetus;

a second ultrasonic transducer included in the housing and constructed to be disposed externally on the abdomen of the expectant mother to receive from the fetus the signals at the particular frequency but modulated by the heart rate of the fetus;

an ultrasonic transducer assembly, including the first and second transducers, constructed to provide for the transmission of the signals at the particular frequency to the fetus and to receive the modulated signals from the fetus;

the transducer assembly including means included in the housing for matching the impedance of the first transducer to the impedance of the abdomen of the expectant mother and means for matching the impedance of the second transducer to the impedance of the abdomen of the expectant mother;

monitor means responsive to the modulated characteristics of the received signal to detect such modulations for the production of indications representing the heart rate of the fetus, the monitor means including means for passing the modulations and rejecting artifact noise;

coupling means releasably connecting the transducer assembly with the monitor means to introduce the modulated signals to the monitor means;

means for providing a carrier signal having a particular frequency;

means for modulating the carrier signals with the signals passing through the monitor means and for transmitting the signals to a distant position;

means at the distant position for demodulating the signals transmitted to the distant position;

means responsive to a change in the disposition of the transducer assembly means to a position removed from the heart of the fetus for providing for at least a particular period of time a signal having a frequency representing the last rate of the heart beat of the fetus before such change in disposition; and

means responsive to a restoration of the transducer assembly means on the abdomen of the expectant mother for restoring the signal having the last rate of the heart beat.