Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/02—Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
  • A61B5/024—Detecting, measuring or recording pulse rate or heart rate
  • A61B5/02411—Detecting, measuring or recording pulse rate or heart rate of foetuses
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/0002—Remote monitoring of patients using telemetry, e.g. transmission of vital signals via a communication network
  • A61B5/0004—Remote monitoring of patients using telemetry, e.g. transmission of vital signals via a communication network characterised by the type of physiological signal transmitted
  • A61B5/0011—Foetal or obstetric data
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/43—Detecting, measuring or recording for evaluating the reproductive systems
  • A61B5/4306—Detecting, measuring or recording for evaluating the reproductive systems for evaluating the female reproductive systems, e.g. gynaecological evaluations
  • A61B5/4343—Pregnancy and labour monitoring, e.g. for labour onset detection
  • A61B5/4362—Assessing foetal parameters
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
  • A61B5/6801—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be attached to or worn on the body surface
  • A61B5/683—Means for maintaining contact with the body
  • A61B5/6831—Straps, bands or harnesses
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/72—Signal processing specially adapted for physiological signals or for diagnostic purposes
  • A61B5/7235—Details of waveform analysis
  • A61B5/7253—Details of waveform analysis characterised by using transforms
  • A61B5/7257—Details of waveform analysis characterised by using transforms using Fourier transforms
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/72—Signal processing specially adapted for physiological signals or for diagnostic purposes
  • A61B5/7203—Signal processing specially adapted for physiological signals or for diagnostic purposes for noise prevention, reduction or removal

Definitions

• This invention relates to new and useful improvements in external fetal heart monitors, and more particularly seeks to provide monitoring of electronically enhanced and processed signals (optionally by Fast Fourier techniques), preferably broadcast from single or multiple, attached, abdominal microphones. Placental blood flow data and abdominal fetal electroencephalographic data may be similarly detected and processed. Other signals such as contraction activity and maternal electrocardiographic data may be simultaneously and similarly processed and/or broadcast.
• the primary field of this invention is medicine, as it relates to embryology and obstetrics, and particularly fetal heartbeat monitoring, primariy during labor but possibly at any time after heartbeat detection.
• the secondary fields of electronic transmission, electronic enhancing, processing, and filtering e.g. using Fourier Transform techniques, and computers are blended with medi cine.
• Patents 3,280,817 and 3,703,168 use analog filters on fetal heart signals.
• Patent 3,187,098 separates fetal and maternal heart signals according to their predominant frequency ranges.
• Patent 3,581,735 shows a method for error detection and correction.
• Patent 3,762,397 discloses demodulation of cardiovascular blood flow sounds and averaging of synchronized signals.
• Patent 3,358,289 shows two signals from two microphones which were combined in the time domain.
• Patent 3,367,323 discloses the use of multiple electrodes for detecting the abdominal fetal electrocardiogram. The resulting signals are combined by a resistor network.
• Patent 3,409,737 discloses the use of several microphones on a single elastic belt, where the fetal signal is derived by analog processing of only one switch-selected microphone signal.
• Patent 4,299,234 discloses a method for the automated combination of two ultrasound and abdominal electrocardiographic fetal heart signals. Other processing techniques include moving average techniques (4,190,866), and the use of both shift registers and autocorrelation (4,037,151). More recently F. Boehm and L. Fields, in Second-generation EFM Is Waiting In The Wings, Contemporary Ob/Gyn, March 1984, p 179, disclose Doppler ultrasound monitors using autocorrelation techniques.
• Patents 4,281,241, 4,356,475, and 4,356,486 show telemetry in surgical applications.
• N. L. Barr, in The Radio Transmission of Physiological Information, The Military Surgeon, vol. 114, no. 2, Feb. 1954, p. 79 discloses telemetry of physiological data.
• Patent 4,378,022 discloses the use of the Fast Fourier Transform in processing a single signal for in vivo analysis of heart valve prostheses.
• the Fast Fourier Transform is a well-known algorithm with many implimentations, cf. A. Aho, J. Hopcroft and J.
• Previous fetal monitors have used four main techniques, namely (1) external detection of fetal heart sounds (fetal "phonocardiograms"), (2) external ultrasound and Doppler ultrasound, (3) external (abdominal) electrocardiogram (fetal ECG), and (4) internal fetal ECG.
• Previous data processing techniques included active audio filters, moving average analysis, and autocorrelation analysis. All of these techniques have disadvantages.
• ECG electrocardiographic
• External phonocardiographic or electrocardiographic (ECG) monitors have failed to obtain a usable signal, thus requiring invasive internal monitors or ultrasound.
• External fetal ECG signals are disrupted by significant electrical activity of the uterus and maternal heart.
• External phonocardiographic monitors may be similarly disrupted by sounds.
• the invention consists of a fetal monitor which uses one or preferably multiple microphones to detect fetal heart signals. These signals as well as auxiliary data on contractions, maternal heartbeat, etc, are optionally but preferably broadcast locally by a lightweight, low power transmitter worn by the mother which allows full mobility during labor monitoring.
• the signals are sent directly or by broadcasting to a processing unit where the signals are rapidly filtered and combined, using primarily digital techniques to produce a filtered and enhanced signal of fetal heartbeat, in synchrony with slightly delayed auxiliary data.
• these techniques produce a direct measure of variability in terms of the range of heart rates present in each sampling cycle. Both the quality of the enhanced signal and the additional measure of variability decrease current use of internal monitors, ultrasound, and Caesarian sections.
• the instant monitor overcomes these difficulties in several ways.
• the signal processing is done by a fast microprocessor and associated circuitry using the Fast
• FFT Fourier Transform
• Each fetal heart signal and a maternal heart signal is amplified (then optionally partially demodulated), filtered to permit accurate sampling, and converted to a digital signal at a sampling rate chosen to completely reject 60 Hz electrical noise.
• the FFT is used to compare the frequencies and respective amplitudes present in each heart signal over the most recent sampling period.
• a subsequent linear or nonlinear, digital filter is applied to emphasize desired frequencies.
• noise du to the maternal heartbeat may be efficiently filtered out from the fetal heart signals.
• the power spectrum (energy at each frequency) of each fetal signal is computed.
• This data is used to combine frequency data from the multiple fetal signals in proportion to their signal quality so as to obtain a single enhanced, filtered signal in the frequency domain.
• the composite power spectrum is then computed to provide data on the range of fetal heart rates.
• the enhanced frequency signal is transformed by the inverse FFT in order to obtain an enhanced, filtered, relatively noise-free fetal heart signal.
• the auxiliary inputs may be presented in real time to signal events such as the onset of contractions, or similarly delayed (and processed if desired) to be in synchrony with the enhanced fetal heart signal.
• the enhanced, filtered fetal heart signal may be further processed using moving average or other techniques to obtain information on instantaneous heart rates, and provides better data on heart rates, variability, acceleration and deceleration than provided by the original fetal heart signals.
• the transform techniques plus filters in the frequency domain combine the performance of active audio filters, sharp notch filtering of 60 Hz noise, and further noise reduction due to implicit time averaging in the frequency domain and an optimal combination of separate signals.
• This noise includes the maternal heartbeat, effects of uterine contractions, and other noise.
• the multiple microphones mean that fetal movemsnt will not prevent obtaining a usable signal of the fetal heartbeat.
• frequency data obtained by the FFT and filters is itself displayed as another indicator of short-term variability.
• Fourier transform techniques offer an additional measure of variability.
• the enhanced signal obtained from the instant monitor allows measurements of relative changes in the strength of the fetal heartbeat in addition to the fetal heart rate.
• the instant fetal monitor thus reduces exposure to internal monitors, ultrasound, and Caesarian sections through more accurate determination of fetal conditions, while enhancing comfort and mobility, which reduce the difficulty and duration of labor.
• the use of multiple signals combined in the frequency domain allows monitoring the abdominal fetal electrocardiogram as well as blood flow in the placenta, in which each major vessel does not move during labor, but offers only a weak signal.
• This method can also be used to detect and enhance other signals in a large variety of medical applications, such as fetal electroencephalography.
• Fig. 1 is a perspective view of a pregnant female , with attached microphones, transmitter unit, and antenna broadcasting to processing and monitor units in accordance with this invention
• Fig. 2 is a more detailed diagrammatic view of the apparatus of Fig. 1;
• Fig. 3 is a detailed diagrammatic view of a microphone unit of Fig 1;
• Fig. 4 is an electronic circuit diagram for the amplifier of Fig. 3;
• Fig. 5 is a detailed electronic circuit diagram of subsequent amplifier stages contained in the transmitter, receiver, and processor of Fig. 1, including filtering and demodulating components;
• Fig. 6 is a detailed diagrammatic view of the transmitter unit of Fig. 2, including a multiplexer;
• Fig. 7 is a detailed diagrammatic view of the receiver unit of Fig. 2;
• Fig. 8 is a detailed diagrammatic view of the demultiplexer of Fig. 7;
• Fig. 9 is a detailed diagrammatic view of an alternate embodiment of the demultiplexer of Fig. 7;
• Fig. 10 is a detailed diagrammatic view of the major electronic components of the processor unit of Fig. 2;
• Fig. 11 is a detailed diagrammatic view of the flow of information in the processor unit shown in Fig. 2;
• Fig. 12 is a detailed diagramnatic view of dual-ported random access memory (RAM) from the processor unit of Fig. 10;
• Fig.13 is a partial detailed diagrammatic view of another dual-ported RAM from the processor unit of Fig. 10;
• Fig. 14 is a graphic view of the representation of two different fetal heart signals, showing differences in variability.
• the main part of the present fetal monitor consists of the electronic (analog and digital) processing devices, as described below. These devices allow the practical use of external monitoring such as the use of one or more fetal microphones as required, and also the use of local broadcasting to facilitate maternal mobility.
• multiple microphones 11 are taped to a woman's abdomen.
• the number of microphones can be varied depending upon circumstances affecting the level of the signal and amount of noise. Such circumstances might include position and depth of the fetus, location of the placenta, amount of external noise, and other factors.
• the signal-to-noise ratio of the overall fetal heart signal depends inversely upon the number of said microphones.
• Several microphones may be combined into a single unit if desired in order to minimise the number of units worn by the mother. Between one and eight microphones may be used, three to six is desirable, and four is preferred. Added microphones give a potentially better signal but require transmission of more signals.
• Microphone denotes a suitable low frequency pressure transducer for detecting the fetal heartbeat or sounds in the placenta, e.g., suitably designed acoustic microphones, photoplethysmographic detectors, etc.
• the positions of these microphones 11 are selected so as to maximize detection of the fetal heart signal during expected maternal and fetal movement, and in the presence of expected noise. This may be done by monitoring the detected fetal heart signals, e.g., by test jacks on the transmitter or processing unit or both. Alternatively, the microphones may be placed in standard positions chosen to allow for movement of the fetus during labor. The microphones may be easily repositioned at any time.
• microphones may be used to detect and monitor placental blood flow, or plural electrodes may be used to detect the abdominal fetal electrocardiograph.
• the microphones 11 convert fetal heart beats to electrical signals which are broadcast by a transmitter 12.
• the transmitter may also broadcast signals from auxiliary devices 15 used to detect contractions, uterine tonus, electromyographic data, intra-uterine pressure, maternal heartbeat, etc.
• auxiliary devices 15 used to detect contractions, uterine tonus, electromyographic data, intra-uterine pressure, maternal heartbeat, etc.
• some noise in the fetal heart signals is due to the maternal heartbeat, which may be detected, processed, and largely eliminated.
• the resulting signal is treated as an auxiliary input up to the processing stage.
• the output from the transmitter containing all of these signals, preferably in a frequency modulated, ultra-high frequency signal, is broadcast by a flexible wire transmitting antenna 14 contained in a belt 13, which also holds the transmitter, worn by the mother.
• the transmitted signal is broadcast to a receiving antenna 16, connected to a receiver 21 and processor 22, the latter enhancing the signal and relaying the enhanced signal to display devices 18.
• the preferred microphones 11 are modified Tapho micorphones of Talbert and Southall containing an improved amplifier which incorporates an active low-pass audio filter.
• the microphone 11 includes a transducer 31 and a microphone amplifier (with self-contained filter) mounted on a circuit board 32, housed in a metal or plastic housing
• the transducer may be a sensitive, low noise, ceramic phono type unit such as the BSR SX6H.
• the transducer is acoustically coupled to the external abdominal wall by a light plastic button 34 in contact with said wall and a rubber coupling 35 connected to the transducer 31 and in contact with the button 34.
• the transducer is clamped between rubber supports 36.
• the housing is lined with acoustic insulation 38 so as to reduce noise. Additional insulation 39 serves as a pad between the housing and the mother's skin.
• the transducer may be attached by adhesive such as double sided tape 40.
• Various bolts (501, and not shown), a metal plate 502 strands 505, and a shielded cable 503 connected to a jack 37 complete the housing.
• the housing 33 may be about 5 cm in diameter and 2.5 cm high, and is designed so that its weight of about 100 gm provides a suitable acoustic reference.
• One or more transducers may be mounted in a single housing.
• a pickup for an external intrauterine pressure sensor may be combined with such a unit to further the number of separate units connected to the mother.
• the circuit diagram for the amplifier mounted on board 32 is also detailed in Fig. 4.
• Pins P1, P2 and P3 represent separate connections of jack 37.
• Pin 1 is connected to a positive voltage source of typically 12 volts
• P2 carries the output signal from the amplifier and P3 is connected to ground.
• Resistors R4 and R5, form a voltage divider so that relative to the DC reference for U1, namely the junction of R4 and R5, both positive and negative DC voltages are available.
• Points Ml and M2 are connected to the two leads of the transducer 31.
• Resistor Rl drains off static charges across the transducer and U1.
• U1 is preferably a low noise field effect transistor (fet) input op amp such as the CA3140, Intersil 7611, or TL071 and is operated here as a non-inverting amplifier to provide high input impedance, essentially that of R1.
• the gain of U1 is thus the 1 plus ratio of resistances R3/ R1.
• a novel added capacitor C1, together with R2 and R3 provides a low pass filter to eliminate noise above the frequency, range of the fetal heart sound. For a 3 decibel (db) -cutoff of about 160 Hz, R3 may be 1 megohm and C1 1000 micro-micro farads or picofarads
• the cutoff frequecy should be in the range 100 - 500 Hz., preferably in the range 120 - 250
• Typical component values may be.
• the ratio R3/R2 should be between 10 to 1 and 400 to 1, and preferably between 50 to 1 and 100 to
• the value of C1 should be chosen to provide the desired cutoff frequency in conjunction with R3.
• This novel design combines the usual active audio filter (6 db/ octave cutoff, here limited to a typical maximum of 20 db) and operational amplifier, using only one op amplifier.
• the flexible attachment of microphones, absence of wires other than those connecting the microphones to the transmitter, and light weight and small size facilitate maternal movement and comfort.
• the use of multiple microphones insures that an adequate filtered, enhanced composite signal can be formed through later processing.
• the composite signal will be superior to any single signal, and no single signal need suffice, an adequate composite signal will be generated whenever one microphone detects a strong, relatively noise-free fetal heartbeat.
• the aggregate signal-to-noise ratio of the fetal heart input signals as well as the signal-to-noise ratio of the final enhanced composite signal depend inversely upon the number of microphones. Provision is made to vary the number as needed depending on circumstances. Referring to Fig. 5, the signal from the microphone amplifier of Fig. 4 is further amplified, filtered, and demodulated. The signal enters via pin P2 and is coupled to the first stage of amplification U2 via capacitor C10.
• Resistor R10 provides a suitable load to the microphone amplifier U1, and minimizes AC hum by providing a relatively low impedance (typically 10 k ohms, the value might range from 2 k to 100 k ohms) path to ground for any induced voltages.
• a relatively low impedance typically 10 k ohms, the value might range from 2 k to 100 k ohms
• Resistor R11 maintains the junction of C10 with inverting input of U2 at DC ground.
• C10 may be a 1 microfarad tantalum capacitor, correctly polarized, and R11 may be 100 k ohms.
• This resistor-capacitor network also eliminates the need for offset null adjustment in the high-gain amplifier U1 and provides high pass filtering to the input signal. In this case the high pass filter has a 6 db per octave roll off below a 3 db cutoff of 3 Hz.
• Other values of C10 and R11 may be used provided that R11 is sufficiently (typically 2 or more times) larger than R10.
• the operational amplifier U2 together with associated resistors R12, R13, and R14, and capacitor C11 is operated as a non-inverting amplifier combined with a low pass filter, as in the microphone amplifier U1.
• the values of C11 and R14 may be chosen so as to provide a 3 db cutoff frequency of 160 Hz and a 6 db per octave rolloff to a gain of 1.
• the cutoff frequency should be the same as that of the microphone amplifier of Fig. 4 in order to provide the smoothest pass-band and best filtering.
• Typical component values (with ranges in parentheses) associated with U2 are:
• the above capacitor and resistors control the gain and filtering of this stage of amplification.
• the resulting filtered, amplified signal from U2 is demodulated by the precision rectifier formed by U3 and U4 and associated components.
• This precision rectifier yields an output signal (junction of R17 and R20) equal to the absolute value of its input signal.
• the design of the precision rectifier is taken from Jung (1983) except that both positive and negative supply voltages are needed to prevent latchup.
• Typical component values and ranges here are: R16, R17 100 k ohms, matched to .1 % (10 k ohms - 1 megohm, as well matched as practical) R18 27 k ohms (10 k - 100 k ohms) R19 47 k ohms (10 k - 100 k ohms) D1 1N914, 1N458A, or similar U3, U4 each 1/2 358 or 1/2 TL072
• This output is then sent through an inverting amplifier U5 operated as an active low passfilter with a cutoff frequency of 15 - 60, and 6 db roll off to a gain of almost 0.
• the cutoff frequency should be less than half of the sampling rate used in later data acquisition in order to prevent aliases.
• filters may be used if desired to provide sharper cutoffs.
• the standard design here is based on Wyss (1984). Typical component values are:
• the output at P12 is sent to the analog data acquisition unit of the processing uni t .
• R22 (2.2 k ohms - 100 k ohms, typically 10 k ohms) provides a low impedance path to ground for any induced voltages on the line from P22.
• This signal is a demodulated, filtered signal obtained from the fetal heart signal (or maternal heart signal) in the range 0 to about 5 volts.
• U6 operates as a comparator to indicate levels.
• Light emitting diode (led) D2 lights whenever the signal exceeds the applied voltage at Pll.
• Two comparators may be used; lighting a green led at adequate signals (above, say, .5 volt), and a red led on peaks (above, say, 4 volts).
• U6 may be a 741 or TL071 type, R22 2.2 k (1 - 10 k) ohms, and D2 any low power led.
• the chip count may be reduced by using double or quad op amplifiers, taking the usual care with signal isolation.
• the usual care should be taken to provide suitable paths to ground for any induced voltages on lines carrying signals, and to provide adequate bypassing.
• the early amplification, through U2 is done before broadcasting, and the demodulation and filtering beginning at U3 is done after receiving the broadcast signal.
• Automatic gain control should be preferably provided in both places. In this way the problem of accurate transmission and reception of a signal modulated by a very low frequency (2-4 Hz) signal is avoided.
• the output from U2 may also be sent to headphones or, preferably with further amplification to a loudspeaker to aid in placing the microphones. If desired the output from U2 may be used to modulate a signal in the range 200 - 1000 Hz., preferably about 700 Hz., using for example, the XR2206 integrated circuit and standard components. This shifts the fetal heart sound to a more readily audible frequency range, which is then sent to headphones or a loudspeaker.
• the transmitter 12 is designed to locally broadcast the plural microphone and auxiliary signals, preferably multiplexed and modulated on a single carrier, to a nearby receiver 21.
• the microphone plugs 37 fit jacks 41 and the auxiliary input plugs fit jacks 42.
• Slowly varying auxiliary inputs may be encoded as audio-frequency signals via optional audio modulators (encoders) 43, or broadcast as received. All these signals from 41 and 42, whether passing through 43 or not, are amplified by amplifiers 44 to obtain suitable amplitudes for later modulation.
• These amplifiers may be the amplifiers of Fig. 5, including the filters therein.
• the demodulation is preferably not done at this stage, but is done after reception of the broadcast.
• the gain of the amplifiers may be set, and optionally, but preferably, controlled by suitable automatic gain controls for calibration purposes and to maintain adequate modulation and need not be readjusted during operation.
• Each modulator strip contains a low-pass audio filter 48, passing signals unattenuated below a cutoff frequency chosen to prevent mutual interference, preferably in the range 250-1000 Hz to balance the needs of adequate bandwidth for later processing and minimum total bandwidth for effective transmission.
• Each strip further contains a local oscillator 49, with the separate intermediate frequencies spaced at suitable intervals, a frequency modulator 50 to minimize noise, a buffer 51 and an output band-pass filter 52 centered about the intermediate frequency.
• the oscillators include suitable frequency standards such as phase-locked loops or crystals, and suitable buffers to maintain frequency stability.
• the filters 48 and 52, and spacing between intermediate frequencies are chosen so that each signal is subject to essentially no interference from other signals (unwanted signals should be at least 20, better at least 30, and preferably at least 40 decibels down in view of the accuracy of later digital processing).
• the required spacing between intermediate frequencies is typically 5-10 times the bandwidth of the filters 48. A wide range of intermediate frequencies is possible.
• the separate modulated intermediate-frequency signals from 52 are combined at a intermediate-frequency combiner 56 into a single relatively broad-band multiplexed intermediate frequency signal.
• the combiner may be a summing amplifier or a hybrid device, together with suitable buffers and intermediate-frequency transformers,
• the multiplexer 46 operates in the audio-fequency range using analogous audio-frequency components at 48, 49, 51, 52 and 56.
• the oscillators 49 operate at audio frequencies spaced by several times the cutoff frequencies of filters 48, using amplitude modulators at 50, and thus obtaining audio frequency signals at different frequencies from the filters 52.
• the output from the audio combiner 56 is then frequency modulated (not shown) on a suitable intermediate-frequency signal obtained from a suitable intermediate-frequency oscillator.
• the intermediate-frequency signal is then heterodyned to a suitable output frequency using a local oscillator 62, mixer 63, buffer/ amplifier 64, and filter 65. This may be done more than once if desired.
• the output from 65 then passes to a low power final amplifier 67 and filter 68, and then to the transmitting antenna 14.
• Output frequencies should be chosen, preferably in the ultra-high frequency range, so that local broadcasting can be obtained with very low power and noise and a very short flexible wire antenna 14.
• the transmitter 12 is enclosed in a small plastic box, which may measure approximately 2 x 7.5 ⁇ 10 cm. attached to a light cloth belt (13 in Fig. 1) worn by the mother.
• the transmitter also contains conventional batteries, switches and controls, and test-points as required for positioning the microphones, maintai nance, etc. For example, it may be desirable to allow for monitoring each amplified input signal for such purposes, or to aid in positioning the microphones 11. No adjustments are needed for normal operation on the patient.
• the transmitted signal from Fig. 6 is picked up by a receiving antenna 16 connected to a receiver 21 in Fig. 7.
• the purpose of the receiver is to detect and demultiplex the signal broadcast from the mother's transmitter, obtaining the separate plural signals sent to the transmitter.
• the receiver contains the standard radio-frequency amplifier 81, mixer 82, oscillator 83, and intermediate frequency amplifier 84 to produce an intermediate frequency signal. (One or more oscillators, mixers, and intermediate frequencies may be used, in which case output from 84 represents the final such signal.)
• the intermediate frequency signal from 84 is then sent to a detector and demultiplexer 86 to recover the separate original input signals.
• a detector and demultiplexer 86 this is accomplished by first splitting the intermediate frequency signal into multiple separate signals, one corresponding to each input, using band-pass filters 88 and isolating amplifiers at either or both of 89 and 90, as desired.
• Each separate signal from 90 is detected at 92 with a suitable detector (FM in case frequency modulation was used).
• Any audio frequency signals used to encode slowly varying inputs at 43 in Fig. 6 are decoded at this point (not shown) to recover the original signals.
• These signals are amplified at amplifiers 93 and carried to the processor 22 by wires 94.
• the amplifiers 93 may be the amplifiers of Fig. 5 or similar units, including the filtering therein. These amplifiers also preferably include demodulators and low-pass filters for preconditioning of the signals. These may be the precision rectifiers and active filters of Fig. 5.
• the signal from I-F amplifier 84 is detected by a suitable FM detector 92, amplified at amplifiers 93, and split into separate component signals using balanced mixers 97, audio-frequency oscillators 98 and filters 99.
• the signals from 99 are then amplified ⁇ if desired at amplifiers 100, and then carried to the processor 22 by leads 94.
• the processor 22 produces a single enhanced fetal hearbeat signal from multiple fetal inputs and one maternal input, and also produces the power spectrum, or range of frequencies present in the fetal heartbeat and their amplitudes, as one measure of variability. It also accepts any other desired signals, such as tonometric, electromyographi c or other data on contractions, maternal electrocardiographic data, etc., corresponding to auxiliary inputs 15.
• These signals are sent to output devices 18 such as loudspeakers, oscilloscopes, strip chart recorders, indicator lights, etc. Further processing to find such data as instantaneous heart rates is also performed and the results displayed at 18.
• output devices 18 show both the fetal heartbeat sound signal, and its frequency spectrum for one measure of variability, as well as auxiliary inputs. It is also possible to receive signals from several transmitters at a central monitoring station.
• Fig. 10 Only major electronic components such as analog t ⁇ digital (A/D) converters, microprocessors and RAM units are shown in Fig. 10. Other components such as latches, buffers, most interfaces and controllers, etc. depend upon the specific major components used, are not shown in Fig. 10, but the components required and their use will be clear to those skilled in the art.
• the processing unit is designed to execute the required processing and programs so that the enhanced fetal heart signal and frequency data lag only slightly behind real time.
• the output signals from the receiver enter the processing unit by wires 94.
• the preferably demodulated fetal heart signals and a maternal heart signal used in the filtering process are then processed.
• Each fetal heart signal as well as the maternal heart signal is separately digitized and sampled at a separate analog data acquisition unit 301.
• Each analog data acquisition unit contains a linear amplifier 123 with signal conditioning circuitry, a sample and hold unit 125, an analog to digital (A/D) converter 127 and an interface 129.
• the units 301 may be combined in an input/output package, together with digital to analog (D/A) converters 181 if desired.
• a single unit 301 may be used to sample several signals provided the signals are first appropriately time-multiplexed, as, for example in the Intel iSBX 311.)
• each signal is first sent to a separate linear amplifier 123.
• the amplifiers may so constructed as to filter out signals at frequencies above the sampling rate, using resistor-capacitor, active, inductor-capacitor, or other similar filters. Active filters may be similar to those of F.ig. 5. This is a standard preconditioning technique familiar to those skilled in the art of digital sampling.
• These signals are then sent to separate sample and hold units 125 under the control of microprocessor 145, which controls the sampling process, via interface 129.
• the A/D converters 127 digitize the signals temporarily stored in the sample and hold units 125, under the control of microprocessor 145 through interface 129.
• each A/D convertor is then sent to a separate dual-ported random access memory (RAM) unit 131 through interface 129 under the control of microprocessor 145.
• RAM random access memory
• the microprocessor may be an 8 bit or preferably a 16 bit unit such as the 68000, 80186, or 16032.
• the outputs from the A/D convertors should be 8-16 and preferably 12-16 bits wide.
• the RAM is most easily configured into words of corresponding numbers of bits. All required timing is controlled by one or more clocks 144.
• Signals from the auxiliary inputs including a second copy of the maternal heart signal if desired, are sampled and stored in similar dual-ported RAM units to slightly delay these signals for synchronization with the processed fetal heart signal.
• the choices of sampling rate, number of samples per cycle, and duration of sampling cycle all contribute to effective operation.
• detection of the fetal heart signal without aliases requires a sampling rate at the Nyquist frequency, at least twice the expected rate of beats, or four times the maximum heart rate, accounting for two beats per cycle. This should be significantly increased, preferably to at least the Nyquist frequency of the fourth and preferably the fifth harmonic of the fetal heart rate to provide accurate measurements and waveforms.
• a sampling rate of at least 1800 samples per minute, and preferably at least 3600 samples per minute should be used. The latter rate implies that even at a fetal heartrate of I80, the fifth harmonic can be detected without aliasing. Even, higher rates such as 7200 samples per second, are better.
• the sampling rate must be above twice 100 Hz, i.e., at least 12000 times per minute.
• Accurate measurement of variability from the enchanced fetal heart signal is also favored by a high sampling rate. For example, an accuracy of +/-1 beat per second requires a sampling rate of at least 3600, and preferably at least 7200 samples per minute.
• sampling rate bears a simple harmonic relation to 60 Hz.
• rates of 1800, 3600, 7200, etc., samples per minute are preferred.
• the sampling rate should have a similar relation with the electrical frequency in use, e.g. 1500, 3000, or 6000 samples per minute for 50 Hz current.
• sampling rate will generally limit the sampling rate to a typical rate of 3600 or 7200 samples per minute, as descri bed bel ow.
• Sampl ing rates of 7200 samples per minute can be obtained and processed with chips such as the 68000 or 80186 or at 14400 with the 68020, coprocessors or dedicated chips.
• the number of samples per sampling cycle must be a power of two.
• the sampling cycle is limited by two considerations.
• the first consideration maximizing a ccur ac y, leads to longer sampling cycles. Simulation suggests that cycles of at least 4 seconds are required, cycles of at least 8 seconds are better and yield an accuracy of about +/-1.5 beats per minute, and cycles of 16 or more seconds are more accurate and thus preferred.
• periods of 16-35 seconds are pref erred . This may be controlled by a switch (not shown) connected to microprocessor 145, or if desired, automatically. In this case, periods as short as 4-9 seconds (and preferably 8-17 seconds) might be selected to measure acceleration or short term variability, and cycles as long as 75 seconds might preferably be used to gather the most accurate data during a contraction. In addition, cycles might be triggered by the start of contractions if desired.
• Microprocessor 145 controls the sampling rate and sampling cycle by controlling the analog data acquisition units 301, the dual-ported RAM units 131, and dual-ported RAM units (not shown) used to delay auxiliary inputs, as described below.
• FIGs. 8 and 9 show three microprocessors 145, 155, and 170, to illustrate three main functions, namely input, processing, and output, more or fewer microprocessors may be used.
• the FFT is illustrated by lines 1000-1900 of the following program, written in Basic, and is an improved novel version of the standard algorithm as found in Horowitz and Sahni (1978), see also Aho, Hopcroft, and Ullman (1974). There are many variants on the FFT algorithm. (The complete program is given here for later reference.)
• T(2) (A(MC,I0,1) + A(MC,10,2)) * (C + S) - VI - V2 1580
• A(MC,I0,1) A(MC,I,1) - T(l) :
• a (MC,I0,2) A(MC,I,2) -
• A(MC,I,1) A(MC,I,1) * FFC(I) :
• a (MC, I, 2) A(MC,I,2) * FFC ( I )
• A(5,I,1) A(5,I,1) * MFC ( I ) :
• a (5, I ,2) A(5,I,2) * MFC ( I )
• DP(2) DP(2) + (A(MC,I,1) + A(MC,I,2)) * (A(5,I,1) - A(5, 1,2)) - V1 - V2 2660
• NEXT I 2670 DP(1> DP(1) / NA 2680
• DP (2) DP (2) / NA 2690 REM FORM ORTHOGONAL COMPLEMENT
• V2 DP (2) * A (5, 1,2)
• T(2) (DP(1) + DP (2)) * (A (5,I,1) + A (5,I,2)) - V1 - V2
• TOTAL FETAL HEART SPECTRA
• 3210 EN EN + PS(MC, I) + PS(MC,KT)
• AX (I) AC (I,1) * AC(I,1) + AC (I,2) * AC (I, 2) + AC(KT,1)
• timer mode control word */ MCR LITERALLY '01100000B' /* mode control word */ /* 01 selects counter 1 10 selects read/write msb 000 selects mode 0 (interrupt) 0 selects hex timer */ COUNTS LITERALLY '00AH'; /* count for timer */
• 105 1 declare bignum real; 106 1 call init$real$math$unit;
• bit reversal accomplished in hardware.
• cos(angle) and sin(angle) are the (j-1)*n/(2*hp)th entries in respective tables in trig fen rom of cos and sin for angles from 0 to (n/2-1)*2*pi/n in steps of 2*pi/n */
• /* module to filter output from fft */ /* illustrates block 321 of fig. 11 */ /*
• Next lines illustrate optional filtering to remove noise in fetal heart spectra due to maternal heartbeat.
• Linear filter is illustrated, a wide choice of linear and nonlinear filters is possible, ffc stores fetal filter coefficients for simple linear filter, mfc stores maternal filter coefficients for simple linear filter */ /* These variables are used for clarity */ /* In the actual implementation multiplication by 0 is replaced by assignment of 0 and multiplication by 1 is not performed */
• 237 2 declare (dp1, v1 , v2, si, en,na, t1 , t2) real; 238 2 declare ax (41) real, op (40) real;
• Each fetal heart signal energy is estimated by adding components of its filtered power spectrum representing a range of 1 to 4 hz (60-240 beats per minute), other ranges may be used.
• */ /* snr is the vector of signal to signal plus noise (total) energies for filtered fetal heart spectra.
• 311 3 end combine; /* end of module to calculate combined, filtered spectrum of fetal heart signals */ 312 2 distribution: do;
• /* angle (j-1)*pi/hp.
• acimag(i) (acimag(i)+acimag(io))/2.0;
• bit reversal accomplished in hardware.
• the bit reversal is accomplished by reversing the order of the seperate address lines in bus 272 of dual-ported ram 163 */
• the PL/M program demonstrates simultaneous data acquisition and processing, including the use of an analog to digital converter (the Intel iSBX 311). Data is gathered for five sampling cycles, and may then be displayed on a computer screen or saved on a disk. In the latter case, data on the fetal heartbeat itself, as opposed or in addition to varaibility data may be thus saved.
• an analog to digital converter the Intel iSBX 311.
• a slight increase in speed may be obtained by indexing arrays used in the Fourier transform and inverse Fourier transform from 0 to n-1 rather than 1 to n.
• the Burrus and Parks algorithm may be used to speed up the Fast Fourier Transform of real data by storing and processing n real data points in an array of n/2 complex numbers. Their algorithm contained an obvious sign error. Here is an implementation of a corrected algorithm, followed by a similar but novel algorithm for the inverse Fast Fourier Transform.
• /* cos (angle) and sin (angle) are the */ /* (j-1)*n/hp th entries in respective tables*/ /* in trig fen rom of cosand sin for angles */ /* from 0 to pi insteps of pi/n */
• variables in all programs may be overlaid when they are no longer needed.
• the bit reversals are easily done in hardware by the addressing scheme of RAM 131.
• n 2 m samples
• m-bit addresses are used, and the digitized ith sample of each sampling cycle is stored in location rev(i-1) where for non-negative j ⁇ n, rev(j) is the binary integer obtained by writing j as an m-bit binary integer, including leading zeros, and then reversing the order of the bits.
• Software controlled switches (not shown) may be used to vary m and n for various sampling cycles without requiring additional processing time.
• a second microprocessor 155 e.g. type 80186, 80286, 68000, 68020, or 16032, reads the digitized microphone outputs (fetal heartbeat signals and optionally one maternal heartbeat signal) from dual-ported RAM units 131.
• This latter processor is run at a high clock speed, 8-16 mHz, in order to perform multiple (typically 4-5) FFTs, digital filtering, one inverse FFT, and some auxiliary computations within the time of one sampling cycle containing 128-8192 or more samples (typically 4.27-68.28 seconds in US versions, 5.12-82 seconds in European versions).
• microprocessor 155 may be supplemented by one or more of dedicated FFT chip 161, dedicated inverse FFT chip 162, arithmetic coprocessor, or another dedicated processor (the last two not shown).
• Box 305 denotes the hardware associated with microprocessor 155.
• a machine language program for the FFT, later inverse FFT, and other computations is stored in ROM 156, and values of the appropriate trigonometric expressions are stored in ROM 157.
• RAM 159 is used for computing.
• the final output from microprocessor 155 is sent to dual-ported RAM 168.
• a 68000 microprocessor running at 10 MHz, performs 100,000 16-bit operations per second.
• the use of other possible chips such as the 80186, 80286, or 16032, coprocessors, or additional processors, depending on the required processing rate, will be clear to skilled artisans.
• the output signals from the FFT each describe the frequencies present in one fetal (or maternal) heartbeat signal from one microphone.
• the fetal heartbeat frequency signals are first separately filtered and then added to produce a single enhanced signal representing the signals present in the fetal heartbeat. These steps are performed by microprocessor 155 and the hardware in box 305 of Fig. 10.
• the first step in filtering consists of multiplying certain components by fractions whose denominators are powers of 2, etc, and deleting other components, particularly the 60 Hz component, which may be readily and completely eliminated because the sampling rate is an integral multiple of 60 Hz. Frequencies in the approximate range .2 to 8-15 Hz should be passed unattenuated to preserve information about the fetal heart rate. Lines 2000-2290 of the program above illustrate this process.
• noise in the fetal heartbeat caused by the maternal heartbeat may be largely removed in a novel way by replacing the fetal heartbeat signals by their projections orthogonal to the maternal signal, or by reducing one or more frequency components present in the maternal signal, or by reducing one or more ranges of frequencies present in the maternal signal. This is illustrated by lines 2500-2900 in the program above.
• Microprocessor 155 then computes the power spectrum, that is, the amount of energy at each frequency, for each separate fetal heart signal.
• the quality of each signal is then evaluated. As illustrated by lines 3000 through 3900 of the above program, this may be done by first computing power spectrum of each signal. The quality is then measured by the ratio of energy in the range 1-4 Hz (other ranges may be used if desired), roughly the rate of the fetal heartbeat, to the total energy, as an analog of the signal to signal plus noise ratio. If desired, another measure of quality may be computed at this point. The filtered signals in the frequency domain are then combined into a single signal, for example by a weighted average, weighting each signal in proportion to their quality, as illustrated by lines 4000-4900 of the above program.
• phase differences may be corrected for the effects of phase differences between the signals by another program if desired (not likely to be needed for fetal heart monitoring).
• One way to correct for phase differences is to dynamically modify the addresses used to write fetal heart signal samples into RAM 131.
• the fetal heartbeat signals could be added directly, weighting each signal in proportion to its average amplitude if desired, and the combined signal could then be filtered with the FFT, digital filters, and inverse FFT as above.
• the filtering techniques could also be applied to auxiliary input signals.
• the inverse FFT is applied to the resulting combined, filtered signal to produce an enhanced, filtered signal of the fetal heartbeat in the time domain in short segments, typically 4.27-82 seconds.
• a novel implimentation of the inverse FFT is illustrated by lines 6000-6900 of the above program, which is specially designed so that the bit reversals form the final step which may again be done in hardware. This computation, except for the bit reversals beginning at line 6280 if desired, may be done either directly by the processor or by the inverse FFT chip 162. .
• Final output from this processor 155 is stored in another dual-ported RAM 168. Note that this RAM is distinct from RAM 159 used by the processor for its own calculations. Also, ports for this RAM are switched at suitable intervals (typically 4.27-82 seconds) by the processor 145.
• a third and final microprocessor 170 reads the RAM 168, using an address bus with the order of separate lines reversed as in RAM 131 to effect the bit reversal s and sends the output at the proper time intervals to digital/analog convertor 181.
• the microprocessor may control addressing without having data pass through- the microprocessor.
• the output of the D/A converters is sent to respective linear amplifier 183 for scaling and isolation purposes.
• the output of this amplifier is the filtered, enhanced fetal heartbeat signal at a slight lag (two sampling cycles, or typically 8-164 seconds in the proposed embodiment).
• This signal is displayed on the fetal heart output display unit 209, part of 18.
• a similar digital output may be provided by sending data from the dual-ported RAM 168 to a suitable interface (not shown) at the sampling rate under the control of microprocessor 170.
• the processor 170 also reads the RAM units (not shown) associated with the other inputs and sends these signals to additional D/A convertors (not shown), linear amplifiers (not ahown), and output units (part of 18).
• the RAM associated with auxiliary inputs is configured so that these signals have the same lag as fetal heartbeat.
• a switch bypasses the circuitry used to delay the auxiliary inputs (analog data acquisition unit, RAM, D/A converter, etc.) for synchrony with the delayed fetal heartbeat to allow undelayed signals from the auxiliary inputs.
• signals from the monitors of contractions, with no delay could be used to initiate controlled breathing or regulate medicine dosage.
• Analog or digital outputs associated with auxiliary inputs are easily provided for displayed on respective display units (part of 18).
• the output from the amplifier 183 consists of an enhanced, filtered (analog) signal of the fetal heartbeat, presented at a short lag behind real time.
• This signal may be further processed (not shown) to compute instantaneous heart rates, beat to beat variability, smoothed beat to beat variability with an algorithm such as the moving average, and may also be processed with autocorrelation techniques.
• a similar digital output may be obtained using a suitable interface (not shown) attached to the output of the D/A converter 181.
• Auxiliary signals such as data on the onset of contractions or intrauterine pressure, are delayed so as to be synchronized with the fetal heartbeat, and presented on the output unit 18.
• acceleration, deceleration and variability may be correlated with the onset and duration of contractions or other data.
• Analog or digital versions may be provided.
• the graphs in Fig. 14 of the frequency output on device 211 show the energy of the different fundamental frequencies or inverse heartrates, which make up the filtered, enhanced fetal heartbeat signal.
• the fetal heart rate is displayed along the horizontal axis and relative energy (or optionally amplitude, or other measure of the size of Fourier coefficients) is displayed along the vertical axis.
• the measure of the size of coefficients is a fixed attribute fo the monitor.
• the average heartrate of 140 in graph 401 is read from the horizontal coordinate 403 of the peak 404.
• Variability corresponds to the width 406 of the spectrum shown at about 15 beats per minute.
• Graph 402 represents the same average heartrate but a smaller variability, about 5 beats per second. This data is updated slightly before the other outputs are obtained, but persists or may be easily held until they are obtained.
• All outputs may be represented on devices such as oscilloscopes, recording devices, or other standard output devices familiar to those skilled in the art. Further processing may also be performed at these points if desired.
• properly synchronozed records combining delayed fetal heart signals, and non-delayed auxiliary signals may be made by using modified strip chart recorders.
• RAM units 131 and 168 The major components of dual -ported RAM units 131 and 168, except for controllers and interfaces, are shown in Figs 10 and 11. Except as noted, the RAM used to store auxiliary inputs is similar and not shown.
• Each RAM unit contains two sets of memory chips, 251 and 256. These chips are configured appropriately to handle 12-16 bit data words and, in the case of RAM 131 and 168, at least one word per sample in each sampling period (typically 512-4096 or more words). This is well within the range of sets of 16K or 64K RAM chips.
• the RAM used to delay auxiliary inputs must be twice as big in order to handle the required time delay of two sampling cycles.
• ports 266 and 271 There are also two ports 266 and 271, respectively.
• a software controlled (via control 276) digital switch 261 which either connects ports 266 and 271 to chips 251 and 256 respectively, or vice versa.
• the connections are made via address lines 252, 257, 267, 272, data lines 253, 258, 268, 273, and control lines 254, 259, 269, 274.
• the address line 272 is inverted so that the bits in each address are reversed to perform the bit reversal or permutation of input in the FFT in hardware.
• microprocessor 145 is connected to switch control 276 and to port 266 for addresses and control, interface 129 is connected to port 266 for data, and microprocessor 155 is connected to port 271 for addresses, data, and control. (Alternatively, if desired, data from interface 129 may pass through microprocessor 145.)
• port 266 is connected to memory chips 251.
• data from the interface 129 is written into memory chips 251.
• port 271 is connected to memory chips 256, but remains idle.
• memory chips 251 contain the digitized fetal heartbeat signals obtained in the first sampling period.
• the signals from each auxiliary input are stored in the corresponding chips 251 in corresponding RAM units (not shown).
• the digital switch 261 in RAM 131 is inverted under the control of microprocessor 145 via switch control 276.
• port 266 is connected to memory chips 256, which store digitized fetal heartbeat signals as described above.
• port 271 is connected to memory chips 251.
• the signals in these chips 251 are read rapidly by microprocessor 155 which performs the FFT algorithm upon this data.
• the first part of the FFT is performed in hardware, requiring no execution time.
• the digital switch 261 is again reversed under the control of microprocessor 145, As in the first interval, fetal heartbeat data is read into chips 251 from the interface, while, by analogy with the second period, microprocessor 155 reads chips 256 and applies the FFT to their data. At the end of each subsequent sampling period, connections and the roles of chips 251 and 256 are similarly reversed.
• RAM 168 of Fig. 13 (only shown where different from Fig. 12) operates similarly.
• Microprocessor 155 is connected to port 266 for data, addresses and control; microprocessor 170 is connected to port 271 for addresses and control, D/A converter 181 and a (digital) interface (not shown) ar e connected to port 271 for data, and microprocessor 145 is connected to switch control 276.
• Microprocessor 155 writes the filtered, enhanced digitized fetal heartbeat data into this RAM at the conclusion of its processing.
• Microprocessor 170 reads data, representing the enhanced fetal heart signal from this RAM into D/A converter 181 sequentially at the original sampling rate to generate the output.
• microprocessor 145 possibly in combination with microprocessor 155, reverses the connections in the switch at the end of each sampling period.
• data can be rapidly written to RAM 168 after processing, while other chips in RAM 168 are read at the sampling rate to generate the output.
• the required connections by sampling periods are:
• the final bit reversal in the inverse FFT is performed by the addressing of RAM 168, requiring no execution time.
• the dual-ported RAM associated with the auxiliary inputs operates similarly, with the exception that connections are reversed after each pair of sampling periods. In this way, this RAM delays signals from the auxiliary inputs by the same amount (2 cycles or 34.14 seconds) as the sum of the delays in the fetal heartbeat RAMs 131 and 163. Note that these delays allow for the required processing time, whereas signals from the auxiliary inputs are not processed. Switching is analogous to that of RAM 131 except for the larger interval between reversals: for example, during sampling periods 1 and 2, port 266 is connected, to chips 251.
• auxiliary signals may be similarly processed.
• signals 94 may be optionally demodulated before further processing (not shown) t ⁇ emphasize the frequencies in the fetal heartbeat.
• the maternal heartbeat is to be used in filtering the fetal heartbeat, the maternal heartbeat is similarly demodulated (not shown).
• the signals are then sent to analog data acquisition units 301, controlled by microprocessor 145.
• the units 301 also contain passive or active audio filters to suppress frequency components in excess of the sampling rate. This yields a less noisy sample and avoids aliases and ringing.
• the analog data acquisition units convert the analog heartbeat signals to digital signals in real time at the specified sampling rate, in the form of 12 to 16 bit wide data.
• the first step 312 consists of a module to compute the FFT of each fetal heart signal (and a maternal heart signal if needed for later filtering), and is illustrated by lines 1000-1900 of the program above (the indicated bit reversals in lines 1200-1400 are performed in addressing RAM 131).
• Output from the FFT consists of the range of frequencies and amount of energy at each frequency present in each fetal heart signal. At a typical sampling rate of 7200 samples per minute, and a sampling period of 2048 samples or about 17.07 seconds, frequencies from .1 Hz through 60 Hz are represented. The outputs from the FFT are called frequency domain fetal heart signals.
• microprocessor 155 possibly with a special purpose chip 161. All of the functions performed by microprocessor 155 are enclosed in a box 306, Fig. 11, using hardware enclosed in a box 305, Fig. 10.
• a single high-speed microprocessor typically 8-16 mHz, 16 or 32 bits
• additional processors, coprocessors, or dedicated chips may be used, optionally in conjunction with additional processors, coprocessors, or dedicated chips.
• Each frequency domain fetal .heartbeat signal is filtered by a module to filter spectra 321, illustrated by lines 2000-2900 of the above program, and executed by Microprocessor 155.
• the spectra first pass through a linear (illustrated by lines 2000-2290) or non-linear filter chosen to pass unattenuated frequency data low frequency data ranging from about .2 to 8-15 Hz, to attenuate higher frequencies as required for noise suppression including suppression of some of the maternal heartbeat signal, and, because of choice of sampling rate, to totally eliminate 60 cycle hum.
• the filtering process may be easily adapted to pass other frequencies.
• the quality of each filtered fetal spectrum is evaluated by a module 316.
• the power spectra are used to compute a measure of the relative quality of each fetal heart frequency domain signal.
• One measure of the quality is the ratio between the amount of energy in the range 60-240 beats per minute (1 to 4 Hz) to the total energy, a rough measure of the signal to noise ratio.
• the portion of the power spectrum of the combined filtered fetal spectra in the range of 60 to 240 beats per minute is computed and smoothed at module 318 as a measure of the distribution of fetal heart rates.
• the resulting signal is then sent to an interface 210 and frequency output devices 211-
• the combined filtered fetal spectrum is processed by the inverse FFT 326, as illustrated by lines 6000-6900 of the above program, to obtain an enhanced, filtered digital fetal heart signal.
• This signal is written into dual ported RAM 163, in such a way, if desired, as to effect the bit reversals illustrated by lines 6280-6330.
• the enhanced, filtered digital fetal heart signal from RAM 168 is read into a digital to analog (D/A) converter 181 at the sampling rate, under the control of microprocessor 170, and then amplified at 183. This yields a filtered, enhanced fetal heart signal (analog) in real time except for a delay of two sampling cycles, between 8 and 164 seconds, and typically about 34 seconds.
• the signal is then sent to suitable output devices 209, and further processed (not shown) to obtain data such as instantaneous heart rates.
• a similar digital signal may also be provided if desired.
• the smoothed, combined power spectrum from module 318 is sent to a buffer 210 to delay it until it is synchronized with the fetal heart signal, and then displayed on a suitable digitally driven frequency output display 211. Again, this signal lags behind the fetal heart rate by two sampling cycles.
• signals from auxiliary inputs may be either displayed as received or delayed to be in synchrony with the delayed fetal heart signal, depending on a switch position.
• the delay is accomplished with dual-ported RAMs analogous to RAM 131 (except that (1) no address line is reversed and (2) twice as much storage is required for a two sampling cycle delay), and D/A convertors and amplifiers such as181 and 183.
• the delayed signal allows comparison between contractions and acceleration, decceleration and variability. Real time tonometric data may be used to alert the mother to the onset of each contraction.
• the sampling cycle is selected in order to satisfy one or more of the following goals.
• a short cycle minimizes the delay, and provides variability data for short time periods.
• a longer cycles may be used to improve signal quality. Cycles lasting from 8 to 41 seconds seem feasible. In all cases the length of a cycle must be a power of two times the interval between samples, and the latter must be harmonica related to 60 Hz in the US and 50 Hz in Europe in order to minimize hum.
• Sampling rates should be as high as practical, with rates of 7200 samples per minute practical using microprocessors such as the 68000, and 240 samples per second or higher using the 68020.
• Placental monitoring may be facilitated by using components of the spectrum of each input to regulate phasing of the inputs by adjusting the addresses used to write digitized input signals into dual-ported RAM 131 to achieve the requisite time delays, and by demodulation if desired.
• a fetal monitor which makes essential use of telemetry and a digital signal processing technique called the FFT.
• FFT techniques and addition of signals in the frequency domain, together with digital frequency domain filtering described above, yields an accurate fetal heart signal as well as an accurate measure of fetal heart rate (after further processing) by filtering out random noise and noise at irrelevant frequencies.
• spectral (frequency) data we obtain the range of fetal heartrates present during the last sampling period, useful information about variability in fetal heartrate, a frequently critical diagnostic indicator.
• this device can be used to monitor placental blood flow, or enhance ultrasound data, should that prove desirable.

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