Patent GB-1477667-A (GB)
Title: Circuit arrangement for producing a pulse sequence corresponding to the foetal cardiac pulses
IPC: A61B5/344
Filed: 1974-06-05
Abstract: 1477667 Measuring foetal cardiac signals HEWLETT-PACKARD GmbH 5 June 1974 [5 June 1973] 20383/74 Heading G1N(E) In an apparatus for producing a signal representing the foetal cardiac rhythm from a composite signal obtained by means of electrodes placed on the mother's abdominal wall, a pair of signal processing channels adapted to select signals produced by the mother and by the foetus, respectively, are used. Means for controlling the foetus channel by reference to the mother's channel which produce blocking signals to inhibit response of the foetus channel to the mother's signal are provided. The absence of an expected foetal signal is detected and an "error signal" is produced. The signals from the electrodes are passed via a preliminary amplifier 1, which reduces the composite signal from the electrodes to a predetermined dynamic range, to a band rejection filter 2 whose centre frequency follows automatically and filters out the "mains" frequency component of the signal. The output of the band-rejection filter 2 is connected to two channels, one channel being provided with a band pass filter 3 which only passes frequencies lower than those contained in the foetal QRS blocks, thereby separating out the signal due to the mother. This signal is directed via a polarity reversal means 4, for ensuring that the dominating component of the mother's signal has a predetermined polarity, to a first trigger circuit 5 to provide a pulse sequence corresponding to the time sequence of the mother's signals. The other channel from the band rejection filter 2 contains a second band pass filter 6 whose pass range is adapted to the foetal signals which have higher frequencies than the mother's signals, in order to separate out the signals from the foetus. The filtered signal then passes via a second polarity reversal means 7 to a second trigger circuit 8 to derive a pulse sequence corresponding to the time sequence of the foetal signals. The second trigger circuit 8 is responsive to "interlocking" signals derived from the pulse sequence signals of the mother in the first channel so that the second trigger circuit 8 will not trigger on response to any signals from the mother's that may have not been filtered out in the second channel. An error detection circuit receives the second trigger circuit output and emits an "error" signal when no foetal signal is received during an expected interval. The expected interval is derived from the period between one or more successive pairs of "earlier" pulses. The "error" signal may be used to "add" a pulse on the foetal signal train or to modify the associated computational or measurement circuit receiving the foetal signal to take the missing signal into account. The description discloses several suitable circuit diagrams for carrying out the operation. Mitigating the effect of a material signal in the foetal channel. Fig. 4 shows part of the trigger circuit in the foetal channel. The circuit comprises a peak detector circuit connected to a main store C6 arranged to be charged through a rectifier D13 by pulses in the foetal channel in the absence of blocking signals. An auxiliary store C7 is connected via switching means T7, T8 to the main store C6. When a delay period, determined by a delay circuit 8H, after peak detection and the main store is discharging and no blocking signal occurs the arrangement of the switches is such that the input of the auxiliary store is connected to an output of the main store. However, when a blocking signal occurs, an output of the auxilliary store C7 is connected by the switches to an input of the main store C6 in response to a blocking signal, in order to enter a function of the valve of the auxillary store in the main store.