Patent GB-1536410-A (GB)
Title: Fetal distress alert system
IPC: A61B5/03
Filed: 1975-12-29
Abstract: 1536410 Fetal distress alert system AMERICAN HOME PRODUCTS CORP 29 Dec 1975 [24 April 1975] 52990/75 Heading G1N(E) In a fetal distress alert system the heart beat rate HR of a fetus and the pressure UP in the maternal uterus are continuously monitored and processed to provide measurement parameters indicative of the condition of the fetus such as heart rate, heart rate variability 17, heart rate deceleration 15 and uterine pressure (both steady-state and peak pressure). The heart parameter processing circuitry is disabled at certain times during a uterine contraction. The measurement parameters are compared with corresponding limits of acceptability and when these limits are exceeded an audio alarm is sounded and a lamp is lit indicating which parameter has exceeded its assigned tolerance for more than a predetermined period. Multi-plex circuitry may be employed so that a number of pregnant women may be monitored simultaneously. The system may also sound an alarm when improper data is received. Error detection The incoming data is checked for error signals such as a "pen-lift signal" PL, Fig. 1, which would be generated by the external monitor placed on the woman (as disclosed in Patent No. 1267680). Also the heart rate signal is monitored to see if it is false by checking if it is above a particular rate (30 beats a minute, Fig. 3 not shown). All data error signals are fed to an error detection circuit 5 which actuates a flip-flop 7 that controls a "buffer channel" enabling circuit 9. The circuit 9 permits or impedes the flow of buffer channel signals from a source 11 thereby controlling the actuation of the fetal heart-rate alert circuits 15, 17, 19 to prevent them processing false signals.  Uterine contraction information The signal representing the pressure in the uterus is supplied via a buffer amplifier 4 to a slope detection circuit 21. Circuit 21 continuously measures the slope of the uterine pressure curve to control a uterine pressure logic circuit 23 causing it to signal circuits 17 and 19 as to the onset of a contraction and to signal circuit 15 as to the peak of a contraction, as well as to signal the termination of a contraction. The buffer amplifier 4 also supplies a steady state uterine pressure (tomes) circuit 25, which monitors uterine pressure between contractions, and which is disabled by a signal from the uterine pressure logic circuit 23 whenever a contraction is occurring Fig. 2 (not shown). A "peak limit" detecting circuit 27, connected as shown, supplies a uterine activity alert circuit 97 which activates an alarm when the uterus pressure parameters are such as to be dangerous to the fetus. Heart rate The heart rate signal may be fed, as shown, Fig. 4, via a sampler 26, and a sample and hold circuit 19 to an amplifier 27 with an offset adjusting voltage 27. The amplifier 27 feeds a pair of comparators 28, 30 which determine whether the heart rate is within predetermined limits. If the heart rate is outside the limits for more than a predetermined period, as set by a delay circuit 32, an alert 37 is set off. Heart rate variability The heart rate variability circuit 17, Fig. 1, is to detect the absence of heart beat irregularities. A constant heart beat for an extended period of time is symptomatic of fetal distress. A differential amplifier 34, Fig. 6, is supplied with heart rate signals from a patient by means of a multi-plexer 120 and a sampler 26 as shown. Variability monitoring is accomplished by employing an accumulator 36 which is incremented by a variability detection network 39 in response to the output of the differential amplifier 34. A variability detection circuit 39 provides an incrementing signal to the variability accumulator 36, the incrementing signal being enabled or inhibited depending upon the output of the differential amplifier 34. Simultaneously, the accumulator 36 is continuously decremented at a constant rate by a variability sensitivity network 38. The output of the accumulator 36 is applied to a variability alert 41, the alert 41 being actuated when the contents of the accumulator 36 fall to a pre-selected "variability" threshold (i.e. zero). Heart deceleration monitoring In this part Fig. 9 (not shown) of the system, the heart rate signal is averaged during each time period between uterus contractions to produce a "heart base line value". This is used in the next contraction with the heart rate signal during one or two predetermined periods following the "peak" of the next contraction. The area between the heart rate amplitude curve during each predetermined period and an extrapolated curve formed from the baseline is determined. If the resulting measurement of the area exceeds a predetermined value the fetus is deemed to be in a distressed condition.