Int JBiomed Comput, 25 (1990) 287-294 Elsevier Scientific Publishers Ireland Ltd.
CRITERIA FOR THE DESIGN SYSTEMS
OF FETAL HEART
G.S. DAWES, M. MOULDEN and C.W.G. REDMAN Nuffield Department of Obstetrics and Gynaecology, John Radcliffe Hospital, Headington, UK-Oxford OX3 9DU (U.K.)
Criteria are described for the automated analysis of fetal pulse intervals, fetal movements and of uterine contractions measured externally, antenatally and interactively on-line, for implementation on a personal computer interfaced to an appropriate fetal monitor, and tested on 10,000 records. Measurements of short and longer term fetal heart rate variation are compared; both are required to identify sinister records. Recall and display of records acquired on the same patient over several weeks has proved a useful diagnostic aid. Keywords: Fetal heart rate; Acceleration; Deceleration; Growth retardation; FHR variation; Fetal movements
Introduction Our original system designed for interactive analysis of the human fetal heart rate antenatally at the bedside was based on the NASCOM microcomputer [l]. From 1982, up to five of these instruments, assembled in the laboratory, were used for routine clinical and research purposes. They were finally phased out in 1987, being replaced by designs based on SAGE microcomputers from 1984 (mainly used for studies in labour) or on APRICOT Xen microcomputers from 1986. One advantage of the new systems was that no electronic assembly was required. The programs, originally in BASIC, were rewritten in PASCAL and ran 20 times faster. The visual display was improved to give up to an hour’s trace using high resolution colour graphics. Yet the primary features were retained, i.e. data were reduced over 3.75 s (l/16-min epochs) and stored in 2 bytes (10 bits for the mean pulse interval, 2001000) ms; 1 bit for fetal movement, present/absent; and 5 bits for the tocodynamometer value, O-31 units). Since the storage format was the same for all three microcomputers, data were readily transferred and we accumulated an archive of more than 10 000 records for reanalysis using programs with new features. As was to be expected, the price of the hardware has dropped over the years. Data Retrieval The change from NASCOM involved a change in the method of data retrieval. 0020-7101/90/$03.50 0 1990 Elsevier Scientific Publishers Ireland Ltd. Published and Printed in Ireland
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The NASCOM system used a separately programmed 6502 chip for this purpose, transferring the data via 10 lines to the NASCOM 280 PI0 for analysis; the timing was done by the 6502 microprocessor. The new systems employ priority interrupts every 100 ms from the central processor unit to address the fetal monitor (HewlettPackard 804OA) via its RS232 port to acquire the current data on pulse interval, autocorrelation function, the tocodynamometer value and fetal movement (indicated by a patient/midwife activated button). The time required for data acquisition is variable, 2-8 ms. The rest of the time is available for analysis; data acquisition continues 10 times a second during which the analysis program is briefly interrupted. Recording Pulse Intervals Originally the fetal heart pulse was identified by a peak detector (Hewlett-Packard 8031A); the time between peaks was measured to 1 ms, and these times were used to calculate the mean pulse interval over l/16 min. In the new system the current best estimate of pulse interval, as determined by autocorrelation, can be sampled up to 10 times a second. If the pulse interval increases from 300 to 600 ms the number of samples per heart beat is increased from three to six, when sampling every 100 ms. This is a potential source of error if the mean value is based on sample number; indeed once the current value is known it is not necessary to sample the pulse interval so frequently. Simultaneous records using the NASCOM and SAGE microcomputers on 50 patients showed no systematic variation in FHR analyses; but with their greater speed the SAGE and.APRICOT systems are used with greater precision, eliminating rounding to the nearest ms in some intermediate calculations. Record Characteristics The next point to consider is in clinical practice whether antenatal FHR records analysed by the microcomputer can be used to identify normal fetal health and its variations and, if so, to determine what features are essential. We can then return to consideration of the hardware and program features. The system is interactive, in that signal loss is continuously evaluated and an audible warning is given if it rises above 30%. Tocodynamometer values are scrutinised and warnings given if they or the recording of fetal movements are deficient. The results of analyses are displayed on the monitor. The printout identifies the number of decelerations (of different sizes), of accelerations, uterine contractions, fetal movements and the duration and amplitude of episodes of high and low heart rate variation, as well as the range of variation overall. Normal values are provided for comparison; values outside the normal are marked with an asterisk. Decelerations associated with uterine contractions are identified. Printed warnings are given when signal loss is excessive, when the record is abnormally flat or decelerative and when short-term variation is grossly reduced (see below). Growth retardation, in the absence of acidaemia, is associated with a decrease in FHR variation . This decrease outside the normal range is not due to prolonga-
Criteria for FHR analysis systems TABLE I
MEAN MINUTE RANGE OF FHR VARIATION IN NORMAL AND ABNORMAL PREGNANCIES (MEAN + S.E.) n Normal pregnancy 28 weeks  Normal pregnancy 40 weeks  Lower limit of normality at 32 weeks (lowest 3%)  Delivered at 32 weeks for antenatal compromise 
Mean range (ms)
39.5 f 1.0
49.0 rt 2.2
31.4 f 1.5
20.6 + 1.2
tion of episodes of low variation (characteristic of quiet sleep postnatally and without movements), but to a general blunting of changes in heart rate about the baseline, even when fetal movements are present . Under these circumstances fetal heart rate variation is decreased well below the third centile (least 3%) of normal. The gross figures are shown in Table I. In such fetuses delivered by section for antenatal compromise at 32 weeks (lowest row of Table I)  the umbilical Pa,o, was decreased to 6.1 & 1.6 (S.E., mmHg) as compared with control groups of 10.0 +: 1.O (delivered at the same age for maternal reason) and 13.O mmHg (caesarean or vaginal delivery near term). The fetuses delivered for compromise were chronically hypoxaemic, since the liquor erythropoietin was raised. They were undergrown and with other characteristic metabolic changes (lower blood glucose), but with no gross change in plasma adenine nucleotides or endorphin concentrations such as occur in asphyxiated infants. The measurement of FHR variation (the mean minute range) used in these analyses excludes decelerations, which are separately identified and characterised. It incorporates accelerations (e.g. of 10 bpm for 15 s or more), whose presence is noted in passing. No special attention is paid to accelerations since they are absent in many long records of infants subsequently delivered in normal health, especially in records before 34 weeks gestation . The mean minute range is a continuous measure of FHR variation down to 1 ms, whereas accelerations only provide a discontinuous limited measure of normality. The mean range of FHR variation about the baseline correlates well (r = 0.97) with the standard deviation or RMS (root mean square deviation) value [S]. Figure 1 illustrates the measurement of the mean minute range. In the I-min section A the rate rises above and falls below the baseline; the range of pulse interval is 476-387 = 89 ms. In section B the rate continually exceeds the baseline; the range is calculated from the baseline, as 438-368 = 70 ms, hence prolonged accelerations
G.S. Dawes et al. Gestation
B 163 bpm (368 ms) N --137
bpm (438 ms)
(476 ms) b
5v f TOGO.
Fig. 1. Diagram to illustrate derivation of the range of fetal pulse interval variation over one minute, A where the rate rises above and falls below the baseline, and B where the rate exceeds the baseline.
are recognised. The mean is calculated from all minutes in the records, excluding those with decelerations (in part or whole). Measures of FHR Variation Obstetricians have discussed three measures of FHR variation, short, medium and long term. Definitions have varied; the casual variation in the length of records often has determined long term variation, and no satisfactory quantitative comparison has been made between these measures and outcome. We have attempted to fill part of this gap. A digital filter was used to fit a baseline about which the range of FHR variation was measured . This separated long term variation (with frequencies < 1 in 10 min) from the remainder in records of 60 min length. The long term variation (i.e. in the baseline) was of no help in discriminating between normal and abnormal traces, with normal or abnormal outcome [4,5]. We have compared medium and short term variation in the FHR, medium being defined as the mean minute range incorporating frequencies from < 16/min to 1 in 10 min. Short term variation was calculated as the mean of successive epochal (l/16min) pulse interval differences. Figure 2 shows a close relation between the two (r = 0.89) in 1000 successive records. The mean values are 7.78 and 42.4 ms for short and medium term variation respectively, but it should be noted that the population contained some traces with abnormally low variation, i.e., with a mean range well below 30 ms.
Criteria for FHR analysis systems
SHORT TERM FHR VARIATION
MEDIUM TERM FHR VARIATION
Fig. 2. Relation between short-term (mean epochal, y axis) and medium term (mean range, x axis) fetal pulse interval variation in 1000 consecutive clinical records (r = 0.89). The lines are 2 S.D. above or below the mean.
The mean minute range covers a relatively wide band of frequencies. Attenuation of either higher or lower frequency components, decreasing the measured variation by up to 40%, did not improve discrimination between records of normal and abnormal pregnancies (e.g. in infants with suspected growth retardation). The discrimination between these groups was statistically better when using medium than short term variation. This is not surprising since medium term variation comprises more information; in absolute terms it is five-fold greater and detects the greater variation associated with fetal movement. However there was one uncommon class of antenatal FHR record in which short term variation was a necessary guide to abnormality, when a sinusoidal rhythm (high medium term variation) was imposed on an otherwise flat trace (abnormally low short term variation). Although the incidence was low, less than 1 in 1000 records, detection of such records is vital. We conclude that both medium and short term measures of FHR variation must be employed. Error Detection and Graphic Display Erroneous decelerations (abrupt falls in heart rate by > 35/min followed by equally abrupt rises attributed to transient detection of the maternal pulse) and dubious decelerations (with > 50% signal loss) occurred in 11% of 2000 records. Their elimination reduced the number of ‘decelerative’ records by more than 40% . The incidence of records with erroneous or doubtful accelerations was the same (11 (r/o);though, since these were not used for record classification, the finding was of less significance. With the SAGE microcomputer, used in labour, both the previous hour’s data
G.S. Dawes et al.
and the current hour’s are displayed. Analyses are based on an hour’s data as soon as available, and are updated every 5 min. The APRICOT system has been used antenatally so far, with analyses every 2 min after the first 10 min. Analysis of an hour’s data, even with resetting the baseline if the trace is flat, and subsequent search for shallow decelerations if no large ones are present (i.e. three sequential analytical passes), is complete in 23 s, and proportionately less for less data. Each disk holds the files on up to 500 records. If a patient has four or more records filed then, when the current record is complete, a synoptic visual display is presented of the last 4 weeks’ records from that patient, with printout if required (Pig. 3). Hence the obstetrician is presented with quantitative information from which a trend in fetal health may be discerned, to supplement that available from other
i;VERALL X IN LOW
x;ifiiii ., ..,_..._..__...,.._..__.._/._.._,_
I*II,,,,, 21 24
1111rtIIIIIIII.I 22 24
Fig. 3. Two examples of printouts, automatically provided, of changing fetal behaviour (upper record, movements/h; middle record, FHR variation; 16wer record, basal FHR; D indicates a decelerative trace) in high risk pregnancies w,ith delivery at 32 weeks (left-hand) and 37 weeks (right-hand); neither was acidaemic.
Criteria for FHR analysis systems
methods: of growth as judged from biparietal diameter or abdominal circumference, of umbilical artery velocity wave-form, of liquor volume, of fetal breathing movements and muscle tone. For, as postnatally, fetal health cannot be judged adequately from a single measure. Design Criteria
Judged by the experience and clinical outcome of this large data set comprising 10 000 records (from an estimated 3600 pregnancies) we believe that it is necessary to measure pulse interval to 1 ms, to provide good estimates of variation. Our decision, 10 years ago, to average the pulse interval over l/16 min was based on three considerations. Firstly, autocorreIation by the HP8040 was over 0.5-I .2 s and this caused a reduction in beat-to-beat variation [ lo]. Ultrasound detection was necessary for data acquisition over a wide range of gestations antenatally, and autocorrelation, with improved transducer design, reduced signal loss substantially . There was no point in trying to measure beat-to-beat variation, because it was normally so low, just over 2 ms [lo, Ill, too near to the limit of accuracy. This important fact is concealed if an index is used, such as that in which variation is divided by the mean heart rate. Averaging the pulse rate over epochs of l/16 min revealed an epochal variation which was substantially greater and in which changes were usefully detected. Secondly, some data reduction was necessary to provide a data series based on unitary time intervals, which made possible the use of a digital filter. Thirdly, data reduction decreased the storage area required and the time for analysis by a factor of 8-9 and, with 960 samples an hour, more closely matched a high resolution visual display (e.g. 800 pixels on the X-axis), or for printing an hour’s data (‘an eye-full’) on A4 paper. We would ask if it is possible to obtain agreement by program designers on this feature of the system, data reduction over l/16 min, with storage in 2 bytes. This system has proved valuable for data transfer within our own institution, with Visser in Holland, Patrick in Canada and our colleagues in Italy. It has worked well antenatally. It could provide a common vehicle to attack the more difficult problem of analysis in labour. It could lead to a common strategy of FHR analysis. Acknowledgements
We thank the Medical Research Council, The Department of Health and Birthright for their support. We also thank our midwives for collecting good quality records, and for their continued help. References 1 2
Wickham PJD, Dawes GS and Belcher R: Development of methods for quantitative analysis of the fetal heart rate, JBiomedEng, 5 (1983) 302-308. Henson GW, Dawes GS and Rednian CWG: Antenatal fetal heart rate variability in relation to fetal acid-base status at caesarean section, Br JObstet Gynaecof, 90 (1983) 516-512.
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294 3 4
Henson GL, Dawes GS and Redman CWG: Characterization of the reduced heart rate variation in growth-retarded fetuses, Br JObstet Gynaecol, 91 (1984) 751-755. Smith JH, Anand KJS, Cotes PM, Dawes GS, Harkness RA, Howlett TA, Rees LH and Redman CWG: Antenatal fetal heart rate variation in relation to the respiratory and metabolic status of the compromised human fetus, BrJ Obstet Gynaecol, 95 (1988) 980-989. Dawes GS, Redman CWG and Smith J: Improvements in the registration and analysis of fetal heart rate records at the bedside, Br J Obstet Gynaecol, 92 (1985) 3 17-325. Smith J, Dawes GS and Redman CWG: Low fetal heart rate variation in normal pregnancy, Br J Obstet Gynaecol, 94 (1987) 656-664.
Dawes GS, Houghton CRS, Redman CWG and Visser GHA: Pattern of the normal human fetal heart rate, Br J Obstet Gynaecol, 89 (1982) 276-284. Dawes GS, Houghton CRS and Redman CWG: Baseline in human fetal heart rate records, Br J Obstet Gynaecol, 89 (1982) 270-275.
Dawes GS, Moulden M and Redman CWG: The incidence of false or doubtful decelerations in antenatal fetal heart rate traces, Am JObstet Gynecol, 162 (1990) 170-173. Lawson GW, Belcher R, Dawes GS and Redman CWG: A comparison of a ultrasound (with autocorrelation) and direct electrocardiogram fetal heart rate detector systems, Am J Obstet Gynecof, 147 (1983) 721-722.
Lawson G, Dawes GS and Redman CWG: A comparison of two fetal heart rate ultrasound detector systems, Am J Obstet Gynecol, 143 (7) (1982) 840-842.