G. S. Dawes

Pattern of the normal human fetal heart rate

Summary. With an improved method for fitting baselines to human fetal heart‐rate traces, the patterns of episodic variations, accelerations and decelerations were similar in 215 64‐min records from normal pregnancies and in 95 with mild hypertension and normal outcome. The change in signal loss with gestational age, by Doppler ultrasound for recording heart rate, was entirely due to the greater loss in episodes of high heart‐rate variation. The changes in the numbers and sizes of accelerations and decelerations with gestational age were described. There were many records which had only one or no acceleration at 28–33 weeks gestation (16.2%) or 34–41 weeks (7.3%). However, only two (0.7%) had episodes of high heart‐rate variation lasting <10 min from 28 weeks onwards. The presence of these episodes, with clusters of fetal movements, is therefore likely to be a better numerical index of normality.

Baseline in human fetal heart-rate records.

Summary. Large variations in the pattern of normal fetal heart‐rate traces near term are described. The presence of flat or bimodal frequency distributions of pulse intervals in these traces complicates the derivation of a baseline by statistical methods. A system is described for deriving an appropriate baseline from which accelerations or decelerations may be measured.

A randomised controlled comparison of betamethasone with dexamethasone: effects on the antenatal fetal heart rate

Objective To compare the effects of maternal administration of betamethasone and dexamethasone on fetal heart rate, using computerised numerical analyses, and to examine the association between changes in short term variation and the timing and indication for delivery.

Characterization of the reduced heart rate variation in growth-retarded fetuses.

Summary. Fetal heart rate (FHR) variation has been studied by computerized numerical analysis in 20 growth‐retarded fetuses and 20 normal fetuses matched for gestational age. FHR variation was significantly reduced in the 14 growth‐retarded fetuses where there was clinical evidence of associated pathology. Rest ‐ activity cycles were assessed by changes in FHR variation and fetal movements. The growth‐retarded fetuses with reduced FHR variation showed the same pattern of rest and activity as normal fetuses but the changes in FHR variation were of lower amplitude. This was observed even in the subgroup of six fetuses with the lowest FHR variation. Thus the unreactive FHR patterns associated with growth retardation do not arise because the fetus spends less time in activity.

Antenatal fetal heart‐rate variability in relation to fetal acid‐base status at caesarean section

Summary. A quantitative comparison was made of fetal heart‐rate (FHR) traces obtained shortly before delivery, and of umbilical artery blood gas values, in 49 women delivered by caesarean section for reasons other than fetal compromise and in 23 women delivered by section for antenatal fetal compromise judged from visual inspection of conventional FHR recordings. The FHR traces in the compromised group of fetuses were abnormal, with low variability and a high incidence of decelerations, but there was no significant difference from the normal group in metabolic acidaemia, as judged by the base excess of umbilical artery blood. The compromised group consisted almost exclusively of infants small‐for‐gestational age. These results suggest that the FHR pattern of the compromised fetuses is not the direct result of intrauterine asphyxia but of some other factor associated with growth retardation.

Criteria for the design of fetal heart rate analysis systems.

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.

Limitations of antenatal fetal heart rate monitors

Erroneous or doubtful decelerations in fetal heart traces were present in 111 of 1000 consecutive antenatal clinical records obtained by monitors with autocorrelation. The incidence was 20% in fetuses less than 30 weeks of gestational age. Their elimination reduced the number of decelerative records by 42%. Erroneous or doubtful accelerations were also present in 11% of records. These errors are caused by the fetal heart rate monitor and may contribute to the high intraobserver and interobserver variation on visual analysis. They can be detected by computer analysis.

Antenatal cardiotocogram quality and interpretation using computers

To test the application in practice of computerized fetal heart rate (FHR) analysis in pregnancy.

Low human fetal heart rate variation in normal pregnancy

The fetal heart rates of 340 normal singleton pregnancies at 30–33 weeks gestation were screened using a microprocessor system on‐line. Eleven fetuses (3·2%) with a heart rate variation less than the 5th centile were identified, of whom 10 were studied longitudinally. At 30–33 weeks the mean minute range of pulse intervals (a measure of fetal heart rate variation) was 314 (SE 1·5) ms compared with 51·0 (SE 3·4) ms in a randomly selected control group. The study group continued to have significantly lower fetal heart rate variation than controls on each of three subsequent occasions until delivery. There were no significant differences between the two groups in fetal outcome, which was good. This demonstrates that a small proportion of normal fetuses have consistently low heart rate variation, and helps to define the lower limit of the normal distribution of fetal heart rate variation. After delivery, there were no significant differences between heart rate or its variation between the two groups. We conclude that the lower prenatal heart rate variation in the study group was a consequence of the uterine environment.

Computerized analysis of episodic changes in fetal heart rate variation in early labor

Fetal heart rate variation in early labor was measured by computerized analysis in cyclic episodes of low or high variation in 136 women at 37 to 42 weeks gestation. The amplitude (mean +/- SE) in episodes of low variation was 20.6 +/- 0.4 milliseconds; in high variation it was 57.3 +/- 1.1 milliseconds. The duration (mean +/- SE) of low episodes (24.3 +/- 1.3 minutes) was less than that of high episodes (45.1 +/- 2.7 minutes) but was sometimes greater than 1 hour. In episodes of low variation the amplitude was less than 5 beats/min long term in 11% and less than 2.5 milliseconds (pulse intervals) short term in 8%; these measures would be predictive of intrauterine death if persistent before birth. We conclude that the use of decreased fetal heart rate variation as a diagnostic sign of acute fetal hypoxemia in labor is incorrect, because changes of this size occur randomly as a consequence of fetal sleep states. There was no significant correlation between fetal heart rate variation over the last hour of labor and umbilical arterial base deficit on delivery.