G.S. Dawes

Diurnal, respiratory, and other rhythms of fetal heart rate in lambs

In lambs during the last third of gestation fetal heart periods were measured to 0.2 msec. and the variation was calculated with a computer in real time. The rate was subject to low-frequency variations in association with breathing movements, with a 2 to 4 hour rhythm of large amplitude and unknown origin, with the time of day, and with increasing gestational age. The calculated standard deviation of heart period bore a semilogarithmic relation to increasing length of observational epoch. There were also multiple rhythms in heart rate variability. The immeidate effect of fetal hypoxia was to increase heart rate variability despite the arrest of fetal breathing movements. The analysis of heart rate variability is examined critically.

Numerical analysis of the human fetal heart rate: The quality of ultrasound records

A method is described for the computerized numerical analysis of fetal heart periods (pulse intervals). It uses a digital filter to separate the record into its high- and low-frequency components and, after removal of baseline variation, identifies accelerations and decelerations of all sizes. It provides an objective method for separating episodes of high heart period variation, normally associated with fetal movements, from episodes of low variation. When Doppler ultrasound is used in the last 10 weeks of gestation, failure time averages 40%. Signal loss is not randomly distributed; it is on average 75% greater during episodes of high heart period variation, although it is not particularly associated with fetal movements as identified by nurse or patient. Nevertheless a comparison of simultaneous direct ECG and ultrasound records shows that the latter provide reasonable statistical measures of heart period variation, and also of accelerations and decelerations provided that signal loss is taken into account. The system thus provides a particularly useful adjunct to the analysis of antenatal human fetal heart rate records.

Diurnal and other cyclic variations in human fetal heart rate near term.

In 10 healthy women near term, fetal heart rate, as monitored from an abdominal electrocardiogram, and its variation were analyzed over 24 hours. During the daytime, this was combined with real-time scanning for fetal breathing and movements. There was an episodic change in high and low pulse (R-R) interval variation with a mean cycle length of 92 minutes. The changes in cycle length were large; around midnight, episodes of high variation lasted up to 6 hours. Episodes of high variation coincided with greater fetal movement (measured only in daytime). Neither low nor high episodes were consistently related to fetal breathing. Both fetal heart rate and its variability showed a diurnal variation, by 11% and 50%, respectively; these were not statistically related. There was a particularly large change in pulse interval variation with peak values around midnight; concomitantly, there was a diurnal variation in the incidence of accelerations of defined size. The implications of these findings are discussed.

System 8000: computerized antenatal FHR analysis.

SYSTEM 8000 is a computerized system for antenatal fetal heart rate (FHR) analysis, with interaction online to ensure good quality recording and to minimize the time required to obtain the necessary information (based on fetal movements and tocodynamometer readings as well as FHR). The equipment consists of a Personal Computer with hard disk, interfaced to a fetal monitor. Software is written in C. An extensive definition is given on most of the functions and parameters as calculated by the system, e.g. record quality, uterine contraction peaks, basal heart rate, variation, decelerations and accelerations. System 8000 is designed to take account of the episodic changes in FHR and fetal movements characteristic of sleep states. Their presence naturally affects the mean FHR variation calculated over, say 20-30 mins. But, as the amplitude of these episodes is attenuated in association with growth retardation and hypoxaemia, the measurements of variation decline. In practice inter- and intra-observer variation is greatest in assessing FHR variation. Yet a decrease in variation is the most reliable index of fetal deterioration. The system measures FHR variation accurately and reproducibly, as well as fetal movements. In the synoptic display these two equally important fetal variables are given their rightful prominence. It has been interesting to observe how rarely the basal FHR changes in fetuses suffering progressive respiratory insufficiency, and how extreme tachycardia (a basal rate greater than 170/min) tends to reflect other fetal problems such as infection or maternal pyrexia.

Short-term variation In abnormal antenatal fetal heart rate records

In a retrospective study the relation of reduced fetal heart rate variation to fetal acidemia was analyzed with a computerized system for numeric analysis. Between 1983 and 1987, 78 pregnancies were identified in which at least one record of the fetal heart rate had very low long-term variation. The outcome was analyzed to determine the numeric criteria of fetal heart rate variation that most efficiently detect a fetus that will die (preterminal) or is dying (terminal). Because fetal compromise was found on occasion to be associated with a slow sinusoidal fetal heart rate rhythm that increased measures of long-term variation. It was necessary to define a new index of short-term fetal heart rate variation (the 1/16 minute epoch-epoch variation). This was closely related to long-term variation (r = 0.9) but provided better detection of preterminal records as judged by metabolic acidemia at delivery or intrauterine death.

Improvements in the registration and analysis of fetal heart rate records at the bedside

Summary. A microprocessor system is described for on‐line analysis of the fetal heart rate detected by conventional Doppler systems. A brief account is given of the instrumentation and program structure. The system has been tested by analysing normal and abnormal antenatal fetal heart rate records. Pulse Doppler with autocorrelation measurement of the fetal pulse interval reduced the signal loss in clinical practice by a factor of 10, to an average of 2.1% in 629 records from uncomplicated pregnancies. Yet it is still necessary to identify signal loss, because it occasionally rises t o unacceptable levels in association with fetal movements or hiccups. Medium‐term measures of fetal heart rate variation (within 16–0.1 cycles/min) varied with gestational age, but were a better index of fetal well‐being than longer‐term measures. The application of the system t o fetal monitoring is illustrated.

NUMERICAL ANALYSIS OF THE NORMAL HUMAN ANTENATAL FETAL HEART RATE

A numerical method for separation of the frequency components of the fetal heart rate, and for identifying episodes of high or low variation, was applied to 196 64‐minute ultrasound records in normal pregnancies during the last half of gestation. Most variables of heart period variation increased with gestation, as did the incidence of accelerations. Before 35 weeks, accelerations of ≥14 beats/minute did not occur in all records. Cyclic episodes of low and high variation (‘unreactive’ and‘reactive’ episodes), associated with rest‐activity cycles, could be identified from 27 weeks onwards. After 36 weeks gestation the length of low variation episodes increased and the variation during these episodes fell. Near term, low variation episodes lasted up to 40 minutes. It is concluded that on scrutiny of fetal heart rate records for evidence of normality, or otherwise, due account should be taken of gestational age.

Measurement of fetal forelimb movements in the lamb in utero

Forelimb movements in the unanesthetized fetal lamb in utero were measured by means of transit-time ultrasound in conjunction with triceps and biceps electromyographic activity. The relationships between forelimb movements and electrocortical activity, hypoxia, and spontaneous labor were studied. There was no evidence of diurnal variation. The amount of time spent moving in low-voltage electrocortical activity with rapid eye movements was significantly less than in other electrocortical states. During acute hypoxia, forelimb movements were much reduced. In active labor near term, the fetal electrocorticogram continued to cycle between high- and low-voltage activity, and the number of movements was reduced; movements were present only during uterine contractions, provided that the fetus was not in low-voltage electrocortical activity with rapid eye movements.

The umbilical circulation

T H E umbilical circulation through the fetal side of the placenta constitutes, for the fetus, a system of the greatest importance, not only because the placenta is the organ of gaseous exchange, but because it is also the single most important determinant of systemic vascular resistance within the fetal circuIation. The purpose of this articIe is to present the evidence for this view and to illustrate some of the interesting consequences which flow from it.

Antenatal fetal heart rate variation in relation to the respiratory and metabolic status of the compromised human fetus

Summary. Three groups of women were delivered by caesarean section before labour: for an abnormal fetal heart rate (FHR) trace (21 cases, group 1), or for maternal deterioration in severe pre‐eclampsia without gross fetal heart rate abnormalities (20 cases, group 2), or to avoid mechanical difficulties in labour at term (30 cases, group 3). The mean gestational ages of the first two groups were 32 weeks with a high proportion of infants small‐for‐gestational‐age. In group 1, FHR variation (mean range of pulse intervals) was less than half (20·6 SE 1·2 ms) of the normal value at the same age (44·4 SE 1·5 ms). This was associated with hypoxaemia (mean umbilical artery Po2 of 6 mmHg at delivery), with evidence of compensation shown by an elevated amniotic fluid erythropoietin. The fetuses were hypoglycaemic and had greater umbilical artery blood alanine concentrations, but no large changes in adenine nucleotide or endorphin plasma concentrations. Although there was a minor degree of respiratory acidaemia at birth, there was not significant metabolic acidaemia. The results demonstrate that the reduced variation of ‘suboptimal’ and ‘decelerative’ fetal heart rate records is associated with fetal hypoxaemia and evidence of nutritional deprivation, but not with asphyxia.