Original Article
Fetal heart rate in relation to its variation in normal and growth retarded fetuses

https://doi.org/10.1016/S0301-2115(99)00162-1Get rights and content

Abstract

Objectives: (1) to assess the relationship of basal fetal heart rate (FHR) with both long term (LTV) and short term (STV) FHR variation in low-risk pregnancies, longitudinally from 24 weeks gestation onwards and (2) to investigate the relationship of FHR with LTV and STV in intrauterine growth retarded (IUGR) fetuses. Study design: Computerised FHR recordings were made in twenty-nine uncomplicated pregnancies (n=224) and in twenty-seven IUGR fetuses who were selected retrospectively from three databases (n=135). Nomograms of FHR variation with FHR and GA were constructed using multilevel analysis. Results and conclusions: There was a strong negative relationship of FHR with both LTV and STV in the control group (R2=53% and 52%, respectively). In the IUGR fetuses, FHR was generally higher than in normal fetuses whereas LTV and STV were lower. The relationship of FHR with LTV and STV in the IUGR group was less strong (for both: R2=18%). Correction of FHR variation for basal FHR in the IUGR fetuses only resulted in a slight reduction in the number of recordings with a variation below the normal range. As it does not improve the recognition of fetuses being considered at the highest risk, such a correction of FHR variation for basal FHR is therefore not necessary. Intrafetal consistency, known to be present in healthy fetuses, was also present in the IUGR fetuses with a low FHR variation.

Introduction

Antenatal fetal heart rate (FHR) monitoring is widely used to assess the fetal condition. FHR variability is known to depend on several factors such as gestational age (GA), basal FHR, hypoxia, fetal behavioural states, cord compression, and the use of medication and social drugs (alcohol, smoking) [1]. Both long term (LTV) and short term (STV) FHR variation have a negative relationship with FHR. FHR variation increases with decreasing FHR and vice versa. Moreover, FHR variation increases with advancing GA [1], [2].

In intrauterine growth retarded (IUGR) fetuses, LTV is about 25% lower than in age-matched appropriately grown fetuses [3]. FHR variation gradually decreases with progressive compromise of the fetal condition [4]. This reduced FHR variation is not caused by a change in the rest-activity cycles [5]. However, it coincides with an increase in basal heart rate [4]. Usually around the same time that FHR variation falls below the norm, decelerations emerge [4], [6]. Ribbert et al. [7] found LTV in IUGR fetuses to be reduced towards the lower level of the normal range about ten days before Caesarean Section (CS) was undertaken for ‘fetal distress’. FHR variation remained fairly constant for some time before a further reduction occurred [7]. Decreased FHR variation and FHR decelerations have been found to be associated with hypoxaemia at CS and at cordocentesis [6], [8], [9], [10]. Abnormal blood flow velocity waveforms from the umbilical artery are present in the majority of fetuses long before the occurrence of fetal distress [7], [11]. The understanding of these temporal changes is important for the management of the IUGR fetus, especially with respect to the timing of delivery [12].

The negative relationship between basal heart rate and its variation in healthy fetuses, and the concomitant changes in rate (up) and variation (down) in IUGR fetuses, suggest that correction for rate may better identify those fetuses who have a reduced FHR variability irrespective of their basal heart rate. By studying FHR in relation to its variation, insight into changes in variation will improve.

The aims of the present study were: (1) to assess the relationship between FHR and FHR variation (LTV and STV) in low-risk pregnancies longitudinally from 24 weeks gestation onwards; and (2) to investigate the same relationship in the IUGR fetus.

Section snippets

Subjects and methods

FHR was monitored longitudinally in 29 singleton fetuses of healthy pregnant women, of which eleven were nulliparous. From 24 weeks GA onwards one hour recordings were made at two-weekly intervals. From 36 weeks till delivery, two hour recordings were made on a weekly basis, but only the first hour was used for the present analysis. The recordings took place at the Leicester General Hospital, Leicester, UK. All women gave their written informed consent. All pregnancies were uncomplicated, and

Results

We analysed a total of 224 recordings from the 29 fetuses of the control group, after exclusion of 14 recordings with a signal loss >30%. FHR was significantly correlated with both LTV and STV. The following models were derived; for LTV: y=−1.15x+208.7 (R=0.73; p<0.0001, n=224) and for STV: y=−0.21x+37.7 (R=0.72; p <0.0001, n=224), where y is LTV (ms) or STV (ms) and x is basal FHR (bpm)).

Multilevel analysis confirmed this strong relationship between FHR and both LTV and STV (p<0.0001) during

Comment

We have previously shown that FHR variation is negatively correlated with basal heart rate in healthy fetuses, both during ‘non-reactive’ FHR patterns (pattern A) and ‘reactive’ FHR patterns (pattern B) [20]. In the present study this was confirmed when the whole one hour recordings were considered. The relationship was strong and about 50% of the differences in FHR variation could be explained by differences in rate (R2=53% for LTV and R2=52% for STV, respectively). This implies that basal

Acknowledgments

The authors like to thank L.S.M. Ribbert, MD PhD (Department of Obstetrics and Gynaecology, St. Antonius Hospital Nieuwegein, The Netherlands), S.V. Koenen, MD and R.H. Stigter, MD (Department of Obstetrics and Gynaecology, University Hospital Utrecht, The Netherlands) for the use of their data, and P. Westers, PhD (Centre for Biostatistics, Utrecht University, The Netherlands) for his statistical advice.

This study was supported by a grant from the Commission of the European Communities

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