Computerized Analysis of Antepartum Cardiotocography: A Review

Abstract

Cardiotocography measures the human fetal heart rate and uterine activity using ultrasound. While it has been a mainstay in antepartum care since the 1960s, cardiotocograms consist of complex signals that have proven difficult for clinicians to interpret accurately and as such clinical inference is often difficult and unreliable. Previous attempts at codifying approaches to analyzing the features within these signals have failed to demonstrate reliability or gain sufficient traction. Since the early 1990s, the Dawes-Redman system of automated computer analysis of cardiotocography signals has enabled robust analysis of cardiotocographic signal features, employing empirically-derived criteria for assessing fetal wellbeing in the antepartum. Over the past 30 years, the Dawes-Redman system has been iteratively updated, now incorporating analyses from over 100,000 pregnancies. In this review, we examine the history of cardiotocography, signal processing methodologies and feature identification, the development of the Dawes-Redman system, and its clinical applications.

Keywords: Antepartum; Cardiotocography; Computerized; Electronic fetal monitoring; Fetal monitoring.

FIGURE 1

Fitting a baseline using the Dawes-Redman method (Table 1). The most frequently occurring pulse interval is 427 ms (∼140 bpm, orange arrow) and occurs in 6.08% of all pulses in the first ten minutes. The frequency of the lower five pulse intervals is less than the frequency of the most frequently occurring pulse interval (seven, green line). The PI sits in the lower 87.5% of the frequency distribution of all pulse intervals. Therefore, the parameters of the bandpass filter applied to this CTG recording would be 427 ± 60ms, or (367,487), corresponding to a FHR range of 123–163 bpm. This is then iteratively updated as the recording progresses. bpm: Beats per minute; CTG: Cardiotocography; FHR: Fetal heart rate; PI: Pulse interval.

FIGURE 2

An example of a complex antepartum CTG signal containing clustered accelerations and decelerations. This signal contains large decelerations, some with >20 lost beats per minute or lasting up to one minute. These patterns were identified with the Dawes-Redman computerized CTG system after baseline fitting. This CTG was performed on a singleton pregnancy at 35 weeks gestation and the fetus was delivered at 40 weeks without any adverse outcomes. bpm: Beats per minute; CTG: Cardiotocography; FHR: Fetal heart rate.

FIGURE 3

Episodes of high variation in a fetus at 37 gestational weeks. The short-term variation was 13.35 ms and the long-term variation 70.60 ms. This record contains three accelerations, no decelerations, and no episodes of low variability. This fetus had normal outcomes. bpm: Beats per minute; CTG: Cardiotocography; FHR: Fetal heart rate.

FIGURE 4

Comparison of a normal antepartum CTG pattern (blue) and a sinusoidal pattern (red) of two fetuses at 39 gestational weeks. The sinusoidal pattern is smooth, regular, and oscillates between 3 and 4 cycles (high frequency sinusoidal pattern) per minute with an amplitude between 5 and 15 bpm around the baseline heart rate. There is reduced baseline variability and no accelerations are present. This baby was acidaemic (pH = 7.07), had low Apgar scores, and required intubation. bpm: Beats per minute; CTG: Cardiotocography.