Fetal ECG waveform – helps differentiating maternal from fetal heart rate

The objective of this retrospective study was to investigate the incidence of heart rate accelerations coinciding with contractions during the second stage of labour when monitored either with abdominal ultrasound (external) or scalp electrode (internal).

Fifty external and fifty internal recordings were obtained from spontaneous or instrumental vaginal deliveries continuously monitored for one hour or more prior to delivery during the second half of 2007.

The assessment of the recordings showed that decelerations were the most common observation for both external and internal recordings, found in 71.9% and 89.1% of recordings respectively. Accelerations were observed in 28.1 % of external and 10.9% of internal recordings. In 11.7 % of the external and in 4% of the internal recording with accelerations, these coincided with contractions.

The findings confirm that fetal heart rate accelerations are more common with external than with internal monitoring, but that they occur in both groups. However, it is more likely to inadvertently record a maternal heart rate as fetal when using an external transducer. The likelihood of observing accelerations coinciding with contractions is less than half when a scalp electrode for internal monitoring is used.

The assessment of the recordings also confirmed that the absence of a p-wave in the ECG waveform helps to differentiate the maternal from the fetal heart rate. When recording the fetal ECG using a scalp electrode and skin electrode placed on the mother’s thigh, it is only the fetal ECG signal that has sufficient strength to show the p-wave. If the maternal heart rate, i.e. ECG, is recorded the p-wave will not be visible in the recording.

This illustration is not from Nurani et al. but shows the identification of a p-wave in the fetal ECG recording.

Abstract – Nurani et al. 2012

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