Cardiotocography (CTG), a screening test for fetal hypoxia during labor, is a continuous graphic record of fetal heart rate (FHR) and uterine contractions. During the second stage of labor, if the abdominal transducer is moved downward to improve the signal quality of the CTG trace, the transducer can pick up sound waves reflected by movements of maternal vessels. This results in recording of maternal heart rate (MHR), which may mimic the FHR and lead to an adverse outcome because the fetus is not monitored during the time of hypoxic stress. Cardiotocography relies on pattern recognition; therefore, clinicians must have a thorough understanding of the features of MHR that differentiate it from FHR during labor.
An external ultrasound transducer is 1 method for recording the FHR, but if it is not placed over the fetal heart, it can record the MHR. Internal recording of the FHR using a scalp electrode is a more accurate method. Fetal electrocardiogram waveforms are obtained by attaching a fetal scalp electrode as well as another reference electrode on the mother’s thigh. A typical CTG records uterine contractions and the baseline rate, baseline variability, accelerations, and decelerations of the FHR. During uterine contractions, the FHR shows decelerations, but the MHR shows accelerations, reflecting a transient contraction related to increased cardiac output.
This retrospective observational study was undertaken to identify the incidence of heart rate accelerations coinciding with uterine contractions that could be maternal during 60 minutes before delivery when monitored with external ultrasound or by fetal scalp electrode.
Data were collected on 100 women whose fetuses had continuous electronic FHR recording for 1 hour before delivery. Fifty CTG recordings each were obtained from an internal scalp electrode and an external abdominal Doppler. Data on uterine contractions were considered to identify accelerations and decelerations in relation to contractions. The time that CTG ended was noted along with the time of delivery. Cardiotocography traces were examined for the presence of accelerations and decelerations and analyzed to determine whether they coincided with uterine contractions. The quality of the CTG recording was also assessed. All women delivered by spontaneous or instrumental vaginal delivery.
Decelerations were the most common observation with external (71.9%) and internal (89.1%) methods of FHR recording. More poor-quality recordings were obtained with the external transducer. In the 60 minutes before delivery, accelerations were seen in 28.1% and 10.9% of external and internal FHR recordings, respectively. Accelerations coinciding with uterine contractions occurred in less than 50% of these cases. Accelerations coinciding with contractions were more commonly observed when monitored with the external device (11.7%) compared with internal monitoring (4.0%).
These results confirm that FHR accelerations are more common with external FHR monitoring than internal monitoring using a scalp electrode. Clinicians should consider the possibility of erroneous recording of MHR if accelerations coincide with uterine contractions during the second stage of labor. The likelihood of observing accelerations coinciding with uterine contractions is less than 50% using internal FHR monitoring compared with external monitoring. Absence of p-wave on the electrocardiogram waveform or simultaneous use of maternal pulse oximetry to record MHR may help clinicians exclude MHR monitoring.