Objective
We sought to evaluate the accuracy of a novel system for measuring fetal heart rate (FHR) and ST-segment changes using noninvasive electrodes on the maternal abdomen.
Study Design
Fetal electrocardiograms were recorded using abdominal sensors from 32 term laboring women who had a fetal scalp electrode (FSE) placed for a clinical indication.
Results
Good-quality data for FHR estimation were available in 91.2% of the FSE segments and 89.9% of the abdominal electrode segments. The root mean square error between the FHR data calculated by both methods over all processed segments was 0.36 beats per minute. ST deviation from the isoelectric point ranged from 0–14.2% of R-wave amplitude. The root mean square error between the ST change calculated by both methods averaged over all processed segments was 3.2%.
Conclusion
FHR and ST change acquired from the maternal abdomen is highly accurate and, on average, is clinically indistinguishable from FHR and ST change calculated using FSE data.
Continuous fetal heart rate (FHR) monitoring during labor is utilized in >85% of labor episodes in the United States and represents the standard of care, although there is scant evidence to demonstrate that the use of the technology improves newborn or maternal outcomes. Encouraging data demonstrate that intrapartum fetal electrocardiogram (fECG) analysis can reduce newborn acidemia, hypoxic ischemic encephalopathy, and cesarean deliveries. However, the only clinically available device for fECG analysis—the STAN monitor from Neoventa (Moindal, Sweden)—requires an invasive fetal scalp electrode (FSE), limiting its use to a subset of pregnant women who are laboring with ruptured membranes and a dilated cervix.
The potential utility of noninvasive fECG for fetal evaluation is significant. However, to date, there has been no systematic study proving that fECG can be extracted noninvasively without distorting important clinical parameters, such as the ST segment. Prior reports have shown the capacity to measure FHR using electrodes on the maternal abdomen, but none have demonstrated the capacity to accurately record the fECG waveform with sufficient fidelity to evaluate the morphology. Recently, we developed a novel real-time signal processing approach for extracting fECG that mitigates many of the issues involved in extracting an accurate and clinically relevant electrocardiogram (ECG). This study evaluates the performance of our technique in extracting FHR variations and ST levels from laboring patients and compares them to invasive scalp electrode data.
Materials and Methods
We recorded data from 32 term laboring women who had an FSE placed for a clinical indication and consented to participate in this study. Enrollment occurred sequentially, and there were no exclusion criteria. The study was conducted during the first and second stages of labor. Demographic information about the studied subjects is summarized in the Table .
| Demographics (n = 32) | Median | Range |
|---|---|---|
| Maternal age at delivery, y | 32 | 19–40 |
| Gestational age at delivery, wk | 40 | 35–41 |
| Nonwhite mother, % | 41 | |
| Multiparous, % | 47 | |
| Birthweight, g | 3459 | 1840–4110 |
| Female baby, % | 50 | |
| 1-min Apgar score | 8 | 2–9 |
| 5-min Apgar score | 9 | 8–9 |
| Body mass index | 30.4 | 21.7–45.1 |
| Epidural during study, % | 97 | |
| Pushing during study, % | 9.4 |
Data were recorded using an E-TROLZ (North Andover, MA) physiologic monitoring platform, which samples 32 channels at 1 kHz. Standard gel-adhesive ECG electrodes (Red-Dot; 3M, St. Paul, MN) were used in a standard configuration developed to maximize the chances of having electrodes adjacent to the fetal heart. The general configuration of the abdominal electrodes is illustrated in Figure 1 . Electrodes were placed based on anatomic landmarks (the umbilicus, xyphoid process, pubic symphysis, axilla, and spine are used to locate electrodes), and as a consequence, the distance between electrodes varied with the maternal abdominal girth. The number of electrodes was arbitrarily chosen based on the capacity of the recording device, and allows for excellent coverage of the maternal abdomen, sides, and back. The specific location of the electrodes is unimportant, as the analysis is done based on the physiologic signal alone, without consideration to the location of each electrode. No patient skin preparation was done prior to electrode placement. Both the abdominal ECG data and the scalp ECG were preprocessed for removal of interference from maternal ECG (mECG), as well as power line contamination and other sources of background electrical noise, including maternal muscle artifact.
