Continuous electronic fetal heart rate monitoring (EFM) is used in the vast majority of all labors in the United States. With the use of EFM categories and definitions from the American College of Obstetricians and Gynecologists, the National Institutes of Health, and the Society for Maternal-Fetal Medicine, clinicians can now better define and communicate tracing assessments. Except for reducing neonatal seizure activity, however, EFM use during labor has not been demonstrated to significantly improve fetal and neonatal outcomes, yet EFM is associated with an increase in cesarean deliveries and instrument-assisted vaginal births.1
The negative predictive value of EFM for fetal hypoxia/acidosis is high, but its positive predictive value is only 30%, and the false-positive rate is as high as 60%.2 Although a false-positive assessment may result in a potentially unnecessary operative vaginal or cesarean delivery, a falsely reassuring strip may produce devastating consequences in the newborn and, not infrequently, medical malpractice liability. One etiology associated with falsely reassuring assessments is that of EFM monitoring of the maternal heart rate and the failure to recognize the tracing as maternal.
In this article, I discuss the mechanisms and periods of labor that often are associated with the maternal heart rate masquerading as the fetal heart rate. I review common EFM patterns associated with the maternal heart rate so as to aid in recognizing the maternal heart rate. In addition, I provide 3 case scenarios that illustrate the simple yet critical steps that clinicians can take to remedy the situation. Being aware of the potential for a maternal heart rate recording, investigating the EFM signals, and correcting the monitoring can help prevent significant morbidity.
CASE 1 EFM shows seesaw decelerations and returns to baseline rate
A 29-year-old woman (G3P2) at 39 weeks’ gestation was admitted to the hospital with spontaneous labor. Continuous EFM external monitoring was initiated. After membranes spontaneously ruptured at 4 cm dilation, an epidural was placed. Throughout the active phase of labor, the fetus demonstrated intermittent mild variable decelerations, and the fetal heart rate baseline increased to 180 beats per minute (BPM). With complete dilation, the patient initiated pushing. During the first several pushes, the EFM demonstrated an initial heart rate deceleration, and a loss of signal, but the heart rate returned to a baseline rate of 150 BPM. With the patient’s continued pushing efforts, the EFM baseline increased to 180 BPM, with evidence of variable decelerations to a nadir of 120 BPM, although with some signal gaps (FIGURE 1, red arrow). The tracing then appeared to have a baseline of 120 BPM with variability or accelerations (FIGURE 1, green arrow) before shifting again to 170 to 180 BPM.
What was happening?
Why does the EFM record the maternal heart rate?
Most commonly, EFM recording of the maternal heart rate occurs during the second stage of labor. Early in labor, the normal fetal heart rate (110–160 BPM) typically exceeds the basal maternal heart rate. However, in the presence of chorioamnionitis and maternal fever or with the stress of maternal pushing, the maternal heart rate frequently approaches or exceeds that of the fetal heart rate. The maximum maternal heart rate can be estimated as 220 BPM minus the maternal age. Thus, the heart rate in a 20-year-old gravida may reach rates of 160 to 180 BPM, equivalent to 80% to 90% of her maximum heart rate during second-stage pushing.
The external Doppler fetal monitor, having a somewhat narrow acoustic window, may lose the focus on the fetal heart as a result of descent of the baby, the abdominal shape-altering effect of uterine contractions, and the patient’s pushing. During the second stage, the EFM may record the maternal heart rate from the uterine arteries. Although some clinicians claim to differentiate the maternal from the fetal heart rate by the “whooshing” maternal uterine artery signal as compared with the “thumping” fetal heart rate signal, this auditory assessment is unproven and likely unreliable.
CASE 1 Problem recognized and addressed
In this case, the obstetrician recognized that “slipping” from the fetal to the maternal heart rate recording occurred with the onset of maternal pushing. After the pushing ceased, the maternal heart rate slipped back to the fetal heart rate. With the next several contractions, only the maternal heart rate was recorded. A fetal scalp electrode was then placed, and fetal variable decelerations were recognized. In view of the category II EFM recording, a vacuum procedure was performed from +3 station and a female infant was delivered. She had Apgar scores of 6 and 8 at 1 and 5 minutes, respectively, and she did well in the nursery.