Fetal heart rate (FHR) monitoring is a widely accepted means of evaluating the status of the fetus during labor. There have been many sporadic reports of misinterpretation of the tracings when the maternal heart rate and not the FHR had been recorded, but the tracing was taken as being that of the FHR. 1–9 Although the maternal electrocardiogram (ECG) signal is approximately 5–10 times stronger than the fetal one, its amplitude is significantly diminished when it is recorded through a fetal scalp electrode, and it is “ignored” in favor of the stronger fetal ECG. However, in cases of fetal death, the maternal ECG signal may be amplified through the fetal scalp electrode and be displayed spuriously as the FHR. 1–3 The possibility of recording the maternal heart rate by an external (ultrasonic) FHR probe has also been reported when the probe was placed over the maternal aorta or great iliac vessels. 6,9–12 Maternal heart rate tracings consistent with fetal bradycardia and/or decelerations of FHR may lead to the performance of an unnecessary emergency cesarean delivery. 5–8 Conversely, maternal tachycardia or accelerations in heart rate 2,4,5 may be falsely reassuring in cases of fetal distress.
Very little is known about maternal heart rate patterns during labor. The purpose of this study was to evaluate maternal heart rate tracings during labor and delivery as recorded by an electronic fetal monitor and to better differentiate them from FHR patterns.
MATERIALS AND METHODS
Thirty healthy women with singleton uncomplicated term pregnancies who were admitted in spontaneous early labor (cervical dilatation less than 3 cm) and fetal vertex presentation between October 1, 1998, and November 30, 1998, were enrolled. They all volunteered and signed informed consent to participate in this prospective study, which was approved by the Institutional Review Board. Exclusion criteria included a past or present cardiovascular or hypertensive disorder, oral temperature greater than or equal to 37.5C or suspected infection, hemoglobin concentration less than 10 gm/dL, a suspect FHR on admission, or any condition in which vaginal delivery was contraindicated. After their recruitment, four women were excluded from the study because of operative delivery (n = 2) and incomplete recordings for the purposes of analysis (n = 2). The remaining 26 women subsequently had uncomplicated labor and spontaneous vertex delivery. The mean maternal age was 27 ± 1 years (± standard error of the mean) and ranged between 19–37 years. Nine women (35%) were nulliparous. The mean (± standard error of the mean) gestational age was 39.6 ± 0.3 weeks. The maternal heart rates and FHRs were recorded continuously and simultaneously from admission until delivery using two separate “fetal” heart rate monitors at a paper speed of 1 cm per minute (Model 151 Fetal Monitor, Corometrics Medical Systems, Inc., Wallingford, CT). The FHR was recorded via an external ultrasonic probe until the membranes ruptured and then through a double-wire spiral electrode attached to the fetal scalp. If the membranes did not rupture spontaneously, amniotomy was carried out as soon as the fetal vertex was 3 cm above the ischial spines (−3 station). The maternal heart rate was recorded by a triple-wire ECG cable with electrodes attached at standard unipolar chest positions. Recordings were made throughout labor, delivery, and for 1 hour after delivery of the placenta. Uterine activity was recorded externally by both monitors, which were time-synchronized. During labor, the study participants received intravenous lactate Ringer at 150 mL per hour, and oral fluid intake was allowed ad libidum. Sixteen women (62%) received narcotic analgesia for pain relief during labor (intravenous pethidine HCl 75 mg and promethazine HCl 25 mg) and ten (38%) underwent epidural analgesia (lidocaine HCl 2% and bupivacaine HCl 0.25%). A fluid bolus of 1000-mL saline was administered before the epidural analgesia. The birth weights and Apgar scores were within normal range.
Representative maternal heart rate and FHR tracings of 30–60 minutes duration were randomly selected from the following divisions of labor and delivery: the latent phase of the first stage of labor (from regular uterine contractions until cervical dilatation of 4 cm), the active phase of the first stage (from cervical dilatation of 4 cm and at least 80% cervical effacement until full dilatation), the second stage of labor (from full cervical dilatation until after delivery of the fetal body), and the “third” stage of labor which, for the purposes of this study, was defined as the period after delivery of the placenta. The representative tracings were quantitatively evaluated (visually) without identifiers in 10-minute windows for the following categories: baseline heart rate, baseline variability (ie, from the segment containing the maximal amplitude of the peak-to-trough oscillations in the recorded baseline heart rate), and the presence of periodic changes (ie, accelerations and decelerations of the heart rate) using guidelines from the National Institute of Child Health and Human Development. 13 In addition, the frequency (number per 20 minutes), mean amplitude (beats per minute), and mean duration (seconds) of the periodic changes in heart rate were determined for each tracing.
Statistical comparisons of baseline heart rate and variability and the presence and quantitative indices of the periodic changes were made between maternal and fetal tracings from each stage of labor. Paired analyses of continuous variables were done by the Wilcoxon signed rank test because maternal-fetal differences did not have a normal distribution. Categoric data were analyzed by the McNemar test. P < .05 was considered as statistically significant.
We performed a study of interobserver agreement among three different investigators on a sample of ten randomly selected maternal heart rate and FHR tracings to confirm the reliability of measurements. A κ score 14 of 1 was obtained for the presence of periodic changes. Kendall's W coefficient of concordance 15 was 0.991 for baseline heart rate, 0.711 for heart rate variability, and 0.853 for frequency of periodic changes. A score of 1 indicated perfect agreement, whereas 0 showed random agreement.
We performed a PubMed (National Library of Medicine, Bethesda, MD) search of English language abstracts from January 1966 to June 2000 using medical subject heading terms “heart rate” and (“pregnancy” or “labor”), limited to “human,” “female,” and “adult.”
Quantitative comparison of maternal heart rate and FHR patterns showed that the mean maternal baseline heart rate was significantly lower and that the mean variability was significantly higher than those of the FHRs during all stages of labor and delivery (Table 1). The prevalence of periodic changes in the maternal heart rate and FHR tracings is described in Table 2. At no time were there any decelerations in heart rate in the maternal tracings, and the proportion of tracings with accelerations was increased as labor advanced. The majority of maternal heart rate accelerations coincided with uterine contractions and/or bearing-down efforts (Figure 1). After delivery, during the “third” stage, only few tracings contained accelerations in the maternal heart rate (Table 2, Figure 2).
In contrast, there were both decelerations and accelerations in the FHR tracings (Table 2). Fetal tracings with accelerations and no decelerations (ie, accelerations only) were observed in decreasing frequency as labor advanced (Table 2). Thus, there were 23% and 12% of FHR tracings with accelerations only during the active phase of the first stage and the second stage of labor, respectively. In contrast, there were 81% and 92% of maternal heart rate tracings with accelerations only during the active phase of the first stage and the second stage of labor, respectively (P < .001). Accelerations in the maternal heart rates occurred more frequently and had a higher mean amplitude than accelerations in the FHRs in the active phase of the first stage and second stage of labor (P < .001, Table 3). The maternal heart rate accelerations were also of a significantly longer mean duration than the FHR accelerations during the second stage.
In this study, changes in the maternal heart rate during labor and delivery were continuously recorded by an electronic “fetal” monitor. This enabled us to characterize the maternal heart rate patterns and to compare them with simultaneously recorded FHR patterns. However, it should be noted that the sample size of this study is a moderate one and that quantitative evaluation was performed visually on 30–60 minute samples of the various stages of labor. A larger sample size and more accurate and objective measurements analyzed by a computerized monitoring system 16 should be used to confirm our findings. Our literature search revealed only one other study on maternal heart rate patterns during labor, that by Odendaal 5 who used a scalp electrode to obtain maternal signals in 30 cases of diagnosed intrauterine fetal death. However, in only one-third of the reported cases was the transmitted maternal signal of high quality and the tracing artifact-free.
Our study produced maternal heart rate tracings with a pattern that closely resembles the fetal one, and the nonsuspicious observer may not be able to discriminate between the two. Indeed, when some of these tracings were presented to obstetricians and midwives, the majority did not suspect a maternal source. Our quantitative evaluation showed that the mean baseline maternal heart rate was significantly lower than the mean baseline FHR throughout all stages of labor and delivery and rarely exceeded 110 beats per minute between contractions. Baseline bradycardia had also been the most consistent finding by others in cases of fetal death and transmission of the maternal ECG via the scalp electrode. 1,2,5,9 Our data showed that the mean variability of the baseline maternal heart rate was significantly greater than that of the FHR baseline during the first and second stages of labor. However, most maternal values in this study fell within the normal range for FHR variability (5–25 beats per minute), and thus this difference may not be large enough to be discriminatory. Only after the delivery of the placenta was a significant drop noted in the mean variability of the baseline maternal heart rate. This change may be accounted for by the withdrawal of external stimuli and the onset of relaxation, tranquillity, and somnolence, which are highly characteristic of this stage.
Our findings demonstrated that periodic changes—usually accelerations in the maternal heart rate—coincide with uterine contractions and bearing-down efforts. Thus, they were noted in the great majority of maternal tracings during the second stage but in very few tracings after delivery of the placenta (the “third” stage). Quantitatively, these accelerations were very similar to FHR accelerations during the latent phase of the first stage but, as labor progressed, they became more frequent, they had higher amplitudes, and they were of longer duration. Accelerations during uterine contractions, simulating FHR accelerations, were also noted by Odendaal 5 in 40% of his cases and in several case reports as well. 4,9 These accelerations in the maternal heart rate probably represent the changes in cardiac output during labor. Basal cardiac output (between uterine contractions) had been shown to increase from prelabor values by 12% during the first stage of labor, 17 and further increases were noted during uterine contractions. 18–20 Cardiac output during contractions increased progressively as labor advanced until the contractions were associated with a mean increase of 34% from basal cardiac output at full cervical dilatation. Although this increase is primarily related to increased cardiac stroke volume early in the first stage of labor, heart rate also increases significantly in the later stages (greater than or equal to 4-cm cervical dilatation). 20 These hemodynamic changes during uterine contractions and the associated accelerations in the maternal heart rate probably result from the displacement of blood from the choriodecidual space and an increase in venous return to the heart, as well as an increase in catecholamines release because of pain and other stimuli. The marked accelerations in the maternal heart rate observed during bearing-down efforts may also represent autonomically mediated increases in heart rate and cardiac contractility, which were recorded during the Valsalva maneuver. 21
Decelerations in the maternal heart rate during labor have been described in isolated case reports 2,6,8 and in only 10% of the cases in Odendaal's study. 5 In our study, however, not one maternal heart rate tracing contained a deceleration pattern. This may be explained by our inclusion criteria (healthy women with uncomplicated pregnancies, labor, and delivery) and a study protocol which ensured adequate hydration. Most of the reported cases of maternal heart rate recordings during labor had included ante- or intrapartum fetal deaths with maternal and/or obstetric complications (ie, placental abruption, infection, etc). It is possible that changes in intravascular volume (due to dehydration, bleeding, or other causes) may result in occasional maternal heart rate decelerations. Alternatively, the marked and sustained accelerations in maternal heart rate during uterine contractions may be erroneously interpreted as baseline tachycardia with late decelerations.
In the present study, differences between maternal and fetal heart rate patterns were most prominent during the second stage of labor. We and others 22 have noted that the majority of FHR tracings in the second stage contain decelerations, and that accelerations without decelerations may be seen in approximately 10% of the tracings. 23 In contrast, more than 90% of our maternal heart rate tracings in the second stage had accelerations coinciding with uterine contractions, whereas no decelerations were observed.
Our study suggests that maternal heart rate and FHR patterns during labor and delivery share strong qualitative and quantitative similarities. In a healthy, well-hydrated woman undergoing uncomplicated labor, the maternal heart rate may be identified by a lower basal heart rate (less than 110 beats per minute) and marked, sustained accelerations coinciding with uterine contractions and bearing-down efforts. However, in cases complicated by dehydration, fever, or anemia, the maternal heart rate pattern may not be distinguishable from the FHR pattern. 3 The presence of heart rate accelerations during uterine contractions in the second stage and the absence of heart rate decelerations may also suggest a maternal rather than a fetal recording. Spurious maternal heart rate tracings may be obtained via an external, ultrasound FHR probe or by a scalp electrode of a dead fetus. The true source may be confirmed by comparison of the tracing with the maternal pulse or a simultaneous maternal ECG recording. Alternatively, the recording can be compared with the FHR as picked up by real-time ultrasound imaging.
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