Characteristics of fetal and maternal heart rate tracings during labor: A prospective observational study

Abstract

BACKGROUND:

Fetal well-being is assured during labor and delivery with the employment of electronic fetal heart monitoring (EFHM). In uncommon instances, maternal heart rate (MHR) instead of fetal heart rate (FHR) can be the source of signals on monitors (signal ambiguity) leading to erroneous interpretation and management. Information about MHR characteristics are comparatively inadequate. We aim to analyze and compare MHR and FHR characteristics during the first and second stages of labor.

METHODS:

A prospective cohort study was conducted in a single tertiary care center during a one year period. Fifty one healthy full term women with singleton pregnancies during labor were enrolled. Uterine contractions, MHR and FHR were recorded simultaneously during both stages of labor by monitors designed for twin gestation.

RESULTS:

When compared to FHR, MHR had significantly lower baseline rate during 1st and 2nd stages (p < 0.0001). It demonstrated also more marked beat-to-beat variability during both stages (p < 0.0001). MHR showed significantly more accelerations (p = 0.03 and p = 0.008) and less decelerations (p < 0.0001 and p = 0.021) during 1st and 2nd stages respectively.

CONCLUSIONS:

All characteristic parameters and patterns produced by FHR could be mimicked by MHR as well, though, at different frequencies. Understanding EFHM patterns suspected to be MHR artefacts and the employment of modern monitors that simultaneously obtain and display FHR and MHR can unmask ambiguity and avert related misinterpretation problems. Similar studies should be conducted in high-risk groups where the potential for fetal hypoxia/acidosis is increased.

Keywords

Cardiotocogram fetal heart rate intrapartum monitor maternal heart rate signal ambiguity

1 Introduction

EFHM has been in clinical use for the past 50 years and is considered the standard of intrapartum care [1]. Its significance stems from two elements: first, it permits surveillance of fetal well-being during labor and delivery and second, it helps early identification of fetal jeopardy, thus, allowing timely interventions by either alleviating harmful exposures or removing the fetus by performing expeditious delivery (including emergent cesarean section) [1]. In some instances, however, a signal ambiguity may occur and this is when monitors mistakenly identify and display MHR as being FHR [2]. Misinterpretation of MHR artefacts can lead to distorted clinical situations, such as, performing an emergent cesarean delivery for a healthy non-distressed fetus, inability to identify intrapartum fetal distress and the delivery of an acidotic severely distressed newborn infant or the inability to identify intrauterine fetal demise (IUFD) at admission to labor and delivery [2 –4]. The objectives of the current study was to analyze and compare simultaneously acquired MHR and FHR characteristics during both stages of labor.

2 Material and methods

This was a prospective cohort study executed between March 1st 2012 and February 28th 2013 at Makassed General Hospital, Beirut, Lebanon. The protocol of the study was approved by the Institutional Review Board and Ethics Committee. A written consent was approved and signed by both the woman and her obstetrician. A total of 51 low-risk women, between 37 and 40 weeks gestation, with singleton pregnancy and vertex presentation were enrolled. All presented with spontaneous labor. None of them had a history of cardiovascular disease, thyroid disorder, hypertension, arrhythmias or fever. FHR was recorded using fetal scalp electrode. Concurrently, MHR was recorded using external ultrasound transducer placed on the left-side of maternal chest at the sternal level while palpating the maternal radial pulse. The monitor used was (Avalon CTS FM 30^®, Philips) designed for twin pregnancy. MHR, FHR, and uterine activity were simultaneously recorded during the first and second stages of labor. Representative parts of the CTG (averaged 30 minutes) during both stages of labor were chosen according to their clarity.

Management during labor was according to the standards of care at our maternity unit. Before epidural anesthesia, all women received one liter of lactated Ringer over one hour. During the study period, all women were at bed rest, had no food or beverage, with intravenous hydration of 120 milliliters of 5% dextrose water per hour.

During both stages of labor, four parameters were recorded and analyzed (the baseline heart rate, the beat-to-beat variability, accelerations and decelerations). Heart rate baseline was defined as the mean rate which is stable over a period of 10 minutes. Beat-to-beat variability was the fluctuation of the heart rate baseline. It was assessed by estimating the difference in beats per minute (bpm) between the highest peak and lowest trough all fluctuations in one minute segments. It was described as minimal (less than 5 bpm), moderate (5–25 bpm) or marked (greater than 25 bpm). Acceleration was specified as the transient increase in heart rate of 15 bpm or more above the baseline and lasting for more than 15 seconds. Deceleration was the transient episode of decreased heart rate below the baseline of more than 15 bpm lasting for at least 15 seconds. It was defined as early, late and variable according to its shape and relation to uterine contraction. These parameters were extracted from both maternal and fetal heart tracings and used for comparison. Due to the inability to place the fetal scalp electrode when cervical dilation was less than 3 cm, the first stage of labor was defined when the cervix was between 3 cm and complete dilation, while second stage of labor was considered the period from full cervical dilation till fetal delivery.

Statistical analysis was performed using SPSS 23 statistical software (IBM SPSS statistics). Continuous variables were expressed as mean±standard deviation, while categorical variables were expressed as number (frequency). Paired analysis for maternal and fetal heart rate baselines was done using Wilcoxon test because they did not have normal distribution. Paired analysis for fetal and maternal beat-to-beat variability, acceleration and deceleration was done using McNemar’s test. A sample size calculation was done according to heart rate acceleration. Assuming that 54% of FHR tracings and 81% of MHR tracings showed accelerations, [5] the study would require a sample of 36 pairs to achieve a power of 80 % and a two sided significance of 5% for detecting a difference of 0.27 between proportions. A p-value of less than 0.05 was considered significant.

3 Results

A total of 51 women were enrolled, of whom 6 were excluded due to incomplete recording or withdrawal from the study. The remaining 45 women and their fetuses provided paired tracings for final analysis during first stage of labor. Nine women did not reach second stage and were delivered by cesarean section due to failure to progress, hence, only 36 paired tracings were analyzed during second stage of labor (Figure 1). This cohort had a mean age of 28±6 years, 29 (64.4%) were multipara, 40 (88.8%) were smokers, 18 (40%) had epidural anesthesia, 43 (95.5%) received oxytocin infusion for labor augmentation at one time during labor.

During the first stage of labor 45 paired tracings were analyzed. FHR had a significantly higher baseline than MHR (136±15 bpm versus 87±13; p-value < 0.0001) and developed more decelerations (44.4% vs. 4.4%; p-value < 0.0001). MHR, on the other hand, developed more accelerations (100% vs 86.7%; p-value = 0.031) and more marked beat-to-beat variability with highly significant difference (80% vs 13.3%; p-value < 0.0001). FHR decelerations coincided with accelerations of MHR in 75% of cases (Table 1, Figure 2).

Fig.1

Flow chart of the women enrollment in the study. FHR: Fetal Heart Rate, MHR: Maternal Heart Rate.

Table 1

Comparison of fetal heart rate and maternal heart rate characteristics during the first stage of labor

Parameter	FHR (N = 45)	MHR (N = 45)	p-value
Heart rate baseline (bpm)	136 + /–15 (110–170)	87 + /–13 (65–130)	<0.0001
Beat-to-beat variability			<0.0001
Minimal + moderate	39 (86.7%)	9 (20%)
Marked	6 (13.3%)	36 (80%)
Acceleration	39 (86.7%)	45 (100%)	0.031
Deceleration	20 (44.4%)	2 (4.4%)	<0.0001

Abbreviations: bpm: beats per minute, FHR: Fetal Heart Rate; MHR: Maternal Heart Rate.

Fig.2

Beat to beat variability. Tracing showing marked beat-to-beat variability of the maternal heart rate (lower panel) compared to the minimal to moderate variability of the fetal heart rate (upper panel) during first stage of labor.

Thirty six paired tracings were analyzed in the second stage of labor. MHR had a significantly lower baseline (88±16 vs. 135±18; p-value < 0.0001), higher marked beat-to-beat variability (83.3% vs. 36%; p-value < 0.0001), and higher acceleration rate (100% vs. 77.8%; p-value = 0.008) than FHR. Although, decelerations of MHR were higher than those in the first stage, these were still significantly lower than FHR decelerations (36.1% vs. 58.3%; p-value = 0.021). FHR decelerations coincided with accelerations of MHR in 90.5% of cases (Table 2, Figure 3). MHR tracings produced the three types of decelerations where two (4.4%) had decelerations in the first stage (one early-type and one variable-type) and 13 (36%) in the second stage (two early-type, nine variable-type and two late-type).

Table 2

Comparison of fetal heart rate and maternal heart rate characteristics during the second stage of labor

Parameter	FHR (N = 36)	MHR (N = 36)	p-value
Heart rate baseline (bpm)	135 + /–18 (110–170)	88 + /–16 (50–140)	<0.0001
Beat-to-beat variability			<0.0001
Minimal + moderate	23 (64%)	6 (16.7%)
Marked	13 (36%)	30 (83.3%)
Acceleration	28 (77.8%)	36 (100%)	0.008
Deceleration	21 (58.3%)	13 (36.1%)	0.021

Abbreviations: bpm: beats per minute, FHR: Fetal Heart Rate; MHR: Maternal Heart Rate.

Fig.3

Fetal heart deceleration and maternal heart acceleration. Tracing showing the coincidence of fetal heart rate deceleration (upper panel) with the maternal heart rate acceleration (lower panel) during uterine contraction in the second stage of labor.

4 Discussion

Intrapartum EFHM is a common practice in most delivery units around the world. Although, no solid evidence exists about decreasing the rate of cerebral palsy, yet, EFHM has become the acceptable standard for surveillance of fetal well-being during labor and delivery [1]. Abundant information has been published about FHR characteristics and categorization during labor while scarce material exists on MHR characteristics which renders interpretation of maternal artefacts difficult for most birth attendants. Many artefacts can appear during monitoring and these usually follow loss of fetal signals especially in obese parturient or with excessive maternal or fetal movements [6]. The lost fetal signals are substituted with signals originating from maternal pelvic vessels. In fact, signal ambiguity during intrapartum monitoring is not uncommon, Paquette et al. upon analyzing 1313 tracings, reported an incidence of 55% of mostly transient incidents, of which 2.7% were major, causing adverse obstetrical outcomes related to misinterpretation [7]. In addition, Pinto et al. found up to 72% of major MHR-FHR ambiguity in 61 tracings acquired during the last hour of labor which was mostly due to MHR acceleration, FHR signal loss or deceleration [8]. These artifacts can be misleading and can result in inappropriate management decisions and possibly adverse obstetrical outcomes.

In this study, changes of MHR during labor and delivery were continuously recorded by external ultrasound transducer. This enabled us to analyze individual MHR parameters and the overall-patterns during the two stages of labor and allowed comparison with simultaneously recorded FHR tracings.

We found that baseline-MHR was significantly lower than that of FHR in both stages which conforms to observations reported by Sherman et al. who used similar methodology on 26 women [5]. Furthermore, we could detect that MHR overlapped with FHR in four women during the first stage (8.9%) and in two women during the second stage (5.6%). Odendaal et al. also reported one MHR tachycardia (3.3%) among 30 women with IUFD [9]. Likewise, Van Veen et al. [10] reported that baseline-MHR was persistently greater than 100 bpm in three women (17%) in first stage, in four women (27%) during the second stage with peak MHR of greater than140 bpm occurring during pushing in 20%. In several case reports, high baseline-MHR caused the confusion and was the direct reason for misinterpretation [11 –14]. Higher MHR baseline was explained by unusual occurrence of cardiac arrhythmia, maternal fever, anxiety and chorioamnionitis [2, 15]. In these cases, other criteria should be used to differentiate FHR from MHR tracings. Even in the absence of the aforementioned conditions, baseline-MHR was found by Towers et al. to increase with advancement of labor. Upon studying 1105 contiguous women during second stage of labor, they noticed that 33.9% had a sustained baseline-MHR of greater than 100 bpm, 18.8% had rates of greater than 110 bpm and 9.1% had rates of greater than 120 bpm [16].

Marked beat-to-beat variability (greater than 25 bpm) was significantly higher in MHR tracings during the first and second stages of labor. In addition, no minimal beat-to-beat variability belonging to MHR tracing was recorded in the first stage and only one was observed in the second stage. Similarly, Sherman et al. noted a significantly higher rate of MHR beat-to-beat variability compared to FHR, however, both values in his study were within the normal range of FHR variability (5–25 bpm) [5].

MHR tracings showed accelerations in response to most first stage and almost all second stage uterine contractions. Similarly, Van Veen et al. also found that MHR acceleration occurred with all contractions in the second stage of labor [10]. During uterine contraction, there is shifting of blood from the chorio-decidual to the intravascular space which in turn increases venous return to the heart. This mechanism, in association with pain- and stress-induced catecholamine discharge, may result in increasing MHR which in turn increases blood flow to the uterus, placenta, and fetus as a compensatory mechanism [17]. These maternal accelerations coincided with the whole duration of uterine contractions (mirror image) and tended to reach higher amplitudes when compared with the less frequent contraction-associated fetal accelerations [5].

FHR decelerations coincided with MHR acceleration in 75% of cases in first and 90.5% in second stage in response to uterine contractions. This coincidental finding, however, cannot be utilized clinically to discern between FHR and MHR as in the usual case scenario only one tracing is available for analysis.

In addition, another interesting finding of our study was that MHR may develop all three types of decelerations, in spite of the low-risk nature of this cohort. Furthermore, MHR tracings were associated with less decelerations than FHR, though, two MHR (4.4%) had decelerations during first stage (one early-type and one variable-type) and 13 (36%) in second stage (two early-type, nine variable-type and two late-type). In the study of Sherman et al, MHR did not show any decelerations [5]. While, Van Veen et al. reported that 10 out of 18 MHR (56%) developed early-type decelerations in the first stage of labor and no decelerations were observed in the second stage [10].

When MHR artefact is suspected, the responsibility of the birth attendant is to verify that the tracing displayed by the monitor is of fetal origin. This can be done by examining maternal radial pulse and if no sizable difference in the rate was elicited, adjusting the external transducer to exactly acquire fetal heart signals under ultrasound guidance would be a sound action. Another plausible act would be applying fetal scalp electrode which is not known to cause ambiguity except in IUFD [9]. A third and even more reasonable idea is to simultaneously monitor MHR with the use of oximeter. Nonetheless, the ultimate action would be the employment of modern-generation-monitors that acquire and display both FHR and MHR.

Limitations of this study was the relatively small sample size and the absence of cases with fetal distress. Furthermore, the study population included only low-risk women which limits extrapolation of results to higher-risk groups.

In this study we could confirm observations reported in the literature [5 , 19]. When compared to FHR, baseline-MHR was usually less than 100 bpm but might attain higher rates while MHR beat-to-beat variability had always higher rates of marked variability. During both stages of labor, MHR might unusually produce any type of decelerations. Furthermore, it frequently produced high accelerations that accompanied the entire duration of the uterine contraction especially during bearing down efforts [5, 19]. All individual characteristic features of FHR usually seen during labor could be produced by MHR as well. Assessment of the overall-pattern can yield more information concerning the origin of the tracings. In fact, several conditions and scenarios can exist where MHR can mimic FHR which attests to the need of abandoning older monitors and relying totally on modern ones [2 , 21]. Further studies on high-risk groups are needed where the potential for fetal hypoxia/acidosis is substantial.

Conflict of interest

The authors report no conflict of interest.

Funding

This work has no funding.

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