Fetal Monitoring in VBAC Labour

For further information, contact Melissa Maimann at Essential Birth Consulting. As published on the Birthrites website http://www.birthrites.org/

Caesarean section rates have risen in the past twenty years to a rate of approximately 20% - 25% in the United States (McMahon, 1998, p.369). Repeat caesarean section is cited as the most common indication for caesarean section (McMahon, 1998, p.369). It is hoped that by promoting vaginal birth after caesarean section, we will halt the increase in the caesarean section rates (McMahon, 1998, p.369). A trial of labour should be encouraged after a previous caesarean section, provided that there are no absolute contraindications to vaginal birth, such as placenta praevia or cephalo-pelvic disproportion (Wing and Paul, 1999, p.836). Due to the greater risk of uterine rupture in women having vaginal births after previous caesarean sections (Menihan, 1999, p.40), it is necessary to monitor the labour to minimise maternal and fetal mortality and morbidity (McMahon, 1998, p.369). The midwife plays a vital role in monitoring the well being of woman and fetus during labour, hence the focus of this options paper will be intrapartum fetal monitoring for women who have had previous caesarean sections.

It is known that changes in the fetal heart rate may signal an impending or actual uterine rupture (Menihan, 1999, p.40), so the monitoring of fetal heart rate is vital to the success of vaginal birth after caesarean section. There is indecision regarding the most appropriate method of intrapartum fetal monitoring in women attempting a vaginal birth after caesarean section owing to a lack of research in the area. Therefore, I have reviewed the literature regarding fetal monitoring in high risk women (including vaginal birth after caesarean section), and fetal monitoring in general. Unfortunately, there is no consensus as yet; fetal monitoring in labour remains a controversial issue.

Fetal bradycardia may be the first sign of an impending uterine rupture (Menihan, 1999, p.40). Late decelerations, variable decelerations, or prolonged decelerations may also occur (Menihan, 1999, pp.40-46). Furthermore, there is loss of variability, and reactivity may be poor (Menihan, 1999, pp.40-46). It is important that intrapartum monitoring enables the detection of these decelerations. The goal of fetal monitoring in labour is to detect fetal hypoxia early, so that interventions may be instituted to prevent a neonatal death (Mahomed, Nyoni, Mulambo, and Jacobus, 1994, p.497; Vintzileos, Nochimson, Guzman, Knuppel, Lake, and Schifrin, 1995, p.149).

Until the 1960s when the electronic fetal heart rate monitor became commercially available, intermittent auscultation was the only method of monitoring the fetal heart rate (Seymour, 1995, p.47). Intermittent auscultation may be performed by the midwife, using a doppler sonic aid, or a pinard stethoscope (Seymour, 1995, p.47). Alternatively, the midwife may monitor the fetal heart rate continuously with an electronic fetal heart rate monitor, either externally, or internally with a fetal scalp electrode. Since these methods rely on the interpretation of changes in the fetal heart rate, it was thought that a more objective assessment of fetal well being may improve outcomes (McNamara and Dildy, 1999, p.671; Greene, 1999, p.641). Fetal well being may be ascertained by obtaining a fetal blood sample and analysing acidity (pH). This is a medical intervention, and will be evaluated as a method of fetal monitoring that complements intermittent auscultation. The remainder of this options paper will describe and evaluate each of the above-mentioned methods of assessing intrapartum fetal well-being.

Intermittent auscultation involves periodically auscultating the fetal heart rate. Gilles, Norman, Dawes, Gee, Rouse, and Newnham (1997, pp.143-148) reviewed methods for intermittent auscultation. They found no consensus regarding appropriate intervals for auscultating the fetal heart rate. In first stage labour, recommendations ranged from auscultation every two hours to every ten minutes, with most sources advocating auscultation every thirty minutes (Gilles et al., 1997, p.145). During second stage labour, guidelines for intermittent auscultation ranged from Ôat intervalsÕ, to every fifteen minutes, to after every contraction (Gilles et al., 1997, p.145). It was generally accepted that auscultation should be performed after every contraction. Intermittent auscultation, as discussed in this options paper, will imply auscultation every thirty minutes during first stage labour, and after every contraction during second stage labour.

The pinard stethoscope was invented during the 1800s for the purpose of auscultating the fetal heart rate (Seymour, 1995, p.47). It is placed firmly on the womanÕs abdomen, at right angles to it, with the midwifeÕs ear in close contact with the stethoscope (Bennett and Brown (eds.), 1999, p.224). The pinard stethoscope is portable and readily available, and is an excellent tool for monitoring the fetal heart rate as long as the midwife is confident in interpreting what is heard (Seymour, 1995, p.47). The only disadvantage of the pinard stethoscope is that only the listener may hear the heart beat (Seymour, 1995, p.47).

Mahomed et al. (1994, pp.497-500) conducted a randomised controlled trial on the effectiveness of different methods of intrapartum monitoring. They found that abnormalities in the fetal heart rate were more reliably detected by doppler sonic aid, compared with a pinard stethoscope. They also found that auscultation with the pinard stethoscope was uncomfortable for the woman as it sometimes required a change of position, and that the woman remain still during auscultation (Mahomed et al., 1994, pp.497-500). Lower apgar scores were more common in the groups monitored with the pinard stethoscope, and neonatal seizures occurred only in the groups monitored with the pinard stethoscope (Mahomed et al., 1994, pp.497-500).

During the late first stage and second stage of labour, contractions are the longest and strongest; theoretically, this period poses the greatest risk of uterine rupture (Arulkumaran, Gibb, Ingermasson, Kitchener, and Ratnam, 1989, cited in Chua and Arulkumaran, 1997, p.7). Anecdotal evidence suggests that auscultation of the fetal heart rate with a pinard stethoscope is often difficult to perform at this time, as the baby has descended into the pelvis. This makes intermittent auscultation difficult to perform, at a time when uterine rupture and possible fetal heart rate abnormalities are the most likely to present. For these reasons, women attempting vaginal birth after caesarean section are best not monitored with the pinard stethoscope as the main method of fetal monitoring.

The doppler sonic aid is the electronic equivalent of the pinard stethoscope, and has the advantage of enabling the woman to hear her baby's heart beat (Seymour, 1995, p.47). It is possible to auscultate the fetal heart rate with the woman in any position, and there are waterproof probes available for use in the shower or bath (Steer, 1999, p.858). In their study, Mahomed et al. (1994, pp.497-500) found that detection of fetal heart rate abnormalities was better with the doppler sonic aid than with the pinard stethoscope, and that the perinatal outcome was no worse than that achieved by intermittent electronic fetal monitoring.

The American College of Obstetricians and Gynecologists (1989, cited in Cibils, 1996, p.1382) recommends that intermittent auscultation and continuous electronic fetal monitoring are equally acceptable methods of fetal monitoring, even in high risk labours. In a Birth Centre study of vaginal birth after caesarean section, Harrington, Miller, McClain, and Paul (1997, pp.304-307) used intermittent auscultation as the main form of fetal monitoring. It was performed during at least one contraction, every fifteen minutes. In both the study and control groups, the average apgar scores were 8.5 at one minute, and 9 at five minutes, and no five minute apgar scores were less than seven (Harrington et al., 1997, p.306). Neonatal outcomes were similar among both study and control groups (Harrington et al. 1997, p.306). These studies demonstrate the safety and acceptability of intermittent auscultation to monitor the fetal heart rate in women attempting a vaginal birth after caesarean section.

Generally, the literature supports intermittent auscultation as a safe method of fetal heart rate monitoring. Enkin, Kierse, Renfrew, and Neilson (1995) conclude that intermittent auscultation is just as effective in preventing intrapartum death as continuous electronic monitoring. Thacker, Stroup, and Peterson (1995, pp.613-620) studied the efficacy and safety of electronic fetal monitoring, and found that neurological consequences occurred in similar frequencies in babies monitored by intermittent auscultation and continuous electronic monitoring. Kripke (1999, p.2421) describes intermittent auscultation as a Òhigh touch, low-techÓ method of lowering the caesarean section rate for fetal distress. Gilles et al. (1997, p.147) suggest that intermittent auscultation may also play an important role in neonatal outcome, as the personal support provided by a midwife during intermittent auscultation of the fetal heart rate may contribute to reduced pain relief requirements and improved progress of labour. These are important aspects of the care of a woman attempting a vaginal birth after caesarean section.

To conclude the literature review of intermittent auscultation, use of the doppler sonic aid improves neonatal outcomes when compared with the pinard stethoscope. Literature comparing use intermittent auscultation and continuous fetal monitoring, even for high risk labours, concludes that intermittent auscultation is at least as effective in preventing neonatal morbidity and mortality. Current and accepted recommendations are for the fetal heart rate to be auscultated every thirty minutes (minimum) in the first stage of labour, and after every contraction in the second stage of labour.

The alternative to intermittent auscultation is to continuously monitor the fetal heart rate internally via a fetal scalp electrode, or externally via doppler ultrasound (Bennett and Brown, 1999, pp.418-419). A tocotransducer, strapped to the fundus of the uterus, is also used to monitor the frequency, intensity, and duration of uterine contractions (Bennett and Brown, 1999, pp. 418-419). This form of monitoring is known as cardiotocography (CTG), and the electronic fetal monitor produces a print-out of fetal heart rate in relation to uterine contractions. The fetal heart response to contractions (and fetal movements) is monitored to determine fetal well being in labour (Bennett and Brown, 1999, p.418). Continuous fetal monitoring was introduced with the hope of detecting early signs of fetal compromise, enabling early intervention to reduce neonatal mortality and morbidity (Boehm, 1999, p.623; Parer and King, 2000, p.982).

Continuous fetal monitoring was seen as an important development in the reduction in neonatal mortality and morbidity, however, proponents of CTG failed to acknowledge the contribution that improved antenatal and neonatal intensive care have made to neonatal well being (Dover and Gauge, 1995, p.18).

In fact, it has been suggested that CTG, as a screening tool, has been far from beneficial for most women. There is a lack of agreed interpretation of fetal heart rate traces (Anonymous, 1997, p.1385; Low, 1999, p.725), with the result of increased intervention in the form of caesarean section and forceps deliveries (Boehm, 1999, p.623). The adverse effects of false positive and false negative CTGs suggests that, as a screening tool for fetal distress in labour, the CTG fails miserably (Low, 1999, p.725).

A study conducted by Vintzileos, Nochimson, Antsaklis, Varvarigos, Guzman, and Knuppel (1995, pp.1021-1024) suggested that CTG was superior to intermittent auscultation in detecting fetal acidaemia at birth. This conclusion was correct, however, the authors failed to state the false positive rate of CTG in their study, as opposed to intermittent auscultation. Cibils, (1996, p.1383) states that over 40% of fetal heart rate patterns are abnormal on CTG, yet Vintzileos, Nochimson, Antsaklis et al. (1995, pp.1021-1024) found that only 8.0% of neonates had acidaemia at birth. Although CTGs were able to accurately detect changes in the fetal heart rate suggestive of acidaemia, there must have also been a substantial number of fetal heart traces suggestive of acidaemia that were in fact perfectly normal. A meta-analysis by Vintzileos, Nochimson, Guzman, et al. (1995, pp.149-155), found that one perinatal death may be prevented by the continuous fetal monitoring of one thousand women in labour (p.154). The authors accept that this would occur at the expense of a higher rate of surgical intervention.

A benefit of continuous CTG monitoring in labour is a reduction in neonatal seizures (Greene, 1999, p.647; Boehm, 1999, p.625) and one minute apgar scores of less than four (Thacker, Stroup, and Peterson, 1995, p.615). However, the authors of these articles conclude that the long term effect of this reduction must be balanced against the increase in caesarean and operative vaginal delivery rates (Thacker et al. 1995, p.619; Boehm, 1999, p.623; Greene, 1999, p.647).

Wing and Paul (1999, p.843) and Scott (1997, p.536) advocate continuous CTG monitoring for women planning a vaginal birth after caesarean section because abnormal fetal heart rate traces are the most common signs of uterine rupture. The incidence of uterine rupture among women planning a vaginal birth after caesarean section is quoted at being between 0.3% and 1.7% (Chua and Arulkumaran, 1997, p.6). Fetal heart rate abnormalities occur in 50%-70% of uterine ruptures (Scott, 1997, p.538), but they also occur in at least 40% of labours with an unscarred uterus (Cibils, 1996, p.1383). The literature failed to address how the midwife or doctor may distinguish fetal distress related to uterine rupture, requiring emergency caesarean section, from fetal heart rate abnormalities resulting from occurrences such as cord compression or head compression (Menihan, 1999, p.45). In fact, Menihan (1999, p.40) states that there is "no single, specific change in fetal heart rate (FHR) pattern predictive of uterine rupture prior to the onset of a profound bradycardia". Furthermore, since abnormal CTG patterns alone cannot accurately distinguish well fetuses from distressed fetuses, I question the accuracy of this form of monitoring in women planning vaginal births after caesarean sections.

A review of the literature suggests that continuous fetal monitoring affords no overall benefit; the reduction in neonatal seizures and low one minute apgar scores occurs at the expense of increased operative deliveries. The options presented thus far are not sufficient enough to conclude that intermittent auscultation is the safest method of fetal monitoring in the woman attempting a vaginal birth after caesarean section. These women require closer monitoring than intermittent auscultation can provide, however, they may suffer unnecessary intervention from the use of continuous monitoring. A compromise is needed.

Fetal blood sampling to ascertain pH (acidity) was developed in the 1960s with the aim of clarifying uncertain CTG patterns (Greene, 1999, p.641). On the basis of CTG patterns alone, false-positive diagnoses of fetal distress are likely to be made (Greene, 1999, p.645). A meta-analysis demonstrated that without access to fetal blood sampling, women who were monitored continuously experienced a four-fold increase in caesarean section rates compared with intermittent auscultation, with no improvement in fetal outcome (Greene, 1999, p.647). When fetal blood sampling was used in conjunction with continuous monitoring or intermittent auscultation, this rise in caesarean section rates was less marked (Greene, 1999, p.647). It is essential that all forms of fetal monitoring be supplemented by fetal blood sampling where indicated, to reduce unnecessary intervention (Steer, 1999, p.859).

Fetal blood sampling has some disadvantages: it is time-consuming to perform (Steer, 1999, p.859), it is unreliable if performed in the presence of oedema or caput succedaneum, and it can only be performed intermittently (Greene, 1999, p.648). However, when it is indicated it may accurately determine fetal acid-base balance in fetuses suspected of compromise on intermittent auscultation of the heart rate. Therefore, it may either confirm the diagnosis of fetal distress, or reassure care givers of fetal well being. Although it is not part of the midwifery management of fetal monitoring, it is capable of complementing intermittent auscultation in women planning vaginal births after previous caesarean sections, thus increasing the safety of vaginal birth after caesarean section, without increasing intervention rates unnecessarily.

In conclusion, the midwifery management of fetal monitoring in women planning vaginal births after caesarean sections is controversial. Standard practice is to continuously monitor the labour using technology that is known to increase operative delivery rates with no proven benefit. On the basis of a literature review, this paper has presented the available options of fetal monitoring. The evidence suggests that even without access to fetal blood sampling, intermittent auscultation is superior to continuous monitoring in correctly identifying fetuses in need of immediate delivery. In the presence of an abnormal fetal heart rate detected by intermittent auscultation, fetal blood sampling may indicate those fetuses that require immediate delivery, or reassure the midwife of fetal well being. Ultimately, the woman needs to be informed of her options for care, and their relative risks and benefits, as she will be the one to experience and live with the consequences (positive or negative) of labour care. This options paper is only a guide, based on the conflicting literature available at this time. Since we cannot say with 100% certainty that one method of monitoring is superior over another, perhaps midwives could best care for women by providing accurate information that facilitates involvement and choice.

Melissa Maimann, Essential Birth Consulting.