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24. Fetal Surveillance in Labour
24.1 Introduction
Labour is a culmination of events which starts with fertilization. It is an arduous process in which the fetus is subjected to strong uterine contractions with a potential for hypoxic insult. Surveillance of fetus in labour is hence crucial but remains challenging.
24.2 The Need for Intrapartum Surveillance
Despite advances in antepartum and intrapartum care, the annual number of stillbirths worldwide remains very high—2.7 million third trimester stillbirths, of which 1.3 million occur in labour [1]. Labour surveillance figures in one of the five priority actions are recommended by these authors to ‘change the curve’ of stillbirths.
More important is the perinatal morbidity in the form of hypoxic ischaemic encephalopathy (HIE) and cerebral palsy (CP) that has been linked to inadequate oxygenation of the fetus during labour. HIE is the short-term neurological dysfunction caused by insufficient fetal oxygenation in labour and cerebral palsy (spastic quadriplegic or dyskinetic type) is the long-term neurological dysfunction associated with it. It is important to note that only 10–20% of cerebral palsy cases are caused by intrapartum hypoxia [2]. The majority are caused by antepartum problems such as infections, metabolic diseases and others. Nevertheless, there is no doubt that labour increases the exposure of the fetus to hypoxic conditions as explained below and fetal surveillance is crucial. The aim of fetal surveillance in labour is early identification of fetuses at risk of hypoxic insult and timely intervention to prevent the same.
24.3 Physiology of Fetal Oxygenation
The fetal oxygen concentration (PO2) is considerably lower than the mother’s (40 mmHg in umbilical vein vs. 95 mmHg in maternal artery). However, there is a higher haemoglobin concentration in fetal blood, and the fetal haemoglobin has a higher affinity for oxygen. The fetal oxygen dissociation curve is shifted to the left and is much steeper compared to the maternal curve. This allows fetus to have higher oxygen content and saturation despite low PO2 and release of more oxygen at tissue level [3].
Fetus gets its energy by aerobic metabolism which depends on a constant supply of glucose and oxygen. Any obstacles in the oxygen supply pathway can lead to fetal hypoxaemia (decreased arterial oxygen concentration) and eventually hypoxia (decreased tissue oxygen concentration). In the absence of oxygen, the fetus tries to produce energy through anaerobic metabolism, but this process is much less efficient and results in production of lactic acid plus accumulation of hydrogen ions in extracellular fluid and within the cells [2]. This eventually leads to cell death.
24.4 Fetal Response to Hypoxia
- (a)
Fetal heart rate increases to increase fetal cardiac output.
- (b)
There is decrease in breathing, movement and tone—To decrease the oxygen consumption.
- (c)
Redistribution of fetal circulation—Preferential flow to the brain, heart and adrenals and decreased to kidneys, gut, lungs and liver.
The American College of Obstetricians and Gynecologists Criteria:
Evidence of a metabolic acidosis in fetal umbilical cord arterial blood obtained at delivery (pH <7.0 and base deficit −12 mmol/L).
Early onset of severe or moderate neonatal encephalopathy in infants born at 34 or more weeks of gestation.
Cerebral palsy of the spastic quadriplegic or dyskinetic type.
Exclusion of other identifiable aetiologies such as trauma, coagulation disorders, infectious conditions or genetic disorders.
24.5 Methods of Intrapartum Fetal Surveillance in Current Usage
In the early twentieth century, auscultation of the fetal heart was an acceptable standard of care. By the 1970s, fetal heart rate tracing by electronic fetal monitoring was thought to be superior to intermittent data through auscultation. Here, we discuss the two most prevalent methods of fetal surveillance—intermittent auscultation (IA) and electronic fetal monitoring (EFM).
24.5.1 Intermittent Auscultation
Fetal heart can be done by various methods—stethoscope, handheld Doppler, intermittent use of an external transducer of the cardiotocography unit or ultrasound machine. IA is deemed to be suitable for low-risk women, provided there is a 1:1 care available to the labouring woman. In this baseline heart rate is assessed by listening and counting the number of beats for 1 min, preferably after a contraction. Maternal pulse should be checked during auscultation to avoid falsely recording the maternal pulse. IA has its advantages as it is less costly, enables a woman to remain mobile (though the same can now be achieved with telemetry in EFM) and requires minimal training. The presence of a healthcare provider continuously by the labouring woman to carry out IA has definite merits. However it has several disadvantages. IA is helpful in assessing the baseline heart rate, rhythm and the presence/absence of accelerations and decelerations. It cannot, however, be used for assessing variability or describing types of decelerations. Subtle changes in the fetal heart rate may be missed. It is difficult to perform in obese women. With rising medicolegal cases involving women in labour, IA provides no objective evidence about the events as there are no paper tracings or recordings available.
24.5.2 Electronic Fetal Monitoring (EFM)
The most common method of intrapartum fetal surveillance worldwide is the use of EFM. Although there are two types of EFM, internal and external, EFM is often synonymous with external fetal monitoring. Changes in Fetal heart rate (FHR) are recorded by an ultrasound transducer attached to the maternal abdomen. Transducers are connected to a CTG machine where it records the fetal heart rate changes and their relationship to the uterine contractions. Continuous EFM during labour has been associated with reduction in neonatal seizures but with no significant difference in cerebral palsy, infant mortality or other standard measures of neonatal wellbeing [4]. In fact a higher incidence of caesarean sections and instrumental deliveries was noted in the EFM group. Consequently, it is strongly recommended that EFM should be resorted to only in high-risk women. These could be antenatal factors such as hypertension, diabetes, fetal indications such as fetal growth restriction or intrapartum indication like induced labour to name a few [3].
24.5.3 Method of EFM
This method uses a cardiotocograph (CTG) to record fetal heart rate (FHR) and hence assess fetal wellbeing. In this, an ultrasound transducer is placed on maternal abdomen after localizing the FHR and simultaneously noting the maternal pulse rate (to avoid false FHR interpretation). Doppler shifts are detected and sent to the computer system that interprets the impulses. Uterine contractions are monitored using a tocodynamometer attached to the uterine fundus which evaluates the increased myometrial tension measured through the abdominal wall. This method provides accurate information on the frequency and the average duration of contractions; but it cannot comment on the intensity or strength of contractions.
24.6 Technical Aspects
24.6.1 Maternal Position During Monitoring
Before delving into the details of CTG, it is important to pay attention to a few technical aspects. Supine position of the mother is known to affect placental perfusion due to the aorto-caval compression, and hence monitoring should be done in sitting, semi-recumbent or left lateral position. Extra care should be taken in women with epidural analgesia while giving top-ups. Those women who are interested in remaining ambulant should be given the option of telemetry, where available.
24.6.2 Paper Speed
The horizontal speed for CTG paper in most countries is 1 cm/min. However some countries such as North America and Japan use 3 cm/min, and a few others such as the Netherlands use 2 cm/min [5]. The healthcare providers need to be familiar with their paper speed to avoid misinterpretation of the CTG trace. A not so uncommon error is when paper speed is erroneously set at 3 cm/min. Here the variability can appear much reduced, to those used to the 1 cm/min setting and unnecessary interventions can occur.
24.6.3 Internal FHR Monitoring
External FHR monitoring uses Doppler ultrasound transducer to detect the movement of cardiac structures. It is reasonably accurate in most situations but is more prone to signal losses particularly in the obese, signal artefacts such ‘double counting’ of FHR during decelerations, especially second stage and inadvertent counting of maternal heart rate. In such situations internal FHR monitoring should be resorted to. Internal FHR monitoring is also advisable where fetal cardiac arrhythmias are suspected. In internal FHR monitoring, a disposable fetal electrode is clipped on to the presenting part (head or breech). This electrode measures the ventricular depolarization cycles by evaluating the time intervals between successive R waves. It is more accurate but more expensive. The membranes should have ruptured before the electrode can be applied. Moreover it is contraindicated in situations with increased risk of transmissible infections such as active herpes lesions; seropositive hepatitis B, C, D and E; or human immunodeficiency virus (HIV). It is also contraindicated where the fetus is likely to be suffering from blood disorders and is relatively contraindicated in preterm infants <32 weeks.
24.6.4 Monitoring of Maternal Heart Rate
Simultaneous documentation of the maternal pulse is a good practice every time the CTG is assessed by a health practitioner. This is inbuilt in some modern-day machines but, where not available, should be done manually. This is particularly essential when the fetus has a low normal baseline (around 100 beats per minute) or when there is maternal tachycardia or in second stage where CTG interpretation becomes difficult (e.g. where fetal accelerations are seen to coincide with contractions).
24.6.5 Monitoring of Twins or Higher Order Gestations
Dual-channel monitors allow simultaneous monitoring of both FHRs in twins. Machines are also available with three channels. If there is difficulty, the presenting twin can be monitored by internal FHR monitoring provided there are no contraindications as highlighted above. Some machines have an inbuilt ability of increasing or decreasing one of the FHRs by 20 beats so as to ensure easy interpretation of the CTG.
24.6.6 Storage of CTG Traces
CTG traces form an important part of the patient records and are important medicolegal documents. At the start of a CTG, the paper speed should be checked. Patient name and hospital number should be documented. Every time a CTG is assessed, date and time, any interventions (e.g. per vaginal examination, epidural top-up), overall impression of the CTG (explained below) and signature and name of the assessor should be documented. These should be stored carefully and where available should be archived in digital files.
24.7 Components of CTG
It is well known that CTG has a 60% false-positive rate and wide interobserver variability [6]. CTG analysis involves evaluation of five basic features—baseline, variability, accelerations, decelerations and contractions.
24.7.1 Baseline
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