Fetal Surveillance in Labour

Intermittent Auscultation

Available evidence suggests that intermittent auscultation (IA) is adequate in low-risk pregnancies. Electronic fetal monitoring (EFM) reduces neonatal convulsions and increases operative interventions, but has not been shown to reduce cerebral palsy. Most national organizations accept IA to be adequate and appropriate for low-risk pregnancies and recommend continuous EFM for high-risk pregnancies. Auscultation of the fetal heart rate (FHR) should be for 1 full minute soon after a contraction, every 15 minutes in the first stage and every 5 minutes in the second stage of labour. If IA cannot be provided as recommended or the mother wishes to have EFM, it should be provided. Low risk before labour may become high risk during labour and this should prompt conversion to EFM.

IA is practised with a fetal stethoscope (Pinard or De Lee) or by using a Doppler device. Increasingly the mother prefers the latter as the family enjoy listening to the FHR. The practice of listening for 15 seconds and multiplying by four to calculate the rate per minute gives rise to erroneous FHR/min because of the possibility of multiplying the error by four. Doppler devices electronically calculate and provide digital display of the FHR and are accurate.

Prior to IA, recording the latest time the woman felt fetal movements (FMs) reassures that the fetus is healthy. The baseline FHR should be auscultated and recorded on admission. Further reassurance is derived by the attendant and mother palpating for FM and recording that event and the auscultated acceleration of the FHR >15 beats above the baseline at the time of FM. As a next step, auscultation should be performed immediately after contraction that should identify any FHR deceleration. Such ‘intelligent auscultation’ may reveal the presence of accelerations with FHR and no decelerations, and is equivalent to a reactive cardiotocograph (CTG) trace. It is likely that the baseline variability will be normal in a reactive CTG with FHR accelerations and FMs.

‘One day whilst examining a patient near term and trying to follow the movements of the fetus with the stethoscope I was suddenly aware of a sound that I had not noticed before; it was like the ticking of a watch. At first I thought I was mistaken, but I was able to repeat the observation over and over again. On counting the beats I found that these occurred 143–148 times per minute and the patient’s pulse was only 72 per minute’. Jacques Alexandre Kergaradec,Memoire sur l’auscultation, appliqué a l’etude de la grossesse. Paris: Mequignon-Marvis; 1822.

The deceleration that returns to the baseline rate before the contraction abates is unlikely to be harmful to the fetus, provided the duration of decelerations is less than the duration of the FHR at the baseline rate so as to generate adequate perfusion. Auscultation with a fetal stethoscope during a contraction is uncomfortable for the mother and there is attenuation of the fetal heart sound with thickening of the contracting myometrium. A Doppler device can be used during and just after a contraction. Most of the ‘harmful’ FHR decelerations are late, or with concerning characteristics (atypical) variable and prolonged decelerations, and should be identified by auscultation immediately after a contraction. Subsequent to the initial ‘intelligent auscultation’ the attendant can listen every 15 minutes in the first stage and every 5 minutes in the second stage of labour for 1 minute just after a contraction. Should there be audible abnormality of the FHR (rise in baseline rate, decelerations) or difficulty in auscultation, or should a high-risk factor become evident in labour (e.g. meconium, bleeding, blood-stained liquor or need for oxytocin augmentation), the process of IA should be converted to continuous EFM. If the FHR is much lower than the baseline rate after a contraction, the FHR should be auscultated soon after the next few contractions to decide whether there are decelerations, to decide on converting to EFM. Modern Doppler machines have the capacity to convert the digital display into an FHR trace for easy identification of accelerations and decelerations. This equipment also has the potential of storing the traces obtained from several patients for review or archiving. Fig. 9.1 shows such a device.

High-Risk Pregnancy and Continuous Efm

EFM provides a continuous recording of the FHR with the use of a transabdominal ultrasound transducer or a fetal scalp electrode after the membranes have ruptured. Those identified as high risk ( Table 9.1 ) during the antenatal period or in labour should be offered continuous EFM. The FHR is recorded on the upper ‘cardio’ channel and the contractions are recorded on the lower ‘toco’ channel of the recording graph paper and this CTG displays the FHR in relation to the contractions.

There are four features in the FHR trace recorded by electronic FHR monitors: baseline rate, baseline variability, accelerations and decelerations. These are described below and are based on the National Institute of Clinical Excellence (NICE) UK guidelines.

Decelerations

Decelerations are a sudden fall of the baseline rate of >15 bpm for >15 seconds. The shapes of the decelerations and relationship to contractions vary. Decelerations indicate a transient stress to the fetus. Based on the shape and timing of decelerations to the contractions, one can identify the cause of the stress.

Early decelerations are ‘mirror images’ of contractions and are associated with head compression in the late first and second stages of labour ( Fig. 9.2 ). There is a slow reduction in FHR as the intensity of contraction increases – the lowest FHR or nadir of the deceleration is at the peak or acme of the contraction. There is slow recovery of the FHR to the baseline rate as the contraction abates and returns to the baseline. Since they are ‘uniform’ decelerations and are reflective of head compression causing vagal stimulation they should be seen only in the late first stage or second stage of labour. Early decelerations are not due to hypoxia and generally they do not decelerate >40 bpm below the baseline rate.

Variable decelerations are ‘nonuniform’ and have a precipitous fall from the baseline rate and a quick recovery back to the baseline FHR. They vary in shape, size and timing in relation to contractions. They are due to cord compression. They can also be due to head compression, being common with malpresentations and malpositions, and when this is the case they do not have the slight increase in the FHR just prior to and at recovery of the FHR, known as ‘shouldering’. Those due to cord compression have a transient slight increase in the baseline FHR just before and after the deceleration (‘shouldering’) due to baroreceptor-mediated responses. These pre and post humps that are described as ‘shouldering’ are shown in Fig. 9.3 . Variable decelerations due to cord compression may be relieved by change of position of the mother or by amnioinfusion.

Atypical variable decelerations ( Fig. 9.4 ). In a pregnancy, more than one mechanism of stress may operate on the fetus. There may be cord compression due to oligohydramnios causing variable decelerations and the same pregnancy may be associated with placental insufficiency, and with contractions the fetus may also exhibit late decelerations. When both mechanisms operate (cord compression and uteroplacental insufficiency at the same time) there may be a variable followed by late deceleration and these are known as ‘combined’ or ‘biphasic’ decelerations or decelerations with characteristics of concern. The merger of the two of these can present as variable deceleration with late recovery of the FHR to the baseline rate, i.e. after the contraction has reached the baseline pressure. Variable decelerations with duration >60 seconds and depth >60 beats, and those with absence of baseline variability during the deceleration and between variable decelerations at the baseline rate, or an overshoot of the returning heart rate, or the absence of shouldering after being initially present, are classified as atypical variable decelerations or decelerations with concerning characteristics. Atypical variable decelerations are considered an abnormal feature in a CTG trace, whilst simple variable decelerations are considered a non-reassuring feature.

Variable decelerations without concerning characteristics develop features that are of concern as seen by increase in depth and duration of the deceleration, increase in baseline rate, reduction in interdeceleration intervals and, finally, reduction in baseline variability, as shown in Fig. 9.5 .

IA soon after a contraction for 1 minute will identify these changes; errors are made when counting is done for 15 seconds and multiplied by four or when listening is not done soon after a contraction as shown in Fig. 9.5 .

Late decelerations start towards the end or soon after the contraction peaks and the rate does not recover until well after the contraction has ceased. There is a lag of greater than 20 seconds after the onset of the contraction and before the onset of deceleration. When blood flow and oxygen supply to the intervillous space are critically reduced, the FHR slows – an effect mediated via chemoreceptors. Typical late decelerations are shown in Fig. 9.6 . The combination of late decelerations (however subtle) with persistent tachycardia and reduced baseline variability is the most pathological FHR pattern and is almost invariably associated with fetal hypoxia ( Fig. 9.7 ).

Fetal Behavioural State in the Cardiotocograph – ‘Cycling’

Nonhypoxic fetuses have alternate ‘ active’ and ‘ quiet’ sleep epochs on the CTG and this is referred to as ‘cycling’. During the active sleep epoch, there are several accelerations and good baseline variability. During the quiet epoch, there are no or occasional accelerations and the baseline variability may be reduced to <5 bpm. The quiet period can be 15–40 minutes and is rarely more than 90 minutes unless influenced by medication. In the late first stage of labour the CTG may show a long, quiet epoch when the head is deeply in the pelvis or after a narcotic is given for pain relief. Occasionally, a healthy fetus that has accelerations and good baseline variability may show segments of reduced variability and shallow decelerations during the quiet epoch but this period does not usually last for >40 minutes and rarely >90 minutes, and seems to be associated with fetal breathing episodes.

The absence of cycling indicates the possibility of CNS depression due to medication, drugs, infection, hypoxia or CNS malformation, or an insult or injury (e.g. brain haemorrhage) that may have already happened. If the trace was reactive and cycling and then becomes pathological, one may be able to identify the time of the insult. If the trace is pathological from the time of admission then it may be CNS depression or it may be due to an insult/injury that may have already taken place and the timing of injury may be difficult to ascertain. A reactive heart rate pattern of a fetus that exhibits cycling from early labour to near full dilatation is shown in Fig. 9.8 .

Cycling with an ‘active’ followed by a ‘quiet’ sleep pattern suggests that the baby is well oxygenated and likely to be nonhypoxic and ‘neurologically normal’. A previously brain-damaged fetus may or may not show cycling but the cord pH is likely to be normal if there are accelerations. Such infants may exhibit signs of neurological damage later on in life.

‘The foetal pulsation is much more frequent than the maternal pulse … being about 130 or 140 in the minute; however, it is not necessarily observed to beat always at this rate … This variation may depend upon a variety of inherent vital causes in the foetus … An obvious explanation, however, is muscular action on the part of the foetus; and we shall very generally observe the pulsation of the foetal heart increased in frequency after such. The external cause which we shall find most frequently to operate on the fetal circulation, is uterine action, particularly when long continued, as in labour’.

• Evory Kennedy

• Observations on Obstetric Auscultation. Dublin: Longman, 1833.