Advances In Intrapartum Fetal Monitoring

Introduction

 


Perinatal mortality rate is an important indicator in determining public health, and hence important for planning health services.

Just over 1 in 200 pregnancies end in a stillbirth and around 1 in 300 babies die in the neonatal period, a devastating event for any parent. Not all these losses are preventable, but continuous surveillance of them is critical to identify trends and problem areas for developing services and provide better care. Furthermore, mortality is but the tip of an iceberg of morbidity.

In 2005 in England, Wales and Northern Ireland there were 668,681 total live births notified to the Office for National Statistics (ONS) and 7225 death notifications to CEMACH: 1193 late fetal losses, 3676 stillbirths and 2356 neonatal deaths. The stillbirth rate was 5.5/1000 and the neonatal mortality rate was 3.5/1000. The perinatal mortality was 8.2/1000.

The main identifiable causes of stillbirths were: severe/lethal congenital anomalies (16%), antepartum haemorrhage (8%) and intrapartum cause (7%). Over 50% of stillbirths remain unexplained using the current classification systems. The main identifiable causes of neonatal deaths were: immaturity (48%), lethal/severe congenital anomalies (22%) and intrapartum causes (9%). Epidemiological data suggest that 10% of cases of cerebral palsy have potential intrapartum causes and, even in some of these there may have been an antenatal component.

To improve the care given to the women in labour and identify fetal compromise early, intrapartum surveillance is important. There are various modalities available for monitoring fetus in labour. The common methods are intermittent auscultation and use of electronic fetal monitor (EFM). The other alternative or adjuvant tests for fetal wellbeing include: Fetal scalp blood sampling, Fetal scalp lactate measurement, Fetal pulse oximetry, Fetal ECG analysis, and Fetal Near infrared spectroscopy (NIRS).

Intermittent Auscultation
Intermittent auscultation is defined as intermittent surveillance of the fetal heart rate during labour, employing either a Pinard stethoscope or a hand-held Doppler ultrasound device.

For a woman who is healthy and has had an otherwise uncomplicated pregnancy, intermittent auscultation is recommended in labour to monitor fetal wellbeing.

In the active stages of labour, intermittent auscultation should occur after a contraction, for a minimum of 60 seconds, and at least every 15 minutes in the first stage and every 5 minutes in the second stage.

Electronic Fetal Monitoring (EFM)
Intrapartum EFM was intended to be a screening tool for intrapartum fetal hypoxia.

In theory, the early detection of hypoxia and prevention of metabolic acidaemia should reduce the incidence of perinatal death, cerebral palsy and neurodevelopmental disability. In a low risk pregnancy, continuous EFM is recommended and should be offered in pregnancies previously monitored with intermittent auscultation if there is evidence on auscultation of a baseline less than 110 bpm or greater than 160 bpm, if there is evidence on auscultation of any decelerations or if any intrapartum risk factors develop.

Continuous EFM should be offered and recommended for high-risk pregnancies and where oxytocin is being used for induction or augmentation of labour.

In clinical practice, Cardiotocographs (CTG) are usually interpreted as a whole, accounting for the summative effect of a number of individual features. CTGs should be reviewed, taking into account maternal and fetal clinical factors and progress of the labour. Interpretation of FHR traces is significantly affected by intra- and interobserver error.

Fetal heart rate features on CTG
* Absence of accelerations with an otherwise normal CTG is of uncertain significance.

Baseline (bpm) Variability (bpm) DecelerationsAccelerations
Reassuring 110-160 </= 5 None present
Non-Reassuring 100-109161-180 <5 for>40but <90min EarlyVariableSingle prolonged deceleration upto 3min
Abnormal <100>180Sinusoidal Pattern for >/=10min <5 for >/=90min Atypical variableLateSingle prolonged deceleration> 3min

Terms explained
Baseline fetal heart rate, expressed in beats per minute (bpm) is the mean level of the FHR when this is stable, excluding accelerations and decelerations, determined over a time period of 5 or 10 minutes.

Baseline variability is the minor fluctuations in baseline FHR occurring at three to five cycles per minute.

Accelerations are transient increases in FHR of 15 bpm or more and lasting 15 seconds or more.

Early decelerations are uniform, repetitive, periodic slowing of FHR with onset early in the contraction and return to baseline at the end of the contraction.

Late decelerations are uniform, repetitive, periodic slowing of FHR with onset mid to end of the contraction and nadir more than 20 seconds after the peak of the contraction and ending after the contraction.

Variable decelerations are variable, intermittent periodic slowing of FHR with rapid onset and recovery.

Atypical variables are variable decelerations with any of the following additional decelerations components:

  • loss of primary or secondary rise in baseline rate
  • slow return to baseline FHR after the end of the contraction
  • prolonged secondary rise in baseline rate
  • biphasic deceleration
  • loss of variability during deceleration
  • continuation of baseline rate at lower level

Prolonged deceleration is an abrupt decrease in FHR to levels below the baseline that lasts at least 60-90 seconds. These decelerations become pathological if they cross two contractions, i.e. greater than 3 minutes.

Sinusoidal pattern is a regular oscillation of the baseline long-term variability resembling a sine wave, lasting at least 10 minutes, has a relatively fixed period of 3-5 cycles per minute and an amplitude of 5-15 bpm above and below the baseline with absent baseline variability.

CTG classification
Normal: A CTG where all four features fall into reassuring category

Suspicious: A CTG whose features fall into one of the non-reassuring categories and the remainder of the features are reassuring

Pathological: A CTG whose features fall into two or more non-reassuring categories or one or more abnormal categories

When evaluating the CTG, one must look at the whole CTG and assess changes such as increasing baseline and not just view the current snapshot.

Discussed below are the specific FHR features and the neonatal outcome:

  • Most FHR features in isolation, with the exception of late decelerations, are poor at predicting poor neonatal outcome.
  • Uncomplicated baseline tachycardia (161-180 bpm) or bradycardia (100-109 bpm) does not appear to be associated with poor neonatal outcome.
  • The predictive value of reduced baseline variability alone is unclear.
  • The presence of FHR accelerations is associated with good outcome.
  • Repeated late decelerations are associated with an increased risk of cerebral palsy, umbilical artery acidosis and an APGAR score of less than seven at five minutes.
  • Reduced baseline variability, together with late or variable decelerations, is associated with an increased risk of cerebral palsy.
  • Atypical variable decelerations alone are associated with an increased risk of umbilical artery acidosis and an APGAR score of less than seven at five minutes.
  • Prolonged decelerations are associated with poor neonatal outcome.

Management based on CTG
In cases where the CTG falls into the suspicious category, conservative measures should be used. The measures include checking the position of the transducer or fetal scalp electrode, checking maternal pulse, temperature, blood pressure, encourage left-lateral position, stop Oxytocin or consider tocolysis using 0.25mg subcutaneous terbutaline if there is evidence of uterine hyperstimulation.

In cases where the CTG falls into the pathological category, conservative measures should be used and fetal blood sampling where appropriate or feasible. In situations where fetal blood sampling is not possible or appropriate then delivery should be expedited.

Fetal scalp blood sampling
All units employing EFM should have ready access to fetal blood sampling facilities. In cases of suspected fetal compromise, fetal blood sampling should be undertaken in the absence of technical difficulties or any contraindications. Fetal blood sampling is contraindicated in maternal infection (e.g. HIV, hepatitis viruses and herpes simplex virus), fetal bleeding disorders (e.g. haemophilia) and prematurity (< 34 weeks). Where there is clear evidence of acute fetal compromise (e.g. prolonged deceleration greater than three minutes), fetal blood sampling should not be undertaken and the baby should be delivered urgently. Fetal blood sampling should be undertaken with the mother in the left-lateral position.

Management based on FBS result (pH)
> 7.25 FBS should be repeated if the FHR abnormality persists
7.21-7.24 Repeat FBS within 30 minutes or consider delivery if rapid fall since last sample
< 7.20 Delivery indicated

FBS has a sensitivity of 93% with a false positive rate of 6% for detecting umbilical artery acidaemia (pH 7.25 or less). Umbilical artery pH below 7.00 is associated with an increase in both short and long-term complications in the neonate (and with cerebral palsy if in combination with a 5-minute Apgar of less than seven).

Fetal scalp lactate measurement
Inadequate oxygen supply results in anaerobic metabolism of glucose, which leads to metabolic acidosis. Anaerobic glycolysis results in acidosis through the production of pyruvate, with some conversion to lactate. The advantages of measuring lactate over pH estimates in fetal blood sampling to detect a hypoxic fetus are: less dilatation of the cervix is needed, fewer scalp punctures are needed, Sampling technique is faster and less fetal blood is needed (5

Categories: ARTICLES

About Author