Fetal monitoring during labour

Changes in FHR or the passage of new meconium-stained liquor (fetal bowel movement) may suggest possibility of fetal hypoxia. These signs can occur in normal labour but more so in high-risk pregnancies and need to be studied to determine the fetal conditions, even with fetal scalp blood sampling if necessary.

Diminishing fetal movements on admission may indicate fetal jeopardy, and cessation of movements may indicate death and hence enquiry about FM should be made on admission in labour.

Intermittent auscultation

The FHR is monitored every 15 minutes for a period of 1 minute soon after a contraction using a handheld Doppler or Pinard fetal stethoscope in the first stage of labour. In the second stage, the FHR is auscultated every 5 minutes or soon after every other contraction. Contractions are monitored by manual palpation over a period of 10 minutes to determine the frequency and duration. The frequency of intermittent auscultation was recommended on the basis that there was no differences in fetal and neonatal outcome in randomised studies that compared IA every 15 minutes for 1 minute after a contraction in the first stage and every 5 minutes in the second stage with electronic fetal monitoring.

Admission CTG or routine CTG using electronic monitoring is not recommended for women classified as a low risk. However, the woman's wishes should be respected after appropriate counselling. The indications for CTG monitoring are given in a table below.

Fetal cardiotocography

EFM enables continuous monitoring of the FHR and the frequency and duration of uterine contractions. The heart rate of a fetus is usually calculated using a Doppler transducer, which is applied externally to the maternal abdomen. The signals that are detected are those of cardiac movements, and what is measured is the time interval between cardiac cycles, which is converted to heart rate. The heart rate can also be measured from the RR wave intervals obtained by a direct application of an electrode to the presenting part.

Uterine activity is recorded either with a pressure transducer applied over the anterior abdominal wall between the fundus and the umbilicus or by inserting a fluid fluid catheter or pressure sensor into the uterine cavity through the cervical canal. External tocography gives an accurate measure of the frequency and duration but only relative information of intrauterine pressure. Accurate measurements of pressure need an intrauterine catheter or transducer, and this is not used as a routine in most centres due to lack of evidence of its clinical benefits.

Baseline heart rate

The definition of normality in the pattern of the FHR is easier than defining what is abnormal.

Normal heart rate varies between 110 and 160 beats/min.

A heart rate faster than 160 is defined as fetal tachycardia and a rate less than 110 is fetal bradycardia.

Baseline variability

The heart rate exhibits variation from the baseline, which is known as baseline variability. Although there is variability on a beat-to-beat basis, the standard fetal monitor averages 3-5 beats and records it as baseline variability on the standard CTG. Paper speed, whether it is 1cm, 2cm or 3cm /min. Baseline variability is due to the millisecond-to-millisecond reaction of the sympathetic and parasympathetic activity on the heart and reflects the integrity of the autonomic nervous system. It is reduced during the fetal sleep phase. Hypoxia, infection and medication can reduce baseline variability. An FHR with a variability of less than 5 beats per minute for >90 minutes is abnormal and may indicate fetal jeopardy.

Transient changes in fetal heart rate

Accelerations

Accelerations are defined as transient abrupt increases in heart rate of more than 15 beats/min for more than 15-seconds and are associated with fetal movements. Accelerations reflect the activity of the somatic nervous systems and are a reassuring sign of a fetus that is not hypoxic.

Decelerations

Decelerations are defined as a decrease in fetal heart rate of more than 15 beats/min for more than 15 seconds. These are defined by their relationship to uterine contractions or by the pathophysiological mechanism that causes them. Some patterns of change are generally considered to have clinical significance in relation to hypoxia.

Early or head compression decelerations

These decelerations are synchronous with uterine contractions - there is a gradual fall and rise of the FHR. The nadir occurs at the peak of the contractions, and the decrease is usually < 40 beats/min. These decelerations are generally due to head compression and are physiological. Hence, they are seen in the late first and second stages of labour.

Late or placental insufficiency decelerations

The onset of the slowing of heart rates occurs >20 seconds after the contractions commences and does not return to normal baseline rate until after the contraction is completed. This is due to placental insufficiency, and with repeated such decelerations, rise in the baseline rate and reduction in baseline variability may be indicated of fetal hypoxia.

Variable or cord compression deceleration

Variable decelerations vary in timing, shape and amplitude - hence their name. They have an initial slight transitory rise in the baseline rate followed by a precipitous fall followed by a quick recovery to the normal baseline/slightly beyond. The slight increase just before the sudden decline and the slight increase is known as shouldering. The heart rate usually falls by more than 40 beas/min and is due to cord compression, which varies with each contraction, giving rise to variable shapes, sizes and timing of decelerations, and are considered not reassuring features. Features suggesting worsening hypoxia are:

• Increase in the depth and duration of the decelerations

• Reduction of inter-deceleration intervals

• Rise in baseline rate

• Reduction in baseline variability

• Additional changes to simple variable decelerations in are called atypical variable decelerations or variable decelerations with 'concerning features' and are abnormal:

  • Slow recovery to baseline

  • Variable followed immediately by late decelerations

The actions taken with an abnormal CTG should be putting the mother in the left lateral position, hydration and stopping oxytocin infusion if suitable. If significant abnormality persist, a fetal blood sample for acid-base may be needed or operative delivery may be needed.

NICE classifies normal, suspicious and pathological fetal heart rate and recommended actions as:

1. Normal

   1. Definition : An FHR trace in which all four features are classified as reassuring

   2. Action : Continue intermittent or continuous monitoring as indicated by risk factors

2. Suspicious

   1. Definition: An FHR trace with one features classified as non-reassuring and the remaining features classified as reassuring

   2. Action: Exclude factors indicating for immediate delivery (cord prolapse, uterine rupture, abruption). Treat dehydration, hyperstimulation, hypotension and change position. Continue CTG

3. Pathological

   1. Definition: An FHR trace with two or more features classified as non-reassuring or one or more classified as abnormal

   2. Action: Exclude factors indicating for immediate delivery (cord prolapse, uterine rupture, abruption). Treat dehydration, hyperstimulation, hypotension and change position. Continue CTG. Deliver if prolonged bradycardia. Either obtain further information on fetal status by fetal blood scalp sampling or deliver.

Specific indications for the use of continuous EFM/CTG

Fetal acid-base balance

Where abnormalities of FHR occur in labour, they may provide an indication of fetal acidosis, but to confirm these findings, the fetal acid-base should be examined. Fetal blood is obtained directly from the scalp through an amnioscope. The instrument is inserted through the cervix, which must be at least 2cm diabetes. The mother is request to lie in the lateral position. The latter is preferred erable to a dorsal or lithotomy position, as it will avoid the risk of inducing supine hypotension. A small stab incision is made in the fetal scalp, and blood is collected into a heparinised capillary tube and analysed in a normal blood gas analyser.

Normal pH lies between 7.25 and 7.35. A pH between 7.20 and 7.25 in the first stage indicates mild acidosis, and sampling should be repeated within the next 30 minutes. If it is <7.20, delivery is recommended unless spontaneous delivery is imminent. If there is a sufficient sample, full blood gas analysis should be performed. Raised PCO₂ with a normal base excess may indicate a respiratory acidosis that may correct itself if the posture of the mother is changed. The degree of metabolic acidosis may also be assed using lactate - this requires smaller blood volumes to measure and can be done using portable handheld devices.