Fetal heart monitoring is recommended as follows:
1. Low risk (low-risk pregnancy and normallabour): intermittent monitoring with a Pinard's stethoscope is said to be as effective as CTG monitoring. Structured intermittent monitoring involves listening immediately after a contraction for a minimum of 60 seconds and repeating this every 15 minutes in the first stage of labour (every 5 minutes in the second stage). It has been suggested that CTG monitoring in the low-risk group may lead to unnecessary intervention and increased anxiety.
2. High risk (high-risk pregnancy and induced or augmented labours): continuous monitoring of the fetal heart rate is recommended using a CTG, which records the fetal heart rate and the uterine contractions (and thus the effect of the latter on heart rate). The monitor uses either an external transducer or a 'clip' applied to the fetal head. It is generally recommended that women with epidural analgesia have continuous fetal monitoring, although there is some evidence to suggest that women who have a mobile (low-dose) epidural may not need this. The need for continuous fetal monitoring during epidural analgesia is related to the cardiovascular instability that may follow administration of large doses of local anaesthetic solutions into the epidural space. There is also evidence that epidural or spinal opioids may cause transient fetal bradycardia.
There are four features of the fetal heart rate that are especially important:
• Baseline rate: normally 110-160 beats/min.
• Variability: normally 5-25 beats/min.
• Accelerations from baseline (>15 beats/min for 15 s): two in 20 minutes are normally present. The significance of absent accelerations on an otherwise normal CTG is uncertain.
• Decelerations from baseline: normally absent. Decelerations are classified as early, variable and late. Early decelerations are synchronous with the contraction; they are benign and may be associated with compression of the fetal head in the pelvis. They mirror the uterine contractions and should be associated with good beat-to-beat variability. Variable decelerations vary in their shape, size and occurrence. They may or may not indicate fetal hypoxia. Late decelerations continue after the contraction has finished and are more ominous, especially if associated with other abnormalities, e.g. reduced variability.
Opioids or other sedative drugs may cause a flat trace with a loss of beat-to-beat variability, which makes interpretation difficult.
Thus there are three categories of pattern of CTG monitoring:
• Normal: all four of the above features are normal ('reassuring').
• Suspicious: one of the following is present (termed 'non-reassuring'):
• Variability: <5 beats/min for 40-90 minutes.
• Variable or early decelerations, or a single deceleration lasting 1-3 minutes.
• Pathological: two or more of the above features are 'non-reassuring' (see above) or one or more of the following is present (termed 'abnormal'):
• Variability: <5 beats/min for >90 minutes.
• Severe variable or late or decelerations, or a single deceleration lasting >3 minutes.
Fetal heart rate monitoring has low specificity and sensitivity. Any trace that causes concern, especially in a high-risk pregnancy, is an indication for a fetal blood sample to be taken, unless there is evidence of acute fetal compromise, in which case urgent delivery is indicated. The CTG trace should be kept for at least 25 years in case of a later medicolegal claim.
Recently, the combination of CTG and fetal electrocardiography (ST waveform analysis) has been used to improve the sensitivity of fetal heart rate monitoring.
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