Cardiotocography is a method of intrapartum fetal surveillance which allows the observer to correlate the frequency, duration and intensity of contractions with variations in fetal heart rate.
The fetal heart rate has a short term (beat to beat) variability. This is the normal irregularity of cardiac rate which results from the continuous interplay between the sympathetic (cardioacceleration) and the parasympathetic (cardiodeceleration) nervous systems.
The typical variability from the baseline is 6-10 beats per minute with values outside this range being of clinical significance. This variability is seen even when there is no stress or stimulation to the fetus. It is a reassuring feature which suggests that the fetus has sufficient reserves to handle the stresses of labour.
Long term variability or periodic changes in the fetal heart rate are transient accelerations or decelerations from the baseline. They usually occur in response to stress, eg. uterine contractions, but can also occur with stimulation, eg. fetal movement. They are classified as:
1. accelerations,
2. early decelerations,
3. late decelerations,
4. variable decelerations.
Each of these abnormal traces provides some indication of the mechanism of the underlying insult (1).
The effects of regional anaesthesia on the cardiotocograph trace are summarised in Table 3.1.