Intrapartum foetal monitoring.

Electronic foetal heart-rate monitoring In current practice, electronic foetal heart-rate monitoring is performed by either the internal or the external method. With the internal method, an ECG electrode is applied directly to the foetus, and the monitor separates the foetal electrocardiogram from the maternal, amplifies the foetal signal and measures the intervals between consecutive foetal heart beats. The external method depends on detection of ultrasound reflected off moving heart structures. In both methods the intervals between consecutive heart beats are converted into displayed instantaneous heart rates. The calculated heart rates are recorded continuously on a strip chart along with. a measure of uterine activity, provided either by ,'m external tochodynamometer or an internal intrauterine pressure catheter (IUPC). Heart-rate tracings are evaluated for the baseline heart rate, heart-rate variability, and periodic changes in rate. The normal baseline rate is between 120 and 160 beats per minute. By convention, baseline rate is measured at a time when no uterine activity is present. Variability may be described as either short-term (beat to beat), or long-term variability. In clinical practice, more significance is assigned to long-term variability. Long-term variability is more easily measured and provides a more accurate indication of fi~etal oxygen reserve than does beat-to-beat variability. Normal long-term variability has an amplitude of at least five beats per minute, occurring at a frequency of three to five cycles per minute. Periodic changes in the foetal heart rate, by definition, are those which occur during and immediatdy following a uterine contraction. Increases in rate~ or accelerations, are for the most part innocuous, and when associated with foetal movements are a good indication of adequate foetal oxygen reserve. Decreases in rate, or decelerations, may be either early, late or variable. Early decelerations begin within five to ten seconds of the onset of contractions, have a uniform shape, and return to the baseline by the end of the contraction. This pattern may be caused by foetal head compression. Early decelerations have no clinical significance and require no intervention, Late decelerations begin late in the contraction phase, the nadir usually occurs more than 20 seconds after the peak of the contraction, and the return to baseline occurs after the end of the contraction. Variable decelerations are the most frequently observed deceleration pattern. They are variable in shape and time of onset. The onset and return to the baseline is usually abrupt and the descent from the baseline frequently reaches below 100 beats per minute. Proper interpretation of alterations in the foetal heart-rate tracings must take into consideration the presence or absence of antepartum conditions detrimental to maternal or foetal health, maternal medications, and intrapartum factors interfering with the normal progress of labour. Changes that occur at any specific time must also be evaluated in terms of previous evidence of a healthy or sick foetus. The following examples illustrate normal and common abnormal patterns of foetal heart tracings. The distance between vertical lines is one minute. The upper panel is the foetal heart rate tracing, in beats per minute, while the lower panel shows the uterine activity (UA).