NUMERICAL FETAL HEART-RATE ANALYSIS - NOMOGRAMS, MINIMAL DURATION OF RECORDING AND INTRAFETAL CONSISTENCY

Objectives To develop references ranges for basal fetal heart rate (FH R) and its long-term (LTV) and short-term (STV) variation; to establis h the minimum recording time required reliably to assess these paramet ers; and to study intrafetal consistency of FHR and its variation thro ughout gestation. Subjects and methods Computerized FHR recordings wer e made longitudinally in 29 uncomplicated pregnancies from 24 weeks of gestation onwards. A total of 224 recordings were studied using multi level analysis. Results Reference ranges were based on 60-min recordin gs and standardized for midday (12.00). Median FHR decreased in a line ar fashion from 144 beats per min (bpm) at 24 weeks of gestation to 13 1 bmp near term. Between 24 and 40 weeks there was a curvilinear incre ase in the median LTV from 34.5 to 54.2 ms and in the median SN from 6 .2 to 9.2 ms. The lower limit of the normal range of FHR variation har dly changed from 30 weeks' gestation onwards. The minimal recording ti me necessary reliably to assess FHR and its variation was at least 30 min before 34 weeks of gestation and at least 40 min thereafter. The l ower limits of the normal ranges were strongly dependent on the record ing length. Intrafetal variance was 19-55 % of the total variance of F HR variation, which indicates a certain intrafetal consistency through out gestation. Conclusions This article gives reference ranges for FHR , data indicate that, for monitoring of trends, each fetus should be i n its own control using recordings of standardized duration and approp riate reference ranges.