[CTG: microfluctuation]

Microfluctuation (MF) of fetal heart rate (FHR) is regarded as the most sensitive parameter for diagnosing the condition of the fetus. The MF can only be crudely quantified with the naked eye. Therefore the following questions arise: 1) How can MF be exactly measured numerically? 2) What interrelationships are there between the MF determined electronically, the basal FHR, the oscillation amplitude (OA) of the FHR and the beat-to-beat variability (beat-to-beat var.) 3) What are the effects of hypoxia and acidosis on these index parameters?

387 intrapartal FHR tracings were registered directly (F-ECG) with the cardiotocograph (HP instruments) via an RS422 interface and stored on diskette. The data were processed further with a computer program we developed ourselves (MATLAB, the MathWorks Inc., USA). The parameters of the fetal acid-base balance were measured in the blood of the umbilical artery (UA) and umbilical vein (UV) with instruments from Radiometer, Copenhagen (ABL 500, ABL 5) and stored off-line with a selection of clinical data and processed further on a laptop (HP, Omnibook XE 3). The fluctuation of the basal FHR was determined on the basis of the following four parameters: the number of high and low points (extrema) per minute (EXT),the mean beat-to-beat variability per minute and the OA (bpm). In order to correlate the MF of the basal FHR with the parameters of the fetal acid-base balance, only the last 30 CTG minutes ante-partum of each tracing were included. All decelerations and optionally in addition all accelerations were electronically deleted from the FHR curve.

Basic statistical values and the distribution of the four index parameters in 5486 minutes of basal FHR were studied: the median of EXT is at 59 and the mean value at 58.9 +/- 13.9 extrema/min. The distribution is normal. The median frequency amounts to 138 bpm, the median OA to 22.2 bpm and the median beat-to-beat variability to 161.7 bpm, respectively. The mean pH value in UA blood was 7.262 +/- 0.064. The acidotic risk (pH, UA < 7.100) reached 1.3 %. There were no pH values below 7.0. With increasing basal FHR, EXT increases highly significantly (r = 0.468, P << 0.0000). EXT decreases highly significantly (r = -0.432, P << 0.0000) with increasing OA. The mean basal frequency shows the best correlation with the base excess in UA blood (r = -0.263, P << 0.0000). Beat-to-beat variability and EXT alone correlate poorly with the actual, pH and BE values (UA). Multiplication of the index parameters leads to an increase of the correlation coefficients when compared with their single values.

With increasing hypoxia and acidosis the four index parameters do show a complex pattern which is characterized by tachycardia, increase of EXT and opening of the OA. A loss of EXT and a reduction of OA seems to be the result of already severe acidosis (pH, UA < 7.000). Using the four parameters of basal FHR alone, there is no possibility to evaluate fetal jeopardization. Numerical combination (e. g., multiplication) of some index parameters ameliorates their prognostic power and should be used in future online scoring procedures.