Technical note

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Abstract
A CMOS [computer memory operation system] microprocessor system with a 2k read-only memory and a 4k random-access memory is constructed. The processor is connected to a commercial cardiotocograph, and it calculates the interval index (II) measuring the long-term variability and the differential index (DI) measuring the short-term variability of human fetal heart rate [FHRV]. The accuracy of the system was tested by calculating the FHRV indices from simultaneously recorded direct FECG [fetal ECG] and abdominal FECG. The correlation between the results of these 2 methods was good. II and DI were calculated serially for 34 fetuses during normal pregnancies. FHRV analysis succeeded in 80-100% of patients from the 20th until the 27th wk of gestation and again from the 35th wk until term. The method was less successful, 35-70%, between gestational weeks 28 and 34. On average, II and DI grew as gestation advanced. II had a greater scatter than DI and it apparently measured mainly fetal arousal level. DI was not below 3 after the 30th wk of gestation. DI may become a new marker of fetal status in antepartum fetal surveillance.