Study on fetal heart rate variation using a probability distribution matrix

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1. We devised a new method using a probability distribution matrix to simultaneously describe variation in the characteristics of fetal heart rate (FHR) and beat-to beat difference (DFHR) between the present and the immediately following FHR.The FHRs were plotted in columns, DFHRs in rows and probabilities in the corresponding elements of the matrix. Analyzed were the age-related changes of FHR data obtained from 743 fetuses at 23-40 weeks' gestation using a pulsed Doppler cardiotocograph with autocorrelation. Three particular probability distribution patterns of FHR vs. DFHR emerged with advance in gestation. The probability distribution matrix presented enables one to condense any amount of FHR data, into one uniform description. This allows analysis of data, en bloc, achieving quantitative inter-group comparison, on an equivalent scale.2. To elucidate the brain segment which would control heart rate changes in human fetuses with advancing gestation, FHR data obtained from 12 anencephalic and 165 uncomplicated fetuses between 25-32 weeks' gestation were analyzed using a probability distribution matrix. There was a critical period between 27-28 and 29-30 weeks' gestation with regard to the developing brain function pertaining to FHR changes. In the early stage, the medulla oblongata plays a role in FHR changes, whereas, in the latter stage, the brain cephalad to the medulla also appears to take on the role of FHR regulator.3. To determine the minimum number of FHRs needed to assess various fetal conditions adequately, analyzed were 9 consecutive two-week interval probability distribution matrices obtained from 743 normal fetuses between 23 and 40 weeks' gestation. The minimum number of FHRs for assessment of the fetus at 23-40 weeks' gestation was almost the same, between 9,000 and 10,000, with 67-75% baseline variability (so called beat-to-beat variability) and 25-33% long-term variability regardless of advance in gestation.