Development of Approximate Entropy of Fetal Heart Rate Varibility during Normal Pregnancy

Yum, Myung Kul; Hwang, Jung Hye; Park, Moon Il

Abstract

Background : The approximate entropy of heart rate variability has recently been used as a tool in evaluating cardiovascular health and physiology and in diagnosing pertubated physiologic status, by quantifying the irregularity and the pattern of the heart rate variability. Therefore approximate entropy of the fetal heart rate variability will be used as an important tool in studying the fetal cardiovascular health and disease, but unfortunately there have been no previous systematic studies of the approximate entropy in normal fetuses. This study's aim has been to present the normal approximate entropy values of the fetal heart rate variability according to the gestational age, to demonstrate the developmental maturation of physiology of the control systems of the heart rate, and to quantify the evolution of the pattern of fetal heart rate variability.
METHODS
Three hundred and thirty seven mothers with normal singleton pregnancies, who were 20 weeks or more of gestational age, were randomly chosen. For more than 40 minutes fetal heart rates and fetal movements were recorded and were downloaded to a computer. From each heart rate data, 5000 points ('total') of heart rate during which fetal movements occured and 1200 points ('rest') during which no fetal movement occured were selected and the approximate entropy was calculated, and then 2 weeks' average of the approximated entropy of the heart rate were calculated and compared to find the maturational differences of them.
RESULTS
The approximate entropy of the rest period showed a tendency to increase with advancing gestation (p=0.0001). It was significantly higher between the 41st-42nd week (0.987 +/-0.017) than the period before the 24 th week (0.864+/-0.032) and between the 25th-26th week (0.868+/-0.022). (p<0.05). The approximate entropy of the total period showed another tendency (p<0.05): from a low level before the 24 th week (0.788+/-0.067) it increased to reach its peak at the 29th-30 th week (0.963+/-0.041) then it gradually decreased again to significantly (p<0.05) lower levels at the 37th-38th week (0.776+/-0.046) and the 39th-40th week (0.786+/-0.047).
CONCLUSIONS
Normal value of approximate entropy, which is bound to be used as an index in evaluating the fetal well-being, has been presented from 20th to 42th week of gestation with an interval of two weeks. The systems that control the heart rate seem to mature consistently as gestation advances, but before the 26th week it is significantly more immature than at term. Fetal heart rate variability evolves from an immature oscillating pattern into a transitional patternless pattern during the second trimester and finally consolidates into a mature accelerational pattern during the third trimester.