Approximate Entropy: A Regularity Measure for Fetal Heart Rate Analysis

Approximate entropy (ApEn), a recently developed mathematical formula quantifying regularity, was applied to fetal heart rate (FHR) data. Three groups were analyzed1) 19 women had normal labors (uncomplicated course of labor, vaginal delivery, no unusual FHR tracings, and 1- and 5-minute Apgar scores of at least 7 and 9, respectively; 2) 15 women had presumed fetal distress (severe cord or late decelerations, bradycardia, or tachycardia; delivery by cesarean with both arterial and venous cord pH above 7.20); and 3) 20 women had acidotic fetuses (both venous and arterial cord pH less than 7.20). Hourly mean (± SD) ApEn values for the three groups wereacidotic fetuses, 0.924 ± 0.235,102 hours; normal fetuses, 1.051 ± 0.145, 97 hours; and nonacidotic “distressed” fetuses, 1.043 ± 0.147, 74 hours. The ApEn values for nonacidotic, presumed distressed fetuses were not significantly different from those of normal fetuses (P >.75). Acidotic fetuses had many more instances of ApEn hourly values less than 0.8 (28%, 29 of 102) than did the normal and the nonacidotic, presumed distressed fetuses combined (5%, nine of 171). The probability that ApEn was less than 0.8 was larger for acidotic fetuses than for the other groups (P