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 analyzed: 1) 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 were: acidotic 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 greater than .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 less than .00003), supporting the hypothesis that extremely regular FHR patterns imply a greater likelihood of acidosis. Significant hourly deviations in ApEn generally corresponded to drug administration or to physiologic changes such as cord compression and its relief. Thus ApEn, a major departure from variability statistics, appears to be able to detect subtle and possibly important differences in heart rate that are not visually apparent.