Decreased Approximate Entropy of Heart Rate Variability in the Hypoxic Ovine Fetus.

Objective: Variability of the fetal heart rate (FHR) is used clinically to assess fetal well being. Approximate entropy (ApEn) is a statistic that quantifies the regularity of a time series. This study was designed to test whether ApEn changed in the FHR of the hypoxic ovine fetus.Methods: Five time-bred ewes at 130 days of gestation were surgically prepared with fetal arterial catheters, fetal electrodes, and a maternal common uterine artery snare occluder. After recovery, a continuous fetal electrocardiogram recording was started, and control blood gas measurements were made. The uterine blood flow was then reduced with the occluder, and blood gas measurements were repeated at fetal pH 7.20 and 7.00. The FHR tracing (1,000 beats) was extracted from the fetal electrocardiogram tracing at the time of each blood gas. For each heart rate tracing, the ApEn was calculated. The significance of changes was assessed using analysis of variance for repeated measures.Results: The snare occluder produced significant fetal hypoxia and acidosis. Although FHR variability was increased, approximate entropy was significantly decreased during periods of hypoxia in the ovine fetus.Conclusions: Approximate entropy of the ovine fetus is directly related to the degree of hypoxia. The decreased ApEn indicates increased regularity in the FHR during hypoxia in spite of the increased variability. ApEn may provide insight into the regulatory feedback mechanisms of the fetal heart rate during periods of hypoxia.