Labor exposes the fetus to repetitive transient hypoxic stress. We assessed whether such events modify the regularity of the fetal inter-beat interval series (fRR), using an in-vivo near-term sheep model, by means of entropy measures. Umbilical cord occlusions (UCO), from partial to complete, were applied to 7 near-term pregnant sheep. Fetal blood samples were collected at intervals of 20 minutes, to quantify pH, lactate and base deficit (“biomarkers”). Fetal ECG recordings were collected with implanted electrodes and used to derive the fRR series. Sample entropy (SampEn), permutation entropy (PE) and conditional PE(cPE) were estimated for fRR patterns of various lengths m, in each 2.5 minutes-long cycle of occlusion and successive recovery. Entropies' changes in time, during the course of the experiment, and their relation with the simultaneous values of the biomarkers were evaluated with Spearman's rank-order correlations. Entropy values decreased during the experimental protocol (rs=−0.62 for SampEn at m = 1, rs=−0.27 for PE at m = 5 and rs=−0.30 for cPE at m = 4; p < 0.05). Correlations with biomarkers were found to be moderate for SampEn (0.49 < |rs| < 0.62; p < 0.05) and weak to moderate for PE and cPE (0.31 < |rs| < 0.54; p < 0.05). Repetitive UCOs changed the regularity of fRR, suggesting a pronounced modulation as first line adaptive response.
