Power spectral analysis of heart rate variability during acute hypoxia in fetal lambs

Backgrounds.  The aim of the current study was to investigate the changes in the power spectral pattern of the heart rate variability of fetal lambs during acute hypoxia and the possible value of power spectral analysis as a quantitative fetal monitoring method. Methods.  Acutely instrumented eight fetal lambs in the third trimester of gestation were subjected to reproducible hypoxia by reducing the maternal placental blood flow with complete obstruction of the maternal abdominal aorta for 60 s. Fetal electrocardiographic data 5 min prior to occlusion, 1 min during occlusion and 1, 5, and 10 min after the removal of occlusion were analyzed using power spectral analysis. Differences among the procedural steps were determined by the Friedman test with multiple comparisons using Duncan's multiple‐range test. Wilcoxon's rank sum test was used for the comparison between low‐and high‐frequency power values at each step. p < 0.05 was considered significant. Results.  Low‐, high‐, total‐frequency power and low‐to‐high frequency ratio all significantly increased with hypoxia compared with the baseline state ( p < 0.05). High‐frequency power remained higher than low‐frequency power during the resting state (baseline state, 5 and 10 min after hypoxia). Conclusions.  Increased low‐frequency power and low‐to‐high frequency ratio during hypoxia reflects increased sympathetic activity compared with the baseline state. Higher high‐frequency power during the resting state compared with low‐frequency power reflects active respiratory movement of the fetal lambs near term and increased parasympathetic activity. It appeared possible that power spectral analysis could serve as a useful quantitative tool to monitor the autonomic changes in fetal lambs during hypoxia.