The relationship between heart rate and asphyxia in the animal fetus.

In order to understand the mechanisms which regulate and control basal heart rate as well as heart rate variability in the fetus, a number of studies have been conducted in fetal animals. Information has been obtained largely from fetal sheep during the last third of gestation, although recently investigators have directed their attention to earlier gestations, such as 80-90 days, with term in the sheep being 145 days. A few studies have focused on the non-human primate fetus for additional information. A variety of techniques has been utilized experimentally to produce hypoxemia and/or acidemia in fetal animals, including umbilical cord compression, maternal hypoxemia, and uterine blood flow restrictions. Evidence to date would indicate that both chemoreceptors and baroreceptors are present and active in the late gestation sheep fetus, giving rise to acute changes in fetal heart rate (FHR) with changes in oxygenation. Factors which must be considered in interpreting alterations in both mean FHR and FHR variability in response to a decrease in oxygenation include: 1) the mechanism giving rise to decrease in oxygen delivery, 2) the duration of hypoxemia and/or asphyxia, 3) the effect of sleep states and behavioural activity, 4) gestational age, and 5) condition of the fetus prior to the decrease in oxygen delivery. Heart rate variability in fetal sheep has been studied extensively and it has been shown that accelerations in the FHR are closely associated with skeletal muscle activity much like the human fetus, suggesting similar control mechanisms in the 2 species. The late gestation sheep fetus consistently and reliably experiences an increase in heart rate variability with acute hypoxemia with and without acidemia until severe acidosis develops. In contrast, the hypoxemic human fetus, prior to labour, generally exhibits a decrease in FHR variability. The explanation for this apparent species difference remains unclear.