Bocking A D
Department of Obstetrics and Gynaecology, University of Western Ontario, London.
Clin Invest Med. 1993 Apr;16(2):166-75.
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.
为了了解调节和控制胎儿基础心率以及心率变异性的机制，已在胎仔动物中进行了多项研究。尽管最近研究人员已将注意力转向更早的孕期，如80 - 90天，绵羊的孕期为145天，但大部分信息是在妊娠最后三分之一期间从胎羊获得的。一些研究聚焦于非人灵长类胎儿以获取更多信息。实验中已运用多种技术在胎仔动物中诱发低氧血症和/或酸血症，包括脐带压迫、母体低氧血症和子宫血流受限。迄今为止的证据表明，化学感受器和压力感受器在妊娠晚期的绵羊胎儿中均存在且活跃，随着氧合变化会引起胎儿心率（FHR）的急性改变。在解释因氧合降低导致的平均FHR和FHR变异性改变时必须考虑的因素包括：1）导致氧输送减少的机制，2）低氧血症和/或窒息的持续时间，3）睡眠状态和行为活动的影响，4）胎龄，以及5）氧输送降低之前胎儿的状况。胎羊的心率变异性已得到广泛研究，结果表明，FHR的加速与骨骼肌活动密切相关，这与人类胎儿非常相似，提示这两个物种具有类似的控制机制。妊娠晚期的绵羊胎儿在有或无酸血症的急性低氧血症情况下，心率变异性会持续且可靠地增加，直至发展为严重酸中毒。相比之下，分娩前低氧血症的人类胎儿通常表现为FHR变异性降低。这种明显的物种差异的原因尚不清楚。
