Blood pressure and cardiovascular reflex development in fetal sheep. Relation to hypoxaemia, weight, and blood glucose.

We measured mean arterial blood pressure (MAP), fetal heart rate (FHR), the baroreflex, weight, blood gases, blood glucose and lactate in chronically instrumented fetal sheep for 14 days from Day 105 to Day 109 gestation, with or without daily administration of 1 h of acute isocapnic hypoxaemia (arterial O2 partial pressure, PaO2, 11-13 mm Hg; 1 mm Hg = 133 Pa). Fetuses subjected to hypoxia showed no significant differences in MAP or FHR v. control fetuses. However, examination of all fetuses together revealed that there were two distinct groups: those showing a rise in MAP over the 14 days ('pressure up' group, PU), and those in which blood pressure did not increase or showed only a slight decrease ('pressure down' group, PD). PU fetuses were proportionately larger than PD fetuses. In contrast to PU fetuses, PD fetuses had lower blood glucose concentration, arterial O2 saturation (SaO2), PaO2, total haemoglobin, haematocrit and oxygen content, and higher lactate concentration, pH and PaCO2. PU fetuses showed a shift of the baroreflex MAP/R-R interval curve to the right, however, the PD group showed a shift upwards from Day 1 to Day 14. The PD group responded to hypoxia with a greater increase in MAP than the PU group. Thus, repeated acute moderate isocapnic hypoxia does not affect development of MAP or FHR in late gestation fetal sheep. However, MAP follows different trajectories in individual fetuses, related to fetal size and the availability of oxygen and/or glucose. Cardiovascular chemoreflexes and baroreflexes are also different, depending on the MAP trajectory. These data indicate an important association between growth and blood pressure development, and also show that differences in growth are associated with changes in cardiovascular control.