Temporal changes in fetal cardiovascular, behavioural, metabolic and endocrine responses to maternally administered dexamethasone in the late gestation fetal sheep

Objective To determine the primary (0-12 h) and secondary (12-24 h) effects of dexamethasone on fetal heart rate, short term heart rate variation, blo od pressure, breathing movements and electrocortical activity, blood gas ex change, metabolism and adrenocortical function in the late gestation sheep fetus. Design Comparison of the effects of a single maternally administered intram uscular injection of dexamethasone (12 mg) with those of saline vehicle fro m 1 h before injection to 24 h post-injection. Fetal cardiovascular and beh avioural parameters were recorded continuously. Fetal and maternal blood sa mples were taken at regular intervals for blood gas, glucose and lactate, c ortisol and adrenocorticotrophin measurements. Sample Sixteen chronically instrumented singleton fetal sheep at 127-133 da ys of gestation (term is about 147 days). Results During the primary phase short term heart rate variation fell (P < 0.001), and this was associated with a transient fall in the incidence of f etal breathing movements, a fall in fetal heart rate and a rise in fetal bl ood pressure. By 12 h there was a significant increase in short term heart rate variation (P < 0.001) and a rise in fetal heart rate, but blood pressu re and fetal breathing movements had returned to normal. Dexamethasone sign ificantly reduced fetal PaO2 throughout most of the experimental period, pa rticularly 1 h post-injection (P < 0.005). Fetal and maternal plasma cortis ol and adrenocorticotrophin concentrations fell significantly from 1 h post -injection. Conclusions The effects of dexamethasone on fetal heart rate variation are more complex than previously described with both a fall and an increase obs erved depending on the time at which heart rate variation was measured afte r injection. Dexamethasone also caused a significant fall in fetal PaO2, an d although this was not to hypoxic levels in normoxic fetuses it does raise questions about the potential impact of dexamethasone on chronically hypox ic fetuses.