Autonomic control of heart rate differs with electrocortical activity and chronic hypoxaemia in fetal lambs

We aimed to determine the effects of the electrocortical (ECoG) cycle on fetal heart rate and its autonomic control under normoxaemic and hypoxaemic conditions. Heart rate was measured and selective pharmacological blockade was used to quantify sympathetic and parasympathetic tone in low voltage (LV) and high voltage (HV) ECoG. We studied 3 groups of fetal lambs: 6 normal-sized, normoxaemic fetuses (control); 5 growth-retarded, normoxaemic carunclectomy fetuses (carunclectomy-normoxaemic); and 5 growth-retarded, hypoxaemic carunclectomy fetuses (carunclectomy-hypoxaemic). We found slower heart rate in LV compared to HV ECoG in all groups. This was explained by greater parasympathetic tone in LV in all groups, and by a complementary change of sympathetic tone in control fetuses. Hypoxaemic fetuses had slower heart rate than normoxaemic fetuses in both ECoG states. This was due to augmented parasympathetic tone (in LV ECoG) and reduced sympathetic tone (in LV and HV ECoG). We conclude that complementary changes of autonomic tone underly the normal variation of fetal heart rate with the ECoG cycle, with the parasympathetic arm dominant in LV and the sympathetic arm dominant in HV ECoG. In chronic hypoxaemia, complementary changes of autonomic tone contribute to slowing of fetal heart rate. Increased parasympathetic tone and decreased sympathetic tone may enhance cardiac efficiency when the oxygen supply is chronically reduced.