Adaptation of cardiovascular responses to repetitive umbilical cord occlusion in the late gestation ovine fetus

1. The impact of repeated umbilical cord occlusion on the normal maturation of fetal heart rate (FHR) and mean arterial pressure (MAP) and the cardiov ascular responses to successive umbilical cord occlusion was investigated o ver a 21 day period in the latter part of gestation. 2. Fifteen chronically instrumented sheep (control group n = 6; occlusion g roup n = 9) were studied for 21 days (113-133 days of gestation, term = 145 days) with umbilical cord occlusions (90 s duration) performed every 30 mi n for 1-4 h each day. On days 1, 9 and 18, FHR, FHR variation and MAP were monitored continuously and fetal arterial blood gases, pH and metabolites w ere measured at predetermined intervals. The baroreflex response to 75-100 mug phenylephrine (I.V.) was tested on days 1 and 18. 3. Basal FHR decreased (Delta FHR: control, 34.6 +/- 3.6 beats min(-1); occ lusion, 36.9 +/- 2.7 beats min(-1)) and MAP increased (Delta MAP: control, 3.1 +/- 1.7 mmHg; occlusion, 5.2 +/- 2.1 mmHg) to a similar extent in contr ol and occlusion groups between days I and 21 of the study. There was a sma ll decline in FHR variation over the 21 clay study in occlusion, but not co ntrol, group fetuses. 4. The magnitude of the fall in FHR decreased and the rise in MAP increased , despite similar changes in blood gases in response to umbilical cord occl usion, over the course of the 21 day study. Despite a significant decline i n the ratio of Delta FHR to Delta MAP on days 9 and IS compared to day 1, t here was no difference between control and occlusion groups in baroreflex s ensitivity. However Delta FHR/DeltaP(O2), an index of chemoreceptor sensiti vity, had decreased by clay 9 and 18 compared to day 1. 5. The cardiovascular responses to umbilical cord occlusion are altered wit h repetitive occlusions during the latter part of gestation, with a decreas e in Delta FHR/Delta MAP, which does not involve changes in baroreflex sens itivity, but may involve changes in chemoreceptor sensitivity. However, rep eated umbilical cord occlusion appears to have no impact on baseline cardio vascular control since there was no change in the normal maturational decre ase in FHR and rise in MAP.