Fetal heart rate variability changes during brief repeated umbilical cord occlusion in near term fetal sheep

Objective To determine whether changes in fetal heart rate variation during repeated umbilical cord occlusions reflect evolving cardiovascular comprom ise in near term fetal sheep. Design Fetal heart rate variation, fetal mean arterial pressure, electroenc ephalogram (EEG) and acid-base status were measured during one minute umbil ical cord occlusions, repeated either every five minutes (1:5 group) or eve ry 2.5 minutes (1:2.5 group) for four hours or until mean arterial pressure fell below 20 mmHg for two successive occlusions. Sample Fourteen chronically instrumented fetal sheep, mean gestation 126.3 (2.6) days. Results Cord occlusion caused variable decelerations with initial sustained hypertension. In the 1:5 occlusion group mean arterial pressure remained e levated throughout, with little change in acid-base status (pH = 7.34 (0.07 ), base deficit = 1.3 (3.9) after 4 hours) and no significant change in fet al heart rate variation. In contrast, in the 1:2.5 group from the third occ lusion there was progressive hypotension during occlusions, severe progress ive metabolic acidaemia (pH 6.92 (0.1), base deficit 17.0 mmol/L (4.7) afte r the last occlusion) and marked EEG suppression (P < 0.01). Fetal heart ra te variation increased with the onset of occlusions (P < 0.05) and then pro gressively fell with continued occlusions. During the last 30 minutes of oc clusions, fetal heart rate variation was severely suppressed in four, but i ncreased in two fetuses, while all six fetuses developed overshoot-instabil ity of fetal heart rate and mean arterial pressure following each occlusion . Conclusions Acute progressive asphyxia was typically associated with an imm ediate, transient increase in fetal heart rate variation. Subsequently vari ation became suppressed in only two-thirds of fetuses during terminal acida emia and hypotension. Fetal heart rate overshoot-instability may be a usefu l marker of fetal decompensation following variable decelerations.