Fetal heart rate overshoot during repeated umbilical cord occlusion in sheep

Objective: To assess the clinical utility of overshoot fetal heart rate (FH R) decelerations by examining their occurrence after umbilical cord occlusi ons of varying frequency and length in near-term fetal sheep. Methods: Fetuses were allocated to the following three groups: 1-minute umb ilical cord occlusion repeated every 5 minutes (1:5 group, n = 8) or every 2.5 minutes (1:2.5 group, n = 8) or 2-minute occlusions repeated every 5 mi nutes (2:5 group, n = 4). Occlusions were continued for 4 hours or until fe tal mean arterial pressure decreased below 20 mmHg during two successive oc clusions. Results: In the 1:5 group, fetuses tolerated 4 hours of occlusion without h ypotension or clinically significant acidosis and overshoot never occurred. In the 2:5 group, fetuses rapidly became hypotensive and acidotic, and occ lusions were terminated at 116.3 +/- 22.9 min (mean a standard deviation). Overshoot was seen after every occlusion, starting with the first occlusion . In the 1:2.5 group, fetuses became progressively acidotic and hypotensive and occlusions were stopped at 183.1 +/- 42.8 min. Overshoot occurred afte r 91.6 +/- 42.5 minutes, at a pH of 7.17 +/- 0.06, base deficit 9.3 +/- 4.5 mmol/L. After the appearance of overshoot there was a more rapid decrease in fetal mean arterial pressure (0.25 [0.21, 0.35, 25-75th percentile] mmHg /minute versus 0.11 [0.03, 0.15] mmHg/minute before overshoot appeared, P < .01). Conclusion: These data suggest that overshoot is related to longer (2-minut e) occlusions or to developing fetal acidosis and hypotension during 1-minu te occlusions. This pattern could have clinical utility, as 1-minute contra ctions are typical of active labor. (C) 2001 by The American College of Obs tetricians and Gynecologists.