Parasympathetic activity is the key regulator of heart rate variability between decelerations during brief repeated umbilical cord occlusions in fetal sheep

Fetal heart rate variability (FHRV) is a widely used index of intrapartum well being. Both arms of the autonomic system regulate FHRV under normoxic conditions in the antenatal period. However, autonomic control of FHRV during labor when the fetus is exposed to repeated, brief hypoxemia during uterine contractions is poorly understood. We have previously shown that the sympathetic nervous system (SNS) does not regulate FHRV during labor-like hypoxia. We therefore investigated the hypothesis that the parasympathetic system is the main mediator of intrapartum FHRV. Twenty-six chronically instrumented fetal sheep at 0.85 of gestation received either bilateral cervical vagotomy (n = 7), atropine sulfate (n = 7), or sham treatment (control, n = 12), followed by three 1-min complete umbilical cord occlusions (UCOs) separated by 4-min reperfusion periods. Parasympathetic blockade reduced three measures of FHRV before UCOs (all P < 0.01). Between UCOs, atropine and vagotomy were associated with marked tachycardia (both P < 0.005), suppressed measures of FHRV (all P < 0.01), and abolished FHRV on visual inspection compared with the control group. Tachycardia in the atropine and vagotomy groups resolved over the first 10 min after the final UCO, in association with evidence that the SNS contribution to FHRV progressively returned during this time. Our findings support that SNS control of FHRV is acutely suppressed for at least 4 min after a deep intrapartum deceleration and takes 5-10 min to recover. The parasympathetic system is therefore likely to be the key mediator of FHRV once frequent FHR decelerations are established during labor.