Lack of evidence for impaired preload or Bezold-Jarisch activation during brief umbilical cord occlusions in fetal sheep.

Abstract

Impaired cardiac preload secondary to umbilical cord occlusion (UCO) has been hypothesized to contribute to intrapartum decelerations, brief falls in fetal heart rate (FHR), through the activation of the Bezold-Jarisch reflex. This cardioprotective reflex increases parasympathetic and inhibits sympathetic outflows triggering hypotension, bradycardia and peripheral vasodilation but its potential to contribute to intrapartum decelerations has never been systematically examined. In this study we performed bilateral cervical vagotomy to remove the afferent arm and the efferent parasympathetic arm of the Bezold-Jarisch reflex. 22 chronically instrumented fetal sheep at 0.85 of gestation received vagotomy (n=7) or sham-vagotomy (control, n=15), followed by three 1-min complete UCOs separated by 4-min reperfusion periods. UCOs in control fetuses were associated with a rapid fall in FHR and reduced femoral blood flow mediated by intense femoral vasoconstriction, leading to hypertension. Vagotomy abolished the rapid fall in FHR (p<0.001), and despite reduced diastolic filling time, increased both carotid (p<0.001) and femoral (p<0.05) blood flow during UCOs, secondary to carotid vasodilation (p<0.01) and delayed femoral vasoconstriction (p<0.05). Finally, vagotomy was associated with an attenuated rise in cortical impedance during UCOs (p<0.05), consistent with improved cerebral substrate supply. In conclusion, increased carotid and femoral blood flows after vagotomy are consistent with increased left and right ventricular output, which is incompatible with the hypothesis that labor-like UCOs impair ventricular filling. Overall, the cardiovascular responses to vagotomy do not support the hypothesis that the Bezold-Jarisch reflex is activated by UCO. The Bezold-Jarisch reflex is therefore mechanistically unable to contribute to intrapartum decelerations.