Fetal heart and surroundings: umbilical cord traction triggers sympathetic heart rate surge in fetal lambs in the artificial womb

Objective: To investigate whether umbilical cord traction triggers a unique umbilico-neuro-cardio reflex via sympathetic autonomic nervous system (ANS) activation, potentially explaining fetal heart rate (FHR) increases such as the double mountain peak sign and unexplained tachycardia outside labor. Design: Experimental physiological study using the EXTrauterine Environment for Neonatal Development (EXTEND) system. Setting: Controlled laboratory environment at The Children's Hospital of Philadelphia with fetal lambs supported by the EXTEND system. Population or sample: Twenty-nine experiments were performed on three fetal lambs: 5 ANS-immature (<110 days gestational age) and 24 ANS-mature (>110 days GA), including subgroups for pharmacological intervention. Methods: Umbilical cord traction was performed in 20 experiments (5 ANS-immature, 15 ANS-mature). To investigate autonomic modulation, nine ANS-mature experiments included administration of either periumbilical capsaicin or intravenous propranolol. FHR changes were measured, and norepinephrine levels were sampled before and after interventions. Main outcome measures: Change in FHR (bpm) and circulating norepinephrine concentrations in response to umbilical cord traction and pharmacologic modulation. Capsaicin also elevated FHR and norepinephrine, while propranolol inhibited both responses. Conclusion: Umbilical cord traction appears to activate an umbilico-neuro-cardiac reflex through sympathetic ANS pathways in ANS-mature fetuses. This mechanism may underlie clinical CTG features such as the double mountain peak sign and fetal tachycardia outside of labor.