Peripheral chemoreflex control of fetal heart rate decelerations overwhelms the baroreflex during brief umbilical cord occlusions in fetal sheep

Abstract

The majority of intrapartum decelerations are widely believed to be mediated by the baroreflex secondary to brief umbilical cord occlusions (UCOs) but this remains unproven. r We examined the responses to brief-UCOs in fetal sheep and compared these to a phenylephrine-stimulated baroreflex in a separate cohort. A further cohort was instrumented with near-infrared spectroscopy to measure cerebral oxygenation during UCO. r The first 3-4 s of the brief-UCOs were consistent with a baroreflex, and associated with a minor fall in fetal heart rate (FHR). Thereafter, the remainder of the FHR decelerations were highly consistent with the peripheral chemoreflex. r The baroreflex is not sufficient to produce deep, rapid decelerations characteristic of variable decelerations and it is therefore likely to be a minor contributor to intrapartum decelerations.

Transient effects of forebrain ischemia on fetal heart rate variability in fetal sheep

“…The delayed rise in MAP during carotid occlusion is additionally consistent with the sympathetic efferent arm of the peripheral chemoreflex, which induces hypertension via peripheral vasoconstriction (36). It is notable that an immediate increase in MAP was not observed during carotid occlusion.…”

Reply to “Letter to the Editor: Bezold–Jarisch reflex in the near-term fetus during labor: a matter of time”

“…First, the key efferent response of the Bezold-Jarisch reflex that would distinguish it from the peripheral chemoreflex is peripheral vasodilation due to sympathetic nervous activity withdrawal (5,6). We have shown using chemical sympathectomy with 6-hydroxy-dopamine that sympathetic nervous system-mediated peripheral vasoconstriction is maintained throughout a prolonged series of repeated UCOs despite the onset of arterial hypotension (6,7).…”

Acute fetal hypoxic bradycardia: solving the chemoreception puzzle

“…The article by Lear et al . (2020) in this issue of The Journal of Phsyiology reports on experiments in fetal sheep (∼125 days gestation) designed to test the comparative roles of baroreceptor‐mediated bradycardia associated with umbilical cord occlusion in utero vs . bradycardia mediated by chemoreception.…”

“…In this report, Lear et al . (2020) found that both short episodes of repeated complete cord occlusions and longer sessions of asphyxia showed an initial hypertensive phase with a clear fetal heart rate (FHR) suppression which lasted only a few seconds. The early response was very similar to the changes in heart rate (FHR) associated with an increase in mean arterial pressure (MAP) during phenylephrine infusion experiments.…”

“…Lear et al . (2020) show that while the baroreflex is alive and well, it is not responsible for prolonged reductions in heart rate that accompany complete cord occlusion. Furthermore, their data suggest that one cannot automatically assume that a depressed fetal heart rate signifies persisting hypertension.…”