to the editor: We thank Lear et al. (1) for their very interesting experimental study on parasympathetic activity between decelerations during brief repeated umbilical cord occlusions (UCOs) in fetal sheep. Using a well-known model of chronically instrumented fetal sheep at 0.85 of gestation, they defined three groups: one receiving bilateral cervical vagotomy, one atropine sulfate, and one sham treatment. The experiment consisted of three 1-min complete UCOs separated by 4-min reperfusion periods. Between UCOs, atropine sulfate and vagotomy receiving groups were associated with marked tachycardia, suppressed measures of fetal heart rate variability (FHRV), and abolished FHRV on visual inspection compared with the control group. They concluded that the parasympathetic system is likely to be the key mediator of FHRV once frequent fetal heart rate (FHR) decelerations are established during labor.
Thus, these results highlight the importance of the parasympathetic tone as a regulator of heart rate variability. Knowledge of fetal physiology during labor is a key point to improve fetal monitoring. Recent FIGO guidelines proposed a specific chapter on fetal discussing the physiology of fetal oxygenation and the main goals of intrapartum fetal monitoring (2). A more physiological approach to interpret FHR during labor is proposed by many authors in their recent reviews on this topic (3–6). Most of this knowledge comes from experimental studies, especially from the studies conducted by Lear et al. (7–9).
To propose a physiological evaluation of fetal well-being during labor, our team has developed a new index, the fetal stress index (FSI), which reflects the parasympathetic activity. Using the same sheep model as proposed by Lear et al. we first assessed the sensitivity and specificity of our index in evaluating parasympathetic activity (10). We directly administered 2.5 mg intravenous atropine, to inhibit parasympathetic tone, and 5 mg propranolol, to block β-receptors of sympathetic activity. Our index, as well as time analysis (root mean square of the successive differences; RMSSD) and spectral analysis [high-frequency (HF) and low-frequency (LF) spectral components obtained via fast Fourier transform] were measured before and after injection. The FSI, HF spectral component, and RMSSD reduced after the atropine injection as observed by Lear et al. (1). Although LF decreased after propranolol administration, FSI, RMSSD, and HF did not change significantly, confirming that these indexes are specific to the parasympathetic nervous system. After this first study, we conducted a labor-like experiment with repeated UCOs every 2.5 min over 3 h (11). Different from the model proposed by Lear et al., we focused on the development of progressive acidosis. We observed a gradual increase in our index, which reflected the role of the parasympathetic activity in the regulation of fetal homeostasis during labor in case of acidosis.
In conclusion, considering the results of the Lear et al. (1, 8, 9) study and our study in the same sheep model, measurement of fetal parasympathetic activity during labor seems to be a new interesting pathway for evaluation of fetal well-being.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the authors.
AUTHOR CONTRIBUTIONS
L.G. and C.G. drafted manuscript; L.G., J.D.J., L.S., and C.G. edited and revised manuscript; L.G., J.D.J., L.S., and C.G. approved final version of manuscript.
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