Abstract
During labor, uterine contractions trigger the response of the autonomic nervous system (ANS) of the fetus, producing sawtooth-like decelerations in the fetal heart rate (FHR) series. Under chronic hypoxia, ANS is known to regulate FHR differently with respect to healthy fetuses. In this study, we hypothesized that such different ANS regulation might also lead to a change in the FHR deceleration morphology. The hypothesis was tested in an animal model comprising nine normoxic and five chronically hypoxic fetuses that underwent a protocol of umbilical cord occlusions (UCOs). Deceleration morphologies in the fetal inter-beat time interval (FRR) series were modeled using a trapezoid with four parameters, i.e., baseline b, deceleration depth a, UCO response time τu and recovery time τr. Comparing normoxic and hypoxic sheep, we found a clear difference for τu (24.8±9.4 vs. 39.8±9.7 s; p < 0.05), a (268.1±109.5 vs. 373.0±46.0 ms; p < 0.1) and Δτ = τu − τr (13.2±6.9 vs. 23.9±7.5 s; p < 0.05). Therefore, the animal model supported the hypothesis that hypoxic fetuses have a longer response time τu and larger asymmetry Δτ as a response to UCOs. Assessing these morphological parameters during labor is challenging due to non-stationarity, phase desynchronization and noise. For this reason, in the second part of the study, we quantified whether acceleration capacity (AC), deceleration capacity (DC), and deceleration reserve (DR), computed through Phase-Rectified Signal Averaging (PRSA, known to be robust to noise), were correlated with the morphological parameters. DC, AC and DR were correlated with τu, τr and Δτ for a wide range of the PRSA parameter T (Pearson's correlation ρ > 0.8, p < 0.05). In conclusion, deceleration morphologies have been found to differ between normoxic and hypoxic sheep fetuses during UCOs. The same difference can be assessed through PRSA based parameters, further motivating future investigations on the translational potential of this methodology on human data.
Highlights
During labor, a fetus might suffer considerable stress due to uterine contractions, causing transient oxygen reduction and head compression, resulting in vagal and sympathetic stimulations
In a study of fetal sheep exposed to repetitive umbilical cord occlusions (UCOs), a model of uterine contractions during labor, we found that there was a high correlation between AC and DC and acid/base balance [10]; AC and DC progressively increased with the severity of the UCOs, suggesting an activation of autonomic nervous system (ANS) of healthy normoxic fetus exposed to acute hypoxemia
These models were characterized by time constants, describing the speed of fetal RR series (FRR) adaptation and we found that healthy normoxic fetuses had longer UCO response times than the time necessary to return to the baseline level [10], suggesting the presence of asymmetric trends in the series during labor
Summary
A fetus might suffer considerable stress due to uterine contractions, causing transient oxygen reduction and head compression, resulting in vagal and sympathetic stimulations. It provides two measures that quantify the average cardiac acceleration (AC) and deceleration (DC) capacities from an inter-beat time interval series (RR). These measures quantify the average RR increase (or decrease) in milliseconds. In a study of fetal sheep exposed to repetitive umbilical cord occlusions (UCOs), a model of uterine contractions during labor, we found that there was a high correlation between AC and DC and acid/base balance [10]; AC and DC progressively increased with the severity of the UCOs, suggesting an activation of ANS of healthy normoxic fetus exposed to acute hypoxemia
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