Effects of β-adrenergic stimulation on fetal heart rate, heart rate variability, and T-wave elevation during brief umbilical cord occlusions in fetal sheep

Circulating catecholamines are critical for fetal adaptation to hypoxia by regulating fetal heart rate (FHR) and promoting myocardial contractility and peripheral vasoconstriction. They have been hypothesized to contribute to changes in FHR variability (FHRV) and T-wave morphology, clinical indexes of fetal well-being during labor. β-Adrenergic blockade with propranolol does not affect FHRV during labor-like hypoxemia and only attenuated the increase in T-wave height between the episodes of hypoxemia. To further investigate the potential role of catecholamines, we investigated whether pharmacological β-adrenergic stimulation could increase FHRV and T-wave elevation during intermittent labor-like hypoxemia. Nineteen chronically instrumented fetal sheep at 0.85 of gestation received isoprenaline hydrochloride ( = 7) or saline (control, = 12), followed by three 1-min complete umbilical cord occlusions (UCOs) separated by 4-min reperfusion periods. Before the UCOs, infusion of isoprenaline increased FHR ( < 0.001), absolute-T/QRS ratio ( < 0.001), and one measure of FHRV [root-mean-square of successive RR interval differences (RMSSD), < 0.05]. UCOs triggered deep FHR decelerations. During UCOs, isoprenaline was associated with increased FHR ( < 0.001) and absolute-T/QRS ratio ( < 0.05), but no effect on T/QRS ratio was observed when normalized to baseline before UCOs (normalized-T/QRS ratio). Between UCOs, isoprenaline increased FHR ( < 0.001) and absolute-T/QRS ratio ( < 0.05) but did not affect normalized-T/QRS ratio or any measures of FHRV. Arterial pressure was not affected by isoprenaline at any point. Our findings indicate that circulating catecholamines regulate FHR but not FHRV during labor-like hypoxemia and promote T-wave elevation between but not during intermittent fetal hypoxemia.