Decelerations are the most common change seen in heart rate during labor. The role of autonomic nervous system (ANS) is well-establish for regulating heart rate (HR) and mean arterial pressure (MAP). Nevertheless, evolution of cholinergic and ß-adrenergic activity remains unclear.
To evaluate the effect of sympathetic or/and parasympathetic blockade during 1-min cord occlusion on fetal hemodynamic response.
Fifteen chronically instrumented fetal sheep received atropine 2.5 mg (n=8), or propranolol 5 mg (n=7), or atropine and propranolol (n=7), or vehicle (n=9), followed by 3 * 1 min of occlusions every 5 minutes.
For control group, HR decrease was immediately observed during 30 seconds with a rising of MAP. Then, a slightly rising of HR and a stable MAP appeared until the end of the occlusion. Blood gases did not change. Cholinergic blockage with atropine attenuated the bradycardia, demonstrated the key role of parasympathetic tone. Atropine exaggerated the initial hypertensive response during the few first seconds with two stages. In contrast, ß-blockade was associated with a greater reduction in HR from the onset of the occlusion and impaired the increase in MAP, confirming the role of parasympathetic activity at the early stage of occlusion and the key role of sympathetic activity after 30 seconds of occlusion. Both injection of atropine and propranolol have showed a gradually HR decline from the second period to the end of occlusion duration, demonstrating a possible role of an unknow mechanism, likely direct myocardium hypoxia.
These data provided a better understanding of fetal regulation during labor.