Autonomic adjustments to severe hypotension in fetal and neonatal sheep.

In fetal sheep, severe hypotension causes heart rate (HR) slowing. Studies during development have also shown that a reflex bradycardia and hypotension can be elicited after chemostimulation with veratridine and is dependent on the age of the animal. In adults, a vagally mediated depressor reflex characterized by bradycardia, hypotension, and withdrawal of efferent sympathetic activity can be observed after stimulation of chemosensitive or mechanosensitive cardiac receptors with veratridine or in circumstances of reduced cardiac filling. This reflex, known as the Bezold-Jarisch reflex, plays a role in disease states such as myocardial ischemia and hemorrhage. The objectives of our study were to determine whether a sympathoinhibitor depressor reflex, along with the bradycardia, is observed during pharmacologically induced hypotension in fetal and newborn lambs. In both fetal and newborn lambs, HR and renal sympathetic nerve activity (RSNA) initially increased (p < 0.05) in response to nitroprusside infusion to reach a maximum value. The range (or "plateau") of mean arterial blood pressure over which maximum RSNA was maintained constant before withdrawal of sympathetic tone started to be observed was significantly (p < 0.05) smaller in fetuses (0.3 +/- 0.3 mm Hg) than newborn (6 +/- 1 mm Hg) lambs. Similarly, the plateau over which maximum HR was maintained before onset of bradycardia was significantly smaller in fetuses (4 +/- 1 versus 11 +/- 2 mm Hg). The mean arterial blood pressure level ("threshold") at which a depressor reflex was triggered was significantly (p < 0.05) lower in fetal than newborn sheep (35 +/- 2 versus 53 +/- 3 mm Hg for HR and 35 +/- 2 versus 57 +/- 2 mm Hg for RSNA). The rates of fall (slopes) for both HR and RSNA were also significantly (p < 0.05) more pronounced in fetuses (1.85 +/- 0.27 and 6.08 +/- 2.45%/mm Hg) than in newborns (1.21 +/- 0.16 and 1.97 +/- 0.32%/mm Hg). Bilateral vagotomy significantly increased the plateau of mean arterial blood pressure over which maximum RSNA and HR were maintained constant. Vagotomy also decreased the threshold for both RSNA and HR and the slope of the RSNA response to the nitroprusside infusion in newborn lambs. Results from this study show that activation of the arterial baroreflex during nitroprusside-induced hypotension is followed by withdrawal of sympathetic tone and bradycardia and that this depressor reflex is more pronounced in late-gestation fetuses than newborn lambs and is significantly attenuated after bilateral vagotomy in newborn lambs.