Hypotension in fetal and newborn lambs; different patterns of reflex heart rate control revealed by autonomic blockade.

Reflex heart rate (HR) responses to hypotension were studied in chronically instrumented fetal and newborn lambs. Studies spanned 106-141 days of gestation and 1-22 days after birth. Brief hypotensive stimuli (10 s) were produced by inflation of a cuff implanted around the inferior vena cava; HR and mean arterial pressure (MAP) were recorded from a carotid arterial catheter. Autonomic nervous control of HR was examined using selective sympathetic beta-adrenergic blockade (propranolol 1 mg/kg), cholinergic blockade (atropine 0.2-0.3 mg/kg), and total autonomic blockade (propranolol plus atropine). In newborn lambs (n = 4) HR increased progressively as MAP was reduced in the range 5-50%. Tachycardia during mild hypotension (less than 15% MAP fall) was due to sympathetic activation as it was abolished by propranolol. During severe hypotension (greater than 30% MAP fall) tachycardia was reduced by selective beta-adrenergic blockade and by cholinergic blockade, and totally abolished by total autonomic blockade; thus withdrawal of vagal tone plus augmentation of sympathetic activity contribute to the increase of HR in response to large MAP falls in the newborn. Fetal lambs (n = 4) responded with tachycardia in mild hypotension (less than 15% MAP fall) but this was reversed when hypotension was severe (greater than 30% MAP fall). The primary tachycardia was due to sympathetic activation and was indistinguishable from the newborn response.(ABSTRACT TRUNCATED AT 250 WORDS)