Increased heart rate response to parasympathetic and beta adrenergic blockade in growth-retarded fetal lambs.

To determine the influence of the autonomic nervous system on the circulation of normal and growth-retarded fetal lambs we measured the responses of heart rate and arterial blood pressure to parasympathetic (atropine 0.2 mg/kg) and beta-adrenergic (propranolol 1.0 mg/kg) blockade in the last quarter of gestation. The heart rate response to parasympathetic blockade increased with gestational age in both normal and growth-retarded fetuses but in the growth-retarded fetuses there was a significantly greater response to parasympathetic blockade than in control fetuses from 121 days' gestation to term. The heart rate response to beta-adrenergic blockade did not change with gestational age in normal fetuses. From 131 days' gestation to term the heart rate response to beta-adrenergic blockade in growth-retarded fetuses was significantly higher than in normal fetuses. The systemic arterial blood pressure responses to parasympathetic or beta-adrenergic blockade were similar in growth-retarded and normal fetuses. These results indicate an increased parasympathetic and beta-adrenergic influence on the heart of fetuses whose growth has been retarded. It may also suggest a more generalized increase in parasympathetic and sympathetic tone affecting other organs and systems in the growth-retarded fetal lambs.