Development of autonomic control of fetal circulation

Abstract

Development of parasympathetic and sympathetic reflexes controlling heart rate, vascular pressures, and blood flows was investigated in fetal lambs weighing 300-5,800 g (65-165 days' gestation). Cardiovascular responses to veratridine injections, atrial stretching, bilateral cervical vagotomy, and cholinergic blockade with atropine were used to test parasympathetic activities. Responses to propranolol and phenoxybenzamine were used to test beta- and alpha-adrenergic activities. Autonomic ganglionic blockade and stimulation provided additional information on both cholinergic and adrenergic systems. Fetal responses to various tests were compared to those of the mother. Results show: a) little parasympathetic tone on resting heart rate and other circulatory functions exists prior to fetal maturity; b) despite the feeble resting tone, the parasympathetic system is capable of exerting significant control when stimulated in both premature and mature fetuses, the capability increases as fetus approaches term; c) alpha- and beta-adrenergic tone in control of resting heart rate and peripheral circulation exists in early fetal life and increases as the fetus reaches maturity, and both adrenergic receptors respond strongly to stimuli in immature, premature, and mature fetuses; d) in immature fetuses, veratridine does not elicit a vagally mediated reflex; instead, it produces a centrally mediated alpha- and beta-adrenergic stimulation; e) the fetal cardiovascular response to any given test is dampened by the existence of the various vascular shunts, the umbilicoplacental circulation and, possibly, by incomplete maturation of vasomotor tone.

Role of Sympathetic Activity in the Generation of Heart Rate and Arterial Pressure Variability in Fetal Sheep

“…00 values of 150 mm Hg (20 kPa)] may have helped prevent the development of fetal hypoxemia despite impaired uterine blood flow. The significant decreases in H R and arterial pressure resulting from ganglionic blockade confirms the findings of previous studies in near-term fetal sheep (22). In preterm fetuses, ganglionic blockade has little effect on systemic hemodynamics.…”

Autonomic control of cardiovascular functions during neonatal development and in adult sheep.

“…1 During the subsequent weeks of neonatal life, as well as in the adult sheep, the average blood pressure fall produced by the same dose of ganglionic blockade ranged from 8% to 12%; these values were significantly different from those observed in the 1st week and in the term fetus (P < 0.01) but were not materially different from each other (Fig. 3).…”

“…4, APRIL 1977 selected for assessing a given autonomic activity was administered through the jugular vein in the selected doses listed in table 1; all agents except trimethaphan were administered as single bolus intravenous injections; trimethaphan was given by constant infusion as previously. [1][2][3][4] (3) A recovery period until all parameters had returned to control values. During the testing and recovery periods, phasic and integrated pressure signals were recorded continuously on an Offner Dynograph; heart rate was obtained from the pulsatile pressure signal through a cardiotachometer; blood respiratory gases and pH were analyzed at frequent intervals.…”

“…The effects produced by the same procedures in the term fetus 1 and adult sheep are presented for comparison.…”

“…During the 1st week of neonatal life the systemic hypotension that followed ganglionic blockade was of the same magnitude as that observed in the full-term fetus. 1 This fact suggests that (1) the ability of the autonomic ganglia to transmit neural impulses to the vascular beds probably reaches full maturity by the time birth becomes imminent, and (2) the vascular systems of both the term fetus and the early newborn lamb have a relatively high neurogenic tone similar in many ways to that existing in neurogenic hypertension or in normotensive pregnant sheep and women.…”