Hypoxia as a Stimulus to Catecholamine Excretion in the Newborn Infant:II. The Effect of Exposure to 10% O₂1

Abstract

The exact mechanisms by which the physiologic responses of the newborn infant to conditions of asphyxia are mediated are still obscure. Moreover, it is not known which disordered parameter (i.e. hypoxia, hypercapnoea, or acidaemia) provides the stimulus for any or all of these changes.Dawes and his co-workers [3, 31 have proposed that under conditions of asphyxia in the newborn lamb, constriction of the ductus arteriosus is due to the endogenous release of sympathetic amines. The evidence for this is inferred from the resultant constriction of the ductus following the administration of adrenaline to the animal.Hypoxia is known to result in a rise in pulmonary artery pressure in both animals [6, 101, and human adults [4, 11, 151. Normally room air contains 20.9% 0,. I n newborn infants exposed for short periods (6 min) to an inspired air content of 10% O,, sufficient to cause an average 5 5 % fall in arterial 0, saturation, a significant Council of Canada. and consistent rise in pulmonary arterial pressure occurred [13]. The intermediary mechanism by which these changes in pulmonary artery pressure are mediated is not known. As it is known that the administration of adrenaline in man will result in an increase in pulmonary arterial pressure [16], it is tempting to implicate the endogenous release of catecholamines as the intermediate controlling mechanism.Adrenal medullary and peripheral sympathetic activation has been demonstrated in newborn infants. Thus, tilting in the erect position (baroreceptor response) significantly increases urinary adrenaline and noradrenaline excretion, while insulininduced hypoglycaemia results in a selective rise of adrenaline only [9]. The efficacy of a reduction in the 0, content of inspired air as a stimulus t o adrenal med-dlmysympathetic secretion is less well established (vide infra).I n a previous study in this laboratory [14] it was suggested that a reduction of the inspired air 0, content to 15% did not result in any significant changes in urinary catecholamine excretion. It may,

The Role of the Adrenal Medulla in the Hypoglycemia of Foetal Malnutrition

“…Since the resting excretion rates for urinary adrenaline are comparable to or larger than those previously reported for the normal newborn [4,11,12], the defect lies not in the resting level but in the inability to increase this under stress.…”

Chemoreceptor reflexes in the new‐born infant: effects of varying degrees of hypoxia on heart rate and ventilation in a warm environment*

“…The tachycardia could be due to increased sympathetic activity. The studies by Stern, Greenberg & Lind (1961) and Stern, Leduc & Lind (1964) who showed no detectable increase in urinary catecholamines in new-born infants breathing 10 and 15 % 02 tend to rule out the possibility -of adrenal medullary stimulation. However, the very small amounts of sympathetic amines released and metabolized at nerve endings make it possible for peripheral sympathetic stimulation to occur without increasing urinary catecholamines.…”

“…The tachycardia could be due to increased sympathetic activity. The studies by Stern, Greenberg & Lind (1961) and Stern, Leduc & Lind (1964) (Cross & Warner, 1951) during the 1st week of life. We believe that this may be due to a decrease in effective pulmonary blood flow and changes in pulmonary compliance, possibly in combination with a reduction in metabolism.…”

Catecholamine Excretion in Spontaneously Occurring Asymptomatic Neonatal Hypoglycemia

“…In the CORNBLATH series, the 3-and 5-h measurements were not made and the observations were discontinued after 6 h. Levels of catecholamine excretion, with means and standard error of the mean, are given in table III. The large patient-to-patient variability has been the experience of others [5,6,11,12,14]. Comparison of the means by groups revealed no significant differences in individual or total catecholamine excretions.…”