Plasma free and sulfoconjugated catecholamines during acute asphyxia in the sheep fetus — relation to cardiovascular parameters

Changes of free and sulfoconjugated catecholamines were measured radioenzymatically during a 5 minute period of acute asphyxia in chronically instrumented sheep fetuses (n = 5). Due to total reduction of uterine blood flow asphyxia as well as metabolic acidosis developed (pH = 6.94 +/- 0.02; Pco2 = 98 +/- 8 mmHG; lactate = 7.1 +/- 0.3 mmol/l). Peak concentrations of free catecholamines were reached after 3 minutes; free NE increased 60-fold, free E 370-fold and free DA 13-fold as compared to control values. Concomitantly, sulfoconjugated catecholamines rose markedly and were 40-fold (NE), 300-fold (E) and 10-fold (DA) higher when compared to the control period. Thus, the results reveal that the fetal sulfoconjugating system is very effective and able to match high concentrations of free catecholamines entering the circulation. After release of occlusion, free and sulfoconjugated catecholamines decreased with a half life time of 2.5 to 4.3 minutes during the initial 10 minutes. A close correlation could be demonstrated between free catecholamines and fetal arterial blood pressure, however, with interference of the effects of desoxygenation on the myocard. Moreover, fetal tachycardia is related to circulating catecholamines, especially E, during the postasphyxial period. Our results suggest, that in unstressed, normoxic fetuses the cardiovascular system is able to maintain basic functions (heart rate and blood pressure) during asphyxia for 5 minutes. In this context, a maximal sympathoadrenal stimulation with secretion of free catecholamines seems to be essential.