Fetal sympathetic activity, transcutaneous PO2, and skin blood flow during repeated asphyxia in sheep

To improve detection of fetal distress, we examined whether increased fetal sympathetic activity during repeated episodes of asphyxia decreases skin blood flow, which can be monitored by recording transcutaneous PO2. Sympathetic activity was assessed by relating catecholamine concentrations in the fetal plasma to blood gas, acid-base, and heart rate variables which are commonly used to determine fetal distress. Fifteen experiments were conducted on 8 anaesthetised fetal sheep in utero between 125 and 145 days of gestation (term is at 147 days). They were subjected to 11 consecutive episodes of asphyxia of 30 (n = 3), 60 (n = 9), or 90 (n = 3) s over 33 min, achieved by arrest of uterine blood flow. Blood samples were drawn at 0, 33, and 60 min to determine arterial blood gases, acid base-balance, and concentrations of lactate, glucose, norepinephrine, and epinephrine. Fetal transcutaneous PO2, relative local skin blood flow, heart rate, arterial blood pressure, and arterial O2 saturation were recorded continuously. Fetal plasma concentrations of norepinephrine and epinephrine increased logarithmically as the duration of repeated asphyxia, anaerobic metabolism, and glucose concentrations increased, and as the mean O2 saturation, transcutaneous PO2, and local skin blood flow decreased. We conclude that during repeated episodes of asphyxia in fetal sheep near term, a significant increase in sympathetic activity can be detected indirectly by transcutaneous PO2 monitoring, because sympathetic activation reduces skin blood flow.