Cardiovascular and catecholamine responses to successive episodes of hypoxemia in the fetus.

The cardiovascular and plasma catecholamine responses to successive episodes of hypoxemia were evaluated in 5 fetal lambs in utero (greater than or equal to 0.7 gestation). Fetal heart rate (HR) and blood pressure (BP) were monitored continuously and specimens obtained for arterial blood gases, plasma norepinephrine (NE) and epinephrine (E) during a baseline control period, during each of 5 hypoxemic episodes and during 4 intervening recovery periods. Hypoxemia (PO2 = 9.8-13.7 Torr) was produced by gradual, controlled constriction of the umbilical cord by a silicone rubber cuff. HR fell from 176 +/- 9 beats/min (means +/- SE) to between 105 +/- 11 and 119 +/- 5 (p less than 0.01) and BP increased from 71 +/- 2/49 +/- 1 mm Hg to between 79 +/- 2/55 +/- 2 and 90 +/- 3/66 +/- 3 (p less than 0.01) during the hypoxic periods. There was no evidence of waning in the magnitude of the hypertensive response over the course of successive hypoxemic episodes. Baseline plasma NE and E rose from 278 +/- 65 and 28 +/- 8 pg/ml, respectively, to between a mean of 4,982 and 17,060 pg/ml for NE and 2,217 to 9,471 pg/ml for E. No attenuation in the rise of NE and E was observed throughout the 5 hypoxic periods. Thus, the late gestation fetus appears capable of sustaining appropriate cardiovascular and catecholamine responses to multiple successive episodes of hypoxemia.