Responses to acute hypoxemia in fetal sheep at 0.6-0.7 gestation

Abstract

A majority of previous studies of fetal responses to acute hypoxemia has focused on the response of the sheep fetus greater than 120 days of gestation when many regulatory systems have been established. To assess the response of younger, less well-developed fetuses, we exposed two groups of fetal sheep (I, 84-91 days; II, 97-99 days gestational age) to acute hypoxemia by giving the ewe a gas mixture containing 9% O2 to breathe. We decreased descending aortic PO2 in both groups of fetuses [I, 24 +/- 6 to 14 +/- 3 (SD) Torr; II, 23 +/- 3 to 12 +/- 4 Torr] by a degree similar to that achieved in previous studies of fetuses greater than 120 days of gestation. Mean arterial blood pressure (I, 31 +/- 6; II, 40 +/- 3 Torr) did not change significantly from control values, and heart rate (I, 224 +/- 27; II, 203 +/- 16 beats/min) increased significantly in group II fetuses with hypoxemia. In group I and II fetuses, as in older fetuses, cerebral, myocardial, and adrenal blood flows, measured by the microsphere technique, increased, and pulmonary blood flow decreased. These responses mature early and are likely local vascular responses to decreases in oxygen content. Combined ventricular output and umbilical-placental blood flow decreased significantly in both groups. Unlike the response of the fetus greater than 120 days, acute hypoxemia did not decrease blood flow to the musculoskeletal and cutaneous circulations (group I only), gastrointestinal, or renal circulations.(ABSTRACT TRUNCATED AT 250 WORDS)

The Instrumented Fetal Sheep as a Model of Cerebral White Matter Injury in the Premature Infant

“…With models of maternal hypoxemia (e.g., high altitude or a low inspired oxygen fraction), fetal oxygenation falls while BP rises transiently, but then returns to normal, and then falls below normal [67,68,96,[103][104][105][106]. Perinatal hypoxic-ischemic encepatholpathy models that involve the entire body frequently use umbilical cord occlusion and are accompanied by reduced systemic fetal arterial oxygen tension and content [70,107,108].…”

Stress Hormone Responses During 24-Hour Hypoxemia in Preterm Goat Fetus

“…Matsuda et al (1992) reported that the cardiovascular and biophysical responses of the preterm sheep fetus to sustained hypoxemia were much less pronounced than those of more mature fetuses, and suggested that these differences might impact both survival and precision of antenatal assessment protocols. Boddy et al (1974) reported fetal tachycardia in response to induced hypoxemia in preterm fetuses (gestational day 100), as did Iwamoto et al (1989), Matsuda et al (1992), and Gleason et al (1990). These findings may relate to a predominance of sympathetic cardiac effects at earlier gestational ages.…”

“…It is unlikely that AVP and norepinephrine could maintain fetal redistribution of blood flow during chronic hypoxemia, given their tachyphylaxic effects (Miyake et al 1991). Iwamoto et al (1989) reported that in younger sheep fetuses, as in older fetuses, cerebral, myocardial, and adrenal blood flow increased, while pulmonary blood flow decreased with acute hypoxemia. These responses mature early and probably represent local vascular responses to decreased oxygen content.…”

“…Unlike the response of fetuses older than gestational day 120, acute hypoxemia did not decrease blood flow to musculoskeletal, cutaneous, gastrointestinal, or renal circulations. Umbilicalplacental blood flow is usually maintained in sheep fetuses older than gestational day 120, whereas blood flow to the fetal body decreases; conversely, the umbilical-placental circulation decreases by 33% in sheep fetuses at gestational day 80-100 (Iwamoto et al 1989). These developmental differences in responses to acute hypoxemia may be related to the rate at which chemoreceptor function, the cardiovascular system, autonomic nervous system, or adrenergic or vasopressin receptor-effector mechanisms mature.…”

“…Relatively few studies (Akagi and Challis 1990b;Carter et al 1998;Iwamoto et al 1989;Matsuda et al 1992;Richardson et al 1996) have been conducted in fetal sheep or goats at such early stages of development. We investigated fetal responses to sustained hypoxemia with pO 2 values around 15 mmHg for a duration of 24 h. Continuous infusion of nitrogen through the maternal tracheal catheter allowed us to rapidly and accurately adjust fetal pO 2 to the desired level.…”