Rebecka L Meyers

Effects of Ductus Venosus Obstruction on Liver and Regional Blood Flows in the Fetal Lamb

ABSTRACT: The ductus venosus allows highly oxygenated blood returning from the umbilical-placental circulation to bypass the liver, and is believed thereby to facilitate preferential distribution of this blood to the fetal brain and heart. To examine this hypothesis, we developed a model that allows acute obstruction of the ductus venosus in chronically catheterized fetal lambs. In seven fetal lambs, a Swann-Ganz catheter was inserted into the inferior vena cava and the balloon tip advanced into the ductus venosus. Control measurements were obtained 1–2 d after surgery, before and during inflation of the balloon in the ductus venosus. At each sample time, radioactive microspheres were injected to determine organ blood flow and the distribution of umbilical venous blood flow. During balloon inflation, the percentage of umbilical venous return passing through the ductus venosus was reduced from 38 ± 15% to 1 ± 0.5%. Umbilical-placental blood flow was unchanged from control values of 181 ± 33 mL/min/kg. Total liver blood flow increased from 346 ± 98 to 553 ± 105 mL/min/100 g. Pressure in the inferior vena cava did not change, but umbilical venous pressure increased from 7.2 ± 2.7 to 8.7 ± 3.5 mm Hg. Total vascular resistance across the liver and ductus venosus increased from 0.013 ± 0.006 to 0.020 ± 0.011 during ductus venosus obstruction. Fetal heart rate, arterial blood pressure, and descending aortic pH and blood gases were unchanged, as was oxygen content in the descending aorta and carotid artery. Organ blood flows, combined ventricular output, and oxygen delivery were also unchanged. In five animals, these studies were repeated during maternal hypoxemia. Similar changes in liver blood flow were observed. Organ blood flows and oxygen delivery were not altered by ductus venosus obstruction during hypoxemia. We conclude that obstruction of the ductus venosus has no effect on regional blood flow distribution or oxygen delivery in normoxemic or hypoxemic animals. Because the hepatic microcirculation has such a high compliance, the ductus venosus is not crucial in regulating umbilical venous return to the central fetal circulation.

Venous and hepatic vascular responses to indomethacin and prostaglandin E1 in the fetal lamb

Evidence is accumulating that prostaglandins affect the tone of the ductus venosus. Therefore prostaglandins might alter the distribution of umbilical venous return between the ductus venosus and liver. We have examined the effect of indomethacin and prostaglandin E1 on the vascular resistance of the umbilical-placental circulation, ductus venosus, and liver in 14 chronically instrumented fetal lambs. We found that indomethacin caused a nearly twofold increase in the vascular resistance of both the ductus venosus and liver. Prostaglandin E1, administered 70 minutes after indomethacin at a dose of 0.41 +/- 0.09 (SEM) micrograms/min per kilogram of fetal weight, decreased umbilical-placental blood flow, increased the vascular resistance of the umbilical arteries and placenta, and reversed the elevation of the vascular resistances of the ductus venosus and liver. Because both indomethacin and prostaglandin E1 affected the resistances of the ductus venosus and liver to the same extent, it is unlikely that prostaglandins of the E series mediate the change in distribution of umbilical venous return between the ductus venosus and liver during fetal stress.

Effect of maternal oxygen administration on fetal oxygenation during graded reduction of umbilical or uterine blood flow in fetal sheep

OBJECTIVE Effects of maternal oxygen administration on fetal blood gases and on oxygen delivery and consumption during reduced uterine and reduced umbilical blood flows were examined. STUDY DESIGN In eight pregnant sheep (gestational age 133 +/- 4 days) flow transducers were applied to a uterine and the common umbilical artery. Graded reductions in uterine and umbilical blood flows were achieved by a hypogastric artery snare and a balloon cuff encircling the umbilical cord. Fetal femoral arterial and umbilical venous oxygen contents and flows were measured at varying flow reductions with the ewe breathing air or oxygen. RESULTS During 75% reduction in umbilical blood flow maternal oxygen administration significantly increased fetal oxygen delivery (6.4 +/- 2.5 to 7.7 +/- 2.3 ml/min/kg) and oxygen consumption (4.3 +/- 1.2 to 5.0 +/- 0.8 ml/min/kg). With similar reduction of uterine flow oxygen administration increased oxygen delivery from 8.3 +/- 2.4 to 12.3 +/- 3.6 and oxygen consumption from 3.3 +/- 0.8 to 4.7 +/- 1.6 ml/min/kg. CONCLUSION Maternal oxygen inhalation improves fetal oxygenation during umbilical but especially during uterine blood flow reduction.