Effects of Birth-Related Events on Central Blood Flow Patterns

Abstract

ABSTRACT. We determined the effects of three components of the birth process on central blood flow patterns in fetal sheep. We instrumented 16 fetal sheep at 133.0 f 1.2 days gestation, inserting various intravascular catheters, intubating the trachea, and placing an inflatable balloon around the umbilical cord. After 2-3 days, we determined central blood flow patterns using radionuclidelabeled microspheres under control conditions, during positive pressure ventilation without oxygenation, during ventilation with 100% Oz, and after umbilical cord occlusion. The foramen ovale right to left shunt was essentially abolished, decreasing from 102 f 48 to 66 f 40 ml/min/ kg with ventilation, and to only 13 f 10 ml/min/kg with oxygenation. The ductus arteriosus right to left shunt decreased progressively, from a control level of 224 2 64 to 6 f 10 ml/min/kg after umbilical cord occlusion. A ductus arteriosus left to right shunt appeared with oxygenation (41 f 26 ml/min/kg) and increased to 65 f 43 ml/ min/kg after cord occlusion. Left ventricular output increased progressively a s a percentage of combined ventricular output (from a control value of 34.8 to 59.5% after cord occlusion), and increased absolutely with ventilation (from 134 f 44 to 211 f 87 ml/min/kg). However, right ventricular output decreased (from a control value of 258 f 75 to 144 f 36 ml/min/kg after cord occlusion) so that combined ventricular output did not change. Although the transition from the fetal to neonatal circulatory pattern was accomplished by simulating these three components of the birth process, none is alone responsible for the large increase in combined ventricular output normally seen a t birth. The adaptation of the fetus to extrauterine life involves numerous processes that occur rapidly. Paramount among these is the change in the pattern of circulation of blood. The fetal circulation is characterized by the presence of central shunts that direct systemic venous return to the systemic arterial circulation ( Fig. 1): the foramen ovale directs the more highly oxygenated umbilical venous blood from the right atrium to the left heart to supply oxygen to the heart, brain, and other organs, and the ductus arteriosus directs the less oxygenated right ventricular output via the descending aorta to the placenta for oxygen uptake. The right ventricle ejects about 50% more blood than the left ventricle in the sheep fetus (1). As part of the transition from fetal to neonatal life, these central blood flow patterns change abruptly. The right to left shunts through the foramen ovale (2) and through the ductus arteriosus (3) cease rapidly, and a transient left to right shunt occurs until the ductus arteriosus closes fully within a few hours after birth (3,4). The two ventricles eject the same volumes once the ductus arteriosus does close. Associated with the increased oxygen requirements after birth, left ventricular output increases 2-to 21/2-fold (5, 6).The specific factors that cause this dramatic change in central blood flow patterns and thei...

In Utero Ventilation Augments the Left Ventricular Response to Isoproterenol and Volume Loading in Fetal Sheep

“…Oxygen content, POz, and Hb oxygen saturation were significantly greater during oxygen ventilation, at levels similar to those described previously in fetal sheep ventilated in utero (3).…”

“…(Pediatr Res 29: 466-472, 1991) The fetal left ventricle functions in an environment of low oxygen demand and in parallel with a dominant right ventricle. Consequently, its output need not be as high as that of the newborn left ventricle (1)(2)(3)(4). At birth, when oxygen demand increases dramatically and the systemic and pulmonary circulations separate, left ventricular output increases threefold (5)(6)(7)(8)(9)(10)(11)(12).…”

“…It is remarkable that both the prematurely born animal (26-28) and human (29), which have not been exposed to prenatal hormone surges and labor to accelerate myocardial maturation, are capable of greatly increasing left ventricular output in response to a patent ductus arteriosus. In addition, recent studies of fetal sheep undergoing oxygen ventilation in utero have shown a doubling of left ventricular output (3,30), and other determinants of output such as septal compliance (31) have been shown to change in the perinatal period.Thus, left ventricular output may be limited in the fetus more because of its "circulatory environment" (the combination of a parallel circulation with central shunts, a dominant hypertrophied right ventricle that ejects more output and fills under similar pressures, a pulmonary vascular bed that is markedly constricted and that returns a very limited amount of blood to the left atrium and ventricle, and a highly compliant fetoplacental vascular bed that can absorb increases in volume that would otherwise increase left ventricular filling volume) than because of myocardial immaturity. For example, the fetal left ventricle has a very limited ability to increase output via the Frank-Starling mechanism (13), with a plateau occurring at 5-7 mm Hg.…”

“…It is remarkable that both the prematurely born animal (26-28) and human (29), which have not been exposed to prenatal hormone surges and labor to accelerate myocardial maturation, are capable of greatly increasing left ventricular output in response to a patent ductus arteriosus. In addition, recent studies of fetal sheep undergoing oxygen ventilation in utero have shown a doubling of left ventricular output (3,30), and other determinants of output such as septal compliance (31) have been shown to change in the perinatal period.…”

“…To ensure a maximal decrease in pulmonary vascular resistance, 100% oxygen was used. The ventilation settings were initially adjusted to deliver an inspiratory:expiratory ratio of 0.8:I, a respiratory rate of 55-60/min, and a tidal volume of about 20-30 mL, similar to that used in our previous experiments (3,39). Positive endexpiratory pressure was applied to counterbalance amniotic pressure.…”