Thomas H. Kirschbaum

Effects of Hyperbaric Oxygen on Uteroplacental and Fetal Circulation

The effects of hyperbaric oxygen (at 3 atmospheres absolute) on uteroplacental and fetal circulations were studied in pregnant ewes near term. The ewe was given spinal anesthesia, the fetus was marsupialized to the abdominal walls to protect the umbilical circulation, and the fetal head was covered with a saline-filled glove to prevent breathing. During hyperbaric oxygenation, maternal arterial blood PO2 rose to 1,300 mm Hg while umbilical vein blood PO2 rose to 300 mm Hg; umbilical arterial Po2 rose to only 50 mm Hg. Maternal and fetal arterial pressures did not change significantly, but uteroplacental and umbilical flows decreased slightly. Ductus arteriosus blood flows decreased strikingly when the oxygen tension of the pulmonary blood rose; net pulmonary blood flow increased markedly because of a decrease in pulmonary vascular resistance produced by oxygen. Ascending aortic flow increased, but effective fetal cardiac output (aortic plus ductus arteriosus flows) decreased. These studies indicate that the fetal pulmonary vascular bed is sensitive to oxygen in that it undergoes vasodilatation when the oxygen tension of the blood passing through it rises; the ductus arteriosus responds to the same stimulus by constricting. Hyperbaric oxygenation seems to establish a circulatory pattern in the fetus similar to that of the early neonatal period.

The dynamics of placental oxygen transfer: I. Effects of maternal hyperoxia in pregnant ewes and fetal lambs☆

Abstract The effects of hyperoxia at one atmosphere on placental oxygen transfer in the pregnant sheep have been studied. Fetal blood pO 2 does not increase proportionately to the maternal increase, but is effectively limited below values of 60 mm. Hg. Net umbilical oxygen transfer is reversibly reduced during maternal hyperoxia. The Krogh equation is inadequate to predict or explain these findings. The need for a model of placental gas diffusion employing terms dealing with blood flow rates, hemoglobin properties, concentrations, and geometric relationships is established.

Oxyhemoglobin dissociation characteristics of human and sheep maternal and fetal blood

Abstract 1.1. Oxyhemoglobin dissociation curves were obtained from in vivo studies of maternal, fetal and neonatal sheep blood. Similar studies using slightly different techniques were performed on human maternal and fetal blood. 2.2. In the sheep, the in vivo analysis results in oxygen dissociation curves that are shifted to the left of those reported by others. The fetal curves lie to the left of maternal with the neonatal in between. In the human, the fetal curve is shifted to the right and is not significantly different from the maternal curve. 3.3. Oxyhemoglobin dissociation characteristics are probably influenced by the use of metabolic antagonists and by the passage of time required for analysis of the blood. These factors may alter the ionic distribution between plasma and erythrocytes and, hence, affect the oxygen affinity of the blood.

Spinal shock and fetal oxygenation

Abstract 1.1. Circulatory shock was induced experimentally in near term pregnant sheep. 2.2. Spinal hypotension was accompanied by a marked reduction in uterine blood flow and a rise in uterine vascular resistance. Uterine oxygen consumption fell only slightly. 3.3. No significant changes occurred in fetal arterial and venous pressures and in fetal oxygen consumption and umbilical flow. 4.4. Administration of Aramine corrected the maternal hemodynamic alterations of spinal shock but did not affect the fetus. Administration of oxygen raised maternal blood pO 2 but failed to either prevent the hemodynamic alterations of spinal shock or to alter fetal pO 2 .

Effects of maternal hypoxia on fetal cardiovascular hemodynamics

Abstract The effects of hypoxia on fetal cardiovascular hemodynamics were monitored in 24 near-term pregnant ewes and their fetal lambs. In one group of animals, effects of hypoxia on fetal effective cardiac output (ductus + aortic flows) were studied; in another group, the effects of hypoxia on pulmonary hemodynamics were investigated. Mild or early hypoxia produces a fall in fetal cardiac output largely because of a decrease in ascending aortic flow; ductus arteriosus flow may increase slightly or remain unchanged. During severe or prolonged hypoxia, the fetal effective cardiac output decreases more because of a fall in both the ductus and ascending aortic flows. The fetal systemic vascular resistance increases during hypoxia. Both severe and moderate hypoxia decrease net pulmonary blood flow (main pulmonary artery-ductus flows) and strikingly increase fetal pulmonary vascular resistance. The right and left ventricular output falls during hypoxia. A new approach for defining maternal and fetal hypoxia has been devised which, in our view, provides a more realistic way of correlating oxygen tension of the blood with cardiovascular changes.

Menaal exercises in placental transfer

M A N Y attempts have been made to determine the mechanisms of transfer of substances between mother and fetus across the placental membranes. The technique most frequently employed is that of inducing changes in the composition of the blood entering the maternal or fetal sides of the placenta and measuring the effects of these changes on the blood leaving the placenta. From the data obtained through this approach, placental exchange mechanisms have been postulated which have varied from concurrent to countercurrent or even multicurrent systems. These conflicting results seem to suggest the existence of a basic defect in the technique of determining the nature of intraplacental transfer mechanisms from extraplacental stresses and measurements. In our view, this approach does not take into consideration the many variables which enter into play simultaneously or independently when a substance is transferred across the placental membrane. The problem is further complicated by the fact that the interrelationship of the anatomic structures

Control of pulmonary and systemic vasomotor tone in the fetus and neonate

Abstract The factors that control the pulmonary and systemic vasomotor tones of the fetus and neonate have been reviewed. In addition, data on the effects of mechanical expansion of the alveoli as well as on those of acid and base infusions on polmonary and systemic hemodynamics in the fetal and neonatal lamb have been presented. The following conclusions can be drawn from these studies: (1) The rise in systemic arterial pressure that follows cord clamping and lung expansion is due to elimination of the low vascular resistance of the placenta. (2) The fall in pulmonary vascular pressure and resistance that follows the first breath is related to both the mechanical action of expanding the alveoli and the effects of oxygen on the vessel walls. (3) The pH and the Pco 2 of the blood do not seem to have any specific action on the pulmonary and systemic hemodynamic changes at birth as long as the Po 2 remains within normal ranges.

A Mathematical Model of Placental Oxygen Transfer.

Abstract : A mathematical model of placental oxygen transfer is used to argue that the experimentally observed control of fetal oxygen concentration in fetal lambs can be explained by the nature of oxyhemoglobin dissociation functions and the differences between these functions in fetal and maternal blood. It is not essential to the explanation to posit any mechanisms responding to a pO2 sensor or any changes in fetal, maternal, or placental vascular structures. Equally unnecessary are assumed alterations in the physical or chemical structure of placental membranes. (Aurthor)

The role of the umbilical sinus in the regulation of placental vascular resistance.

The role of the umbilical sinus – ductus venosus complex in the regulation of umbilical venous and placental vascular resistance was investigated in 12 near term exteriorized fetal lambs before and af

Effects of maternal-fetal blood exchange transfusion in fetal lambs

Abstract An attempt to evaluate experimentally the effects of intrauterine fetal exchange transfusion on placental function is reported. In a series of acute experiments carried out on pregnant sheep and their fetal lambs, maternal-fetal cross transfusion, sufficient significantly to increase the content of maternal blood in the fetus, was successfully carried out. No significant changes in fetal oxygen consumption or umbilical vein blood flow rates were noted. Significant fetal acidosis and an increase in fetal blood P O 2 , with the ewe breathing either air or oxygen, were seen. The implications are discussed.