Effects of Hyperbaric Oxygen on Uteroplacental and Fetal Circulation

Abstract

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 Po 2 rose to 1,300 mm Hg while umbilical vein blood Poo rose to 300 mm Hg; umbilical arterial Po 2 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.From the

Role of the Alpha2-Adrenoceptors on the Pulmonary Circulation in the Ovine Fetus

“…The potent pulmonary vasodilation observed with UK 14,304 further indicates that selective activation of ␣ 2 -adrenoceptors decreases PVR. As observed with most pulmonary endotheliumdependent vasodilator stimuli in the fetus (acetylcholine, prostaglandins, shear stress, bradykinin), the pulmonary vascular response to UK 14,304 was transient (2,3). The mechanisms opposing prolonged pulmonary vasodilation are uncertain but may include time-dependent decreased ability to sustain production or effectiveness of endogenous vasodilators or enhanced production of vasoconstrictors.…”

“…PVR decreases dramatically during the normal transition from the fetal to neonatal circulation at birth. Three main factors contribute to the increase of Q p during this transition: ventilation of the lung (1), increased O 2 (2), and hemodynamic forces, such as increased shear stress (3). Vasoactive mediators released from the endothelium, such as NO, play a major role in the regulation of acute changes in vascular tone in the perinatal lung, and in many cases, modulate the pulmonary vascular response to these birth-related stimuli (4).…”

Responses of Small Intrapulmonary Arteries to Vasoactive Compounds in the Fetal and Neonatal Lamb: Norepinephrine, Epinephrine, Serotonin, and Potassium Chloride

“…Third generation vessels were significantly reduced in sensitivity (log molar concentration which produced 50% of the maximum r e s p o n~e -E C~~) to norepinephrine a t d 1 (5.89 + 0.12 log molar) and 7 (5.90 + 0. 21) At birth, the neonatal lung is still undergoing maturation and development: multiplication of intraalveolar vessels, remodeling of vessels under 250 pm, and extension of smooth muscle into the alveolar region occur (1)(2)(3)(4)(5)(6)(7)(8). Physiologically, the lungs adapt to extrauterine life with a progressive decrease in pulmonary vascular resistance and increase in blood flow to perform the vital function of gas exchange.…”

The uterine and maternal placental blood flow during hyperoxia

“…RORKE et al [10] in humans and several investigators using animals [5,4,1] have demonstrated that an increase of maternal oxygen tension causes only a disproportionately small increase of fetal oxygen tension. Such an effect might be explained by the fact that the placental venous P Oz increases only moderately and thus that the pressure head for oxygen diffusion is not elevated in proportion to maternal arterial P O2 .…”

“…The distribution of the blood flow between the myometrium and the maternal part of the placenta in particular, does not appear to have been investigated. KIRSCHBAUM et al [8] found that the uterine flow in anesthetized pregnant ewes was changed only minimally by acute Inhalation of pure oxygen (Method: electromagnetic flowmeter), while ASSALI et al [1] demonstrated a decrease of the uterine blood flow when the ewes were breathing hyperbaric oxygen, using the electromagnetic flow-meter method. BATTAGLIA et al [3] on the other hand found np significant effect of acute changes of the arterial oxygen pressure upon uterine blood flow (4-aminoantipyrine method).…”