Effect of Exteriorization of the Sheep Fetus on Its Cardiovascular Function

Abstract

Exteriorized sheep or goat fetuses, with placental circulation continuing, have been extensively used for measurements of fetal cardiovascular function. However, the effects of this procedure on fetal cardiovascular function have not been assessed. We inserted vinyl catheters into an umbilical vein, an umbilical or fetal hindlimb artery, and a hindlimb vein. We measured umbilical arterial and venous blood gas tensions, umbilical blood flow using the steadystate diffusion antipyrine method, and fetal arterial and umbilical venous pressures before and after delivery by cesarean section. Following exteriorization, placental vascular resistance consistently increased and umbilical blood flow decreased. Umbilical venous Po 2 increased, but umbilical arterial and venous pH and Pco L . and arterial Po 2 did not change significantly. Thus exteriorization of the fetus produces major changes in the fetal circulation, but these changes are not indicated by any major alterations in fetal blood gases and pH.ADDITIONAL KEY WORDS fetal exteriorization fetal blood gas tensions placental vascular resistance electromagnetic flowmeter umbilical blood flow antipyrine method• Since Huggett's (1) initial description, exteriorized sheep and goat fetuses have been extensively used for the measurement of fetal cardiovascular function. Various modifications of the original method have been adopted, but they still necessitate removal of the fetus from the uterus. Although much information about fetal physiology has been derived from these experiments, no one has studied the effects of the procedure on the fetus. We have used techniques recently developed for studying the fetus in utero (2, 3) to assess how exteriorization of the fetus may affect some aspects of fetal cardiovascular function and fetal blood gas status. Method Studies were performed on 10 unregistered pregnant sheep with fetuses of gestational ages ranging from 92 to 147 days as determined from nomograms published by Barcroft (4). Following a period of fasting of 12 to 24 hours, hyperbaric low spinal analgesia was given and the ewe was blindfolded and placed on her right side on an operating table. A vinyl catheter was placed in a branch of the maternal femoral artery. We then made a small incision in the ewe's left flank and placed vinyl catheters in the umbilical and lower limb veins and umbilical or hindlimb artery as we have described previously (3). The catheters were then brought out through the incision, and the abdominal incision was closed. Umbilical blood flow was measured in utero by application of the Fick principle, using our modification (5) of the original steady-state diffusion method of Meschia and co-workers (6). Continuous infusion of about 12 mg of antipyrine/min was begun after the abdominal incision was closed; a recovery period of at least an hour was allowed before umbilical flow was first measured.Umbilical venous and arterial and maternal arterial pH, blood gas tensions and blood pressures were monitored intermittently throughout the study....

Cardiopulmonary Function in Very Preterm Lambs during Liquid Ventilation

“…for prenatal carotid blood gas tensions (Pa02 = 25.3 -1 1.9 mmHg, Paco2 = 20.4 + 2.7 mmHg) and acid base status (pH = 7.29 f 0.02, HC03-= 13.3 + 4.3 mEq/liter) fell within the range previously reported in fetuses of similar gestational age (14). It has been recognized that exteriorization of the fetus leads to some differences in placental gas exchange (5). Figure 1 illustrates sequential Pao2, Paco2, pH and, rectal temperature data in lamb #50681 after exteriorization delivery, and during 165 min of liquid ventilation.…”

Maturational Changes in Myocardial Contractile State of Newborn Lambs

“…Finally, acute surgical exteriorization of the fetus is associated with a decline in umbilical flow and related parameters of ventricular function (13). Nevertheless, contractile indices obtained after exteriorization of fetuses but before simulation of birth were stable and compared favorably with those obtained in older lambs (Gps I1 to IV), suggesting that acute exteriorization did not alter these measurements of contractile state in our fetuses to any great degree.…”

“…In addition, we considered the potential effects of anesthesia, thoracotomy, and instrumentation on neonatal ventricular function. In contrast to the marked depressant effect of barbiturates on myocardial function (13,32), alpha chloralose has been shown to induce only minimal cardiovascular depression (32). Further, in the doses utilized, alpha chloralose does not alter oxygen consumption in the newborn lamb (3).…”

“…The newborn heart has been shown to be structurally and functionally immature (7,13,26). Despite this immaturity, hemodynamic alterations associated with the birth process result in a sudden persisting elevation in cardiac output in the first week of postnatal life (1,17).…”

Fetal Cardiovascular and Catecholamine Responses to Hypoxemia after Chemical Sympathectomy

“…Utilizing splanchnic nerve sections, Comline and Silver (4,5) have shown that sympathetic innervation of the adrenal gland is an important mediator of E release in the late gestation fetus; however, concern persists about these findings because of the acute nature of these experiments performed in exteriorized and anesthetized fepl lambs (12). Furthermore, little information exists about the response of fetal lambs in utero to hypoxemia after a more generalized reduction in sympathetic neuronal tone.…”