Christine Roman

Relationship between Brain Blood Flow and Carotid Arterial Flow in the Sheep Fetus

ABSTRACT: The present study investigates whether changes in total brain blood flow can be reliably estimated by changes in carotid arterial blood flow in fetal and perinatal lambs. We therefore compared carotid arterial blood flow, measured with implanted transit-time ultrasound transducers, with brain blood flow, measured by radioactive microspheres in fetal lambs during normal oxygenation and during pulmonary ventilation with oxygen, with Po: ranging from levels normal for the healthy fetus to levels normally seen postnatally. Cerebral perfusion pressure was modified over a wide range to alter brain blood flow: it was decreased by balloon occlusion of the brachiocephalic trunk and increased by a balloon occluder around the aortic isthmus. Carotid arterial blood flow and brain blood flow were closely related (r = 0.97, p < 0.0001). The relationship was not altered at different levels of oxygenation. However, measurements during higher cerebral perfusion pressures, obtained during aortic isthmus occlusion, had a negative influence on the agreement between carotid arterial blood flow and brain blood flow. When excluding values obtained by aortic isthmus occlusion, changes of 20% or more in brain blood flow could be predicted with carotid arterial blood flow within a confidence limit of 95%. Blood flow measurements in the carotid artery may be useful to estimate changes in brain perfusion.

Circulating prostaglandin E2 concentrations and patent ductus arteriosus in fetal and neonatal lambs

We used pregnant sheep and their fetuses as well as newborn lambs (with and without severe respiratory distress due to prematurity) to study the differences in plasma clearance rate, production rate, and circulating concentrations of immunoreactive PGE2. Fetal PGE2 concentrations were significantly higher than simultaneous maternal concentrations. After delivery by cesarean section, all newborn animals were paralyzed and mechanically ventilated. The PGE2 concentrations fell in those lambs that required only minimal ventilatory support (FIO2 < 0.25) and were similar to maternal concentrations by two to three hours. Newborn lambs that developed severe respiratory distress (FIO2 < 0.55) continued to have concentrations that were even greater than fetal concentrations. The elevated PGE2 concentrations in severely distressed lambs were due not only to a decreased plasma clearance rate but also to an increased production rate of PGE2. Since PGE2 appears to maintain the patency of the ductus arteriosus in the fetus and preterm neonate, we examined the patency of the ductus arteriosus in 3-hour-old newborn lambs by radioactive microsphere injections. The ductus was more widely patent in lambs with higher concentrations of PGE2. The increased circulating concentrations of PGE2 in newborn lambs with severe respiratory distress may contribute to the pathogenesis of patent ductus arteriosus by exerting an additional vasodilatory effect on the vessel.

Cardiovascular effects of patent ductus arteriosus in preterm lambs with respiratory distress

We created a model for studying the cardiovascular and pulmonary effects of patent ductus arteriosus (PDA) in premature lambs with respiratory distress. In 47 fetal lambs at 129 to 133 days gestation (term, 145 days), we infiltrated the ductus arteriosus with formalin and placed a mechanical occluder about it so that its patency could be regulated. Two days later the lambs were delivered, given sheep surfactant, paralyzed, and their lungs mechanically ventilated. These premature lambs could more than double their left ventricular output when challenged with increasing degrees of left-to-right shunts through the PDA. This was accomplished by an increase in stroke volume, not by an increase in heart rate. During the 40-minute observation period, there was no change in dynamic compliance or functional residual capacity while the ductus was patent. When the ductus was patent, there was a significant increase in arterial PaO2 (even with small left-to-right shunts) and a decrease in PaCO2 (with large shunts). Despite the hearts ability to handle the increased volume load of a PDA, there were significant alterations in individual organ blood flows, resulting from a combination of decreased perfusion pressure and localized vasoconstriction. The abdominal organs had significant reductions in blood flow even with small PDA shunts. This decrease in organ blood flow may explain some of the pathophysiologic manifestations of PDA in preterm infants.

Effect of endothelium-derived relaxing factor inhibition on theumbilical-placental circulation in fetal lambs in utero

OBJECTIVE The purpose of this study was to examine whether basal endothelium-derived relaxing factor release contributes to regulation of resting umbilical-placental vascular resistance. STUDY DESIGN Because N omega-nitro-L-arginine selectively inhibits the synthesis of nitric oxide, a major endothelium-derived relaxing factor, we investigated the effects of N omega-nitro-L-arginine on umbilical-placental vascular resistance in 10 fetal lambs in utero. We inserted catheters and fitted an umbilical artery electromagnetic flow transducer around the common umbilical artery to measure umbilical blood flow and catheterized the left umbilical arterial hypogastric branch to allow selective umbilical-placental infusion (60 minutes) of pH-matched saline solution (control) or N omega-nitro-L-arginine. RESULTS In seven normal fetal lambs, N omega-nitro-L-arginine increased umbilical-placental vascular resistance and arterial pressures and decreased umbilical blood flow (p less than 0.05); percentage changes from baseline were 50.8% +/- 18.3%, 40.3% +/- 8.1%, and -9.9% +/- 6.4%, respectively. In three mildly asphyxiated (compromised) fetuses, these changes were 101.4% +/- 28.7%, 31.2% +/- 4.8%, and -37.9% +/- 12.0%. CONCLUSION These data support the hypothesis that the basal endothelium-derived relaxing factor release plays a role in regulating resting umbilical-placental vascular resistance.

Increased shunt through the patent ductus arteriosus after surfactant replacement therapy

Instillation of surfactant into the trachea of preterm infants with respiratory distress syndrome is associated with a 90% incidence of patent ductus arteriosus. We studied the effects of surfactant therapy on the ductus arteriosus in 12 preterm lambs. Flow across the ductus arteriosus and systemic blood flow were calculated from radioactive microsphere injections. All developed respiratory failure (pH less than 7.1, Paco2 greater than 60) by 30 minutes after birth. Between 30 and 60 minutes after birth, six lambs were treated with tracheal instillation of 50 mg/kg surfactant lipid. By two hours after birth, treated lambs differed significantly from controls in pH (7.27 +0.02 vs 6.97 +0.08) and Paco2 (43.3 +4.1 vs 85 + 15). There were no differences in Pao2 or PGE2 concentrations or ductus arteriosus resistance, but there was a significantly larger shunt through the ductus arteriosus in treated lambs. This increased shunt resulted from the significant drop in pulmonary vascular resistance and not from a change in patency of the ductus arteriosus. Surfactant replacement may require interventions directed specifically at the patent ductus arteriosus in sick preterm infants.

PGE2 is a more potent vasodilator of the lamb ductus arteriosus than is either PGI2 or 6 keto PGFα

It has been shown in vitro that the lamb ductus arteriosus forms prostaglandins PGE2, PGF2alpha, 6 keto PGF1alpha (and its unstable precursor PGI2). In this study the relative potencies of these endogenous prostaglandins were investigated on isolated lamb ductus arteriosus preparations contracted by exposure to elevated PO2 and indomethacin. All the prostaglandins (except PGF2alpha) relaxed the vessel. This is consistent with the hypothesis that endogenous prostaglandins inhibit the tendency of the vessel to contract in response to oxygen. Only PGE2, however, relaxed the vessel at concentrations below 10(-8)M. PGI2 and 6 keto PGF1alpha had approximately 0.001 and 0.0001 times the activity of PGE2. Although PGE2 has been observed to be a minor product of prostaglandin production in the lamb ductus arteriosus, the tissues marked sensitivity to PGE2 might make it the most significant prostaglandin in regulating the patency of the vessel.

Perinatal Myocardial DNA and Protein Changes in the Lamb: Effect of Cortisol in the Fetus

Myocardial growth during fetal life is accomplished by proliferation of the number of myocytes (hyperplasia). Shortly after birth, normal growth of the heart is predominantly due to increase in cell size (hypertrophy), and myocytes largely lose the capability to replicate. This change is characterized by a decrease in myocardial DNA concentration and an increase in protein/DNA concentration ratio. Among many of the events associated with birth is an increase in plasma cortisol concentrations in the few days before delivery of the fetus. To determine the possible role of cortisol in the postnatal change in myocardial growth, we measured DNA and protein concentrations in the free walls of the left (LV) and right (RV) ventricles in normal fetal lambs, normal newborn lambs, and in fetal lambs in which cortisone was infused for 72-80 h into the left coronary artery, which we showed does not perfuse the RV free wall. Normally, fetal RV DNA is higher than LV DNA concentration, and DNA/protein ratio is lower in RV than in LV. It is suggested that this could be related to the greater load on the RV. Postnatally, protein concentrations increase progressively, but DNA remains the same in both ventricles, and protein/DNA ratios increase. Cortisol, infused to achieve normal prenatal levels in LV myocardium, markedly decreases LV DNA without affecting RV DNA concentrations. The present study indicates that cortisol inhibits myocyte replication and that cortisol simulates the change in myocardial growth pattern normally occurring after birth. It raises concerns regarding prenatal administration of glucocorticoids to mothers to mature the fetal lungs before preterm delivery.

Role of Nitric Oxide in the Regulation of the Cerebral Circulation in the Lamb Fetus during Normoxemia and Hypoxemia

The influence of nitric oxide (NO) blockade on resting tone and on hypoxia-induced vasodilatation of the cerebral vascular bed was examined in chronically instrumented lamb fetuses. Total (Qbrain-tot) and regional brain blood flow were measured using radioactive microspheres. NO blockade was achieved by N omega-nitro-L-arginine (NNLA) infusion into the carotid artery via a lingual artery. Fetal cerebral blood flow and cerebral vascular resistance (Rcer) were determined during normoxemia and hypoxemia and before and during infusion of L-arginine. During normoxemia, the brain blood flow decreased, and the resistance increased significantly after NNLA infusion (Qbrain-tot from 129 +/- 25 to 89 +/- 26 ml/100 g/min, p < 0.05; Rcer from 0.46 +/- 0.03 to 0.80 +/- 0.09 mm Hg/ml/100 g/min, p < 0.05). During hypoxemia before NNLA infusion, Qbrain-tot increased (from 129 +/- 25 to 187 +/- 56 ml/100 g/min, p < 0.05), and Rcer decreased (from 0.46 +/- 0.03 to 0.39 +/- 0.07 mm Hg/ml/100 g/min, p < 0.05). This vasodilatory response was largely blocked after NNLA (Qbrain-tot 143 +/- 45 ml/100 g/min; Rcer 0.58 +/- 0.07 mm Hg/ml/100 g/min). The response to hypoxemia was restored after infusion of L-arginine (Qbrain-tot 180 +/- 47 ml/100 g/min). The resting tone of the cerebral vascular bed of the lamb fetus is under NO control, and NO mediates the cerebral vasodilatory response to hypoxia in the lamb fetus.

Factors determining the loss of ductus arteriosus responsiveness to prostaglandin E.

The ductus arteriosus of the newborn infant varies in its ability to dilate when doses of prostaglandin E (PGE) are administered. We infused PGE2 into 15 late-gestation newborn lambs to determine which factors regulate the ability of the ductus arteriosus to respond to PGE. PGE2 dilated the ductus in 10 lambs (responders); in five other lambs, despite similar PGE2 concentrations, there was no effect (nonresponders). The measured ductus resistance after the PGE2 infusion was directly related to the ductus resistance before the infusion. Responders had a larger left-to-right shunt through the ductus before the PGE infusion than nonresponders. Ductus that were isolated and studied in vitro from lambs that were nonresponsive to PGE2 (in vivo) were limited in their ability to relax with PGE2 as well as to actively contract with oxygen and indomethacin. This generalized loss of ductus responsiveness was directly related to the amount of the left-to-right shunt through the lumen of the ductus. These observations are consistent with the hypothesis that the constriction of the ductus arteriosus after birth limits the ability of the ductus to respond to PGE therapy.

Prostaglandin E2—Mediated Relaxation of the Ductus Arteriosus Effects of Gestational Age on G Protein-Coupled Receptor Expression, Signaling, and Vasomotor Control

Background—In the preterm newborn, a patent ductus arteriosus is in large part a result of the increased sensitivity of the immature ductus to prostaglandin E2 (PGE2). PGE2 acts through 3 G protein–coupled receptors (EP2, EP3, and EP4) that activate both adenyl cyclase and KATP channels. We explored these pathways to identify the mechanisms responsible for the increased sensitivity of the immature ductus to PGE2. Methods and Results—We measured EP receptor content (mRNA and protein), receptor binding, cAMP production, and isometric tension in rings of ductus taken from immature (65% gestation) and mature (95% gestation) sheep and baboon fetuses. Ductus relaxation and cAMP generation were augmented in response to selective EP receptor agonists in the immature ductus. 8-Br-cAMP, a stable cAMP analogue, produced greater relaxation in the immature ductus. In the presence of a selective protein kinase A inhibitor, Rp-8-CPT cAMPS, the developmental differences in sensitivity to PGE2 could no longer be demonstrated. EP2, EP3, and EP4 receptor densities were higher in immature ductus, despite similar receptor mRNA and protein contents at the 2 gestational ages. In contrast, forskolin and NaF, direct activators of adenyl cyclase and Gs, respectively, elicited comparable increases in cAMP in both age groups. KATP channel inhibition also had similar effects on PGE2-induced relaxation in both age groups. Conclusions—Two mechanisms explain the increased sensitivity of the immature ductus to PGE2: (1) increased cAMP production because of increased binding of PGE2 to the individual EP receptors and (2) increased potency of cAMP on protein kinase A–regulated pathways.