Robert S Green

The Onset of Breathing at Birth Stimulates Pulmonary Vascular Prostacyclin Synthesis

Summary: The purpose of the present study was to determine if pulmonary prostacyclin synthesis was stimulated by spontaneous onset of breathing by unanesthetized fetuses at birth. Cannulae were implanted and flow cuffs placed in fetal lambs and goats (0.93 term). Fetuses were delivered by cesarean section at 0.95 term and began breathing spontaneously. Prostacyclin in blood was determined by radioimmunoassay of its hydrolysis product, 6-ketoprostaglandin F1α using methods that produced the same values in duplicate samples as did gas chromatography with electron capture detection. Fetal pulmonary prostacyclin production (left lung) [(left pulmonary venous concentration — pulmonary arterial concentration) × left pulmonary blood flow] was undetectable [−1.7 ± 1.0 (SEM) ng PGI2·kg−1·min−1] and fetal pulmonary vascular resistance (left lung) high (5.1 ± 0.9 mm Hg · kg·min · ml−1). Pulmonary prostacyclin production increased to 30.1 ± 12.3 ngPGI2 · kg−1 · min−1 and pulmonary vascular resistance declined to 0.5 ± 0.1 mm Hg · kg · min · ml−1 15 min after-birth. Pulmonary vascular resistance remained low even though pulmonary prostacyclin production fell 2-5 h after birth. These results, coupled with earlier studies using indomethacin to inhibit prostaglandin synthesis, support the hypothesis that pulmonary prostacyclin synthesis participates in the decline of pulmonary vascular resistance that accompanies the onset of ventilation at birth, but may be less important in maintenance of low pulmonary vascular resistance once reduced pulmonary vascular tone has been established.

Effects of indomethacin upon cerebral hemodynamics of newborn pigs.

ABSTRACT: Treatment of unanesthetized newborn pigs with indomethacin trihydrate (5 ± 1 mg/kg, intravenous) decreased cerebral blood flow uniformly throughout the brain by 18-28% without changing cardiac output, arterial pressure, or arterial blood gases and pH. Breathing 10% O2, 9% CO2 with the balance N2 (hypoxia/hypercapnia) caused cerebral blood flow to increase from 102 ± 12 to 218 ± 19 ml/100 g-min. Intravenous administration of indomethacin during hypoxia/hypercapnia caused a uniform decrease in cerebral flow throughout the brain to levels (94 ± 5 ml/100 g-min) indistinguishable from those when the piglet was breathing ambient air. Further, 2.5 h later, the cerebral hyperemia caused by hypoxia/hypercapnia was attenuated markedly (129 ± 19 ml/100 g-min). Vehicle treatment did not alter resting cerebral blood flow or cerebral hyperemia in response to hypoxia/hypercapnia. Measurements of 6-keto-prostaglandin F1α, thromboxane B2, and prostaglandin E2 demonstrated that intravenously administered indomethacin crossed the blood-brain barrier of newborn pigs in sufficient quantity to inhibit prostanoid release into the cerebrospinal fluid passing over the surface of the brain. The mechanism by which indomethacin reduces cerebral blood flow and attenuates cerebral hyperemia cannot be determined from the present experiments. We conclude that intravenous administration of indomethacin decreases cerebral blood flow and attenuates cerebral hyperemia induced by severe, combined hypoxia/hypercapnia in newborn pigs.

Indomethacin reduces the risks of severe intraventricular hemorrhage

A prospective, random selection, double-blind clinical trial was carried out to determine the efficacy of indomethacin in preventing periventricular-intraventricular hemorrhage (PV-IVH). Babies who were born in our institution, had birth weights less than or equal to 1500 gm, and had no PV-IVH or grade 1 PV-IVH were given either placebo (n = 70) or indomethacin (n = 71), 0.2 mg/kg intravenously at 6 hours of age and 0.1 mg/kg at 18 and 30 hours. Two major outcomes were determined: the development of grades 2 to 4 PV-IVH and the development of severe PV-IVH (i.e., hemorrhages with blood filling greater than 50% of the ventricles and in some cases with associated parenchymal echodensities). Grades 2 to 4 PV-IVH occurred in 16 (23%) of the indomethacin group and 27 (39%) of the placebo group (p less than 0.03). The incidence of severe PV-IVH was 3% in the indomethacin-treated babies and 14% in the control group (p less than 0.02). The influence of other perinatal factors on the incidence of grades 2 to 4 or severe PV-IVH was determined by stepwise logistic regression. Placebo use, early grade 1 PV-IVH, lower birth weight, and higher fraction of inspired oxygen at 6 hours of life were associated with higher estimated odds of the development of grades 2 to 4 PV-IVH. Placebo use, male gender, lower 5-minute Apgar score, and a large base deficit were predictive of severe PV-IVH. Estimated odds ratios of severe PV-IVH with placebo use and male gender were 11.25:1 and 9:1, respectively. Thus indomethacin prophylaxis reduced the relative risk of grades 2 to 4 PV-IVH and severe PV-IVH, but other perinatal variables contributed significantly to the overall risk of PV-IVH.

Relationship between mean airway pressure, cardiac output, and organ blood flow with normal and decreased respiratory compliance

We investigated the relation between blood flow and mean airway pressure in two groups of anesthetized newborn piglets. The first group had normal respiratory compliance; the second group had pulmonary surfactant depleted by repeated saline lavage, which decreased static respiratory compliance by 42%. In the normal group, cardiac output decreased linearly from 292 +/- 43 mL/min/kg at 5 cm H2O airway pressure to 134 +/- 37 ml/min/kg at 20 cm H2O airway pressure, a drop of 43% (r2 = 0.79). Blood flow to the heart, kidney, and intestines had a similar decline, but brain, hepatic artery, and adrenal flow were constant. Mean arterial blood pressure did not decrease significantly until the highest airway pressure was reached, whereas sagittal sinus pressure increased as mean airway pressure increased. In contrast, the surfactant-depleted group maintained cardiac output up to a mean airway pressure of 15 cm H2O. At 20 cm H2O, cardiac output fell to 40% of the original value. Blood flow to the heart and kidneys fell at a mean airway pressure of 20 cm H2O; intestinal blood flow decreased beginning at 10 cm H2O. As in the normal piglets, brain, hepatic arterial, and adrenal blood flow were not affected by increasing ventilation pressure. Our data show that positive pressure ventilation in the neonate has important cardiovascular effects that are blunted when respiratory compliance is decreased. More important, because cardiac output decreased prior to a significant decline in arterial blood pressure, these data suggest that in a clinical setting considerable cardiovascular alterations can occur before a decline in arterial blood pressure is detected.

Studies on Group B β-Hemolytic Streptococcus. II. Effects on Pulmonary Hemodynamics and Vascular Permeability in Unanesthetized Sheep

Summary: To study the effects of Group B β-hemolytic Streptococcus on the pulmonary circulation and lung fluid balance, live and heat-killed bacteria and their toxin were infused into an awake sheep lung-lymph preparation. In every case, the response was biphasic; there was an initial period of marked pulmonary hypertension and high flow of protein-poor lymph associated with tachypnea, chills, and fever. A second phase followed during which pulmonary vascular pressures returned to near baseline and remained stable, but lymph flow remained high. The changes seen in the initial phase resemble the previously reported response to mechanically increased pulmonary vascular pressure and suggest that the increase in fluid filtration is secondary to increased microvascular pressure. During the second phase after toxin infusion, the increase in lung lymph flow was paralleled by an increase in lymph protein clearance. This cannot be accounted for by the hemodynamic changes alone and suggests that the permeability of lung microvascular walls to protein was increased. It is concluded that group B β-hemolytic streptococcal toxin in the sheep model causes pulmonary hypertension and increased pulmonary vascular permeability.Speculation: The effects caused by group G β-hemolytic Streptococcus toxin on the pulmonary circulation may be relevant to the pathogenesis of the respiratory distress and shock seen in newborns with this infection. Further understanding of the effects of this toxin could provide means for therapeutic intervention.

Mechanisms of stimulation of pulmonary prostacyclin synthesis at birth

In order to investigate the mechanism behind ventilation-induced pulmonary prostacyclin production at birth, chloralose anesthetized, exteriorized, fetal lambs were ventilated with a gas mixture that did not change blood gases (fetal gas) and unventilated fetal lungs were perfused with blood containing increased O2 and decreased CO2. Ventilation with fetal gas (3%O2, 5%CO2) increased net pulmonary prostacyclin (as 6-keto-PGF1 alpha) production from -5.1 +/- 4.4 to +12.6 +/- 7.6 ng/kg X min. When ventilation was stopped, net pulmonary prostacyclin production returned to nondetectable levels. Ventilation with gas mixtures which increased pulmonary venous PO2 and decreased PCO2 also stimulated pulmonary prostacyclin production, but did not have greater effects than did ventilation with fetal gas. In order to determine if increasing PO2 or decreasing PCO2 could stimulate pulmonary prostacyclin production independently from ventilation, unventilated fetal lamb lungs were perfused with blood that had PO2 and PCO2 similar to fetal blood, blood with elevated O2, and blood that had PO2 and PCO2 values similar to arterial blood of newborn animals. Neither increased O2 nor decreased CO2 in the blood perfusing the lungs stimulated pulmonary prostacyclin synthesis. We conclude that the mechanism responsible for the stimulation of pulmonary prostacyclin production with the onset of ventilation at birth is tissue stress during establishment of gaseous ventilation and rhythmic ventilation.

Studies on group B beta-hemolytic Streptococcus. I. Isolation and partial characterization of an extracellular toxin.

Summary: To initiate an investigation into the biochemistry and mechanism of group B β-hemolytic Streptococcus virulence, bacterial cultures grown in suspension were centrifuged, and the bacteria and media were subjected to extensive fractionation. Each fraction was assayed for physiologic activity by repeated intravenous infusion into adult unanesthetized sheep. Pulmonary artery pressure, arterial Po2, and rectal temperature were monitored. The media fraction, but not the bacteria, contained a component (molecular weight, 2 X 106) composed of 84% carbohydrate and 16% protein with physiological activity. Two mg quantities, when infused, caused the pulmonary artery pressure to increase 100%, Po2 to decrease by 20% and chills and fever. The active component could be degraded by hot phenol-water extraction into a pure polysaccharide (molecular weight, 200,000). This lower molecular weight polysaccharide retained the identical physiologic properties when infused in the sheep. The degraded component precipitated with group B-specific antiserum.This study demonstrates that, in the sheep, a pure polysaccharide is the physiologically active part of a high-molecular-weight toxin synthesized by group B β-hemolytic Streptococcus type III and that this component has a different carbohydrate composition from the group B capsular antigen.Speculation: The clinical syndrome associated with group B β-hemolytic Streptococci in early onset disease is caused by the interactions of an extracellular bacterial component and a specific target tissue in the infected infant.

Effects of indomethacin on cardiac output distribution in normal and asphyxiated piglets

We determined the effect of breathing 9% CO2/10% O2/81% N2 (asphyxia) on cardiac output distribution (microspheres) in 4-5 day old unanesthetized, chronically instrumented piglets prior to and following intravenous indomethacin administration. Thirty minutes of asphyxia caused PaCO2 to increase from 35 +/- 2 mmHg to 66 +/- 2 mmHg, PaO2 to decrease from 73 +/- 4 mmHg to 41 +/- 1 mmHg, and pH to decrease from 7.52 +/- 0.05 to 7.21 +/- 0.07. Arterial pressure was increased slightly but cardiac output was not changed significantly. Asphyxia caused blood flow to the brain, diaphragm, liver, heart, and adrenal glands to increase while causing decreases in blood flow to the skin, small intestine, and colon. Blood flows to the stomach and kidneys tended to decrease, but the changes were not significant. Treatment with indomethacin during asphyxia did not alter arterial pressure or cardiac output but decreased cerebral blood flow to the preasphyxiated level and decreased adrenal blood flow about 20%. Indomethacin did not alter blood flow to any other systemic organ. At this time the piglet was allowed to breathe air for 2.5 hr undisturbed. Two and a half hours after indomethacin administration, blood flows to all organs returned to the preasphyxia control levels with the exception of cerebral blood flow which was reduced (93 +/- 13 to 65 +/- 7 ml/100 g X min). Three hours after indomethacin administration, the cerebral hyperemia caused by asphyxia was less (134 +/- 17 ml/100 g X min) than prior to indomethacin (221 +/- 15 ml/100 g X min). Indomethacin did not alter the asphyxia-induced changes to any other systemic organ.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of sodium chloride on pregnant sheep with reduced uteroplacental perfusion pressure.

This study investigated the effects of NaCl supplementation (5 mEq/kg/day) on the arterial pressure of pregnant and nonpregnant sheep with and without reduction of uteroplacental perfusion pressure. In pregnant sheep receiving NaCl supplementation during the third trimester, reduction of aortic pressure caudal to the kidneys to 65% of the upstream pressure (occlusion) caused a progressive increase in mean arterial pressure from 89 +/- 3 to 110 +/- 3 mm Hg over 2 weeks. Occlusion was accompanied by a decrease in urine flow. Six of seven sheep died or were killed because of severe respiratory distress. No abnormalities were detected in nonpregnant sheep or pregnant sheep receiving NaCl supplementation only. Pregnant sheep that were occluded but received no supplementary NaCl did not become hypertensive but aborted about 2 weeks after occlusion. These results indicate that reduction of uteroplacental perfusion pressure causes hypertension in NaCl-supplemented pregnant sheep but not in sheep receiving a normal, low sodium diet.

Indomethacin does not alter the circulating catecholamine response to asphyxia in the neonatal piglet

Abstract: The response of circulating catecholamines to asphyxia in unanesthetized, spontaneously breathing neonatal piglets was measured before and after treatment with indomethacin. Prior to treatment with indomethacin, baseline levels [geometric mean, pg/ml (95% confidence limits)] of D, E, and N were 162 (99-266), 174 (52-579), and 380 (286-506), respectively. Inhalation of 10% O2/9% CO2 for 20 min caused significant increases in arterial levels of all three catecholamines to 389 (230-659, 1514 (993-2306), and 3802 (2731-5293), respectively. Treatment with indomethacin (5 mg/kg, intravenous) did not significantly alter either baseline levels of the catecholamines or the levels after 20 min of the asphyxiating gas. In time control piglets, baseline levels and the response to asphyxia were similar before and after placebo. These results suggest that the circulating catecholamine response to asphyxia of the neonatal piglet is independent of the prostaglandin system.