Characteristics of pulmonary hypertension in preterm neonates.

OBJECTIVE: Characteristics of preterm infants who develop pulmonary hypertension (PHT) and their response to inhaled nitric oxide (iNO) are not well described. Our objective was to identify risk factors for PHT in infants <37 weeks gestational age (GA) and to evaluate their response to iNO. STUDY DESIGN: A retrospective chart review was conducted in infants <37 weeks GA born from July/2000 to October/2005 who had an echocardiographic diagnosis of PHT in the first 4 weeks of life. A comparison non-PHT group was generated matched for GA and birth date. Data on prenatal and postnatal characteristics, response to iNO and mortality were collected. RESULTS: Low Apgar scores, preterm premature rupture of membranes, oligohydramnios, pulmonary hypoplasia and sepsis were independently predictive of PHT. Mortality was significantly higher in the PHT group (26.2% versus 4.1%; P<0.0001) compared to the control group. Low birth weight, severe intraventricular hemorrhage and male sex were significantly associated with death in infants with PHT. Thirty-seven percent (23/61) of infants with PHT were treated with inhaled NO. Infants < 29-week GA had poor response to iNO and the response to iNO increased with GA (P<0.02). CONCLUSIONS: Low Apgar scores, oligohydramnios and pulmonary hypoplasia are associated with the development of PHT in premature infants. The percentage of infants responding to iNO increases with advancing GA.

Ductus venosus flow velocity in newborn lambs during increased pulmonary artery pressure.

The aim of the present study was to assess with ultrasound the ductus venosus flow velocity in newborn lambs with increasing pulmonary artery pressures and to evaluate whether this is a useful method to detect elevated pulmonary artery pressure. The ductus venosus flow velocity was studied with pulsed-wave Doppler echocardiography in nine newborn lambs < or = 30 h old. The lambs were anesthetized, mechanically ventilated, and instrumented to measure mean airway pressure and pulmonary artery and arterial blood pressures. A vascular occluder was placed around the main pulmonary artery. With mean pressures ranging from 20 to 50 mm Hg in the pulmonary artery, the ductus venosus flow velocity was examined. In seven lambs, the mean portal pressure and central venous pressure were also measured. With a stepwise increase in the pulmonary artery pressure, the minimum ductus venosus flow velocity during atrial systole decreased to a reversed flow, and the duration of this reversed flow component increased. The systolic forward peak flow velocity signal also gradually decreased. No changes were detected in the mean central venous or in the portal pressure with increasing pulmonary artery pressure or changes in ductus venosus flow. The flow velocity in the ductus venosus, which is higher than in other precordial veins, shows a reduction and even reversal of the nadir and an increase of the duration of reversed flow during atrial systole as a response to increased pulmonary artery pressure. Thus, Doppler ultrasound of the ductus venosus flow velocity may be a useful noninvasive diagnostic supplement to detect pulmonary hypertension of the newborn.

The cGMP-specific phosphodiesterase inhibitor E4021 dilates the pulmonary circulation.

We investigated the pulmonary vascular effects of E4021, a potent inhibitor of cGMP-specific phosphodiesterase, in control late-gestation fetal lambs, and in newborn lambs with persistent pulmonary hypertension (PPHN) after prenatal ligation of the ductus arteriosus. E4021 alone significantly relaxed fifth-generation pulmonary arteries isolated from control fetal lambs, an effect completely blocked after inhibition of nitric oxide synthase (NOS). In contrast, E4021 did not relax pulmonary arteries isolated from hypertensive lambs. Pretreatment with E4021 (10(-7) M) significantly enhanced relaxations to the NO donor S-nitrosyl-acetyl-penicilamine (SNAP) in arteries from both control and hypertensive lambs. In control, fully instrumented fetal lambs, infusions of E4021 (31 microgram/min) selectively dilated the pulmonary circulation, an effect again blocked after inhibition of NO synthase. Further studies were performed in newborn lambs with PPHN to study the vascular effects of E4021 alone, and in combination with inhaled NO. E4021 alone (1 to 100 microgram/kg/min) decreased pulmonary artery pressure (Ppa) in a dose-dependent fashion, and had minimal effect on systemic pressure. At the highest dose (100 microgram/kg/min), the dilation was selective for the pulmonary circulation. In subsequent protocols, E4021 (10 microgram/kg/min) significantly decreased Ppa and pulmonary vascular resistance (PVR), but these pulmonary vascular effects were not enhanced after NO inhalation at 0.5 or 5 ppm. We speculate that the lack of enhancement was due to the dramatic effects of E4021 alone. Potent, specific phosphodiesterase inhibitors such as E4021 may prove to be useful in the treatment of PPHN.

Pulmonary and systemic effects of the phosphodiesterase inhibitor dipyridamole in newborn lambs with persistent pulmonary hypertension.

Nitric oxide (NO) relaxes vascular smooth muscle by increasing the intracellular concentration of cGMP. In the pulmonary circulation, cGMP is inactivated by specific phosphodiesterases (PDE5). Dipyridamole, a clinically approved drug, has inhibitory activity against PDE5 and has been reported to augment the response to inhaled NO in persistent pulmonary hypertension of the newborn (PPHN). We wished to determine whether dipyridamole alone, or in combination with NO, can be used to treat a newborn lamb model of PPHN. In newborn lambs with PPHN, dipyridamole infused at 0.02 mg/kg/min for 45 min alone, or in combination with 5 ppm of inhaled NO for the final 15 min, significantly decreased pulmonary and systemic blood pressure, decreased pulmonary vascular resistance, and increased pulmonary blood flow. There was no significant difference between the pulmonary vascular effects of 5 ppm NO alone compared with the effects of NO combined with dipyridamole. In control lambs, the 45-min infusion of dipyridamole did not change pulmonary pressure whereas systemic pressure decreased by 28 +/- 3%. These systemic effects in control lambs persisted 90 min after discontinuing the dipyridamole infusion. Systemic arteries isolated from both control and PPHN lambs were significantly more sensitive to dipyridamole than pulmonary arteries. We conclude that dipyridamole has significant hemodynamic effects in both the pulmonary and systemic circulations of newborn lambs with pulmonary hypertension as well as in the systemic circulation of newborn control lambs. The pronounced effects of dipyridamole on the systemic circulation limits its utility as an adjunct to inhaled NO in the treatment of PPHN.

High-frequency partial liquid ventilation in respiratory distress syndrome: hemodynamics and gas exchange.

Partial liquid ventilation using conventional ventilatory schemes improves lung function in animal models of respiratory failure. We examined the feasibility of high-frequency partial liquid ventilation in the preterm lamb with respiratory distress syndrome and evaluated its effect on pulmonary and systemic hemodynamics. Seventeen lambs were studied in three groups: high-frequency gas ventilation (Gas group), high-frequency partial liquid ventilation (Liquid group), and high-frequency partial liquid ventilation with hypoxia-hypercarbia (Liquid-Hypoxia group). High-frequency partial liquid ventilation increased oxygenation compared with high-frequency gas ventilation over 5 h (arterial oxygen tension 253 +/- 21.3 vs. 17 +/- 1.8 Torr; P < 0.001). Pulmonary vascular resistance decreased 78% (P < 0.001), pulmonary blood flow increased fivefold (P < 0.001), and aortic pressure was maintained (P < 0.01) in the Liquid group, in contrast to progressive hypoxemia, hypercarbia, and shock in the Gas group. Central venous pressure did not change. The Liquid-Hypoxia group was similar to the Gas group. We conclude that high-frequency partial liquid ventilation improves gas exchange and stabilizes pulmonary and systemic hemodynamics compared with high-frequency gas ventilation. The stabilization appears to be due in large part to improvement in gas exchange.

Heterogeneous distribution of soluble guanylate cyclase in the pulmonary vasculature of the fetal lamb.

BACKGROUND: Vascular segments in the fetal lung differ anatomically and functionally from one another. At birth, the nitric oxide (NO) pathway plays an integral role in reducing pulmonary vascular resistance through a marked vasodilation. However, the contributions of each vascular segment to this dilation are unclear. We sought to determine the distribution of soluble guanylate cyclase (sGC), the enzyme NO activates to induce vasodilation across the pulmonary vasculature. METHODS: Pulmonary airspaces were expanded with freezing compound and the pulmonary arterial tree was infused with barium sulfate gelatin. Soluble guanylate cyclase was localized by immunohistochemistry across the pulmonary vasculature of four near-term fetal lambs and its immunoreaction product was assessed by a semiquantitative method. The physiologic response of fourth- and fifth-generation arteries and veins isolated from age-matched lambs to NO was measured using standard tissue bath techniques. RESULTS: Clear differences in sGC immunostaining were present throughout the pulmonary vasculature: very weak to absent in large arteries accompanying bronchi, but intensely positive for veins. This pronounced staining for sGC in preacinar veins correlated with a 100-fold greater sensitivity to NO in veins compared to arteries of the same generation. The percentage of arteries staining positively approached 100% at the level of respiratory bronchioles and alveoli. CONCLUSIONS: These findings suggest that the increased response to NO in preacinar veins compared to that of arteries is in part due to increased sGC within venous vascular smooth muscle. Furthermore, intense staining within distal arteries implies a greater role for NO-mediated vasodilation within these segments.

cGMP inhibition of endothelin-stimulated inositol phosphate production in the fetal lamb pulmonary artery.

Endothelin-1 (ET-1) stimulates inositol phosphate production in vascular smooth muscle. In the present study, interactions between cyclic GMP (cGMP), cyclic AMP (cAMP) and ET-1 in fetal lamb pulmonary arteries were investigated using phosphoinositide hydrolysis studies and tissue bath techniques. ET-1 was found to be a potent vasoconstrictor of these vessels, with an EC50 of 15.8 nM. ET-1 stimulated total inositol phosphate (IP) production; basal IP production was 68 cpm/mg vs. 247 cm/mg with 1 microM ET-1. 8-bromo-cGMP (2 mM) significantly increased the threshold of ET-1 concentration for pulmonary artery contraction, but had no effect on IP production. Zaprinast (a selective type V phosphodiesterase inhibitor, 60 microM) did not affect ET-1-induced contractility or IP production. IBMX (0.5 mM), a non-specific phosphodiesterase inhibitor, inhibited the potent and maximal effects of ET-1 in arterial contraction and decreased ET-1-stimulated IP production by 49%, while forskolin had a lesser effect in the tissue bath and no effect on IP production. Thus, 8-bromo-cGMP and IBMX alter the contractile effects of ET-1 in the fetal pulmonary artery and IBMX also inhibits inositol phosphate production. The cross-talk mechanisms of these agents require further investigation.

Models of persistent pulmonary hypertension of the newborn (PPHN) and the role of cyclic guanosine monophosphate (GMP) in pulmonary vasorelaxation.

At birth, a marked decrease in pulmonary vascular resistance allows the lung to establish gas exchange. Persistent pulmonary hypertension of the newborn (PPHN) occurs when this normal adaptation of gas exchange does not occur. We review animal models used to study the pathogenesis and treatment of PPHN. Both acute models, such as acute hypoxia and infusion of vasoconstrictors, and chronic models of PPHN created both before and immediately after birth are described. Inhaled nitric oxide is an important emerging therapy for PPHN. We review nitric oxide receptor mechanisms, including soluble guanylate cyclase, which produces cGMP when stimulated by nitric oxide, and phosphodiesterases, which control the intensity and duration of cGMP signal transduction. A better understanding of these mechanisms of regulation of vascular tone may lead to safer use of nitric oxide and improved clinical outcomes.

Partial liquid ventilation and nitric oxide in congenital diaphragmatic hernia.

PURPOSE: In congenital diaphragmatic hernia (CDH) there is immature lung development with a resulting clinical picture of pulmonary hypoplasia, surfactant deficiency, and pulmonary hypertension. Pulmonary hypoplasia and surfactant deficiency both have been successfully treated using partial liquid ventilation (PLV). Pulmonary hypertension associated with CDH has proven difficult to treat, but inhaled nitric oxide, which is a potent highly selective pulmonary vasodilator, may have potential. The aim of this study was to assess PLV in CDH and to document the effect of nitric oxide when administered through perfluorocarbon. METHODS: This study using the lamb CDH model consisted of two groups; a conventional mechanically ventilated (CMV) group and a PLV group. At 1 and 3 hours, nitric oxide (80 ppm) was given for 15 minutes. Data collected included blood gases, pulmonary function tests, pulmonary and systemic blood pressure. RESULTS: After 30 minutes of ventilation, blood gases in the PLV group were all significantly improved (P < .001): pH, CMV 6.92 +/- 0.15 versus PLV 7.24 +/- 0.11; P(CO2), CMV 139 +/- 26 mmHg versus PLV 52 +/- 11 mmHg; P(O2), CMV 26 +/- 15 mmHg versus PLV 184 +/- 60 mmHg. In addition, there was a significant increase in dynamic compliance and a reduction in pulmonary hypertension. Nitric oxide was only efficacious in the PLV group, causing a further increase in oxygenation and a decrease in pulmonary hypertension. These effects were reversed when the nitric oxide was stopped. CONCLUSION: This study shows that PLV both improves gas exchange and pulmonary mechanics in CDH and allows the effective delivery of nitric oxide to reduce the pulmonary hypertension associated with CDH.

Pulmonary endothelial NO synthase gene expression is decreased in fetal lambs with pulmonary hypertension.

Nitric oxide (NO), produced by endothelial (e) NO synthase (NOS), is critically involved in the cardiopulmonary transition from fetal to neonatal life. We have previously shown that NO-dependent relaxation is attenuated in intrapulmonary arteries from fetal lambs with pulmonary hypertension (PHT) created by prenatal ligation of the ductus arteriosus. In the present study, we determined whether this is due to altered pulmonary eNOS expression. eNOS and neuronal NOS (nNOS) protein expression were assessed in lungs from near-term control lambs and PHT lambs that underwent ductal ligation 10 days earlier. eNOS protein expression was decreased 49% in PHT lung. In contrast, nNOS protein abundance was unchanged. NOS enzymatic activity was also diminished in PHT vs. control lung (60 +/- 3 vs. 110 +/- 7 fmol.mg protein-1.min-1, respectively). Paralleling the declines in eNOS protein and NOS enzymatic activity, eNOS mRNA abundance was decreased 64% in PHT lung. Thus pulmonary eNOS gene expression is attenuated in the lamb model of fetal PHT. Because NO modulates both vasodilation and vascular smooth muscle growth, diminished eNOS expression may contribute to both the abnormal vasoreactivity and the excessive muscularization of the pulmonary circulation in fetal PHT.

Inhaled nitric oxide enhances oxygenation but not survival in infants with alveolar capillary dysplasia.

A complex vascular abnormality in the lungs, termed alveolar capillary dysplasia (ACD) and misalignment of the lung vessels, has been recently recognized in some infants with persistent pulmonary hypertension. These infants die despite maximal medical support including extracorporeal membrane oxygenation (ECMO). Inhaled nitric oxide has been reported to improve oxygenation in neonates with persistent pulmonary hypertension of the newborn, and may allow some infants to avoid the need for ECMO. We identified five infants who had received inhaled nitric oxide to treat refractory hypoxemia caused by persistent pulmonary hypertension of the newborn, and who subsequently died and had autopsy confirmation of ACD. Each infant received care at a different medical center. In each patient, inhaled NO increased the arterial partial pressure of oxygen dramatically. Despite initial clinical improvement, the response to NO was not sustained in any patient. As responsiveness was lost, each infant with ACD required inhaled NO concentrations of 80 ppm or higher to sustain oxygenation. Each infant died, four after extensive periods of ECMO support. This experience demonstrates that a short-term improvement after inhalation of nitric oxide does not lead to long-term survival in ACD. Further, in three infants the diagnosis of ACD was established by lung biopsy before death. Increasing awareness of this clinical entity may allow for the avoidance of costly, invasive procedures such as ECMO until more specific therapies become available.

Inhaled nitric oxide and persistent pulmonary hypertension of the newborn. The Inhaled Nitric Oxide Study Group.

BACKGROUND: Persistent pulmonary hypertension of the newborn causes systemic arterial hypoxemia because of increased pulmonary vascular resistance and right-to-left shunting of deoxygenated blood. Inhaled nitric oxide decreases pulmonary vascular resistance in newborns. We studied whether inhaled nitric oxide decreases severe hypoxemia in infants with persistent pulmonary hypertension. METHODS: In a prospective, multicenter study, 58 full-term infants with severe hypoxemia and persistent pulmonary hypertension were randomly assigned to breathe either a control gas (nitrogen) or nitric oxide (80 parts per million), mixed with oxygen from a ventilator. If oxygenation increased after 20 minutes and systemic blood pressure did not decrease, the treatment was considered successful and was continued at lower concentrations. Otherwise, it was discontinued and alternative therapies, including extracorporeal membrane oxygenation, were used. RESULTS: Inhaled nitric oxide successfully doubled systemic oxygenation in 16 of 30 infants (53 percent), whereas conventional therapy without inhaled nitric oxide increased oxygenation in only 2 of 28 infants (7 percent). Long-term therapy with inhaled nitric oxide sustained systemic oxygenation in 75 percent of the infants who had initial improvement. Extracorporeal membrane oxygenation was required in 71 percent of the control group and 40 percent of the nitric oxide group (P=0.02). The number of deaths was similar in the two groups. Inhaled nitric oxide did not cause systemic hypotension or increase methemoglobin levels. CONCLUSIONS: Inhaled nitric oxide improves systemic oxygenation in infants with persistent pulmonary hypertension and may reduce the need for more invasive treatments.

The normal pulmonary vascular transition at birth.

Although the normal pulmonary vascular transition at birth takes place quickly in the delivery room, it has its basis in the complex structural and biochemical development of the lung. We are only beginning to understand the stimuli that initiate and mediate the transition, as well as their interrelationships. Comprehension of normal structure and function is the foundation that will enable us to understand how this process is impaired in the baby born with congenital diaphragmatic hernia.

Effect of methylene blue on refractory neonatal hypotension.

Excess nitric oxide is a mediator of the hypotension in septic shock. Nitric oxide dilates vascular smooth muscle through activation of soluble guanylate cyclase. We report the increase in blood pressure caused by methylene blue (MB), a soluble guanylate cyclase inhibitor, in five neonates with presumed septic shock unresponsive to colloids, inotropic agents, and corticosteroids. MB was given intravenously at a dose of 1 mg/kg during a 1-hour period. MB increased blood pressure in each patient (average, 33% +/- 20%). Blood pressure subsequently decreased to near baseline values in three patients, who then received a second infusion of MB. Blood pressure again increased in these patients. Three of five patients were weaned from inotropic support within 72 hours. Three of five patients survived and were discharged home. We suggest that MB increased blood pressure in these neonates with refractory hypotension.

Surfactant rescue in the fetal lamb model of congenital diaphragmatic hernia.

Surfactant therapy given before the onset of ventilation (surfactant prophylaxis) has been shown to improve oxygenation, ventilation, and pulmonary hemodynamics in the lamb model of congenital diaphragmatic hernia (CDH). The aim of this study was to assess the efficacy of surfactant administered after the onset of ventilation ("surfactant rescue"). Ten lambs with surgically created CDH were instrumented, at full term, to measure pulmonary blood flow and pulmonary vascular resistance (PVR). Catheters also were positioned for monitoring of systemic blood pressure and arterial blood gases. The animals were delivered and pressure-ventilated according to a standard protocol (PIP, 30 cm; PEEP, 4 cm; respiratory rate, 60 breaths per minute). After 30 minutes of ventilation, five animals received an intratracheal dose of calf lung surfactant extract (50 mg/kg). The animals were studied for 4 hours. Surfactant rescue had no discernible effect on Pco2, Pco2, or pH. There was an increase in pulmonary blood flow, but it was not significant. The dramatic improvement in oxygenation, ventilation, and pulmonary blood flow found with prophylactic surfactant cannot be reproduced when surfactant is administered as rescue therapy. This indicates that the surfactant is not being delivered adequately, the lungs have already incurred significant barotrauma, and/or the surfactant is being inactivated by alveolar protein. Therefore, the authors suggest that when exogenous surfactant therapy is being considered for the fetus or newborn with CDH, it should be administered as early as possible, preferably before the infant's first breath. Prenatal diagnosis and delivery in a tertiary care center would facilitate this optimum management.

Surfactant decreases pulmonary vascular resistance and increases pulmonary blood flow in the fetal lamb model of congenital diaphragmatic hernia.

INTRODUCTION: Experiments using animal models of neonatal respiratory distress syndrome have shown a decrease in pulmonary vascular resistance (PVR) with surfactant replacement, whereas studies with the lamb model of congenital diaphragmatic hernia (CDH) have demonstrated improvement in oxygenation and lung mechanics with this therapy. The aim of the present study was to measure the effects of surfactant replacement therapy on the pulmonary hemodynamics of the lamb model of CDH. METHODS: Ten lambs with surgically created CDH and five control lambs were instrumented at term, with the placental circulation intact. Ultrasonic flow probes were positioned around the main pulmonary artery and the common origin of the left and right pulmonary arteries to record total lung and main pulmonary artery blood flow. Catheters were inserted to record systemic, pulmonary, and left atrial pressure. Five CDH animals received 50 mg/kg of surfactant by tracheal instillation just before delivery. All 15 animals were then ventilated for 4 hours. RESULTS: Correcting the surfactant deficiency in the CDH lamb resulted in a significant increase in pulmonary blood flow, a decrease in PVR, and a reduction in right-to-left shunting. These improvements in hemodynamics were associated with a significant improvement in gas exchange over 4 hours. CONCLUSION: The fetal lamb model of CDH has elevated PVR in comparison to controls. Prophylactic surfactant therapy reduces this resistance and dramatically increases pulmonary blood flow while reducing extrapulmonary shunt. A surfactant deficiency may be partially responsible for the persistent pulmonary hypertension in neonates with CDH.

Perfluorocarbon-associated gas exchange improves pulmonary mechanics, oxygenation, ventilation, and allows nitric oxide delivery in the hypoplastic lung congenital diaphragmatic hernia lamb model.

OBJECTIVES: To determine the efficacy of perfluorocarbon-associated gas exchange and the effects of inhaled nitric oxide during perfluorocarbon-associated gas exchange in the congenital diaphragmatic hernia lamb model. DESIGN: Prospective, nonrandomized, controlled, nonhuman trial. SETTING: Animal research facility. SUBJECTS: Fetal lambs of 16 time-dated pregnant ewes, at 80 days gestation (term 140 to 145 days). MEASUREMENTS AND MAIN RESULTS: The congenital diaphragmatic hernia lamb model was created in 16 animals. Twelve animals survived to be studied. All animals were mechanically ventilated for 4 hrs with a time-cycled, pressure-limited ventilator. Perfluorocarbon-associated gas exchange was started after 15 mins of ventilation (n = 6). Blood gases were analyzed at 30 mins and then hourly. The perfluorocarbon-associated gas exchange animals had dynamic compliance and tidal volumes measured. After 1 hr, inhaled nitric oxide (80 parts per million) was delivered to the perfluorocarbon-associated gas exchange animals for 10 mins. All blood gas parameters, including pH (6.72 +/- 0.06 vs. 7.14 +/- 0.07), PCO2 (186 +/- 12 vs. 70.5 +/- 16.7 torr [24.8 +/- 1.6 vs. 9.5 +/- 2.1 kPa]), and PO2 (48 +/- 17 vs. 156 +/- 48 torr [6.4 +/- 2.3 vs. 20.8 +/- 6.4 kPa]) were significantly improved in the perfluorocarbon-associated gas exchange-treated group at 4 hrs (p < .005). Dynamic compliance (0.13 +/- 0.02 vs. 0.32 +/- 0.06 mL/cm H2O/kg) and tidal volume (3.5 +/- 0.35 vs. 7.22 +/- 0.61 mL/kg) were also significantly (p < .001) increased in the perfluorocarbon-associated gas exchange group. In the perfluorocarbon-associated gas exchange animals, nitric oxide caused a significant (p < .05) increase in oxygenation and a reduction in pulmonary hypertension. This effect was reversed by stopping the inhaled nitric oxide. CONCLUSIONS: Perfluorocarbon-associated gas exchange significantly improved gas exchange, dynamic compliance, and tidal volumes. Furthermore, inhaled nitric oxide can be effectively delivered during perfluorocarbon-associated gas exchange to reduce pulmonary hypertension and enhance oxygenation.

The cGMP phosphodiesterase inhibitor zaprinast enhances the effect of nitric oxide.

We investigated the effect of zaprinast (M&B 22948), a specific cGMP phosphodiesterase inhibitor, on pulmonary arteries isolated from lambs with persistent pulmonary hypertension following prenatal ligation of the ductus arteriosus. Relaxations to sodium nitroprusside, which donates nitric oxide inside the smooth muscle cell, were significantly decreased in pulmonary arteries from ligated lambs. Pretreatment with 3 x 10(-5) M zaprinast restored them to levels close to those observed in untreated arteries from control animals. Further studies in intact newborn lambs were then conducted under three experimental conditions: (1) NO inhalation at 6 ppm, (2) zaprinast infusion at 0.05 mg/kg/min, and (3) combination therapy of zaprinast infusion in addition to inhaled NO at 6 ppm. Combined therapy with NO and zaprinast decreased the pulmonary artery pressure (34.3 +/- 3%) and pulmonary vascular resistance (64 +/- 7%) and increased pulmonary blood flow (88 +/- 34%) and postductal PaO2 (287 +/- 34%) to a significantly greater extent than NO alone, zaprinast alone, or the sum of these two responses, indicating a true synergistic effect. Zaprinast pretreatment also markedly increased the duration of pulmonary vasodilation to nitric oxide. There was no effect on systemic blood pressure with the combined therapy. We conclude that zaprinast pretreatment significantly enhances the effect of sodium nitroprusside on isolated pulmonary arteries, as well the effect of inhaled NO at 6 ppm in newborn lambs with persistent pulmonary hypertension. We speculate that phosphodiesterase inhibition may increase the response rate to NO or allow the use of much lower inhaled concentrations of NO.

Combined effects of nitric oxide and hyperoxia on surfactant function and pulmonary inflammation.

NO and its derivative ONOO- are potent free radicals that can cause cell damage, especially in the presence of O2. To determine the potential pulmonary toxicities of nitric oxide (NO) and peroxynitrite (ONOO-) in vitro, Survanta (2.5 mg/ml) was exposed to ONOO- (0.3-8 mM) in the presence of two different buffering systems (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid and phosphate buffer) and minimum surface tension (MST) was determined with an oscillating bubble surfactometer. Significant increases in MST were seen only with exposure to 8 mM ONOO-, indicating that in vitro, high concentrations of ONOO- can inhibit natural surfactant function. The in vivo effects of NO and hyperoxia were then studied in four groups of newborn piglets ventilated for 48 h with 21% O2, 100% O2, 21% O2 and 100 ppm NO, or with 90% O2 and 100 ppm NO. Five animals served as an untreated control group. Bronchoalveolar lavage fluid (BAL) obtained at 48 h was subjected to centrifugation and the surfactant pellet was reconstituted to 5 mg phospholipid/ml. Significant increases in MST were seen in surfactant from piglets ventilated with NO and 90% O2, compared with either untreated controls or piglets ventilated with 21% O2 for 48 h (P < 0.05, analysis of variance). Significant increases in neutrophil chemotactic activity (NCA) of BAL were also found in the NO and O2 group (P < 0.05), with significant positive interaction between NO and O2 found (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)