Prostaglandin D2 inhibits hypoxic pulmonary vasoconstriction in neonatal lambs

Prostanoids and phosphodiesterase inhibitors in experimental pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease with a poor prognosis, characterized by intimal lesions, medial hypertrophy, and adventitial thickening of precapillary pulmonary arteries. Several approved therapies are currently available for the treatment of PAH, of which intravenous epoprostenol is the best explored over the past decade. Newly available oral endothelin receptor antagonists, although clinically efficacious, bear the risk of liver toxicity in a significant portion of patients. Substances that stimulate the formation of the second messengers cyclic adenosine monophosphate (cAMP) or guanosine monophosphate (cGMP) have proved useful in the treatment of various forms of pre-capillary pulmonary hypertension. These second messengers of the endogenous vasodilator mediators that include prostacyclin and nitric oxide (NO) are hydrolyzed by cyclic nucleotide phosphodiesterases (PDEs), a class of enzymes from which 11 isoforms have been characterized. This chapter highlights developments in the treatment of experimental pulmonary hypertension with special attention to prostanoids and PDE inhibitors. We summarize findings for the acute vasodilatory as well as chronic effects of prostanoids, PDE inhibitors, or combinations of both, in animal models of pulmonary hypertension.

Pathophysiology of Neonatal Respiratory Distress Syndrome

Neonatal respiratory distress syndrome (RDS) remains one of the major causes of neonatal mortality and morbidity despite advances in perinatal care. The initial management of infants with RDS has almost become 'too routine' with little thought about the pathophysiological processes that lead to the disease and how the clinician can use the existing therapeutic interventions to optimize care. The transition from fetus to infant involves many complex adaptations at birth; the most important is the function of the lungs as a gas exchange organ. Preterm surfactant-deficient infants are less well equipped to deal with this transition. Optimum gas exchange is achieved through matching of ventilation and perfusion. In RDS, ventilation may be affected by homogeneity of the airways with atelectasis and over distension, as hyaline membranes block small airways. In turn this contributes to the inflammation that becomes bronchopulmonary dysplasia. Exogenous surfactant given early, particularly with positive end-expiratory pressure and, where necessary, gentle ventilation, would seem to be the optimum way to prevent atelectasis. How this can be achieved in neonates after surfactant therapy is explored through a review of the normal physiology of the newborn lung and how this is affected by RDS. The therapeutic interventions of resuscitation, exogenous surfactant, ventilation and inhaled nitric oxide are discussed in relation to their effects and what are currently the optimum ways to use these. It is hoped that with a better understanding of the normal physiology in the newborn lung, and the effects of both disease and interventions on that physiology, the practising clinician will have a greater appreciation of management of preterm infants with, or at risk of, RDS.

Relative effects of cyclooxygenase and nitric oxide synthase inhibition on vascular resistances in neonatal lamb lungs

Effective attenuation of pulmonary vasoconstriction is essential during early postnatal development when increased pulmonary vascular resistance (PVR) may lead to a resumption of right-to-left shunting across fetal channels. In addition, modulation of venous resistance contributes to normal lung fluid balance. This study was designed to identify the relative modulating effects of endothelium-derived nitric oxide (EDNO) and dilator prostaglandins (PG) on normoxic and hypoxic pulmonary vasomotor tone in young newborns. Total and segmental PVR were measured using inflow-outflow and double occlusion techniques in isolated lungs of 6-h-old lambs studied under control conditions or after blocking PG and/or EDNO synthesis with indomethacin and/or N omega-nitro-L-arginine, respectively. During normoxia, both indomethacin and N omega-nitro-L-arginine were required to increase total PVR, but EDNO appeared to have the greater modulating effect. Indomethacin markedly enhanced hypoxic pulmonary vasoconstriction of large and small arteries and small veins, whereas N omega-nitro-L-arginine caused a lesser, but significant, increase in hypoxic pulmonary vasoconstriction of small arteries and veins, suggesting that dilator PG played the dominant modulating role during hypoxia. In addition, PG synthesis appeared to be enhanced after inhibition of EDNO synthesis. In contrast, indomethacin caused a decrease in venous resistance, suggesting that constrictor prostanoids had a greater effect than dilator PG on this segment. EDNO had a modest modulating effect on venous resistance in these lungs. These data suggest that dilator PG and EDNO exert complementary effects in attenuating total and segmental PVR during normoxia and hypoxia in 6-hold lamb lungs.

Production of pulmonary vasodilation by tolazoline, independent of nitric oxide production in neonatal lambs

OBJECTIVE

To determine whether tolazoline reduces pulmonary vascular resistance (PVR) by means of endogenous nitric oxide production.

DESIGN

Thirty newborn lambs (2 to 7 days of age) were anesthetized with pentobarbital, and their lungs were ventilated through an endotracheal tube. Intravascular catheters were placed in the left ventricle, descending aorta, right atrium, and pulmonary artery for continuous monitoring of intravascular pressures. Cardiac output was measured with radiolabeled microspheres. Arterial carbon dioxide pressure and pH were maintained in a normal range throughout the experiments. Animals were randomly assigned to the following groups: group 1, lungs ventilated with a hypoxic gas mixture and administered tolazoline; group 2, given N omega-nitro-L-arginine (L-NA) (5 mg/min intravenously for 60 minutes) and tolazoline; group 3, given L-NA with hypoxia and tolazoline. Acetylcholine (0.5 microgram/kg) was injected into the right atrium to assess pulmonary nitric oxide synthase activity before and after the L-NA infusion. Data were analyzed by analysis of variance.

RESULTS

L-NA inhibited the acetylcholine-induced reduction in mean pulmonary artery pressure (MPAP) by more than 75%. Hypoxia and L-NA increased both MPAP and PVR. Tolazoline produced immediate reductions in both MPAP and PVR in all three groups (group 1, 27% +/- 3% and 50% +/- 5%; group 2, 34% +/- 5% and 50% +/- 6%; and group 3, 31% +/- 4% and 46% +/- 5%, respectively).

CONCLUSIONS

These results suggest that tolazoline produces vasodilation independent of nitric oxide production. Understanding the mechanism by which tolazoline produces pulmonary vasodilation may provide insight into the clinical use of this drug and information regarding other potential endogenous mediators of pulmonary vasomotor tone in the neonate.

Treatment of severe persistent pulmonary hypertension of the newborn with magnesium sulphate

Eight of nine newborn infants with severe persistent pulmonary hypertension of the newborn (PPHN), and a predicted mortality of 100%, and one infant with a predicted mortality greater than 94% based on alveolar-arterial oxygen tension difference [A-a)DO2) were treated with magnesium sulphate (MgSO4) as a life saving therapy after they failed to improve with conventional treatment. Magnesium at high serum concentrations decreases pulmonary pressures and is a muscle relaxant and sedative. Diluted MgSO4.7H2O solution (200 mg/kg) was given intravenously over 20-30 minutes. No changes in the treatment were made after MgSO4. Mean serum magnesium concentration was maintained between 2.88 and 5.67 mmol/l by continuous intravenous infusion (six infants). Baseline arterial oxygen tension (PaO2) and haemoglobin oxygen saturation had mean (SD) values of 4.66 (1.8) kPa and 60.4 (29.7)% respectively, which started to increase one hour after MgSO4 infusion, and increased significantly at six hours to 12.04 (7.07) kPa and 91.8 (10.88)% respectively. Arterial carbon dioxide tension (PaCO2) decreased and pH increased significantly after one hour compared with the baseline value. PaO2 increases are probably secondary to a decrease in pulmonary vascular resistance and pressure, decrease in a right to left shunt, better ventilation:perfusion ratio, and PaCO2 decrease and pH rise. Seven infants survived (77.8%). These results demonstrate the beneficial effect of magnesium in the management of PPHN when other accepted treatment fails, is contraindicated, or not available.

Pulmonary hypertension in lambs with congenital diaphragmatic hernia: vasodilator prostaglandins, isoprenaline, and tolazoline

After antenatal induction of diaphragmatic hernias in fetal lambs, prostaglandins D2, E1, and I2 were compared to tolazoline, or isoprenaline, for the treatment of pulmonary hypertension. When rendered hypoxic, these, and normal lambs, showed an increase in pulmonary artery pressure, a decrease in systemic pressure, and a decrease in pulmonary blood flow. All of the drugs altered that response, but to different degrees. None of the drugs tested was consistently successful in reversing the adverse affects of hypoxia, but prostaglandin D2 came closest to the ideal vasodilator, decreasing the pulmonary artery pressure in all seven hypoxic lambs having a diaphragmatic hernia. There was a concomitant increase in pulmonary blood flow in six; in the remaining lamb the decrease in blood flow induced by the hypoxia was arrested. At the same time, there was an increase in systemic artery pressure in three, the decrease was arrested in two, but the decrease continued in the other two. Isoprenaline was a more effective drug than tolazoline, producing an increase in pulmonary blood flow in five of the seven lambs, with minor decreases in systemic pressure in five. Tolazoline improved blood flow in three of six lambs (not all lambs survived the full study), with a marked decrease in systemic pressure in four of them. Prostaglandin D2 seems to be a useful drug for the treatment of patients having diaphragmatic hernias and pulmonary hypertension, and warrants further study. Isoprenaline was the most effective of the readily available drugs tested in this animal model.

The transitional circulation: physiology and anesthetic implications

Over the past decade, several aspects of the physiology of the transitional circulation have been elucidated. The transitional circulation may be viewed as a process divided into four phases. The concept of these phases underscores the fact that the normal transitional circulation should be viewed as an orderly process and not a single event. Within these phases, several mechanisms seem to be involved in the control of pulmonary vascular resistance (PVR). Yet even these mechanisms do not completely explain the process of the normal transition, and very little is understood about why the transition occasionally fails. No doubt other as yet undescribed mechanisms also play a role. Much work remains to be done in the study of the normal and abnormal transitional circulation. The profound hypoxia that characterizes infants with failed transitional circulation from any cause is due to a persistently high PVR, causing right-to-left shunting at the ductal and foramental levels. Clinical care of these infants is based on efforts to simultaneously decrease PVR and increase systemic vascular resistance (SVR). Appropriate measures include the use of supplemental oxygen, hyperventilation, alkalinization, and sedation to decrease PVR and intravenous (IV) fluids and pressors to increase SVR. The rapidly fluctuating nature of the physiologic processes that cause failure of the transitional circulation must be kept in mind when caring for these infants, both during initial stabilization in the delivery room and while administering anesthesia for surgical repair of congenital defects.

Dose-dependent effects of prostaglandin D2 on hemodynamics, renal function, and blood gas analyses

Dose-response effects of prostaglandin D2 (0.125, 0.25, 0.5, and 0.75 micrograms/kg/min) infused intravenously in pentobarbital-anesthetized dogs were studied with particular reference to renal, pulmonary, and systemic effects. Another group receiving the vehicle alone served as controls. Prostaglandin D2 administration resulted in a significant dose-dependent increase in renal artery flow, urine output, creatinine clearance, plasma renin activity, sodium excretion, potassium excretion, and pulmonary artery pressure. A significant decrease occurred in renal resistance and arterial PO2. There were no appreciable changes in mean arterial pressure, heart rate, hematocrit, platelet count, arterial pH, and PCO2. In the vehicle control group, all other parameters remained relatively stable, except for some increase in the mean arterial pressure, plasma renin activity, and potassium excretion. The results of this study suggest that prostaglandin D2 administered intravenously at levels lower than those required to produce adverse pulmonary and systemic effects will improve the renal blood flow and function.

Characteristics, mortality, and outcome of higher-birth weight infants who require intensive care

The improved outcome of small infants who have received intensive care is well documented; however, the mortality and morbidity of infants weighing greater than or equal to 2500 gm who require intensive care have not been emphasized. During a 2-year period these infants accounted for 41% of admissions and 34% of all deaths in our nursery. The most common diagnoses were pulmonary disease (32%), asphyxia (22%), congenital anomalies (18%), infant of diabetic mother (10%), hematologic disease (9%), and infection (4%). Mortality was 11% with 50% of the deaths from lethal malformations, 26% from asphyxia, 13% from infection, and 11% from miscellaneous causes. One half of the deaths were potentially preventable. According to developmental follow-up, over 90% of the survivors were developing normally. Thus, while outcome for survivors is usually good, mortality remains excessively high. This large and understudied group of infants requires increased investigative emphasis.