Thomas A. Parker

Effects of inhaled nitric oxide on pulmonary edema and lung neutrophil accumulation in severe experimental hyaline membrane disease.

To determine the effects of inhaled NO (iNO) on pulmonary edema and lung inflammation in experimental hyaline membrane disease (HMD), we measured the effects of iNO on pulmonary hemodynamics, gas exchange, pulmonary edema, and lung myeloperoxidase (MPO) activity in extremely premature lambs (115 d of gestation, 0.78 term). In protocol 1, we measured the effects of iNO (20 ppm) on lung vascular endothelial permeability to 125I-labeled albumin(indexed to blood volume using 57Cr-tagged red blood cells) during 1 h(n = 10) and 3 h (n = 14) of conventional mechanical ventilation with Fio2 = 1.00. In comparison with controls, iNO improved pulmonary hemodynamics and gas exchange, but did not alter lung weight-to-dry weight ratio or vascular permeability to albumin after 1 or 3 h of mechanical ventilation. To determine whether low dose iNO (5 ppm) would decrease lung neutrophil accumulation in severe HMD, we measured lung MPO activity after 4 h of mechanical ventilation with or without iNO (protocol 2). Low dose iNO improved gas exchange during 4 h of mechanical ventilation (Pao2 at 4 h: 119 ± 35 mm Hg iNO versus 41 ± 7 mm Hg control,p < 0.05), and reduced MPO activity by 79% (p < 0.05). We conclude that low dose iNO increases pulmonary blood flow, without worsening pulmonary edema, and decreases lung neutrophil accumulation in severe experimental HMD. We speculate that in addition to its hemodynamic effects, low dose iNO decreases early neutrophil recruitment and may attenuate lung injury in severe HMD.

Prolonged endothelin A receptor blockade attenuates chronic pulmonary hypertension in the ovine fetus.

Based on past studies of an experimental model of severe intrauterine pulmonary hypertension, we hypothesized that endothelin-1 (ET-1) contributes to high pulmonary vascular resistance (PVR), hypertensive lung structural changes, and right ventricular hypertrophy (RVH) caused by prolonged closure of the ductus arteriosus. To test this hypothesis, we studied the effects of BQ 123, a selective ET(A) receptor antagonist, after ligation of the ductus arteriosus in utero. In 19 late gestation fetal lambs (126+/-3 d; 147 d, term) we ligated the ductus arteriosus at surgery, and treated animals with either BQ 123 (1 mg/d) or vehicle (0.1% DMSO, HTN) in the pulmonary artery for 8 d. Chronic BQ 123 treatment attenuated the rise in mean pulmonary artery pressure (PAP) 8 d after ductus arteriosus ligation (78+/-2, HTN vs. 70+/-4 mmHg, BQ 123, P < 0.05). To study the effects of ET(A) blockade at birth, 15 animals were delivered by cesarean section and ventilated with 10% oxygen (O2), 100% O2 and inhaled nitric oxide (NO). Lambs treated with BQ 123 had lower PVR after delivery during ventilation with 10% O2, 100% O2, and inhaled NO (HTN vs. BQ 123, P < 0.05 for each intervention). Acute BQ 123 treatment (2 mg/30 min) lowered PVR in three HTN animals ventilated with 100% O2 and inhaled NO (P < 0.05). Chronic BQ 123 treatment prevented the development of RVH as determined by the ratio of the right ventricle/left ventricle + septum (0.79+/-0.03, HTN vs. 0.57+/-0.06, BQ 123, P < 0.05) and attenuated the increase in wall thickness of small pulmonary arteries (61+/-2, HTN vs. 50+/-2%, BQ 123, P < 0.05). In summary, chronic intrauterine ET(A) receptor blockade decreased PAP in utero, decreased RVH and distal muscularization of small pulmonary arteries, and increased the fall in PVR at delivery. We conclude that ET(A) receptor stimulation contributes to the pathogenesis and pathophysiology of experimental perinatal pulmonary hypertension.

The pulmonary circulation in bronchopulmonary dysplasia

Abnormalities of the pulmonary circulation are increasingly being recognized as a major contributor to the high morbidity and mortality of bronchopulmonary dysplasia. Historically, studies have focused on the importance of pulmonary hypertension to the pathophysiology of BPD, with the assumption that pulmonary vascular abnormalities are a secondary consequence of primary injury to the airspace. Recent studies suggest, however, that abnormalities of the pulmonary vasculature, including altered growth and structure, may directly contribute to the abnormal alveolarization that characterizes the condition. In this article, we briefly outline mechanisms of pulmonary vascular injury in infants at risk of BPD. We then focus on the recognition and management of pulmonary hypertension in these infants. Finally, we review how disordered pulmonary vascular growth may contribute to the pathogenesis of BPD and emphasize the importance of the reciprocal development of the airspace and the pulmonary circulation.

In Vivo Evidence for a Myogenic Response in the Fetal Pulmonary Circulation

In vitro studies have suggested that pulmonary arteries can exhibit a myogenic response and that this myogenic response may be potent during the perinatal period. However, whether a myogenic response can be demonstrated to exist in vivo and the potential role of the myogenic response on the regulation of pulmonary blood flow during fetal life is unknown. We hypothesized that an acute increase in pulmonary artery pressure resulting from partial compression of the ductus arteriosus (DA) in the fetus may simultaneously activate two opposing responses: 1) blood flow-induced vasodilation (owing to shear stress); and 2) pressure-induced vasoconstriction (owing to the myogenic response). To test this hypothesis, we studied the hemodynamic response to partial DA compression with and without inhibition of shear stress-induced vasodilation by nitric oxide synthase blockade in chronically prepared late-gestation fetal lambs. Without inhibition of nitric oxide synthase, pulmonary vascular resistance progressively decreased by 39 ± 5% during the DA compression period (p < 0.05). In contrast, DA compression after nitric oxide synthase inhibition caused left pulmonary artery blood flow to initially increase and then steadily decrease toward a plateau value, and caused pulmonary vascular resistance to progressively increase by 28 ± 4% above baseline (p < 0.05). The plateau value of pulmonary vascular resistance was reached in less than 5 min after the onset of DA compression. Left pulmonary artery blood flow after 10 min of partial DA compression did not change with the rise in pulmonary artery pressure; plateau values of pulmonary vascular resistance increased linearly with the increase in pulmonary artery pressure. These results support the hypothesis that the perinatal lung circulation has a potent myogenic response, and that this response may be masked in vivo under physiologic conditions by nitric oxide synthase activity. We speculate that the myogenic response may become a predominant regulatory mechanism of pulmonary vascular resistance when endothelium-dependent vasoreactivity is impaired, such as in persistent pulmonary hypertension of the newborn.

K+-channel blockade inhibits shear stress-induced pulmonary vasodilation in the ovine fetus.

To determine whether K+-channel activation mediates shear stress-induced pulmonary vasodilation in the fetus, we studied the hemodynamic effects of K+-channel blockers on basal pulmonary vascular resistance and on the pulmonary vascular response to partial compression of the ductus arteriosus (DA) in chronically prepared late-gestation fetal lambs (128-132 days gestation). Study drugs included tetraethylammonium (TEA; Ca2+-dependent K+-channel blocker), glibenclamide (Glib; ATP-dependent K+-channel blocker), charybdotoxin (CTX; preferential high-conductance Ca2+-dependent K+-channel blocker), apamin (Apa; low-conductance Ca2+-dependent K+-channel blocker), and 4-aminopyridine (4-AP; voltage-dependent K+-channel blocker). Catheters were inserted in the left pulmonary artery (LPA) for selective drug infusion and in the main pulmonary artery, aorta, and left atrium to measure pressure. An inflatable vascular occluder was placed around the DA. LPA flow was measured with an ultrasonic flow transducer. Animals were treated with saline, high- or low-dose TEA, Glib, Apa, CTX, CTX plus Apa, or 4-AP injected into the LPA. DA compression caused a time-related decrease in pulmonary vascular resistance in the control, Glib, Apa, CTX, CTX plus Apa, and low-dose TEA groups but not in the high-dose TEA and 4-AP groups. These data suggest that pharmacological blockade of Ca2+- and voltage-dependent K+-channel activity but not of low-conductance Ca2+- and ATP-dependent K+-channel activity attenuates shear stress-induced fetal pulmonary vasodilation.To determine whether K+-channel activation mediates shear stress-induced pulmonary vasodilation in the fetus, we studied the hemodynamic effects of K+-channel blockers on basal pulmonary vascular resistance and on the pulmonary vascular response to partial compression of the ductus arteriosus (DA) in chronically prepared late-gestation fetal lambs (128-132 days gestation). Study drugs included tetraethylammonium (TEA; Ca2+-dependent K+-channel blocker), glibenclamide (Glib; ATP-dependent K+-channel blocker), charybdotoxin (CTX; preferential high-conductance Ca2+-dependent K+-channel blocker), apamin (Apa; low-conductance Ca2+-dependent K+-channel blocker), and 4-aminopyridine (4-AP; voltage-dependent K+-channel blocker). Catheters were inserted in the left pulmonary artery (LPA) for selective drug infusion and in the main pulmonary artery, aorta, and left atrium to measure pressure. An inflatable vascular occluder was placed around the DA. LPA flow was measured with an ultrasonic flow transducer. Animals were treated with saline, high- or low-dose TEA, Glib, Apa, CTX, CTX plus Apa, or 4-AP injected into the LPA. DA compression caused a time-related decrease in pulmonary vascular resistance in the control, Glib, Apa, CTX, CTX plus Apa, and low-dose TEA groups but not in the high-dose TEA and 4-AP groups. These data suggest that pharmacological blockade of Ca2+- and voltage-dependent K+-channel activity but not of low-conductance Ca2+- and ATP-dependent K+-channel activity attenuates shear stress-induced fetal pulmonary vasodilation.

Lung Volume Recruitment in Lambs during High-Frequency Oscillatory Ventilation Using Respiratory Inductive Plethysmography

Monitoring lung volume is important in the treatment of acute hypoxemic respiratory failure. However, there are no tools available for lung volume measurement to guide ventilator management during high-frequency oscillatory ventilation (HFOV) and during dynamic changes in conventional ventilation (CV). We studied the performance of a new respiratory inductive plethysmograph (RIP) with modified software. We measured Δ changes in lung volume above end-expiratory volume (VRIP) during HFOV and studied whether changes in VRIP parallel changes in mean airway pressure. Calibration of the plethysmograph was made by serial injections of a known gas volume in six term (140 d gestation) and eight preterm (125 d gestation) lambs. Linear regression analysis of the relationship between injected gas volume and VRIP showed strong correlation (r2 = 0.93–1.00 term animals, r2 = 0.86–1.00 preterm animals). The pressure volume curves from the calibration with the injected gas volumes also correlated well with the pressure volume curves extrapolated from changes in VRIP. Lung hysteresis was clearly demonstrated with RIP after changes in mean airway pressure during HFOV and after changes in positive end-expiratory pressure during CV. We conclude that measurements of lung volume in term and preterm lambs by use of modified RIP correlate well with changes in mean airway pressure during HFOV, with static pressure volume curves and with changes in positive end-expiratory pressure during CV. We speculate that this technique may provide clinically useful information about changes in lung volume during HFOV and CV. However, evaluation of the precision and chronic stability of RIP measurements over prolonged periods will require further studies.

Acute intrauterine pulmonary hypertension impairs endothelium-dependent vasodilation in the ovine fetus.

To determine whether acute pulmonary hypertension in utero alters fetal pulmonary vascular reactivity, we compared pulmonary vasodilation with an endothelium-dependent agonist, acetylcholine, with that of an endothelium-independent agonist, 8-bromo-guanosine 3′,5′-cylic monophosphate. Acute pulmonary hypertension was produced in chronically prepared, late-gestation fetal lambs by 3 repeated 30-minute partial occlusions of the ductus arteriosus (DA). The first DA compression increased LPA blood flow from 80 ± 10 to 180 ± 21 mL/min (p < 0.01) and decreased pulmonary vascular resistance. In contrast, LPA blood flow did not change and pulmonary vascular resistance increased by 25% during the third period of DA compression. Pulmonary vasodilation during acetylcholine infusion after serial DA compressions was decreased in comparison with the acetylcholine-induced vasodilator response achieved during the baseline period (fall in pulmonary vascular resistance = -49 ± 7% (baseline) versus -25 ± 5% after repeated DA compressions; p < 0.05). In contrast, the vasodilator response to 8-bromo-guanosine 3′,5′-cylic monophosphate remained intact. To determine whether decreased nitric oxide (NO) production may contribute to altered vasoreactivity after acute pulmonary hypertension, repeated DA compressions were performed after treatment with a nonspecific NO synthase inhibitor (nitro-arginine). NO synthase inhibition blocked the pulmonary vasodilation during the first DA compression period, and repeated DA compressions after NO synthase inhibition did not further alter the hemodynamic response to DA compression. These findings support the hypothesis that brief hypertension due to DA compression impairs endothelium-dependent pulmonary vasodilation in the fetus, and that this may be due to decreased NO production.

High-frequency oscillatory ventilation and partial liquid ventilation after acute lung injury in premature lambs with respiratory distress syndrome

Objective Conventional mechanical ventilatory support (CV) contributes to lung injury in premature lambs with respiratory distress syndrome, a disease that is characterized by progressive deterioration of gas exchange and increased lung inflammation. Lung recruitment strategies, such as high-frequency oscillatory ventilation (HFOV) and partial liquid ventilation (PLV), improve gas exchange and attenuate lung inflammation when instituted immediately after birth. However, whether these recruitment strategies are effective as rescue treatment after established lung injury is unknown. To determine the separate and combined effects of HFOV and PLV when initiated after the establishment of acute lung injury in severe respiratory distress syndrome, we studied the effects of these strategies on gas exchange and histologic signs of acute lung injury in premature lambs. Design Animals were intubated, treated with surfactant and ventilated with 1.00 Fio2 for 4 hrs. After 2 hrs, animals were either continued on CV (controls) or treated with one of three strategies: HFOV; CV + PLV; or HFOV + PLV. The response to low-dose inhaled nitric oxide (5 ppm) was measured in each group at the end of the study. Setting An animal laboratory affiliated with University of Colorado School of Medicine. Subjects A total of 20 premature lambs at 115–118 days of gestation (term = 147 days). Measurements and Main Results In comparison with control animals, each of the rescue therapies improved Pao2 after 1 hr of treatment. The HFOV and HFOV + PLV groups had higher Pao2 than CV + PLV or CV alone (p < .05). Mean airway pressure (P aw) was lower in the PLV groups during CV or HFOV compared with their controls (p < .05). Inhaled NO improved Pao2 in all groups; however, the increase in Pao2 was greatest in the HFOV + PLV group (p < .05). Histologic examination and myeloperoxidase assay were not different between groups. Conclusion We conclude that each lung recruitment strategy improved oxygenation in premature lambs with established lung injury.

Endothelin A Receptor Blockade Decreases Pulmonary Vascular Resistance in Premature Lambs with Hyaline Membrane Disease

Endothelin (ET)-1 is a potent vasoconstrictor peptide that modulates basal pulmonary vascular resistance (PVR) in the normal ovine fetus and contributes to high PVR after chronic intrauterine pulmonary hypertension. Although high PVR is present in premature lambs with severe hyaline membrane disease (HMD), whether ET-1 plays a role in the pathophysiology of experimental HMD is unknown. To test the hypothesis that ET-1 activity contributes to high PVR in the premature lamb with HMD, we studied the hemodynamic effects of a selective ETA receptor antagonist, BQ 123, in 10 animals (gestational age 125 d; 147 d = term). After baseline measurements, animals were intubated, treated with surfactant (Infasurf), and mechanically ventilated with a fraction of inspired oxygen of 1.00 for 8 h. Animals were treated with continuous infusions of either BQ 123 (1 mg/h; treatment group, n = 5) or 1% DMSO (control; n = 5). Plasma ET-1 levels progressively increased during prolonged ventilation with hyperoxia (0.8 ± 0.1 pg/mL, baseline to 6.8 ± 2.5 pg/mL, 8 h, p < 0.05). In comparison with control lambs, BQ 123 treatment caused a sustained reduction in pulmonary vascular resistance (0.55 ± 0.04 mm Hg mL-1 min-1, control versus 0.18± 0.04 mm Hg mL-1 min-1, BQ 123, p < 0.05), increased left pulmonary artery blood flow (70 ± 12 mL/min, control versus 194 ± 28 mL/min, BQ 123, p < 0.05), and increased arterial PaO2 (53 ± 14 mm Hg, control versus 174 ± 71 mm Hg, BQ 123, p < 0.05) 8 h after the onset of ventilation. We conclude that circulating levels of ET-1 increase after delivery of premature lambs with severe HMD, and that selective ETA receptor blockade causes sustained improvement in hemodynamics in severe experimental HMD. These studies suggest that ET-1 contributes to the hemodynamic abnormalities in this model of pulmonary hypertension and severe HMD.

Vascular endothelial growth factor causes pulmonary vasodilation through activation of the phosphatidylinositol-3-kinase-nitric oxide pathway in the late-gestation ovine fetus.

Vascular endothelial growth factor (VEGF) causes vasodilation in adult models of peripheral vascular disease and myocardial ischemia through the acute release of nitric oxide (NO). However, the hemodynamic effects of VEGF and its effects on NO production have not been studied in the developing lung circulation. We hypothesized that VEGF causes fetal pulmonary vasodilation, and that its actions are mediated through the release of endogenous NO. We performed surgery in 16 fetal lambs (125–135 d gestation; term = 147 d), and placed catheters in the main pulmonary artery, aorta, and left atrium to measure pressures. An ultrasonic flow transducer was placed on the left pulmonary artery (LPA) to measure blood flow, and a catheter was placed in the LPA for local drug infusion. Pulmonary vascular resistance in the left lung was calculated as pulmonary artery pressure minus left atrial pressure divided by LPA flow. Fetal lambs were treated with brief infusions of recombinant human VEGF (dose, 0.5–2.0 μg) into the LPA. Recombinant human VEGF infusions acutely increased LPA flow by up to 3-fold (p < 0.02) and decreased pulmonary vascular resistance by 65% (p < 0.05) in a dose-related fashion, without affecting aortic pressure or heart rate. To determine the mechanism of VEGF-induced vasodilation, we studied the effects of nitro-l-arginine, an NO synthase inhibitor, and LY294002, a phosphatidylinositol-3-kinase inhibitor, on the response to VEGF. We found that pretreatment with either nitro-l-arginine or LY294002 completely inhibited the vasodilator response to recombinant human VEGF (p < 0.005). These findings suggest that recombinant human VEGF causes fetal pulmonary vasodilation, and that this response is likely mediated by the release of NO through activation of phosphatidylinositol-3-kinase.