Development of the pulmonary vasculature in newborn lambs: structure-function relationships

Care of the critically ill neonate with hypoxemic respiratory failure and acute pulmonary hypertension: framework for practice based on consensus opinion of neonatal hemodynamics working group

Circulatory transition after birth presents a critical period whereby the pulmonary vascular bed and right ventricle must adapt to rapidly changing loading conditions. Failure of postnatal transition may present as hypoxemic respiratory failure, with disordered pulmonary and systemic blood flow. In this review, we present the biological and clinical contributors to pathophysiology and present a management framework.

Structure and composition of pulmonary arteries, capillaries, and veins

The pulmonary vasculature comprises three anatomic compartments connected in series: the arterial tree, an extensive capillary bed, and the venular tree. Although, in general, this vasculature is thin-walled, structure is nonetheless complex. Contributions to structure (and thus potentially to function) from cells other than endothelial and smooth muscle cells as well as those from the extracellular matrix should be considered. This review is multifaceted, bringing together information regarding (i) classification of pulmonary vessels, (ii) branching geometry in the pulmonary vascular tree, (iii) a quantitative view of structure based on morphometry of the vascular wall, (iv) the relationship of nerves, a variety of interstitial cells, matrix proteins, and striated myocytes to smooth muscle and endothelium in the vascular wall, (v) heterogeneity within cell populations and between vascular compartments, (vi) homo- and heterotypic cell-cell junctional complexes, and (vii) the relation of the pulmonary vasculature to that of airways. These issues for pulmonary vascular structure are compared, when data is available, across species from human to mouse and shrew. Data from studies utilizing vascular casting, light and electron microscopy, as well as models developed from those data, are discussed. Finally, the need for rigorous quantitative approaches to study of vascular structure in lung is highlighted.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Cavopulmonary assist: (em)powering the univentricular fontan circulation

Since the Fontan/Kreutzer procedure was introduced, evolutionary clinical advances via a staged surgical reconstructive approach have markedly improved outcomes for patients with functional single ventricle. However, significant challenges remain. Early stage mortality risk seems impenetrable. Serious morbidities - construed as immutable consequences of palliation - have hardly been addressed. Late functional status is increasingly linked to pathophysiologic consequences of prior staged procedures. As more single-ventricle patients survive into adulthood, Fontan failure is emerging as an intractable problem for which there is no targeted therapy. Incremental solutions to address these ongoing problems have not had a measurable impact. Therefore, a fundamental reconsideration of the overall approach is reasonable and warranted. The ability to provide a modest pressure boost (2 to 6 mmHg) to existing blood flow at the total cavopulmonary connection can effectively restore more stable biventricular status. This would impact not only treatment of late Fontan failure, but also facilitate early surgical repair. A realistic means to provide such a pressure boost has never been apparent. Recent advances are beginning to unravel the unique challenges that must be addressed to realize this goal, with promise to open single-ventricle palliation to new therapeutic vistas.

Upregulation of vascular calcium channels in neonatal piglets with hypoxia-induced pulmonary hypertension

Inhibition of voltage-gated, L-type Ca(2+) (Ca(L)) channels by clinical calcium channel blockers provides symptomatic improvement to some pediatric patients with pulmonary arterial hypertension (PAH). The present study investigated whether abnormalities of vascular Ca(L) channels contribute to the pathogenesis of neonatal PAH using a newborn piglet model of hypoxia-induced PAH. Neonatal piglets exposed to chronic hypoxia (CH) developed PAH by 21 days, which was evident as a 2.1-fold increase in pulmonary vascular resistance in vivo compared with piglets raised in normoxia (N). Transpulmonary pressures (DeltaPtp) in the corresponding isolated perfused lungs were 20.5 +/- 2.1 mmHg (CH) and 11.6 +/- 0.8 mmHg (N). Nifedipine reduced the elevated DeltaPtp in isolated lungs of CH piglets by 6.4 +/- 1.3 mmHg but only reduced DeltaPtp in lungs of N piglets by 1.9 +/- 0.2 mmHg. Small pulmonary arteries from CH piglets also demonstrated accentuated Ca(2+)-dependent contraction, and Ca(2+) channel current was 3.94-fold higher in the resident vascular muscle cells. Finally, although the level of mRNA encoding the pore-forming alpha(1C)-subunit of the Ca(L) channel was similar between small pulmonary arteries from N and CH piglets, a profound and persistent upregulation of the vascular alpha(1C) protein was detected by 10 days in CH piglets at a time when pulmonary vascular resistance was only mildly elevated. Thus chronic hypoxia in the neonate is associated with the anomalous upregulation of Ca(L) channels in small pulmonary arteries in vivo and the resulting abnormal Ca(2+)-dependent resistance may contribute to the pathogenesis of PAH.

Postnatal changes in pulmonary vein responses to endothelin-1 in the normal and chronically hypoxic lung

The response of pulmonary arteries to endothelin-1 (ET-1) changes with age in normal pigs and is abnormal in pulmonary hypertension. The purpose of this study was to determine if the same is true of the pulmonary veins. We studied the wall structure and functional response to ET-1 in pulmonary veins from normal pigs from fetal life to adulthood and from pigs subjected to chronic hypobaric hypoxia either from birth for 3 days or from 3 to 6 days of age. In isolated normal veins, the contractile response decreased by 40% between late fetal life and 14 days of age with a concomitant twofold increase in endothelium-dependent relaxant response. The ETA antagonist BQ-123 reduced the contractile response significantly more in newborn than older animals, whereas the ET-B antagonist BQ-788 had no effect in fetal animals and maximally increased contraction at 14 days of age. Hypoxic exposure significantly increased pulmonary vein smooth muscle area and contractile response to ET-1. The relaxation response was impaired following hypoxic exposure from birth but not from 3 to 6 days of age. The ETA antagonist BQ-123 decreased contractile and increased dilator responses significantly more than in age-matched controls. Thus pulmonary veins show age-related changes similar to those seen in the pulmonary arteries with a decrease in ETA-mediated contractile and increase in ET-B-mediated relaxant response with age. Contractile response was also increased in hypoxia as in the arteries. This study suggests that pulmonary veins are involved in postnatal adaptation and the pathogenesis of pulmonary hypertension.

Induction of pulmonary angiogenesis by adenoviral-mediated gene transfer of vascular endothelial growth factor

BACKGROUND

We hypothesized that gene transfer of vascular endothelial growth factor (VEGF) mediated by an adenovirus vector might induce pulmonary artery angiogenesis in a lamb model of pulmonary artery hypoplasia.

METHODS

Thirteen fetal lambs had left pulmonary artery banding at 106 days of gestation. Following birth, 3 groups were divided: VEGF group (n = 5) and beta-GAL group (n = 4) received an adenoviral vector encoding respectively for human VEGF165 and for galactosidase A. A control group (n = 4) had neither gene nor virus. Viral suspensions were selectively instilled in the left bronchus 6.5 days after birth. Five nonoperated lambs constituted the normal group. Euthanasia was performed at 30 days of age. Gene transfer was confirmed by blue coloration of left lung obtained with Xgal solution in an additional experiment. Histomorphometric evaluation was performed. All groups were compared with ANOVA test and paired test was used to compare right and left lung in each animal.

RESULTS

Left lung was similarly hypoplastic in all operated lambs. Left pulmonary artery hypoplasia present in all operated groups was significantly less pronounced in VEGF group. The number of pleural arteries was similarly increased in left lung of all operated lambs. Left lung arterial density was higher in VEGF group than in all other groups. The percentage of parenchyma of left lung was lower in beta-GAL group than in all others, partially returned to normal in VEGF group.

CONCLUSIONS

In this model, transbronchial VEGF gene transfer induces pulmonary angiogenesis, proximal pulmonary artery growth and contributes to lung parenchyma recovery.

Modulation of pulmonary vasomotor tone in the fetus and neonate

The high pulmonary vascular resistance (PVR) of atelectatic, hypoxic, fetal lungs limits intrauterine pulmonary blood flow (PBF) to less than 10% of combined right and left ventricular output. At birth, PVR decreases precipitously to accommodate the entire cardiac output. The present review focuses on the role of endothelium-derived nitric oxide (NO), prostacyclin, and vascular smooth muscle potassium channels in mediating the decrease in PVR that occurs at birth, and in maintaining reduced pulmonary vasomotor tone during the neonatal period. The contribution of vasodilator and vasoconstrictor modulator activity to the pathophysiology of neonatal pulmonary hypertension is also addressed.

Mediators of alkalosis-induced relaxation of piglet pulmonary veins

Pulmonary venous constriction leads to significant pulmonary hypertension and increased edema formation in several models using newborns. Although alkalosis is widely used in treating neonatal and pediatric pulmonary hypertension, its effects on pulmonary venous tone have not previously been directly measured. This study sought to determine whether alkalosis caused pulmonary venous relaxation and, if so, to identify the mediator(s) involved. Pulmonary venous rings (500-microm external diameter) were isolated from 1-wk-old piglets and precontracted with the thromboxane mimetic U-46619. Responses to hypocapnic alkalosis were then measured under control conditions after inhibition of endothelium-derived modulator activity or K(+) channels. In control rings, alkalosis caused a 34.4 +/- 4.8% decrease in the U-46619-induced contraction. This relaxation was significantly blunted in rings without functional endothelium and in rings treated with nitric oxide synthase or guanylate cyclase inhibitors. However, neither cyclooxygenase inhibition nor voltage-dependent, calcium-dependent, or ATP-dependent K(+)-channel inhibitors altered alkalosis-induced relaxation. These data suggest that alkalosis caused significant dilation of piglet pulmonary veins that was mediated by the nitric oxide-cGMP pathway.

Blunting of pulmonary but not systemic vasodilator responses to dobutamine in newborn lambs

Little is known about the changes in systemic and pulmonary vascular responses to dobutamine that occur during postnatal development. To address this question, vascular pressures and cardiac output were measured in anesthetized 1- to 2-d-old (n = 6), 7- to 10-d-old (n = 7), and 6- to 8-wk-old lambs (n = 6) while dobutamine was infused incrementally in the dose range of 0.5-40 microg x kg(-1) x min(-1). Dobutamine reduced pulmonary vascular resistance in the three age groups (all p<0.005). However, although this reduction reached a plateau over the infusion range of 15-40 microg x kg(-1) x min(-1) at all ages, its magnitude was less (p<0.02) in 1- to 2-d-old lambs (24+/-2%) than in 7- to 10-d-old (41+/-7%) or 6- to 8-wk-old lambs (42+/-6%). Over the same infusion range, dobutamine produced a fall in systemic vascular resistance (all p<0.005) that was proportionally similar in 1- to 2-d-old (51+/-3%), 7- to 10-d-old (48+/-3%), and 6- to 8-wk-old lambs (42+/-6%). In separate subgroups of 1- to 2-d-old lambs, pulmonary and systemic responses to dobutamine were not affected by pretreatment with selective alpha1-, beta1-, or beta2-adrenoceptor antagonists. Taken together, these findings suggest that the pulmonary but not systemic vasodilator response to dobutamine in lambs is blunted in the initial days after birth, and that this pulmonary effect is not directly related to the adrenoceptor-stimulating properties of dobutamine.

Maturational changes in ovine pulmonary vascular responses to inhaled nitric oxide

Developmental changes in modulation of pulmonary vasomotor tone by endothelium-derived nitric oxide (EDNO) may reflect maturational differences in endothelial synthesis of and/or vascular smooth muscle response to nitric oxide. This study sought to determine whether pulmonary vascular sensitivity and responsiveness to nitric oxide change during newborn development, and whether this is related to changes in guanylate cyclase activity. Pulmonary artery dose-responses to inhaled nitric oxide (iNO, 0.25-100 parts per million) were measured in hypoxic, indomethacin-treated, isolated lungs from 1-day (1-d)- and 1-month (1-m)-old lambs. The lungs of 1-m-old lambs were ventilated with 4% (oxygen) O2, and lungs of 1-d-old lambs were ventilated with either 4% or 7% O2 in order to achieve similar stimuli or vasomotor tone. Cyclic guanosine monophosphate (cGMP) concentrations in the perfusate were measured at iNO concentrations of 0, 5, and 100 parts per million (ppm). Basal and stimulated pulmonary guanylate cyclase activity was also measured in lung extracts in vitro. The effects of iNO were similar in both 1-d groups, even though baseline hypoxic tone was significantly higher in 1-d lungs ventilated with 4% O2 than with 7% O2. Furthermore, both the 1-d 7% O2 and 1-d 4% O2 lungs exhibited greater responsiveness and sensitivity to iNO than 1-m lungs. Perfusate cGMP concentrations and soluble guanylate cyclase activity were higher under stimulated than basal conditions, but neither differed statistically between 1 d and 1 m. These data suggest that pulmonary vascular responsiveness and sensitivity to nitric oxide decrease with age, but the mechanisms underlying these maturational changes require further investigation.

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.

Pathophysiology, Diagnosis, and Management of Pulmonary Hypertension in Infants and Children

Pulmonary hypertension (PH) may occur as a primary process or as a complication of several diseases. In the pediatric population, PH secondary to congenital heart disease, chronic hypoxemia, or acute respiratory failure is more common than primary PH. Regardless of etiology, PH may lead to significant morbidity or mortality as a consequence of right-to-left shunting across cardiovascular channels or right heart failure. In this review, PH is defined in terms of the determinants of pulmonary blood flow: pulmonary artery pressure, downstream pressure, and pulmonary vascular resistance. Research addressing both normal developmental changes in pulmonary vascular resistance and abnormal pulmonary vascular reactivity is then reviewed, followed by a discussion of the etiologies of PH in children. Some of the more common clinical presentations of PH are presented focussing on the differences seen between patients with and without intracardiac communications. Assessment of the severity of PH using both noninvasive (electrocardiogram, echocardiogram, magnetic resonance imaging) and invasive (cardiac catheterization, lung biopsy) techniques is then discussed. Treatment of PH is presented, focussing on restoration of adequate pulmonary blood flow through use of both conventional and newer vasodilator therapies. The review concludes by noting the limits to our understanding of the pathogenesis and therapy of PH.

Timing of repair of congenital diaphragmatic hernia requiring extracorporeal membrane oxygenation support

Treatment of congenital diaphragmatic hernia (CDH) has undergone a revolutionary change in philosophy, from previous urgent repair to the present practice of stabilization and delayed repair. However, when extracorporeal membrane oxygenation (ECMO) is required, many people believe that the risk of postoperative pulmonary hypertension (PPHN) mandates hernia repair while on ECMO. This report details the experience in two ECMO centers with stabilization, ECMO if required, and CDH repair post-ECMO. All CDH patients symptomatic in the first hour of life with a gestational age of at least 34 weeks during the period were reviewed retrospectively. Standard criteria were used to select patients for ECMO. High-frequency jet or oscillating ventilators and nitric oxide were not routinely available throughout the study period, but were used in some of the more recent patients. A total of 60 patients presented to the two centers; 24 cases were stabilized with conventional management, repair of the CDH was done elective, and survival was 100%. Eight patients were referred after having repair elsewhere; six survived (75%). The two deaths were attributable to associated lethal lesions--complex cyanotic heart disease and alveolar capillary dysplasia. Eight patients who required ECMO were managed with the intention of repairing the defect on ECMO. Four survived (50%). Two patients died before repair. Twenty patients were managed with ECMO, with the intention of repairing the defect after decannulation. Overall survival was 13 (65%), deaths were caused by pre-ECMO hypoxia, pulmonary insufficiency, and associated cardiac disease. No patient had recurrent pulmonary hypertension after late repair.(ABSTRACT TRUNCATED AT 250 WORDS)

Differing effects of acute and prolonged alkalosis on hypoxic pulmonary vasoconstriction

Animal studies and clinical pediatric practice have shown that acute alkalosis attenuates hypoxic pulmonary vasoconstriction (HPV). However, increased intracellular pH appears to enhance pulmonary vasoreactivity. We therefore hypothesized that prolonged alkalosis augments HPV. This study compares the effects of acute and prolonged alkalosis on HPV in isolated perfused lungs of 1-month-old lambs (n = 5) and the hypoxic responses of 300- to 500-microns diameter segments of pulmonary arteries (n = 7) from mature cats at control pH and after 30 min of alkalosis. In isolated lamb lungs, normocarbic (5% CO2) hypoxia (4% O2) increased the total pressure gradient (delta PT) by 6.0 +/- 2.7 (SEM) mm Hg (p < or = 0.05). Acute hypocarbia (3% CO2) increased the perfusate pH to approximately 7.52 and significantly decreased the hypoxic delta PT to normocarbic, normoxic (28% O2) levels. Subsequent exposure to normoxia (while maintaining alkalosis) further decreased delta PT. However, re-exposure to hypoxia after 60 min of normoxic alkalosis significantly increased delta PT by 11.6 +/- 1.6 mm Hg (p < or = 0.05) to a level similar to that seen during normocarbic hypoxia. The increased hypoxic reactivity (i.e., change in pressure between normoxia and hypoxia) during prolonged alkalosis was due to enhanced HPV of the small vessels within the middle segment of the pulmonary circuit, as defined by an inflow-outflow occlusion technique (p < or = 0.05). The occlusion data also suggested that most of this increase occurred in small arteries. Moreover, the hypoxic response of isolated small arteries from the cat was increased almost threefold (p < or = 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Intratracheal administration of pulmonary vasodilator agents

We compared intravenous and intratracheal administration of histamine (0.4 and 1.6 micrograms/kg, respectively) and nitroglycerin (5.0 and 20.0 micrograms/kg, respectively) in seven hypoxemic 2 week old lambs, during right lung only perfusion, to see if intratracheal administration could limit their vasodilator action to the pulmonary vessels. The hemodynamic variables: pulmonary artery pressure (Ppa), left atrial pressure (Pla), pulmonary blood flow per kilogram (Q/kg), and aortic pressure (Pao) were measured at baseline and in each experimental state, then pulmonary vascular resistance (PVR) and systemic vascular input resistance (SVR) were determined. We found that intravenous histamine showed some pulmonary vasodilator selectivity in that it caused a 19% decrease of Ppa from baseline (P less than 0.002), a 23% decrease of PVR from baseline (P less than 0.002), and an 8% decrease of SVR from baseline (P less than 0.05). Intratracheal histamine produced smaller effects, decreasing Ppa by 11% from baseline (P less than 0.02), and PVR by 14% from baseline (P less than 0.02), while SVR was unaffected. Intravenous nitroglycerin decreased cardiac output by 16% from baseline (P less than 0.02), and also decreased SVR by 8% while producing a small increase in PVR. Intratracheal nitroglycerin caused a similar 17% (P less than 0.01) decrease in cardiac output, and again an increased PVR but a decreased SVR. This study confirms that histamine has some intrinsic pulmonary vasodilator selectivity. Furthermore, the data suggest that intratracheal administration may accentuate pulmonary selectivity by lessening systemic effects. Nitroglycerin, on the other hand, had untoward hemodynamic effects in the presence of hypoxia.

Isolated pulmonary resistance vessels from fetal lambs. Contractile behavior and responses to indomethacin and endothelin-1

A method was developed for recording isometric tension from isolated small arteries (mean internal diameter, 169 microns) and veins (mean internal diameter, 273 microns) of the term fetal lung and was then applied to the study of the mechanisms controlling perinatal pulmonary hemodynamics. The specific purpose was to determine whether the activity of the prostaglandin synthetic system in vessels is conditioned by the oxygen tension and the mode of action of endothelin-1. Both preparations appeared structurally intact and, after normalizing their lumen diameter to either the transmural pressure in vivo (artery) or the contractile capacity of the vessel in vitro (vein), generated force to the activating solution (5 mM Ca2+ in K+ Krebs' solution) in excess of the expected performance under physiological conditions. Treatment with indomethacin (2.8 microM) had no effect on arteries preequilibrated at low PO2 (21 +/- 1 mm Hg); however, the same treatment contracted (approximately 45% of the response to activating solution) arteries at either an intermediate (40 +/- 0.8 mm Hg) or high (70 +/- 0.9 mm Hg) PO2. Endothelin-1 contracted both arteries and veins in a concentration-dependent manner, the threshold being lower with veins (1-10 versus 10-100 pM). Endothelin-1 constriction was also seen in arteries whose tone had been raised with a thromboxane A2 analogue, whereas in thromboxane-treated veins constriction was preceded by a modest relaxation over the range of 1-1,000 pM. The findings with indomethacin lead us to infer that small pulmonary arteries are endowed with a prostaglandin-relaxing mechanism that becomes functional on raising the PO2 from fetal to neonatal levels. Endothelin-1 is a constrictor regardless of the level of intrinsic tone, suggesting a possible role of the peptide in maintaining elevated pulmonary vascular tone in the fetus.