Fetal lamb pulmonary hypoplasia: pulmonary vascular and myocardial abnormalities

Neonatal cardiopulmonary transition in an ovine model of congenital diaphragmatic hernia

OBJECTIVE

Infants with a congenital diaphragmatic hernia (CDH) are at high risk of developing pulmonary hypertension after birth, but little is known of their physiological transition at birth. We aimed to characterise the changes in cardiopulmonary physiology during the neonatal transition in an ovine model of CDH.

METHODS

A diaphragmatic hernia (DH) was surgically created at 80 days of gestational age (dGA) in 10 fetuses, whereas controls underwent sham surgery (n=6). At 138 dGA, lambs were delivered via caesarean section and ventilated for 2 hours. Physiological and ventilation parameters were continuously recorded, and arterial blood gas values were measured.

RESULTS

DH lambs had lower wet lung-to-body-weight ratio (0.016±0.002vs0.033±0.004), reduced dynamic lung compliance (0.4±0.1mL/cmH₂O vs1.2±0.1 mL/cmH₂O) and reduced arterial pH (7.11±0.05vs7.26±0.05), compared with controls. While measured pulmonary blood flow (PBF) was lower in DH lambs, after correction for lung weight, PBF was not different between groups (4.05±0.60mL/min/gvs4.29±0.57 mL/min/g). Cerebral tissue oxygen saturation was lower in DH compared with control lambs (55.7±3.5vs67.7%±3.9%).

CONCLUSIONS

Immediately after birth, DH lambs have small, non-compliant lungs, respiratory acidosis and poor cerebral oxygenation that reflects the clinical phenotype of human CDH. PBF (indexed to lung weight) was similar in DH and control lambs, suggesting that the reduction in PBF associated with CDH is proportional to the degree of lung hypoplasia during the neonatal cardiopulmonary transition.

Role of GABA receptors in fetal lung development in rats

Fluid accumulation is critical for lung distension and normal development. The multi-subunit γ-amino butyric acid type A receptors (GABA_A) mainly act by mediating chloride ion (Cl⁻) fluxes. Since fetal lung actively secretes Cl⁻-rich fluid, we investigated the role of GABA_A receptors in fetal lung development. The physiological ligand, GABA, and its synthesizing enzyme, glutamic acid decarboxylase, were predominantly localized to saccular epithelium. To examine the effect of activating GABA_A receptors in fetal lung development in vivo, timed-pregnant rats of day 18 gestation underwent an in utero surgery for the administration of GABA_A receptor modulators into the fetuses. The fetal lungs were isolated on day 21 of gestation and analyzed for changes in fetal lung development. Fetuses injected with GABA had a significantly higher body weight and lung weight when compared to phosphate-buffered saline (control)-injected fetuses. GABA-injected fetal lungs had a higher number of saccules than the control. GABA increased the number of alveolar epithelial type II cells as indicated by surfactant protein C-positive cells. However, GABA decreased the number of α-smooth muscle actin-positive myofibroblasts, but did not affect the number of Clara cells or alveolar type I cells. GABA-mediated effects were blocked by the GABA_A receptor antagonist, bicuculline. GABA also increased cell proliferation and Cl⁻ efflux in fetal distal lung epithelial cells. In conclusion, our results indicate that GABA_A receptors accelerate fetal lung development, likely through an enhanced cell proliferation and/or fluid secretion.

Mechanisms of action of the congenital diaphragmatic hernia-inducing teratogen nitrofen

Congenital diaphragmatic hernia (CDH) is a developmental anomaly that results in significant mortality and morbidity. The underlying etiology is poorly understood. Insights will arise from an understanding of the mechanisms by which the teratogen nitrofen induces CDH in rodent models. In this study, we use in vitro cell assays in conjunction with whole animal rodent studies to test hypotheses regarding nitrofen's mechanism of action. The first component examined the interaction of nitrofen with various aspects of the retinoid signaling pathway including uptake proteins, binding proteins, receptors, conversion, and degradation enzymes. The second component examined the interactions of nitrofen and vitamins A, C, and E to test the hypothesis that nitrofen was functioning as an antioxidant to interfere with retinoid signaling. Third, we performed a series of experiments examining the interaction of nitrofen and thyroid signaling. Collectively, the data suggest that the primary aspect of retinoid signaling affected by nitrofen is via inhibition of the rate-limiting enzymes controlling retinoic acid synthesis. Retinoid signaling perturbations do not appear to involve oxidative effects of nitrofen. Any substantial roles of nitrofen-induced perturbations of thyroid hormone signaling or receptor function are not supported.

Vitamin A deficiency (VAD), teratogenic, and surgical models of congenital diaphragmatic hernia (CDH)

Congenital diaphragmatic hernia (CDH) is a congenital malformation that occurs with a frequency of 0.08 to 0.45 per 1,000 births. Children with CDH are born with the abdominal contents herniated through the diaphragm and exhibit an associated pulmonary hypoplasia which is frequently accompanied by severe morbidity and mortality. Although the etiology of CDH is largely unknown, considerable progress has been made in understanding its molecular mechanisms through the usage of genetic, teratogenic, and surgical models. The following review focuses on the teratogenic and surgical models of CDH and the possible molecular mechanisms of nitrofen (a diphenyl ether, formerly used as an herbicide) in both induction of CDH and pulmonary hypoplasia. In addition, the mechanisms of other compounds including several anti-inflammatory agents that have been linked to CDH will be discussed. Furthermore, this review will also explore the importance of vitamin A in lung and diaphragm development and the possible mechanisms of teratogen interference in vitamin A homeostasis. Continued exploration of these models will bring forth a clearer understanding of CDH and its molecular underpinnings, which will ultimately facilitate development of therapeutic strategies.

Congenital diaphragmatic hernia: a retinoid-signaling pathway disruption during lung development?

Congenital diaphragmatic hernia (CDH) usually occurs sporadically. The prognosis remains poor, with a 50% perinatal mortality rate. Most deaths result from hypoxemia due to lung hypoplasia and abnormal development of pulmonary vasculature that results in persistent pulmonary hypertension. Our current understanding of the pathogenesis of CDH is based on an assumption linking herniation of abdominal viscera into the thorax with compression of the developing lung. Pulmonary hypoplasia, however, can also result from reduced distension of the developing lung secondary to impaired fetal breathing movements. Moreover, a nitrofen-induced CDH model shows that lung hypoplasia precedes the diaphragmatic defect, leading to a "dual-hit hypothesis." Recent data reveal the role of a retinoid-signaling pathway disruption in the pathogenesis of CDH. We describe the clinical and epidemiological aspects of human CDH, the metabolic and molecular aspects of the retinoid-signaling pathway, and the implications of retinoids in the development of the diaphragm and the lung. Finally, we highlight the existing links between CDH and disruption of the retinoid-signaling pathway, which may suggest an eventual use of retinoids in the treatment of CDH.

Changes in fetal lung distension alter expression of vascular endothelial growth factor and its isoforms in developing rat lung

Vascular endothelial growth factor A (VEGF-A) is essential for normal pulmonary vascular and parenchymal development. Changes in fetal lung distension profoundly affect lung growth and maturation, including vascular development. To define developmental lung expression of VEGF-A and its receptors and investigate effects of changes in fetal lung distension, we studied fetal rats at embryonic day (ED) 16, 19, and 22, postnatal rats at postnatal day (PD) 5, 10, and 21, and adult rats. We used reverse transcriptase PCR to measure mRNA expression for VEGF-A isoforms (VEGF-A(120), (-144), (-164), and (-188)) and VEGF-A receptors, Flt-1 and Flk-1. With advancing development, mRNA content increased only for VEGF-A(188) (p < 0.05) and for Flt-1 (p < 0.02) and Flk-1 (p < 0.005). As a percentage of total VEGF-A mRNA, VEGF-A(188) (15% at ED 16) increased to become the dominant isoform at PD 21 (40%, p < 0.005) and adulthood; in contrast, there were decreases in both VEGF-A(144) (p < 0.05) and (-120) (p < 0.005). VEGF-A protein was expressed in alveolar epithelium (type I and II cells) and interstitium. Increasing fetal lung distension by tracheal occlusion (TO) accelerated the normal maturational pattern of VEGF-A isoforms and increased VEGF-A protein; decreasing fetal lung distension by congenital diaphragmatic hernia (CDH) retarded the normal developmental pattern and decreased VEGF-A protein. Neither TO nor CDH consistently affected Flt-1 or Flk-1 mRNA content. These results show that mechanical factors significantly affect lung VEGF-A expression and suggest that VEGF-A mediates previously described changes in lung vascular and parenchymal development caused by CDH and by TO.

Effect of lung hypoplasia on birth-related changes in the pulmonary circulation in sheep

Lung hypoplasia (LH) is a serious cause of neonatal compromise, but little is known of its functional effects on the pulmonary circulation. Our aim was to characterize birth-related changes in the pulmonary circulation of newborn lambs with LH and to compare them with alterations in respiratory function. LH was induced in six ovine fetuses by the creation of a tracheo-amniotic shunt as well as amniotic fluid drainage starting at 105.6+/-1.5 (mean+/-SEM) days of gestation (term approximately 147 d). At 139.9+/-0.3 d, fetuses were exteriorized under anesthesia to implant vascular catheters and an ultrasonic flow probe around the left pulmonary artery. The lambs then were delivered and ventilated for 2 h, during which systemic and pulmonary artery pressures, left pulmonary blood flow, and measures of respiratory function were recorded. At autopsy, lungs were weighed and volume was measured at 20 cm H2O. In LH lambs, lung weight was 25% lower and respiratory system compliance was 30% lower than in controls. Mean pulmonary blood flow in LH lambs was 42% lower and pulmonary vascular resistance was 138% higher than in controls. Morphometry showed that volume density of pulmonary arteries in LH was 30% lower than in controls. We conclude that, in this LH model, changes in ventilatory indices were proportional to the change in lung size, whereas changes in the pulmonary circulation were greater than the change in lung size and were associated with reduced density of pulmonary arteries. LH severely impairs normal adaptation of the pulmonary circulation in the perinatal period.

Airway smooth muscle changes in the nitrofen-induced congenital diaphragmatic hernia rat model

In the fetal rat, nitrofen induces congenital diaphragmatic hernia (CDH) and pulmonary vascular remodeling similar to what is observed in the human condition. Airway hyperactivity is common in infants with CDH and attributed to the ventilator-induced airway damage. The purpose of this study was to test the hypothesis that airway smooth muscle mechanical properties are altered in the nitrofen-induced CDH rat model. Lungs from nitrofen-exposed fetuses with hernias (CDH) or intact diaphragm (nitrofen) and untreated fetuses (control) were studied on gestation d 21. The left intrapulmonary artery and bronchi were removed and mounted on a wire myograph, and lung expression, content, and immunolocalization of cyclooxygenases COX-1 and COX-2 were evaluated. Pulmonary artery muscle in the CDH group had significantly (p < 0.01) lower force generation compared with control and nitrofen groups. In contrast, the same generation bronchial smooth muscle of the CDH and nitrofen groups developed higher force compared with control. Whereas no differences were found in endothelium-dependent pulmonary vascular muscle tone, the epithelium-dependent airway muscle relaxation was significantly decreased (p < 0.01) in the CDH and nitrofen groups. The lung mRNA levels of COX-1 and COX-2 were increased in the CDH and nitrofen groups. COX-1 vascular and airway immunostaining, as well as COX-1 and COX-2 lung protein content, were increased in the CDH group. This is the first report of airway smooth muscle abnormalities in the nitrofen-induced fetal rat model of CDH. We speculate that congenital airway muscle changes may be present in the human form of this disease.

Diaphragm defects occur in a CDH hernia model independently of myogenesis and lung formation

Congenital diaphragmatic hernia (CDH) is a significant clinical problem in which a portion of the diaphragmatic musculature fails to form, resulting in a hole in the diaphragm. Here we use animal models of CDH to test two hypotheses regarding the pathogenesis. First, the origin of the defect results from the malformation of the amuscular mesenchymal component of the primordial diaphragm rather than with the process of myogenesis. Second, the defect in the primordial diaphragmatic tissue is not secondary to defects in the developing lung. In c-met(-/-) mouse embryos, in which diaphragm muscle fibers do not form because of a defect in muscle precursor migration, the amuscular substratum forms fully. We show that a defect characteristic of CDH can be induced in the amuscular membrane. In Fgf10(-/-) mouse embryos that have lung agenesis we show that the primordial diaphragm does not depend on signals from lung tissue for proper development and that diaphragmatic malformation is a primary defect in CDH. These data suggest that the pathogenesis of CDH involves mechanisms fundamentally different from previously proposed hypotheses.

Influence of congenital heart disease on survival in children with congenital diaphragmatic hernia

OBJECTIVE

We sought to assess outcome in patients with CDH and HD to determine if LHR is also predictive of outcome in this subset of patients.

STUDY DESIGN

We carried out a retrospective review (April 1996-October 2000) of patients with isolated CDH (n = 143, 82.2%) and patients with HD (n = 31, 17.8%) to determine the incidence of additional anomalies, survival to term, CDH repair, cardiac repair, and survival to discharge. Survival based on LHR was analyzed in a subset of fetuses.

RESULTS

The risk of death from birth to last follow-up was 2.9 times higher for patients with CDH plus HD than for patients with CDH alone (P <.0001). Of 11 patients with CDH plus HD who had CDH repair (5 of whom also had HD repair), 5 survived. All 10 patients with an LHR <1.2 died; 3 of 6 with an LHR >1.2 survived (Fisher exact test, P =.04).

CONCLUSION

Heart disease remains a significant risk factor for death in infants with CDH. The LHR helps predict survival in this high-risk group of patients.

Fetal pulmonary artery diameters and their association with lung hypoplasia and postnatal outcome in congenital diaphragmatic hernia

OBJECTIVE

We hypothesized that fetal branch pulmonary artery (PA) diameters indirectly reflect lung mass and are associated with postnatal outcome in cases of isolated congenital diaphragmatic hernia (CDH).

STUDY DESIGN

We retrospectively reviewed echocardiograms of fetuses with CDH, measuring branch PA diameters and other echocardiographic parameters. Antenatal parameters were correlated with postmortem lung weights in 5 fetuses after pregnancy termination. Fetal echocardiographic measures were correlated with outcome variables in 29 live-born infants with CDH to identify antenatal indices associated with postnatal death and respiratory morbidity.

RESULTS

Antenatal branch PA size correlated with postmortem lung weights from 5 terminated fetuses (r = 0.87). In 26 cases of left CDH in which the fetus continued to term, the ipsilateral branch PA diameter was significantly smaller than the contralateral branch PA diameter at presentation (P <.001). In these fetuses, a larger contralateral PA diameter was associated with worse postnatal survival (P =.049). Among survivors with left CDH, the main PA z score and the discrepancy between right and left PA diameters correlated positively with duration of supplemental oxygen requirement (P =.019 and P =.022, respectively) and ventilation (P =.036 and P =.012, respectively). Serial antenatal studies in 8 of 10 cases revealed progressive ipsilateral PA hypoplasia.

CONCLUSION

Antenatal branch PA size correlates with postmortem lung weight. A larger contralateral PA, and significant branch PA discrepancy and larger main PA diameter, best correlate with postnatal death and respiratory morbidity, respectively. Progressive ipsilateral PA hypoplasia suggests progressive in utero lung hypoplasia in cases of CDH.

[Congenital hernia of the diaphragm. A retrospective study of 123 cases recorded in the Neonatal Medicine Department, URHC in Lille between 1985 and 1996]

BACKGROUND

During the last ten years, new therapeutic strategies have been used in order to improve the management of congenital diaphragmatic hernia (CDH). CDH is associated with pulmonary hypoplasia, abnormal pulmonary vascular reactivity and pulmonary immaturity. Between 1985 and 1990, mechanical hyperventilation and early surgery were provided systematically. Since 1991, the management of CDH in our institution has involved a preoperative stabilization with exogenous surfactant replacement, gentle ventilation, high-frequency oscillation, nitric oxide or extracorporeal membrane oxygenation.

PURPOSE

To analyse the impact of the new therapeutic strategy on the survival and outcome of newborns with CDH.

METHODS

Retrospective review of all infants with CDH admitted to our institution from 1985 through 1996. Mortality and morbidity were compared between period I (1985-1990) and period II (1991-1996).

RESULTS

Between 1985 and 1996, 123 neonates were admitted to our Neonatal Department. Nine of them had another severe congenital malformation and were excluded from the study. Survival was 23% (12/52) in period I and 56% (35/62) in period II (p < 0.001). In period II, complications were more frequent among survivors in whom an extracorporeal membrane oxygenation was required (13 infants): bronchopulmonary dysplasia 77% (10/13), gastroesophageal reflux 61% (8/13), and hypotrophy 61% (8/13).

CONCLUSION

These data demonstrate a significant improvement in survival in CDH since the implementation of new therapeutic modalities. Nevertheless, a significant morbidity exists among the infants who survive a severe respiratory failure.

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.