Tracheal ligation does not correct the surfactant deficiency associated with congenital diaphragmatic hernia

Temporary tracheal occlusion causes catch-up lung maturation in a fetal model of diaphragmatic hernia

BACKGROUND

The lungs of infants born with diaphragmatic hernia are hypoplastic, immature, and surfactant-deficient. Tracheal occlusion in utero, which is being proposed as antenatal treatment of diaphragmatic hernia by promoting compensatory lung growth, decreases surfactant production as well, through loss of type II pneumocytes. The authors studied whether temporary tracheal occlusion might cause 'catch-up' lung growth and maturation, without negative effects of prolonged tracheal occlusion on the surfactant system.

METHODS

Diaphragmatic hernia was created in time-dated fetal lambs (65 to 75 days). At 108 days, the trachea was occluded with an embolectomy catheter (DH + TO, n = 6). After day 14, the balloon was deflated. Six congenital diaphragmatic hernia (CDH) fetuses were left unobstructed (DH). For comparison, a group of fetuses without diaphragmatic hernia were subjected to prolonged tracheal ligation (TL; 4-week tracheal ligation, n = 3). Unoperated littermates (n = 8) were used as controls (CTR). All were killed near term. Lung tissue was processed for light and electron microscopy (computerized stereologic morphometry). Type II pneumocytes were identified with antisurfactant protein B antibody.

RESULTS

Four animals in DH + TO and four in DH survived to term. Lung fluid volume (LFV) at 108 days was 5.2 +/- 4.4 mL in DH and 24.6 +/- 6.8 mL in controls (P < .05, Student t test). In DH + TO, LFV increased ninefold (to 48.3 +/- 13.3 mL) by 1 week postocclusion, suggesting accelerated lung growth. At term, lung weight to body weight ratio (LW/BW) was higher in TL (9.85% +/- 1.81%) than in CTR (3.55% +/- 0.56%; P < .05, analysis of variance); LW/BW and parenchymal volume tended to be greater in DH + TO than in DH, and air-exchanging parenchymal volume in DH + TO was similar to CTR (v a 50% reduction in DH), indicating some degree of hyperplasia after temporary occlusion. Pneumocyte II numerical density was decreased more than 10-fold in TL (60 +/- 22 v 826 +/- 324 in CTR, P < .001; it was slightly lower in DH + TO than in CTR, but individual type II pneumocyte cell volume was greater in the latter, and they appeared more mature than in DH (increased granulation by light microscopy, fewer glycogen granules, and abundant lamellar bodies by electron microscopy). Surfactant was also seen in the air spaces in DH + TO and CTR; it was absent in unobstructed CDH and in TL.

CONCLUSIONS

Temporary tracheal occlusion in utero does not cause the dramatic decrease in type II pneumocytes seen after prolonged occlusion. Although only minimal increase in lung volume is seen in CDH, catch-up parenchymal growth and maturation occur, most notably in the surfactant-producing system.

Single-port tracheoscopic surgery in the fetal lamb

BACKGROUND/PURPOSE

Endoscopic fetal surgery could help avoid many of the problems associated with open fetal surgery, but the use of multiple ports may be too traumatic to the membranes. The authors describe a single-port technique of tracheoscopic surgery in the fetus.

METHODS

Time-dated pregnant ewes (95 to 105 days; term, 145 days) underwent midline laparotomy under general halothane anesthesia. A 5-mm-diameter balloon-tipped cannula was introduced in the uterus by Seldinger technique. A 1.2-mm semirigid mini-endoscope, fitted inside a 9F, 20 degrees curved sheath, was introduced under continuous, low-pressure irrigation, inside the fetus' mouth, and advanced into the trachea.

RESULTS

Endotracheal procedures, including temporary (n = 11) and permanent balloon tracheal occlusion (n = 30) and placement of a barbed guide wire for endotracheal occlusion device insertion (n = 12), were performed by introducing a 1-mm diameter instrument alongside the telescope. These were successfully performed in 52 of the 53 fetuses. The rigidity of the telescope allowed controlled access to the pharynx; its curve allowed full tracheobronchial endoscopy with the fetus in utero.

CONCLUSIONS

The present technique marries the control and optical quality of a rigid endoscope with the physiological curve only a flexible instrument could offer until now. The types of procedures performed with this technique illustrate its potential as a research tool; the size (1.2-mm diameter), shape, and optical qualities of the telescope should make clinical applications possible.

Lung growth and maturation after tracheal occlusion in diaphragmatic hernia

Tracheal occlusion (TO) was performed at 120 d of gestation by noninvasive endoscopic technique using a releasable latex balloon, in fetal lambs with diaphragmatic hernia (DH) established at 85 d. The lungs were studied at 139 d in five fetuses with DH + TO, five fetuses with DH only, and six control fetuses. Fluid retention consecutive to TO allowed fetal lungs to grow. Histological pulmonary structure was more mature in DH + TO than in DH alone. The growth-inducing effect of TO was however incomplete, with an increased protein/DNA ratio. Tissue phospholipids were increased, but this was not reflected in the surfactant compartment. The major surfactant component, disaturated phosphatidylcholine, was reduced to 58% of its control value in DH, and further reduced to 17.5% of its control value in DH + TO. The proportion of surfactant protein B immunoreactive cells, assumed to represent the proportion of type II cells, was increased in DH (27% of all parenchymal cells), and reduced in DH + TO (7.8%) as compared with control fetuses (15%). In conclusion, although noninvasive tracheal occlusion in utero is feasible and may partly compensate the adverse effects of DH on lung organogenesis, it reduces the number of type II cells and induces a dramatic surfactant deficit. Using this technique in human fetuses requires careful consideration until further evaluation of lung functional characteristics has been achieved in this experimental model.

Arterio-venous extracorporeal membrane oxygenation of fetal goat incubated in artificial amniotic fluid (artificial placenta): influence on lung growth and maturation

PURPOSE

The authors attempted to achieve extrauterine support of goat fetuses using arteriovenous extracorporeal membrane oxygenation (A-V ECMO) via umbilical vessels, incubated in a warm bath. They hypothesized that this extrauterine support system affected both the lung growth and maturation of goat fetuses.

METHODS

Four goat fetuses at about 125 days' gestation (term, 150 days) were placed in this fetal ECMO system. Their four twin fetuses with similar body weights were harvested and examined for baseline data before ECMO (pre-ECMO data). The mean duration of A-V ECMO was 138.8+/-66.9 hours (range, 87 to 237 hours). Tracheal ligation was performed on all four fetuses to prevent the efflux of fetal tracheal fluid and to collect a sample of tracheal fluid daily. The surfactant and electrolytes of the tracheal fluid were analyzed in pre-ECMO (twin fetuses) and during ECMO. The surfactant of lung tissue, and lung weight were analyzed in pre-ECMO (twin fetuses) and after ECMO. Plasma cortisol and T3 levels were also assayed as hormonal factors that affected lung maturation in pre-ECMO and during ECMO.

RESULTS

The phospholipid, phosphatidylcholine, and lecithin-sphingomyelin ratio of the tracheal fluid on day 5 during ECMO (15.0+/-7.1 mg/dL, 53.4+/-26.5%, and 6.2+/-7.1) were elevated above pre-ECMO (10.0+/-4.6 mg/dL, 43.0+/-0.6%, and 4.0+/-2.9), but there were no significant differences. The phosphatidylcholine and disaturated phosphatidylcholine of the lung tissue after ECMO were 72.3+/-15.4 mg/g and 19.2+/-8.2 mg/g, which were significantly higher than pre-ECMO (53.7+/-13.9 mg/g and 11.6+/-4.1 mg/g). The Cl- and K+ of the tracheal fluid were 123.5+/-6.2 mEq/L and 3.7+/-0.7 mEq/L on day 1 during ECMO, which were significantly lower than pre-ECMO (141.8+/-2.9 mEq/L and 6.8+/-1.1 mEq/L), but they then recovered to pre-ECMO levels. The wet and dry lung weights after ECMO were 87.8+/-18.0 g and 6.3+/-1.9 g, which were significantly higher than pre-ECMO (49.3+/-5.9 g and 4.2+/-1.3 g). Plasma cortisol levels and T3 levels during ECMO were significantly higher than pre-ECMO. Electron microscopy demonstrated a higher increase of mature type 11 cells in the lungs after ECMO than pre-ECMO.

CONCLUSION

This fetal A-V ECMO system in the bath showed a production of surfactant, the maintenance of ion transport by the pulmonary epithelium, an increase in lung weight, and an increase in mature type II cells, resulting in lung growth and maturation.

Deleterious effect of tracheal obstruction on type II pneumocytes in fetal sheep

It was previously shown that tracheal obstruction accelerated fetal lung growth and eventually reversed the pulmonary hypoplasia in experimental diaphragmatic hernia. We have successfully developed a reversible tracheal obstruction technique in fetal sheep using balloon occlusion and showed that 3 wk of obstruction induced significant lung growth of the same magnitude as the tracheal ligation. The purpose of this study was to examine the effects of 1 and 3 wk of tracheal occlusion on the alveolar cell population with specific attention to the type II pneumocytes. We first showed that 1 wk of occlusion induced a significant increase in lung weight and in alveolar surface area. We then used the surfactant protein C (SP-C) mRNA as a specific marker of differentiated type II pneumocytes. Total RNA was isolated from fetal sheep lung with or without tracheal occlusion, and Northern blots were hybridized with a cDNA probe specific for the sheep SP-C. The results show a dramatic decrease in SP-C mRNA expression (8.8-fold, p < 0.01). In situ hybridization showed a marked decrease in the density of cells expressing SP-C, as well as the amount of SP-C mRNA expressed by the cells. The effect was present as early as 1 wk of occlusion. The sparseness of type II pneumocytes was further confirmed by electron microscopy. We thus conclude that tracheal obstruction causes a profound decrease in the number of type II pneumocytes in the lungs. Given the crucial role of type II pneumocytes in surfactant production, we could speculate that, if tracheal occlusion is able to accelerate lung growth, the final product is probably surfactant-deficient.

Tracheal ligation: the dark side of in utero congenital diaphragmatic hernia treatment

Currently there are two in utero procedures that have been proposed for the treatment of Congenital diaphragmatic hernia (CDH); reduction of the herniated viscera with repair of the diaphragmatic defect (CDH repair) and stimulation of lung growth by ligation of the fetal trachea (CDH + TL). Recent studies have shown that CDH + TL may result in a significant surfactant deficiency. The aim of this study was to compare the postnatal lung function of these two interventions using the fetal lamb model of CDH. CDH was created in 14 lambs at 78 days' gestation. At 110 days, seven lambs had their trachea ligated through a transverse neck incision and seven had repair of their diaphragmatic defect via a left subcostal incision. At term the lambs were instrumented with the umbilical circulation intact, then delivered and ventilated to a standard protocol for 4 hours. Pulmonary hemodynamics and blood gas levels were measured and compared every 30 minutes. Four lambs in the CDH repair group and five lambs in the CDH + TL group survived to be studied. After the initial data were analyzed, a further group of CDH + TL lambs (n = 4) were studied. In this group a replacement dose of surfactant (Infasurf, Ony Inc, Buffalo, NY) was administered. These initial results cast doubt on tracheal ligation as an in utero therapy for CDH, and indicate that the lung produced by this intervention is not physiologically normal as previously thought. However, the function of these lungs can be normalized if the surfactant deficiency is corrected. If this improvement can be maintained and there is recovery of the endogenous surfactant system, then in utero tracheal ligation may become a viable treatment for fetal CDH.

Tracheal ligation increases cell proliferation but decreases surfactant protein in fetal murine lungs in vitro

Tracheal occlusion affects both fetal lung growth and maturation. The authors used a murine in vitro whole organ culture model to investigate these effects. The authors hypothesized that tracheal ligation would increase lung growth by increasing cell proliferation and would change surfactant protein synthesis in this system. Lungs were removed from day 14 gestation murine fetuses (term, 21 days). Tracheas were ligated and explants cultured in chemically defined, serum-free media for 1, 3, 4, 5, 7, or 14 days. DNA synthesis and cell division were assessed using a 5-bromo-2'-deoxy-uridine (BrdU) incorporation assay. Surfactant proteins A and B, markers of lung maturity, were detected using immunohistochemistry. Ligated lungs showed more BrdU-labeled cells per 1,000 x field (cells/hpf) at every time point. Ligated lungs on day 1 showed 27% more cells/hpf than unligated, on day 3, 21% more, on day 5, 54% more, on day 7, 60% more, and on day 14, 123% more (P < .05). In contrast, ligated lungs showed significantly less staining for surfactant proteins A and B than did unligated lungs. The authors conclude that tracheal ligation increases cell division but decreases surfactant protein in fetal murine lungs in vitro. These data suggest that although tracheal occlusion increases lung growth, it may decrease or delay lung maturation. This model provides a powerful tool for investigating the mechanisms underlying fetal lung development and tracheal occlusion-induced pulmonary hyperplasia.

Physiology and pathophysiology of lung development

Over the past few years, there has been a considerable improvement in our understanding of the normal development of the fetal lung and its regulation. These advances have occurred mostly through increased knowledge of molecular biological mechanisms of growth and differentiation. These advances have also resulted in an improvement in our comprehension of the pathological basis of various pulmonary diseases. As a result of this new and improved knowledge, new and innovative therapeutic modalities are being introduced into clinical practice. The introduction of surfactant therapy into the clinical setting was one such milestone in neonatal respiratory management. Pulmonary surfactant is responsible for stabilising alveoli during normal respiration, thereby preventing atelectasis or alveolar flooding. Disease processes which result in an insufficiency in surfactant, such as respiratory distress syndrome (RDS) or congenital diaphragmatic hernia (CDH), generally carry a very high mortality rate. Exogenous surfactant administration reduces both the mortality and morbidity associated with RDS and its sequelae, and its efficacy in the treatment of CDH is now being evaluated clinically. Moreover, laboratory studies suggest that surfactant therapy may be used in combination with other treatments, such as tracheal occlusion to promote lung growth in CDH, in order to achieve a maximal effect in these complex, multifactorial lung diseases.

Correction of congenital diaphragmatic hernia in utero VIII: Response of the hypoplastic lung to tracheal occlusion

Most fetuses with congenital diaphragmatic hernia (CDH) diagnosed before 24 weeks' gestation die despite optimal postnatal care. In fetuses with liver herniation into the chest, prenatal repair has not been successful. In the course of exploring the pathophysiology of CDH and its repair in fetal lambs, the authors found that obstructing the normal egress of fetal lung fluid enlarges developing fetal lungs, reduces the herniated viscera, and accelerates lung growth, resulting in improved pulmonary function after birth. They developed and tested experimentally a variety of methods to temporarily occlude the fetal trachea, allow fetal lung growth, and reverse the obstruction at birth. The authors applied this strategy of temporary tracheal occlusion in eight human fetuses with CDH and liver herniation at 25 to 28 weeks' gestation. With ongoing experimental and clinical experience, the technique of tracheal occlusion evolved from an internal plug (two patients) to an external clip (six patients), and a technique was developed for unplugging the trachea at the time of birth (Ex Utero Intrapartum Tracheoplasty [EXIT]). Two fetuses had a foam plug placed inside the trachea. The first showed dramatic lung growth in utero and survived; the second (who had a smaller plug to avoid tracheomalacia) showed no demonstrable lung growth and died at birth. Two fetuses had external spring-loaded aneurysm clips placed on the trachea; one was aborted due to tocolytic failure, and the other showed no lung growth (presumed leak) and died 3 months after birth. Four fetuses had metal clips placed on the trachea. All showed dramatic lung growth in utero, with reversal of pulmonary hypoplasia documented after birth. However, all died of nonpulmonary causes. Temporary occlusion of the fetal trachea accelerates fetal lung growth and ameliorates the often fatal pulmonary hypoplasia associated with severe CDH. Although the strategy is physiologically sound and technically feasible, complications encountered during the evolution of these techniques have limited the survival rate. Further evolution of this technique is required before it can be recommended as therapy for fetal pulmonary hypoplasia.