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Abstract Endoscopic laser ablation of placental vessels is the most commonly performed fetal operation today. Herein, we review the pathophysiology of twin-to-twin transfusion syndrome and the challenges of its treatment. Pediatric surgeons, with their knowledge of fetal and congenital pathology, and their technical expertise with minimally invasive surgery, can be of great benefit to the patient and the medical team.

Background/purpose: Late-gestation lung remodeling is associated with alveolar type II cell apoptosis early in the saccular stage (day 28 in fetal rabbits). Intrauterine tracheal occlusion (TO), a potent stimulus of fetal lung growth and maturation, significantly increases type II cell apoptosis. The aim of this study was to determine the effect of fetal TO on the spatiotemporal expression of key apoptosis-related signaling molecules., Methods: Tracheal occlusion of fetal rabbits was performed at gestational day 25 (term, 31 days), and apoptotic gene expression was studied between days 26 and 28., Results: At days 26 and 27, the protein levels of Fas and Fas-ligand (FasL) in lung lysates were similar in TO fetuses and sham-operated controls. At day 28, however, synchronous with the onset of TO-induced pulmonary distension and type II cell apoptosis, the FasL protein content was 8-fold higher in TO lungs compared with controls (P < .01), whereas Fas levels were comparable. In contrast, Bax and Bcl-2 protein levels were similar in TO and control fetuses at all time-points. TO significantly increased the cellular concentration of immunoreactive FasL in type II cells and bronchial epithelial Clara cells. Furthermore, bronchoalveolar lavage fluid (BAL) from TO fetuses at day 28 induced significantly more type II cell apoptosis in vitro compared with control BAL, an effect that was inhibited by neutralizing anti-FasL antibody., Conclusions: Our findings show that TO results in time-specific increase of both cellular and soluble FasL in fetal lungs and implicate the Fas/FasL pathway as a pivotal autocrine and/or paracrine regulator of TO- induced type II cell apoptosis.

Background/purpose: Fetal tracheal occlusion (TO) causes accelerated lung growth. However, prolonged TO is associated with a decline in the type II cell number. Type II cell function after TO is unclear. Herein, the authors examine type II cell function after TO and the role of tracheal fluid., Methods: Fetal lambs (term, 145 days) underwent TO at 122 days. Tracheal pressure was recorded daily. In one group of animals (TF; n = 6), lung fluid was aspirated, measured, and reinfused daily. In their respective twins, NS group, lung fluid was replaced milliliter per milliliter with normal saline (NS; n = 6). At death near term, lung weight was obtained, and tissues were processed for stereologic volumetry. Type II cells were quantitated using antisurfactant protein B immunohistochemistry. Surfactant protein B-mRNA expression was studied by Northern analysis. Wilcoxon signed rank test and single factor analysis of variance (ANOVA) were used for statistical analysis (P<.05 was significant)., Results: In both experimental groups, intratracheal pressure rose from 1.9+/-1.0 torr to 3.7 to 4.8 torr by day 1, and remained constant thereafter. Lung fluid volume increased from 11.9+/-4.2 on day 0 to 36.8+/-8.0 mL/kg in TF, and to 28.4+/-9.3 mL/kg in NS by day 1 (P<.05). At death, lung weight/body weight ratio was higher in TF (5.45% +/- 0.91%) than in NS (4.40% +/- 0. 67%) or control (3.83%+/-0.58%; P<.05). Type II numerical density was substantially reduced after TO: 57.7+/-12.8 x 10(6)/mL (TF) and 45.0 +/-25.9 x 10(6)/mL (NS), versus 82.3+/-13.6 x 10(6)/mL in controls. Ultrastructurally, remaining type II cells in TF were enlarged and engorged with lamellar bodies; in NS, they were smaller and contained fewer lamellar bodies. Surfactant protein B mRNA expression was significantly decreased in NS, but not in TF, compared with controls., Conclusions: Type II cell function as well as overall lung growth are stimulated by TO. Lung growth after TO is therefore not unavoidably detrimental to type II cells. After isobaric saline exchange of lung fluid, type II cell function is severely inhibited, confirming the role of tracheal fluid composition in type II stimulating type II cell function.

Background/purpose: Prolonged tracheal occlusion (TO) accelerates lung growth but impairs surfactant production. Short-term TO results in less lung growth but preserves type II cell function. The authors studied the effects of short-term TO on lung physiology in diaphragmatic hernia., Methods: Diaphragmatic hernia was created in 9 fetal lambs at 90 to 95 days. Five were left uncorrected (CDH), 4 underwent 2-week TO (108 to 122 days; CDH + TO). Five unoperated lambs served as controls. Near-term (136 days) fetuses were ventilated for 90 to 150 minutes. Pulmonary arterial pressure, postductal blood gases, quasistatic compliance, total lung capacity (TLC), and lung weight to body weight (LW/BW) were measured., Results: There was an overall survival rate of 89% at full term. Short-term occlusion did not induce lung growth (TLC and LW/BW, 6.07 +/- 2.92 mL/kg and 0.022 +/- 0.008 in CDH, 4.86 mL/kg and 0.019 +/- 0.005 in CDH + TO, 10.81 +/- 3.55 mL/kg and 0.036 +/- 0.006 in controls, respectively). However, pulmonary hypertension in CDH (47.4 +/- 12.32/35.8 +/- 12.19 torr) was corrected by short-term occlusion (20.2 +/- 4.0/16.0 +/- 4.8 torr in CDH + TO, P< .05, single-factor analysis of variance [ANOVA]; similar to control). Best pO2 and pCO2 improved after occlusion (CDH, 48.6 +/- 6.7 torr and 107.1 +/- 34.3 torr, respectively; CDH + TO, 101.5 +/- 16.3 torr and 81.9 +/- 2.4 torr; control, 291.4 +/- 4.7 torr and 37.7 +/- 17.3), as did oxygenation index (P < .05, CDH vCDH + TO; CDH, 97.2 +/- 23.0; CDH + TO, 28.7 +/- 3.1; control, 5.6 +/- 0.6)., Conclusions: Short-term TO corrects pulmonary hypertension and improves gas exchange in fetal lambs with diaphragmatic hernia despite failure to produce accelerated lung growth. Inducing lung maturation and correcting the physiological derangement in diaphragmatic hernia may be more important than achieving lung growth alone.

Background/purpose: During fetal development, the mammalian lung undergoes progressive parenchymal involution. Intrauterine tracheal occlusion induces accelerated architectural maturation of the fetal lungs associated with depletion of the surfactant-producing type II cells. This study investigates the spatiotemporal pattern of apoptosis during normal fetal lung development and its modulation in tracheal occlusion-induced accelerated fetal lung growth., Methods: Fetal rabbit lungs were studied at 25 to 31 days' gestational age (DGA; term, 31 DGA), corresponding to late pseudoglandular through terminal air sac stages of fetal lung development. Intrauterine tracheal ligation (TL) was performed at 24 DGA. TL fetuses were monitored until 29 DGA, a time-point previously shown to coincide with significant type II cell depletion. Apoptotic cells were identified by light and electron microscopy, as well as terminal deoxynucleotidyl transferase-mediated dUTP-FITC nick-end labeling (TUNEL). Epithelial (type I and II) cell apoptosis was studied by TUNEL labeling in conjunction with antisurfactant protein and anticytokeratin immunohistochemistry. DNA fragmentation was analyzed by gel electrophoresis. Sham-operated littermates served as controls., Results: The number of apoptotic cells progressively increased with advancing lung growth and architectural maturation (apoptotic index [Al] 1.2 +/- 0.7 x 10(-3) at 25 DGA v 4.2 +/- 1.4 x 10(-3) at 31 DGA; P< .05, analysis of variance). In TL fetuses, the apoptotic rate was significantly higher than in non-TL fetuses from the third postligation day on, coinciding with the onset of significantly increased airspace distension (Al 4.9 +/- 1.3 x 10(-3) in TL v2.6 +/- 0.4 x 10(-3) in controls at 29 DGA; P< .05, Student's ttest). Apoptosis occurred in parenchymal cells and in isolated cells within the airspaces. The apoptotic activity of type II cells was significantly higher in TL fetuses than C fetuses at 29 DGA (type II Al 25.5 +/- 6.3 x 10(-3) in TL v2.3 +/- 0.8 x 10(-3) in C; P< .001). Electron microscopic studies confirmed the presence of apoptotic nuclei in interstitial macrophages and in degenerating intraluminal type II cells. DNA analysis showed nucleosomal bands., Conclusions: Normal fetal lung development is associated with a progressive increase of epithelial and interstitial apoptotic activity, a process enhanced by TL. Tracheal occlusion induces a significant increase of type II cell apoptosis, which likely contributes to the observed type II cell depletion after TL. We speculate that fetal type II cell apoptosis after TL may be induced by mechanical distension (stretch) of the airspaces.

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.

Fetal tracheal ligation increases lung growth in utero, making it potentially applicable for antenatal treatment of diaphragmatic hernia. This phenomenon has been ascribed to increased intratracheal pressure, which activates as yet unidentified pulmonary stretch receptors. The purpose of this study was to determine whether the composition of lung fluid has any effect on fetal lung development after tracheal obstruction. Six sets of fetal lamb twins underwent tracheal ligation with placement of intratracheal catheters at 122 days' gestation (term, 145 days). In group 1 (n = 6), tracheal fluid was aspirated daily, measured, and replaced with equal volumes of saline. Their respective twins (group 2, n = 6) had daily reinfusion of their own tracheal aspirates. Intratracheal pressure was recorded daily in both groups. Unobstructed fetal lambs (n = 7) were used as negative controls. Animals were killed on postoperative day 14 (136 days). Lungs were weighed, perfusion fixed at 25 cm H2O, and processed for standard morphometric analysis. Intratracheal pressure remained between 3 and 5 torr in both experimental groups throughout the entire postoperative period. In all 12 experimental fetuses, tracheal ligation resulted in an almost threefold increase in lung fluid volume by day 1; a slight decrease at a mean of 2.4 days; and a second surge from day 4 on. Lung fluid volume was significantly higher in group 2 than in group 1 at all measured time points (P < .05, Wilcoxon rank sum test) except on days 3, 4, and 8 (P = .06). Lung weight per body weight (LW/BW) at delivery was 0.045 +/- 0.008 in group 1, not significantly different from unobstructed controls (0.038 +/- 0.006). LW/BW in group 2 was 0.055 +/- 0.010, significantly larger than either group 1 or control (P < .05, single factor analysis of variance). Air space fraction was comparable between the three groups. Alveolar numerical density was significantly lower in groups 1 and 2 than in unobstructed controls (P < 0.05). Replacement of tracheal fluid with saline inhibits the lung hypertrophy seen after tracheal ligation. This phenomenon therefore appears more dependent on tracheal fluid growth factors than on increased intratracheal pressure after obstruction. The immediate decrease in net lung fluid production after saline exchange suggests that these humoral factors play an important role in the initiation of lung cell proliferation.