Effect of lung fluid composition on type II cellular activity after tracheal occlusion in the fetal lamb.

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.

Short-term tracheal occlusion corrects pulmonary vascular anomalies in the fetal lamb with diaphragmatic hernia.

BACKGROUND: Sustained fetal tracheal occlusion (TO) results in accelerated lung growth but causes severe type II cell depletion. Temporary TO fails to cause lung growth in a congenital diaphragmatic hernia (CDH) model but preserves type II cells and corrects pulmonary hypertension. Herein, we study the pulmonary vascular changes caused by temporary TO. METHODS: CDH was created in 12 fetal lambs (65-70 d; term, 145 days). In 6 lambs, the trachea was occluded for 2 weeks (CDH + TO; 108-122 d). Animals were killed at 136 days. The lungs were processed with elastin stains and anti-alpha-smooth muscle actin antibody. Partial or circumferential presence of inner and outer elastic lamina was used to determine muscularization of pulmonary arterioles. The percent of medial wall thickness was plotted against vessel diameter for each group. RESULTS: Lung weight/body weight was smaller in lambs with CDH (1. 35% +/- 0.56%) and CDH + TO (1.70% +/- 0.34%) than in control lambs (3.55% +/- 0.56%; P <.05, single-factor analysis of variance). The smallest muscularized vessel was 113 +/- 50 microm, and the largest nonmuscularized vessel was 138 +/- 49 microm in lambs with CDH, significantly different from control lambs (185 +/- 69 microm and 350 +/- 116 microm, respectively) and lambs with CDH + TO (185 +/- 97 microm and 245 +/- 100 microm, respectively; P <.05). In lambs with CDH, only 25% of vessels of less than 60 microm were nonmuscularized, compared with 81% in control lambs (P <.05) and 74% in lambs with CDH + TO.Conclusions. Temporary tracheal occlusion, from 108 to 122 days, corrects the abnormal muscularization of pulmonary arterioles seen in CDH. These morphometric findings parallel physiologic results at birth and further suggest that short-term occlusion, which preserves surfactant-producing type II pneumocytes without lung growth, may be sufficient to improve neonatal outcome of diaphragmatic hernia.

Late-gestation tracheal occlusion in the fetal lamb causes rapid lung growth with type II cell preservation.

BACKGROUND: Fetal tracheal occlusion (TO) results in varying degrees of lung growth. This study examines whether gestational age influences lung growth response following TO. MATERIALS AND METHODS: Fetal lambs (term = 145 days) underwent TO early (108 days, n = 6) or late (122 days, n = 6) in gestation. Aspirated lung fluid volume (LFV) and intratracheal pressure (ITP) were recorded daily. Two weeks after TO, the fetuses were sacrificed. Lung growth was assessed by lung weight and stereologic volumetry. Type II cellular density was assessed by computer-assisted morphometry using antisurfactant protein B antibody. RESULTS: After early TO, ITP remained below 2 mm Hg for all but one of the first 5 days. In late TO, ITP rose to 4.8 +/- 1.7 mm Hg by Day 1 and remained elevated. LFV remained lower after early than after late TO (P < 0.05) for 8 days. Thereafter, pressure and volume reached similar levels in both TO groups; both were significantly higher than their respective controls (P < 0.05). Parenchymal fraction (1 - air-space fraction) was significantly smaller after late TO (22.8 +/- 1.2%) than after early TO (31.3 +/- 0.5%). Type II density was 38.0 +/- 12.4 x 10(6)/mL after early TO and 84.0 +/- 24.3 x 10(6)/mL in control (P < 0.05); the difference between late TO and control was not significant. CONCLUSIONS: Late tracheal occlusion in fetal lambs caused more rapid lung growth than earlier TO, although ultimate lung size was similar in both groups. Late TO also resulted in greater air-space fraction and better preservation of the type II cell population than early TO. Late-gestation tracheal occlusion may therefore be preferable to prolonged occlusion initiated earlier.

Short-term tracheal occlusion in fetal lambs with diaphragmatic hernia improves lung function, even in the absence of lung growth.

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.