Role of fetal breathing movements in control of fetal lung distension

The upper airway parameters: the potential diagnostic clues for congenital intrathoracic lesions

BACKGROUND

The diagnosis of congenital intrathoracic lesions still has limitations. The airway development was influenced by intrathoracic factors. Whether the diagnostic value of the upper airway parameters in congenital intrathoracic lesions has not been confirmed.

OBJECTIVES

We aimed to compare fetal upper airway parameters between normal fetuses and fetuses with intrathoracic lesions, and we tried to verify its diagnostic value in intrathoracic lesions.

METHODS

This was an observational case-control study. In the control group, 77 women were screened at 20-24 weeks' gestational age, 23 were screened at 24-28 weeks' gestational age, and 27 were screened at 28-34 weeks' gestational age. In the case group, 41 cases were enrolled (6 cases of intrathoracic bronchopulmonary sequestration, 22 of congenital pulmonary airway malformations, and 13 of congenital diaphragmatic hernia). Fetal upper airway parameters (tracheal width, the narrowest lumen width, and width of the subglottic cavity and laryngeal vestibule) were measured using ultrasound equipment. The correlations between fetal upper airway parameters and gestational age, and the differences in fetal upper airway parameters between cases and controls, were analyzed. The standardized airway paraments were acquired, and their potential diagnostic value for congenital intrathoracic lesions were analyzed.

RESULTS

The fetal upper airway parameters of both groups were positively correlated with the gestational age: The control group, tracheal width (R² = 0.569, p < 0.001), narrowest lumen width (R² = 0.429, p < 0.001), subglottic cavity width (R² = 0.551, p < 0.001), laryngeal vestibule width (R² = 0.349, p < 0.001). The case group (tracheal width R² = 0.474, p < 0.001) narrowest lumen width (R² = 0.425, p < 0.001), subglottic cavity width (R² = 0.623, p < 0.001), laryngeal vestibule width (R² = 0.347, p < 0.001). Fetal upper airway parameters of the cases group were smaller than those of the controls group. The tracheal width in fetuses with congenital diaphragmatic hernia was the smallest among the other case groups studied. The standardized tracheal width has the best diagnostic value for congenital intrathoracic lesions in the standardized airway paraments (the area under the ROC curve was 0.894), and has a high diagnostic value for congenital pulmonary airway malformations and congenital diaphragmatic hernia (the area under the ROC curve was 0.911 and 0.992, respectively).

CONCLUSION

Fetal upper airway parameters differ between normal fetuses and fetuses with intrathoracic lesions, and might offer potential diagnostic clues for congenital intrathoracic lesions.

Impact of fetal treatments for congenital diaphragmatic hernia on lung development

The extent of lung hypoplasia impacts the survival and severity of morbidities associated with congenital diaphragmatic hernia (CDH). The alveoli of CDH infants and in experimental models of CDH have thickened septa with fewer type II pneumocytes and capillaries. Fetal treatments of CDH-risk preterm birth. Therefore, treatments must aim to balance the need for increased gas exchange surface area with the restoration of pulmonary epithelial type II cells and the long-term respiratory and neurodevelopmental consequences of prematurity. Achievement of sufficient lung development in utero for successful postnatal transition requires adequate intra-thoracic space for lung growth, maintenance of sufficient volume and appropriate composition of fetal lung fluid, regular fetal breathing movements, appropriate gas exchange area, and ample surfactant production. The review aims to examine the rationale for current and future therapeutic strategies to improve postnatal outcomes of infants with CDH.

Airway mechanical compression: its role in asthma pathogenesis and progression

The lung is a mechanically active organ, but uncontrolled or excessive mechanical forces disrupt normal lung function and can contribute to the development of disease. In asthma, bronchoconstriction leads to airway narrowing and airway wall buckling. A growing body of evidence suggests that pathological mechanical forces induced by airway buckling alone can perpetuate disease processes in asthma. Here, we review the data obtained from a variety of experimental models, including in vitro, ex vivo and in vivo approaches, which have been used to study the impact of mechanical forces in asthma pathogenesis. We review the evidence showing that mechanical compression alters the biological and biophysical properties of the airway epithelium, including activation of the epidermal growth factor receptor pathway, overproduction of asthma-associated mediators, goblet cell hyperplasia, and a phase transition of epithelium from a static jammed phase to a mobile unjammed phase. We also define questions regarding the impact of mechanical forces on the pathology of asthma, with a focus on known triggers of asthma exacerbations such as viral infection.

Effect of lung hypoplasia on the cardiorespiratory transition in newborn lambs

Newborns with lung hypoplasia (LH) commonly have limited respiratory function and often require ventilatory assistance after birth. We aimed to characterize the cardiorespiratory transition and respiratory function in newborn lambs with LH. LH was induced by draining fetal lung liquid in utero [110-133 days (d), term = 147d, n = 6]. At ~133d gestation, LH and Control lambs (n = 6) were instrumented and ventilated for 3 h to monitor blood-gas status, oxygenation, ventilator requirements, and hemodynamics during the transition from fetal to newborn life. Lambs with LH had significantly reduced relative wet and dry lung weights indicating hypoplastic lungs compared with Control lambs. LH lambs experienced persistent hypercapnia and acidosis during the ventilation period, had lower lung compliance, and had higher alveolar-arterial differences in oxygen and oxygenation index compared with Control lambs. As a result, LH lambs required greater respiratory support and more supplemental oxygen. Following delivery, LH lambs experienced periods of significantly lower pulmonary artery blood flow and higher carotid artery blood flow in association with the lower oxygenation levels. The detrimental effects of LH can be attributed to a reduction in lung size and poorer gas exchange capabilities. This study has provided greater understanding of the effect of LH itself on the physiology underpinning the transition from fetal to newborn life. Advances in this area is the key to identifying improved or novel management strategies for babies with LH starting in the delivery room, to favorably alter the fetal-to-newborn transition toward improved outcomes and reduced lifelong morbidity.NEW & NOTEWORTHY Current clinical management of newborns with lung hypoplasia (LH) is largely based on expert opinion rather than scientific evidence. We have generated physiological evidence for detrimental effects of LH on hemodynamics and respiratory function in newborn lambs, which mimics the morbidity observed in LH newborns clinically. The unfavorable consequences of LH can be attributed to a reduction in lung size and poorer gas exchange capabilities.

Congenital diaphragmatic disease: An unusual presentation in adulthood. Case report

INTRODUCTION

Congenital diaphragmatic disease is a quite common condition that usually occurs in the neonatal period, and the diagnosis of congenital diaphragmatic disease in adulthood is rare.

CASE PRESENTATION

A 64-years-old Caucasian woman was admitted in emergency at our Department, due to a bowel obstruction and dyspnea. A CT-scan showed a diaphragmatic herniation in the left area, with malposition of dilated transverse and descending colon in the chest. An emergency laparatomy was performed, showing a toxic megacolon, in the absence of a true diaphragmatic hernia, and a left diaphragm and left liver hypoplasia. An intraoperative bronchoscopy revealed concomitant hypoplasia of the left lung. A subtotal colectomy with ileo-rectal anastomosis was performed. The postoperative course was uneventful. Histological examination demonstrated hyperplasia of the muscularis mucosae of the colon and cytoplasmic vacuolization of the Auerbach plexus ganglia. The karyotype genetic analysis excluded concomitant microdeletion or duplication syndromes.

DISCUSSION

To our knowledge, this seems to be the first reported case of toxic megacolon in a patient with congenital hypoplasia of the left bronchial-lung system, of the left liver, and of the left diaphragm.

CONCLUSION

The correct development of the diaphragm is essential for the neighboring organs. The observed clinical pattern could be related to a partial modification of neural crest cell detachment or migration, which could be responsible for bowel and diaphragm defects, even though it was not included in typical neural crest cell syndromes. Further researches should be performed in order to define the sporadic or syndromic source of these multiorgan defects.

Microfluidic chest cavities reveal that transmural pressure controls the rate of lung development

Mechanical forces are increasingly recognized to regulate morphogenesis, but how this is accomplished in the context of the multiple tissue types present within a developing organ remains unclear. Here, we use bioengineered 'microfluidic chest cavities' to precisely control the mechanical environment of the fetal lung. We show that transmural pressure controls airway branching morphogenesis, the frequency of airway smooth muscle contraction, and the rate of developmental maturation of the lungs, as assessed by transcriptional analyses. Time-lapse imaging reveals that branching events are synchronized across distant locations within the lung, and are preceded by long-duration waves of airway smooth muscle contraction. Higher transmural pressure decreases the interval between systemic smooth muscle contractions and increases the rate of morphogenesis of the airway epithelium. These data reveal that the mechanical properties of the microenvironment instruct crosstalk between different tissues to control the development of the embryonic lung.

Pulmonary hypoplasia

To survive the transition to extrauterine life, newborn infants must have lungs that provide an adequate surface area and volume to allow for gas exchange. The dynamic activities of fetal breathing movements and accumulation of lung luminal fluid are key to fetal lung development throughout the various phases of lung development and growth, first by branching morphogenesis, and later by septation. Because effective gas exchange is essential to survival, pulmonary hypoplasia is among the leading findings on autopsies of children dying in the newborn period. Management of infants born prematurely who had disrupted lung development, especially at the pre-glandular or canalicular periods, may be challenging, but limited success has been reported. Growing understanding of stem cell biology and mechanical development of the lung, and how to apply them clinically, may lead to new approaches that will lead to better outcomes for these patients.

Quantifying stretch and secretion in the embryonic lung: Implications for morphogenesis

Branching in the embryonic lung is controlled by a variety of morphogens. Mechanics is also believed to play a significant role in lung branching. The relative roles and interactions of these two broad factors are challenging to determine. We considered three hypotheses for explaining why tracheal occlusion triples branching with no overall increase in size. Both hypotheses are based on tracheal occlusion blocking the exit of secretions. (H1) Increased lumen pressure stretches tissues; stretch receptors at shoulders of growing tips increase local rate of branching. (H2) Blocking exit of secretions blocks advective transport of morphogens, leading to (H2a) increased overall concentration of morphogens or (H2b) increased flux of morphogens at specific locations. We constructed and analyzed computational models of tissue stretch and solute transport in a 3D lung geometry. Observed tissue stresses and stretches were predominantly in locations unrelated to subsequent branch locations, suggesting that tissue stretch (H1) is not the mechanism of enhancement of branching. Morphogen concentration in the mesenchyme (H2a) increased with tracheal occlusion, consistent with previously reported results. Morphogen flux at the epithelial surface (H2b) completely changed its distribution pattern when the trachea was occluded, tripling the number of locations at which it was elevated. Our results are consistent with the hypothesis that tracheal occlusion blocks outflow of secretions, leading to a higher number of high-flux locations at branching tips, in turn leading to a large increase in number of branching locations.

Tracheal occlusion alters pulmonary circulation in the fetal lamb with normally developing lungs

BACKGROUND

Tracheal occlusion (TO) promotes fetal lung growth through an increase in intraluminal pressure. Although evidence suggests that fetal TO (FETO) decreases the occurrence of pulmonary hypertension in severe congenital diaphragmatic hernia, controversies on its effect on the pulmonary circulation remain. Therefore, we investigated the effects of FETO on the lung hemodynamics in a chronically catheterized fetal lamb model.

METHODS

Fifteen pregnant ewes were operated on between 125 and 128 days of gestation (term: 145 days). Catheters and ultrasonic flow transducer were placed through a left thoracotomy in the lamb fetus to determine aortic, pulmonary and left atrial pressures, and left pulmonary artery blood flow. A balloon was positioned between the carina and vocal cords under fetoscopic control. The animals were assigned to either control (n=6) or FETO (n=9) groups. TO was performed by inflating the balloon. We studied the acute effects of temporary (2-h) and prolonged (4-day) TO on basal pulmonary vascular tone and on the pulmonary vascular reactivity to acetylcholine and to increased fetal oxygen tension.

RESULTS

We found that left pulmonary blood flow (LPA) increased and pulmonary vascular resistance (PVR) decreased by 20% during brief TO (p<0.05). After balloon deflation, LPA blood flow further increased by 40%, and PVR decreased by 50% compared to baseline values (p<0.05). In contrast, no change in LPA blood flow or PVR was observed during prolonged TO. Moreover, the vasodilator responses to acetylcholine and to increased fetal PaO2 were blunted during TO.

CONCLUSIONS

These data indicate that antenatal tracheal occlusion promotes active pulmonary vasodilation, which is partly blunted by the mechanical effects of elevation of the intraluminal pressure.

Sculpting organs: mechanical regulation of tissue development

The ramified architectures of organs such as the mammary gland and lung are generated via branching morphogenesis, a developmental process through which individual cells bud and pinch off of pre-existing epithelial sheets. Although specified by signaling programs, organ development requires integration of all aspects of the microenvironment. We describe the essential role of endogenous cellular contractility in the formation of branching tubes. We also highlight the role of exogenous forces in normal and aberrant branching.

ROCK2 is involved in accelerated fetal lung development induced by in vivo lung distension

Lung development is strongly influenced by its state of distension. For instance, increasing distension induced by fetal tracheal occlusion (TO) stimulates lung development. In contrast, oligohydramnios (OH) reduces lung distending forces and results in lung hypoplasia. We hypothesize that Rho/Rho-associated kinase (ROCK) pathway plays an important role as mechanosensor in vivo acting either directly or indirectly in the translation of increased distension into acceleration of lung growth. TO was done in fetal mice sacrificed either 3 or 24 hr later; in a subset of dam, fasudil, a specific ROCK inhibitor, or vehicle was injected intra-peritoneally. OH was done by puncture of the amniotic sac. ROCK2 protein levels were assessed by Western blot and immunohistochemistry (IHC); lung development was assessed by measuring the generation of distal respiratory airway. Significant differences were found in ROCK2 protein levels between TO and Sham-TO at 3 and 24 hr, but not for ROCK1. Indeed, IHC revealed that ROCK2 staining was sparse and restricted to a few mesenchymal cells in Sham-TO, whereas it was strong in acini of TO lungs. OH lungs expressed lower levels of ROCK2 in the acini when compared to untouched controls. In fasudil-treated animals, the degree of lung development following TO was significantly lower than in the group injected with vehicle. At the dose regimen used, fasudil did not affect normal lung development, as observed in the untouched animals. In summary, ROCK2 protein levels was affected by the degree of lung expansion and blunting ROCK activity abolished the response to increased lung distension, suggesting that ROCK is a key regulator in TO-induced accelerated lung development.

Partial pulmonary embolization disrupts alveolarization in fetal sheep

Background

Although bronchopulmonary dysplasia is closely associated with an arrest of alveolar development and pulmonary capillary dysplasia, it is unknown whether these two features are causally related. To investigate the relationship between pulmonary capillaries and alveolar formation, we partially embolized the pulmonary capillary bed.

Methods

Partial pulmonary embolization (PPE) was induced in chronically catheterized fetal sheep by injection of microspheres into the left pulmonary artery for 1 day (1d PPE; 115d gestational age; GA) or 5 days (5d PPE; 110-115d GA). Control fetuses received vehicle injections. Lung morphology, secondary septal crests, elastin, collagen, myofibroblast, PECAM1 and HIF1α abundance and localization were determined histologically. VEGF-A, Flk-1, PDGF-A and PDGF-Rα mRNA levels were measured using real-time PCR.

Results

At 130d GA (term ~147d), in embolized regions of the lung the percentage of lung occupied by tissue was increased from 29 ± 1% in controls to 35 ± 1% in 1d PPE and 44 ± 1% in 5d PPE fetuses (p < 0.001). Secondary septal crest density was reduced from 8 ± 0% in controls to 5 ± 0% in 1d PPE and 4 ± 0% in 5d PPE fetuses (p < 0.05), indicating impaired alveolar formation. The deposition of differentiated myofibroblasts (23 ± 1% vs 28 ± 1%; p < 0.001) and elastin fibres (3 ± 0% vs 4 ± 0%; p < 0.05) were also impaired in embolized lung regions of PPE fetuses compared to controls. PPE did not alter the deposition of collagen or PECAM1. At 116d GA in 5d PPE fetuses, markers of hypoxia indicated that a small and transient hypoxic event had occurred (hypoxia in 6.7 ± 1.4% of the tissue within embolized regions of 5d PPE fetuses at 116d compared to 0.8 ± 0.2% of tissue in control regions). There was no change in the proportion of tissue labelled with HIF1α. There was no change in mRNA levels of the angiogenic factors VEGF and Flk-1, although a small increase in PDGF-Rα expression at 116d GA, from 1.00 ± 0.12 in control fetuses to 1.61 ± 0.18 in 5d PPE fetuses may account for impaired differentiation of alveolar myofibroblasts and alveolar development.

Conclusions

PPE impairs alveolarization without adverse systemic effects and is a novel model for investigating the role of pulmonary capillaries and alveolar myofibroblasts in alveolar formation.

Lung organogenesis

Developmental lung biology is a field that has the potential for significant human impact: lung disease at the extremes of age continues to cause major morbidity and mortality worldwide. Understanding how the lung develops holds the promise that investigators can use this knowledge to aid lung repair and regeneration. In the decade since the "molecular embryology" of the lung was first comprehensively reviewed, new challenges have emerged-and it is on these that we focus the current review. Firstly, there is a critical need to understand the progenitor cell biology of the lung in order to exploit the potential of stem cells for the treatment of lung disease. Secondly, the current familiar descriptions of lung morphogenesis governed by growth and transcription factors need to be elaborated upon with the reinclusion and reconsideration of other factors, such as mechanics, in lung growth. Thirdly, efforts to parse the finer detail of lung bud signaling may need to be combined with broader consideration of overarching mechanisms that may be therapeutically easier to target: in this arena, we advance the proposal that looking at the lung in general (and branching in particular) in terms of clocks may yield unexpected benefits.

Identification of cellular processes that are rapidly modulated in response to tracheal occlusion within mice lungs

Lung development progresses through a process reliant on mechanical cell stretch. However, this process is not well defined at the molecular level. Our goal was to globally analyze the transcriptome of fetal mouse lungs following short periods of tracheal occlusion (TO) to identify cellular processes that are rapidly modulated in response to intraluminal stretch increase. Serial analysis of gene expression (SAGE) was used to examine the global transcriptomic response of mouse prealveolar stage lungs to in vivo TO at 1 and 3 h. SAGE results were extended by histo- and immunochemical examination. Based on the 97 TO-modulated transcripts identified, our results further point out that continuous stretch in developing lungs leads directly to rapid and highly specific phenotypic modifications in a significant proportion of pulmonary cells. We conclude that intraluminal stretch increase during prealveolar stage of lung development induces a critical transition of pulmonary cells phenotype in which there is an increase in alpha-smooth muscle actin (alpha-SMA)-containing cells along with a relative decrease in lipid-containing cells.

Exploiting mechanical stimuli to rescue growth of the hypoplastic lung

Impaired lung development afflicts a range of newborns cared for by paediatric surgeons. As a result the speciality has led in the development of surgical models that illustrate the biomechanical regulation of lung growth. Using transgenic mutants, biologists have similarly discovered much about the biochemical regulation of prenatal lung growth. Airway smooth muscle (ASM) and its prenatal contractility airway peristalsis (AP) represent a novel link between these areas: ASM progenitors produce an essential biochemical factor for lung morphogenesis, whilst calcium-driven biomechanical ASM activity appears to regulate the same. In this invited paper, I take the opportunity both to review our recent findings on lung growth and prenatal ASM, and also to discuss mechanisms by which ASM contractility can regulate growth. Finally, I will introduce some novel ideas for exploration: ASM contractility could help to schedule parturition (pulmonary parturition clock) and could even be a generic model for smooth muscle regulation of morphogenesis in similar organs.

Development of mechanics and pulmonary reflexes

The mechanical properties of the respiratory system are paramount in converting neural output into ventilation. The highly compliant chest wall of the newborn results in chest distortion and volume loss during inspiration and, as the chest is also unable to resist the inward recoil of the lung, there is a reduction in lung volume at end expiration (functional residual capacity) and a tendency for alveoli to collapse. Vagal innervation of the lungs and airways is responsible for eliciting various reflexes that result in the dynamic modification of respiratory mechanics and an improvement in ventilation. From the first breath, the newborn increases the frequency of augmented breaths to improve lung compliance and prolongs the expiratory time constant in order to increase the amount of air remaining in the lung at end expiration and help prevent lung collapse. This review examines the respiratory mechanics of the mammalian neonate at birth and during early development together with the vagal reflexes that are responsible for the dynamic modification of respiratory mechanics in order to ensure that effective gas exchange occurs from birth.

Volume and secretion rate of lung liquid in the final days of gestation and labour in the fetal sheep

1. Most of the liquid that fills the lung of the fetal sheep in late gestation is cleared by the end of labour. Clearance of this liquid has a beneficial effect on postnatal gas exchange and therefore represents an important adaptation for postnatal life. Despite its importance, there is disagreement about whether clearance begins prior to labour, or occurs entirely within labour. 2. To address this issue, we made serial determinations of lung liquid volume by indicator dilution during late gestation and labour in the fetal sheep. 3. Regression analysis demonstrated that lung liquid volume exhibited a plateau level in the near-term fetus before it began to decline. Two models provided a fit to the decline in volume. In one, lung liquid clearance occurred in two linear phases, the first beginning 70 h before the study was terminated when the ewe was in advanced labour, the second occupying the last 8 h of the study period. In the initial phase, average lung liquid volume fell from 38.3 to 26.4 ml x kg(-1) before a rapid decline in the second phase reduced the volume to 13.8 ml x kg(-1). An exponential decay model was also found to fit the data; this showed a gradual decline in lung liquid volume in the 2 days preceding onset of labour, followed by a much more rapid decline within labour. 4. The rate of lung liquid secretion also declined in two linear phases, both of which commenced earlier than the changes in lung liquid volume. An exponential decay model also gave a significant fit to the data, but the fit was significantly weaker than that achieved with the two-slope model. 5. We conclude that clearance of lung liquid begins well before commencement of labour in the full term fetal sheep, and then accelerates once labour is established. In our study, lung liquid volume fell even in the absence of reabsorption of liquid across the pulmonary epithelium, indicating that outflow of liquid through the trachea must have occurred at a rate in excess of the secretion rate.

Mechanical stretch promotes alveolar epithelial type II cell differentiation

Functional maturation of pulmonary alveolar epithelial cells is crucial for extrauterine survival. Mechanical distension and mesenchymal-epithelial interactions play important roles in this process. We hypothesized that mechanical stretch simulating fetal breathing movements is an important regulator of pulmonary epithelial cell differentiation. Using a Flexercell Strain Unit, we analyzed effects of stretch on primary cultures of type II cells and cocultures of epithelial and mesenchymal cells isolated from fetal rat lungs during late development. Cyclic stretch of isolated type II cells increased surfactant protein (SP) C mRNA expression by 150 +/- 30% over controls (P < 0.02) on gestational day 18 and by 130 +/- 30% on day 19 (P < 0.03). Stretch of cocultures with fibroblasts increased SP-C expression on days 18 and 19 by 170 +/- 40 and 270 +/- 40%, respectively, compared with unstretched cocultures. On day 19, stretch of isolated type II cells increased SP-B mRNA expression by 50% (P < 0.003). Unlike SP-C, addition of fibroblasts did not produce significant additional effects on SP-B mRNA levels. Under these conditions, we observed only modest increases in cellular immunoreactive SP-B, but secreted saturated phosphatidylcholine rose by 40% (P < 0.002). These results indicate that cyclic stretch promotes developmentally timed differentiation of fetal type II cells, as a direct effect on epithelial cell function and via mesenchymal-epithelial interactions. Expression of the SP-C gene appears to be highly responsive to mechanical stimulation.

The compromised intra-uterine environment: implications for future lung health

1. Epidemiological studies of infants, children and adults indicate that prenatal compromises that restrict fetal growth and cause low birthweight increase the risk of respiratory deficiencies after birth. 2. It is apparent that the lung has a limited ability to recover from early developmental compromises and that altered development can permanently impair lung architecture. 3. Lung development in utero can be adversely affected by factors associated with fetal growth restriction, namely fetal hypoxaemia, reduced substrate supply and hypercortisolaemia. 4. We have conducted a series of studies of respiratory development in chronically catheterized ovine fetuses and postnatal lambs in which growth restriction was induced during late gestation by embolizing the umbilico-placental vascular bed, a technique that replicates key aspects of human placental insufficiency. 5. During late gestation, restricting the growth of the ovine fetus did not alter lung weight or lung liquid secretion or volume when each factor was related to bodyweight, but it did lead to increased lung DNA concentrations and an increased thickness of the air-blood barrier. Expression of pulmonary surfactant proteins A, B and C were not altered and, hence, it was unlikely that surfactant protein synthesis had been impaired by growth restriction. 6. When growth restriction continued to term, lambs were born with a low birthweight and remained small compared with controls for 8 weeks after birth. Low-birthweight lambs were mildy hypoxaemic and compliances of their lungs and chest wall were, respectively, decreased and increased relative to controls. Pulmonary surfactant proteins A, B and C were not deficient, indicating that decreased lung compliance most likely had a structural basis.

Lung growth for beginners

Lung growth occurs as a series of tightly regulated events commencing in the embryo and continuing post-natally. It depends on a number of factors, including developmental, genetic and environment ones. Abnormalities of any of these factors may have a profound influence on lung growth. The causes of developmental abnormalities of the lung such as lung cysts and congenital diaphragmatic hernia are poorly understood, but may result from a combination of genetic and environmental factors. Normal fetal breathing movements and an adequate balance between the production of fetal lung fluid and drainage of this fluid are both essential for normal fetal lung growth. It seems that fetal breathing movements are necessary to maintain sufficient pressure within the airways and perhaps to directly stimulate lung growth via induction of mitogenic activity. The volume of intra-pulmonary fluid is regulated by the resistance of the upper airway and by contractions of the diaphragm. Increased drainage of the amniotic fluid, another essential factor for normal lung growth and development, will result in marked pulmonary hypoplasia as may occur with pre-term rupture of the membranes and with fetal renal disease. Perhaps the most important factor for adverse lung growth is pre-term delivery of the infant from any cause including intra-uterine infection. Both ante- and post-natal factors, including mechanical ventilation and oxygen therapy, will affect normal alveolization. In this review, particular attention is paid to breathing movements and the balance between fluid production and drainage.

Effects of prostaglandin E2 on renal function and lung liquid dynamics in foetal sheep

1. The aim of the present study was to determine the effects of prolonged prostaglandin E2 (PGE2) administration on the function of the foetal kidneys and lungs in order to gain a greater understanding of the role played by PGE2 in the control of foetal fluid balance. By studying the effects of PGE2 at two gestational ages, we have also been able to examine the influence of age. 2. We studied the effects of 26 h PGE2 infusion on foetal sheep at a mean (+/- SEM) of 120.0 +/- 0.6 (n = 6) and 139.0 +/- 0.8 (n = 4) days of gestation. In both groups, foetal urine production was significantly inhibited throughout the infusion period (P < 0.05). In younger, but not older foetuses, urine production returned to control values within 24 h of ending the infusion (P < 0.05). This PGE2-induced anti-diuresis was associated with foetal hypoxaemia and acidaemia, a reduction in free water clearance and an increase in foetal plasma arginine vasopressin concentrations (P < 0.05). 3. During PGE2 infusions, foetal breathing movements were inhibited, the effect being greater and more sustained in older foetuses (P < 0.05). 4. Infusions of PGE2 led to increased lung liquid production at both ages (P < 0.05); lung liquid volumes were reduced in older foetuses (P < 0.05), but were unchanged in younger foetuses. The reduction in lung liquid volume in older foetuses may have been due to inhibition of foetal breathing. 5. We conclude that increased circulating levels of PGE2 have profound effects on foetal renal and lung function which, if sustained, could compromise foetal lung development and perinatal well-being.

Massive decline in lung liquid before vaginal delivery at term in the fetal lamb

OBJECTIVE

Our aim was to determine the volume of liquid remaining in the lungs of the fetal lamb just before a normal vaginal delivery at term to assess the extent to which an excess of liquid in the airspaces might contribute to the respiratory morbidity that accompanies elective cesarean delivery.

STUDY DESIGN

The volume of liquid in the future airspace of the lungs was determined at the end of labor in eight fetal lambs at term from the dilution of an impermeable tracer (125I-labeled human serum albumin) mixed into the liquid. This volume was compared with that measured in a second group of 10 fetal lambs studied 7 days before the expected date of delivery (term = 147 days).

RESULTS

The volume of lung liquid present at the end of labor was 6.8 +/- 1.0 ml x kg(-1) (n = 8) compared with 28.2 +/- 1.8 ml x kg(-1) (n = 10) in the second group of lambs studied before the onset of labor at 140 days of gestation.

CONCLUSION

Our results indicate that the bulk (>75%) of the liquid that fills the lungs of the fetal lamb at 140 days of gestation is cleared at some time before normal term birth, suggesting that the adverse respiratory impact of elective cesarean delivery may be largely explained by denying the fetus this important adaptive mechanism.

Cortisol pretreatment enhances the lung growth response to tracheal obstruction in fetal sheep

We have investigated whether cortisol pretreatment of sheep fetuses will result in a greater liquid accumulation within the lung and a greater lung growth response to obstruction of the fetal trachea. Chronically catheterized fetal sheep received either 1) a cortisol infusion at an increasing dose (1.5-4.0 mg/day) from days 118 to 127 of gestation; the fetal trachea was then obstructed from days 128 to 131 of gestation (n = 4); 2) a saline infusion from days 118 to 127 of gestation; the fetal trachea was then obstructed from days 128 to 131 of gestation (n = 4); or 3) a saline infusion from days 118 to 127 of gestation with no period of tracheal obstruction (control; n = 4). Fetal tracheal pressures were measured from days 128 to 131 of gestation, whereas lung liquid secretion rates and volumes were measured on days 118, 128, and 131 of gestation. On day 131 of gestation, all fetuses were given an intravenous injection of [3H]thymidine and were killed 8 h later. Cortisol pretreatment increased the volume of liquid that accumulated within the fetal lung from 69.5 +/- 4.1 to 96.1 +/- 14.1 ml/kg after 3 days of tracheal obstruction. Similarly, cortisol pretreatment significantly enhanced the increase in lung DNA content from 257.4 +/- 11.0 to 309.1 +/- 16.3 mg/kg after 3 days of tracheal obstruction. We conclude that pretreatment of fetuses with cortisol increases the volume of liquid that accumulates after tracheal obstruction and, as a result, increases the fetal lung growth response to tracheal obstruction.

Fetal pulmonary development: the role of respiratory movements

The lung develops before birth as a collapsible, liquid-filled, organ. Throughout the later stages of gestation the fetal lungs are maintained at a level of expansion that is considerably greater than the level achieved as a result of passive equilibration between lung recoil and the chest wall. Fetal breathing movements (FBM) are a feature of normal fetal life and, as such, are used clinically in the assessment of fetal wellbeing. By opposing lung recoil, FBM help to maintain the high level of lung expansion that is now known to be essential for normal growth and structural maturation of the fetal lungs. During 'apnoeic' periods between successive episodes of FBM, active laryngeal constriction has the effect of opposing lung recoil by resisting the escape of lung liquid via the trachea. The prolonged absence or impairment of FBM is likely to result in a reduced mean level of lung expansion which can lead to hypoplasia of the lungs. There is clinical evidence, disputed by some, that the absence of FBM exacerbates the effects of other factors that are associated with lung hypoplasia, such as premature rupture of fetal membranes and oligohydramnios. Even in the absence of such factors, prolonged or repeated reductions or abolition of FBM may contribute to impairments of fetal lung development; FBM can be inhibited by fetal hypoxaemia, hypoglycaemia, maternal alcohol consumption, maternal smoking, intra-amniotic infection and maternal consumption of sedatives or narcotic drugs. Abnormal growth of the fetal lungs has relevance for postnatal respiratory health as it is now recognised that there may be only a limited capacity after birth for the restoration of normal pulmonary architecture following impaired intra-uterine lung development.

Patterns of fetal perinasal fluid flow in cases of congenital diaphragmatic hernia

OBJECTIVE(S)

Our purpose was to expand the previous reported series of observations of fetal perinasal fluid flow in cases of antenatally diagnosed congenital diaphragmatic hernia, characterize the timing parameters of the fetal breath cycle, and define the relationship of fetal perinasal fluid flow and the diaphragmatic component of fetal breathing movements. Our hypothesis was that characteristics of diaphragm-related and nondiaphragm-related perinasal fluid flow and other breath cycle characteristics differ in cases of congenital diaphragmatic hernia compared with controls.

STUDY DESIGN

Fetal perinasal fluid flow velocity and fetal chest wall movements were studied in 24 cases of uncomplicated pregnancy, and flow was studied in 24 cases of antenatally diagnosed congenital diaphragmatic hernia at gestational ages ranging from 30 to 41 weeks. The examination of fetal perinasal fluid flow velocity was performed with use of an ultrasonography system applying color flow and spectral Doppler analysis. Breath-to-breath interval, time of inspiration, time of expiration, and peak inspiratory and expiratory velocities were determined for each type of perinasal flow.

RESULTS

The study revealed that the time of expiration in cases of congenital diaphragmatic hernia at 30 to 36 and 37 to 41 weeks of gestation was significantly shorter than in cases of uncomplicated pregnancy. The ratio of time of inspiration and breath-to-breath interval in cases of diaphragmatic hernia was approximately 30% higher (p = 0.001) at 30 to 36 weeks of gestation than in cases of uncomplicated pregnancy. The study also showed that in cases of congenital diaphragmatic hernia the expiratory peak velocity ratio at 30 to 36 weeks of gestation was significantly lower than in cases of uncomplicated pregnancy.

CONCLUSIONS

We conclude that by Doppler ultrasonography measurements of fetal perinasal fluid flow, in cases of congenital diaphragmatic hernia, we can evaluate the timing parameters of fetal diaphragm-related breath cycles, the relationship of intraalveolar and intraamniotic pressures, and fetal upper respiratory tract resistance. Fetuses with diaphragmatic hernia spent significantly more time with diaphragm-nonrelated perinasal flow than did fetuses in cases of uncomplicated pregnancy, which can cause the increased loss of lung liquid and consequently be associated with pulmonary insufficiency in the early neonatal period.

Lung liquid production rates and volumes do not decrease before labor in healthy fetal sheep

Previous studies have suggested that the volume and production rate of fetal lung liquid decrease late in gestation, before the onset of labor, in preparation for the clearance of lung liquid at birth. In contrast, our earlier studies have not shown a decrease in lung liquid volume near term, although these studies were not continued to the onset of labor. Our aim was to determine the changes in lung liquid volume and production rate in fetal sheep during the last 2 wk of gestation up to the onset of labor at term (approximately 147 days). In eight chronically catheterized fetal sheep, the volume and production rate of fetal lung liquid were measured at 130, 135, and 140 days of gestation and then on every 2nd day until the onset of labor. Labor was detected by monitoring uterine muscle activity and intrauterine pressure changes. On the day of labor onset, which occurred at 147 +/- 1 days of gestation, fetuses weighted 5.0 +/- 0.2 kg. The volume of fetal lung liquid was 40.4 +/- 2.7 ml/kg at 19 +/- 1 days before labor onset and had not significantly changed by 0.7 +/- 0.2 days (44.8 +/- 5.1 ml/kg) before labor. Similarly, lung liquid production rates at 19 +/- 1 days before labor (5.1 +/- 1.8 ml.h-1.kg-1) were not significantly different from those at 0.7 +/- 0.2 days before labor (3.4 +/- 0.7 ml.h-1.kg-1). We conclude that, in healthy ovine fetuses, lung liquid volumes and production rates do not decrease before the onset of labor. Our results indicate that the entire volume of fetal lung liquid (approximately 222.5 +/- 36.6 ml) must be cleared after the onset of labor.

Effect of gestational age on the increase in fetal lung growth following tracheal obstruction

The growth response of the fetal lung to increased expansion was compared at two gestational ages. In fetal sheep, lung expansion was increased by occluding the trachea for 48 h at either 112-114 days (younger fetuses) or 125-127 days (older fetuses) of gestation (term is approximately 145 days). After 24 h of tracheal occlusion, the volumes of liquid that could be drained from the lungs were increased by 64.7 and 158% above control in younger and older fetuses respectively; the volumes were not increased further after 48 h. In younger fetuses, 48 h of tracheal occlusion increased (p < .05) fetal lung wet weights (21% above control) and protein contents (43% above control) but not DNA contents. In older fetuses, 48 h of tracheal occlusion increased (p < .05) fetal lung wet weights (61% above control), protein contents (41% above control), and DNA contents (22% above control). However, 48 h of tracheal occlusion did not alter total lung hydroxyproline content at either age, resulting in a reduction in the hydroxyproline/protein ratio of the fetal lungs. The results suggest that the lung growth response to tracheal occlusion is greater at 125-127 days of gestation than at 112-114 days of gestation, probably due to a greater accumulation of lung liquid and hence a greater increase in lung expansion, in older fetuses.

Fetal lung liquid: a major determinant of the growth and functional development of the fetal lung

1. During fetal life the lung develops as a liquid-filled organ. This liquid is produced by the fetal lung and leaves via the trachea from where it is either swallowed or enters the amniotic sac. Fetal lung liquid plays a crucial role in the growth and development of the lungs by maintaining them in a distended state. It is now recognized that the retention of liquid within the future airways is required to maintain the lungs at an appropriate level of expansion in order to stimulate their growth. Indeed, it is likely that most, if not all, of the conditions and malformations that lead to inadequate growth of the fetal lung do so by reducing the volume of lung liquid and hence the degree of lung expansion. 2. The volume of fetal lung liquid is principally regulated by the resistance to lung liquid efflux through the fetal upper airway and by the presence of diaphragmatic activity associated with fetal breathing movements (FBM). During non-breathing periods, the relatively high resistance offered by the upper airway to the efflux of lung liquid opposes the loss of liquid from the lung, thereby maintaining fetal lung expansion. During episodes of FBM, when the larynx is actively dilated and the resistance to lung liquid efflux is reduced, lung liquid leaves the lungs at an increased rate. However, selective inhibition of diaphragmatic muscle activity in the foetus leads to a reduction in lung liquid volume, rather than an increase. This finding indicates that during periods of FBM, rhythmical contractions of the diaphragm retard the loss of lung liquid and help to maintain lung expansion when the upper airway resistance is reduced. It is now apparent that the maintenance of lung expansion by FBM is the basis for their role in promoting fetal lung growth. 3. Successful transition from intra-uterine to extra-uterine life is dependent upon the clearance of liquid from the fetal lungs at the time of birth so that the lungs may effectively function as an organ of gas exchange.(ABSTRACT TRUNCATED AT 400 WORDS)