Decline in lung liquid volume before labor in fetal lambs

The rising incidence and impact of non-medically indicated pre-labour cesarean section in Latin America

In the poorest populations of the world the difficulties of performing a surgical procedure lead to extremely low cesarean section rates associated with very high perinatal mortality. Meanwhile the proportion of births by cesarean section has been increasing for several decades in many areas of the world, reaching alarmingly high rates especially in Latin America. This review aims to describe this secular trend. The causes of the increase in cesarean deliveries are analyzed with a multidimensional approach, trying to recognize the reasons behind the choice of the route of delivery. We are facing a shift in the guiding paradigm, leaving the classical biomedical one. Advantages and disadvantages of non-medically indicated cesarean section are being sought, comparing it with the results of vaginal delivery, with special emphasis on the short- and long-term morbidity of the newborn. Several variables involved in this controversial topic are considered, with the objective of stimulating critical thoughts about the medical, bioethical and social aspects of the increasing trend in the cesarean section rate.

Prevention and Treatment of Respiratory Distress Syndrome in Preterm Neonates

Respiratory distress syndrome (RDS) impacts a high proportion of preterm neonates, resulting in significant morbidity and mortality. Advances in pharmacotherapy, specifically antenatal corticosteroids and postnatal surfactant therapy, have significantly reduced the incidence and impact of neonatal RDS. Antenatal corticosteroids accelerate fetal lung maturation by increasing the activity of enzymes responsible for surfactant biosynthesis, resulting in improved lung compliance. Maternal antenatal corticosteroid treatment has improved survival of preterm neonates and lowered the incidence of brain injury. After birth, exogenous surfactant administration improves lung compliance and oxygenation, resulting in reductions in the incidence of pneumothorax and of death. Future research will identify the optimal surfactant product, timing of the initial dose, and mode of delivery.

Factors relating caesarean section to persistent pulmonary hypertension of the newborn

BACKGROUND

Several studies have clearly demonstrated a significantly higher incidence of persistent pulmonary hypertension of the newborn (PPHN) in neonates delivered by caesarean section (CS) compared to those delivered vaginally. The pathophysiological factors underlying the link between CS and PPHN are still poorly understood. In this review, we describe the mechanisms that could explain the association between CS delivery and subsequent PPHN, as well as potential preventive measures.

DATA SOURCES

A literature search was conducted by electronic scanning of databases such as PubMed and Web of Science using the key words "persistent pulmonary hypertension of the newborn", "caesarean section", "iatrogenic prematurity", "oxidative stress", "late preterm", "labor" and "vasoactive agents".

RESULTS

Iatrogenic prematurity, higher rates of late preterm delivery and lack of physiological changes of labor play an important role in the association between CS and PPHN. CS delivery also results in limited endogenous pulmonary vasodilator synthesis and lower levels of protective anti-oxidants in the neonates. In addition, CS delivery exposes infants to a higher risk of respiratory distress syndrome and its concomitant increase in endothelin-1 levels, which might indirectly lead to a higher risk of developing PPHN. We believe that neonates delivered by CS are exposed to a combination of these pathophysiological events, culminating in an endpoint of respiratory distress, hypoxia, acidosis, and delayed transition and thereby increased risks of PPHN. The use of antenatal corticosteroids prior to elective CS in late preterm deliveries, promoting accurate informedconsent process, delaying elective CS to 39 weeks of gestation or beyond and antenatal maternal anti-oxidant supplementation could potentially mitigate the effects of CS delivery and minimize CS-related PPHN.

CONCLUSIONS

The link between CS delivery and PPHN is complex. In view of the rising rates of CS worldwide, there is an urgent need to further explore the mechanisms linking CS to PPHN and experimentally test therapeutic options in order to allow effective targeted interventions.

Effect of oronasopharyngeal suction on arterial oxygen saturation in normal, term infants delivered vaginally: a prospective randomised controlled trial

Oronasopharyngeal suction (ONPS) with a suction bulb at birth is a traditional practice in the initial management of healthy infants in Iran and many other countries. The purpose of this study was to compare the effects of oronasopharyngeal suction (ONPS) with those of no suction in normal, term newborns delivered vaginally. A total of 170 healthy term infants of first and single uncomplicated pregnancies, with clear amniotic fluid, vaginal delivery and cephalic presentation, enrolled in the trial during labour. Newborns were randomised into one of the two groups, according to the use of the ONPS procedure. Arterial oxygen saturation (SaO2) levels, heart rates, blood gases of umbilical cord and Apgar scores were determined. The mean SaO2 values over the first and fifth min of birth were similar in the two groups. The maximum time to reach SaO2 of ≥ 92% was shorter in the no suction group. There were no statistically significant differences in the mean of heart rates, respiratory rates and Apgar scores between the groups. Apgar scores at 5 and 10 min were between 8 and 10 for all infants, respectively. Newborns receiving suction showed a statistically significant, lower mean partial carbon dioxide pressure (PCO2) and a significantly higher partial oxygen pressure (PO2) of umbilical artery. Although the differences were statistically significant, these were not considered clinically significant because values remained within normal ranges. According to this study, ONPS is not recommended as a routine procedure in normal, term infants delivered vaginally.

Establishing functional residual capacity in the non-breathing infant

The transition to newborn life critically depends upon lung aeration and the onset of air-breathing, which triggers major cardiovascular changes required for postnatal life, including increases in pulmonary blood flow. Recent imaging studies indicate that lung aeration and functional residual capacity (FRC) recruitment results from inspiratory efforts, which create transpulmonary pressure gradients. During inspiration, these pressure gradients drive airway liquid movement through the conducting and into the distal airways where it crosses the pulmonary epithelium and enters the surrounding tissue. Although this process can occur rapidly (within 3-5 breaths), liquid clearance from lung tissue is much slower, resulting in oedema and increased interstitial tissue pressures, facilitating liquid re-entry into the airways at FRC. Whereas this liquid may be cleared during the next inspiration, liquid re-entry at FRC can be opposed by Na(+) reabsorption, oncotic pressures and expiratory braking manoeuvres. Recognition that transpulmonary pressure gradients mainly drive airway liquid clearance at birth has provided a clearer understanding of how this process may be facilitated in very preterm infants. In particular, it underpins the rationale for providing respiratory support that initially focuses on moving liquid through tubes (airways) rather than air. As the viscosity of liquid is much greater than air, the resistance to moving liquid is ≈ 100 times greater than air, necessitating the use of higher pressures or longer inflation times. Although it is unclear how this strategy could be safely applied clinically, it is clear that end-expiratory pressures are required to create and maintain FRC in preterm infants.

Clinical implication of lung fluid balance in the perinatal period

At birth, lung fluid produced during fetal life must be cleared immediately and efficiently before the first breath takes place, in order for infants to achieve a normal and successful transition from prenatal to postnatal life. Postnatal lung fluid resorption is mediated through activation of airway epithelial sodium channels (ENaC). The observation that ENaC expression is a gestational age-dependent process contributes to our understanding of the development of respiratory distress in both term and preterm infants due to impaired clearing of fluid from their lungs. As fluid absorption, mediated by ENaC activity, in postnatal life has a significant biological role in preventing respiratory distress, any strategy that enhances ENaC activity can potentially help to decrease its incidence and associated morbidity.

Radiographic assessment of pulmonary fluid clearance in healthy neonatal foals

We characterized the kinetics of postnatal equine lung using sequential thoracic radiography. The aim was to establish the earliest time when normal foals have clear, radiolucent lung fields, and to characterize the pattern of this clearance. Both right-to-left and left-to-right thoracic radiographs were acquired in lateral recumbency at peak inspiration within the first 30 min after birth and thereafter at 1, 2, 3, 4, 6, 8, 12, 24, 48, and 72 h. Radiographs were interpreted by three observers. The overall assessment of radiographic lung clearance was followed by the evaluation of individual lung quadrants to document changes in pulmonary radiographic patterns over time. We concluded that thoracic images in a healthy foal older than 4 h should be characterized by clear lungfields and that after this time distinctions between physiologic and pathologic conditions can be made. The ventral lung cleared first, presumably due to the greater flexibility of the thoracic wall in this anatomic region.

Elective cesarean section: its impact on neonatal respiratory outcome

Physiologic events in the last few weeks of pregnancy coupled with the onset of spontaneous labor are accompanied by changes in the hormonal milieu of the fetus and its mother, resulting in preparation of the fetus for neonatal transition. Rapid clearance of fetal lung fluid is a key part of these changes, and is mediated in large part by transepithelial sodium reabsorption through amiloride-sensitive sodium channels in the alveolar epithelial cells, with only a limited contribution from mechanical factors and Starling forces. This article discusses the respiratory morbidity associated with elective cesarean section, the physiologic mechanisms underlying fetal lung fluid absorption, and potential strategies for facilitating neonatal transition when infants are delivered by elective cesarean section before the onset of spontaneous labor.

Expulsion of liquid from the fetal lung during labour in sheep

Effective gas exchange after birth requires clearance of most of the liquid filling the lung during gestation. To date the focus has been on active Na(+) transport from lung lumen to interstitium, but Na(+) transport begins only close to delivery, making it an unlikely mechanism for clearing the bulk of fetal lung liquid. We hypothesised that fetal trunk muscle contractions, known to occur in labour, are involved in lung liquid clearance. We measured maternal uterine contractions, fetal tracheal flow directly and fetal electromyograms in thoracic and abdominal muscles. During labour in five fetal sheep, brief flow pulses were observed in the trachea, most of which expelled a small volume of lung liquid. Tracheal flow pulses were associated with fetal muscle contractions 89% of the time, which were associated on 91% of occasions with uterine contractions. Our results suggest that liquid contained in the fetal lung is cleared before and during labour as a result of fetal muscular effort, perhaps stimulated by uterine contractions.

Respiratory transition in infants delivered by cesarean section

One of the biggest challenges a newborn faces after birth is the task of making a smooth transition to air breathing. This task is complicated by the fact that fetal lungs are full of fluid which must be cleared rapidly to allow for gas exchange. Respiratory morbidity as a result of failure to clear fetal lung fluid is not uncommon, and can be particularly problematic in some infants delivered by elective cesarean delivery (ECS). Given the high rates of cesarean deliveries in the USA and worldwide, the public health and economic impact of morbidity in this subgroup is considerable. Whereas the occurrence of birth asphyxia, trauma, and meconium aspiration is reduced by elective Cesarean delivery, the risk of respiratory distress secondary to transient tachypnea of the newborn, surfactant deficiency, and pulmonary hypertension is increased. It is clear that physiologic events in the last few weeks of pregnancy coupled with the onset of spontaneous labor are accompanied by changes in the hormonal milieu of the fetus and its mother, resulting in preparation of the fetus for neonatal transition. Rapid clearance of fetal lung fluid is a key part of these changes, and is mediated in large part by transepithelial Na reabsorption through amiloride-sensitive Na channels in the alveolar epithelial cells, with only a limited contribution from mechanical factors and Starling forces. This chapter discusses the physiologic mechanisms underlying fetal lung fluid absorption and explores potential strategies for facilitating neonatal transition when infants are delivered by ECS before the onset of spontaneous labor.

Physiology of fetal lung fluid clearance and the effect of labor

Respiratory morbidity in near term (> or =34 and <37 weeks) infants delivered spontaneously or by elective cesarean section (ECS) has been well documented in the literature, and accounts for a significant number of admissions to intensive care units among these neonates. Given the high rates of near-term deliveries in the USA and worldwide, the public health and economic impact of morbidity in this subgroup is considerable. Causes of respiratory distress include transient tachypnea of the newborn (TTNB), surfactant deficiency, pneumonia, and pulmonary hypertension. There is considerable evidence that physiologic events in the last few weeks of pregnancy coupled with the onset of spontaneous labor are accompanied by changes in the hormonal milieu of the fetus and its mother, resulting in rapid maturation and preparation of the fetus for delivery and neonatal transition. A surge in endogenous steroids and catecholamines accompanies term gestation and spontaneous vaginal delivery, and is responsible for some of the maturational effects. Rapid clearance of fetal lung fluid clearance plays a key role in the transition to air breathing. The bulk of this fluid clearance is mediated by transepithelial sodium reabsorption through amiloride-sensitive sodium channels in the alveolar epithelial cells with only a limited contribution from mechanical factors and Starling forces. Disruption of this process can lead to retention of fluid in air spaces, setting the stage for alveolar hypoventilation. When infants are delivered near-term, especially by cesarean section (repeat or primary) before the onset of spontaneous labor, the fetus is often deprived of these hormonal changes, making the neonatal transition more difficult. This chapter discusses the physiologic mechanisms underlying fetal lung fluid absorption and explores potential strategies for facilitating neonatal transition.

Oxytocin-induced labor augments IL-1beta-stimulated lung fluid absorption in fetal guinea pig lungs

We tested the hypothesis that oxytocin-induced labor augmented IL-1beta-induced/-stimulated lung fluid absorption in preterm guinea pig fetuses. IL-1beta was administered subcutaneously daily to timed-pregnant guinea pigs for 3 days with and without simultaneous cortisol synthesis inhibition by metyrapone. At day 3, oxytocin was administered, and fetuses were delivered by abdominal hysterotomy at 61 and by oxytocin-induced birth at 68 days gestation. Delivered fetuses were instilled with isosmolar 5% albumin into the lungs, and lung fluid movement was measured over 1 h by mass balance. Lung fluid absorption was induced in 61-day and stimulated in 68-day gestation lungs by IL-1beta. Labor induction by oxytocin augmented IL-1beta-induced/-stimulated lung fluid absorption. Metyrapone pretreatment did not affect oxytocin-induced/-stimulated lung fluid absorption, while completely blocking IL-1beta-induced/-stimulated fluid absorption. Fetal lung fluid absorption, when present, was always propranolol and amiloride sensitive, suggesting that beta-adrenoceptor stimulation and amiloride-sensitive sodium channels were critical for fluid absorption. Epithelial sodium channel and Na-K-ATPase subunit expressions were both increased by IL-1beta, but not further by oxytocin. Our results indicate that IL-1beta release into the maternal blood circulation positively affects lung maturation due to the IL-1beta-induced release of cortisol and thus prepares the lungs for the epinephrine surge associated with labor.

Interferon-gamma regulates ClC-2 chloride channel in lung epithelial cells

Epithelial Cl(-) channels mediate Cl(-) and fluid secretion in the lung. In cystic fibrosis, aberrant Cl(-) secretion is one of the major causes for lung fluid imbalance. Regulation of Cl(-) channels is therefore an important issue in the lung. IFN-gamma regulates Na(+) and Cl(-) channels and fluid transport in the lung, but the mechanisms involved in these regulations are not clear. In expression studies, we found that IFN-gamma increased ClC-2 transcripts in Calu-3 cells. Studies of the promoter identified a minimal promoter which interacts with transcription factors Sp1 and Sp3. However, reporter gene assays showed that IFN-gamma did not activate the promoter. Instead, IFN-gamma significantly increased ClC-2 transcript stability. Using Ussing chamber experiments, we demonstrate that IFN-gamma activates a pH-regulated and Cd(2+)-sensitive short circuit current, characteristic properties of the ClC-2 Cl(-) channel. These data suggest that IFN-gamma activates ClC-2 channel activity in lung epithelial cells via mRNA stabilization.

IL-1β stimulates alveolar fluid absorption in fetal guinea pig lungs via the hypothalamus-pituitary-adrenal gland axis

We tested the hypothesis that interleukin (IL)-1β-induced cortisol synthesis stimulates alveolar fluid clearance in preterm fetuses. IL-1β was administered subcutaneously daily to timed-pregnant guinea pigs for 3 days with and without simultaneous cortisol synthesis inhibition by metyrapone. Fetuses were obtained by abdominal hysterotomy at 61 and 68 days gestation and instilled with isosmolar 5% albumin in the lungs, and alveolar fluid movement was measured over 1 h from the change in alveolar protein concentration. Alveolar fluid clearance was induced at 61 days gestation and stimulated at 68 days gestation by IL-1β, which both were attenuated by cortisol synthesis inhibition. Plasma ACTH and cortisol concentrations were increased by IL-1β at both gestational ages, and metyrapone reduced cortisol concentrations. IL-1β was mostly low or undetectable in both fetal and maternal blood. Prenatal alveolar fluid clearance, when present as well as IL-1β induced, was always propranolol and amiloride sensitive, suggesting that β-adrenoceptor stimulation and amiloride-sensitive Na+channels were critical for fluid absorption. IL-1β increased lung β-adrenoceptor density at gestation day 61, and cortisol synthesis inhibition attenuated the increased β-adrenoceptor density. Epithelial Na+channel and Na+-K+-ATPase subunit expressions were both increased by IL-1β and attenuated by cortisol synthesis inhibition. These results may explain why babies delivered preterm after intrauterine inflammation have a reduced risk of developing severe respiratory distress.

Inhibition of CLC-2 chloride channel expression interrupts expansion of fetal lung cysts

Normal lung morphogenesis is dependent on chloride-driven fluid transport. The molecular identity of essential fetal lung chloride channel(s) has not been elucidated. CLC-2 is a chloride channel, which is expressed on the apical surface of the developing respiratory epithelium. CLC-2-like pH-dependent chloride secretion exists in fetal airway cells. We used a 14-day fetal rat lung submersion culture model to examine the role of CLC-2 in lung development. In this model, the excised fetal lung continues to grow, secrete fluid, and become progressively cystic in morphology (26). We inhibited CLC-2 expression in these explants, using antisense oligonucleotides, and found that lung cyst morphology was disrupted. In addition, transepithelial voltage (V(t)) of lung explants transfected with antisense CLC-2 was inhibited with V(t) = -1.5 +/- 0.2 mV (means + SE) compared with -3.7 +/- 0.3 mV (means + SE) for mock-transfected controls and -3.3 +/- 0.3 mV (means + SE) for nonsense oligodeoxynucleotide-transfected controls. This suggests that CLC-2 is important for fetal lung fluid production and that it may play a role in normal lung morphogenesis.

Developmental Regulation of Lung Liquid Transport

The developing distal lung epithelium displays an evolving liquid transport phenotype, reflecting a changing and dynamic balance between Cl- ion secretion and Na+ ion absorption, which in turn reflects changing functional requirements. Thus in the fetus, Cl--driven liquid secretion predominates throughout gestation and generates a distending pressure to stretch the lung and stimulate growth. Increasing Na+ absorptive capacity develops toward term, anticipating the switch to an absorptive phenotype at birth and beyond. There is some empirical evidence of ligand-gated regulation of Cl- transport and of regulation via changes in the driving force for Cl- secretion. Epinephrine, O2, glucocorticoid, and thyroid hormones interact to stimulate Na+ absorption by increasing Na+ pump activity and apical Na+ conductance (GNa+) to bring about the switch from net secretion to net absorption as lung liquid is cleared from the lung at birth. Postnatally, the lung lumen contains a small Cl--based liquid secretion that generates a surface liquid layer, but the lung retains a large absorptive capacity to prevent alveolar flooding and clear edema fluid. This review explores the mechanisms underlying the functional development of the lung epithelium and draws upon evidence from classic integrative physiological studies combined with molecular physiology approaches.

Estimation of lung liquid production in fetal sheep with blue dye dextran and radioiodinated serum albumin

Lung liquid production and reabsorption rates and lung volumes were measured in 99 fetal sheep (119-148 days of gestation) by indicator-dilution methods with the simultaneous use of blue dye dextran (BDD) and radioiodinated serum albumin (RISA). There were no significant differences between rates of lung liquid production or reabsorption by the two methods (n = 71 pairs; paired t-test; Wilcoxon test; ANOVA); this was equally true for rates in milliliters per hour or milliliters per kilogram body weight per hour and was independent of age. Volumes measured by both methods showed a close linear relationship (r = 0.97; for slope P < 0.0001; n = 99), whether expressed as milliliters or milliliters per kilogram body weight. Either method could give the higher volume. Values differed by only approximately 4%, independent of age or parameter (ml or ml/kg body wt; volumes regressed to original volume, or as measured in untreated control hours). However, this small difference was significant by paired t-test or Wilcoxon test when all data were combined irrespective of age; it was not significant after allowance for gestational age (two-way ANOVA). Both indicators showed the same increase in lung volume toward birth and the same fall when related to body weight (slopes significant P = 0.0003-0.0004; r = 0.93). Two-way ANOVA showed that the declines were significant (P = 0.003). The data suggest that 1) there was no significant difference in production or reabsorption rates measured by BDD or RISA, 2) differences in volumes measured by the two indicators were only significant if gestational age was ignored and were too small to have physiological importance, and 3) although BDD and RISA each may have methodological weaknesses, for purposes of measuring lung liquid volumes both are sufficiently accurate and reproducible to obtain meaningful physiological results.

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.

Alveolar fluid clearance in late-gestational guinea pigs after labor induction: mechanisms and regulation

We tested the hypothesis that labor-induced epinephrine release would stimulate alveolar fluid clearance in preterm fetuses. Preterm fetuses were obtained by cesarean section from timed-pregnant guinea pigs at 61-69 days postconception. Fetal guinea pigs were euthanized and placed on continuous positive airway pressure oxygenation, and an isosmolar 5% albumin solution was instilled. Alveolar fluid clearance was measured over 1 h. The fetal lung began to absorb fluid at 64-66 days postconception, and at birth, alveolar fluid clearance quadrupled. Baseline alveolar fluid clearance when present was sensitive to propranolol inhibition and depended on beta-adrenergic stimulation. Measurements of plasma epinephrine in fetal animals confirmed high epinephrine levels in 66- to 69-day postconception fetuses. Prenatal alveolar fluid clearance when present was highly amiloride sensitive, suggesting that amiloride-sensitive Na+ channels were critical. Oxytocin-induced labor initiated an amiloride- and propranolol-sensitive net alveolar fluid clearance in 61-day-gestation animals. Moreover, oxytocin induced significant epinephrine release in all fetuses. These results have clinical implications for infants delivered by cesarean section before the onset of labor. Use of pharmacological agents to induce labor may reduce the occurrence and severity of perinatal respiratory distress.

Errors in estimating lung liquid volume in fetal lambs when using radiolabeled serum albumin and blue dextran

Fetal lung liquid volume is usually determined by using radio-iodinated serum albumin (RISA) or blue dextran (BD) as volume tracers. We tested the reliability of both tracers at 124 (G124) and 142 days of gestation (G142; term = G147) when the labels were employed simultaneously. We measured the proportion of label bound reversibly to the lung, or apparently lost from the lung compartment, by washing out the lung with saline and 5% albumin. At G124, volume estimates with the two labels were similar. At G142, the volume estimate with BD (36.3 +/- 8.7 ml/kg of body wt) was higher (P < 0. 05) than with RISA (22.3 +/- 3.5 ml/kg). This difference resulted from reversible binding of BD, because 5% albumin washout released 38.5 +/- 4.0% of the BD added at the start of the experiment but a lesser amount of RISA (9.8 +/- 0.7%; P < 0.05). At G142, when RISA was used alone, its reversible binding was 1.3 +/- 0.2%. Background absorbance increased during experiments, giving rise to an apparent increase in BD concentration. We conclude that RISA is an effective tracer for lung liquid volume determination in the fetal lamb, whereas our findings of substantial epithelial binding of BD and large changes in background absorbance demonstrate that, under the conditions of our experiments, BD is a poor tracer close to term.

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.

Nitric oxide decreases lung liquid production in fetal lambs

To examine the effect of nitric oxide on fetal lung liquid production, I measured lung liquid production in fetal sheep at 130 +/- 5 days gestation (range 122-137 days) before and after intrapulmonary instillation of nitric oxide. Thirty-one studies were done in which net lung luminal liquid production (JV) was measured by plotting the change in lung luminal liquid concentration of radiolabeled albumin, an impermeant tracer that was mixed into the lung liquid at the start of each study. To see whether changes in JV might be associated with changes in pulmonary hemodynamics, pulmonary and systemic pressures were measured and left pulmonary arterial flow was measured by an ultrasonic Doppler flow probe. Variables were measured during a 1- to 2-h control period and for 4 h after a small bolus of isotonic saline saturated with nitric oxide gas (10 or 100%) was instilled into the lung liquid. Control (saline) instillations (n = 6) caused no change in any variable over 6 h. Nitric oxide instillation significantly decreased JV and increased pulmonary blood flow; these effects were sustained for 1-2 h. There was also a significant but transient decrease in pulmonary arterial pressure. Thus intrapulmonary nitric oxide causes a significant decrease in lung liquid and is associated with a decrease in pulmonary vascular resistance. In a separate series of experiments either amiloride or benzamil, which blocks Na+ transport, was mixed into the lung liquid before nitric oxide instillation; still, there was a similar reduction in lung liquid production. Thus the reduction in lung liquid secretion caused by nitric oxide does not appear to depend on apical Na+ efflux.

Developmental changes in water permeability across the alveolar barrier in perinatal rabbit lung

Lung fluid is reabsorbed rapidly at birth to permit alveolar respiration. We reported previously that expression of aquaporins (AQP) 1, 4, and 5 in rat lung increased just after birth. The hypothesis was tested that the increased AQP expression is associated with increased osmotic water permeability (Pf) between the airspace and capillary compartments. Pf was measured in isolated perfused fetal and newborn rabbit lungs using a pleural surface fluorescence method (Carter, E.P., M.A. Matthay, J. Farinas, and A.S. Verkman. 1996. J. Gen. Physiol. 108:133-142). In response to perfusate osmolality increase from 300 to 600 mosM, initial rates of osmotic equilibration were 1.13+/-0.13 mosM/s at 0-12 h after birth, increasing to 1.52+/-0.19 mosM/s at 12-24 h, and 1.83+/-0.10 mosM/s at 24-84 h. Corresponding Pf values (in cm/s x 10(-2)), computed from d[mosM]/dt and alveolar surface-to-volume ratios, were 1.03+/-0.11 (0-12 h), 1.51+/-0.16 (12-24 h), and 1.88+/-0.09 (24-84 h). Pf was relatively low in prenatal (1.22-1.27, fetal days 29 and 31) and adolescent (1.25+/-0.08, 21-d) rabbit lungs. To test for involvement of molecular water channels, measurements were made of Arrhenius activation energy (Ea), mercurial inhibition, diffusional water permeability (Pd), and AQP expression. Temperature-dependence measurements showed a 25% decrease in Ea for Pf in lungs < 1 d vs. 4 d. Pf was decreased 30% by 0.5 mM HgCl2 in < 1-d lungs and 44% in 4-d lungs. Pd was 1.0 x 10(-)5 cm/s and did not change when Pf was increased by 75%. RNase protection assay showed increased transcript expression in the first 24 h after birth for rabbit isoforms of AQP1 and AQP4. These results provide the first functional data on water permeability in perinatal lung. The increased water permeability after birth may facilitate the maintenance of dry alveoli.

Oronasopharyngeal suction at birth: effects on arterial oxygen saturation

The effect of oronasopharyngeal suction (ONPS) on arterial oxygen saturation (SaO2) is described in a controlled study of 30 normal term newborn infants. In 15 of them, ONPS was performed immediately after birth. The SaO2 value was recorded through a pulse oximeter. The ONPS group had a significantly lower SaO2 between the first and the sixth minutes of life and took longer to reach 86% and 92% saturation. According to this study, ONPS should not be performed as a routine procedure in normal, term, vaginally born infants.

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 lung liquid volume on respiratory performance after caesarean delivery in the lamb

1. The volume of liquid in the lungs of the fetal lamb is reported to fall in the final days of gestation and during labour itself. We aimed to test the hypothesis that this fall in liquid volume adapts the lungs for air breathing and pulmonary gas exchange. 2. In twelve chronically catheterized fetal lambs we measured lung liquid volume at 140 days gestation (term is 147 days) and then delivered the fetuses by Caesarean section under maternal spinal anaesthesia. In five fetuses we removed approximately half the liquid contained in the lungs just before delivery (experimental group) while the remaining seven fetuses were delivered without change to their lung liquid (control group). 3. Lambs born with reduced lung liquid volume improved their arterial blood gas and acid-base status more quickly than lambs born without alteration to lung liquid. 4. Carotid arterial blood gas values in the first 60 min of postnatal life were significantly related to the volume of liquid present in the lungs at birth, with higher arterial partial pressure of oxygen (Pa,02) and arterial oxygen saturation (Sa,02) and lower arterial partial pressure of carbon dioxide (Pa,CO2) levels being associated with lower lung liquid volumes. 5. We conclude that postnatal gas exchange is enhanced by a reduction in the volume of liquid remaining in the lungs when breathing starts.

In utero lung growth of fetal sheep with diaphragmatic hernia and tracheal stenosis

The observation that tracheal ligation produces pulmonary hyperplasia even in animals with surgically induced diaphragmatic hernia (DH) has led to rapid application of the technique to human fetuses with DH. The aim of this study was to determine how rapidly fetal lung volume increases after creation of a high-grade tracheal stenosis in fetal sheep with surgically created DH. Twenty-three fetal sheep were prepared with a left thoracotomy at 90 days' gestational. Six had creation of a DH with tracheal stenosis (DHTS) over an 18-gauge cannula, which was then removed. Ten had DH alone, and seven control animals (CT) had a thoracotomy without DH. Thirty days later, vascular and tracheal loop catheters were inserted in all animals and tunneled out the ewes' flank. Between 125 and 140 days' gestation, lung volumes and lung liquid production were measured in awake, unanesthetized animals using a standard double-marker dilution technique. Average lung volumes (in milliliters) were 150.9 +/- 13.9 for CT, 29.3 +/- 4.4 for DH, and 414.5 +/- 88 for DHTS (p < 0.01). Mean lung liquid production varied from 6.00 +/- 2.23 mL/h in DH animals before 130 days to 16.69 +/- 8.29 mL/h in DHTS animals after 135 days' gestation. DH animals had lower lung liquid production (8.51 +/- 1.4 mL/h) than CT (12.4 +/- 0.8 mL/h) or DHTS animals (12.4 +/- 2.2 mL/h)(P < .01). The rate constant gamma (h-1) for lung liquid production was significantly higher in DH animals than in either CT or DHTS animals (P < .01). Tracheal stenosis in this model causes rapid lung growth before 130 days' gestation. The authors speculate that short periods of incomplete stenosis might reverse the pulmonary hypoplasia associated with DH. To achieve this goal, the timing and duration of treatment and the optimal degree of stenosis must be defined.

Swallowing of lung liquid and amniotic fluid by the ovine fetus under normoxic and hypoxic conditions

OBJECTIVE

The lungs of the mammalian fetus secrete large volumes of fluid daily. The purpose of this study was to estimate the fraction of the lung liquid that is swallowed as it exits the fetal trachea versus that which enters the amniotic fluid under normoxic and hypoxic conditions.

STUDY DESIGN

In chronically catheterized fetal sheep at 119 to 133 days' gestation the volume of fluid swallowed by the fetus was monitored five times per day for three consecutive 24-hour periods: control, hypoxia, and recovery. The Na+, K+, and Cl- concentrations of the swallowed fluid, lung liquid, and amniotic fluid were measured simultaneously. The fraction of the swallowed fluid that originated from the lungs or amniotic fluid was calculated from 24-hour average compositions and the assumption that the fetus swallowed only amniotic fluid and lung liquid.

RESULTS

During the control, hypoxia, and recovery periods the fetuses swallowed 264 +/- 43 (SE), 92 +/- 23, and 271 +/- 24 ml/kg of fetal weight per day, respectively. As determined from Cl- concentrations, this swallowed fluid was composed of 17.7% +/- 2.7%, 24.8% +/- 5.8%, and 11.9% +/- 3.4% lung liquid, respectively, with the remainder being amniotic fluid. Throughout the three 24-hour observation periods there was an inverse relationship between the net 24-hour swallowed volume and the fraction of the swallowed fluid that originated from the lungs. Calculations based on Na+ concentrations yielded essentially the same results with slightly more scatter, whereas calculations based on K+ concentrations were unreliable.

CONCLUSIONS

(1) Chloride concentrations provide the best of the three index values for a compositional analysis of fluids swallowed by the fetus. (2) Under normoxic conditions around 18% of swallowed fluid is derived from the fetal lungs. (3) On the basis of published fluid secretion rates for the fetal lung, an average of 50% of the liquid that exits the fetal trachea is swallowed and the rest mixes with the amniotic fluid.

Influence of labor and route of delivery on the frequency of respiratory morbidity in term neonates

OBJECTIVES

The purpose of this study is to compare the frequency of respiratory morbidity (RDS-Type I, RDS-Type II and PFC) in term neonates relative to three different delivery modes (cesarean delivery without labor, cesarean in labor, vaginal delivery).

METHODS

A case-control study was performed consisting of 692 maternal/neonate pairs at term gestation with either a documented mature lung profile at an obstetrical age > or = 37 weeks' gestation or a Dubowitz age > or = 38 weeks' gestation.

RESULTS

Overall, the frequency of neonatal respiratory morbidity was 5.1%. Neonatal respiratory morbidity was observed in 23 (12.4%) of 186 cases when cesarean delivery was performed prior to the onset of labor; in 10 (5.6%) of 177 cases when the cesarean delivery was performed after labor had ensued; and in 2 (0.6%) of 329 cases when the fetus was delivered vaginally (P < 0.001).

CONCLUSION

The frequency of respiratory morbidity in term neonates is influenced by labor and route of delivery, and it is highest when cesarean delivery is performed prior to the onset of labor.

Effects of hormones on potential difference and liquid balance across explants from proximal and distal fetal rat lung

1. Fetal rat tracheas and lung buds form liquid-filled cysts in submersion culture. The volume that accumulates in cysts is driven by active Cl- secretion. 2. We examined the effects, on these explants, of hormones that induce liquid absorption by fetal sheep lung in vivo. Explants were impaled with microelectrodes to measure potential difference (PD). Liquid was estimated from explant weight. 3. Water/dry weight ratio of lung buds and tracheas after 8 days in culture averaged 12 and 22. Exposure of triiodothyronine (T3) and hydrocortisone (HC) followed by a physiological dose of adrenaline on day 7 for 24 h or by a maximal dose of terbutaline on day 8 for 4 h induced a 35% decrease in water/dry weight ratio of distal buds but not tracheas. No hormone, or combination of two hormones, affected the ratio for tracheas or lung buds. 4. Basal PDs of tracheas (18.9 mV) and lung buds (3.7 mV) were increased about 50% by terbutaline. The terbutaline response was inhibited by bumetanide, but not by amiloride injected into the cysts. 5. T3 and HC pretreatment reduced basal PD by one-third. Subsequent exposure to terbutaline raised the PD of hormone-pretreated lung buds by more than 150%, a response that was blocked by amiloride, but was antagonized minimally by bumetanide. Responses of hormone-pretreated tracheas were not different from those of untreated tracheas. 6. We conclude that: (a) absorption of liquid from lung buds is driven by an amiloride-sensitive process (active Na+ transport?) and (b) only distal lung contributes to the adrenaline-sensitive reabsorptive process required for perinatal adaptation to air breathing.

Epithelial ion transport in the fetal and perinatal lung

The lung is a complex organ whose intrauterine development depends on many factors, one of which is a continuous secretion of large volumes of Cl(-)-enriched fluid by the pulmonary epithelium. At birth this fluid must be cleared, and it is now known that this process depends in large part on active Na+ transport by the pulmonary epithelium. Only recently has it been possible to culture some of the different lung epithelial cells so that it is possible to investigate the role of individual epithelial cell types, their individual cellular transport mechanisms, and how these are affected by developmental lung maturity.

Growth retardation and the development of the respiratory system in fetal sheep

In an experimental model of fetal growth retardation which involves the reduction of placental mass in ewes, we have investigated the effects of intrauterine deprivation on aspects of structural development of the trachea and lungs of fetal sheep (140 days gestation). We have also measured the volume of luminal liquid aspirated from the lungs and the phospholipid content of this liquid as an index of pulmonary surfactant production. The effects of growth retardation are evident in the trachea where the structural development of the mucosal and submucosal layers has been affected. Abnormal aspects of development include the frequent lack of a ciliated border on epithelial cells in the mucosal layer and the reduction in the extent of the folds usually characteristic of this layer in near term fetal sheep. Although the fetal lungs are smaller in growth retardation (P less than 0.01) they are appropriate for fetal weight and their structural development does not appear to have been retarded. In contrast, lung liquid volume is significantly reduced in relation to lung weight in growth retarded fetuses and the concentration of phospholipids in lung liquids is also reduced (P less than 0.01).

Ion transport in cultured fetal and adult rabbit tracheal epithelia

Transepithelial potential difference (Vte), short circuit current (Isc), and transepithelial resistance (Rte) were measured to assess Na+ and Cl- transport in cultured tracheal epithelial cells from fetal and adult rabbits. Cultured fetal and adult cells developed differentiated morphology as determined by electron microscopy. Base-line Vte, Isc, and Rte were equivalent in late gestation (28-30 days) and adult cultures. Amiloride (10(-4) M) (Na+ transport inhibitor) reduced Isc by 7.4 +/- 1.2 microA/cm2 (24%) in adult cultures but only by 1.8 +/- 0.7 microA/cm2 (7%) in fetal cultures (P less than 0.001). Furosemide (10(-4) M) (Cl- transport inhibitor) diminished Isc by 16.5 +/- 3.2 microA/cm2 (53%) in adult cultures and 18.8 +/- 2.8 microA/cm2 (74%) in fetal cultures (P less than 0.01). Epinephrine (10(-6) M) stimulated Isc by 24.2 +/- 5.8 microA/cm2 (72%) in adult cells but only by 7.0 +/- 1.1 microA/cm2 (32%) in fetal cells (P less than 0.05). After epinephrine exposure, furosemide-sensitive Isc increased to 41.5 +/- 7.7 microA/cm2 in adult cells but remained unchanged in fetal cells (P less than 0.001). Our results suggest that fetal cells absorb less Na+ and secrete more Cl- than adult cells and that the Cl- secretory response to epinephrine is greater in adult than in fetal cells.