Clearance of liquid from lungs of newborn rabbits

Monitoring of the Healthy Neonatal Transition Period with Serial Lung Ultrasound

Ultrasound has been used to observe lung aeration and fluid clearance during the neonatal transition period, but there is no consensus regarding the optimal timing of lung ultrasound. We aimed to monitor the trend of the serial lung ultrasound score (LUS) and extended LUS (eLUS) throughout the neonatal transition period (≤1, 2, 4, 8, 24, and 48 h after birth), assess any correlation to the clinical presentation (using the Silverman Andersen Respiratory Severity Score (RSS)), and determine the optimal time of the ultrasound. We found both LUS and eLUS decreased significantly after 2 h of life and had similar statistical differences among the serial time points. Although both scores had a positive, moderate correlation to the RSS overall (Pearson correlation 0.499 [p < 0.001] between LUS and RSS, 0.504 [p < 0.001] between eLUS and RSS), the correlation was poor within 1 h of life (Pearson correlation 0.15 [p = 0.389] between LUS and RSS, 0.099 [p = 0.573] between eLUS and RSS). For better clinical correlation, the first lung ultrasound for the neonate may be performed at 2 h of life. Further research is warranted to explore the clinical value and limitations of earlier (≤1 h of life) lung ultrasound examinations.

Technology in the delivery room supporting the neonatal healthcare provider's task

Very preterm infants are a unique and highly vulnerable group of patients that have a narrow physiological margin within which interventions are safe and effective. The increased understanding of the foetal to neonatal transition marks the intricacy of the rapid and major physiological changes that take place, making delivery room stabilisation and resuscitation an increasingly complex and sophisticated activity for caregivers to perform. While modern, automated technologies are progressively implemented in the neonatal intensive care unit (NICU) to enhance the caregivers in providing the right care for these patients, the technology in the delivery room still lags far behind. Diligent translation of well-known and promising technological solutions from the NICU to the delivery room will allow for better support of the caregivers in performing their tasks. In this review we will discuss the current technology used for stabilisation of preterm infants in the delivery room and how this could be optimised in order to further improve care and outcomes of preterm infants in the near future.

Higher CPAP levels improve functional residual capacity at birth in preterm rabbits

BACKGROUND

Preterm infants are commonly supported with 4-8 cm H₂O continuous positive airway pressures (CPAP), although higher CPAP levels may improve functional residual capacity (FRC).

METHODS

Preterm rabbits delivered at 29/32 days (~26-28 weeks human) gestation received 0, 5, 8, 12, 15 cm H₂O of CPAP or variable CPAP of 15 to 5 or 15 to 8 cm H₂O (decreasing ~2 cm H₂O/min) for up to 10 min after birth.

RESULTS

FRC was lower in the 0 (6.8 (1.0-11.2) mL/kg) and 5 (10.1 (1.1-16.8) mL/kg) compared to the 15 (18.8 (10.9-22.4) mL/kg) cm H₂O groups (p = 0.003). Fewer kittens achieved FRC > 15 mL/kg in the 0 (20%), compared to 8 (36%), 12 (60%) and 15 (73%) cm H₂O groups (p = 0.008). While breathing rates were not different (p = 0.096), apnoea tended to occur more often with CPAP < 8 cm H₂O (p = 0.185). CPAP belly and lung bulging rates were similar whereas pneumothoraces were rare. Lowering CPAP from 15 to 5, but not 15 to 8 cm H₂O, decreased FRC and breathing rates.

CONCLUSION

In all, 15 cm H₂O of CPAP improved lung aeration and reduced apnoea, but did not increase the risk of lung over-expansion, pneumothorax or CPAP belly immediately after birth. FRC and breathing rates were maintained when CPAP was decreased to 8 cm H₂O.

IMPACT

Although preterm infants are commonly supported with 4-8 cm H₂O CPAP at birth, preclinical studies have shown that higher PEEP levels improve lung aeration. In this study, CPAP levels of 15 cm H₂O improved lung aeration and reduced apnoea in preterm rabbit kittens immediately after birth. In all, 15 cm H₂O CPAP did not increase the risk of lung over-expansion (indicated by bulging between the ribs), pneumothorax, or CPAP belly. These results can be used when designing future studies on CPAP strategies for preterm infants in the delivery room.

Increased airway liquid volumes at birth impairs cardiorespiratory function in preterm and near-term lambs

Respiratory distress is relatively common in infants born at or near-term, particularly in infants delivered following elective cesarean section. The pathophysiology underlying respiratory distress at term has largely been explained by a failure to clear airway liquid, but recent physiological evidence has indicated that it results from elevated airway liquid at the onset of air-breathing. We have investigated the effect of elevated airway liquid volumes at birth on cardiorespiratory function in preterm and near-term lambs. Preterm (130 ± 0 days gestation, term ~147 days gestation; n=13) and near-term (139 ± 1 days gestation; n=13) lambs were instrumented (to measure blood pressure, blood flow and blood gas status) and at delivery airway liquid volumes were adjusted to mimic levels expected following vaginal delivery (Controls; ~7mL/kg) or elective caesarean section with no labour (elevated liquid; EL; 37mL/kg). Lambs were delivered, mechanically ventilated and monitored for blood gas status, oxygenation, ventilator requirements, blood flows (carotid artery and pulmonary artery) and blood pressure during the first few hours of life. Preterm and near-term EL lambs had poorer gas exchange and required greater ventilatory support to maintain adequate oxygenation. Pulmonary blood flow was reduced and carotid artery blood flow, mean arterial blood pressure and heart rate were reduced in EL near-term but not preterm lambs. These data provide further evidence that greater airway liquid volumes at birth adversely effects newborn cardiorespiratory function, with the effects being greater in near-term newborns.

Assessment of Postnatal Pulmonary Adaption in Bovine Neonates Using Electric Impedance Tomography (EIT)

Several aspects of postnatal pulmonary adaption in the bovine neonate remain unclear, particularly the dynamics and regional ventilation of the lungs. We used electric impedance tomography (EIT) to measure changes in ventilation in the first 3 weeks of life in 20 non-sedated neonatal calves born without difficulty in sternal recumbency. Arterial blood gas variables were determined in the first 24 h after birth. Immediately after birth, dorsal parts of the lungs had 4.53% ± 2.82% nondependent silent spaces (NSS), and ventral parts had 5.23% ± 2.66% dependent silent spaces (DSS). The latter increased in the first hour, presumably because of gravity-driven ventral movement of residual amniotic fluid. The remaining lung regions had good ventilation immediately after birth, and the percentage of lung regions with high ventilation increased significantly during the study period. The centre of ventilation was always dorsal to and on the right of the theoretical centre of ventilation. The right lung was responsible for a significantly larger proportion of ventilation (63.84% ± 12.74%, p < 0.00001) compared with the left lung. In the right lung, the centrodorsal lung area was the most ventilated, whereas, in the left lung, it was the centroventral area. Tidal impedance changes, serving as a surrogate for tidal volume, increased in the first 3 weeks of life (p < 0.00001). This study shows the dynamic changes in lung ventilation in the bovine neonate according to EIT measurements. The findings form a basis for the recognition of structural and functional lung disorders in neonatal calves.

Increased end-expiratory pressures improve lung function in near-term newborn rabbits with elevated airway liquid volume at birth

Approximately 53% of near-term newborns admitted to intensive care experience respiratory distress. These newborns are commonly delivered by cesarean section and have elevated airway liquid volumes at birth, which can cause respiratory morbidity. We investigated the effect of providing respiratory support with a positive end-expiratory pressure (PEEP) of 8 cmH₂O on lung function in newborn rabbit kittens with elevated airway liquid volumes at birth. Near-term rabbits (30 days; term = 32 days) with airway liquid volumes that corresponded to vaginal delivery (∼7 mL/kg, control, n = 11) or cesarean section [∼37 mL/kg; elevated liquid (EL), n = 11] were mechanically ventilated (tidal volume = 8 mL/kg). The PEEP was changed after lung aeration from 0 to 8 to 0 cmH₂O (control, n = 6; EL, n = 6), and in a separate group of kittens, PEEP was changed after lung aeration from 8 to 0 to 8 cmH₂O (control, n = 5; EL, n = 5). Lung function (ventilator parameters, compliance, lung gas volumes, and distribution of gas within the lung) was evaluated using plethysmography and synchrotron-based phase-contrast X-ray imaging. EL kittens initially receiving 0 cmH₂O PEEP had reduced functional residual capacities and lung compliance, requiring higher inflation pressures to aerate the lung compared with control kittens. Commencing ventilation with 8 cmH₂O PEEP mitigated the adverse effects of EL, increasing lung compliance, functional residual capacity, and the uniformity and distribution of lung aeration, but did not normalize aeration of the distal airways. Respiratory support with PEEP supports lung function in near-term newborn rabbits with elevated airway liquid volumes at birth who are at a greater risk of suffering respiratory distress.NEW & NOTEWORTHY Term babies born by cesarean section have elevated airway liquid volumes, which predisposes them to respiratory distress. Treatments targeting molecular mechanisms to clear lung liquid are ineffective for term newborn respiratory distress. We showed that respiratory support with an end-expiratory pressure supports lung function in near-term rabbits with elevated airway liquid volumes at birth. This study provides further physiological understanding of lung function in newborns with elevated airway liquid volumes at risk of respiratory distress.

Feasibility and Effect of Physiological-Based CPAP in Preterm Infants at Birth

Background: Preterm infants are commonly supported with 5-8 cmH₂O CPAP. However, animal studies demonstrate that high initial CPAP levels (12-15 cmH₂O) which are then reduced (termed physiological based (PB)-CPAP), improve lung aeration without adversely affecting cardiovascular function. We investigated the feasibility of PB-CPAP and the effect in preterm infants at birth. Methods: Preterm infants (24-30 weeks gestation) were randomized to PB-CPAP or 5-8 cmH₂O CPAP for the first 10 min after birth. PB-CPAP consisted of 15 cmH₂O CPAP that was decreased when infants were stabilized (heart rate ≥100 bpm, SpO₂ ≥85%, FiO₂ ≤ 0.4, spontaneous breathing) to 8 cmH₂O with steps of ~2/3 cmH₂O/min. Primary outcomes were feasibility and SpO₂ in the first 5 min after birth. Secondary outcomes included physiological and breathing parameters and short-term neonatal outcomes. Planned enrollment was 42 infants. Results: The trial was stopped after enrolling 31 infants due to a low inclusion rate and recent changes in the local resuscitation guideline that conflict with the study protocol. Measurements were available for analysis in 28 infants (PB-CPAP n = 8, 5-8 cmH₂O n = 20). Protocol deviations in the PB-CPAP group included one infant receiving 3 inflations with 15 cmH₂O PEEP and two infants in which CPAP levels were decreased faster than described in the study protocol. In the 5-8 cmH₂O CPAP group, three infants received 4, 10, and 12 cmH₂O CPAP. During evaluations, caregivers indicated that the current PB-CPAP protocol was difficult to execute. The SpO₂ in the first 5 min after birth was not different [61 (49-70) vs. 64 (47-74), p = 0.973]. However, infants receiving PB-CPAP achieved higher heart rates [121 (111-130) vs. 97 (82-119) bpm, p = 0.016] and duration of mask ventilation was shorter [0:42 (0:34-2:22) vs. 2:58 (1:36-6:03) min, p = 0.020]. Infants in the PB-CPAP group required 6:36 (5:49-11:03) min to stabilize, compared to 9:57 (6:58-15:06) min in the 5-8 cmH2O CPAP group (p = 0.256). There were no differences in short-term outcomes. Conclusion: Stabilization of preterm infants with PB-CPAP is feasible but tailoring CPAP appeared challenging. PB-CPAP did not lead to higher SpO₂ but increased heart rate and shortened the duration of mask ventilation, which may reflect faster lung aeration.

Respiratory mechanics during initial lung aeration at birth in the preterm lamb

Despite recent insights into the dynamic processes during lung aeration at birth, several aspects remain poorly understood. We aimed to characterize changes in lung mechanics during the first inflation at birth and their relationship to changes in lung volume. Intubated preterm lambs (gestational age, 124-127 days; n = 17) were studied at birth. Lung volume changes were measured by electrical impedance tomography (V_LEIT). Respiratory system resistance (R₅) and oscillatory compliance (C_x5) were monitored with the forced oscillation technique at 5 Hz. Lambs received 3-7 s of 8 cmH₂O of continuous distending pressure (CDP) before delivery of a sustained inflation (SI) of 40 cmH₂O. The SI was then applied until either C_x5 or the V_LEIT or the airway opening volume was stable. CDP was resumed for 3-7 s before commencement of mechanical ventilation. The exponential increases with time of C_x5 and V_LEIT from commencement of the SI were characterized by estimating their time constants (τC_x5 and τV_LEIT, respectively). During SI, a fast decrease in R₅ and an exponential increase in C_x5 and V_LEIT were observed. C_x5 and V_LEIT provided comparable information on the dynamics of lung aeration in all lambs, with τC_x5 and τV_LEIT being highly linearly correlated (r² = 0.87, P < 0.001). C_x5 and V_LEIT decreased immediately after SI. Despite the standardization of the animal model, changes in C_x5 and R₅ both during and after SI were highly variable. Lung aeration at birth is characterized by a fast reduction in resistance and a slower increase in oscillatory compliance, the latter being a direct reflection of the amount of lung aeration.

Issues in cardiopulmonary transition at birth

The transition from fetal to newborn life involves a complex series of physiological events that commences with lung aeration, which is thought to involve 3 mechanisms. Two mechanisms occur during labour, Na⁺ reabsorption and fetal postural changes, and one occurs after birth due to pressure gradients generated by inspiration. However, only one of these mechanisms, fetal postural changes, involves the loss of liquid from the respiratory system. Both other mechanisms involve liquid being reabsorbed from the airways into lung tissue. While this stimulates an increase in pulmonary blood flow (PBF), in large quantities this liquid can adversely affect postnatal respiratory function. The increase in PBF (i) facilitates the onset of pulmonary gas exchange and (ii) allows pulmonary venous return to take over the role of providing preload for the left ventricle, a role played by umbilical venous return during fetal life. Thus, aerating the lung and increasing PBF before umbilical cord clamping (known as physiological based cord clamping), can avoid the loss of preload and reduction in cardiac output that normally accompanies immediate cord clamping.

Effective ventilation: The most critical intervention for successful delivery room resuscitation

Lung aeration is the critical first step that triggers the transition from fetal to postnatal cardiopulmonary physiology after birth. When an infant is apneic or does not breathe sufficiently, intervention is needed to support this transition. Effective ventilation is therefore the cornerstone of neonatal resuscitation. In this article, we review the physiology of cardiopulmonary transition at birth, with particular attention to factors the caregiver should consider when providing ventilation. We then summarize the available clinical evidence for strategies to monitor and perform positive pressure ventilation in the delivery room setting.

Birth delivery method affects expression of immune genes in lung and jejunum tissue of neonatal beef calves

Background

Caesarean section is a routine veterinary obstetrical procedure employed to alleviate dystocia in cattle. However, CS, particularly before the onset of labour, is known to negatively affect neonatal respiration and metabolic adaptation in humans, though there is little published information for cattle. The aim of this study was to investigate the effect of elective caesarean section (ECS) or normal trans-vaginal (TV) delivery, on lung and jejunal gene expression profiles of neonatal calves.

Results

Paternal half-sib Angus calves (gestation length 278 + 1.8 d) were delivered either transvaginally (TV; n = 8) or by elective caesarean section (ECS; n = 9) and immediately euthanized. Lung and jejunum epithelial tissue was isolated and snap frozen. Total RNA was extracted using Trizol reagent and reverse transcribed to generate cDNA. For lung tissue, primers were designed to target genes involved in immunity, surfactant production, cellular detoxification, membrane transport and mucin production. Primers for jejunum tissue were chosen to target mucin production, immunoglobulin uptake, cortisol reaction and membrane trafficking. Quantitative real-time PCR reactions were performed and data were statistically analysed using mixed models ANOVA. In lung tissue the expression of five genes were affected (p < 0.05) by delivery method. Four of these genes were present at lower (LAP, CYP1A1, SCN11α and SCN11β) and one (MUC5AC) at higher abundance in ECS compared with TV calves. In jejunal tissue, expression of TNFα, Il-1β and 1 l-6 was higher in ECS compared with TV calves.

Conclusions

This novel study shows that ECS delivery affects the expression of key genes involved in the efficiency of the pulmonary liquid to air transition at birth, and may lead to an increased inflammatory response in jejunal tissue, which could compromise colostral immunoglobulin absorption. These findings are important to our understanding of the viability and management of neonatal calves born through ECS.

Lung hypoplasia in newborn rabbits with a diaphragmatic hernia affects pulmonary ventilation but not perfusion

BackgroundA congenital diaphragmatic hernia (DH) can result in severe lung hypoplasia that increases the risk of morbidity and mortality after birth; however, little is known about the cardiorespiratory transition at birth.MethodsUsing phase-contrast X-ray imaging and angiography, we examined the cardiorespiratory transition at birth in rabbit kittens with DHs. Surgery was performed on pregnant New Zealand white rabbits (n=18) at 25 days' gestation to induce a left-sided DH. Kittens were delivered at 30 days' gestation, intubated, and ventilated to achieve a tidal volume (V_t) of 8 ml/kg in control and 4 ml/kg in DH kittens while they were imaged.ResultsFunctional residual capacity (FRC) recruitment and V_t in the hypoplastic left lung were markedly reduced, resulting in a disproportionate distribution of FRC into the right lung. Following lung aeration, relative pulmonary blood flow (PBF) increased equally in both lungs, and the increase in pulmonary venous return was similar in both control and DH kittens.ConclusionThese findings indicate that nonuniform lung hypoplasia caused by DH alters the distribution of ventilation away from hypoplastic and into normally grown lung regions. During transition, the increase in PBF and pulmonary venous return, which is vital for maintaining cardiac output, is not affected by lung hypoplasia.

Lung liquid clearance in preterm lambs assessed by magnetic resonance imaging

BACKGROUND

Postnatal adaptation requires liquid clearance and lung aeration. However, their relative contribution to the expansion of functional residual capacity (FRC) has not been fully investigated. We studied evolution of lung liquid removal and lung aeration after birth in preterm lambs.

METHODS

Lung liquid content and lung volume were assessed at birth and every 30 min over 2 h using magnetic resonance imaging (MRI) in three groups of lambs delivered by cesarean: preterm, late preterm, and late preterm with antenatal steroids. Lung function and mechanics of the respiratory system were also measured.

RESULTS

Lung liquid content increased by approximately 30% in the preterm group (P < 0.05), whereas it did not change significantly in the late preterm lambs. Antenatal steroids induced a 50% drop in the lung liquid content (P < 0.05). Total lung volume increased in all groups (P < 0.05) but was higher in the late preterm + steroids group relative to other groups (P < 0.05). Compliance and resistances of the respiratory system were significantly correlated with lung liquid content (P < 0.05).

CONCLUSION

FRC expansion results mainly from an increase in lung volume rather than a decrease in lung liquid in preterm and late preterm lambs. Antenatal steroids promote FRC expansion through increases in lung volume and liquid clearance.

Vagal denervation inhibits the increase in pulmonary blood flow during partial lung aeration at birth

KEY POINTS

Lung aeration at birth significantly increases pulmonary blood flow, which is unrelated to increased oxygenation or other spatial relationships that match ventilation to perfusion. Using simultaneous X-ray imaging and angiography in near-term rabbits, we investigated the relative contributions of the vagus nerve and oxygenation to the increase in pulmonary blood flow at birth. Vagal denervation inhibited the global increase in pulmonary blood flow induced by partial lung aeration, although high inspired oxygen concentrations can partially mitigate this effect. The results of the present study indicate that a vagal reflex may mediate a rapid global increase in pulmonary blood flow in response to partial lung aeration.

ABSTRACT

Air entry into the lungs at birth triggers major cardiovascular changes, including a large increase in pulmonary blood flow (PBF) that is not spatially related to regional lung aeration. To investigate the possible underlying role of a vagally-mediated stimulus, we used simultaneous phase-contrast X-ray imaging and angiography in near-term (30 days of gestation) vagotomized (n = 15) or sham-operated (n = 15) rabbit kittens. Rabbits were imaged before ventilation, when one lung was ventilated (unilateral) with 100% nitrogen (N₂ ), air or 100% oxygen (O₂ ), and after all kittens were switched to unilateral ventilation in air and then ventilation of both lungs using air. Compared to control kittens, vagotomized kittens had little or no increase in PBF in both lungs following unilateral ventilation when ventilation occurred with 100% N₂ or with air. However, relative PBF did increase in vagotomized animals ventilated with 100% O₂ , indicating the independent stimulatory effects of local oxygen concentration and autonomic innervation on the changes in PBF at birth. These findings demonstrate that vagal denervation inhibits the previously observed increase in PBF with partial lung aeration, although high inspired oxygen concentrations can partially mitigate this effect.

Effect of betamethasone, surfactant, and positive end-expiratory pressures on lung aeration at birth in preterm rabbits

Antenatal glucocorticoids, exogenous surfactant, and positive end-expiratory pressure (PEEP) ventilation are commonly provided to preterm infants to enhance respiratory function after birth. It is unclear how these treatments interact to improve the transition to air-breathing at birth. We investigated the relative contribution of antenatal betamethasone, prophylactic surfactant, and PEEP (3 cmH2O) on functional residual capacity (FRC) and dynamic lung compliance (CDL) in preterm (28 day GA) rabbit kittens at birth. Kittens were delivered by cesarean section and mechanically ventilated. FRC was calculated from X-ray images, and CDL was measured using plethysmography. Without betamethasone, PEEP increased FRC recruitment and CDL Surfactant did not further increase FRC, but significantly increased CDL Betamethasone abolished the benefit of PEEP on FRC, but surfactant counteracted this effect of betamethasone. These findings indicate that low PEEP levels are insufficient to establish FRC at birth following betamethasone treatment. However, surfactant reversed the effect of betamethasone and when combined, these two treatments enhanced FRC recruitment irrespective of PEEP level.

Respiratory transition in the newborn: a three-phase process

We propose that the respiratory transition at birth passes through three distinct, but overlapping phases, which reflect different physiological states of the lung. Accordingly, respiratory support given to infants should be optimised to suit the underlying physiological state of the lung as it passes through each phase. During the first phase, the airways are liquid-filled and so no pulmonary gas exchange can occur. Respiratory support should, therefore, be focused on clearing the gas exchange regions of liquid. In the absence of gas exchange, little or no CO2will accumulate within the airways and, therefore, interrupting inflation pressures to allow the lung to deflate and exhale CO2is unnecessary. This is the primary rationale for administering a sustained inflation at birth. During the second phase, the gas exchange regions are mostly cleared of liquid, allowing pulmonary gas exchange to commence. However, the liquid cleared from the airways resides within the tissue during this phase, which increases perialveolar interstitial tissue pressures and the risk of liquid re-entry back into the airways. As a result, respiratory support should be optimised to minimise alveolar re-flooding during expiration, which can be achieved by applying an end-expiratory pressure. The third and final phase occurs when the liquid is eventually cleared from lung tissue. Although gas exchange may be restricted by lung immaturity, injury and inflammation during this phase, considerations of how fetal lung liquid can adversely affect lung function are no longer relevant.

Increase in pulmonary blood flow at birth: role of oxygen and lung aeration

Lung aeration stimulates the increase in pulmonary blood flow (PBF) at birth, but the spatial relationships between PBF and lung aeration and the role of increased oxygenation remain unclear. Using simultaneous phase-contrast X-ray imaging and angiography, we have investigated the separate roles of lung aeration and increased oxygenation in PBF changes at birth using near-term (30 days of gestation) rabbit kits (n = 18). Rabbits were imaged before ventilation, then the right lung was ventilated with 100% nitrogen (N2), air or 100% O2 (oxygen), before all kits were switched to ventilation in air, followed by ventilation of both lungs using air. Unilateral ventilation of the right lung with 100% N2 significantly increased heart rate (from 69.4 ± 4.9 to 93.0 ± 15.0 bpm), the diameters of both left and right pulmonary axial arteries, number of visible vessels in both left and right lungs, relative PBF index in both pulmonary arteries, and reduced bolus transit time for both left and right axial arteries (from 1.34 ± 0.39 and 1.81 ± 0.43 s to 0.52 ± 0.17 and 0.89 ± 0.21 s in the left and right axial arteries, respectively). Similar changes were observed with 100% oxygen, but increases in visible vessel number and vessel diameter of the axial arteries were greater in the ventilated right lung during unilateral ventilation. These findings confirm that PBF increase at birth is not spatially related to lung aeration and that the increase in PBF to unventilated regions is unrelated to oxygenation, although oxygen can potentiate this increase.

X-ray specks: low dose in vivo imaging of lung structure and function

Respiratory health is directly linked to the structural and mechanical properties of the airways of the lungs. For studying respiratory development and pathology, the ability to quantitatively measure airway dimensions and changes in their size during respiration is highly desirable. Real-time imaging of the terminal airways with sufficient contrast and resolution during respiration is currently not possible. Herein we reveal a simple method for measuring lung airway dimensions in small animals during respiration from a single propagation-based phase contrast x-ray image, thereby requiring minimal radiation. This modality renders the lungs visible as a speckled intensity pattern. In the near-field regime, the size of the speckles is directly correlated with that of the dominant length scale of the airways. We demonstrate that Fourier space quantification of the speckle texture can be used to statistically measure regional airway dimensions at the alveolar scale, with measurement precision finer than the spatial resolution of the imaging system. Using this technique we discovered striking differences in developmental maturity in the lungs of rabbit kittens at birth.

Antenatal hypoxia and pulmonary vascular function and remodeling

This review provides evidence that antenatal hypoxia, which represents a significant and worldwide problem, causes prenatal programming of the lung. A general overview of lung development is provided along with some background regarding transcriptional and signaling systems of the lung. The review illustrates that antenatal hypoxic stress can induce a continuum of responses depending on the species examined. Fetuses and newborns of certain species and specific human populations are well acclimated to antenatal hypoxia. However, antenatal hypoxia causes pulmonary vascular disease in fetuses and newborns of most mammalian species and humans. Disease can range from mild pulmonary hypertension, to severe vascular remodeling and dangerous elevations in pressure. The timing, length, and magnitude of the intrauterine hypoxic stress are important to disease development, however there is also a genetic-environmental relationship that is not yet completely understood. Determining the origins of pulmonary vascular remodeling and pulmonary hypertension and their associated effects is a challenging task, but is necessary in order to develop targeted therapies for pulmonary hypertension in the newborn due to antenatal hypoxia that can both treat the symptoms and curtail or reverse disease progression.

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.

The role of lung inflation and sodium transport in airway liquid clearance during lung aeration in newborn rabbits

BACKGROUND

Recent phase-contrast X-ray imaging studies suggest that inspiration primarily drives lung aeration and airway liquid clearance at birth, which questions the role of adrenaline-induced activation of epithelial sodium channels (ENaCs). We hypothesized that pressures generated by inspiration have a greater role in airway liquid clearance than do ENaCs after birth.

METHODS

Rabbit pups (30 d of gestation) were delivered and sedated, and 0.1 ml of saline (S) or amiloride (Am; an ENaC inhibitor) was instilled into the lungs before mechanical ventilation. Two other groups (30 d of gestation) were treated similarly but were also given adrenaline (S/Ad and Am/Ad) before mechanical ventilation.

RESULTS

Amiloride and adrenaline did not affect functional residual capacity (FRC) recruitment (P > 0.05). Amiloride increased the rate of FRC loss between inflations (Am: -5.2 ± 0.6 ml/kg/s), whereas adrenaline reduced the rate of FRC loss (S/Ad: -1.9 ± 0.3 ml/kg/s) as compared with saline-treated controls (S: -3.5 ± -0.6 ml/kg/s; P < 0.05).

CONCLUSION

These data indicate that inspiration is a major determinant of airway liquid clearance and FRC development during positive pressure ventilation. Although ENaC inhibition and adrenaline administration had no detectable effect on FRC development, ENaC may help to prevent liquid from re-entering the airways during expiration.

Contribution of stretch to the change of activation properties of muscle fibers in the diaphragm at the transition from fetal to neonatal life

The transition from fetal to postnatal life involves clearance of liquid from the lung and airways, and rapid formation of a functional residual capacity. Despite the importance of the diaphragm in this process, the impact of birth on the mechanical and functional activity of its muscle fibers is not known. This study determined the contractile characteristics of individual “skinned” diaphragm fibers from 70 days (0.47) gestation to after birth in sheep. Based on differential sensitivity to the divalent ions calcium (Ca²⁺) and strontium (Sr²⁺), all fibers in the fetal diaphragm were classified as “fast,” whereas fibers from the adult sheep diaphragm exhibited a “hybrid” phenotype where both “fast” and “slow” characteristics were present within each single fiber. Transition to the hybrid phenotype occurred at birth, was evident after only 40 min of spontaneous breathing, and could be induced by simple mechanical stretch of diaphragm fibers from near-term fetuses (∼147 days gestation). Both physical stretch of isolated fibers, and mechanical ventilation of the fetal diaphragm in situ, significantly increased sensitivity to Ca²⁺ and Sr²⁺, maximum force generating capacity, and decreased passive tension in near-term and preterm fetuses; however, only fibers from near-term fetuses showed a complete transition to a “hybrid” activation profile. These findings suggest that stretch associated with the transition from a liquid to air-filled lung at birth induces physical changes of proteins determining the activation and elastic properties of the diaphragm. These changes may allow the diaphragm to meet the increased mechanical demands of breathing immediately after birth.

Lymphatic ontogeny and effect of hypoplasia in developing lung

The pulmonary lymphatic vasculature plays a vital role in maintaining fluid homeostasis required for efficient gas exchange at capillary alveolar barriers and contributes to lung fluid clearance at birth. To further understanding of pulmonary lymphatic function at birth, lineage-tracing analysis of mouse lung was used. Lineage analysis confirmed that lymphatic endothelial cells (LEC) bud from extrapulmonary lymphatics and demonstrated that LEC migrate into developing lung along precise pathways. LEC cluster first in the primary bronchovascular region then along the secondary broncho-arterial regions and along veins. Small lymphatic vessels in distal lung develop from LEC that have migrated into lung mesenchyme from the extrapulmonary lymphatics. Finally, proximal and distal lymphatics remodel to form vessels with lumens in stereotypical locations. Loss of function analysis with lung-specific expression of a secreted form of the extracellular domain of vascular endothelial growth factor receptor-3 (dnR3) caused significant embryonic pulmonary lymphatic hypoplasia with fourfold reduction in distal LEC. Lung-specific expression of dnR3 did not affect blood vascular development, overall lung organogenesis or lymphatic development in other organs. Neonatal mice with pulmonary lymphatic hypoplasia developed respiratory distress with significantly increased mortality. During the transition to air breathing, lymphatic hypoplasia adversely affected fetal lung fluid clearance as determined by wet/dry weight analysis and morphometric analysis of bronchovascular cuffing and mesenchymal thickening. Surfactant synthesis was unaffected. Together, these data demonstrate that lung lymphatics develop autonomously and that pulmonary lymphatic hypoplasia is detrimental to survival of the neonate due to impaired lung fluid clearance.

Imaging lung aeration and lung liquid clearance at birth using phase contrast X-ray imaging

The transition to extra-uterine life at birth is critically dependent on airway liquid clearance to allow the entry of air and the onset of gaseous ventilation. We have used phase contrast X-ray imaging to identify factors that regulate lung aeration at birth in spontaneously breathing term and mechanically ventilated preterm rabbit pups. Phase contrast X-ray imaging exploits the difference in refractive index between air and water to enhance image contrast, enabling the smallest air-filled structures of the lung (alveoli; < 100 microm) to be resolved. Using this technique, the lungs become visible as they aerate, allowing the air-liquid interface to be observed as it moves distally during lung aeration. Spontaneously breathing term rabbit pups rapidly aerate their lungs, with most fully recruiting their functional residual capacity (FRC) within the first few breaths. The increase in FRC occurs mainly during individual breaths, demonstrating that airway liquid clearance and lung aeration is closely associated with inspiration. We suggest that transpulmonary pressures generated by inspiration provide a hydrostatic pressure gradient for the movement of water out of the airways and into the surrounding lung tissue after birth. In mechanically ventilated preterm pups, lung aeration is closely associated with lung inflation and a positive end-expiratory pressure is required to generate and maintain FRC after birth. In summary, phase contrast X-ray imaging can image the air-filled lung with high temporal and spatial resolution and is ideal for identifying factors that regulate lung aeration at birth in both spontaneously breathing term and mechanically ventilated preterm neonates.

Inspiration regulates the rate and temporal pattern of lung liquid clearance and lung aeration at birth

At birth, the initiation of pulmonary gas exchange is dependent on air entry into the lungs, and recent evidence indicates that pressures generated by inspiration may be involved. We have used simultaneous plethysmography and phase-contrast X-ray imaging to investigate the contribution of inspiration and expiratory braking maneuvers (EBMs) to lung aeration and the formation of a functional residual capacity (FRC) after birth. Near-term rabbit pups (n = 26) were delivered by cesarean section, placed in a water plethysmograph, and imaged during the initiation of spontaneous breathing. Breath-by-breath changes in lung gas volumes were measured using plethysmography and visualized using phase-contrast X-ray imaging. Pups rapidly (1-5 breaths) generate a FRC (16.2 +/- 1.2 ml/kg) by inhaling a greater volume than they expire (by 2.9 +/- 0.4 ml.kg(-1).breath(-1) over the first 5 breaths). As a result, 94.8 +/- 1.4% of lung aeration occurred during inspiration over multiple breaths. The incidence of EBMs was rare early during lung aeration, with most (>80%) occurring after >80% of max FRC was achieved. Although EBMs were associated with an overall increase in FRC, 34.8 +/- 5.3% of EBMs were associated with a decrease in FRC. We conclude that lung aeration is predominantly achieved by inspiratory efforts and that EBMs help to maintain FRC following its formation.

Developmental regulation of lumenal lung fluid and electrolyte transport

In the fetus, there is a net secretion of liquid (LL) by the lung as a result of active transport of chloride ions. The rate of secretion and the resulting volume of LL are vital for normal lung growth but how volume is sensed and how secretion may be regulated are still unknown. Towards term under the influence of thyroid and adrenocorticoid hormones, the epithelial sodium channel (ENaC) is increasingly expressed in the pulmonary epithelium. Adrenaline released by the fetus during labour activates ENaC and produces rapid absorption of liquid in preparation for air breathing; absence of ENaC is incompatible with survival. There may be other mechanisms involved in aiding liquid clearance including changes in epithelial permeability, an effect of oxygen on both ENaC and Na/K ATPase and perhaps the influence of additional hormones on ENaC activity. Some time after birth there are further developmental changes with the appearance of other cation channels (CNG1 and perhaps NSCC) which contribute to the liquid absorptive side of the balance existing across the epithelium between secretion and absorption to produce essentially almost no net liquid movement in the postnatal lung. The evidence for these processes is discussed and areas of uncertainty indicated.

Thyroid hormone rapidly stimulates alveolar Na,K-ATPase by activation of phosphatidylinositol 3-kinase

PURPOSE OF REVIEW

Nongenomic actions of 3,3',5-triiodo-L-thyronine (T3) occur quite rapidly usually via activation of signaling cascades. In this review, we focus on recent advances made in the understanding of activation of the phosphatidylinositol 3-kinase pathway by T3 in alveolar epithelial cells, resulting in upregulation of Na,K-ATPase hydrolytic activity and potential physiological significance of this finding.

RECENT FINDINGS

T3 stimulates the Src family of kinases. Activation of Src-kinase and phosphatidylinositol 3-kinase/protein kinase B is required for the T3-induced stimulation of alveolar epithelial Na,K-ATPase activity in rat alveolar epithelial cells. The stimulation does not require transcription. This T3-sensitive Na,K-ATPase stimulation in rat alveolar epithelial cells is switched on late in gestation. In skin fibroblasts phosphatidylinositol 3-kinase is also involved in the nongenomic T3 stimulation of ZAK1-4alpha protein expression, an endogenous calcineurin inhibitor.

SUMMARY

T3 plays an important role in cell survival and differentiation. Nongenomic regulation of phosphatidylinositol 3-kinase and downstream molecules by T3 is being recognized in different tissues. Upregulation of alveolar Na,K-ATPase is one such molecule, which plays an important role in removal of edema fluid from the alveolar space. These effects are rapid and do not require direct nuclear gene transcription.

Imaging lung aeration and lung liquid clearance at birth

Aeration of the lung and the transition to air-breathing at birth is fundamental to mammalian life and initiates major changes in cardiopulmonary physiology. However, the dynamics of this process and the factors involved are largely unknown, because it has not been possible to observe or measure lung aeration on a breath-by-breath basis. We have used the high contrast and spatial resolution of phase contrast X-ray imaging to study lung aeration at birth in spontaneously breathing neonatal rabbits. As the liquid-filled fetal lungs provide little absorption or phase contrast, they are not visible and only become visible as they aerate, allowing a detailed examination of this process. Pups were imaged live from birth to determine the timing and spatial pattern of lung aeration, and relative levels of lung aeration were measured from the images using a power spectral analysis. We report the first detailed observations and measurements of lung aeration, demonstrating its dependence on inspiratory activity and body position; dependent regions aerated at much slower rates. The air/liquid interface moved toward the distal airways only during inspiration, with little proximal movement during expiration, indicating that trans-pulmonary pressures play an important role in airway liquid clearance at birth. Using these imaging techniques, the dynamics of lung aeration and the critical role it plays in regulating the physiological changes at birth can be fully explored.

Fluid and electrolyte management in the very low birth weight neonate

Fluid and electrolyte management in the very low birth weight infant is critical to survival. The amount of fluid present in the plasma, interstitial fluid, and cellular fluid changes throughout the fetal and neonatal period, presenting a challenging situation. One of the many factors influencing fluid requirements is the insensible water loss by mechanisms such as evaporation. Low birth weight infants are especially susceptible to this due to their large body surface area and immature skin, often resulting in hypernatremia and the complications associated with it. However, some infants may experience hyperkalemia, hyperglycemia, and/or hyponatremia, resulting in various other complications. Careful monitoring is essential in deciding how to manage these infants. This article aims to discuss the management of fluid and electrolytes in very low birth weight infants and address ways to decrease the morbidity and mortality associated with the imbalances in fluid and electrolytes seen in this population.

Current concepts on the pulmonary surfactant in infants

Surfactant has been a main topic of neonatology in the last 20 years. Many studies have been conducted since the discovery of its role in the pathogenesis of respiratory distress syndrome and the knowledge on its composition and metabolism has become complex. In this article we review the current concepts of its metabolism, ways of acting, properties of its proteins and activities other than the ability of reducing surface tension within the lung as a basis to understand the development of disease in case of its deficiency.

Perinatal changes in pulmonary vascular endothelial function

The pulmonary endothelium plays a crucial role in lung development and function during the perinatal period. Its 2 most important functions at this time are to help reduce pulmonary vascular resistance (PVR) in order to permit the entire cardiac output to pass through the lungs for the first time and to facilitate the clearance of lung fluid. In response to changes in environmental factors such as oxygen tension, blood flow, circulating cytokines, and growth factors, the endothelium synthesizes and/or extracts many vasoactive mediators such as endothelin-1 (ET-1), norepinephrine, angiotensin 1, thromboxane, prostacyclin (PGI(2)), and the endothelial-derived relaxing factor nitric oxide (NO). The endothelium acts as a transducer conveying information about environmental changes to the underlying smooth muscle cells (SMCs), which helps regulate their reactivity and pulmonary vascular tone. The endothelial layer also acts as a barrier, regulating the exchange of fluids and nutrients between blood components and the surrounding tissues. The purpose of this review is to demonstrate the importance of structural and functional changes in the pulmonary endothelium during the perinatal period and explain their role in the regulation of the pulmonary circulation in health and disease. We also highlight signalling pathways of some of the most important endothelium-derived factors and indicate potential targets for pharmacological intervention.

Improvement of lung compliance during postnatal adaptation correlates with airway sodium transport

RATIONALE

Fetal lung liquid secretion is coupled with chloride transport into the lung lumen. The postnatal clearance of lung liquid is dependent on osmotic force generated by active sodium absorption.

OBJECTIVE

To study the interaction between airway epithelial sodium transport and postnatal lung function.

METHODS

We determined lung compliance and nasal transepithelial potential difference as a measure of airway ion transport and epithelial sodium channel gene expression in 41 healthy newborn infants during the first 50 h after birth.

MEASUREMENTS AND MAIN RESULTS

Lung compliance improved significantly during the study period, whereas nasal potential difference remained constant. There was a significant decrease in the expressions of beta and gamma subunits of the epithelial sodium channel. A positive correlation existed between amiloride-sensitive nasal potential difference measured at 1-4 h of age and lung compliance at 21-27 h of age. We found no correlation between the molecular data and functional measurements.

CONCLUSIONS

An important part of pulmonary adaptation takes place during the first hour after birth. The improvement of lung compliance continues over the first postnatal days and coincides with down-regulation of epithelial sodium channel beta and gamma subunit expression.

Incomplete protection by prophylactic surfactant against the adverse effects of large lung inflations at birth in immature lambs

OBJECTIVE

To investigate whether preceding surfactant instillation prevents the harmful effect of large lung inflations at birth in immature lambs, and, if not, to find out for how long the immature lung remains sensitive to large inflations.

DESIGN

In an exploratory study, 12 preterm lambs given surfactant at birth were randomized to receive five large lung inflations at four different times: at birth just before or immediately after surfactant treatment; at 10 min; or at 60 min of age. In a confirmatory study, 10 pairs of preterm lamb twins were all given surfactant before the first breath. One lamb in each pair was randomised to receive large lung inflations immediately after surfactant while the other twin underwent similar inflations at 10-15 min of age.

SETTING

Animal laboratory.

EXPERIMENTAL ANIMALS

Anaesthetized lambs delivered by cesarean section at a gestational age of 127 days.

INTERVENTIONS

Surfactant supplementation at birth. Five sustained lung inflations of 16 ml/kg at different times in relation to surfactant instillation. Pressure-limited mechanical ventilation for 4 h.

MEASUREMENTS AND RESULTS

The response to surfactant was assessed by ventilator settings, lung mechanics and lung histology. Preceding surfactant supplementation did not prevent the adverse effect of large lung inflations at birth on ventilatory efficiency and lung mechanics, but seemed to protect against severe lung injury. No adverse effect was seen from large lung inflations given at 10 min of age or later.

CONCLUSION

Prophylactic surfactant supplementation does not fully protect against the harmful effect of large lung inflations during a short sensitive period immediately after birth.

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.

Postnatal glucocorticoids induce alpha-ENaC formation and regulate glucocorticoid receptors in the preterm rabbit lung

At birth, lung fluid clearance is coupled to Na+ transport through epithelial Na+ channels (ENaC) in the distal lung epithelium. We evaluated the effect of postnatal glucocorticoids (GC) on lung alpha-ENaC expression in preterm 29-day gestational age (GA) fetal rabbits. Postnatal treatment of 29-day GA fetuses with 0.5 mg/kg of dexamethasone (Dex) iv resulted in a 2- and 22-fold increase in lung alpha-ENaC mRNA expression compared with saline-treated fetuses after 8 and 16 h, respectively. Lung alpha-ENaC protein levels in Dex-treated fetuses were also elevated compared with saline-treated counterparts. The extravascular lung water (EVLW)/dry lung tissue weight ratios of 29-day GA fetuses treated with either saline or Dex decreased over 24 h compared with that observed at birth; however, at 24 h, the EVLW/dry lung tissue weight ratios of saline- and Dex-treated fetuses were similar. Dex-induced alpha-ENaC mRNA and protein levels were attenuated by glucocorticoid receptor (GCR) antagonist RU-486 in fetal distal lung epithelial cells isolated from 29-day GA fetuses, indicating that GC-dependent augmentation of lung alpha-ENaC requires the presence of functional GCR. Lung GCR mRNA expression and protein levels were elevated in 29-day GA fetuses compared with fetuses at earlier GA. Exposure of 29-day GA fetuses to Dex for 16 h caused a 2.1-fold increase in lung GCR mRNA expression, but GCR protein levels were decreased in Dex-treated fetuses after 24 h. We conclude that postnatal treatment of preterm 29-day GA fetal rabbits with GC results in an elevation of lung alpha-ENaC accompanied by an autoregulation of pulmonary GCR.

Interstitial albumin concentration measured during growth of perivascular cuffs in liquid-filled rabbit lung

The growth rate and albumin concentration of interstitial fluid cuffs were measured in isolated rabbit lungs inflated with albumin solution (3 g/dl) to constant airway (Paw) and vascular pressures for up to 10 h. Cuff size was measured from images of frozen lung sections, and cuff albumin concentration (Cc) was measured from the fluorescence of Evans blue labeled albumin that entered the cuffs from the alveolar space. At 5-cmH2O Paw, cuff size peaked at 1 h and then decreased by 75% in 2 h. The decreased cuff size was consistent with an osmotic absorption into the albumin solution that filled the vascular and alveolar spaces. At 15-cmH2O Paw, cuff size peaked at 0.25 h and then remained constant. Cc rose continuously at both pressures, but was greater at the higher pressure. The increasing Cc with a constant cuff size was modeled as diffusion through epithelial pores. Initial Cc-to-airway albumin concentration ratio was 0.1 at 5-cmH2O Paw and increased to 0.3 at 15 cmH2O, a behavior that indicated an increased permeability with lung inflation. Estimated epithelial reflection coefficient was 0.9 and 0.7, and equivalent epithelial pore radii were 4.5 and 6.1 nm at 5- and 15-cmH2O Paw, respectively. The initial cuff growth occurred against an albumin colloid osmotic pressure gradient because a high interstitial resistance reduced the overall epithelial-interstitial reflection coefficient to the low value of the interstitium.

Physiology of the alveolar surface network

The alveolar surface network (ASN) is the totally fluid intraacinar conformation of the alveolar surface liquid (ASL) continuum circulating, both in series and in parallel, through ultrathin (to <7 nm) molecular conduits formed by appositions of unit bubbles of alveolar gas. The ASN is the analogue of foam in vitro. Appositions of unit bubble films, namely foam films, include (a) bubble-to-bubble at the alveolar entrance, across alveolar ducts, and at pores of Kohn ('classical foam films'); (b) bubble-to-epithelial cell surface ('cell-surface foam film'); and (c) bubble-to-open surface liquid layer of the terminal conducting airways ('surface foam film'). These appositions of monolayer bubble films create (a) 'macrochannels' ('pressure points', 'reservoirs') that modulate ASL transfers, volume and flow throughout the acinus and between acinar surface and both the interstitium and the terminal conducting airways surfaces, and (b) 'microchannels' along the broadest surfaces of the appositions. 'Microchannels', which are expectedly bilayer, serve several functions, including (a) virtually frictionless orientation of unit bubbles and ASL to fill the acinar air space; (b) virtually unrestricted diffusion of respiratory gases; (c) architectural support ('infrastructure') against the 'mass' and 'recoil' force of the interstitium; and (d) provision of 'gate' and 'bridge' dynamics that further modulate and direct ASL circulation. The physiological and anatomical boundary between acinar ASN and the bubble-free open liquid surfaces of the conducting airways is marked by the surface foam film. The ASN operates as outlined above in all regions of the lung, at all lung volumes, beginning at the onset of air-breathing at birth and continuing throughout life. Reports of its discovery (Pulmonary Physiology of the Fetus, Newborn and Child (1975) 116; Pediatr. Res. 12 (1978) 1070) and subsequent confirmatory research including the adult lung are summarized in this review by progressive development of each function. These functions, which are normal for a relatively dry foam such as the ASN (where gas:liquid volume ratio is >99:1) cannot be duplicated by the conventional theories and models of an open 'alveolar lining layer'. The unfortunate research technologies upon which these theories and models have been formulated have, indeed, obfuscated recognition of the ASN in vivo. They are also presented and critiqued in this review.

Hormonal modulation of Na(+) transport in rat fetal distal lung epithelial cells

Isolated rat fetal distal lung epithelial (FDLE) cells were cultured (approximately 48 h) on permeable supports in medium devoid of hormones and growth factors whilst P(O2) was maintained at the level found in either the fetal (23 mmHg) or the postnatal (100 mmHg) alveolar regions. The cells became incorporated into epithelial layers that generated a basal short-circuit current (I(SC)) attributable to spontaneous Na(+) absorption. Cells at neonatal P(O2) generated larger currents than did cells at fetal P(O2), indicating that this Na(+) transport process is oxygen sensitive. Irrespective of P(O2), isoprenaline failed to elicit a discernible change in I(SC), demonstrating that beta-adrenoceptor agonists do not stimulate Na(+) transport under these conditions. However, isoprenaline did elicit cAMP accumulation in these cells, indicating that functionally coupled beta-adrenoceptors are present. Further experiments showed that isoprenaline did increase I(SC) in cells treated (24 h) with a combination of tri-iodothyronine (T(3), 10 nM) and dexamethasone (200 nM). Studies of basolaterally permeabilised cells showed that these hormones are essential for the isoprenaline-evoked increase in the apical membrane's Na(+) conductance (G(Na)), whereas isoprenaline-evoked changes in apical Cl(-) conductance (G(Cl)) can occur in both control and hormone-treated cells. Irrespective of their hormonal status, FDLE cells thus express beta-adrenoceptors that are functionally coupled to adenylate cyclase, and allow beta-adrenoceptor agonists to modulate the apical membrane's anion conductance. However, T(3) and dexamethasone are needed if these receptors are to exert control over G(Na). These hormones may thus play an important role in the functional maturation of the lung by allowing beta-adrenoceptor-mediated control over epithelial Na(+) channels in the apical plasma membrane.

Hypoxemia and low Crs in vagally denervated lambs result from reduced lung volume and not pulmonary edema

Vagal denervation performed in the intrathoracic region in newborn lambs leads to hypoxemia and decreased respiratory system compliance (Crs), which could result from atelectasis and/or pulmonary edema. The objective of the present study was to quantify the relative roles of alveolar derecruitment and pulmonary edema as underlying cause(s) of respiratory failure. Vagal denervation was performed in the intrathoracic region and below the recurrent laryngeal nerves in six newborn lambs within 24 h of birth, whereas six were sham operated. Pre- and postinflation Crs was measured to investigate the presence of alveolar derecruitment. Pulmonary edema was assessed with lung wet-dry-to-wet and lung tissue wet-to-dry ratios, total protein, and FITC-BSA recovery in lung tissue and bronchoalveolar lavage. Compared with that in the sham-operated animals, Crs was significantly lower in vagally denervated animals. However, postinflation, pulmonary system compliance obtained by quasi-static lung inflation and deflation to 30 cmH2O showed no significant difference between the sham-operated and denervated lambs. The lung wet-dry-to-wet and lung tissue wet-to-dry ratios, total protein, and FITC-BSA recovery in lung tissue and bronchoalveolar lavage were similar in denervated and sham-operated groups. We provide evidence that reduced lung volume and not pulmonary edema is associated with intrathoracic vagal denervation and is the likely underlying mechanism for hypoxemia and low Crs.

Ca(2+)-dependent stimulation of alveolar fluid clearance in near-term fetal guinea pigs

We investigated the importance of changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) for amiloride-sensitive alveolar fluid clearance (AFC) in late-gestational guinea pigs. Fetal guinea pigs of 61, 68, and 69 days (term) gestation were investigated under normal conditions and after oxytocin-induced preterm labor. AFC or alveolar fluid secretion was measured using an impermeable tracer technique. At 61 days gestation there was net secretion of fluid into the lungs, and at birth the lungs cleared 49 +/- 7% of the instilled fluid volume over 1 h. Induction of preterm labor with oxytocin induced AFC at 61 days gestation. When present, AFC was inhibited or reversed to net fluid secretion by amiloride (10(-3) M). Inhibition of membrane Ca(2+) channels by verapamil (10(-4) M) or depletion of intracellular Ca(2+) by thapsigargin (10(-5) M) reduced AFC when net AFC was evident. Amiloride lacked an inhibitory effect on AFC when instilled with verapamil or thapsigargin. The results indicate that AFC via amiloride-sensitive pathways develops during late gestation, and that inducing preterm labor precociously may activate such pathways. Our results suggest that Ca(2+) may act as a second messenger in mediating catecholamine-stimulated AFC.

Decreased pulmonary vascular permeability in aquaporin-1-null humans

The molecular determinants of water permeability in the human lung are incompletely defined. Aquaporins (AQP) are water-specific membrane channel proteins. AQP1 is present in endothelial cells in the lung, including those in the vascular plexus around the airways. Rare individuals have been identified who are deficient in AQP1. High-resolution computed tomography scans of the lung were used to evaluate the response to i.v. fluid challenge in two unrelated AQP1-null individuals and five normal controls. The airways and pulmonary vessels were measured at baseline and after i.v. administration of 3 liters of saline. Increases in airway wall thickness after fluid administration reflect peribronchiolar edema formation. Both control and AQP1 null subjects had approximately a 20% increase in pulmonary vessel area in response to saline infusion, suggesting similar degrees of volume loading. Control subjects had a 44% increase in the thickness of the airway wall, consistent with peribronchiolar edema formation. In marked contrast, airway wall thickness did not change in AQP1-null subjects in response to saline infusion. These studies indicate that AQP1 is a determinant of vascular permeability in the lung, and demonstrate a role for aquaporins in human pulmonary physiology.

Lung Morphometry after Repetitive Antenatal Glucocorticoid Treatment in Preterm Sheep

Antenatal glucocorticoids are thought to be less effective when delivery occurs more than 7 d after initiation of treatment; therefore, repeat courses are often administered. We examined lung structure after single or repetitive antenatal glucocorticoid injections in fetal sheep. Pregnant ewes received single or repetitive doses of 0.5 mg/kg betamethasone at 7-d intervals by maternal or fetal injection, beginning at D104 or D114 with delivery at D125, D135, or D146 gestation (term = 150 d). Changes in lung structure were more pronounced after repetitive versus single injections. Repetitive fetal or maternal injections beginning at D104 (delivery at D125) resulted in comparable structural changes: alveolar volume increased by 50 to 80%, alveolar numerical density decreased by 30 to 40%, and pleural and interlobular septal volumes decreased by as much as 70%. Similar changes were seen in animals delivered at D135 after repetitive maternal injections beginning at D114. There were no structural differences between control and repetitive betamethasone animals when delivery was delayed until D146, indicating that betamethasone induced structural changes were reversible.

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.

Antenatal endotoxin and glucocorticoid effects on lung morphometry in preterm lambs

In utero inflammation may accelerate fetal lung maturation but may also play a role in the pathogenesis of chronic lung disease. We examined the impact of endotoxin, a potent proinflammatory stimulus, on structural and functional maturation of preterm sheep lungs. Date bred ewes received 20 mg Escherichia coli endotoxin or saline by ultrasound guided intra-amniotic injection at 119 d gestation. A comparison group of animals received 0.5 mg/kg betamethasone, a known maturational agent, at 118 d gestation. Lambs were delivered by cesarean section at 125 d (term = 150 d) and ventilated for 40 min. Lung function data are reported elsewhere. Total and differential white cell counts were performed on amniotic fluid and fetal lung fluid samples. Morphometric analyses were performed on inflation fixed right upper lobes. Total cell count increased slightly but not significantly in both amniotic fluid and fetal lung fluid. Both endotoxin and betamethasone had similar effects on alveolarization: average alveolar volume increased by approximately 20% and total alveolar number decreased by almost 30%. Both treatments led to thinning of alveolar walls, although this was statistically significant in the betamethasone-treated group only. Although antenatal endotoxin leads to striking improvements in postnatal lung function, this may be at the expense of normal alveolar development.

Mechanics of the respiratory system in healthy newborn calves using impulse oscillometry

Arterial blood gases, acid-base balance and respiratory function tests using impulse oscillometry (IOS) were performed on 40 clinically healthy newborn calves during the first 24 hours of life to evaluate their respiratory adaptation to extrauterine life. Gas exchange efficiency of the lung was significantly improved with time and was accompanied by the correction of the mixed acidosis observed at birth and by significant changes in respiratory mechanics. Major changes were detected within the first 6 hours. The significant decrease in resistance (R) and the increase in reactance (X) with time, demonstrate the improvement in respiratory mechanics of both upper and lower airways, and reflect the increase in lung volume, the improved lung tissue elasticity and/or distribution of the ventilation. Respiratory mechanical, arterial blood gases and acid-base balance data provided in this study describe a successful respiratory adaptation to extrauterine life in healthy newborn calves.

Effects of prenatal steroids on water and sodium homeostasis in extremely low birth weight neonates

OBJECTIVE

We sought to determine if prenatal steroid (PNS) treatment affects water and sodium (Na) balance in extremely low birth weight infants (<1000 g).

METHODS

PNS treatment enhances lung maturation in preterm infants and induces maturation of renal tubular function and adenylate cyclase activity in animals. We compared water and Na homeostasis for the first week of life in those infants whose mothers received steroids before delivery (PNS: n = 16) to those who did not (nonsteroid group [NSG]: n = 14). The data were collected prospectively, but PNS treatment was not given in a randomized manner. Fluids were initiated at 100 to 125 mL/kg/d and adjusted every 8 to 12 hours to allow a daily weight loss of </=4% of birth weight and to maintain normal serum electrolytes. Weight, serum and urine electrolytes, and urine output were frequently measured and fluid intake was adjusted by increasing the amount of free water to achieve these goals.

RESULTS

When using our fluid management protocol, the percent weight loss in both groups was equivalent during each of the 7 days (15% PNS vs 17% NSG maximum loss) as well as the cumulative urine output at 1 week of age (663 mL/kg/wk PNS vs 681 mL/kg/wk NSG). PNS infants had a higher urine output on the first 2 days of life and a lower daily fluid intake for the first week. PNS infants also had significantly less insensible water loss for each of the first 4 days of life. The PNS group had a significantly lower mean peak serum Na of 138 +/- 1 mmol/L vs 144 +/- 2 mmol/L and none had a peak serum Na >150 mmol/L compared with 36% of the NSG infants. PNS infants had a higher cumulative Na excretion at day 2 of life (10 +/- 2 mmol/kg vs 6 +/- 1 mmol/kg) but a less negative cumulative Na balance at 1 week (-10 mmol/kg vs -14 mmol/kg).

CONCLUSION

PNS treatment was associated with lower estimated insensible water loss, a decreased incidence of hypernatremia, and an earlier diuresis and natriuresis in extremely low birth weight neonates. We speculate that PNS effects these changes through enhancement of epithelial cell maturation improving skin barrier function. PNS treatment may also enhance lung Na, K-ATPase activity leading to an earlier postnatal reabsorption of fetal lung fluid increasing extracellular volume expansion to help prevent hypernatremia.