Changes in non-electrolyte permeability of alveoli and the absorption of lung liquid at the start of breathing in the lamb

The physiology of neonatal resuscitation

PURPOSE OF REVIEW

As the infant's physiology changes dramatically after birth, modern neonatal resuscitation approaches should detect and be modified in response to these changes. This review describes the changes in respiratory physiology at birth and highlights approaches that can assist these changes.

RECENT FINDINGS

To better target assistance given to infants at birth, the changes in lung physiology have been classified into three phases. The first phase involves lung aeration. As little or no gas exchange can occur, assistance should focus on airway liquid clearance. During the second phase, as airway liquid resides in lung tissue, assistance should focus on minimizing the complications associated with lung edema. The third phase occurs whenever the liquid is cleared from the tissue and respiratory mechanics stabilize. Although more traditional approaches are most effective during this phase, this is not the case for the first two phases. Furthermore, the glottis actively adducts during apnea in newborns and so noninvasive respiratory support requires the infant to be breathing so that the glottis will open.

SUMMARY

The respiratory support provided to infants at birth should match the infant's changing physiology during transition, which requires a more sophisticated approach and equipment than current recommendations.

Efeitos da corticoterapia materna nos valores hemogasométricos de cordeiros nascidos a termo e prematuros

RESUMO: O objetivo do estudo foi avaliar as variáveis hemogasométricas de cordeiros nascidos a termo e prematuros do nascimento às 48 horas de vida. Foram constituídos quatros grupos experimentais: PN (cordeiros nascidos de parto normal, n=15, média de 146 dias de gestação); PNDEX (cordeiros nascidos de parto normal, cujas mães receberam 16 mg de dexametasona aos 141 de gestação, n=8, média de 143 dias de gestação); PRE (cordeiros prematuros nascidos de cesarianas aos 138 dias de gestação, n=10) e PREDEX (cordeiros prematuros nascidos de cesarianas aos 138 dias de gestação, cujas mães receberam 16 mg de dexametasona dois dias antes do parto, n=9). Imediatamente após o nascimento, os cordeiros de todos os grupos apresentaram quadro de acidose respiratória (pH baixo e pCO2 elevada), com maior ênfase nos animais prematuros. A concentração de HCO3 - diminuiu entre 15 e 60 minutos de vida, principalmente nos grupos PRE e PREDEX, com posterior aumento no M24h. Os valores de diferença de base foram menores nos cordeiros prematuros, os quais apresentaram respiração abdominal, intensa dispneia e grande quantidade de líquido pulmonar. A estabilização do equilíbrio ácidobase ocorreu em todos os animais ao longo das primeiras 24 horas de vida. A dexametasona teve influência positiva sobre a condição clínica dos animais prematuros, resultando em adequada ventilação e perfusão tecidual, o que garantiu maior taxa de sobrevivência.

Does epithelial sodium channel hyperactivity contribute to cystic fibrosis lung disease?

  Airway epithelia absorb Na+ through the epithelial Na+ channel (ENaC) and secrete Cl- through the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel. This balance maintains sufficient airway surface liquid hydration to permit efficient mucus clearance, which is needed to maintain sterility of the lung. Cystic fibrosis (CF) is a common autosomal recessive inherited disease caused by mutations in the CFTR gene that lead to the reduction or elimination of the CFTR protein. CF is a multi-organ disease that affects epithelia lining the intestines, lungs, pancreas, sweat ducts and vas deferens, among others. CF lungs are characterized by viscous, dehydrated mucus, persistent neutrophilia and chronic infections. ENaC is negatively regulated by CFTR and, in patients with CF, the absence of CFTR results in a double hit of reduced Cl-/HCO3- and H2O secretion as well as ENaC hyperactivity and increased Na+ and H2O absorption. Together, these effects are hypothesized to trigger mucus dehydration, resulting in a failure to clear mucus. Rehydrating CF mucus has become a recent clinical focus and yields important end-points for clinical trials. However, while ENaC hyperactivity in CF airways has been detected in vivo and in vitro, recent data have brought the role of ENaC in CF lung disease pathogenesis into question. This review will focus on our current understanding of the contribution of ENaC to CF pathogenesis.

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.

Resuscitation and critical care of neonatal calves

Neonatal morbidity and mortality are major economic concerns in both beef and dairy cattle in the United States. In both beef and dairy most calf death occurs in the early neonatal period, particularly in calves born following dystocia. This article focuses on the resuscitation of calves after delivery and highlights some therapeutic points for the care of critical calves.

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.

The permeability of lung capillary and alveolar walls as determinants of liquid movements in the lung

The lungs of the exteriorized fetal lamb offer an unusually good opportunity for measuring the permeability of lung capillary and alveolar walls, because access to plasma, interstitial fluid and alveolar liquid can be obtained without seriously disturbing their physiological relationships. Although there are probably some differences in permeability between fetal and air-breathing lungs, the underlying pattern appears to be similar. Large water-soluble molecules, including proteins, can penetrate capillary but not alveolar walls. The latter appear to be impermeable to molecules with diffusion radii greater than 0.5-0.6 nm. Pore theory can be used to characterize these permeabilities and to obtain estimates of osmotic reflection coefficients for various water-soluble substances. From them, predictions can be made of the bulk movements of liquid to be expected in a variety of circumstances, including the adaptation of the lungs at birth, lung oedema and drowning in sea water or fresh water.

Effect of lung inflation on alveolar permeability to solutes

The alveolar epithelium separates gas and liquid phases in the lung. Osmotic forces available for separating these phases will be determined by the solute permeability of the epithelium. The relative rates of diffusion of several simultaneously studied solutes of known molecular size have been used for measuring the equivalent pore radius across the alveolar epithelium in vivo in the perinatal period and in adult animals. In perinatal sheep, the equivalent pore radius increases from 0.5 nm in the fetal state to 1.5-4.5 nm during the first minutes of spontaneous ventilation. Postnatal animals, 12-60 hours of age, have pore radii of 0.7-1.4 nm. Static inflation of the lungs, in fetal lambs, produced pressure-dependent increases in pore radii. In adult sheep and rabbits the measured radius of equivalent pores across the epithelium in vivo was positively correlated with the degree of inflation of the lungs (P less than 0.005). Measurements varied from 0.5 nm at low levels of inflation to large leaks at high levels. These experiments indicate that solute permeability across the alveolar epithelium is a dynamic function of the inflation of the lung, rather than a static feature.

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.

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.

Delayed clearance of fetal lung liquid and sodium transport--genetic predisposition not evident yet

The epithelial Na+ channel (ENaC) contributes to the clearance of fetal lung liquid. In premature infants, low ENaC activity and low expression level of alpha-ENaC have been associated with respiratory failure. The polymorphism in the alpha-ENaC gene remains to be studied as a factor explaining the variation in the incidence of transient tachypnoea or respiratory distress syndrome in the newborn.

CONCLUSION

The study of genetic predisposition to common multifactorial diseases requires analyses of large, well-defined cohorts for representative variants of relevant candidate genes.

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.

Invited review: Clearance of lung liquid during the perinatal period

At birth, the distal lung epithelium undergoes a profound phenotypic switch from secretion to absorption in the course of adaptation to air breathing. In this review, we describe the developmental regulation of key membrane transport proteins and the way in which epinephrine, oxygen, glucocorticoids, and thyroid hormones interact to bring about this crucial change in function. Evidence from molecular, transgenic, cell culture, and whole lung studies is presented, and the clinical consequences of the failure of the physiological mechanisms that underlie perinatal lung liquid absorption are discussed.

Significance of respiratory dynamics of the lung tissue in pulmonary drug permeation

There are two main objectives in this study. One is to investigate the roles of respiration, i.e., the dynamic change of the lung tissue, on drug transport across the air-blood barrier. The others is to establish the quantitative relationship between the effect of respiration and the physicochemical properties of drugs. To achieve these objectives, progesterone and a group of its hydroxy derivatives with varying hydrophilicity were used as the model drugs and their permeation kinetics studies were conducted under simulated respiratory dynamics using the in vitro pulmonary permeation system developed earlier in this laboratory. The physiological respiratory dynamics were successfully simulated and found to enhance significantly the transpulmonary permeation of progesterone and its hydroxy derivatives through bullfrog lung membrane, a model air-blood barrier. The extent of enhancement in the rate of drug permeation was observed to depend on the pattern of pressure application. As a pressure of the same magnitude was applied, the respiratory pressure was found to have a greater effect than a constant pressure. The results suggested that respiration has increased not only the surface area of lung membrane for permeation, but also dramatically affected the permeability of the lung membrane. Furthermore, the enhancement in permeation rate produced by respiration was observed to be in a linear correlation with the hydrophilicity of penetrants. The effect of variation in various respiratory parameters on drug permeation was also evaluated, and the results suggested that the dynamic change of the lung tissue generated by respiration plays an important role in the transpulmonary permeation of drugs. A possible mechanism involved could be attributed to the formation of transitional pores in lung membrane during the dynamic process of respiration. Therefore, it is necessary to bear in mind and take the respiratory dynamics into consideration for studying the transpulmonary permeation of drugs, especially when the drug has hydrophilic characteristics.

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.

Intratracheal endotoxin causes systemic inflammation in ventilated preterm lambs

Intratracheal endotoxin causes acute inflammation in the adult lung, and injurious styles of mechanical ventilation can result in systemic inflammation derived from the lungs. We asked how ventilated premature and near-term lungs responded to intratracheal endotoxin and if systemic inflammation occurred. Lambs delivered at 130 d gestational age (GA) were treated with surfactant or surfactant plus endotoxin (0.1 mg/kg or 10 mg/kg) (Escherichia coli, serotype O55:B5) and were ventilated for 6 h. Both endotoxin doses resulted in impaired gas exchange and systemic inflammation in the preterm lambs. Lambs at 141 d GA (term 146 d) were given either 10 mg/kg intratracheal endotoxin, 10 mg/kg endotoxin plus high tidal volume ventilation for the first 30 min of life, or 5 microg/kg endotoxin given intravenously. Endotoxin alone (10 mg/kg) caused lung inflammation but no systemic effects after 6 h of ventilation. Lambs given 10 mg/kg endotoxin plus high tidal volume ventilation or 5 microg/kg endotoxin intravenously had decreased gas exchange and systemic inflammation. Endotoxin was detected in the plasma of lambs at 130 d GA but not at 141 d GA. Inflammation in the lungs was more severe in preterm animals. Mechanical ventilation of the endotoxin-exposed preterm lung resulted in systemic effects at a low endotoxin dose and without high tidal volume ventilation.

Significance of ion transport during lung development and in respiratory disease of the newborn

Active ion transport plays a critical role in the liquid movement across the fetal and perinatal lung epithelium. The fetal lung liquid production is coupled with active secretion of Cl- into the luminal space. The potential for fluid absorbing mechanisms related to active Na+ transport from the apical to the basolateral side of the epithelium appears near the end of gestation. At birth there is a dramatic change of environment with commencement of air-breathing, sudden increase in oxygen partial pressure (PO2) and profound changes in the pulmonary circulation. A concurrent switch from fluid secretion to maintenance of low amounts of alveolar fluid is another major physiological adjustment taking place in the perinatal distal lung epithelium. The fluid-absorbing mechanism is a result of a well-synchronized co-operation between the basolateral membrane Na-K-ATPase and the apical membrane Na+ channels and it promotes salt and water movement from the airspace. Inability of the fetal lung epithelium to switch from fluid secretion to Na+ transport-dependent absorption seems to be an important factor adversely contributing to the respiratory distress of the newborn premature infant.

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.

Pulmonary technetium-99m diethylene triamine penta-acetic acid aerosol clearance as an index of lung injury

Although the clearance of an inhaled nebulised radioaerosol has long been employed as a measure of lung function, this test has not found favour in nuclear medicine units to the extent that might originally have been anticipated. In this review the theoretical basis of solute transfer is considered and the measurement of radioaerosol clearance discussed. Thereafter the various clinical applications of alveolar permeability measurement are outlined. Finally, possible reasons for the restricted clinical use of the diethylene triamine penta-acetic acid clearance technique are considered. It is concluded that the technique should provide a rapid screening evaluation of the HIV+ve patient presenting predominantly with chest symptoms.

Simulated uterine contractions facilitate fetal and newborn respiratory behavior in rats

We tested the hypothesis that sensory and nonsensory factors associated with birth stimulate respiratory behavior in the fetal and newborn rat. Late gestation (Day 21) rat fetuses were externalized from the uterus with intact umbilical connections to the dam and exposed to stimuli normally associated with labor and delivery. Onset and maintenance of respiratory movements were monitored. In the first experiment, rat fetuses were exposed to either: (i) simulated uterine contractions; (ii) cooling (26 degrees C); (iii) umbilical cord occlusion; or (iv) air heated to intrauterine temperature (37.5 degrees C). Subjects were videotaped for 1 h, and respiratory movements counted during tape review. Fetuses in each group showed some respiratory behavior although compression significantly elevated respiratory rate compared to other experimental conditions. All subjects in each group were respiring after 1 h, except for pups that received umbilical cord occlusion. The 100% attrition rate of the cord occlusion-alone group was reversed by combining cord occlusion with compression, or with compression and cooling, but not by combining cord occlusion with cooling. Simulated birth pups (i.e., those exposed to compression, cooling and umbilical cord occlusion) and normal, vaginally delivered pups breathed at identical rates and showed a similar pattern of postpartum breathing. These results suggest that mechanical stimulation of the fetus associated with uterine contractions plays a critical role in the maintenance of continuous respiration at birth. Possible mechanisms for the facilitative effects of compression on perinatal breathing are discussed.

Physiologic mechanisms of adaptation in the fetal calf at birth

At the moment the umbilicus separates from the fetus during the birthing process, several physiologic functions formerly supported by the dam must become operational in the neonate to ensure survival of the calf. These functions include the ability to maintain adequate oxygen saturation of blood, to regulate acid-base balance, to engage endogenous metabolic pathways for energy production, and to preserve body temperature within critical limits. This article reviews cardiopulmonary function, acid-base balance, and energy metabolism in the fetus and neonate.

Weakness in the newborn calf

The peak time period for the average beef producer to experience the majority of calf losses has consistently been from the time of birth through the first seven days of life. Weakness is a principal clinical sign of diseases or conditions responsible for mortality including birth trauma, prematurity or dysmaturity, congenital malformations, metabolic defects, intrauterine infection, anoxia or hypoxia, hypothermia, starvation, extremes in birth weight, and post-natal infection. This article discusses anoxia/hypoxia and septicemia in greater detail because of their involvement as a common cause of weakness in the newborn calf.

Fluid dynamics during initial aeration of mature fetal lung and effect of temperature

Air volume-pressure (VP) curves were recorded simultaneously on pairs of mature rabbit fetuses from the same litter with one member of the pair at 37 degrees C and the other at 22 degrees C. Intrasaccular bubbles, formed primarily during inflation, were assessed for stability and surface tension (gamma). Average air flow rates (dV/dt) were calculated from the VP data. In separate experiments, liquid VP curves were recorded at 37 degrees and 22 degrees C: maximal liquid V was matched to maximal air V at 37 degrees and 22 degrees C, respectively. Fetal pulmonary liquid (FPL) viscosity (eta) and density (rho) were determined by standard methods. Both the effect of temperature on lung mechanics as reported previously, and the reliability of the rabbit model were confirmed in the paired fetuses. Analysis of fluid dynamics revealed that of the six parameters relevant to initial inflation-deflation of FPL-filled lungs, liquid rho, distensibility (recoil), and gamma were not altered significantly by temperature increase from 22 degrees to 37 degrees C. Enhanced lung mechanics at 37 degrees C (including enhanced inflation at lower P, higher maximal V, increased production of intrasaccular bubbles, and higher V at end-deflation) was primarily due to lowering of FPL eta at the higher temperature which appears to have an effect by augmenting bulk liquid flow and liquid drainage. Lower eta increases bulk flow through airways directly. Consequent recruitment and distention of these conducting units effectively increases radius (r) and further enhances flow. (The ultimate "brake" to airways flow at both temperatures is counter P from gamma at air/liquid menisci.).(ABSTRACT TRUNCATED AT 250 WORDS)

Lung to blood passage of albumin and a nonapeptide after intratracheal instillation in the young developing pig

The passage via the lower respiratory tract of the macromolecule, bovine serum albumin (BSA, mol. wt = 67000) and the nonapeptide, 1-deaminocysteine-8-D-arginine vasopressin (dDAVP, mol. wt = 1067), was assessed for 240 h after intratracheal instillation in young, developing pigs. The plasma levels of immunoreactive BSA increased with time, reaching a maximum between 36-86 h after instillation, and the main portion of BSA in plasma appeared intact, as assessed by crossed immunoelectrophoresis. The dDAVP levels reached a maximum after 1-3 h. The time to reach maximal plasma levels increased with the age of the pig for BSA, while no such effect was observed for dDAVP. The total lung passage, as calculated by dividing the area under the plasma level-time curve obtained after intratracheal instillation with that obtained after intravenous administration (elimination curve), showed no significant differences with age for BSA (21.8 +/- 9.0% in the newborn, 15.1 +/- 8.0% in the 2-day-old and 16.1 +/- 4.2% in the 70-day-old pigs). For dDAVP, a significant decrease in the total passage was obtained with age, from 74.6 +/- 9.4% in the newborn pigs to 44.1 +/- 13.3%, in the 2-day-old pigs (P < 0.001), and to 23.6 +/- 7.1% (P < 0.01) in the 70-day-old pigs. The study showed that the marker molecules, BSA and dDAVP, passed over the porcine lung into the blood circulation in high amounts, apparently inversely related to their molecular size. The total lung passage of dDAVP, but not that of BSA, was affected by changes with age, indicating that these two markers might traverse the lung epithelium via different routes that are differently affected during postnatal development.

Lung albumin recovery in surfactant-treated preterm ventilated lambs

Preterm ventilated animals and infants with respiratory distress syndrome (RDS) develop proteinaceous alveolar edema. To study the effect of postnatal age on intravascular radiolabeled albumin accumulation into lungs, preterm lambs at 132 days gestational age were ventilated after treatment with sheep surfactant or cow surfactant extract for periods as long as 24 h. Lambs not treated with surfactant were studied for only 5 h because of severe respiratory failure. All lambs were given radiolabeled albumin by intravascular injection 1 h before they were killed, and the net recovery of the labeled albumin was measured in the lung tissue and air space as quantified by alveolar lavage. Net 1-h radiolabeled albumin recoveries in the lungs decreased from 5 to 6% soon after birth to 0.9% at 24 h in the surfactant-treated groups (p less than 0.01). At 3 h there was less labeled albumin recovery by alveolar lavages in lambs treated with sheep surfactant than in control lambs and lambs treated with cow surfactant extract (p less than 0.05). Protein in alveolar washes from lambs treated with cow surfactant extract exceeded that in lambs treated with sheep surfactant at 3 h (p less than 0.05), but protein recoveries had decreased to similar values by 24 h, indicating a net clearance of air-space protein. These studies demonstrate a sixfold decrease in net albumin accumulation from birth to 24 h of age despite continued ventilation and oxygen exposure of the premature lamb lungs.

Pathogenesis of respiratory failure in the preterm infant

Respiratory failure in the preterm results from not only surfactant deficiency but also the immaturity of a number of other elements that have a structural basis. Airway, alveolar, fluid clearance, and epithelial and endothelial barrier functions also are important to lung function. Immaturities in these lung elements have identifiable adverse consequences for lung function such as pulmonary interstitial emphysema and pulmonary edema. The maturation of each of these elements appears to be achievable by agents such as corticosteroids, and maturation will result in an improved response to surfactant treatments. While surfactant treatments can improve respiratory failure by minimizing lung injury, other aspects of lung immaturity continue to contribute to respiratory compromise in the preterm. A thorough understanding of respiratory failure in the newborn depends on a better appreciation of the contribution of immaturity of the different structural elements of the lung on lung function.

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.

Influx of urea during bronchoalveolar lavage depends on the permeability of the respiratory membrane

In 6 healthy controls and 23 patients with pulmonary diseases the influx of urea during bronchoalveolar lavage was measured by comparing the concentrations of albumin and urea in the sequential samples recovered. It varied between -28 and 151 mumol/l. In the bronchoalveolar lavage fluid and serum we measured alpha-2-macroglobulin (A2M) and ceruloplasmin (CP). The bronchoalveolar lavage fluid to serum ratios were calculated (QCP and QA2M). QA2M/QCP was taken as a measure of the respiratory membrane permeability; it varied between 0.05 and 0.53. Influx of urea during lavage was higher according as the QA2M/QCP ratio was higher. We conclude that concentrations of substances in the epithelial lining fluid calculated with the urea correction method have to be corrected for the influx of urea.

Immunochemical and immunohistochemical evaluation of lung permeability in ventilated newborn rabbits

These experiments were designed to quantify the vascular-to-alveolar leakage albumin in the neonatal lung and to analyze the distribution of leaking airspaces in the lung parenchyma. Immediately after delivery, newborn rabbits with gestational age 27-29 days received an intravenous injection of human albumin as a marker and were ventilated for 15 min with standardized tidal volume (10 ml/kg). After the period of ventilation the lungs were either lavaged via the airways or fixed for histological studies. The median amount of albumin in lung lavage fluid, determined by immunodiffusion, was 4.8% of the injected dose after 27 days, 1.3% after 28 days, and 0.4% after 29 days of gestation; it was inversely correlated with the compliance of the respiratory system (r = -0.78; p less than .001). Immunohistochemical examination of lung section revealed that the leak was not diffuse; even in animals with gestational age 27 days it involved only a median of 48% of total alveoli. The median amount of alveoli containing the label fell to 6% after 28 days and to 0% after 29 days gestation, correlating inversely with the compliance of the respiratory system (r = -0.53; p less than 0.01). We suggest that our experimental model is useful for histological demonstration of serum proteins leaking into the airpaces under experimental conditions and for evaluating the effect of therapeutic regiments on neonatal lung permeability.

Effect of lung collapse on alveolar surfactant in rabbits subjected to unilateral pneumothorax

To determine whether atelectasis might modify lung surfactant, we injected N2 into the right pleural space of adult rabbits. Daily, under sedation, pleural gas volume and pressure were measured and adjusted to 20 ml/kg and 0 to +2 cm H2O with N2. On the sixth day, pHa, PaCO2, PaO2, and FRC were measured. Pressure-volume diagrams or bronchoalveolar lavages (BAL) were performed separately on right and left lungs. Surfactant subfractions were obtained from BAL fluid, and total protein, LDH, and cell counts were determined. Phospholipid (PL) was assayed in lung homogenate, BAL fluid, and subfractions, and PL composition was determined on the largest BAL subfraction (P4). On the sixth day the pleural gas volume was 19.7 +/- 2.7 (SD) ml/kg, and PaO2 and FRC were significantly decreased. Air volume in excised right lungs at 30 cm H2O was 13.1 +/- 2.8 (SE) ml/kg with pneumothorax (PN) and 22.8 +/- 1.9 (SE) ml/kg in controls. Total PL was decreased 43% in BAL and 59% in P4 of collapsed lungs. Phosphatidylglycerol to phosphatidylinositol (PI) plus phosphatidylserine (PS) ratio of P4 was substantially decreased in both lungs of PN animals. Cell counts, LDH, and protein in BAL did not suggest inflammation or epithelial damage. We conclude that pneumothorax decreases the quantity of alveolar surfactant in the collapsed lung and alters its phospholipid composition toward the fetal pattern in both lungs, possibly due in part to the proliferative response of the lungs to pneumothorax.

Lung water and vascular permeability-surface area in premature newborn lambs with hyaline membrane disease

Extravascular lung water and vascular permeability-surface area products were measured with a multiple indicator dilution method in 6 premature lambs with hyaline membrane disease 1-5 hours following delivery by cesarean section. The indicators used were 51Cr-labelled erythrocytes, 125I-albumin, 3H-water, and 14C-urea. Results were compared with previously obtained data in newborn lambs without hyaline membrane disease also delivered by cesarean section. Extravascular lung water was significantly higher in lambs with hyaline membrane disease [23.2 +/- 1.0 (SEM) vs. 10.7 +/- 1.4 ml/kg body wt]. Vascular permeability-surface area products for 14C-urea were significantly lower in lambs with hyaline membrane disease (0.30 +/- 0.10 vs 0.78 +/- 0.11 ml/s per kg). It is concluded that extravascular lung water is high in lambs with hyaline membrane disease. Permeability-surface area products for 14C-urea is low in lambs with hyaline membrane disease, which probably indicates a decrease in detectable surface area for exchange due to derecruitment or hypoperfusion of pulmonary exchange vessels in edematous and hypoxic areas of the lungs.

Air opening pressure in fetal lungs

The volume of pulmonary fluid (PF) in fetus decreases at late gestation and with initiation of labor. In addition there is a massive release of surfactant in PF during labor. In postmaturity the reduction in PF continues till the lungs become virtually fluid-free. Furthermore there is a loss of surface activity with prolongation of gestation. The present study was conducted to determine the role of the volume and surface activity of pulmonary fluid on the opening pressure (OP) in fetal lungs. Fetal (term and post term) and degassed newborn lungs were inflated with air and OP recorded. The lowest OP was noted in lungs of term fetuses delivered after labor. As the lung became dryer, either after birth or during postmaturity, the OP increased. The OP was greater in postmature compared to degassed postnatal lungs. Instillation of minute volumes of isotonic saline in post term to restore the volume of pulmonary fluid to that of term lungs decreased the OP. Thereafter as the volume of intrapulmonary fluid was increased beyond that of term, either in term or post term lungs, the OP was also increased. The angle of deflection from fluid-removed to fluid-instilled segments of the OP-volume curve was more acute in the fetal lung that in the adult. These studies suggest that a reduction in the volume of pulmonary fluid at birth beyond that of term fetus (FRC) may not be beneficial for newborn and that massive release of surfactant in PF during labor serves to enhance opening of air saccules.

Corticosteroids and surfactant change lung function and protein leaks in the lungs of ventilated premature rabbits

Fetal rabbits were treated with corticosteroids by maternal administration for 48 h before delivery at 27 d gestational age. The treated and control rabbits were placed on ventilator-plethysmographs so that ventilation could be adjusted by regulation of tidal volumes to 10-13 ml/kg body wt. [125I]albumin was mixed with fetal lung fluid at birth, alternate rabbits from each litter were treated with Surfactant-TA, and [131I]albumin was injected intravascularly. The movement of the labeled albumins into and out of the alveolar wash and lung tissue was measured after 30 min of ventilation. Corticosteroid treatment (total dose, 0.2 mg/kg betamethasone) significantly decreased the protein leak across the endothelium (P less than 0.001) but increased the protein leak across the epithelium (P less than 0.001). Surfactant treatment decreased both the endothelial and epithelial leaks, and the combination of surfactant and corticosteroid treatments decreased endothelial leaks to 29% of control values and increased compliance more than either treatment alone. The 48-h corticosteroid treatment did not increase alveolar surfactant pool sizes. Corticosteroids significantly changed lung protein leaks independently of surfactant, and improved the response of the preterm lung to surfactant treatments.

Leakage of protein in the immature rabbit lung; effect of surfactant replacement

Immature newborn rabbits, delivered on day 27 of gestation, were ventilated artificially for 60 min, with or without previous treatment with natural surfactant. Insufflation pressure was adjusted to maintain an average tidal volume of about 10 ml/kg. All animals received, before the onset of ventilation, 125I-labeled albumin via the airways and 131I-labeled albumin intravenously. At the end of the experiment 3.1 +/- 1.3% (means +/- SD) of the 131I-albumin had permeated into the alveolar compartment of control animals; the corresponding figures for surfactant-treated animals were 1.7 +/- 0.8% (P less than 0.002). In control animals only 18.2 +/- 4.4% of the 125I-albumin could be recovered from the airspaces after 60 min, whereas 69.9 +/- 14.6% of this label was recovered in surfactant-treated animals (P less than 0.002). Alveolar wash samples from control animals also contained significantly increased activity of surfactant inhibitor, as evaluated with pulsating bubble. The bidirectional flux of protein, including surfactant inhibitor, was thus significantly decreased in these immature lungs by surfactant replacement.

Measurement of net transplacental transfer of fluid to the fetal sheep

If fetal drinking activity is prevented and it is assumed that in the latter third of gestation the fetus is capable of maintaining itself in fluid balance, then the net amount of fluid gained across the placenta by the fetus is equal to the amount of fluid lost from the fetus, by routes other than the placenta, plus fluid deposited in growing tissues minus the amount of water produced as a result of oxidative metabolism. Net transplacental transfer of fluid to the fetus over a 3 h period was measured in eight chronically catheterized fetal sheep in which drinking activity was prevented by ligating the oesophagus. Urine and lung liquid flow rates were measured. In the latter third of gestation, these are the only significant sources of fluid loss from these fetuses during the 3 h experimental period. Water produced as a result of oxidative metabolism was calculated, as was the amount of fluid deposited in growing tissues during the course of the experiment. The weight of the fetus at the beginning of the experiment and the change in weight that occurred during the experiment was calculated by measuring the weight of the fetus at death (within 30 h) and applying an equation which describes the body weight-gestation age relationship for merino sheep. Net transplacental fluid transfer was 0.40 +/- 0.09 ml min-1 kg-1 (range 0.30-0.54 ml min-1 kg-1). Fetal urine flow rate averaged 0.30 +/- 0.11 ml min-1 kg-1. It was 72.8 +/- 10.0% of the volumes used to calculate net transplacental fluid transfer to the fetus. Lung liquid flow rate was 0.079 +/- 0.039 ml min-1 kg-1. It was 20.2 +/- 9.2% of the volumes used to calculate net fluid intake. The amount of fluid deposited as a result of tissue growth was 0.023 +/- 0.001 ml min-1 kg-1; it was 5.94 +/- 1.1% of the volumes used in the equation, while the production of water as a result of metabolism was 3.9 X 10(-3) ml min-1 kg-1 (Conrad & Faber, 1977) and constituted 1.01 +/- 0.22% of the volumes used in the equation. This method of measuring net transplacental fluid transfer to the fetus can be used to measure fetal fluid intake over relatively short periods of time. It also means that the effects of disturbances in maternal fluid and electrolyte balance on fluid transfer to the fetus can be studied and quantitated.

The effect of age on pulmonary epithelial permeability in unanesthetized lambs

Pulmonary epithelial permeability was measured 1) in unanesthetized sheep, and 2) longitudinally in growing lambs. Awake sheep were intubated and a solution of 51Cr-ethylenediaminetetraacetic acid and 125I-antipyrine was instilled in the intrathoracic trachea via the nasotracheal tube. Arterial blood was drawn 1-25 minutes after the instillation. The ratios of the counts of 51Cr to 125I at 7, 10, and 13 min were calculated and averaged for each animal. Data from six adult sheep showed that the mean +/- SE of the permeability ratio was 0.012 +/- 0.003 and was reproducible over three months. When measured twice within two hours, the second ratio was significantly higher than the first (0.021 +/- 0.004 vs. 0.011 +/- 0.003, n = 6, P less than 0.05). One hour of general anesthesia with methoxyflurane did not alter the permeability ratio significantly. Ten lambs were studied longitudinally 10 hours and 5, 10, 20, and 30 days after delivery. Within the first 24 hours of life the permeability ratio was 0.035 +/- 0.007, significantly greater (P less than 0.05) than the adult value. At five days there was no significant difference between lambs and adult sheep. Throughout the first month of life, the permeability ratio in lambs remained at at the adult level. In summary, 1) pulmonary epithelial permeability can be assessed in awake lambs and adult sheep, 2) the permeability ratio is reproducible for three months but should not be measured within two hours, 3) newborn lambs have a significantly higher ratio than older lambs or adult sheep, and 4) the ratio reaches the adult level within five days after birth.

Fetal lung liquid regulation by neuropeptides

The fetal lung, filled with fluid during intrauterine life, is recognized as a site of fluid production, significantly contributing to amniotic fluid volume. To assess possible hormonal control of lung liquid production, we studied the effect of intravenous vasopressin or vasotocin on lung liquid production in chronically catheterized fetal sheep. Both vasopressin and vasotocin infusion resulted in a significant decrease in lung fluid production (38% and 40%, respectively) when compared to that in control animals receiving saline infusion. There was no significant change in lung fluid osmolality, sodium, or potassium. These findings suggest that lung liquid production in utero may be regulated by the fetus and that vasopressin or vasotocin may be of physiologic importance in lung fluid dynamics during the third trimester of ovine pregnancy and perhaps during parturition.

Edema formation in the lungs and its relationship to neonatal respiratory distress

Pulmonary edema is an important feature of many newborn lung diseases, including respiratory distress from severe perinatal asphyxia, heart failure, hyaline membrane disease, pneumonitis from group B beta-hemolytic streptococcus, and chronic lung disease (bronchopulmonary dysplasia). Neonatal pulmonary edema often results from increased filtration pressure in the microcirculation of the lungs. This occurs during sustained hypoxia, in left ventricular failure associated with congenital heart disease or myocardial dysfunction, following excessive intravascular infusions of blood, colloid, fat, or electrolyte solution, and in conditions that increase pulmonary blood flow. Low intravascular protein osmotic pressure from hypoproteinemia may predispose infants to pulmonary edema. Hypoproteinemia is common in infants who are born prematurely. Large intravascular infusions of protein-free fluid further decrease the concentration of protein in plasma and thereby facilitate edema formation. Lymphatic obstruction by air (pulmonary interstitial emphysema) or fibrosis (long-standing lung disease) also may contribute to the development of edema. Bacteremia, endotoxemia, and prolonged oxygen breathing injure the pulmonary microvascular endothelium and cause protein-rich fluid to accumulate in the lungs. The risk of neonatal pulmonary edema can be reduced by several therapeutic measures designed to lessen filtration pressure, increase plasma protein osmotic pressure, and prevent or reduce the severity of lung injury.

Functional anatomy and volume-pressure characteristics of immature lungs

The mechanical behavior of immature rabbit fetal lungs in situ was assessed by air and saline volume-pressure diagrams. All lungs were in their natural fetal state, i.e., filled with fetal pulmonary fluid, prior to inflation. Anatomic correlates were determined by continuous stereomicroscopic monitoring of the lungs. We found the following to be characteristic of immature lungs: (1) Tissue retractive forces are similar to adults. (2) Fetal lungs are not 'plastic' above functional residual capacity. (3) Initial aeration is by 'axial filling' in which airways are distended several times their resting size. (4) Invariably, peripheral rather than central saccules are the first to be aerated and saccules are recruited by both pressure- and time-dependent processes. (5) Pressure-dependence is related to surface forces and terminal orifice size, while time-dependent processes include orifice enlargement, liquid flow through terminal conduits, and the formation of very short-lived, labile bubbles. (6) 'Opening pressure' inflection in the VP diagram is not coincidental with, but follows the onset of saccular aeration. (7) Negative compliance at the onset of deflation is due to saccular enlargement and recruitment. (8) Hysteresis is due to tissue conformational characteristics at high pressures and air entrapment at low pressures. (9) Surface tension cannot be measured reliably from the saline and air VP diagrams.

Lung mechanics during spontaneous ventilation in premature and fullterm rabbit neonates

Rabbit neonates were tracheotomized on day 27--30 of gestation, and lung mechanics during spontaneous ventilation was registered with a fluid-filled esophagus catheter for pressure recording and a pneumotachograph connected to the tracheal cannula via a Fleisch tube. The first breath of all neonates had a very long inspiratory phase (less than or equal to 1.12 sec), with insufflation pressures amounting to 42 cm H2O in the mature animals. The expiration phase of the first breath was usually 'passive', i.e. there was no positive expiratory esophagus pressure. In animals delivered on day 27--28 there was, as a rule, no difference between inspiratory and expiratory tidal volume during the first few breaths, whereas such difference was regularly registered in mature animals; this phenomenon probably corresponds to the establishment of FRC. Dynamic lung compliance increased with both gestational and postnatal age, up to 60 min after delivery. Shortly after birth, pulmonary resistance was higher in immature animals than in fullterm ones, but at later intervals, there was no difference between survivors of various gestational age.

Hypoxia in the newborn infant

Hypoxia in newborn infants is becoming much easier to prevent, detect and treat. Nevertheless the successful management of potentially hypoxic fetuses and newborn infants remains the major challenge to all physicians concerned with perinatal care. What is at stake is not only that sick infants should survive, but equally or more importantly that the survivors should be normal children. Recent follow-up studies show that this aim can, with few exceptions, now be achieved (Stewart and Reynolds, 1974; Davies and Stewart, 1975; Durbin et al, 1976).