Respiratory distress syndrome of the newborn infant

Effort and work-of-breathing parameters strongly correlate with increased resistance in an animal model

BACKGROUND

Effort of Breathing (EOB) calculations may be a reliable alternative to Work of Breathing (WOB) calculations in which Respiratory Inductance Plethysmography (RIP) replaces spirometry. We sought to compare EOB and WOB measurements in a nonhuman primate model of increasing extrathoracic inspiratory resistance simulating upper airway obstruction (UAO).

METHODS

RIP, spirometry, and esophageal manometry were measured in spontaneously breathing, intubated Rhesus monkeys utilizing 11 calibrated resistors randomly applied for 2-min. EOB was calculated breath-by-breath as Pressure Rate Product (PRP) and Pressure Time Product (PTP). WOB was calculated from the Pressure-Volume curve based on spirometry (WOBSPIR) or RIP flow (WOBRIP).

RESULTS

WOB, PRP and PTP showed similar linear increases when exposed to higher levels of resistive loads. When comparing WOBSPIR to WOBRIP, a similar strong correlation was seen for both signals as resistance increased and there were no statistically significant differences.

CONCLUSION

EOB and WOB parameters utilizing esophageal manometry and RIP, independent of spirometry, showed a strong correlation as a function of increasing inspiratory resistance in nonhuman primates. This allows several potential monitoring possibilities for non-invasively ventilated patients or situations where spirometry is not available.

IMPACT

EOB and WOB parameters showed a strong correlation as a function of increasing inspiratory resistance in nonhuman primates. There was a strong correlation between spirometry-based WOB versus RIP-based WOB. To date, it has remained untested as to whether EOB is a reliable alternative for WOB and if RIP can replace spirometry in these measurements. Our results enable additional potential monitoring possibilities for non-invasively ventilated patients or situations where spirometry is not available. Where spirometry is not available, there is no need to apply a facemask post extubation to a spontaneously breathing, non-intubated infant to make objective EOB measurements.

Optimization of Variable Ventilation for Physiology, Immune Response and Surfactant Enhancement in Preterm Lambs

Preterm infants often require mechanical ventilation due to lung immaturity including reduced or abnormal surfactant. Since cyclic stretch with cycle-by-cycle variability is known to augment surfactant release by epithelial cells, we hypothesized that such in vivo mechanotransduction improves surfactant maturation and hence lung physiology in preterm subjects. We thus tested whether breath-by-breath variability in tidal volume (V_T) in variable ventilation (VV) can be tuned for optimal performance in a preterm lamb model. Preterm lambs were ventilated for 3 h with conventional ventilation (CV) or two variants of VV that used a maximum V_T of 1.5 (VV1) or 2.25 (VV2) times the mean V_T. V_T was adjusted during ventilation to a permissive pCO₂ target range. Respiratory mechanics were monitored continuously using the forced oscillation technique, followed by postmortem bronchoalveolar lavage and tissue collection. Both VVs outperformed CV in blood gas parameters (pH, SaO₂, cerebral O₂ saturation). However, only VV2 lowered PaCO₂ and had a higher specific respiratory compliance than CV. VV2 also increased surfactant protein (SP)-B release compared to VV1 and stimulated its production compared to CV. The production and release of proSP-C however, was increased with CV compared to both VVs. There was more SP-A in both VVs than CV in the lung, but VV2 downregulated SP-A in the lavage, whereas SP-D significantly increased in CV in both the lavage and lung. Compared to CV, the cytokines IL-1β, and TNFα decreased with both VVs with less inflammation during VV2. Additionally, VV2 lungs showed the most homogeneous alveolar structure and least inflammatory cell infiltration assessed by histology. CV lungs exhibited over-distension mixed with collapsed and interstitial edematous regions with occasional hemorrhage. Following VV1, some lambs had normal alveolar structure while others were similar to CV. The IgG serum proteins in the lavage, a marker of leakage, were the highest in CV. An overall combined index of performance that included physiological, biochemical and histological markers was the best in VV2 followed by VV1. Thus, VV2 outperformed VV1 by enhancing SP-B metabolism resulting in open alveolar airspaces, less leakage and inflammation and hence better respiratory mechanics.

In vitro study on work of breathing during non-invasive ventilation using a new variable flow generator

OBJECTIVE

In an attempt to reduce the work of breathing (WOB) and the risk of respiratory failure, preterm infants are increasingly treated with nasal synchronised biphasic positive airway pressure (BPAP) via the Infant Flow SiPAP system. However, the relatively high resistance of the generator limits the pressure amplitude (PA) and pressure build-up (PB) of this system. This in vitro study investigates the impact of a new generator with improved fluid mechanics on the WOB, PA and PB during BPAP.

METHODS

Using a low compliance lung model, WOB, PA and PB, were measured during BPAP using the old and the new Infant Flow generators. Airway resistance (tube sizes 2.5 mm, 3.0 mm and 3.5 mm), nasal interface sizes (small, medium and large) and four different ventilator settings were used to mimic different clinical conditions.

RESULTS

Compared with the old generator, the new generator significantly reduced the WOB between 10% and 70%, depending on the measurement configuration. The maximum PA was higher when using the new (6-7 cm H2O) generator versus the old (3-4 cm H2O) generator. During the first 100 ms of inspiration, the new generator reached between 33% and 40% of the peak pressure compared with 11-20% for the old generator.

CONCLUSIONS

This in vitro study shows that a new generator of the Infant Flow SiPAP device results in a significant reduction in WOB and an increase in PA and PB during BPAP. The results of this study need to be confirmed under variable clinical conditions in preterm infants.

Fetal lung maturity testing: the end of an era

Respiratory distress syndrome is a major cause of neonatal morbidity and mortality that is most commonly caused by a deficiency in lung surfactant in premature infants. Therefore, laboratory tests were developed to measure the presence and/or concentration of lung surfactant in amniotic fluid in order to estimate maturity of the fetal lung. Although these tests were once widely employed, their utilization by physicians has decreased in recent years. Several studies have shown that demonstration of a mature fetal lung index by antenatal testing does not improve neonatal outcomes. Instead, decreased respiratory and nonrespiratory morbidities are most highly correlated with gestational age of the fetus. Therefore, fetal lung maturity testing may have passed the point of being clinically useful.

Optimal time interval between a single course of antenatal corticosteroids and delivery for reduction of respiratory distress syndrome in preterm twins

OBJECTIVE

The objective of the study was to investigate the effect of a single course of antenatal corticosteroid (ACS) therapy on the incidence of respiratory distress syndrome (RDS) in preterm twins according to the time interval between ACS administration and delivery.

STUDY DESIGN

We performed a retrospective cohort study of twins born between 24 and 34 weeks of gestation from November 1995 to May 2011. Subjects were grouped on the basis of the time interval between the first ACS dose and delivery: the ACS-to-delivery interval of less than 2 days (n = 166), 2-7 days (n = 114), and more than 7 days (n = 66). Pregnancy and neonatal outcomes of each group were compared with a control group of twins who were not exposed to ACS (n = 122). Multiple logistic regression analysis was used to examine the association between the ACS-to-delivery interval and the incidence of RDS after adjusting for potential confounding variables.

RESULTS

Compared with the ACS nonexposure group, the incidence of RDS in the group with an ACS-to-delivery interval of less than 2 days was not significantly different (adjusted odds ratio [aOR], 1.089; 95% confidence interval [CI], 0.524-2.262; P = .819). RDS occurred significantly less frequently when the ACS-to-delivery interval was between 2 and 7 days (aOR, 0.419; 95% CI, 0.181-0.968; P = .042). However, there was no significant reduction in the incidence of RDS when the ACS-to-delivery interval exceeded 7 days (aOR, 2.205; 95% CI, 0.773-6.292; P = .139).

CONCLUSION

In twin pregnancies, a single course of ACS treatment was associated with a decreased rate of RDS only when the ACS-to-delivery interval was between 2 and 7 days.

Jamming dynamics of stretch-induced surfactant release by alveolar type II cells

Secretion of pulmonary surfactant by alveolar epithelial type II cells is vital for the reduction of interfacial surface tension, thus preventing lung collapse. To study secretion dynamics, rat alveolar epithelial type II cells were cultured on elastic membranes and cyclically stretched. The amounts of phosphatidylcholine, the primary lipid component of surfactant, inside and outside the cells, were measured using radiolabeled choline. During and immediately after stretch, cells secreted less surfactant than unstretched cells; however, stretched cells secreted significantly more surfactant than unstretched cells after an extended lag period. We developed a model based on the hypothesis that stretching leads to jamming of surfactant traffic escaping the cell, similar to vehicular traffic jams. In the model, stretch increases surfactant transport from the interior to the exterior of the cell. This transport is mediated by a surface layer with a finite capacity due to the limited number of fusion pores through which secretion occurs. When the amount of surfactant in the surface layer approaches this capacity, interference among lamellar bodies carrying surfactant reduces the rate of secretion, effectively creating a jam. When the stretch stops, the jam takes an extended time to clear, and subsequently the amount of secreted surfactant increases. We solved the model analytically and show that its dynamics are consistent with experimental observations, implying that surfactant secretion is a fundamentally nonlinear process with memory representing collective behavior at the level of single cells. Our results thus highlight the importance of a jamming dynamics in stretch-induced cellular secretory processes.

Expression of Transcription Factor GATA-6 in Alveolar Epithelial Cells Is Linked to Neonatal Lung Disease

BACKGROUND

Premature birth and respiratory distress syndrome (RDS) are risk factors for disturbed lung development and bronchopulmonary dysplasia (BPD). The molecular mechanisms related to prematurity and BPD remain largely unknown. Epithelial expression of the transcription factor GATA-6 has been implicated in normal and abnormal murine lung development.

OBJECTIVES

The possible involvement of GATA-6 in the normal development and in RDS and BPD was investigated in the human and baboon lung.

METHODS

Immunohistochemistry was used to study the expression of GATA-6 and thyroid transcription factor 1 in lung specimens from different age groups of human and baboon fetuses and newborns with lung disease. Furthermore, the regulatory role of TGF-β₁ in GATA-6 expression was investigated in human pulmonary epithelial cell lines using RT-PCR.

RESULTS

GATA-6 expression increased in the developing human airway epithelium along with advancing gestation, but diminished to negligible at birth. In RDS, GATA-6 expression was enhanced at 5-7 days after birth, and decreased thereafter. In BPD, the expression of GATA-6 in alveolar epithelial cells was low. These results were confirmed and extended using an established baboon model of prematurity. The in vitro experiments revealed that TGF-β₁ induces GATA-6 and thyroid transcription factor 1 expression in lung epithelial cells.

CONCLUSIONS

Our results suggest that the expression of GATA-6 at the early stages of the preterm lung may be related to impaired postnatal alveolar development.

Biomarker tests for fetal lung maturity

The production of surfactant is a key step in fetal lung development. Surfactant decreases alveolar surface tension, thereby preventing alveolar collapse and allowing efficient gas exchange. The lack of adequate amounts of lung surfactant results in respiratory distress syndrome. Tests that assess surfactant concentrations in amniotic fluid are good predictors of infants that will not develop respiratory distress syndrome. The most frequently used test to assess fetal lung maturity (TDx FLM II) will not be available after December 2011. Therefore, we review the currently available tests for fetal lung maturity including lecithin:sphingomyelin ratio, phosphatidyl glycerol, surfactant:albumin ratio and lamellar body counts. Herein, we discuss their clinical utility and consider a suitable replacement for the future.

Respiratory distress syndrome in calves

Respiratory disease syndrome (RDS) is a condition of neonatal calves in which insufficient oxygen uptake and increased retention of carbon dioxide result in respiratory acidosis. This condition is more common in premature calves and seems to be associated with a deficiency of surfactant. Although there is no uniform definition of RDS, clinical signs appear as tachypnea and expiration accentuated by an abdominal lift and expiratory grunt, and they occur in association with characteristic blood gas changes. This article discusses the pathophysiology of RDS in calves, along with the clinical findings, diagnosis, and treatment options.

Positive end-expiratory pressure enhances development of a functional residual capacity in preterm rabbits ventilated from birth

The factors regulating lung aeration and the initiation of pulmonary gas exchange at birth are largely unknown, particularly in infants born very preterm. As hydrostatic pressure gradients may play a role, we have examined the effect of a positive end-expiratory pressure (PEEP) on the spatial and temporal pattern of lung aeration in preterm rabbit pups mechanically ventilated from birth using simultaneous phase-contrast X-ray imaging and plethysmography. Preterm rabbit pups were delivered by caesarean section at 28 days of gestational age, anesthetized, intubated, and placed within a water-filled plethysmograph (head out). Pups were imaged as they were mechanically ventilated from birth with a PEEP of either 0 cmH2O or 5 cmH2O. The peak inflation pressure was held constant at 35 cmH2O. Without PEEP, gas only entered into the distal airways during inflation. The distal airways collapsed during expiration, and, as a result, the functional residual capacity (FRC) did not increase above the lung's anatomic dead space volume (2.5 ± 0.8 ml/kg). In contrast, ventilation with 5-cmH2O PEEP gradually increased aeration of the distal airways, which did not collapse at end expiration. The FRC achieved in pups ventilated with PEEP (19.9 ± 3.2 ml/kg) was significantly greater than in pups ventilated without PEEP (−2.3 ± 3.5 ml/kg). PEEP greatly facilitates aeration of the distal airways and the accumulation of FRC and prevents distal airway collapse at end expiration in very preterm rabbit pups mechanically ventilated from birth.

Pulmonary Vascular Endothelial Growth Factor-C in Development and Lung Injury in Preterm Infants

RATIONALE

In mice, vascular endothelial growth factor-C (VEGF-C) plays an important role in development of the lymphatic system and in pathogenesis of pulmonary inflammation. Its role in development of the lymphatic system in human lung and in lung injury in newborns remains unclear.

OBJECTIVES

We studied the role of VEGF-C in developing human lung, and in acute and chronic lung injury in preterm infants.

METHODS

Included in the immunohistochemistry study were 10 fetuses, 15 control neonates without primary lung disease, 15 preterm infants with respiratory distress syndrome, and 8 infants with bronchopulmonary dysplasia. Tracheal aspirate fluid samples of intubated very-low-birth-weight infants during Postnatal Weeks 1-5 were analyzed with ELISA.

RESULTS

Bronchiolar staining for VEGF-C was observed in all 48 samples. Alveolar epithelial staining was seen in most fetuses (8/10). In addition, staining was observed in alveolar macrophages in bronchopulmonary dysplasia (4/8), and late respiratory distress syndrome (2/7). VEGF receptor-3 (VEGFR-3) staining was observed in lymphatic endothelium adjacent to vascular endothelium. VEGF-C was expressed consistently in tracheal aspirate fluid, being highest during the first 2 postnatal days. Antenatal administration of glucocorticoids was associated with higher VEGF-C in tracheal aspirate fluid.

CONCLUSIONS

The pattern of pulmonary VEGF-C and VEGFR-3 protein expression and consistent VEGF-C protein appearance in tracheal aspirate fluid in human preterm infants indicate a role for VEGF-C in the physiologic development of the lymphatic system of the lung.

Fetal lung maturity

Respiratory distress syndrome of the newborn infant caused by immaturity of the fetal lung continues to be a clinical problem. Measurement of pulmonary surfactant production is the most effective way to evaluate pulmonary maturity. Since the first fetal lung maturity test was described more than two decades ago, advances in methodology have produced diagnostically sensitive tests that are both rapid and precise. Unfortunately, currently available tests continue to demonstrate low diagnostic specificity and remain poor predictors of fetal lung immaturity. We review the background, methodology, pre-analytical and analytical concerns, and clinical performance of various fetal lung maturity assays, and discuss the appropriate use and interpretation of these tests.

Predictors of successful extubation of preterm low-birth-weight infants with respiratory distress syndrome

OBJECTIVE

The aim of the study was to measure pulmonary mechanics in infants with respiratory distress syndrome before extubation and to correlate pulmonary function values with successful extubation.

DESIGN

Clinical study.

SETTING

Neonatal intensive care unit.

PATIENTS

Fifty-one infants (birth weight, 1158.6 +/- 150.6 g; gestational age, 29.1 +/- 2.0 wks).

INTERVENTIONS

Ventilation and daily ventilatory management.

MEASUREMENTS AND MAIN RESULTS

Of the 51 infants studied, 35 (60.8%) were successfully extubated, whereas 16 (39.2%) required reintubation and mechanical ventilation within 72 hrs after extubation. All patients met the clinical and biochemical criteria for extubation. Variables of artificial ventilation before extubation were minimal in all the studied cases (Fio(2) </=0.4, inspiratory pressure </=20 cm H(2)O, ventilatory rate, </=10/min). Pulmonary mechanics were measured before extubation using a noninvasive, mobile VenTrak measuring station.

RESULTS

Significant differences in pulmonary function values between the groups were found. Lower resistance of airways and work of breathing and higher dynamic compliance, tidal volume, and minute ventilation before extubation were associated with successful extubation.

CONCLUSION

On the average, tidal volume values of >6 mL/kg, minute ventilation of >309 mL/kg/min, work of breathing of <0.172 J/L, dynamic compliance of >/=1 mL/cm H(2)O/kg, and resistance of airways of </=176 cm H(2)O/L/sec predicted successful extubation. We recommend measurement of pulmonary function as an assessment tool in determining readiness for extubation.

Matrix metalloproteinases-2, -8, and -9 and TIMP-2 in tracheal aspirates from preterm infants with respiratory distress

OBJECTIVES

Matrix metalloproteinases (MMPs) are a family endoproteinases that act in degradation of extracellular matrix and basement membranes. The development of bronchopulmonary dysplasia (BPD) is characterized by early pulmonary inflammation, increased microvascular permeability, and subsequently by disordered repair. The aims of our study were to characterize the presence and molecular weight forms of MMP-2, -8, and -9 and their specific inhibitor, tissue inhibitor of metalloproteinases (TIMP)-2, in lungs of preterm infants during the early postnatal period and to determine whether levels of these MMPs and TIMP-2 in tracheal aspirate fluid (TAF) are associated with acute or chronic lung morbidity of the preterm infant.

METHODS

TAF samples were collected from 16 intubated preterm infants (gestational age 27.0 +/- 2.0 weeks; birth weight 875 +/- 246 g) during their first 5 postnatal days. The presence and molecular weight forms of MMPs and TIMP-2 were identified by Western immunoblotting, and their levels were evaluated by densitometric scanning.

RESULTS

MMP-8 in TAF was higher in infants who needed treatment with surfactant (25.4 +/- 6.3 vs 10.6 +/- 1.5 arbitrary unit/secretory component of immunoglobulin A [AU/SC]) and in whom BPD developed (N = 6; 27.6 +/- 5.2 vs 15.1 +/- 5.0 AU/SC). TIMP-2 levels were lower in infants with initial arterial to alveolar oxygen tension ratios <0.22 (2.7 +/- 1.1 vs 16.8 +/- 7.4 AU/SC) and in infants needing mechanical ventilation for >1 week (5.2 +/- 2.1 vs 22.8 +/- 11.7 AU/SC).

CONCLUSIONS

In preterm infants, an imbalance between pulmonary MMP-8 and TIMP-2 participates in the acute inflammatory process in respiratory distress syndrome and may contribute to the development of chronic lung injury.

Participation of carnitine palmitoyltransferase in the synthesis of dipalmitoylphosphatidylcholine in rat alveolar type II cells

We have investigated the role of carnitine palmitoyltransferase (EC 2.3.1.21) in pulmonar type II pneumocyte, a lung cell responsible for the synthesis of surface active lipids. Adult type II pneumocytes were isolated from rat lung and purified by differential adherence. When these lung cells were incubated with radioactive palmitate, the percentage of radioactivity recovered into dipalmitoylphosphatidylcholine (DPPC), a major surface active lipid, was almost 60% with respect to total phosphatidylcholine (PC) molecular species. Cellular lysates from type II pneumocytes contained detectable amount of carnitine palmitoyltransferase (CPT) activity (1 nmol/min/mg). Most of the CPT activity found in these cells could be inhibited by incubating them for 60 min with 5 microM tetradecylglycidic acid (TDGA), a specific and irreversible CPT inhibitor of the malonyl-CoA sensitive CPT isoform (CPT I). TDGA treatment of adult type II pneumocytes caused a significant reduction in the incorporation of radioactive palmitate into PC, though this effect did not seem to be specific for DPPC. TDGA affected the incorporation of radioactive palmitate at the sn2 rather than the sn1 position of the glycerol backbone of PC. The incorporation of radioactive palmitate into DPPC was also observed when these lung cells were incubated with palmitate-labeled palmitoyl-L-carnitine. Our data suggest that type II pneumocyte CPT may play an important role in remodelling PC fatty acid composition and hence DPPC synthesis.

Vascular endothelial growth factor in human preterm lung

Endothelial cell damage is characteristic for respiratory distress syndrome and development of chronic lung disease. Vascular endothelial growth factor (VEGF) is an endothelial mitogen that takes part in the growth and repair of vascular endothelial cells. We measured VEGF in 189 tracheal aspirate samples (TAF), and in 24 plasma samples from 44 intubated preterm infants (gestational age, 27.3 +/- 2.0 wk; birth weight, 962 +/- 319 g) during their first postnatal week. VEGF in TAF increased from 25 +/- 12 pg/ml (mean +/- SEM) on Day 1 to 526 +/- 120 pg/ml on Day 7 (mean concentrations, 106 +/- 25 pg/ml on Days 1 to 3 and 342 +/- 36 pg/ml on Days 4 to 7). In plasma, mean concentration of VEGF during the first week was 48 +/- 6 pg/ml, with no increase observed. In TAF, higher VEGF was found in patients born to mothers with premature rupture of the membranes, or chorionamnionitis, whereas preeclampsia of the mother was associated with lower VEGF (all p < 0.05). In TAF, no correlations existed between VEGF and gestational age or birth weight, but a correlation existed between lecithin/sphengomyelin ratio and VEGF (p < 0.05). During Days 4 to 7 patients developing bronchopulmonary dysplasia (BPD) had lower VEGF in TAF than did those surviving without BPD (235 +/- 31 versus 383 +/- 50; p < 0.05). VEGF increased rapidly in the lungs of the preterm infant during the first days of life. VEGF may be indicative of pulmonary maturity and may participate in pulmonary repair after acute lung injury.