Albumin mixed with meconium attenuates pulmonary dysfunction in a newborn piglet model with meconium aspiration

Meconium Influences Pulmonary Short-Chain Fatty Acid Concentration in Porcine Meconium Aspiration Model

Introduction

The factors influencing meconium aspiration syndrome (MAS) severity remain poorly understood. In a piglet model of MAS, we hypothesized the respiratory microbiome would reflect the bacterial signature of meconium with short-chain fatty acid (SCFA) accumulation as a byproduct of bacterial fermentation.

Methods

Cesarean section at approximately 115-day term was performed on two sows. Male (9) and female (3) piglets were delivered, instrumented, anesthetized, and randomized into a Control (n = 6) or MAS group (n = 6). MAS received a meconium slurry (3 mL/kg) aspiration injury. Experimental animals were monitored continuously, ventilated, and resuscitated for 24 h. BALF was collected for 16S microbiome sequencing and SCFA analysis by gas chromatography. Effects of SCFAs on A549 alveolar pulmonary epithelial in vitro cell viability and inflammation were assessed.

Results

The MAS group had significantly higher fluid and vasopressor requirements than the Control group (p < 0.05) though both groups developed lung injury. The meconium microbiome demonstrated a difference in genus proportions as compared with the BALF of the Control and MAS groups. The MAS group had a relative increase in propionic acid-forming bacteria and higher BALF concentrations of propionic acid (0.6 ± 0.2 mmol/kg) than the Control group (0.2 ± 0.2 mmol/kg; p > 0.05). Propionic acid was associated with decreased pulmonary epithelial cell viability and an upregulated pro-inflammatory response.

Conclusions

Meconium may host a microbiome with byproducts that interact with the pulmonary epithelium and influence lung injury severity in MAS.

An Unsettled Promise: The Newborn Piglet Model of Neonatal Acute Respiratory Distress Syndrome (NARDS). Physiologic Data and Systematic Review

Despite great advances in mechanical ventilation and surfactant administration for the newborn infant with life-threatening respiratory failure no specific therapies are currently established to tackle major pro-inflammatory pathways. The susceptibility of the newborn infant with neonatal acute respiratory distress syndrome (NARDS) to exogenous surfactant is linked with a suppression of most of the immunologic responses by the innate immune system, however, additional corticosteroids applied in any severe pediatric lung disease with inflammatory background do not reduce morbidity or mortality and may even cause harm. Thus, the neonatal piglet model of acute lung injury serves as an excellent model to study respiratory failure and is the preferred animal model for reasons of availability, body size, similarities of porcine and human lung, robustness, and costs. In addition, similarities to the human toll-like receptor 4, the existence of intraalveolar macrophages, the sensitivity to lipopolysaccharide, and the production of nitric oxide make the piglet indispensable in anti-inflammatory research. Here we present the physiologic and immunologic data of newborn piglets from three trials involving acute lung injury secondary to repeated airway lavage (and others), mechanical ventilation, and a specific anti-inflammatory intervention via the intratracheal route using surfactant as a carrier substance. The physiologic data from many organ systems of the newborn piglet—but with preference on the lung—are presented here differentiating between baseline data from the uninjured piglet, the impact of acute lung injury on various parameters (24 h), and the follow up data after 72 h of mechanical ventilation. Data from the control group and the intervention groups are listed separately or combined. A systematic review of the newborn piglet meconium aspiration model and the repeated airway lavage model is finally presented. While many studies assessed lung injury scores, leukocyte infiltration, and protein/cytokine concentrations in bronchoalveolar fluid, a systematic approach to tackle major upstream pro-inflammatory pathways of the innate immune system is still in the fledgling stages. For the sake of newborn infants with life-threatening NARDS the newborn piglet model still is an unsettled promise offering many options to conquer neonatal physiology/immunology and to establish potent treatment modalities.

Experimental Models of Acute Lung Injury in the Newborns

Acute lung injury in the preterm newborns can originate from prematurity of the lung and insufficient synthesis of pulmonary surfactant. This situation is known as respiratory distress syndrome (RDS). In the term neonates, the respiratory insufficiency is related to a secondary inactivation of the pulmonary surfactant, for instance, by action of endotoxins in bacterial pneumonia or by effects of aspirated meconium. The use of experimental models of the mentioned situations provides new information on the pathophysiology of these disorders and offers unique possibility to test novel therapeutic approaches in the conditions which are very similar to the clinical syndromes. Herewith we review the advantages and limitations of the use of experimental models of RDS and meconium aspiration syndrome (MAS) and their value for clinics.

Restoring pulmonary surfactant membranes and films at the respiratory surface

Pulmonary surfactant is a complex of lipids and proteins assembled and secreted by the alveolar epithelium into the thin layer of fluid coating the respiratory surface of lungs. There, surfactant forms interfacial films at the air-water interface, reducing dramatically surface tension and thus stabilizing the air-exposed interface to prevent alveolar collapse along respiratory mechanics. The absence or deficiency of surfactant produces severe lung pathologies. This review describes some of the most important surfactant-related pathologies, which are a cause of high morbidity and mortality in neonates and adults. The review also updates current therapeutic approaches pursuing restoration of surfactant operative films in diseased lungs, mainly through supplementation with exogenous clinical surfactant preparations. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Meconium impairs pulmonary surfactant by a combined action of cholesterol and bile acids

Mechanisms for meconium-induced inactivation of pulmonary surfactant as part of the meconium aspiration syndrome in newborn infants, to our knowledge, are not clearly understood. Here we have studied the biophysical mechanisms of how meconium affects surface activity of pulmonary surfactant and whether the membrane-perturbing effects of meconium can be mimicked by exposure of surfactant to a mixture of bile acids and cholesterol. Surface activity of pulmonary surfactant complexes purified from animal lungs was analyzed in the absence and in the presence of meconium in standard surface balances and in a captive bubble surfactometer. We have also evaluated accumulation of surfactant at the air-liquid interface by what we believe to be a novel microtiter plate fluorescent assay, and the effect of meconium components on surfactant membrane fluidity using Laurdan fluorescence thermotropic profiles and differential scanning calorimetry thermograms. Rapid interfacial adsorption, low surface tension upon film compression, efficient film replenishment upon expansion, and thermotropic properties of surfactant complexes are all adversely affected by meconium, and, in a similar manner, they are affected by cholesterol/taurocholate mixtures but not by taurocholate alone. We conclude that inhibition of surfactant by meconium can be mimicked by a bile salt-promoted incorporation of excess cholesterol into surfactant complexes. These results highlight the potential pathogenic role of cholesterol-mobilizing agents as a crucial factor resulting in cholesterol induced alterations of structure and dynamics of surfactant membranes and films.

Albumin lavage does not improve the outcome of meconium aspiration syndrome

OBJECTIVE

Meconium aspiration syndrome is still a serious condition with high mortality and morbidity. No specific treatment is yet available, although surfactant is known to reduce the need for extracorporeal membrane oxygenation and surfactant lavage has shown promising results in animal studies. Our group has previously shown reduced oxygenation index in an experimental model of meconium aspiration syndrome in newborn pigs when mixing albumin with meconium before endotracheal instillation. Lung compliance increased when albumin was instilled after meconium as a rescue. The aim of this study was to combine the effect of albumin and lavage.

METHODS

Sixteen newborn pigs (six in the meconium-albumin group, six in the meconium group, and four control animals) were anesthetized and tracheotomized. Meconium 4 mL/kg was instilled endotracheally. After five minutes, albumin 15 mL/kg was instilled in the meconium-albumin group followed by endotracheal suctioning. The observation time was six hours. Respiratory and hemodynamic parameters were measured. The terminal complement complex and proinflammatory cytokines were analyzed in plasma.

RESULTS

Oxygenation index, ventilatory index, and the terminal complement complex (sC5b-9) increased significantly in both groups, but significantly more in the meconium-albumin group. Compliance decreased, but significantly more in the meconium-albumin group. The terminal sC5b-9 complex increased in both groups, but significantly more in the meconium-albumin group. Tumor necrosis factor-alpha, interleukin (IL)- 1beta, and IL-6 increased significantly in both groups.

CONCLUSION

Albumin-lavage did not improve the outcome of experimental meconium aspiration syndrome.

Toxic effects of different meconium fractions on lung function: new therapeutic strategies for meconium aspiration syndrome?

To review and summarize experimental data examining the effects of different fractions of meconium, and to test the effect of albumin on meconium aspiration both as prophylactic and rescue treatment. Newborn piglets 2 to 5 days of age were made hypoxic and then instilled meconium or fractions of meconium intratracheally. Meconium-added albumin and albumin instilled after meconium were also tested. Lung function and inflammatory cytokines were measured. Both the lipid- and water-soluble fractions induce inflammation in the lungs with elevation of inflammatory cytokines. When meconium was mixed with albumin, the inflammatory effects of meconium were significantly ameliorated. Rescue therapy with intratracheal albumin 5 min after the meconium aspiration syndrome was induced also improved lung function. These results indicate that at least part of the symptoms seen in the meconium aspiration syndrome could be prevented by blocking the active substances of meconium such as bile acids and free fatty acids.

Neuroprotective effects of the nonpsychoactive cannabinoid cannabidiol in hypoxic-ischemic newborn piglets

To test the neuroprotective effects of the nonpsychoactive cannabinoid cannabidiol (CBD), piglets received i.v. CBD or vehicle after hypoxia-ischemia (HI: temporary occlusion of both carotid arteries plus hypoxia). Nonhypoxic-ischemic sham-operated piglets remained as controls. Brain damage was studied by near-infrared spectroscopy (NIRS) and amplitude-integrated electroencephalography (aEEG) and by histologic assessment (Nissl and FluoroJadeB staining). In HI+vehicle, HI led to severe cerebral hemodynamic and metabolic impairment, as reflected in NIRS by an increase in total Hb index (THI) and a decrease in the fractional tissue oxygenation extraction (FTOE); in HI+CBD the increase of THI was blunted and FTOE remained similar to SHAM. HI profoundly decreased EEG amplitude, which was not recovered in HI+vehicle, indicating cerebral hypofunction; seizures were observed in all HI+vehicle. In HI+CBD, however, EEG amplitude recovered to 46.4 7.8% baseline and seizures appeared only in 4/8 piglets (both p < 0.05). The number of viable neurons decreased and that of degenerating neurons increased in HI+vehicle; CBD reduced both effects by more than 50%. CBD administration was free from side effects; moreover, CBD administration was associated with cardiac, hemodynamic, and ventilatory beneficial effects. In conclusion, administration of CBD after HI reduced short-term brain damage and was associated with extracerebral benefits.

Meconium induced IL-8 production and intratracheal albumin alleviated lung injury in newborn pigs

We have recently shown that albumin added to meconium before intratracheal instillation in newborn pigs limits detrimental effect on the lungs and reduces increase of IL-8. The aim of this study was to test the effect of albumin instillation as rescue treatment in meconium aspiration syndrome (MAS). MAS was induced in hypoxic piglets by lung instillation of meconium (MAS I = 675 mg/kg, n=12; MAS II=540 mg/kg, n=14). Morbidity and mortality differed (MAS I, dead=7/12; MAS II, dead=5/14). MAS groups were randomized to postmeconium instillation of either bovine albumin (30%, 1.4 mL/kg; MAS I, n=6; MAS II, n=7) or isotonic saline (9 mg/mL, 1.4 mL/kg; MAS I, n=6; MAS II, n=7). The controls (n=4) were tested by sequential instillation of saline (9 mg/mL, 5 mL/kg) and albumin (30%, 1.4 mL/kg). Lung function and gas exchange deteriorated significantly after instillation of meconium [oxygenation index (OI): MAS I, +814%; MAS II, +386%; ventilation index (VI): MAS I, +256%; MAS II, +162%; compliance: MAS I, -53%; MAS II, -44%]. Increases of tracheal IL-8 correlated to deterioration of lung function were 10- (MAS I) and 5-fold (MAS II) (p <0.001). Lung compliance was higher in albumin instillation versus saline instillation (MAS I, p=0.008; MAS II, p=0.002). Albumin did not influence intergroup differences in IL-8, hemodynamics, OI, or VI. MAS-induced IL-8 increases correlated with deterioration of lung function (OI, VI, and compliance). Rescue treatment with albumin in meconium aspiration improved lung compliance in piglets and may represent a new therapeutic approach to MAS.

Complement activation reflects severity of meconium aspiration syndrome in newborn pigs

Meconium aspiration syndrome (MAS) is a serious condition in newborns, associated with a poorly characterized inflammatory reaction. The aim of this study was to investigate a possible role for complement in pulmonary pathophysiology and systemic inflammation in experimental MAS. MAS was induced by instillation of meconium into the lungs of 12 hypoxic piglets. Six controls received saline under otherwise identical conditions. Hemo- and lung dynamics were recorded for 5 h. Plasma complement activation, revealed by the terminal sC5b-9 complex (TCC), and cytokines were measured by enzyme immunoassays. TCC increased substantially in MAS animals compared with controls (p <0.0005). The increase in TCC correlated with lung dysfunction: closely with oxygenation index (r=0.51, p <0.0001) and ventilation index (r=0.64, p < 0.0001) and inversely with lung compliance (r=-0.22, p=0.05). IL-1beta and tumor necrosis factor-alpha increased significantly in MAS animals compared with the controls (p=0.004 and 0.008, respectively). The cytokine increase occurred later than TCC and showed correlations with lung dysfunction similar to TCC. IL-10 did not discriminate between MAS animals and controls (p=0.32). Finally, the subgroup of MAS animals that died (n=5) had substantially higher TCC concentration compared with the surviving MAS animals (n=7; p <0.0005). TCC increased substantially in MAS and was closely correlated to lung dysfunction. Complement activation preceded cytokine release, which may suggest a primary role for complement in the pathophysiology of MAS.

Intratracheal albumin reduces interleukin-8 in tracheobronchial aspirates in piglets after meconium aspiration

Meconium aspiration induces pulmonary inflammation and reduces surfactant function. We hypothesized that albumin mixed with meconium attenuates pulmonary inflammation and improves surfactant function after meconium aspiration. We measured the concentration of free fatty acids (FFA) in the meconium (110 mg dry weight/mL) and added albumin to provide a molar FFA:albumin ratio of 1:1. Newborn piglets, 0-2 day of age, artificially ventilated and exposed to hypoxemia by ventilation with 8% O2, were randomized to group A receiving meconium (n = 12), or group B receiving meconium + albumin (n = 12), 3 ml/kg intratracheally. The animals were reoxygenated for 8 h. Reoxygenation was started when mean arterial blood pressure was < 20 mm Hg or base excess was < -20 mmol/L. During 8 h of reoxygenation the interleukin-8 concentrations in tracheobronchial aspirates increased 5-fold more in the meconium vs. the meconium + albumin groups (93 +/- 56 vs. 18 +/- 4 pg/mL, p < 0.005). There were no differences between the groups for tumor necrosis factor alpha in tracheobronchial aspirates, recruitment of inflammatory cells in the airspaces or surfactant function in bronchoalveolar lavage fluid. In conclusion, albumin significantly decreased interleukin-8 concentrations in tracheobronchial aspirates after meconium aspiration.

Meconium is a potent activator of complement in human serum and in piglets

Meconium aspiration syndrome (MAS) is a clinical condition in the newborn infant with a significant morbidity and mortality. The complex pathophysiology of MAS, leading to both pulmonary and systemic complications, is characterized by an incompletely understood inflammatory reaction. Treatment is symptomatic, mainly limited to airway cleaning and ventilatory support. In this study, we show for the first time that meconium is a potent activator of complement, a key mediator of inflammation. In vitro, meconium activated the alternative complement pathway in human umbilical cord serum as judged by a substantial increase in the alternative pathway convertase C3bBbP. The activation proceeded through C3 (C3bc) and the terminal C5-9 pathway (terminal SC5b-9 complement complex), whereas the classical and lectin pathways were not activated (C1rs-C1-inhibitor complexes and C4bc). The lipid fraction, containing, e.g. free fatty acids, and the water fraction, containing, e.g. bile acids, contributed equally to the complement activation. A blocking antibody to factor D (alternative pathway) completely inhibited the meconium-induced complement activation, whereas blocking antibodies to mannose-binding lectin (lectin pathway) and C2 (classical and lectin pathway) had no effect. In vivo, meconium induced systemic complement activation in a piglet model of MAS, paralleling the increase in lung dysfunction. In conclusion, meconium is a potent activator of the complement system both in vitro and in vivo. Complement may be important in the pathogenesis of MAS, and specific complement inhibition might be a possible treatment approach in MAS.

Innovative neonatal ventilation and meconium aspiration syndrome

Respiratory failure remains a major cause of morbidity and mortality in the neonatal population. Infants with hypoxemic respiratory failure because of meconium aspiration syndrome (MAS), persistent pulmonary hypertension of the newborn (PPHN), and pneumonia/sepsis have a potential for increased survival with extracorporeal membrane oxygenation (ECMO). Other treatment options previously limited to inotropic support, conventional ventilatory management, respiratory alkalosis, paralysis and intravenousvasodilators have been replaced by high-frequency oscillatory ventilation (HFOV), surfactant, and inhaled nitric oxide (iNO). HFOV has been advocated for use to improve lung inflation while potentially decreasing lung injury through volutrauma. Other reports describe enhanced efficacy of HFOV when combined with iNO. Subsequent to studies reporting surfactant deficiency or inactivation may contribute to neonatal respiratory failure exogenous surfactant therapy has been implemented with apparent success. Recent studies have shown that iNO therapy in the neonate with hypoxemic respiratory failure can result in improved oxygenation and decreased need for ECMO. In this article, the authors place in context of a system-based strategy the prenatal, natal and postnatal management of babies delivered through meconium stained amniotic fluid (MSAF) so that adverse outcomes are minimized, and the least number of babies require innovative ventilatory support. At Pennsylvania Hospital, over a six-year period (1995 to 2000), 14.5% (3370/23,175 of live births babies were delivered with MSAF. These data show that 4.6% (155/3370) of babies with MSAF sustained MAS. Overall, 26% (40/155) of babies with MAS needed ventilatory support (or 0.17% of all live-births); of these only 20% (8/40 or 0.035% of live births) needed innovative ventilatory support. None died or needed ECMO. These data describe the impact of a system-based approach to prevent and manage adverse outcomes related to MSAF at regional Level III perinatal center.