Hypoxic respiratory failure in the late preterm infant

Hypoxic respiratory failure in late preterm infants has received increased attention in the last decade, and while the incidence is low, it accounts for a significant number of admissions to neonatal ICUs because of the large number of late preterm births in the United States and worldwide. Causes of respiratory distress include transient tachypnea of the newborn, surfactant deficiency, pneumonia, and pulmonary hypertension. The physiologic mechanisms underlying delayed transition caused by surfactant deficiency and poor fetal lung fluid absorption have been reviewed recently elsewhere. This article focuses on the less-explored problem of severe hypoxic respiratory failure in the late preterm infant and discusses potential strategies for management.

Surfactant Therapy for Meconium Aspiration Syndrome

Meconium aspiration syndrome (MAS) is an important cause of respiratory distress in the term infant. Therapy for the disease remains problematic, and newer treatments such as high-frequency ventilation and inhaled nitric oxide are being applied with increasing frequency. There is a significant disturbance of the pulmonary surfactant system in MAS, with a wealth of experimental data indicating that inhibition of surfactant function in the alveolar space is an important element of the pathophysiology of the disease. This inhibition may be mediated by meconium, plasma proteins, haemoglobin and oedema fluid, and, at least in vitro, can be overcome by increasing surfactant phospholipid concentration. These observations have served as the rationale for administration of exogenous surfactant preparations in MAS, initially as standard bolus therapy and, more recently, in association with therapeutic lung lavage. Bolus surfactant therapy in ventilated infants with MAS has been found to improve oxygenation in most studies, although there are a significant proportion of nonresponders and in many cases the effect is transient. Pooled data from randomised controlled trials of surfactant therapy suggest a benefit in terms of a reduction in the requirement for extracorporeal membrane oxygenation (relative risk 0.48 in surfactant-treated infants) but no diminution of air leak or ventilator days. Current evidence would support the use of bolus surfactant therapy on a case by case basis in nurseries with a relatively high mortality associated with MAS, or the lack of availability of other forms of respiratory support such as high-frequency ventilation or nitric oxide. If used, bolus surfactant should be administered as early as practicable to infants who exhibit significant parenchymal disease, at a phospholipid dose of at least 100 mg/kg, rapidly instilled into the trachea. Natural surfactant or a third-generation synthetic surfactant should be used and the dosage repeated every 6 hours until oxygenation has improved. Lung lavage with dilute surfactant has recently emerged as an alternative to bolus therapy in MAS, which has the advantage of removing surfactant inhibitors from the alveolar space in addition to augmenting surfactant phospholipid concentration. Combined animal and human data suggest that lung lavage can remove significant amounts of meconium and alveolar debris, and thereby improve oxygenation and pulmonary mechanics. Arterial oxygen saturation inevitably falls during lavage but has been noted to recover relatively rapidly, even in infants with severe disease. Several randomised controlled trials of surfactant lavage in MAS are underway, and until the results are known, lavage must be considered an unproven and experimental therapy.

Surfactant lavage and replacement in meconium aspiration syndrome with pulmonary hemorrhage

We reported two case studies of meconium aspiration syndrome (MAS) with pulmonary hemorrhage in which we applied surfactant lavage and replacement. Surfactant lavage and replacement of MAS with pulmonary hemorrhage appears to be effective and safe adjunctive therapy. The relatively small replacement doses of surfactant required may be attributed to surfactant lavage followed by surfactant replacement. We suggest that surfactant lavage and replacement of MAS with pulmonary hemorrhage should be further investigated by a randomized controlled trial.

Effects of surfactant treatment on gas-exchange and clinical course in near-term newborns with RDS

OBJECTIVE

To determine if there is a relationship between acute improvement in pulmonary gas exchange and surfactant use in near-term (35-39 weeks' gestation) infants with respiratory distress syndrome (RDS).

METHODS

We examined retrospectively the records of 54 infants admitted during a 15 month period who were 35 or more weeks of gestation, and who demonstrated all the diagnostic features of RDS. Data analyzed included: birth weight; gestational age; Apgar scores; calculated alveolar to arterial oxygen gradient (AaDO2) and oxygenation index (0I); hours of life at intubation; surfactant administration; complications with surfactant administration; use of inotropic medications.

RESULTS

A total of 30 of infants were treated with intubation and surfactant administration. Mean FiO2 at the time of surfactant administration was 0.96. The AaDO2 decreased from 64.0 +/- 14.8 kPa to 41.8 +/- 22.5 kPa by 6 hrs (p < 0.0001). There was no relationship between age at treatment (6-89 hr) and response to treatment as measured by changes in AaDO2, FiO2, or oxygenation index (OI).

CONCLUSIONS

Near-term infants with severe RDS often respond to exogenous surfactant, suggesting a functional deficiency of endogenous surfactant at a "late" stage in their disease process. We speculate that delayed treatment may still be effective in these patients.

Surfactant and surfactant inhibitors in meconium aspiration syndrome

Surfactant indices and inhibitors were measured in lung lavage fluid from 8 infants with meconium aspiration syndrome (MAS) who were receiving mechanical ventilation and 11 healthy control subjects. Surfactant phospholipid and surfactant protein A content in MAS was not different from that of control subjects, but concentrations of total protein, albumin, and membrane-derived phospholipid were elevated. All infants with MAS had hemorrhagic pulmonary edema. These findings reinforce the notion of MAS as a toxic pneumonitis with epithelial disruption and proteinaceous exudation.

Multicenter study of surfactant (beractant) use in the treatment of term infants with severe respiratory failure. Survanta in Term Infants Study Group

OBJECTIVE

The purpose of this study was to determine whether surfactant (beractant) administration to term newborns in respiratory failure and at risk for requiring extracorporeal membrane oxygenation (ECMO) treatment would significantly reduce the incidence of severe complications through 28 days of age and the need for ECMO.

STUDY DESIGN

A multicenter (n = 44), randomized, double-blind, placebo-controlled trial was conducted. Infants weighing 2000 gm or more with gestational ages of 36 weeks or greater were stratified by diagnosis (meconium aspiration syndrome, sepsis, or idiopathic persistent pulmonary hypertension of the newborn) and oxygenation index (15 to 22, 23 to 30, 31 to 39) and then randomly assigned to receive four doses of beractant, 100 mg/kg (n = 167), or air placebo (n = 161) before ECMO treatment and four additional doses during ECMO, if ECMO was required. The incidence of untoward effects (including hemorrhagic, neurologic, pulmonary, renal, cardiovascular, infectious, metabolic, and technical complications) occurring before and after randomization and through 28 days of age or discharge were recorded.

RESULTS

The two treatment groups were comparable in baseline parameters, including birth weight, sex, gestational age, oxygenation index, and primary diagnosis. There was no difference in the incidence of severe complications. The need for ECMO therapy was significantly less in the surfactant group than in the placebo group (p = 0.038); this effect was greatest within the lowest oxygenation index stratum (15 to 22; p = 0.013).

CONCLUSIONS

Use of surfactant, particularly in the early phase of respiratory failure, significantly decreases the need for ECMO in the treatment of term newborns with respiratory failure, without increasing the risk of complications.

Combined effects of sphingomyelin and cholesterol on the hydrolysis of emulsion particle triolein by lipoprotein lipase

Sphingomyelin (SM) is one of the major lipids in lipoproteins. However, its function in lipoprotein metabolism is unknown. In an attempt to understand the role that this lipid plays in modulation of lipoprotein lipase (LPL)-mediated hydrolysis, triolein-based emulsion particles containing 15% (physiological concentration) and 30% of the phospholipid content as SM together with phosphatidyl choline were used as substrate for the enzyme. Using a continuous fluorescence displacement assay to measure triglyceride (triolein) hydrolysis, it is shown that LPL activity was not modified by physiological concentrations of SM. However, under these assay conditions the presence of 30% SM inhibited LPL hydrolysis. SM and cholesterol (a normal component of the lipoprotein surface monolayer) become closely associated in phospholipid monolayers and bilayers. Incorporation of cholesterol into emulsion particles containing only PC increased LPL activity, but this increase was reduced by the additional presence of a physiological concentration (15%) of SM. These model studies suggest that the ratio, cholesterol:SM, in the monolayer may regulate the hydrolytic activity of the LPL. The production of ceramide by sphingomyelinase pre-treatment of emulsion particles containing SM leads to a two- to three-fold increase in LPL activity. This effect was dependent on sphingomyelinase concentration and time of pre-incubation and was not seen with cholesterol containing substrates. The ability of apolipoprotein CII to enhance LPL-catalysed triolein hydrolysis was not affected by the presence of SM; however, the stimulatory effect of this apolipoprotein was attenuated by pre-treatment of emulsion particles with sphingomyelinase. In summary, physiological concentrations of SM can inhibit the hydrolysis of cholesterol-containing emulsion particles; while pre-treatment of SM containing emulsion particles with sphingomyelinase in the absence of cholesterol can increase LPL-mediated triglyceride hydrolysis.

Surfactant content in children with inflammatory lung disease

OBJECTIVE

To determine surfactant profiles of tracheal secretions in mechanically ventilated children with respiratory failure secondary to bacterial pneumonia, viral pneumonitis, adult respiratory distress syndrome (ARDS), and cardiopulmonary bypass.

DESIGN

Prospective, cohort study.

SETTING

Tertiary, multidisciplinary, pediatric intensive care unit.

PATIENTS

One hundred twenty pediatric patients with respiratory failure requiring mechanical ventilation.

INTERVENTIONS

Routine tracheal aspirates were collected from children with bacterial pneumonia, viral pneumonitis, ARDS, postcardiopulmonary bypass, and a postsurgical control group. Samples were obtained on days 1, 2, 3, after every week of intubation and on the day of extubation.

MEASUREMENTS AND MAIN RESULTS

The tracheal aspirates were analyzed by high-performance liquid chromatography for lecithin/sphingomyelin rations and by enzyme-linked immunosorbent assay for surfactant proteins A and B. Lung compliance and the oxygenation index were measured on each day of sample collection. On day 1, patients with bacterial pneumonia, viral pneumonitis, and ARDS had decreased lecithin/sphingomyelin ration (p < .001), and those patients with bacterial pneumonia and viral pneumonitis had decreased surfactant protein A/protein concentration (p < .001). The lecithin/sphingomyelin ratios and surfactant protein A/protein concentration were significantly different among the groups (p < .001), with the bacterial pneumonia and viral pneumonitis groups having higher lecithin/sphingomyelin ratios and increased surfactant protein concentrations before extubation. Pulmonary compliance was lower and the oxygenation index was higher than controls (p < .001) in patients with bacterial pneumonia, viral pneumonitis, and ARDS. Pulmonary compliance was correlated weakly with lecithin/sphingomyelin ratio (r2 = .11, p < .001) and surfactant protein A/protein concentration (r2 = .03, p < .05). Surfactant protein B was similar in the diagnostic groups. Surfactant content in tracheal secretions from cardiopulmonary bypass patients was equivalent to controls.

CONCLUSION

Abnormal tracheal aspirate surfactant phospholipids and surfactant protein A were noted in children with bacterial pneumonia, viral pneumonitis, and ARDS, but not in children on cardiopulmonary bypass.