Delayed compliance increase in infants with respiratory distress syndrome following synthetic surfactant

Development of a New Dry Powder Aerosol Synthetic Lung Surfactant Product for Neonatal Respiratory Distress Syndrome (RDS) - Part II: In vivo Efficacy Testing in a Rabbit Surfactant Washout Model

PURPOSE

Surfactant therapy incorporates liquid bolus instillation via endotracheal tube catheter and a mechanical ventilator in preterm neonates with respiratory distress syndrome (RDS). Aerosolized surfactants have generated interest and conflicting data on the efficacy of phospholipid (PL) dose requirements. We developed and characterized a synthetic lung surfactant excipient enhanced growth (SLS-EEG) dry powder aerosol product. In this study, we compare the in vivo performance of the new aerosol product with standard-of-care liquid instillation.

METHODS

Juvenile rabbits were sedated, anesthetized, intubated, and ventilated. Endogenous surfactant was depleted via whole lung lavage. Animals received either a standard dose of liquid Curosurf (200 mg PL/kg) instilled via a tracheal catheter, SLS-EEG powder aerosol (60 mg device loaded dose; equivalent to 24 mg PL/kg), or sham control. Gas exchange, lung compliance, and indices of disease severity were recorded every 30 min for 3.5 h and macro- and microscopy images were acquired at necropsy.

RESULTS

While aerosol was administered at an approximately tenfold lower PL dose, both liquid-instilled and aerosol groups had similar, nearly complete recoveries of arterial oxygenation (PaO2; 96-100% recovery) and oxygenation index, and the aerosol group had superior recovery of compliance (P < 0.05). The SLS-EEG aerosol group showed less lung tissue injury, greater uniformity in lung aeration, and more homogenous surfactant distribution at the alveolar surfaces compared with liquid Curosurf.

CONCLUSIONS

The new dry powder aerosol SLS product (which includes the delivery strategy, formulation, and delivery system) has the potential to be a safe, effective, and economical alternative to the current clinical standard of liquid bolus surfactant instillation.

Surfactant replacement therapy

Surfactant replacement therapy (SRT) has a proven role in the treatment of neonatal respiratory distress syndrome and severe meconium aspiration syndrome in infants, and may have a role in the treatment of pediatric patients with ARDS. Although newer delivery mechanisms and strategies are being studied, the classic surfactant administration paradigm consists of endotracheal intubation, surfactant instillation into the lung, and stabilization with mechanical ventilation followed by extubation when stable on low respiratory support. Currently, this surfactant administration procedure is bundled into Current Procedural Terminology (CPT) codes used when providing intensive care. A specific CPT code for surfactant administration is scheduled to be introduced in 2007. This article reviews clinical issues in SRT and the practice management considerations necessary to provide this care.

Lung function tests in neonates and infants with chronic lung disease: lung and chest-wall mechanics

This is the fifth paper in a review series that summarizes available data and critically discusses the potential role of lung function testing in infants and young children with acute neonatal respiratory disorders and chronic lung disease of infancy (CLDI). This review focuses on respiratory mechanics, including chest-wall and tissue mechanics, obtained in the intensive care setting and in infants during unassisted breathing. Following orientation of the reader to the subject area, we focused comments on areas of enquiry proposed in the introductory paper to this series. The quality of the published literature is reviewed critically with respect to relevant methods, equipment and study design, limitations and strengths of different techniques, and availability and appropriateness of reference data. Recommendations to guide future investigations in this field are provided. Numerous different methods have been used to assess respiratory mechanics with the aims of describing pulmonary status in preterm infants and assessing the effect of therapeutic interventions such as surfactant treatment, antenatal or postnatal steroids, or bronchodilator treatment. Interpretation of many of these studies is limited because lung volume was not measured simultaneously. In addition, populations are not comparable, and the number of infants studied has generally been small. Nevertheless, results appear to support the pathophysiological concept that immaturity of the lung leads to impaired lung function, which may improve with growth and development, irrespective of the diagnosis of chronic lung disease. To fully understand the impact of immaturity on the developing lung, it is unlikely that a single parameter such as respiratory compliance or resistance will accurately describe underlying changes. Assessment of respiratory mechanics will have to be supplemented by assessment of lung volume and airway function. New methods such as the low-frequency forced oscillation technique, which differentiate the tissue and airway components of respiratory mechanics, are likely to require further development before they can be of clinical significance.

Effect of lung water content, manipulated by intratracheal furosemide, surfactant, or a mixture of both, on compliance and viscoelastic tissue forces in lung-lavaged newborn piglets

OBJECTIVE

To study the impact of lung water content and its reduction by a topically applied diuretic on respiratory and lung tissue mechanics in comparison with surfactant administration in surfactant-deficient newborn piglets with lavage-induced lung injury.

DESIGN

Controlled, randomized study.

SETTING

Animal research facility.

SUBJECTS

Newborn piglets. TREATMENT Piglets were surfactant depleted by lung lavage and, after a pretreatment period, randomly treated with intratracheal furosemide, furosemide and surfactant, or with surfactant alone.

MEASUREMENTS AND MAIN RESULTS

Dynamic compliance (C(DYN)), static compliance (C(ST)), stress-adaptation pressures (P(DIFF)) and post mortem lung water content were determined. Static compliance in the furosemide-surfactant group was not significantly higher than in the surfactant group. At the end of the study, C(ST) did not differ between the three groups because C(ST) in the furosemide group had increased to values similar to those of the surfactant-containing treatment groups: C(ST) F+S: 0.73 +/- 0.2 mL/cm H2O/kg body weight (BW); C(ST) S: 0.61 +/- 0.11 mL/cm H2O/kg BW; and C(ST) F: 0.60 +/- 0.19 mL/cm H2O/kg BW). Compliance was inversely and P(DIFF) was directly correlated to lung water (LW) content (C(ST) vs. LW: r2 = .59, p = .001; C(DYN) vs. LW: r2 = .49, p = .006; P(DIFF) vs. LW: r2 = .37, p = .059), independent of the type of treatment. Changes in C(ST) and C(DYN) were inversely related to changes in P(DIFF). Intrapulmonary furosemide was more rapidly absorbed when administered to the surfactant-depleted lung alone compared with the mixture with surfactant, and intrapulmonary furosemide had a rapid systemic effect.

CONCLUSION

Although the combination of surfactant with a diuretic failed to increase respiratory compliance to a significantly larger extent than surfactant alone, furosemide at the end of the study increased respiratory compliance to a level similar to surfactant-containing treatments. Lung water content and, to a lesser extent, the absence or presence of surfactant appeared to determine lung mechanics, and its impact on lung mechanics was similar to surfactant administration.

Effects of mean and swing pressures on piston-type high-frequency oscillatory ventilation in rabbits with and without acute lung injury

To determine whether low mean airway pressure (MAP) and/or stroke volume (SV) settings cause lung injury during piston-type high-frequency oscillatory ventilation (HFOV), we investigated the influence of various combinations of MAP and SV on the amplitude of the pressure swing at four different sites in the normal lung of rabbits. We also examined the effects of these factors on progression of lung injury in lavaged surfactant-deficient lungs. We measured changes in the mean pressure (MP) and swing pressure (SP) during HFOV at MAPs ranging between 5-30 cm H2O in combination with SVs ranging from 5-30 mL in 13 rabbits at four different sites: 1) the proximal airway, 2) the distal end of the endotracheal tube, 3) the bronchi, and 4) the pleural space. Lung lavage was performed in 8 rabbits and differences in MP and SP between normal and surfactant-deficient rabbits were investigated. In the remaining 5 rabbits, lungs were lavaged and subjected to two trials of sustained inflation to 30 cm H2O for 15 s to reverse atelectasis, and the resulting SP was measured. In normal lungs, SP increased at the bronchial and pleural sites as MAP was increased. Alterations in SV did not affect MP in normal or lavaged lungs. In the lavages, surfactant-deficient lungs at MAPs < or = 15 cm H2O, there were significant increases in SP at the distal end of the endotracheal tube and the bronchial sites. SP decreased to the prelavaged level following sustained inflation to 30 cm H2O for 15 s. We conclude that low MAP settings are insufficient to open alveoli in the low-compliance lung and allow for development of atelectasis rather than air trapping. SP was markedly increased in the presence of atelectasis, possibly leading to excessive expansion of the airway. In the clinical setting, such overexpansion of the distal airways may contribute to lung injury. Our findings suggest that physicians should use caution in reducing MAP during piston-type HFOV until lung compliance has normalized, especially in infants with respiratory distress syndrome.

Tracheal aspirate surface tension in babies with hyaline membrane disease: effects of synthetic surfactant replacement

Our objective was to determine changes in surface tension of tracheal aspirate over the first 4-5 days of life in babies with hyaline membrane disease, with and without synthetic surfactant replacement. Tracheal aspirates were collected prior to and for 96-108 hr after initiation of a randomized double-blind trial of synthetic surfactant (EXOSURF Neonatal) or air-treated control patients. Using the captive bubble technique, we measured minimum surface tension (initial adsorption, first quasi-static compression, dynamic cycling at 30 cpm, second quasi-static compression and 5 min after quasi-static compressions) in 39 surfactant-treated and 44 control babies. We also compared minimum surface tension with the respiratory support provided. Twelve hours after one dose of synthetic surfactant, minimum surface tension on first quasistatic compression decreased significantly from 20.9+/-1.4 to 17.6+/-1.3 mN/m compared to air-treated babies, who did not show any change. Reduction in minimum tracheal aspirate surface tension on first quasi-static compression and during dynamic cycling over 48-60 hr occurred more rapidly in surfactant-treated babies. Ventilator support did not correlate with minimum tracheal aspirate surface tension. We conclude that treatment of babies with synthetic surfactant improved tracheal aspirate minimum surface tension within 12 hr of the first dose and for the next 48-60 hr.

Spiking Survanta with synthetic surfactant peptides improves oxygenation in surfactant-deficient rats

The hypothesis that the in vivo function of Survanta (Beractant) can be improved by supplementation with synthetic surfactant peptides B and C was tested in a surfactant-deficient rat model. Full length surfactant protein-B (SP-B1-78) (B) and palmitoylated surfactant protein-C (SP-C1-35) (C), and synthetic KL4 peptide were added to Survanta after extraction, creating extracted Survanta (ES) with 1% B, 2% B, and 2% B plus 1% C, or mixed with Survanta without extraction, creating modified Survanta (S) with 2% B, 2% B plus 1% C, and 2% KL4. Adult rats were ventilated with 100% oxygen, tidal volumes (VT) of 7.5 ml/kg and a rate of 60/min, and were lavaged until the PaO2 dropped below 80 mm Hg, when 100 mg/kg of surfactant was instilled. After 15 to 60 min of ventilation, pressure-volume (P-V) curves were generated in situ. Instillation of ES or S with 2% B plus 1% C led to the greatest increase in oxygenation, closely followed by ES and S with 2% B, and more distantly by S plus 2% KL4. TLC was comparable among the ES and S groups, but greater than that of air-placebo controls. These data suggest that spiking of Survanta with synthetic SP-B and SP-C increased oxygenation more effectively than B or KL4 alone in this surfactant-deficient rat model.

Changes in lung volume, compliance and oxygenation in the first 48 hours of life in infants given surfactant

Changes in lung volume (functional residual capacity) compliance and oxygenation in the first 48 h of life in infants given surfactant replacement therapy were studied. Fourteen infants, median gestational age 30 weeks (range 26-35), who were given two doses of an artificial surfactant (Exosurf) at approximately 2 h of age and 12 h later, were examined. Compared to baseline, functional residual capacity, compliance of the respiratory system and oxygenation, significantly improved at 24 and 48 hours, but specific compliance (compliance/functional residual capacity) decreased. The greater changes, however, were experienced between baseline and 24 h. These data suggest that the elevation of FRC over the first 48 hours of life may be due to increased distension as well as recruitment of alveoli.

One-year outcome in 232 premature infants with birth weights of 750 to 1249 grams and respiratory distress syndrome randomized to rescue treatment with two doses of synthetic surfactant or air placebo. Canadian Exosurf Neonatal Study Group

A randomized, double-blind, placebo-controlled trial was performed in 13 hospitals in Canada to assess whether two rescue doses of a synthetic surfactant (Exosurf Neonatal) would reduce mortality and morbidity rates in neonates with respiratory distress syndrome who weighed from 750 to 1249 gm. As part of the original trial design, double-blind follow-up evaluations were performed at 1-year adjusted age. A total of 118 patients who received air placebo and 114 patients who received synthetic surfactant were evaluated at 1 year. Growth and development in the two groups were equivalent. Scores on the Bayley Scales of Infant Development were within the normal range for both groups (mental development index, 90 +/- 22 vs 92 +/- 22; psychomotor development index, 81 +/- 19 vs 87 +/- 22 for the air placebo and synthetic surfactant groups, respectively). However, in both groups the proportion of infants with any impairment (air placebo group, 43 of 118 (36%); synthetic surfactant group, 41 of 114 (36%) and severe impairment (air placebo group, 29 of 118 (25%); synthetic surfactant group, 21 of 114 (18%)) was substantial. We conclude that two rescue doses of synthetic surfactant in infants with RDS who weighed 750 to 1249 gm had no detrimental effect on developmental outcome or late morbidity. No long-term benefits to 12-months corrected age were observed with the use of surfactant in this weight class. Larger studies or meta-analyses of existing trials will be required to determine if there are any late outcome advantages associated with rescue use of synthetic surfactant in infants weighing 700 to 1249 gm.

Partial liquid ventilation in premature lambs with respiratory distress syndrome: efficacy and compatibility with exogenous surfactant

OBJECTIVE

To determine the efficacy of partial liquid ventilation (PLV) by means of a medical-grade perfluorochemical liquid, perflubron (LiquiVent), in premature lambs with respiratory distress syndrome (RDS). Further, to determine the compatibility of perflubron with exogenous surfactant both in vitro and in vivo during PLV.

DESIGN

Prospective, randomized, controlled study, with in vitro open comparison.

SUBJECTS

Twenty-two premature lambs with RDS.

INTERVENTIONS

In vitro assays were conducted on three exogenous surfactants before and after combination with perflubron. We studied four groups of lambs, which received one of the following treatment strategies: conventional mechanical ventilation (CMV); surfactant (Exosurf) plus CMV; PLV; or surfactant plus PLV.

MEASUREMENTS AND MAIN RESULTS

In vitro surface tension, measured for three exogenous surfactants, was unchanged in each animal after exposure to perflubron. Lung mechanics and arterial blood gases were serially measured. All animals treated with PLV survived the 5 hours of experiment without complication; several animals treated with CMV died. During CMV, all animals had marked hypoxemia and hypercapnia. During PLV, arterial oxygen tension increased sixfold to sevenfold within minutes of initiation, and this increase was sustained; arterial carbon dioxide tension decreased to within the normal range. Compliance increased fourfold to fivefold during PLV compared with CMV. Tidal volumes were increased during PLV, with lower mean airway pressure. Resistance was similar for both CMV and PLV; there was no difference with surfactant treatment.

CONCLUSIONS

We conclude that PLV with perflubron improves lung mechanics and gas exchange in premature lambs with RDS, that PLV is compatible with exogenous surfactant therapy, and that, as a treatment for RDS in this model, PLV is superior to the surfactant studied.

Exosurf rescue surfactant improves high ventilation-perfusion mismatch in respiratory distress syndrome

OBJECTIVE

To assess ventilation/perfusion (VA/Q) mismatch of the high type, following rescue surfactant therapy for respiratory distress syndrome.

HYPOTHESIS

Surfactant therapy reduces such mismatch.

DESIGN

Randomized, double-blind, placebo-controlled study, assessing VA/Q with the arterial-alveolar difference of CO2 tension (P(a-A)CO2). This difference was determined with capnometry and arterial blood gases, using the equation: P(a-A)CO2 equals arterial CO2 minus alveolar CO2 partial pressure.

SETTING

A level III nursery.

PATIENTS

Ten intubated infants with respiratory distress syndrome.

INTERVENTION

Infants were randomized to each receive two doses of surfactant or two doses of air placebo.

RESULTS

P(a-A)CO2 improved after surfactant and worsened after placebo (P = 0.0021), comparing slopes of 12-hr regression lines. A similar pattern occurred with oxygenation. These changes in P(a-A)CO2 and in oxygenation were minimally correlated within the surfactant group.

CONCLUSION

Exosurf rescue surfactant reduced VA/Q mismatch of the high type, over several hours.

Early changes in lung function and response to surfactant replacement therapy

Dynamic respiratory system compliance (Cdyn) was measured in 44 preterm babies before, immediately after, and for 96 h following administration of artificial surfactant (Exosurf). There was no significant change in Cdyn for the whole group over the entire study period. Subdivision into three groups on the basis of Cdyn prior to surfactant revealed a significant and sustained deterioration in lung function in those babies with the highest starting compliance and a significant and sustained improvement in those with the lowest compliance. Inspired oxygen and alveolar/arterial oxygen gradient also exhibited significant differences with least improvement in the babies with the best initial lung function and most improvement in the babies with worst initial lung function and most improvement in the babies with worst initial lung function. Despite clear initial differences in clinical status, neither long-term oxygen requirements nor the incidence of chronic lung disease differed significantly between the three groups. We conclude that the response of an individual baby to the administration of surfactant is, in part, determined by the lung function before surfactant is administered. Babies with higher initial lung compliance are more likely to deteriorate after administration and caution should be used before selection of such babies for surfactant treatment.