Prediction of the acute response to surfactant therapy by pulmonary function testing

Surfactant

Exogenous pulmonary surfactant, widely used in neonatal care, is one of the best-studied treatments in neonatology, and its introduction in the 1990s led to a significant improvement in neonatal outcomes in preterm infants, including a decrease in mortality. This chapter provides an overview of surfactant composition and function in health and disease and summarizes the evidence for its clinical use.

Current surfactant use in premature infants

Exogenous surfactant therapy has been a significant advance in the management of preterm infants with RDS. It has become established as a standard part of the management of such infants. Both natural and synthetic surfactants lead to clinical improvement and decreased mortality, with natural surfactants having additional advantages over currently available synthetic surfactants. The use of prophylactic surfactant administered after initial stabilization at birth to infants at risk for RDS has benefits compared with rescue surfactant given to treat infants with established RDS. In infants who do not receive prophylaxis, earlier treatment (before 2 hours) has benefits over later treatment. The use of multiple doses of surfactant is a superior strategy to the use of a single dose, whereas the use of a higher threshold for retreatment seems to be as effective as a low threshold. Adverse effects of surfactant therapy are infrequent and usually not serious. Long-term follow-up of infants treated with surfactant in the neonatal period is reassuring. In the future we are likely to see the development of new types of surfactants. Further research is required to determine the optimal use of surfactant in conjunction with other respiratory interventions.

Therapeutic aerosols and airway secretions

Despite common misconceptions, airway mucus is not an impermeable barrier that entraps and clears all inhaled material. To be therapeutically effective, an aerosol medication must efficiently deposit in the airway and then diffuse or translocate across the mucous barrier while retaining bioactivity. Characteristics of aerosols that affect their ability to penetrate the mucous barrier include particle charge, solubility, and size. Aerosol medications can also change the biophysical properties of secretions and influence how rapidly mucus is cleared. Surfactant is probably responsible for the displacement of inhaled particles through the mucous layer (Schürch et al., 1990). As well, exogenous surfactant has been shown to increase tracheal mucociliary clearance in anesthetized dogs (de Sanctis et al., 1994) and to improve the mucociliary clearability of secretions from babies with neonatal respiratory distress syndrome (Rubin et al., 1992). Using measurements of sputum-substrate contact angle and interfacial tension, cystic fibrosis (CF) sputum had been shown to have an abnormally high adhesion tension, (Girod et al., 1992). We assessed the in vitro effects of synthetic surfactant (Exosurf, Burroughs-Wellcome, Research Triangle Park, NC) on the physical and transport properties of sputum from 15 patients with CF and 30 patients with stable chronic bronchitis (CB). The concentration of ExosurfTM used was 13.5 mg of DPPC/ml corresponding to the reconstituted concentration for aerosol administration. The sputum was divided so that aliquots each were treated with Exosurf and amphibian Ringer's solution layered on the sputum at a concentration of 1:5 v/v for 15 min at 24 degrees C. There was a reduction in spinnability (p < 0.0001) in CF sputum from baseline and a fall in adhesion tension (0.05 < p < 0.1) with Exosurf treatment. Potentially important increases in both mucociliary and cough clearability were noted without associated changes in sputum rheology. In bronchitis sputum, surfactant reduced adhesiveness (p < 0.01), but this was not associated with improved clearability. Adhesive forces are involved in ciliary coupling and cough transport. This raises the possibility of using surfactant as a mucokinetic agent in the therapy of chronic suppurative lung disease.

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.

The surface and transport properties of meconium and reconstituted meconium solutions

Passage of meconium in utero and subsequent pulmonary aspiration of meconium admixed with amniotic fluid is a major cause of neonatal respiratory distress. Airway clearance is the first defense of the lung, and clearance is dependent on the bulk physical (rheologic) as well as the surface properties of airway material. We therefore evaluated the surface adhesive properties and the transport properties of freshly passed meconium and of two dilutions of reconstituted, blended, meconium as used to mimic the effect of meconium passage into the amniotic fluid in animal models of meconium aspiration syndrome. Reconstituted and fresh meconium had similar physical and transport properties, including an extremely high interfacial (adhesion) tension and very poor transportability by either airflow or cilia. The similarities between the freshly passed and reconstituted meconium suggest that the latter is an adequate substitute for use in animal models of meconium aspiration syndrome. The high adhesiveness of meconium suggests a potential role for surfactant administration as an adhesive to improve airway clearance after meconium aspiration.

Factors affecting the neonatal response to artificial surfactant

OBJECTIVE

To define the individual neonatal response to the artificial surfactant, Exosurf, and factors that may influence the response.

METHODOLOGY

Eighty-two consecutive, preterm neonates with respiratory distress syndrome, who received Exosurf at < 12 h of age were studied. Their response was categorized from the graphical change in the oxygenation index with postnatal age, for 12 h after each of two doses of surfactant and assessed independently by two observers. Clinical factors were analysed for their effect on the four pre-defined categories of response, namely: none; mild; good: relapsed; and good: sustained.

RESULTS

Within the first 12 h, 11% of the neonatas showed no response, 5% a mild response and 84% a good response, but 34% relapsed. By 24 h, 6% still showed no response (all died), 11% showed a mild response and 83% a good response, of whom half relapsed. At 24 h, no response was significantly associated with low gestational age and asphyxia, mild response with less severe lung disease. According to the response there was a gradation in the risk of death during the first week.

CONCLUSIONS

The response to Exosurf can be individually and reproducibly categorized and demonstrated that 83% of neonates had a good response but half relapsed. No response was associated with extreme prematurity and asphyxia.

Lung function testing--useless in ventilated newborns?

Several methods have been used for lung function testing in the ventilated newborn. The interest in the field has been stimulated by the recent appearance of commercially available equipment for assessment of mechanical parameters and of functional residual capacity in this group. Nevertheless, lung function testing is rarely used as a clinical routine, even such simple variables as tidal volume and minute ventilation. Among the many possible reasons for this condition, the fragile nature of the infants and the hands-off policy usually exercised, the difficulties in measuring flow accurately, and the complexity of the present methods deserve special attention. In order to change this situation more work needs to be done to elucidate basal physiology of the ventilated lung and the relationships between ventilator settings, lung function and side-effects in different conditions. If then sufficiently simple, safe and accurate methods to assess the most important functions can be offered, lung function testing would be likely to become a useful component of routine care in future neonatal intensive care.