Experimental findings on the ventilatory management after surfactant therapy

The use of surfactant replacement therapy in infants suffering from respiratory distress syndrome has significantly reduced mortality. However, the lack of an optimal strategy for ventilatory management of these patients results in a high incidence of chronic lung disease, which is related to ventilator-induced injury and alveoarization arrest. In addition to lung protective ventilation strategies (i.e. lowering tidal volume), lung volume recruitment maneuvers have been introduced to expand intrathoracic lung volume and to take on unventilated lung regions. Such a ventilation strategy could improve the results of surfactant therapy, maybe allowing a better redistribution of the surfactant.

Changes in clinical practice, such as prenatal steroid administration, use of surfactants, new ventilator strategies and aggressive management of the patent ductus arteriosus, have improved in the survival of very immature newborns. This has not affected the incidence of BPD, but it has certainly changed the clinical manifestations of the disease. As a result, a redefinition of the problem is needed and a new therapeutical approach has to be developed with a major regard to long term consequences of the disease.

The study presented in this article has evaluated the effects of surfactant instillation on regional lung ventilation in an animal model of acute lung injury while applying two different ventilation strategies to the postsurfactant period. The novelty of this analysis lies in the use of a radiation-free imaging technique: electrical impedance tomography (EIT). EIT can detect pulmonary gas volume changes in restricted lung regions, thus allowing a fine functional estimation of the different pulmonary areas.

Lung injury was induced in sixteen newborn piglets and a first recruitment maneuver was performed in all of them, in order to assess an optimum positive end-expiratory pressure (PEEP) which was set as ventilation pressure. Thereafter they were all treated with surfactant, administered either in an "open lung" (i.e. through an endotracheal tube with ventilation) or in a "closed" lung (i.e. without endotracheal tube and ventilation). Then, they were divided into two groups: i) the Recruitment group, in which a recruitment maneuver was performed again, settling a new PEEP value for the mechanical ventilation ii) the No Recruitment group, for which ventilation went on at the previously assessed PEEP value.

During the experiment, regional lung ventilation was monitored by EIT in each animal, in 64 regions of interest in both lungs, at six different times. The accurate analysis of the researchers, comparing the Recruitment with the No Recruitment group, let to the following results:

• both groups of animals had an improvement in ventilation after the first recruitment maneuver;
• the different way of surfactant administration did not vary the ventilation parameters.
• The distribution of ventilation in the Recruitment group was homogeneous between the two lungs;
• the No Recruitment animals had a ventilation asymmetry, with persistent non ventilated areas. Therefore, they were exposed to a ventilator-induced injury.

In conclusion, the results of this experimental study indicate that early lung volume recruitment after surfactant therapy might have an influence on spatial distribution of regional lung ventilation. Thus, the combination of the two treatments needs a further exploration in prevision of clinical applications.

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